Electrolysis of carbon dioxide to carbon monoxide, through which the greenhouse gas could be effectively utilized, using solid oxide electrolysis cells is now attracting much interest. Here, we show for the first time that the redox-stable Sr2Fe1.5Mo0.5O6−δ (SFM) ceramic electronic-ionic conductor can be used as the electrocatalyst to electrolyze and convert 100% CO2 to CO without using any safe gases like H2 and CO. SFM maintained its cubic structure and had an electrical conductivity of 21.39 S cm–1 at 800 °C in 1:1 CO–CO2 atmosphere. Its surface reaction coefficient for CO2 reduction is 7.15 × 10–5 cm s–1 at 800 °C. Compared with those reported for the typical oxide ceramic electrodes, high electrochemical performance has been demonstrated for single phase SFM cathode using 100% CO2 as the feeding gas. For example, a current density of 0.71 A·cm–2 was obtained using a fuel cell supported on LSGM (La0.9Sr0.1Ga0.8Mg0.2O3−δ) electrolyte operated at 800 °C and an applied voltage of 1.5 V. The electrolysis performance was further improved by using SFM–Sm0.2Ce0.8O2−δ composite cathode, and the current density increased to 1.09 A·cm–2 under the same operation conditions. Durability test at 800 °C for 100 h demonstrated a relatively stable performance for CO2 electrolysis under harsh conditions of 100% CO2 without safe gas and above 1 A cm–2 current density, which is seldom achieved in the literature but highly desirable for the commercial application, indicating that SFM is a highly promising ceramic fuel electrode for CO2 electrolysis.Keywords: CO2 reduction; Fuel electrode; Mixed ionic and electronic conductor; Solid oxide electrolysis cells; Sr2Fe1.5Mo0.5O6−δ;

•CaO particles improve oxygen reduction reaction activity of LSCF electrocatalyst.•CaO enhances the oxygen surface exchange coefficient by a factor up to 20.•CaO nanoparticles reduce LSCF electrode resistance and improve cell performance.The oxygen reduction reaction (ORR) on lanthanum strontium cobalt ferrite (LSCF) catalyst is critical for intermediate temperature solid oxide fuel cells (SOFCs). The reaction rate can be effectively improved by addition various nanoparticles including electrocatalysts such as Pd, Ag and mixed electronic-ionic conductors and electrolytes like samaria doped ceria (SDC). This work shows that ORR rate can also be improved by CaO, which is neither catalyst nor conductor. The CaO nanoparticles are deposited to porous LSCF electrodes using the infiltrating technique. No obvious reaction between CaO and LSCF is detected with X-ray diffraction analysis, indicating that CaO is chemically compatible with LSCF in the intermediate-temperature SOFC operation conditions. Impedance spectrum analysis demonstrates that the CaO nanoparticles can effectively reduce the interfacial polarization resistances for both single phase LSCF electrodes and LSCF-SDC composite electrodes. In addition, CaO nanoparticles can improve the peak power densities and reduce the total electrode resistances of single cells consisting of NiO-SDC anodes, SDC electrolytes, and LSCF based cathodes. Further, CaO can increase the oxygen surface exchange coefficient as demonstrated with electrical conductivity relaxation measurement. The improving factor is comparable to those for Rh and Pd catalysts, suggesting it is effective to increase ORR rate by infiltrating CaO nanoparticles.

Cobalt oxide is usually used as a dopant to improve the catalytic activity of Mn and Fe based perovskites such as lanthanum strontium ferrite (LSF) for solid oxide fuel cells. This work presents the catalytic improvement by decorating LSF surface with cobalt oxide, Co3O4, using infiltration technique. X-ray diffraction and high resolution electron microscopy analysis indicate that Co3O4 is chemically compatible with LSF at the intermediate-temperature range for solid oxide fuel cells. Electrochemical impedance spectrum demonstrates substantial reduction in interfacial polarization resistance, from 0.22 to 0.083 Ωcm2 at 700 °C when 5.84 wt. % Co3O4 is infiltrated into the bare LSF electrodes. Further analysing the impedance spectrum with distribution of relaxation time calculation suggests that the performance improvement is associated with the charge-transfer processes of the surface reaction. Meanwhile, the electrochemical conductivity relaxation measurement shows that Co3O4 particles can improve the surface reaction kinetics, increasing the oxygen surface exchange coefficient by a factor of about 5 at 700 °C. In addition, Co3O4 particles can increase the peak power density of the single cells from 0.865 Wcm−2 to 1.3 Wcm−2 at 800 °C with LSF based cathodes. The results clearly demonstrate an alternative mean to use cobalt oxide in improving the catalytic performance of Mn and Fe based perovskites.

Strontium doped lanthanum cobalt ferrite (LSCF) is a widely applied electrocatalyst for the oxygen reduction reaction (ORR) in solid-oxide fuel cells (SOFCs) operated at intermediate temperatures. Sr surface segregation in long-term operation has been reported to have contradicting effects that either degrade or improve the reaction. Thus, it is critical to understand the mechanism of surface Sr compounds on ORR kinetics. This work aims to verify the effect and propose the mechanism by decorating SrCO3 nanoparticles using the infiltration method. Electrochemical conductivity relaxation measurements show that SrCO3 particles improve the chemical oxygen surface exchange coefficient by up to a factor of 100. The electrochemical performance is significantly improved by the infiltration of SrCO3, which is comparable to those obtained by typical electrocatalysts including precious metals such as Pd and Rh. Distribution of relaxation time (DRT) analysis shows that the performance enhancement is strongly related to the improved kinetics of charge transfer and oxygen incorporation processes. Density functional theory calculations show that the surface SrCO3 reduces the O2 dissociation energy barrier from 1.01 eV to 0.33 eV, thus enhancing the ORR kinetics, possibly through changing the charge density distribution at the LSCF–SrCO3 interface.

Solid oxide electrolysis cells (SOECs) can directly convert CO2 to CO and O2 that are important building blocks for chemical production and other applications. However, the use of SOECs for direct CO2 electrolysis has been hampered mainly due to the absence of a stable, highly catalytically active and cost effective cathode (fuel electrode) material. Here we report a ceramic SOEC cathode material of perovskite-structured Sr1.9Fe1.5Mo0.4Ni0.1O6−δ for direct CO2 electrolysis. By annealing at 800 °C in H2, homogeneously dispersed nano-sized NiFe alloy nanoparticles are exsolved from the Sr1.9Fe1.5Mo0.4Ni0.1O6−δ perovskite lattice. The exsolved NiFe nanoparticles significantly enhance the chemical adsorption and surface reaction kinetics of CO2 with the cathode. SOECs with the novel cathode have demonstrated a peak current density of 2.16 A cm−2 under an applied voltage of 1.5 V at 800 °C and have demonstrated stable direct CO2 electrolysis performance during 500 h of operation under current density above 1 A cm−2 at 800 °C.

Bismuth has been doped into mixed ionic–electronic conducting La1.75Sr0.25NiO4+δ (LSN) with the 2D K2NiF4-type structure to evaluate its influence on various properties of the host material, which include its potential use as a SOFC cathode. X-ray powder diffraction indicates that LSN retains its tetragonal structure after doping with 5 mol% bismuth to form La1.65Bi0.1Sr0.25NiO4+δ (LSN–Bi). Bismuth doping profoundly lowers (by ∼150 °C) the sintering temperature of LSN. Both LSN and LSN–Bi show excellent compatibility with electrolytes yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) in terms of thermal expansion and chemical reactivity (<900 °C). The electrical conductivity of both materials is metallic like and reaches values of 99.3 S cm−1 and 100.3 S cm−1 at 550 °C for LSN and LSN–Bi, respectively. The data from electrical conductivity relaxation (ECR) measurements demonstrate that the substitution of lanthanum by bismuth enhances the chemical diffusion coefficient (Dchem) and surface exchange coefficient (kchem) by factors of 2–3. The faster kinetics of oxygen transport exhibited by LSN–Bi relative to parent LSN is reflected by a lower polarization resistance of the former when the electrode performance of both materials is compared in symmetric cells. The corresponding values at 700 °C are 4.2 Ω cm2 and 0.61 Ω cm2 for LSN and LSN–Bi, respectively. High peak power densities are achieved (328 mW cm−2 and 131 mW cm−2 at 700 and 600 °C, respectively), when LSN–Bi is incorporated as the cathode in a fuel cell operated with humidified hydrogen as the fuel and air as the cathode gas. The material is considered a promising candidate for further study.

Oxygen reduction at PrBaCo2O5+δ-Sm0.2Ce0.8O1.9 (PBC-SDC) composite might take place at the PBC-gas two-phase interface (2PB) as well as the PBC-SDC-gas three-phase boundary (3PB). This work investigates oxygen reduction process at the 3PB of PBC-SDC composite using electrical conductivity relaxation technique. The obtained results indicate the reaction at 3PB is much more facile than that at 2PB, which contributes up to 80% of the total oxygen incorporated to the composite when 60% wt. % SDC is introduced. The reaction rate constant at 3PB shows non-linear dependence on 3PB length, suggesting that the oxygen incorporation reaction is not confined at 3PB. It is possible that the oxygen species adsorbed on PBC transfer to the surface of SDC via a spillover phenomenon to combine with the oxygen vacancies from SDC. The introduction of SDC results in increased chemical oxygen ion diffusion coefficient by a factor up to 3.37, and chemical oxygen surface exchange coefficient by a factor up to 3.06 when 60 wt. % SDC is used. The enhanced oxygen transport properties are also demonstrated with symmetrical cells using electrochemical impedance spectroscopy. The interfacial polarization resistance is reduced from 0.281 Ω cm2 for a bare PBC electrode to 0.107 Ω cm2 at 700 °C for a PBC-SDC composite electrode with 60 wt. % SDC.

There is increasing interest in converting CO2/H2O to syngas via solid oxide electrolysis cells (SOECs) driven by renewable and nuclear energies. The electrolysis reaction is usually conducted through Ni–YSZ (yttria stabilized zirconia) cermets, state-of-the-art fuel electrodes for SOECs. However, one obvious problem for practical applications is the usage of CO/H2 safe gas, which must be supplied to maintain the electrode performance. This work reports a safe gas free ceramic electrode for efficient CO2/H2O electrolysis. The electrode has a heterogeneously porous structure with Sr2Fe1.5Mo0.5O6−δ (SFM) electrocatalyst nanoparticles deposited onto the inner surface of the YSZ scaffold fabricated by a modified phase-inversion tape-casting method. The nanostructured SFM–YSZ electrodes have demonstrated excellent performance for CO2–H2O electrolysis. For example, the electrode polarization resistance is 0.25 Ω cm2 under open circuit conditions while the current density is 1.1 A cm−2 at 1.5 V for dry CO2 electrolysis at 800 °C. The performance is comparable with those reported for the Ni–YSZ fuel electrodes, where safe gas must be supplied. In addition, the performance is up to one order of magnitude better than those reported for other ceramic electrodes such as La0.75Sr0.25Cr0.5Mn0.5O3−δ and La0.2Sr0.8TiO3+δ. Furthermore, the electrode exhibits good stability in the short-term test at 1.3 V for CO2-20 vol% H2O co-electrolysis, which produces a syngas with a H2/CO ratio close to 2. The reduced interfacial polarization resistance, high current density, and good stability show that the nanostructured SFM–YSZ fuel electrode is highly effective for CO2/H2O electrolysis without using the safe gas, which is critical for practical applications.

The solid oxide fuel cell (SOFC) is an environmentally-friendly, highly efficient, and fuel adaptable electrochemical conversion device. The Cu–CeO2 material has been recognized as a promising anode material for SOFCs. Although Cu is not a good enough oxidation catalyst, and has a relatively low catalytic activity, a combination of Cu and CeO2 can strengthen catalytic activity and overcome the problems associated with either Cu or CeO2 individually. Our density functional theory (DFT) calculations illustrate that a Cu cluster supported on CeO2(111) suppresses the formation of interface oxygen vacancies, while also enhancing the catalytic activity, and reduces the energy barrier of the H2 oxidation reaction process compared to that of stoichiometric CeO2(111). The three phase boundary (TPB) pathway with the highest energy barrier of 0.836 eV is obviously much lower than the stoichiometric CeO2(111) equivalent with the highest energy barrier of 2.399 eV. Experimentally, temperature programmed reduction (TPR) experiments establish that Cu particles can reduce the reduction reaction temperature for ceria and increase the amount of reduced ceria. In addition, a dramatic comparison between pure ceria and Cu-modified ceria through electrical conductivity relaxation (ECR) experiments quantitatively demonstrates that Cu particles greatly improve the reaction kinetics with the specific oxygen surface exchange coefficient increasing from 1.012 × 10−4 cm s−1 of the bare ceria to 12.180 × 10−4 cm s−1 of the Cu-modified ceria (CeO2–Cu80), which agrees well with the results of the theoretical calculations.

•CuO is used as synergistic catalyst for oxygen reduction reaction on LSCF.•Enhanced rate is attributed to CuO surface and LSCF-CuO-gas boundaries.•The contribution of CuO to incorporated oxygen could reach 78%.•The reduced resistance of LSCF is related to oxygen surface process.•LSCF and CuO could remain chemical compatible in testing condition.This work presents the effect of dispersed copper oxide (CuO) nanoparticles on the oxygen reduction reaction (ORR) on a typical solid oxide fuel cell (SOFC) electrocatalyst, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF). The ORR kinetics were enhanced by a factor up to 4 at 750 °C as demonstrated by electrical conductivity relaxation measurements used to determine the chemical surface exchange coefficient, kchem. The value of kchem increased from 2.6 × 10−5 cm s−1 to 9.3 × 10−5 cm s−1 at 750 °C when the LSCF surface was coated with submicron CuO particles. The enhanced kchem was attributed to additional reactions that occur on the CuO surface and at the LSCF-CuO-gas three-phase boundaries (3PBs) as suggested by the kchem dependence on CuO coverage and 3PB length. This enhancement was further demonstrated by the introduction of CuO nanoparticles into LSCF electrodes. CuO infiltrated electrodes reduced the interfacial polarization resistance from 2.27 Ω cm2 to 1.5 Ω cm2 at 600 °C and increased the peak power density from 0.54 W cm−2 to 0.72 W cm−2 at 650 °C. Electrochemical impedance spectroscopy indicated that the reduced resistance was due to the shrinkage of the low frequency arc, which is associated with the electrochemical surface exchange reaction.

•This approach is free of adjusting parameter.•The DRT does not contain pseudo peaks.•Discontinuities in DRT can be captured.•Well-established algorithm is available.A new Tikhonov regularization approach without adjusting parameters is proposed for reconstructing distribution of relaxation time (DRT). It is capable of eliminating the pseudo peaks and capturing discontinuities in the DRT, making it feasible to resolve the number and the nature of electrochemical processes without making assumptions.

•Conductivity of nanosized LSM depends strongly on the fabricating parameters.•The intrinsic conductivity is estimated with the analytical model.•The impregnated electrode with 5 vol.% LSM exhibits the highest performance.•Ohmic resistance of the impregnated LSM electrode is negligible.Strontium-doped lanthanum manganite (LSM) nanoparticles are deposited onto porous yttria-stabilized zirconia frameworks via an ion impregnation/infiltration process. The apparent conductivity of the impregnated LSM nanostructure is investigated regarding the fabricating parameters including LSM loading, heat treatment temperature, heating rate, and annealing at 750 °C for 400 h. Besides, the conductivity, the intrinsic conductivity as well as Bruggeman factor of the impregnated LSM is estimated from the apparent conductivity using the analytical model for the three-dimensional impregnate network. The conductivity increases with LSM loading while the interfacial polarization resistance exhibits the lowest value at an optimal loading of about 5 vol.%, which corresponds to the largest three-phase boundary as predicted using the numerical infiltration methodology. At the optimal loading, the area specific ohmic resistance of the impregnated LSM is about 0.032 Ω cm2 at 700 °C for a typical impregnated cathode of 30 μm thick. It is only 5.5% of the cathode interfacial polarization resistance and 3.3% of the total resistance for a single cell consisting of a Ni-YSZ support, a 10 μm thick electrolyte and a 30 μm thick cathode, demonstrating that the ohmic resistance is negligible in the LSM impregnated cathode for SOFCs.

La0.4Bi0.4Sr0.2FeO3-δ (LBSF) has previously been demonstrated to show the highest electrochemical performance in a series of bismuth doped lanthanum strontium ferrite La0.8-xBixSr0.2FeO3-δ where 0 ≤ x ≤ 0.8 as the cathode for intermediate-temperature solid oxide fuel cells. The cobalt-free electrocatalyst LBSF is further investigated in the present study using thermogravimetric analysis, oxygen temperature-programmed desorption method, iodometric titration and high-temperature X-ray diffraction refinement methods to reveal its structural properties including oxygen non-stoichiometry coefficient (δ), valence state of Fe, and lattice parameters at the different temperatures. In addition, the oxygen reduction process on the single phase LBSF is explored using the distribution of relaxation time method based on the electrochemical impedance spectroscopy measurements conducted in oxygen partial pressure from 0.01 to 1.0 atm. The LBSF cathode electrochemical performance is effectively improved by cooperating Sm0.2Ce0.8O1.9 (SDC), an oxygen ion conductor, resulting in interfacial polarization resistance less than 0.1 Ω cm2 at 700 °C when SDC is used as the electrolyte.

•The I–V curves over 2PB and 3PB can be derived from the ECR method.•The polarization resistance over 2PB and 3PB can also be derived.•The change in oxygen partial pressure corresponds to an electromotive force.•The current density can be obtained with the surface reaction rate.The oxygen incorporation process is investigated in a new insight based on the electrical conductivity relaxation (ECR) technique. A theoretical method is proposed to qualitatively evaluate the electrochemical performance for the oxygen reduction reaction on the electrocatalyst for solid oxide fuel cells (SOFCs). In this method, the change in oxygen partial pressure to conduct the ECR process corresponds to an electromotive force while the surface reaction rate to a current density. Accordingly, the polarization resistance as well as the current–voltage relation can be derived for the oxygen reduction reaction occurred on the electrocatalyst-gas two-phase interface and at the electrocatslyst-electrolyte-gas three-phase boundary. The method is demonstrated with our previously experimental results on the composite system consisting of lanthanum strontium cobalt ferrite and samaria-doped ceria, which is a typical composite cathode for intermediate-temperature SOFCs.

•Hydrogen electrode with pore gradient is fabricated for SOEC.•An additional graphite layer is employed to remove the relative dense skin layer.•The asymmetric structure can eliminate the steam starvation.Asymmetric structure with pore gradient is fabricated as the hydrogen electrode for millimeter tubular solid oxide electrolysis cells (SOECs). The gradient electrode is achieved with a graphite-assistant phase inversion process using an additional graphite layer to remove the relative dense skin layer. The electrode has demonstrated higher gas permeability than that with normal composite structure. Consequently, the steam starvation (concentration polarization) is successfully eliminated when SOEC is operated at relatively high voltage and low steam concentration. In addition, reduced interfacial polarization resistance and elevated current density are achieved for the tubular SOEC with the unique electrode.

Efficient electrocatalyst for oxygen reduction reaction (ORR) is crucial for the performance improvement of fuel cells and metal-air batteries. However, catalyst with high activity, easy fabrication process, and low cost is still a daunting challenge. In this work, low-cost BaCO3 nanorods have been demonstrated as efficient electrocatalysts toward the ORR in alkaline media for the first time. The activity of BaCO3 nanorods can be further enhanced by hybridizing with reduced graphene oxide (BaCO3/rGO). The mechanism of ORR on the surface of BaCO3 catalyst was investigated via in situ electrochemical Raman spectroscopy (in situ EC-Raman). Our findings suggest that the barium ions on the surface of catalyst play a key role in the adsorption of oxygen molecules and the formation/decomposition of intermediates. This work provides an important insight into the catalytic activity of BaCO3 for ORR, which can serve as a guide for the design of alkali-earth metal-carbonate-based catalyst.

The oxygen release kinetics of mixed-conducting Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O2−δ (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of the SDC phase volume fraction using electrical conductivity relaxation (ECR) under reducing atmospheres, extending our previous work on the oxygen incorporation kinetics of these composites under oxidizing conditions. Gas mixtures of H2/H2O and CO/CO2 were used to control step changes in the oxygen partial pressure (pO2) in the range 10−24 to 10−20 atm. At the conditions of the experiments, oxygen re-equilibration is entirely controlled by the surface exchange kinetics. A model is developed which allows deconvolution of the effective time constant of the relaxation process in terms of the intrinsic contributions of the components to oxygen surface exchange and synergetic contributions caused by heterogeneous interfaces. The oxygen surface exchange kinetics under H2/H2O atmosphere is found to be a weighted average of the intrinsic contributions of SFM and SDC phases. No evidence is found for an enhanced exchange rate at the SFM–SDC–gas triple phase boundaries (TPB). Synergetic contributions arise under CO/CO2 atmosphere, enhancing the rate of oxygen surface exchange up to a factor of 2.4. The obtained results are discussed in terms of the surface microstructure of the composites from image analysis. Overall, the results of this and our previous study confirm that the triple phase boundaries in SFM–SDC composites significantly accelerate the oxygen incorporation kinetics under oxidizing conditions, but only modestly, or even negligibly, influence the oxygen release kinetics under reducing conditions.

•BaCO3 nanoparticles can greatly enhance high-temperature oxygen reduction.•It could greatly reduce the low frequency resistance as well as increase oxygen surface exchange coefficient.•Its catalytic improving factor is higher than those reported for Pd, Rh and Pt.BaCO3 nanoparticles are demonstrated as outstanding electrocatalysts to enhance the high temperature oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFCs). BaCO3 nanoparticles are formed from thermal decomposition of barium acetate, Ba(Ac)2 infiltrated to porous cathode skeleton and shows good chemical compatibility with cathode materials. BaCO3 nanoparticles can greatly reduce the area specific resistance (ASR) of typical SOFC cathode materials, including La0.8Sr0.2FeO3 -δ (LSF), La0.6Sr0.4Co0.2Fe0.8O3 -δ (LSCF) and La0.8Sr0.2MnO3 -δ (LSM). For example at 700 °C, ASR for LSF on yttria-stabilized zirconia (YSZ) electrolyte decreases from 2.95 Ω cm2 to 0.77 Ω cm2 when 12.9 wt.% BaCO3 nanoparticles are deposited on the surface of the porous LSF electrode. Impedance spectra analysis shows that the decrease in ASR mainly comes from the reduction of the low frequency resistance. Furthermore, BaCO3 nanoparticles are found to greatly enhance the oxygen chemical exchange coefficient. Most importantly, it has been found that the catalytic activity of BaCO3 nanoparticles is even higher than those of the precious metals such as Pd, Rh, Pt and Ag, infiltrated into LSF, LSCF and LSM electrodes supported on YSZ electrolytes.

Exploring chemically stable and high proton conductive electrolyte materials is a key to develop commercial solid oxide fuel cells operating at intermediate temperatures. BaCeO3 based oxides show good proton conductivity at intermediate temperatures; nevertheless, their large Brønsted basicity makes them prone to react with acidic gases such as CO2, and thus, chemically unstable in the operating atmospheres of SOFCs. This problem largely hampers the practical application of these oxides. In this work, we report a new strategy to reduce the basicity of BaCeO3 based oxides by using F−, with higher electronegativity, to partially substitute for O2− ions. The new compound of F-doped BaCe0.8Sm0.2O2.9 (BCSF) has demonstrated significant improved chemical stability in CO2 containing atmosphere with no loss of proton conductivity. With a 39 μm-thick BCSF electrolyte, the peak power density of single cell was 420 mW cm−2 at 700 °C in humidified H2/air system, approximately 20% higher than that using a BaCe0.8Sm0.2O2.9 electrolyte. Especially, within a 146 h test, negligible degradation in open circuit voltage (OCV) was observed using our new BCSF electrolyte.

Directly converting CO2 to hydrocarbons offers a potential route for carbon-neutral energy technologies. Here we report a novel design, integrating the high-temperature CO2–H2O co-electrolysis and low-temperature Fischer–Tropsch synthesis in a single tubular unit, for the direct synthesis of methane from CO2 with a substantial yield of 11.84%.

The oxygen incorporation kinetics of Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O1.9 (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of SDC phase volume fraction using electrical conductivity relaxation. It is shown that the oxygen re-equilibration kinetics in the range of oxygen partial pressure (pO2) from 0.01 to 1 atm is limited by the surface exchange rate. The effective surface exchange coefficient of the composites is found to increase profoundly upon increasing the phase volume fraction of the oxide electrolyte phase SDC. The results are interpreted to reflect the synergistic oxygen incorporation at the SFM–SDC–gas triple phase boundaries (TPBs), which occurs in addition to the direct incorporation via the surface of the perovskite mixed conductor SFM. Already at a SDC phase volume fraction of 0.105, the uptake of oxygen via the synergistic TPB route (referred to as route III), following a step change in the surrounding pO2, comprises more than 75% of the overall uptake of oxygen by the composite. It is further concluded that under the conditions of the experiments the two-phase SFM–SDC boundaries allow for a facile exchange of oxygen ions between both involved phases.

Bismuth is doped to lanthanum strontium ferrite to produce ferrite-based perovskites with a composition of La0.8-xBixSr0.2FeO3-δ (0 ≤ x ≤ 0.8) as novel cathode material for intermediate-temperature solid oxide fuel cells. The perovskite properties including oxygen nonstoichiometry coefficient (δ), average valence of Fe, sinterability, thermal expansion coefficient, electrical conductivity (σ), oxygen chemical surface exchange coefficient (Kchem), and chemical diffusion coefficient (Dchem) are explored as a function of bismuth content. While σ decreases with x due to the reduced Fe4+ content, Dchem and Kchem increase since the oxygen vacancy concentration is increased by Bi doping. Consequently, the electrochemical performance is substantially improved and the interfacial polarization resistance is reduced from 1.0 to 0.10 Ω cm2 at 700 °C with Bi doping. The perovskite with x = 0.4 is suggested as the most promising composition as solid oxide fuel cell cathode material since it has demonstrated high electrical conductivity and low interfacial polarization resistance.Keywords: bismuth doping; cathode; lanthanum strontium ferrite; solid oxide fuel cells; transport properties

•Tortuosity factor of the 3D infiltrate network is calculated.•Analytical model for the tortuosity factor of infiltrate network is developed.•Strategies to decrease the tortuosity factor of infiltrate network are suggested.•Intrinsic conductivity of the infiltrate network can be resolved.The tortuosity factor is calculated by solving Laplace's equation of electrostatic potential within the numerically constructed three-dimensional infiltrate network of nanostructured solid oxide cell electrodes. Based on Bruggeman approach, an analytical model is proposed to calculate the tortuosity factor and effective conductivity as a function of infiltration loading. The intrinsic conductivity of infiltrate network can be resolved from its effective conductivity using the analytical model. Good agreement is found between the numerical and analytical models and the experimental data in literature. Parametric study for the effects of backbone microstructure, nanoparticle size and aggregation of infiltrate suggests practical strategies to decrease the tortuosity factor of infiltrate network.

•LSCF–YSZ cathode supported tubular SOFCs are successfully developed.•The tubular cell has a low polarization resistance of 0.33 Ω cm2 at 750 °C.•The tubular cell has a peak power density of 0.55 W cm−2 at 750 °C.•It is feasible to fabricate cathode supported SOFC with the impregnation method.Tubular solid oxide fuel cells (SOFCs) are developed with thick (∼0.50 mm) La0.6Sr0.4Co0.2Fe0.8O3 (LSCF)–(Y2O3)0.08(ZrO2)0.92 (YSZ) cathode substrates and thin (6.9 μm) film YSZ electrolytes. LSCF is introduced to the thick porous YSZ substrate by impregnating technique, resulting in nanostructured LSCF particles, which are formed at 800 °C. The relatively low sintering temperature has effectively eliminated the possible solid–state reaction, which occurs between YSZ and LSCF above 900 °C. The nanostructured electrocatalyst promotes the electrochemical activity, resulting in total interfacial polarization resistance of 0.33 Ω cm2 at 750 °C for the single cell. At the same temperature, the cell has achieved a peak power density of 0.55 W cm−2, much higher than those reported for the cathode supported tubular SOFCs (about 0.2 W cm−2). The improved performance demonstrates the feasibility of fabricating cathode supported tubular SOFCs with highly active catalysts such as LSCF, Sm0.5Sr0.5CoO3, Ba0.5Sr0.5Co0.8Fe0.2O3 and PrBaCo2O5.

•A graphite-assisted phase inversion process is developed to fabricate tubular SOFCs.•The process is successful in fabricating anode substrates with porosity gradient.•Concentration polarization loss is eliminated with the graded structure.Tubular solid oxide fuel cells (SOFCs) are fabricated using a modified phase inversion process to obtain anode structure with graded pore distribution. The novel structure is achieved using an additional graphite layer to control the phase separation reaction in the ceramic layer and to remove the skin layer, which always presents in phase inversion process. The graded anode can effectively eliminate the concentration polarization loss at high current density as observed for the anode with the skin layer. In addition, improved peak power density is obtained with the graded-anode based cell, demonstrating that the modified process is promise in fabricating tubular SOFCs.

This work investigates the ionic conductivity of nano-sized samaria-doped ceria (SDC), which is often deposited in the electrodes of solid oxide fuel cells to enhance their electrochemical performance by extending the three-phase boundary (TPB) length. The SDC nano-particles are fabricated via an ion impregnation/infiltration method using porous ceria as the backbone and samarium-cerium nitrate solution as the precursor. The apparent conductivity, which is determined with electrochemical impedance spectroscopy, increases with SDC loading and reaches 8.40 × 10−4 Scm−1 at 700 °C for the loading of 25.1 wt.%. A model is developed to calculate the conductivity of the impregnated phase, which has a porosity of 50.4%. The nano-sized SDC conductivity at 700 °C is 9.82× 10−3 Scm−1, lower than 2.09× 10−2 Scm−1 for the bulk SDC prepared from the same precursor. Considering the Bruggeman factor, the conductivity of a dense impregnated SDC is estimated to be 5.88× 10−2 Scm−1, higher than the bulk material. The impedance for the impregnated SDC is characterized by much smaller grain-boundary contribution than the grain-interior, which is quite different with the bulk SDC.

•The chemically-induced stresses in SDC electrolytes for SOEC is evaluated.•The mechanical stability dependence on the operating T, P and V are explored.•The fracture preference of doped-ceria electrolytes in SOEC mode is explained.Doped-ceria is an attractive electrolyte material for solid oxide electrolysis cells (SOECs) operated at intermediate temperatures. However, ceria is highly prone to break down under high applied voltages and low oxygen partial pressures at the fuel side. This phenomenon is analyzed for the typical Sm0.2Ce0.8O1.9−δ electrolyte based on the chemically-induced stress, which is caused by the inhomogeneous distribution of oxygen non-stoichiometry throughout the thickness of electrolyte plate. The sensitivities of the maximum tensile stresses are explored under typical SOEC operating parameters such as temperature, applied voltage and oxygen partial pressure. Varying from short-circuit of solid oxide fuel cell (SOFC) mode to high voltage of SOEC conditions, the applied voltage sharpens the maximum tensile stress by seven times and raises the minimum permitted oxygen partial pressure at the cathode-electrolyte interface by a factor of 104.5 at most. The analysis results indicate that a ceria-based electrolyte under SOEC conditions denotes a definite trend of collapse at 700 °C even 600 °C, suggesting the inapplicability of doped-ceria electrolyte in SOEC mode.

•Impregnating SSC greatly reduce the resistance of SSC–SDC on BZCY electrolyte.•Impregnating SSC reduces the resistance associated with the surface diffusion step.•Performance enhancement is significant at reduced temperatures.Sm0.5Sr0.5CoO3−δ–Ce0.8Sm0.2O2−δ (SSC–SDC) composites, which are often used as the cathodes for solid oxide fuel cells (SOFCs) with oxygen-ion conducting electrolytes, have been recently shown to be also applicable in SOFCs based on proton conductors such as BaZr0.1Ce0.7Y0.2O3−δ (BZCY). The electrochemical performances of blank SSC–SDC electrodes on BZCY electrolytes are substantially improved in this work by impregnating SSC nanoparticles additionally. When the loading increases, the interfacial polarization resistance of the symmetric cell decreases gradually at first, notably when it exceeds 14 wt.%, and to the lowest value at about 22 wt.%. Furthermore, impregnating SSC reduces the low-frequency-arc resistance that corresponds to the surface exchange step. In addition, impregnating SSC reduces the activation energy for oxygen reduction from 1.14 to 0.70 eV, thus resulting in significantly improvement on electrode performance at the reduced temperatures for SOFCs based on proton conductors.

Perovskite Sr1–xCexCoO3−δ (0.05 ≤ x ≤ 0.15) have been prepared by a sol–gel method and studied as cathodes for intermediate temperature solid oxide fuel cells. As SOFC cathodes, Sr1–xCexCoO3−δ materials have sufficiently high electronic conductivities and excellent chemical compatibility with SDC electrolyte. The peak power density of cells with Sr0.95Ce0.05CoO3−δ is 0.625 W cm–2 at 700 °C. By forming a composite cathode with an oxygen ion conductor SDC, the peak power density of the cell with Sr0.95Ce0.05CoO3−δ-30 wt %SDC composite cathode, reaches 1.01 W cm–2 at 700 °C, better than that of Sm0.5Sr0.5CoO3-based cathode. All these results demonstrates that Sr1–xCexCoO3−δ (0.05 ≤ x ≤ 0.15)-based materials are promising cathodes for an IT-SOFC.Keywords: composite cathode; intermediate temperature; oxygen-reduction reaction; solid oxide fuel cell; Sr1−xCexCoO3−δ;

•Sr2Ti2xNi1−xMo1−xO6 (x = 0, 0.1, 0.3, 0.5, 0.7) perovskites are synthesized.•The strong Ti–O bond is held responsible for the enhanced structural stability.•First-principle calculations are performed on the conduction properties.•Cells with Sr2TiNi0.5Mo0.5O6 anode show remarkable carbon tolerance.Ti doping is found to increase the stability of Sr2NiMoO6 perovskite oxides in reducing atmosphere. The composition Sr2TiNi0.5Mo0.5O6 (STNM) is further evaluated as a potential oxide anode for solid oxide fuel cells (SOFCs). Electrical conductivity, thermal expansion coefficient, surface exchange coefficient, chemical diffusion coefficient, and its electrochemical performance in single cells with La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolytes are investigated. STNM exhibits a high conductivity of 17.5 S cm−1 at 800 °C at anodic atmosphere. The material shows good chemical and thermal expansion compatibilities with LSGM. To investigate the effect of Ti doping on the conduction properties, first-principle calculations are performed using the Vienna Ab initio Simulation. The strong Ti–O bond is held responsible for the enhanced structural stability of STNM under humidified H2 atmospheres, relative to that of the undoped system. The remarkable cell performance with both H2 and dry CH4 as the fuel indicates the potential ability of STNM to be used as SOFC anodes. These results obtained indicate that Sr2TiNi0.5Mo0.5O6 is a promising material for use as anode for intermediate temperature SOFCs.Ti-doped molybdenum-based perovskites, Sr2Ti2xNi1−xMo1−xO6 (x = 0, 0.1, 0.3, 0.5 and 0.7) have been synthesized by the sol–gel assisted combustion method. The remarkable cell performance with both H2 and dry CH4 as the fuel and the stability during operation indicated the potential ability of Sr2TiNi0.5Mo0.5O6 to be used as SOFC anodes.

•Application of impregnation/infiltration in anodes is reviewed.•Introducing nanoparticles to anodes by impregnation/infiltration is reviewed.•Novel anodes by impregnation/infiltration are reviewed.•Improved carbon and sulfur tolerance by impregnation/infiltration is reviewed.•Enhancing anodic activity by impregnation/infiltration is reviewed.The future commercialization and application of solid oxide fuel cell (SOFC) technologies requires the development of novel anode materials with excellent performance and stability at intermediate-temperatures with various fuels including hydrogen, syngas and particularly hydrocarbons. Whether by modifying the state-of-the-art Ni based anodes, or through exploring alternative metal cermet or ceramic based materials, wet impregnation/infiltration is shown to be one of the most effective approaches for both cell fabrication and performance optimization. This paper reviews most of the progress reported in the literature committed to the fabrication and optimization of SOFC anodes by wet impregnation for low temperature and/or hydrocarbon operation. The optimization of traditional nickel based anodes by adding excellent catalyst, the replacement of nickel by other inert metal or ceramic species, and some metal supported designs with impregnated catalyst are all presented and discussed, mainly focusing on the cell performance, redox and thermal stability, long-term reliability, carbon and sulfur tolerance of the anodes.

•H2 oxidation is studied using ECR technique by increasing pH2.•H2 oxidation is remarkably improved by introducing Pt particles to SDC surfaces.•The catalytic activity increases almost linearly with Pt–SDC–gas boundary length.•The contribution of H2 oxidation from the SDC–gas interface is as small as 4.3%.Electrical conductivity relaxation (ECR) method is utilized to determine H2 oxidation process on Sm0.2Ce0.8O1.9 (SDC) with scattered Pt or Au particles by increasing the H2 partial pressure in the gas stream. For SDC with Au particles, the reaction occurs only at the SDC–gas interface. When Pt is introduced, the surface exchange kinetics can be remarkably improved whereas the contribution from the ceria–gas interface can be as small as 4.3%. The improvement increases linearly with the Pt–SDC boundary length, demonstrating that H2 oxidation pathway is dominated by the Pt–SDC–gas three-phase boundaries.Hydrogen oxidation on samaria-doped ceria (SDC) with scattered Pt particles is studied with the electrical conductivity relaxation method. A new coefficient, kPt, is introduced to show the enhanced reaction kinetics. kPt increases almost linearly with the SDC–Pt boundary length, demonstrating that H2 oxidation pathway is dominated by the Pt–SDC–gas three-phase boundaries.

BaCeO3 based ceramics have demonstrated high proton conductivity and have been extensively investigated as electrolytes for solid oxide fuel cells (SOFCs). However, these materials are not chemically stable and are prone to reaction with CO2 in air at the typical SOFC operating conditions. This work presents a novel strategy to improve the chemical stability of BaCeO3 based ceramics to CO2 in air while still maintaining high proton conductivity by introducing barium chloride precursor in the powder synthesis process. As an example, BaCe0.8Sm0.2O3 − δ with Cl doping has demonstrated significant enhancement in chemical stability in the presence of CO2 in air without compromising its high proton conductivity.Highlights► Chemical stability of BCS against CO2 in air is enhanced by Cl doping. ► No significant effects on electrochemical performance are made by Cl addition. ► The doping of Cl can be easily achieved by using BaCl2 as the precursors.

Co is doped to Sr2Fe1.5Mo0.5O6 to enhance its electrochemical activity as the cathode for intermediate-temperature solid oxide fuel cells. Pure cubic perovskites of Sr2Fe1.5−xCoxMo0.5O6 (SF1.5−xCxM, x = 0, 0.5, 1) are synthesized using a glycine-nitrate combustion progress. The average thermal expansion coefficient varies from 15.8 to 19.8 × 10−6 K−1. The electrical conductivity increases while its activation energy decreases with increasing Co content. X-ray photoelectron spectroscopy analysis demonstrates mixed valences of Fe, Co and Mo, suggesting small polaron hopping mechanism. Electrical conductivity relaxation (ECR) measurement shows that the surface exchange coefficient increases about two orders of magnitude when the content increases from x = 0 to x = 1.0, i.e. from 2.55 × 10−5 to 2.20 × 10−3 cm s−1 at 750 °C. ECR also exhibits that chemical diffusion coefficient increases with Co content. Density Functional Theory calculation demonstrates that oxygen vacancy formation energy decreases with Co content, suggesting high oxygen vacancy concentration at high Co content. Impedance spectroscopy on symmetric cells consisting of SF1.5−xCxM electrodes and La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolytes shows that Co doping is very effective in reducing the interfacial polarization resistance, from 0.105 Ω cm2 to 0.056 Ω cm2 at 750 °C. These results suggest that Co doping into Sr2Fe1.5Mo0.5O6 can substantially improve its electrochemical performance.Highlights► Co is doped to Sr2Fe1.5Mo0.5O6 double perovskite by glycine-nitrate progress. ► Electronic conductivity is increased by Co doping. ► Oxygen surface exchange kinetics is enhanced by Co doping. ► Interface polarization resistance is decreased by Co doping.

Nano-structured electro-catalyst of layered-structure cobaltite PrBaCo2O5+x (PBC) has been developed as cathode for solid oxide fuel cells (SOFCs) and excellent electrochemical activity towards oxygen reduction has been achieved. PBC nano-particles are deposited into porous samaria-doped ceria (SDC) backbones with an impregnation method. The fabrication processing parameters including composition of precursor solution, PBC loading, and firing temperature have been investigated to optimize the cathode microstructure and further to minimize the cathode interfacial polarization resistance, leading to a cathode interfacial polarization resistance of only 0.082 Ω cm2 at 600 °C, much lower than those reported for pure PBC electrode (0.86 Ω cm2), PBC–SDC composite cathode (0.25 Ω cm2) or various other impregnated cobaltite cathodes such as Sm0.5Sr0.5CoO3−δ (0.25 Ω cm2), La0.5Sr0.5CoO3−δ (0.31 Ω cm2), and La0.6Sr0.4Co0.2Fe0.8O3−δ (0.24 Ω cm2). The novel nano-structured PBC electrochemical reaction mechanism has found to be similar to that of a conventional PBC cathode. However, both oxygen ion incorporation and charge transfer steps are greatly accelerated for the novel nano-structured PBC cathode, suggesting that the impregnation process is very effective in fabricating layered-structure cobaltite electro-catalyst for intermediate-temperature SOFC cathode with enhanced electrode performance.Highlights► Microstructure of the PBC impregnated cathode has been optimized. ► The impregnated PBC cathode has achieved an ASR as low as 0.08 Ω cm2 at 600 °C. ► Impregnated PBC shows similar reaction mechanism to that of a pure PBC cathode. ► Power density of 600 mW cm−2 at 600 °C is gained for cells with impregnated PBC.

Chemical oxygen surface exchange coefficient (Kex) of La0.6Sr0.4Co0.8Fe0.2O3 − δ (LSCF) coated with samaria-doped ceria (SDC) particles has been investigated using the electrical conductivity relaxation method. It has been found that adding doped ceria to LSCF results in an increase of a factor of 10 in surface exchange rate, suggesting that oxygen incorporation at the LSCF/SDC/gas boundary sites is facile. Furthermore, Kex of the SDC coated LSCF increases with the conductivity of SDC rather than the composition of the SDC, inferring that SDC supplies additional free oxygen vacancies for the surface exchange reaction. These results demonstrate that introducing doped ceria to La1 − xSrxCo1 − yFeyO3 − δ can substantially enhance the oxygen surface incorporation process for applications such as solid oxide fuel cell cathodes as well as oxygen separation membranes.Graphical abstractThe oxygen exchange process of La0.6Sr0.4Co0.8Fe0.2O3 − δ (LSCF) is significantly enhanced by coating samaria-doped ceria (SDC) particles. The enhancement is related to the SDC loading as well as the conductivity of SDC. The oxygen surface exchange coefficient, Kex, increases from 2.7 × 10−5 cm s−1 at 750 °C for bare LSCF (LSCF with no SDC coating) to 4.57 × 10−4 cm s−1 for LSCF coated with 0.86 mg cm−2 SDC. In addition, Kex of SDC coated LSCF increases almost linearly with the conductivity of SDC, indicating that doped ceria can significantly enhance the oxygen exchange kinetics of LSCF.Highlights► Surface exchange coefficient has significantly increased for SDC coated LSCF. ► Surface exchange coefficient of SDC coated LSCF depends on the SDC loading. ► Exchange coefficient of SDC coated LSCF increases with ionic conductivity of SDC.

A novel method is presented to determine the particle–particle fracture of yttria stabilized zirconia (YSZ), which is crucial in predicting the thermal cycle properties of solid oxide fuel cells (SOFCs). The method is demonstrated by determining the Weibull and normal distribution parameters via resistivity variation of YSZ–Al2O3 composites undergoing thermal cycle processes. A straightforward approach is developed to relate YSZ mechanical property with its conductivity based on fracture statistics distributions and percolation theory. By the measurement of the conductivity change in thermal cycles, the fracture between YSZ particles caused by thermal stress can be statistically “counted”, approaching these parameters with a statistical principle, and offering a possible way to understand particle–particle fracture in microscale, and to predict the effect of microstructure change using electric signals. Finally, this method offers a potential to precisely forecast the performance degradation in the different thermal cycle processes for SOFC components such as doped ceria electrolytes and perovskite electrodes.Highlights► A simple approach is proposed to determine the thermal stress of YSZ. ► Particle–particle fracture of YSZ is determined using electric signals. ► The method can be used to obtain mechanical properties of other SOFC components. ► Fracture can be predicted for cycle with various temperature ranges.

For solid oxide fuel cells with proton-conducting electrolytes, oxygen is reduced as well as water is formed at the so-called H-TPB where proton, oxygen and electron are available. Proton conductor cooperation to the cathode can thus increase the H-TPB length while oxygen-ion conductor could not. However, previous reports show that oxygen-ion conductor can also significantly increase the cathode performance, suggesting different cathodic mechanism, which is proposed in this work. Oxygen is reduced at O-TPB where oxygen-ion, oxygen and electron meet while H2O is formed at the electrode–electrolyte interface. Experimental investigation reveals that the cathodic reactions are primarily limited by the diffusion of Oad− at O-TPB and oxygen-ion transport within the electrode whereas water formation at the interface is not a limiting step. It is further exhibited that Sm0.5Sr0.5CoO3−δ electrocatalyst cooperated with SDC (Ce0.8Sm0.2O2−δ), an oxygen-ion conductor, show even higher cathodic performance than that with BCS (BaCe0.8Sm0.2O3−δ), a proton conductor, when BCS is used as the electrolyte.Highlights► Reaction model is proposed for H-SOFC cathodes with oxygen-ion conductors. ► For these cathodes, oxygen is reduced at O-TPB while water is formed at interface. ► These cathodes might be beneficial for H-SOFCs during actual operation.

The molecular-beam spectrometric technique coupled with tunable synchrotron vacuum ultraviolet photoionization is applied to reveal the catalytic decomposition of methane over Ni-based composites with and without impregnated nano-sized samaria-doped ceria (SDC) particles. It is shown that the coating of SDC nanoparticles not only decreases the decomposition temperature, but also increases the conversion ratio, thus indicating that those impregnated SDC nanoparticles are highly catalytically active for methane decomposition. In addition, C2H4 is observed when the impregnated Ni-SDC composites are used as the catalyst, suggesting that SDC coating also suppresses carbon deposition at the anodes of solid oxide fuel cells.

Electrochemical reaction at the cathodes of solid oxide fuel cells has typically been proposed to include a series of elementary steps occurring in the electrode, especially on the electrode surface. However, the electrode performance depends critically on the properties of both the electrode and electrolyte materials. This work assumes oxygen vacancy/ion transport cross the electrode–electrolyte interface as an elementary step to demonstrate the electrolyte effect on electrode performance. With this assumption, the electrode interfacial polarization resistance, Rp, can be theoretically related to the electrolyte conductivity, σ  , with a general formula, Rp∝σlPO2n, where PO2PO2 is the oxygen partial pressure at the cathode, l and n are the controlling parameters corresponding to various elementary steps occurred at the electrode–electrolyte interface as well as on the electrode. The assumed elementary step is experimentally confirmed by analyzing the electrochemical impedance spectra of symmetric cells of porous La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) electrodes on samaria-doped ceria (SDC) electrolytes with different conductivities as a result of various dopant contents. The high frequency resistance, which can be fitted to a Warburg-type element, increases linearly with the electrolyte resistivity, clearly demonstrating that this process corresponds to the transport of oxygen vacancy at the electrode–electrolyte interface, from the electrolyte to the electrode.Highlights► An elementary step is proposed regarding electrolyte effect on cathode reaction. ► Electrode performance is theoretically linked with electrolyte conductivity. ► The model is experimentally validated with LSCF cathodes.

Metal-supported solid oxide fuel cells (SOFCs) are usually four-layer structure consisting of the metal support, the anode, the electrolyte and the cathode. This communication reports a simplified three-layer design without the anode interlayer. The novel design is demonstrated by co-firing yttria-stabilized zirconia electrolytes and 430L stainless steel substrates, where Ni and doped ceria are impregnated to increase the catalytic activity toward electrochemical oxidation. Peak power density as high as 246 mW cm−2 is obtained at 700 °C, and good tolerance to complete redox cycles is also initially demonstrated, suggesting that this design is feasible for high performance metal-supported SOFCs.Highlights► A simplified three-layer design is proposed for metal-supported SOFCs. ► The design is demonstrated with SS430 as the anode-substrate layer. ► The cells exhibit comparable high performance with the state-of-the-art 4-layer ones. ► Good tolerance to complete redox cycles is demonstrated with this design.

Electrolyte effects on the oxygen surface exchange coefficients of strontium-doped lanthanum manganates (LSM) are investigated using electrical conductivity relaxation technique. Introducing electrolytes can significantly reduce the re-equilibration time, demonstrating the substantial promotion in the surface exchange kinetics. The coefficient at 1000 °C increases from 9.00 × 10−5 cm s−1 for pure LSM to 2.45 × 10−4 cm s−1 for LSM coated with yttria-stabilized zirconia, and further increases to 7.92 × 10−4 cm s−1 for LSM with samaria-doped ceria. The nearly one order of magnitude increase in the coefficient demonstrates that introducing electrolytes can effectively increase the electrochemical performance of solid-oxide-fuel-cell cathodes by enhancing the surface exchange reaction in addition to extending the reaction sites.Highlights► Introducing electrolyte can increase the oxygen surface exchange coefficient of LSM. ► The coefficient increases from 9.00 × 10−5 to 7.92 × 10−4 cm s−1 when doped ceria is coated. ► The coefficient increment depends on electrolyte conductivity rather than catalytic activity.

Micro-tubular proton-conducting solid oxide fuel cells (SOFCs) are developed with thin film BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) electrolytes supported on Ni-BZCYYb anodes. The substrates, NiO-BZCYYb hollow fibers, are prepared by an immersion induced phase inversion technique. The resulted fibers have a special asymmetrical structure consisting of a sponge-like layer and a finger-like porous layer, which is propitious to serving as the anode supports for micro-tubular SOFCs. The fibers are characterized in terms of porosity, mechanical strength, and electrical conductivity regarding their sintering temperatures. To make a single cell, a dense BZCYYb electrolyte membrane about 20 μm thick is deposited on the hollow fiber by a suspension-coating process and a porous Sm0.5Sr0.5CoO3 (SSC)-BZCYYb cathode is subsequently fabricated by a slurry coating technique. The micro-tubular proton-conducting SOFC generates a peak power density of 254 mW cm−2 at 650 °C when humidified hydrogen is used as the fuel and ambient air as the oxidant.

Bi0.5Sr0.5MnO3 (BSM), a manganite-based perovskite, has been investigated as a new cathode material for intermediate-temperature solid oxide fuel cells (SOFCs). The average thermal-expansion coefficient of BSM is 14 × 10−6 K−1, close to that of the typical electrolyte material. Its electrical conductivity is 82–200 S cm−1 over the temperature range of 600–800 °C, and the oxygen ionic conductivity is about 2.0 × 10−4 S cm−1 at 800 °C. Although the cathodic polarization behavior of BSM is similar to that of lanthanum strontium manganite (LSM), the interfacial polarization resistance of BSM is substantially lower than that of LSM. The cathode polarization resistance of BSM is only 0.4 Ω cm2 at 700 °C and it decreases to 0.17 Ω cm2 when SDC is added to form a BSM–SDC composite cathode. Peak power densities of single cells using a pure BSM cathode and a BSM–SDC composite electrode are 277 and 349 mW cm2 at 600 °C, respectively, which are much higher than those obtained with LSM-based cathode. The high electrochemical performance indicates that BSM can be a promising cathode material for intermediate-temperature SOFCs.

Impregnated nanoparticles are very effective in improving the electrochemical performance of solid oxide fuel cell (SOFC) anodes possibly due to the extension of reaction sites and/or the enhancement of catalytic activity. In this work, samaria-doped ceria (SDC), pure ceria, samaria, and alumina oxides impregnated Ni-based anodes are fabricated to compare the site extending and the catalytic effects. Except for alumina, the impregnation of the other three nano-sized oxides could substantially enhance the performance of the anodes for the hydrogen oxidation reactions. Moreover, single cells with CeO2 and Sm2O3 impregnated anodes could exhibit as great performance as those with SDC impregnated anodes. When the impregnation loading reached the optimal value, 1.7 mmol cm−3, these cells exhibit very high performance, with peak power densities around 750 mW cm−2. The high performance of CeO2 and Sm2O3 impregnated anodes demonstrates that the improved performance are mainly attributed to the significantly improved electrochemical activities of the anodes, but not to the extension of triple-phase-boundary, and wet impregnation is indeed an alternative and effective technique to introduce these nano-sized catalytic active oxides into the anode configuration of SOFCs to enhance cell performance, stability and reliability.Highlights► CeO2 and Sm2O3 impregnated anodes show great performance as SDC impregnated ones. ► Improved performance is mainly attributed to the enhanced electrochemical activity. ► The activity of Sm2O3 impregnated anodes may derive from a hydrogen spillover effect. ► Wet impregnation is an effective approach to design anodes for low temperature SOFCs.

A percolation theory based model considering particle size and its distribution is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model calculation agrees excellently with 3D numerical reconstruction results, suggesting great validity of prediction. Moreover, it is also consistent well with experiment for real LSM (lanthanum strontium manganite)–YSZ (yttria-stabilized zirconia) electrodes with different composition, especially in range from 40:60 to 60:40 wt.% LSM:YSZ. The model can explicitly capture the effects of particle size, distribution, and electrode composition on several basic microstructure features and electrochemical properties of composite electrodes, such as coordination numbers and percolation probability, total and active three-phase boundary length, and interfacial polarization resistance. The model is further used to estimate LSM–YSZ electrode performance with the particle size and distribution of the source materials. The estimation generally coincides with the experiment, showing great potential in predicting power density based on the particle parameters of source materials for SOFCs.

Cermet anodes composed of Ni and Dy2O3 have been applied as anode for intermediate-temperature solid oxide fuel cells. Although the oxide ion conductivity of Dy2O3 is negligible compared to that of doped ceria (DCO), Ni–Dy2O3 cermet anodes have displayed very high performance comparable to that of the commonly used Ni–SDC cermet anode. Temperature programmed reduction study suggests that the catalytic activity of the Ni–Dy2O3 cermet might be originated from the hydrogen adsorption ability on the Dy2O3 surface, promoting hydrogen spillover process, and consequently enhancing the electrochemical oxidation of the fuel. Further, the electrochemical catalytic activity of Au-based cermet anode confirms that hydrogen adsorption capability is as important as oxygen-ion conductivity for the ceramic component in a cermet anode.

A film percolation model is proposed for composite electrodes of solid oxide fuel cells (SOFCs). The model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of electrochemical devices such as SOFCs. The model can be used to estimate electrode properties, such as percolation probability, active three-phase boundary length and interfacial polarization resistance. Compared with the classic percolation theory, which is particularly applicable to 3D-infinite bulks, the model can explicitly capture the effects of thinly layered nature of composite electrodes, and describes a cross-over between 2D-infinite films and 3D-infinite bulks. It also permits the prediction within whole electrode composition range, and can be easily applied in SOFC modeling.Highlights► A film percolation model is developed to predict the percolation properties of 2D-infinite structures which represent the structural characteristics of composite electrodes of SOFCs. ► The model describes a cross-over between 2D-infinite films and 3D-infinite bulks. ► The distribution of active three-phase boundaries within electrodes can be calculated using this model. ► The film percolation model permits the prediction within whole electrode composition range.

Sr2Fe1.5Mo0.5O6 is a mixed ionic and electronic conductor which can be applied as both cathode and anode material in intermediate-temperature solid oxide fuel cells. To enhance its cathode performance, Ce0.8Sm0.2O1.9 nanoparticles are deposited using ion impregnation technique. Impedance spectroscopy shows that the nanoparticles are very effective in reducing the interfacial polarization resistance from 0.27 Ω cm2 to 0.11 Ω cm2 at 750 °C. Electrical conductivity relaxation (ECR) measurement demonstrates that the surface exchange coefficient of Sr2Fe1.5Mo0.5O6 is significantly improved by the nanoparticles, from 10− 5 to 10− 3 cms− 1 at 750 °C. The results suggest that doped ceria nanoparticles are very promising in enhancing the surface exchange processes and consequently can greatly improve the electrode performance.Depositing Ce0.8Sm0.2O1.9 nanoparticles to Sr2Fe1.5Mo0.5O6 can increase the surface exchange coefficient by two orders of magnitude as determined with electrical conductivity relaxation measurement; from 1.4 × 10− 5 to 1.0–2.0 × 10− 3 cm s− 1 at 750 °C. This demonstrates that doped ceria nanoparticles are very promising in enhancing the electrode kinetic processes and consequently can greatly improve the electrode performance.Research Highlights►The impregnated nanoparticles increase the surface exchange coefficient by two orders of magnitude. ►The enhancement of nanoparticles on electrode kinetics has been demonstrated using electrical conductivity relaxation method. ►The interfacial polarization resistance decreases from 0.27 to 0.11 Ω cm2 at 750 °C by Ce0.8Sm0.2O1.9 nanoparticles.

BaZr0.1Ce0.7Y0.2O3 − δ (BZCY), an intermediate temperature proton conductor, has been applied as an electronic blocking material for microtubular solid oxide fuel cells (SOFCs) using doped ceria electrolyte. Bi-layer electrolyte consisting of 3 μm thick BZCY and 10 μm thick Sm0.2Ce0.8O1.9 (SDC) are successfully deposited on anode substrates with 1.0 mm diameter using phase inversion, suspension-coating and co-firing techniques. At 700 °C, open circuit voltage increases from 0.72 V for the cells with single SDC electrolyte to 0.97 V for those with bi-layer electrolyte, demonstrating that BZCY can effectively prevent the internal shorting in doped ceria electrolyte. Peak power density of 246 mW cm−2 has been achieved at 700 °C with the microtubular SOFCs based on the bi-layer electrolyte.In this communication, BaZr0.1Ce0.7Y0.2O3 − δ (BZCY), which is a proton conductor, has been used as the electronic blocking material of samaria-doped ceria (SDC) electrolyte for microtubular solid oxide fuel cells (SOFCs). Open circuit voltages are substantially increased, from 0.72 V at 700 °C to 0.97 V when BZCY is applied as the electronic blocking layer, demonstrating that BZCY can be an alternative novel electronic blocking material. Peak power density of 246 mW cm−2 has been achieved at 700 °C with microtubular cells based on BZCY–SDC electrolyte, suggesting that the bi-layer electrolyte is attractive for applications in micro-tubular SOFCs.Research highlights► BZCY is used as a new electron blocking material for SDC electrolyte. ► The OCV value of BZCY–SDC electrolyte is promoted, compared to SDC electrolyte. ► Peak power density of 246 mW cm–2 is obtained at 700 °C with a bi-layer electrolyte cell.

Ni–LnOx (Ln = Dy, Ho, Er, Yb and Tb) cermets are investigated as the anodes of intermediate-temperature solid oxide fuel cells using ceria-based electrolyte to seek insights into the properties and electrocatalytic activity of these lanthanide oxides, whose oxygen ion conductivity is negligible. They have displayed similar electrochemical activity which is comparable to, if not higher than, those of the commonly Ni-doped ceria cermets. The anode performance has been found to depend strongly on cermet composition and porosity. Temperature programmed reduction study and EIS analysis under different hydrogen partial pressure suggest that the catalytic activity of the Ni–LnOx cermets might be originated from the hydrogen adsorption ability on the LnOx surface, promoting hydrogen spillover process, and consequently enhancing the electrochemical oxidation of the fuel.Graphical abstractIf a anode consists of an oxide with negligible oxygen-ion conductivity, the H2 oxidation reaction is limited to the physical interface between the electrolyte and Ni, where the H2, Ni, and SDC electrolyte phases meet, i.e. path A. Accordingly, it is reasonable to consider that the high performance of Ni–LnOx (Ln = Dy, Ho, Er, Yb, Tb) anodes is only related to the high catalytic activity of LnOx. Temperature programmed reduction shows that the catalytic activity might originate from its capability of hydrogen adsorption. The presence of Ni/LnOx might promote hydrogen spillover processes, and consequently, enhance the electrochemical oxidation of the fuel. This reaction mechanism via the spillover process is illustrated by the reaction path B.Highlights► LnOx (Ln = Dy, Ho, Er, Yb, Tb) are firstly investigated as the ceramic components in Ni-based cermet anodes for SOFCs. ► The high performance might be caused by LnOx capability of hydrogen adsorption. ► Hydrogen adsorption capability is as important as oxygen-ion conductivity for the ceramic component in a composite anode.

Iron doped layered structured perovskites, PrBaCo2−xFexO5+δ (x = 0, 0.5, 1.0, 1.5 and 2.0), are evaluated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The effects of dopant content are investigated on their structural and electrochemical properties including crystalline structure, oxygen nonstoichiometry, stability in presence of CO2, compatibility with electrolytes, thermal expansion coefficient, electrical conductivity, and cathodic interfacial polarization resistance. The lattice parameter and oxygen nonstoichiometry content, δ, at room temperature increase, whereas the conductivity, thermal expansion coefficient, and cathodic performance decrease with increasing iron content, x. PrBaCo2−xFexO5+δ exhibit excellent stability at 700 °C in atmosphere consisting of 3% CO2 and 97% air, show good chemical compatibility with doped ceria electrolytes at 1000 °C, but react readily with yttria-stabilized zirconia at 700 °C. Even with a Co-free PrBaFe2O5+δ as the electrode, a symmetrical cell demonstrates area specific resistance of 0.18 Ω cm2 at 700 °C with samaria-doped ceria electrolyte. The resistance is lower than those for typical Co-free electrodes reported in the literatures, suggesting that PrBaCo2−xFexO5+δ are potential promising cathode materials for IT-SOFCs.Research highlights► PrBaCo2-xFexO5+δ is pretty stable under atmosphere containing 3% CO2, and the stability is not affected by Fe content. ► Iron doping of PrBaCo2-xFexO5+δ is effective to reduce the thermal expansion coefficient, which is beneficial for SOFC systems in maintaining long-term stability and enduring thermal cycle. ► Even with a Co-free PrBaFe2O5+δ as the electrode, the resistance is lower than those for typical Co-free electrodes reported in the literatures, suggesting that PrBaCo2-xFexO5+δ are potential promising cathode materials for IT-SOFC.

Ni-LnOx cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnOx is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm2O3 cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm2O3 anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm2O3 anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm2O3 and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm2O3 anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm−2 at 700 °C, and the interfacial polarization is about 0.7 Ω cm2.Highlights► Ni-Sm2O3 are primarily investigated as the anodes for SOFCs with ScSZ electrolytes. ► An anode interlayer is introducing to increase the fuel cell stability. ► The cell performance is substantially improved by depositing the Ni-SDC interlayer.

Melilite type ceramics ABC3O7 such as La1.54Sr0.46Ga3O7.27 are a new class of oxide conductors where the conductivity is carried out through interstitial oxygen ions. This work presents the attempt to replace the A-site element La with the other lanthanide elements and Y, resulting in various Ln1 + xSr1 − xGa3O7 + x/2 ceramics, in which Ln = La, Pr, Nd, Sm, Eu, Gd, Dy, Yb, Y, and 0.1 < x < 0.54. X-ray diffraction analysis shows that the melilite structure could be formed when the replacement is conducted with most lanthanides but not Yb and Y. Impedance spectroscopy demonstrates that the conductivity decreases dramatically with the decreasing of Ln3+ size and the charge-carrier concentration. These results suggest that, as an interstitial oxide ion electrolyte, La1.54Sr0.46Ga3O7.27 is the most promising ceramic in the Ln1 + xSr1 − xGa3O7+x/2 melilite family since La3+ has the largest ionic radius of the lanthanide elements.Research Highlights► Melilite Ln1 + xSr1 – xGa3O7 + x/2 (Ln=lanthanide elements) electrolytes are synthesized. ► The melilite structure can be formed with most lanthanides. ► La1.54Sr0.46Ga3O7.27 shows the highest conductivity in the melilite family.

The cermet anodes for solid oxide fuel cells (SOFCs) usually consist of Ni and an oxygen ion conductor such as yttria-stabilized zirconia (YSZ) and doped ceria (DCO). In this work, Ni–LnOx cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnOx is not an oxygen ion conductor, are primarily investigated as the anodes for intermediate-temperature SOFCs. The electrochemical performances of the Ni–LnOx anodes are characterized using single SOFCs with Sm0.5Sr0.5CoO3 composite cathodes and Gd0.1Ce0.9O1.95 electrolytes. When humidified H2 is used as the fuel and ambient air as the oxidant, Ni–CeO2 and Ni–Gd2O3 anodes have exhibited very high performance, which is comparable to that of Ni–DCO anodes, the state-of-the-art electrodes for intermediate-temperature SOFCs with ceria electrolytes. The performance is further improved by increasing the anode porosity; peak power density up to 730 mW cm−2 and total interfacial polarization resistance down to 0.12 Ω cm2 are achieved for Ln = Sm, Eu, Ce, and Gd. The low interfacial polarization resistance and high power densities might be related to the high catalytic activity of LnOx and the optimized microstructures by increasing the porosity. These results suggest a promising alternative to the conventional anodes for SOFCs.

Despite the intense interest in solid oxide fuel cells, many details of their durability remain a mystery. Here, we present the insight see on electrode degradation in thermal cycle processes. Our model interprets the degradation to the stresses induced by thermal expansion mismatch of the electrocatalyst and electrolyte in a composite electrode that undergoes a temperature change. Such stresses might break the particle–particle interfaces (grain boundaries), thus reduce oxygen-ionic conductivity, electronic conductivity, and three-phase boundaries within the electrode, and consequently, degrade its performance. The model formulates the degradation rate as a function of cycle number, thermal expansion coefficient, composition, and particle size, providing a remarkable ability to balance thermal expansion restriction and catalytic activity of electrode materials, to optimize the electrode structure and composition, and to predict thermal-cycle durability. The model explicitly demonstrates that, in addition to their excellent electrochemical activity, nanostructured electrodes exhibit exceptional durability in thermal cycle processes.

A particle-layer model is developed to quantitatively evaluate the electrochemical parameters of conventional composite cathodes (CCCs) and impregnated composite cathodes (ICCs) for solid oxide full cells (SOFCs). In this model, these cathodes are considered as a construction composed of particle layers. The parameters such as interfacial polarization resistance, three-phase boundary (TPB) length, and effective electrode thickness are formulated as a function of effective TPB resistivity, ionic resistivity, and cathode structure characteristics including electrode composition, porosity, particle size of electrocatalyst and electrolyte, and thickness. In addition, the model can be used as a convenient tool to estimate the effective TPB resistivity when the interfacial polarization resistance is available or experimentally determined. Furthermore, the ICC and CCC electrodes are theoretically compared. It is confirmed that the electrochemical performance can be significantly enhanced and small effective thickness can be reached by using ICC structure, compared with CCC, due to the remarkable enlargement of TPB length. The model also provides some strategies to design a high performance cathode.

Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices working efficiently and environmental friendly. SOFC requires a functional cathode with high electrocatalytic activity for the electrochemical reduction of oxygen. The electrode is often fabricated at high temperature to achieve good bonding between the electrode and electrolyte. The high temperature not only limits material choice but also results in coarse particles with low electrocatalytic activity. Nano-structured electrodes fabricated at low temperature by an infiltration/impregnation technique have shown many advantages including superior activity and wider range of material choices. The impregnation technique involves depositing nanoparticle catalysts into a pre-sintered electrode backbone. Two basic types of nano-structures are developed since the electrode is usually a composite consists of an electrolyte and an electrocatalyst. One is infiltrating electronically conducting nano-catalyst into a single phase ionic conducting backbone, while the other is infiltrating ionically conducting nanoparticles into a single phase electronically conducting backbone. In addition, nanoparticles of the electrocatalyst, electrolyte and other oxides have also been infiltrated into mixed conducting backbones. These nano-structured cathodes are reviewed here regarding the preparation methods, their electrochemical performance, and stability upon thermal cycling.

The correlations of the microstructures and the electrical properties of high reactive Ce0.8Sm0.2O1.9 (SDC) powders, synthesized via an optimal carbonate coprecipitation method, were investigated. Microstructure of the SDC ceramics sintered at 900–1400 °C showed uniform grain and small grain size, compared with those prepared with various methods under similar sintering conditions. These features may be related to high conductivity (σ600 °C = 0.022 S cm−1) and low activation energy for conduction (0.66 eV). AC impedance spectra were involved to resolve grain interior and grain boundary resistance. Grain boundary contribution to the total resistance showed the values below 1/2 at 200–450 °C, suggesting low grain boundary effect. The motion enthalpy for the grain interior conduction decreased while the association enthalpy increased with sintering temperature up to 1300 °C, which might be possibly originated from the increase in lattice parameters with the sintering temperature.

The electrochemical characteristics of the solid oxide fuel cell (SOFC) cathodes prepared by infiltration of (La0.85Sr0.15)0.9MnO3−δ (LSM) nanoparticles into porous Y0.5Bi1.5O3 (YSB) backbones are investigated in terms of overpotential, interfacial polarization resistance, and single cell performance obtained with three-electrode cell, symmetrical cell, and single cell, respectively. X-ray diffraction confirms the formation of perovskite LSM by heating the infiltrated nitrates at 800 °C. The electrical conductivity of the electrode measured using Van der Pauw method is 1.67 S cm−1, which is acceptable at the typical SOFC operating temperatures. The single cell with the LSM infiltrated YSB cathode generates maximum power densities of 0.23, 0.45, 0.78, and 1.13 W cm−2 at 600, 650, 700, and 750 °C, respectively. The oxygen reduction mechanism on the cathode is studied by analyzing the impedance spectra obtained under various temperatures and oxygen partial pressures. The impedance spectra under various cathodic current densities are also measured to study the effect of cathodic polarization on the performance of the cathode.

Low-temperature solid oxide fuel cells (SOFCs) operated at a temperature of 500 °C and below are developed by modifying the microstructures of single cells consisting of Ni-cermet anodes, doped ceria electrolytes and strontium-doped samaria cobaltite cathodes. The cell microstructure is optimized by varying the starting powder firing temperature, so that the doped ceria electrolytes have a high sinterability, reducing the spin-coating cycles to decrease the electrolyte thickness to approximately 9 μm, adopting a two-step sintering process so that the electrolytes consist of small grains and have a high density; while the anodes are composed of small particles and have high porosity. In particular, the two-step sintering process depresses the co-firing temperature, thus enhancing the electrolyte conductivity and reducing the electrode polarization resistance. Outstanding performance with peak power density of 476, 319, and 189 mW cm−2 at 500, 450, and 400 °C is achieved with a typical single cell comprising a 9-μm-thick Sm0.2Ce0.8O1.9 (SDC) electrolyte, a Ni-SDC porous anode, and a Sm0.5Sr0.5CoO3−δ-Sm0.2Ce0.8O1.9 (SSC-SDC) composite cathode. A durability test over 110 h maintained a power density of approximately 150 mW cm−2 at 400 °C, suggesting optimization of the microstructure has promise for enhancing the performance of low-temperature SOFCs.

A novel anode consisting of Ni and Sm2O3 with negligible oxygen-ion conductivity was developed for intermediate-temperature solid oxide fuel cells (SOFCs). Its triple phase boundary length is pretty small compared with the conventional Ni-samaria doped ceria (SDC) anode, of which SDC is one of the electrolytes having high oxygen-ion conductivity. Even so, single cells with Ni–Sm2O3 anodes generated peak power density of 542 mW cm−2 at 600 °C, comparable to, if not higher than those with the Ni–SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni–Sm2O3 exhibited lower interfacial polarization resistance than Ni–SDC. The high electrochemical performance, which might be related to the high catalytic activity of Sm2O3 and the unique microstructures of the Ni–Sm2O3, suggests a viable alternative to the conventional anodes for SOFCs.

Taking Y2O3 stabilized Bi2O3 (YSB) as an example, bismuth oxide-added (La,Sr)MnO3 (LSM) is evaluated as a cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs) with 8 mol% Y2O3 stabilized ZrO2 (YSZ) electrolytes. YSB was added to LSM cathodes using an impregnation method, dramatically improving the electrode performance. The interfacial polarization resistance Rp, at 700 °C for the electrode coated with 50 wt.% of YSB is 0.14 Ω cm2, which is only 0.2% of the value for a pure LSM electrode. The high oxygen ionic conductivity and the catalytic activity of YSB, as well as the favorable electrode microstructure are likely reasons for the dramatic reduction of Rp. The YSB-added LSM cathodes also exhibited lower overpotential and higher exchange current density than the pure LSM cathode. Moreover, these electrodes show much lower Rp than that of parallel-fabricated LSM electrodes with samaria-doped-CeO2 as well as other LSM-based electrodes reported in the literature, demonstrating the superiority of the of YSB as the ionic conduction component in composite LSM electrodes. The superior performance of the single cell further demonstrates that the bismuth oxide-added LSM cathode is an excellent candidate for IT-SOFCs.

A novel nano-network of Sm0.5Sr0.5CoO3−δ (SSC) is successfully fabricated as the cathodes for intermediate-temperature solid oxide fuel cells (SOFCs) operated at 500–600 °C. The cathode is composed of SSC nanowires formed from nanobeads of less than 50 nm thus exhibiting high surface area and porosity, forming straight path for oxygen ion and electron transportation, resulting in high three-phase boundaries, and consequently showing remarkably high electrode performance. An anode-supported cell with the nano-network cathode demonstrates a peak power density of 0.44 W cm−2 at 500 °C and displays exceptional performance with cell operating time. The result suggests a new direction to significantly improve the SOFC performance.

Bismuth oxide based oxygen ion conductors are incorporated into (La,Sr)MnO3 (LSM), the classical cathode material for solid oxide fuel cells (SOFC), to improve the cathode performance. Yttria-stabilized bismuth oxide (YSB) is taken as an example and is impregnated into a preformed porous LSM frame, forming a highly active cathode for intermediate-temperature SOFCs (IT-SOFCs) with doped ceria electrolytes. X-ray diffraction indicates that YSB is chemically compatible with LSM at intermediate temperatures below 800 °C. The impregnated YSB particles are nanosized and are deposited on the surface of the framework. Significant performance improvement is achieved by introducing nanosized YSB into the LSM electrodes. At 600 °C, the interfacial polarization resistance under open-circuit conditions for electrodes impregnated with 50% YSB is only 1.3% of the original value for a pure LSM electrode. The resistance is further reduced dramatically when current is passed through. In addition, the YSB impregnated LSM electrodes has the highest electrochemical performance among those based on LSM. Single cell with 25% of YSB impregnated LSM cathode generates maximum power density of 300 mW cm−2 at 600 °C, indicating the promise of using LSM-based electrodes for IT-SOFC.

La1−xSrxMnO3 (LSM) has been widely developed as the cathode material for high-temperature solid oxide fuel cells (SOFCs) due to its chemical and mechanical compatibilities with the electrolyte materials. However, its application to low-temperature SOFCs is limited since its electrochemical activity decreases substantially when the temperature is reduced. In this work, low-temperature SOFCs based on LSM cathodes are developed by coating nanoscale samaria-doped ceria (SDC) onto the porous electrodes to significantly increase the electrode activity of both cathodes and anodes. A peak power density of 0.46 W cm−2 and area specific interfacial polarization resistance of 0.36 Ω cm2 are achieved at 600 °C for single cells consisting of Ni-SDC anodes, LSM cathodes, and SDC electrolytes. The cell performances are comparable with those obtained with cobalt-based cathodes such as Sm0.5Sr0.5CoO3, and therefore encouraging in the development of low-temperature SOFCs with high reliability and durability.

A geometric micro-model and experiment development are presented for electrolyte-coated anodes with high performance in solid oxide fuel cells. The anodes are based on electron conducting frameworks, where fine, oxygen-ion conducting inclusions are introduced via an ion impregnation process. The model shows that the length of triple-phase-boundary (TPB) increases with the loading of the coated electrolyte, and is dependent only on the loading before a maximum loading for monolayer coverage is obtained. The maximum loading increases with the porosity of the framework. As a result, the prolonged TPB length can be achieved by increasing the porosity and the loading. In the experimental study, Ni was used as the electron conductor, and samaria-doped ceria (SDC) was employed as the electrolyte to form anode-supported single cells. The cell performance was evaluated using humidified hydrogen as the fuel. The peak power density increased with SDC loading to a maximum value and decreased when the loading was further increased. The highest peak power density of the cells whose anodes were prepared with 10, 20 and 30 wt.% pore former was 571, 631 and 723 mW cm−2, corresponding to 508, 564 and 648 mg cm−3 of SDC loading, respectively. The experimental results are in good agreement with the model prediction. Therefore, this work demonstrates theoretically and experimentally that optimization of the porosity and electrolyte loading is critical for further improving the performance of electrolyte-coated anodes.

K2NiF4 structured La2Co0.8Ni0.2O4 + δ is an oxygen overstoichiometric oxide with high oxygen diffusion and oxygen surface exchange coefficients at temperature range 450–800 °C. In this work, composites consisting of La2 − xSrxCo0.8Ni0.2O4 + δ (LSCN, x = 0, 0.4, 0.8, 1.2, 1.6) and Ce0.9Gd0.1O1.95 (GDC) have been investigated as the cathodes for low-and-intermediate temperature solid oxide fuel cells (SOFCs). AC impedance spectroscopy on symmetric cells indicated that among the series of LSCN–GDC composites, La1.2Sr0.8Co0.8Ni0.2O4 + δ-based electrode had the lowest interfacial polarization resistance, which was 1.36 Ω cm2 at 600 °C when the electrode was not activated. Significant activation effect was observed with a single cell when current treatment was performed at 200 mA cm− 2 within 30 min. The single cell with La1.2Sr0.8Co0.8Ni0.2O4 + δ–GDC as the cathode generated power density up to 350 mW cm− 2 at 600 °C. In addition, the performance was pretty stable when a constant output voltage of 0.5 V was set for 36 h. These results suggest that La2 − xSrxCo0.8Ni0.2O4 + δ could be promising materials as the cathodes for SOFCs that operated at low-and-intermediate temperatures.

LSM (La0.85Sr0.15MnO3−δ)–SDC (Sm0.2Ce0.8O1.9) composites, which are based on LSM matrices embedded with SDC particles, were fabricated with an ion-impregnation technique as cathodes for intermediate and low temperature solid oxide fuel cells(SOFCs). Effect of heat-treatment temperatures for both LSM matrices and SDC particles was investigated on cathodic interfacial polarization resistances. When the matrix was fired in the range of 800–1000 °C, low temperature resulted in high specific surface area, and therefore low resistance. On the contrary, high temperature enhanced the bonding between the particles and matrices, and consequently caused increased electrochemical performance when the impregnated SDC was heated at temperature from 700 °C to 800 °C. With the impregnated LSM–SDC composite as the cathode, a single cell generated 138 mW/cm2 at 600 °C when humidified H2 was used as the fuel, suggesting that it is possible to use LSM as the cathodes for SOFCs that operated at low temperature when the microstructure is further optimized.

The electrical conductivity at intermediate temperature of 150–250 °C and the activation energy for conductivity of composite proton conductors, 2NH4PO3–(NH4)2Mn(PO3)4 and 2NH4PO3–(NH4)2SiP4O13, were investigated as a function of water vapor pressure, PH2OPH2O. The activation energy decreased linearly with the natural logarithm of PH2OPH2O, indicating that water is chemically adsorbed to the electrolytes. The decrease in activation energy is possibly caused by formation of hydrogen bonds between the adsorbed water and the electrolytes. In addition, the pre-exponential factor of Arrhenius equation, σ0, increased with PH2OPH2O. This suggests that the adsorbed water may generate additional mobile protons for the composite electrolyte. Therefore, the enhancement in the electrical conductivity of a NH4PO3-based electrolyte in a water-vapor rich atmosphere originates possibly from the decrease in activation energy as well as the increase in mobile proton concentration.

Ni–Cu alloy-based anodes, Ni1−xCux (x = 0, 0.05, 0.2, 0.3)–Ce0.8Sm0.2O1.9 (SDC), were developed for direct utilization of biomass-produced gas in low-temperature solid oxide fuel cells (LT-SOFCs) with thin film Ce0.9Gd0.1O1.95 electrolytes. The alloys were formed by in situ reduction of Ni1−xCuxOy composites synthesized using a glycine-nitrate technique. The electrolyte films were fabricated with a co-pressing and co-firing technique. Electrochemical performance of the Ni1−xCux–SDC anode supported cells was investigated at 600 °C when humidified (3% H2O) biomass-produced gas (BPG) was used as the fuel and stationary air as the oxidant. With Ni–Cu alloys as anodes, carbon deposition was substantially suppressed and electrochemical performance of the cells was sustained for much longer periods of time. For example, the power export of a Ni–SDC supported cell was only 50% of the initial value (200 mW cm−2 at 0.5 V) after 20 min, while Ni0.8Cu0.2–SDC supported cells could maintain 90% of the initial power density (250 mW cm−2 at 0.5 V) over a period of 10 h. The improved performance of the Ni–Cu alloy-based anodes is worth considering in developing SOFCs fueled directly with dilute hydrocarbons such as gases derived from biomass.

A new proton-conductive composite of NH4PO3–(NH4)2Mn(PO3)4 was synthesized and characterized as a potential electrolyte for intermediate temperature fuel cells that operated around 250 °C. Thermal gravimetric analysis and X-ray diffraction investigation showed that (NH4)2Mn(PO3)4 was stable as a supporting matrix for NH4PO3. The composite conductivity, measured using impedance spectroscopy, improved with increasing the molar ratio of NH4PO3 in both dry and wet atmospheres. A conductivity of 7 mS cm−1 was obtained at 250 °C in wet hydrogen. Electromotive forces measured by hydrogen concentration cells showed that the composite was nearly a pure protonic conductor with hydrogen partial pressure in the range of 102–105 Pa. The proton transference number was determined to be 0.95 at 250 °C for 2NH4PO3–(NH4)2Mn(PO3)4 electrolyte. Fuel cells using 2NH4PO3–(NH4)2Mn(PO3)4 as an electrolyte and the Pt–C catalyst as an electrode were fabricated. Maximum power density of 16.8 mW/cm2 was achieved at 250 °C with dry hydrogen and dry oxygen as the fuel and oxidant, respectively. However, the NH4PO3–(NH4)2Mn(PO3)4 electrolyte is not compatible with the Pt–C catalyst, indicating that it is critical to develop new electrode materials for the intermediate temperature fuel cells.

NiO–SDC(Ce0.8Sm0.2O1.9) composites were synthesized using gel-casting technique with nitrate precursors. The composite oxides were formed after the gels were fired at 350 °C in flowing air. No reaction between NiO and CeO2 occurs as analyzed using X-ray diffraction (XRD). The average particle size was about 50 nm and specific surface area was 10 m2 g−1, when the powder was fired at 900 °C for 2 h. NiO–SDC cermets were prepared by firing the composites at 1350 °C. The electrical conductivities and porosities of the cermets were measured, for example, 360 S cm−1 at 600 °C for a cermet with 30% porosity, 35 vol.% Ni, and 35 vol.% SDC. And the electrochemical performance of Ni–SDC cermet as an anode was investigated using a fuel cell with 25-μm thick SDC electrolyte and Sm0.5Sr0.5CoO3-SDC cathode. Maximum power density was 618 mW cm−2 and 491 mW cm−2 at 650 and 600 °C, respectively, inferring high catalytic activity of the Ni–SDC anode. Impedance measurements on the fuel cell at open circuit showed that the interfacial polarization resistance of the Ni–SDC anode was negligible compared to the resistance of cathode and electrolyte.

Two systematic electrolytes of Ce1−xGdxO2−x (GDC) and Ce1−xSmxO2−x (SDC) (x=0–0.25) were synthesized using an oxalate coprecipitation process. Dependence of a, unit cell parameter versus dopant concentration, x, of Gd3+ and Sm3+ ions show that these solid solutions obey Vegard’s rule as a=5.4121+0.0525x for GDC and a=5.4117+0.1237x for SDC, respectively. Electrical conductivity reached maximum at x=0.15 in the temperature range of 400–850 °C for both kinds of doped-ceria electrolyte membranes. A single cell was made for the measurement of open circuit voltage. The results show that the open circuit voltages are greatly influenced by ionic transference number of the electrolyte, gaseous fuel composition and cathode membrane material.

Although a lot of work has been done on electroless nickel coating, work on preparing asymmetric nickel membranes on porous ceramic supports is rarely reported. In this work, microfiltration nickel membranes are prepared on porous alumina supports with electroless plating. A new approach technique, sol–gel process is used to activate the alumina supports. The coated membranes were investigated by means of scanning electron microscopy and gas permeation test. Membrane thickness and amount of nickel increase with plating time. Membrane pore size decreases greatly as electroless coating is processed for 15 min. After 15-min coating the pore size decreases slightly. For a 90-min electroless plated membrane, the thickness reaches 4.5 μm and the mean pore radius is 0.13 μm with a narrow distribution.

Bismuth has been doped into mixed ionic–electronic conducting La1.75Sr0.25NiO4+δ (LSN) with the 2D K2NiF4-type structure to evaluate its influence on various properties of the host material, which include its potential use as a SOFC cathode. X-ray powder diffraction indicates that LSN retains its tetragonal structure after doping with 5 mol% bismuth to form La1.65Bi0.1Sr0.25NiO4+δ (LSN–Bi). Bismuth doping profoundly lowers (by ∼150 °C) the sintering temperature of LSN. Both LSN and LSN–Bi show excellent compatibility with electrolytes yttria-stabilized zirconia (YSZ) and samaria-doped ceria (SDC) in terms of thermal expansion and chemical reactivity (<900 °C). The electrical conductivity of both materials is metallic like and reaches values of 99.3 S cm−1 and 100.3 S cm−1 at 550 °C for LSN and LSN–Bi, respectively. The data from electrical conductivity relaxation (ECR) measurements demonstrate that the substitution of lanthanum by bismuth enhances the chemical diffusion coefficient (Dchem) and surface exchange coefficient (kchem) by factors of 2–3. The faster kinetics of oxygen transport exhibited by LSN–Bi relative to parent LSN is reflected by a lower polarization resistance of the former when the electrode performance of both materials is compared in symmetric cells. The corresponding values at 700 °C are 4.2 Ω cm2 and 0.61 Ω cm2 for LSN and LSN–Bi, respectively. High peak power densities are achieved (328 mW cm−2 and 131 mW cm−2 at 700 and 600 °C, respectively), when LSN–Bi is incorporated as the cathode in a fuel cell operated with humidified hydrogen as the fuel and air as the cathode gas. The material is considered a promising candidate for further study.

The oxygen incorporation kinetics of Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O1.9 (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of SDC phase volume fraction using electrical conductivity relaxation. It is shown that the oxygen re-equilibration kinetics in the range of oxygen partial pressure (pO2) from 0.01 to 1 atm is limited by the surface exchange rate. The effective surface exchange coefficient of the composites is found to increase profoundly upon increasing the phase volume fraction of the oxide electrolyte phase SDC. The results are interpreted to reflect the synergistic oxygen incorporation at the SFM–SDC–gas triple phase boundaries (TPBs), which occurs in addition to the direct incorporation via the surface of the perovskite mixed conductor SFM. Already at a SDC phase volume fraction of 0.105, the uptake of oxygen via the synergistic TPB route (referred to as route III), following a step change in the surrounding pO2, comprises more than 75% of the overall uptake of oxygen by the composite. It is further concluded that under the conditions of the experiments the two-phase SFM–SDC boundaries allow for a facile exchange of oxygen ions between both involved phases.

The oxygen release kinetics of mixed-conducting Sr2Fe1.5Mo0.5O6−δ–Sm0.2Ce0.8O2−δ (SFM–SDC) dual-phase composites has been investigated, at 750 °C, as a function of the SDC phase volume fraction using electrical conductivity relaxation (ECR) under reducing atmospheres, extending our previous work on the oxygen incorporation kinetics of these composites under oxidizing conditions. Gas mixtures of H2/H2O and CO/CO2 were used to control step changes in the oxygen partial pressure (pO2) in the range 10−24 to 10−20 atm. At the conditions of the experiments, oxygen re-equilibration is entirely controlled by the surface exchange kinetics. A model is developed which allows deconvolution of the effective time constant of the relaxation process in terms of the intrinsic contributions of the components to oxygen surface exchange and synergetic contributions caused by heterogeneous interfaces. The oxygen surface exchange kinetics under H2/H2O atmosphere is found to be a weighted average of the intrinsic contributions of SFM and SDC phases. No evidence is found for an enhanced exchange rate at the SFM–SDC–gas triple phase boundaries (TPB). Synergetic contributions arise under CO/CO2 atmosphere, enhancing the rate of oxygen surface exchange up to a factor of 2.4. The obtained results are discussed in terms of the surface microstructure of the composites from image analysis. Overall, the results of this and our previous study confirm that the triple phase boundaries in SFM–SDC composites significantly accelerate the oxygen incorporation kinetics under oxidizing conditions, but only modestly, or even negligibly, influence the oxygen release kinetics under reducing conditions.

The solid oxide fuel cell (SOFC) is an environmentally-friendly, highly efficient, and fuel adaptable electrochemical conversion device. The Cu–CeO2 material has been recognized as a promising anode material for SOFCs. Although Cu is not a good enough oxidation catalyst, and has a relatively low catalytic activity, a combination of Cu and CeO2 can strengthen catalytic activity and overcome the problems associated with either Cu or CeO2 individually. Our density functional theory (DFT) calculations illustrate that a Cu cluster supported on CeO2(111) suppresses the formation of interface oxygen vacancies, while also enhancing the catalytic activity, and reduces the energy barrier of the H2 oxidation reaction process compared to that of stoichiometric CeO2(111). The three phase boundary (TPB) pathway with the highest energy barrier of 0.836 eV is obviously much lower than the stoichiometric CeO2(111) equivalent with the highest energy barrier of 2.399 eV. Experimentally, temperature programmed reduction (TPR) experiments establish that Cu particles can reduce the reduction reaction temperature for ceria and increase the amount of reduced ceria. In addition, a dramatic comparison between pure ceria and Cu-modified ceria through electrical conductivity relaxation (ECR) experiments quantitatively demonstrates that Cu particles greatly improve the reaction kinetics with the specific oxygen surface exchange coefficient increasing from 1.012 × 10−4 cm s−1 of the bare ceria to 12.180 × 10−4 cm s−1 of the Cu-modified ceria (CeO2–Cu80), which agrees well with the results of the theoretical calculations.

There is increasing interest in converting CO2/H2O to syngas via solid oxide electrolysis cells (SOECs) driven by renewable and nuclear energies. The electrolysis reaction is usually conducted through Ni–YSZ (yttria stabilized zirconia) cermets, state-of-the-art fuel electrodes for SOECs. However, one obvious problem for practical applications is the usage of CO/H2 safe gas, which must be supplied to maintain the electrode performance. This work reports a safe gas free ceramic electrode for efficient CO2/H2O electrolysis. The electrode has a heterogeneously porous structure with Sr2Fe1.5Mo0.5O6−δ (SFM) electrocatalyst nanoparticles deposited onto the inner surface of the YSZ scaffold fabricated by a modified phase-inversion tape-casting method. The nanostructured SFM–YSZ electrodes have demonstrated excellent performance for CO2–H2O electrolysis. For example, the electrode polarization resistance is 0.25 Ω cm2 under open circuit conditions while the current density is 1.1 A cm−2 at 1.5 V for dry CO2 electrolysis at 800 °C. The performance is comparable with those reported for the Ni–YSZ fuel electrodes, where safe gas must be supplied. In addition, the performance is up to one order of magnitude better than those reported for other ceramic electrodes such as La0.75Sr0.25Cr0.5Mn0.5O3−δ and La0.2Sr0.8TiO3+δ. Furthermore, the electrode exhibits good stability in the short-term test at 1.3 V for CO2-20 vol% H2O co-electrolysis, which produces a syngas with a H2/CO ratio close to 2. The reduced interfacial polarization resistance, high current density, and good stability show that the nanostructured SFM–YSZ fuel electrode is highly effective for CO2/H2O electrolysis without using the safe gas, which is critical for practical applications.

Strontium doped lanthanum cobalt ferrite (LSCF) is a widely applied electrocatalyst for the oxygen reduction reaction (ORR) in solid-oxide fuel cells (SOFCs) operated at intermediate temperatures. Sr surface segregation in long-term operation has been reported to have contradicting effects that either degrade or improve the reaction. Thus, it is critical to understand the mechanism of surface Sr compounds on ORR kinetics. This work aims to verify the effect and propose the mechanism by decorating SrCO3 nanoparticles using the infiltration method. Electrochemical conductivity relaxation measurements show that SrCO3 particles improve the chemical oxygen surface exchange coefficient by up to a factor of 100. The electrochemical performance is significantly improved by the infiltration of SrCO3, which is comparable to those obtained by typical electrocatalysts including precious metals such as Pd and Rh. Distribution of relaxation time (DRT) analysis shows that the performance enhancement is strongly related to the improved kinetics of charge transfer and oxygen incorporation processes. Density functional theory calculations show that the surface SrCO3 reduces the O2 dissociation energy barrier from 1.01 eV to 0.33 eV, thus enhancing the ORR kinetics, possibly through changing the charge density distribution at the LSCF–SrCO3 interface.