•The slow diffusion process of CO2 is the rate-determining step for the whole reaction process of co-electrolysis.•The ASR of pure CO2 electrolysis is about three times that of H2O.•The CO addition in the inlet gas was not favorable for the co-electrolysis process.Flexible nuclear power for synthetic fuel production through high temperature co-electrolysis technology (HTCE) using solid oxide electrolysis cell (SOEC) has recently received increasing international interest in the large-scale, highly efficient and carbon-neutral energy storage field. It is of great importance to enhancing the understanding of co-electrolysis process and the related mechanism. In this paper, CO2 behavior and its effect on the performance of SOEC were examined by the electrochemical characterization and impedance analysis to determine the proper operating conditions, such as H2O, CO2, H2, CO, operation temperature and electrolysis current. The polarization mechanism is also investigated by the experimental and modeling results. It was found that the electrolysis of CO2 is much harder than that of H2O, and the ASR of pure CO2 electrolysis is about three times that of H2O. When the CO2 content decreases from 50% to 10%, the ASR decreases from 1.59 to 0.90 Ω cm2. Increasing the H2O content could also improve the electrolysis efficiency to some degree, while the CO addition in the inlet gas was not favorable for the process. Mechanism study shows that the diffusion impedance of CO2 should be the restricted step for the polarization energy loss.

•Micro-channel structure is a promising electrode design for SOECs due to its high stability.•Recent progress on the preparation of micro-channel structure by freeze casting is reviewed.•Factors influencing the preparation process are discussed in detail.Freeze casting, a near net shaping technique to fabricate porous materials with directional microstructures, has a very good development prospect. This review elaborates the applications of freeze casting technology on the preparation of robust YSZ (yttria stabilized zirconia) scaffolds in solid oxide electrolysis cells (SOECs), with the goal of reducing the polarization resistance caused by gas diffusion. In this article, the freeze casting process of YSZ support and its solidification principles are summarized. Meanwhile, the critical factors influencing the pore morphology and distribution, such as the dispersion mediums, additives, freezing conditions, and solid content, are included and discussed. The further development, application and prospect of freeze casting technology are proposed.

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H2O/CO2 and metal–air batteries.

High-temperature solid oxide electrolysis cells (SOECs) are advanced electrochemical energy storage and conversion devices with high conversion/energy efficiencies. They offer attractive high-temperature co-electrolysis routes that reduce extra CO2 emissions, enable large-scale energy storage/conversion and facilitate the integration of renewable energies into the electric grid. Exciting new research has focused on CO2 electrochemical activation/conversion through a co-electrolysis process based on the assumption that difficult CO double bonds can be activated effectively through this electrochemical method. Based on existing investigations, this paper puts forth a comprehensive overview of recent and past developments in co-electrolysis with SOECs for CO2 conversion and utilization. Here, we discuss in detail the approaches of CO2 conversion, the developmental history, the basic principles, the economic feasibility of CO2/H2O co-electrolysis, and the diverse range of fuel electrodes as well as oxygen electrode materials. SOEC performance measurements, characterization and simulations are classified and presented in this paper. SOEC cell and stack designs, fabrications and scale-ups are also summarized and described. In particular, insights into CO2 electrochemical conversions, solid oxide cell material behaviors and degradation mechanisms are highlighted to obtain a better understanding of the high temperature electrolysis process in SOECs. Proposed research directions are also outlined to provide guidelines for future research.

•A comprehensive overview of energy related CO2 conversions.•Fundamentals of CO2 including molecular structure, thermodynamics and kinetics.•Mechanisms and features of five typical conversion technologies are summarized.•Economic feasibility analysis of various CO2 conversion technologies.•Challenges, future outlook and perspectives on the energy related conversion of CO2.CO2 conversion to produce useful fuels/chemicals is a promising route for reducing CO2 emission as well as for exploring the promising energy storage method. To facilitate the research and development of CO2 conversion, this paper provides a comprehensive overview of CO2 conversions using advanced materials/nanomaterials and technologies for the production of useful fuels/chemicals. The molecular structure, thermodynamics and kinetics of CO2 are reviewed for the understanding of fundamentals and explaining why C=O double bonds are difficult to break. The mechanisms and features of various conversion technologies are summarized and classified into enzymatic, mineralization, photochemical/photo-electrochemical, thermochemical as well as electrochemical processes. In particular, by comparing electrochemical conversion technologies at low and high temperatures, CO2 conversion at intermediate temperatures is emphasized. Furthermore, the economic feasibility for CO2 utilization is analyzed with cases for the production of various chemicals/fuels. The technical and application challenges of CO2 conversion into useful fuels/chemicals are also summarized, composing mainly of: insufficient fundamental understanding, low product selectivity, as well as low efficiency and stability. To overcome these challenges, future research directions are proposed in this review.Main approaches for carbon dioxide reduction [1,2]. (a) Controlling the polymorphism of CaCO3 to generate value-added mineral carbonation products [3]; 2 × 3 supercells (b) and EELS elemental mapping (c) of the core/shell Cu/SnO2 catalyst [4]; the reverse water gas shift (RWGS) reaction on Pd-Ni nanoalloys (d) and EDX analysis (e) of this nanoalloys on fuel-electrode substrate [5]; 2D in-situ XRD map (f) and EDX analysis and corresponding STEM image (g) of 50Fe2O3/MgAl2O4 used in super-dry reforming of CH4 [6]; Artificial photosynthesis for HCOOH generation from CO2 (h) [7]; Behaviors of an oxygen vacancy defect (VO), a bridging hydroxyl (OHb) and an absorbed CO2 molecule on reduced TiO2 (110) surface (i) and STM image of this surface after CO2 adsorption (j) [8]. LT, Low-temperature, HT, High-temperature. Reproduced with permission from Ref. [3–8]. Copyright 2017 and 2017, American Chemical Society; Copyright 2016 Elsevier B.V.; Copyright 2016, American Association for the Advancement of Science; Copyright 2012 and 2011, American Chemical Society, respectively.Download high-res image (484KB)Download full-size image

High Temperature Electrolysis (HTE) through a solid oxide electrolytic cell (SOEC) had been receiving more and more attentions recently because of its high conversion efficiency (45–59%) and its potential usage for large-scale hydrogen or synthetic fuels production. One of the key technologies associated with SOEC fabrication was to prepare dense yttria-stabilized zirconia (YSZ) electrolyte film on the surface of hydrogen electrode. A novel screen-printing method was developed to fabricate gas-tight YSZ films on porous NiO-YSZ to reduce ohmic resistance of electrolytes and improve electrochemical performance of cells in this paper. The effects of pre-calcining temperature of cathodes, numbers of printing layers and sintering temperature of YSZ films were investigated in detail. SEM and EIS analyses revealed that the selected process parameters had significant influences on the microscopic morphology of YSZ electrolyte film, the OCVs and power density of the prepared cells. After optimization, a 10 μm dense YSZ film was prepared successfully on porous NiO-YSZ support with an OCV of 1.069 V and the electrolysis current density up to 0.681 A/cm2 at 1.50 V and 850 °C.Highlights► A ceramic power method was developed to fabricate gas-tight YSZ on porous Ni/YSZ. ► Effects of process conditions were investigated in detail. ► A 10 μm dense YSZ film was prepared successfully on porous Ni/YSZ support. ► The electrolysis current density can be up to 0.681 A/cm2 at 1.50 V and 850 °C.

High-temperature steam electrolysis (HTSE) systems using solid oxide electrolysis cells (SOECs) provided a promising method for highly efficient large-scale hydrogen production, which was one of the most potential hydrogen production technologies to meet the hydrogen economy demand in the future.The physical properties and microstructures of supporting cathodes are crucial for the performances of the entire SOECs. For this reason, four different pore formers (polymethyl methacrylate (PMMA), potato starch, ammonium oxalate, ammonium carbonate) were considered for its optimization. Their influence on the amount of porosity and on the pore shape and distribution as well as the effect on the electronic conductivity was analyzed. The results showed that PMMA was the most promising pore former, which had high porosity and uniform pore size distribution. The optimum weight percent concentration was 10%, correspondingly, porosity was 45% and electronic conductivity was 6726S cm−1, which was suitable for supporting cathodes for SOEC application. The pore former of potato starch was better than the inorganic pore formers of ammonium oxalate and ammonium carbonate. The optimum weight percent concentration was 10%, correspondingly, porosity was 40% and electronic conductivity was 5827S cm−1, which was not suitable for supporting cathodes for SOEC application, while, It was suitable for supporting anodes for SOFC application.

Hydrogen generation through high temperature steam electrolysis (HTSE) using solid oxide electrolysis cells (SOEC) has recently received increasingly international interest in the large-scale, highly efficient nuclear hydrogen production field. The research and development of HTSE technology was initiated in INET of Tsinghua University from 2005 as one of the approaches in National Key Special Projects for HTGR which aims at promoting highly efficient and sustainable application of nuclear process heat in the future. In the past three years, the research team mainly focused on preliminary investigation, feasibility study, equipment development and fundamental research. Currently, two bench-scale equipments for the study of HTSE process and SOEC components have been designed and constructed. In addition, the research group made rapid progress in the development of novel anode materials, effective microstructure control of cathodes and theoretically quantitative analysis of hydrogen production efficiency through HTSE coupled with HTGR.

Nickel oxide and yttria-stabilized zirconia (NiO–YSZ) composite powders were synthesized by a new situ-combustion method in this paper. The adding amount of CO(NH2)2 was calculated by the combustion reaction equation. The products were characterized by X-ray diffraction, field emission scanning electronic microscope and electrochemical impedance spectra (EIS). The results showed that the products were well crystallized with NiO coating on YSZ particles. The optimized ratio of CO(NH2)2 to Ni(NO3)2 was 2:1. A single solid oxide electrolysis cell made from NiO–YSZ composite cathode with the powder prepared at optimized ratio 2:1 exhibited better performance than other samples with the electrolytic voltage of 0.98 V. The electrolytic cell was operated steadily at 900 °C for 2 h with the current of 0.33 A cm−2 when the stream of 80% H2O + 20% H2 was input. EIS analysis indicated that H2O adsorption and diffusion of the Ni–YSZ electrode were the limited step in the whole electrolysis reaction.

High-temperature steam electrolysis (HTSE), a reversible process of solid oxide fuel cell (SOFC) in principle, is a promising method for highly efficient large-scale hydrogen production. In our study, the overall efficiency of the HTSE system was calculated through electrochemical and thermodynamic analysis. A thermodynamic model in regards to the efficiency of the HTSE system was established and the quantitative effects of three key parameters, electrical efficiency (ηel), electrolysis efficiency (ηes), and thermal efficiency (ηth) on the overall efficiency (ηoverall) of the HTSE system were investigated. Results showed that the contribution of ηel, ηes, ηth to the overall efficiency were about 70%, 22%, and 8%, respectively. As temperatures increased from 500 °C to 1000 °C, the effect of ηel on ηoverall decreased gradually and the ηes effect remained almost constant, while the ηth effect increased gradually. The overall efficiency of the high-temperature gas-cooled reactor (HTGR) coupled with the HTSE system under different conditions was also calculated. With the increase of electrical, electrolysis, and thermal efficiency, the overall efficiencies were anticipated to increase from 33% to a maximum of 59% at 1000 °C, which is over two times higher than that of the conventional alkaline water electrolysis.

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) was synthesized successfully by a novel citric acid–nitrate combustion method and employed as the anode of solid oxide electrolysis cells (SOEC) for hydrogen production for the first time in this paper. The crystal structure, chemical composition and electrochemical properties of BSCF were investigated in detail. The results showed that BSCF is in good stoichiometry of Ba0.5Sr0.5Co0.8Fe0.2O3−σ formation. ASR of BSCF/YSZ is only 0.077 Ω cm2 at 850 °C, remarkably lower than the commonly used oxygen materials LSM as well as the current focus materials LSC and LSCF. Also, BSCF electrode exhibited much better performance than LSM under both SOEC and SOFC operating modes. The hydrogen production rate of BSCF/YSZ/Ni-YSZ can be up to 147.2 mL cm−2 h−1, about three times higher than that of LSM/YSZ/Ni-YSZ, which indicates that BSCF could be a very promising candidate for the practical application of SOEC technology.

High-temperature solid oxide electrolysis cells (SOECs) are advanced electrochemical energy storage and conversion devices with high conversion/energy efficiencies. They offer attractive high-temperature co-electrolysis routes that reduce extra CO2 emissions, enable large-scale energy storage/conversion and facilitate the integration of renewable energies into the electric grid. Exciting new research has focused on CO2 electrochemical activation/conversion through a co-electrolysis process based on the assumption that difficult CO double bonds can be activated effectively through this electrochemical method. Based on existing investigations, this paper puts forth a comprehensive overview of recent and past developments in co-electrolysis with SOECs for CO2 conversion and utilization. Here, we discuss in detail the approaches of CO2 conversion, the developmental history, the basic principles, the economic feasibility of CO2/H2O co-electrolysis, and the diverse range of fuel electrodes as well as oxygen electrode materials. SOEC performance measurements, characterization and simulations are classified and presented in this paper. SOEC cell and stack designs, fabrications and scale-ups are also summarized and described. In particular, insights into CO2 electrochemical conversions, solid oxide cell material behaviors and degradation mechanisms are highlighted to obtain a better understanding of the high temperature electrolysis process in SOECs. Proposed research directions are also outlined to provide guidelines for future research.