•Pt/CN-doped yields superior catalytic activity to methanol oxidation reaction.•The −OH species on carbon promotes strip off of CO-intermediates on Pt.•A strong interfacial interaction exists between Pt and support.•2D carbon extends the interface, favoring both Pt utilization and mass transfer.Nitrogen-doped ordered mesoporous carbon (NOMC) is studied as the support to synthesize a Pt/CN-doped catalyst for methanol oxidation reaction (MOR). The effects of carbon dimension and metal loading are investigated by nitrogen ad/desorption isotherms, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Both TEM and XRD results show that platinum nanoparticles (Pt NPs) are highly dispersed on NOMC with a uniform and narrow distribution, while the dimension of NOMC has a considerable effect on the dispersion of Pt. For 10 wt% metal loading, the average diameter of Pt NPs on 2D-NOMC is 1.40 nm; smaller than that on 3D-NOMC (1.90 nm). XPS results reveal that a strong electronic interaction exists between Pt and NOMC, indicating the formation of Pt–N bonds at the interface. Such an interaction gets more pronounced in the case of low Pt loadings and low-dimensional supports; the reason is that the 2D support is more accessible to load Pt and the metal-support interface is better developed at low metal loadings.The electrochemical results are well correlated with the physiochemical characterizations. 1) The electrochemically active surface area of Pt is higher on the 2D-support than that on the 3D-one, confirming the better dispersion of Pt on NOMC-2D. 2) A positive shift in the potential is observed for the adsorption of oxygen-containing species onto Pt, which is indicative of the charge transfer from Pt to the support and the formation of Pt–N bonds. 3) Both onset and peak potentials are negatively shifted by ca. 50 mV for MOR on 10 wt% Pt/NOMC-2D, as compared with the commercial Pt/XC-72. The ratio of forward to backward current, a measure of poisoning tolerance, is 1.1 on Pt/NOMC-2D and 0.80 on Pt/XC-72R. This enhancement can be attributed to the bifunctional mechanism at the Pt/CN-doped interface. In MOR, CO-like intermediate species on Pt can be effectively stripped off by the adjacent active –OH species on carbon, generated at lower potentials.Download high-res image (190KB)Download full-size image

•An ultrathin (1.0 nm thick) 2D nitrogen-doped hierarchically porous carbon (2DNHPC) film is developed.•It is revealed that 2DNHPC is featured by extremely high aspect ratio and bimodal pore distribution.•2DNHPC yields high ORR limiting current, showing the role of low dimensional structure in mass transfer.•2DNHPC has comparable ORR activity with commercial Pt catalyst in both alkaline and acid media.An ultrathin (thickness 1.0 nm) 2D nitrogen-doped hierarchically porous carbon (2DNHPC) film is developed by the nanocasting method; for comparison, a 3D nitrogen-doped ordered mesoporous carbon (3DNOMC) is also synthesized. Characterizations reveal that 2DNHPC is featured by an extremely high aspect ratio (several hundred) and a bimodal pore distribution. Such a 2D hierarchically porous structure is found to facilitate both the mass transfer of the reactive species and the utilization of active site in the electrode. First, 2DNHPC yields a larger limiting current than does 3DNOMC for the oxygen reduction reaction (ORR), revealing the key role of the low dimensional structure to facilitate the mass transfer. Second, at the loading of 500 μg cm−2, 2DNHPC shows the same kinetic current with 3DNOMC, indicating that the two catalysts have the same active site and turnover frequency. In comparison, at a lower loading of 250 μg cm−2, the kinetic current of 2DNHPC remains unchanged, which however seriously deteriorates for 3DNOMC. This result strongly highlights the effect of the carbon dimension on the utilization efficiency of the active site. Finally, it is noted that 2DNHPC yields a comparable ORR electrocatalytic activity and long-term stability with commercial Pt catalyst in both alkaline and acid media.Download high-res image (160KB)Download full-size image

•Interpenetrated nonporous MOF used as precursor to synthesize N-doped hierarchically mesoporous carbon.•Enriched metal ions in MOF are effective in generating mesopores as a pore-forming agent in final carbon.•Carbon yields superior electrocatalytic activity and remarkable anion-insensitivity for ORR.Nitrogen-doped carbon materials with hierarchically mesoporous structure are synthesized in the present work via the pyrolysis of an interpenetrated non-porous metal-organic framework (MOF), viz. [Zn2(TPT)(BDC)2]·H2O (SCUT-11, TPT = tris(4-pyridyl)triazine, BDC = 1,4-benzenedicarboxylate), as the precursor. X-ray diffraction reveals that the synthesized metal-organic framework (MOF) is of high purity of the crystalline phase, and its structure follows our previously reported SCUT-11. This triply-interpenetrated MOF features high density of Zn cations in their interwoven packing structure, which act as effective pore-forming agent to generate mesopores in final carbon. Physicochemical characterizations reveal that the resultant carbon has high specific surface area and bimodal mesopore size distribution, which originate from the removal of metal oxide and/or metal zinc. These textual features favour both oxygen mass transfer and accessibility of catalytically active sites. Electrochemical results confirm that the resultant carbon, synthesized by pyrolysis at 900 °C, shows a superior oxygen reduction reaction (ORR) activity, which is associated with high onset and half-wave potential up to 1.0 and 0.88 V, respectively. Further investigation suggests that the as-synthesized carbon catalyst exhibits a remarkable insensitivity towards anions, like sulphate and phosphate, compared with the Pt counterpart. The above features make this carbon catalyst promising to be widely used in different fuel cell types.Download high-res image (168KB)Download full-size image

A novel ultrathin silica film with semi-ordered fingerprint-like mesopores is synthesized with the aid of the dual templates of graphene oxide (go) and tri-block copolymer P123. The formation is proposed to proceed by a co-operative assembly mechanism. As an application, the silica film is used to synthesize a 2D nitrogen-doped mesoporous carbon film, which yields superior kinetic and diffusion-limit currents than the commercial Pt/C catalyst.

•Interaction between SBA-15 and carbon precursor promotes the nitrogen doping.•Nanoconfinement affects composition/structure of N-doped carbon during pyrolysis.•Electrocatalytic activity/selectivity is dramatically improved in this way.The effect of the pore structure of SBA-15, which is tuned by varying the hydrothermal temperature (90–150 °C), is investigated on nitrogen-doped ordered mesoporous carbon. It is found that the pore structure of SBA-15 yields effects on both the composition and the pore structure of carbon, which thereby affecting its electrocatalytic activity for the oxygen reduction reaction (ORR). XPS reveals that the nitrogen content of the template-free carbon is 2.79 at.%, while dramatically increases to 4.48, 5.01 and 4.16 at.% for the three template-aided carbon catalysts with SBA-15 synthesized at 90, 130 and 150 °C, respectively. Nitrogen ad/desorption isotherms show that the specific surface area of the template-free carbon is 257 m2 g−1, which increases to 724, 731 and 691 m2 g−1 for the above three carbons. Such changes correlate well with the electrochemical results. RRDE results show that the template-aided carbon catalysts yield better activity and selectivity for the ORR than does the template-free one. And the best performance is achieved for the carbon catalyst with SBA-15 synthesized at 130 °C, which coincidentally has the highest nitrogen content and surface area.

In this work, the effect of pH on a nitrogen-doped ordered mesoporous carbon catalyst for the oxygen reduction reaction (ORR) is extensively investigated. Electrochemical methods, including cyclic voltammetry (CV), rotating ring-disk electrode (RRDE), and cathodic stripping voltammetry, are applied to investigate the electrochemical behavior in electrolyte solutions of different pHs (0–2, 7, 12–14). The CV result reveals that nitrogen-doped carbon has a variety of enriched reversible redox couples on the surface, and the pH has a significant effect. Whether these redox couples are electrochemically active or inactive to the ORR depends on the electrolyte used. In acid media, an oxygen molecule directly interacts with the redox couple, and its reduction proceeds by the surface-confined redox-mediation mechanism, yielding water as the product. Similarly, the first electron transfer in alkaline media is achieved by the surface-confined redox-mediation mechanism at the higher potentials. With decreasing potential, another parallel charge transfer process by the outer-sphere electron transfer mechanism gets pronounced, followed by parallel 2-e and 4-e reduction of oxygen. The proposed mechanisms are well supported by the following electrochemical results. At high potentials, the Tafel slope remains unchanged (60–70 mV dec–1) at all investigated pHs, and the reaction order of proton and hydroxyl ions is found to be 1 and −0.5, respectively, in acid and alkaline media. The electron transfer number is ∼4 at high potentials in both acid and alkaline media; however, at higher pHs, it shows a considerable decrease as the potential decreases, indicating the change in the reaction pathway. Finally, the nitrogen-doped carbon catalyst shows performance in alkaline media superior to that in acid media. Such a gap in performance is rationalized by considering the chemical change in the surface at different pH values.Keywords: active site; electrocatalytic activity; nitrogen-doped carbon; ordered mesoporous carbon; oxygen reduction reaction; pH effect

In this work, high-performance LiFePO4/C materials are successfully prepared by a spray drying-carbothermal method. The effect of the carbon source (viz. inorganic, organic and their hybrid) is extensively investigated on the microstructure and electrochemical property of the composite materials. Vulcan XC-72, an inorganic carbon additive, can remarkably improve the electronic conductivity of the composite material; however, the resultant large crystallite size and low specific surface area limit the Li ion diffusion and thereby lower the electrochemical performance. As a typical organic carbon additive, glucose yields mesoporous LiFePO4/C particles with a much higher specific surface area and smaller crystallite size; however, the electronic conductivity of the pyrolyzed carbon is insufficiently high for the real applications. By using the dual hybrid carbon sources, the synthesized LiFePO4/C material features a high electronic conductivity, large specific surface area, small grain size, and the formation of mesopores. In line with these advantages, the composite yields the best electrochemical performance with the discharge capacity of 168.1 mAh g−1 at 0.1 C, which is approximate to the theoretical capacity. This material also performs well at high rates with discharge capacities of 122.0 and 93.0 mAh g−1 at 5.0 and 10.0 C, respectively.Graphical abstractHigh-performance LiFePO4/C composite materials were synthesized by using hybrid organic/inorganic carbon sources, featuring a mesoporous nature, large specific surface area, small grain size and decent electronic conductivity.Highlights► We prepare the LiFePO4/C materials by a facile and scalable method. ► Carbon source has a complex and coupled effect on LiFePO4/C. ► Inorganic carbon source benefits the electronic conductivity of LiFePO4/C. ► Organic carbon source benefits the Li ion diffusion of LiFePO4/C. ► Dual carbon sources yield the best electrochemical performance and rate capacities.

Hemin has been reported to be an effective electrocatalyst for mediating the oxygen reduction reaction. In this work, the stability of hemin/C is extensively investigated in both acid and alkaline media by the electrochemical methods. It is found that the pristine hemin/C yields significant change in the composition and the electrochemical features when it undergoes the potential cycling in acid media. In comparison, the catalyst shows superior stability in alkaline media. The pyrolysis can improve the stability of the hemin/C catalyst by removing the organic groups in hemin; however, the heat treatment cannot prevent the metal ion loss in acid media. Finally, the acid-leaching experiment reveals that the active center for the 4-electron reaction tends to get lost in acid, indicating that the iron metal ion should be involved in catalyzing the 4-electron reduction reaction. Furthermore, the XPS result indicates that the element N is also involved in the active center. Therefore, it can be concluded that the Fe–N contributes to the active center for the complete reduction of oxygen in alkaline media.Highlights►Pyrolysis cannot mitigate the demetallization of hemin in acid media. ►Hemin is electrochemically stable in alkaline media. ►Fe–N is involved in the active center for the 4-e reduction reaction of oxygen.

A miniature air-breathing twin-cell stack is designed and evaluated for direct formic acid fuel cell (DFAFC) applications. The stack consists of two face-to-face single cells with one shared fuel reservoir. This particular design has advantages in volume reduction relative to single cells in series connection. The performance, stability and reproducibility of the stack are investigated extensively for practical fuel cell applications. A maximum power density of 44.5 mW cm−2 is obtained with 5.0 M formic acid solution as the fuel and Pt catalysts in electrodes. It is also found that the stack yields high stability and reproducibility when discharged at a constant current of 20 mA. The output voltage can be maintained at 1.14 V for about 5 h by feeding 3.5 ml 5.0 M formic acid solution and the performance can almost be reproduced when the fresh fuel is injected.

Carbon-supported hemin (hemin/C) was investigated as a nonprecious metal electrocatalyst mediating the oxygen reduction reaction (ORR). The effect of the heat treatment and metal content on the electrocatalytic activity was extensively studied. We found that the electrocatalytic activity was significantly improved after the heat treatment, and the best performance was achieved at the treatment temperature of 600 °C. Physico-chemical characterizations indicated that the heat treatment temperature yielded a coincident effect on the composition and microstructure of the catalyst. X-ray photoelectron spectroscopy (XPS) revealed that the atomic ratio of Fe to N was 1:4.7 for pristine hemin/C, which amounted to the highest value of 1:1.9 for hemin/C heat-treated at 600 °C. Nitrogen ad/desorption analysis showed that the pyrolysis at 600 °C resulted in the largest microporous surface area. On the basis of the above findings, we suggest that FeN2 be the active center in hemin/C and the micropore play an important role in catalyzing the ORR. Furthermore, we also found that the electrocatalytic activity of hemin/C was improved with increasing the metal content. More inspiringly, we report that the optimized hemin/C electrocatalyst could yield a comparable electrochemical performance with the commercial Pt catalyst in alkaline media.

We electrodeposited noble metal (palladium, platinum) nanowires into the hydrophilic pores of Nafion membrane for mitigating the problem of methanol crossover in direct methanol fuel cells (DMFCs). The DMFC performance result shows that the composite membranes yield lower rate of methanol crossover and better cell performance than the pure Nafion® membrane. At low current densities, the Pd nanowire incorporated Nafion membrane shows the best performance. In comparison, the highest performance is achieved at higher current densities with the Pt nanowire modified Nafion membrane. Based on the above findings, we suggest that for the Pd nanowire incorporated Nafion membrane, the mechanism for the suppression of the methanol crossover is mainly the blocking effect due to the ‘narrowed’ hydrophilic channels in Nafion membrane. For the Pt nanowire modified Nafion membrane, the mechanism includes both increasing the membrane tortuosity and so-called ‘on-way consumption’ of methanol on the Pt nanowires deposited into the Nafion membrane when the fuel cell is discharging.

•Nitrogen-doped carbon catalyst with decent activity for hydrogen evolution reaction.•Dopant nitrogen atoms act as the active site for hydrogen evolution reaction.•Electrocatalytic activity is superior in acid vs. alkaline media.•Hydrogen evolution reaction proceeds by Volmer-Heyrovsky mechanism.•Hydrogen underpotential deposition is possible on the nitrogen-doped carbon.The nature of active sites and mechanism of hydrogen evolution reaction (HER) on the nitrogen-doped carbon catalyst is extensively investigated, by combining physicochemical and electrochemical methods. Two carbon catalysts, with the same chemical nature but different nitrogen content, are employed in this investigation. Electrochemical methods are applied to investigate the electrochemical behavior at different pH values (1.0–2.0, 12.0–13.0). It is found that increasing nitrogen content has a positive effect on the electrocatalytic activity, and therefore, the doped nitrogen atoms should be the active sites. The kinetic current, normalized by the surface nitrogen content, is found to be the same for the two catalysts, confirming the above claim. As such, HER is proposed to proceed on these active sites by the Volmer-Heyrovsky mechanism. The electrochemical tests reveal that the electrocatalytic activity closely relies on the solution pH, which is due to the chemical evolution of the active site in different solutions. In acid media, the electrocatalytic activity increases with the concentration of proton, and the Tafel slope is ca. 120 mV dec−1. It is proposed that the electrochemical desorption of proton on the doped nitrogen atoms is the rate determining step (r.d.s.). In alkaline media, the electrocatalytic activity increases with pH, because the increase in pH dramatically enhances the basicity or the surface charge density, thereby facilitating charge transfer and improving activity. In alkaline media, Tafel analysis shows that Heyrovsky step is the rate determining.Download high-res image (67KB)Download full-size image