For promoting the development of fuel cells and metal-air batteries, it is imperative to replace rare and expensive precious-metal catalysts with sustainable catalysts composed of earth-abundant elements. Herein, we reported the preparation of polyaniline derived N- and O-enriched high surface area hierarchical porous carbon catalysts that can be regarded as metal-free catalysts for oxygen reduction reaction (ORR). Using perchloric acid (HClO4) as oxidant and pore-forming agent, the N- and O-enriched high surface area hierarchical porous carbon catalysts were fabricated by electrochemical polymerization of aniline, followed by high temperature carbonization of the resulting polyaniline (PANI) precursors. The obtained carbon catalysts exhibited excellent catalytic activity for ORR with high ORR onset potential and high kinetic current density, and good electrochemical stability. These superior electrochemical properties of the N- and O- enriched high surface area hierarchical porous carbon catalysts were closely related to their high surface area and high content of pyridinic nitrogen and graphitic nitrogen.

In this paper, a novel accelerated test method was proposed to analyze the durability of MEA, considering the actual operation of the fuel cell vehicle. The proposed method includes 7 working conditions: open circuit voltage (OCV), idling, rated output, overload, idling-rated cycle, idling-overload cycle, and OCV-idling cycle. The experimental results indicate that the proposed method can effectively destroy the MEA in a short time (165 h). Moreover, the degradation mechanism of MEA was analyzed by measuring the polarization curve, CV, SEM and TEM. This paper may provide a new research direction for improving the durability of fuel cell.

To understand the origin of its catalytic activity, the oxygen reduction reaction (ORR) on zirconia with different phases is investigated by the first principles method. The free energy diagrams for an associative mechanism are firstly given to identify the rate-determining step for the ORR on the (111) surface of monoclinic, tetragonal and cubic phase zirconia. The calculation results show that the first electron transfer is the rate-determining step for the ORR on monoclinic and cubic phase zirconia, whereas the last electron transfer is the rate-limiting step for the ORR on tetragonal phase zirconia. The d-electron partial density of states for the active zirconium atoms is calculated to explore the relationship between the ORR catalytic activity of zirconia with different phases and their electronic properties. The calculated results show that the catalytic activity of zirconia for ORR is strongly dependent on the d-orbital occupation of the active zirconium atoms.

The electrocatalytic activity toward oxygen reduction reaction is studied on the perovskite oxide La1-xSrxMnO3, as prepared under different firing temperatures. X-ray diffraction shows that three different crystal phases featuring tetragonal, cubic, and orthorhombic symmetries form with increasing crystallinities. The electrocatalytic activity is characterized by cyclic voltammetry and linear sweeping voltammetry for the three phases of La1-xSrxMnO3. We find that the tetragonal phase has the best catalytic activity among the three crystal phases, with the largest onset potential of 0.147 V. The synergistic effect between the volume per unit cell and crystallinity is indicated to account for the good catalytic activity of the tetragonal phase.

Iridium dioxide with different morphologies (nanorod and nanogranular) is successfully prepared by a modified sol-gel and Adams methods. The catalytic activity of both samples for oxygen reduction reaction is investigated in an alkaline solution. The electrochemical results show that the catalytic activity of the nanogranular IrO2 sample is superior to that of the nanorod sample due to its higher onset potential for oxygen reduction reaction and higher electrode current density in low potential region. The results of Koutecky-Levich analysis indicate that the oxygen reduction reaction catalyzed by both samples is a mixture transfer pathway. It is dominated by four electron transfer pathway for both samples in high overpotential area, while it is controlled by two electron transfer process for both samples in low overpotential area.

•Nitrogen was doped into La2Zr2O7 nanoparticles by the sol-gel process combined with ammonolysis.•The ORR activity of N-doped La2Zr2O7 is considerably enhanced.•Density functional theory (DFT) calculations indicate that the band gap was reduced in N-doped La2Zr2O7.Nitrogen shows a positive influence on the ORR activity of certain catalysts, such as N-doped carbon and N-doped ZrO2. In this paper, we report a method to dope nitrogen into La2Zr2O7 nanoparticles by the sol-gel process combined with ammonolysis. XPS results indicate that the nitrogen content in La2Zr2O7 is approximately 0.92 at.%. Compared to the ORR activity of La2Zr2O7, ORR activity of the N-doped La2Zr2O7 is considerably enhanced. Density functional theory (DFT) calculations indicate that the band gap is reduced in N-doped La2Zr2O7, which may be one of the factors that contribute to the improvement.

The enhanced electrochemical stability of the synthesized hybrid catalyst has been demonstrated by the introduction of the synergistic effect between carbon powder additive and the prepared catalyst. Single crystal IrO2 nanorod (SC-IrO2NR) catalyst was prepared by a sol-gel method. The structure and performance of the catalyst sample were characterized by X-ray diffraction spectroscopy (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), rotating disk electrode (RDE) and cyclic voltammetry (CV) measurements. XRD patterns and TEM images indicate that the catalyst sample has a rutile IrO2 single crystal nanorod structure. The onset potential for oxygen reduction reaction (ORR) of the SC-IrO2NR-carbon hybrid catalyst specimen is 0.75 V (vs. RHE) in RDE measurement. CV and RDE test results show that the SC-IrO2NR-carbon hybrid catalyst has a better electrochemical stability in comparison with the commercial Pt/C catalyst, with attenuation ratios of 17.67% and 44.60% for the SC-IrO2NR-carbon hybrid catalyst and the commercial Pt/C catalyst after 1500 cycles, respectively. Therefore, in terms of stability, the SC-IrO2NR-carbon hybrid catalyst has a promising potential in the application of the proton exchange membrane fuel cell.

In this paper, the effect of shutoff sequences of hydrogen and air on the degradation behaviours of proton exchange membrane fuel cells (PEMFCs) is investigated with two different shutdown procedures. After one of the gases is shut off, the dummy load is applied to consume the residual gas in the flow field. Theoretical analysis and experimental tests indicate that different gas shutoff sequences have great effect on the oxygen permeation rate across the membrane during introducing the dummy load, resulting in producing the H2/O2 interface at the anode in the next startup process. Electrochemical techniques, including the measurement of polarization curves, cyclic voltammetry, and cross-sectional scanning electron microscopy (SEM) of membrane electrode assemblies (MEAs) and transmission electron microscopy (TEM) of Pt/C catalyst are employed to evaluate the performance decay of PEMFCs after 1500 startup and shutdown cycles. The results show that the case if air is shut off firstly would significantly alleviate both the performance decay and the decrease in the electrochemically active surface area, resulting in an improvement in the durability of PEMFCs.Graphical abstractSchematic images of two models, in case A air was shut off firstly and in case B hydrogen was shut off firstly with the formation process of the H2/O2 interface at the anode.Highlights► Catalyst layer degradation was reduced by shutting off air prior to hydrogen. ► Applying dummy load caused different oxygen permeation rates in shutdown processes. ► A theoretic model was used to analyse O2 permeation for different shutoff processes.

We propose an analytical model to predict the effective binary oxygen diffusivity of the porous gas diffusion layer (GDL) in proton exchange membrane fuel cells (PEMFCs). In this study, we consider the fractal characteristics of the porous GDL as well as its general microstructure, and we adopt the Bosanquet equation to derive effective diffusivity. The fractal characterization of GDL enables us to model effective diffusivity in a continuous manner while taking into account the effect of pore size distribution. Comparison to two other theoretical models that are generally accepted in the simulation of PEMFCs shows similar trends in all three models, indicating that our proposed model is well founded. Furthermore, the predicted effective binary oxygen diffusivities of two samples show that after treatment with polytetrafluoroethylene (PTFE), the effective binary diffusivity of the GDL decreases. Based on the parametric effect analysis, we conclude that effective binary diffusivity is negatively correlated with tortuosity fractal dimension but positively correlated with the fractal dimension of pore area, porosity, or mean pore diameter. The proposed model facilitates fast prediction of effective diffusivity as well as multi-scale modeling of PEMFCs and thus facilitates the design of the GDLs and of PEMFCs.

In this paper, a carbon-supported binary FeCo–N/C catalyst using tripyridyl triazine (TPTZ) as the complex ligand was successfully synthesized. The FeCo–TPTZ complex was then heat-treated at 600 °C, 700 °C, 800 °C, and 900 °C to optimize its oxygen reduction reaction (ORR) activity. It was found that the 700 °C heat-treatment yielded the most active FeCo–N/C catalyst for the ORR. XRD, EDX, TEM, XPS, and cyclic voltammetry techniques were used to characterize the structural changes in these catalysts after heat-treatment, including the total metal loading and the mole ratio of Fe to Co in the catalyst, the possible structures of the surface active sites, and the electrochemical activity. XPS analysis revealed that Co–Nx, Fe–Nx, and C–N were present on the catalyst particle surface. To assess catalyst ORR activity, quantitative evaluations using both RDE and RRDE techniques were carried out, and several kinetic parameters were obtained, including overall ORR electron transfer number, electron transfer coefficient in the rate-determining step (RDS), electron transfer rate constant in the RDS, exchange current density, and mole percentage of H2O2 produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was ∼3.88, with H2O2 production under 10%, suggesting that the ORR catalyzed by FeCo–N/C catalyst is dominated by a 4-electron transfer pathway that produces H2O. The stability of the binary FeCo–N/C catalyst was also tested using single Fe–N/C and Co–N/C catalysts as baselines. The experimental results clearly indicated that the binary FeCo–N/C catalyst had enhanced activity and stability towards the ORR. Based on the experimental results, a possible mechanism for ORR performance enhancement using a binary FeCo–N/C catalyst is proposed and discussed.

In this paper, carbon-supported cobalt–tripyridyl triazine (Co–TPTZ) complexes were synthesized by a simple chemical method, then heat-treated at 600, 700, 800, and 900 °C to optimize their activity for the oxygen reduction reaction (ORR). The resulting catalysts (Co–N/C) all showed strong catalytic activity toward the ORR, but the catalyst heat-treated at 700 °C yielded the best ORR activity. Co–N/C catalysts with several Co loadings – 0.64, 2.0, 2.96, 3.33, 5.28, and 7.18 wt% – were also synthesized and tested for ORR activity. X-ray diffraction and energy dispersive X-ray analysis were used to characterize these catalysts in terms of their structure and composition. To achieve further quantitative evaluation of the catalysts in terms of their ORR kinetics and mechanism, rotating disk electrode and rotating ring-disk electrode techniques were used with the Koutecky–Levich theory to obtain several important kinetic parameters: overall ORR electron transfer number, electron transfer coefficiency in the rate-determining step (RDS), chemical reaction rate constant, electron transfer rate constant in the RDS, exchange current density, and mole percentage of H2O2 produced in the catalyzed ORR. The overall electron transfer number for the catalyzed ORR was determined to be ∼3.5 with 14% H2O2 production, suggesting that the ORR catalyzed by Co–N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process; based on these measurements, an ORR mechanism is proposed based on the literature and our understanding, to facilitate further investigation. The stability of a Co–N/C catalyst was also tested by fixing a current density to record the change in electrode potential with time. For comparison, two other catalysts, Fe–N/C and TPTZ/C, were also tested for stability under the same conditions as the Co–N/C catalyst. Among these three, the 5 wt% Co–N/C was most stable.

Novel electro-catalyst based on phthalocyanine stabilized Pt colloids has been developed for methanol electro-oxidation. Water soluble Cu2+ phthalocyanine functioned with sulfonic groups were selected as catalyst supports because of the relatively high catalytic activity of Pt catalyst and nearly the same catalytic selectivity complex with Cu-phthalocyanine, compared to others that chelated with Fe, Co and Ni ions. The as-resulting Pt–CuTsPc catalysts have average particle size of 2 nm and narrow size distribution. With the assistance of CuTsPc supports, the methanol electro-oxidation activity and poison tolerance of Pt catalyst have a significant increase. If/Ib ratio (anodic peak current density, forward to backward) of the Pt–CuTsPc/C catalysts also has obvious increase to 2.5, from value of 0.8 for pure Pt/C catalyst. The reaction Tafel slope of Pt-CuTsPc/C catalysts is 56.6 mV dec−1, much smaller than that of the Pt/C catalyst. The transient current density on Pt–CuTsPc/C at 0.60 V is enhanced to 650% of that on the Pt/C catalyst while the enhancement factor R for comparison of steady-state current obtained on Pt–CuTsPc/C and Pt/C catalyst varies between 111% and 534% in the potential region of 0.3–0.75 V.

The catalytic activity for methanol electro-oxidation on CoPc-Pt/C co-catalysts, prepared by impregnation method, was studied in details through electrochemical methods. Cyclic voltammetry (CV) result demonstrates that CoPc has higher forward anodic peak current density and jf/jb value (forward anodic peak current density/backward anodic peak current density) than Pt/C. Chronoamperometry (CA) analysis indicates that CoPc-Pt/C exhibits both excellent transient current density and stable current density for methanol electro-oxidation compared with Pt/C. Two main mechanisms related to the promotion of catalytic activity are as follows: CoPc-Pt/C has the activity of tolerance to carbonaceous intermediates, thus inhibiting the self-poisoning of catalysts; CoPc-Pt/C owns prominent intrinsic catalytic activity indicated by the apparent activation energy for methanol oxidation on CoPc-Pt/C, which is 18 kJ/mol, less than that on Pt and PtRu catalysts as reported.

The process of heat transfer within porous media is usually considered as a transport through large numbers of straight channels with uniform pore sizes. For the prediction of effective thermal conductivity of gas diffusion layer (GDL), morphological properties such as the tortuosity of channels and pore-size distribution of this porous layer should be considered. Thus in this article, novel parallel and series-parallel prediction models of effective thermal conductivity for the GDL in proton exchange membrane fuel cell (PEMFC) have been derived by fractal theoretical characterization of the real microstructure of GDL. The prediction of fractal parallel model for carbon paper, a basal material of the GDL, is in good agreement with the reference value supplied by Toray Inc. The prediction results from the proposed models are also reasonable because they are distributed between the upper and lower bounds. Parametric effect has been investigated by using the presented models in dimensionless formalism. It can be concluded that dimensionless effective thermal conductivity (k′effk′eff) has a positive correlation with effective porosity (ɛ) or the pore-area fractal dimension (Dp) when ks/kg < 1; whereas it has a negative correlation with ɛ or Dp when ks/kg > 1 and with tortuous fractal dimension (Dt) whether ks/kg < 1 or not. Furthermore, these fractal models have been modified by considering the effect of polytetrafluoroethylene (PTFE) incorporated into the pore spaces of carbon paper, and the corresponding model prediction shows that there is an increase in the effective thermal conductivity due to the filling of PTFE that has high thermal conductivity.

This paper describes an investigation of the role of nickel phthalocyanine-tetrasulfonic acid (NiPcTs) for methanol electro-oxidation on a Pt/C catalyst. Cyclic voltammetry (CV) revealed that NiPcTs has no catalytic activity in methanol or CO electro-oxidation. However, methanol electro-oxidation occurs faster on a Pt/C catalyst modified with NiPcTs than on the original Pt/C catalyst. CO stripping results demonstrated that NiPcTs promotes electro-oxidation of adsorbed CO (COads) on the Pt/C catalyst, which is likely to be responsible for the enhancement of the methanol electro-oxidation rate. The promotion effect of NiPcTs is attributable to its ability to modify the electron density of the Pt surface. The electron deficiency of Pt0 in the NiPcTs-Pt/C catalyst is shown by the shift of the Pt04f peak to higher binding energies in the X-ray photoelectron spectrum.

A novel Nafion/SiO 2 nanocomposite membrane has been developed by combining the self-assembly route and Nafion/SiO 2 hybrids. Nafion-stabilized SiO 2 nanoparticles were synthesized by using the self-assembly route. The hydrolysis procedure has significant influence on the size and distribution of SiO 2 nanoparticles and the quality and performance of the composite membranes. Best results were obtained on the Nafion/SiO 2 composite membrane recast from Nafion−SiO 2 (5 wt %) nanoparticles with a H 2O:TEOS ratio of 4:1. The Nafion/SiO 2 composite membrane prepared by the self-assembled Nafion–SiO 2 nanoparticles shows remarkable durability as compared to the Nafion/SiO 2 composite membrane prepared by the conventional sol−gel method and pure Nafion 212 membrane, most likely due to the intimate interface between the self-assembled Nafion–SiO 2 nanoparticles and Nafion polymeric matrix. The tensile strength and proton conductivity of the self-assembled Nafion/SiO 2 nanocomposite membranes are 27.5 MPa and 0.090 S/cm, respectively, close to that of the pure Nafion 212 membrane. The λ H 2O/SO 3H of the self-assembled Nafion/SiO 2 composite membrane at 60 °C is 7.02, ∼5 times higher than 1.33 of the Nafion 212 membrane. Very different from the pure Nafion 212 membrane, the PEM fuel cells assembled with a self-assembled Nafion/SiO 2 nanocomposite membrane show good performance and negligible performance decay when the RH of the inlet gases changes from 100% to 50% under a constant current load of 600 mA cm −2 at 60 °C. This work demonstrates the promising potential of the self-assembled Nafion/SiO 2 composite membranes for the development of high-performance and high-stability PEM fuel cells with high tolerance to RH variations at elevated high temperatures.

•PANINF/CP is prepared by a simple and fast electrochemical polymerization method.•PNANINFs grow predominantly perpendicular to CP forming a vertically aligned array.•The specific capacitance of PANINF/CP as high as of 531 F g−1 is achieved.A simple and fast electrochemical polymerization method is used to prepare polyaniline nanofibers supported on carbon paper. The composition and morphology of the obtained composites are characterized by infrared spectroscopy, X-ray diffraction and scanning electron microscopy. It is found that an aligned polyaniline nanofiber array is uniformly supported on carbon paper substrate. This unique array architecture provides good electrochemical supercapacitor performances for the obtained composites. A specific capacitance as high as of 531 F g−1 at a current density of 3 mA cm−2 is achieved. The research data show that acceptable electrochemical properties can be attained by controlling the structure of supercapacitor electrode.