Simulation of a polymer electrolyte fuel cell electrode

A detailed one dimensional dynamic model of a gas diffusion electrode as part of a complete fuel cell model is presented. Various effects of parameter changes are considered. Comparison of experimental results and simulation is performed and a new approach to simulation of a complete current voltage curve is discussed.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Liquid water transport in a mixed-wet gas diffusion layer of a polymer electrolyte fuel cell

After PTFE treatment, a gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) features mixed wettability, which substantially impacts liquid water transport and associated mass transport losses. A pore-network model is developed in this work to delineate the effect of GDL wettability distribution on pore-scale liquid water transport in a GDL under fuel cell operating conditions. It is found that in a mixed-wet GDL liquid water preferentially flows through connected GDL hydrophilic network, and thereby suppresses the finger-like morphology observed in a wholly hydrophobic GDL. The effect of GDL hydrophilic fraction distribution is investigated, and the existence of an optimum hydrophilic fraction that leads to the least mass transport losses is established. The need for controlled PTFE treatment is stressed, and a wettability-tailored GDL is proposed.     

A calorimetric analysis of a polymer electrolyte fuel cell and the production of H2O2 at the cathode

A calorimeter has been constructed and used to measure the total heat production of a single polymer electrolyte fuel cell that is operated on hydrogen and oxygen at 50 °C and 1 bar. The cell had a SolviCore Catalyst Coated Backing and Nafion membranes 112, 115 and 117. We report that the total heat production plus the power production corresponds to the enthalpy of formation of water for cell potentials above 0.55 V. For cell potentials less than 0.55 V, we measured a linear decrease in the reaction enthalpy with decreasing cell potential. This effect was obtained independently of membrane thickness and current density. We propose therefore that the main power loss at low cell potentials and the inflection point in the polarisation curve is due to hydrogen peroxide formation at the cathode. The total heat production was decomposed into reversible and irreversible effects (non-ohmic and ohmic). The non-ohmic part was evaluated using Tafel plots. We show that it is possible to determine the overpotential of an electrode also from its thermal signature.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells

The influence of the structure and composition of the diffusion layer on polymer electrolyte fuel cell (PEFC) cathode performance was investigated. Electrodes were prepared with different poly-tetrafluoroethylene (PTFE) content in the diffusion layer and maintaining a constant composition for the catalytic layer with a low-Pt loading (0.11 mg cm⁻²). Electrodes were characterized by Hg-intrusion porosimetry, scanning electron microscopy and electrochemical techniques (cyclic voltammetry, galvanostatic polarization and ac-impedance spectroscopy).     

Pore-network modeling of liquid water transport in gas diffusion layer of a polymer electrolyte fuel cell

A pore-network model is developed to study the liquid water movement and flooding in a gas diffusion layer (GDL), with the GDL morphology taken into account. The dynamics of liquid water transport at the pore-scale and evolution of saturation profile in a GDL under realistic fuel cell operating conditions is examined for the first time. It is found that capillary forces control liquid water transport in the GDL and that liquid water moves in connected clusters with finger-like liquid waterfronts, rendering concave-shaped saturation profiles characteristic of fractal capillary fingering. The effect of liquid coverage at the GDL–channel interface on the liquid water transport inside GDL is also studied, and it is found that liquid coverage at the GDL–channel interface results in pressure buildup inside the GDL causing the liquid water to break out from preferential locations.     

基于电极修饰质子交换膜燃料电池自增湿研究进展

自增湿对于质子交换膜燃料电池的实际应用具有重要意义。本文从电极组分和结构修饰这一角度出发,介绍了近年来质子交换膜燃料电池自增湿研究的一些重要进展和发展趋势。首先介绍了基于催化层修饰实现质子交换膜燃料电池自增湿的发展状况,指出吸湿性催化剂开发是实现高效自增湿催化层的关键;其次介绍了基于气体扩散层修饰和电极结构改进实现质子交换膜燃料电池自增湿的研究进展,分析了两种方式各自的优缺点,讨论了其后续的发展方向;最后针对现有自增湿工艺存在的问题,提出了未来的研究方向和重点,对这类自增湿研究的发展趋势及应用前景进行了展望,指出吸湿性催化剂研发以及多种工艺协同互补将是今后自增湿质子交换膜燃料电池发展的重要方向。     

The use of the heteropoly acids, H3PMo12O40 and H3PW12O40, for the enhanced electrochemical oxidation of methanol for direct methanol fuel cells

Polarization and electrochemical impedance spectroscopy experiments were performed on a direct methanol fuel cell (DMFC) incorporating the heteropoly acids (HPAs) phosphomolybdic acid, H₃PMo₁₂O₄₀, (HPMo) or phosphotungstic acid, H₃PW₁₂O₄₀, (HPW) in the anode Pt/C catalyst layer. Both HPW-Pt and HPMo-Pt showed higher performance than the Pt control at 30 psig of backpressure and at ambient pressure. Anodic polarizations were also performed, and Tafel slopes were extracted from the data between 0.25 V and 0.5 V. At 30 psig, Tafel slopes of 133 mV/dec, 146 mV/dec, and 161 mV/dec were found for HPW-Pt, HPMo-Pt and the Pt control, respectively. At 0 psig, the Tafel slopes were 172 mV/dec, 178 mV/dec, and 188 mV/dec for HPW-Pt, HPMo-Pt and the Pt control. An equivalent circuit model, which incorporated constant phase elements (CPEs), was used to model the impedance data. From the impedance model it was found that the incorporation of HPAs into the catalyst layer resulted in a reduction in the resistances to charge transfer. This shows that these two heteropoly acids do act as co-catalysts with platinum for methanol electrooxidation.     

CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells

The electrochemical activity of carbon-supported cobalt-palladium alloy electrocatalysts of various compositions have been investigated for the oxygen reduction reaction in a 5 cm² single cell polymer electrolyte membrane fuel cell. The polarization experiments have been conducted at various temperatures between 30 and 60 °C and the reduction performance compared with data from a commercial Pt catalyst under identical conditions. Investigation of the catalytic activity of the CoPd_x PEMFC system with varying composition reveals that a nominal cobalt-palladium atomic ratio of 1:3, CoPd₃, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to the commercial Pt catalyst. The ORR on CoPd₃ has a low activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate-determining step is a chemical step following the first electron transfer step and may involve the breaking of the oxygen bond. The CoPd₃ catalyst also exhibits excellent chemical stability, with the open circuit cell voltage decreasing by only 3% and the observed current decreasing by only 10% at 0.8 V over 25 h. The CoPd₃ catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.     

Realising a reference electrode in a polymer electrolyte fuel cell by laser ablation

This work presents a new concept for realising a reference electrode configuration in a PEM fuel cell by means of laser ablation. The laser beam is used to evaporate a small part of the electrode of a catalyst-coated membrane (CCM) to isolate the reference electrode from the active catalyst layer. This method enables the simultaneous ablation of the electrodes on both sides of the CCM because the membrane is transparent for the laser beam. Therefore, a smooth electrode edge without electrode misalignment can be realised. A test fuel cell was constructed which together with the ablated CCM enables the separation of the total cell losses during operation into the cathode, anode and membrane overpotentials in PEFC as well as in DMFC mode. The methanol tolerance of a selenium-modified ruthenium-based catalyst (RuSe _x ) was investigated under real fuel cell conditions by measuring polarisation curves, electrochemical impedance spectroscopy (EIS) and current interrupt measurements (CI).     

Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells

Polymer electrolyte fuel cell (PEFC) electrodes were prepared by applying different porous gas diffusion half-layers (GDHLs) onto each face of a carbon cloth support, followed by the deposition of a catalyst layer onto one of these half-layers. The performance of PEFCs in H₂/air operation using cathodes with GDHLs presenting different characteristics were compared. The best result was obtained using cathodes with GDHLs having polytetrafluorethylene (PTFE) contents of 30 wt % in the gas side and 15 wt % in the catalyst side. This behaviour was explained in terms of a better water management within the cell.     

Heteropolyacid in glass electrolytes for the development of H2/O2 fuel cells

The scope of the present work was to investigate and evaluate the electrochemical activity of H₂/O₂ fuel cells based on the influence of a heteropolyacid glass membrane with a Pt/C electrode at low temperature. A new trend of sol-gel derived PMA (H₃PMo₁₂O₄₀) heteropolyacid-containing glass membranes inherent of a high proton conductivity and mechanical stability, was heat treated at 600 °C and implemented to H₂/O₂ fuel cell activities through electrochemical characterization. Significant research has been focused on the development of H₂/O₂ fuel cells using optimization of heteropolyacid glasses as electrolytes with Pt/C electrodes at 30 °C. A maximum power density of 23.9 mW/cm² was attained for operation with hydrogen and oxygen, respectively, at 30 °C and 30% humidity with the PMA glass membranes (4-92-4 mol%). Impedance spectroscopy measurements were performed on a total ohmic cell resistance of a membrane-electrode-assembly (MEA) at the end of the experiment.     

Investigation of the response of separate electrodes in a polymer electrolyte membrane fuel cell without reference electrode

The paper presents electrochemical measurements carried out in a PEMFC with a view to determining the separate kinetics of the electrode reactions. For this purpose, the separate response of one electrode (anode or cathode) was magnified by dilution of the reacting gas, respectively hydrogen and oxygen, and comparison of the experimental data in the form of steady voltage-current variations and impedance spectra. Experiments were carried out at 60 °C and ambient pressure. Water management was thoroughly controlled so that the gases leaving the cell had the same relative humidity in all experiments of one series. Hydrogen oxidation, although rapid, corresponds to overpotentials up to 50 mV at high dilution rates and current densities. Assuming a Tafel–Volmer mechanism, the exchange current density of the anode reaction at the Pt surface is of the order of 1 mA cm⁻². The two techniques employed led to Tafel slopes of oxygen reduction ranging from 120 to 150 mV/decade, with an exchange current density near 1 μA cm⁻², in good agreement with published data.     

Multi-layer configuration for the cathode electrode of polymer electrolyte fuel cell

This paper conducts a one-dimensional theoretical study on the electrochemical phenomenon in the dual-layer cathode electrode of polymer electrolyte fuel cells (PEFCs) with varying sub-layer thicknesses, and further extends the analysis to a triple-layer configuration. We obtain the explicit solution for a general dual-layer configuration with different layer thicknesses. Distributions of the key quantities such as the local reaction current and electrolyte overpotential are exhibited at different ratios of the ionic conductivities, electrochemical kinetics, and layer thicknesses. Based on the dual-layer approach, we further derive the explicit solutions for a triple-layer electrode. Sub-layer performances are plotted and compared. The results indicate that the layer adjacent to the electrolyte membrane may contribute a major part of the electrode faradic current production. The theoretical analysis presented in this paper can be applied to assist electrode development through complicated multi-layer configuration for cost-effective high performance electrodes.     

Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties

The lithium deposition-dissolution process in solid polymer electrolytes containing Al₂O₃ filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF₄ and α-Al₂O₃ filler by using a dry process, where the surface of α-Al₂O₃ was beforehand modified by a wet process. The exchange current densities, i₀, of the lithium electrode process in P(EO)₂₀LiBF₄ with and without Al₂O₃ filler were determined by a micro-polarization method. The temperature dependence of i₀ provided similar values for activation energy, ca. 25 and 70 kJ mol⁻¹ in both temperature regions above and below 60 °C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al₂O₃ surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al₂O₃ filler.     

Direct synthesis of H2O2 acid solutions on carbon cathode prepared from activated carbon and vapor-growing-carbon-fiber by a H2/O2 fuel cell

Direct synthesis of H₂O₂ acid solutions was studied using a gas-diffusion cathode prepared from activated carbon (AC), vapor-growing-carbon-fiber (VGCF) and poly-tetra-fluoro-ethylene (PTFE) powders, with a new H₂/O₂ fuel cell reactor. O₂ reduction to H₂O₂ was remarkably enhanced at the three-phase boundary (O₂(g)-electrode(s)-acid(l)) at the [AC + VGCF] cathode. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk acid solutions were essential for H₂O₂ accumulation. Synergy of AC and VGCF was observed for the H₂O₂ formation. RRDE and cyclic voltammetry studies indicated that the surface of AC functioned as the active phase for O₂ reduction to HO₂, and VGCF functioned as an electron conductor and a promoter to convert HO₂ to H₂O₂. A maximum H₂O₂ concentration of 353 mM (1.2 wt%) was accomplished under short-circuit conditions (current density 12.7 mA cm⁻², current efficiency 40.1%, geometric area of cathode 1.3 cm², reaction time 6 h).