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.     

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.     

Effects of flow channel geometry on cell performance for PEM fuel cells with parallel and interdigitated flow fields

A 3D numerical model was developed to explore the effects of the cathode flow channel configuration on the local transport phenomena and cell performance for parallel and interdigitated flow fields in proton exchange membrane (PEM) fuel cells. The effect of liquid water formation on the reactant transport is taken into account in the model. For operating voltages greater than 0.7 V, the electrochemical reaction rates are low with a small amount of oxygen consumption and liquid water production, and all cell designs provide sufficient oxygen for the electrochemical reactions. Thus, the flow channel aspect ratio and the flow channel cross-sectional area have little effect on the cell performance. For operating voltages lower than 0.7 V, as the operating voltage decreases the electrochemical reaction rates gradually increase with a large amount of oxygen consumption and liquid water production, so the cell performance is strongly dependent on the flow field design. For the parallel flow field design, lower flow channel aspect ratios and flow channel cross-sectional area areas improve liquid water removal, thus, decreasing both improves cell performance. However, the interdigitated design has an optimal aspect ratio of 1.00 and an optimal cross-sectional area of 1.000 mm × 1.000 mm.     

Copper alloy bipolar plates for polymer electrolyte membrane fuel cell

Bipolar plates are one of the critical componets of the fuel cell. Excellent corrosion resistance and good conductivity are some of the desirable properties for better operation of fuel cells. Corrosion studies of copper–beryllium alloy (C-17200) were conducted in corrosion simulated fuel cell environment by chronoamperometry. The alloy selected was observed to show good corrosion resistance. The selected alloy was tested in an actual fuel cell. Standard parallel flow field design with 16 cm² surface area was machined on end plates. The single cell showed good performance without degradation of MEA and bipolar plates for 100 h at a constant load of 1 A. Characterization of membrane electrode assembly (MEA) and plates revealed no poisoning or degradation. Calculated cell resistance from the polarization curve was 0.0836 Ω which is comparable with SS316L under similar conditions.     

Effect of ambient conditions on performance and current distribution of a polymer electrolyte membrane fuel cell

The performance and current distribution of a free-breathing polymer electrolyte membrane fuel cell (PEMFC) was studied experimentally in a climate chamber, in which temperature and relative humidity were controlled. The performance was studied by simulating ambient conditions in the temperature range 10 to 40 °C. The current distribution was measured with a segmented current collector. The results indicated that the operating conditions have a significant effect on the performance of the fuel cell. It was observed that a temperature gradient between the fuel cell and air is needed to achieve efficient oxygen transport to the electrode. Furthermore, varying the air humidity resulted in major changes in the mass diffusion overpotential at higher temperatures.     

Optimum condition of membrane electrode assembly fabrication for PEM fuel cells

The aim of this research was to study the effect of fabrication factors on the performance of MEA of a PEM fuel cell. The MEA was prepared by using 5 cm² of porous electrodes with Pt loading 1 mg/cm² and Nafion 115 membrane from Electrochem Co. Ltd. The studied factors were temperature, pressure and time of compression in the range of 130–150 ‡C, 50–100 kg/cm² and 1–5 minutes, respectively. A 2^k factorial design was conducted in this study. The results showed that interaction between pressure and temperature and interaction between temperature and time of compression have significant effects on the performance of the MEA. With low pressure, but high temperature and long compression time, current density is increased. The results showed that the optimum condition was 65 kg/cm², 137 ‡C and 5.5 min of compression time. It was also found that the force of 69 kg-cm for assembling the single cell gave the best performance.     

Experimental and numerical study on improvement performance by wave parallel flow field in a proton exchange membrane fuel cell

The performance and operation stability of proton exchange membrane fuel cells (PEMFCs) are closely related to the transportation of reactants and water management in the membrane electrode assembly (MEA) and flow field. In this paper, a new three-dimensional wave parallel flow field (WPFF) in cathode was designed and analyzed throughout simulation studies and an experimental method. The experimental results show that the performance of PEMFC with WPFF outperforms that of PEMFC with straight parallel flow field (SPFF). Specifically, the peak power density increased by 13.45% for the PEMFC with WPFF as opposed to PEMFC with SPFF. In addition, the flow field with area of 11.56 cm² was formed by the assembly of transparent end plate used for cathode and the traditional graphite plate used for anode. To understand the mechanism of the novel flow field improving the performance of PEMFC, a model of PEMFC was proposed based on the geometry, operating conditions and MEA parameters. The thickness of gas diffusion layers (GDL), catalytic layers (CL) and proton exchange membrane were measured by scanning electron microscope. The simulation result shows that compared with SPFF, the WPFF based PEMFC promote the oxygen transfer from flow channel to the surface of CL through GDL, and it was beneficial to remove the liquid water in the flow channel and the MEA.     

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     

聚醚砜-聚乙烯吡咯烷酮高温聚合物电解质膜及燃料电池堆性能研究

基于磷酸掺杂聚苯并咪唑膜（PA/PBI）的高温聚合物电解质膜燃料电池具有高的输出功率和优异的稳定性,然而PBI膜昂贵的价格和复杂的制备工艺限制了高温聚合物电解质膜燃料电池的商业化应用。本研究以成本低和制备工艺简单的聚醚砜-聚乙烯吡咯烷酮（PES-PVP）膜的商业化应用为目标,小规模制备了幅宽为40 cm的PES-PVP复合膜,证实了流延法放大制备PES-PVP复合膜的可行性。PES-PVP膜中每个PVP重复单元的吸附量达4.9个磷酸（PA）分子,且在180℃的质子电导率达85 mS·cm^-1。此外,尺寸为165 cm²的PA/PES-PVP高温膜电极在150℃的输出功率达0.19 W·cm^-2@0.6 V,与同尺寸的商业化PA/PBI高温膜电极的输出功率相当,并在近3000 h的寿命测试中展示出良好的稳定性。最后,将PA/PES-PVP高温膜电极（单片有效面积200 cm²）组装高温膜燃料电池短堆,其中基于3片膜电极的短堆展现出良好的电堆启停稳定性;基于20片膜电极电堆的峰值功率达1.15 kW。以上结果表明所制备的PA/PES-PVP是一种性能优良、价格便宜的高温聚合物电解质膜材料,并且基于该膜材料组装的高温聚合物电解质膜电池和电堆性能优异。本研究工作为高温聚合物电解质膜燃料电池关键材料和电堆的国产化提供了研究基础。     

Analysis of high temperature polymer electrolyte membrane fuel cell electrodes using electrochemical impedance spectroscopy

An electrochemical impedance spectroscopy (EIS) study of electrodes in a phosphoric acid loaded polybenzimidazole (PBI) membrane fuel cell is reported. Using EIS, the effect of electrode parameters such as Pt catalyst wt%, acid doping in PBI and PTFE baesd electrodes and catalyst heat treatment on kinetic and mass transport characteristics is characterised. The influence of cell parameters of current load, temperature and oxidant gas on response is demonstrated and interpreted using an equivalent circuit model. For polarisable electrodes under small to medium steady-state current operation, the model was capable of identifying electrodes with the best kinetic or mass transport behaviour and classifying behaviour in terms of relative performance.     

Effect of hydrophobicity and pore geometry in cathode GDL on PEM fuel cell performance

The effect of hydrophobic and structural properties of a single/dual-layer cathode gas diffusion layer on mass transport in PEM fuel cells was studied using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fraction of hydrophilic surface and the average pore diameter in the cathode gas diffusion layer. Deposition of a hydrophobic microporous layer reduces the average pore diameter in the macroporous substrate. It also increases the hydrophobic surface, which improves the mass transport of the reactant. The optimized hydrophobicity and pore geometry in a dual-layer cathode GDL leads to an effective water management, and enhances the oxygen diffusion kinetics.     

Effect of hydrogen partial pressure on a polymer electrolyte fuel cell performance

We first investigated the effect of partial pressure of hydrogen (H₂) on the performance of polymer electrolyte fuel cells (PEFCs) by controlling the ratio of hydrogen and nitrogen (N₂). The cell performance with Pt/C anode was significantly decreased with reduction of the partial pressure of H₂ in the presence of carbon monoxide (CO), while the performance variation was negligible in the absence of CO. Severe CO poisoning on Pt/C electrode at low partial pressure of H₂ might be attributed to the hindering effect by N₂ and CO. On the other hand, PtRu/C anode showed consistent power performance even at low partial pressure of H₂.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.     

Corrosion resistance and sintering effect of carbon supports in polymer electrolyte membrane fuel cells

The corrosion resistance of carbon black, carbon nanofiber and carbon nanocage used as catalyst supports in fuel cells was investigated by monitoring CO₂ emission using on-line mass spectrometry when 1.4 V was applied for 30 min. The changes associated with the carbon corrosion were assessed through electrochemical methods. In general, graphitized carbon supports were more corrosion-resistant than amorphous carbon black. However, the degree of graphitization did not directly correlate with higher resistance to corrosion. Hydrophobicity was critical in enhancing resistance to corrosion. When sintering of Pt particles was considered, carbon nanocages were more resistant than nanofibers. The present findings thus indicate that the carbon nanocage is an appropriate catalyst support in fuel cell systems.     

Characteristics and performance of membrane electrode assemblies with operating conditions in polymer electrolyte membrane fuel cell

The degradation behavior of a membrane-electrode assembly (MEA) was investigated in accelerated degradation tests under constant voltage (0.8 V and 0.7 V) and load cycling (from open circuit voltage to 0.35 V) conditions. Changes in the structural and electrochemical characteristics of MEA after the durability tests give information as to the degradation mechanism of MEAs. The results of cyclic voltammogram and postmortem analysis by X-ray diffraction and high resolution-transmission electron microscopy indicate that the cathode catalyst layers of the MEAs showed no extreme degradation under constant voltage mode, whereas MEAs under repetition of load cycling mode showed very severe degradation after 280 h. However, the single cell performance of the MEA under repetition of load cycling mode was higher than under constant voltage mode. In addition, although the Pt band in the membrane of the MEA under repetition of load cycling mode was observed by field emission scanning electron microscopy, it did not affect the ohmic resistance.     

Computational fluid dynamics simulation of polymer electrolyte membrane fuel cells operating on reformate

Carbon monoxide (CO) can extremely diminish the polymer electrolyte membrane fuel cell (PEMFC) performance since it is preferentially absorbed on the platinum catalyst layer blocking and reducing the number of catalyst sites available for the hydrogen oxidation reaction. To gain a good insight of CO poisoning characteristics so as to provide a remedial solution for CO-poisoned PEMFCs, a two-dimensional, isothermal, and single phase CO poisoning numerical model taking into account the transport phenomena, electrochemical reactions and multi-component gas mixture transport is developed for such purpose. Linear and bridged-bonded adsorbed CO modes were considered to occur in parallel on the highly dispersed nano-crystalline Pt/C and PtRu/C catalysts. By performing computational fluid dynamics numerical simulations, this study clearly demonstrates the CO poisoning mechanisms and characteristics of PEMFCs. The numerical results obtained are in reasonably good agreement with the experimental data showing the predictive capability of the model.     

Impact of channel wall hydrophobicity on through-plane water distribution and flooding behavior in a polymer electrolyte fuel cell

Through-plane liquid accumulation, distribution and transport inside polymer electrolyte fuel cell (PEFC) components were analyzed as a function of channel wall hydrophobicity with the use of high-resolution neutron imaging. Neutron images were taken with polytetrafluoroethylene (PTFE) coated and uncoated flow channel walls. Anode to cathode liquid distribution was analyzed for each case at low and high current conditions over 20 min of operation. The form and amount of liquid water inside the channels and diffusion media (DM) were compared for hydrophobically coated channels and hydrophilic channels, and a primary liquid transport-flooding mechanism is suggested for each case. The location and value of maximum water storage in DM at low and high current operation were analyzed and slopes of water mass versus distance curve were calculated to compare the significance of capillary liquid flow and phase-change-induced flow within the diffusion media. A significant effect of CL|MPL and MPL|DM interfaces on liquid transport and flooding is found through the analysis of micro-porous layer (MPL) water content and saturation profile along the CL|MPL and MPL|DM interface region.     

Steady-state multiplicity in the autohumidification polymer electrolyte membrane fuel cell

“Ignition/extinction” phenomena and steady-state multiplicity were discovered in an autohumidification polymer electrolyte membrane fuel cell. At steady state, the water produced by the fuel cell reaction is balanced by water removal by the flowing reactant gas streams. Ignition, corresponding to a high fuel cell current, arises from positive feedback between the water produced by the reaction and the transport of protons in the membrane. A critical level of membrane hydration is required for ignition; insufficient membrane hydration will extinguish the fuel cell current. This new autocatalytic mechanism has an interesting analogy to the autothermal reactor.