This article outlines recent progress in electrocatalysts and membranes for polymer electrolyte fuel cells (PEFCs), as well as various evaluation methods for these materials, including characterization of structural and physical properties, commonly used electrochemical techniques, and advanced in situ methods. Particular attention has been paid towards matters that require consideration when using specific techniques and the uniqueness of particular evaluation methods.

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A membrane electrode assembly (MEA) was successfully prepared by electrophoretic deposition (EPD) process onto the inorganic-organic composite membrane composed of three-dimensionally ordered macroporous (3DOM) silica and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) gel polymer. An ethanol suspension of carbon powders with Pt catalyst and ionomer was utilized to the EPD process. The catalyst layers fabricated by the EPD process were well-attached to both sides of the 3DOM composite membrane and those thicknesses were easily controlled by the EPD duration. The obtained MEA exhibited higher cell performance than an ordinary one prepared by decal transfer process, due to improvement in the contact between the 3DOM composite membrane and catalyst layers.

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Carbon supports of electrocatalysts have recently attracted much attention for improving the performance of polymer electrolyte fuel cells (PEFCs). In this study, the PEFCs were formed using activated carbon loaded with platinum of various pore structures in the cathode catalyst layer. The performance of the PEFC at the relative humidity of 100% was dependent on the specific surface area and the mean pore diameter. The behavior was correlated with the results obtained using rotating disk electrodes in a previous study. The PEFC performance was also examined in a low-humidity condition and found to be also dependent on the pore structure.

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In order to improve the stability and the proton conductivity of sulfonated polyimide ionomer membranes, effect of cross-linking has been investigated. The cross-linking moieties have been introduced into the ionomer structure by applying 2 mol% of trifunctional monomer in the polymerization reaction. Tough, flexible, and processable membranes were obtained by solution casting. Among the three cross-linking agents investigated, tris(aminoethyl)amine (TE) was the most effective to improve the membrane properties. Having high ion exchange capacity (2.33 meq/g), the cross-linked SPI-5TE showed good stability to the oxidative and hydrolytic degradation, superior mechanical strength, and high proton conductivity at low humidity conditions. Low methanol permeation of the cross-linked SPI-5TE membrane than the uncross-linked SPI-5 and Nafion 112 membranes has been confirmed.

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This communication reports on the fabrication of membrane electrode assemblies (MEAs) for low temperature direct methanol fuel cells (DMFCs) using PVA-PAMPS proton-conducting semi-IPN membranes. New types of proton conducting polymers PAMPS, PVA/PAMPS and PVA/PAMPS/PEGDCE of similar chemical nature were tested as both the catalyst ink and the binder for MEA fabrications in place of traditional Nafion solutions. MEAs fabricated with PVA/PAMPS binder achieved not only the improved cell performance but also a good durability comparable to those fabricated with Nafion solution. Performance in DMFC mode with PVA/PAMPS/PEGDCE proton-conducting semi-IPN membrane showed a power density of 19.5 mW cm⁻² at 25°C and increased to 49.7 mW cm⁻² at 80°C with a metal loading on the anode of 2 mg cm⁻².

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The proton conductivity of a polymer electrolyte membrane has been measured in the film thickness direction by employing an Au/polymer electrolyte membrane/Au sandwiched cell by varying the electrode area, pressure of the sandwiched cell, and applied voltage. As a result, the optimized electrode area, gauge pressure and applied voltage were found to be 0.015–0.070 cm², 0.8–1.5×10⁵ Pa, and 10–50 mV, respectively. By using Nafion 117 as a polymer electrolyte membrane, we measured the proton conductivity in the film thickness direction and compared it to that in the film surface direction evaluated by a conventional four-probe method, which resulted in the same magnitude of 0.090 S cm⁻¹.

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This paper describes the phenomena of methanol permeation through a membrane electrode assembly (MEA) incorporating a proton exchange membrane (PEM) from anode to cathode under the proton transmission to clarify the methanol crossover, which diminishes the direct methanol fuel cell power. By using the MEA-installed single cell with a feed of a methanol-water solution to the anode and without any feed to the cathode, we measured (i) the hydrogen evolution rate and (ii) the methanol-water solution permeation rate, both observed at the cathode by single-cell electrolysis operation. As a result, when the current density of the electrolysis increased, the rates of hydrogen evolution and methanol-water permeation proportionally increased. Based on the observations, protons generated at the anode were transformed to hydrogen at cathode, accompanied by the generation of the methanol-water solution at the cathode. In addition, the methanol concentration of the methanol-water solution was found to be the same as that fed to the anode. According to these, protons solvated with methanol and water are transmitted through the PEM to generate the methanol-water solution at the cathode, which leads to a methanol crossover.

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The deterioration of the solid polymer electrolyte membrane employed for the polymer electrolyte fuel cells (PEFCs) during the operations is the urgent problem to be solved for the practical use of PEFCs. In the present study, to investigate the deterioration mechanisms, the morphological change of the membrane which was deteriorated chemically in Fenton media was studied by observing the spectral change of the confined water molecules in the membrane by ¹H NMR spectroscopy. For untreated membranes, the polymer structure is thermally converted at around 358 K, so that the mobility of water became higher and the chemical environments became more homogeneous. However, it was revealed that the deteriorated membrane could not take such a specific structure that enables the higher conductivity of the membrane.

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The performance of a type of microtubular direct methanol fuel cell (DMFC) was measured to investigate the effects of the concentration and feeding rate of methanol solution and the loading amount of a Pt-Ru anode catalyst. The half-cell polarization test showed that the optimum loading amount of the Pt-Ru catalyst was 15 mg cm⁻² in a methanol concentration range of 1 to 5 M. For the single cell, a maximum power density of 13.1 mW cm⁻² was achieved with 5 M methanol solution and air at room temperature. When a pump was used for feeding methanol, the power density increased up to 21.3 mW cm⁻² at a feeding rate of 0.1 mL min⁻¹. In addition, the flexibility test exhibited that the single-cell performances did not change, though the tube was bent to about 90 degrees.

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In order to better understand the cathode catalyst degradation of a polymer electrolyte membrane fuel cell, electrochemical measurements of oxygen reduction reaction (ORR) at a 47 wt% platinum-loaded carbon (Pt/C) have been conducted using porous microelectrode techniques before and after the polarization for 15 hours. This technique enables an evaluation of the ORR at the Pt/C powder electrocatalyst without any use of binder materials such as Nafion. In the ORR polarization curve at the Pt/C-containing microelectrode, a limiting current is clearly observed in the oxygen saturated 0.5 mol dm⁻³ sulfuric acid. The Tafel plot analysis revealed only a 60 mV/decade slope. After a 15-hour polarization of the Pt/C at +0.7, +0.8, and +0.9 V vs. RHE in an oxygen atmosphere, the following electrochemical measurement demonstrated that (i) the ORR i-E curve completely shifted in the cathodic direction by about 10 mV, (ii) the electrochemical surface area measured in a nitrogen atmosphere by cyclic voltammography decreased to 90-94%, and (iii) the limiting current density decreased to 85-81% at 0.7 V vs. RHE.

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The support effect of carbon nanotubes (CNTs) on the performance of CO tolerant electrocatalysts for PEFC was studied using CNTs with and without defect preparation, carbon black, and fishbone-type CNTs. Pt-Ru/defect-free CNTs revealed very high CO tolerance under 100 ppm CO in the half-cell system of hydrogen oxidation. The catalytic activity was maintained under 100 ppm level CO with good reproducibility. On the other hand, the hydrogen oxidation current on Pt-Ru/defective CNTs, Pt-Ru/fishbone-type CNTs and Pt-Ru/VulcanXC-72C decreased largely with increasing concentration of CO up to 100 ppm. It is thus considered that the carbon substrates significantly affect the CO tolerance of anode electrocatalysts in PEFC. This is ascribed to the flat interface between CNTs and metal catalysts, at which the electron transfer occurs, and this interface would modify the catalytic properties of Pt-Ru particles.

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Water flooding in a cathode gas diffusion layer (GDL) is one of the critical problems of polymer electrolyte fuel cell (PEFC) because liquid water condensed in the porous GDL blocks oxygen transport to the active reaction sites. In this study, we experimentally investigate state of liquid water in a porous electrode of PEFC cathode using an optical diagnostic system and show that it is affected by operating parameters and GDL thickness. Furthermore, microscopic behavior of condensed water inside a cathode GDL of operated PEFC is also predicted by using the GDL with a groove.

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Low cost metals are quite attractive as bipolar plate of polymer electrolyte fuel cell (PEFC). However, low cost metals easily corrode due to the absence of good passivation films. In order to give high electrical conductivity and corrosion resistance to low cost metals such as carbon steel (S25C), carbon-coat was carried out by plasma-assisted chemical vapor deposition. Carbon-coated samples were characterized by XRD, Raman spectroscopy, and interfacial contact resistance. Based on the results of polarization measurement, the combination of carbon-coat and electroless nickel plating was found to be effective for decreasing of anodic dissolution of base metal.

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Platinum depositions have been observed in the electrolyte membrane of polymer electrolyte fuel cells (PEFCs) as in other types of fuel cells; PEFCs, MCFCs. The platinum solubility in the membrane must be a key issue to improve and guarantee the durability of PEFCs. In this basic study, the platinum solubility has been determined in an acidic medium as a function of temperature, pH, and potential which was controlled by the oxygen partial pressure. The solubility was 3.0×10⁻⁶ mol dm⁻³ at 23°C in 1 mol dm⁻³ H₂SO₄ under air, and increased with temperature and the decrease in pH. The platinum solubility slightly increased with the oxygen partial pressure in oxygen–nitrogen mixtures. However, the solubility in nitrogen was much smaller than the mixtures. Based on these results, the dissolution of platinum in an acidic medium would follow the acidic dissolution mechanism.

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The CO-tolerance of a carbon-supported Pt/Mo-oxide (Pt/MoOx/C) anode in proton-exchange membrane fuel cells was examined by electrochemical impedance spectroscopy. A pseudo-inductive loop was clearly observed only for a Pt/MoOx/C anode at low anodic polarizations, which indicates the onset of the electro-oxidation of adsorbed CO (CO_ad) on the Pt/MoOx/C catalyst. The CO-tolerance of Pt/MoOx/C at low anodic polarizations is mainly ascribed to the combination of the electro-oxidation of CO_ad and the removal of CO by the water-gas shift reaction depending on the operating conditions.

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Platinum-free air cathode catalysts were prepared by heat-treating transition metal hexacyanometallate precursors under an inert atmosphere. The catalytic activity for oxygen reduction was examined with the floating electrode and rotating ring-disk electrode techniques. Among several Pt-free catalysts based on 3d-transition elements, catalysts containing cobalt or copper with iron exhibited high activity toward oxygen reduction, and the catalyst containing copper and iron showed very low generation of hydrogen peroxide during oxygen reduction. A possibility of the cost-effective air cathode catalysts for fuel cells substituting for Pt-based catalysts is discussed therefrom.

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Ta-C-N electrocatalysts were prepared by reactive R.F. sputtering with heat treatment in the temperature range from 70 to 800°C. The Ta-C-N was found to have high electrochemical stability in 0.1 mol dm⁻³ H₂SO₄. The catalytic activity for the oxygen reduction reaction of the sputtered Ta-C-N increased with the increasing heat treatment temperature. In particular, the current of the ORR on the Ta-C-N with the heat treatment temperature of 800°C was observed at about 0.73 V vs. RHE. It was found that the crystallinity of the Ta-C-N would affect the catalytic activity for the ORR.

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New CO tolerant anode catalysts for reformate type polymer electrolyte fuel cells are proposed that are superior to state-of-the-art Pt-Ru metal alloy catalysts. Composite catalysts based on platinum and organic metal complexes were fabricated by a new procedure where platinum precursor, metal complex and carbon black powder were mixed and then heat-treated in Ar atmosphere at 400°C. Catalysts were evaluated in the anode half cell or single fuel cell at 70°C with H₂ containing various levels of CO. Compared with commercial Pt-Ru/C catalysts, new anode catalysts proved to be very promising both in initial and long term performances.

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A Pt-Ru nanoparticle catalyst was prepared using mesoporous carbon, CMK-3, as a support, and the catalytic activity was compared to that on Ketjen carbon black, KB. The DMFC performances of MEA prepared with these catalysts were also measured and compared with each other. The catalysts deposited on CMK-3, Pt-Ru/CMK-3, had a narrow size distribution around 2.5nm and highly distributed on the surface of the support. Pt-Ru/CMK-3 showed a higher mass activity for methanol oxidation based on a cyclic voltammogram, but a lower DMFC performance than that of Pt-Ru/KB. It was considered that the catalyst layer with CMK-3 had a lower electric conductivity and/or unfavorable microstructure to prepare a large number of active reaction sites in the layer.

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Proton exchange membranes for use in fuel cells were prepared by our original ion-track technology, which involves (i) the swift heavy ion irradiation of polyvinylidene fluoride films and subsequent chemical etching to obtain cylindrical pores, and (ii) the filling of proton-conducting polymer chains into the etched pores by γ-ray-induced graft polymerization. We found that the membranes possessed one-dimensional straight proton conducting pathways parallel to the ion-beam incident axis. Such restricted structures probably led to less water uptake and lower methanol permeability compared to a commercially-available Nafion membrane.

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Raman spectra of a platinum-dispersed carbon (Pt/C) electrode in H₂SO₄ aqueous solution were measured at room temperature under constant electrode potential. Raman bands for disordered carbon at around 1360 and 1610 cm⁻¹ were examined in-situ with increasing time and electrode potential. A decrease in the peak intensities of these Raman bands and a peak shift of the 1610 cm⁻¹ band were observed. These changes were probably due to a change in surface structure of carbon via carbon corrosion assisted by a platinum catalyst.

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In our previous paper, we have shown that platinum dispersed polyaniline (PAn) electrodes exhibit high catalytic activity for the electrochemical oxidation of methanol. In this paper, the catalytic activity of Pt dispersed PAn electrode towards the reduction of oxygen was studied to develop highly performed direct methanol fuel cell. Voltammetric measurements showed that the oxygen reduction currents of Pt modified PAn (Pt/PAn) electrodes are apparently higher (about twice under optimum conditions) than those of Pt dispersed carbon electrodes. The effect of pre-deposition of iron onto PAn surface was studied to reduce the amount of expensive platinum. When total amount of Pt and Fe was kept at constant, the catalytic activity of Pt (25%)/Fe (75%)/PAn electrode was slightly higher than that of Pt (100%)/PAn electrode. This indicates that about 80% of Pt can be replaced by inexpensive iron.

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The temperature dependence of the methanol oxidation reaction (MOR) rate at various Pt alloy electrodes was investigated by using a thin layer flow cell in a wide temperature range from 20 to 120°C in 1 M CH₃OH+0.1 M HClO₄ solution under pressurized operation. At 60°C and 0.55 V vs. RHE, the MOR current densities at Pt-Sn and Pt-Ti were about 4 times higher than that at pure Pt, although it was still lower than Pt-Ru. Such enhanced MOR activities at Pt alloyed with non-precious metals were ascribed to the modified electronic structure of Pt skin layer formed on the alloy surface, facilitating the oxidation of weakly adsorbed CO. However, these alloys lost high MOR activities once heated at the high temperature (>80°C) in the acid electrolyte solution, probably due to the thickening of Pt skin layer by a considerable dissolution of non-precious metal components from the alloys.

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We have investigated new Al-carbon bipolar plate materials for a polymer electrolyte fuel cell (PEFC). Recently, many developments for bipolar plate materials have been founded in PEFC studies. More improvements are required in advancements for low corrosion rate, high conductivity and reduced cost. We propose a new concept for bipolar plate materials, specifically Al-carbon hybrid materials. This hybrid includes thin film carbon technology and ordered interfacial technology. The Al-carbon hybrid materials proposed in this paper have low corrosion rate, high conductivity and low cost. We present the electrochemical and physical properties of Al-carbon hybrid bipolar plate materials for PEFC. The corrosion behavior of Al-carbon hybrid materials was investigated using the Tafelextrapolation method and inductively coupled plasma spectrometry (ICP). In spite of local battery-effect reactions occurring within some Al alloy materials, the proposed Al-carbon hybrid bipolar plate materials have good performance for corrosion resistance and conductivity.

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Anodic oxidation of ethanol was investigated on a platinum (Pt) microparticle electrocatalyst. The Pt catalyst was chemically deposited on a poly(N-methylpyrrole)/Nafion composite film formed on a glassy carbon (GC) support. The initial specific activity of the electrocatalyst, which was evaluated by the anodic current per Pt-loading observed on a cyclic voltammogram, was improved by modifying the GC surface with the polymeric composite film. The optimum thickness of the polymeric film gave the highest activity for ethanol oxidation. The modification of the GC support with the polymeric composite was also effective to depress the activity fading with the duration of the anodic polarization at a constant potential.

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Ethanol oxidation on platinum-ruthenium binary (PtRu) electrocatalyst loaded on a composite of poly(N-methylpyrrole) and Nafion (PNMPY) has been investigated. Two kinds of methods, chemical reduction and electrodeposition, were applied to load PtRu metallic species on catalyst supports. The PtRu electrocatalyst prepared by the former method exhibited higher initial activity for ethanol oxidation per unit weight of Pt than that prepared by the latter method. The dependence of Ru/(Pt+Ru) ratio on catalytic properties was examined for the electrocatalysts prepared by the electrodeposition method. The initial activity increased as the increase of Ru/(Pt+Ru) ratio. In contrast, the long-term activity showed rather complicated dependence toward Ru/(Pt+Ru) ratio.

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The proton conductivity of the reinforced perfluorinated membrane was measured under various conditions of humidity and temperature using a four-electrode impedance method. The conductivity of the membrane was over 8×10⁻² S cm⁻¹ with 70% relative humidity at 70°C; it varied strongly with both humidity and temperature. A stable conductivity condition was observed when the relative humidity was over 20%. From the results, the activation energy was calculated to be less than 20 kJ mol⁻¹ at any humidity. These results suggested that the Grothuss-type proton conduction might be dominant for the proton migration inside the reinforced membrane at a wide range of humidity.

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The effects of the ion exchange capacity (IEC), of the perfluorosulfonic acid polymers, on the performance of PEFC has been investigated quantitatively by a simplified analysis method. It was determined that the IEC of the membrane influences the mass transfer overpotential of the oxygen reduction reaction (ORR). Whereas, it was determined that the IEC of the catalyst coating ionomer influences the activation overpotential of the ORR.

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A durability test of a PEFC single cell using perfluorinated ionomer membrane as an electrolyte was carried out at 80°C under low humidification. The observed voltage drop under low humidification was mostly reversible up to 5200 h. However, hydrogen crossover increased with time and fluoride-ion was continuously detected in drain water, which indicated that membrane degradation proceeded steadily during the test under low humidification. Fluoride-ion release rate decreased with increasing current density, which suggested that hydrogen peroxide was formed at the anode catalyst layer upon direct combustion of hydrogen with crossover oxygen. Drain water analysis revealed that large amounts of sulfate ions and Fe(II or III) ions were released in addition to fluoride ions when the cell was temporarily operated under full humidification. From this fact, it was concluded that accumulation of impurities and hydrogen peroxide under low humidification is a reason why low humidification enhances membrane degradation.

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A new design of metal separator coated with corrosion-resistive and electronically conductive carbon/resin composite layers has been developed. A stainless steel flat plate was coated with a thin composite layer, followed by a formation of ribs of the similar composite on the layer as the gas flow field. We found that the electric conductivity was improved by blending carbon fiber with Ketjen black in the carbon/resin composite. The lowest resistivity of 20.6 mΩ cm was obtained at total carbon content of 50 vol.% with blend ratio of the carbon fiber of 75 vol.%. The composite exhibited a good stability in hot water at 90°C for 2000 h, releasing very small amount of ionic impurities.

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Gold nanoparticles supported on Pt-Sn/C and Pt/C (nano-Au/Pt-Sn/C and nano-Au/Pt/C, respectively) were prepared by the gas-phase grafting method and their catalytic activities for methanol oxidation were evaluated by steady-state polarization measurement. X-ray diffraction measurement revealed that gold nanoparticles had a diameter of ca. 4 nm, and X-ray photoelectron spectroscopy revealed that gold nanoparticles deposited on Pt-Sn/C and Pt/C were in the zero-oxidation state. Electrochemical measurements performed in an aqueous solution of 1 M (M=mol dm⁻³) HClO₄ and 1 M CH₃OH demonstrated that the addition of gold nanoparticles to Pt-Sn/C catalyst gave higher catalytic activities for methanol oxidation than Pt-Sn/C catalyst without gold nanoparticles. In contrast, nano-Au/Pt/C and Pt/C showed almost identical catalytic activities. These results showed that the catalytic activities of gold nanoparticles were influenced by the supporting materials.

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To investigate the effect of fuel cell operating conditions on catalyst agglomeration of a platinum catalyst, several single cell life tests were carried out under different conditions. It was found that catalyst agglomeration was accelerated by higher temperature operation, while it was not accelerated by higher current density (∼1.5 A/cm²) nor by higher potential (∼0.85 V) when the operating temperature was 100°C. Catalyst agglomeration was much more severe in the cathode, and the surface area of the platinum catalyst was decreased from 58.9 m²/g to 30.6 m²/g after a life test for 4000 hours. Because there was no sign of platinum dissolution/precipitation in the ionomer phase and there was a peak tail in the direction of larger particles in the particle distribution after 4000 hours, this catalyst agglomeration may be caused by a crystallite migration mechanism and a coalescence growth mechanism. In addition, to investigate the degradation of the CO-tolerance performance of a platinum-ruthenium catalyst, 5000-hour life tests and several life tests for shorter periods were carried out. It was found that the CO-tolerance performance was gradually decreased with time and that one of the reasons for the degradation was considered to be decomposition of the platinum-ruthenium alloy. It was also found that an air-bleeding technique could reduce the loss of cell performance due to the degradation of the CO-tolerance performance but may slightly accelerate the catalyst material degradation.

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The Nafion membrane impregnated with various room-temperature ionic liquids have been prepared and characterized as the electrolytes for polymer electrolyte fuel cells (PEFCs) operated at elevated temperature (100∼200°C). The continuous proton transportation in the membrane was confirmed under dry hydrogen atmosphere at 160°C. The ionic conductivity of the membrane was determined by both DC and AC measurements under dry hydrogen atmosphere and found to reach the order of 10⁻² S cm⁻¹ at 200°C without humidification. It is suggested that the high ionic conductivity of the membrane is caused mainly by free ionic liquids in the membranes. The continuous operation of the PEFC using the membrane was found to be possible without humidification at 120°C. However, the overpotential at the cathode was found to be large compared with the anode probably due to the sluggish electrode reaction and/or the slow diffusion of HTFSI in the membranes.

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AMPS polymer membranes containing various amounts of water were fabricated to simulate the material degradation of in-service membranes in PEFC. The life time of PEFC was measured along with measurements of tensile properties and swelling of the membranes. It was concluded that (1) the failure of PEFC is caused by fracture of the AMPS membrane; (2) the tensile strength of the AMPS membrane decreases with increasing water content due to a softening of the membrane; and (3) a high concentration of the cross-linker molecules is very effective for improving the mechanical strength of the membrane and the resistive force against the swelling, which results in the longer durability of PEFC.

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The nano-composite of the phosphoric acid derivative of fullerene ([PO(OH)₂]₂CC₆₀) with SiO₂ was synthesized using the in-situ reaction from [PO(OH)₂]₂CC₆₀ with tetraethoxysilane (TEOS). The nano-composite was characterized using FT-IR measurements, and its morphology was observed using SEM. The humidity dependence of their proton conductivity for both [PO(OH)₂]₂CC₆₀ and its nano-composite was investigated. The proton conductivity of the nano-composite is increased more than one order of magnitude higher than that of [PO(OH)₂]₂CC₆₀ under low relative humidity. This is probably because the more proton conductive sites are generated by SiO₂. However its proton conductivity is lower in the humidity range above 60%, probably because the number of the highly dissociative protons from phosphoric acid is decreased after being combined with silica.

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Meniscus formation and hydrogen oxidation on partially immersed Nafion^® coated and uncoated electrodes were investigated as a model of the reaction sites in polymer electrolyte fuel cells. Hydrogen oxidation current was measured at 0.4 V with raising the electrode, and simultaneously the optical microscope observation was carried out. The intrinsic meniscus developed from the boundary between Nafion^®-coated (Pt-loaded) and the uncoated parts. A liquid thin film, which would have sub-micron order thickness, was observed above the intrinsic meniscus on the uncoated electrode under hydrogen atmosphere after development of the intrinsic meniscus was terminated. At the same time, hydrogen oxidation current was further increased. Therefore, the liquid thin film behaved as an effective supermeniscus on the uncoated electrode. On Nafion^®-coated electrode, hydrogen oxidation current was small in the intrinsic meniscus region; however, it became larger with an increase in the exposed area of Nafion^®-coated part to the gas phase. This fact showed that hydrogen oxidation reaction occurred mainly in the exposed Nafion^®-coated part. The current distribution in the supermeniscus region was discussed, and it was found that the effective area is controlled by the ohmic drop in the Nafion^® coating.

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In this fundamental study on corrosion of the carbon catalyst support in a PEMFC cathode, corrosion rates of electrodes made of carbon powder were investigated by measuring CO₂ generation. The corrosion rate was affected by not only the retention potential but also by the presence of dissolved oxygen in the electrolyte. The corrosion rate showed two stages; initial corrosion was observed for several hours, and was followed by steady-state corrosion. Even in this latter stage, change to a lower potential induced a peak in the corrosion rate that exceeded the previous steady value just after the potential was restored. Corrosion was also accelerated by platinum catalyst.

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A transparent optical cell for a polymer electrolyte fuel cell (PEFC) was used to measure the water distribution in the cathode during cell operation. The distribution of water droplets could be clearly observed in the cathode. The results show that the transition point to two-phase flow in the cathode flow field moved towards the cathode inlet when the cathode gas utilization rate was increased. This was compared with the results of a calculation based on a two-dimensional cell model and good agreement between the two was obtained. It was also observed that at high current densities, water droplets were swept away from the flow field by the shear force of the cathode gas.

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