Currently, carbon black is widely used as an electrocatalyst support for polymer electrolyte fuel cells (PEFCs). However, electrochemical oxidation leads to degradation of this material. In contrast, tin oxide (SnO 2 ) is electrochemically stable even under strongly acidic conditions, and relatively high electronic conductivity can be achieved by doping with niobium (Nb-SnO 2 ), compared with other metal oxides. In this study, Nb-SnO 2 is composited with various conductive carbon fillers, including vapor-grown carbon fibers (VGCF), carbon nanotubes (CNT), and graphitized carbon black (GCB), followed by platinum nanoparticle decoration. These nanocomposite electrocatalysts are incorporated into membrane electrode assemblies (MEAs) and tested under PEFC operational conditions. The resulting fuel cells achieve high initial I-V performance up to 0.742 V at 0.2 A cm −2 (80 • C), as well as excellent cycling durability. In particular, MEAs fabricated with Pt/Nb-SnO 2 /VGCF cathode electrocatalysts exhibit remarkable durability, with only a 12.1% drop in cell voltage at 0.2 A cm −2 over 60,000 start-stop cycles, and a 42.9% drop over 400,000 load potential cycles, corresponding to the lifetime of a fuel cell vehicle (FCV). Platinum-decorated metal oxide electrocatalysts can simultaneously realize high catalytic activity and extended durability, not only in ex-situ half-cell measurements, but also in full cell conditions.
Novel catalyst-integrated gas diffusion electrodes (GDEs) for polymer electrolyte membrane water electrolysis (PEMWE) cells are presented, in which porous titanium microfiber sheets are etched in NaOH to generate a nanostructured TiO2 surface, followed by arc plasma deposition (APD) of iridium nanoparticles. The porous titanium sheet acts as a gas diffusion layer (GDL); the nanostructured TiO2 surface acts as a catalyst support with large surface area; and the iridium nanoparticles act as the electrocatalyst. The performance of these unique GDEs in PEMWE cells was optimized by etching in different NaOH concentrations to vary the nanostructure of the TiO2; and by varying the Ir loading via the number of APD pulses. The current-voltage characteristics and the durability of the optimized GDEs were comparable to those reported in the literature using conventional Ir-based electrocatalysts, and electrolysis was achieved with current density up to 5 A cm−2. The main advantages of this catalyst-integrated GDE include the very low iridium loading (i.e. around 0.1 mg cm−2, or just one-tenth of the loading typically used in conventional PEMWEs); high electrolysis current density; the fabrication of stacks with fewer components; and the fabrications of thinner stacks. This could ultimately lead to smaller and lower cost PEMWE systems.
Carbon black is difficult to be used as the catalyst support material for polymer electrolyte membrane water electrolysis (PEMWE) due to carbon corrosion at a high potential. In order to prepare iridium-based nano-size electrocatalysts similar to PEFC, SnO2 and TiO2 are considered as possible catalyst support materials with high potential stability. In this study, we prepare (i) iridium-decorated SnO2 supported on Vapor-Grown Cabon Fiber (IrO2/Sn(Nb)O2/VGCF) and (ii) carbon-free Ir/TiOx/Ti sheet where Ti sheet with naturally oxide surface is used as the catalyst support and the gas diffusion layer (GDL). Ir nanoparticles on each metal oxide support could be prepared, confirmed by electron microscopy. The Ir/TiOx/Ti electrode exhibited higher IV characteristics with a lower Ir loading than the IrO2/Sn(Nb)O2/VGCF electrode.
Carbon black support is generally used for PEFC electrocatalysts, while their durability is an important technological issue caused by e.g. carbon corrosion. Metal oxide supports have been studied as alternatives to carbon black. TiO2 has also been considered as an alternative support material due to its stability under both cathode and anode conditions. Here, a Ti-based sheet-form electrode is developed, consisting of Pt electrocatalysts, and a porous metallic Ti sheet acting as both catalyst support and gas diffusion layer (GDL). Ti-based nanofibers with a diameter of several nm were formed on the surface of the porous Ti sheet via NaOH treatment. Pt catalysts were successfully decorated on such nanostructure to construct a three-dimensional network structure in a carbon-free all-in-one electrode for PEFCs.
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