This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.This work addresses current challenges in catalyst development for proton exchange membrane water electrolyzers (PEM-WEs). To reduce the amount of iridium at the oxygen anode to levels commensurate with large-scale application of PEM-WEs, high-structured catalysts with a low packing density are required. To allow an efficient development of such catalysts, activity and durability screening tests are essential. Rotating disk electrode measurements are suitable to determine catalyst activity, while accelerated stress tests on the MEA level are required to evaluate catalyst stability.
In this study, a commercial IrO2/TiO2 catalyst (75 wt% Ir, named “Benchmark”) for the oxygen evolution reaction (OER) is compared to a newly developed IrO(OH)x/TiO2 catalyst (45 wt% Ir, named “P2X”). Due to its lower Ir packing density and higher OER activity vs the Benchmark catalyst (440 vs 12 A gIr
−1 at 1.43 ViR-free), the P2X catalyst shows an improved PEM (proton exchange membrane) water electrolyzer performance at ≈9 times reduced Ir loading, however, only if a platinum-coated porous transport layer (PTL) at the anode is used. While the performance of membrane electrode assemblies (MEAs) with the Benchmark catalyst is unaffected when using an untreated titanium PTL, MEAs with the P2X catalyst perform poorly, which can be attributed to a contact resistance at the anode/PTL interface due to the low electrical conductivity of the P2X catalyst (0.7 S cm−1) vs the Benchmark catalyst (416 S cm−1) and the passivation of the Ti-PTL. A heat treatment procedure is used to transform the amorphous IrO(OH)x of the P2X catalyst into crystalline IrOx and, hence, increases its electrical conductivity. The optimum temperature for heat treatment to maximize electrical conductivity, OER activity and MEA performance will be evaluated.
Lowering the iridium loading at the anode of proton exchange membrane (PEM) water electrolyzers is crucial for the envisaged GW-scale deployment of PEM water electrolysis. Here, the durability of a novel iridium catalyst with a low iridium packing density, allowing for low iridium loadings without decreasing the electrode thickness, is being investigated in a 10-cell PEM water electrolyzer short stack. The anodes of the membrane electrode assemblies (MEAs) of the first five cells utilize a conventional iridium catalyst, at loadings that serve as benchmark for today’s industry standard (2 mgIr/cm2). The last five cells utilize the novel catalyst at 8-fold lower loadings (0.25 mgIr/cm2). The MEAs are based on Nafion® 117 and are tested for 3700 h by load cycling between 0.2 and 2.0 A/cm2, with weekly polarization curves and impedance diagnostics. For both catalysts, the performance degradation at low current densities is dominated by an increase of the overpotential for the oxygen evolution reaction (OER), whereby the OER mass activity of the novel catalyst remains ≈4-fold higher after 3700 h. The temporal evolution of the OER mass activities of the two catalysts will be analyzed in order to assess the suitability of the novel catalyst for industrial application.
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