In this study, the influence of catalyst loading on the performance of a proton exchange membrane (PEM) water electrolyzer is investigated (Nafion 212 membrane; IrO 2 /TiO 2 (anode) and Pt/C (cathode)). Due to the fast kinetics of the hydrogen evolution reaction (HER) on platinum (Pt), the Pt loading on the cathode can be reduced from 0.30 mg Pt cm −2 to 0.025 mg Pt cm −2 without any negative effect on performance. On the anode, the iridium (Ir) loading was varied between 0.20-5.41 mg Ir cm −2 and an optimum in performance at operational current densities (≥1 A cm −2 ) was found for 1-2 mg Ir cm −2 . At higher Ir loadings, the performance decreases at high current densities due to insufficient water transport through the catalyst layer whereas at Ir loadings <0.5 mg Ir cm −2 the catalyst layer becomes inhomogeneous, which leads to a lower electrochemically active area and catalyst utilization, resulting in a significant decrease of performance. To investigate the potential for a large-scale application of PEM water electrolysis, the Ir-specific power density (g Ir kW −1 ) for membrane electrode assemblies (MEAs) with different catalyst loadings is analyzed as a function of voltage efficiency, and the consequences regarding catalyst material requirements are discussed. PEM water electrolysis could provide electrolytic hydrogen for large-scale energy storage and mobility in a future energy scenario based on renewable energy sources. Currently, only a small share of the global hydrogen demand is served by PEM electrolysis due to the relatively high costs associated with this technology.

In this study, the influence of ionomer content in IrO 2 /TiO 2 anode electrodes for a proton exchange membrane (PEM) electrolyzer is investigated (Nafion 212 membrane; 2.0 mg Ir cm −2 / 0.35 mg Pt cm −2 (anode/cathode)) and the contributions of ohmic losses, kinetic losses, proton transport losses in the electrodes, and mass transport losses to the overall cell voltage are analyzed. Electrolysis tests are performed with an in-house designed high pressure electrolyzer cell at differential pressure up to 30 bar. The best performance is obtained for an ionomer content of 11.6 wt% and a cell voltage of 1.57 V at 1 A cm −2 and less than 2 V at 6 A cm −2 (ambient pressure, 80 • C). Performance losses at lower ionomer contents are the result of a higher proton conduction resistance. For higher ionomer contents, on the other hand, performance losses can be related to a filling of the electrode void volume by ionomer, leading to a higher O 2 mass transport resistance, an increased electronic contact resistance, and the electronic insulation of parts of the catalyst by ionomer. At high pressure operation, the performance corrected by the shift of the Nernst voltage increases with H 2 pressure and we propose a new explanation for this effect. In the course of the transition from fossil-based to renewable energy sources, hydrogen technology has gained considerable attention during the past decades. Proton exchange membrane (PEM) electrolyzers are well suited to be coupled with intermittent energy sources such as wind and solar and could provide electrolytic hydrogen for long-term energy storage or fuel cell mobility. At the moment, the large-scale application of PEM electrolyzers is still hindered by their high capital costs.1,2 One attempt to overcome this challenge is to increase the H 2 output by operating an electrolyzer at current densities much higher than the values typically reported in the literature (1-2 A cm −2 ). 2 Recent publications have shown that current densities of 5 A cm −2 and higher are possible. 3,4 Another factor that can be economically beneficial is the operation at high pressure because it allows direct storage of H 2 without subsequent mechanical compression. However, high-pressure operation leads to more demanding materials requirements, imposes additional safety precautions, and reduces the faradaic efficiency due to a higher gas permeation through the membrane. 5,6 It was reported that an operating pressure of 30-45 bar could be a good compromise, 7 with differential pressure operation ( p O 2 ≈ ambient pressure) being more efficient than balanced pressure operation ( p O 2 ≈ p H 2 ).8 However, increasing the current density and the operating pressure of an electrolyzer will increase the cell voltage, leading to a lower overall voltage efficiency and thus higher operating costs. Since the latter are, along with the capital costs, one of the main cost drivers for large-scale applications, 9 minimizing cell voltage at high current densities and pressures is essential for economic competitiveness...

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.