Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings

PEM water electrolyzers are promising candidates for large-scale energy storage in combination with renewable energy sources. Typically, high loadings of platinum group metal (PGM) catalysts are used, but since most currently installed systems are relatively small (kW range), catalyst costs are minor compared to other system costs. However, for large-scale applications, the contribution of balance-of-plant costs will be much lower and catalyst costs will become a major cost contributor [1]. Increasing prices due to a higher demand of PGMs might further amplify this trend in the future [2]. Therefore, a reduction of catalyst loading is essential to improve the economic competitiveness of PEM electrolyzers. There have been efforts to reduce PGM loadings, and previous studies show that high performance is possible even at PGM loadings of ~0.5 mg PGM cm -2 [3,4]. However, there are only few studies which systematically analyze the effect of catalyst loading on the voltage losses of an electrolyzer [5,6]. In this study, we investigate the influence of reduced catalyst loadings on both the anode and cathode of a PEM electrolyzer and analyze the resulting changes in performance and voltage loss contributions. MEAs based on a Nafion® 212 membrane, carbon-supported platinum catalysts (Pt/C) for the hydrogen evolution reaction (HER), and an IrO 2 /TiO 2 catalyst (Umicore) for the oxygen evolution reaction (OER) were fabricated with different catalyst loadings. A reduction of the Pt loading on the cathode from 0.32 to 0.02 mg Pt cm -2 was achieved by using a catalyst with a lower Pt content of 4.8 wt% instead of 46.1 wt% Pt, while keeping the electrode thickness similar (~10 μm). Even though the loading was reduced by more than an order of magnitude, the performance remained essentially unchanged (see red and blue curve in Figure 1a), which is mainly due to the fast kinetics of the HER [7]. On the anode, the Ir loading was reduced from 1.6 to 0.4 mg Ir cm -2 by decreasing the electrode thickness from ~8 to 2 μm. The resulting increase in cell voltage (see green curve in Figure 1a) is in part due to the lower electrochemically active surface area, i.e., due to an OER kinetic loss of ~30 mV. Unfortunately, we observe an additional increase in contact resistance for this very thin electrode, which we ascribe to its inhomogeneity. Contributions of proton and mass transport losses for the different electrodes will be analyzed as well. Figure 1b shows the PGM-specific p