Platinum Is Not Just Platinum: Experimental Analysis on the Effectiveness and Stability of Different Gas Recombination Catalysts for PEM Water Electrolysis

Polymer electrolyte membrane (PEM) water electrolyzers provide a promising solution for storing and transporting renewable energy through the production of green hydrogen. However, an issue that still needs to be solved is small portions of the produced hydrogen and oxygen gases permeating through the cell membrane to the opposite electrode side during operation. This so called gas crossover poses problems regarding both efficiency and safety of the cell, since the hydrogen permeating to the anode is not only no longer available as product gas but also induces critical safety issues when it is combined with oxygen at the anode [1,2]. Therefore, crossover mitigation strategies are essential to achieve higher cathode pressures of about 30 bar. One of the most promising strategies are gas recombination catalysts (GRCs), which are used to recombine permeating hydrogen with oxygen to water at catalyst particles inside of the membrane [3]. In this study, we examine different platinum-based recombination catalysts: platinum black, platinum nanoparticles, platinum on a carbon support, and a reference catalyst coated membrane (CCM) without GRC. Each CCM is measured at 30 bar cathode pressure with a test protocol involving fast voltage cycles, OCV steps, and constant current holds at low and high current densities. Figure 1 shows the different results for each CCM at begin of life (BoL). All CCMs with a GRC have a substantially lower crossover than the reference CCM. Interestingly, the nanoparticles did not perform as well as anticipated, which is likely due to the agglomeration of the nanoparticles during the manufacturing process. Equally interesting is the good performance of the carbon-supported platinum, which may be attributed to an increase in the active surface area. Besides developing new GRCs and their initial performance, long-term stability is crucial. Therefore, we stressed all of these CCMs with the above described protocol for 1500 h to examine their long-term performance regarding electrochemical and gas crossover characteristics. Figure 2 shows the development of the hydrogen in oxygen content over time for selected GRCs at two different current densities. We observe a substantial rise in hydrogen crossover in the first 250 h. This can probably be understood as some kind of conditioning phase without actual degradation. After this, a new degradation phase is reached with a much slower degradation rate. Ex-situ imaging, SEM and XRF, will bring further insights into the comparison of pristine and degraded CCMs in terms of a possible migration of the platinum inside the CCM. References: [1] M. Schalenbach et al (2013) Int. J. Hydrogen Energy 38 14921–14933, DOI 10.1016/j.ijhydene.2013.09.013 [2] H. Ito et al (2016) Int. J. Hydrogen Energy 41 20439–20446, DOI 10.1016/j.ijhydene.2016.08.119 [3] C. Klose et al (2018) J. Electrochem. Soc. 165 F1271, DOI 10.1149/2.1241814jes Figure 1