Iridium on Conductive Support: Towards Low Iridium Anodes in PEM Electrolyzers

Abstract

Commercialization of proton exchange membrane (PEM) electrolyzers for green hydrogen production have been recently achieved, but with a limited scale of low gigawatts (GW).1 Large-scale and sustainable deployment of PEM electrolyzers will face the challenges of scarcity and high cost of iridium (Ir) used in the anode to catalyze oxygen evolution reaction (OER). A 1 MW PEM electrolyzer stack currently uses ~0.4 kg of Ir based on an Ir loading of 1.5 mg/cm2, which contributes ~$60k cost per stack.2 Moreover, the Ir production has been about 8 tons/year in recent years.3 This can only support an annual production of 5 GW PEM electrolyzer if assuming 25% Ir is available for PEM electrolyzers with the same Ir loading. Therefore, lowering the Ir loading in PEM electrolyzers is urgently needed to meet the rapid expansion of the PEM electrolysis market.Several groups including Plug have developed supported Ir catalysts to lower the Ir loading by a factor of 5 without sacrifice in efficiency.4-8 However, all support used by far is non- or poorly electrically conductive. The conductivity of the electrodes relies solely on the surface IrOx, which sets stringent limits on the catalyst/electrode development, especially with low Ir contents. Here we first argue from fundamental aspects that the catalysts/electrodes with Ir on conductive support can be free of these limits. We further show that platinum (Pt) and titanium diboride (TiB2) powders are feasible candidates as conductive support for Ir-based OER catalysts. We demonstrated a TiB2 supported IrOx (IrOx/TiB2) catalyst synthesized via wet chemistry deposition without post heat treatment combines a mass activity towards OER with high conductivity. Its conductivity of ~30 S/cm2 is comparable to that of Vulcan carbon, and ~105 times that of the counterpart IrOx/W-TiO2 (W-TiO2 represents commercial tungsten doped TiO2 nanoparticles). Meanwhile, the IrOx/TiB2 catalyst shows a mass activity comparable to that of the counterpart IrOx/W-TiO2, twice that of commercial Ir black, and 50 times that of a commercial IrO2/TiO2 catalyst in acidic solution. Durability test showed that the Ir dissolution of the IrOx/TiB2 in acidic solution holding at 2 V for 100 hours is comparable to that of Ir black. Characterization of the IrOx/TiB2 showed small hydrous IrOx nanoparticles (1-2 nm) uniformly distributed on the surface of TiB2 nanoparticles (~58 nm) with an Ir content of ~33±7 wt%. Membrane electrode assembly evaluation on the IrOx/TiB2 catalyst is undergoing. The results will be reported and discussed. References (1) IEA, World Energy Outlook, 2022. https://iea.blob.core.windows.net/assets/830fe099-5530-48f2-a7c1-11f35d510983/WorldEnergyOutlook2022.pdf (accessed 2023-02-12).(2) Mittelsteadt, C. (Invited) Ir Strangelove: Or How I Learned to Stop Worrying and Embrace the PEM. ECS Meeting s 2022, MA2022-01, 1335-1335.(3) Seeking Alpha Home Page. https://seekingalpha.com/article/4399727-sibanye-should-benefit-from-hydrogen-wars-thanks-to-iridium-exposure (accessed 2023-02-12).(4) Böhm, D.; Beetz, M.; Gebauer, C.; Bernt, M.; Schröter, J.; Kornherr, M.; Zoller, F.; Bein, T.; Fattakhova-Rohlfing, D. Highly conductive titania supported iridium oxide nanoparticles with low overall iridium density as OER catalyst for large-scale PEM electrolysis. Applied Materials Today 2021, 24, 101134.(5) Pham, C. V.; Bühler, M.; Knöppel, J.; Bierling, M.; Seeberger, D.; Escalera-López, D.; Mayrhofer, K. J. J.; Cherevko, S.; Thiele, S. IrO2 coated TiO2 core-shell microparticles advance performance of low loading proton exchange membrane water electrolyzers. Appl. Catal. B‐Environ. 2020, 269, 118762.(6) Zhao, S.; Stocks, A.; Rasimick, B.; More, K.; Xu, H. Highly Active, Durable Dispersed Iridium Nanocatalysts for PEM Water Electrolyzers. J. Electrochem. Soc. 2018, 165, F82-F89.(7) Oakton, E.; Lebedev, D.; Povia, M.; Abbott, D. F.; Fabbri, E.; Fedorov, A.; Nachtegaal, M.; Copéret, C.; Schmidt, T. J. IrO2-TiO2: A high-surface-area, active, and stable electrocatalyst for the oxygen evolution reaction. ACS Catal. 2017, 7, 2346-2352.(8) Lewinski, K. A.; van der Vliet, D.; Luopa, S. M. NSTF advances for PEM electrolysis-the effect of alloying on activity of NSTF electrolyzer catalysts and performance of NSTF based PEM electrolyzers. ECS Trans 2015, 69, 893.