(Invited, Digital Presentation) Nanostructured Thin Film (NSTF) Iridium Catalyst Powder for Proton Exchange Membrane Water Electrolyzers

Proton exchange membrane water electrolyzers (PEMWEs) are electrochemical devices which generate hydrogen (H 2 ) gas from water and electrical energy feedstocks. PEMWEs produce H 2 renewably and carbon-free when the electricity is from renewable sources, and are a pathway to enable deep decarbonization across multiple industrial and energy sectors[1]. However, commercial deployment of PEMWEs is currently limited to megawatt-scale due to relatively higher H 2 production costs and capital costs than hydrocarbon reforming [2]. The higher costs are due in part to the use of significant quantities of expensive materials (Pt and Ir electrocatalysts and perfluorinated ionomers), insufficient operating performance and durability, and high manufacturing costs. Additionally, commercial PEMWEs additionally use high Ir loadings [3] for the oxygen evolution reaction (OER), and the limited abundance of Ir [4] may limit PEMWE annual deployment of those technologies to gigawatt (GW) scale. 3M Nanostructured Thin Film (NSTF) PEMWE OER powder catalysts and electrodes are a unique approach to address the cost and Ir utilization barriers noted above. NSTF catalysts [5] are comprised of nm-scale catalyst metal thin films on a high aspect ratio inert support (Fig. A). NSTF Ir OER catalysts enable high efficiency and high durability due to high OER mass activity and intrinsic resistance to dissolution, imparted by the unique agglomerated thin film catalyst structure. NSTF OER electrodes [6] consist of a dispersed matrix of NSTF catalyst powder particles within a perfluorosulfonic acid (PFSA) ionomer binder (Fig. B), which have high catalyst utilization due to the high electronic conductivity of the primary catalyst particles. One of the key challenges associated with development of OER catalysts and electrodes is the lack of qualified accelerated stress tests (ASTs) to enable rapid assessments of durability under conditions relevant for end-use. The challenge is in part magnified by the long lifetime requirements of 80,000 hours and low required decay rates of single microvolts per hour, which traditionally has required long testing times and multiple replicates to obtain needed statistical significance. Additionally, evaluations of durability have often occurred under steady state testing with fixed current densities, which do not reflect anticipated use profiles when integrated with renewables such as wind and solar with significant power production variability over time. Lastly, operation at increased stack power densities is considered a key strategy to reduce stack capital costs and Ir requirements on a gram per kW basis. In this paper, we will report recent work on our durability assessment of NSTF OER powder catalysts and electrodes under aggressive testing protocols with low catalyst loadings relevant for PEM electrolyzers at large scale. Assessments included steady state durability tests, an accelerated stress test, and a protocol intended to simulate integration with a wind variable renewable energy (VRE) load profile. An example of results from the wind VRE protocol are summarized in Figs. C and D. The wind VRE protocol generated by Alia et al. [7] was modified from voltage control to current control and the maximum current density was scaled to 4.5A/cm 2 . The protocol was applied to a 3M laboratory CCM comprising a 0.20 mg/cm 2 of 78wt% Ir/NSTF powder catalyst OER electrode, 0.09 mg/cm 2 of 78wt% Pt/NSTF powder hydrogen evolution reaction (HER) electrode, and a 100 micron thick PEM (800EW 3M PFSA). After 500 hours of the wind VRE protocol, the cell performance was essentially unchanged (1mV voltage decrease at 2A/cm 2 ). S. Dept. of Energy “H2@Scale”, https://www.energy.gov/eere/fuelcells/h2scale . S. Dept. of Energy H2USA Model, Current Forecourt Hydrogen Production v. 3.101. Ayers et al., Catalysis Today 262 121-132 (2016). Babic et al., Electrochem. Soc. 164 F387 (2017). Debe et al., ECS Trans. 45 (2) 47-68 (2012). Steinbach et al., 2019 U.S. DOE Annual Merit Review, Project ta026. Alia et al., Electrochem. Soc. 166 F1164 (2019). Figure 1