In commercial electrolysis, the cost of electricity input drives the cost of hydrogen production. Therefore, electrolysis is typically run at high catalyst loading and constant power input over long periods of time. Lowering water-splitting hydrogen production costs to a level comparable to steam methane reformation requires: coupling electrolysis with low-cost power input (wind, solar) to reduce feedstock costs; and dropping catalyst loading to reduce the capital cost at lower capacity.[1,2] While minimal durability loss is seen in commercial electrolyzers, catalyst losses can be masked by high loading (several mg cm^‒2). At low loading, however, these losses become more apparent.[3]

In this study, electrolyzer durability was evaluated at low iridium-anode loading (0.1‒0.5 mg cm^‒2) and with different power inputs (potentials, intermittency). Higher loading tended to delay the onset of durability losses; in contrast, a loading of 0.1 mg cm_elec^‒2 resulted in incremental but immediate loss when exposed to high potential. Increasing the upper potential limit appeared to increase iridium dissolution and migration and resulted in higher performance losses. This trend was expected and iridium dissolution is anticipated to be a primary factor in electrolyzer loss at low loading.[4] Introducing cycling (square/triangle wave), however, significantly increased the rate of performance decay and was less expected from half-cell tests and even though less time was spent at elevated potential.[3] Changing the rate of potential increase (saw tooth profiles) confirmed that rapid input increases accelerated loss and may be due to localized potential spikes occurring within the catalyst layer. A variety of system control-based mitigation strategies have been evaluated for lessening durability losses when handling intermittent power sources.

Several iridium catalyst types (oxides, surface areas) have been tested in half- and single-cells. Catalyst development efforts often focus on metallic- or hydroxide-based iridium structures, due to higher activity in ex-situ tests. These performance advantages, however, largely disappear in single-cell tests and may be due to surface/near-surface oxidation during conditioning protocols. Metallic/hydroxide durability losses further tend to be larger in both half- and single-cells and may be related to the kinetics of iridium metal/hydroxide/oxide dissolution.

In single-cell testing, we have focused on how loading, test parameters, and catalyst type affect proton exchange membrane-based electrolyzer durability. These tests have significant implications on lowering the cost of electrolysis-based hydrogen production and on coupling electrolysis with renewable power inputs.

[1] H2 at Scale: Deeply Decarbonizing our Energy System. Presented at Annual Merit Review, U.S. Department of Energy; Washington, DC, June 6−10, 2016. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf.

[2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. Available at the following: http://www.nrel.gov/docs/fy16osti/65023.pdf.

[3] Alia, S. M.; Rasimick, B.; Ngo, C.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Xu, H.; Pivovar, B. S. J. Electrochem. Soc. 2016, 163, F3105−F3112.

[4] Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J. P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A.; Mayrhofer, K. J. J. Cat. Today 2016, 262, 170.