Activity and Durability of Iridium Nanoparticles in the Oxygen Evolution Reaction

Although electrochemical water splitting currently represents a small amount of the US hydrogen supply, its role is expected to grow with the emergence of a hydrogen-based infrastructure.[1] Electrolysis is reasonably cost-competitive with a variety of technologies, including batteries (solid state and flow), pumped hydro, and compressed air.[2] When coupled with renewable energy sources, such as wind and solar, electrochemical water splitting can produce hydrogen using a nearly carbon free pathway.[3] The commercial progress of electrolyzers is limited in part by the cost of the catalyst layer. The oxygen evolution reaction is the focus of electrocatalyst development due to slow kinetics relative to hydrogen evolution; iridium nanoparticles are commonly used to catalyse oxygen evolution, with platinum added to ensure conductivity throughout the catalyst layer.

Extended surface nanostructures have previously been developed as electrocatalysts in hydrogen fuel cells, where they have produced an order of magnitude higher specific activity than nanoparticles in oxygen reduction and demonstrated a potential benefit in accelerated stress tests.[4] Extended nanostructures have recently been developed for the oxygen evolution reaction, where a similar specific activity improvement was observed in rotating disk electrode (RDE) half-cells. Catalysts formed by spontaneous galvanic displacement are ideally situated, being able to take advantage of the specific activities generally associated with the catalyst type, while significantly improving upon the traditionally low surface areas of extended surfaces. This approach has been used in forming iridium-nickel and iridium-cobalt nanowires.[5]

While nanoparticles have modest site-specific activity in oxygen reduction and evolution, nanoparticle evolution catalysts are at a particular disadvantage with regards to surface area, since carbon supports cannot be used at high potentials. Iridium-cobalt and iridium-nickel nanowires have been synthesized with surface areas of 70 m² g_Ir^‒1, more than double the nanoparticles. In RDE half-cells, the extended nanostructures produce an oxygen evolution mass activity 7 to 8 times greater throughout the kinetic region. Accelerated stress tests were completed by potential holds and cycling over a variety of conditions to study catalyst degradation and establish standardized testing protocols. Acid treatment has been used to remove excess nickel and cobalt template, improving catalyst durability. Following accelerated stress tests in RDE half-cells, iridium-cobalt and iridium-nickel nanowires exceed the mass activity of nanoparticles by more than an order of magnitude.

Membrane electrode assembly (MEA) fabrication of these materials has produced electrolyzers with higher performance than the benchmark device. Recent improvements in MEA performance suggest that the promise of extended surface catalysts observed in half-cells can be realized in the application.

[1] R. Forgie, G. Bugosh, K.C. Neyerlin, Z. Liu, P. Strasser, Electrochemical and Solid-State Letters, 13 (2010) B36-B39.

[2] K. Harrison, M. Peters, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review13/pd031_harrison_2013_o.pdf, 2013.

[3] K. Harrison, M. Peters, C. Ainscough, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/progress13/ii_a_2_harrison_2013.pdf, 2013.

[4] S.M. Alia, Y.S. Yan, B.S. Pivovar, Catalysis Science & Technology, 4 (2014) 3589-3600.

[5] H. Xu, High-Performance, Long-Lifetime Catalysts for Proton Exchange Membrane Electrolysis, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review14/pd103_xu_2014_o.pdf, 2014.

Figure 1. Surface areas (x-axis) and site-specific oxygen evolution activities (y-axis) of iridium-cobalt nanowires, iridium-nickel nanoparticles, and iridium nanoparticles prior to (red, blue, black) and following (dark red, dark blue, grey) durability testing in RDE half cells. Arbitrary mass activities (1, 4, 7 A mg_Ir^‒1) are included as solid black lines.