Activity and Durability of Iridium Nanoparticles in Oxygen Evolution

Abstract

Electrolysis is near parity as an energy storage process when compared to the cost of competing technologies, including batteries, pumped hydro, and compressed air.[1] When coupled with renewable energy sources, such as wind and solar, electrochemical water splitting can also produce hydrogen using a nearly carbon free pathway.[2] Iridium nanoparticles are commonly used as electrocatalysts in acidic electrolyzers due to moderate stability and activity in the oxygen evolution reaction. The activity and durability of iridium nanoparticles has been studied in rotating disk electrode half-cells to develop methods for establishing electrochemical baselines. Mercury underpotential deposition has been used to determine the surface areas and specific activities of iridium catalysts.[3] This evaluation method has proved beneficial to those used previously since similar surface areas are obtained prior to and following exposure to high potential and the irreversible oxidation of surface iridium. Through mercury adsorption, the intrinsic activities of oxygen evolution catalysts can be evaluated and used to direct future electrocatalyst development. The activities of iridium nanoparticle have been evaluated over a wide range of potentials in the kinetic region and transport limitations generally become a factor at potentials greater than 1.5 V vs a reversible hydrogen electrode. Initial optimization of electrode coating processes has further provided a robust benchmark in oxygen evolution. Accelerated stress tests have been applied in rotating disk electrode half-cells using potential cycling and potential holds. At moderate potential, activity losses are generally attributed to losses in surface area by nanoparticle agglomeration. Although bubble formation may destabilize the catalyst layer, similar experiments in hydrogen peroxide and formic acid oxidation have suggested that this loss is unique to the higher potentials in oxygen evolution. At elevated potential, heavy activity losses are observed and are increasingly due to iridium dissolution into the electrolyte. This study establishes rotating disk electrode testing protocols for iridium in oxygen evolution. The observed losses in accelerated stress tests have significant implications on acidic electrolyzers, particularly at low iridium loadings. [1] K. Harrison, M. Peters, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review13/pd031_harrison_2013_o.pdf, 2013. [2] 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. [3] S.P. Kounaves, J. Buffle, Journal of The Electrochemical Society, 133 (1986) 2495-2498.