Mercury Underpotential Deposition to Determine the Electrochemical Surface Areas of Iridium and Iridium Oxide
Authors: Alia Shaun M , Kocha Shyam S. , Pivovar Bryan S 
Published: February 2020

Within the United States, hydrogen is a major chemical commodity for the production of ammonia in agriculture and the upgrading of crude oil in transportation. Hydrogen in the US is produced largely from natural gas by steam methane reformation.[1] Although electrochemical water splitting currently accounts for a small amount of hydrogen production, it is expected to have a larger role in the future, particularly when using renewable energy sources as input. Studies in proton exchange membrane (PEM) electrolysis are usually focused on catalysts in the oxygen evolution reaction, due to the high overpotential relative to the hydrogen evolution reaction. PEM electrolyzers typically use iridium (Ir) or Ir oxide as the anode catalyst, due to reasonable activity and stability. Although platinum (Pt) and ruthenium (Ru) are often examined as potential alternatives, Pt requires a higher overpotential and Ru is prone to dissolution at elevated potential.[2, 3]

Determining the electrochemical surface area (ECA) of Ir allows for the quantification of the number of sites available to participate in oxygen evolution. It also allows for the evaluation of site-specific activity, or site quality. ECAs are of interest in Ir catalyst development, to compare different catalyst types and direct future research. ECAs are also of interest in durability studies to evaluate the modes of Ir degradation, and to determine whether activity losses are due to deteriorating surface area or site quality.

The ECAs of Ir catalysts have typically been evaluated using hydrogen underpotential deposition, carbon monoxide oxidation, or capacitance. These methods are generally limited to predurability measurements and catalyst type: hydrogen underpotential deposition and carbon monoxide for Ir metals; capacitance for Ir oxides. Hydrogen underpotential deposition and carbon monoxide oxidation can be used to determine the ECA of Ir metals, but not Ir oxides, and are sensitive to surface oxides formed on Ir metals following oxygen evolution characterization and durability testing. To date, no method is available to determine the ECAs of Ir and Ir oxide, prior to and following durability testing. Mercury underpotential deposition is presented in this study as an alternative, able to produce reasonable ECAs on Ir and Ir oxide nanoparticles, and able to produce similar ECAs prior to and following characterization in oxygen evolution. This method was previously developed for the study of polycrystalline Ir, and expanded in this study to include nanoparticle catalysts, oxides, and oxygen evolution relevant testing condition.[4]

Figure 1. Cyclic voltammograms of (a) Ir nanoparticles and (b) Ir oxide nanoparticles in 0.1 m perchloric acid (blue) and 0.1 m perchloric acid containing 1 mm mercury nitrate (red).

[1] A. Milbrandt, M. Mann, in: U.S. Department of Energy (Ed.), http://www.nrel.gov/docs/fy09osti/42773.pdf, 2009.

[2] T. Reier, M. Oezaslan, P. Strasser, ACS Catalysis, 2 (2012) 1765-1772.

[3] M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, National Association of Corrosion Engineers, Houston, Texas, 1974.

[4] S.P. Kounaves, J. Buffle, Journal of The Electrochemical Society, 133 (1986) 2495-2498.

Figure 1