(Invited) Understanding the Electrochemical Conversion Processes from Iridium Black to Iridium Oxides and Implications on Their Activity and Degradation Toward Oxygen Evolution

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Iridium oxides (IrOx) are by far the only electrocatalysts that can be used in the anodes of commercial proton exchange membrane water electrolyzers (PEMWEs) due to their sufficient activity and stability toward acidic oxygen evolution reaction (OER). Nanoscale Ir powders (Ir black) are often used in PEMWEs since they convert to IrOx at high potential during PEMWE operation. The types of the IrOx formed and its morphology dictate the activity and durability of the anodes. It is thus imperative to understand the electrochemical conversion processes between Ir black and IrOx that may help advancing the anodes of PEMWEs.Here we reported a study on the conversion processes from Ir black to IrOx in acidic media by conducting cyclic voltammetry (CV) analysis, in combination with complementary characterization techniques. We found that Ir black converted to hydrous iridium oxides (IrOx·nH2O) in dilute acid solution when the lower potential limit is below 0.2 V (all potentials are versus reversible hydrogen electrode) as well as the higher potential limit is beyond 1.2 V.1,2 Rationalization of the observation led us to propose oxidation of Ir forming anhydrous IrOx at <1.2 V via the place-exchange mechanism, followed by hydration of the anhydrous IrOx at >1.2 V. The hydration process is water molecules filling the oxygen vacancies of IrOx forming coordinated water: IrOx + nH2O → IrOx·nH2O. The hydration of IrOx moved toward the core of Ir nanoparticles layer-by-layer only if the IrOx layers get reduced at <0.2 V in association with cleavage of Ir-O-Ir bonds (or removal of lattice oxygen), leaving new oxygen vacancies on the inner side of the IrOx layers to be filled by water molecules on the next cycling. While multiple layers of IrOx·nH2O can be formed via this process, only the outer layers contribute to OER activities with an ultrahigh inherent OER activity. Holding at high voltage led to degradation of IrOx·nH2O, as well as dehydration followed by crystallization. We accordingly proposed that OER is a water consuming process with the rate determining step not being water adsorption (hydration), which leads to dehydration of IrOx·nH2O: 2IrOx·nH2O + 2H2O → 2IrOx + (2+n)O2 + (4+2n)H+ + (4+2n)e-. The competing dehydration (OER) and hydration processes causes the degradation and recovery of IrOx·nH2O. On the other hand, Ir black converted mainly to anhydrous IrOx in concentrated acid solution1 and in membrane electrode assemblies (MEAs), in association with a different degradation mode from that in dilute acid. These complex conversions among metallic Ir, anhydrous/amorphous IrOx, IrOx·nH2O, and crystalline IrO2 are made more complicated with the presence of H2 gas (Figure 1). The implications of occurrences of these conversions on the activity and degradation of Ir-based anodes of PEMWEs will be discussed. References (1) Rand, D. A. J.; Woods, R. Cyclic voltammetric studies on iridium electrodes in sulphuric acid solutions: Nature of oxygen layer and metal dissolution. J. Electroanal. Chem. Interf. Electrochem. 1974, 55, 375-381. (2) Pickup, P. G.; Birss, V. I. A model for anodic hydrous oxide growth at iridium. J. Electroanal. Chem. Interf. Electrochem. 1987, 220, 83-100. Figure 1