An iridium oxide nanoparticle electrocatalyst under oxygen evolution reaction conditions was probed in situ by ambient-pressure X-ray photoelectron spectroscopy. Under OER conditions, iridium undergoes a change in oxidation state from Ir(IV) to Ir(V) that takes place predominantly at the surface of the catalyst. The chemical change in iridium is coupled to a decrease in surface hydroxide, providing experimental evidence which strongly suggests that the oxygen evolution reaction on iridium oxide occurs through an OOH-mediated deprotonation mechanism.

Chemical analysis of solid-liquid interfaces under electrochemical conditions has recently become feasible due to the development of new synchrotron radiation techniques. Here we report the use of "tender" X-ray Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) to characterize a thin film of Ni-Fe oxyhydroxide electrodeposited on Au as the working electrode at different applied potentials in 0.1 M KOH as the electrolyte. Our results show that the asprepared 7 nm thick Ni-Fe (50% Fe) film contains Fe and Ni in both their metallic as well as oxidized states, and undergoes further oxidation when the sample is subjected to electrochemical oxidation-reduction cycles. Metallic Fe is oxidized to Fe 3+ and metallic Ni to Ni 2+/3+ . This work shows that it is possible to monitor the chemical nature of the Ni-Fe catalyst as function of potential when the corresponding current densities are small. This allows for operando measurements just above the onset of OER; however, current densities as they are desired in photoelectrochemical devices (~1-10 mA cm -2 ) could not be achieved in this work, due to ohmic losses in the thin electrolyte film. We use a two-dimensional model to describe-the spatial distribution of the electrochemical potential, current density and pH as a function of the position above the electrolyte meniscus, to provide guidance towards enabling the acquisition of operando APXPS at high current density. The shifts in binding energy of water with applied potential predicted by the model is in good agreement with the experimental values.3

An iridium oxide nanoparticle electrocatalyst under oxygen evolution reaction conditions was probed in situ by ambient-pressure X-ray photoelectron spectroscopy. Under OER conditions, iridium undergoes a change in oxidation state from Ir IV to Ir V that takes place predominantly at the surface of the catalyst. The chemical change in iridium is coupled to a decrease in surface hydroxide, providing experimental evidence which strongly suggests that the oxygen evolution reaction on iridium oxide occurs through an OOH-mediated deprotonation mechanism.

Molybdenum sulfide structures, particularly amorphous MoS 3 nanoparticles, are promising materials in the search for cost-effective and scalable watersplitting catalysts. Ex situ observations show that the nanoparticles exhibit a composition change from MoS 3 to defective MoS 2 when subjected to hydrogen evolution reaction (HER) conditions, raising questions regarding the active surface sites taking part in the reaction. We tracked the in situ transformation of amorphous MoS 3 nanoparticles under HER conditions through ambient pressure X-ray photoelectron spectroscopy and performed density functional theory studies of model MoS x systems. We demonstrate that, under operating conditions, surface sites are converted from MoS 3 to MoS 2 in a gradual manner and that the electrolytic current densities are proportional to the extent of the transformation. We also posit that it is the MoS 2 edge-like sites that are active during HER, with the high activity of the catalyst being attributed to the increase in surface MoS 2 edge-like sites after the reduction of MoS 3 sites.