Catalytic mechanisms at electrode surfaces guide the development of electrochemically controlled energy storing reactions and chemical synthesis. The intermediate steps of these mechanisms are challenging to identify experimentally but are critical to understanding the speed, stability, and selectivity of product evolution for a given energy input. One of the most studied reactions is the water-splitting reaction which breaks apart H₂O into H₂ and O₂. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct the energy intensive part, the oxygen evolution reaction (OER), into its component parts. The first step of this oxidative reaction, which is to release a proton and take an electron from water, has often been thought of as a theoretical “descriptor” that differentiates the OER activity of materials: if this is hard to do, then the material is less energy efficient and if it is easy to do, then a later, bond-forming step limits the energy efficiency. Yet, the formation free energy of this step had yet to be measured experimentally. Here, I will begin by summarizing the work done previously by the group: the structure and kinetics of forming these first reaction intermediates (Ti-OH^*) by their optical and vibrational signatures upon driving OER with an ultrafast photo-trigger of the electrode surface (SrTiO₃). I will then discuss the most recent work, which shows that by separating the reaction steps in time and optically detecting these intermediates created upon hole-transfer from the valence band, we can determine their formation free energy¹. In addition to the optical spectroscopy, we employ reaction isotherm methodology borrowed from surface studies of reactions. By tuning the pH, we obtain a Langmuir isotherm on the driven surface (Figure 1) regulated by the proton transfer that increases surface hydroxylation for higher pH prior to photo-triggering OER. From the midpoint of this isotherm, one can obtain the formation free energy of Ti-OH^* from valence band holes. Such an application of isotherms is new in that the product is not the final one, for which one generally knows the reaction free energy, but rather an intermediate of a reaction. Overall, the innovation of the work is to utilize kinetic measurements to obtain the calculated free energy of an intermediate reaction step at a material surface.

¹I. Vinogradov, S. Singh, H. Lyle, M. Paolino, A. Mandal*, J. Rossmeisl, T. Cuk*, Nature Materials, 2022, 21, 88.

Figure 1: The time-resolved optical data as a function of pH show how the population of intermediates generates a reaction isotherm (black trace). The emission (in blue) counts the intermediate Ti-OH* population.