Coupled Effects of Temperature, Pressure, and pH on Water Oxidation Thermodynamics and Kinetics

Commercial water-splitting electrolyzers operate at elevated temperature and pressure. Here, we develop a general framework for describing the coupled effects of temperature, pressure, and pH on various phenomena relevant to the oxygen evolution reaction (OER) involved in water splitting. These include water evaporation, water autoionization, and oxygen dissolution. We also consider important variables such as species free energies, species activities, OER standard potentials (E_OER⁰), and rate constants. We apply our model to (Ni,Fe)OOH, a promising electrocatalyst, to study in detail OER thermodynamics and kinetics under realistic operating conditions. We show that an increase in temperature makes water oxidation thermodynamics more favorable withE_OER⁰decreasing from 1.24 V at 10 °C to 1.18 V at 90 °C. Even this small reduction plays a significant role in accelerating OER kinetics beyond the conventional Arrhenius-type increase in the reaction rate with temperature. Using a recently developed microkinetic model, we show that a room-temperature OER current density of ∼10 mA/cm²translates to ∼997 mA/cm²at 90 °C at a fixed potential of 1.51 V versus the reversible hydrogen electrode (RHE). We infer that catalysts on which the room-temperature rate-determining step involves oxygen as a product are favorable for high-temperature operation. Notably, for optimal OER kinetics at elevated temperatures and fixed potentials versus the RHE, the electrolyzer must maintain a pH level less than the standard pH (corresponding to 1 M OH^-concentration). Our model predicts an optimal alkali concentration as low as 0.15 mM at 90 °C, with implications for the design of environmentally benign processes. Moreover, we show that a pH of 14.0, often used at room temperature, is physically unachievable at high temperatures. We also demonstrate the mild effect of pressure on the OER potential, with the latter increasing from 1.18 V at 1 bar to 1.21 V at 100 bar, at a fixed temperature of 90 °C. We find the effect of pressure on OER kinetics at fixed potentials to be negligible and indicative of the benefits of maintaining high pressure to produce compressed oxygen (and hydrogen). Our work shows how electrochemical water splitting under operating conditions currently used industrially compares to the process under laboratory conditions.