Fundamentals of Water Electrolysis

Abstract

Electrocatalytic water-splitting is one of the most ideal and effective ways to produce hydrogen with high purity. The water-splitting reaction is known with two half reactions: the water oxidation reaction (or oxygen evolution reaction, OER) and the water reduction reaction (or hydrogen evolution reaction, HER). The Tafel slope values can be used to analyze the reaction route on a specific catalyst. As widely accepted, Tafel slopes of 120, 40 and 30 mVdec⁻¹ were observed for the Volmer, Heyrovsky and Tafel determining rate steps, respectively. The free energy of hydrogen adsorption (DGH) associated with Hads is a key descriptor to estimate the reaction. If DGH is rather positive, the Hads will be bound strongly with the electrode surface and the initial Volmer step react easily, but the subsequent Tafel or Heyrovsky step will become difficult. If DGH is negative and small, Hads will bear a weak interaction with the electrode surface, which results in a slow Volmer step limiting the overall turnover rate. On the anode part, the sluggish kinetics of OER will prevent the total reaction from being applied in practice. This is because OER involves four electrons transfer and requires O-O bond formation and thus high energy to overcome the barrier. Generally, the standard thermodynamic voltage of water splitting is 1.23 V for driving the whole reaction. Nevertheless, the multi-steps of electron transfer process are involved in the reaction, requiring additional energy to accelerate electron transfer, and overcome kinetics barrier. The additional energy is to keep the reaction proceeding at appropriate water-splitting rate (evaluated as ‘current density’). To reduce energy usage and overpotential, the electrocatalysts with high catalytic activities are frequently employed in water electrolysis. As far, noble metals based electrocatalysts, such as IrO₂/RuO₂ for OER and Pt for HER, are now widely used in the research field as well as industry applications. However, their high costs bring a heavy burden for their large-scale deployment. In recent years, various low-cost materials have been demonstrated with great potential as efficient and stable electrocatalysts. For instance, transition metal sulfides, selenides, carbides, borides, nitrides, phosphides, and nanocomposites are utilized in HER; transition metal oxides, hydroxides, phosphates and polymeric carbon nitride are employed in OER. Although these materials have performed excellent HER or OER properties in their respective alkaline or acidic media, the water-splitting performance is usually unsatisfactory when organized the materials in the same electrolyte. Namely, the HER catalysts produce mass hydrogen but poor OER efficiency in acidic media; the OER catalysts present ideal OER performance but less hydrogen production in alkaline media. How to integrate outstanding HER and OER performance of a catalyst in one cell has been an important problem for determining whether it can be a qualified bifunctional catalyst and overcome kinetics barrier.