(Invited, Digital Presentation) Neutral pH Water Electrolyzer: Can It Become a Disruptive Technology for Green Hydrogen Production?

Abstract

Cost reduction for green hydrogen production is urgently needed to realize the carbon neutral society. Low temperature water electrolysis is operated in extreme pH conditions; polymer electrolyte membrane (PEM) electrolyzer (acidic) or alkaline electrolyzer. In such electrolyzer, substantial cost must be spent for electrode materials, system corrosion tolerance, ion conductive membrane and/or securing safety operation. Water electrolyzer using near-neutral pH environment may become a disruptive technology to produce green hydrogen driven by renewable electricity. This is essentially because a variety of materials can be applied to the electrolysis system in electrode and substance, and most significantly the electrolyte is naturally unharmful to human beings. In the past, the study on neutral pH electrolyzer was limited due to inevitable low efficiency compared to abovementioned counterparts. To identify the limiting factors associated with near neutral pH electrolysis, a rigorous microkinetic analysis of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was performed in various electrolyte conditions. The electrocatalytic performance is dependent substantially on the buffered conditions, which involves the kinetics of the local H+/OH− supply by the buffering action, mitigating the local pH shift observed in unbuffered conditions. Our study herein demonstrated thorough “electrolyte engineering” approach that can maximize the efficiency by maximizing reactant flux, mitigating pH gradient, reducing solution resistance, and regulating the crossover of gaseous molecules. Specifically suppressing the crossover of gaseous molecules by regulating their diffusion flux can be achieved. A simple hydrophilized mechanically stable glass-filter was sufficient to suppress the permeation of gas bubbles. We also found that a trade-off between the exchange current density and the Tafel slope with respect to the pKa and pH of the electrolyte disclosed the optimal pKa(=pH) and buffer identity to minimize the HER/OER overpotential at a given current density. Our preliminary results show that overall performance for water splitting under near neutral pH is already competitive with the performance to those operated at extreme pH conditions. High current density and excellent stability are achieved by the inexpensive electrolyzer system at near neutral pH.