Electrolyte Engineering Applying Concentrated Chloride Ions with Mixed Buffer Solutions for a Versatile High-Productivity Water-Splitting System

Seawater electrolysis is an attractive way for green hydrogen production; however, it faces challenges in efficiency loss because of the overpotentials in the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), high concentration overpotential, and high ohmic potential (iR) drop. Here, our electrolyte engineering approach led to the introduction of a highly conductive Cl–-containing borate/carbonate mixed buffer electrolyte. At a borate/carbonate molar ratio of 1.0, this electrolyte has a new apparent pKa (pKa,app) of pH 9.8. While a typical water electrolysis system removes Cl– to avoid competitive Cl– oxidation, we intentionally utilized concentrated Cl– to improve conductivity, reaching around 50 S m–1 at 353 K, making the value competitive with 30 wt % KOH (∼130 S m–1). In this mixed buffer electrolyte with Cl–, the performances for HER using RuNiOxHy/Ni felt and for OER using CoFeOxHy/Ti felt were, respectively, optimized by electrolyte engineering, tuning the concentration of cations and operating pH. The two electrodes, highly conductive electrolytes, and newly adopted polyethersulfone separator led to a zero-gap cell that worked stably at 2.00 V and 500 mA cm–2 with 106 mV iR loss and unity gas faradaic efficiency for 80 h under non-extreme pH conditions. This study provides a new design of electrolyte engineering for seawater splitting.