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

Abstract

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 pK a (pK a,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 RuNiO x H y /Ni felt and for OER using CoFeO x H y /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.

MOF-Derived Hollow Heterostructure MnCo2O4@Co9S8 Nanocages as a High-Performance Bifunctional Electrode for Water Electrolysis and Flexible Supercapacitor

“…To our knowledge, Pt-coated Ti felt has been widely used as a hydrogen evolution electrode due to its excellent properties (Figure S12). As shown in Figure b, the voltage (1.83 V) required for the MnCo 2 O 4 @Co 9 S 8 -10//Pt-coated Ti felt electrolyzer to generate the 100 mA cm –2 current density is almost the same as that of 1 M KOH (1.79 V) at 1 M KOH + 0.5 M NaCl. The operating durability of the cell is an important index for evaluating the performance of the electrolyzer .…”

Recent advances of bifunctional electrocatalysts and electrolyzers for overall seawater splitting

“…42 researchers. [43][44][45][46][47] Considering the challenges of OER, some researchers hope to decouple the OER reaction and replace it with another easier anode reaction, thereby boosting H 2 production. Fig.…”

“…42 Buffer solutions like 1 M KOH (pH = 14), 0.1 M borate (pH = 9.2), carbonate (pH = 8.6), and 0.1 M phosphate (pH = 7) have been attempted by various researchers. 43–47 Considering the challenges of OER, some researchers hope to decouple the OER reaction and replace it with another easier anode reaction, thereby boosting H 2 production. Fig.…”

The Effect of Electrolytes on the Kinetics of the Hydrogen Evolution Reaction

“…(a) (b) Seawater electrolysis, a key process for green hydrogen production, encounters many challenges, including the concentration overpotential and iR drop [3]. Aiming to enhance the efficiency of seawater electrolysis, a recent study by [127] introduced a novel mixed buffer electrolyte with a 1:1 borate/carbonate molar ratio with a new apparent pK a (pK a,app ) of pH 9.8. The electrolyte deliberately incorporates concentrated Cl − , achieving a conductivity of about 50 S m −1 at 353 K, which is on par with 30 wt % KOH (~130 S m −1 ).…”