High Current Density Oxygen Evolution in Carbonate Buffered Solution Achieved by Active Site Densification and Electrolyte Engineering

Abstract

High current density reaching 1 A cm À 2 for efficient oxygen evolution reaction (OER) was demonstrated by interactively optimizing electrolyte and electrode at non-extreme pH levels. Careful electrolyte assessment revealed that the state-of-the-art nickel-iron oxide electrocatalyst in alkaline solution maintained its high OER performance with a small Tafel slope in Kcarbonate solution at pH 10.5 at 353 K. The OER performance was improved when Cu or Au was introduced into the FeO xmodified nanostructured Ni electrode as the third element during the preparation of electrode by electrodeposition. The resultant OER achieved 1 A cm À 2 at 1.53 V vs. reversible hydrogen electrode (RHE) stably for 90 h, comparable to those in extreme alkaline conditions. Constant Tafel slopes, apparent activation energy, and the same signatures from operando Xray absorption spectroscopy among these samples suggested that this improvement seems solely correlated with enhanced electrochemical surface area caused by adding the third element.

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

“…At pH 9.8, the difference in the Tafel slope value between the two distinct regions was minimum, indicating that the highest buffer capacity suppressed the further local pH shifting. This Tafel slope tendency matches previously reported theoretical and experimental ones considering buffer capacity in a single buffer system, , thus indicating that utilizing the electrolyte p K a is critical for efficient water splitting, irrespective of single or mixed systems.…”

Oxidized Copper and Molybdenum Species Exclusively Boosting Electrocatalytic Hydrogen Evolution in Non-Extreme pH Carbonate Buffer Electrolyte

“…To minimize the resistance loss in the overall cell performance, zero-gap configuration has been commercially utilized, , with the OER anode and HER cathode in direct contact with a gas separator. Combining the NiMoO x (Cu) cathode and a previously developed anode, the Cu-modified FeO x catalyst deposited on nanostructured Ni foam (FeO x (Cu)/ECA-Ni foam), zero-gap configuration was developed (Figure b). Because the minimization of resistance loss is crucial in overall water electrolysis, the dense electrolyte of 3.0 mol kg –1 carbonate buffer, highly conductive cathode substrate of Cu mesh, and gas separator of hydrophilic polyether sulfone (PES) were used in the zero-gap water electrolysis (see Figure S9 and Table S3 for a detailed discussion).…”

“…As the anode in overall water electrolysis, the FeO x (Cu) electrocatalyst was deposited onto electrochemically activated Ni foam (ECA-Ni foam) by following a previous report …”

“…Our previous study developed a highly efficient OER anode using only earth-abundant metals . In a carefully selected carbonate buffer solution at pH 10.5 and 353 K, the developed anode exhibited an OER performance comparable to extremely alkaline pH counterparts . In this context, the development of HER cathode functioning in such carbonate buffer solutions is highly desirable to realize the non-extreme pH water electrolyzer on a practical scale.…”

“…It should be mentioned that we define non-extreme pH as a pH range where H + and OH – concentrations are insufficient to avoid diffusion limitation for their reduction and oxidation, respectively, even at the industrially required minimum current density of 100 mA cm –2 (i.e., 2.3 < pH < 11.7, according to calculation with the Levich equation and a Nernst diffusion layer of 10 μm, a reported value at a current density of hundreds mA cm –2 ). − In this pH range, inexpensive/abundant materials such as iron and copper are thermodynamically stable, suggesting the possibility of using such materials in the electrolyzer. Our previous study developed a highly efficient OER anode using only earth-abundant metals . In a carefully selected carbonate buffer solution at pH 10.5 and 353 K, the developed anode exhibited an OER performance comparable to extremely alkaline pH counterparts .…”