Electrochemical Cycling‐Induced Amorphization of Cobalt(II,III) Oxide for Stable High Surface Area Oxygen Evolution Electrocatalysts

Abstract

The activity of electrocatalysts critically depends on the chemical coordination around the active sites. Amorphous materials have short‐range atomic ordering while their crystalline counterparts have both short and long‐range ordering. Traditional synthesis of amorphous materials, involving quenching from high temperatures is unsuitable as it results in less porosity and surface area. In this context, room‐temperature syntheses of high surface area amorphous materials with high activity are desirable. Here, we contrast two electrochemical synthesis procedures for generating high surface area amorphous Co3O4 at room temperature via electrochemical ion intercalation/deintercalation and surface oxidation/reduction cycles and evaluate their performance for electrocatalytic oxygen evolution reaction (OER). In the first approach, Li‐ion is used for the intercalation/deintercalation (Li/D−Li) cycles in Co3O4, which leads to expansion and contraction of structure, inducing amorphization of Co3O4 by the pulverization of crystal structure in non‐aqueous medium. In the second approach, rapid electrochemical surface oxidation/reduction (Ox/Red) of Co3O4 in the aqueous medium leads to the formation of a metastable amorphous structure. The OER specific activity (activity per unit electrochemical surface area) for Li/D−Li−Co3O4 is ∼3.5 times and Ox/Red‐ Co3O4 induced amorphization is ∼2.5 times higher than their crystalline Co3O4. The superior OER metrics of both the room‐temperature amorphization techniques are rationalized via the increase in the ratio of Co2+/Co3+ obtained from the Co‐2p XPS spectra. Further, the decrease in overall polarization resistance per site for the OER reaction for both amorphous samples were analyzed from the Tafel plot and electrochemical impedance spectroscopy (EIS). In Li/D−Li−Co3O4, the Li‐ion intercalation in bulk Co3O4 structure generates higher bulk‐oxygen vacancies leading to higher conductivity and reduction in overall charge‐transport resistance for electrocatalyst. On the other hand, Ox/Red‐ induced amorphization is restricted to the surface or near‐surface only with the formation of a small amount of metallic Co which hampers the OER.

Competing Effect of Co3+Reducibility and Oxygen-Deficient Defects Toward High Oxygen Evolution Activity in Co3O4Systems in Alkaline Medium

“…However, the thickness of the amorphous layer is highly nonuniform across the n-Co 3 O 4 nanosheet aggregates. During the OER reaction, Co 2+ ions in the T d site gets oxidized to Co 3+ ions followed by change in their coordination from T d to O h causing surface amorphization via di-μ-O­(H)-bridged Co 3+/4+ O h , and is the cause of fast catalytic degradation observed in the amorphous Co 3 O 4 system . In the case of n-Co 3 O 4 , the presence of higher surface concentration of Co 3+ ions coupled with the smaller particle size and sheet nature may favor the formation of larger amorphous layer, which can be responsible for fast electrocatalyst degradation during OER.…”

Progress and Challenge of Amorphous Catalysts for Electrochemical Water Splitting

“…Typically, chemical processes introduce defects and disorders via chemical reactions, leading to a partial or complete phase transition or change in coordination environment. Doping, , intercalation, reduction, electrochemical transformation, , surface modification, and ion exchange are common means to chemically transform crystalline to amorphous materials. We highlight some interesting cases as follows.…”

Phosphorus‐Based Electrocatalysts: Black Phosphorus, Metal Phosphides, and Phosphates

“…The surface area can be increased through fabricating a desired nanostructure or introducing a level of controlled porosity. [ 39 ] Because the ECSA is linearly proportional to the geometric double layer capacitance ( C dl ), C dl is currently one of the most popular methods for measuring the ECSA of a catalyst, which can be estimated using cyclic voltammetry (CV). [ 40 ] Other experimental techniques like Brunauer–Emmett–Teller (BET) and atomic force microscopy (AFM) methods have been adopted as well for measuring the ECSA of electrocatalysts, which have been summarized and compared in a review.…”