Atom Surface Dynamics of Manganese Oxide under Oxygen Evolution Reaction-Like Conditions Studied by In Situ Environmental Transmission Electron Microscopy

Abstract

Hydrogen production by electrochemical water splitting is limited by the sluggish oxygen evolution reaction (OER). In order to improve our understanding of the underlying mechanisms, information about the atomic surface structure of the active state of the electrode is required. Here, we present environmental transmission electron microscopy studies of Ca-birnessite (K0.20Ca0.21MnO2.21·1.4H2O) electrodes under conditions close to those of the OER. Remarkably, in H2O vapor, a highly dynamic state of the surface and subsurface develops with a thickness of the formed dynamic layer of up to 0.6 nm, which is absent in O2 and inert gases. Electron beam-induced effects are carefully studied, showing high stability of the material against radiation damage in high vacuum until a dose rate of 42,000 e–/(Å2 s). In contrast, in H2O, the dynamic surface layer develops and forms a stationary state even at low dose rates, down to 5000 e–/(Å2 s). Electron energy-loss spectroscopy reveals an increase in the Mn oxidation state in H2O and in O2 ambient. Our results are interpreted as the formation of a few-angstrom-thick, dynamic, and hydrated surface layer of birnessite in H2O, with an increased Mn valence state. Such a dynamic surface layer with a flexible Mn coordination and valence state might be optimal for oxygen evolution due to the higher effective interaction volume beyond the surface area and a flexible bond coordination of partially hydrated Mn species.

Mechanistic Understanding of Water Oxidation in the Presence of a Copper Complex by In Situ Electrochemical Liquid Transmission Electron Microscopy

“…This radiolysis process can generate different products such as hydrated protons (H 3 O + ), solvated electrons (e h – ), and radical species, for example, OH • and H • . The radiolysis products can change the local chemical environment in the irradiated area, which leads to changes in the local electrochemical conditions in the EC-LTEM experiments. , By using a suitable combination of the electron dose rate, proper buffering capacity in the applied phosphate electrolyte to keep the pH in the desired range, and a reasonable flow rate of the electrolyte in the EC-LTEM holder, we managed to run the EC-LTEM experiment below the threshold to minimize the electron beam effect.…”

Structural Variations of Metal Oxide‐Based Electrocatalysts for Oxygen Evolution Reaction

“…[ 196 ] Operando TEM which integrates electrochemical liquid flow cell with conventional TEM equipment can provide unique insight into the interfacial reaction process as well as the real‐time structural evolution of OER catalysts in electrochemical reactions. [ 197–199 ] Figure a shows the common operando electrochemical TEM holder. [ 200 ] There is a disposable electrochemical chip comprising of working (glass carbon), reference (Pt wire) and counter (Pt wire) electrode positioned on the tip of the holder.…”

Insights into Heterogeneous Catalysts under Reaction Conditions by In Situ/Operando Electron Microscopy

“…One example is the ETEM study of the surface dynamics of Ca‐birnessite electrode, i.e., K 0.20 Ca 0.21 MnO 2.21 ·1.4H 2 O, under several conditions (H 2 O and O 2 ) relevant to the OER ( Figure 12 A). [ 232 ] In high vacuum, electron beam‐induced radiation damage is minimized by controlling electron dosage. In H 2 O, there is a dynamic hydrated surface layer of birnessite in sub‐nm thickness, which facilitates an optimal oxygen evolution via an effective interaction volume and Mn coordination flexibility.…”