Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

Abstract

As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported during the past few decades because of their high OER activities. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after the OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during the OER for these electrocatalysts, has not been understood yet; therefore a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale, is urgently needed. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoS x into crystallized CoOOH in the OER has been in situ captured: irreversible conversion of CoS x to crystallized CoOOH is initiated on the surface of the electrocatalysts with a morphology change via Co(OH) 2 intermediate during the OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR), and other ex situ technologies. Moreover, on the basis of this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoS x has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in the OER measurement can provide insights into the fundamental understanding of the mechanism for the OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting.

Cation‐Modulated HER and OER Activities of Hierarchical VOOH Hollow Architectures for High‐Efficiency and Stable Overall Water Splitting

“…An extraordinarily low onset potential is calculated as 1.425 V (η 10 ) from VOOH‐Fe sample, which was significantly lower than the other four electrodes (VOOH‐5Fe: 1.472 V, VOOH‐1Fe: 1.477 V, VOOH‐10Fe: 1.479 V, and VOOH‐0Fe: 1.484 V). To the best of our knowledge, 195 mV overpotential versus 10 mA cm −2 is the best value reported for V or Fe (oxy)hydroxides and transcends many of the previously reported nonprecious metal OER catalysts, including Fe 0.5 V 0.5 OOH, Fe 0.33 Co 0.67 OOH, MoFe:Ni(OH) 2 /NiOOH, Co‐V hydr(oxy)oxide, F‐CoOOH/NF, CoS x /N‐doped graphene, etc. (Table S3, Supporting Information).…”

The Role of Non‐Metallic and Metalloid Elements on the Electrocatalytic Activity of Cobalt and Nickel Catalysts for the Oxygen Evolution Reaction

“…Through a combination of operando and ex‐situ characterization techniques, Fan et al. successfully observed time‐resolved conversion of CoS x into CoOOH as the active OER catalyst . Time‐resolved TEM images showing structural transformation of CoS x at selected time intervals during the OER and the corresponding electron diffraction patterns are shown in the Figures (a–c) and (d–f), respectively.…”

Identification of Key Reversible Intermediates in Self‐Reconstructed Nickel‐Based Hybrid Electrocatalysts for Oxygen Evolution

“…However, according to reported observations toward sulfide or phosphide, it is reasonable to suspect the real active species of NiNPS towards oxygen evolution, because the NiNPS are thermodynamically unstable with higher enthalpy compared to the oxide during the harsh oxidation process . To this end, ex situ TEM and in situ Raman were performed to unravel the true electrocatalytically active species of NiNPS.…”

“…However,a ccording to reported observations toward sulfide or phosphide, [32][33][34][35] it is reasonable to suspect the real active species of NiNPS towards oxygen evolution, because the NiNPS are thermodynamically unstable with higher enthalpy compared to the oxide during the harsh oxidation process. [16,36] To this end, ex situ TEM and in situ Raman were performed to unravel the true electrocatalytically active species of NiNPS.A ss hown in the TEM and HRTEM images of NiNPS after 2CVs in Figure 3a,b,acontinuous amorphous shell of about 5nmisobserved around the initial NiNPS nanoparticles,d ecorated with some dispersive crystalline NiO domains less than 5nm.…”