Structural Changes of Spinel MCo₂O₄ (M = Mn, Fe, Co, Ni, and Zn) Electrocatalysts during the Oxygen Evolution Reaction Investigated by In Situ X-ray Absorption Spectroscopy

Abstract

A comparison of the electrochemical and physicochemical behavior of cobalt-based oxides with spinel structure MCo 2 O 4 (M = Mn, Fe, Co, Ni, and Zn) was conducted to investigate the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity of the oxygen evolution reaction (OER) in an alkaline solution. Various spinel MCo 2 O 4 electrocatalysts were synthesized by a facile microwave-assisted synthesis and low-temperature annealing. The overpotential of these MCo 2 O 4 electrocatalysts for the OER is comparable to the reported overpotentials of catalysts based on cobalt oxides. From the findings, the catalytic activity of OER decreases in the order of ZnCo 2 O 4 > NiCo 2 O 4 > FeCo 2 O 4 > Co 3 O 4 > MnCo 2 O 4 . It was revealed that the active sites are controlled by the balance of M 3+ /M 2+ cation distribution in octahedral and tetrahedral sites and bythe bond strength between M and oxygen atoms at the catalyst surface from the direct combination of in situ X-ray absorption fine structure (XAFS) spectroscopy with the electrochemical experiments to track the oxidation state and the structural changes of electrocatalysts before and after the exposure to the OER conditions. This study provides insights into the effects of cation distributions on the OER activity and demonstrates a promising method for determining the fundamental mechanism of cationsubstituted cobalt oxides for OER.

Strong Chiroptical Activity in Cobalt Oxide/Hydroxide Nanoparticles Passivated by Chiral Nonthiol Amino Acid Proline

“…The Co K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were conducted on the beamline 9C at the Photon Factory (PF) synchrotron radiation facility (KEK, Japan) . For the XANES/EXAFS measurements, the sample powder was diluted by mixing with BN to obtain a pressed disk pellet with the diameter of 10 mm.…”

Recent progress in ZnCo2O4and its composites for energy storage and conversion: a review

“…Nevertheless, other metal ions like Ni and Mn, when inserted into Co 3 O 4 to form NiCo 2 O 4 and MnCo 2 O 4 , respectively, can suppress the catalytic activity for the OER, 261 due to occupation of tetrahedral and octahedral sites in the Co 3 O 4 spinel structure. In addition, M. Harada, F. Kotegawa, & M. Kuwa 16 demonstrated that the active sites are controlled by the balance of M 3+ /M 2+ cation distribution in O h and T d sites and by the bond strength between M and oxygen atoms at the electrocatalyst surface before and after the exposure to OER conditions, where the catalytic activity of the OER decreases in the order of ZnCo 2 O 4 > NiCo 2 O 4 > FeCo 2 O 4 > Co 3 O 4 > MnCo 2 O 4 .…”

“…Its multifunctionality and excellent electrochemical properties are closely related to its structure which presents a regular spinel structure where Zn 2+ only replaces Co 2+ in the T d sites in Co 3 O 4 , leaving the Co 3+ content in the O h sites unchanged, while Ni and Mn mainly occupy O h sites in NiCo 2 O 4 and MnCo 2 O 4 . 16 In fact, the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity for the OER in an alkaline solution has been studied, and a comparison of the electrochemical and physicochemical behavior of MCo 2 O 4 (where M = Mn, Fe, Co, Ni, and Zn) was made by M. Harada, F. Kotegawa, & M. Kuwa. 16 Interestingly, their catalytic activity for the OER follows the order: ZnCo According to the authors, the active sites for the OER are M 3+ species in the octahedral site, and their activities are significantly dependent on the Co 3+ /Co 2+ and M 3+ /M 2+ content ratios in the octahedral site as demonstrated according to XPS and in situ X-ray absorption fine structure (XAFS) measurements, demonstrating the importance of the presence of Zn 2+ ions in ZnCo 2 O 4 .…”

“…16 In fact, the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity for the OER in an alkaline solution has been studied, and a comparison of the electrochemical and physicochemical behavior of MCo 2 O 4 (where M = Mn, Fe, Co, Ni, and Zn) was made by M. Harada, F. Kotegawa, & M. Kuwa. 16 Interestingly, their catalytic activity for the OER follows the order: ZnCo According to the authors, the active sites for the OER are M 3+ species in the octahedral site, and their activities are significantly dependent on the Co 3+ /Co 2+ and M 3+ /M 2+ content ratios in the octahedral site as demonstrated according to XPS and in situ X-ray absorption fine structure (XAFS) measurements, demonstrating the importance of the presence of Zn 2+ ions in ZnCo 2 O 4 . 16 Complementarily, ZnCo 2 O 4 is a promising energy storage material which shows advantageous properties, including low cost, low-toxicity, different morphologies, high electrical conductivity, 17,18 and high theoretical capacity in comparison with unitary ZnO and CoO and binary Co 3 O 4 .…”

“…16 Interestingly, their catalytic activity for the OER follows the order: ZnCo According to the authors, the active sites for the OER are M 3+ species in the octahedral site, and their activities are significantly dependent on the Co 3+ /Co 2+ and M 3+ /M 2+ content ratios in the octahedral site as demonstrated according to XPS and in situ X-ray absorption fine structure (XAFS) measurements, demonstrating the importance of the presence of Zn 2+ ions in ZnCo 2 O 4 . 16 Complementarily, ZnCo 2 O 4 is a promising energy storage material which shows advantageous properties, including low cost, low-toxicity, different morphologies, high electrical conductivity, 17,18 and high theoretical capacity in comparison with unitary ZnO and CoO and binary Co 3 O 4 . 17 Inspired by the above considerations, and despite being the second most reported cobaltite, as far as we know, there is no review work summarizing recent progress in ZnCo 2 O 4 in energy applications.…”