In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction

Abstract

In-situ x-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems, with the ability to elucidate the chemical nature of electrocatalysts under reaction conditions. In this study, we perform in-situ XAS measurements on a bifunctional manganese oxide (MnOx) catalyst with high electrochemical activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Using x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS), we find that exposure to an ORR-relevant potential of 0.7 V vs. RHE produces a disordered Mn3II,III,IIIO4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs. RHE, relevant to the OER, we observe an oxidation of approximately 80% of the catalytic thin film to form a mixed MnIII,IV oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged Mn3II,III,IIIO4. XAS and electrochemical characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidation states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. This result suggests that the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, we hypothesize that the MnIII,IV oxide, rather than Mn3II,III,IIIO4, is the phase pertinent to the observed OER activity.

Benchmarking the Stability of Oxygen Evolution Reaction Catalysts: The Importance of Monitoring Mass Losses

“…The MnO x film was evaluated using the Mn 3 s multiplet splitting and Mn 2p1 = 2 distance between the main peak and its satellite. [55,56] The former was found to be 4.7 eV and the latter 11.4 eV, suggesting that the stoichiometry is consistent with MnO 2 , which is also the most stable surface for Mn under OER conditions. [57] The RuO 2 film was evaluated based on the position and area of the Ru 3d lines, which matched with RuO 2 literature references.…”

Efficient Water Splitting Cascade Photoanodes with Ligand‐Engineered MnO Cocatalysts

“…Therefore, most of the previously reported results related to BiVO 4 ‐based photoanodes13, 14, 16, 19, 47, 51, 52, 53 were measured in sulfite oxidation condition to show photo‐electrochemical properties of BiVO 4 ‐based electrodes independently of its poor water oxidation kinetics, as shown in Table S2 (Supporting Information). The photo‐electrochemical current densities of the BiVO 4 ‐based photoanodes4, 13, 14, 16, 17, 19, 20, 21, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 77 were plotted as a function of potential versus RHE. Thus, we measured PEC properties of BiVO 4 ‐based anodes under sulfite oxidation to figure out the effect of ligand engineering.…”

“…Specifically, under neutral condition, manganese oxide nanoparticles exhibited superior performance to previously reported transition metal–based catalysts 32, 33. Several researches show that Mn‐based oxides have superior catalytic activity at neutral pH and are highly active in water oxidation compared to low activity oxides, as shown in Table S1 (Supporting Information) 31, 32, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49. As compared to other OECs, Mn‐based oxides showed good catalytic properties (Tafel slope and overpotential) in the neutral pH condition for water oxidation.…”

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

“…For another type of Mn oxide catalyst electrodeposited on a Au/Ti electrode suitable for in situ X-ray absorption spectroscopy (XANES and EXAFS) under electrocatalytic conditions, Jaramillo and coworkers have identified distinct structural phases present under water oxidation and oxygen reduction conditions for one and the same electrodeposit [58,59]. The XANES spectrum of the Mn oxide catalyst film measured under O 2 reduction conditions (0.7 V RHE, pH 13), shown in Fig.…”