Manganese Oxide as an Inorganic Catalyst for the Oxygen Evolution Reaction Studied by X‐Ray Photoelectron and Operando Raman Spectroscopy

Abstract

Manganese oxide (MnOx) is considered a promising material for the oxygen evolution reaction (OER) to replace noble metal catalysts in water splitting. The improvement of MnOx requires mechanistic and kinetic knowledge of the four‐electron transfer steps of the OER. X‐ray photoelectron spectroscopy, a widely used tool to characterize the electronic structure of thin films, is used in combination with surface‐enhanced Raman spectroscopy to gain a deeper knowledge of the different mixed MnOx types and their respective change in chemical composition. Using Raman spectroscopy during electrochemical measurements, all samples were found to reveal Birnessite‐type MnO2 motifs in alkaline media at an applied potential. Their activity correlates with two shifting Raman active modes, one of them being assigned to the formation of MnIII species, and one to the expansion of layers of MnO6 octahedra. A special activation treatment leads independent of the starting material to a highly amorphous mixed‐valence oxide, which shows the highest OER activity.

Importance of Nickel Oxide Lattice Defects for Efficient Oxygen Evolution Reaction

“…Considering that Raman spectroscopy is not surface sensitive, thin films allow the evaluation of the complete structural properties of a material by surface sensitive (XPS) and deeper penetrating techniques. However, the surface of the substrate must be prepared to allow for surface enhancement to produce an intense scattering signal of surface bound species in Raman spectroscopy. , …”

“…The electrochemical oxidation of Ni II (OH) 2 to Ni III OOH and even further must be analyzed in situ as this process is reversible, and the system reverts to a Ni II (OH) 2 state at zero applied potential. For the rational design of OER catalysts, in situ / operando characterization techniques are becoming increasingly popular to better understand the active sites and reaction pathways. − Raman spectroscopy is particularly suitable for this purpose as visible light provides strong vibrational signals for typical catalytically active sites such as metal oxides and is applicable under electrochemical conditions through a transparent window. , In addition, X-ray photoelectron spectroscopy (XPS) is a powerful tool in catalysis research to study the surface layers (<10 nm) of a sample and to determine the chemical composition of solids . It allows quantification of the oxidation state in a mixed-valent system, mostly applied to catalytic systems directly after preparation and again after electrochemical cycling (aEC) .…”

Electrochemically Fabricated Superhydrophilic/Superaerophobic Manganese Oxide Nanowires at Discontinuous Solid–Liquid Interfaces for Enhanced Oxygen Evolution Performances

“…A strong peak at 620 cm −1 and other distinct peaks at 583, 496, and 286 cm −1 were observed, and they preliminarily indicated that the nanowires existed as an MnO phase. [41] The MnO component was further confirmed via XPS. A common pathway for confirming the oxidation state of manganese in oxides is to assess the distance between the Mn 2p 1/2 peak and its satellite (ΔE2p 1/2 ) and the magnitude of ΔE3s.…”

Reimagining the eg1 Electronic State in Oxygen Evolution Catalysis: Oxidation‐State‐Modulated Superlattices as a New Type of Heterostructure for Maximizing Catalysis

“…Among the various metal oxides, birnessite has been a subject of interest of our group [ 23–31 ] and those of others'. [ 17,20,32–37 ] It is a layered manganese oxide material that consists of negatively charged Mn III/IV O 2 sheets with interstitial hydrated countercations to balance the negative charge of the MnO 2 sheets ( Figure 2 ). Past studies have led to a general consensus that there is a correlation between high catalytic activity and a high concentration of Mn III in birnessite and other manganese oxides.…”