Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation

Abstract

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in MnO) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400-600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnO films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches MnO (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnO films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA·cm at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnO electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4-0.5 mA·cm were observed. The thicknesses and surface morphologies of the CaMnO films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnO were evaluated, the results demonstrate that Ca incorporation can enhance the OER activity of MnO electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

Manganese(ii) phosphate nanosheet assembly with native out-of-plane Mn centres for electrocatalytic water oxidation

“…The 2p 3/2 satellite peak of Mn III (d4) will appear at higher binding energy, overlapping with its 2p 1/2 peak 48. In addition, the energy splitting value of Mn 3s will decrease significantly from Mn II to Mn III because of unpaired 3d electrons in Mn II 49. The XPS splitting of the Mn 3s peaks of our catalyst is 6.2 eV, which is a signature value of Mn( ii ).…”

Phase and Morphology Transformation of MnO2 Induced by Ionic Liquids toward Efficient Water Oxidation

“…The difference between the Mn 2p 1/2 and Mn 2p 3/2 peaks for both MnO 2 -0IL (β-MnO 2 ) and MnO 2 -0.5IL (α-MnO 2 ) samples is ∼11.7 eV (Figure c,d), confirming their ballpark MnO 2 status. However, it has been well-known that from the Mn 2p peaks alone, it is difficult to determine the valence of Mn due to the overlap in binding energy values, the complex multiplet splitting, the satellite-loss features, as well as the asymmetry of the spectra. − Instead, analysis on the Mn 3s doublet splitting (Δ E 3s ) enables a more precise assessment (Figure e,f). The Mn 3s peak splitting is caused by the electron exchange in the 3s–3d orbitals of Mn upon photoelectron ejection, which is more sensitive to the oxidation state of manganese than that of Mn 2p. − For convenience, the estimation of AOS for Mn from Δ E 3s can follow an empirical equation, AOS Mn = 9.67–1.27Δ E 3s , developed by Beyreuther et al, and the obtained AOS numbers of Mn are listed in Table S2, in which the AOS of MnO 2 -0.5IL (α-MnO 2 ) is lower than that of MnO 2 -0IL (β-MnO 2 ), showing more characteristics of Mn 3+ .…”

A Double‐Buffering Strategy to Boost the Lithium Storage of Botryoid MnOx/C Anodes

“…The binding energy peaks of Mn 2p 1/2 and 2p 3/2 in Figure e are located at 653.2 and 641.7 eV, respectively, suggesting a ballpark oxidation state between Mn 2+ (MnO) and Mn 3+ (Mn 2 O 3 ). However, it has been well known that it is difficult to determine the valence number of Mn from the Mn 2p peaks alone due to the overlap in binding energy values, the complex multiplet splitting, the satellite‐loss features, as well as the asymmetry of the spectra . Instead, analysis on the Mn 3s doublet splitting (Δ E 3s ) enables a more precise assessment (Figure f and Figure S7a, Supporting Information).…”