Cosputtered Calcium Manganese Oxide Electrodes for Water Oxidation

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO₃ 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 Mn₂O₃) 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 CaMn_xO_y 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 Mn₃O₄ (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 CaMn_xO_y 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^-2 at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMn_xO_y 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^-2 were observed. The thicknesses and surface morphologies of the CaMn_xO_y 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_x were evaluated, the results demonstrate that Ca²⁺ incorporation can enhance the OER activity of MnO_x 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.