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 degrees 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 degrees C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnxOy films are amorphous even when heated to 600 degrees 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 similar to+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnxOy films were OER-inactive. In contrast, annealing at 400 or 600 degrees C resulted in an increase of similar to 15-fold in the current densities, which reached j congruent to 1.5 mA.cm(-2) at OER overpotentials of eta congruent to 550 mV in cyclic voltammetry (CV) sweeps. For the same eta, annealed CaMnxOy electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j congruent to 0.4-0.5 mA.cm(-2) were observed. The thicknesses and surface morphologies of the CaMnxOy 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 MnOx were evaluated, the results demonstrate that Ca2+ incorporation can enhance the OER activity of MnOx 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.