Beneficial effect of Fe addition on the catalytic activity of electrodeposited MnO_x films in the water oxidation reaction

We report on a fast and simple protocol for the electrodeposition of Fe-MnO_x films used as catalysts for the water oxidation (WO) reaction at neutral pH, and showing the beneficial effect of iron in terms of both activity and stability of the catalyst. While most electrodeposited MnO_x WO catalysts are obtained starting from Mn(II) precursors, the proposed protocol consists in the galvanostatic cathodic deposition of Fe-MnO_x onto conductive FTO glasses, using KMnO₄ and Fe(NO₃)₃ as Mn and Fe precursors, respectively. In the absence of Fe, the Tafel slope drastically increases from 103 to 270 mV dec⁻¹ when passing from low to high overpotentials. The slope change, instead, is progressively reduced when the Fe precursor is added to the deposition solution and a constant slope of 105 mV dec⁻¹ is obtained in the whole overpotential range with an optimal Fe concentration of 1.00 mM, Accordingly, Electrochemical Impedance Spectroscopy (EIS) shows that Fe addition improves both charge transfer and transport properties of the electrodeposited films. In particular, a five-fold decrease in charge transfer resistance at the catalyst/electrolyte interface was observed, suggesting a more facile oxygen evolving kinetics for Fe containing samples. Furthermore, the lower the iron content, the lower the film stability, as pointed out by chronopotentiometric measurements, and confimed by FESEM analysis and EIS as measured both before and after water electrolysis experiments. To establish structure activity relationships, an extended characterization of the electrodeposited films was carried out by means of Raman Microscopy, Transmission Electron Microscopy, UV–vis and X-ray Photoelectron Spectroscopies. The ensemble of the characterization results suggests that Fe³⁺ ions are actually incorporated within the electrodeposited film, with limited effects on the final Fe-MnO_x structure, consisting in a defective MnO₂ birnessite-type structure with significant fraction of surface Mn³⁺ species.