Electrochemical deposition of manganese oxide–phosphate–reduced graphene oxide composite and electrocatalysis of the oxygen evolution reaction

Abstract

A composite of manganese oxide and reduced graphene oxide (rGO) is prepared in a single step electrochemical reduction process in a phosphate buffer solution for studying as an electrocatalyst for the oxygen evolution reaction (OER). The novel composite catalyst, namely, MnO_x–Pi–rGO, is electrodeposited from a suspension of graphene oxide (GO) in a neutral phosphate buffer solution containing KMnO₄. The manganese oxide incorporates phosphate ions and deposits on the rGO sheet, which in turn is formed on the substrate electrode by electrochemical reduction of GO in the suspension. The OER is studied with the MnO_x–Pi–rGO catalyst in a neutral phosphate electrolyte by linear sweep voltammetry. The results indicate a positive influence of rGO in the catalyst. By varying the ratio of KMnO₄ and GO in the deposition medium and performing linear sweep voltammetry for the OER, the optimum composition of the deposition medium is obtained as 20 mM KMnO₄ + 6.5% GO in 0.1 M phosphate buffer solution of pH 7. Under identical conditions, the MnO_x–Pi–rGO catalyst exhibits 6.2 mA cm⁻² OER current against 2.9 mA cm⁻² by MnO_x–Pi catalyst at 2.05 V in neutral phosphate solution. The Tafel slopes measured for OER at MnO_x–Pi and MnO_x–Pi–rGO are similar in magnitude at about 0.180 V decade⁻¹. The high Tafel slopes are attributed to partial dissolution of the catalyst during oxygen evolution. The O₂ evolved at the catalyst is measured by the water displacement method and the positive role of rGO on catalytic activity of MnO_x–Pi is demonstrated.