Surface Oxidation of Stainless Steel: Oxygen Evolution Electrocatalysts with High Catalytic Activity

The cheap stainless commodity steel AISI 304, which basically consists of Fe, Ni, and Cr, was surface-oxidized by exposure to Cl2 gas. This treatment turned AISI 304 steel into an efficient electrocatalyst for water splitting at pH 7 and pH 13. The overpotential of the anodic oxygen evolution reaction (OER), which typically limits the efficiency of the overall water-splitting process, could be reduced to 260 mV at 1.5 mA/cm2 in 0.1 M KOH. At pH 7, overpotentials of 500–550 mV at current densities of 0.65 mA/cm2 were achieved. These values represent a surprisingly good activity taking into account the simplicity of the procedure and the fact that the starting material is virtually omnipresent. Surface-oxidized AISI 304 steel exhibited outstanding long-term stability of its electrocatalytic properties in the alkaline as well as in the neutral regime, which did not deteriorate even after chronopoteniometry for 150 000 s. XPS analysis revealed that surface oxidation resulted in the formation of Fe oxide and Cr oxide surface layers with a thickness in the range of a few nanometers accompanied by enrichment of Cr in the surface layer. Depending on the duration of the Cl2 treatment, the purity of the Fe oxide/Cr oxide mixture lies between 95% and 98%. Surface oxidation of AISI 304 steel by chlorination is an easy and scalable access to nontoxic, cheap, stable, and efficient electrocatalysts for water splitting.