Conversion of inert cryptomelane-type manganese oxide into a highly efficient oxygen evolution catalyst via limited Ir doping

Abstract

The oxygen evolution reaction (OER) is a critical half reaction for energy storage techniques and is regarded as a major challenge due to its sluggish kinetics and complex reaction mechanism. The traditional OER catalysts, such as IrO₂, RuO₂ and their binary or ternary oxides, have finite large-scale commercial applications due to their significant cost and rareness. Here, we hydrothermally synthesized cry-Ir by doping Ir into non-OER active cryptomelane-type manganese oxide to significantly reduce the Ir mass ratio by 60.3% from 85.7% in IrO₂ to 34% in the developed catalyst, along with higher OER performance with a lower onset potential and 10 times higher specific mass activity. The special tunnel structure of cryptomelane plays an important role in promoting its OER activity through facilitating water molecular insertion into the tunnel. We combined Raman, XPS and TEM mapping to confirm that no IrO₂ composite is present on the cry-Ir surface. The XPS and XAS spectra indicate substitution of Ir⁴⁺ on the Mn³⁺ site and the presence of more 5d states in the Ir site compared to IrO₂. The differences in VBS spectra between cry-Ir, IrO₂ and cry-Mn indicate that the electronic structure of Ir sites is modified when Ir substitutes Mn³⁺ sites. Thus, this special tunnel structure and modified Ir electronic structure in cry-Ir are responsible for the outstanding OER performance. Our studies provide an approach for designing effective Ir-based OER catalysts whilst significantly reducing the consumption of precious elements.