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