Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Abstract

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrO_<i>x</i>) due to its superior performance and excellent stability. However, large scale applications using IrO_<i>x</i> are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHf_<i>x</i>O_<i>y</i>) electrocatalyst for OER. The IrHf_<i>x</i>O_<i>y</i> electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrO_<i>x</i> electrocatalyst. The highest intrinsic mass activity of the IrHf_<i>x</i>O_<i>y</i> catalyst in acid conditions was calculated as 6950 A g_IrOx^-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrO_<i>x</i> and IrHf_<i>x</i>O_<i>y</i> catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.