Formation of Porous Nanotubes for Simultaneous Improvements of Material Usage Efficiency and Catalytic Performance of Iridium-Based Water Oxidation Reaction Catalysts

Iridium (Ir) is the most widely used material for oxygen evolution reaction (OER) catalyst due to its great catalytic performance. However, Iridium is a very expensive element because it is one of the rarest elements on earth. Therefore, it is necessary to minimize Iridium consumption whilst maintaining the highest possible catalytic performance. One of common strategies to achieve a high catalytic performance is enlarging surface area by forming nanostructures; however, such nanostructures often limit mass transport of both reactants (liquid electrolyte) and products (oxygen gas), nullifying the benefit gained from the large surface area. Here, we developed a porous nanotube catalyst structure which can maximize its performance with a very small amount of Iridium and allow facile mass transport via pores formed on the sidewall of the nanotubes. The synthesis involves the formation of vertically aligned ZnO nanowires that serve as a template. Electrodeposition of Ni follows to conformally coat the ZnO nanowires. Careful control of wet etching conditions results in the complete removal of ZnO and partial removal of Ni shells forming pores on the Ni nanotubes. Finally, Ir is electrodeposited on the porous Ni nanotubes. The porosity of the structure can be controlled to optimize the catalytic performance. It can also be used as a catalyst template for other elements that need high performance while using a small amount of the material. The details of the synthesis and the electrochemical measurements of OER using our nanostructured Ir-based catalysts will be presented.