Ir is one of the most efficient oxygen evolution reaction (OER) catalysts; however, it is also one of the rarest and most expensive elements. Therefore, it is highly desirable to develop Ir catalysts with nanostructures that reduce Ir consumption by maximizing the surface‐to‐volume ratio without limiting the mass transport of reactants and products of reactions. Ir OER catalysts on a template that consisted of porous nanotubes (PNTs) based on Ni are fabricated. The Ir/Ni PNTs offer multiple benefits, including high catalytic performance (potential of 1.500 V vs. reversible hydrogen electrode (RHE) at an operating current density of 10 mA cm−2 and Tafel slope of 44.34 mV decade−1), minimal use of Ir (mass activity of 3273 A g−1 at 1.53 V vs RHE), and facile mass transport through the NT‐sidewall pores (stable operation for more than 10 h). The Ir/Ni PNTs are also applied to a tandem device, consisting of a Cu(In,Ga)Se2‐based photocathode and halide perovskite photovoltaic cell, for unassisted water splitting. A solar‐to‐hydrogen conversion efficiency that exceeded 10% is also demonstrated, which is nearly 1% point greater than when a planar Ir film is used as the anode instead of Ir/Ni PNTs.

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.