Highly Ordered 1-D IrO2 Electrode Synthesized with Titanium Oxide Nanotube Array for OER
Authors: Yan Changfeng , Lu Zhuoxin , Shi Yan , Wang Zhida , Guo Changqing , Tan Hongyi 
Published: February 2020

One-dimensional structure with high surface area, oriented charge transfer and facile mass transfer is a promising solution to reduce the loading amount of noble metal as catalyst in PEM water electrolysis. In this work, TiO2 nanotube array (TNTA) is applied to decorate IrO2 to construct highly ordered 1-D electrode.
Firstly, TNTA, synthesized by anodizing in hydrofluoric solution, is used as substrate as its great stability in both acidic and oxidizing environment. By considering the insufficient conductivity of TiO2, modification, including calcining in hydrogen or cathodic polarization, is conducted to improve its conductivity. Both the method can reduce part of Ti4+ to Ti3+ to achieve improving conductivity. By comparison, cathodic polarization treated TNTA performs higher carrier density without hydrogen brittleness. Iridium oxide is electro-deposited onto TNTA. By increasing the conductivity of TNTA, IrO2/reduced-TNTA shows a three order of magnitude higher OER activity than IrO2/pristine-TNTA. Reduced-TNTA is predictably unstable in oxidizing environment. By increasing the coverage of IrO2, the stability of IrO2/reduced-TNTA increase as the surface layer of IrO2 will isolate the unstable reduced-TNTA from oxidizing species generated during OER to prevent decrease of substrate conductivity.
Secondly, as a compact layer of IrO2 on the surface of reduced-TNTA is essential to its stability, TNTA is used as sacrificed template to fabricate one-dimensional IrO2 nano-array electrode. By removing TNTA, the mass-ratio surface area of IrO2 is double. Hot-press treatment followed by two-step etching of TNTA is conducted to transfer the IrO2 nano-array to Nafion membrane. With 1500 nanometer long TNTA prepared in hydrofluoric sodium solution is used as template, iridium oxide nanotube arrays and hollow nanowire arrays is achieved by applying different scan rate in deposition. The nanotube array is consisted with intact nanotube and porous nanotube surrounding it, which is corresponding to the structure of template. A mechanism of the scan rate controlling deposition is proposed. The deposition of iridium oxide on TNTA can be divided into the deposition near the mouth and the deposition along the tube wall. With continuously depositing, the concentration in the nanotube will gradually decrease and form a gradient. While the concentration near the mouth keep constant as its sufficient diffusion of precursor from the bulk electrolyte. So, if the duration of deposition in a single cycle is long enough, iridium will be preferentially deposited at the mouth of TNTA. And once the cover layer on the top surface is formed, the diffusion into the tube will be limited and further deposition will only lead to the thickening of the cover layer. As a result, with slow scan rate, the cover layer will shell the mouth of TNTA before the constant layer of iridium is deposited throughout the template, and results in nanotube arrays. And with fast scan rate, the deposition is more even, and the hollow nanowire arrays is obtained. Electrochemical study shows that the current density of iridium oxide nano-arrays in OER is positive correlated to its surface area, and it shows better performance of mass-ratio activity than commercial iridium oxide nanoparticle as anode in MEA.