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Reducing the cost of PEM water electrolyser is still a big challenge for renewable energy storage. Though there are some reports which have achieved very low Ir loading (0.1~0.2mg/cm2) in membrane electrode assemblies (MEA) for PEM water electrolyser, the other issues such as the performance and stability of MEA, the reduction of Pt loading in cathode and the scalability of preparation method of MEA still need further investigation for developing low cost and high performance PEM water electrolyser. In this work, to develop high performance anode with low Ir loading, highly active Iridium oxohydroxides were supported on conductive titanium metal nanosphere as the anode catalyst. The diatmeter of metal Ti nanosphere ranges from 60-200nm, and the Iridium oxohydroxides loaded on Ti support shows a particle size between 1.5-3nm. The OER activity of the catalyst with various Ir loading was characterized, and the catalyst with 10% Ir loading exhibited highest Ir mass activity (1700A/g @η=0.37V), it is about 6 times as the commercial IrO2. Increasing the loading of Ir will lead to the decline of Ir mass activity, this may be due to the limited surface area of Ti nanosphere. The stability of the catalyst (Ir loading 10%) was characterized by CV scans between 0.4-1.5V (vs.RHE) in 0.5M H2SO4. After 2000 cycles, the active sites of IrOx declined only 18%, and the catalyst does not show an obviously rising in resistance from EIS characterization, indicating a good stability both on active sites and support. The MEAs using such IrOx/Ti as the anode were prepared by conventional CCM method, and the Pt loading (20% wt. Pt/C) in cathode was reduced to about 0.2mg/cm2. The anode with various Ir loading and membrane with different thickness were determined at atmosphere under 80 degrees. The electrolyser with Ir loading 0.13mg/cm2 in anode, Pt loading 0.18mg/cm2 in cathode and N212 membrane achieved 1.623V at 1A/cm2 and 1.778V at 2A/cm2, which exhibits a good performance with such low noble metal loading. Further decreasing the Ir loading in anode catalyst layer will lead to performance degradation, but the rising of voltage was only 50-60mv at 1A/cm2 when Ir was reduced to 0.08 mg/cm2. With respect of the durability of MEAs, an electrolyser using Nafion 117 as the membrane, and 0.08 mg /cm2 Ir in MEA was tested under 1A/cm2. After 850h, the voltage increased from 1.79V to 1.83V, and the degradation rate is below 50μV/h, this suggests a good stability of supported Iridium oxohydroxides, and the stability of electrolyser with N212 membrane is now ongoing. Figure 1

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.

The effective, stable and low cost anodic catalyst or electrode for OER is still a big challenge for water electrolysis in acid condition. Many efforts were devoted to develop suitable supports for depositing Ir and Ru metal or oxides to obtain an enhanced activity or stability for OER. In this work, self-doped TiO2 nanotube arrays (sTNTA) which has a higher electrical conductivity than TNTA was chosen as the support, it was fabricated by a simple electrochemical reduction of TNTA in neutral electrolyte. Then IrO2 nano particles were deposited on it by electrochemical pulse deposition method to form an OER electrode. The cyclic voltammetric behavior, electrical conductivity and micro structure of sTNTA prepared under different reduction potential were characterized to obtain an optimal performance for depositing IrO2, the OER activity and stability of new electrode were also determined. The obtained sTNTA shows an increased current in CV test than TNTA, which is due to the increased carrier density. As the reduction potential move to more negative level (from -1.5V to -1.9V vs. Ag/AgCl), the carrier density was found increased for 3 order of magnitudes, and XPS characterization confirm the existence of Ti3+ which formed in electrochemical reduction process, and sTNTA prepared under -1.9V shows the best electrochemical performance. After depositing IrO2, IrO2/sTNTA exhibited higher OER activity than IrO2/TNTA, according to the CV and EIS test, this can be ascribed to the enhanced electrical conductivity of support, which results in more active sites and lower charge transfer resistance of the electrode. The OER stability of IrO2/sTNTA was tested under constant current of 5mA/cm2, IrO2/TNTA and IrO2/Ti which prepared by electrochemical deposition on Ti foil were also tested for comparison. A higher stability of IrO2/sTNTA electrode was observed than the other two electrodes, and this may be due to the interaction between the IrO2 and sTNTA. XPS studies show a clear peak shift of Ir4f in IrO2/sTNTA, indicating a possible lower valence of Iridium, and such lower oxidation state of Iridium may be response for the enhanced OER stability. In summary, our studies have demonstrated the possibility of self-doped TNTA as a potential anodic support for OER. Comparing with the TNTA, the high electrical conductivity of sTNTA facilitate the charge transfer rate and thus improve the OER activity. Additionally, a higher OER stability was observed for IrO2/sTNTA electrode, and this may be due to possible catalyst-support interactions between IrO2 and self-doped TNTA. Figure 1