Development and Evaluation of Ir Based Anode Electrocatalysts for Water Electrolysis

Abstract

Introduction Hydrogen production through PEM type water electrolysis followed by hydrogen storage and use has been focused on as one of the efficient uses of renewable energy. Although PEM type water electrolyzer has an advantage on reduced ohmic resistance between two electrodes, it has also a disadvantage that common carbon supports cannot be used in order to disperse small catalyst particles under the high potential condition over 1.5 V at the anode. That leads to low active surface area of anode electrocatalysts, resulting in high cost. Therefore, the purpose of this study is to develop carbon-free porous anode electrocatalysts with high active surface area. So far, our group has been working on mesoporous carbon materials for PEFC studies1, and we have found that such a porous structure is kept even after being built into the device and can improve the mass transfer within catalyst layers. Then, basic concepts of mesoporous carbon are applied to this study. We propose development of carbon-free porous metal catalysts to enhance the performance of water electrolyzer. Experimental method Ir(acac)3 and Pluronic F127 were used as a metal precursor and a template to form a porous structure, respectively. A mixture of the metal precursor, solvent, and surfactant was heated up by step-by-step treatments in N2 or Air atmosphere to make the porous structure. After the fundamental material characterizations of the resulting Ir material, electrochemical measurements were performed using a common half-cell setup in the acidic solution. Oxygen evolution reaction (OER) activity was evaluated as anode electrocatalysts. Also, membrane electrode assembles (MEAs) were developed, and then a single cell performance was evaluated. Results and discussion Based on nitrogen sorption measurements and a SEM image as shown in Figure 1, the porous structure consisted by aggregation of particles was confirmed for the obtained Ir material. The resulting pore diameter was mostly smaller than 70 nm. When OER activity of porous Ir material was measured and compared with commercial IrO2 electrocatalyst (Figure 2), initial OER activity was lower than IrO2. This was due to fact that porous Ir material is mainly composed by Ir metal, which was confirmed by XRD. Then, the surface of porous Ir was activated via electrochemical oxidation. As a result, activated porous Ir/IrOx showed equivalent OER activity to commercial IrO2 as seen in Figure 2. Even though OER activity was just equivalent to commercial IrO2 as powder, the difference was seen when electrocatalysts were built into MEAs. The details of IV characteristics will be discussed. Furthermore, we have also worked on durability analyses against potential fluctuation derived by renewal energy, which will be also discussed. Reference (1) Hayashi, et al., Electrochim. Acta, 53, 6117-6125 (2008). Figure 1