GDL-Integrated Electrodes with Ir-Based Electrocatalysts for Polymer Electrolyte Membrane Water Electrolysis

Abstract

Introduction Polymer Electrolyte Membrane Water Electrolysis (PEMWE) is one of the promising technologies for renewable energy storage. High surface area carbon (e.g. carbon black, carbon nanotube etc.) is known as a good catalyst support material, but severe carbon corrosion occurs at the PEMWE anode. Metal oxides such as tin oxide and titanium oxide are considered as alternative support materials to carbon [1], [2]. We have developed Ti-based sheet-form electrodes consisting of noble metal electrocatalysts for Polymer Electrolyte Fuel Cell (PEFC) and PEMWE [3], [4]. This Ti sheet acts as Gas Diffusion Layer (GDL), current collector, and support material. We prepared two types of GDL-integrated electrodes with Ir-based electrocatalysts by the Arc Plasma Deposition (APD) and the evaporation-to-dryness process. A higher water electrolysis performance was obtained for electrodes prepared by the APD compared to those by the evaporation-to-dryness process. We also evaluated the durability of the electrodes prepared by the APD. Experimental Ti sheet was etched with 1 M NaOH solution at 60°C for 1 h to obtain Ti sheet with a high surface area. Thereafter, it was washed in 0.01 M HNO3 solution and in deionized water at room temperature for 30 min and 10 min to remove NaOH. Heat treatment was made at 400°C and 500°C in 5%H2-N2 for 30 min. Ir-based nanoparticles were deposited on the Ti sheet after heat treatment at 400°C by the APD. Another electrode with Ir-based nanoparticles was prepared via the evaporation-to-dryness process. Ir-based nanoparticles were decorated on the Ti sheets after heat treatment at 500°C by dipping the Ti sheet into H2IrCl6 solution and evaporating-to-dryness process, followed by heat treatment at 500°C in air. The microstructure of these two types of electrodes was observed by Field-Emission Scanning Electron Microscopy (FE-SEM) and Scanning Transmission Electron Microscopy (STEM). Electrochemical measurements were made for the half-cell and the single-cell (MEA). Results and discussion Figure 1 shows a SEM image of a Ti sheet. Figure 2 shows the microstructure of Ir-based electrocatalysts on the Ti sheet prepared by the evaporation-to-dryness process. From the SEM image, it was observed that Ir-based nanoparticles with a diameter of 2 to 3 nm were supported on the Ti sheet as previously reported for the electrode prepared by the APD [4]. Oxygen Evolution Reaction (OER) activity of these electrodes was measured with a half-cell setup. The electrode prepared by the APD exhibited higher OER activity, and sufficient I-V characteristics with a single-cell setup. Durability of the electrode prepared by the APD was evaluated by the durability evaluation protocol simulating practical operating conditions for the use with solar and wind power, as Muto et al suggested [5]. Figure 3 shows I-V characteristics before and after the durability test. Remarkable performance degradation was confirmed after the durability test. It might be considered that Ir could not be oxidized sufficiently to stable Ir-oxides. Therefore, heat treatment for the electrode prepared by the APD was also made in air. Figure 3 shows I-V characteristics of this electrode prepared by the APD after heat treatment before and after the durability test. Initial I-V performance was declined after heat treatment, but the durability was improved. One of the explanations for the stability improvement of the cell is that Ir could be oxidized sufficiently by this heat treatment. The GDL-integrated electrode consisting of highly dispersed Ir-based nanoparticles with sufficient I-V characteristics and the durability may be prepared by adjusting heat treatment conditions. Preparing Ir-based nano-sized electrocatalysts could reduce precious Ir loading in the PEMWE anode.