Observation of Oxygen Bubble Growth near Anode Catalyst Electrode in PEM Water Electrolyzer

Abstract

Water electrolysis technology is expected to be essential to build a hydrogen energy system. In particular, proton exchange membrane water electrolyzer (PEMWE) has attracted much attention because of its advantages such as the ability to downsize the equipment and produce high-purity hydrogen [1]. Improving the electrolysis efficiency of PEMWE is needed for future commercialization. One of the serious problems to improve the electrolysis efficiency is the oxygen concentration overpotential [2]. The oxygen concentration overpotential is considered to be not only caused by the generated oxygen bubbles but also by the dissolved oxygen near the anode catalyst electrode. Therefore, it is necessary to investigate the concentration distribution of dissolved oxygen near the anode catalyst electrode and to clarify the mechanism of the oxygen concentration overpotential. Although there are some reports about oxygen bubbles in PEMWE [3], there are few reports which investigate the dissolved oxygen. In this study, due to investigate the existence of the dissolved oxygen, the oxygen bubble behavior near the anode catalyst electrode was observed by using a high-speed camera and measured the oxygen bubble size change.A single cell with a 1 cm2 active area was used and a transparent acrylic plate was set to the side face of the cell for observing bubbles. Nafion-117 was used as the PEM, and IrOx and Pt/C were used as the anode and the cathode catalysts, respectively. A sintered Ti with Pt plating was used for the anode porous transport layer (PTL) and a carbon paper (TGP-H-060) was used for the cathode PTL. A high-speed camera (Photron, FASTCAM Mini AX100) and two LED lighting apparatuses (Hayashi-Repic, LA-HDF108DA) were used for visualizing. The resolution was 1024 × 1024 pixel, the pixel size was 3.2 μm, and the framerate was 4000 fps. An electrochemical measurement system (Bio-Logic, SP-300) was used for the power supply. Visualization was performed during electrolysis at 30 mA/cm2 with current control.From the observation of oxygen bubble behavior, the diameter of oxygen bubbles continued to increase until the bubbles left a certain distance even after detached from the anode catalyst. It was considered that the generated oxygen dissolved into the water and there were oxygen supersaturated areas near the anode catalyst electrode.Reference[1] M. Carmo et al, Int. J. Hydrogen Energy, 38 (12), 4901-4934 (2013)[2] J. Kai et al, J. Electrochem . Soc., 166 (4), F246-F254 (2019)[3] P. J. Kim et al, J. Electrochem . Soc., 167, 124522 (2020)AcknowledgmentsThis study was based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and also partly supported by JSPS KAKENHI Grant Number 19J20495.