IrOx/Nafion Catalyst for Oxygen Evolution: Effect of Surface Oxide on Activity and Stability

Abstract

High catalyst loading is a major barrier for large-scale production of proton exchange membrane water electrolysis (PEMWE) despite its advantages over other electrolysis technologies. [1]. [2,3] Herein, a low iridium loading composite catalyst layer of iridium oxide/Nafion (IrOx/Nafion) is developed using reactive spray deposition technology (RSDT). In this process, the catalyst synthesis and catalyst-coated membrane (CCM) fabrication are combined into one step. The activity and stability for oxygen evolution reaction (OER) are tested through rotating disk electrode (RDE) measurements and membrane electrode assembly (MEA), respectively. The iridium particles has an average size of ~2 nm resulting in very high surface area and superior activity over planar and electrodeposited IrO2 thin films. In this study, two flame conditions (F-1 and F-2) are used for synthesizing the iridium particles and a correlation between the surface oxide layer and the electrochemical performance is observed. Based on preliminary results of X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), we hypothesize that the iridium particles exhibit core-shell structure with a metallic iridium core and mixed iridium (IV and III) oxides shell. Higher flame temperature and longer residence time in the flame (F-2 condition) increases the amount of surface oxide and forms a thicker oxide layer. The oxide composition of the particle can be determined from XPS results (Figure 1 a and b) employing the fitting model proposed in reference [4]. The oxide composition for sample F-1 and F-2 are 54% and 90%, respectively. On the other hand, electron diffraction pattern from TEM analysis shows a metallic phase, which suggests the oxides may reside only on the surface since XPS is surface sensitive. Electrochemical measurements have shown that the iridium particles with higher oxide content improves the OER mass activity by ~3 fold over particles with lower oxide content. Furthermore, the catalyst stability in electrolysis test is improved from 250 hours to 4500 hours with the higher oxide content. Detailed surface analysis is carried out using XPS and high-resolution TEM to obtain the chemical state or change of iridium in particles synthesized under different flame conditions and to investigate the role of surface oxide in the enhancement of activity and stability. Figure 1. XPS of iridium particles synthesized in F-1 (a) and F-2 condition (b) fitted with asymmetric Doniach-Sunjic line shape.