Stabilization of platinum catalyst surface-treated by atomic layer deposition of cobalt for polymer electrolyte membrane fuel cells

Abstract

To improve the stability of the platinum/carbon catalyst widely used in polymer electrolyte membrane fuel cells, cobalt was deposited via plasma-enhanced atomic layer deposition (PEALD), which enables the uniform deposition of a high-purity thin film even on the porous structure of a cathode. After single cells were fabricated using the prepared Co-Pt/C cathodes, an accelerated stress test (AST) was performed to evaluate the stability of the Co-Pt/C catalysts. When cathodes obtained by 10 and 20 cycles of Co deposition (Co-ALD 10 and Co-ALD 20, respectively) were used, the electrochemically active area of the Pt catalyst particles was reduced because of Co deposition on the cathode. Consequently, the initial performance and electrochemical surface area (ECSA) decreased compared with those of a cathode without Co (Co-ALD 0). However, the decrease in performance and ECSA after the AST were smaller, confirming that Co deposition via the designed PEALD process improved the stability of the Pt/C catalyst. To determine the cause of this stability improvement, the cathode catalysts before and after the AST were compared using high-resolution transmission electron microscopy (HRTEM). The HRTEM images of the Co-Pt/C catalyst after the AST showed that the Pt catalyst particles were distributed relatively uniformly on the carbon support. The particle size of the Co-Pt/C catalyst was smaller than that of the bare Pt/C catalyst, on which Co was not deposited. These results indicate that a sufficiently thin Co layer deposited via PEALD immobilizes adjacent Pt catalyst particles and inhibits the decomposition of Pt catalyst particles that may occur during the AST. The stability of the Pt/C catalyst was improved through this mechanism. In addition, the Co layer can be deposited via short-term deposition even at low temperatures using plasma-state reaction gases in contrast to the high-temperature and long-term heat treatment methods of the existing alloy catalyst manufacturing strategies.