In this study, to enhance the stability of the cathode platinum (Pt) catalyst in polymer electrolyte membrane fuel cells, cerium oxide (CeOx) was deposited by plasma-enhanced atomic layer deposition (PEALD) process on the Pt catalyst sputtered on the cathode. A change in the peak power density loss after an accelerated stress test (AST) during I-V measurement of the membrane-electrode assembly according to the number of cycles was observed, which confirmed stability improvement. In polymer electrode membrane fuel cells (PEMFCs), free radicals lead to degradation of the performance and stability of catalysts; we used CeOx to prevent these problems. CeOx acts as a free radical scavenger through the redox reaction of Ce3+/4+ ions in the cell test and prevents oxidative hydroxyl and hydroperoxyl radical attack created in the reaction between hydrogen peroxide and released cations. By preventing oxidation, the stability was improved without decreasing the performance. Therefore, the improvement of stability through plasma-enhanced atomic layer deposition CeOx encapsulation can be considered a promising strategy for PEMFC catalysts.
Summary
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.
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