Contribution of Electrocatalyst Support to PEM Oxidative Degradation in an Operating PEFC

Abstract

The contribution of the electrocatalyst support to polymer electrolyte membrane (PEM) oxidative degradation in an operating polymer electrolyte fuel cell was investigated. A corrosion-resistant non-carbon catalyst support based on mixed ruthenium and silicon oxides (RuO 2 -SiO 2 ; RSO) was compared against a benchmark carbon-based support (Vulcan XC 72; C). The rates of in-situ reactive oxygen species (ROS) generation (Pt/C: 9.0 ± 0.20 × 10 −5 s −1 ; Pt/RSO: 5.9 ± 0.19 × 10 −5 s −1 ) and macroscopic PEM degradation measured ex-situ as the fluoride emission rate (FER; Pt/C: 2.7 ± 0.32 × 10 −5 ppm cm -2 s -1 ; Pt/RSO: 2.5 ± 0.31 × 10 −5 ppm cm -2 s -1 ) were significantly lower for platinum supported on RSO than for platinum supported on carbon. There was an excellent correlation between the in-situ ROS generation rate and the FER, thereby confirming the causal relationship between ROS generation and PEM degradation. The lower rate of ROS generation over RSO and Pt/RSO was attributed to a lower net rate of electrochemical H 2 O 2 generation during the oxygen reduction reaction (ORR). Rotating ring-disk electrode experiments confirmed that the net electrochemical H 2 O 2 generation rate on Pt/RSO was about twice lower than that on Pt/C. Kinetic parameters estimated for the ORR supported a direct 4e -pathway on both Pt/RSO (with i k of 4.5 mAcm -2 , n = 3.97 and a Tafel slope of 64 mVdec -1 ) and Pt/ C (with i k of 4.1 mAcm -2 , n = 3.93 and a Tafel slope of 68 mVdec -1 ). In conjunction with its high corrosion-resistance, this finding further illustrates the viability of RSO (and analogs such as ruthenium-titanium oxide) as outstanding PEFC electrocatalyst supports. Polymer electrolyte fuel cells (PEFCs) have attracted a lot of interest as electrochemical energy conversion sources for multiple applications in the automotive, stationary power, portable power, and military sectors due to their high efficiency, modularity and environmental benefits.1-3 The commercialization of PEFCs, particularly for the automobile sector, has been hindered by high cost and insufficient component durability. 2,4 The high cost of component materials, such as platinum (Pt) electrocatalyst 5,6 and Nafion electrolyte membranes are among the main factors influencing the cost of PEFCs.4,7-9 Recent advances in the design of PEFCs enable high performance with very low total Pt loadings (ca. 0.15 mg Pt cm -2 ) and using very thin polymer electrolyte membranes (PEMs; e.g. Nafion 211, ca. 25 μm, or even lower thickness variants). 4,10,11 However, the long-term durability of PEFC components still remains a major concern. The key issues affecting PEFC durability include electrode degradation via carbon corrosion, 1,12-15 Pt dissolution, 5,16-19 catalyst sintering 20,21 and electrolyte degradation via PEM oxidative degradation.2,3,22 Carbon corrosion and PEM degradation are key degradation modes that need to be mitigated. Carbon corrosion is primarily caused by oxidation of carbon at higher electrode potentials that are encountered during transient...

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