Mitigation of PtCo/C Cathode Catalyst Degradation via Control of Relative Humidity

Abstract

Maintaining the high performance of proton-exchange membrane fuel cells (PEMFC) over the course of its lifetime is a key enabling factor for its successful commercialization as a primary power source in zero-emission transportation applications. In this context, it is important to mitigate the degradation of PtCo-alloy based cathode catalysts used for oxygen reduction reaction (ORR). PtCo-alloy catalysts exhibit high activity at beginning-of-life (BOL) which tends to decrease during operation due to loss of electrochemical surface area (ECSA) and dissolution-contamination related effects of the Co-alloying component. Here, we demonstrate the use of relative humidity (RH) of the inlet gases as a controllable parameter to mitigate the degradation of PtCo-alloy catalyst degradation. We employ a catalyst-specific voltage cycling accelerated stress test (AST) durability protocol as a function of inlet RH to degrade PtCo catalysts. A series of in situ electrochemical diagnostics and ex situ characterizations have been carried out to investigate the catalyst layer characteristics at end-of-test (EOT). Our results show that at sub-saturated conditions of durability protocol operation, PtCo catalyst sustains higher EOT H2/air performance due to better retention of ECSA and smaller impact of Co2+ dissolution/contamination.

Improving Durability of Fuel Cells with Platinum-rich Alloy Cathode Catalysts

“…We have previously shown using Electron Probe Microanalysis (EPMA) of MEA cross-sections after voltage cycling AST protocol that the Co leached from the cathode catalyst nanoparticles is largely observed to be present in the membrane. 5,21 Leached Co is predicted to be present in the membrane as Co 2+ cations ionically bound to the SO 3 ˉanions in the PFSA side chains thereby displacing two H + in the process. Figure 8 shows the EPMA cross-sectional profiles of Co for the various PtCo-alloy catalyst MEAs after 30,000 AST cycles.…”

“…Movement of Co 2+ cations into the cathode increases with various MEA operating conditions such as higher current densities, temperature, and higher relative humidity (RH) or higher electrode water content etc. 5,17,21 2+ partitioning between the electrode and the membrane. 17 They reported a critical Co 2+ concentration of ∼44% for a 25 μm thick membrane above which a sharp performance drop was observed.…”

“…Alternately, a system-level mitigation strategy has also been recently demonstrated wherein the operation of fuel cell at sub-saturated relative humidity was shown to lead to decrease in ECSA loss and Co dissolution. 21,23 In this article, we investigate the role of Pt to Co atomic ratio on achieving a balance between a higher initial activity and improved durability. Pt x Co compositions with x = 3, 5, 7 were studied in comparison to a pure Pt catalyst.…”

Evaluation of functional layers thinning of high temperature polymer electrolyte membrane fuel cells after long term operation

“…There are many studies that try to understand the mechanisms behind such degradation processes. A lot of these studies focus on the catalyst degradation, [10][11][12][13] since catalyst particles agglomeration or dissolution are well known phenomena that decrease the active surface area of the electrode and impact the cell performance. Other studies focus on the carbon corrosion, especially on the cathode, where the higher potential makes the oxidation of the C support to CO 2 .…”

Effect of Engineered Cracks in Catalyst Layers on PEMFC Catalyst Layer Durability

“…2 This enhanced durability requirement affects catalyst layers (CLs) (catalysts, [3][4][5][6] supports, 7,8 and ionomers [9][10][11] ), membranes, [12][13][14] gas diffusion layers, 15,16 and flow fields. 17 While many of the degradation mechanisms can be suppressed using system operation strategies, operating conditions are also directly correlated to PEMFC performance 18 and hence limits the practical applicability of such strategies. Thus, materials development (particularly in CL structure) remains critical for reducing system cost, providing fault tolerance, and improving lifetime.…”