Paul Taichiang Yu

The Impact of Carbon Stability on PEM Fuel Cell Startup and Shutdown Voltage Degradation

Conventional carbon MEAs and graphitized carbon MEAs were evaluated for the resistance to carbon corrosion and startup/shutdown durability in this paper. Graphitized carbon MEAs show higher resistance to carbon corrosion than conventional carbon MEAs by a factor of 35 at a point where 5% weight loss had occurred. A graphitized carbon MEA yielded lower degradation rate than that of a conventional carbon MEA by a factor of 5 after 1,000 startup/shutdown cycles. The kinetics of carbon corrosion over both conventional carbon MEAs and graphitized carbon MEAs were measured, and carbon corrosion during startup/shutdown was explained and modeled. The model results correlate to what we have measured from our startup/shutdown durability test. Overall, MEAs with corrosion resistant carbon supports are one of major materials approaches to mitigate cell voltage degradation due to fuel cell startup/shutdown. We believe that a combination of corrosion resistant materials and system operating mitigation strategies is the path to attain the strict automotive durability targets.

Carbon-Support Requirements for Highly Durable Fuel Cell Operation

Owing to its unique electrical and structural properties, high surface area carbon has found widespread use as a catalyst support material in proton exchange membrane fuel cell (PEMFC) electrodes. The highly dynamic operating conditions in automotive applications require robust and durable catalyst support materials. In this chapter, carbon corrosion kinetics of commercial conventional-carbon-supported membrane electrode assemblies (MEAs) are presented. Carbon corrosion was investigated under various automotive fuel cell operating conditions. Fuel cell system start/stop and anode local hydrogen starvation are two major contributors to carbon corrosion. Projections from these studies indicate that conventional-carbon-supported MEAs fall short of meeting automotive the durability targets of PEMFCs. MEAs made of different carbon support materials were evaluated for their resistance to carbon corrosion under accelerated test conditions. The results show that graphitized-carbon-supported MEAs are more resistant to carbon corrosion than nongraphitized carbon materials. Fundamental model analyses incorporating the measured carbon corrosion kinetics were developed for start/stop and local hydrogen starvation conditions. The combination of experiment and modeling suggests that MEAs with corrosion-resistant carbon supports are promising material approaches to mitigate carbon corrosion during automotive fuel cell operation.

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion

Carbon-support corrosion causes electrode structure damage and thus electrode degradation. This chapter discusses fundamental models developed to predict cathode carbon-support corrosion induced by local H2 starvation and start–stop in a proton-exchange-membrane (PEM) fuel cell. Kinetic models based on the balance of current among the various electrode reactions are illustrative, yielding much insight on the origin of carbon corrosion and its implications for future materials developments. They are particularly useful in assessing carbon corrosion rates at a quasi-steady-state when an H2-rich region serves as a power source that drives an H2-free region as a load. Coupled kinetic and transport models are essential in predicting when local H2 starvation occurs and how it affects the carbon corrosion rate. They are specifically needed to estimate length scales at which H2 will be depleted and time scales that are valuable for developing mitigation strategies. To predict carbon-support loss distributions over an entire active area, incorporating the electrode pseudo-capacitance appears necessary for situations with shorter residence times such as start–stop events. As carbon-support corrosion is observed under normal transient operations, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling and incorporating mechanisms that can quantify voltage decay with carbon-support loss.

Catalyst Degradation Mechanisms in PEM and Direct Methanol Fuel Cells

While much attention has been given to optimizing initial fuel cell performance, only recent research has focused on the various materials degradation mechanisms observed over the life-time of fuel cells under real-life operating conditions. This presentation will focus on fuel cell durability constraints produced by platinum sintering/dissolution, carbon-support oxidation, and membrane chemical and mechanical degradations. Over the past 10 years, extensive R&D efforts were directed towards optimizing catalysts, membranes, and gas diffusion layers (GDL) as well as combining them into improved membrane electrode assemblies (MEAs), leading to significant improvements in initial performance of H2/air-fed proton exchange membrane fuel cells (PEMFCs) and methanol/air-fed direct methanol fuel cells (DMFCs). 3 While the required performance targets have not yet been met, current PEMFC and DMFC performance are close to meeting entry-level applications and many prototypes have been developed for field testing. This partially shifted the R&D focus from performance optimization to more closely examining materials degradation phenomena which limit fuel cell durability under real-life testing conditions. The predominant degradation mechanisms are sintering/dissolution of platinum-based cathode catalysts under highly dynamic operating conditions, dissolution of ruthenium from DMFC anode catalysts, the oxidation of carbon-supports of the cathode catalyst during fuel cell startup and shutdown, and the formation of pinholes in proton exchange membranes if

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Catalysts: Life-Limiting Considerations

Material degradation mechanisms of carbon-supported platinum-based electrocatalysts in proton-exchange membrane fuel cells (PEMFCs) are reviewed in the context of automotive applications. Transient operations such as idling, sitting at open-circuit voltage, load cycling, and startup/shutdown pose significant durability concerns over the state-of-the-art catalysts in the expected lifetime range. High cathode potentials and potential cycling can cause damage to both the catalyst and the carbon support, leading to the growth of platinum particles and the loss of platinum into the ionomer phase, as well as carbon-support corrosion. Startup and shutdown are the most damaging transient operations in PEMFC systems, which can cause significant cathode electrode carbon-support corrosion. PEMFC performance decays dramatically when the electrode structure is damaged due to carbon corrosion.