Temperature Effects on PEM Fuel Cells Pt∕C Catalyst Degradation

Abstract

Operating proton exchange membrane ͑PEM͒ fuel cells at relatively high temperatures provides benefits that include greater tolerance to CO, easier management of water, and improved efficiency of heat rejection. At the same time, the degradation of fuel cell components is accelerated at higher cell temperatures, resulting in shortened cell lifetimes. In this study, we investigated temperature effects on cathode Pt/C catalyst durability. Cathode degradation was accelerated using square-wave potential cycling between 0.87 and 1.2 V ͑vs reference hydrogen electrode͒. More rapid Pt/C degradation was observed at higher temperatures, resulting in lower performance, loss of Pt catalyst electrochemically active surface area, and deposition of Pt in the membrane. Each has been quantified at 40, 60, and 80°C. More durable catalysts with improved microstructures are needed for commercial applications of fuel cells, and alternative accelerated tests must be developed for catalyst systems operating at higher temperatures.Proton exchange membrane ͑PEM͒ fuel cells show promise as the power source in future transportation systems, offering high energy-conversion efficiency without greenhouse gas emissions. We expect that the best vehicle designs will use fuel cell and battery hybrid-power systems. The system architecture, its operation, and the approach to power management will affect the life of both the battery and the fuel cell. For systems operated continuously, the electrode potential is nearly constant and generally low enough so that dissolution of platinum is not a first-order concern. For hybrid systems, and particularly for transportation applications, the electrode potential is frequently cycled and platinum dissolution and migration is prevalent. In addition to the high number of potential cycles expected for fuel cell vehicles, two further research objectives that affect this mode of degradation have been identified: reduced precious metal loadings to lower system cost and higher temperatures of operation for improved heat rejection. With the relatively high loadings ͑0.5 mg/cm 2 ͒ of catalysts common today, substantial loss in electrochemically active platinum may be tolerable. In contrast, if platinum loadings are decreased an order of magnitude, the electrode will be much less robust. Thus, a more complete understanding of the physics and chemistry of platinum dissolution and migration and the development of more stable catalyst structures are indispensable.Through fundamental understanding of the mechanisms of degradation, the broader objective of our research is to guide the development of new materials and to identify strategies to mitigate these failure modes in vehicle systems. An essential element is the development of physics-based models. Therefore, our aim is to not only to quantify the effect of temperature on the rate of degradation but to provide data to validate mathematical modeling of platinum dissolution, diffusion, and deposition under cyclic conditions. These models will help solidify our underst...

The effect of humidity and oxygen partial pressure on degradation of Pt/C catalyst in PEM fuel cell

“…Operation at higher temperatures allows for better heat rejection and for simpler fuel-cell stack design. However, accelerated cathode catalyst degradation at high temperatures was observed in our previous study [4,5]. In Borup et al's brief study of Pt particle growth during durability tests [6], the growth rates were found to be accelerated at high cell potentials, increased cell temperatures, and high relative humidity conditions.…”

A review of polymer electrolyte membrane fuel cell durability test protocols

“…A variety of investigations into catalyst degradation have been conducted [63][64][65][66][67][68][69][70]. Catalyst ASTs described in the literature generally simulate duty cycle induced catalyst degradation by potential cycling from a lower potential, in the range of 0.1-0.7 V, to increased potential, such as OCV, 1.0, or 1.2 V. Some examples of these studies are given in Table 2.…”

Empirical membrane lifetime model for heavy duty fuel cell systems

“…As the voltage cycles below 0.9 V, removal of the PtO surface causes instability of Pt-Pt bonds in the first and second atomic layers of the catalyst, exposing them to dissolution [96]. High potentials during fuel cell start-up and shut-down also lead to corrosion of catalyst carbon support material, especially close to MEA outlets and at high temperatures [97], [98]. Carbon in the cathode catalyst layer is electrochemically oxidized at cathode potentials above 1.1 V, due to the reverse current mechanism [95], [99] allowing unsupported Pt particles to sinter, coalesce into larger particles, and/or migrate into the membrane.…”