Modeling proton-exchange-membrane fuel cell performance/degradation tradeoffs with chemical scavengers

Victoria M Ehlinger,Andrew R Crothers,Ahmet Kusoglu,Adam Z Weber

doi:10.1088/2515-7655/abb194

Victoria M Ehlinger, Andrew R Crothers + Show 2 more

Open Access

https://doi.org/10.1088/2515-7655/abb194

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Abstract

One of the primary limiting factors for proton-exchange-membrane (PEM) fuel-cell lifetime is membrane degradation driven by operational stressors such as generation of highly reactive radical species, which result in cell failure and voltage decay. To extend the lifetime of the membrane, cerium ions are added to the membrane to mitigate the effects of chemical degradation by scavenging radicals produced by crossover of reactant gases across the PEM. Although cerium has shown to be very effective at reducing chemical degradation during PEM fuel cell operation, the cerium ions also lead to a decrease in performance due to changes in the membrane transport properties and possible site blockage in the catalyst layers. In this paper, a full-cell, transient performance and durability model is presented in which a micro-kinetic framework accounts for gas crossover induced degradation and concentrated-solution theory describes transport in the PEM. The transport model takes into account the coupled nature of the electrochemical driving forces that cause transport of cerium ions, protons, and water. The cell model predicts the migration of cerium out of the membrane and into the catalyst layers and its impact on performance. A comparison between dilute-solution-theory and concentrated-solution-theory models shows how water management in the cell also effects cerium distribution, where higher relative humidity leads to better retention of cerium in the membrane. A voltage loss breakdown shows that cerium leads to performance losses in the cell both by decreasing proton activity and by modifying transport properties of water and protons through the membrane. Transient simulations show that the optimal tradeoff between performance and durability metrics is reached at low cerium concentrations in the membrane (less than 1% of membrane sulfonic acid sites occupied by cerium for our analysis). Finally, analysis of membrane thickness and catalyst layer thickness as design parameters shows that thicker membranes and thinner catalyst layers best optimize both performance and durability.

Highlights

With increasing interest in proton-exchange-membrane fuel cells (PEMFCs) for medium- and heavy-duty applications, the research needs shift towards improving lifetime and durability to enable commercialization
The thicker membranes increase lifetime because the crossover gases are slower to permeate the membrane, while the thicker catalyst layers decrease lifetime due to the increased reaction rate due to a greater availability of reaction sites for radical formation. These results show that an optimal tradeoff between performance and durability requires thicker membranes and thinner catalyst layers, while considering limitations due to local losses and flooding that are not included in this model
A full-cell, transient durability model with a microkinetic framework for degradation and concentrated-solution-theory based transport and mitigating effects of cerium ion was developed for a proton-exchange-membrane fuel cell

Summary

Introduction

With increasing interest in proton-exchange-membrane fuel cells (PEMFCs) for medium- and heavy-duty applications, the research needs shift towards improving lifetime and durability to enable commercialization. During PEMFC operation, a combination of mechanical and chemical stressors occur that lead to loss of performance, or even failure of the proton-exchange-membrane (PEM). Chemical degradation results from the oxidative attack of hydroxyl radicals to the chemical bonds in the ionomer’s fluorocarbon backbone and side-chains [6,7,8]. A concentrated-solution-theory model is used to model the transport of cerium through the membrane and catalyst layers This approach allows important consideration of the effects of cerium on water and proton transport and identifies two phenomena that cause performance losses when cerium is added to the fuel-cell membrane. The model approach is described, including concentrated-solution-theory approach for cerium transport, the effect of cerium on reaction kinetics, and chemical degradation and mitigation kinetics. A study of the trade-offs between performance and durability is accomplished along with a sensitivity analysis with respect to membrane thickness

Theoretical

Cerium-ion impact on reaction kinetics

Results and discussion

Conclusions