In-Situ Modeling of Chemical Membrane Degradation and Mitigation in Ceria-Supported Fuel Cells

Abstract

Ceria-supported membrane electrode assemblies (MEAs) have recently been proposed to address chemical membrane degradation in polymer electrolyte fuel cells. Although ceria is known to effectively protect the membrane at open circuit voltage (OCV) conditions, its effectiveness has not been demonstrated for cell voltages below OCV and associated conditions relevant for field operation. In the present work, a comprehensive, transient in situ chemical degradation model for ceria stabilized MEAs is developed and applied to investigate the mitigation effectiveness of ceria additive. At high cell voltages, abundant Ce 3+ ions are available in the membrane to quench hydroxyl radicals which is the primary mitigation mechanism observed at OCV conditions. However, the mitigation is suppressed at low cell voltages, where electromigration drives Ce 3+ ions into the cathode catalyst layer (CL). Without an adequate amount of Ce 3+ in the membrane, the hydroxyl radical scavenging is significantly reduced, leading to a ten-fold reduction in mitigation effectiveness at cell voltages below 0.7 V. The simulated results also suggest that significant ceria precipitation may occur in the cathode CL due to the increased local Ce 3+ concentration at low to medium cell voltages. Ceria-supported MEAs may therefore experience higher rates of chemical membrane degradation at low cell voltages than at OCV. Hydrogen powered polymer electrolyte fuel cells (PEFCs) generally use perfluorosulfonic acid (PFSA) ionomer membranes to separate the two electrodes in the membrane electrode assembly (MEA). Their high proton conductivity at low temperatures, relatively low reactant permeation, and superior electrical insulation lead to high fuel cell performance. However, the ionomer membrane can be degraded in the fuel cell environment which reduces its stability and limits its lifetime.1,2 Chemical degradation initiates the overall degradation processes 3,4 and further damages the membrane when combined with mechanical stress, strain, and fatigue induced by hygrothermal fluctuations in the MEA. [5][6][7][8][9][10][11][12] The resulting physical damage in the form of cracks and holes eventually causes hydrogen leaks across the membrane which is considered one of the main lifetime limiting failure modes in fuel cells. 4,[13][14][15][16] The primary chemical degradation in PEFCs is caused by reactions of reactive radicals with the PFSA ionomer membrane.17-21 For instance, hydrogen peroxide (H 2 O 2 ) can be generated via the twoelectron oxygen reduction reaction (ORR) resulting in hydroxyl radical (·OH) formation due to decomposition of hydrogen peroxide in the presence of metal contaminants. 22,23 The hydroxyl radical formed is highly reactive and can attack the ionomer membrane in terms of side chain cleavage and unzipping 18,20,21,24,25 which is responsible for the deteriorated physicochemical properties observed in degraded membranes. Radical scavenging is therefore proposed to mitigate the chemical damage by quenching the radicals before they attack t...

Holistic approach to chemical degradation of Nafion membranes in fuel cells: modelling and predictions

“…Simulations show that more than 99% of the cerium ions reside in the trivalent state and hence, are active for quenching OH, the most destructive oxygen species. This is well in line with the modelling results by Gubler et al 101 and Wong and Kjeang, 45,103 who also found that the dominant oxidation state in the cerium system is +III.…”

“…When ceria is incorporated into the membrane, it dissolves under operating conditions and produces both trivalent and tetravalent cerium ions. 45,99 A detailed scheme describing how CeO 2 mitigates membrane degradation by scavenging ROS was presented by Prabhakaran et al 47 The most stable oxidation states +III and +IV form a redox couple (E 0 (Ce 3+ /Ce 4+ ) = À1.44 V Fig. 13 The calculated fluoride emission rate (averaged over 100 h) is plotted as function of t 99 .…”

“…101 Table 7 gives an overview of the kinetic data for the reactions involving Ce x+ species. 45,83,[95][96][97][98]102 Wong and Kjeang 45 estimated the same activation energy for reactions R 24 -R 27 in their model to describe the membrane degradation in ceriasupported fuel cells.…”

“…42 Under operating conditions, Ce 4+ is also reduced to Ce 3+ electrochemically. 45 Without doubt, these reactions influence the equilibrium of Ce 3+ and Ce 4+ and hence, also the radical scavenging.…”

“…This powerful model allows monitoring and prediction of experimentally accessible data like time traces of chemical species involved 33 and fluoride emission rates (FERs). [35][36][37] Furthermore, we discuss the implications of iron containing ORR catalysts [38][39][40] for membrane degradation and the use of cerium agents [41][42][43][44][45][46][47] as radical scavengers to protect the chemical integrity of the membrane.…”