Contamination Mechanisms of Proton Exchange Membrane Fuel Cells - Mass Transfer Overpotential Origin
Authors: Bethune Keith , St-Pierre Jean , LaManna Jacob M , Hussey Daniel S , Jacobson David L 
Published: December 2020

Proton exchange membrane fuel cells (PEMFCs), fed with oxygen from ambient air and hydrogen, spontaneously produce power. However, key materials, gas diffusion layers, catalysts, ionomers, and membranes, are sensitive to a multitude of other species that may be inadvertently introduced into PEMFCs even at parts per million (ppm) or less levels. For a commercially relevant catalyst loading of 0.1 mg Pt cm–2 and a severe case such as 100 ppm acetylene in air, the temporary cell voltage loss exceeds 80 % [1]. Exposure to species such as bromomethane is even more problematic because the cell voltage cannot be restored without a specific recovery procedure [2]. The development of preventive and recovery procedures to improve durability is facilitated by contamination mechanisms. Although mass transfer losses were often observed in the presence of organic species [1], the commonest contaminants either in air or released by system components [3], the origin of this effect is currently unclear. It may stem from adsorption on carbon [4] or platinum [5] respectively modifying surface hydrophobicity and liquid water management or reducing the catalyst active surface, which increases the real current density.
Several diagnostic techniques able to either characterize water management or oxygen mass transfer were used to evaluate PEMFCs contaminated with propene or methyl methacrylate. Neutron radiographs were obtained for both in-plane and through-plane cell configurations to record changes in liquid water distribution [6,7]. Polarization curves were measured with O2, 21 % O2 in He, and air to derive concentration, molecular diffusion and ionomer permeability mass transfer overpotentials [8]. Overall oxygen mass transfer coefficients were acquired by first operating PEMFCs fed with 1 % to 7 % O2 in He, N2 or CO2 streams under limiting current conditions. Subsequently, a mathematical model was used to fit average limiting current data as a function of oxidant flow rate to extract the overall oxygen mass transfer coefficient. Separation of the overall mass transfer coefficient into molecular and Knudsen diffusion, and ionomer permeability contributions was conducted by plotting mass transfer resistance data (inverse of the mass transfer coefficient) either as a function of the O2 diluent molecular weight or O2 concentration and extrapolating linear correlations to the origin. The original separation method [9] was improved by taking advantage of the insensitivity of Knudsen diffusion to variations in O2 concentration. Propene and methyl methacrylate were selected because they have largely different solubility in water (surrogate hydrophobicity measure), minimize test durations and simplify test procedures (fast contamination and recovery kinetics), and facilitate data interpretation (minimal current distribution and effective concentration changes).
Experimental data demonstrated that changes in liquid water distributions were subtle and cannot explain mass transfer losses induced by contaminants. In contrast, ionomer permeability overpotentials and mass transfer coefficients revealed major increases in the presence of both contaminants. These observations support the notion that propene and methyl methacrylate contaminants adsorb on the Pt catalyst, decrease the active area, and bring the real current density closer to the limiting value. Experimental data analyses based on a contamination model that include catalyst surface processes for both O2 and contaminant (adsorption, reaction, desorption) [10] confirmed this hypothesis. The proposed contamination mechanism is similar to the increase in oxygen mass transfer overpotential induced by a decrease in Pt catalyst loading [11].
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