Alejandro Franco

Transient Multiscale Modeling of Aging Mechanisms in a PEFC Cathode

In this paper we propose a mechanistic model of the electrochemical processes in a polymer electrolyte fuel cell cathode, focusing on aging phenomena. The proposed model is based on a nonequilibrium thermodynamics approach previously developed by us, describing the transient response to current perturbations of an electrochemical double layer at the catalyst/Nafion-electrolyte interface. It describes the internal dynamics of the electrochemical double layer, taking into account the coupling between the transport of protons and Pt 2+ in the diffuse layer, as well as the carbon-supported Pt coarsening, the Pt oxidation, the oxygen reduction reaction, and the water dipoles adsorption in the inner layer. This continuous nanoscale interfacial model is coupled with a microscale model of the oxygen transport through the impregnated Nafion layer and is designed to be coupled with a continuous microscale model of electron, proton, and Pt 2+ transports through the membrane-cathode assembly thickness. The model allows analysis of cathodic potential sensitivity to the operating conditions, the initial Pt loading, and to the temporal evolution of the electrochemical activity from aging mechanisms. In particular, the influence of simulated time on the impedance spectra pattern is studied.

Diffuse Charge Effects in Fuel Cell Membranes

Netherlands. Materials Innovation Institute (M2i) (Strategic Research Programme project MC3.05236)

Mechanistic Investigations of NO2 Impact on ORR in PEM Fuel Cells: a Coupled Experimental and Multi-scale Modeling Approach

Atmospheric air is the most convenient oxidant for proton exchange membrane fuel cells (PEMFC) applications thanks to its permanent availability. Depending on the location, air quality can be variable and this can be a problem especially for transport applications which requires on board purification. Networks of air quality monitoring have been set up to analyze the main pollutants in urban, industrial and rural areas. Analysis of this data shows that NO2 is one of the main pollutants, with a maximum quantity of 0.3 ppm for heavy urban traffic [1-2]. While most of the studies concern the impact of hydrogen pollutant on the PEMFC performance, the effect of air impurities has not been much investigated. Concerning NO2, very few experimental studies showed that PEMFC performance decreases rapidly and significantly under several ppm levels [3-5]. It was also observed that the reversibility of the impact strongly depends on the pollution duration: it is reversible for short time (few hours) and becomes irreversible for longer durations. Several hypotheses were proposed to explain performance degradation. NO2 is believed not only to interact with the catalyst but also with the protonic groups of the membrane. It was assumed that reactions can involve several species such as HNO3, HNO2, NO2 and NO3. However, as the best of our knowledge, modeling of the detailed implicated reactions is quasi-inexistent in literature. Indeed, a mathematical model, based on the empirical Butler-Volmer approach was recently proposed by St-Pierre et al. to simulate the effect of NO2 contaminant in air [6]. In that the impact of the NO2 intermediates on the ORR (and thus on the PEMFC cathode potential) is neglected. Only the presence of NO adsorbates (coming from the NO2 dissociative adsorption) is assumed on the catalyst surface. But, in realistic conditions, a strong competition it is expected to occur between the adsorbates of the ORR and the NO2 intermediates. We have recently proposed a multiscale modeling framework of PEMFC electrochemical processes [7-12], to understand the detailed mechanisms underlying the MEA contamination and materials degradation under automotive operating conditions. This approach scales up ab initio data into interconnected elementary kinetic models accounting for: 1) the MEA physicochemistry at the spatial nanoscale (detailed HOR and ORR electrocatalysis, steric and electrochemical double layer effects...) and microscale (ionic and reactants transfers), 2) the intrinsic electrodes nanomaterials degradation (catalyst oxidation/dissolution, carbon support corrosion...) and PEM chemical aging. Based on new experimental results and our multi-scale modeling approach, this paper presents new insights on the the interaction mechanisms of NO2 with the ORR. The impact of some parameters such as temperature, NO2 concentration and current density was studied on single cells. As shown in Figure 1, the impact is reversible until at least 4 ppm. Performance decrease level and rate are more important when NO2 concentration increases. A very good stability of the performance was observed after the initial drop. Besides these expected results, an untypical phenomenon of “voltage wave” was observed. This wave arrives earlier and lasts shorter when NO2 concentration increases. Figure 2 shows the effect of the current density: the voltage wave is shorter and comes earlier when the current density increases. Come back to initial performance under pure air is also faster when current density is high. In particular, for intermediate current density value (0.6 A/cm2), the return to initial performances is characterized by two steps. The ”wave” behavior of the potential is explained in this paper on the basis of our ab intiio-based kinetic model, highlighting the importance of accounting for the competitive adsorption of ORR and NO2 intermediates as well as the surface reactivity between the different adsorbates.