An Isothermal Model for Predicting Performance Loss in PEMFCs from Balance of Plant Leachates

Abstract

A model is presented for the contamination of a proton exchange membrane fuel cell (PEMFC), including adsorption onto the Pt catalyst, absorption into the membrane, and ion exchange with ionomeric components. Model predictions for three sources of voltage loss account for the two-dimensional, time-dependent contamination along the channel and into the membrane. The model is developed by considering the well-known concepts of Langmuir adsorption, partition coefficients, plug flow reactors (PFRs), and dimensionless analysis. The phenomena are shown to be controlled by three important dimensionless groups: a Damköhler number for the contamination reaction rate, a capacity ratio, and a coverage ratio for each contamination mechanism. These groups show how to scale ex situ equilibrium data for in situ predictions. The model predictions are shown to be reasonable when compared to in situ experiment data once ex situ data are used to provide reaction and equilibrium parameters. The predictions enable estimation of tolerance limits for leachates according to each mechanism. For typical parameters, the predicted voltage loss in the electrode ionomer by an ion-exchange mechanism shows slower reaction rates but greater performance losses than other mechanisms. Analysis of the potential for commercialization of proton exchange membrane fuel cells (PEMFCs) shows that the cost of balance of plant (BOP) materials can be as much as 30% of the system cost.1 One opportunity to decrease these costs is to use off-the-shelf materials rather than custom-made materials, if leachates from less expensive materials would not affect performance and life-time.2 This loss in performance is related to the contamination of the membrane 3 as well as the electrodes. Many previous studies for contamination [3][4][5][6][7][8][9][10][11][12][13] in PEMFCs have shown the sensitivity of performance to low levels of contamination. For example, cation leachates from gaskets or seals, from NH 3 in the fuel, or from catalyst metals 3-9 are well known contaminants of the membrane through an ion-exchange mechanism. Catalyst contamination by CO through an adsorption mechanism from feed gas contaminants is so well-known that those studies are too numerous to list here. Examples are also known for SO 2 , 10,11 for H 2 S, 12 and even for details of the reverse water-gas shift reaction of CO 2 . 13The fundamental mechanism of contamination from organic contaminants such as aniline, ε-caprolactam, and diaminotoluene (DAT), [14][15][16][17][18] which have been identified as possible system contaminants, have been studied only briefly and recently, and it has been shown that their impact on PEMFC performance depends on their functional groups. For example, the ion-exchange reaction of the amine group in aniline with the Nafion membrane/ionomer causes a decrease in the number of available proton sites, thereby hindering the transport of protons and increasing ohmic losses. Aniline is also adsorbed on the carbonsupported platinum catalyst through interactions betw...

Effect of System Contaminants on the Performance of a Proton Exchange Membrane Fuel Cell

“…These results have the same trends as ex-situ adsorption isotherm of 2,6-DAT reported by Cho et al 38 The maximum value for θ 1 reported in Figure 7 Figure 9. The rate of ionomer poisoning during contamination γ ic with 2,6-DAT.…”

The Contamination Behavior of System Derived Organic Model Compounds on PEMFC Based upon Functional Groups

“…These parameters can be used for the prediction of performance loss in PEMFCs using the previously developed model. 53,83 Interestingly, the maximum coverages for aniline, benzene, and DATs (i.e., 0.55-0.65) were equal within the accuracy of these data. Note that as described in Ref.…”

“…Note that as described in Ref. 83, the coverage (θ) and equilibrium concentration (K eq ) of contaminant A at the interface (C * A ) between the gas diffusion layer and the catalyst layer can be expressed by the Langmuir adsorption isotherm as below: where B is the maximum coverage measured by ex-situ isotherms; C A * is the concentration of contaminant A at the midpoint of the electrode [mol/cm 3 ]; C AS is the average concentration of the adsorbed contaminant A on the Pt sites [mol/cm 2 ]; C s sis the surface concentration of Pt sites [mol/cm 2 ]; and K eq is the equilibrium constant for the Langmuir isotherm [cm 3 /mol].…”

“…Predicting the voltage loss.-Next, to assess and develop a prediction model 83 The assumption of no OCV change by contamination can be justified for a low concentration of contaminants. The resistance is related to the membrane conductivity (κ), which can be determined from the ex-situ membrane conductivity results shown in Figure 4.…”

“…The method of the scale was introduced in previous studies. 53,83 Furthermore a comparison for properties of different Nafion membranes (NRE 211, N115, and N117) that are used in this study is listed in Appendix. Two samples of each cation solution were prepared to check reproducibility.…”