Modeling the cation transport in an operating polymer electrolyte fuel cell (PEFC)

Abstract

A computational fluid dynamics model is developed to investigate the multicomponent cation transport in polymer electrolyte membranes and to predict the performance degradation of the polymer electrolyte fuel cell (PEFC) due to the cationic contamination. A Maxwell–Stefan approach is implemented by modifying the Nernst–Planck equations to model the multicomponent cationic species transport in the membrane. Langmuir isotherms are used to model the non-ideal species adsorption in the membrane. Cation transport model shows good agreement with the experimental data found in the literature. Following the validation of the cation transport model, it is incorporated in a PEFC model framework, which solves for mass, momentum, species and charge conservations. Both fuel side and air side impurities are considered in analyses, which show that for air side impurities current density drops from 0.9 to 0.7 A/cm² whereas for fuel side impurities current density drops to impractical values as low as 0.05 A/cm², at the steady-state. Effect of cationic impurity on water transport in the membrane is also investigated and found that cathode dry-out occurs due to decreased water generation in case of fuel side contamination.

Introduction

Owing to their environment friendly operation with low pollutant levels, quiet operation and the potential to reduce fossil fuel dependency, PEFCs are soaring as a very promising candidate for an alternative power generation method amidst the discussions of renewable energy and global warming.

Although PEFCs are becoming commercially available, the improvement of the technology relies on overcoming certain issues. Other than relatively high cost, reliability and durability of PEFCs still remain among the biggest hurdles. Reliability requires sustaining a robust operation and having a stable electrochemical performance as well as a structural integrity without mechanical or chemical deterioration. However, a PEFC may be required to operate in hazardous environments that may cause chemical, electrochemical or mechanical degradation sometimes to the extent of the complete failure of the cell.

For example, PEFCs employed in transportation applications rely on ambient air, which can be rich of salt suspensions. Such an environment is expected to have an impact on the cell performance as the fuel cell does not use pure air but that is contaminated with sea salt containing NaCl. Other than salt particles suspended in the air, a broad range of impurities can be found in the PEFC air and fuel streams as a result of residuals of the fuel reforming process and metal particles due to the corrosion in the stack and balance of plant materials. These impurities are then adsorbed in the ionomer in cationic forms. Cationic contamination of PEFCs may cause serious degradation in cell performance, which sometimes can be recoverable depending on the operating conditions and the exposure time, but many times it is irreversible [1], [2].

There have been efforts in characterizing the effects of cationic contaminants in PEFC operation [3], [4], [5], [6], as well as characterizing the properties of the cations in the membrane [7], [8], [9], [10], [11], [12], [13]. However, there is a little work in the literature describing the performance degradation mechanisms of the cationic contaminants. The first attempts are by Okada who developed a simple model to outline the effects of contaminants on the water transport in the membrane for both anode side [14] and cathode side [15] impurities. These studies neglect the transport of the cations and assume an infected zone in the membrane where a prescribed profile (either linear or constant) for cations is considered. Sodaye et al. [16] used Maxwell–Stefan approach to model the cationic exchange in a polymer electrolyte membrane in the presence of two different foreign cations. They only focus on the competitive transport of different cations in the membrane and do not consider fuel cell operation. Kienitz et al. developed one dimensional models for a fuel cell membrane [17] and a half cell [18] and predicted the cation concentration profiles in the studied domains. Their model assumes constant current density and does not provide an expression to describe the reaction kinetics; hence it does not relate the effects of cations on the performance loss substantially. Greszler et al. [19] developed a model for a hydrogen pump to investigate the effects of local proton concentration on the reversible potential of the reaction. Their study is significant as they put forward the thermodynamic aspects of the effects of contamination the first time. The most rigorous study up to now is provided by Weber and Delacourt [20] who used concentrated solution theory to model the interactions between each cation and water while they neglect the interactions between two cation pairs in a polymer electrolyte membrane. The model is utilized to predict the operation of a hydrogen pump with cationic impurities.

Albeit their individual strengths in explaining the cation transport in the polymer electrolyte membrane, all of the models cited above have some simplifications and none of them considers a complete fuel cell with both anode and cathode reactions. None of these models incorporate the coupled transport phenomena related to oxygen, hydrogen and water either. Since the transport properties of the contaminants depend on fuel cell operational variables such as water content and temperature, a comprehensive model requires a good representation of the transport phenomena in the full cell operation. All of the above studies are one dimensional only. In order to have an accurate representation of the fuel cell operation two-dimensional effects need to be accounted for, since 1-D models are not capable of predicting the water transport inside a PEFC. Due to the water accumulation under the bipolar plate rib area, there is a significant water concentration gradient in the in-plane direction (i.e. channel to the current collector) in addition to the through-plane direction (i.e. from anode to cathode). The gradient along the flow direction can be neglected if the fuel flow rates are high enough which minimizes the variations in the flow direction.

In this study we present a modeling framework to elucidate the cation transport in the polymer electrolyte membrane as well as to determine the mechanisms leading to performance degradation of a PEFC. We present the equations to describe the cation transport and validate them with the available experimental data. We then incorporate these equations into our fuel cell model, which solves for the transport phenomena pertaining to the fuel cell operation that impact the cation transport significantly. With our fuel cell model, we explain how the foreign cations affect the performance degradation of an operating fuel cell.

In this study we present the following for the first time:

• Cation transport model is incorporated in a complete fuel cell model with catalyst layers (CLs) and gas diffusion layers (GDLs); a complete representation of the fuel cell transport phenomena is aimed so that the effects of fuel cell variables on the cation transfer is accounted more accurately.

• Cation uptake in the catalyst layers; competitive site occupation between the protons and the foreign cations in the catalyst layers is represented in the model.

• Two-dimensional effects in description of the cation transfer; we investigate the effects of water accumulation under the current collector rib.