(Invited) Modeling of Carbon Dioxide Exposure and Mitigation in Hydroxide Exchange Membrane Fuel Cells

Abstract

Despite rapid advancements in performance and durability of hydroxide exchange membrane fuel cells (HEMFCs), the performance loss caused by carbon dioxide in air is unacceptable for transportation applications1. In this paper an expanded model2 of carbon-dioxide-induced performance loss will be presented. The model predicts performance loss under dynamic and steady-state conditions, considering effects of temperature, carbon dioxide concentration, fuel and air stoichiometry, anode recycle, and down-the-channel concentration gradients. There are two major causes of performance loss upon CO2 exposure: lowered anode pH and reduced ionic conductivity. CO2 reacts with hydroxide to form carbonate and bicarbonate, which are pushed to the anode by the ionic potential gradient. At the anode, these anions accumulate, lowering the pH. At a sufficiently low anode pH, bicarbonate decomposes, establishing a steady state. The pH gradient shifts the half-cell potentials of the anode and cathode reactions, which reduces the cell voltage by an amount that depends on the transference number of hydroxide, but can be approximated as 70 mV per pH unit over most of the pH range. Accumulation of bicarbonate also causes an increase in Ohmic losses due to reduced ionic conductivity, but this effect is generally secondary to the concentration polarization caused by the pH gradient. The anode pH is determined primarily by the kinetics or thermodynamics of bicarbonate decomposition, which in practice, means that even very high values of ionomer conductivity cannot mitigate the performance loss under CO2. Figure 1 shows the modeled performance loss caused by 400 ppm CO2 in air for HEMFCs with ionomer hydroxide conductivity values of 5-40 S/m. Except at very low current density, increasing hydroxide conductivity does not render the HEMFC less sensitive to CO2. Figure 2 shows the transient performance loss caused by CO2 uptake at 1 A/cm2 and a range of CO2 concentrations from 0.1 to 10 ppm. At 1 ppm, performance drops by 10 mV in the first 0.5 h and 30 mV at steady state. Even at 0.1 ppm, the modeled performance loss is 10 mV at steady state. These simulations provide a first indication of the level of CO2 removal required for systems operating on air and should be supplemented with experimental measurements. The model results lead to the disappointing conclusion that a CO2-tolerant HEMFC cell is unlikely. However, modeling also supports the viability of CO2 removal using electrochemical pumping, which could be significantly more compact and less costly than conventional scrubbing. Proof-of-concept experiments will be presented, and future prospects will be discussed. References S. Gottesfeld et al., J. Power Sources, 375, 170–184 (2018).B. P. Setzler and Y. Yan, Meet. Abstr. , MA2017-01, 1654 (2017). Figure 1