Electrochemical Pumping for Carbon Dioxide Removal in Automotive Hydroxide Exchange Membrane Fuel Cell Systems

Abstract

Hydroxide exchange membrane fuel cells (HEMFCs) suffer unacceptable efficiency losses when exposed to the carbon dioxide in ambient air1. Despite rapid progress on power density and stability, the need to remove carbon dioxide from the air feed of the fuel cell has been a major roadblock for HEMFCs, especially in space-constrained automotive systems. Conventional sorbent-based carbon dioxide removal systems are complex and bulky, with multiple beds and careful thermal integration required to operate continuously. Herein, we propose the electrochemical carbon dioxide pump2,3 (ECP) as a compact, efficient, and highly effective method for carbon dioxide removal in HEMFC systems (Figure 1). Model-based projections indicate than an optimized ECP could remove 99.9% of CO2 from air in an active area half as large as the HEMFC it feeds, powered by a continuous 2% purge of hydrogen from the HEMFC anode. The module would contribute zero parasitic electrical load, add <15 kPa pressure drop, and be packaged efficiently to further minimize volume. The cost contribution to a 100-kW automotive HEMFC system would be less than $2 kW-1 at large scale. The ECP operates as a H2-air HEMFC at low current density (e.g. 20 mA cm-2). Given the low concentration of CO2, even a low current density generates a large excess of hydroxide anions. When in excess, hydroxide reacts rapidly with CO2 to produce carbonate and bicarbonate anions, which are electrochemically transported to the anode. At the anode, bicarbonate accumulates, lowering the pH and triggering its own decomposition. In this way, steady-state CO2 transport is rapidly established, with performance usually controlled by CO2 diffusion and reaction with hydroxide. The performance of a single-cell 25 cm2 ECP is shown in Figure 2. With the first-order processes of CO2 diffusion and chemical reaction with hydroxide as the rate-limiting steps, the cell shows an exponential relationship between residual CO2 and the inverse of the air flow rate. As a consequence of this behavior, the ECP area required for a given rated flow is proportional to the logarithm of residual CO2 level, and very high removal fractions are possible. In addition to results and theory, a technoeconomic roadmap for ECPs in automotive HEMFC systems will be presented. Without optimization, an ECP could add the cost and bulk of a second electrochemical stack. However, such a design would deliver electrochemical performance in the ECP that is orders of magnitude greater than necessary to remove only 400 ppm of CO2. The true potential of the ECP is realized with a distinct architecture that sacrifices unnecessary electrochemical performance for rapid CO2 transport, compact packaging, and low cost. References Ziv, W. E. Mustain, and D. R. Dekel, ChemSusChem, 11, 1136–1150 (2018).Winnick, Adv. Electrochem. Sci. Eng., 1, 205–248 (1990).A. Rigdon et al., J. Electrochem. Energy Convers. Storage, 14, 020701 (2017). Figure 1