Development of Electrochemically-Driven CO2 Separator for Hydroxide Exchange Membrane Fuel Cells in Transportation Applications

Abstract

Hydrogen fuel cell vehicles are a promising, emerging alternative to the internal combustion engine but the commercialized proton exchange membrane fuel cells (PEMFCs) are comparatively expensive. Hydroxide exchange membrane fuel cells (HEMFCs) are a potentially lower cost hydrogen fuel cell technology under development; unfortunately, ambient levels of CO2 in air reduce HEMFC performance and impede HEMFCs from becoming competitive with the incumbent fuel cell technology.1 Current levels of atmospheric CO2 have been shown to decrease fuel cell performance up to 200 mV.2 The hydroxide produced during the electrochemical reaction in a HEMFC reacts readily with CO2 in air at the cathode due to its acid-base reaction and transports CO2 across the hydroxide exchange membrane. At the anode, the bicarbonates build up lowering the local pH until CO2 evolution is favorable, concurrently the pH gradient saps cell voltage. However, this transport results in the effective separation of CO2 from air. By optimizing a HEMFC for CO2 capture, rather than power production, an electrochemically-driven CO2 separator (EDCS) was developed using a poly(aryl piperidinium) membrane.3 The continuous and compact EDCS has the potential to enable HEMFCs to operate efficiently with ambient air in automotive applications.This work will demonstrate the ability of the EDCS to effectively and continuously remove CO2 from air at ambient levels using minimal hydrogen flow to power the separation. This work will explore the effect of operating conditions such as flow rates, current density, and relative humidity on CO2 separation performance with the innovative EDCS. A carbon-ionomer interlayer between the catalyst layer and membrane was added to improve CO2 capture by creating an accessible volume for hydroxide and CO2 gas to react. Additionally, various design features of the cell were investigated to improve the mass transport such as flow fields and gas diffusion layers. The CO2 removal performance of a single-cell 25 cm2 EDCS is shown in Figure 1. Currently, the optimal design configuration of the EDCS has achieved 98% CO2 removal for 100 hours (Figure 2) with the potential to improve CO2 removal as well as processed gas throughput by further development of the module’s design.