Direct Formic Acid Fuel Cell: Anode Flow Field Designed Two-Phase Removal of Carbon Dioxide

Abstract

Direct Formic Acid Fuel Cells (DFAFC) are a potential substitute in portable power devices surpassing the capacity of conventional batteries in everyday items such as cell phones and laptops. The challenge of maintaining high liquid phase formic acid (FA, HCOOH) concentrations within the catalyst layer at high current densities is due to the gaseous by-product carbon dioxide formation (HCOOH→ CO2 +2H+ + 2e-). The two-phase FA/CO2 results in mass transport limitations. Previous studies have shown enhanced current density performance when the anode flow field is designed to flow the FA in a singular serpentine pattern[1], and the gaseous CO2 is removed through a semi-permeable separator near site of formation[2]. Fig.1 shows the current serpentine anode flow field’s geometry in ANSYS Fluent. The CO2 forms along the diffusion media//anode catalyst layer and disperses through the semi-permeable layer, after traversing the flow channel. The initial flow field prototype with a semi-permeable separator has issues with liquid FA break through. Improved sealing surface designs were investigated to improve the DFAFC’s efficiency. Mass transport of CO2 in the current semi-permeable anode flow field design results in a “barrier” of higher velocity in between the layers, Fig. 1. In addition, an eddy current appears at the point of entrance into the chamber, potentially re-circulating the CO2 bubbles and hindering removal. Further modifications of the anode flow field, based on the present Computational Fluid Dynamics (CFD) model, are underway to eliminate the barrier and remove the eddy. The objects of the CFD modeling is to realize needed flow distribution improvements on future anode flow field, resulting in a higher rate of CO2 mass transport through the semi-permeable layer. Experimental validation of prototypes will be performed on the resulting model driven design optimizations. [1] Limjeerajarus, Nuttapol, and Patcharawat Charoen-Amornkitt. "Effect of Different Flow Field Designs and Number of Channels on Performance of a Small PEFC." International Journal of Hydrogen Energy, 2015, 40, 7144. [2] S. Saeed, A. Pistono, J. Cisco, C. S. Burke, M. Mench, C. Rice, Fuel Cells, 2017, 17, 48. Figure 1