Abstract
Ion exchange membranes are widely used in fuel cells to physically separate two electrodes and functionally conduct charge-carrier ions, such as anion exchange membranes and cation exchange membranes. The physiochemical characteristics of ion exchange membranes can affect the ion transport processes through the membrane and thus the fuel cell performance. This work aims to understand the ion transport characteristics through different types of ion exchange membrane in direct formate fuel cells. A one-dimensional model is developed and applied to predict the polarization curves, concentration distributions of reactants/products, distributions of three potentials (electric potential, electrolyte potential, and electrode potential) and the local current density in direct formate fuel cells. The effects of the membrane type and membrane thickness on the ion transport process and thus fuel cell performance are numerically investigated. In addition, particular attention is paid to the effect of the anion-cation conducting ratio of the membrane, i.e., the ratio of the anionic current to the cationic current through the membrane, on the fuel cell performance. The modeling results show that, when using an anion exchange membrane, both formate and hydroxide concentrations in the anode catalyst layer are higher than those achieved by using a cation exchange membrane. Although a thicker membrane better alleviates the fuel crossover phenomenon, increasing the membrane thickness will increase the ohmic loss, due to the enlarged ion-transport distance through the membrane. It is further found that increasing the anion-cation conducting ratio will upgrade the fuel cell performance via three mechanisms: (i) providing a higher ionic conductivity and thus reducing the ohmic loss; (ii) enabling more OH− ions to transport from the cathode to the anode and thus increasing the OH− concentration in the anode catalyst layer; and (iii) accumulating more cations in the anode and thus enhancing the formate-ion migration to the anode catalyst layer for the anodic reaction.
Highlights
Fuel cells that can convert the chemical energy stored in fuels into electricity are promising power devices
The membrane type determines which type of charge-carrier ions to transport between two electrodes: anions (OH− ions) for AEMs and cations (K+ ions) for CEMs (An and Chen, 2017)
When an AEM is employed in direct formate fuel cells (DFFCs), the OH− ions produced in the cathode will transport to the anode and the OH− concentration in the anode catalyst layer (ACL) is higher than that achieved by using the CEM
Summary
Fuel cells that can convert the chemical energy stored in fuels into electricity are promising power devices. Direct formate fuel cells (DFFCs) possess several important advantageous characteristics: (i) formate oxidation reaction (FOR) is facile in alkaline medium (Li and Zhao, 2011); DFFCs intrinsically exhibit a faster anode kinetics as compared to other types of direct liquid fuel cell; (ii) the theoretical voltage can reach as high as 1.45 V, which is 0.24 V higher than direct methanol fuel cells (Shukla et al, 2002), 0.31 V higher than direct ethanol fuel cells (Li, 2016) and 0.46 V higher than direct ethylene glycol fuel cells (An et al, 2010); (iii) formate can serve to store the energy that is collected from other alternative energy technologies during their productions, e.g., electrochemical productions using solar power and wind power, as well as photoelectrochemical production using solar energy (Vo et al, 2015); and (iv) formate can be completely oxidized into water and carbon dioxide, which results in a high electron transfer rate of 100%. The performance of AEM-DFFCs (41 mW cm−2 @ 40◦C and 106–267 mW cm−2 @ 60◦C with oxygen oxidant)
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