Pore Designing for the Anode Porous Electrode of the Direct Formic Acid Fuel Cell
Authors: Tsujiguchi Takuya , Fujiwara Kakeru , Osaka Yugo , Kodama Akio , Yoshida Kouya 
Published: December 2024

Direct formic acid fuel cells (DFAFCs) have received much attention since the formic acid used as their fuel is considered to be one of the promising energy carrier. Although it can generate higher power density comparing to the direct methanol fuel cell, direct ethanol fuel cell and the other direct liquid fuel cell., not only the activation overpotential but also the mass transport overpotential in anode should be reduced to achieve the higher power density [1]. Comparing to the polymer electrolyte fuel cell (PEFC) using gas hydrogen as a fuel, the mass transport of the formic acid is much slower since it is a liquid which shows high viscosity and low diffusion coefficient. Moreover, CO2 is generated by the electrode reaction, and the generated CO2 inhibits a formic acid supplying to the catalyst layer since it should be excess to the channel thorough the catalyst layer and gas diffusion layer. Therefore, in order to improve the mass transport rate in anode porous layer, pore properties, such as pore diameter, porosity, shape, and amount of pore etc., should be appropriately designed to enhance the formic acid supplying and exhausting the CO2. In this context, the pore controlling the size and amount was designed and added to the conventional diffusion layer and catalyst layer of DFAFC in order to clarify the appropriate pore properties for the DFAFC anode porous layer in this study.
As shown in Figure, the 80-200 μm pore was generated to the carbon paper (CP) by the needle punching machine. For the catalyst layer, the polystyrene fiber (PSF) having 1-5 μm in diameter and 10-40 μm in length, and polystyrene beads (PB) having 1-10μm of diameter were used as a pore former. Catalyst coated membrane (CCM) was prepared by direct painting of Pd/C for anode and Pt/C for the cathode using ultrasonic spraying system [2]. Using CP with adding pore as a diffusion layers for anode, DFAFC was fabricated and tested under 5 M formic acid and 60 oC. For the pore added CL, the CCM containing PSF and PB were prepared by similar procedure to the CCM without pore former. After fabricating the CCM, MEA was dipped in to the ethyl acetate to remove the pore former and make the pore.
It was found that power density increased by adding the pore with enhancing the mass transport rate. In the case of the pore added to the diffusion layer, improvement of the power density by adding the pore was significant in the case using hydrophobic CP. Based on the result from the pore added to the CL, it was found that there were appropriate amount and pore size. From these results, it was found that the adding the pore to the porous electrode could contribute to improve the mass transport in the anode porous layer. Moreover, it was suggested that the improvement in the mass transport was given by two size and/or wide pore size distribution since the gas and liquid was transported separately through the different size of pore, i.e., the gas was easy to excess through a large pore due to a lower capillary force and liquid was easy to supply through a small pore due to a higher capillary force.
Acknowledgement
This study was supported by Kakenhi 22H01858, 24H00478, and NEDO JPNP2004.
Reference
[1] Tsujiguchi et al., Electrochimica Acta, 197, 32–38 (2016)
[2] Zainoodin, A. M.et al., Int. J. Hydrogen. Energy., 43(12), 6413–6420 (2018).

Figure 1