The Effect of CO2 Bubble Distribution on Power Generation Performance of a Direct Formic Acid Fuel Cell

Abstract

Introduction Recently, formic acid is expected as one of the energy career for renewable energy. Especially, direct formic acid fuel cells (DFAFCs) which use formic acid as fuel, have received considerable attention since they can generate higher power density than the other direct liquid fuel cells [1]. However, CO2 gas is generated in the anode reaction when DFAFCs are operating. If CO2 bubbles stay in the anode gas diffusion layer (GDL) the mass transport resistance in GDL increases and the power generation performance of DFAFCs decreases with the lapse of time. In this study, we have investigated the effect of CO2 bubble distribution on power generation performance by visualizing the CO2 bubble distribution in DFAFCs during power generation using X-ray CT. Experimental A single cell with 1 cm2 of active area was used. We used NR-212 as proton exchange membrane, PEM. Pd/C was used for anode catalyst and Pt/C was used for cathode catalyst. Moreover, two types of GDL with different structures of carbon paper and carbon cloth were used. The voltage sweep was performed at a sweep rate of -5 mV / sec until the cell voltage turned from the open circuit voltage to 0 V, and the cell voltage and current density during the sweep were recorded. A three-dimensional measurement X-ray CT apparatus (TDM-1000H-Ⅱ (2K), Yamato Scientific) was used for visualizing CO2 bubble distribution in DFAFCs. It took 15 minutes to visualize and the spatial resolution was 2.7 μm. Results and discussion From the measurement of the power generation characteristics, it was found that the maximum current density is higher with carbon cloth GDL than with carbon paper GDL, despite the lower open circuit voltage. In addition, using carbon paper GDL, the gradient of polarization curve became steep in the high current density region. These phenomena are generally considered to be caused by mass transport, and in this experiment it is thought that this is probably due to the effect of bubbles in the anode GDL. Therefore, in order to investigate the influence of bubbles in the anode GDL, visualization was performed. Fig. 1 shows the CO2 bubble distribution in the anode GDL at high current density operation using each GDL. These figures are three-dimensional representation of the bubble distribution in the anode GDL averaged over 15 minutes of visualization time. Furthermore, these are top views seen from the vertically upward direction of the GDL plane, the white part indicates that bubbles exist. As shown in Fig. 1, using carbon paper GDL, bubbles were present under most of the rib and also present under the channel, which was thought to cause the power generation performance degradation due to the mass transport. On the other hand, with the carbon cloth GDL, there were almost no bubbles under the channel, and bubbles and voids were concentrated under the rib. It is considered that in the carbon cloth, the bubbles were concentrated in part and the discharge path of the bubbles was formed, so that the bubbles were efficiently discharged and the power generation performance did not decrease even in the high current density region. Reference 1) Y. Zhu, Z. Khan, R.I. Masel, Journal of Power Sources, 139, 15-20 (2005) Figure 1