Basic Performance of Tubular Molten Carbonate-Type Direct Carbon Fuel Cells

Abstract

In order to resolve energy issues, novel technologies for more effective utilization of carbon resources such as biomass and organic waste are needed. Direct carbon fuel cells (DCFCs) are fuel cells utilizing solid carbon as fuel. The total reaction can be expressed as follows. C(s) + O2(g) = CO2(g) (1)DCFCs as well as typical fuel cells are expected to achieve relatively high efficiency even on a small scale. Due to utilizing solid fuel, storage and transportation of fuel are easy. However, conventional DCFCs had two big problems such as the way to continuously supply solid fuel and low cell performance. We have proposed novel tubular molten-carbonate type direct carbon fuel cells (TMC-DCFCs) [1] to resolve those problems. This study investigated basic performance of TMC-DCFCs. We have already developed tubular molten carbonate-type fuel cells (TMCFCs) with comparatively high robustness and good durability against impurities [2,3], and the manufacturing method of them were applied for TMC-DCFCs. Each cell component of TMC-DCFCs was almost same with typical MCFCs. Cathode was NiO-3%MgO, electrolyte matrix was LiAlO2, anode was Ni-2%AlCr and they were formed by slurry coating methods. Electrolyte was 60%Li2CO3-40%Na2CO3 molten carbonate. Activated carbon powders (MD: 43 µm) or carbonized wood powders (MD: 31 µm) was used as solid carbon fuel. A tubular cell was inserted into the mixture of solid carbon fuel and molten carbonate same with electrolyte in the weight ratio of 80/20. In this paper, we refer to the mixing ratio between solid carbon and molten carbonate as “the carbon/carbonate ratio”. Carbonized wood powders were sawmill residues from conifer carbonized at 623 K and included 50.3% of volatile matter, 46.3% of fixed carbon and 3.4% of ash. Therefore, the net fixed carbon/carbonate ratio was 65/35 when using carbonized wood powders. Single cell tests of continuous power generation were conducted for 2 hours and about 0.72 V of cell voltages with 230 mA cm-2 of current densities, which corresponded to over 160 mW cm-2 of power densities, were achieved at 1073 K in both cases using activated carbon and carbonized wood (Fig. 1). TMC-DCFCs had high cell performance and could sufficiently utilize carbonized wood powders as fuel. These results suggested that the change of the carbon/carbonate ratio was permissible to some extent and a small amount of ash hardly affected the cell performance. On the other hand, it is presumed that the contact between the anode and solid carbon might be poor when the carbon/carbonate ratio is too large or the flooding might occur when the carbon/carbonate ratio is too small. Hence, effects of the mixing ratio and ash contents should be investigated in more detail. Furthermore, the stack test using two cells connected in series was conducted at 973 K. The container filled with the fuel mixture was separated into two sections by the alumina board to be electrically insulated. Results of the stack test will be reported at our presentation.