Development of a Low Temperature, Molten Hydroxide Direct Carbon Fuel Cell

Abstract

The performance and characterization of a batch, direct carbon fuel cell (DCFC), employing molten hydroxide electrolytes, is presented. Particular attention was focused on reducing the operating temperature of the system to minimize corrosion of both fuel cell materials and fuel carbon. Constant power was achieved over a current density range of 37 to 92 mA/cm2 (200 to 500 mA) with NaOH electrolyte at 550 °C, and up to 170 mA/cm2 with a NaOH/KOH (54/46 mol %) eutectic. The voltage decreased with temperature, becoming unstable at ≤400 °C. Electrochemical impedance spectroscopy (EIS) measurements showed that charge-transfer resistance, Rp, is more temperature-dependent than the ohmic resistance, Ro, with Rp decreasing by 55% for the anode and 82% for the cathode with an increase in temperature from 370 °C to 500 °C. Stable operation was achieved at temperatures as low as 400 °C with the hydroxide eutectic electrolyte. This was attributed to the higher concentration of superoxide ions in the eutectic, as identified with cyclic voltammetry. Using a three-electrode system, it was found that the anode overpotential was significantly greater than that of the cathode at low temperatures during galvanostatic polarization.