Carbon Surface Characteristics after Electrochemical Oxidation in a Direct Carbon Fuel Cell Using a Single Carbon Pellet and Molten Carbonates

This study investigates carbon surface characteristics after discharge in a direct carbon fuel cell using molten carbonates. In the experiment, the top surface of a single carbon pellet is pressed on the Au electrode submerged in molten carbonates and discharge is started at a constant current. First, the effect of the press force of the carbon pellet on voltage was investigated. With a decreasing press force, voltage reduction was observed at a high current, whereas the voltage remained constant at a low current. Then, after the pellet was washed, its top and side surfaces after discharge were observed and analyzed. In Auger electron spectroscopy spectra, Na peaks were not detected on the carbon surface after discharge, indicating that the washing of the pellet was sufficient to remove the carbonates. On the top surface that was in contact with the electrode, surface roughness parameters decreased after discharge, indicating that the surface became smooth after discharge. In addition, changes in surface Morphologies were observed after discharge; smooth hemispherical particles were observed after discharge, whereas angular particles and edges were observed before discharge. Moreover, cleavages were observed after discharge. Meanwhile, on the side surface that was not in contact with the electrode, the only observed changes were thermal cracks caused by exposure to high-temperature molten carbonates. The results suggest that hemispherical particles were formed after discharge because of the combination of press force and electrochemical oxidation on the flat Au electrode, which caused edges to be smoothed during discharge. Observations indicated that the triple-phase boundary (carbon, electrolyte, and anode) played an important role in changing carbon surface morphologies during discharge.