Investigation of the cathode polarization and carbon deposition in a molten carbonate direct carbon fuel cell

Molten carbonate direct carbon fuel cells (MC-DCFCs) are among the most promising devices for high-efficiency energy conversion and clean power generation from coal. Many studies have focused on the anode performance, while little attention was paid to the cathode. In the present study, we comprehensively investigate the cathode polarization performance, revealing the reactions taken place and the impact of several important operation parameters on reaction rate. The results show that the reduction of inputted gases is the primary reaction at moderate cathodic polarization of a gold electrode, which could benefit from incorporation of an optimal O2/CO2 molar ratio of 1/2, increase of the gas flux or reaction temperature to a certain extent. Interestingly, with a sufficient overpotential applied, an unexpected carbon deposition phenomenon on the cathode is observed. To give an insight into this reaction, several material characterization techniques and electrochemical tests are conducted to analyze the composition and formation conditions of the deposit, respectively. It shows that the carbon deposition is resulted from the reduction of carbonate ions in the electrolyte, which occurs when the cathode potential reaches a critical value of −1.5 V, corresponding to a current density of −32 mA cm−2. To avoid the contamination of the cathode surface by the carbon deposition when operating an MC-DCFC, a feasible strategy is using a sufficiently large cathode to keep the cathodic current density as well as overpotential below the critical value.