Molten carbonate composition effects on carbon electro-oxidation at a solid anode interface

The electrochemical oxidation of graphite is investigated to determine the relative influence of the composition of a molten carbonate electrolyte on a solid anode. The binary eutectics of sodium, lithium and potassium carbonates were investigated as well as ternary mixtures including the eutectic and combinations of systematically varied lithium content. It was seen that the combination of cations included in the carbonate melt can influence the electrochemical performance of graphite with binary combinations performing better than ternary in all cases. Very little change in the mechanistic behavior was observed through tafel analysis; instead it is proposed that the observed kinetic effects are a combination of the catalytic role of all metal cations present as well as the intercalation of lithium into the solid graphite electrode. The effect of lithium intercalation is suggested to lead to a change in the graphite surface polarity which affects product gas bubble formation and dislodgement at the solid anode. Increased lithium concentration, and it is assumed intercalation, encourages smaller bubbles to form which are dislodged at a faster rate than surfaces with less lithium intercalation in the ternary electrolyte. A threshold lithium concentration is reached above approximately 65 % where the electrochemical behavior is significantly changed and bubble formation behavior is no longer observed.