Analysis of the carbon anode in direct carbon conversion fuel cells

The total electrochemical efficiency of a direct carbon fuel cell with molten carbonate electrolyte is dominated by the product of coulombic efficiency (electron yield (n) per carbon atom, divided by 4) and voltaic efficiency (ratio of cell voltage to theoretical voltage). The voltaic efficiency is acceptably high (70–80%) for many atomically-disordered carbon materials. High coulombic efficiency is more difficult to achieve but ranges from below 50% at low current densities in porous material to 100% in certain monolithic and particulate carbon anodes at high current densities where substantially pure CO2 is the product gas. We find evidence for two competing anode reactions associated with distinct low- and high polarization segments, respectively: (1) a charge-transfer controlled, linear–polarization reaction occurring predominately within pores, proportional to specific area, and tending toward low efficiency by co-production of CO and CO2; and (2) a flow-dependent reaction occurring on the exterior surface of the anode, requiring > 100 mV polarization and tending to produce CO2. Based on this interpretation, high electrochemical efficiency of a carbon fuel cell is expected with anodes made of atomically disordered ("turbostratic") carbon that have negligible porosity, or with anodes of disordered carbon for which interior pores are intentionally blocked with an impervious solid material, such as an inert salt or readily carbonized pitch.