Determination of Efficiency in Direct Carbon Fuel Cells

Expressions for total efficiency of a carbon fuel cell reduce to a simple product of voltage efficiency (V(i)/V°) and carbon efficiency (defined as n/4, where n is the number of equivalents of charge associated with reaction of one mole of carbon). Examination of steady state polarization curves led us to propose that separate reactions predominate in two distinct regions of the curve: there is a low potential segment in which rate is proportional to total pore area, and a high potential segment where rate is a function of external flow conditions but not internal area. We make use of the Yongdan Li model for 4-electron net anodic oxidation of carbon to interpret the shift between the two segments as accompanied by shift in location of the reaction from the interior pores to the external surface of the anode, where n = 4. Total efficiency may be maximized using anodes fabricated from atomically disordered carbon by confining current to the exterior surface by means of filling the pores with inert material.