Role of Transport Phenomena in the Direct Oxidation of Solid Fuels

Direct carbon fuel cells have shown promise for stationary power generation by utilizing the direct oxidation of a solid carbon fuel source at the anode. In laboratory settings, researchers have reported up to 300mA/cm2 of current density from these cells types which suffer from mass transport losses. This paper studies the surface properties of the solid fuel source, and describes the process of CO2 evolution through an analogy to pool boiling. In nucleate boiling (a subset of pool boiling) vapor bubbles grow from nucleation sites where gas are trapped in micro-cavities on the surface. Carbon surfaces possess these same features, entrapping gases when they are immersed in a liquid electrolyte. The geometric shape and departure radius of these bubbles are shown to be different depending on the surface wetting characteristics. A force balance is performed to equilibrate the buoyant and surface tension forces as a function of contact angle, showing that CO2 gas bubbles requires significant volume to depart the carbon surface, a problem which robs the fuel source of electrochemical surface area. To confirm these findings, ANSYS FLUENT is utilized to show the growth rate and shape of CO2 bubbles on the surface of carbon. The results show that carbon particles inside the anode of a direct carbon fuel source can be encircled by a film of CO2 gas rendering them inactive.