Recent insights concerning DCFC development: 1998–2012

Abstract

We present an overview of recent developments of the Direct Carbon Fuel Cell (DCFC) cell and system technology which we believe are key to the worldwide renewal of interest in the DCFC during the last ten years. The importance of understanding and exploiting the co-production of CO and CO2 are examined. A distinction must be made between, on the one hand, the tendency toward chemical and electrochemical equilibrium and on the other hand the complex effects of chemical and electrochemical inhibition. The tendency toward equilibrium may be very active in the DCFC anode, resulting in high CO/CO2 ratios at high temperature and/or at low current density, consistent with the Boudouard equilibrium. The complex inhibitive effects tend to produce predominantly CO2 at moderate temperature and moderate current density. If the DCFC anode is allowed to come close to equilibrium, electrochemical production of CO may result. It is accompanied by a large increase in entropy compensated by absorption of thermal energy. This approach to equilibrium may be desirable, for example, in energy conversion systems where the absorption of thermal energy can be ensured via solar collectors. In that case, the product CO may be electrochemically converted or used for chemical or heating value. Such systems can reach an efficiency of greater than 80%. On the other hand, by inhibiting the Boudouard equilibrium either within the reaction mechanism or in the gas product in contact with carbon, it is possible to promote, even at relatively high temperature (700–750 °C), the 4-electron conversion of carbon to CO2, resulting in very high conversion efficiency (70–80%). Recent work has pinpointed the conditions under which a DCFC operating at high Coulombic efficiency can be realized. Such a power source of very high energy density, provided it also has sufficient power density, may compete with commercially available batteries and fuel cells in electrical storage and conversion applications. The molecular structure characteristics and kinetics yielding favorable conditions for this type of operation are discussed in detail, together with the optimal operating conditions for this mode of DCFC application.