Modeling the anodic half-cell of a low-temperature coal fuel cell.

Herein we examine the reduction of Fe ions by subbituminous coal (SBC) having a composition CH0.81O0. 22 in 5m H2SO4 at 100 8C. The reaction proceeds with a maximum rate constant of 8 ) 10 4 cms , which is 15–160 times greater than those reported previously. We used this reaction in the anode compartment of a prototype of a coal fuel cell that operated at 100 8C and at atmospheric pressure with VO2 + as the model oxidant. The cell produced a maximum current (power) density of 5 A (0.6 W) per liter of a slurry of electrolyte and coal, and 100 mA (11 mW) at the scale at which we carried out experiments (19 mL). Attempts to develop a process for generating electricity directly from the oxidation of coal date back to the late 19th century. Recent work on coal-powered fuel cells has been motivated by two facts: 1) the worldwide reserves of coal significantly exceed those of other fossil fuels, and 2) the environmental benefits of converting coal into electricity directly in a fuel cell rather than by generating steam in turbines have been perceived as significant. The oxidation of carbon electrochemically has a large activation energy (26 kcalmol ), and proceeds at room temperature at detectable rates only at an overpotential of greater than 1 V. Two strategies have been used to overcome the intrinsically slow kinetics of the oxidation of carbon in a coal fuel cell. One strategy is to oxidize a carbonaceous fuel (e.g., graphite) electrochemically at high temperatures (> 600 8C)—the majority of recent research on coal fuel cells has been based on this approach. Molten carbonates are typically used as electrolytes in high-temperature cells; molten hydroxides, borates and silicates, and solid oxides have also been used. One of the most advanced examples of a direct carbon–air fuel cell operated at 630 8C for 500 h with molten KOH/ NaOH as the electrolyte, a graphite anode that also served as the fuel, and a Fe–Ti alloy as the cathode at which O2 was reduced. The cell produced current and power densities of 30 AL 1 (270 mAcm 2 of anode) and 2.5 WL 1 (0.12 Wcm 2 of anode), respectively. The high operating temperature of this class of cells tends to result in their corrosion, particularly in the presence of oxygen. Other technical problems with these cells include: 1) the high resistivity of molten salts (up to 100 Wcm); 2) the consumption of carbon by reaction with CO2 to produce CO; [7] and 3) the clogging of cells by ash and other components of the fuel that are not oxidized. The high resistivity and poor mechanical stability of carbon fuels such as bituminous coal make them impractical as consumable anodes in coal fuel cells—the use of coal slurries in molten electrolytes requires vigorous stirring. Zecevic et al. have argued that high-temperature coal fuel cells cannot be made practical because of these problems. An alternative strategy for overcoming the slow kinetics of the direct electrochemical oxidation of coal is based on the observation that several transition-metal ions, including Fe, Sn, Cu, and Ce, oxidize coal slurries with an activation energy of 7–16 kcalmol , which is significantly lower than that for the direct electrooxidation of coal. 13] Although the electrolysis of coal slurries in the presence of transition-metal ions has been studied extensively, 12,13] we are not aware of any coal-powered fuel cells that use a secondary redox couple (e.g., Fe/Fe). This type of cell—in which metal ions would be reduced by the coal and reoxidized at the anode electrochemically—has been considered impractical because of the [*] Dr. D. B. Weibel, Dr. R. Boulatov, A. Lee, Dr. R. Ferrigno, Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University Cambridge, MA 02138 (USA) Fax: (+1)617-495-9857 E-mail: gwhitesides@gmwgroup.harvard. edu [] These authors contributed equally to the work.