Direct electrochemical conversion of carbon: systems for efficient conversion of fossil fuels to electricity
The direct electrochemical conversion of carbon involves discharge of suspensions of reactive carbon particles in a molten salt electrolyte against an oxygen (air) cathode. (Figure 1). The free energy and the enthalpy of the oxidation reaction are nearly identical. This allows theoretical efficiencies ({Delta}G(T)/{Delta}H) to approach 100% at temperatures from 500 to 800 C. Entropy heat losses are therefore negligible. The activities of the elemental carbon and of the carbon dioxide product are uniform throughout the fuel cell and constant over discharge time. This stabilizes cell EMF and allows full utilization of the carbon fuel in a single pass. Finally, the energy cost for pyrolysis of hydrocarbons is generally very low compared with that of steam reforming or water gas reactions. Direct electrochemical conversion of carbon might be compared with molten carbonate fuel cell using carbon rather than hydrogen. However, there are important differences. There is no hydrogen involved (except from trace water contamination). The mixture of molten carbonate and carbon is not highly flammable. The carbon is introduced in as a particulate, rather than as a high volume flow of hydrogen. At the relatively low rates of discharge (about 1 kA/m{sup 2}), the stoichiometric requirements for carbon dioxide by the cathodic reaction may be met by diffusion across the thin electrolyte gap. We report recent experimental work at LLNL using melt slurries of reactive carbons produced by the thermal decomposition of hydrocarbons. We have found that anodic reactivity of carbon in mixed carbonate melts depends strongly on form, structure and nano-scale disorder of the materials, which are fixed by the hydrocarbon starting material and the conditions of pyrolysis. Thus otherwise chemically pure carbons made by hydrocarbon pyrolysis show rates at fixed potentials that span an order of magnitude, while this range lies 1-2 orders of magnitude higher than the current density of graphite plate electrodes. One carbon materials was identified which delivered anode current densities of 1 kA/m{sup 2} at 0.8 V (i.e., 80% efficiency, based on the standard enthalpy of carbon/oxygen reaction, and assuming full conversion), which we believe to be sufficiently great to allow practical application in fuel cell arrays. Since the hydrocarbon starting materials are ''ash free,'' entrainment of ash into the melt is not limiting. Finally, the use of fine carbon particulates in slurries avoids cost and logistics of carbon electrode manufacture and distribution.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15006377
- Report Number(s):
- UCRL-JC-138900; TRN: US200409%%59
- Resource Relation:
- Conference: Fuel Cell 2000 Seminar, Portland, OR (US), 11/02/2000; Other Information: PBD: 10 Aug 2000
- Country of Publication:
- United States
- Language:
- English
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