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Article

Performance of a First-Generation, Aqueous-Alkaline Biocarbon Fuel Cell

Teppei Nunoura,^†^‡ Kiyoshi Dowaki,^†^§ Chihiro Fushimi,^†^# Stephen Allen,^† Erika Mészáros, and Michael Jerry Antal, Jr.*^†

Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822 and Chemical Research Center, Institute of Materials and Environmental Chemistry, Hungarian Academy of Sciences, P.O. Box H-1525, Budapest, Hungary

Ind. Eng. Chem. Res., 2007, 46 (3), pp 734–744

DOI: 10.1021/ie061202s

Publication Date (Web): January 3, 2007

†

University of Hawaii at Manoa.

‡

Current address: Environmental Science Center, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, Chiba 277-8581, Japan.

Current address: Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan.

Current address: Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan.

⊥

Current address: Hawaii Pacific University, 45-045 Kamehameha Highway, Kane'ohe, HI 96744-5297.

∇

Chemical Research Center, Institute of Materials and Environmental Chemistry, Hungarian Academy of Sciences.

To whom correspondence should be addressed. E-mail address: mantal@hawaii.edu.

Abstract

Because the carbon fuel cell has the potential to convert the chemical energy of carbon into electric power with an efficiency approaching 100%, there has been keen interest in its development for more than a century. A practical carbon fuel cell requires a carbon feed that conducts electricity and is highly reactive. Biocarbon (carbonized charcoal) satisfies both these criteria, and its combustion does not contribute to climate change. In this paper, we describe the performance of an aqueous-alkaline biocarbon fuel cell that generates power at temperatures of 500 K. Thermochemical equilibrium favors the reduction of oxygen on the cathode at temperatures of <500 K, whereas the chemical kinetics of the oxidation of carbon by hydroxyl anions in the electrolyte demands temperatures of >500 K. Nevertheless, an aqueous-alkaline cell operating at 518 K and 35.8 bar was able to realize an open-circuit voltage of 0.57 V, a short circuit current density of 43.6 mA/cm², and a maximum power of 19 mW, using a 6 M KOH/1 M LiOH mixed electrolyte with a catalytic silver screen/platinum foil cathode and an anode composed of 0.5 g of compacted corncob charcoal previously carbonized at 950 °C. A comparison of temperature-programmed desorption (TPD) data for the oxidized biocarbon anode material with prior work suggests that the temperature of the anode was too low: carbon oxides accumulated on the biocarbon without the steady release of CO₂ and active sites needed to sustain combustion. Consequently, the open-circuit voltage of the cell was less than the expected value (1 V). Carbonate ions, formed in the electrolyte as a product of the reaction of CO₂ with hydroxyl ions, can halt the operation of the cell. We show that the carbonate ion is not stable in hydrothermal solutions at 523 K and above; it decomposes via the release of CO₂ and the formation of hydroxyl anion. Consequently, it should be possible to regenerate the electrolyte through the use of reaction conditions similar to those used in the fuel cell. We believe that substantial improvements in performance can be realized from an aqueous-alkaline cell whose cathode is designed to operate at temperatures significantly below 500 K, and whose biocarbon anode operates at temperatures significantly above 500 K.