The aim of this project is the development of an aqueous-alkaline/carbonate biocarbon fuel cell which performs well while realizing electrolyte invariance by exploiting electrochemical reactions that are favored at temperatures near 300 °C.
Broader impacts. Very large quantities of lignocellulosic residues (e.g. corncobs, coconut shells) accompany the production of bioethanol and biodiesel fuels. These residues can be efficiently and quickly converted into biocarbons. Carbon fuel cells can generate electricity from these biocarbons -as well as from coal, and other fossil carbons-with a theoretical thermodynamic efficiency of 100%. A recent EPRI study indicates that carbon fuel cells have the potential to convert biocarbons into electrical power at a system level efficiency of about 60%, which is over 20% higher than the efficiencies realized by current state-of-the-art integrated gasification combined cycle (IGCC) or advanced pulverized coal power generation systems. Thus the production of biocarbon can complement the production of bioethanol and biodiesel in a biomass refinery that also produces electricity at a very high efficiency. Other impacts include the training of two BS and two MS students, the involvement of Hawaii Pacific University (HPU) faculty, and the development and inclusion of new electrochemical engineering course material in the UH and HPU curricula. In view of the fact that a college degree in chemical engineering is not offered in the State of Hawaii, these impacts have special significance.
Intellectual merit. This project is based on two hypotheses. 1) At temperatures approaching 300 °C the aqueous-alkaline/carbonate biocarbon fuel cell will offer an open circuit voltage (OCV) of about 1 V and a steady, maximum power density that exceeds 100 mW/cm2. 2) During operation the composition of the electrolyte will evolve towards an equilibrium mixture of hydroxide and carbonate ions that afterwards will be invariant (i.e. stable).
The cathode of this cell resembles that of a Bacon fuel cell, where oxygen in air is reduced to hydroxide ion over a silver catalyst. Thermodynamic analyses indicate that the cathode should perform well at temperatures approaching 300 °C. Likewise, thermodynamic analyses indicate that at these temperatures both the hydroxide ion and the carbonate ion (formed by the reaction of CO2 with hydroxide ion) should vigorously oxidize the carbon anode and release electrons; thereby generating power at high efficiency.
This project has three objectives: 1) to characterize the oxidation behavior of anodic char-coal in the aqueous-alkaline/carbonate environment of the fuel cell at temperatures near 300 °C; 2) to characterize the stability of the electrolyte, together with the catalytic effects of differing electrolytes on the anodic and cathodic reactions at temperatures near 300 °C; and 3) to characterize the performance of the biocarbon anode as a working electrode in a setup that includes a counter electrode, and flow of the electrolyte through a heat exchanger bridge to a reference electrode maintained at system pressure but at a much lower temperature.
With two important exceptions, a direct carbon fuel cell is similar to a hydrogen fuel cell. These exceptions are: the direct carbon fuel cell employs carbon (e.g. charcoal from a grocery store) for fuel, not hydrogen; and thermodynamics permits the direct carbon fuel cell to generate electricity from its carbon fuel at 100% efficiency. Both of these advantageous exceptions are worthy of note. Charcoal is a renewable fuel that can be produced sustainably and is so safe that cub scouts use it to do BBQ. The second exception unveils the holy grail of energy conversion processes: the direct carbon fuel cell has the potential to generate power with unbeatable efficiency. Because of these advantages, worldwide research aimed at developing a practical direct carbon fuel cell has increased dramatically during the past decade. Unfortunately, numerous hurdles beset the development of the direct carbon fuel cell. Our work emphasized the development of an aqueous-alkaline cell that would operate at moderate temperatures between 200 and 300 °C and elevated pressures. This cell is similar to the hydrogen fuel cell that powered the early Apollo missions. The chief hurdle for such cells was believed to be the carbon-dioxide induced formation of potassium bicarbonate crystals (similar to baking soda), that subsequently precipitate and degrade the alkalinity of the electrolyte. We showed that these crystals are not produced at temperatures above 150 °C; instead they form at lower temperatures when the cell is cooled slowly. If the cell is quickly cooled and depressurized, the electrolyte is stable and no crystals are formed. This finding has considerable significance for the development of aqueous-alkaline fuel cells, and also has implications for chemistry related to carbon sequestration and climate change. Another hurdle for the cell is the extremely corrosive nature of the electrolyte. We have tested several cell designs that delivered promising results at first, but suffered unacceptable corrosion during their operation. Although our NSF grant has now concluded, we intend to continue testing our most promising cell design (see Figures 1 and 2) during 2013 with the hope of overcoming problems associated with corrosion that limit its long-term performance. If we are successful, we will submit proposals to continue our work. During the past four years this NSF grant enabled three Associate Professors, one Post Doc from Japan, three graduate students, and three undergraduates to experience research involving fuel cells and biofuel production. Among these, the grant enabled one MSE candidate to initiate a research project at the Norwegian University of Science and Technology (NTNU) in Trondheim, Norway during the summer of 2011. Because our collaboration with NTNU involves research of interest to the Dow Corning Corporation, Dow Corning provided us with needed materials for study at NTNU. Also, the grant enabled collaboration with chemists of the Hungarian Academy of Sciences in Budapest, who performed definitive analyses of crystals taken from the direct carbon fuel cell. Finally, with full support from Spain two faculty members of the University of Zaragoza involved themselves with this project. One Spanish faculty member completed and published two papers concerning the thermodynamic properties of various electrolytes and pure water at the elevated temperatures and pressures present in the direct carbon fuel cell. The second Spanish professor became an expert on the use of LabView software and modernized our data acquisition programs. In addition to the publications already associated with this grant (see attached list), its products included one MSE thesis, and one honors BS thesis.
