Thermally Regenerative Fuel Cells
Over half of the energy that is generated by a diesel-powered, internal combustion engine is lost as waste heat, performing no work to propel the vehicle forward or power its auxiliary components. [Demirdoven, N., Deutch, J., Science, 2004, 305, 974-976.] In order to rectify this situation, we have proposed the introduction of a thermally regenerative fuel cell (TRFC), which would capture some of that waste heat and convert it to electricity. The heat generated by the engine would be used to dehydrogenate a hydrogen carrier (XH2) on a metal catalyst bed to produce hydrogen (H2 (g)) and the dehydrogenated carrier (X). The hydrogen is then directed to a polymer electrolyte membrane fuel cell (PEM-FC) in order to generate electricity. At the fuel cell cathode, X would recombine with protons and electrons, regenerating XH2 to be recycled within the system (see below). Extreme chemoselectivity of both the dehydrogenation and hydrogenation reactions is required to allow for a long carrier lifetime. A PEM resistant to degradation in the presence of X and XH2 that can satisfy the required energy demands is also a necessity. Extensive heterogeneous catalyst and XH2 screenings have been completed to address the former issue, while for the latter, different polymers have been identified and tested for their usefulness in such a system. The Jessop group is currently investigating these empediments.
A schematic diagram of the thermally regenerative fuel cell system proposed, where XH2 is the hydrogen carrier, and X, the dehydrogenated carrier.
Further Reading
Hydrogen Storage
One major factor preventing widespread use of automotive fuel cells is the lack of a viable on-board method for hydrogen storage. While many methods have been proposed, such as compressed hydrogen, metal hydrides, reversibly hydrogenated liquids, and reactive chemical hydrides, each method has its own critical drawbacks.
Reversibly hydrogenated liquids (XH2) are compounds such as saturated cyclic compounds that can be dehydrogenated over a heterogeneous catalyst, inside the vehicle, giving several equivalents of H2 plus the unsaturated cyclic product (X). While the H2 would go into the fuel cell, the unsaturated product would be stored. At the gas station, the vehicle would offload the unsaturated product while onloading more of the saturated compound. The unsaturated product would be sent back to a factory for re-hydrogenation. While this concept has been known for almost 40 years (originally with cyclohexane as the liquid), none of the proposed saturated liquids have been satisfactory in all respects.
As part of the effort to develop a solution for on-board hydrogen storage, our long term goal is to optimize the structure of reversible organic hydrogen-storage liquids so that they meet the following five requirements:
2. have an enthalpy of dehydrogenation low enough that the dehydrogenation is thermodynamically favored at as low a temperature as possible (at least below 180 °C);
3. be liquid and nonvolatile from -40 °C to the dehydrogenation temperature;
4. have a hydrogen storage capacity > 6% by weight and 45 g H2 per litre of liquid; and
5. be stable against thermal or catalytic decomposition at operating temperatures. Our strategies for achieving these goals will be revealed in our publications as they appear.
The proposed hydrogen for use of a reversibly hydrogenated liquid for the vehicular fuel cell hydrogen source.
Further Reading
» D. Wechsler, B. Davis, P. G. Jessop, "The Dehydrogenation of Combined Organic and Inorganic Hydrogen Storage Carrier", Can. J. Chem., 2010, 88, 548-555.
» D. Wechsler, Y. Cui, D. Dean, B. Davis, P. G. Jessop, "Production of H2 from Combined Endothermic and Exothermic Hydrogen Carriers," J. Am. Chem. Soc., 2008, 130, 17195-17203.
» Y. Cui, S. Kwok, A. Bucholtz, B. Davis, R. A. Whitney, P. G. Jessop, "The Effect of Substitution on the Utility of Piperidines and Octahydroindoles for Reversible Hydrogen Storage," New J. Chem., 2008, 32, 1027-1037.
