The vision of a 'hydrogen economy', was driven by concerns over the long-term supply of fossil fuels and the adverse environmental consequences of their use. Hydrogen as derived from renewable resources would be the ideal fuel but its widespread use has been restricted by the cost of a new energy infrastructure and the current technical limitations in storing the gas. These issues could be avoided by conveying the hydrogen as an easily transportable organic liquid carrier wherein it's 'contained' by a reversible catalytic hydrogenation process. The concept is viable for stationary H2 storage but not for transportation-as in a fuel cell vehicle, because of the need to provide for the endotherm of an on-board dehydrogenation reactor. We and others have suggested employing the liquid carrier as the direct feed to a PEM fuel cell, where it would undergo an anodic dehydrogenation that is not thermodynamically limited, along with furnishing electric power. In principle the process may be reversed, either by operating the fuel cell in reverse with an input of electricity or a catalytic hydrogenation of the recovered 'spent' liquid. The direct fuel cell/liquid carrier combination thus provides a means for electrical energy storage. The maximum storage capacity is determined the Gibbs free energy change between the 'fresh' and 'spent' liquid fuels and is not limited to H2 -reversible systems but would apply to any reversibly electrochemically oxidizable fuel. The practical challenges are in devising the direct fuel cell and identifying redox ' fuel pairs' that could be environmentally acceptable and cost effective in large scale use. As a starting point we used a Danish Power System PEM fuel cell ,(Pt 1.5 mg/cm2 electrodes), with a phosphoric acid and 12-silicotungstate impregnated polyimidazole(PBI) membrane,operating at 130-180 deg.C using (as a base case) humidified methylcyclohexane as the feed .The cell operated with an open circuit potential of 0.8V and provided a maximum power density of 5mW/cm2 at 0.6V. By NMR, toluene and two as yet unidentified products were detected in the cell's effluent. A maximum energy storage capacity of 1.29 kWh/L is estimated for the methlcyclohexane/toluene 'fuel pair' .In continuing studies we are using a similar fuel cell configuration with perhydrodibenzyltoluene, a much less volatile liquid with a wide liquid range as the fuel. Dibenzyltoluene,the expected product is a relatively low cost industrial heat transfer fluid.

From hydrogen containment to electrical energy storage

256th National Meeting (2018)

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