Due to the intermittency of many renewable energy sources, their potential to significantly participate in the overall energy mix is strongly dependent on our ability to identify and develop efficient and cheap energy storage technologies¹. Liquid fuels are an appealing option with energy densities typically 10-100x higher than batteries. Ammonia, in particular, has many attributes that make it the ideal energy storage compound including high energy density and an existing infrastructure for transportation. The highly developed, large-scale production of ammonia via Haber-Bosch process is not suitable for energy storage applications due to the high-temperatures and pressures used. Theory² and experiment³ suggest that a more suitable approach is the production of ammonia from nitrogen and water via electrosynthesis whereby small-scale reactors could enable distributed ammonia production closer to the consumer and be more compatible with energy inputs from intermittent renewable energy resources - improvements that could dramatically reduce the energy and carbon intensity of ammonia production and distribution.

Prior research pursuing the electrosynthesis of ammonia has focused primarily on the use of proton conducting electrolytes at high temperatures, >600˚C or low temperatures, <100˚C due to materials limitations in the intermediate temperature range. The intermediate temperature range is particularly advantageous as lower temperatures favor the thermodynamics of the forward reaction to form NH₃, while higher temperatures aid in the breaking of the incredibly stable di-nitrogen bond (946 kJ/mol). In order to access this intermediate temperature range of operation, we have developed a composite SnP₂O₇/Nafion based electrolyte material that exhibits conductivity on the order of 20 mScm^-1 at 200˚C. We have demonstrated a maximum ammonia production rate of ~1x10^-8molcm^-2s^-1 at 200˚C using N₂ and H₂ as reactant gases and Pt-based electrodes. This talk will discuss the factors influencing production rates and current efficiencies of these intermediate temperature devices.

References

1. J. B. Goodenough, H. D. Abruna and M. V. Buchanan, 2007.
2. T. H. Rod, A. Logadottir and J. K. Nørskov, The Journal of Chemical Physics 112 (12), 5343 (2000).
3. G. Marnellos and M. Stoukides, Science 282 (1998).

Acknowledgements

This project is supported by ARPA-E under Award No. DE-AR0000687.