by Emil Morhardt
Nitrogen fertilizer, crucial for growing commercial crops, is based on ammonia made in factories using the energy- and CO2-intensive Haber-Bosch process; hydrogen is stripped off natural gas using steam, then reacted with nitrogen in the air. The process uses repeated cycling at high temperature and pressure, and consumes 2% of the world’s energy production. Stuart Licht and colleagues at George Washington University noticed, however, that a recently developed fuel cell using ammonia as a fuel and producing electricity as an output might be run in reverse: electricity in, ammonia out, with a whole lot less temperature and pressure (and energy) required. Even better, it wouldn’t need natural gas as a hydrogen source—with its attendant CO2 production—being able to get it from air and steam at a temperature lower than a household oven baking bread and at ambient pressure. Furthermore only simple materials would be required; molten sodium and potassium hydroxide (inexpensive commodity chemicals), nickel electrodes, and an iron oxide catalyst, all in a single pot.
After considerable experimentation with different temperatures, voltages, forms of iron oxide, and concentrations of the two hydroxides they were able to produce ammonia and excess hydrogen with 35% efficiency (35% of the energy in the electricity they needed was effectively converted to ammonia).
They figure that as they develop the system further, the heat can be supplied directly from solar thermal energy, simultaneously increasing the pressure for a more efficient level of electricity use in the process, all requiring no fossil fuels. Furthermore, there is apparently plenty of reason to believe there might be more-effective catalysts, and that various additives to the molten hydroxide mix might help as well. This bodes well for substantially decreasing the amount of fossil fuel energy needed to produce nitrogen fertilizer, as well as its cost.
Licht, S., Cui, B., Wang, B., Li, F.-F., Lau, J., Liu, S., 2014. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3. Science 345, 637-640. http://www.sciencemag.org/content/345/6197/637.short