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GWU team demonstrates relatively efficient electrochemical process for low-GHG production of ammonia
8 August 2014
A team at George Washington University led by Stuart Licht has developed a relatively efficient electrochemical process for the production of ammonia from water and nitrogen, without the need for an independent hydrogenation step (and thus the associated carbon-intensive steam reforming of methane as the hydrogen source). The process, reported in the journal Science, electrolyzes air and steam in a molten hydroxide salt with a nanostructured iron oxide–derived catalyst (nano-Fe2O3).
At 200 °C in an electrolyte with a molar ratio of 0.5 NaOH/0.5 KOH, ammonia is produced at 1.2 V under 2 milliamperes per centimeter squared (mA cm-2) of applied current at coulombic efficiency of 35% (35% of the applied current results in the six-electron conversion of N2 and water to ammonia, and excess H2 is cogenerated with the ammonia). At 250 °C and 25 bar of steam pressure, the electrolysis voltage necessary for 2 mA cm−2 current density decreased to 1.0 V.
Although the suspension is only stable for a few hours, the protocol points to a way to produce ammonia from purely renewable resources.
The well-established Haber-Bosch synthesizes ammonia for fertilizer and other uses via the hydrogenation of nitrogen from the atmosphere; the hydrogen required is produced primarily through a separate process of steam reformation, which consumes 3 to 5% of the world’s natural gas production and releases large quantities of CO2 to the atmosphere.
The ammonia hydrogenation reaction is separate from the steam-reforming reaction (Eq. 2) that generates the hydrogen. Renewable energy–driven water splitting could provide an alternative H2 source, but economic, non–CO2-emitting sources of H2 have yet to be proven on the industrial scale. Although ammonia hydrogenation is exothermic, it is kinetically disfavored at ambient temperature and pressure. In the Haber-Bosch process, this kinetic limitation is overcome via an iron-based catalyst, repeated cycling, high pressure, and elevated temperature. The last-named conditions are energy-intensive and consume 2% of the world’s energy production.
… By effectively reversing the NH3 fuel cell, we present an electrochemical pathway to produce ammonia from air and steam at 200 °C with simple materials (molten hydroxide, Ni electrodes, and nano-Fe2O3), in one pot without a separator.—Licht et al.
In this study, the team also introduced a solar thermal water self-pressurizing, low electrolysis energy path system.
There is ample room for advances of this pathway. Fe2O3 was utilized as the reactive surface, whereas today’s Haber-Bosch catalysts use Fe2O3 or ruthenium-based catalysts with a wide variety of carefully optimized additives which may also improve this electrochemical process.—Licht et al.
Stuart Licht, Baochen Cui, Baohui Wang, Fang-Fang Li, Jason Lau, and Shuzhi Liu (2014) “Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3” Science 345 (6197), 637-640 doi: 10.1126/science.1254234
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