Producing ammonia with small-scale electrochemical reactors

A TEAM of MIT chemical engineers has developed an electrochemical process for producing ammonia that reduces emissions and could allow decentralised production of ammonia in remote areas.

The production of ammonia from nitrogen and hydrogen involves an energy-intensive reaction owing to the strength of the N₂ triple bond, which is very difficult to break. Ammonia is typically produced via the Haber-Bosch process, which is responsible for around 1.8% of global CO₂ emissions, according to the Royal Society. The process typically uses hydrogen produced from fossil fuels, and requires temperatures of 300–500^oC and pressures of 200–300 bar.

Ammonia is mainly used to produce fertiliser but since it requires large manufacturing facilities, transporting it to remote regions such as sub-Saharan Africa makes the cost two to three times higher than elsewhere in the world. The team at MIT wanted to devise a small-scale ammonia production method that could be used in remote areas to make fertiliser, without the high emissions and harsh conditions of the Haber-Bosch process.  

“In the future, if we envision how we want this to be used someday, we want a device that can breathe in air, take in water, have a solar panel hooked up to it, and be able to produce ammonia. This could be used by a farmer or a small community of farmers,” said Karthish Manthiram, Assistant Professor of Chemical Engineering at MIT and the senior author of the study.

MIT

Electrochemical production of ammonia has been performed in other studies, but there have been challenges that have been difficult to overcome. Reactions involving nitrogen and hydrogen in electrolytes containing non-aqueous solvents are slow due to their low solubility. Carbon fibre gas diffusion electrodes improve the transport in aqueous solutions by allowing good contact between the gas, electrolyte, and catalyst. However, if a non-aqueous solvent such as tetrahydrofuran (THF) is used, the electrodes become flooded.

The MIT team developed a way to use a gas diffusion electrode in a non-aqueous solution to improve the rate of ammonia production. They used gas diffusion electrodes made of a stainless steel cloth (SSC), which don’t become flooded by the electrolyte. The SSC anode is coated in platinum, and the SSC cathode is coated in lithium, which acts as a catalyst. The hydrogen is produced in a separate electrochemical cell from the splitting of water, and is fed into the ammonia-producing cell at the anode.

The ammonia-producing cell uses an electrolyte of ethanol dissolved in THF. The ethanol is used to form ethoxide at the cathode and in the process the nitrogen becomes reduced to ammonia. At the anode, protons are produced from oxidised hydrogen molecules, and then the ethoxide is converted back into ethanol.

The ammonia production rate is a record for electrochemical production, however the energy efficiency is still only 1.4–2.8%, so it cannot yet compete with the energy efficiency of the Haber-Bosch process, which is around 50–80%.

“The largest source of energy losses in our system is ionic resistance, which we are working to overcome by developing new electrolytes which are more conductive,” said Manthiram. “The lithium-mediated process has a lower energy efficiency than Haber-Bosch. However, there is some indication that lower energy efficiencies may be techno-economically tolerated for a process which operates at ambient conditions, like the one we have developed; our process is driven by voltage, unlike Haber Bosch, which requires temperature and pressure.”

“There is much research to do to fully realise this vision, but our system which achieves higher rates and selectivities for producing ammonia is a step in that direction.”

Nature Catalysis http://doi.org/dxhj