Enzyme process makes ammonia and electricity

CHEMISTS at the University of Utah, US, have developed an enzyme-driven fuel cell process which can generate ammonia at room temperature, and produce a small electrical current.

Conventionally, ammonia, a vital agricultural fertiliser, is produced by the Haber-Bosch process, which uses high temperatures and pressures, and while effective, is very energetically expensive, using around 1% of the world’s total energy supplies. The team at Utah, led by chemistry and materials science and engineering professor Shelley Minteer and postdoctoral researcher Ross Milton, believe that their enzyme process could one day offer a more sustainable alternative.

Minteer and Milton’s fuel cell process works at room temperature and pressure – unlike the 250 atm and 500?C of the Haber-Bosch process. Like all fuel cells, Minteer and Milton’s has an anode and a cathode. Made from carbon paper and placed in separate chambers, the anode is coated with nitrogenase enzymes, while the cathode is coated in hydrogenase.

Hydrogenase oxidises molecular hydrogen to H⁺ ions, while the electrons pass into the anode and to the cathode via a circuit. The nitrogenase then reduces N₂ with the H⁺ ions and electrons created by the hydrogenase, to make the ammonia, NH₃. In an enzymatic fuel cell, electron transfer between the electrode and the enzyme can be very inefficient, so the reaction is facilitated by the use of methyl viologen (N,N’-dimethyl-4,4’-bipyridinium) as an electron donor. The movement of the electrons creates a very small amount of current.

Passing 60 mC of charge across the fuel cell generates around 286 nmol of ammonia per mg of enzyme and around 230 mV of current.

“The real thing is not the quantity of ammonia produced, but that it's possible to make electricity at the same time,” said Milton.

Clearly, there are challenges to overcome before the process can be scaled up. Nitrogenase is very sensitive to the presence of oxygen, so better ways to tackle this must be found. In addition, the reaction still requires ATP, a molecule found in living cells and used as an energy source. Milton says that if the reaction could be re-engineered so that ATP is no longer necessary, it would take the process “to the next level”.

Angewandte Chemie International Edition DOI: 10/bzd4