Ammonia with extra electricity

Along with machinery and chemical pesticides, the Haber-Bosch process has been instrumental for the development of modern agriculture, producing the ammonia required to fertilize the world’s crops. A rising global population is only making its role even more critical.

But this all comes at the expense of energy consumption and global warming, because the Haber-Bosch process involves passing molecular hydrogen (H₂) and nitrogen (N₂) over an iron-based catalyst at high temperatures and pressures. Combined with the large volume of ammonia (NH₃) that is required, this explains why the Haber-Bosch process currently accounts for over 1% of global energy use. And a rising global population will only make matters worse.

Unless, that is, scientists develop a novel, more efficient process for producing ammonia from molecular nitrogen and hydrogen, which is just what a team from the US, Ireland and Spain have succeeded in doing. But they have also gone one step further, because their novel bio-based process doesn’t just consume less energy than the Haber-Bosch process, it actually generates energy while producing ammonia.

This is because their method utilizes an enzymatic fuel cell (EFC). Like a microbial fuel cell (MFC; see Releasing the power within), an EFC breaks down a compound at an anode to generate electrons and protons that travel to the cathode, where they combine to produce a new compound. The protons travel through a proton-exchange membrane (PEM), while the electrons travel round an external circuit to generate electricity. As would be expected, the only difference between an MFC and an EFC is that the compound is broken down by microbes in an MFC and by extracted enzymes in an EFC.

To develop an EFC that can produce ammonia, the team, led by Shelley Minteer at the University of Utah, needed to employ nitrogenases. These are the only enzymes known to be able to reduce molecular nitrogen to ammonia, and are produced by nitrogen-fixing bacteria. Although other research groups have looked at developing EFCs for producing ammonia, they have struggled to find an electron mediator, required to transfer electrons between the enzyme and the anode or cathode, that works well with nitrogenases. Minteer and her team have now managed to find just such an electron mediator: a compound called methyl viologen.

This compound is incorporated into both the anode and cathode chambers of the EFC, which, as usual, are separated by a PEM. Nitrogenase is then added to the cathode chamber and hydrogenase, an enzyme that breaks down hydrogen into electrons and protons, to the anode chamber. Minteer and her team derived the nitrogenase from a nitrogen-fixing soil bacterium and the hydrogenase from a sulfate-reducing bacterium.

When hydrogen is pumped into the anode chamber, it is broken down by the hydrogenase, with methyl viologen transferring the released electrons to the anode.  The electrons then travel to the cathode via an external circuit, generating electricity, while the released protons pass through the PEM into the cathode chamber. Here, the nitrogenase uses the protons and electrons to convert a pumped supply of nitrogen into ammonia, with methyl viologen transferring the electrons from the cathode to the nitrogenase.

As reported in a paper in Angewandte Chemie, Minteer and her team found that their EFC could produce around 2.1μmol of ammonia per milligram of nitrogenase while also generating a current of 48μA. Now while these tiny yields of ammonia and electricity are of little practical use, they do demonstrate that the process works. “The real thing is not the quantity of ammonia produced, but that it's possible to make electricity at the same time,” says team member Ross Milton, also at the University of Utah.

This work is also at an early stage and there is a lot of scope for improving the efficiency of the EFC. For example, by incorporating the enzymes into the surface of the anode and cathode rather than having them free in the chambers.

The views represented here are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. or of the SCI.