University of Utah researchers have used enzymes to generate ammonia at room temperature, a process that also generates a small electrical current.
The method is published in Angewandte Chemie International Edition.
Professor Shelley Minteer and postdoctoral student Ross Milton have only been able to produce small quantities of ammonia so far, but their method could lead to a less energy-intensive source of ammonia, which is used globally as a fertilizer.
“It’s a spontaneous process, so rather than having to put energy in, it’s actually generating its own electricity,” Minteer said in a statement.
To make ammonia, which consists of one nitrogen atom and three hydrogen atoms, chemists break the bond that holds two nitrogen atoms together, and then reduce the nitrogen, or add electrons and protons to it in the form of hydrogen.
Ammonia is produced on an industrial scale using the Haber-Bosch process, where hydrogen and nitrogen is pumped over beds of metal catalysts at pressures up to 250 times atmospheric pressure and temperatures up to 500 degrees Celsius. The process currently produces nearly 500 million tons of ammonia every year.
In biology, conversion of gaseous nitrogen to ammonia – nitrogen fixation – is accomplished several ways, including through nitrogenases enzymes, which are the only known enzymes to reduce nitrogen to ammonia. Nitrogenase is rarely studied in fuel cell applications, because the enzyme is not commercially available and must be handled in an oxygen-free environment.
Minteer and Milton proposed a fuel cell system that replicated the biological process of nitrogen fixation, using nitrogenase and hydrogenase, an enzyme provided by Minteer’s collaborators at the Instituto de Catalisis y Petroleoquimica in Spain, to strip electrons from hydrogen gas and provide them to the nitrogen-reducing reaction.
According to the University of Utah, the cell consists of two compartments, connected via carbon paper electrodes. In one vial, hydrogen gas is oxidised by hydrogenase and electrons are carried to the anode. In the other, electrons come off the cathode and are combined with nitrogen, via nitrogenase, to create ammonia.
The electrons move from the anode to the cathode via a circuit. Protons (oxidized hydrogen atoms) travel through a membrane between the anodic and cathodic chambers, supplying the hydrogen atoms needed to synthesise ammonia.
The movement of the electrons creates current, and is the source of the small amount of electrical power generated by the reaction.
Several challenges remain before Minteer and Milton’s small-scale process can find application at an industrial scale. One is the oxygen sensitivity of nitrogenase, another is the requirement of chemically-expensive ATP, a source of energy in cells and in nitrogen fixation.
Milton said that re-engineering the reaction to circumvent the need for ATP would take the fuel cell “to the next level.” Until then, he says, the most notable and impactful aspect of this work is the production of ammonia without the massive energy drain characteristic of the industry standard process.
“The real thing is not the quantity of ammonia produced, but that it’s possible to make electricity at the same time,” Milton says.