Flipping the switch on ammonia production: Process generates electricity instead of consuming energy

February 3, 2017
A diagram of a hydrogenase-nitrogenase ammonia producing fuel cell. Credit: Ross Milton

Nearly a century ago, German chemist Fritz Haber won the Nobel Prize in Chemistry for a process to generate ammonia from hydrogen and nitrogen gases. The process, still in use today, ushered in a revolution in agriculture, but now consumes around one percent of the world's energy to achieve the high pressures and temperatures that drive the chemical reactions to produce ammonia.

Today, University of Utah chemists publish a different method, using enzymes derived from nature, that generates at room temperature. As a bonus, the reaction generates a small electrical current. The method is published in Angewandte Chemie International Edition.

Although chemistry and materials science and engineering professor Shelley Minteer and postdoctoral scholar Ross Milton have only been able to produce small quantities of ammonia so far, their method could lead to a less energy-intensive source of the ammonia, used worldwide as a vital fertilizer.

"It's a spontaneous process, so rather than having to put energy in, it's actually generating its own electricity," Minteer says.

How to make ammonia

Both the Haber-Bosch process (named also for Carl Bosch, who scaled up the process for industrial production) and the process developed by Minteer and Milton are based on fundamental chemical principles. To make ammonia, which consists of one and three hydrogen atoms, chemists must break the strong bond that holds two nitrogen atoms together, and then reduce the nitrogen, or add electrons and protons to it in the form of hydrogen. In the Haber-Bosch process, hydrogen and are pumped over beds of metal catalysts, which aid the reaction, at pressures up to 250 times atmospheric pressure and temperatures up to 500 degrees Celsius (932 F). The process currently produces nearly 500 million tons of ammonia every year.

Assembled H2/N2 fuel cell. Carbon paper electrodes are inserted into the anodic (left hand side) and cathodic (right hand side) chambers of the fuel cell. Credit: Ross Milton.

Simulating a cell

In biology, conversion of gaseous nitrogen to ammonia is called nitrogen "fixation" and is accomplished through several pathways, including through enzymes called nitrogenases. Used by some bacteria, nitrogenases 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.

"One of the things my group does well is designing the interface between the enzyme and the electrode, so the enzymes can communicate with electrode surfaces," Minteer says.

Minteer and Milton envisioned a fuel cell system that replicated the biological process of , using nitrogenase and hydrogenase, an enzyme graciously provided by Minteer's collaborators at the Instituto de Catalisis y Petroleoquimica in Spain, to strip electrons from and provide them to the nitrogen-reducing reaction.

The cell consists of two compartments, connected via carbon paper electrodes. In one vial, hydrogen gas is oxidized 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.

Ross Milton works in the glove box that keep nitrogenase in an oxygen-free environment. Credit: Paul Gabrielsen/University of Utah

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 needed to synthesize ammonia.

The movement of the electrons creates current, and is the source of the small amount of electrical power generated by the reaction.

Scaling up

Several challenges remain to be overcome 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 fixation. Milton says that re-engineering the reaction to circumvent the need for ATP would take the "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 .

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

Explore further: Method found for pulling hydrogen from ammonia for use as clean fuel

More information: Ross D. Milton et al, Bioelectrochemical Haber-Bosch Process: An Ammonia-Producing H/NFuel Cell, Angewandte Chemie International Edition (2017). DOI: 10.1002/anie.201612500

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11 comments

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dirk_bruere
not rated yet Feb 03, 2017
So where does the energy to make ammonia come from?
Eikka
5 / 5 (2) Feb 03, 2017
So where does the energy to make ammonia come from?


From the energy required to produce hydrogen and nitrogen.

With the Haber process, the hydrogen and the heat and pressure used to drive the reaction comes from a steam reformer that converts natural gas. The energy and raw material input is in the form of methane.
barakn
3 / 5 (2) Feb 03, 2017
I'm a little dismayed to see that the researcher thinks that it will be possible to eliminate the use of ATP, another hidden source of energy. If nature couldn't find a way to make the reaction work with ATP, he won't either. This will never be turned into an industrial process.
Cusco
3 / 5 (2) Feb 03, 2017
"This will never be turned into an industrial process." - barakn

Fortunately Carl Bosch ignored the people who said the same thing in the early 20th century.
Whydening Gyre
5 / 5 (1) Feb 03, 2017
Anybody ever have to clean horse stalls that haven't been cleaned in a while? Plenty of ammonia there. Nature found a pretty efficient way to produce it - Urine, straw and a little time...
humy
not rated yet Feb 04, 2017
Anybody ever have to clean horse stalls that haven't been cleaned in a while? Plenty of ammonia there. Nature found a pretty efficient way to produce it - Urine, straw and a little time...

Currently, without the Haber process, there just isn't enough horse piss and shit in the world to provide enough nitrogen fertilizer for the world crops.
ugosugo
not rated yet Feb 04, 2017
A "simple" thermodynamic estimation should allow to guess whether this process would be indeed less energetically costly. Mainly, to check if the energy barrier, to split nitrogen, via the enzyme (+ATP) is much lower than with metal catalyst+pressure+heat. The fact, that in nature, nitrogen fixation is accomplished only by specialized bacteria (even plants have to accept them as symbionts) and it is not widespread make me think that the overall energetic cost is pretty much the same, i.e. very high!
big_hairy_jimbo
not rated yet Feb 05, 2017
The energy comes from ATP, SO they need to either replace ATP as the energy provider, OR add in more biological pathways. To me it seems they are just repeating what nature does, so if you can add in the ATP production side of things you are on your way. ATP production is costly, but what if you add a synthetic leaf like system? The leaf captures sunlight and causes an electron avalanche. Use the avalanche (as nature does) to produce ATP. Then of course you need other metabolites as an input for the manufacture of ATP. So keep adding systems until you arrive at a synthetic organism that can utilise cheap food stocks and sunlight to manufacture ammonia. The idea here is to fine tune everything and make a bio reactor that also makes electricity. Nature can and WILL be improved upon. After all, nature gave humans intelligent minds, which eventually worked out how to put those minds on the moon. The human mind is a marvellous machine.
Eikka
not rated yet Feb 05, 2017
but what if you add a synthetic leaf like system?


Or what if you just grew methanogenic bacteria and put the resulting gas through the Haber process like before?

TrollBane
not rated yet Feb 05, 2017
This just begs for a link to the classic gag... "Ammonia! Ammonia!" http://www.conden...659_.htm
big_hairy_jimbo
not rated yet Feb 09, 2017
@Eikka, I thought the whole Idea was to avoid the Energy expensive Haber process?

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