Phosphonium salt boosts electrochemical Haber-Bosch reaction

A phosphonium salt speeds the delivery of protons and boosts the performance of an electrochemical Haber-Bosch process to record levels (Science, 2021 DOI: 10.1126/science.abg2371).

The Haber-Bosch process is one of the most important and energy-consuming chemical reactions in the world. If it could be done electrochemically, the process could be powered by renewable electricity instead of fossil fuels, burned to provide the high temperatures and pressures in most Haber-Bosch reactors. The electrochemical process directly reduces nitrogen to ammonia. It first activates dissolved nitrogen gas with lithium ions at the cathode, forming lithium nitride, an unstable, transient species. Then, protons produced at the anode replace lithium and convert lithium nitride to ammonia.

For decades, this process has been studied only in labs, suffering from low efficiency and slowness. Bryan H. R. Suryanto, Alexandr N. Simonov, Douglas R. MacFarlane, and their colleagues at Monash University identified one target for improvement: if protons could move faster from the anode to the cathode, ammonia production could speed up.

Typically, ethanol serves as a proton shuttle. But ethanol molecules diffuse slowly across the cell and get consumed in the process. Effectively, that means ammonia is synthesized from ethanol, which is not sustainable, Simonov said.

Instead, the team turned to phosphonium salts. Lithium nitride is a strong, proton-seeking base, and phosphonium cations are known to give up a proton from the phosphorus atom’s neighboring carbon atoms to form ylides, molecules with opposite charges on adjacent atoms, MacFarlane explained. If the ylide could in turn pick up protons from the anode, it would regenerate the phosphonium cation. The phosphonium cations thus had the potential to be a recyclable proton shuttle, driven by their charge to move quickly towards the negatively charged cathode.

The researchers tested their idea using trihexyltetradecylphosphonium salt in a simple electrochemical cell. Over a 20-hour experiment, the cell produced ammonia at a rate of 53 nmol per second per square centimeter of the electrode’s surface area with 69% faradaic efficiency, a measure of how efficiently electrons are converted to products in an electrochemical reaction. In comparison, the previous high record was 30 nmol s^–1 cm^–2 with 35% faradaic efficiency, which was sustained for under an hour (Nat. Catal., 2020 DOI: 10.1038/s41929-020-0455-8).

These “much-improved” performances represent important steps forward, said Karthish Manthiram at the Massachusetts Institute of Technology, who led the 2020 study. “To get this kind of performance at this duration is really very special.” More importantly, the novel demonstration that phosphonium ions could be effective proton shuttles “opens up a lot of new space,” Manthiram said.

When the researchers extended the reaction time to 93 hours, the overall performance dropped closer to that from Manthiram’s study. Still, the fact the reaction lasted 93 hours was “remarkable,” Manthiram says.

One likely reason for the performance loss over time is buildup of ammonia gas in the system, explained MacFarlane. The researchers opted for a fixed-volume cell configuration for this proof-of-concept study. The researchers are now working to test the process on a larger scale using a flow setup, which would continuously remove ammonia and avoid this problem. Another “elephant-in-the-room” problem that plagues the field more broadly is the use of tetrahydrofuran as a solvent, said Suryanto. Tetrahydrofuran is electrochemically unstable and polymerizes over time, slowing diffusion during long-term experiments.