Electrochemical Fixation of N2

NH₃ is a promising carbon-free energy carrier with high energy density and a well established infrastructure. The industrial Haber-Bosch process for NH₃ synthesis from dissociated N₂ and H₂:

N₂ + 3H₂ ↔ 2NH₃

requires high pressures, temperatures, and large installations, and is thus illsuited for decentralized production using, e.g. photo-voltaics or wind turbines.
The enzyme nitrogenase produces ammonia under ambient conditions by catalyzing the direct addition of protons and electrons to N₂:

N2 + 8H⁺ + 8e- ↔ 2NH₃ + H₂

We will search for new catalyst that fixate N₂ and electrochemically adsorb protons and electrons to form NH₃.

To find a catalyst that will bind or dissociate N₂ at ambient conditions and allow the addition of protons and electrons to form and desorb NH₃, while keeping hydrogen evolution low/non-existing.

The vision is to construct an electrochemical cell which converts N₂ into NH₃ at reasonably low temperatures of 20 - 400°C

Cathode reaction:
N₂ + 6H⁺ + 6e^- ↔ 2NH₃

Anode reaction(s):
H₂ ↔ 2H⁺ + 2e^-
3H₂O ↔ 6e^- + 6H⁺ +1.5O₂

where the second anode reaction is the dream scenario of producing NH₃ directly from sustainable electricity, H₂O and N₂.

Two materials properties are highly important for the cell efficiency:

• A dry, i.e. absolutely water free, electrolyte with high proton conduction.
• A cathode which preferentially binds N2 and adsorbs protons and electrons.

• Surface science experiments on well characterized single crystal samples in a controlled environment.
• Theoretical screening studies on transition metal alloys, sulfides and nitrides.

A Theta probe XPS (fig.) for fast and non-destructive determination of surface chemistry and depth profiling will be coupled directly to an electrochemical cell to allow contamination-free sample transfer between these two units.

• Computational screening methods (DFT) to search for mixed metals, nitrides and sulfides, which enable non-dissociative adsorption of N₂.
  
• Investigate the role of steps and nitrogen vacancies (fig.) to search for materials favoring N₂ adsorption/dissociation and reduction to NH₃, rather than the competitive H₂ evolution reaction.

• Establishment of fundamental processes by combining single crystal experiments with DFT based computational screening.
• Construction and testing of an electrochemical cell containing a La_0.95Sr_0.05H_0.05PO₄ proton conducting electrolyte and commercially available electrodes.
• Synthesis of a dry electrolyte with higher proton conductivity.
• Design of new cathode materials with preferential N₂ binding.

Principal investigators:

• Thomas Bligaard
• Ib Chorkendorff
• Søren Dahl
• Sebastian Horch
• Tejs Vegge
• Mogens Mogensen

• [1] Rod, Logadóttir, Nørskov; J. Chem. Phys. 112 (2000)
• [2] Nørskov, Rossmeisl, Logadóttir, Lindqvist, Kitchin, Bligaard, Jónsson; J. Phys. Chem. B 108 (2004)
• [3] Skúlason, Bligaard, Rossmeisl, Logadóttir, Nørskov, Jónsson; in preparation