Electrifying the Haber–Bosch

Joule https://doi.org/10.1016/j.joule.2019.10.006 (2019)

The Haber–Bosch process, where nitrogen and hydrogen molecules react to form ammonia (N₂ + H₂ → NH₃), accounts for 1.4% of global carbon dioxide emissions and consumes 1% of the world’s total energy production. Hydrogen is produced onsite via methane steam reforming (CH₄ + H₂O → CO + 3H₂) in combination with the water–gas shift reaction (CO + H₂O → CO₂ + H₂). Nitrogen and hydrogen combine in a high-pressure reactor with the aid of an iron-based catalyst. Given the energy- and carbon-intensive requirements of the process, alternatives must be sought to fulfil the enormous global demand for ammonia through a significantly greener approach.

Credit: Elsevier

Now, Vasileios Kyriakou and colleagues have designed a protonic ceramic membrane reactor that consists of Ni-BaZr_0.7Ce_0.2Y_0.1O_3–x and VN-Fe porous electrodes, respectively as the anode and cathode, separated by a BaZr_0.8Ce_0.1Y_0.1O_3–x solid electrolyte membrane. The reactor operates at ambient pressure and temperatures between 550 and 650 °C. In the anode compartment the CH₄ + H₂O reactant mixture is converted to CO₂ and H⁺. The latter is transported to the cathode compartment through the membrane, where it reacts with lattice nitrogen from the cathode to form NH₃ via a Mars–van Krevelen mechanism — nitrogen vacancies are healed with N₂ gas. The researchers achieve a maximum ammonia rate of 68 mmol_NH3 h^–1 m^–2 at 0.63 V and 600 °C, with a Faradaic efficiency to ammonia of 5.5%. Higher Faradaic efficiencies up to 14% are obtained at lower currents, at the expense of lower NH₃ rates.

The protonic ceramic membrane reactor is combined with a protonic ceramic fuel cell, fed with the leftover H₂ from the reactor plus ambient air at 550 °C, which recovers part of the electricity consumed in the synthetic process and furthermore purifies N₂ for its use in the reactor. The researchers conclude by performing energy consumption and CO₂ emissions analyses to explore the viability of their overall process. They find that at 0.3 V, the cathode catalyst’s Faradaic efficiency to NH₃ must increase to at least 35% to consume less energy than a conventional Haber–Bosch plant (500 kJ mol_NH3^–1). CO₂ emissions could be cut by half at Faradaic efficiencies higher than 75%.