Joule (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-BaZr0.7Ce0.2Y0.1O3–x and VN-Fe porous electrodes, respectively as the anode and cathode, separated by a BaZr0.8Ce0.1Y0.1O3–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 mmolNH3 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 molNH3–1). CO2 emissions could be cut by half at Faradaic efficiencies higher than 75%.