Gas Diffusion Electrodes for Electrocatalytic Oxidation of Gaseous Ammonia: Stepping Over the Nitrogen Energy Canyon

As ammonia continues to gain more and more interest as a promising hydrogen carrier compound, so does the electrochemical ammonia oxidation reaction (AmOR). To avoid the liberation of H₂ in a reverse Haber-Bosch reaction under release of the energetically more favorable N₂, we propose the oxidation of ammonia to value-added nitrite (NO₂ ^-), which is usually obtained during the Ostwald process. We investigated the anodic oxidation of gaseous ammonia directly supplied to a gas diffusion electrode (GDE) using a variety of compositionally different multi-metal catalysts coated on Ni foam under the simultaneous formation of H₂ at the cathode. This will double the amount of H₂ per ammonia molecule while applying a lower overpotential than that required for water electrolysis (1.4-1.8 V vs. RHE at 50 mA ⋅ cm^-2). A selectivity study demonstrated that some of the catalyst compositions were able to produce significant amounts of NO₂ ^-, and further investigations using the most promising catalyst composition Ni_f_AlCoCrCuFe integrated within a GDE demonstrated up to 88 % Faradaic efficiency for NO₂ ^- at the anode coupled to close to 100 % Faradaic efficiency for the cathodic H₂ production.