Vanadium Nitride Is an Efficient Hydrogen-Diffusive Cathode for Green Ammonia Electrochemical Synthesis by Protonic Solid Oxide Electrolysis Cells
Naoki Kamitani, Seongwoo Jeong, H. Habazaki, Yoshitaka Aoki
Abstract
H 2 O–N 2 coelectrolysis by protonic solid oxide cells (H + -SOCs) holds great promise as it enables the sustainable production of pure ammonia gas from air and water through renewable electricity. This investigation highlights the exceptional hydrogen ion diffusivity and limited hydrogen evolution reaction (HER) activity of VN 0.9, establishing it as a valuable cathode material for H + -SOCs. The model cells were fabricated by sputter-depositing the dense VN x film cathode ( x = 0.9 and 1.1) on BaZr 0.4 Ce 0.4 Y 0.1 Yb 0.1 O 3-δ (BZCYYb4411) bulk proton electrolytes. The hydrogen pumping measurements proved that the VN 0.9 cathode allowed the bulk diffusion of the hydrogen ions while effectively retarding hydrogen evolution at its surfaces. Hence, coelectrolysis with the Ru-loaded VN 0.9 cathode achieved an ammonia Faraday efficiency and production rate of 12% and 3.8 × 10 –9 mol cm –2, respectively, which were 2 orders of magnitude higher than those of the coelectrolytic H + -SOCs using metal cathodes. Integration of electrochemical measurements and mass spectroscopy established that the desorption of ammonia products was the rate-controlling step of the ENRR on the Ru-loaded VN 0.9 cathode. The Ru-loaded VN 0.9 cathode enabled continuous ammonia synthesis at an average rate of 2.1 × 10 –9 mol cm –2 over 50 h with periodic rest at open-circuit voltage for the catalyst reactivation by the release of chemisorbed NH 3 .