Transition metal nitrides for the electrocatalysis of the electrochemical ammonia synthesis

Kurzfassung

The direct electrochemical ammonia synthesis (EAS) from water and nitrogen using renewable energy shows great potential for future decentralized production of indispensable green ammonia and may, at least partially, supersede the fossil fuel-based Haber-Bosch process in the future [1]. Material innovation for the conversion of nitrogen to ammonia via the nitrogen reduction reaction (NRR) is one cornerstone for the successful development of the EAS technology. To date, only Lithium-mediated NRR has been demonstrated to show sufficient production rate and faradaic efficiency for a prospective application [2], while the reaction in aqueous electrolyte, independent of the studied material class, suffers from false-positive results due to low productions rates, low Faradaic efficiencies and complex contamination issues [3]. Several catalyst systems are currently investigated for NRR in aqueous electrolytes. Transition metal nitrides (TMN) may offer an energetic advantage as electrocatalyst, because the NRR is expected to be catalyzed via the Mars-van-Krevelen mechanism (MvK) on TMNs. Here the protonation of a lattice nitrogen atom forms the first ammonia molecule. The resulting N-vacancy is filled by molecular nitrogen where one N adatom is subsequently protonated to form the second ammonia molecule. Thus, the N2 activation step occurs at the N-vacancy site and avoids the surface adsorption and dissociation of N2. Specific facets of Zr, Cr, V and Nb were described as stable, active and selective TMN catalysts for the NRR based on theoretical catalyst screening [4]. However, current literature studies state contradictory results and require the differentiation of genuine activity and non-catalytic decomposition of TMN catalysts [5], which is currently lacking. In our work, we address the synthesis of Zr-based TMNs, the electrochemical evaluation in a gas diffusion electrode setup and the trace analysis of ammonia by ion chromatography [6]. Only consideration of all three interconnected steps can give an indication of genuine NRR activity, where structural characterization after NRR experiments elucidates the stability of the material. Special interest is given to the role of surface oxynitrides and their ratio to the nitride phase for electrocatalytic NRR. Comparison of nanoparticulate catalysts to surface engineered thin film catalyst as well as insights from first principles simulation delivers a comprehensive characterization of the electrosynthesis of ammonia by TMNs.