The direct electrochemical ammonia synthesis (EAS) from water and nitrogen using renewable energy shows great potential for future decentralized production of indispensable green ammonia [1]. However, identification of catalyst materials that genuinely catalyze the conversion of nitrogen to ammonia via the nitrogen reduction reaction (NRR) in aqueous electrolyte is the bottleneck for the successful development of 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 focus on the electrochemical evaluation of the NRR dependent on different electrode morphologies of Zr-based TMNs with trace analysis of ammonia established by ion chromatography technique [6]. Comparison of nanoparticulate ZrN catalysts [7] processed as gas diffusion electrode (GDE) to surface engineered model thin film catalyst [8] as well as insights from first principles simulation delivers a comprehensive analysis of material properties and experimental conditions for the electrosynthesis of ammonia. Detailed structural characterization after NRR experiments by x-ray photoelectron spectroscopy and dedicated chemical dissolution experiments elucidate the stability of the material as an important prerequisite of electrocatalytic NRR.