Ketjenblack-Supported and Unsupported ZrO2–ZrN Nanoparticle Systems for Enabling Efficient Electrochemical Nitrogen Reduction to Ammonia

Abstract

Artificial N₂ fixation via the electrocatalytic nitrogen (N₂) reduction reaction (NRR) has been recently promoted as a rational route toward reducing energy consumption and CO₂ emission as compared with the traditional Haber–Bosch process. Nevertheless, optimizing NRR relies on developing highly efficient electrocatalysts. Herein, we report on the reliable and reproducible synthesis of two promising electrocatalysts in either the presence or absence of Ketjenblack (KB), namely, ZrO₂–ZrN@KB and ZrO₂–ZrN systems, synthesized through the nitriding of Zr. Both materials had never previously been considered for NRR, to the best of our knowledge. Nevertheless, both of these electrocatalysts incorporated a combination of tetragonal ZrO₂, ZrON, and cubic ZrN and showed excellent activity and durability toward NH₃ formation. Moreover, the maximum NH₃ production rate of 84.1 μg h^–1 mg^–1 at −0.7 V vs a reversible hydrogen electrode (RHE) was achieved with the ZrO₂–ZrN electrocatalyst with an impressive Faradaic efficiency of 21.2% at −0.6 V vs RHE, indicating a high selectivity associated with the NRR. Additionally, the catalysts demonstrated excellent stability during the electrolysis process and recycling tests. We postulate that the combination of exposed active sites of ZrN and ZrO₂ likely contributes to the enhanced NRR performance attributed to ZrO₂–ZrN.