Conductive nanocrystalline diamond (NCD) films for electrocatalytic nitrate reduction

The electrocatalytic nitrate (NO 3 − ) reduction to ammonia (NH 3 ) offers a solution for simultaneous environmental remediation and sustainable ammonia production. However, the rational design of nanocrystalline diamond (NCD) film electrodes balancing high activity and long-term stability remains a critical challenge. Herein, the defect chemistry and surface architecture of NCD films were tailored by adjusting the H 2 /Ar gas ratio in a mixed atmosphere of N 2 , CH 4 , Ar, and H 2 using hot-filament chemical vapor deposition (HFCVD). A series of NCD films with sp 3 /sp 2 ratios of 0.71, 1.14, 1.83, 3.12, and 6.14 were prepared by varying the H 2 /Ar gas ratio, corresponding to electrical conductivities of 18, 8.62, 7.94, 6.1, and 4.18 S/m, respectively. The sp 2 carbon phase serves as the primary active site for nitrate adsorption and electron transfer via its conjugated π-bond network. In addition, the sp 3 carbon phase provides structural stability through its robust tetrahedral framework. The electrode with the sp 3 /sp 2 ratio of 0.71 (H0) achieved the highest faradaic efficiency (FE) of 75.0 % for ammonia synthesis at −1.5 V (vs. RHE), whereas the electrode with the sp 3 /sp 2 ratio of 1.14 (H5) exhibited a slightly lower FE of 65.2 % but superior long-term stability, maintaining a steady faradaic efficiency over 50 h of operation. The optimal sp 3 /sp 2 ratio window (0.7–1.1) balances electron transport efficiency (sp 2 -dominated) and structural durability (sp 3 -dominated), revealing a synergistic interplay between defect-mediated conductivity and chemical robustness. Investigation of the mechanism reveals that optimal H 2 /Ar ratios selectively etch sp 2 impurities while preserving conductive pathways, creating a synergistic interplay between defect-mediated electron transport and chemically robust sp 3 frameworks. This study establishes a scalable strategy for engineering high-performance diamond electrocatalysts, bridging the gap between activity and durability in environmental electrochemistry.