Titanium-, Nitrogen-Doped Carbon Flowers Catalyze Electrochemical Nitrate Reduction Reaction to Ammonia

An emerging design heuristic for electrochemical nitrate reduction (NO3RR) catalysts is synthesizing electron-deficient sites to facilitate binding of electron-rich NO3-. However, this rule has rarely been applied to metal-, nitrogen-doped carbon (MNC) catalysts. Titanium (Ti), with low electronegativity and high NO3RR reactivity, is a compelling MNC candidate. To date, atomically dispersed TiNx motifs have eluded synthesis due to the strong oxophilicity of Ti. Here, we leverage nitrogen-rich carbon flowers (CF) to overcome synthetic challenges and produce Ti-, N-doped carbon flower (TiCF) catalysts. Advanced materials characterization demonstrates that TiCF catalysts are a mixed phase material with 3/4 of Ti atoms in TiO2-like nanoparticles and 1/4 of Ti atoms in novel, atomically dispersed TiNx sites. TiCF achieves 61 ± 7% NH3-selectivity at -0.70 V vs RHE and 14 ± 5 mA/cm2 to NH3 formation (|jNH3|) at -0.85 V vs RHE in (0.1 M NaOH + 0.1 M NaNO3 + 0.45 M Na2SO4) electrolyte. Control studies show both CF morphology and Ti sites are essential for high NO3RR activity. Density functional theory calculations attribute the NO3RR reactivity to TiNx, which facilitates multiple bond formation with surface intermediates to promote favorable NH3 synthesis pathways. Thus, TiCF exhibits 60× higher |jNH3| values than bulk Ti and NH3 yield rates (>0.06 mmol NH3/h/cm2) that are competitive with state-of-the-art MNC catalysts (e.g., FeNC, CuNC). TiCF introduces a new class of Ti electrocatalysts, advancing the MNC design space and sustainable NH3 production.