Tuning Electrolyte Composition to Modulate Catalytic Performance in Molecularly Catalyzed Electrochemical Nitrate Reduction

In molecular electrocatalysis, the turnover frequency (TOF) plateaus at applied potentials beyond the half-wave potential of the catalyst (Ecat/2), limiting the achievable catalytic rate for a given overpotential (η). This phenomenon remains a longstanding challenge for multielectron, multiproton reactions including the electrochemical nitrate reduction reaction (NO3RR), which generally requires large overpotentials for ammonia production (ηNO3−/NH3 > 1.5 V). Thus, strategies to lower ηNO3−/NH3 while increasing TOF are needed for a more energy-efficient NO3RR. In this study, we investigate the molecular electrocatalyst [CoIII(DIM)Br2]ClO4 (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene), which selectivity catalyzes NO3RR to NH3. Through electroanalytical studies, we demonstrate that electrolyte pH and counteranion concentration can be synergistically tuned to lower ηNO3−/NH3 (up to 0.50 V) while increasing TOF (up to 5.2×). We therefore tailor electrolyte composition to achieve a TOF of (5.3 ± 0.4) s–1 at −0.13 VRHE (ηNO3−/NH3 ≈ 0.85 V) in a solution of (0.01 M KBr + 80 mM KNO3 + 2 mM [CoIII(DIM)Br2]ClO4) adjusted to pH 11.1. This catalytic performance surpasses both the activities and overpotentials of state-of-the-art NO3RR molecular electrocatalysts. Our results reveal that even in complex reactions such as NO3RR (8 e– and 10 H+ transfers to produce NH3), electrolyte composition provides a simple handle for promoting catalytic performance.