Tuning electrolyte composition subverts scaling relationships in molecular catalysis of electrochemical nitrate reduction

In molecular electrocatalysis, a log-linear tradeoff between turnover frequency (TOF) and overpotential (η) is frequently observed and limits catalytic performance. This scaling relationship is reflected in the molecularly catalyzed electrochemical nitrate reduction reaction (NO3RR), which generally requires large overpotentials for ammonia production (η > 1.5 V). Thus, strategies to lower η while increasing TOF are needed for 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 bromide concentration can be synergistically tuned to lower η (up to 0.50 V) while increasing TOF (up to 5.2x). We therefore tailor electrolyte composition to achieve a TOF of (5.3 ± 0.4) s−1 at −0.13 V vs. RHE (η ≈ 0.85 V), surpassing both the catalytic activities and overpotentials of state-of-the-art NO3RR molecular electrocatalysts. Our results reveal that tuning electrolyte composition can help overcome the TOF-η tradeoff in molecular electrocatalysis.