Assessing electrolyte effects on the kinetics and thermodynamics of 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). This phenomenon limits the achievable catalytic rate for a given overpotential (η) and remains a longstanding challenge for multielectron, multiproton reactions including the electrochemical nitrate reduction reaction (NO3RR). In this work, we explore the impact of electrolyte composition on the performance of the molecular electrocatalyst [CoIII(DIM)Br2]ClO4 for NO3RR, with a particular focus on operating in alkaline electrolytes which facilitates NH3 recovery. Electroanalytical studies reveal that bromide concentration modulates the redox behavior of the CoIII/II and CoII/I couples through ligand (de)coordination such that lower bromide concentrations promote active catalyst formation. Meanwhile, both TOF and Ecat/2 remain independent of electrolyte pH because neither protons nor water are involved in the rate-limiting step of catalysis. [CoIII(DIM)Br2]ClO4 therefore enables operation in alkaline electrolytes, which can be leveraged for reactive separation processes to yield a pure ammonia product. In a solution of (0.01 M KBr + 80 mM KNO3 + 2 mM [CoIII(DIM)Br2]ClO4) adjusted to pH 11.1, we achieve a TOF of (5.3 ± 0.4) s−1 at −0.13 V vs. RHE (η ≈ 0.65 V), outperforming the catalytic activities (TOF ≈ 10−1 to 10 s−1) and overpotentials (typically > 1.5 V) of state-of-the-art NO3RR molecular electrocatalysts. These findings highlight how aqueous electrolyte composition can be tuned to benefit catalytic activity and support operation of molecularly catalyzed NO3RR within an industrially relevant pH range.