(Invited) Electrokinetics and Operational Stability of the Ammonia-Selective Nitrate Reduction Catalyst Co (DIM)
Authors: Liu Matthew Junjie , Miller Dean , Tarpeh William Abraham 
Published: September 2023

Haber-Bosch fertilizer production has enabled large-scale food production; however, the process demands exorbitant energy inputs while yielding direct atmospheric carbon emissions, and the resulting anthropogenic perturbations to the global nitrogen cycle threaten water quality. Enabling a sustainable food-energy-water nexus requires feeding a growing population while minimizing environmental impacts. Molecular catalysis of aqueous electrochemical nitrate reduction (NO3RR) to ammonia can simultaneously reduce nitrogen pollution and electrify ammonia manufacturing. While several Co-N4 complexes have been explored as molecular catalysts for aqueous NO3RR, such as CoIII–cyclam and CoIII–protoporphyrin,1,2 the metal complex we focus on in this talk is the cobalt–centered macrocycle [Co(DIM)Br2]+ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene: abbreviated as Co(DIM)) because it remains the only water-soluble catalyst that has been reported to yield ammonia from NO3RR with near full selectivity (> 95%).3–5

50 years have passed since Co(DIM) was synthesized in 1972. Yet despite the promise Co(DIM) holds for advancing progress towards molecular-catalyst-enabled wastewater-NO3RR, the few existing studies on Co(DIM) focus primarily on the structural and electronic properties of Co(DIM) that contribute to selective nitrate and nitrite conversion.5–8 A dearth of experimental investigations prevents the rational design and implementation of NO3RR molecular platforms such as Co(DIM). From an electroanalysis standpoint, kinetic benchmarking of Co(DIM)-mediated NO3RR would contextualize the performance of Co(DIM) relative to state-of-the-art molecular electrocatalysts and quantitatively facilitate optimization goals for next-generation NO3RR molecular catalysts. While turnover frequency (TOF) vs. overpotential (η) relationships are reported for HER, ORR, and CO2RR molecular electrocatalysts, to date, such a relationship has not been reported for any molecular NO3RR electrocatalyst. And salient to any molecular electrocatalyst, studies on the operational stability of Co(DIM) are needed to provide greater insight into the conditions that contribute to catalyst instability and thus strategies to avoid catalyst degradation.

In this work, we propose an ECEC mechanism that governs the kinetics of Co(DIM)-mediated NO3RR. From first-principles, we solve a 1D reaction-diffusion model that relates catalytic current to applied potential and validate the model through fitting experimental, voltammetric data. Through foot-of-the-wave and plateau current analysis, we evaluate the turnover frequency (TOF) vs. overpotential (η) relationship for Co(DIM)-mediated NO3RR. These methods provide approximations for TOF0, a measure of intrinsic kinetics, and TOFmax, the TOF achievable by Co(DIM) in the absence of interfacial nitrate depletion and catalyst degradation. TOF values from controlled potential electrolysis (CPE) experiments are consistently lower than TOFmax, and we identify two key reasons for the discrepancy. First, interfacial substrate depletion due to mass transfer limitations prevents TOFmax from being achieved during CPE. Second, catalyst degradation in alkaline environments, as demonstrated via characterization of the electrode (XPS, SEM-EDX, and XRD) and electrolyte (UV-VIS, IC), leads to loss in activity. Through in situ CVs and ex situ rinse tests (CPE involving electrodes with surface immobilized degradation products and no Co(DIM) in solution), we demonstrate that the degradation products likely play a small catalytic role compared to Co(DIM), highlighting the importance of maintaining the structural integrity of Co(DIM) during CPE operation. This work addresses several critical knowledge gaps surrounding Co(DIM)-mediated NO3RR in order to meet the scale and urgency of electrochemically remediating nitrogen emissions and producing ammonia.

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