Identifying and Breaking Scaling Relations in Molecular Catalysis of Electrochemical Reactions

Abstract

Improving molecular catalysis for important electrochemical proton-coupled electron transfer (PCET) reactions, such as the interconversions of H/H, O/HO, CO/CO, and N/NH, is an ongoing challenge. Synthetic modifications to the molecular catalysts are valuable but often show trade-offs between turnover frequency (TOF) and the effective overpotential required to initiate catalysis (η). Herein, we derive a new approach for improving efficiencies-higher TOF at lower η-by changing the concentrations and properties of the reactants and products, rather than by modifying the catalyst. The dependence of TOF on η is shown to be quite different upon changing, for instance, the pK of the acid HA versus the concentration or partial pressure of a reactant or product. Using the electrochemical reduction of dioxygen catalyzed by iron porphyrins in DMF as an example, decreasing [HA] 10-fold lowers η by 59 mV and decreases the TOF by a factor of 10. Alternatively, a 10-fold decrease in K(HA) also lowers η by 59 mV but only decreases the TOF by a factor of 2. This approach has been used to improve a catalytic TOF by 10 vs the previously reported scaling relationship developed via synthetic modifications to the catalyst. The analysis has the potential to predict improved efficiency and product selectivity of any molecular PCET catalyst, based on its mechanism and rate law.

Combining scaling relationships overcomes rate versus overpotential trade-offs in O 2 molecular electrocatalysis

“…In contrast, using acetic acid (AcOH, pK a = 23.5) gave markedly improved catalysis: a faster TOF max (170 s −1 ) at less than half the  eff (0.54 V). This result contradicts previously derived log(TOF max )/ eff relationships, which always predict that a lower overpotential will give a slower rate, as seen for iron tetra-arylporphyrins (see the Supplementary Materials) (7). Catalysis with 1 and trifluoroacetic acid (TFAH), salicylic acid (SalOH), or benzoic acid (BzOH) similarly show improvements in both TOF max and  eff versus the [DMF-H + ] point ( Fig.…”

“…This unique distinction is due to the highly cationic nature of 1, which enhances binding of anionic conjugate bases but not neutral ones (see the Supplementary Materials). Further supporting the importance of the cationic ligand, no notable change in catalyst E 1/2 was reported when iron tetraphenylporphyrin-an analog without the cationic trimethylanilinium groups-was combined with similar buffers under similar conditions (7).…”

“…1A, blue dashed line), a previously unrealized goal in molecular electrocatalysis (5,13,14). The following sections describe a model that explains this inverse relationship as the sum of two known kinetic/thermodynamic scaling relationships (7).…”

“…Cyclic voltammograms (CVs) of 1 in MeCN containing 0.1 M tetra-n-butylammonium tetrafluoroborate electrolyte ([n-Bu 4 N][BF 4 ]) showed three reversible redox features (Fe III/II , Fe II/I , and Fe I/0 (10). Solutions containing 1, O 2 , and buffered acid (1:1 [HA]/[A − ]) showed a large, irreversible current at the Fe III /Fe II reduction potential, indicative of catalysis (5,7,11). All CV experiments were buffered (1:1 acid-to-conjugate base) to define the ORR equilibrium potential and  eff , which was calculated using Eq.…”

“…All CV experiments were buffered (1:1 acid-to-conjugate base) to define the ORR equilibrium potential and  eff , which was calculated using Eq. 1 (5,7,11).…”