Identifying and Breaking Scaling Relations in Molecular Catalysis of Electrochemical Reactions

Improving molecular catalysis for important electrochemical proton-coupled electron transfer (PCET) reactions, such as the interconversions of H⁺/H₂, O₂/H₂O, CO₂/CO, and N₂/NH₃, 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_eff). Herein, we derive a new approach for improving efficiencies-higher TOF at lower η_eff-by changing the concentrations and properties of the reactants and products, rather than by modifying the catalyst. The dependence of TOF on η_eff is shown to be quite different upon changing, for instance, the pK_a 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_eff by 59 mV and decreases the TOF by a factor of 10. Alternatively, a 10-fold decrease in K_a(HA) also lowers η_eff by 59 mV but only decreases the TOF by a factor of 2. This approach has been used to improve a catalytic TOF by 10⁴ 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.