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

Abstract

The development of advanced chemical-to-electrical energy conversions requires fast and efficient electrocatalysis of multielectron/multiproton reactions, such as the oxygen reduction reaction (ORR). Using molecular catalysts, correlations between the reaction rate and energy efficiency have recently been identified. Improved catalysis requires circumventing the rate versus overpotential trade-offs implied by such "scaling relationships." Described here is an ORR system-using a soluble iron porphyrin and weak acids-with the best reported combination of rate and efficiency for a soluble ORR catalyst. This advance is achieved not by "breaking" scaling relationships but rather by combining two of them. Key to this behavior is a polycationic ligand, which enhances anionic ligand binding and changes the catalyst E 1/2 . These results show how combining scaling relationships is a powerful way toward improved electrocatalysis.

Using nature’s blueprint to expand catalysis with Earth-abundant metals

“…For example, an Fe complex(62) catalyzes the asymmetric transfer hydrogenation of ketones with performance superior to that of Ru catalysts, and a Co complex catalyzes the asymmetric hydrogenation of the C=C bond of enamides. (63) In addition to modifications of the ligands bound directly to the metal (primary coordination sphere), the environment of molecular catalysts can be tuned by positioning secondary coordination sphere substituents, such as Lewis acids,(64) positively charged groups, (65)(66)(67) hydrogen bond donors, (68) and pendant amines functioning as proton relays(69-73) ( Figure 5A) proximal to the EAM center. These strategies have enhanced the rates of molecular EAM catalysis of electrochemical H2 evolution,(69-71) H2 oxidation, (70,71,73) CO2 reduction, (65) and O2 reduction.…”

“…These strategies have enhanced the rates of molecular EAM catalysis of electrochemical H2 evolution,(69-71) H2 oxidation, (70,71,73) CO2 reduction, (65) and O2 reduction. (74) Since the redox reactivity involves coupling of electron flow and bond rearrangement, the secondary sphere substituents must be precisely positioned to foster optimal cooperativity. For example, the rates of proton-coupled electron transfer (75,76) can be sensitive to sub-angstrom level changes in the proton-donor acceptor distance.…”

Recent Developments in Metalloporphyrin Electrocatalysts for Reduction of Small Molecules: Strategies for Managing Electron and Proton Transfer Reactions

“…Such an investigation of ORR electrocatalysis in CH 3 CN using a series of Fe‐porphyrins ( 1 , 3 , 4 , 5 , 8 – 14 ) revealed that the logarithm of the rates of O 2 reduction varied linearly with overpotential [25] . That observation led to the proposal and development of “scaling relationships” that provide a framework for rapidly identifying optimal ORR parameters for different catalyst families and for comparing the performance of catalysts studied under different conditions [26–28] . This simple, predictive approach is not without controversy [29] .…”

“…[25] That observation led to the proposal and development of "scaling relationships" that provide a framework for rapidly identifying optimal ORR parameters for different catalyst families and for comparing the performance of catalysts studied under different conditions. [26][27][28] This simple, predictive approach is not without controversy. [29] Further explorations of scaling relationships in buffered CH 3 CN using the cationic compound 15 showed that anionic buffers (e. g., benzoic and acetic acid derivates) can be used to vary the catalyst formal reduction potential (E 1/2 ) and therefore the ORR overpotential.…”

Beyond the Active Site: Mechanistic Investigations of the Role of the Secondary Coordination Sphere and Beyond on Multi-Electron Electrocatalytic Reactions and Their Relations Between Kinetics and Thermodynamics

“…Recently, a series of Fe tetraphenylporphyrin (FeTPP) derivatives has been prepared by the Mayer group to investigate their catalytic mechanisms for ORR and the effect of the SCS through judicious designs of porphyrin ligands. [16][17][18][19][20] All reported rate constants for these complexes were determined by FOWA, which they referred to as the rate-limiting step for the first proton transfer as shown in Scheme 1 (eq (5)ORR) following sequential EC steps. However, re-examinations of voltammograms and FOWA plots suggest that the rate constant determined by FOWA is not the rate-limiting step and instead should be assigned to the first fast chemical step as suggested in the Table 1.…”