Active-Site Modulation in an Fe-Porphyrin-Based Metal–Organic Framework through Ligand Axial Coordination: Accelerating Electrocatalysis and Charge-Transport Kinetics

Abstract

The construction of artificial solar fuel generating systems requires the heterogenization of large quantities of catalytically active sites on electrodes. In that sense, metal–organic frameworks (MOFs) have been utilized to assemble unpreceded concentration of electrochemically active molecular catalysts to drive energy-conversion electrocatalytic reactions. However, despite recent advances in MOF-based electrocatalysis, so far no attempt has been made to exploit their unique chemical modularity in order to tailor the electrocatalytic function of MOF-anchored active sites at the molecular level. Here, we show that the axial coordination of electron-donating ligands to active MOF-installed Fe-porphyrins dramatically alters their electronic properties, accelerating the rates of both redox-based MOF conductivity and the electrocatalytic oxygen reduction reaction (ORR). Additionally, electrochemical characterizations show that in multiple proton-coupled electron transfer reactions MOF-based redox hopping is not the only factor that limits the overall electrocatalytic rate. Hence, future efforts to enhance the efficiency of electrocatalytic MOFs should also consider other important kinetic parameters such as the rate of proton-associated chemical steps as well as mass-transport rates of counterions, protons, and reactants toward catalytically active sites.

A Porphyrinic Zirconium Metal–Organic Framework for Oxygen Reduction Reaction: Tailoring the Spacing between Active-Sites through Chain-Based Inorganic Building Units

“…A close packing could further enhance the overall charge mobility via the hopping mechanism. 30 − 32 As a proof of concept, the electrochemical ORR catalyzed by PCN-226 was tested.…”

“… 27 − 29 Metalloporphyrin complexes exhibit strong interactions with O 2 and offer a good opportunity by serving as a channel for electron transfer via a hopping mechanism. 30 − 32 Moreover, the rich chemistry and the high electrochemical stability of metalloporphyrin complexes endow porphyrinic MOFs as designable and durable electrocatalysts. Recently, such MOFs have been successfully utilized for the electrocatalytic hydrogen evolution reaction (HER), 33 , 34 CO 2 reduction reaction (CO 2 RR), 35 − 37 oxygen evolution reaction (OER), 38 − 40 and oxygen reduction reaction (ORR), 41 − 44 etc.…”

Circumventing Scaling Relations in Oxygen Electrochemistry Using Metal–Organic Frameworks

“…MOFs) where the 3-D structure of the active site can be intentionally designed to circumvent the *OOH vs. *OH scaling relations. Other factors that impact the efficacy of MOF electrocatalysts, such as charge transport, [62][63][64][65] counterion and substrate diffusion, 64,65 explicit solvation, 47 etc. represent ongoing research directions in the group.…”

From Molecules to Porous Materials: Integrating Discrete Electrocatalytic Active Sites into Extended Frameworks

“…Such strategies can be extrapolated to framework materials. 121 , 122 Future investigations into the role of trapped molecular additives and ions inside the pores of framework materials and their effects on the electrocatalytic behavior can provide new fundamental knowledge in catalyst design.…”