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.

Redox-active Metal−Organic Frameworks (MOFs) are considered as promising platforms for assembling high quantities of solutionaccessible molecular catalysts on conductive surfaces, toward their utilization in electrochemical solar fuel related reactions. Nevertheless, slow redox hopping-based conductivity often constitutes a kinetic bottleneck hindering the overall electrocatalytic performance of these systems. In this work, we show that, by a systematic control of MOF defect site density, one can modulate the spatial distribution of post synthetically installed molecular catalyst and hence accelerate charge transport rates by an order of magnitude. Moreover, the improved MOF conductivity also yields an enhancement in its intrinsic electrocatalytic activity. Consequently, these results offer new possibilities for designing efficient MOF-based electrocatalytic systems.

Electrochemically active Metal-Organic Frameworks (MOFs) have been progressively recognized for their use in solar fuel production schemes. Typically, they are utilized as platforms for heterogeneous tethering of exceptionally large concentration of molecular electrocatalysts onto electrodes. Yet so far, the potential influence of their extraordinary chemical modularity on electrocatalysis has been overlooked. Herein, we demonstrate that, when assembled on a solid Ag CO 2 reduction electrocatalyst, a non-catalytic UiO-66 MOF acts as a porous membrane that systematically tunes the active sites immediate chemical environment, leading to a drastic enhancement of electrocatalytic activity and selectivity. Electrochemical analysis shows that the MOF membrane improves catalytic performance through physical and electrostatic regulation of reactants delivery towards the catalytic sites. The MOF also stabilizes catalytic intermediates via modulation of active sites secondary coordination sphere. This concept can be expanded to a wide range of proton-coupled electrochemical reactions, providing new means for precise, molecularlevel manipulation of heterogeneous solar fuels systems.

We show that a MOF-converted compound can act as highly active co-catalyst in a water splitting photo-electrochemical cell.

Carbon nitrides (CN) have emerged as promising photoanode materials for water-splitting photoelectrochemical cells (PECs). However, their poor charge separation and transfer properties, together with slow wateroxidation kinetics, have resulted in low PEC activity and instability, which strongly impede their further development. In this work, these limitations are addressed by optimizing the charge separation and transfer process. To this end, a nickel-iron based metal-organic framework, Ni/Fe-MIL-53, is deposited, that acts as an oxygen evolution pre-catalyst within the CN layer and incorporate reduced graphene oxide as an electron acceptor. Upon electrochemical activation, a uniform distribution of highly active oxygen evolution reaction (OER) catalysts is obtained on the porous CN surface. Detailed mechanistic studies reveal excellent hole extraction properties with high OER catalytic activity (83% faradaic efficiency) and long-term stability, up to 35 h. These results indicate that the decrease in performance is mainly due to the slow leaching of the catalyst from the CN layer. The CN photoanode exhibits a reproducible photocurrent density of 472 ± 20 µA cm −2 at 1.23 V versus reversible hydrogen electrode (RHE) in 0.1 m KOH, an exceptionally low onset potential of ≈0.034 V versus RHE, and high external quantum yield.