Oxidation State Dependent Conjugation Controls Electrocatalytic Activity in a Two-Dimensional Di-Copper Metal-Organic Framework

Abstract

Molecularly defined two-dimensional conjugated metal-organic frameworks combine properties of molecular and material based electrocatalyst, enabling tunable active site and bandgap design. The rational optimization of these systems requires an understanding of the complex interplay between the various metal sites and their influence on catalysis. For this purpose, copper-phthalocyanine-based two-dimensional conjugated metal-organic framework (CuPc-CuO4 2D c-MOF) films with an edge-on layer orientation were transferred to nickel-nitrilotiacetic acid (Ni-NTA)-functionalized graphite electrodes and analyzed via electrochemical resonance Raman spectroscopy. With the help of density functional theory (DFT) calculations for the first time a detailed assignment of the vibrational bands for different Cu oxidation states could be achieved and correlated to their electrocatalytic activity in respect to oxygen reduction (ORR). Potential dependent Raman spectroscopy made it furthermore possible to determine the redox potentials of the Cu in the CuPc moieties and the Cu-catecholate nodes individually with ECuPc = −0.04 V and ECuO4 = +0.33 V versus Ag|AgCl. Electrocatalytic ORR, however, was only observed below −0.2 V where both Cu units were present in their respective CuI state. DFT calculations of bandgaps and density of states (DOS) showed a significant decrease in bandgap and increase in π-conjugation upon transition from the inactive mixed CuII/CuI state to the active CuI/CuI state, suggesting that slow electron transfer in the mixed state limits ORR catalysis. Our results indicate that the coupling between metal oxidation changes and π-conjugation of the 2D c-MOF is a key parameter toward achieving electrocatalytic activity.