Green synthesis of a redox-active riboflavin-integrated Ni-MOF and its versatile electrocatalytic applications towards oxygen evolution and reduction, and HMF oxidation reactions

The post-synthetic modification (PSM) of metal–organic frameworks (MOFs) with redox-active molecules under mild conditions is a highly challenging approach to modify the inherent properties of MOFs without altering their crystallinity and other characteristics. Here, we prepared a single crystal Ni-MOF with a two-dimensional rod-like morphology. Furthermore, we utilized the PSM technique to incorporate redox-active riboflavin (Rbf) in the Ni-MOF using a green and facile mechanochemical method under solvent-free conditions. The Rbf-doped Ni-MOF (Rbf-Ni-MOF) showed a 4-fold increase in conductivity compared to the pristine Ni-MOF. We employed the Rbf-Ni-MOF as a multifunctional electrocatalyst towards the oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and oxidation of 5-hydroxymethylfurfural (HMF). As an OER electrocatalyst, the Rbf-Ni-MOF delivered a high current density of 580 mA cm−2, which is around 26-fold greater than those of Rbf and the pristine Ni-MOF. The prepared Rbf-Ni-MOF showed the lowest Tafel slope value of 0.039 V dec−1 with a minimum overpotential (0.220 V) at the benchmark value of 10 mA cm−2. Interestingly, the Rbf-Ni-MOF also showed an enhanced ORR activity with a minimum onset potential of 0.89 V and a larger current density of 4.85 mA cm−2, which are comparable with those of Pt-based standard electrocatalysts. In addition, the Rbf-Ni-MOF efficiently catalyzed the oxidation of HMF to 2,5-furan dicarboxylic acid (FDCA) with more than 95% faradaic efficiency. DFT analysis proved that the incorporation of Rbf in the Ni-MOF alters its band structures and induces a certain conductivity, which helps the Rbf-Ni-MOF display enhanced electrocatalytic activity. Hence, this approach provides new insights into the PSM method for modulating the electrical and redox properties of MOFs in order to employ them as electrode materials for energy applications.