A versatile iron [1-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole](3) metal complex redox active material for energy conversion and storage systems

A novel metal complex redox material is synthesized via coordinating the iron metal site with 1-(naphthalen-2-ylmethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (npbi) ligands, herein named as Fe2+/3+ [npbi](3). The as-synthesized Fe2+/3+ [npbi](3) redox material has a good redox potential (0.43 V vs. NHE), low Gibbs free energy (1.86 eV), low reorganization energy (1.90 eV), high diffusion coefficient and less driving force in an electrolyte system for dye sensitized solar cell (DSC) and supercapacitor (SC) applications to accomplish huge quasi-reversible redox operations between the electrode/electrolyte interfaces of the devices. Interestingly, the DSC with the Fe2+/3+ [npbi](3) electrolyte and an organic dye achieved a conversion efficiency of 2.96% under 1 sun illumination. This high power efficiency is attributed to a potential matching of this redox system with the organic dye, which can diminish the recombination reactions and improve the open circuit voltage (OCV) of the DSC. At the same time, the addition of Fe2+/3+ [npbi](3) in a symmetric SC with a H2SO4 electrolyte and graphene oxide electrodes contributes to an additional Faradaic reaction (electro-sorb mechanism) at the electrode/electrolyte interface. This results in the corresponding SC displaying a high energy density of 29.79 W h kg(-1) at a power density of 719 W kg(-1) and long charge/discharge stability for 11 000 cycles with 97.4% capacity retention respectively. Overall, this study offers new insights on designing appropriate transition metal complex redox materials for both DSC and SC applications with boosted efficiencies.