Nickel-Based Two-Electron Redox Shuttle for Dye-Sensitized Solar Cells in Low Light Applications

Dye-sensitized solar cells (DSCs) are important to indoor solar powered devices and energy sustainable buildings because of their remarkable performance under indoor/ambient light conditions. Triiodide/iodide (I3–/I–) has been used as the most common redox mediator in DSCs because of its desirable kinetic properties and multielectron redox cycle. However, the low redox potential, corrosiveness, competitive visible light absorption, and lack of tunability of this redox mediator limit its performance in many DSC devices. Here we report a class of transition metal complex redox shuttles which operate on a similar multielectron redox cycle as I3–/I– while maintaining desirable kinetics and improving on its limitations. These complexes, nickel dithiocarbamates, were evaluated as redox shuttles in DSCs, which exhibited excellent performance under low light conditions. The recombination behavior of the redox shuttles with electrons in TiO2, dye regeneration behavior, and counter electrode electron transfer resistance were studied via chronoamperometry and electrochemical impedance spectroscopy (EIS). Further, DSC devices were studied with the Ni-based redox shuttles via incident photon-to-current conversion efficiencies (IPCEs) and current–voltage (J–V) curves under varied light intensities. The Ni-based redox shuttles showed up to 20.4% power conversion efficiency under fluorescent illumination, which was higher than I3–/I–-based devices (13%) at similar electrolyte concentrations. Taken together, these results show that nickel dithiocarbamate redox shuttles have faster rates of dye regeneration than the I3–/I– shuttle but suffer from faster recombination of photoinjected electrons with oxidized Ni­(IV) species, which decrease photovoltages

Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Herein, we investigated the redox cycle of nickel(II) bis(diethyldithiocarbamate), NiII(dtc)2, for potential use as a multielectron storage catholyte in nonaqueous redox flow batteries (RFBs). Previous studies have shown that the unique redox cycle of NiII(dtc)2 offers 2e– chemistry upon oxidation from NiII → NiIV but 1e– chemistry upon reduction from NiIV → NiIII → NiII. Electrochemical experiments presented here show that the addition of as little as 10 mol % ZnII(ClO4)2 to the electrolyte consolidates the two 1e– reduction peaks into a single 2e– reduction where [NiIV(dtc)3]+ is reduced directly to NiII(dtc)2. This catalytic enhancement is believed to be due to ZnII removal of a dtc– ligand from a NiIII(dtc)3 intermediate, resulting in more facile reduction to NiII(dtc)2. The addition of ZnII also improves the 2e– oxidation, shifting the anodic peak negative and decreasing the 2e– peak separation. H-cell cycling experiments showed that 97% Coulombic efficiency and 98% charge storage efficiency was maintained for 50 cycles over 25 h using 0.1 M ZnII(ClO4)2 as the supporting electrolyte. If ZnII(ClO4)2 was replaced with TBAPF6 in the electrolyte, the Coulombic efficiency fell to 78%. The use of ZnII to increase the reversibility of 2e– transfer is a promising result that points to the ability to use nickel dithiocarbonates for multielectron storage in RFBs