Non-aqueous redox flow batteries (NAqRFBs) have recently received considerable attention as promisinghigh energy density,low cost grid-level energy storage technologies. Despite these attractive features,NAqRFBsare still at an early stage of development and innovative designtechniques are necessary to improve performance and decrease costs. In this work, we investigate multi-electron transfer, commonion exchangeNAqRFBs. Common ion systemsdecrease thesupporting electrolyte requirement, which subsequently improves active material solubility and decreases electrolytecost.Voltammetric and electrolytic techniques are used to study the electrochemical performance and chemical compatibility of model redox active materials,iron (II) tris(2,2'-bypridine) tetrafluoroborate(Fe(bpy) 3 (BF 4) 2) and ferrocenylmethyl dimethyl ethyl ammonium tetrafluoroborate (Fc1N112-BF 4). These results helpdisentangle complex cycling behavior observed in flow cell experiments.Further, a simple techno-economic modeldemonstrates the cost benefits of employing common ion exchange NAqRFBs, afforded by decreasing the salt and solventcontributions to total chemical cost.This study highlights two new concepts, common ion exchange and multi-electron transfer, for NAqRFBs through a demonstration flow cell employing model active species. In addition, the compatibility analysis developed for asymmetric chemistries can apply to other promising species, including organics, metal coordination complexes (MCCs) and mixed MCC/organic systems, enabling the design of low cost NAqRFBs.

Anatase phase TiO2 and related alloyed TiO2:N and TiO2:Nb congeners have been prepared by sol–gel processing techniques. The coalloyed TiO2:(Nb,N)-1 composition, in which niobium substitutes for titanium on the cation sublattice and nitrogen appears in either chemisorbed or interstitial sites as well as substitutes for oxygen on the anion sublattice, has also been prepared. EPR spectroscopy performed on the coalloyed material at 4 K shows that the bulk material contains minor impurities of Ti3+ and F + centers. Annealing this compound under oxygen oxidizes the material to give TiO2:(Nb,N)-2, which is EPR silent. All alloyed compositions show surface areas of 41–68 m2/g, different from the 2 m2/g for TiO2. In addition, the monoalloyed compounds show band gaps that are not significantly different than that of the parent TiO2 composition (3.2 eV), whereas the coalloyed compound TiO2:(Nb,N)-1 shows a significantly lower energy absorption edge of 2.0 eV. Each composition was tested for its ability to photodegrade methylene blue (MB) dye catalytically, and the coalloyed composition TiO2:(Nb,N)-1 shows a 7-fold increase in rate (1.203 h–1) compared to the parent TiO2 phase. The oxygen annealed version, TiO2:(Nb,N)-2, shows a rate of only 0.234 h–1, leading to the conclusion that bulk Ti3+ and/or F + centers serve as catalytic sites for MB degradation.

Visible-light-absorbing compounds that generate active oxygen species in water are needed to maximize the rate of organic dye degradation under incident solar radiation. In this study, a series of Ti 1−(5x/4) Nb x O 2−y−δ N y compounds, herein denoted as TiO 2 :(Nb,N)-x, with varying mole percentage of niobium substituting for titanium (x = 1−30) was prepared by a sol−gel process followed by nitridation under flowing ammonia. All compositions crystallized in the anatase structure, as determined by powder X-ray diffraction, and diffuse reflectance UV−vis spectroscopy showed that the indirect band gap ranged from 2.37 eV (x = 1) to 2.20 eV (x = 30). X-ray photoelectron spectroscopy revealed that the mole percentage of substitutional nitrogen in the compounds was a linear function of the mole percentage of niobium present. NbN-25 and -30 compounds exhibited a 6-fold increase in the rate of methylene blue dye degradation (k = 0.779 and 0.759 h −1 , respectively) compared to lower-molepercentage niobium samples NbN-1 and -5 (k = 0.115 and 0.146 h −1 , respectively).