A reversible Fe /Fe redox couple of an azamacrocyclic complex is evaluated as an electrolyte with a pH-tunable potential range for aqueous redox-flow batteries (RFBs). The Fe complex is formed by 1,4,7-triazacyclononane (TACN) appended with three 2-methyl-imidazole donors, denoted as Fe(Tim). This complex exhibits pH-sensitive redox couples that span E (Fe /Fe )=317 to -270 mV vs. NHE at pH 3.3 and pH 12.8, respectively. The 590 mV shift in potential and kinetic inertness are driven by ionization of the imidazoles at various pH values. The Fe /Fe redox is proton-coupled at alkaline conditions, and bulk electrolysis is non-destructive. The electrolyte demonstrates high charge/discharge capacities at both acidic and alkaline conditions throughout 100 cycles. Given its tunable redox, fast electrochemical kinetics, exceptional stability/cyclability, this complex is promising for the design of aqueous RFB catholytes and anolytes that utilize the earth-abundant element iron.
A family of four polyoxovanadate-alkoxide (POV-alkoxide) clusters was prepared and electrochemical techniques were used to evaluate diffusion coefficients and electron transport across a range of oxidation states. Synthetic routes were developed to increase the alkyl chain length of the [V 6 O 7 (OR) 12 ] cores, increasing R from the previously reported R = CH 3 , C 2 H 5 to R = C 3 H 7 , C 4 H 9 . Whereas increasing chain length may enhance solubility, such modifications may also hinder diffusion and electron transfer by shielding the core, thus we experimentally determined these parameters using both cyclic voltammetry and rotating disk voltammetry. Increasing the alkyl chain length of the POV-alkoxide nanostructures from methoxide to butoxide changes the solubility from 0.205 to 0.297 M in acetonitrile. Although some variations in diffusion coefficients and heterogeneous electron transfer rate constants were observed across the suite of oxidation states from species to species, they range from 0.14 × 10 −5 cm 2 /s to 2.24 × 10 −5 cm 2 /s for D 0 and 0.56 × 10 −3 cm/s to 209.00 × 10 −3 cm/s for k het . An increased chain length did not result in lower diffusion coefficients. Thus, we conclude that between C1 and C4 chains, no shielding of the redox core occurs, nor is transport through solution systematically hindered.
An all-organic redox flow battery (RFB) employing a fluorescent boron-dipyrromethene (BODIPY) dye (PM567) was investigated. In a RFB, the stability of the electrolyte in all charged states is critically linked to coulombic efficiency. To evaluate stability, bulk electrolysis and cyclic voltammetry (CV) experiments were performed. Oxidized and reduced, PM567 does not remain intact; however, the products of bulk electrolysis evolve over time to show stable redox behavior, making the dye a precursor for the active species of an RFB. A theoretical cell potential of 2.32 V was predicted from CV experiments with a working discharge voltage of approximately 1.6 V in a static test cell. Mass spectrometry was used to identify the products of bulk electrolysis. Related experiments were carried out using ferrocene and cobaltocenium hexafluorophosphate as redox-stable benchmarks to further explain the stability results. The coulombic efficiency of a model cell using PM567 as a precursor for charge carriers stabilized around 73 %.
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