Metal coordination complexes in nonaqueous redox flow batteries

The ongoing search for new electroactive materials for next-generation redox flow batteries has within the last decade encompassed metal–ligand coordination chemistry. Here, we review the handful of metal coordination complexes proposed as redox flow battery electrolytes. We highlight examples with careful ligand design, driving research towards higher energy density storage materials. Metal coordination complexes designed to be highly soluble not only in the initial redox state but also in all charged states accessed during the battery cycle give particularly impressive performances. Optimisation of flow cell conditions for metal coordination complexes remains largely unexplored, with most complexes screened in nonideal H-cell experiments with little investigation into membrane and electrode materials

Characterisation of the ferrocene/ferrocenium ion redox couple as a model chemistry for non-aqueous redox flow battery research

The simple ferrocene/ferrocenium ion (Fc/FcBF4) redox couple was examined as a model chemistry for non-aqueous redox flow battery research. Its properties were fully characterised using voltammetry, flow-cell battery cycling, and UV–vis spectroscopy to validate flow-cell performance. Fc demonstrates facile kinetics and high stability of its oxidation states, making the Fc/FcBF4 redox couple a useful low-cost model chemistry, despite its limited 0.16 M solubility in acetonitrile. By use of ‘single redox couple cycling’, in which only the Fc/FcBF4 redox couple is battery cycled, the high capacity retention of Fc at 10 mM concentration was demonstrated; 80% capacity retention after 200 cycles (7.8 days). The mechanism for the capacity loss was investigated and diagnosed to occur via FcBF4 decomposition in the electrolyte, which proceeds irrespective of battery cycling

Application of the dianion croconate violet for symmetric organic non-aqueous redox flow battery electrolytes

Redox active organic molecules (ROMs) are promising candidates for redox flow battery (RFB) energy storage due to their high sustainability and low cost. Herein, the pseudooxocarbon derivative croconate violet (Croc2-) is applied as a novel symmetric ROM in acetonitrile electrolyte, whereby Croc2- is used as both the battery posolyte and negolyte, with a 1.82 V cell potential and ≃1 M solubility. Characterisation of the dianion Croc2- is given by way of voltammetry and battery cycling techniques to demonstrate the high number of oxidation states accessible by Croc2-, thus highlighting a high intrinsic capacity for a low molecular weight ROM. The stability of Croc2- and its charged radical states is investigated to assess the viability of the symmetric design, and an undesirable radical disproportionation mechanism of the Croc•3- oxidation state is identified to account for poor capacity retention. Asymmetric battery experiments of a Croc2- posolyte with 2,1,3-benzothiadiazole or tetracyanoquinodimethane negolytes gave improved battery performance, indicating that Croc2- is a promising anionic ROM posolyte

Misreported non-aqueous reference potentials:The battery research endemic

Often the evaluation of energy storage systems which use non-aqueous media requires the use of three-electrode electrochemical cells that employ a stable and reliable reference electrode. The existence of such a reference electrode is a non-trivial matter which has led to extensive misuse and misreporting across the field of battery research. This commentary highlights the current challenges in non-aqueous referencing which are commonly overlooked and offers best practices for acknowledging and accounting for such challenges when reporting data which heavily relies on having accurate reference potentials

Nonenzymatic determination of glucose on electrodes prepared by directed electrochemical nanowire assembly (DENA)

The analytical characteristics of gold nanowires prepared by direct electrochemical synthesis were studied with the use of the amperometric determination of glucose as an example. The applicability of the gold nanowires to the determination of glucose in a neutral medium (a phosphate buffer solution with pH 7.2) over a concentration range from 1 × 10–4 to 5 × 10–3 M at a detection potential of +0.35 V was shown. It was found that the sensitivity of a nonenzymatic sensor for the determination of glucose on the gold nanowires was high: 3.7 × 10–4 A M–1 m–2. The limit of detection was 3.3 × 10–5 M