The application of novel organic redox materials is a plausible pathway towards technoeconomic energy storage targets due to their low cost and sustainable design. Their operation in non-aqueous redox flow batteries affords researchers the opportunity to innovate, design and optimise these new chemistries towards practical energy densities. Despite this, the identification of high capacity organics which also display long-term stability is inherently challenging due to the high reactivity of molecular radicals.

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 2 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.

A single species redox flow battery employing a new class of cobalt(II) complexes with 'tunable' tridentate azole-pyridine type ligands is reported. Four structures were synthesised and their electrochemical, physical and battery characteristics were investigated as a function of successive substitution of the ligand terminal pyridyl donors. The Co(II/I) and Co(III/II) couples are stable and quasi-reversible on gold and glassy carbon electrodes, however redox potentials are tunable allowing the cobalt potential difference to be preferentially increased from 1.07 to 1.91 V via pyridine substitution with weaker σ-donating/ π-accepting 3,5-dimethylpyrazole groups. The charge-discharge properties of the system were evaluated using an H-type glass cell and graphite rod electrodes. The complexes delivered high Coulombic efficiencies of 89.7-99.8 % and very good voltaic efficiencies of 70.3-81.0 %. Consequently, energy efficiencies are high at 63.1-80.8 %, marking an improvement on other similar non-aqueous systems. Modification of the ligands also improved solubility from 0.18 M to 0.50 M via pyridyl substitution with 3,5-dimethylpyrazole, though the low solubility of the complexes limits the overall energy capacity to between 2.58 to 12.80 W h L-1 .

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Five metal complexes of the dithiolene ligand maleonitriledithiolate (mnt2−) with M=V, Fe, Co, Ni, Cu were studied as redox‐active materials for nonaqueous redox flow batteries (RFBs). All five complexes exhibit at least two redox processes, making them applicable to symmetric RFBs as single‐species electrolytes, that is, as both negolyte and posolyte. Charge–discharge cycling in a small‐scale RFB gave modest performances for [(tea)2Vmnt], [(tea)2Comnt], and [(tea)2Cumnt] whereas [(tea)Femnt] and [(tea)2Nimnt] (tea=tetraethylammonium) failed to hold any significant capacity, indicating poor stability. Independent negolyte‐ and posolyte‐only battery cycling of a single redox couple, as well as UV/Vis spectroscopy, showed that for [(tea)2Vmnt] the negolyte is stable whereas the posolyte is unstable over multiple charge–discharge cycles; for [(tea)2Comnt], [(tea)2Nimnt], and [(tea)2Cumnt], the negolyte suffers rapid capacity fading although the posolyte is more robust. Identifying a means to stabilize Vmnt 3−/2− as a negolyte, and Comnt 2−/1−, Nimnt 2−/1−, and Cumnt 2−/1− as posolytes could lead to their use in asymmetric RFBs.

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In this work, the dithiolene complex iron(III) bis‐maleonitriledithiolene [Fe(mnt)2] is characterised and evaluated as a homogeneous CO2 reduction catalyst. Electrochemically the Fe(mnt)2 is reduced twice to the trianionic Fe(mnt)23− state, which is correspondingly found to be active towards CO2. Interestingly, the first reduction event appears to comprise overlapping reversible couples, attributed to the presence of both a dimeric and monomeric form of the dithiolene complex. In acetonitrile Fe(mnt)2 demonstrates a catalytic response to CO2 yielding typical two‐electron reduction products: H2, CO and CHOOH. The product distribution and yield were governed by the proton source. Operating with H2O as the proton source gave only H2 and CO as products, whereas using 2,2,2‐trifluoroethanol gave 38 % CHOOH faradaic efficiency with H2 and CO as minor products.

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