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suspensions containing a nanoscale conducting carbon network that was formed by Ketjen black in alkyl carbonate/LiPF6 solution, with micrometre-scale particles of electrode material (e.g. LiCoO2) distributed throughout the network. The flowable suspensions that were demonstrated had up to 12 M active material concentration. A full-cell test using intermittent flow mode in which a single volume of the semi-solid suspensions was pumped into the cell, discharged and later displaced by a new volume gave a first demonstration of a fully operational semi-solid flow cell. Suspensions containing LiCoO2 were utilised at the cathode (20 vol% (10.2 M), 1.5% Ketjen black) and suspensions containing Li4Ti5O12 were active at the anode (10 vol% (2.3 M), 2% Ketjen black), both in 1 M LiPF6 in dimethyl carbonate. CE of 73% and 80% were achieved in the first and second cycles. A calculated theoretical energy density for a semi-solid system utilising LiCoO2 and Li4Ti5O12 with 40 vol% solid in each suspension at an average discharge voltage of 2.35 V was 397 Wh L-1. More generally, it was estimated that optimised semi-solid flow cells utilising lithium intercalation compounds could have energy densities of 300 to 500 Wh L-1. However, higher viscosity electrolytes are associated with increased parasitic energy losses, owing to the additional energy that is required to pump the electrolyte. 6.5 Redox active polymers for RFBs The use of organic molecules or polymers as the active materials in RFBs is also gaining prominence in RFB research as such species can be synthesised from inexpensive organic raw materials. An all-polymer RFB utilising a polymer bearing the TEMPO moiety at the cathode and a viologen-based polymer at the anode was presented by Janoschka et al. [90]. The TEMPO unit (2,2,6,6-tetramethylpiperidine-1-oxyl) is a heterocyclic, stable radical nitroxide molecule that can be oxidised to an oxammonium cation, while viologens are -bipyridine derivatives that can be reduced to monocationic radicals. Both polymers also contained quaternary ammonium units to aid solubility. A study of the redox properties of the polymers by CV indicated the TEMPO-containing polymer underwent oxidation to TEMPO+ at 0.7 V vs. Ag/AgCl while the viologen polymer underwent a reduction at around -0.4 V vs. Ag/AgCl which corresponded to the formation of a monovalent radical Viol species. The electrolyte for the all-polymer battery was an aqueous sodium chloride solution and a low-cost cellulose-based dialysis membrane was used to prevent crossover of the polymeric species but allow the movement of ions between half-cells. Page 31 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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