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concentration or the molecular weight of a dissolved polymer increases, so too does the viscosity of a polymer solution. While for any RFB a higher concentration of active species leads to a greater energy density, increasing the concentration of a polymer solution may be problematic as viscous solutions increase the energy cost of pumping the electrolytes. Thus the advantage of the use of a size-exclusion membrane in a polymer RFB could be counterbalanced by the viscosity that is intrinsic to polymer solutions. The use of TEMPO and viologen as small organic molecules in a RFB is an alternative to TEMPO and viologen-based polymers. For instance, a RFB that used aqueous solutions of TEMPTMA (N,N,N-2,2,6,6-heptamethylpiperidinyl oxy-4-ammonium chloride) (a TEMPO derivative) and methyl-viologen ( -dimethyl-4,4-bipyridinium dichloride) had an energy density of 38 Wh L-1 for a theoretical cell voltage of 1.4 V and excellent capacity retention over 100 cycles [118]. Winsberg et al. presented a polymer-based RFB that utilised two polymers bearing the boron-dipyrromethene (BODIPY) unit [119] in propylene carbonate as solvent. CV of the monomer containing the BODIPY group showed two redox reactions at -1.51 V vs. AgNO3/Ag and 0.69 V vs. AgNO3/Ag which corresponded to reduction of the monomer to BODIPY- and oxidation to BODIPY+ respectively. The BODIPY monomer was copolymerised with either TASt ((vinylbenzyl)trimethylammonium perchlorate) or TEGSt ((vinyl-benzyl)-triethylene glycol monomethyl ether) to generate two different polymers that could be used as the anolyte and catholyte in cell testing. Two static cell tests were performed. One employed poly(BODIPY-co-TEGSt) as anolyte with a further polymer that bore the TEMPO group used as the catholyte (poly(TEMPO-co-PEGMA). The second cell utilised poly(BODIPY-co-TASt) and poly(BODIPY-co-TEGSt) as the catholyte and anolyte respectively. A size-exclusion membrane separated the half-cells and a solution of 0.5 M tetrabutylammonium perchlorate (Bu4NClO4) in propylene carbonate was used as the supporting electrolyte. The TEMPO/BODIPY battery outperformed the all-BODIPY battery, with an average discharging voltage of 1.82 V and VE EE of 89% and 88% respectively. A CE of 99% was achieved over 100 cycles, with 70% of the initial discharge capacity preserved. The all-BODIPY system had a charging plateau at 2.06 V while the mean discharge voltage was 1.28 V leading to VE EE of 62% and 55% respectively, with CE reaching 89%. The energy density of the electrolyte was 0.5 Wh L-1 for both polymer RFBs. Page 33 of 63PDF Image | Redox Flow Batteries Concepts Chemistries
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