Electrochimica Acta, Vol.246, 156-164, 2017
Reduction potential tuning of first row transition metal M-III/M-II (M = Cr, Mn, Fe, Co, Ni) hexadentate complexes for viable aqueous redox flow battery catholytes: A DFT study
We systematically investigated the tuning of the reduction potentials (E-0) for the first row transition metal (M = Cr, Mn, Fe, Co, Ni) complexes with the functionalized 1,4,7-Triazacyclononane-N,N',N-triacetate (TCTA) ligands by means of DFT calculations. To predict reliable E-0, the modified UB3LYP functional and equilibrium concept between high and low spin states were utilized. The functional groups -NH2, -CN, -F, and -NO2 were attached to the carbon atoms carefully selected by considering the LUMO and steric hindrance. Based on firmed accuracy of DFT calculations, finally we obtained the calculated E-0 for a series of complexes. It was found that electron donating group such as -NH2 will cause a negative shift of E-0 while electron withdrawing groups have the opposite effect. The overall trend of the calculated E(0)s according to ligand modifications were predicted to have the order as -NH2 < Pristine < -F approximate to -CN < -NO2. In addition, optimized geometries, LUMO, vertical electron attachment and energy components constituting E-0 were discussed in detail to assist the further understanding for E(0)s. Consequently, we suggested that 16 complexes can play a role as an electrolyte in aqueous redox flow battery. They can be classified into 5 groups having similar E-0 ranges: Group I (- 0.6 V similar to -0.7 V), Group II (around 0.0 V), Group III (around 0.3 V), Group IV (0.6 similar to 0.8 V) and Group V (1.1 similar to 1.2 V). Especially, it is expected that [MnLF], [MnLCN] and [NiLNH2] can be used as promising catholyte candidates possibly possessing high E(0)s which almost reach to the reduction potential limitation 1.25 V in aqueous redox flow battery. Our systematic approach to tune E-0 can be applied to the design of other complexes via rational ligand modification. (C) 2017 Elsevier Ltd. All rights reserved.