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Title Reduction potential tuning of first row transition metal MIII/MII (M= Cr, Mn, Fe, Co, Ni) hexadentate complexes for viable aqueous redox flow battery catholytes: A DFT study
Posted by Francis Kirby Burnea
Authors Francis Kirby Bokingo Burnea, Hu Shi, Kyoung Chul Ko, Jin Yong Lee
Publication date 2017/8/20
Journal Electrochimica Acta
Volume 246
Pages 156-164
Publisher Pergamon
Abstract We systematically investigated the tuning of the reduction potentials (E0) 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 E0, the modified UB3LYP functional and equilibrium concept between high and low spin states were utilized. The functional groups single bondNH2, single bondCN, −F, and single bondNO2 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 E0 for a series of complexes. It was found that electron donating group such as single bondNH2 will cause a negative shift of E0 while electron withdrawing groups have the opposite effect. The overall trend of the calculated E0s according to ligand modifications were predicted to have the order as −NH2 < Pristine < −F ≈ −CN < −NO2. In addition, optimized geometries, LUMO, vertical electron attachment and energy components constituting E0 were discussed in detail to assist the further understanding for E0s. 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 E0 ranges: Group I (- 0.6 V ∼ − 0.7 V), Group II (around 0.0 V), Group III (around 0.3 V), Group IV (0.6 ∼ 0.8 V) and Group V (1.1 ∼ 1.2 V). Especially, it is expected that [MnLF], [MnLCN] and [NiLNH2] can be used as promising catholyte candidates possibly possessing high E0s which almost reach to the reduction potential limitation 1.25 V in aqueous redox flow battery. Our systematic approach to tune E0 can be applied to the design of other complexes via rational ligand modification.
Index terms / Keywords Redox flow battery; Metal complexes; DFT; Redox potential; Catholyte