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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 NH 2 , CN, −F, and NO 2 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 NH 2 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 −NH 2 < Pristine < −F ≈ −CN < −NO 2 . 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 ∼ − 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 [MnL F ] , [MnL CN ] and [NiL NH2 ] 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. [ABSTRACT FROM AUTHOR]