Our main results are summarized as follows. First, in this proof-of-concept study, we proposed the concept 'cavity-free QED-ET reaction' and demonstrated that QED effects can significantly influence ET rates in the absence of cavities. The proposed concept is implicitly supported by several previous experimental studies. Note that the concept of the cavity-free QED-ET reaction is fundamentally different from that of polariton chemistry because it accounts for the effect of infinite one-photon states in the absence of strong light-matter interactions. Second, we established a unified theory to describe both radiative and nonradiative ET. The explicit expression of the Marcus ET rate (kMarcus) and the radiative ET rate (kQED) enables quantitative analyses of the interplay between electrostatic and electrodynamic effects on ET reactions. Third, we developed the concept of ET overlap-the spectral overlap of the ET radiation spectrum F(ωph) × ρph(ωph) and vibrational transition DOS ρvib(ωph) (Fig. )-which contributes to the qualitative understanding of the radiative ET mechanism. Fourth, we demonstrated the energy gap power law kET ≈ kQED ~ |ΔE+Er| 3 in the highly negative Marcus inverted region (QED region) and proposed the concept 'QED-induced barrier-free ET reaction'. The former can be employed to identify the existence of cavity-free QED effects, and the latter can be used to design barrier-free redox reactions. Our results provide a simple but useful guide for exploiting QED effects to influence ET reactions, and the proposed mechanism does not require strong light-matter interactions, polariton formation, or confined cavities. Finally, we believe that this work will stimulate further investigations on the basic principles and practical applications of QED-based chemical reactions.