where h is the Planck constant and λ is the reorganization energy of the redox mediator. Many outer-sphere electrode reactions are non-adiabatic due to weak electronic coupling, which also applies to our system as a result of the low DOS of graphite. The standard heterogeneous electron transfer rate constant k 0 (in cm s -1 ) is a special case of k ET at E = E 0' , integrated over all the distances from the electrode, so from Eq. ( ) we have: