I am working on fabricating an electrocatalytic device to convert CO2 to CH4. I am trying to use the B-V equation to calculate current density.

(j0 = 0.014 mA/cm2; αc = 0.25; n = 8; T = 298 K; η = -2.4V) Because of the high overpotential, the equation can be written as j ≈ −j0​ * exp[(−αc*n*F*η) / (RT​)].

The calculated result showed an extremely low current density compared with the measured value (-100 mA/cm2).

I am wondering why there is such a big difference. Did I overlook or make something wrong in this process?