Enzymatic conversion of CO2 to formate was carried out in the cathodic cell of a two-chamber electrochemical apparatus where NAD(+) was reduced on the surface of a Copper foam electrode. Formate dehydrogenase (FDH) was used as the biocatalyst in both free form and immobilized on the modified electrospun polystyrene nanofibers (EPSNF). The fabricated EPSNF were modified by a multistage procedure including acid treatment, silanization followed by activation with glutaraldehyde. The effects of regenerated NADH concentration and time of enzymatic reaction on the formate production in the both systems were studied. The results indicated that the EPSNF immobilized FDH had a desirable activity, long-term storage stability (41% after 20 days) and reusability after eight cycles of successive reactions (53% of the initial activity). Moreover, it was revealed that the increase of cofactor concentration at the early times of reaction was favorable to the formate production. However, an inhibitory effect was observed at higher concentrations of NADH, and the optimum values of 0.45mM and 0.51mM were obtained for the maximum enzyme activity by the free and immobilized enzymes respectively. The produced formate at the optimum cofactor concentration after 300 min was 0.61mM and 0.31mM for the free and immobilized enzyme systems. Finally, it can be concluded that the presented process is a promising approach to the enzymatic conversion of CO2.