Among the many strategies proposed to convert generated CO2 into value-added chemicals and energy carriers involves its electrochemical conversion using bioelectrochemical systems (BES)[1]. The advantage of this new technology lies in the possibility of exploiting the enzyme properties catalysing the reduction reaction with a high selectivity and specificity toward products with a low overpotential applied.

In this work, we present a novel BES, where the NAD-dependent enzyme Formate Dehydrogenase from Thiobacillus sp. KNK65MA [2] is expressed by heterologous production in E. coli BL21 (DE3) and deposited on a nanostructured mesoporous support of Titanium Nitride (TiN) realized by Pulsed Laser Deposition. Thanks to this method, we can realize a nanostructured support with high surface area and tree-like morphology. By optimizing the synthesis parameters, it is possible to obtain a nanostructured, hierarchical support that maximizes the available surface area for catalyst absorption and enhances the bio-interface between enzyme and inorganic electrode. We quantify the amount of immobilized enzymatic catalyst on the nanostructure through standard enzymatic assays, demonstrating that the nanostructuration of the TiN support increases the surface area available for enzyme immobilization, achieving a maximum enzyme adsorption of 59 µg cm^-2 for the BES. The TiN support is firstly characterized electrochemically, to verify its mechanical and chemical stability in the electrolyte solution. Subsequently, the enzymatic electrosynthesis of formic acid from CO2 is investigated at different applied potentials, showing a productivity for formic acid that ranges from 1.5 to 3.7 mmol mg^-1_enzyme h^-1 according to the applied overpotentials. Finally, post-catalysis characterization of the hybrid system shows that the amorphous nanostructure does not undergo any important modifications in its morphology and composition, thus demonstrating the mechanical stability of the TiN scaffold.

This performance, which is unparalleled in previous studies involving enzymes of the FDH family immobilized on inorganic supports, stems from a combination of are the best for inorganic support-immobilized enzymes of the FDH family, achieved thanks to the high reducing activity of TsFDH and the high contact area offered by the nanostructured TiN support, and demonstrates the feasibility and as well as the potential for a biotechnological device in terms offeaturing product specificity and stability.