Numerous homogeneous and heterogeneous catalytic systems have been proposed to induce the CO₂ reduction and, depending on reaction conditions (applied potential, strength of the electrolyte, local pH, local concentration of CO₂ and the catalyst applied), various products can be obtained. Because the CO₂ molecule is very stable, its electroreduction would be characterized by large over-potentials. In this respect, behavior of carbon dioxide resembles that of dioxygen. In both cases, the limiting steps involve protonation of the respective species.

A crucial problem in designing effective electrocatalytic systems for the reduction of  carbon dioxide is the necessity to cope with the another competitive process, namely the simultaneous hydrogen evolution. Mechanisms of reductions of both carbon dioxide and protons are dependent not only on the proton availability and mobility at the electrocatalytic interface but also on the presence the adsorbed hydrogens at the active catalytic sites. In this respect, numerous metallic catalysts (e.g. Cu, Pd, Pt) have been proposed.

Alternate approaches to efficient electroreduction of carbon dioxide my refer to utilization of biological systems. Recently, whole cell biocatalysts and microbial electrocatalysts have been considered. Microorganisms can form very stable biofilms well-adhering to different solid surfaces. We propose here a hybrid (biofilm-based organic-inorganic) support or matrix for catalytic (noble metal) nanoparticles. The system was obtained by consecutive deposition of the porous polyaniline-supported bacterial biofilm (Yersinia enterocolitica) and multi-walled carbon nanotubes. Although this biofilm does not exhibit appreciable activity toward reduction of carbon dioxide, its structure (while being robust) contains open water channels existing between the bacterial micro-colonies thus permitting unimpeded flow of the aqueous electrolyte. The system’s performance will also be discussed with respect to the oxygen reduction.

To appreciate effective reduction of carbon dioxide, instead of conventional Pd nanoparticles, nanosized Pd immobilized within supramolecular assemblies of tridentate Schiff-base ligands or supported onto hybrid biofilm-based matrices have been considered. Reduction of carbon dioxide begins now at less negative potentials and is accompanied by significant enhancement of the CO₂-reduction current densities. Among important issues are specific interactions between nitrogen coordinating centers and metallic palladium sites. interface. The above activating phenomena will also be verified during oxygen reduction.  

Through intentional and controlled combination of metal oxide semiconductors, we have been able to drive effectively photo-electrochemical reduction of carbon dioxide. The combination  of titanium (IV) oxide (TiO₂) and copper (I) oxide (Cu₂O) has been explored toward the reduction of carbon (IV) oxide (CO₂) before and after sunlight illumination. Application of the hybrid system composed of both above-mentioned oxides resulted in high current densities originating from photoelectrochemical reduction of carbon dioxide mostly to methanol (CH₃OH) as demonstrated upon identification of final products. The role of TiO₂ is not only in stabilizing the interface: the oxide is also expected to prevent the recombination of charge carriers. Possibility of further decoration with traces of the conventional noble-metal-based systems (mentioned above) will be also addressed.