Electroreduction is an attractive way for converting CO₂ to energetically relevant molecules due to the mild conditions of temperature and pressure used in these processes. CO₂ reduction to formic acid is also of great interest since the latter is a very promising hydrogen carrier¹. Therefore, there is a need in the development of cheap and efficient electrocatalysts able to perform the CO₂ reduction reaction at low overpotential for the further development of energetically efficient CO₂ conversion systems². One of the most attractive metal catalyst for this reaction is copper due to its unique ability to reduce CO₂ into a wide class of compounds. The way the catalyst is synthesized can dramatically tune the selectivities. Thus, a careful control of the catalyst surface properties as well as the supporting material are critical to reach high current densities and high faradaic yields. In this study, we discuss on the role of the carbon support morphology on the global performances of this electrochemical reaction. Copper nanoparticles are supported on several carbon morphologies, from amorphous (Vulcan X72R) to highly organized mesoporous structures (CMK3, FDU15). The nucleation and growth of the nanoparticles are also investigated by using the microwave assisted polyol synthesis and a one pot soft templating method. Electron tomography allowed evidencing that copper nanoparticles can be embedded inside the mesoporous structure while keeping the pore structure unaffected (Fig. 1a), thus ensuring efficient diffusion path for CO₂ access to the catalytic sites. Cu nanoparticles supported onto mesoporous carbon are found to be ideal candidates to increase the active electrode surface area and to reach current densities in the range of 10 mA cm^-2 (Fig. 1b) in a complete electrolyser configuration. The activity of the different electrocatalysts is following the trend of the exposed metallic surface area, as determined by a novel methodology³ based on the surface statistics on a truncated octahedron cluster structure. Nevertheless, the competitive water reduction reaction leading to hydrogen levels off the faradaic yield for formic acid to ca. 24%. This intrinsic limitation is related to the onset potentials of the two competitive reactions, as evidenced by a coupled electrochemical and mass spectroscopic study. The strategy of alloying Cu with another metal to improve formic acid faradaic yield is also discussed.

References

1. Lu, X.; Leung, D. Y. C.; Wang, H.; Leung, M. K. H.; Xuan, J., Electrochemical Reduction of Carbon Dioxide to Formic Acid. ChemElectroChem 2014, 1, 836-849.

2. Tatin, A.; Comminges, C.; Kokoh, B.; Costentin, C.; Robert, M.; Savéant, J.-M., Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials. Proceedings of the National Academy of Sciences 2016, 113, 5526-5529.

3. Le Valant, A.; Comminges, C.; Can, F.; Thomas, K.; Houalla, M.; Epron, F., Platinum Supported Catalysts: Predictive CO and H2 Chemisorption by a Statistical Cuboctahedron Cluster Model. The Journal of Physical Chemistry C 2016.in press (doi 10.1021/acs.jpcc.6b09241).