High-surface area metal foam catalysts for the electrochemical CO₂ reduction reaction (ec-CO₂RR) are deposited on planar model substrates and technical supports (e.g. 2D mesh or 3D sponges). The metal deposition is typically carried from acidified plating baths at particularly high current densities (e.g. -3 A/cm²) where the metal deposition is superimposed on the hydrogen evolution reaction (HER). Gas bubbles formed on the support surface serve as geometrical template for the metal deposition leaving highly porous metal foams behind.^1,2 Pore density and the particular side-wall morphology of these foams can be fine-tuned not only by the current density, metal ion concentration in solution but also by the use of particular polymeric plating additives (e.g. polyethylene-imines, PVPs, polymerizates of imidazole and epichlorohydrin). The activity and product selectivity of the electrochemical reduction of CO₂ on Cu foam catalysts strongly depend on the resulting pore size distribution of the Cu catalyst. The maximum in the C-C coupling efficiency (FE_C2 = 55%) observed for pore size dimensions in the range of 50 to 100 µm. Such morphology dependence of the ec-CO₂RR product distribution will be discussed not only on the basis of the availability of certain reactive surface sites and facets but also on the basis of a temporal trapping of gaseous intermediates (in particular CO and C₂H₄) inside the mesoporous catalyst material. Mild thermal annealing (300°C, 1h) of the Cu metal foams activates them for the production of C2 and C3 alcohols (ethanol, n-propanol). Identical location (IL) HR-SEM, HR-TEM, white-light interferometry and 3D optical microscopy are applied to probe morphological changes on different length scales that are induced by the foam activation (e.g. by thermal annealing) and the ec-CO₂RR itself. This study indicates that the catalyst morphology might undergo drastic changes under operando conditions.

[1] Shin, H. C.; Liu, M. L., Chem. Mater. 2004, 16 (25), 5460-5464.

[2] Shin, H. C.; Dong, J.; Liu, M. L., Adv. Mater. 2004, 16 (3), 237-240.