Porous Ag-based Electrodes for the Electrochemical Reduction of CO2: aMAZEing catalysts

The electrochemical reduction of CO2, combined with CO2 capture, is relevant to decrease the amount of CO2 in the atmosphere, but catalysts improvement is needed for industrial application. Typically, transition metals are used in this reaction, but achieving a high activity, selectivity and stability remains challenging. In this thesis we investigated how (bi)metallic porous catalysts influence the activity and selectivity of the catalyst. After providing the theoretical background, we described the effect of electrodeposition conditions used in synthesis of templated porous Ag. We showed that variations in the electrodeposition structure lead to changes in the microstructure of the catalyst, with the main difference being the formation of so-called flat or coral structures. However, these changes are not decisive for the catalytic performance: the nanostructure created by the PMMA sphere template is dominant in steering the catalytic performance to more CO. After catalysis the catalysts only changed on the 10 nm scale. We proved this happened during the more negative applied potentials. Knowing that the electrodeposition conditions do not influence the catalytic performance significantly, the pore size was varied using PMMA templates with different sphere diameters. This lead to porous Ag catalysts with pore sizes between ~100 and ~400 nm. We found that the intrinsic catalytic performance depends on the pore size: when changing the pore size from ~100 to ~300 nm the CO production increases, for larger pore sizes it stabilizes. Using FIB-SEM we reconstructed and quantitively analyzed the porous structures. We found that a lower tortuosity and less additional potential drops together can explain the increase in CO production. As larger pore sizes have a higher intrinsic CO production, we used the porous Ag with 372 nm pores to study bimetallic templated porous Cu/Ag catalysts. We prepared catalysts with a well-defined Cu location with respect to the porous Ag: either we deposited Cu on top of and inside the porous structure using electrodeposition, or placed Cu on top of the porous Ag using sputter coating. FIB-SEM in combination with SEM-EDX confirmed the different Cu spatial distributions. Interestingly, the bimetallic Cu/Ag catalysts showed improved ethylene and ethanol production compared to monometallic Ag and Cu references. This effect persisted during six hours of catalysis, despite the loss of Cu. Finally, we developed a novel synthesis route to make nanostructured AgxCu10-x starting from trimetallic AlAgCu. Before leaching out Al, we added a quenching step to suppress undesired crystalline phases. Using this method, we were able to synthesize nanostructured AgxCu10-x with high control over the Ag:Cu ratio, covering the full range of x = 0 to x = 10. The AgCu structures had surface areas up to 25 m2/g with Ag and Cu mixed on the 10 nm scale. The structures were used for the e-CO2 reduction reaction, showing that the selectivity depends on the Cu content. An optimum in ethylene production was found between 50 and 70 at% Cu. Besides, the atomic ratios of Ag and Cu did not change significantly after catalysis, indicating that these catalysts are stable.