Electrochemical CO₂ Reduction to Two-Electron Products

Abstract

Climate change remains a major global challenge and with CO2 emissions still increasing, a rapid transition towards renewables is required. Power-2-X and in this context electrochemical CO2 reduction offers a promising solution to several issues related to this. First of all, it provides a means of storing excess energy from intermittent renewable energy sources in chemical bonds. At the same time, it also provides a viable synthesis route for the production of CO2 neutral fuels we can use in our existing energy infrastructure. The technology still needs significant improvements both in terms of the activity and efficient use of the electrons supplied, but also in terms of improving selectivity towards desired products such as ethanol. This thesis studies the formation of two-electron products in CO2 reduction, to understand what guides the activity and selectivity on the different metals. We map out the selectivity for different metals and identify palladium as a clear outlier. In addition, Pd is able to produce both two-electron products with high selectivity and even switch between them across a quite narrow potential range, which makes it interesting to us as a fundamental study of what guides CO2R product selectivity. We find that the entire Pd electrode undergoes structural changes in the presence of the electrochemical environment, forming a highly intercalated palladium hydride (PdH) structure. This changes the properties of the material and thus its ability to reduce CO2. It has been proposed that the selectivity towards formate is driven by the *OCO binding motif, as opposed to *CO2 driving CO production. This is not seen on PdH, instead formate is formed via a (surface) hydrogenation step. Furthermore, the electrochemical environment introduces strong differences in the driving forces between the two products, with the formate pathway initially being favored followed by a switch due to strong stabilization with potential of the CO pathway. Next, we move on to take a more general look at the structural and environmental factors that affect the CO2R activity. We construct a general model used to explore the possible CO2 binding motifs. We distinguish the structural effects of chemical bonding through surface hybridization from the environmental effects of changing CO2 binding through interactions between the electric field and the surface dipole. While the *OCO motif is initially destabilized by the field upon activation, ultimately both motifs benefit from the electric potential. The relative *OCO/*CO2 stability becomes a competition between the hybridization/chemical bonding, which appears to favor *OCO on our model Cu(211) surface, and dipole-field interaction/-electrostatic effects, which favors *CO2. Thus, given the right surface *OCO may become more stable even at slightly negative potentials. This was however not found to be the case for the post-transition metals known to produce formate.Finally, as the other works of this thesis shows, the electric field is of paramount importance in CO2 reduction, and with this in mind, we probe methods to intrinsically improve this. Specifically, we study whether curvature-induced field enhancements are large enough to drive the improved activity we observe. We find however, that the electric field effect is convoluted with structure changes in the most active systems. Ultimately, we find that the field enhancement associated with even high-curvature surfaces are negligible. Instead, we attribute the improved activity to increases in site density of step sites. The experimental data also verifies the theoretical hypothesis, that a region limited by *COOH formation exists at low overpotential, with a different potential response to that of CO2 adsorption