Transition Metal-Based Catalysts Modified with p-Block Elements for the Electrochemical Reduction of CO2

Abstract

Avoiding the most serious effects of climate change is one of the greatest technical and socio-political challenges of our time. At the present rate of anthropogenic CO2 emissions, in less than two decades mankind will lose the opportunity to limit the global temperature increase to 2 °C (as stated in the Paris Agreement), a fact that makes carbon dioxide an urgent target for recycling efforts. In this context, the combination of the electrochemical reduction of CO2 (eCO2RR) with carbon-neutral energy sources opens the door for the valorization of carbon dioxide as a medium for energy storage and as a source for the production of building blocks in a fossil fuel-free chemical industry. In particular, the efficient reduction of CO2 to CO would provide a versatile compound for the production of liquid fuels and plastics by established industrial processes. However, a key challenge for the eCO2RR on its way toward technological viability is the development of highly active and robust electrocatalysts capable of targeting a single CO2 reduction product and of inhibiting the competing hydrogen evolution reaction (HER) in aqueous media. Theoretical insights indicate that the key to unlocking breakthrough advances in catalytic performance for the eCO2RR lies in breaking the sub-optimal scaling relations between reaction intermediates that exist on transition metal surfaces. In this context, this thesis work is aimed at exploring the emergence of synergistic interactions in multicomponent materials as a design strategy for the development of improved eCO2RR catalysts, with an emphasis on understanding how the introduction of p-block elements modulates the activity and selectivity of transition metal-based catalysts for this reaction. First steps are aimed at evaluating whether the intrinsic selectivity for CO of silver electrodes can be enhanced by an interaction with indium, which is a poor HER catalyst. To this end, a comprehensive set of Ag-In electrocatalysts with different architectures is synthesized and tested, ranging from bulk intermetallic compounds to Ag nanoparticles supported on In2O3 and In(OH)3. Bulk Ag9In4 and AgIn2 alloys prepared by a succession of electrodeposition and annealing steps show a catalytic performance very similar to that of pure In, which is attributed to the surface enrichment of In in these materials. In contrast, the supported catalysts exhibit an enhanced current efficiency for CO at moderate overpotential, evidencing a synergistic effect between the metal nanoparticles and the oxidic supports. This effect is particularly marked with In(OH)3 as support, which unlike In2O3 is characterized by a kinetic impairment toward reduction to metallic In under eCO2RR conditions. In a following step, this approach is extended to the study of Cu-In catalysts. In this regard, the structure of Cu-In nanoalloys prepared by the in situ reduction of the CuInO2 delafossite and of In(OH)3-supported Cu nanoparticles evolves substantially over several electrocatalytic cycles, in parallel with an increase in the activity and selectivity for CO evolution. The detailed characterization of this process reveals that this evolving behavior is caused by the progressive segregation of Cu and In in these materials, resulting in the formation of a heterogeneous nanostructure of Cu-rich cores embedded within an In(OH)3 shell-like matrix. In addition, the presence of In(OH)3 in the equilibrated catalysts is shown to play a key role in ensuring a high selectivity toward CO. In the second part of this thesis, the modulating effect of p-block elements on the properties of Cu-based catalysts is further explored, along with an investigation of alternative synthetic routes. A Cu2O catalyst prepared by a solvothermal route shows enhanced performance compared to a commercially available material and to oxide-derived Cu bulk electrodes, highlighting the capacity of a simple and potentially scalable synthesis to prepare a Cu-based powder catalyst that can be readily incorporated into gas diffusion electrodes in view of practical applications. Moreover, the addition of a suitable precursor to the synthesis medium enables the preparation and comparison of Cu2O catalysts modified with other elements. Indium- and tin-promoted catalysts show even higher activity and selectivity toward CO compared to pristine Cu2O. On the other hand, the incorporation of aluminum into Cu2O negatively affects the production of CO but enhances the selectivity toward more reduced products, such as ethylene and alcohols, which based on detailed characterization is ascribed to a stabilizing effect of aluminum on copper(I) species under reaction conditions. Modification with sulfur results in a striking shift of the selectivity exclusively toward formate, implying the inhibition of the CO pathway that is characteristic of Cu-based catalysts. Sulfur-modified Cu electrocatalysts show profound surface restructuring under reaction conditions, attaining a surface configuration which is independent of the initial sulfur content of the fresh material. Apart from the fundamental interest that such a radical change in selectivity represents, sulfur-modified Cu outperforms all practical earth-abundant and non-toxic electrocatalysts reported to date for the production of formate via the eCO2RR. Despite recent advances, further progress in the design of more efficient eCO2RR catalysts would be greatly boosted by an expansion of the fundamental understanding. In this context, the last part of this thesis focuses on the development of a versatile photolithography-based microfabrication process for structured electrodes with controlled geometry and composition. This approach is aimed at gaining insights into the role of interfaces in multicomponent electrocatalysts and at deriving clear structure-performance relationships that can push forward the development of this class of materials. The microfabrication process is applied to the Cu-In system, and structured electrodes are produced in which well-defined arrays of microsized In2O3 and In islands are deposited on Cu and Cu2O substrates by the patterning of a sacrificial photoresist layer. This approach confirms the crucial role of the Cu2O substrate in attaining the synergistic effect, and the control over the geometry uncovers the fundamental dependence of the CO evolution activity on the interfaces between the components. Overall, the work in this thesis demonstrates that the introduction of p-block elements is a powerful strategy to modify the catalytic properties of transition metal-based eCO2RR catalysts typically used in this reaction, and the microfabrication of structured electrodes is shown as a powerful experimental platform for the rationalization of synergistic interactions and dynamic phenomena in multicomponent catalysts for CO2 reduction.