Browsing by Author "Suominen, Milla, Dr., Aalto University, Department of Chemistry and Materials Science, Finland"

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Engineering Sn- and Bi-based Electrocatalysts and Electrodes for Efficient Carbon Dioxide to Formate Production
(2024) Shi, Junjie
School of Chemical Engineering | Doctoral dissertation (article-based)
Electrochemical CO2 reduction reaction (CO2RR) is a feasible method for storing extra electricity produced by intermittent renewable energy sources while converting waste to valuable products. Converting CO2 to formate is one of the most viable and cost-effective CO2RR processes. Both Sn and Bi are appealing candidates for accelerating formate production due to their promising selectivity, minimal environmental impact, and great abundance, but the process suffers from low activity. Moreover, the active sites remain unclear under cathodic potentials when starting with SnOx and BiOx. In this thesis, their CO2RR activities are first improved by several strategies and the active sites and reaction mechanisms are further studied. The first synthesis strategy is depositing SnO2 nanoparticles on carbon. This material efficiently catalyzes CO2RR and shows higher activity compared to that of SnO2 nanoparticles on TiO2. The evolution of SnO2 nanoparticles under cathodic potentials when they are grown on these two supports are then investigated. Furthermore, atomistic simulated models create a relationship between SnOx with different oxygen content and their CO2RR activities. With density functional theory (DFT) analysis, the metallic Sn is found to be the most favorable site for formate production. At the same time, SnOx with oxygen atoms trapped in their center is capable of CO generation. In the second strategy, nitrogen-doped carbon (NC) as a support is proved to improve the catalytic performance of a binder-free Bi2O3 electrode effectively in the beginning, but instability and unwanted enhancement in hydrogen production at high overpotentials appear soon. By employing hydrophobicity modification with polytetrafluoroethylene on NC before Bi2O3 growth, the adverse effects are overcome. It minimizes flooding and encourages both gaseous and liquid reactant transport with the coexistence of both the hydrophobic substrate and the hydrophilic active sites, thus optimizing active site use at the triple-phase interface. The last strategy is doping. In this work, Mn-doping is introduced to enhance the activity of a binder-free Bi2O3 electrode. Then, the change of active materials during CO2RR is elaborated using in situ Raman and other characterization techniques. Both doped and undoped Bi2O3 are found to change to Bi2(CO3)O2 in contact with KHCO3 and then reduce to metallic Bi under cathodic potentials. The effects of Mn-doping on electronic structure and reaction pathways are investigated using DFT. Moreover, the importance of substrates' hydrophobicity for long-term stability is shown.