Cobalt-based Nanocatalysts for Electrochemical Energy Conversion
[eng] Cobalt-based nanomaterials are an important class of electrocatalysts that own abundant and diverse d-orbital electronic structures, and can exhibit excellent HER, OER, EOR catalytic activity and stability in energy conversion. Despite some elemental cobalt catalysts having been recognized as...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2022 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/189857 |
| Acceso en línea: | https://hdl.handle.net/2445/189857 http://hdl.handle.net/10803/675684 |
| Access Level: | acceso abierto |
| Palabra clave: | Electroquímica Catàlisi Conversió directa de l'energia Nanotecnologia Electrochemistry Catalysis Direct energy conversion Nanotechnology |
| Sumario: | [eng] Cobalt-based nanomaterials are an important class of electrocatalysts that own abundant and diverse d-orbital electronic structures, and can exhibit excellent HER, OER, EOR catalytic activity and stability in energy conversion. Despite some elemental cobalt catalysts having been recognized as potential candidate catalysts in some reactions, the performance of elemental cobalt is far from ideal, especially in the field of electrocatalysis. Thus, composite cobalt-based nanocatalysts have been developed by introducing other materials such as carbon to increase surface area and electrical conductivity, and transition metal elements that allow a fine-tuning of the Co electronic energy levels. In my thesis, I detail the synthesis and performance of four cobalt-based nanocatalysts, towards three key electrochemical energy conversion processes. The thesis is divided into 5 chapters. Chapter 1 introduces the relevant electrochemical energy conversion technologies and cobalt-based nanocatalysts to explain the motivation of this thesis. In Chapter 2, amorphous ultrathin MoCoxOy nanosheets with excellent OER catalytic performance and prepared through an ion etching and pyrolysis assisted strategy. The incorporation of molybdate ions not only modifies the material architecture to generate two-dimensional layered nanosheet structures, but also regulates the local coordination environment and electronic structure of the cobalt oxide. Besides, the amorphous nanosheet architecture of the material favours the presence of surface defects and coordinatively unsaturated sites, boosting the exposure of active sites and further improving catalytic activity. Finally, using several pieces of evidence revealed the incorporation of molybdenum to enable a LOM, which is in part responsible for the excellent OER performance obtained. In Chapter 3, cobalt-iron oxide nanosheets containing SO42- anionic groups were produced from the etching and partial cation exchange of cobalt-based ZIF-67 with an ammonium iron sulfate. The salt breaks the polyhedral structure of ZIF-67, yielding porous assemblies of nanosheets, containing controlled amounts of iron and sulfate ions. The material composition and crystal structure can be adjusted to optimize its OER performance. The excellent performance is associated with the presence of the three elements, cobalt, iron and sulfate ions, and to the porous and amorphous structure of the material. In situ Raman spectroscopy analysis correlated with XPS data probes the material to be further oxidized to an oxohydroxide phase. It is further demonstrated here, that CoFexOy-SO4 samples catalyse the OER through an effective LOM mechanism. In Chapter 4, I detail the engineering of ultrathin CoMoP nanosheets using the ZIF-67 as a self-sacrificial template. CoMoP nanosheets exhibit outstanding performance towards HER and OER in alkaline media, which I associate with the proper transport properties and electronic energy levels provided by their composition and their porous nanosheet structure. This work provides a suitable strategy to synthesize high-performance Co-MoP electrocatalysts with abundant exposed active sites and effective avenues for charge and electrolyte transport, and it can be employed to further tune the structure and composition of other 2D nanostructures with optimized performance towards OWS and other electrocatalytic reactions. In Chapter 5, a new solution-based method was developed to produce Ni1-xCoxSe2 NPs with tuned metal ratios. The electrochemical performance of the materials was tested in alkaline ethanol electrolytes. CoSe2 showed enhanced OER activity and NiSe2 demonstrated a more efficient EOR. The incorporation of Co to the NiSe2 structure resulted in the highest EOR activities. DFT calculations showed that the presence of Co improved ethanol adsorption and decreased the barrier for ethanol dehydrogenation. This work provides a cost-effective approach to the high Faradaic efficiency electrochemical reforming of ethanol with acetate coproduction. Finally, the main conclusion of this thesis and some perspectives for future work are presented. |
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