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Functional design of nanocomposites and their applications in energy conversion and storage

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Universitat de Barcelona. Facultat de Física

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[eng] This thesis focuses on the functional design and preparation of different kind of nanocomposites and their applications in energy conversion and storage technologies, particular in OER, HER, H2O2ER and LSBs. For the different applications nanocomposite materials are designed, engineered and tested. First, nanocomposite materials are designed based on the requirements for various properties of the material, including conductivity, catalysis, adsorption performance, band structure, specific surface area, material stability etc. Then the composites nanocomposites are engineered using methods like NPs coating, self-assembly and in-situ growth method. Finally, as prepared functional nanocomposites are tested in specific application. Results from the tests are frequently used as feedback for the design and engineering tasks, thus optimizing the final material. An understanding of the correlation between the designed nanocomposite structure and the specific application performance allows a more rational design of functional nanocomposites.
This thesis consists of five chapters. The first chapter presents a general introduction to the synthetic strategies and applications of nanocomposites. Chapter 2 details the preparation of CoFe2O4 NPs and their incorporation on Ni foams (NFs). The coating process was optimized for large electrode areas, ensuring a proper distribution of the NPs on the NF that allowed overcoming the electrical conductivity limitations of oxide NPs. We were able to produce CoFe2O4-coated NFs having 10 cm2 geometric surface areas with overpotentials below 300 mV for the OER at a current density of 50 mA/cm2. The CoFe2O4-coated NFs were also tested in a photovoltaic-electrolyzer coupled system and achieved a conversion efficiency of solar to chemical up to 13%. 
In chapter 3, the preaparation of 2D/2D heterojunction of TiO2 nanosheets/ultrathin g-C3N4 through the electrostatic self-assembly method is detailed. The obtained nanocomposites were applied for the photocatalytic HER under simulated solar light, presenting high performance and good stabilities. Compared with g-C3N4 and pure TiO2 nanosheets, this 2D/2D TiO2/g-C3N4 heterojunction exhibited ultra-high charge separation and transport properties and obvious improvement in photocatalytic performance. 
In chapter 4, the oxygen doping of highly dispersed Ni-loaded g-C3N4 nanotubes produced through an in situ growth method is detailed. The obtained nanocomposites were applied for photocatalytic hydrogen peroxide evolution under visible light. The hollow structure of the tubular g-C3N4 provides a large surface with a high density of reactive sites and efficient visible light absorption during photocatalytic reaction. Furthermore, oxygen doping and the Ni loading of the Ni/g-C3N4 composite catalyst displays a superior ability to separate photogenerated charge carriers and a high selectivity to the two-electron process during the ORR. The optimized composition, Ni4%/O0.2tCN, displays a H2O2 production rate of 2464 mol g-1·h-1, and achieves an apparent quantum yield (AQY) of 14.9% at 420 nm. 
Chapter 5 details the synthesis of colloidal CoFeP nanorods, tubular g-C3N4 and nanocomposites of CoFeP@t-CN. The as prepared CoFeP@t-CN composites were employed as sulfur hosts for LSBs. Density functional theory (DFT) calculations and experimental data confirmed that CoFeP@CN composites are characterized by a suitable electronic structure and charge rearrangement that allows them to act as a Mott-Schottky catalyst to accelerate LiPS conversion. Besides, the tubular geometry of CoFeP@CN composites facilitates the diffusion of Li ions, accommodates volume change during the reaction, and offers abundant lithiophilic/sulfiphilic sites to effectively trap soluble LiPS. As a result, S@CoFeP@CN electrodes deliver high initial capacities of 1607 mAh g−1 at 0.1 C, superior rate performance of 630 mAh g−1 at 5 C, and remarkable cycling stability with 90.44% capacity retention over 700 cycles. We further produced coin cells with high sulfur loading, 4.1 mg cm−2, and pouch cells with 0.1 Ah capacity to validate their superior cycling stability.

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[spa] Esta tesis consta de cinco capítulos. El primer capítulo presenta principalmente los antecedentes, la motivación y los objetivos de esta tesis. El trabajo experimental realizado para lograr esos objetivos se muestra en los capítulos 2-5. Dentro del marco general de diseño, preparación y optimización de los nanocompuestos, el trabajo presentado en esta tesis contiene principalmente los siguientes aspectos: (1) Diseño de compuestos basados en nanomateriales funcionales y en aplicaciones específicas; (2) De acuerdo con el diseño del material, se llevó a cabo la preparación de materiales nanocompuestos; (3) Pruebas de rendimiento de los nanocompuestos preparados y luego, emplear los resultados para verificar nuestras conjeturas de diseño. En el capítulo dos, se desarrolló un electrodo compuesto por una esponja de níquel recubierta con nanopartículas de CoFe2O4 para evaluar su actividad catalítica en la reacción de evolución de oxígeno. El aumento en la actividad electrocatalítica OER fue atribuido a la combinación de las partículas de CoFe2O4 de tamaño nanométrico y la estructura macroporosa 3D de la esponja de níquel. En el capítulo tres, se produjeron heterouniones 2D/2D basadas en nanohojas de TiO2/g-C3N4 ultradelgado para estudiar sus efectos en la reacción fotocatalítica de evolución hidrógeno, cuyo incremento en el rendimiento de la reacción se atribuyó a los efectos integrados de la morfología única 2D/2D y el mecanismo de transferencia de heterounión tipo II. En el cuarto capítulo, se preparó un compuesto de nanotubos g-C3N4 cargados con Ni altamente disperso para investigar su actividad en la reacción de evolución de peróxido de hidrógeno. La combinación de nanopartículas de níquel y la estructura tubular del g-C3N4 promueve la separación efectiva de cargas y la alta eficiencia de conversión del peróxido de hidrógeno, y lo que finalmente produce la conversión eficiente de la energía luminosa en energía química. En el capítulo cinco, se prepararon nanovarillas de CoFeP soportadas en g-C3N4 tubular para baterías de litio-azufre. La combinación de CoFeP y g-C3N4 acelera la conversión de LiPS, facilita la difusión de iones de Li, y se adapta al cambio de volumen ofreciendo abundantes sitios litiófilos/sulfófilos, resultando en mayor estabilidad en el ciclado y capacidad de velocidad.

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Functional design of nanocomposites and their applications in energy conversion and storage