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Universitat de Barcelona. Facultat de Física

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[eng] The thesis is focused on the development and enhancement of the electrochemical properties of the carbon based supercapacitors and pseudocapacitors. To overcome the capacitance loss at the condition of fast charging in the carbon-based supercapacitors, a metal-oxide embedded porous carbon nanofiber with a 3-D electrode architecture is designed. This electrode reduces the electrode resistance and at the same time increases the associated values of capacitance at high rates. The investigation also indicates an essential role in the concentration of the metal oxide precursor towards the electrochemical behavior of the electrodes. This correlation could be useful to design better electrodes for supercapacitor, functioning with better energy and power density capabilities. Whereas, in the case of the water-based pseudocapacitors, it is shown that they suffer from low voltages. Two strategies were used to overcome this issue. 
(i)	Exploring and improving the electrode material based non-carbon materials. In this regard, new materials from the family of MXenes are introduced, to achieve higher cell voltages. Under this frame, a new 2-D MXene based on Molybdenum Vanadium Carbide is proposed and its electrochemical characteristics were investigated. According to its characteristics, its coupling with 2-D Titanium Carbide MXene exhibits a higher cell voltage. The investigation reveals that the charge storage in 2-D molybdenum vanadium carbide MXene has the dependence on the type of electrolyte cations. For the case in point, small size monovalent cations, such as lithium and sodium ions, demonstrate lower hindrance to the charge storage, while large size monovalent potassium ions and bivalent magnesium ions suffer from hindrance effect, causing them to have lower charge storage than lithium and sodium ions. Therefore, the selection of appropriate electrolyte ions especially in the case of MXene based materials appears to be important, which is here found to be with the protonic and sodium ion based electrolytes. 
(ii)	the proposed approach is based on the use of water-based super-concentrated salt solutions which are promising electrolytes to contribute to widening the cell voltage of aqueous pseudocapacitors. Likewise, besides this, it is also proposed that the coupling of 2-D Titanium Carbide MXene with the tunnel structures of Manganese Oxide using this super-concentrated electrolyte water in salt can enable a high voltage aqueous pseudocapacitive energy storage device. The investigation using this approach reveals that the concentration of the salt electrolyte plays a significant role in the values of charge storage in 2-D titanium carbides. Although an extremely high concentration of salt electrolytes widens the potential window, the electrolyte ions in such high concentration face difficulty to insert within the 2-D layers of titanium carbide MXene. On the contrary, the use of low concentrated salt solutions is not recommended, as they provide narrow potential windows. Consequently, during the cell assembling using super-concentrated electrolytes, a moderate concentration of salt electrolyte needs to be taken into attention. On this way, both wider potential window and high charge storage, can be achieved with pseudocapacitive materials like 2-D titanium carbides MXenes. The crystallographic tunnel size of manganese oxide plays a vital role in the charge storage. For instance, tunnel structures, both smaller and larger than the size of the electrolyte ions store fewer charges. As both of these tunnel phases of manganese oxide face difficulty for the insertion of the electrolyte ions. Therefore, manganese oxide with adequate tunnel size needs to be taken into account. Besides this, it is also essential to consider the electronic conductivity of the manganese oxide phase, as high electronic conductivity allows it to store more charges during the condition of fast charging. In regards of the cell assembly, after considering the above-mentioned understanding the practice of applying the voltage-hold test to determine the realistic cell voltage is helpful, as the cell assembled with such realistic voltages permits the cell to have long cycle life. 
Besides these understanding, remarkable performances were witnesses with the technologies developed in this thesis. For example: (i) the carbon-based electric double layer supercapacitor shows faster responses than the existing carbon-based supercapacitors, (ii) the pseudocapacitors shows high volumetric capacitances (> 35 F cm-3) than carbon-based supercapacitors. Besides this, pseudocapacitors also exhibit higher cells voltages than the existing pseudocapacitors. The pseudocapacitor cells developed in this exhibits high electrochemical stability (> 95 %) over thousands of cycles. Furthermore, the pseudocapacitor is more favorable than EDLCs in applications as they provide slower self-discharges than EDLCs. The above understanding, such as the selection of the electrode, electrode processing and the cell assembly is a tool for designing better supercapacitors.

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[spa] La tesis se centra en el desarrollo del conocimiento orientado y conducido a la mejora de las propiedades electroquímicas de los supercapacitores, ya que sufren bajos valores de densidad de energía. Este inconveniente limita a los supercapacitores en las aplicaciones donde son necesarios tanto alta potencia como densidad de energía. Entonces, en este escenario, se identificaron dos problemas principales importantes: (a) las limitaciones de rendimiento del supercapacitor debido a la condición de carga rápida, y (b) el bajo voltaje de celda de los pseudocapacitores en electrolitos acuosos en comparación con los electrolitos orgánicos. Para superar la limitación de rendimiento en el primer problema, se muestra una alternativa original a través del electrospinning para diseñar nanofibras de carbono porosas con incrustaciones de óxido metálico con una arquitectura de electrodo 3D que contribuyen a reducir la resistencia del electrodo y al mismo tiempo aumentan los valores asociados de capacidad. La investigación indica un papel esencial en la concentración del precursor de óxido metálico hacia el comportamiento electroquímico de los electrodos. Esta correlación podría ser útil para diseñar mejores electrodos para supercapacitadores, funcionando con mejores capacidades de densidad de energía y potencia. En lo que respecta al problema relacionado con los bajos voltajes celulares en el pseudocapacitor acuoso, en lugar de utilizar materiales basados en carbono más estándar, se toma una metodología en términos de exploración y mejora basada en las propiedades del material del electrodo. Así, se introducen nuevos materiales de la familia de MXenes, para lograr voltajes de celda más altos. Bajo este marco, se propone un nuevo MXene 2-D basado en carburo de vanadio y molibdeno y se han investigado sus características electroquímicas. De acuerdo con sus características, su acoplamiento con carburo de titanio 2-D MXene exhibe un voltaje más alto en una celda pseudocapacitiva todo en MXene. Además de esto, el problema del bajo voltaje de la celda también se resuelve aplicando otro enfoque basado en la modificación del electrólito. El enfoque propuesto se basa en el uso de soluciones salinas superconcentradas a base de agua que son electrolitos prometedores en la ampliación del voltaje celular de los pseudocapacitores acuosos. Del mismo modo, también se propone que el acoplamiento del carburo de titanio 2-D MXene con las estructuras del túnel de óxido de manganeso utilizando este electrolito súper concentrado o agua en sal permite lograr una celda de pseudocapacitador acuoso de alto voltaje. En conjunto, la estrategia presentada a través de esta tesis en términos de preparación de electrodos, selección de materiales, ensamblaje celular y su evaluación de las propiedades electroquímicas es una herramienta para diseñar supercapacitores con mejores capacidades de energía y potencia.