Optimization and degradation of supercapacitors in aqueous and super-concentrated “water-in-salt” electrolytes

Abstract

[eng] Supercapacitors are electrochemical energy storage devices. Its energy comes from the electrostatic accumulation of ionic charge on the surface of an electrode, compensated by an opposite electrical charge on the inner surface of the electrode, for this reason they are also called electrochemical double layer capacitors. Since no chemical reactions are involved, these devices can be charged and discharged rapidly. This makes them ideal for high-power applications such as powering camera flashes, starting engines, or managing power peaks in electrical grids. They are also used for energy recovery in regenerative braking systems of trains or electric vehicles. Although capacitors provide great power, their energy density is an order of magnitude lower than that of batteries, which limits their extensive use. One of the objectives of this thesis is to increase the energy, power, and energy efficiency of capacitors based on activated carbon electrodes in aqueous electrolytes. To this end, different strategies have been addressed to increase capacitance and voltage, which are directly related to the increase in energy, as well as to reduce resistance, which is related to performance and lifespan. Using the aqueous electrolyte 1 M KOH, the following aspects have been studied: 1) optimizing the mass balance between the positive and negative electrodes to improve energy efficiency and cycle life, 2) thermally treating the electrodes at temperatures below the melting point of the binder to increase energy, 3) investigating the impact of two membranes, fibreglass, and polypropylene, on rate-capacitance performance. Optimizations conducted in Chapter 3 using YP50 as electrode show that the positive to negative mass ratio should be adjusted to 0.6, the thermal treatment should be set to 240 ºC, and fibreglass membranes should be used to achieve a better capacitor response. In Chapter 4, we used super-concentrated aqueous solutions based on potassium acetate, called “water-in-salt” electrolytes, to increase the voltage of the devices. Concentrations from 1 m to 32 m (mol/Kg) have been studied, verifying an increase in the potential window up to 1.8 V and finding a compromise between capacitance, rate capability, cycle life, and cost for the 27 m KAc electrolyte. The use of the 27 m KAc electrolyte showed a capacitance of 26.3 F/g and 19.6 F/g when increasing the scan rate from 5 to 100 mV/s, with a retention of 74.5%, a capacitance of 13 F/g over 10,000 charge cycles at 1 A/g, and the estimated electrolyte cost was €4.42 to produce 1,000 cells. In chapter 6 we prepared hybrid capacitors, composed of a capacitive activated carbon electrode and a faradaic electrode using lithium oxides. Lithium titanium oxide (LTO), commonly used as anode for its well-defined phase transition around 1.6 V vs Li/Li+, was used as the negative electrode and YP50 as the positive. Lithium iron phosphate (LFP), commonly used as cathode for its well-defined phase transition around 3.5 V vs Li/Li+, was used as the positive electrode and YP50 as the negative. The YP50/LTO capacitor with the 1 M LiPF6 organic electrolyte showed a high capacitance of 30.2 F/g at a low rate of 0.6 mV/s and 7.7 F/g at a medium rate of 20 mV/s in a potential window of 2 V. The LFP/YP50 capacitor showed with the organic electrolyte 1 M LiPF6 a capacitance of 20.9 F/g at a low rate of 0.1 mV/s and 8.7 F/g at a medium rate of 20 mV/s in a potential window of 2.9 V and with the super-concentrated water-in-salt electrolyte 32 m KAc + 6 m LiAc it showed a very high capacitance of 52.4 F/g at 0.1 mV/s and 13.3 F/g at 10 mV/s in a potential window of 1.8 V. The other objective of this thesis, discussed in chapter 5, is the study of degradation mechanisms involving irreversible reactions in aqueous capacitor components, which reduce its performance. These mechanisms are accelerated by operating conditions such as temperature, humidity, and working potential window. We studied the degradation mechanism of activated carbon capacitors in two aqueous electrolytes: super-concentrated "water-in-salt" 27 m KAc and dilute 1 M KOH. To accelerate their degradation, we have performed an electrochemical characterization of the cells by means of a series of 10,000 charge and discharge cycles at 1 A/g in a potential window of 2 V, which, being larger than the stability window of the electrolytes, ensures a rapid degradation. Once the cells were degraded, we proceeded to disassemble and study with SEM, EDS, FTIR, XRD and XPS methods. A reassembly of the cells with 27 m KAc was also performed after degradation, changing the electrolyte and the membrane and maintaining the electrodes and current collectors, cycled again and studied with the method of failure mode and effects analysis (FMEA) to identify and evaluate the potential failure modes, analyse their causes and its severity and create strategies to mitigate them. The main failure modes analysed are related to the decomposition of the electrolytes, leading to water splitting and the generation of hydrogen and oxygen. Oxygen would oxidize the current collectors and the electrodes, increasing the resistance and reducing the electrical conductivity and capacitance. The oxidation of the activated carbon electrodes would generate CO2, which together with O2 and H2 would block the pores of the electrodes, making electrical contact difficult and reducing capacitance. The decomposition of the electrolytes would limit their stability and cause their precipitation on the electrodes, as well as decomposition of the electrode binder with loss of cohesion.

[spa] Los supercapacitores almacenan energía acumulando carga iónica en la superficie de un electrodo, permitiendo una rápida carga y descarga. Aunque ideales para aplicaciones de alta potencia, su densidad de energía es inferior a la de las baterías. Esta tesis busca mejorar la energía, potencia y eficiencia de supercapacitores con electrodos de carbono activado en electrolitos acuosos, explorando diversas estrategias para aumentar capacitancia, voltaje y reducir resistencia. Usando el electrolito acuoso 1 M KOH, se estudió la optimización del balance de masa entre electrodos, el tratamiento térmico de los electrodos y el impacto de las membranas de fibra de vidrio y polipropileno en el rendimiento. Se encontró que el mejor rendimiento se lograba con una proporción de masa de 0.6, un tratamiento térmico a 240ºC y el uso de membranas de fibra de vidrio. Además, se investigaron soluciones acuosas súper concentradas basadas en acetato de potasio, conocidas como electrolitos "agua en sal", para aumentar el voltaje de los dispositivos. El electrolito 27 m KAc mostró una ventana de potencial de 1.8 V y buena retención de capacitancia. Se lograron capacitancias de hasta 26.3 F/g a 5 mV/s y 19,6 F/g a 100 mV/s, con una retención del 74.5% tras 10,000 ciclos de carga a 1 A/g. Se desarrollaron capacitores híbridos utilizando un electrodo capacitivo de carbono activado y un electrodo faradaico. El capacitor YP50/LTO con electrolito 1 M LiPF6 mostró una capacitancia de 30.2 F/g a 0.6 mV/s y 7.7 F/g a 20 mV/s en una ventana de 2 V. El capacitor LFP/YP50 con electrolito súper concentrado (32 m KAc + 6 m LiAc), logró una capacitancia de 52.4 F/g a 0.1 mV/s y 13.3 F/g a 10 mV/s en una ventana de 1.8 V. Otro objetivo de la tesis fue estudiar los mecanismos de degradación de los componentes de los capacitores en electrolitos acuosos. Algunas celdas fueron reensambladas con electrolito fresco y membrana nueva y vuelto a degradar. Después de la degradación electroquímica acelerada se caracterizaron los componentes mediante SEM, EDS, FTIR, XRD y XPS. También se realizó un análisis de modos y efectos de fallos (FMEA), identificando la degradación de los electrolitos y la generación de gases que bloquean los poros de los electrodos reduciendo la capacitancia y conductividad eléctrica, oxidando los colectores de corriente y los electrodos con producción de CO2.