Development of Redox Electrolytes and their application in new electrochemical energy storage devices

Resumen

Transportation and energy production activities based on fossil fuels have important environmental impacts. In order to minimize them, alternatives to fossil fuel vehicles and an efficient use of renewable energy resources are needed. Energy storage can provide a safe and cost-effective solution to the inherent irregular energy supply from those sources and to the massive implementation of the electric vehicle. Thus, remarkable investments are being done in the development of electrochemical energy storage technologies, such as supercapacitors and batteries, due to their high efficiency, variety of power and energy rates, long cycle life and low maintenance. One of the main challenges in this field is to improve their energy density while reducing their cost. With respect to supercapacitors, the development of redox electrolytes for hybrid devices is becoming one of the hottest topics, since they are an easier alternative than the complex development of new electrode materials. In this way, by simple dissolution of active species into the supporting electrolyte, important parameters such as capacity, energy and power density, can be enhanced. Regarding Redox Flow Batteries, different approaches are being investigated such as the replacement of vanadium species by organic redox molecules or the substitution of problematic ion-selective membranes by less expensive separators. Despite all the scientific efforts, many challenges still remain to be faced both in supercapacitors (SCs) and Redox Flow Batteries (RFB). This thesis addresses some of them and aims to provide new insights to the matter. In this thesis, we aim to contribute to the field of electrochemical energy storage by developing new organic redox electrolytes with application in different devices. Specifically, the formulation of new redox electrolytes containing commercially available organic molecules has been studied, as well as their electrochemical characterization. Thus, important parameters such as solubility, redox potential, reversibility, kinetics and diffusion coefficients were determined. Moreover, the electrochemical performance of these new redox electrolytes in devices such as hybrid supercapacitors and in an innovative concept of Membrane-Free Redox Flow Battery was analyzed with the main goal of improving their performance. This PhD thesis is organized in 6 different chapters whose content is described below: Chapter 1 consists on an introduction in which the state-of-art and the current main challenges of the electrochemical energy storage devices, especially those containing redox electrolytes, are commented. In chapter 2 the main objectives of this PhD thesis are described as well as the initial hypothesis and the scientific questions that are pursued to be answered. Chapters 3, 4 and 5 contained the main scientific results related to the challenges and the objectives expounded previously, including the specific methodology and bibliography in each chapter. In chapter 3, the effect of using a organic redox electrolyte based on ionic liquids on a hybrid SC was investigated. Thus, a solution of 0.4 M para-Benzoquinone (pBQ) in the ionic liquid (IL) N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PYR14TFSI) was used as a redox electrolyte in hybrid supercapacitors. Two carbons with very different textural properties, Pica carbon and Vulcan carbon, were used as electrode material. Electrochemical performance of these energy storage systems was investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (CD). Unlike SCs with pure IL electrolyte, new battery-like features appeared in the CV curves and CD profiles. This electrochemical performance, associated with the faradaic contribution of the redox electrolyte, results in a significant improvement of the electrochemical performance of the hybrid system. For Vulcan carbon with low specific surface area (SBET=240 m2·g-1), specific capacitance (Cs) and specific real energy (Ereal) values as high as 70 F·g-1 and 10.3 Wh·kg-1 were obtained at 5 mA·cm-2 with hybrid SC operating at 3 V. This represents an increment of 300 % in Cs and Ereal with respect to the SC based on pure PYR14TFSI. For high surface area carbon such as Pica (SBET=2410 m2·g-1), the addition of the redox quinone molecule resulted in a moderate enhancement, reaching values of 156 F·g-1 and 30 Wh·kg-1 under the same experimental conditions (36 % and 10 % increment, respectively). Moreover, the energy storage mechanism of these hybrid devices was assessed in detail by Electrochemical Impedance Spectroscopy (EIS). In this thesis, EIS was used for the first time to analyse the energy storage mechanism in supercapacitors with non-aqueous electrolytes with organic redox molecules dissolved, as a function of the frequency and bias voltage. In addition, a simple electrical model was developed to describe the behaviour of the supercapacitor with a very good correlation with the experimental results. This investigation provides useful information delving into the behaviour of the devices which is fundamental for their future practical applications. In chapter 4, a new concept of Membrane-Free RFB that relies on the immiscibility of redox electrolytes and where the vanadium species are replaced by organic molecules was presented. It was demonstrated that the biphasic system formed by one acidic solution and one ionic liquid, both containing dissolved quinoyl redox-active species, behaves as a reversible battery without any membrane or separator. This proof-of-concept of Membrane-Free Battery exhibits an open circuit voltage (OCV) of 1.4 V with a high theoretical energy density of 22.5 Wh·L-1 and it is able to deliver 90 % of its theoretical capacity. Moreover, this battery shows excellent long-term performance with coulombic efficiency close to 100 % and energy efficiency of 70 % upon cycling. In addition, the versatility of this concept was investigated. Hence, the electrochemical performance of 10 immiscible redox electrolytes based on different solvents such as propylene carbonate, 2-butanone or neutral media and containing different organic molecules such as TEMPO or substituted anthraquinones was explored. Those showing promising electrochemical performance were paired and used as anolyte and catholyte in Membrane-Free RFBs that exhibited different performance. For instance, a 50 % improved OCV (2.1 V), an operating voltage of 1.75 V and 35 % higher power density compared with the first battery were revealed in batteries with substituted anthraquinones. Moreover, cycling-life was evaluated over 300 cycles for a battery containing propylene carbonate-based anolyte achieving excellent (85 %) capacity retention. Thus, these results demonstrated the huge versatility and countless possibilities of this new Membrane-Free RFB concept which can be tailored to several applications with different requirements. With the aim of extending the applicability of this new concept beyond the aqueous-nonaqueous combination, the feasibility of using Aqueous Biphasic Systems (ABS) was investigated in chapter 5. ABS in which the two immiscible phases are aqueous are a successful tool employed in extraction/separation processes. In this chapter, ABS containing one redox organic molecule selectively dissolved in each phase were proposed as redox electrolytes for Membrane-Free RFB for the first time. In this concept, the cross-contamination typically occurring through the ineffective membranes is determined by the partition coefficients of the active molecules between the two phases. Thus, a series of ABS with different compositions and the partition of several redox organic molecules on these systems were tested. In addition, the electrochemistry of these redox-active immiscible phases was evaluated. Combining thermodynamics and electrochemistry, several redox ABS that might be used in Total Aqueous Membrane-Free RFB were proposed exhibiting theoretical battery voltages as high as 1.6 V. Among them, the ABS based on the polymer PEG1000 and containing methylviologen (MV) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) as active species was selected to become an unprecedented Total Aqueous Membrane-Free Battery. When connected electrically, this redox-active ABS becomes a Membrane-Free Battery with an open circuit voltage (OCV) of 1.23 V, high peak power density (23 mW·cm-2) and excellent long-cycling performance (99.99 % capacity retention over 550 cycles). Moreover, essential aspects of this technology such as the crossover, controlled here by partition coefficients, and the inherent self-discharge phenomena were addressed for the first time. These results point out the potential of this pioneering Total Aqueous Membrane-Free RFB as a new energy storage technology. Finally, this thesis includes a chapter 6 in which general conclusions and future work are explained. In the appendix A the scientific contributions of this work are gathered. In the appendix B, the figures, tables and acronyms used in this thesis are listed.