Resumen de Organic Anolytes Design and Membrane Modification Through Efficient Redox Flow Batteries
Human development has brought about advancements in technology, healthcare, and human rights, contributing to an improved quality of life. However, this progress has also led to certain challenges, such as the ever-increasing demand for resources. To address the energy demand, the utilization of renewable energy sources has emerged as a clear solution, offering eco-friendly and virtually unlimited energy options. Solar and wind energy are among the most developed renewable energy sources. However, the intermittent nature of wind and solar power poses a significant challenge.
Energy storage has become crucial to store surplus energy generated by renewable sources for later use. Electrochemical Energy Storage (EES), particularly Redox Flow Batteries, have emerged as a versatile solution for energy storage, ranging from domestic applications to large-scale storage. Redox Flow Batteries offer fast response times, long lifetimes, reduced environmental impact, low cost, and high round-trip efficiency. The scientific community has dedicated significant attention to the development of this technology, resulting in a growing number of research papers, patents, projects, and companies.
Vanadium Redox Flow Batteries (VRFB) currently represent the state-of-the-art technology, with many companies and large-scale batteries utilizing this system. However, challenges persist, including the limited availability and high cost of vanadium electrolyte, the precipitation of V2O5 at high temperatures (> 40 ºC), low solubility at low temperatures, and the volatile price of vanadium. Recognizing the European Union's reliance on China, Australia, and Africa for 85% of the vanadium supply required for VRFB, the European Commission has been funding research projects to develop new organic earth-abundant electrolytes as potential substitutes for vanadium electrolyte in Redox Flow Batteries.
This thesis has received funding from the HIGREEW project, which aims to develop and validate a redox flow battery utilizing a new water-soluble, low-cost organic electrolyte. The project also focuses on optimizing low-resistance membranes and fast electrode kinetics to achieve high energy density and long-term durability. The thesis is structured around three key components: the investigation of organic electrolytes (Chapter 1), the development of ion exchange membranes (Chapter 2), and the testing of single-cell RFB systems (Chapter 3).
- Organic electrolyte: In this chapter, we focused on the synthesis and electrochemical characterization of 2,2'-bipyridinium anolytes as the initial candidates for the project, in collaboration with CIC EnergiGUNE. DFT calculations were employed to investigate the impact of structural parameters and the incorporation of various substituents on their electrochemical properties, including redox potential and kinetic constants. We developed a predictive model based on Natural Bond Orbitals (NBO) delocalization and Atomic Dipole Corrected Hirshfeld (ADCH) charge distribution of the radical-reduced species to understand and predict the stability and performance of the 2,2'-bipyridinium anolytes in the battery. Our findings identified compound 4 as the most stable and promising anolyte for further testing.
Additionally, during my international research stay in Turku, Finland, we successfully synthesized and electrochemically characterized a novel triazine anolyte for multiple electron storage. This new triazine anolyte exhibited three reversible redox processes for 4 electrons at -0.47 V, -0.62 V, and -0.82 V vs Ag|AgCl (3 M KCl), demonstrating fast kinetics, high solubility (>0.9 M), and diffusion coefficients. These properties make the new derivative a compelling candidate for multiple electron storage in RFB. Moreover, we observed a strong correlation between the solubility of the new triazine and the salt concentration, suggesting a significant interaction between the solubilizing group (-SO3-) and the pyridinium moieties. The details of this work on the organic electrolyte can be found in Chapter 1.
- Ion Exchange Membrane: The primary objective of our research group within the HIGREEW consortium was to select, characterize, and modify commercially available ion exchange membranes (IEM) that meet the requirements of HIGREEW in terms of permeability, cost, and performance. To achieve this, we conducted electrochemical characterization of various commercial membranes, evaluating important parameters such as swelling ratio, water uptake, ion exchange capacity, ionic conductivity, and permeability. Based on this initial screening, we identified the most promising membranes: two cationic membranes (FS-950 and E-630(K)) and two anionic membranes (FAA-3-50 and FAA-3-30PE).
Subsequently, these selected membranes were subjected to modification through a cost-effective and scalable in situ polymerization method using pyrrole and aniline. The modification of membranes with polypyrrole resulted in a significant reduction in permeability while maintaining a reasonable level of resistance. The modified membranes were thoroughly analyzed and characterized to understand the impact of the modification on transport phenomena. A detailed account of the selection, characterization, and modification processes can be found in Chapter 2.
- Single-cell results: First, a comprehensive study was conducted to establish a strong theoretical foundation for redox flow batteries and align it with the experimental techniques used for battery characterization. This provided a solid background for the subsequent research. The anolytes developed in Chapter 1 underwent thorough evaluation, yielding the following corroborations: i) The predicting model and experimental results regarding the stability of bipyridinium anolytes were found to align. Compound 4 exhibited the highest stability, with a capacity decay of 0.16% per day compared to the unsubstituted 2,2'-bipyridinium (Compound 2) which showed a capacity decay of 0.75% per day. Furthermore, a detailed characterization of the plausible degradation pathway for the bipyridinium anolyte was achieved.
ii) In this same chapter, the (SPr)34TpyTz anolyte demonstrated the ability to store multiple electrons without significant capacity decay. The concentration of the supporting electrolyte was found to have a significant effect on the solubility of the reduced state. A battery using 100 mM of the triazine anolyte against 100 mM K4[Fe(CN)6] showed no capacity decay and achieved an energy efficiency of approximately 75% under appropriate conditions. Increasing the concentration to 200 mM of (SPr)34TpyTz against 300 mM K4[Fe(CN)6] resulted in a small capacity decay of 0.44% per day and no crossover within a 14-day period. Furthermore, the system was investigated using the third and fourth electrons, revealing a significant capacity decay and an increase in electrolyte pH, suggesting a degradation mechanism involving the protonation of the reduced triazine.
