Resumen de Ion exchange membranes for aqueous organic redox flow batteries
Ion exchange membranes (IEMs) have been reported in the last century as materials for many applications resulting in a large technical revolution. The potential application of membranes has been presented in this thesis where membranes are used as materials for electrochemical energy storage. The interesting properties of these materials have been promoted by several research works looking for potentially exploitable usage in today¿s membrane technology, especially for RFB applications. Herein, in this thesis, a set of commercial membrane materials have been considered for their evaluation and study. The research was funded by the European Union Horizon 2020 research and innovation program under grant agreement 875613 (HIGREEW Affordable High-performance Green REdox floW batteries). In this regard, the research was done in line with the HIGREEW needs and requirements towards the development and implementation of a feasible Aqueous Organic Redox Flow Battery (AORFB).
The evaluation of these membrane materials is of remarkable importance since they largely determine the efficiency of the device and its economic viability. The major functions are preventing the cross-mixing of the electroactive compounds between the half-compartments while allowing the passage of the ionic species needed to complete the electric circuit when the battery is operating. For their feasibility, some important parameters should be considered including ion conductivity, permeability, electrolyte/water uptake, degree of swelling, ion exchange capacity, and the cost of membranes. The state-of-the-art membranes within this technology are governed by NafionR membranes which are particularly expensive. However, to overcome the major economic barriers, different materials have been considered such as (i) cation and anion exchange membranes (CEMs and AEMs, respectively), (ii) non-ionic membranes, and (iii) porous separators for their possible implementation in the HIGREEW battery prototype.
From this evaluation, it was evidenced that not all the materials possess all the specific requirements for an ideal application and every single parameter should be carefully considered when selecting a material for a feasible application. HIGREEW project relies on organic active materials for the development of sustainable and cost-effective RFBs at neutral pH. Herein, the membrane should provide the best performance addressed to this emerging technology. Therefore, a selective criterion was created as a tool to select the best materials and foresee the membrane performance on the battery prototype. The selection consists of analyzing the potential materials using the permeability of an electroactive organic compound and ionic conductivity at neutral pH (NaCl 1 M, in this thesis). By following this methodology, two AEMs (FAA-3-50 and FAA-3-PE-30) and two CEMs (FS-950 and E-630(K)) were preselected as potential materials to be implemented in the AORFB prototype.
Concerning the relevance of crossover in RFBs, membrane modifications can help in reducing the permeation of the active species preserving the battery performance. The membrane modification proposed in this thesis consists of a chemical in situ polymerization of Py monomer by immersion method using FeCl3 as initiator. In the first step, the membrane was impregnated with the monomer covering the surface and slipping through the holes of the membrane. In the second step, the chemical polymerization takes place immediately when soaking in the solution containing the oxidizing agent. Then, the polymer formed filled these holes that hindered the redox organic molecules crossing through the membrane while enhancing the mechanical properties of the composite membrane.
The modified membranes were deeply characterized to illustrate the effect of pyrrole polymerization effect on membrane properties. SEM and EDX analysis were helpful to determine the homogeneity of the polymerization process over the membrane surface and to determine whether the PPy segments were hosted inside the pores of membranes. Some fundamental properties were evaluated such as permeability, membrane exchange capacity, and hydrophilicity, among others. The symbiotic study between spectroscopic characterization with the analysis of the most fundamental properties of membranes was key to determining the role of PPy in the inner structure of membranes and their behavior. From this evaluation, modified membranes showed negligible effect on the electrical conductivity, and modified-AEMs were found to have optimal properties, showing an increase in ion exchange capacity while maintaining excellent mechanical stability and unaltered permselectivity. Additionally, the diffusion boundary layer of these AEMs was slightly extended, which suggests a greater double-layer stability for ion exchange processes than in the case of CEMs. By contrast, modified CEMs showed worse capacity exchange and lower double-layer due to the positive charges of PPy segments formed upon polymerization (doped polymer), which was confirmed by Infrared and Diffuse Reflectance spectroscopies.
Since a membrane is necessary to physically separate the positive side from the negative side while allowing the passage of ions, some other studies on modified membranes have been explored based on thermodynamical considerations (the phenomenological approach), that is, by structural and generalized conductivity considerations and its implications in terms of physicochemical characteristics. From this evaluation, Transport Structural Parameters (TSP) have been obtained from the electrolyte concentration dependencies (NaCl, in this thesis) which was useful for the description of membranes and the effect of PPy incorporation. This evaluation reinforced modified-AEMs as a good material since its incorporated PPy phase enhanced their membrane conductivity and exchange capacity while reducing the interstice regions where the crossover can be easily addressed. By contrast, in the case of CEMs, their PPy phase hindered some of the possible exchangeable regions leading to a more depleted membrane exchange capacity although the interstice regions were successfully reduced.
Concerning the research done on the electrolyte optimization and definition for the project, a DFT study about methyl viologen derivatives has been performed and, the selected derivatives were electrochemically characterized towards new organic anolyte compounds for their implementation in an AORFB system. The symbiosis between theoretical and experimental characterization served to determine the effect of the ring size, geometry, and electron density on the physicochemical properties of the materials to understand the stability of the reduced species in terms of electronic delocalization and the importance of the molecular design on the stability of electrolyte for AORFB when selecting a good candidate, anolyte in this case. Our studies were corroborated in a single-cell performance and the obtained results were in good agreement with the predicted stability.
However, despite all efforts done on membrane and electrolyte optimization, the project lines were conducted to another electrolyte definition where (i) the methyl viologen derivatives were not considered due to economic and environmental issues, and (ii) CEMs were positioned as the only alternative. In this way, the electrolyte consisted of an (SPr)2V and ammonium ferrocyanide compounds as anolyte and catholyte, respectively, and NH4Cl as a supporting electrolyte. Therefore, the membranes needed for this new system required CEMs as a separator and the selected membranes were FS-950 and E-630(K) and their respective modifications with PPy.
Finally, this thesis has been complemented with a 3-month research stay at KTH Royal Institute of Technology in Stockholm (Sweden) allowing the obtention of international thesis recognition. During this stay, I was working on the electrochemical characterization of new membrane compositions for the Vanadium RFB system in collaboration with the University of Lund (Lund, Sweden). The results of this research revealed that a novel membrane composition with 60% of zwitterionic content showed a remarkable performance compared to the N212 membrane suggesting good chemical stability and a durable membrane that may be seriously considered for a practical application prospect in VRFB technology.
The results of the research described in this Ph.D. thesis were disseminated through several scientific publications (five articles and two book chapters) and various presentations at both national and international conference contributions (twelve oral and five poster communications).
