Development of Bipolar Membranes by Electrospinning and Electrospraying - Relevance to Acid-Base Flow Battery Applications
Emad Al-Dhubhani is a PhD student in the department Membrane Science & Technology. (Co)Promotors are prof.dr.ir. W.M. de Vos from the faculty Science & Technology and dr. M. Saakes and dr. M. Tedesco from Wetsus.
The heightened recognition of the consequences stemming from modified natural carbon cycle due to greenhouse gas emissions has been amplified by the intersection of global warming and geopolitical transformations. The concentration of carbon dioxide (CO2) in the Earth's atmosphere has experienced a significant increase, rising from 260 parts per million (ppm) to 417 ppm. It has been determined that energy-related sources of emissions are responsible for 70 % of this increase. As a result, governments worldwide are consistently striving to substitute fossil fuel resources with those of intermittent renewable energy sources. In order to facilitate this significant transition, it is imperative to make progress in pertinent technologies and materials. Electro-membrane applications, including flow batteries, fall within the purview of this transitional phase.
Chapter 1 includes an introduction to the thesis, basics of the concept and theoretical underpinnings of the acid-base flow battery system and bipolar membrane fabrication are discussed. The chapter expounds upon the challenges and potential areas for further investigation that remain in connection to the research conducted in the other chapters of the thesis.
The objective of Chapter 2 was to assess and compare the material properties and electrochemical efficiency of five commercially available bipolar membranes. The work focused on examining the performance of these membranes under both reverse bias conditions, which involves water dissociation, and forward bias conditions, which involves acid-base neutralization. The outcomes of the characterization process have facilitated the identification of specific Bipolar Membranes (BPMs) that exhibit superior performance under forward bias, rendering them more appropriate for experimentation in an Acid-Base Flow Battery (ABFB).
The BPMs that were characterized exhibited varying performance with respect to water dissociation across different concentrations of NaCl solution. Furthermore, it was observed that the efficiencies of the BPMs were enhanced with an increase of the applied current density, which suggested a reduced impact of co-ion leakage. The capability of the BPMs to neutralize acid and base under a forward bias is an aspect that holds significant importance yet is often overlooked. In the given circumstances, it was observed that BPMs displayed distinct characteristics. Notably, Fumasep and Weifang BPMs demonstrated the most elevated open circuit voltages and the least voltage reduction at higher (forward-bias) currents. The observed behavior may be attributed to a greater degree of selectivity exhibited by AEL and CEL with respect to H+ and OH-, respectively.
The utilization of reversible water dissociation in bipolar membranes (BPMs) has led to the development of acid-base flow batteries, which show potential as a viable option for energy storage. For this concept to be viable on a larger scale, it is imperative that the bipolar membrane (BPM) exhibits consistent and reliable performance when subjected to both reverse and forward bias. This is particularly crucial in cases where there is a low water dissociation potential during reverse bias and a low voltage drop during forward bias. The present study's outcomes have led us to choose Fumasep and Weifang BPMs for subsequent evaluation in the ABFB. Additionally, our findings indicated that it is advantageous to conduct the flow battery at elevated current density while charging to mitigate the impact of co-ion leakage, and at reduced current density while discharging to prevent abrupt voltage declines. The round-trip efficiency was observed to increase up to 65 % by tuning the discharge-to-charge current density ratio, specifically when utilizing a discharge-to-charge ratio of 60:240 A/m2. It is imperative to advance the BPM technology to enhance the energy density and round-trip efficiency of the ABFB, thereby enabling it to effectively rival established flow battery systems such as the vanadium redox flow batteries. Enhanced selectivity in BPM could facilitate the operation of ABFB under advantageous circumstances, including elevated charge-discharge current densities and concentrated solutions exceeding 1M.
Chapter 3 presented a systematic study of the impact of a 2D and 3D junction on the water dissociation rate of a bipolar membrane. This study was conducted using identical anion and cation polymeric materials. The 2D and 3D bipolar membranes were realized through the utilization of a dual electrospinning fabrication technique, along with the implementation of chemically durable SPEEK and FAA-3 substances as viable options for the cation and anion exchange layers. The study revealed that the water dissociation process was accelerated by the 3D junction in comparison to the 2D junction, independent of any catalyst augmentation. The presented study aimed to showcase the versatility of the electrospinning methodology by exploring the incorporation of a polymeric water dissociation catalyst, namely poly (4-vinylpyrrolidine), in both the 2D and 3D junctions. This work reported a higher catalytic activity of poly (4-vinylpyrrolidine) on the rate of water splitting for electrospun bipolar membranes (BPM) with a 3D junction, as compared to bipolar membranes with a 2D junction, under galvanostatic conditions in 0.5 M NaCl and 2M NaCl solutions. This phenomenon can be rationalized by the intricate interlinking of the polymer fibers of the anion and cation in three dimensions, leading to an intended augmentation in the junction's specific surface area. Ultimately, the open-circuit voltages of all produced BPMs were assessed through the utilization of 0.5 M HCl and 0.5 M NaOH solutions. The electrosprayed poly (4-vinylpyrrolidine) layer at the 2D junction resulted in increased open-circuit values for the BPMs. The approach presented in this chapter was more effective and resilient than the conventional casting process that results in a 2D catalytic junction. Furthermore, this technique enabled the integration of diverse polymeric materials as catalysts for the water splitting process.
The fabrication of multiple bipolar membranes (BPMs) with varying 3D junction thicknesses was achieved through the utilization of electrospinning techniques as presented in Chapter 4. The deposition time required for the nano/microfibers at the junction was adjusted to produce the desired thickness. The presented investigation involved the production of four distinct BPMs, each possessing a 3D junction thickness of approximately 4, 8, 17, and 35 µm. The primary objective was to assess the impact of the junction thickness on the electrospun hot-pressed BPMs' performance.
The water dissociation curves of bipolar membranes (BPMs) indicated a decrease in voltage for BPMs with greater thickness. This phenomenon was attributed to the three-dimensional expansion of the interfacial contact area between cation exchange and anion exchange fibers, which led to an increase in the effective water dissociation reaction area. The increase of BPM thickness from 4 µm (BPM I) to 35 µm (BPM IV) led to a reduction of 32% in the BPM water dissociation overpotential.
The assessment of the presented effectiveness was demonstrated by proficient generation of hydrochloric acid (HCl) and sodium hydroxide (NaOH) with a notable efficiency level within the range of 90-100 %. Additionally, an evaluation of the BPM's effectiveness during the water association process indicated a significant decrease in voltage from its designated open circuit voltage (OCV) as a result of substantial leakage of hydroxide ions (OH-) and protons (H+) through the respective layers. For BPMs to function optimally in both dissociated and associated water modes, it was imperative to devise BPMs that possessed anion exchange and cation exchange layers that exhibited exceptional selectivity.
In Chapter 5, a novel methodology for the fabrication of BPMs through the hybrid electrospinning/electrospraying approach was presented. Through the combination of electrospinning techniques for ion exchange polymers and electrospraying methods for dispersed catalyst nanoparticles, the production of bipolar membranes (BPMs) was demonstrated with certain polymeric stability, increased junction-specific surface area, and the potential for incorporating diverse catalyst materials. Furthermore, the utilization of MCM-41 (silica nanoparticles) as a novel porous catalyst material was investigated as the principal catalyst for water dissociation in the BPM.
In this study, several bipolar membranes (BPMs) were synthesized with different loadings of MCM-41 catalyst. The MCM-41 silica nanoparticles were deposited in layers directly adjacent to the 3D junction of the BPM, as well as solely on either the cation exchange or anion exchange side of the junction. The current-voltage curves for water dissociation of the recently developed bipolar membranes exhibited a noteworthy enhancement in the electrochemical performance of the bipolar membrane, as a result of decreased transmembrane voltage. The electrochemical performance of a benchmarked commercial BPM manufactured by Fumatech was surpassed by a BPM with an optimal catalyst (MCM-41) loading of 0.13 mg/cm2, which recorded an overpotential of 280 mV at 1000 A/m2. Additionally, the results obtained from the analysis of open circuit voltage and water formation characterization provided clear evidence of the advantageous nature of the catalyst when the BPM was operated in the forward bias mode. It is noteworthy that the voltage drop remained above 0.2V at 500 A/m2, which is a significant characteristic for the utilization of fuel cells and flow batteries.
The study in Chapter 5 showcased progress in the field of BPM development by presenting a new BPM architecture that utilized hybrid electrospinning-electrospraying techniques and a novel catalyst known as MCM-41. It is anticipated that the selection of a catalyst in BPM that is specifically designed for the functions of water dissociation and water formation will become a prevalent trend in the advancement of high-performance BPM.
Finally, the thesis is concluded with Chapter 6 where general conclusions of the thesis are discussed. In addition, some of the interesting results of attempts to incorporate different polymers as potential water dissociation catalysts are also discussed and analyzed. The chapter also includes an outlook for upscaling the BPM fabrication, where the technical and economical parts are also covered. Other significant aspects of the BPM development are also delved onto, like BPM durability, future BPM relation to acid-base flow battery and potential advanced extensive BPM fabrication technologies.
