The aim of this paper is to investigate the use of 3D membrane junctions obtained by electrospinning to improve the catalytic process in bipolar membranes (BPMs) used in water dissociation processes. For this purpose, bipolar membranes with 2D junctions manufactured by lamination are compared with 3D entangled junctions manufactured by electrospinning, using the same polymers for the anionic and cationic layer. In addition, the use of the P4VP polymer as a catalyst for the reaction has been investigated.
Bipolar membranes (BPMs) are a type of membrane that allows ion exchange. BPMs are used to dissociate water in many electrodialysis-based processes, resulting in hydroxides (OH-) and protons (H+) from a neutral solution. Industrial applications of BPMs include the production and purification of acids and bases, CO2 capture, flow batteries and fuel cells, among others.
BPMs are composed of two adjacent charged layers: an anion exchange layer (AEL) and a cation exchange layer (CEL), which are bonded by different techniques. The contact surface between the two exchange layers is called the membrane junction. This junction is generally two-dimensional (2D) and is where the electrocatalytic process of water dissociation takes place.
Conventional BPMs are manufactured by alternating successive layers of anion and cation exchange, which are bonded or pressed together at high pressures and temperatures. BPMs can also be made by melting the different anion and cation exchange layers in a controlled and sequential manner.
Current BPMs have certain limitations, such as low water dissociation rate and poor stability under adverse conditions. To overcome these limitations, the authors of this publication propose to fabricate the membrane junctions of the BPMs by electrospinning. In this case, nanofibers of different polymeric materials used in the anionic and cationic layers are deposited at the membrane junction to increase the contact region between the exchange layers. This generates a three-dimensional (3D) entangled and open structure that facilitates the transport of water within the BPM, thereby increasing the efficiency of the dissociation process. In addition, they also propose the use of polymeric catalysts, particularly P4VP.
Materials and methods
Two different types of polymers were used to manufacture the membranes. The polymer used as cation exchanger was SPEEK (sulphonated poly(ether ether ketone)). The polymer used as an anion exchanger was FAA-3. In both cases, the solvent used was DMAc (dimethylacetamide). On the other hand, there are different catalysts that can be used together with BPM to accelerate the water dissociation process. In this work, the use of P4VP (poly(4-vinylpyrrolidine)) as a catalyst has been investigated by incorporating it into the membrane junction of some of the BPMs manufactured.
Specifically, 5 different types of BPMs have been manufactured in this research work:
– 2 BPMs with 2D junction by lamination and hot pressing: one without catalyst, and the other with P4VP electro-stretched into the bond before lamination.
– 3 BPMs with 3D entangled junction by electrospinning and subsequent hot pressing: one without catalyst, one with 7.7% wt of P4VP in the FAA-3 fibers at the juction, and one with 15% wt of P4VP in the FAA-3 fibers, both at the junction and at the AEL.
The electrospinning equipment used to manufacture the BPMs with 3D bonding was a Fluidnatek LE-50 equipped with two independent emitters mounted on two translation axes for the simultaneous electrospinning of the SPEEK and FAA-3 solutions. The equipment also incorporated an environmental control unit (ECU) to establish and precisely maintain the required temperature and humidity conditions during the process.
The fabricated membranes were morphologically characterized using SEM (scanning electron microscopy) images and EDS (energy dispersive X-ray spectroscopy). The characterization of the electrochemical properties was carried out using a homemade five-compartment test cell that allowed the measurement of current efficiency and OCV (open-circuit voltage).
The obtained results show that the electrospinning fabricated BPMs with 3D junction offer a higher performance in the water dissociation process compared to the BPMs with laminated 2D junctions, since the entangled 3D juctions offer a higher specific exchange surface for dissociation. On the other hand, the results also show that the use of a polymer catalyst such as P4VP improves the performance of the water dissociation process due to the increase in the number of additional active sites for the reaction.
The increased performance and stability in the water dissociation process offered by 3D BPMs fabricated by electrospinning make them ideal candidates for applications such as desalination, hydrogen production and energy storage.
ACS Appl. Energy Mater. 2021, 4, 3724-3736