Layer-by-Layer Modification of Electrospun Bipolar Membranes

Student thesis: Master

Abstract

Because of global warming and climate change a worldwide shift is seen from burning fossil fuels to the use of renewable energy. To reach the goal of becoming completely climate neutral by 2050, the share of renewables is expected to increase drastically. This leads to the inevitable electrification of the industry, where electricity becomes the main driving force for existing and emerging industrial processes. In many of such processes, bipolar membranes (BPMs) could play a promising role. BPMs are a type of ion exchange membrane that consist of a cation exchange layer (CEL) and an anion exchange layer (AEL), which are separated by an interface layer (IL) or bipolar junction. BPMs are able to facilitate the water dissociation (WD) reaction in the IL when a sufficient potential is applied. The in situ production of H+ and OH- ions enables BPMs to maintain a stable pH gradient across the membrane, making them a useful tool in many electrochemical applications, such as in flow batteries or water electrolysis. However, the currently available commercial BPMs lack the efficiency and mechanical stability required for these high current density operations. In this regard, electrospinning in combination with the incorporation of WD catalysts is suggested to improve both properties. Preparation of BPMs with electrospinning creates an entangled network between the CEL and AEL, which improves the mechanical properties, while it enhances WD by providing alternative pathways for the WD products to leave the junction. In this study, BPMs are fabricated using electrospinning, followed by hot pressing. The material used for the CEL is sulfonated poly(ether ether ketone) (SPEEK) and for the AEL the commercially available FAA-3 is used. The WD catalysts are introduced to the IL by layer-by-layer (LbL) modification of the electrospun SPEEK. With the LbL method, polyelectrolyte multilayers (PEMs) are deposited by alternating adsorption of polyethyleneimine (PEI) and polyacrylic acid (PAA). The growth of PEI and PAA multilayers can be controlled by varying external factors, such as the polyelectrolyte concentration, ionic strength and pH. The multilayer growth was monitored on a silicon wafer via optical fixed angle reflectometry. At a pH of 6.5, linear growth was observed for low ionic strengths (5 and 50 mM NaCl). Coating at a high ionic strength (500 mM NaCl) resulted in exponential growth of the multilayer, due to the screening of charges by salt ions and interdiffusion of the polyelectrolyte chains. This lead to significantly larger total adsorption values after deposition of the same number of layers. The pH could be used to vary the ionization degree of PEI and PAA. The ionization degree affects the diffusivity of PEI and PAA and dictates the type of multilayer growth. It was found that the multilayers grew linearly at high ionization degrees, while exponential growth was observed for low ionization degrees. Especially the ionization degree of PAA determined the growth behavior. The catalytic effect of PEI and PAA multilayers on WD was assessed electrochemically by recording polarization curves and measuring the permselectivity. The results showed that incorporation of low amounts of PEI and PAA already resulted in a significant reduction of the WD onset potential when compared to an electrospun BPM without catalyst. The best WD performance was found in BPMs with the highest catalyst loadings and where the PEI and PAA multilayers showed exponential growth. Similar results were observed during the permselectivity measurements, showing that the combination of electrospinning and the LbL modification of the IL with PEI and PAA multilayers is promising for the development of high performance BPMs.
Date of Award28 Feb 2024
Original languageEnglish
SupervisorD.C. (Kitty) Nijmeijer (Supervisor 1), Zandrie Borneman (Supervisor 2), M.T. (Thijs) de Groot (Supervisor 2) & H.J.M. (Menno) Houben (Supervisor 2)

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