Enhanced proton conductivity of sulfonated poly(ether ether ketone) incorporating oxidized polyvinyl alcohol for high-performance proton exchange membranes

Materials for Renewable and Sustainable Energy https://doi.org/10.1007/s40243-025-00305-x (2025) 14:31 ORIGINAL PAPER Enhanced proton conductivity of sulfonated poly(ether ether ketone) incorporating oxidized polyvinyl alcohol for high‑performance proton exchange membranes Mohamed A. Ben Ali1 · Mohamed A. Ben Moussa2 · Souhib Umer Ilyas1 · Rizwan Nasir1 · Dorra Ghorbel3,4 · Sherif M. A. S. Keshk5 Received: 28 October 2024 / Accepted: 18 March 2025 © The Author(s) 2025 Abstract Alternative proton exchange membranes (PEMs) with high proton conductivity must be fabricated at reasonable costs to qualify as commercially used proton-exchange membrane fuel cells (PEMFCs). As a result, composite membranes containing sulfonated poly(ether ether ketone) (SPEEK) blended with various quantities of partially oxidized polyvinyl alcohol (OPVA) at 5 wt%, 10 wt%, and 20 wt% were developed for PEMs. At room temperature, the water uptake capacities of the SPEEK membranes containing 5, 10, and 20 wt% OPVA were 45%, 75%, and 109%, respectively. Correspondingly, the proton conductivities of SPEEK containing 5, 10, and 20 wt% OPVA were 22, 48, and 80 mS cm−1 at 110 °C, respectively. Compared with prestine SPEEK, OPVA/SPEEK have greater strength, stiffness, and thermal stability. The characterization results indicated that the strong hydrogen bond network that evolved between OPVA and SPEEK provided more jump sites for proton transfer. This study confirmed that OPVA/SPEEK membranes are effective as proton exchange membranes. Graphical abstract Keywords Sulfonated polyether ether ketone · Oxidized polyvinyl alcohol · Proton conductivity Introduction The energy sector desperately needs green and ecologically acceptable options. Fossil fuels, as energy sources, negatively impact the environment and human health, particularly energy [1–3]. Additionally, they emit greenhouse gases. Fuel cells are an excellent solution to energy issues because they produce only clean exhaust, such as heat and water [4]. Extended author information available on the last page of the article Hydrogen is considered the fuel of the future, and its energy can be tapped using fuel cells. Therefore, fuel cells help reduce the need for fossil fuels and other conventional fuels [5]. Fuel cells can be categorized based on their electrolytes [6]. Proton-exchange membrane fuel cells (PEMFCs) have attracted widespread attention because of their low operating temperatures and adaptability to stationary and mobile applications [7]. To improve PEMFCs, it is important to manufacture a unique proton exchange membrane (PEM) Vol.:(0123456789) 31 Page 2 of 11 with high proton conductivity, low cost, and excellent thermal and mechanical stability [8]. Sulfonated poly(ether ether ketone) (SPEEK), a nonfluorinated polymer, is deemed reliable for PEM applications because of its excellent thermooxidative resistance, exceptional chemical stability, good thermal stability, and low cost [9]. Increasing the degree of sulfonation (DS) of SPEEK enhances membrane swelling and proton conductivity but decreases mechanical stability since the sulfonated groups are plastic [10]. The DS of SPEEK can be readily regulated through temperature, acid concentration, and reaction duration, all of which impact its functionality [11–13]. However, over time, the stability of the SPEEK membrane decreases owing to the breakdown of the sulfonic acid group, which increases the inflation of the membrane [14, 15]. Additionally, increasing the operating temperature beyond 80 °C is often good for increasing fuel cell performance, including increasing the CO tolerance of the electrodes and facilitating water management. However, it lowers the water content of the membranes. Moreover, SPEEK membranes have significantly decreased proton conductivity at low relative humidity due to destruction of the water network and proton conducting channels, which deteriorates fuel cell performance. Consequently, the creation of new membrane materials and techniques for increasing proton conductivity in SPEEK-based PEMs is being attempted because the initial goal was to improve the performance of SPEEK membranes. Presently, competitive options exist to establish interconnected proton-conducting pathways for increased proton conductivity in SPEEK-based PEMs. These are hybrid membranes made of the SPEEK matrix and organic or inorganic fillers, and they can be affordable, easy to process, and perform well [10–15]. Solution casting was used to synthesize nanoparticles of graphene oxide (GO) grafted with 3-aminopropyl trimethoxy silane, which were integrated into the SPEEK matrix to form a composite PEM. The synthesized nanocomposite membranes had improved proton conductivity and water retention [7]. At 120 °C, the SPEEK membrane with 2 wt% amine-functionalized GO exhibited a proton conductivity of 11.32 mS cm−1, which was 2.45 times greater than that of the pure SPEEK membrane [14]. The explanation for this was that the SPEEK membranes had evenly spread amine-functionalized GO nanoparticles, creating new pathways for proton transport. Furthermore, composite SPEEK membranes were developed for PEM applications through the blending of 5 and 10 wt% sulfo ethyl cellulose (SEC). Compared with those of pure SPEEK, the composite SPEEK membranes had better water absorption and thermal stability [15]. Furthermore, compared with pure SPEEK, the composite SPEEK membranes had higher proton conductivity, reaching as high as 110 mS cm−1 above 100 °C. Conversely, the highest proton conductivity of up to 100 mS cm−1 was observed at 70 °C when microcrystalline cellulose (MCC) Materials for Renewable and Sustainable Energy (2025) 14:31 was incorporated into SPEEK via solution casting [16]. On the other hand, the proton conductivity of the GO layer between the SPEEK-polyvinyl alcohol (PVA) matrix membranes increased with temperature, increasing from 1 mS cm−1 at 30 °C to 8.3 mS c m−1 at 130 °C [17]. The PVA/ SPEEK-based membranes were fabricated via the addition of colloidal silica. At 80 °C, the PVA/SPEEK-based membranes doped with 1, 5, and 10 wt% silica had proton conductivities of 250, 560, and 650 mS cm−1, respectively [18]. The development of hydrophilic ionic channels and free-capacity holes to increase proton mobility is a novel and effective way to achieve high proton conductivity [19]. Consequently, less activation energy was required for the transfer of a proton inside the polymer matrix. More ketone groups were introduced into the PVA backbone when PVA was oxidized, decreasing the possibility of hydrogen bonding (condensed capacity) with an increase in free-capacity holes. This structural change in PVA has several impacts, such as a lower viscosity in aqueous media, a faster dissolution rate, and decreased crystallinity [20–22]. Therefore, the carbonyl groups of the oxidized polyvinyl alcohol (OPVA) were thought to be reactive sites for interactions with the sulfonic acid groups of SP (...truncated)