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Yayın Biopolymer electrolytes and composites based on chitosan for electrochemical processes: developing technologies, device integration, and ion transport mechanisms(Elsevier Ltd, 2026) Yılmazoğlu, Mesut; Kouka, Tarek; Güzel, İlkay; Çoban, Ozan; Okkay, Hikmet; El Messaoudi, Noureddine; Messali, MouslimChitosan, a biopolymer with multifunctionality that occurs naturally from chitin, was found to be an efficacious high-potential platform for building green polymer electrolytes and electrochemical device composite materials. Its inherent properties like dense functional groups, biocompatibility, film-forming nature, and ease of chemical modification, favorably position it as a reliable substitute for conventional synthetic polymers. This review encompasses chitosan-based biopolymer electrolytes and composites, the mechanism of ionic conductance, structural tuning, and their incorporation into high-performance electrochemical devices. The review places particular importance on recent strategies pursued for enhancing ionic conductivity, mechanical stability, and electrochemical performance by chemical functionalization, blending, and nanomaterial inclusion. Particular focus is placed on ion dynamic awareness, proton and cation conducting channels, and polymer–filler interaction for charge transportation optimization. Application domains of fuel cell, battery, supercapacitor, and bioelectronic devices are comprehensively discussed with focus placed on both the achievements and ongoing challenges of chitosan systems. Finally, the review challenges issues of durability, scalability, and sustainability and outlines directions for future material engineering and technology integration. Bridging the gap between fundamental knowledge and real-world applications, this review article serves to illustrate the potential of chitosan-based electrolytes and composites to propel next-generation green and high-performance electrochemical technologies.Yayın Free volume impact on ionic conductivity of PVdF/GO/PVP solid polymer electrolytes via positron annihilation approach(Elsevier Ltd, 2025) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Tekay, Emre; Çoban, Ozan; Veziroğlu, Sümeyye; Yumak Yahşi, Ayşe; Tav, Cumali; Yahşi, UğurThis study reports the effects of free volume (FV) profiles on the ionic conductivities of PVdF/GO/PVP ternary polymer electrolytes using positron annihilation lifetime spectroscopy (PALS). The electrolytes were characterized by various tests such as FTIR, XRD, TGA-DTG, SEM, contact angle and DMA. FV profiles were evaluated by o-Ps lifetime (τ₃), intensity (I3) and FV fractions (fυ). PVdF exhibits a proton conductivity of 2.1 × 10⁻5 S/m at 80 °C. However, the introduction of GO leads to a decrease in conductivity, with PVdF/GO showing 1.7 × 10⁻5 S/m at 80 °C. The presence of PVP in PVdF/GO/PVP10 and PVdF/GO/PVP30 creates new FV spaces via hydrogen bonds and intermolecular interactions, expanding hydrophobic areas and increasing I₃ values. PVP's high mobility and positive charge density reduce the τ₃ values. In contrast, I₃ and fυ values decrease in PVdF/GO/PVP50, accompanied by a significant drop in τ₃ values and the proton conductivity and dielectric constant peak at 6.1 × 10⁻2 S/m and 77.38, respectively. High PVP concentration may enhance interactions within the polymer matrix, forming a dense structure that, despite reduced FV, maintains or enhances proton mobility through alternative conduction pathways and increased polarization. This study emphasizes the balance of FV and dielectric behavior for efficient electrochemical processes.Yayın Graphene oxide-induced free volume effects in proton conductive chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes: a positron annihilation lifetime study(Springer New York, 2025) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Tekay, Emre; Çoban, Ozan; Veziroğlu, Sümeyye; Yumak Yahşi, Ayşe; Tav, Cumali; Yahşi, UğurIn this study, chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes containing ionic liquid (IL) and graphene oxide (GO) as conductivity-enhancing additives were prepared and characterized in detail in terms of free volume (FV) parameters, mechanical strength, and proton conductivity. FV parameters were determined by positron annihilation lifetime spectroscopy (PALS) and correlated with dynamic mechanical analysis (DMA), ionic conductivity, and dielectric measurements. According to PALS analyses, the CS-5 sample (4 wt% GO) exhibited the highest FV fraction (2.45% at 40 °C) with a corresponding ortho-positronium lifetime (τ₃) of 2.12 ns, and FV hole radius (R) of 2.96 nm. This microstructural expansion was directly associated with the maximum proton conductivity of 1.64 × 10⁻3 S/m at 1 MHz, demonstrating the role of GO in facilitating continuous proton pathways. Moreover, CS-5 achieved superior structural reinforcement, with a high-temperature storage modulus of 12.38 MPa, confirming that GO simultaneously enhances both conduction and mechanical stability. The strong correlation between FV fraction and proton conductivity experimentally validates the FV-based conduction theory, offering mechanistic evidence for the design of next-generation polymer electrolytes. These results highlight that tuning FV distribution through GO optimization provides a reliable strategy for advancing solid-state biopolymer electrolytes in energy storage and environmental applications.Yayın High-performance PVdF-HFP/PEG-IL composites: the combined effects of PEG and ionic liquid on proton conductivity and dielectric characteristics(Elsevier Ltd, 2025) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Çoban, OzanThis study explores the influence of varying polyethylene glycol (PEG) concentrations on the properties of PVdF-HFP/PEG-IL polymer composites through comprehensive characterization techniques, including FTIR, SEM, TGA, DMA, XRD and the detailed assessments of proton conductivity, dielectric properties, and relaxation dynamics. In terms of conductivity, the addition of PEG markedly improves proton conductivity. The PVdF-HFP/PEG40-IL composite exhibits the highest conductivity, reaching 1.96 × 10⁻2 S/m at 1 MHz and 300 K, and increasing to 4.27 × 10⁻2 S/m at 420 K. Dielectric properties show that the dielectric constant (ε′) increases with PEG content at low frequencies but decreases at higher frequencies due to reduced ionic polarization. Notably, PVdF-HFP/PEG40-IL achieves a dielectric constant of 3.39 × 106 at 20 Hz, which decreases to 30.34 at 1 MHz. Dielectric loss (ε'') also rises with temperature, with PVdF-HFP/PEG40-IL demonstrating the highest dielectric loss, indicative of superior proton conduction and polarization capabilities. Relaxation dynamics, as evidenced by tanδ, reveal that relaxation time significantly decreases with both increased PEG content and temperature, dropping from 1.06 × 10⁻4 s to 2 × 10⁻6 s as PEG concentration increases from 10 % to 40 %. This reduction in relaxation time correlates with enhanced proton conductivity and faster dipole relaxation, indicating PEG effect as a plasticizer that reduces polymer viscosity and improves ion transport. In conclusion, incorporating PEG into PVdF-HFP-IL composites leads to substantial improvements in proton conductivity, dielectric properties, and relaxation dynamics. The results highlight the crucial role of PEG in optimizing the performance of polymer electrolyte composites, making them effective candidates for advanced energy storage and conversion applications.Yayın Proton conductivity and dielectric studies on chitosan/polyvinyl alcohol blend electrolytes: synergistic improvements with ionic liquid and graphene oxide(Elsevier B.V., 2024) Yılmazoğlu, Mesut; Okkay, Hikmet; Abacı, Ufuk; Çoban, OzanThis study investigates the impact of ionic liquid, 1-methylimidazolium tetrafluoroborate (IL) and graphene oxide (GO) on the performance of chitosan/polyvinyl alcohol (CS/PVA)-based composite electrolytes. Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) confirm the successful incorporation of IL and GO, affecting the structural and morphological properties of the electrolytes. Thermogravimetric analysis (TGA) reveals enhanced thermal stability in GO-doped samples, with increased residual weight at high temperatures, while IL addition leads to higher initial weight loss due to its hygroscopic nature. Ionic conductivity measurements demonstrate that the CS/PVA/IL-GO(4.0) composite achieves the highest proton conductivity of 1.76 × 10?3 S/m at 300 K and 1 MHz, surpassing other samples and aligning with top values reported in literature. Dielectric studies show a significant increase in dielectric constant to 9.55 × 104 at 300 K and 20 Hz for CS/PVA/IL-GO(4.0), attributed to enhanced dipole alignment and polarization effects. The loss tangent analysis indicates the shortest relaxation time of 2.07 × 10?4 s for CS/PVA/IL-GO(4.0), correlating with its superior proton conductivity. These findings highlight the potential of CS/PVA/IL-GO electrolytes for advanced energy storage and conversion applications, suggesting further research into GO dispersion and long-term stability for optimized performance in practical devices.
