The increasing CO2 emissions from fuel combustion necessitate a transition to sustainable energy sources. The EU aims to drastically reduce greenhouse gas emissions by 2050, emphasizing renewable energy. Salinity gradient power (SPG), utilizing the energy released when mixing solutions of different salinity, is a promising, constant energy source. Reverse electrodialysis (RED) is a leading SPG technology employing ion-exchange membranes (IEMs) to generate electricity from salinity gradients. Current polymeric IEMs have limitations in performance, especially with concentrated and multivalent solutions. Two-dimensional (2D) nanomaterials, such as graphene oxide (GO), offer potential advantages due to their tunable properties and surface charge effects on ion selectivity. This study focuses on developing a scalable method for producing high-performance GO membranes for RED applications, addressing the challenges of maintaining high selectivity while reducing ionic resistance and improving mechanical stability.

Graphene oxide (GO) membranes have shown promise in ion selectivity due to their surface functional groups, offering a cost-effective alternative to conventional polymer membranes. Existing research, however, often involves small-scale experimental setups. The ion sieving mechanism in GO membranes is governed by both size and charge exclusion, with the interlayer distance and surface properties influencing transport. Studies have shown that cations can tune anion transportation through GO membranes, and GO can be functionalized to create anion-exchange membranes. While some small-scale all-graphene-based RED experiments have demonstrated high power density, scaling up the production of high-performance GO membranes remains a challenge.

This research employed the doctor blade technique for scalable GO membrane fabrication, producing large-area (14.5 cm x 14.5 cm) homogeneous membranes. The study systematically investigated various parameters to optimize membrane properties. Membrane stability was assessed by immersing them in highly concentrated solutions (H2SO4 10 M, NaOH 10 M, NaBr 5 M, NaCH3COO 5 M, and NaI 10 M) for extended periods. The impact of membrane thickness on stability was also evaluated. To improve mechanical properties, various binders (PEO, PVA, PVP, PVDF, chitosan, and SPEEK) were incorporated into the GO membranes. The morphology of the membranes was characterized using FESEM. UV-light irradiation was used as a green reduction method to control the degree of GO reduction, affecting interlayer spacing and hydrophobicity. XPS and FTIR spectroscopy were employed to investigate changes in surface chemistry after UV treatment. XRD was used to determine interlayer distances in both dry and wet states, with and without the presence of NaCl. Contact angle measurements determined the hydrophobicity of the membranes. Electrochemical characterization included permselectivity measurements with and without ion transport number and junction potential corrections, and ionic resistance measurements using EIS. Different concentration gradients and cation types were tested to assess the membrane's performance in a wide range of conditions. The lateral size of the GO flakes was also investigated through centrifugation to separate different size ranges. Sulfonated polyether ether ketone (SPEEK) was synthesized by dissolving PEEK in sulfuric acid, followed by precipitation and washing. A home-designed two-chamber cell was used for electrochemical measurements, validated using a commercial membrane.

The doctor blade technique successfully produced large-area, homogeneous GO membranes. 20 µm thick membranes showed excellent stability in various harsh solutions. Composite membranes with PVA and PVP exhibited the highest mechanical resistance, easily handled in both wet and organic solvents. UV-light irradiation effectively reduced GO, decreasing interlayer distance and improving permselectivity, with a saturation point at 10 min. FESEM analysis showed uniform flake stacking, while XPS and FTIR confirmed GO reduction after UV treatment. XRD measurements showed decreasing interlayer distance with increasing UV-reduction time, reaching a saturation point at 20 min. Contact angle measurements showed increased hydrophobicity with increased reduction time. Permselectivity reached a maximum of 96%, showing selectivity towards monovalent ions (Na+ and K+). Permselectivity decreased at higher concentration gradients and with more dilute diluted streams, likely due to the reduced Donnan potential and membrane swelling. Thicker membranes (30 µm) exhibited better ion exclusion due to lower ion permeation and reduced swelling. Larger GO flakes led to higher tortuosity and slower ion transport, while smaller flakes increased nano-capillaries, enhancing ion penetration. Composite membranes with binders demonstrated improved permselectivity due to increased mechanical stability. GO-20 displayed an ionic resistance of 4.6 Ω cm², significantly lower than previously reported GO-based membranes. Ionic resistance increased with thickness and reduction time and was reduced to 3.9 Ω cm² after 90 min incubation. PVA increased membrane resistance due to non-homogeneous distribution and pore shrinking, while PVP and SPEEK had little effect. The cation transference number was 0.97 at a fivefold concentration gradient, achieving a maximum power density of 0.54 W m².

The findings demonstrate a successful approach for producing scalable, high-performance GO-based IEMs for RED applications. The use of the doctor blade technique, coupled with optimization of membrane thickness, UV reduction, binder selection, and flake size, significantly improved both permselectivity and ionic resistance. The low ionic resistance achieved is comparable to commercial membranes, representing a significant advancement in GO-based IEM technology. The high permselectivity and cation transference number indicate excellent performance in RED applications. The impact of different concentration gradients on permselectivity highlights the importance of further investigation into optimizing membrane behavior across various salinity differences.

This study successfully fabricated scalable, high-performance graphene oxide-based ion-exchange membranes for reverse electrodialysis. Key improvements include high permselectivity (up to 96%), low ionic resistance (4.6 Ω cm²), and enhanced mechanical stability through binder addition. UV-light reduction proved an effective and green method for optimizing membrane properties. Future research could explore further optimization of membrane structure and composition to enhance performance and investigate the long-term durability and fouling resistance of these membranes in realistic RED applications.

The study primarily focused on monovalent ions, and further research is needed to assess the performance with multivalent ions. The long-term stability and fouling resistance of these membranes under real-world conditions require further investigation. While the doctor blade technique offers scalability, industrial-scale production and cost-effectiveness need to be evaluated.