Cellulose acetate/MXene composite nanofiber-based cation exchange membrane for electrochemical energy generation

Abstract

The need to search for alternative energy resources without risk of depletion and pollution is increasing because fossil fuels are running out and it releases environmental pollutant during power generation. As part of efforts to solve this problem, research on the renewable energy systems that generate power from natural resources such as the sun or wind without using oil, coal, and gas are being actively conducted. Representative examples include solar power, wind power, tidal power, hydropower, geothermal power, and biomass. However, since most of these technologies use natural energy resources, they are greatly affected by time and climate. Because of these basic limitations, it not only cannot produce electricity for 24 hours but also difficult to produce as much electricity as desired by the conventional renewable energy system. Thus, this type of renewable energy-based power generation system has the inconvenience that an energy storage device must be accompanied. From a longer-term perspective, an effective renewable energy system which is not affected by time and climate is needed, and it is a salinity gradient power system that meets requirements. Salinity gradient power is a method of generating electricity through a sequential physical or electrochemical process by transport water or ions using the feed water with different concentrations and a semi-permeable membrane. Thus, electricity can be generated regardless of time and climate. Reverse electrodialysis (RED) and pressure-retarded osmosis (PRO) are representative technology of the salinity gradient power, and this study focused on RED. The principle of the RED system is that electricity is produced through an oxidation/reduction reaction at both electrodes with chemical potential which generated by the separation of ions from high salinity solution (HSS) and low salinity solution (LSS) supplied to a stack cell in which cation exchange membrane (CEM) and anion exchange membrane (AEM) are alternately arranged. It can be considered that the resource is almost infinite because the RED system uses HSS and LSS as an energy source. And, since it is a method of producing electricity through an electrochemical reaction, pollutants such as CO2, SOX, NOX which generated by burning fossil fuels are not generated. In addition, RED is not a form of a physical power generation system that requires operating a turbine such as PRO thereby it is can be applied to all sizes from small to large. RED is still a stage in which research and development are required on several factors, and among them, intensive research on ion exchange membranes (IEMs) is needed. In the RED system, the IEM is an important element involved in separating ions to create the chemical potential which makes a voltage in the electrochemical system. IEMs for RED has not yet been widely adopted, and most of them are the membrane for electrodialysis (ED). Since ED-only membranes usually require stable mechanical properties and high ion adsorption character, the focus is on making them thick and high ion exchange capacity (IEC). However, the membrane for RED must be thin because ions need to pass through the membrane and move to the opposite side, and characteristics such as areal resistance (AR) and permselectivity should be satisfied along with IEC. In this study, therefore, a composite CEM was fabricated through the introduction of novel material and structural engineering as part of developing an IEM for RED. The conventional IEMs are either thick or consist of a layered structure in which an ion exchange layer and a support layer are overlapped. The composite CEM fabricated in this study is a form in which the skeleton is filled with an ionomer, motivated by the concrete structure constituting the building. Thus, the thickness can be made thin while maintaining basic mechanical properties, which is advantageous for ion transport. The cellulose acetate (CA) and MXene composite were electrospun to produce a nanofiber-shaped skeleton. When the physical properties of the nanofibers were checked according to the MXene content, CA/MXene(5wt%) showed the best result, thereby Nafion was filled in the as-prepared CA/MXene(5wt%) to fabricate Nafion@CA/MXene(5wt%) composite CEM. Nafion@CA/MXene(5wt%) showed better mechanical properties than layered Nafion//CA/MXene(5wt%) or recasted Nafion membrane in a tensile test, confirming the effect of introducing concrete structure. And, Nafion@CA/MXene(5wt%) exhibited the physical/electrochemical properties of water uptake 18.7%, AR 2.53 Ω cm2, IEC 1.21 meq g‒1, and permselectivity 92.7%. Besides, when Nafion@CA/MXene(5wt%) CEM was applied to the RED system, a power density of 1.9 W m‒2 was obtained, and the result was superior to that of the commercial membranes. In particular, the effect of MXene was confirmed because it exhibited better results than the recasted Nafion, and the effect of concrete structure was checked by comparing RED performance with the layered Nafion//CA/MXene(5wt%) membrane.