Fabrication of Fully Sustainable Electrospun Fiber-Reinforced 3D Carbon from Waste as Scalable Electrodes in Energy Storage Materials

Carbon fibers for the new batteries have been of interest for a long time based on their advantageous shape that provides stability, flexibility and a high surface area, lightweight, and high electrical conductivity. Most of the current reported and commercial carbon fibers are derived from using polyacrylonitrile, a synthetic polymer obtained from crude oil. Due to the finite nature and environmental impact of sourcing and refining crude oil, it is necessary to find sustainable alternatives to ensure the filling of the demand in the future.<br/>Fully sustainable alternatives have been reported based on the use of biopolymers. A promising candidate as precursor for carbon fibers is lignin, the main byproduct of the paper and pulp industry and the second most abundant biopolymer that is rich in aromatic carbon groups. Carbonizing lignin therefore, results in a high yield of carbon fibers. To ensure enough entanglement of the molecular chains, that allows the formation of stable fibers, the high molecular weight fraction of lignin is extracted, allowing the fabrication of pure lignin fibers without the use of additional synthetic polymers as carrier.<br/>One technique to fabricate fibers that have been of particular interest is electrospinning. Through the use of a high voltage field, the electrostatic forces draw a polymer solution to fibers in submicron diameter ranges, lower than any of the other commercially used techniques to fabricate fibers. Due to the enormous surface area to volume ratio, these fibers have been at the center of interest in various applications, including energy storage systems and (electro)catalysis. In addition to the relatively simple setup, electrospinning is the ideal technique to explore the properties of new materials in a laboratory scale. In larger scaled energy storage systems, such as flow batteries, electrospun fibers show a disadvantage due to the limited thickness of the fiber mat can reach compared to other spinning processes due to the relatively small fabrication scale and process that uses the electric field.<br/>To overcome this issue, we created a 3D structure based on lignin fibers through the lyophilization of the electrospun lignin fibers. The structure was obtained through a sonification process of the fibers in an aqueous solution of potato-based protein, another byproduct of the potato starch industry.<br/>n addition to enhancing stability in its dimensions, the soluble protein increases the mass of the resulting carbon, as well as the introduction of heteroatom doping of nitrogen and sulfur within the carbon for electrocatalytic improvement. After the lyophilization process, the combination of the lignin fibers and the potato protein form a fiber-reinforced sponge structure, an ideal precursor for fully sustainable 3D carbon structures, and adjustable in size and dimensions.<br/>It was shown that with the ideal ratio of protein to lignin fibers, a full coverage of the fiber could be achieved, and in addition, the carbon shows an improvement in the electrochemical catalysis of V⁴⁺/V⁵⁺ redox reaction, important for the use of the redox-flow battery. In comparison to the unsustainable, commercial felts that are predominantly used in the past, the sustainable alternative 3D freeze-dried carbon shows significantly better performance in cyclic voltammetry measurements and electrochemical impedance spectroscopy (EIS), and could further improve the performance of an asymmetrical flow battery.