Wood-derived lignin-based fibers as supercapacitor electrodes

Licentiate thesis, 2022

Today, in order to replace fossil energy sources with renewable energy sources such as solar and wind, reliable energy storage systems that can provide power regardless of the intermittent nature of the energy sources must be created. Especially with the future’s rising energy demands the development of such energy storage systems from green-collar materials with the least negative environmental impact is pressing. Transforming the major cheap
and replenishable forest resource, wood, to carbon materials with desirable morphologies can potentially be used as supercapacitors (SC) electrodes with long cycle life and higher power density than batteries today.
Forest materials are abundant, but their extraction to manufacturing hold practicality issues due to yet not established procedures. Active research has focused on advancing lignin-based electrospun carbon fibers (ELCFs) and activated carbons with simple, high-yielding mass production units. The ECLF is self-standing and flexible, making them a prospective candidate for flexible and wearable electronics. As of today, the materials face shortcomings such as low electrical conductivity and poor mechanical stability post thermal carbonization especially if the spinning discards fossil based secondary polymers. Research on optimized fractionated high molecular weight lignin solutions from black liquor - an industrial paper and pulp industry byproduct - have improved their spinnability. Turning these lignin-based materials to commercial utilization requires more investigation and understanding of the
materials.
This thesis discusses the electrochemical performance of lignin fibers as highly reliable supercapacitor electrode material. Grafting the right amount of beneficial functional groups on the ELCF surface by low-power oxygen plasma treatment, the properties of the electrode-electrolyte interface significantly improved the wettability, increased active sites favorable for pseudocapacitance, reduced diffusion limitation, thus enhancing its electrochemical storage ability. Quite often, the surface functional groups have a detrimental impact on a device’s electrochemical performance such as increased resistance, low power performance, low stability, and high self-discharge rate. However, the non-invasive nature of the conducted plasma treatment made a remarkable improvement in the capacitive performance in KOH aqueous medium without compromising power and energy performance metrics. Preliminary quantification performed to understand the charge storage behavior in other aqueous electrolytes H2SO4 and Li2SO4 are also revealed. Furthermore, the observation of enhanced electrochemical performance via applying a voltage of 1.2 V and 10 000 charge-discharge cycles is discussed.
With the competition of supercapacitors energy storage ability with batteries, efforts have been taken to make thick electrodes to boost energy density. Electrodes with high areal mass loading in supercapacitor maximize the packing density of the electroactive electrode materials while lowering the manufacturing cost by reducing the number of inactive material layers. Herein, the fabrication and electrochemical performance of 180-280 μm thick activated carbon (AC) electrodes with 2 wt% of hair-like carbonized lignin carbon fibers (LCF) as conductive agent alongside carbon black in the electrode matrix was assessed. In the resulting electrodes, the LCF inclusions into the AC matrix increased flexibility and contributed to improved capacitances due to better conductivity in the electrodes. The reduced resistances suggest that LCFs act as an intermediate layer among AC particles and serve as conductive pathways, facilitating electronic conductivity of more AC particles in deeper layers. Considering the biologically hazardous nature of other commonly used binders like polytetrafluoroethylene, and polyvinylidene fluoride, environmentally friendly binder microfibrillated cellulose (MFC) binder was successfully used to fabricate freestanding electrodes.