A green, though black, sustainable and low-cost carbon material-charcoal produced from wood-is developed for electricity storage. Charcoal electrodes are fabricated by ball-milling charcoal and adding protein nanofibril binders. The charcoal electrode presents a capacitance of 360 F g(-1) and a conductivity of 0.2 S m(-1). A pair of redox peaks is observed in the cyclic voltammetry and assigned to originate from quinone groups. Compared with other wooden electrodes, these charcoal electrodes display better cycling stability with 88% capacity retention after 1000 cycles. Their discharge capacity is 2.5 times that of lignosulfonate/graphite hybrid electrodes.

Two-dimensional (2D) transition metal carbides and carbonitrides, called MXenes, with metallic conductivity and hydrophilic surfaces, show great promise as electrode materials for supercapacitors. A major drawback of 2D nanomaterials is the re-stacking of the nanosheets, which prevents full utilization of surface area and blocks the access of the electrolyte. In this study, a free-standing nanocomposite paper electrode is realized by combining Mo1.33C MXene and positively charged biopolymer lignin (the second most abundant biopolymer in nature, L-DEA). The self-assembled layered architecture with alternating polymer and MXene flakes increases the interlayer space to promote ion transport, and with combining charge storage capability of the lignin derivative and MXene in an interpenetrating MXene/L-DEA nanocomposite, which offers an impressive capacitance of 503.7 F g(-1). Moreover, we demonstrate flexible solid-state asymmetric supercapacitors (ASCs) using Mo1.33C@L-DEA as the negative electrode and electrochemically exfoliated graphene with ruthenium oxide (EG@RuOx) as the positive electrode. This asymmetric device operates at a voltage window of 1.35 V, which is about two times wider than that of a symmetric Mo1.33C@L-DEA based supercapacitor. Finally, the ASCs can deliver an energy density of 51.9 Wh kg(-1) at a power density of 338.5 W kg(-1), with 86 % capacitance retention after 10000 charge-discharge cycles.

Currently there is an urgent need to reduce the use of fossil fuels, and efficient sustainable energy harvesters from sun and wind have been developed and are widely used for electricity generation. Storage of electrical energy is accordingly necessary to accommodate the time varying supply of wind and solar electricity. Quinones (Q) are attractive as energy storage materials due to their high theoretical charge density and the renewable and abundant source – biomass. Plant-based biomass materials – such as lignin and humic acids – contain redox active Q-groups that potentially could be used for electricity storage instead of simply burning the biomass, which releases CO2, CH4, NOx, and SOx. Lignin accounts for 20-30% of the biomass weight and contains a sizable fraction of Q-structures. However, utilization of lignin for large scale energy storage is still a challenging task, as lignin is electrically insulating and conductive materials are required to get access to the generated electrons in the bulk. Various relatively expensive materials, such as conductive polymers and various carbon materials (carbon nanotubes, active carbon, graphene, etc.) have been combined with lignin, resulting in hybrid materials for energy storage. However, as the scale required for production of charge storage devices is huge it is of outmost importance to reduce the cost and therefore investigate low-cost conductive materials. In this thesis, common graphite flakes are combined with the lignin derivative lignosulphonate (LS) via a solvent free ball-milling process, followed by treatment with water and resulting in a paste that can be processed into electrodes. Similarly, humic acid derived from peat, lignite that contains a large amount of Q-groups is also fabricated into electrode with graphite via the ball-milling process. In order to further reduce the impact on environment during the extraction of Q-materials from biomass, barks that contain as much as 30% of lignin are directly used for energy storage via co-milling with pristine graphite to generate the biomass/graphite hybrid material electrodes. However, larger weight fraction of Q are required to further improve the electrochemical performance of these electrodes and Q chemicals (QCs) that also originate from biomass are introduced to fabricate the QCs/graphite electrodes with an increased capacity. Additionally, self-discharge mechanism is studied on the LS/graphite hybrid material electrodes, which provides instructions to achieve a low self-discharge rate.

Overall, this study has brought us one step forward on the establishing of scalable, sustainable, and cost-effective energy storage systems using aqueous electrolytes.