Biocarbon Meets Carbon—Humic Acid/Graphite Electrodes Formed by Mechanochemistry

Abstract

Humic acid (HA) is a biopolymer formed from degraded plants, making it a ubiquitous, renewable, sustainable, and low cost source of biocarbon materials. HA contains abundant functional groups, such as carboxyl-, phenolic/alcoholic hydroxyl-, ketone-, and quinone/hydroquinone (Q/QH2)-groups. The presence of Q/QH2 groups makes HA redox active and, accordingly, HA is a candidate material for energy storage. However, as HA is an electronic insulator, it is essential to combine it with conductive materials in order to enable fabrication of HA electrodes. One of the lowest cost types of conductive materials that can be considered is carbon-based conductors such as graphite. Herein, we develop a facile method allowing the biocarbon to meet carbon; HA (in the form of a sodium salt) is mixed with graphite by a solvent-free mechanochemical method involving ball milling. Few-layer graphene sheets are formed and the HA/graphite mixtures can be used to fabricate HA/graphite hybrid material electrodes. These electrodes exhibit a conductivity of up to 160 S·m−1 and a discharge capacity as large as 20 mAhg−1. Our study demonstrates a novel methodology enabling scalable fabrication of low cost and sustainable organic electrodes for application as supercapacitors.

Renewable and Scalable Energy Storage Materials Derived from Quinones in Biomass

“…[14] For thin-layer electrode, or if only the adsorbed species on the electrode contribute to the current, there is no diffusion limitation from the electroactive species and Ip is proportional to the scan rate but not to the square root of scan rate. As observed in paper I in this thesis, [17] the lignin/graphite hybrid material electrodes display a changing Ep and a growing peak separation at increasing potential scan rate, indicating the electrode redox reactions to be quasi-reversible at high scan rate; while for the humic acid/graphite hybrid material electrode in paper II, [18] Ep and the peak separation do not vary as the scan rate increases. It is due to the limited amount of redox active species confined in the electrode and the redox reactions are not limited by the diffusion process.…”

“…Other properties of these HA/graphite electrodes with different stoichiometry (2/1, 4/1, 7/1, 10/1, (w/w)) are also investigated: as the stoichiometry of HA/graphite increase, the conductivity of these electrodes decrease from 159 S•m -1 to only 0.34 S•m -1 , which are much lower than that of LS/graphite hybrid electrodes; the mass ratio of HA and graphite in the pellets increase from 1.17 to 4.85, which explains the decreasing and low conductivity. [18] Moreover, there is a smaller fraction of Q/QH2 groups in HA than that in LS, these HA electrodes thus display no obvious redox peaks in CV (Figure 4.4 a) and a discharge capacity range of 17-20 mAhg -1 (Figure 4.4 b). Interestingly, the conductivity does not display a high impact on the discharge capacity of the HA/graphite electrodes even with a conductivity of 0.34 S•m -1 .…”

“…Dependent on the pH, HA display a protonated or deprotonated form with a HA salt formed at higher pH and an acid at lower pH; at neutral pH (water), the solubility of HA salt is much better than that of HA, thus HA is employed in the form of a sodium salt for the electrodes fabrication. [18] It demonstrates a mixture of HA and graphite in the pellets and graphite is exfoliated into few-layer graphene in the presence of HA during the ball-milling process. Similarly, the aromatic structures in HA interact with graphite via π-π interaction that enables the exfoliation.…”

“…Similar to lignin, HA is also electrically insulating thus conductive materialgraphiteis employed to combine with HA to fabricate the HA/graphite hybrid material electrodes. [18] The processing method used is similar to that of LS/graphite hybrid material electrodes in paper Imechanical milling -that also makes these HA/graphite hybrid material electrodes scalable and cost-effective. Though the discharge capacity of these HA/graphite electrodes (20 mAhg -1 ) is lower than that of the LS/graphite electrodes (35 mAhg -1 ), it renders another sustainable, abundant and low cost Q-source for EES.…”

“…Based on the fabrication processing of LS/graphite hybrid material electrodes, similar method is used to generate the HA/graphite hybrid material electrodes in paper II. [18] This preparation process contains aqueous treatment of the HA/graphite mixture that helps to disperse the graphene layers, and the solubility of HA in water is vital to obtain a homogeneous film. Dependent on the pH, HA display a protonated or deprotonated form with a HA salt formed at higher pH and an acid at lower pH; at neutral pH (water), the solubility of HA salt is much better than that of HA, thus HA is employed in the form of a sodium salt for the electrodes fabrication.…”