Organic Small-Molecule Electrodes: Emerging Organic Composite Materials in Supercapacitors for Efficient Energy Storage

Abstract

Organic small molecules with electrochemically active and reversible redox groups are excellent candidates for energy storage systems due to their abundant natural origin and design flexibility. However, their practical application is generally limited by inherent electrical insulating properties and high solubility. To achieve both high energy density and power density, organic small molecules are usually immobilized on the surface of a carbon substrate with a high specific surface area and excellent electrical conductivity through non-covalent interactions or chemical bonds. The resulting composite materials are called organic small-molecule electrodes (OMEs). The redox reaction of OMEs occurs near the surface with fast kinetic and higher utilization compared to storing charge through diffusion-limited Faraday reactions. In the past decade, our research group has developed a large number of novel OMEs with different connections or molecular skeletons. This paper introduces the latest development of OMEs for efficient energy storage. Furthermore, we focus on the design motivation, structural advantages, charge storage mechanism, and various electrode parameters of OMEs. With small organic molecules as the active center, OMEs can significantly improve the energy density at low molecular weight through proton-coupled electron transfer, which is not limited by lattice size. Finally, we outline possible trends in the rational design of OMEs toward high-performance supercapacitors.

Donor–acceptor molecular architecture involving carbazole/pyrazine/anthraquinone units for efficient supercapacitor applications

“…26,27 Acceptor anthraquinone (AQ) and their polymers have been extensively utilized in energy storage applications due to their redox properties. 28–30 Thus, in the design of DTCz-Pyz-AQ, we demonstrate that the combination of one donor DTCz subunit and fused acceptor subunits such as Pyz-AQ showed an excellent voltage profile. Moreover, three-electrode SC and two-electrode symmetric supercapacitor (SSC) cells were fabricated using DTCz-Pyz-AQ as an organic material on a graphite foil surface.…”

Green and Sustainable Hydroxyquinone Molecule Electrodes Prepared for Efficient Energy Storage

“…In the electrochemical process, the linear slope of the high-frequency domain half-arc and the low-frequency domain is an obvious resistance phenomenon. The imaginary part of the impedance is almost perpendicular to the real part, indicating its ideal capacitance characteristics …”

“…The imaginary part of the impedance is almost perpendicular to the real part, indicating its ideal capacitance characteristics. 57 As we all know, the diameter of the semicircle represents the charge-transfer resistance (R ct ), and the smaller the radius of the arc, the smaller the R ct . Owing to low R ct , HAQ-rGO(0.5:1) has almost no significant semicircle.…”

Asymmetric Supercapacitors based on 1,10‐phenanthroline‐5,6‐dione Molecular Electrodes Paired with MXene

“…[30] Moreover, a higher redox potential can be achieved in the small molecules with electron withdrawing groups since the electron affinity increases when the electron cloud around the redox center becomes attractive. [31] Based on the above considerations, 1,10-Phenanthroline-5,6dione (PD) with π-π conjugated structure is a promising candidate for organic molecule electrode materials due to a heterocyclic quinone compound containing two carbonyl groups and two pyridine N sites, which can achieve twoelectron transfer. [32] The presence of pyridine N is equivalent to an electron withdrawing group (À NO 2 ) on the benzene ring, consequently moving the redox potential of the PD molecule towards a more positive potential.…”

“…[32] The presence of pyridine N is equivalent to an electron withdrawing group (À NO 2 ) on the benzene ring, consequently moving the redox potential of the PD molecule towards a more positive potential. [31,33] This suggests that an OME prepared by immobilizing PD molecule on the graphene surface is well-suited as a typical positive material. As such, assembling an ASC with MXene as the negative electrode and PD-based OME as the positive electrode is likely to have a larger voltage window.…”