Highly boosting the supercapacitor performance by polydopamine-induced surface modification of carbon materials and use of hydroquinone as an electrolyte additive

Revealing Energy Density in Porous Carbon Supercapacitors Using Hydroquinone Sulfonic Acid as Cathodic and Alizarin Red S as Anodic Redox Electrolytes

Exploration of innovative strategies aiming to boost energy densities of supercapacitors without sacrificing the power density and long-term stability is of great importance. Herein, highly porous nitrogen-doped carbon spheres (NPCS) are decorated onto the graphite sheets (GSs) through a hydrothermal route, followed by a chemical activation. The capacitive performance of the NPCS is then enhanced by hydroquinone sulfonic acid (HSQA) incorporation in both cathodic electrolyte and electrode materials. Later, NPCS are decorated with polypyrrole (PPY), in which HSQA takes a versatile role as conjugated polymer dopant and cathodic redox additive. The capacitive performance of the negative electrodes is enhanced by incorporating of alizarin red S (ARS) as anodic redox additive. Finally, PPY(HQSA)@NPCS-GS//NPCS-GS asymmetric supercapacitor is assembled and tested in dual redox electrolyte system containing HQSA-cathodic and ARS-anodic electrolytes. This device delivers a remarkable energy density of 60.37 Wh kg-1, which is close or even better than lead acid batteries. Thus, the present work provides a novel pathway to develop high energy supercapacitors using redox active electrolytes for next-generation energy storage applications.

Implantable anti-biofouling biosupercapacitor with high energy performance

Biofluidic open-type supercapacitors offer significant advantages over batteries in implantable electronics. However, poor energy storage in bioelectrolytes and performance degradation owing to electrode biofouling remain challenges and hamper their implementation. In this study, we present a flexible polydopamine (PDA)-infiltrated carbon nanotube (CNT) yarn (PDA/CNT) supercapacitor with high performance in biofluids, encapsulated by a hydrogel-barrier circular knit that provides anti-biofouling protection. Infiltration of the biopolymer PDA provide a hydrophilic coating to obtain a hydrophobic CNT electrode under aqueous conditions and an energy density 250-fold higher than that of the pristine CNT in the biofluid. The PDA/CNT supercapacitor exhibited remarkable energy performance in biological fluids in terms of the maximum areal capacitance (503.91 mF cm-2), energy density (274 μWh/cm2), and power density (25.52 mW cm-2). Moreover, it demonstrated negligible capacitance loss after 10,000 repeated charge/discharge cycles and bending tests. To prevent biofouling, the PDA/CNT electrode was encapsulated in an agarose-coated circular knit that allows free movement of the electrolyte. Notably, implanting an encapsulated PDA/CNT supercapacitor into the abdominal cavity of rat resulted in stable in vivo energy storage performance without biofouling for 21 d, and the charged supercapacitor was used successfully to power a light-emitting diode in vivo.

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

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.

Biosourced quinones for high-performance environmentally benign electrochemical capacitors via interface engineering

Biosourced and biodegradable organic electrode materials respond to the need for sustainable storage of renewable energy. Here, we report on electrochemical capacitors based on electrodes made up of quinones, such as Sepia melanin and catechin/tannic acid (Ctn/TA), solution-deposited on carbon paper engineered to create high-performance interfaces. Sepia melanin and Ctn/TA on TCP electrodes exhibit a capacitance as high as 1355 mF cm-2 (452 F g-1) and 898 mF cm-2 (300 F g-1), respectively. Sepia melanin and Ctn/TA symmetric electrochemical capacitors operating in aqueous electrolytes exhibit up to 100% capacitance retention and 100% coulombic efficiency over 50,000 and 10,000 cycles at 150 mA cm-2 (10 A g-1), respectively. Maximum power densities as high as 1274 mW cm-2 (46 kW kg-1) and 727 mW cm-2 (26 kW kg-1) with maximum energy densities of 0.56 mWh cm-2 (20 Wh kg-1) and 0.65 mWh cm-2 (23 Wh kg-1) are obtained for Sepia melanin and Ctn/TA.

Dynamics of Photogenerated Charge Carriers in Inorganic/Organic S-Scheme Heterojunctions

Step-scheme heterojunctions formed between two firmly bound photocatalysts facilitate charge separation due to interfacial charge transfer, which is usually illustrated by the gain or loss of electrons in the constituent photocatalysts characterized by in situ irradiated X-ray photoelectron spectroscopy. This technique provides a steady-state view of charge distribution but overlooks the transient and complex dynamics of charge transfer, trapping, and recombination. To provide a molecular-level and dynamic view of these processes, we investigated the behaviors of photogenerated charge carriers within an inorganic/organic TiO₂/polydopamine S-scheme heterojunction using ultrafast transient absorption spectroscopy and time-resolved photoluminescence spectroscopy. We found the interfacial charge transfer within the step-scheme heterojunction occurred at a smaller shorter time scale than recombination, leading to efficient charge separation. Moreover, the charge-discharge property of polydopamine induces electron backflow, which should be avoided in practical photocatalytic applications. The composite showed higher photocatalytic H₂O₂-production activities due to faster H₂O₂ formation and suppressed H₂O₂ decomposition.