Performance evaluation of conductive additives for activated carbon supercapacitors in organic electrolyte

Electroactive Ionic Polymer of Intrinsic Microporosity for High-Performance Capacitive Energy Storage

Here, an ionic polymer of intrinsic microporosity (PIM) as a high-functioning supercapacitor electrode without the need for conductive additives or binders is reported. The performance of this material is directly related to its large accessible surface area. By comparing electrochemical performance between a porous viologen PIM and a nonporous viologen polymer, it is revealed that the high energy and power density are both due to the ability of ions to rapidly access the ionic PIM. In 0.1 m H2SO4 electrolyte, a pseudocapacitve energy of 315 F g-1 is observed, whereas in 0.1 m Na2SO4, a capacitive energy density of 250 F g-1 is obtained. In both cases, this capacity is retained over 10 000 charge-discharge cycles, without the need for stabilizing binders or conductive additives even at moderate loadings (5 mg cm-2). This desirable performance is maintained in a prototype symmetric two-electrode capacitor device, which has >99% Coloumbic efficiency and a <10 mF capacity drop over 2000 cycles. These results demonstrate that ionic PIMs function well as standalone supercapacitor electrodes and suggest ionic PIMs may perform well in other electrochemical devices such as sensors, ion-separation membranes, or displays.

Understanding Sorption of Aqueous Electrolytes in Porous Carbon by NMR Spectroscopy

Ion adsorption at solid-water interfaces is crucial for many electrochemical processes involving aqueous electrolytes including energy storage, electrochemical separations, and electrocatalysis. However, the impact of the hydronium (H3O+) and hydroxide (OH-) ions on the ion adsorption and surface charge distributions remains poorly understood. Many fundamental studies of supercapacitors focus on non-aqueous electrolytes to avoid addressing the role of functional groups and electrolyte pH in altering ion uptake. Achieving microscopic level characterization of interfacial mixed ion adsorption is particularly challenging due to the complex ion dynamics, disordered structures, and hierarchical porosity of the carbon electrodes. This work addresses these challenges starting with pH measurements to quantify the adsorbed H3O+ concentrations, which reveal the basic nature of the activated carbon YP-50F commonly used in supercapacitors. Solid-state NMR spectroscopy is used to study the uptake of lithium bis(trifluoromethanesulfonyl)-imide (LiTFSI) aqueous electrolyte in the YP-50F carbon across the full pH range. The NMR data analysis highlights the importance of including the fast ion-exchange processes for accurate quantification of the adsorbed ions. Under acidic conditions, more TFSI- ions are adsorbed in the carbon pores than Li+ ions, with charge compensation also occurring via H3O+ adsorption. Under neutral and basic conditions, when the carbon's surface charge is close to zero, the Li+ and TFSI- ions exhibit similar but lower affinities toward the carbon pores. Our experimental approach and evidence of H3O+ uptake in pores provide a methodology to relate the local structure to the function and performance in a wide range of materials for energy applications and beyond.

Black goes green: single-step solvent exchange for sol-gel synthesis of carbon spherogels as high-performance supercapacitor electrodes

Nanoporous carbon materials with customized structural features enable sustainable and electrochemical applications through improved performance and efficiency. Carbon spherogels (highly porous carbon aerogel materials consisting of an assembly of hollow carbon nanosphere units with uniform diameters) are desirable candidates as they combine exceptional electrical conductivity, bespoke shell porosity, tunability of the shell thickness, and a high surface area. Herein, we introduce a novel and more environmentally friendly sol-gel synthesis of resorcinol-formaldehyde (RF) templated by polystyrene spheres, forming carbon spherogels in an organic solvent. By tailoring the molar ratio of resorcinol to isopropyl alcohol (R/IPA) and the concentration of polystyrene, the appropriate synthesis conditions were identified to produce carbon spherogels with adjustable wall thicknesses. A single-step solvent exchange process from deionized water to isopropyl alcohol reduces surface tension within the porous gel network, making this approach significantly time and cost-effective. The lower surface tension of IPA enables solvent extraction under ambient conditions, allowing for direct carbonization of RF gels while maintaining a specific surface area loss of less than 20% compared to supercritically dried counterparts. The specific surface areas obtained after physical activation with carbon dioxide are 2300-3600 m2 g-1. Transmission and scanning electron microscopy verify the uniform, hollow carbon sphere network morphology. Specifically, those carbon spherogels are high-performing electrodes for energy storage in a supercapacitor setup featuring a specific capacitance of up to 204 F g-1 at 200 mA g-1 using 1 M potassium hydroxide (KOH) solution as the electrolyte.

Activated carbons derived from polyethylene terephthalate for coin-cell supercapacitor electrodes

We successfully prepared activated carbon derived from polyethylene terephthalate (PET) via carbonization and subsequent activation under various conditions and applied it as active material for supercapacitors. In the activation, we used CO2 for physical activation or KOH for chemical activation and varied the activation temperature from 600 °C to 1,000 °C. We found that CO2 activation is unsuitable because of insufficient pore formation or low activation yield. Interestingly, PET-derived activated carbon obtained using KOH (PETK) at 700 °C-900 °C exhibited higher specific surface areas than YP50f, which is a commercial activated carbon. Furthermore, some PETKs even displayed a dramatic increase in crystallinity. In particular, the PET-derived activated carbon prepared at 900 °C with KOH (PETK900) had the highest retention rate at a high charge-discharge rate and better durability after 2500 cycles than YP50f. Furthermore, employing the same process that we used with the PET chips, we successfully converted waste PET bottles into activated carbon materials. Waste PET-derived activated carbons exhibited good electrochemical performance as active material for supercapacitors. We thus found chemical activation with KOH to be an appropriate method for manufacturing PET-derived activated carbon and PETKs derived both from PET chips and waste PET have considerable potential for commercial use as active materials for supercapacitors.

Electronic Supplementary Material

Supplementary material is available for this article at 10.1007/s11814-023-1466-3 and is accessible for authorized users.

Nanostructured Thick Electrode Strategies toward Enhanced Electrode-Electrolyte Interfaces

This article addresses the issue of bulk electrode design and the factors limiting the performance of thick electrodes. Indeed, one of the challenges for achieving improved performance in electrochemical energy storage devices (batteries or supercapacitors) is the maximization of the ratio between active and non-active components while maintaining ionic and electronic conductivity of the assembly. In this study, we developed and compared supercapacitor thick electrodes using commercially available carbons and utilising conventional, easily scalable methods such as spray coating and freeze-casting. We also compared different binders and conductive carbons to develop thick electrodes and analysed factors that determine the performance of such thick electrodes, such as porosity and tortuosity. The spray-coated electrodes showed high areal capacitances of 1428 mF cm^-2 at 0.3 mm thickness and 2459 F cm^-2 at 0.6 mm thickness.

MnCO3 on Graphene Porous Framework via Diffusion-Driven Layer-by-Layer Assembly for High-Performance Pseudocapacitor

Diffusion-driven layer-by-layer (dd-LbL) assembly is a simple yet versatile process that can be used to construct graphene oxide (GO) into a three-dimensional (3D) porous framework with good mechanical stability. In particular, the oxygen functional groups on the GO surface are well retained, providing nucleation sites for further chemical reactions to be performed upon. Therefore, such a scaffold should serve as a promising starting material for creating a wide range of 3D graphene-based composites while maintaining a high accessible surface area. Herein, we demonstrate the use of the porous GO macrostructure derived from dd-LbL assembly for the preparation of graphene-MnCO₃ hybrid structures. MnCO₃ is a newly reported pseudocapacitive material for supercapacitors; however, its electrochemical performance is hampered by its low electrical conductivity and poor chemical stability. Through reaction between KMnO₄ and GO during a hydrothermal process, the surface of the porous scaffold was rendered with uniform MnCO₃ nanoparticles. With the reduced graphene oxide (rGO) sheets serving as the conductive backbone, the resultant MnCO₃ nanoparticles exhibited a capacitance of 698 F g^-1 at a charge/discharge current of 0.5 mA (320 F g^-1 for the combined rGO and MnCO₃ composite). Furthermore, the electrode maintained 77% of its initial capacity even after 5000 cycles of charge/discharge tests at 20 mA.

A Review of Redox Electrolytes for Supercapacitors

Supercapacitors (SCs) have attracted widespread attention due to their short charging/discharging time, long cycle life, and good temperature characteristics. Electrolytes have been considered as a key factor affecting the performance of SCs. They largely determine the energy density based on their decomposition voltage and the power density from their ionic conductivity. In recent years, redox electrolytes obtained a growing interest due to an additional redox activity from electrolytes, which offers an increased charge storage capacity in SCs. This article summarizes the latest progress in the research of redox electrolytes, and focuses on their properties, mechanisms, and applications based on different solvent types available. It also proposes potential solutions for how to effectively increase the energy density of the SCs while maintaining their high power and long life.

Solution Self-Assembly of an Alternating Copolymer toward Hollow Carbon Nanospheres with Uniform Micropores

Controllable preparation of porous hollow carbon spheres (HCSs) has attracted considerable attention due to their potential applications, e.g., in energy conversion and storage. We report for the first time the synthesis of narrowly size-distributed HCSs with uniform micropores in the wall, through a simple template-free approach, which employs the solution self-assembly of an alternating copolymer (poly(9,9'-bis(4-glycidyloxyphenyl)fluorene-alt-2,3-dihydroxy-butylene dithioether) (P(BGF-a-DHBDT))). This alternating copolymer first self-assembled into previously undocumented hollow polymeric spheres (HPSs) in an N,N-dimethylformamide (DMF)/H₂O solvent mixture. After the cross-linking of the BGF segments in the spheres, the stabilized HPSs (CL-HPSs) were carbonized at 800 °C under N₂ atmosphere, yielding porous HCSs with uniform micropores of very narrow size distribution (0.4-0.8 nm) in the wall, benefiting from the uniform DHBDT block length in the alternating copolymer. Through KOH activation, which made the internal pores fully interconnected, uniform micropores (0.5-1.0 nm) of a narrow size distribution were retained within the activated HCSs (A-HCSs), while their specific surface areas (SSAs) were much increased to 2580 m² g^-1. As a proof of concept, the A-HCSs were applied as electrode materials of supercapacitors. They exhibited superior electrochemical performance with a high specific capacitance (292 F g^-1 at 0.2 A g^-1), good rate capability, and outstanding cycling stability with no apparent capacitance loss after 10 000 cycles.