Capacitive mechanism of oxygen functional groups on carbon surface in supercapacitors

Phosphorus-oxygen groups drive surface-confined growth of high-valence amorphous transition-metal oxides on carbon surface for oxygen evolution reaction

Beyond the morphological study, the key to optimizing the performance of carbon-supported oxides in oxygen evolution reaction (OER) lies in phase engineering and electronic structure regulation. In this study, the surface-confined growth of high-valence amorphous transition-metal oxides on carbon surface is achieved by introducing phosphorus-oxygen groups, which strongly adsorb transition-metal ions and inhibit atomic rearrangement during heat treatment. Experimental characterization revealed that the oxides remained amorphous over a wide temperature range (up to 550 °C). Density functional theory (DFT) calculations and electronic structure analysis revealed the high electron-withdrawing capacity of the phosphorus-oxygen groups, which led to an increase in the valence state of transition metals. The obtained catalysts show excellent OER performance due to the matched OH- adsorption and deprotonation, requiring an overpotential of 251 mV to reach 10 mA cm-2. This study highlights the importance of phase engineering and electronic structure regulation in designing carbon-supported oxides, providing a universal method to achieve amorphization and electronic modulation of transition-metal oxides.

Deoxygenated lignin carbon aerogel with enhanced electrochemical performance in organic systems for supercapacitor applications

Porous lignin carbon materials have attracted considerable attention for use as supercapacitor electrode materials. However, their electrochemical performances can be significantly affected by the presence of oxygen heteroatoms on their surfaces. In this study, oxygen heteroatoms were partially eliminated from a lignin-derived carbon aerogel through deoxygenation treatment using hydrazine hydrate. The relationship between the surface chemical composition and the textural structure of the carbon aerogel was examined, and the electrochemical behavior resulting from its use as a carbon electrode in a symmetric supercapacitor was evaluated, both before and after deoxygenation. Simple deoxygenation of the carbon aerogel reduced the surface oxygen content from 10.45 to 4.08 %, resulting in an ultrahigh specific surface area (3356.2 m2 g-1) and average pore size of 2.16 nm. In an organic electrolyte consisting of a 1 M Et4NBF4 acetonitrile solution, an exceptional specific capacitance (182.93 F g-1 at a current density of 1 A g-1) and excellent stability (capacitance retention of 96.63 % after 10,000 cycles) were achieved for the deoxygenated carbon aerogel. Additionally, at a high power density of 18,660 W kg-1, the material demonstrated a high specific energy density of 40.66 Wh kg-1. This remarkable energy-storage capability was attributed to deoxygenation, which resulted in an enhanced specific surface area, a suitable pore size distribution, and an excellent wettability toward the organic electrolyte. This study therefore offers a flexible technique for enhancing the functionality of lignin-based porous carbon materials, and paves the way for the high-value use of lignin in the new energy sector.

Supercapacitor Performance of Activated Carbon from Eucommia Ulmoides Oliver Wood Optimized by the Activation Method

Amid the growing demand for sustainable energy storage, biomass-derived porous carbons have emerged as eco-friendly alternatives to conventional electrode materials. This study shows that activated carbon prepared by one-step activation exhibits an enhanced specific surface area and pore volume. The optimum parameter for ameliorating the structural and electrochemical properties is 60 min of microwave heating. The specific surface area, pore volume, and mesopore volume of the resulting activated carbon (EUAC1-60) achieve 1589.0 m2/g, 0.82 cm3/g, and 0.28 cm3/g, respectively. EUAC1-60 exhibits an exceptional defect degree with an I D/I G value of 0.92 and can provide ample active sites for ion storage. The electrochemical investigation shows that the EUAC1-60 electrode has the highest specific capacitance of 232.92 F/g at a current density of 0.2 A/g. In addition, continuous cycling performance at a current density of 1 A/g validates its exceptional stability with capacitance retention of 89.90% and Coulombic efficiency of 117.21% after 10,000 cycles. The zinc ion hybrid supercapacitor with the EUAC1-60 cathode and Zn foil anode displayed an excellent energy density performance of 95.58 W h/kg at a power density of 64,800 W/kg. This research presents an innovative approach to the fabrication of high-performance activated carbon electrode materials from Eucommia Ulmoides Oliver, demonstrating its promising potential in supercapacitor applications.

Optimization studies on output stabilization time and graphene oxide concentration in graphene-based flexible micro-supercapacitor

Miniature energy storage devices are vital for developing flexible and wearable electronics. This paper discusses the fabrication of flexible laser-induced graphene-based micro-supercapacitors (MSCs) using graphene oxide (GO) coated polyimide film as the precursor for laser scribing. The areal capacitance of the MSCs was assessed daily after applying a H2SO4/polyvinyl alcohol (PVA) gel electrolyte. The capacitance displayed a substantial increase in the early days before stabilizing at a consistent value. The stabilization time was evaluated through systematic experimentation conducted over ten consecutive days. The experiments showed that the capacitance stabilized after six days. Various concentrations of GO were used to assemble the MSCs, and their performance was evaluated to determine the optimal concentration. The electrochemical impedance spectroscopy revealed that the supercapacitor fabricated with the optimum concentration of GO exhibited the lowest resistance. The optimized MSC displayed an areal capacitance of 10.07 mF cm-2at a current density of 13µA cm-2. The device could maintain a reliable output at different bending states and retain 87.9% of its original capacitance after 5000 charge-discharge cycles, highlighting its suitability for flexible and self-powered systems.

Dielectric barrier discharge low-temperature plasma modification of bamboo charcoal for supercapacitor applications

Biochar is commonly utilized as an electrode material in supercapacitors. However, the conventional carbonization process often results in macromolecular compounds, which obstruct the porous structure of carbon materials, thereby reducing their capacitance. Dielectric barrier discharge low-temperature plasma (DLTP) is a technology that transforms gases into highly excited states, utilizing high-energy particles for enhanced energy applications. This study investigated the effects of DLTP on the electrochemical performance of bamboo charcoal (BC), utilizing bamboo shavings (BS) as the carbon source. The results indicated that the specific capacitance of BC varied under different atmospheric conditions, input voltages, and treatment durations, thereby achieving a maximum increase of 144 F/g. Furthermore, when combined with KOH activation, DLTP modification further enhanced the specific capacitance of BC to 237 F/g. The DLTP treatment enhanced the specific surface area and the types of functional groups in BC, thereby leading to a significant enhancement of its electrochemical properties.

Reversible Molecule Interactions Enable Ultrastretchable and Recyclable Ionogels for Wearable Piezoionic Sensors

Ionogel-based piezoionic sensors feel motions and strains like human skin relying on reversible ion migrations under external mechanical stimulus and are of great importance to artificial intelligence. However, conventional ion-conductive polymers behave with degraded electrical and mechanical properties after thousands of strain cycles, and the discarded materials and devices become electronic wastes as well. Here, we develop ultrastretchable ionogels with superior electrical properties via the mediation of metal-organic frameworks, whose properties are attributed to reversible molecule interactions inside the material system. Ionogels present excellent mechanical properties with breaking elongation as high as 850%, exceeding most previously reported similar materials, and the high conductivity enables further application in sensor devices. In addition, our ionogels display superior recyclability because of the reversible physical and chemical interactions inside material molecules, which are eco-friendly to the environment. As a result, the ionogel-based piezoionic sensors deliver high sensitivity, flexibility, cyclic stability, and signal reliability, which are of great significance to wearable applications in human-motion detections such as throat vibration, facial expression, joint mobility, and finger movement. Our study paves the way for ultrastretchable and eco-friendly ionogel design for flexible electrochemical devices.

Oxygen-Assisted Supercapacitive Swing Adsorption of Carbon Dioxide

We report on the supercapacitive swing adsorption (SSA) of carbon dioxide at different voltage windows in the presence of oxygen using activated carbon electrodes, and deliquescent, aqueous electrolytes. The presence of O2 in the CO2/N2 gas mixture results in an up to 11 times higher CO2 adsorption capacity with 3 M MgBr2 (at 0.6 V) and up to 4-5 times higher adsorption capacity with 3 M MgCl2 (at 1 V). A tradeoff between high CO2 adsorption capacities and lower coulombic efficiencies was observed at voltages above 0.6 V. The energetic and adsorptive performance of the electrodes in the presence of oxygen below 0.5 V was similar to the performance with a CO2/N2 mixture without oxygen at 1 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the electrodes demonstrate that the specific capacitance increases while the diffusion resistance decreases in the presence of oxygen. Oxygen concentrations ranging between 5-20 % give similar energetic and adsorptive performance. The electrodes exhibit stable performance for up to 100 cycles of operation.

Enhancing electrochemical carbon dioxide capture with supercapacitors

Supercapacitors are emerging as energy-efficient and robust devices for electrochemical CO2 capture. However, the impacts of electrode structure and charging protocols on CO2 capture performance remain unclear. Therefore, this study develops structure-property-performance correlations for supercapacitor electrodes at different charging conditions. We find that electrodes with large surface areas and low oxygen functionalization generally perform best, while a combination of micro- and mesopores is important to achieve fast CO2 capture rates. With these structural features and tunable charging protocols, YP80F activated carbon electrodes show the best CO2 capture performance with a capture rate of 350 mmolCO2 kg-1 h-1 and a low electrical energy consumption of 18 kJ molCO2-1 at 300 mA g-1 under CO2, together with a long lifetime over 12000 cycles at 150 mA g-1 under CO2 and excellent CO2 selectivity over N2 and O2. Operated in a "positive charging mode", the system achieves excellent electrochemical reversibility with Coulombic efficiencies over 99.8% in the presence of approximately 15% O2, alongside stable cycling performance over 1000 cycles. This study paves the way for improved supercapacitor electrodes and charging protocols for electrochemical CO2 capture.

Carbon Corrosion Induced by Surface Defects Accelerates Degradation of Platinum/Graphene Catalysts in Oxygen Reduction Reaction

Graphene supported electrocatalysts have demonstrated remarkable catalytic performance for oxygen reduction reaction (ORR). However, their durability and cycling performance are greatly limited by Oswald ripening of platinum (Pt) and graphene support corrosion. Moreover, comprehensive studies on the mechanisms of catalysts degradation under 0.6-1.6 V versus RHE (Reversible Hydrogen Electrode) is still lacking. Herein, degradation mechanisms triggered by different defects on graphene supports are investigated by two cycling protocols. In the start-up/shutdown cycling (1.0-1.6 V vs. RHE), carbon oxidation reaction (COR) leads to shedding or swarm-like aggregation of Pt nanoparticles (NPs). Theoretical simulation results show that the expansion of vacancy defects promotes reaction kinetics of the decisive step in COR, reducing its reaction overpotential. While under the load cycling (0.6-1.0 V vs. RHE), oxygen containing defects lead to an elevated content of Pt in its oxidation state which intensifies Oswald ripening of Pt. The presence of vacancy defects can enhance the transfer of electrons from graphene to the Pt surface, reducing the d-band center of Pt and making it more difficult for the oxidation state of platinum to form in the cycling. This work will provide comprehensive understanding on Pt/Graphene catalysts degradation mechanisms.

Large-Area Fabrication of Porous Graphene Networks on Carbon Fabric via Millisecond Photothermal Processing of Polyaniline for Supercapacitors

Supercapacitors demonstrate promising potential for flexible, multi-functional energy storage devices; however, their widespread adoption is confronted by fabrication challenges. To access a combination of desirable device qualities such as flexibility, lightweight, structural stability, and enhanced electrochemical performance, carbon fiber (CF) can be utilized as a current collector, alongside graphene as an electrochemically active material. Yet achieving a cost-effective, large-scale graphene production, particularly on CF, remains challenging. Here, a rapid (<1 min) photothermal approach is developed for the large-scale production of graphene directly onto CF, utilizing polyaniline (PANI) as a polymer precursor. The in situ electropolymerization of PANI on CF facilitates its rapid synthesis on large areas, followed by conversion into graphene networks, enabling the binder-free fabrication of supercapacitor devices. These devices exhibit an areal capacitance of 180 mF cm-2 (at 2 mA cm-2 in 1 m H2SO4), an order of magnitude higher than other fabric-based devices. Moreover, the devised photothermal strategy allows for one-step preparation of supercapacitor devices on areas exceeding 100 cm-2, yielding an absolute areal capacitance of 4.5 F. The proportional increase in capacitance with device area facilitates scaling and indicates the commercial viability of this approach for low-cost, energy-efficient, and high-throughput production of lightweight, high-performance graphene-based multi-functional supercapacitor devices.

Molecularly-regulating oxygen-containing functional groups of ramie activated carbon for high-performance supercapacitors

Effectively managing oxygen-containing functional groups (OCFGs) within activated carbon and methodically elucidating their intricate types and proportions are essential for considerably improving the electrochemical performance of carbon-based supercapacitors. Herein, we designed a ZnCl2-based molecular regulation strategy to introduce OCFGs into ramie-activated carbon (RAC), managing different OCFGs and revealing their structure-activity relationship with electrochemical performance. Thus, this regulated RAC, with a 3.5-fold enhancement in advantageous OCFGs (a-OCFGs: CO and COO), exhibits a supreme specific capacitance of 286.4F g-1 at 1 A/g and an excellent capacitance retention rate of 89.7 % at 20 A/g in an aqueous electrolyte, considerably surpassing that of nonregulated RAC (212.0F g-1 and 81.9 %). This confirms that a-OCFGs provide ample ion-storage accommodation and suppress solvent electronic oxidation, thereby enhancing electrochemical performance. Furthermore, its electrochemical performance is competitive with that of the commercial YP-50F (129.2F g-1 at 1 A/g). Therefore, this work not only highlights the contributions of specific OCFGs to high electrochemical performance but also designs a promising commercial electrode material to meet the demands of OCFGs-adequate carbon-based energy storage devices.

Advanced High-Energy All-Solid-State Hybrid Supercapacitor with Nickel-Cobalt-Layered Double Hydroxide Nanoflowers Supported on Jute Stick-Derived Activated Carbon Nanosheets

Developing efficient, lightweight, and durable all-solid-state supercapacitors is crucial for future energy storage systems. The study focuses on optimizing electrode materials to achieve high capacitance and stability. This study introduces a novel two-step pyrolysis process to synthesize activated carbon nanosheets from jute sticks (JAC), resulting in an optimized JAC-2 material with a high yield (≈24%) and specific surface area (≈2600 m2 g-1). Furthermore, an innovative in situ synthesis approach is employed to synthesize hybrid nanocomposites (NiCoLDH-1@JAC-2) by integrating JAC nanosheets with nickel-cobalt-layered double hydroxide nanoflowers (NiCoLDH). These nanocomposites serve as positive electrode materials and JAC-2 as the negative electrode material in all-solid-state asymmetric hybrid supercapacitors (HSCs), exhibiting remarkable performance metrics. The HSCs achieve a specific capacitance of 750 F g-1, a specific capacity of 209 mAh g-1 (at 0.5 A g-1), and an energy density of 100 Wh kg-1 (at 250 W kg-1) using PVA/KOH solid electrolyte, while maintaining outstanding cyclic stability. Importantly, a density functional theory framework is utilized to validate the experimental findings, underscoring the potential of this novel approach for enhancing HSC performance and enabling the large-scale production of transition metal-based layered double hydroxides.

A unique choline nitrate-based organo-aqueous electrolyte enables carbon/carbon supercapacitor operation in a wide temperature window (-40°C to 60°C)

Some drawbacks of aqueous electrolytes, such as freezing at low temperatures and extensive evaporation at high temperatures, restrict their industrial viability. This article introduces a stabilized neutral aqueous choline nitrate electrolyte with a 10 vol.% methanol additive that improves the temperature stability of the electrolyte via enhanced hydrogen bonding with the choline cation and water and maintains the good state of health of the supercapacitor cells under extreme operating conditions. The symmetric carbon/carbon supercapacitor in 5 mol/kg choline nitrate + 10 vol.% methanol (σ = 76 ms/cm at 25°C) exhibits 103 F/g at room temperature during galvanostatic charge/discharge up to 1.5 V, which decreases to 78 F/g at -40°C due to the suppressed Faradaic reactions occurring at the carbon electrode. However, under similar charge/discharge conditions, the capacitance increases to 112 F/g when the supercapacitor operates at 60°C. This capacitance increase at high temperatures is due to the Faradaic reactions related to enhanced hydrogen adsorption and desorption. The most remarkable aspect of the proposed supercapacitor is its ability to maintain capacitance and power performance during high voltage floating at 1.5 V at three tested temperatures (-40°C, 24°C, and 60°C).

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.

Construction of hierarchical NiCo2 S4 /2D-Carbyne nanohybrid onto nickel foam for high performance supercapacitor and non-enzymatic electrochemical glucose sensor applications

Synthesising and designing pseudocapacitive material with good electrochemical and electrocatalytic behaviour is essential to use as supercapacitor as well as non-enzymatic glucose sensor electrode. In this work, NiCo2 S4 nanoparticles decorated onto the 2D-Carbyne nanosheets are achieved by the solvothermal process. The as-prepared NiCo2 S4 @2D-Carbyne provides rich reaction sites and better diffusion pathways. On usage as an electrode for supercapacitor application, the NiCo2 S4 @2D-Carbyne exhibits the specific capacitance of about 2507 F g-1 at 1 A g-1 . In addition, the fabricated hybrid device generates an energy density of 52.2 Wh kg-1 at a power density of 1.01 kW kg-1 . Besides, the glucose oxidation behaviour of NiCo2 S4 @2D-Carbyne modified GCE has also been performed. The diffusion of glucose from the electrolyte to the electrode obeys the kinetic control process. Furthermore, the fabricated NiCo2 S4 @2D-Carbyne non-enzymatic glucose sensor exhibits a limit of detection of about 34.5 μM with a sensitivity of about 135 μA mM-1  cm-2 . These findings highlight the need to design and synthesis electrode materials with adequate electrolyte-electrode contact, strong structural integrity, and rapid ion/electron transport.

Cation desolvation-induced capacitance enhancement in reduced graphene oxide (rGO)

Understanding the local electrochemical processes is of key importance for efficient energy storage applications, including electrochemical double layer capacitors. In this work, we studied the charge storage mechanism of a model material - reduced graphene oxide (rGO) - in aqueous electrolyte using the combination of cavity micro-electrode, operando electrochemical quartz crystal microbalance (EQCM) and operando electrochemical dilatometry (ECD) tools. We evidence two regions with different charge storage mechanisms, depending on the cation-carbon interaction. Notably, under high cathodic polarization (region II), we report an important capacitance increase in Zn2+ containing electrolyte with minimum volume expansion, which is associated with Zn2+ desolvation resulting from strong electrostatic Zn2+-rGO interactions. These results highlight the significant role of ion-electrode interaction strength and cation desolvation in modulating the charging mechanisms, offering potential pathways for optimized capacitive energy storage. As a broader perspective, understanding confined electrochemical systems and the coupling between chemical, electrochemical and transport processes in confinement may open tremendous opportunities for energy, catalysis or water treatment applications in the future.

Co-pyrolysis of chicken feathers and macadamia nut shells, a promising strategy to create nitrogen-enriched electrode materials for supercapacitor applications

Global food waste emits substantial quantities of nitrogen to the environment (6.3 Mtons annually), chicken feather (CF) waste is a major contributor to this. Pyrolysis, in particular co-pyrolysis of nitrogen-rich and lignocellulosic waste streams is a promising strategy to improve the extent of pyrolytic nitrogen retention by incorporating nitrogen in its solid biochar structure. As such, this biochar can serve as a precursor for nitrogen-enriched activated carbons for application in supercapacitors. Therefore, this study investigates the co-pyrolysis of CF with macadamia nut shells (MNS) to create nitrogen-rich activated carbons. Co-pyrolysis increased nitrogen retention during pyrolysis from 9 % to 18 % compared to CF mono-pyrolysis, while the porosity was maintained. After removing undesirable inorganic impurities by dilute acid washing, this led to a specific capacitance of 21F/g using a scan rate of 20 mV/s. Finally, cycling stability tests demonstrated good stability with 73 % capacitance retention after 10 000 cycles.

Synthesis of Sustainable Lignin Precursors for Hierarchical Porous Carbons and Their Efficient Performance in Energy Storage Applications

Lignin-derived porous carbons have great potential for energy storage applications. However, their traditional synthesis requires highly corrosive activating agents in order to produce porous structures. In this work, an environmentally friendly and unique method has been developed for preparing lignin-based 3D spherical porous carbons (LSPCs). Dropwise injection of a lignin solution containing PVA sacrificial templates into liquid nitrogen produces tiny spheres that are lyophilized and carbonized to produce LSPCs. Most of the synthesized samples possess excellent specific surface areas (426.6-790.5 m2/g) along with hierarchical micro- and mesoporous morphologies. When tested in supercapacitor applications, LSPC-28 demonstrates a superior specific capacitance of 102.3 F/g at 0.5 A/g, excellent rate capability with 70.3% capacitance retention at 20 A/g, and a commendable energy density of 2.1 Wh/kg at 250 W/kg. These materials (LSPC-46) also show promising performance as an anode material in sodium-ion batteries with high reversible capacity (110 mAh g-1 at 100 mA g-1), high Coulombic efficiency, and excellent cycling stability. This novel and green technique is anticipated to facilitate the scalability of lignin-based porous carbons and open a range of research opportunities for energy storage applications.

Fabrication of Multi-Layered Paper-Based Supercapacitor Anode by Growing Cu(OH)2 Nanorods on Oxygen Functional Groups-Rich Sponge-Like Carbon Fibers

This work addresses the challenges in developing carbon fiber paper-based supercapacitors (SCs) with high energy density by focusing on the limited capacity of carbon fiber. To overcome this limitation, a sponge-like porous carbon fiber paper enriched with oxygen functional groups (OFGs) is prepared, and Cu(OH)2 nanorods are grown on its surface to construct the SC anode. This design results in a multi-layered carbon fiber paper-based electrode with a specific structure and enhanced capacitance. The Cu(OH)2 @PCFP anode exhibits an areal capacitance of 547.83 mF cm-2 at a current density of 1 mA cm-2 and demonstrates excellent capacitance retention of 99.8% after 10 000 cycles. Theoretical calculations further confirm that the Cu(OH)2 /OFGs-graphite heterostructure exhibits higher conductivity, facilitating faster charge transfer. A solid-state SC is successfully assembled using Ketjen Black@PCFP as the cathode and KOH/PVA as the gel electrolyte. The resulting device exhibits an energy density of 0.21 Wh cm-2 at 1.50 mW cm-2 , surpassing the performance of reported Cu(OH)2 SCs. This approach, combining materials design with an understanding of underlying mechanisms, not only expands the range of electrode materials but also provides valuable insights for the development of high-capacity energy storage devices.

A hydrophobic alloy-coated Zn anode for durable electrochromic devices

To mitigate Zn corrosion, dendrite growth and hydrogen evolution reactions (HER) in Zn-anode based electrochromic devices, hydrophobic CuZn5 alloy was coated on Zn@CuZn with lower nucleation potential, high coulombic efficiency, inhibited HER, and prolonged reversibility, enabling improved switching kinetics and cycling stability in an electrochromic Zn@CuZn||Prussian Blue (PB) device.

Nitrogen-Doped Cellulose-Derived Porous Carbon Fibers for High Mass-Loading Aqueous Supercapacitors

Biomass-based porous carbon with renewability and flexible structure tunability is a promising electrode material for supercapacitors. However, there is a huge gap between experimental research and practical applications. How to maintain good electrochemical performance of high mass-loading electrodes and suppress the self-discharge of supercapacitors is a key issue that urgently needs to be addressed. The structure regulation of electrode materials such as heteroatom doping is a promising optimization strategy for high mass-loading electrodes. In this work, nitrogen-doped cellulose-derived porous carbon fibers (N-CHPCs) were prepared by a facile bio-template method using cotton cellulose as raw material and urea as dopant. The prepared N-CHPCs have high specific surface area, excellent hierarchical porous structure, partial graphitization properties and suitable heteroatom content. The assembled high mass-loading (12.8 mg cm-2 ; 245 μm) aqueous supercapacitor has excellent electrochemical performance, i. e., low open-circuit voltage attenuation rate (21.39 mV h-1 ), high voltage retention rate (78.81 %), high specific capacitance (295.8 F g-1 at 0.1 A g-1 ), excellent area capacitance (3.79 F cm-2 at 0.1 A g-1 ), excellent cycling stability (97.28 % over 20,000 cycles at 1.0 A g-1 ). The excellent performance of high mass-loading N-CHPCs is of great significance for their practical applications in advanced aqueous supercapacitors.

Unlocking the full potential of citric acid-synthesized carbon dots as a supercapacitor electrode material via surface functionalization

This research paper investigates the effect of functionalizing the surfaces of citric acid-synthesized carbon dots (CDs) with hyperbranched bis(methylol)propionic acid (bis-MPA) polyester hydroxyl polymers (HBPs) on their performance as electrode materials in a supercapacitor. Two types of HBPs with 16 and 64 peripheral hydroxyl groups were used to functionalize the CDs' oxygen-enriched surface. Here, CDs were used as electrode materials for the first time in symmetric supercapacitors without a composite material, and how surface modification affects the capacitance performance of CDs was investigated. Our results showed that the functionalization of green-emitting CDs with HBP resulted in the successful passivation of surface defects, which improved their stability and prevented further oxidation. The CDs with HBP passivation exhibited excellent electrochemical performance, with a high specific capacitance of 32.08 F g-1 at 0.1 A g-1 and good rate capability, indicating a faster ion transport rate at high current densities. Experimental EPR spectra of functionalized and non-functionalized CDs reveal distinct changes in g-factor values and line widths, confirming the impact of dangling bonds and spin-orbit interactions. The observed broader linewidth indicates a wider range of electron spin resonances due to energy-level splitting induced by spin-orbit coupling. The excellent electrochemical performance of CDs with HBP passivation can be attributed to the presence of oxygen-containing surface functional groups such as hydroxyl and carboxyl groups on their surfaces, which enhance the conductivity and charge transfer reactions. These results suggest that functionalization with polar HBPs is a promising strategy to enhance the electrochemical performance of CDs in supercapacitor applications.

Introducing micropores into carbon nanoparticles synthesized via a solution plasma process by thermal treatment and their charge storage properties in supercapacitors

Carbon materials synthesized via a solution plasma process (SPP) have recently shown great potential for various applications. However, they mainly possess a meso-macroporous structure with a lack of micropores, which limits their applications for supercapacitors. Herein, carbon nanoparticles (CNPs) were synthesized from benzene via SPP and then subjected to thermal treatment at different temperatures (400, 600, 800, and 1000 °C) in an argon environment. The CNPs exhibited an amorphous phase and were more graphitized at high treatment temperatures. A small content of tungsten carbide particles was also observed, which were encapsulated in CNPs. An increase in treatment temperature led to an increase in the specific surface area of CNPs from 184 to 260 m2 g-1 through the development of micropores, while their meso-macropore structure remained unchanged. The oxygen content of CNPs decreased from 14.72 to 1.20 atom% as the treatment temperature increased due to the degradation of oxygen functionality. The charge storage properties of CNPs were evaluated for supercapacitor applications by electrochemical measurements using a three-electrode system in 1 M H2SO4 electrolyte. The CNPs treated at low temperatures exhibited an electric double layer and pseudocapacitive behavior due to the presence of quinone groups on the carbon surface. With increasing treatment temperature, the electric double layer behavior became more dominant, while pseudocapacitive behavior was suppressed due to the quinone degradation. Regarding cycling stability, the CNPs treated at high temperatures (with a lack of oxygen functionality) were more stable than those treated at low temperatures. This work highlights a way of introducing micropores into CNPs derived from SPP via thermal treatment, which could be helpful for controlling and adjusting their pore structure for supercapacitor applications.

Retrospective insights into recent MXene-based catalysts for CO2 electro/photoreduction: how far have we gone?

The electro/photocatalytic CO₂ reduction reaction (CO₂RR) is a long-term avenue toward synthesizing renewable fuels and value-added chemicals, as well as addressing the global energy crisis and environmental challenges. As a result, current research studies have focused on investigating new materials and implementing numerous fabrication approaches to increase the catalytic performances of electro/photocatalysts toward the CO₂RR. MXenes, also known as 2D transition metal carbides, nitrides, and carbonitrides, are intriguing materials with outstanding traits. Since their discovery in 2011, there has been a flurry of interest in MXenes in electrocatalysis and photocatalysis, owing to their several benefits, including high mechanical strength, tunable structure, surface functionality, high specific surface area, and remarkable electrical conductivity. Herein, this review serves as a milestone for the most recent development of MXene-based catalysts for the electrocatalytic and photocatalytic CO₂RR. The overall structure of MXenes is described, followed by a summary of several synthesis pathways classified as top-down and bottom-up approaches, including HF-etching, in situ HF-formation, electrochemical etching, and halogen etching. Additionally, the state-of-the-art development in the field of both the electrocatalytic and photocatalytic CO₂RR is systematically reviewed. Surface termination modulation and heterostructure engineering of MXene-based electro/photocatalysts, and insights into the reaction mechanism for the comprehension of the structure-performance relationship from the CO₂RR via density functional theory (DFT) have been underlined toward activity enhancement. Finally, imperative issues together with future perspectives associated with MXene-based electro/photocatalysts are proposed.

Novel and flexible asymmetric supercapacitors based on NiCo2O4 nanosheets coated on Al and Cu tapes for wearable devices applications

The binary metal oxides show advantages in energy storage devices. Specifically, nickel cobaltite (NiCo2O4) materials showed promising pseudocapacitive properties, high electrical conductivity and large surface area by virtue of their effective porous structure. NiCo2O4 nanosheets were hydrothermally grown in this work over flexible tapes of Aluminum (Al) and Copper (Cu). A nanosheets structure obtained of NiCo2O4 as confirmed by SEM and AFM images. The measured thickness by 3D profilometer of NiCo2O4 nanosheets based Al framework found to be 4.3 µm compared to 8.4 µm thick of film based-Cu framework. Asymmetric supercapacitor prepared from graphite and NiCo2O4 electrodes separated by filter paper. Acidic aqueous electrolyte of H2SO4 and basic aqueous electrolyte of KOH were employed to verify the cyclic activity and electrochemical reaction of asymmetric prepared supercapacitor devices. The basic KOH electrolyte shows a high stability and better charge transfer/ionic diffusion compared to the acidic H2SO4 electrolyte in particular for NiCo2O4 film-based Cu framework. The energy density and power density values were 0.9 W h kg−1 and 66.45 W kg−1, respectively. The highest specific capacity (in F.g−1) = 10.09 coincides with NiCo2O4/Cu supercapacitor in the basic KOH electrolyte. The charge storage in the supercapacitor system of NiCo2O4 and graphite can be ascribed in the form of Faradic charge transfer and capacitive non-faradic double layer, respectively.

Switchable Polyacrylonitrile-Copolymer for Melt-Processing and Thermal Carbonization-3D Printing of Carbon Supercapacitor Electrodes with High Capacitance

Polyacrylonitrile (PAN) represents the most widely used precursor for carbon fibers and carbon materials. Carbon materials stand out with their high mechanical performance, but they also show excellent electrical conductivity and high surface area. These properties render carbon materials suitable as electrode material for fuel cells, batteries, and supercapacitors. However, PAN has to be processed from solution before being thermally converted to carbon, limiting its final format to fibers, films, and non-wovens. Here, a PAN-copolymer with an intrinsic plasticizer is presented to reduce the melting temperature and avoid undesired entering of the thermal carbonization regime. This plasticizer enables melt extrusion-based additive manufacturing (EAM). The plasticizer in the PAN-copolymer can be switched to increase the melting temperature after processing, allowing the 3D-melt-printed workpiece to be thermally carbonized after EAM. Melt-processing of the PAN copolymer extends the freedom-in-design of carbon materials to mold-free rapid prototyping, in the absence of solvents, which enables more economic and sustainable manufacturing processes. As an example for the capability of this material system, open meshed carbon electrodes are printed for supercapacitors that are metal- and binder-free with an optimized thickness of 1.5 mm and a capacitance of up to 387 mF cm^-2 .

Fibril-Type Textile Electrodes Enabling Extremely High Areal Capacity through Pseudocapacitive Electroplating onto Chalcogenide Nanoparticle-Encapsulated Fibrils

Effective incorporation of conductive and energy storage materials into 3D porous textiles plays a pivotal role in developing and designing high-performance energy storage devices. Here, a fibril-type textile pseudocapacitor electrode with outstanding capacity, good rate capability, and excellent mechanical stability through controlled interfacial interaction-induced electroplating is reported. First, tetraoctylammonium bromide-stabilized copper sulfide nanoparticles (TOABr-CuS NPs) are uniformly assembled onto cotton textiles. This approach converts insulating textiles to conductive textiles preserving their intrinsically porous structure with an extremely large surface area. For the preparation of textile current collector with bulk metal-like electrical conductivity, Ni is additionally electroplated onto the CuS NP-assembled textiles (i.e., Ni-EPT). Furthermore, a pseudocapacitive NiCo-layered double hydroxide (LDH) layer is subsequently electroplated onto Ni-EPT for the cathode. The formed NiCo-LDH electroplated textiles (i.e., NiCo-EPT) exhibit a high areal capacitance of 12.2 F cm-2 (at 10 mA cm-2 ), good rate performance, and excellent cycling stability. Particularly, the areal capacity of NiCo-EPT can be further increased through their subsequent stacking. The 3-stack NiCo-EPT delivers an unprecedentedly high areal capacitance of 28.8 F cm-2 (at 30 mA cm-2 ), which outperforms those of textile-based pseudocapacitor electrodes reported to date.

Bio-Phenolic Resin Derived Porous Carbon Materials for High-Performance Lithium-Ion Capacitor

In this article, hierarchical porous carbon (HPC) with high surface area of 1604.9 m2/g is prepared by the pyrolysis of rubberwood sawdust using CaCO3 as a hard template. The bio-oil pyrolyzed from the rubber sawdust, followed by the polymerization reaction to form resole phenolic resin, can be used as a carbon source to prepare HPC. The biomass-derived HPC shows a three-dimensionally interconnected morphology which can offer a continuous pathway for ionic transport. The symmetrical supercapacitors based on the as-prepared HPC were tested in 1.0 M tetraethylammonium tetrafluoroborate/propylene carbonate electrolyte. The results of electrochemical analysis show that the HPC-based supercapacitor exhibits a high specific capacitance of 113.3 F/g at 0.5 A/g with superior rate capability and cycling stability up to 5000 cycles. Hybrid lithium-ion capacitors (LICs) based on the HPC and Li4Ti5O12 (LTO) were also fabricated. The LICs have a maximum energy density of 113.3 Wh/kg at a power density of 281 W/kg. Moreover, the LIC also displays a remarkable cycling performance with a retention of 92.8% after 3000 cycles at a large current density of 0.75 A/g, suggesting great potential application in the energy storage of the LIC.

A Simple Route to Produce Highly Efficient Porous Carbons Recycled from Tea Waste for High-Performance Symmetric Supercapacitor Electrodes

High-performance porous carbons derived from tea waste were prepared by hydrothermal treatment, combined together with KOH activation. The heat-treatment-processed materials possess an abundant hierarchical structure, with a large specific surface of 2235 m² g⁻¹ and wetting-complemental hydrophilicity for electrolytes. In a two-electrode system, the porous carbon electrodes’ built-in supercapacitor exhibited a high specific capacitance of 256 F g⁻¹ at 0.05 A g⁻¹, an excellent capacitance retention of 95.4% after 10,000 cycles, and a low leakage current of 0.014 mA. In our work, the collective results present that the precursor crafted from the tea waste can be a promising strategy to prepare valuable electrodes for high-performance supercapacitors, which offers a practical strategy to recycle biowastes into manufactured materials in energy storage applications.

A Low-Temperature Dehydration Carbon-Fixation Strategy for Lignocellulose-Based Hierarchical Porous Carbon for Supercapacitors

Lignocellulose-based hierarchical porous carbon is a very promising electrode material for supercapacitors, but lower volumetric energy density and yield have hindered its practical applications. Herein, a low-temperature dehydration carbon-fixation method using NH₄ Cl as modification reagent was developed to prepare rice husk-based hierarchical porous carbon (RHPC) with high volumetric performance and yield. The RHPC-N electrode exhibited a higher volumetric specific capacitance of 134.4 F cm^-3 than that of the RHPC electrode (98.4 F cm^-3 ) in 1 m Et₄ NBF₄ /propylene carbonate electrolyte. The volumetric energy density (28.8 Wh L^-1 ) of the RHPC-N electrode was 37.1 % higher than that of the RHPC electrode (21.0 Wh L^-1 ), which greatly enhanced the practical application potential of RHPC in supercapacitors. Moreover, the yield of RHPC increased 1.2 times by this method, which greatly improved the production capacity and reduced the cost. This research establishes a simple and highly efficient method to improve the volumetric energy density and the yield of lignocellulose-based hierarchical porous carbon.

Origins of Electron-Transfer Regime in Persulfate-Based Nonradical Oxidation Processes

Persulfate-based nonradical oxidation processes (PS-NOPs) are appealing in wastewater purification due to their high efficiency and selectivity for removing trace organic contaminants in complicated water matrices. In this review, we showcased the recent progresses of state-of-the-art strategies in the nonradical electron-transfer regimes in PS-NOPs, including design of metal and metal-free heterogeneous catalysts, in situ/operando characterization/analytical techniques, and insights into the origins of electron-transfer mechanisms. In a typical electron-transfer process (ETP), persulfate is activated by a catalyst to form surface activated complexes, which directly or indirectly interact with target pollutants to finalize the oxidation. We discussed different analytical techniques on the fundamentals and tactics for accurate analysis of ETP. Moreover, we demonstrated the challenges and proposed future research strategies for ETP-based systems, such as computation-enabled molecular-level investigations, rational design of catalysts, and real-scenario applications in the complicated water environment. Overall, this review dedicates to sharpening the understanding of ETP in PS-NOPs and presenting promising applications in remediation technology and green chemistry.

Organic–inorganic hybrid ferrocene/AC as cathodes for wide temperature range aqueous Zn-ion supercapacitors

Organic and inorganic materials have their own advantages and limitations, and new properties can be displayed in organic–inorganic hybrid materials by uniformly combining the two categories of materials at small scale.

Valorization of Albedo Orange Peel Waste to Develop Electrode Materials in Supercapacitors for the Electric Industry

This work proposes the use of albedo of orange peel in generation of carbon for applications in supercapacitors. For this, a comparison of compositional and electrochemical properties present in the carbons obtained of albedo, flavedo, and the complete orange peel was carried out. The morphology and composition of carbons obtained were analyzed by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray (EDX), X-Ray Diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The synthetized carbons were not subjected to the activation process by chemical compounds to relate only the properties of orange peel parts with their electrochemical behaviour. All samples were tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The carbon obtained of albedo presented a superior specific capacitance (210 F/g) of the rest samples. The value of albedo-based carbon capacitance is comparable with works presented in the literature that used a whole orange peel with chemical activators. In this way, it is possible to obtain large capacitances using only a part of orange peel (albedo). Thus, the importance of this study is that the albedo can be proposed as a material applied to electrodes for supercapacitors while the flavedo can be used in food industry or for oil extraction.

Mesoporous Manganese Oxide/Lignin-Derived Carbon for High Performance of Supercapacitor Electrodes

This study explores the modification of lignin with surfactants, which can be used as a template to make mesoporous structures, and can also be used in combination with manganese oxide to produce manganese oxide/lignin-derived carbon. Organosolv extraction, using ethanol (70%) at 150 °C, was carried out to extract lignin from oil palm wood. Lignin was then mixed with Pluronic F-127, with and without Mn(NO₃)₂, and then crosslinked with acidic formaldehyde, resulting in a carbon precursor-based modified lignin. Carbonization was carried out at 900 °C to produce lignin-derived carbon and manganese oxide/lignin-derived carbon. The characterization materials included Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope-energy dispersive X-ray (SEM-EDX) mapping, X-ray diffraction (XRD), and N₂-sorption analysis. FTIR curves displayed the vibration bands of lignin and manganese oxide. SEM images exhibited the different morphological characteristics of carbon from LS120% (lignin with a Pluronic surfactant of 120%) and LS120%Mn20% (lignin with a Pluronic of 120% and Mn oxide of 20%). Carbon LS120% (C-LS120%) showed the highest specific surface area of 1425 m²/g with a mean pore size of 3.14 nm. The largest mean pore size of 5.23 nm with a specific surface area of 922 m²/g was exhibited by carbon LS120%-Mn20% (C-LS120%-Mn20%). C-LS120%Mn20% features two phases of Mn oxide crystals. The highest specific capacitance of 345 F/g was exhibited by C-LS120%-Mn20%.

Mesoporous onion-like carbon nanostructures from natural oil for high-performance supercapacitor and electrochemical sensing applications: Insights into the post-synthesis sonochemical treatment on the electrochemical performance

Although onion-like carbon nanostructures (OLCs) are attractive materials for energy storage, their commercialization is hampered by the absence of a simple, cost-effective, large-scale synthesis route and binder-free electrode processing. The present study employs a scalable and straightforward technique to fabricate sonochemically tailored OLCs-based high-performance supercapacitor electrode material. An enhanced supercapacitive performance was demonstrated by the OLCs when sonicated in DMF at 60 °C for 15 min, with a specific capacitance of 647 F/g, capacitance retention of 97% for 5000 cycles, and a charge transfer resistance of 3 Ω. Furthermore, the OLCs were employed in the electrochemical quantification of methylene blue, a potential COVID-19 drug. The sensor demonstrated excellent analytical characteristics, including a linear range of 100 pM to 1000 pM, an ultralow sensitivity of 64.23 pM, and a high selectivity. When used to identify and quantify methylene blue in its pharmaceutical formulation, the sensor demonstrated excellent reproducibility, high stability, and satisfactory recovery.

Highly efficient metal-free electro-Fenton degradation of organic contaminants on a bifunctional catalyst

Heterogeneous electro-Fenton (e-Fenton) is a promising technology for the treatment of persistent organic pollutants, in which H₂O₂ is produced via 2e^- oxygen reduction and is simultaneously activated into •OH by the iron-based catalyst. This system often suffers from the inevitable metal dissolution in the acidic or even neutral environment, leading to poor pH adaptation and low stability. In this work, we designed a metal-free e-Fenton system, using O-doped carbon nanotubes (OCNTs) as the bifunctional metal-free cathode for the degradation of organic pollutants. The system showed the excellent e-Fenton performance under neutral conditions with the phenol degradation kinetic constant of 0.071 min^-1, and the corresponding TOC removal was 76.6% within 300 min. It also exhibited excellent performance for actual coking wastewater treatment with the specific energy consumption of 7.4 kW h kg^-1 COD^-1, which was lower than that reported heterogeneous electro-Fenton system (9.2-14.4 kW h kg^-1 COD^-1). The in-situ metal-free e-Fenton system could be regarded as a promising strategy for actual wastewater treatment.

A Comparative Study of the Influence of Nitrogen Content and Structural Characteristics of NiS/Nitrogen-Doped Carbon Nanocomposites on Capacitive Performances in Alkaline Medium

Supercapacitors (SCs) have been regarded as alternative electrochemical energy storage devices; however, optimizing the electrode materials to further enhance their specific energy and retain their rate capability is highly essential. Herein, the influence of nitrogen content and structural characteristics (i.e., porous and non-porous) of the NiS/nitrogen-doped carbon nanocomposites on their electrochemical performances in an alkaline electrolyte is explored. Due to their distinctive surface and the structural features of the porous carbon (A-PVP-NC), the as-synthesized NiS/A-PVP-NC nanocomposites not only reveal a high wettability with 6 M KOH electrolyte and less polarization but also exhibit remarkable rate capability (101 C/g at 1 A/g and 74 C/g at 10 A/g). Although non-porous carbon (PI-NC) possesses more nitrogen content than the A-PVP-NC, the specific capacity output from the latter at 10 A/g is 3.7 times higher than that of the NiS/PI-NC. Consequently, our findings suggest that the surface nature and porous architectures that exist in carbon materials would be significant factors affecting the electrochemical behavior of electrode materials compared to nitrogen content.