An activated carbon with high capacitance from carbonization of a resorcinol–formaldehyde resin

Molecular Level Modulation of Anthraquinone-containing Resorcinol-formaldehyde Resin Photocatalysts for H2 O2 Production with Exceeding 1.2 % Efficiency

Designing polymeric photocatalysts at the molecular level to modulate the photogenerated charge behavior is a promising and challenging strategy for efficient hydrogen peroxide (H₂ O₂ ) photosynthesis. Here, we introduce electron-deficient 1,4-dihydroxyanthraquinone (DHAQ) into the framework of resorcinol-formaldehyde (RF) resin, which modulates the donor/acceptor ratio from the perspective of molecular design for promoting the charge separation. Interestingly, H₂ O₂ can be produced via oxygen reduction and water oxidation pathways, verified by isotopic labeling and in situ characterization techniques. Density functional theory (DFT) calculations elucidate that DHAQ can reduce the energy barrier for H₂ O₂ production. RF-DHAQ exhibits excellent overall photosynthesis of H₂ O₂ with a solar-to-chemical conversion (SCC) efficiency exceeding 1.2 %. This work opens a new avenue to design polymeric photocatalysts at the molecular level for high-efficiency artificial photosynthesis.

Electrochemical properties of an activated carbon xerogel monolith from resorcinol-formaldehyde for supercapacitor electrode applications

Activated carbon xerogel monoliths were prepared from resorcinol and formaldehyde via a catalyst-free and template-free hydrothermal polycondensation reaction, followed by pyrolysis and activation. The ratio of resorcinol (R) to distilled water (W) was varied to afford an interconnected pore structure with controlled pore size, while the pyrolysis temperature was optimized to give high specific surface area. Activation was carried out at 700 °C after soaking the samples in 6 M KOH aqueous solution. The same process, called "heat treatment", was also carried out without soaking in KOH for comparison. The weight loss upon pyrolysis, activation and heat treatment and the weight gain via KOH soaking were measured. Field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and an N2 sorption instrument were utilized for characterization. Additionally, electrochemical properties were evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) with a 3-electrode system, while a 2-electrode system was also employed for selected samples. The highest specific capacitance of 323 F g-1 via GCD at 1 A g-1 was obtained at the R/W ratio of 45 and with 500 °C pyrolysis. In addition, this sample also exhibited 89.4% retention at 20 A g-1 in the current density variation and 100% retention in 5000 cycling tests.

Investigating the Role of the Catalyst within Resorcinol-Formaldehyde Gel Synthesis

Resorcinol-formaldehyde (RF) gels are porous materials synthesized via a sol-gel reaction and subsequently dried, producing structures with high surface areas and low densities-properties that are highly attractive for use in various applications. The RF gel reaction takes place in the presence of a catalyst, either acidic or basic in nature, the concentration of which significantly impacts final gel properties. The full extent of the catalyst's role, however, has been subject to debate, with the general consensus within the field being that it is simply a pH-adjuster. The work presented here explores this theory, in addition to other theories postulated in the literature, through the synthesis and analysis of RF gels catalysed by mixtures of relevant compounds with varying concentrations. The relationship between catalyst concentration and initial solution pH is decoupled, and the individual roles of both the cation and the anion within the catalyst are investigated. The results presented here point towards the significance of the metal cation within the RF gel reaction, with similar structural properties observed for gels synthesized at constant Na+ concentrations, regardless of the initial solution pH. Furthermore, through the use of alternative cations and anions within catalyst compounds, the potential effects of ions on the stabilization of macromolecules in solution are explored, the results of which suggest a 'Hofmeister-like' series could be applicable within the catalysis of RF gel reactions.

Aqueous Al-Ion Supercapacitor with V2O5 Mesoporous Carbon Electrodes

A high-performance, low-cost, aqueous Al-ion supercapacitor was fabricated based on nanostructured V₂O₅ impregnated mesoporous carbon microspheres (MCM/V₂O₅) electrodes and Al₂(SO₄)₃ electrolyte for efficient energy storage. MCM/V₂O₅ composites exhibit high dispersion of nanostructured V₂O₅ in a mesoporous carbon matrix, beneficial to fast reversible redox reactions with a short diffusion path. The corresponding capacitor illustrates the distinguishable redox behavior, most likely due to the Al³⁺ intercalation/deintercalation leading to the reduction/oxidation of V⁵⁺/V⁴⁺. It delivers a high-energy density of 18.0 Wh kg^-1 at 147 W kg^-1 and a long cycling lifespan with over 88% capacitance retention over 10 000 cycles. The competitive performance can be ascribed to the integration of the electric double layer capacitance provided from MCM with pseudocapacitance contributed by nanostructured V₂O₅. This work offers the possibilities of high-performance aqueous capacitors based on trivalent Al-ion as guest species, providing new directions for future development of supercapacitors.

Economic and High Performance Phosphorus-Carbon Composite for Lithium and Sodium Storage

Porous carbon derived from rice hulls has potential for application in phosphorus-carbon composites as high capacity anode materials for lithium-ion and sodium-ion batteries. The native composition of rice husks produces a porous carbon with a unique doped structure, as well as an efficient pore and channel structure, which may facilitate high and stable lithium storage. After cycling for over 100 cycles, the composite delivered a capacity of about 1293 mAh g^-1, as well as a coulombic efficiency of nearly 99% at the current density of 130 mA g^-1 with a capacity density of 1.43 mAh cm^-2. High specific discharge capacities were maintained at different current densities (∼2224, ∼1895, ∼1642, and ∼1187 mAh g^-1 _composite at 130, 260, 520, and 1300 mA g^-1, respectively). This study may offer a golden opportunity to change the humble fate of rice hulls, and also pave the way toward successful battery application for phosphorus-carbon composite anode materials.

Creating 3D Hierarchical Carbon Architectures with Micro-, Meso-, and Macropores via a Simple Self-Blowing Strategy for a Flow-through Deionization Capacitor

In this work, 3D hierarchical carbon architectures (3DHCAs) with micro-, meso-, and macropores were prepared via a simple self-blowing strategy as highly efficient electrodes for a flow-through deionization capacitor (FTDC). The obtained 3DHCAs have a hierarchically porous structure, large accessible specific surface area (2061 m(2) g(-1)), and good wettability. The electrochemical tests show that the 3DHCA electrode has a high specific capacitance and good electric conductivity. The deionization experiments demonstrate that the 3DHCA electrodes possess a high deionization capacity of 17.83 mg g(-1) in a 500 mg L(-1) NaCl solution at 1.2 V. Moreover, the 3DHCA electrodes present a fast deionization rate in 100-500 mg L(-1) NaCl solutions at 0.8-1.4 V. The 3DHCA electrodes also present a good regeneration behavior in the reiterative regeneration test. These above factors render the 3DHCAs a promising FTDC electrode material.

Effects of graphene oxide addition on the synthesis and supercapacitor performance of carbon aerogel particles

Effects of graphene oxide addition on the synthesis of carbon aerogel particles with morphologies of spheres, irregular-semispheres and wrinkle-capsules were studied, and the specific capacitances of resulted carbon particles were discussed.

Nanomorphology-dependent pseudocapacitive properties of NiO electrodes engineered through a controlled potentiodynamic electrodeposition process

Three nickel oxide (NiO) electrodes of different nanostructures have been successfully engineered through a controlled potentiodynamic electrodeposition process, which show a noticeable effect on pseudocapacitive properties of NiO electrodes.

Understanding performance limitation and suppression of leakage current or self-discharge in electrochemical capacitors: a review

Self-discharge is known to have considerable adverse effects on the performance and application of electrochemical capacitors (ECs). Thus, obtaining an understanding of EC self-discharge mechanism(s) and subsequent derivation and solution of EC models, subject to a particular mechanism or combination of mechanisms during charging, discharging and storage of the device, is the only way to solve problems associated with EC self-discharge. In this review, we summarize recent progress with respect to EC self-discharge by considering the two basic types, electric double-layer capacitors (EDLC) and pseudocapacitors, and their hybrids with their respective charge storage mechanisms, distinguishable self-discharge mechanisms, charge redistribution and charge/energy loss during self-discharge. It was clearly observed that most of the voltage reduction is not purely due to the self-discharge effect but is basically due to redistribution of charge carriers deep inside pores and can therefore be retrieved from a capacitor during long-time discharging. Tuning the self-discharge rate is therefore feasible for single-walled carbon nanotube (SWNT) ECs and can be achieved by simply adjusting the surface chemistry of the nanotubes. The effects of surface chemistry modification on EC self-discharge are very important in studying and suppressing the self-discharge process and will benefit potential applications of ECs with respect to energy retention. Self-discharge can be averted by the use of redox couples that are transformed to insoluble species via electrolysis and adsorbed onto the activated carbon electrode in redox-couple EDLCs, thus transforming the EDLC electrolyte into a material that can store charge. Self-discharge in ECs can also be successfully suppressed by utilizing an ion-interchange layer (ion-exchange membrane), separator or CuSO4 mobile electrolyte that can be converted into an insoluble species by electrolysis during the charge/discharge process. This will help in producing a modern-day blueprint for ECs with high capacitance and improved energy sustainability.

Low-cost flexible supercapacitors with high-energy density based on nanostructured MnO2 and Fe2O3 thin films directly fabricated onto stainless steel

The facile and economical electrochemical and successive ionic layer adsorption and reaction (SILAR) methods have been employed in order to prepare manganese oxide (MnO2) and iron oxide (Fe2O3) thin films, respectively with the fine optimized nanostructures on highly flexible stainless steel sheet. The symmetric and asymmetric flexible-solid-state supercapacitors (FSS-SCs) of nanostructured (nanosheets for MnO2 and nanoparticles for Fe2O3) electrodes with Na2SO4/Carboxymethyl cellulose (CMC) gel as a separator and electrolyte were assembled. MnO2 as positive and negative electrodes were used to fabricate symmetric SC, while the asymmetric SC was assembled by employing MnO2 as positive and Fe2O3 as negative electrode. Furthermore, the electrochemical features of symmetric and asymmetric SCs are systematically investigated. The results verify that the fabricated symmetric and asymmetric FSS-SCs present excellent reversibility (within the voltage window of 0-1 V and 0-2 V, respectively) and good cycling stability (83 and 91%, respectively for 3000 of CV cycles). Additionally, the asymmetric SC shows maximum specific capacitance of 92 Fg(-1), about 2-fold of higher energy density (41.8 Wh kg(-1)) than symmetric SC and excellent mechanical flexibility. Furthermore, the "real-life" demonstration of fabricated SCs to the panel of SUK confirms that asymmetric SC has 2-fold higher energy density compare to symmetric SC.

Mn3O4 nanoparticles anchored to multiwall carbon nanotubes: a distinctive synergism for high-performance supercapacitors

A modified hydrothermal (MHT) route has been adopted for the synthesis of a MWCNT/Mn₃O₄ composite to attain enhanced supercapacitive performance.

Architectured morphologies of chemically prepared NiO/MWCNTs nanohybrid thin films for high performance supercapacitors

The preparation of nanostructured metal oxide decorated on multiwalled carbon nanotubes (MWCNTs) nanohybrid films through simple, scalable, additive-free, binderless, and cost-effective route has fascinated significant attention not only in fundamental research areas but also its commercial applications, in order to reduce the growing environmental pollution and the cost of electrode fabrication. Here, we report the fabrication of highly flexible electrode with NiO/MWCNTs nanohybrid thin films directly on stainless steel substrate using successive ionic layer adsorption and reaction (SILAR) method. The impact of ratio of adsorption and reaction cycles on structural, surface areas and electrochemical properties of NiO/MWCNTs nanohybrids was investigated. X-ray diffraction measurements confirm the hybridization and face centered cubic (FCC) crystal structure of NiO in NiO/MWCNTs nanohybrids. In addition, these nanohybrids exhibit excellent surface properties such as uniform surface morphology, good surface area, pore volume, and uniform pore size distribution. The electrochemical tests demonstrate the highest specific capacitance of 1727 F g(-1) at 5 mA cm(-2) of current density with 91% capacitance retention after 2000 cycles. In addition, the Ragone plot confirms the better power and energy densities for all NiO/MWCNTs nanohybrids. The attractive electrochemical capacitive activity revealed by NiO/MWCNTs nanohybrid electrode proposes that it is an auspicious respondent for future energy storage application.

Porous 3D graphene-based bulk materials with exceptional high surface area and excellent conductivity for supercapacitors

Until now, few sp² carbon materials simultaneously exhibit superior performance for specific surface area (SSA) and electrical conductivity at bulk state. Thus, it is extremely important to make such materials at bulk scale with those two outstanding properties combined together. Here, we present a simple and green but very efficient approach using two standard and simple industry steps to make such three-dimensional graphene-based porous materials at the bulk scale, with ultrahigh SSA (3523 m²/g) and excellent bulk conductivity. We conclude that these materials consist of mainly defected/wrinkled single layer graphene sheets in the dimensional size of a few nanometers, with at least some covalent bond between each other. The outstanding properties of these materials are demonstrated by their superior supercapacitor performance in ionic liquid with specific capacitance and energy density of 231 F/g and 98 Wh/kg, respectively, so far the best reported capacitance performance for all bulk carbon materials.

Capacitive energy storage in nanostructured carbon-electrolyte systems

Securing our energy future is the most important problem that humanity faces in this century. Burning fossil fuels is not sustainable, and wide use of renewable energy sources will require a drastically increased ability to store electrical energy. In the move toward an electrical economy, chemical (batteries) and capacitive energy storage (electrochemical capacitors or supercapacitors) devices are expected to play an important role. This Account summarizes research in the field of electrochemical capacitors conducted over the past decade. Overall, the combination of the right electrode materials with a proper electrolyte can successfully increase both the energy stored by the device and its power, but no perfect active material exists and no electrolyte suits every material and every performance goal. However, today, many materials are available, including porous activated, carbide-derived, and templated carbons with high surface areas and porosities that range from subnanometer to just a few nanometers. If the pore size is matched with the electrolyte ion size, those materials can provide high energy density. Exohedral nanoparticles, such as carbon nanotubes and onion-like carbon, can provide high power due to fast ion sorption/desorption on their outer surfaces. Because of its higher charge-discharge rates compared with activated carbons, graphene has attracted increasing attention, but graphene had not yet shown a higher volumetric capacitance than porous carbons. Although aqueous electrolytes, such as sodium sulfate, are the safest and least expensive, they have a limited voltage window. Organic electrolytes, such as solutions of [N(C2H5)4]BF4 in acetonitrile or propylene carbonate, are the most common in commercial devices. Researchers are increasingly interested in nonflammable ionic liquids. These liquids have low vapor pressures, which allow them to be used safely over a temperature range from -50 °C to at least 100 °C and over a larger voltage window, which results in a higher energy density than other electrolytes. In situ characterization techniques, such as nuclear magnetic resonance (NMR), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), and electrochemical quartz crystal microbalance (EQCM) have improved our understanding of the electrical double layer in confinement and desolvation of ions in narrow pores. Atomisitic and continuum modeling have verified and guided these experimental studies. The further development of materials and better understanding of charged solid-electrolyte interfaces should lead to wider use of capacitive energy storage at scales ranging from microelectronics to transportation and the electrical grid. Even with the many exciting results obtained using newer materials, such as graphene and nanotubes, the promising properties reported for new electrode materials do not directly extrapolate to improved device performance. Although thin films of nanoparticles may show a very high gravimetric power density and discharge rate, those characteristics will not scale up linearly with the thickness of the electrode.

Advances in tailoring resorcinol-formaldehyde organic and carbon gels

An overview on the preparation and properties of resorcinol-formaldehyde (RF) organic and carbon gels reveals the fascinating and remarkably flexible properties of RF carbon and organic gels and how these properties are related to the synthesis and processing conditions. The structural properties can be easily tailored by rigidly controlling such conditions. However, slight variations in some conditions may cause drastic variations in the structural characteristics, and hence properties. Therefore, the effects of different conditions must be well-understood before attempting to tailor organic or carbon gels to specific applications. The most important factors that affect the properties of an organic gel are the precursor concentrations, the catalyst type and concentration, the time and temperature of curing, and the drying method. In addition to these factors, characteristics of activated carbon gels also depend on the pyrolysis temperature and the activation method. These conditions impact the structural and performance characteristics significantly.