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Abstract In this study, the method of hydrothermal synthesis is introduced to prepare the titanium dioxide (i.e.,TiO 2) tetragonal prism array on the conducting plane of fluorine-doped tin oxide (i.e., FTO), and then the polyaniline (i.e., PANI) is coated on the surface of TiO 2 array by normal chemical oxidation to form the PANI/TiO 2 shell/core nanoarray. The array architecture of PANI/TiO 2 composite is further confirmed by SEM, XPS, XRD, UV–vis and Raman spectroscopy. The regular array structure of PANI/TiO 2 can reduce the resistance of ionic diffuse and charge transfer via optimizing the ionic diffusion to obtain a lower impedance and higher specific capacitance. Compared with the individual component (PANI or TiO 2), the PANI/TiO 2 /FTO electrode possesses a higher specific capacitance as the supercapacitor electrode material. The maximum specific capacitances of 633 F/g at the rate of 10 mV/s and 781 F/g at the current density of 1 A/g are obtained in the PANI/TiO 2 /FTO electrode and it is superior or close to individual PANI or PANI-based materials. In addition, the TiO 2 array can undertake some mechanical deformation in the redox process without destroying the electrode material to enhance the cyclic stability of PANI/TiO 2 /FTO electrode. The capacitance retention of PANI/TiO 2 /FTO electrode keeps 75% of its initial value which is higher than that of the PANI/FTO electrode (65%) after 2000 cycles. Therefore, such array architecture composite can be promisingly used as the electrode material for electrochemical capacitive energy storage. Graphical abstract The array architecture of PANI/TiO 2 shell/core composite was successful prepared onto the conducting plane of ITO. The PANI/TiO 2 shell/core array can reduce the resistance of ionic diffuse and charge transfer via optimizing the ionic diffusion to obtain lower impedance and a higher specific capacitance. The maximum specific capacitance of 633 F/g at rate of 10 mV/s and 781 F/g at 1 A/g are obtained in the PAN/TiO 2 /FTO electrode and it is superior or close to individual PANI or PANI-based materials. Image 10723 Highlights • The PANI/TiO 2 shell/core array was successful prepared onto the conducting plane of the ITO. • The specific capacitance of 781 F/g was obtained when PANI/TiO 2 acted as the electrode. • The cycling stability with capacitance retentions of 75% after 2000 cycles. • The PAN/TiO 2 shell/core array can reduce the resistance of ionic diffuse and charge transfer. [ABSTRACT FROM AUTHOR]

Abstract Carbon Xerogel electrodes' materials were prepared by polycondensation of resorcinol and formaldehyde. They show accommodation of anions in preference to cations by their porous structure. In addition, their selectivity toward anion electroadsorption was found to be pH dependent, with an increase in electrodes' capacitance in acidic solutions. This dependence of the capacitance on the pH of the solution is explained by the presence of functional carboxylic surface groups in the Xerogel carbon micropores, determined by Boehm titration to be 3.26 meq/g. These functional surface groups give rise to the Donnan exclusion effect, whereby they cause partial repulsion of anions from the microporous structure. When the pH of the electrolyte solution is acidic (<2), most of these acidic functional surface groups are in their non-ionized state, and hence anions can freely penetrate the Xerogel carbon microporous structure. The pH dependent capacitance of these carbons was utilized to examine the feasibility of energy extraction from mixing of saline and acidic solutions following the four basic steps of capacitive mixing processes – charge, solution exchange, discharge, and another solution exchange. As opposed to what we expected, a potential rise was observed when the solution was exchanged from acidic to saline, rather than during the other exchange, from saline to acidic solution. It appears that upon exchange of the solutions from saline to acidic, the expected potential rise resulting from the change in the electrodes capacitance, is cancelled by the new quinone-hydroquinone equilibrium that is established on the electrodes by their surface groups. The ability of these surface redox functional groups to store charge provides a basis for energy extraction from the neutralization of acidic solutions, as described herein. [ABSTRACT FROM AUTHOR]

Abstract Metal–organic frameworks (MOFs), which combine organic and inorganic components, are promising materials for energy storage. The conjugated structure of MOFs offers excellent electrical conductivity and charge storage performance. In this study, we introduce Zn-p–phenylenediamine (Zn-pPDA) MOFs for a first time in energy storage devices, and we also use a green and facile synthesis method for the preparation of the materials. Multi-layered Zn-pPDA MOF nanostructures with a thickness of 1–5 μm and a width of ∼20 nm are synthesised using liquid–liquid interfacial reactions. Their MOF complexes are used to fabricate positive electrodes for supercapacitors. The MOF supercapacitors exhibit high energy and power densities of approximately 62.8 Wh/kg and 4500 W/kg, respectively. Additionally, excellent cyclability with ∼96% capacitive retention after 2000 cycles is obtained. We show that the synthesis method can open a new pathway for formulating pPDA-ligand-based MOFs, and that the Zn-pPDA MOF is a highly promising material for future energy storage devices. [ABSTRACT FROM AUTHOR]

Abstract In this work, we report the use of pseudocapacitive RuO 2 in a porous carbon-based substrate as a composite electrode for high-performance capacitive deionization (CDI). RuO 2 was electrodeposited onto inexpensive activated carbon (AC) via cyclic voltammetry to optimize the composite electrode (denoted as RuO 2 (20)-AC) fabrication. The electrochemical measurements indicate that the composite electrode with a specific surface area of 576 m2/g and hydrophilicity yields an improved specific capacitance and good cycling stability. The notably enhanced performance is attributed to the presence of RuO 2 , which allows rapid Faradaic charge-transfer reactions as well as pseudocapacitive charge storage. These results confirm that incorporating RuO 2 onto an AC electrode effectively reduces the electrical resistance and enhances the charge efficiency. Furthermore, batch-mode CDI experiments were conducted at 1.2 V in a 5 mM NaCl solution. As evidenced, the RuO 2 (20)-AC composite has a promising salt adsorption capacity of 11.26 mg/g, which is 3.7-fold higher than that of pristine AC. Therefore, using the RuO 2 (20)-AC composite as the cathode, an enhanced desalination performance can be achieved through a mixed capacitive-Faradaic process, resulting from two removal mechanisms of capacitive electrosorption and pseudocapacitive redox reactions. This work provides an efficient strategy to utilize RuO 2 on porous carbon-based substrates to improve CDI performance. Highlights • RuO 2 /activated carbon composite electrodes were prepared for CDI. • Electrodeposited RuO 2 showed good pseudocapacitive charge storage. • The metallic conductivity of RuO 2 ensures rapid ion transport. • The composite had a superior salt electrosorption capacity. • The combination of capacitive electrosorption and Faradaic reactions enhanced the CDI performance. [ABSTRACT FROM AUTHOR]

Abstract A biomass-derived activated carbon with a systematic control over the pore size distribution is used to decode the effect of pore size distribution on charge dynamics in organic acetonitrile-based supercapacitors. Distinct trends in the high-current capacitance of the positive and negative electrodes are revealed by isolating the ion-specific accessible pore width and specific surface area from the total values calculated on the basis of low-temperature nitrogen adsorption/desorption isotherms. A size match between ions and pores for each separate electrode is established to maximize gravimetric capacitance under high current load. Most importantly, the high-current gravimetric capacitance demonstrates the existence of an optimum micropore width depending on polarization as well as no need for wide micropores or mesopores for ensuring rapid capacitive response. [ABSTRACT FROM AUTHOR]

• Sub-10 nm SnO 2 -In 2 O 3 heterostructures was fabricated by ALD technique. • The thin-film of heterostructure could significantly increase the performance of ESC. • The heterostructure electrode delivered a high specific capacitance of 757 F g−1. • Results demonstrate that thin-film electrode is promising for high-performance ESCs. Fabrication of thin-film heterostructured electrodes with thickness below 10.0 nm is of great importance for the development of high-performance electrochemical supercapacitors (ESCs) with great capabilities. This is because such electrodes with uniform thicknesses enable fast electrochemical responses during charging/discharging ascribed to their short charge and ion diffusion lengths. In this study, two-dimensional (2D) SnO 2 -In 2 O 3 heterostructures with a thickness of ∼7.84 nm was fabricated on Au/SiO 2 /Si wafers by the atomic layer deposition (ALD) technique for ESCs application. Various spectroscopic and microscopic analyses demonstrated the formation of 2D SnO 2 -In 2 O 3 heterojunction with high purity, crystallinity and homogeneous morphology. Cyclic voltammetry (CV) measurements revealed the pseudocapacitive Faradaic redox reactions with good reversibility for 2D SnO 2 -In 2 O 3 heterostructure. The heterostructured electrodes demonstrated excellent super-capacitive performance with high specific capacitance (C s) up to 1048.25 and 757 F g−1 at the scan rate and the current density of 10 mV s−1 and 12 A g−1, respectively. Accordingly, a high energy density of 51.55 Wh kg−1 is attained at 12 A g−1. The high super-capacitive performance of these heterostructures can be attributed to the improved redox activity and short ion diffusion length of the SnO 2 -In 2 O 3 nanohybrid thin-film. The electrodes also demonstrated great electrochemical stability with the C s retention of 96.15% even after 5000 charge/discharge cycles. [Display omitted] [ABSTRACT FROM AUTHOR]

• The first high entropy oxides based multifunctional electrode material is synthesized by simple grinding technique. • Nano-sized (< 10 nm) high entropy oxides are incorporated in surface functionalized biowaste derived activated carbon. • High salt adsorption capacity of 241 mg g−1 and cycle repeatability upto 20 cycles is demonstrated. • The excellent desalination capacity is attributed to the combined electric-double layer and pseudocapacitive mechanism. Nano-sized high entropy oxides (particle size < 10 nm) are expected to possess enhanced stability, electrochemical properties and increased potential in versatile applications than bulk metal oxides. In this study, nano-sized high entropy oxides incorporated surface functionalized coconut shell derived activated carbon (HEO/f-CSAC) is synthesized, characterised and used as an advanced multifunctional material for electrochemical energy storage and desalination. Attributed to its unique microstructure and extraordinary properties, HEO/f-CSAC shows an excellent salt adsorption capacity of 241 mg g−1 at 1.2 V in 500 mg L−1 concentration of NaCl solution, which is very high as compared to previous reports. The reason behind high salt adsorption capacity is its outstanding specific capacitance of 147.5 F g−1 at current density 1 A g−1. Moreover, the assembled lab-scale CDI unit demonstrates extremely good performance upto 20 adsorption-desorption cycles due to the long cycle life with good capacitance retention of fabricated HEO/f-CSAC based two-electrode supercapacitor. Additionally, a red light-emitting diode illumination by connecting three supercapacitors in series, indicates the high efficiency and potential of HEO/f-CSAC for real life energy-related application. These results prove that HEO/f-CSAC can be implemented for water desalination with enhanced supercapacitive performance manifesting its multifunctionality for energy and environmental applications. [Display omitted] [ABSTRACT FROM AUTHOR]

• The polypyrrole precursors were prepared by the bubble-templated polymerization. • HNS-CNA-1 possesses a high active N content and abundant holes in nanosheets. • The HNS-CNA-1 electrode provides a capacitance of 477 F g−1 at 1 A g−1 in H 2 SO 4. • The HNS-CNA-1 SC delivers the energy density of 71.7 Wh kg−1 at 1 kW kg−1. Holey graphene with unique in-plane holes at the basal plane of graphene, has shown extraordinary promise for high-rate supercapacitors (SCs), due to its hole architecture that can shorten ion diffusion paths and facilitate electrolyte ion penetration. However, holey graphene is usually fabricated through a chemical etching procedure to create holes, which requires rigorous etching conditions and a cumbersome purification process. We propose a novel holey N/S-codoped carbon nanosheet aerogel (HNS-CNA) with honeycomb-like structure, which can be fabricated through an unprecedented bubble-templated polymerization of pyrrole and the subsequent carbonization of the holey polypyrrole nanosheet hydrogel (H-PPyNH) precursors. The bubble-templated route to H-PPyNH is considered to be promising for large-scale commercialization, due to the superiority of this cost-effective solution procedure. With the voltage window of 2 V, the symmetric HNS-CNA-1 SC delivers a high specific capacitance of 129 F g−1 at 1 A g−1, cycling stability of 90.7% (after 10,000 cycles) and energy density of 71.7 Wh kg−1 at 1 kW kg−1, attributed to abundant holes and N/S co-doping characteristics, which can facilitate electrolyte ion diffusion and introduce pseudocapacitance respectively. This holey N/S-codoped carbon nanosheet aerogel opens up a facile approach to promising electrode materials for highly-efficient capacitive energy storage. [Display omitted] [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an emerging technology as a sustainable low energy process for desalination of brackish water. Activated carbon electrodes are often used in the CDI devices. Electrosorption capacity was found to be improved on asymmetric electrode configuration using activated carbon cloth doped with fluorine due to redistribution of electric potential. This led to improvement in desalination performance up to 12.4 mg/g for a desalination cycle of 6 min employing an asymmetric fluorinated electrode as cathode (ACC//F-ACC). A relatively high charge efficiency of 77 % was obtained representing 92 % charge efficiency neglecting the leakage currents. Furthermore, the ion adsorption rate was found to improve substantially due to an increased surface conductivity of the fluorinated electrode confirmed by Mott-Schottky analysis. Energy consumption during desalination of 1000 ppm sodium chloride solution of 0.71kWh/m3 for symmetric electrode configuration was found to reduce by 36 % upon employing asymmetric configuration. This study shows some of the benefits of asymmetric configuration to achieve an optimal operation of CDI device, as well as improvements related to energy consumption. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Highly hydrophilic and holey g-C 3 N 4 (GCN) nanosheets are prepared by one-step method. • GCN nanosheets are used as the conductive substrate to grow flaccid NiO nanofibers. • The 1.8 V NiO/GCN||N-rGO delivers ~3-times more specific capacitance over NiO||N-rGO ASSASC device. • After 6000 GCD, NiO/GCN||N-rGO shows ~13.4% more specific capacitance retention over NiO||N-rGO ASSASC device. • At high power density, NiO/GCN||N-rGO delivers ~6-times more energy density over NiO||N-rGO ASSASC device. Herein, we report a modified polyol condensation based synthesis of highly-hydrophilic, lowly-crystalline and holey-ultrathin g-C 3 N 4 (GCN) nanosheets, which are further used as the conducting matrix to interfacially grow NiO nanofibers. The physicochemical studies show restricted crystal growth of NiO on GCN, ultra-dispersive nature of NiO/GCN heteronanocomposite in water, and chemical interaction between NiO and GCN. Thorough electrochemical analyses in 3-electrode setup confirm lower equivalent series resistance, charge transfer resistance and diffusion resistance of NiO/GCN heteronanocomposite as compared to pristine NiO. The NiO/GCN heteronanocomposite offers ~2 times more specific capacitance and higher rate capacitance as compared to the pristine NiO. Further, 1.8 V all-solid-state asymmetric supercapacitor (ASSASC) devices are fabricated by using NiO/GCN heteronanocomposite and pristine NiO as the positive electrode materials, and N-rGO as the negative electrode material, and the supercapacitive charge storage efficiencies of the devices are systematically compared. Results show that, the respective mass-specific capacitance and rate capacitance offered by NiO/GCN||N-rGO is ~2 and ~3 times more than that offered by pristine-NiO||N-rGO ASSASC device. The NiO/GCN||N-rGO also offers ~2.7 and ~6 times more energy densities at respective power densities of ~3200 and ~6900 W kg–1, over pristine-NiO||N-rGO ASSASC device. Further, the NiO/GCN||N-rGO retains ~13% more specific capacitance as compare to the pristine-NiO||N-rGO ASSASC device, after 6000 charge-discharge cycles. The largely improved supercapacitive charge storage efficiency of NiO/GCN heteronanocomposite is ascribed to GCN-prompted surface functionality, lowly-impeded electron transport, and greater electrolyte access to the majority of redox-active sites, during high-rate supercapacitive charge storage process. Image, graphical abstract [ABSTRACT FROM AUTHOR]

As promising precursor of functional carbon materials, coal tar pitch was selected as a precursor to prepare porous carbon nanosheets (PCN). Then MnO 2 was incorporated into porous carbon nanosheets to form MnO 2 @PCN by hydrothermal treatment. Both PCN and MnO 2 @PCN showed high specific surface areas (1605 m2/g and 836 m2/g) and nanosheets structures with the thickness of 30 ± 5 nm and 60 ± 10 nm, respectively. When PCN and MnO 2 @PCN were used as electrode materials of supercapacitors, PCN displayed ideal electrical double layer capacitive characteristics and excellent rate performance, but a narrow operating voltage window and relatively low specific capacitance (225 F/g at 0.5 A/g) in 1 M Na 2 SO 4 aqueous; while the MnO 2 @PCN possessed high-rate performance, low charge transport resistance, extended the operating voltage window (−0.2 V–0.8 V) and improved the specific capacitance (377 F/g at 0.5 A/g). Furthermore, the asymmetric supercapacitors fabricated with MnO 2 @PCN as cathode and PCN as anode also exhibited high specific capacitance (87.8 F/g at 0.5 A/g), superior energy density (31.13 Wh/kg at 193.6 W/kg) and good cyclic ability (88.3% after 5000 cycles). This work provided an alternative preparation of high-performance porous carbon composite for resource utilization of coal tar pitch. Image 1 [ABSTRACT FROM AUTHOR]

In the context of low impedance electron transfer in electrochemical charge storage devices, stepped-surfaced solids are important because they provide low work function and suitable interface for electron transfer. Contextually, in this work, for the first time, a carbonate-assisted synthesis method is adopted to fabricate severely stepped-surfaced porous NiCo 2 O 4. Further, N-doped defective rGO (ND-rGO) wrapped NiCo 2 O 4 is assembled, and the electron bedding effect of ND-rGO on the supercapacitive charge storage performance of NiCo 2 O 4 is thoroughly studied. The physicochemical studies authenticate restricted crystal growth of NiCo 2 O 4 on ND-rGO and chemical bonding between ND-rGO and NiCo 2 O 4. Detailed electrochemical studies using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques show redox-mediated charge storage in NiCo 2 O 4 , ND-rGO-induced enhanced high-rate supercapacitive charge storage and lower charge-transfer resistance, equivalent series resistance and Warburg resistance in NiCo 2 O 4 /ND-rGO heteronanocomposite. Further, for the first time, Bi 2 O 3 was used as a super compatible negative electrode material to fabricate asymmetric supercapacitor (ASC) devices based on pristine NiCo 2 O 4 and NiCo 2 O 4 /ND-rGO, and in depth electrochemical characterizations show redox-capacitive charge storage behaviour in both the devices. The NiCo 2 O 4 /ND-rGO||Bi 2 O 3 demonstrates 2-times more areal capacitance at extreme rate condition, lower iR drop, very low charge transfer resistance, solution resistance and ion-diffusion resistance, as compared to NiCo 2 O 4 ||Bi 2 O 3 ASC device. The NiCo 2 O 4 /ND-rGO||Bi 2 O 3 ASC device also retains a very high 93.1% of specific capacitance after 5000 GCD cycles at an extreme applied current density of 32 mA cm−2. Further, the NiCo 2 O 4 /ND-rGO||Bi 2 O 3 offers twice the energy density as compared to NiCo 2 O 4 ||Bi 2 O 3 ASC device, at elevated power density up to ∼10,000 Wh kg−1. The enhanced supercapacitive performance of NiCo 2 O 4 /ND-rGO||Bi 2 O 3 ASC device is ascribed to electron bedding effect of wrapped ND-rGO which provides a continuous conducting network, ND-rGO induced improved wettability, activated NiCo 2 O 4 |ND-rGO interface, and excellent electromechanical properties of NiCo 2 O 4 /ND-rGO heteronanocomposite. Image 1 • N-doped defective-rGO (ND-rGO) wrapped stepped-surfaced-NiCo 2 O 4 is prepared using (NH 4) 2 CO 3 as a novel precipitating agent. • Wrapped ND-rGO acts as a virtual electron bed for unimpeded electron transfer during supercapacitive charge storage process. • Bi 2 O 3 is used as a negative electrode material for the fabrication of NiCo 2 O 4 /ND-rGO||Bi 2 O 3 ASC device. • The NiCo 2 O 4 /ND-rGO||Bi 2 O 3 ASC device shows enhanced capacitance and lower charge transfer & ion-diffusion resistance. • At high power density, the ND-rGO/NiCo 2 O 4 ||Bi 2 O 3 delivers 200% more energy density than the NiCo 2 O 4 ||Bi 2 O 3 ASC device. [ABSTRACT FROM AUTHOR]

CoMoO 4 with oxygen vacancy (CoMoO 4-x) porous nanosheet is prepared by hydrothermal synthesis and hydrogenation reduction method. The CoMoO 4-x porous nanosheet has a higher specific surface area together with a more diverse pore size distribution than CoMoO 4 nanosheet. The density functional theory calculation result exhibits that CoMoO 4-x has lower energy band gap (0.041 eV) than CoMoO 4 (1.683 eV). The electronic density of states plots reveals that CoMoO 4-x has more electron state distribution at Fermi energy level than CoMoO 4. CoMoO 4-x porous sheet achieves higher specific capacitance (1989 g−1) than CoMoO 4 (917 F g−1) at 2 mA cm−2. It also achieves superior capacitance retention rate (91.3%) than CoMoO 4 (83.9%) with the current density increasing from 2 to 20 mA cm−2. The introduction of oxygen vacancy can increase the carrier density, accelerate the electron transfer, promote the electrical conductivity, and accordingly strengthen the redox reactivity of CoMoO 4-x. An asymmetric supercapacitor is also constructed by using CoMoO 4-x as positive electrode and activated carbon as negative electrode. It achieves high energy density of 62.3 W h kg−1 at a power density of 800 W kg−1, together with a good cyclic life. Both experimental measurement and theorical calculation are applied to prove the promotive role of oxygen vacancy in supercapacitive performance of CoMoO 4-x. [ABSTRACT FROM AUTHOR]

While Electrical Double-Layer Capacitors (EDLC's) have been proven to achieve round-trip efficiencies of over 95%, electrical efficiencies of Capacitive Deionization (CDI) systems are still far from those values. Since this issue seems to be crucial for CDI to commercially compete with other water treatment processes including Reverse Osmosis (RO) it is essential that these efficiencies be improved. In this work, the electrical response of a commercial EDLC's and a CDI system are compared. Results of asymmetric constant current experiments reveal that the main electrical inefficiency occurs during the discharge stage (i.e. in CDI this refers as salt desorption or electrode's regeneration). Here, we propose an operational mode of CDI focused on improving performance of regeneration by simply conducting this stage in a more concentrated solution (3.5M NaC) brine water. The experimental findings suggest that this operational strategy led to 1) an increase from ≈40% to 75% in the round-trip efficiency of CDI, approaching (although admittedly not reaching) those of EDLC's; 2) a drop in ohmic resistance in the discharge cycle of 92% (from 19.77 Ω to 1.45 Ω) consequently increasing the amount of energy that could be potentially recovered from 0.24 J to 0.50 J, and thus, reducing significantly the energy consumption of the process; 3) a wider range of deionization/regeneration current density ratios (0.1–1.25) in which high efficiency values (65–75%) are still maintained. In general, we find that this operational mode might have practical relevance as it implies that CDI systems may be operated with a higher degree of flexibility allowing them to be coupled more readily with renewable energy technologies. Furthermore, the use of a highly concentrated electrolyte (i.e. not deionized water) during regeneration offers an innovative solution for brine concentration. Image 1 • The electrical performance of a CDI flow-cell and an EDLC were compared. • Low round-trip efficiency of CDI cell was mainly associated to discharge step. • New operational mode based on electrolyte replacement was successfully tested. • Round-trip efficiency of CDI can be boosted up to 75%. • Wider range of charge/discharge current density ratios can be used. [ABSTRACT FROM AUTHOR]