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Potassium‐ion hybrid capacitors have attracted increasing attention due to good energy density, high power density, and low cost. Ti3C2Tx‐MXene is considered as a promising anode material for K ion storage. However, undesirable stacking issues decrease its exposed area and breeds sluggish K ion transport. Herein, a facile spray‐lyophilization strategy is proposed to construct stacking‐resistant Ti3C2Tx with 3D structures. As‐prepared Ti3C2Tx hollow spheres/tubes present stack resistance, a large specific surface area, and a short ion diffusion pathway. When serving as an anode material, it shows enhanced capacity and thickness‐independent rate performance compared to 2D Ti3C2Tx. After 10 000 cycles, a specific capacity of 122 mAh g−1 is obtained at 1 A g−1. Systematic kinetics analyses demonstrate the significance of concentration polarization on the electrode's rate ability. Furthermore, a 3D Ti3C2Tx‖hierarchical porous activated carbon (HPAC) K‐ion hybrid capacitor is assembled and displays remarkable energy and power densities with energy retention of 100% after 10 000 cycles at 1 A g−1. Following this strategy, other 3D structures from nanosheets can also be obtained, such as 3D Ti3C2Tx microtubes and graphene oxide nanoscrolls. This study provides a viable approach to solve the stacking issues of 2D nanosheets to promote the application of 2D materials. [ABSTRACT FROM AUTHOR]

Summary: Sodium‐ion hybrid capacitors are a prospective energy storage device candidate that couples the superiorities of battery‐type and capacitive storage mechanisms. In this study, we fabricate a composite of NiCo2O4 nanowires with carbon tubular bundles (CTBs) via a facile hydrothermal and annealing procedure. The density functional theory (DFT) calculations are performed to evaluate the bonding strength between the two components of the composite, the binding energy of the NiCo2O4 is calculated to be −0.952 J m−2, indicating that the NiCo2O4 nanowires can be stabilized on both sides of the carbon tubular bundles, which leads to a good cycling performance. Moreover, the composite in this work exhibits a metallic property because of the introduction of carbon material. As expected, when used for sodium storage, the NiCo2O4/CTBs shows a high capacity of 298 mA h g−1 at 1 A g−1 and high capacity retention of 92% after 500 cycles, which are superior than the bare NiCo2O4 electrode. Consequently, the sodium‐ion hybrid capacitor is also assembled with NiCo2O4/carbon tubular bundles and commercial activated carbon, which achieves high energy density of 99 Wh kg−1 in a wide potential range from 0.5 V to 4.0 V. [ABSTRACT FROM AUTHOR]

Fe-N-C catalysts were prepared through metal-assisted polymerization method. Effects of carbon treatment, Fe loading, nitrogen source, and calcination temperature on the catalytic performance of the Fe-N-C for HO electroreduction were measured by voltammetry and chronoamperometry. The Fe-N-C catalyst shows optimal performance when prepared with pretreated active carbon, 0.2 wt.% Fe, paranitroaniline (4-NA) and one-time calcination. The Fe-N-C catalyst displayed good performance and stability for electroreduction of HO in alkaline solution. An Al-HO semi-fuel cell was set up with Fe-N-C catalyst as cathode and Al as anode. The cell exhibits an open-circuit voltage of 1.3 V and its power density reached 51.4 mW cm at 65 mA cm. [ABSTRACT FROM AUTHOR]

Abstract: Direct carbon fuel cells are promising power sources using solid carbon directly as fuel. Their performances significantly depend on the electrooxidation activity of carbon fuel. Electrooxidation of activated carbon particulates in molten Li2CO3–K2CO3 was investigated by potentiodynamic and potentiostatic method. Results indicated that the electrooxidation performance of activated carbon was significantly enhanced by pre-soaking with Li2CO3–K2CO3 and by treatment with HF, HNO3 and NaOH, respectively. The onset potential negatively shifted by around 100 mV and the current density increased by around 50 mA cm−2 after pre-soaking. The non-oxidant acids (HF) treatments are more effective than oxidant acid (HNO3) and base (NaOH) treatments. HF treated activated carbon exhibited the highest activity among all the samples. The enhancement in electrooxidation performance can be closely correlated with the increase in surface area and porosity caused by acid and base treatments. [Copyright &y& Elsevier]

Composites of LiFePO4 and activated carbon (LFP-AC) were prepared by a ball milling method. Their morphologies were investigated by scanning electronic microscopy and energy dispersive spectroscopy. Hybrid supercapacitors were assembled using the LFP-AC composites as the positive electrode and Li4Ti5O12 as the negative electrode (LFP-AC/Li4Ti5O12). Effects of the positive electrode composition (ratio of LiFePO4 to AC) on the performance of LFP-AC/Li4Ti5O12 were investigated via constant current charge-discharge tests. The results demonstrated that the specific capacity and the specific power of the LFP-AC/Li4Ti5O12 hybrid supercapacitor significantly depend upon the content of LiFePO4 in the LFP-AC positive electrode. At 0.5 A g-1 charge-discharge current, the specific capacity of the LFP-AC/Li4Ti5O12 with 30 wt. % LiFePO4 and 70 wt. % AC in the positive electrode is 42% higher than that of AC/Li4Ti5O12, and the two supercapacitors have the same specific power. After 500 charge-discharge cycles at 1.0 A g-1, the specific capacity of the LFP-AC/Li4Ti5O12 with 80 wt. % LiFePO4 and 20 wt. % AC in the positive electrode is 53% higher than that of AC/Li4Ti5O12. The hybrid device stores electric energy via both the double-layer electrostatic adsorption and the intercalation/deintercalation of lithium ions. [ABSTRACT FROM AUTHOR]

Abstract: Ni(OH)2 nanosheets are directly grown on nickel foam by a simple template-free growth process. Their microstructure and surface morphology are studied by X-ray diffraction spectroscopy and scanning electron microscopy. The XRD and SEM results show that Ni(OH)2 has a β-phase structure and covers the nickel foam skeleton with nanosheets. This β-Ni(OH)2/Ni-foam electrode exhibits a high specific electric quantity of 790.3 C g−1 approaching the theoretical value (1040.6 C g−1) and high electrochemical activity. Asymmetric supercapacitor has been fabricated successfully using β-Ni(OH)2/Ni-foam nanosheets as positive electrode and activated carbon as negative electrode in a KOH aqueous electrolyte. The electrochemical capacitances of this supercapacitor are investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. An asymmetric supercapacitor AC/6 mol L−1 KOH/β-Ni(OH)2/Ni-foam could be cycled reversibly in the high-voltage region of 0–1.6 V and displays intriguing performances with a specific capacitance of 105.8 F g−1 and high energy density of 36.2 W h kg−1. Importantly, this asymmetric supercapacitor device exhibits an excellent long cycle life along with 92% specific capacitance retained after 1000 cycles. [Copyright &y& Elsevier]

Binary transition metal sulfides are considered to be promising electrode materials for high-performance supercapacitors because of their ultrahigh electron conductivity and superior electrochemical activity compared to the unitary transition metal sulfides. In this work, copper cobalt sulfides nanowire arrays on nickel foam with different element ratios and porosities are prepared by using a two-step hydrothermal method, and their electrochemical performance is explored. Benefited from the high electrical conductivity, rich active sites and optimized microstructure, the optimized copper cobalt sulfides (CuCo 2 S 4) electrode can achieve a ultrahigh specific capacitance of 2446.6 F·g−1 at 1 A·g−1 and favorable rate performance. The asymmetric supercapacitor assembled with the CuCo 2 S 4 nanowire array and activated carbon (AC) electrodes displays a high energy density of 33.4 Wh·kg−1 at a power density of 751.5 W·kg−1 can be obtained. Additionally, the assembled asymmetric cell shows a favorable capacitance retention of 78% after charging and discharging for 10,000 cycles at a high current density of 4 A·g−1. • The CuCo 2 S 4 electrode achieves an ultrahigh specific capacitance of 2446 F·g−1 at 1 A·g−1. • The CuCo 2 S 4 electrode has unique microstructure and exhibits good rate performance. • The CuCo 2 S 4 /AC-ASC displays high energy density of 33.4 Wh·kg−1 at power density of 751.5 W·kg−1. [ABSTRACT FROM AUTHOR]

The flexible solid-state supercapacitor (FSSC) has attracted many attentions due to its feature of high power density with stable cycling performance, as well as lightweight and flexible features and is expected as an ideal candidate power supply for portable and wearable electronics devices. In general, the electrode plays an important role in determining the overall performance of the FSSC. Therefore, to meet the requirement of practical applications, a simple, facile and environment friendly strategy to prepare deformable/flexible electrodes with high energy density is the target issue to be considered. Herein, we report a generalized “soaking-recrystallization-calcination” method to enhance the capacitance performance of commercialized carbon fiber cloth. Thanks to the in-situ etching process to increase the surface area and the synchronous heteroatom doping to induce pseudocapacitive behavior, the resultant activated carbon fiber cloth sample exhibits an ultra-high areal capacitance up to 362 mF cm −2 , is about 1448 fold enhancement than that of fresh carbon fiber cloth calcined at 750 °C. Meanwhile, the assembled FSSC based on the activated carbon fiber cloth exhibits a maximum volumetric energy density of 0.35 mWh cm −3 and gravimetric energy density of 0.84 Wh kg −1 . Such results represent a novel and promising direction to prepare high performance flexible electrode for FSSC application. [ABSTRACT FROM AUTHOR]

In this work, Co(OH) 2 buds direct growth on Ni foam (Co(OH) 2 buds/Ni foam) is prepared via a hydrothermal method. Their structure and morphologies are characterized by using X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. Result shows that the Co(OH) 2 buds are assembled by several nanorods and uniformly covered on the surface of Ni foam. Due to its unique structure, the Co(OH) 2 buds/Ni foam electrode shows high capacitive performance and long cycle life. The specific capacitance of Co(OH) 2 buds/Ni foam is as high as 2041 F g −1 at a current density of 3 mA cm −2 in 6 M KOH electrolyte, and 811 F g −1 even at a high current density of 60 mA cm −2 . The capacitance of the Co(OH) 2 buds/Ni foam electrode remained 72.4% after 1000 cycles at 18 mA cm −2 . An asymmetric supercapacitor was successfully assembled, in which Co(OH) 2 buds/Ni foam and AC used as positive and negative electrode, respectively. The energy density of the AC//Co(OH) 2 buds/Ni foam supercapacitor reaches to 20.3 W h kg −1 at a power density of 90.6 W kg −1 . Our result shows that the Co(OH) 2 materials are promising candidate for electrochemical energy applications. [ABSTRACT FROM AUTHOR]

K-ion batteries (KIBs) are widely considered as a promising alternative to Li-ion batteries (LIBs) due to the cost effective and earth abundant features of K. However, identifying a potential anode for KIBs showing good capacity and stability is still challenging. Because traditional graphite suffers from poor stability owing to the large volume expansion during the insertion/extraction of K ions. Non-graphite carbon is a good candidate because of the disordered nature. A facile high temperature pyrolysis method was adopted to obtain bio-derived carbon materials. Among different kinds of biomass precursors, the porous dandelion seed with nitrogen doping shows the best electrochemical performance with a specific capacity of 168 mAh g−1. The nitrogen-doped carbon presents good K ion adsorption capability and enhanced wettability, which is demonstrated in Fig.S1. Moreover, after introducing activation agent (KOH) in the carbonization process, the capacity in improved to 243 mAh g−1. Meanwhile, the as-prepared carbon material delivers good capacity retention of 91% after 100 cycles. Therefore, the hierarchically porous doped carbon derived from renewable biomass precursor shows great economical potential as KIBs anode. Unlabelled Image • The bio-derived carbon activated by KOH maintain maintain an integrated porous microstructure. • The heteroatoms-doped bio-derived carbon exhibits high specific capacity as K ions anode. • The abundant bio-mass precursor enables large scale preparation of the carbon materials. [ABSTRACT FROM AUTHOR]

MnO 2 is regarded as an ideal material of supercapacitor since its low-cost, environment friendly and high specific capacitance but hindered by its poor electrical conductivity. Developing a composite electrode that combines nano-structure MnO 2 with a conductive skeleton such as carbon materials could make up for the shortcomings. Here, porous activated carbon (PAC) is synthesized by using low-cost wheat bran as biomass carbon precursor and a mixture of NaCl/ZnCl 2 as combined solvent-porogen. The resultant PAC sample presents a hierarchical porous structure and large specific surface area up to 1058 m2 g−1. Afterwards, MnO 2 nanosheets decorated PAC (MnO 2 @PAC) is prepared via an in-situ hydrothermal deposition. It is a key finding that the ion/electron transfer kinetics of MnO 2 @PAC could be effectively improved by the addition of hierarchical porous carbon. Thus, the MnO 2 @PAC electrode displays a high specific capacitance (258 F g−1 at 1 A g−1) and superior rate performance (82.8% capacitance retention with the current density ranging from 1 A g−1 to 20 A g−1). Furthermore, an asymmetric supercapacitor is assembled by employing the MnO 2 @PAC as the positive electrode and PAC as negative electrode, which exhibits a high energy density of 32.6 Wh kg−1 and as well as 93.6% capacity retention at over 10,000 charge/discharge cycles. • Porous activated carbon (PAC) is synthesized by a controllable molten salt method. • MnO 2 nanosheets were deposited on the PAC through a simple hydrothermal reaction. • The MnO 2 @PAC displays high specific capacitance and outstanding stability. • The PAC//MnO 2 @PAC asymmetric supercapacitor exhibits energy density of 32.6 Wh kg−1. [ABSTRACT FROM AUTHOR]