No abstract

Owing to excellent metallic conductivity, hydrophilic surfaces, and surface redox properties, a two-dimensional (2D) metal carbide of Ti3C2T x -MXene could serve as a promising pseudocapacitive electrode material for energy storage devices. Meanwhile, the 2D reduced graphene oxide (rGO) combining with the hierarchical cubic spinel nickel–cobalt bimetal oxide (NiCo2O4) nanospikes could control ion diffusion for charge storage, thereby facilitating the improvement of the energy density of a supercapacitor. As per the strategy, the pseudocapacitive 2D Ti3C2T x was loaded on a flexible acid-treated carbon fiber (ACF) backbone to prepare a Ti3C2T x /ACF negative electrode by a convenient drop-casting method. Meanwhile, 2D rGO was deposited on ACF by a simple dip-dry process, which was further decorated by the spinel NiCo2O4 nanospikes using a hydrothermal method to obtain a NiCo2O4@rGO/ACF positive electrode. The fabricated Ti3C2Tx/ACF electrode exhibited an excellent specific capacitance of 246.9 F/g (197.5 mF/cm2) at 4 mA/cm2 along with 96.7% capacity retention over 5000 charge/discharge cycles, whereas the NiCo2O4@rGO/ACF electrode showed a specific capacitance of 1487 F/g (458.3 mA h/g) at 3 mA/cm2 with a cycling stability of 88.2% over 10 000 charge/discharge cycles. As a result, a flexible all-solid-state hybrid supercapacitor (FHSC) device using the pseudocapacitive Ti3C2T x /ACF on the negative side with a widespread voltage window and the battery-type NiCo2O4@rGO/ACF on the positive side with high electrochemical activity delivered an excellent volumetric capacitance of 2.32 F/cm3 (141.9 F/g) at a current density of 5 mA/cm2 with a high-energy density of 44.36 Wh/kg (0.72 mWh/cm3) at a power density of 985 W/kg (16.13 mW/cm3) along with a cycling stability of 90.48% over 4500 charge/discharge cycles. Therefore, the pseudocapacitive 2D Ti3C2T x /ACF negative electrode could replace carbon-based electrodes and a combination of it with the battery-type NiCo2O4@rGO/ACF positive electrode should be a promising way to step up the energy density of a supercapacitor.

Conversion of CO 2 into valuable chemicals via electrochemical CO 2 reduction reaction (CO 2 RR) is a promising technology to alleviate the energy crisis and the greenhouse effect. Herein, low-cost wood biomass was applied as the carbon source to prepare nitrogen (N)-doped carbon electrocatalysts for the conversion of CO 2 to CO and further as the cathode material for Zn−CO 2 batteries. By virtue of N-doping and assistance of FeCl 3 , a cedar biomass-derived three-dimensional (3D) N-doped graphitized carbon with a high N-doping content (5.38%), an ultrahigh specific surface area (1673.6 m 2 g −1 ), rich nanopores, and sufficient active N sites was successfully obtained, which exhibited super CO 2 RR activity with a high faradaic efficiency of 91% at a low applied potential of 0.56 V (vs RHE) and a long-term stability for at least 20 h. Furthermore, a Zn−CO 2 battery with it as the cathode material delivered a stable open circuit voltage of 0.79 V, a peak power density of 0.51 mW cm −2 at 2.14 mA cm −2 , and a maximum faradaic efficiency to CO of 80.4% at 2.56 mA cm −2 , indicating that it could be applied in a practical process by using CO 2 to generate power with the production of CO. Density functional theory calculations revealed that pyridinic N could more effectively decrease the free energy barriers for CO 2 RR and boost the reaction. This work not only revealed a facile approach to convert waste biomass into N-doped-graphitization carbon as valuable CO 2 RR electrocatalysts but also provided a new strategy to achieve "carbon solving carbon's problem".

In present investigation, we have prepared a nanocomposites of highly porous MnO2 spongy balls and multi-walled carbon nanotubes (MWCNTs) in thin film form and tested in novel redox-active electrolyte (K3[Fe(CN)6] doped aqueous Na2SO4) for supercapacitor application. Briefly, MWCNTs were deposited on stainless steel substrate by “dip and dry” method followed by electrodeposition of MnO2 spongy balls. Further, the supercapacitive properties of these hybrid thin films were evaluated in hybrid electrolyte ((K3[Fe(CN)6 doped aqueous Na2SO4). Thus, this is the first proof-of-design where redox-active electrolyte is applied to MWCNTs/MnO2 hybrid thin films. Impressively, the MWCNTs/MnO2 hybrid film showed a significant improvement in electrochemical performance with maximum specific capacitance of 1012 Fg−1 at 2 mA cm−2 current density in redox-active electrolyte, which is 1.5-fold higher than that of conventional electrolyte (Na2SO4). Further, asymmetric capacitor based on MWCNTs/MnO2 hybrid film as positive and Fe2O3 thin film as negative electrode was fabricated and tested in redox-active electrolytes. Strikingly, MWCNTs/MnO2//Fe2O3 asymmetric cell showed an excellent supercapacitive performance with maximum specific capacitance of 226 Fg−1 and specific energy of 54.39 Wh kg−1 at specific power of 667 Wkg−1. Strikingly, actual practical demonstration shows lightning of 567 red LEDs suggesting “ready-to sell” product for industries.

The current problem of the relatively low energy density and cycling stability of supercapacitors can be effectively addressed by designing an all-solid-state high performance asymmetric supercapacitor (ASSCs) using chemically deposited beta-manganese oxide (β-MnO2) as a positive active electrode and orthorhombic-tin sulfide (O-SnS) as a negative electrode material on a stainless steel (SS) substrate with a poly­(vinyl alcohol)-lithium perchlorate (PVA-LiClO4) solid gel polymer electrolyte and as a separator. Time-dependent surface morphological modification and its subsequent effect on electrochemical performance of the O-SnS negative electrode has been examined. Electrodes prepared at deposition time periods of 120, 240, and 360 min provide specific surface areas (SSA) of 36.7, 78.3, and 65.6 m2 g–1, respectively. Superior nanostructures of both the β-MnO2 and O-SnS electrodes offer high specific capacitance (C s) of 994 and 1203 F g–1 at 5 mV s–1 and energy and power densities (ED and PD) of 83.3, 69.2 Wh kg–1 and 10 and 1.8 kWh kg–1, respectively. A fabricated ASSCs device exhibits C s of 122 F g–1, ED of 29.8 Wh kg–1, and PD of 1.25 kWh kg–1, with capacity retention of 95.3% up to 5000 charge–discharge cycles. Impressively, such two series assembled ASSCs can light up light-emitting diodes (LEDs) after 30 s of charge.