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[en] This work describes the application of a new electrochemical technique for online monitoring of electrosynthesis using fast Fourier transform continuous cyclic voltammetry (FFTCCV). The electrochemical system included two compartments, a batch electrosynthesis cell and a flow analysis system. In the electrosynthesis cell, the formation of 4-nitrocatechol (4NC) took place via reaction of o-benzoquinone (o-BQ) with nitrite ion. In the flow cell, FFTCCV was used for monitoring the concentration of catechol at a gold ultramicroelectrode. Each measurement was done at the potential range −1.00–0.40 V at a scan rate of 10 V/s for 8000 s, in which the electrode signal was calculated by integrating the current in the selected potential range. Also, using FFTCCV technique, an intermediate was detected and used to find the mechanism of the electrochemical reaction. The redox peak of 4NBQ to 4NC was observed at +0.27 V, whereas the peaks of 4-nitrosocatechol (4NSC) and the intermediate were detected at −0.39 V. The results showed that the used electrochemical system is very useful for online monitoring and investigation of fast electrochemical reactions.
[en] A comprehensive research coupling experiment and computation has been performed to understand the phase transition of Na3SbS4 and to synthesize cubic Na3SbS4 (c-Na3SbS4), a high temperature phase of Na3SbS4 that is difficult to be preserved when cooled down to ambient temperature. The formation of c-Na3SbS4 is verified by Rietveld refinement, nuclear magnetic resonance spectroscopy as well as electrochemical impedance spectroscopy. Unlike tetragonal Na3SbS4 (t-Na3SbS4) appearing phase transition at high temperature, c-Na3SbS4 is stable not just at room temperature but also sustaining thermal cycling up to at least 200 °C. Both experiment and theoretical calculation reveal that the ionic conductivity of c-Na3SbS4 is higher than that of t-Na3SbS4, though the values are in the same order of magnitude. Both structures allow fast ion transport. All-solid-state cells with c-Na3SbS4 solid electrolyte demonstrate superior Coulombic efficiency, high specific capacity, and relatively good cycling stability. Na3SbS4 solid electrolyte is promising for all-solid-state sodium-ion batteries.
[en] Highlights: • An ultrasensitive non-enzymatic glucose sensors based on controlled petal-like CuO nanostructure were obtained. • The best as-prepared CuO electrode (Nafion/CuO8/GCE) showed an excellent sensitivity of 2634.44 μA cm−2 mM−1 in within a wide linear range from 0.5 μM to 2.67 mM. • The Nafion/CuO8/GCE electrode also shows excellent limit of detection, fast response time, good reproducibility and long duration stability. • The outstanding properties of Nafion/CuO8/GCE electrode may ascribe to (i) the morphologies (ii) strong binding of Tween 80 (iii) rough surface. - Abstract: In this work, an ultrasensitive and highly stable non-enzymatic glucose sensor is developed with CuO nanostructures through a facile, low price, wet-chemistry route. Three kinds of CuO nanostructures are obtained by kinetic control. The best as-prepared CuO electrode with rough petal-like structure exhibits a quantitative sensitivity as high as 7546.37 μA cm−2 mM−1 and an ultra low limit detection 0.259 μM, which are much better than some previously reported. The rough petals might augment the contact area for kinetic mass transfer between CuO nanostructures and the electrode in an alkaline environment, which significantly improves the performances of electro-oxidation of glucose. Furthermore, the as-fabricated non-enzymatic glucose sensors have also shown excellent anti-interference property, fast response time, good reproducibility and long duration stability. Thus, as-fabricated CuO electrode can be a promising material in the application of non-enzymatic glucose sensors.
[en] The redox reaction of vanadium ions (V2+↔3+↔4+↔5+) on well-developed nitrogen-doped ordered mesoporous carbon (NOMC) was extensively investigated in different electrolyte solutions by electrochemical methods. It is found that both the electronic structure modulated by nitrogen doping and the enriched electrochemically active functional groups on NOMC favor the three electrochemical transitions between the adjacent couples, viz. V2+↔3+↔4+↔5+, as compared with Vulcan XC72 carbon black. Salient findings are as follows. First, the transition of V3+↔2+ is the same on the two distinctly different carbons, which indicates that this reaction is an outer-sphere charge transfer reaction. The concomitant hydrogen evolution reaction makes NOMC unsuitable to be used as a negative electrode material in flow batteries. Second, the transition of V5+↔4+ shows a quasi-reversible behavior, indicating that NOMC can be used as a positive electrode material. Simulation of cyclic voltammogram (CV) reveals that the standard rate constant and the adsorption equilibrium constant are (7.0 ± 0.9)*10−3 cm s−1 and 0.70 ± 0.09 (both V4+ and V5+), respectively. Third, the transition of V4+↔3+ is recognized in the CV curve, which proceeds in a quasi-reversible reaction. The preceding adsorption of the symmetrical ions (V3+) is found to play a key role in determining the kinetics. Finally, for the two latter transitions, the content of dopant nitrogen yields a negligible effect on the electrochemical activity, excluding the possibility of its direct involvement in electrocatalysis. The above findings not only reveal the applicability of the nitrogen-doped carbon to be used as an electrode material in flow batteries, but also offer an in-depth understanding of the reaction mechanism of vanadium redox couples.
[en] Bifunctional polymer binders featured with both strong binding and superior polysulfide trapping properties are highly desired for the fabrication of sulfur cathodes with suppressed polysulfide shuttling in Li–S batteries. In this paper, we have explored the potential of a quaternary ammonium cationic polymer, polydiallyldimethylammonium (PDADMA-X; X = T, B, P, and Cl) with different counter anions (TFSI–, BF4−, PF6−, and Cl−, respectively) as the bifunctional binder. We have also revealed the dramatic effects of the counter anion on the performance of the cationic polymer binder. PDADMA-X's containing the former three weakly associating anions have been demonstrated to show polysulfide adsorption capability. In particular, PDADMA-T having the largest, least interacting TFSI– anion shows the optimum performance, with strong binding strength and the best polysulfide adsorption capability. Relative to commercial PVDF and PDADMA-X's of other counter anions, it offers sulfur cathodes with lowered polarization, higher discharge capacity, significantly better capacity retention, and improved cycling stability. With its convenient synthesis from commercially available PDADMA-Cl, cationic PDADMA-T having the TFSI– anion is a promising bifunctioal binder for sulfur cathodes in practical Li-sulfur batteries.
[en] Highlights: • A simple in situ hydrothermal process has been proposed for an effective carbon coating and preparation of mesoporous LFP composite. • The in situ carbonization method influences the preferred crystal orientation growth. • By suitable choice of the precursors, the dispersity and homogeneity of the residual carbon coating can be regulated. • The enhancement in electrochemical performance is ascribed to the synergistic interactions between LFP, TA and glycerol. - Abstract: An in situ hydrothermal synthesis process was explored to prepare nano-sized high performance lithium iron phosphate carbon composite (LFPin/C) as active cathode material for lithium ion batteries. Tartaric acid (TA), as chelating agent and carbon source, was added into glycerol/water solution forming a homogeneous precursor. The mixture was transferred to a hydrothermal reactor to take full advantage of the synergistic interaction between both organic compounds in the synthesis of LFPin/C. For comparison, the properties of the ex-situ synthesized LFPex/C composite, obtained by addition of TA after the hydrothermal step, were evaluated. Results of comparative experiments show that the in situ method is capable to improve the homogeneity and dispersity of the residual carbon in combination with calcination at 600 °C for 3 h. Cyclic voltammetry and electrochemical impedance spectroscopy showed that the in situ generated carbon matrix with embedded LiFePO4 particles can reduce the charge transfer resistance. LFPin/C features a large specific surface area of 22.3 m2 g−1 and uniform particle size distribution with a typical size in the range of 20–40 nm. In line with these advantages, the composite yields excellent cycle stability with initial discharge capacities of 166.1, 140 and 104 mAh g−1 at rates of 0.1 C, 2 C and 10 C, respectively.
[en] Highlights: • We report a facile strategy to prepare sandwiched graphene/polyaniline electrodes. • The flexible supercapacitor shows large specific capacitance of 1506.6 mF/cm2. • After 1000 bending cycles, the capacitance of the device remains 95.8%. - Abstract: Mechanically flexible supercapacitors with fast charge-discharge capacity, high stability and good mechanical properties are essential for powering flexible electronics devices. In this paper, we report a simple process to obtain composite electrode consisting of graphene and polyaniline on stainless steel fabric. The all solid-state flexible supercapacitors based on the composite electrode exhibit a maximal specific capacitance of 1506.6 mF/cm2, and capacitance retention of 92% after 5000 charge-discharge cycles. Furthermore, the bending test shows that the flexible supercapacitor maintains 95.8% of the original capacitance after 1000 bending cycles. The high capacitive performance and anti-bending property of the supercapacitor are ascribed to the favourable microstructure of graphene/polyaniline composite and high flexibility of stainless steel fabric. Our work demonstrates a novel type of high-performance flexible electrode, which will boost the development of flexible and wearable electronics and integrated fabric power devices.
[en] Highlights: • A single step, low temperature and time consuming method adopted to prepare material. • Solvents viscosity, saturated vapor pressure, and polarity influenced on morphology. • Nanoflake encouraged large charge storage than polyhedron flower morphology. • Battery-type charge storage process confirmed using Power's law. • Assembled device showed only 15% capacitance loss over 4000 cycles. - Abstract: Layered materials provide good electrochemical performance, but insufficient rate capability, which is the main issue in energy storage. Herein, we propose a facile synthesis of Co2(CO3)(OH)2 nanoflakes and polyhedron flowers supported on Ni foam, a novel binder-free electrode. Power law revealed that Co2(CO3)(OH)2 stores charge by a battery-type mechanism at the peak potential. The nanoflakes store more internal surface charge than the polyhedron-flower, which was confirmed via Trasatti plot. Benefiting from amorphous nanostructure, unique morphology and high surface area, the nanoflakes shows good performance. The areal capacitance (2111 mF cm−2), rate capability (80%), and energy density (0.152 mWh cm−2) are comparable to recent reports. The results suggest that the amorphous Co2(CO3)(OH)2 nanoflakes are a suitable cathode candidate for the supercapattery. The assembled supercapattery (ASC) provides high specific capacitance (91 F g−1), high energy density (26.22 Wh kg−1 at power density 828 W kg−1), and long cycle life (specific capacitance retention of 85% over 4000 cycles). The ASC device shows good potential in the field of energy storage devices.
[en] We study the electrokinetic transport behavior of water molecules and ions in hydrophobic graphene nanochannels with variable surface charge densities as well as the interfacial water structure based on detailed molecular dynamics simulations. The interfacial water structure, described by the water density, hydrogen bonding, diffusion, distribution of the OH bond and dipole orientations, is strikingly influenced by the surface charge sign and density. We find anomalous electrokinetic effects which are related to the distribution of counterions close to the surface, ion-specific effects and the interfacial water structure. On a negatively charged graphene layer, the attraction of Na+ ions towards the surface enhances the interfacial friction. In contrast, if the surface is positively charged, high surface charge density triggers an anomalous enhancement of electroosmotic flow, accompanied by an abrupt change of the interfacial water structure. At high surface charge densities, the mobility of the interfacial water at the positively charged surfaces is suppressed more strongly compared to the negatively charged surface. Our results reveal new electrokinetic phenomena by the comparison of negatively and positively charged surfaces.
[en] It is important to prepare an environmental-friendly and earth-abundant electrocatalyst with excellent performance and superior stability for efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a facile precursor route is developed to synthesize CoP nanostructure with different morphologies just by controlling the hydrothermal reaction temperature and time. The precursor Co(CO3)0.5OH·0.11H2O (CHCH) nanowire (NW) and uniform flower-like microball (MB) with exposed (001) facet were firstly successfully prepared at hydrothermal reaction temperature of 115 °C for 3 h and 10 h, respectively. And then a phosphorization treatment of CHCH was performed to get CoP NWs and flower-like CoP MBs assembled with ordered NWs that exposed (002) facets. The CoP MBs exhibit a lower over-potential of 105.3 mV at the current density of 10 mA cm−2, and a smaller Tafel slope of 53.5 mV·dec−1 in 0.5 M H2SO4 solution for HER. Furthermore, this hierarchical structure could possess its good activity for 40,000 s. The catalyst also shows an excellent activity with the overpotential of 97.3 mV at 10 mA cm−2, and a Tafel slope of 84.3 mV·dec−1 in 1 M KOH solution for HER. The density functional theory (DFT) calculations further reveal that hydrogen adsorbed on all P sites of CoP could result in smaller adsorption energy (0.085 eV), which is comparable to Pt. So, all P sites are the main active sites for HER. Also, these flower-like CoP MBs could display a good electrocatalytic activity for OER. Compared to disordered CoP NWs, the flower-like CoP MBs exhibit a better HER and OER performance because this hierarchical structure possessed two merits, that is, the fast vectorial electron transfer along the ordered NWs and improved inherent activity of each active site due to the exposed (002) facet. This low-cost and high-activity hierarchical structure is an efficient catalyst for HER and OER.