Retaining soluble polysulfides in the sulfur cathodes and allowing for deep redox are essential to develop high-performance lithium–sulfur batteries. The versatile textures and physicochemical characteristics of abundant biomass offer a great opportunity to prepare biochar materials that can enhance the performance of Li–S batteries in sustainable mode. Here, we exploit micro-/mesoporous coconut shell carbon (CSC) with high specific surface areas as a sulfur host for Li–S batteries. The sulfur-infiltrated CSC materials show superior discharge–charge capacity, cycling stability, and high rate capability. High discharge capacities of 1599 and 1500 mA h g–1 were achieved at current rates of 0.5 and 2.0 C, respectively. A high reversible capacity of 517 mA h g–1 was retained at 2.0 C even after 400 cycles. The results demonstrate a high retention and a deep lithiation of the CSC-confined sulfur. The success of this strategy provides insights into seeking high-performance biochar materials for Li–S batteries from abundant bioresources.Keywords: biomass; coconut shell carbon; lithium−sulfur battery; sulfur cathode; sulfur retention;

•Both composition and morphology of NiCu nanoalloys are tunable with current density.•Electrocatalytic activity highly depends on composition and morphology.•Dendritic nanopearl chains in 1:1 atomic ratio show best electrocatalytic activity.•Difference in activity among alloys is more significant in acidic than basic media.The nickel-copper nanoalloys with tunable composition and morphology were prepared by galvanostatic deposition on copper substrate, in order to investigate the relationship between alloy composition and electrocatalytic activity. Both the composition and morphology of the NixCuy nanoalloys are highly dependent on the applied current density. The atomic ratio of Ni to Cu in the alloys changes from 1:9 to 3:1, with the increase of current density from 10 to 100 mA cm− 2. The difference in electrocatalytic activity among these nanoalloys was evaluated through the hydrogen evolution reaction (HER) in 1.0 M H2SO4 and 1.0 M KOH. The composition-dependence of the electrocatalytic activity of the alloys is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH. By tuning the composition of NixCuy alloys, 13.5 and 5.7 times increase in exchange current density of the HER was achieved in 1.0 M H2SO4 and 1.0 M KOH, respectively. Meanwhile, 4.5 and 2.0 times decrease in charge transfer resistance was observed in the same two media. The best electrocatalytic activity to the HER was always achieved on the nanoalloy with a 1:1 atom ratio and a single crystal (111) plane. This favorable nanoalloy is composed of four-level dendritic nanochains. The results demonstrate that galvanostatic method can tune not only the composition but also the morphology of nanoalloys, both being important for nanoscale design of industrial electrocatalysts.Download high-res image (151KB)Download full-size image

A highly conductive sulfur cathode is crucial for improving the kinetic performance of a Li–S battery. The encapsulation of sulfur in porous nanocarbons is expected to benefit the Li+ migration, yet the e– conduction is still to be improved due to a low graphitization degree of a conventional carbon substrate, especially that pyrolyzed from carbohydrates or polymers. Aiming at facilitating the e– conduction in the cathode, here we propose to use ketjen black, a highly graphitized nanocarbon building block to form a conductive network for electrons in a biomass-derived, hierarchically porous carbon sponge by a easily scaled-up approach at a low cost. The specifically designed carbon host ensures a high loading and good retention of active sulfur, while also provides a faster electron transmission to benefit the lithiation/delithiation kinetics of sulfur. The sulfur cathode prepared from the carbon network shows excellent cycling and rate performance in a Li–S battery, rendering its practicality for emerging energy storage opportunities such as grids or automobiles.Keywords: electronic conductivity; ketjen black; lithium−sulfur battery; porous carbon; sodium alginate; sulfur cathode

•Coconut shell nanocarbon (CSC) prepared for photocatalysis of H2 production.•A H2 evolution rate as high as 1679.5 μmol h−1.•Abundant nanopores and surface oxygen-containing groups on CSC.•CSC facilitating electron transfer kinetics.•CSC promoting separation of the photoinduced electron–hole pairs.Coconut shell carbon (CSC) nanosheets were applied to support CdS quantum dots (≤5 nm) and Pt nanoparticles to form a composite Pt/CdS/CSC catalyst for the visible-light-driven photocatalytic H2 production from water. The H2 evolution rate on Pt/CdS/CSC is as high as 1679.5 μmol h−1, which is significantly enhanced as compared with that on Pt/CdS without CSC (636.2 μmol h−1). Electrocatalytic experiments indicate a highly efficient electron transfer on the CSC nanosheets, which may be due to the presence of the abundant nanopores (<4 nm) and surface oxygen-containing groups behaving as the charge capture traps. The unique electron transfer flexibility of CSC facilitates the separation of the photoinduced electron–hole pairs on CdS/Pt/CSC in the photocatalytic process. This is the main origin for the significantly enhanced photocatalytic performance of CdS/Pt/CSC. It is believed that the findings from this study will provide useful clues for designing efficient biochar-based catalysts for visible-light-driven photocatalysis.

•Cyclic voltabsorptometry peaks assigned to redox of Cu–glycine complexes.•Nanofoil electrode adaptable for use in cyclic voltabsorptometry of metals.•High electrochemical stability of Cu(I)–glycine complex.•Difficult formation of Cu(II)–glycine complex due to kinetic hindrance.Real time detection of the electrode products as a function of potential during the electrolysis is desirable for fundamental analyses of the electrode processes. In the present work, cyclic voltabsorptometry was used to investigate the electrochemical behavior of copper in glycine solutions at acidic, neutral and alkaline pH levels. A long-optical-path thin-layer cell was used for the spectroelectrochemical measurements. Electro-oxidation of the deposited copper nanofoil leads to formation of Cu(I)–glycine complex with high electrochemical stability. At a high concentration of glycine, all the deposited copper can be directly converted into soluble Cu(I)–glycine complex in the KCl-containing solutions without accumulation of the intermediate CuClads. The Cu(I)–glycine complex is also evidenced to be an intermediate in the cupric complex reduction to copper. This Cu(I) complex can be destroyed at relatively positive potentials where the evolution of chlorine occurs, especially in acidic and neutral solutions. This work proves that cyclic voltabsorptometry is adaptable for the analysis of the potential-dependent formation and conversion of the light-absorbing metal complexes.

A highly sensitive electrode for sensing glucose has been fabricated by electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole on a solid carbon paste substrate, and subsequent electrodeposition of multi-layer stacked copper particles as an outer surface. The individual copper particles are characterized by a large number of edges and corners of crystallites. Their preferred orientation {111} is parallel to the electrode surface. The conductive polymer interlayer results in an increase of the particle nucleation density and a further decrease of the polarization overpotential for direct (enzyme-free) oxidation of glucose in 0.1 M NaOH solution. A well-shaped voltammetric peak can be observed at around 0.3–0.5 V (vs. SCE, depending on scan rate) that is due to glucose oxidation. This potential is much lower than the one required for Cu(III) formation. A bulk electrolysis experiment using a thin-layer electrochemical cell confirmed the assumption that that glucose undergoes 2-electron oxidation. The mechanism of glucose oxidation in the absence of Cu(III) is discussed. The electrode exhibits a very high sensitivity (slope) of 3.31 mA cm−2 mM−1, and the detection limit is 2 μM (at an SNR of 3). Features of the new sensor include the ease of fabrication, its high stability and good selectivity.

•A simple electrodeposition approach to grow multi-laminated copper particles.•Cu particles with preferentially oriented {111} planes parallel to the substrate.•Solid carbon paste coated with PAMT film as a support of Cu catalyst.•Excellent catalytic activity to methanol oxidation in alkaline electrolytes.•PAMT interlayer facilitating electron transfer kinetics of methanol oxidation.A simple electrodeposition approach to grow multi-laminated copper particles on two conductive substrates is presented. Morphological and structural characterization was performed using SEM and XRD. The copper crystallites are preferentially oriented with {111} planes parallel to the substrate surfaces, providing an optimum interface for methanol oxidation. There are a large number of edges, corners, and atomic steps around individual multi-laminated nanostructured particles. The excellent electrocatalytic activity of the particles to methanol oxidation in alkaline solutions is demonstrated by cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The presence of the conductive poly(2-amino-5-mercapto-1,3,4-thiadiazole) interlayer between the Cu particles and the carbon paste substrate results in larger specific surface areas of the particles and smaller charge-transfer resistances of methanol oxidation reaction in the lower potential range. Such an anisotropic laminated structure of non-noble metal nanomaterials deserves further investigation for finding a suitable alternative to noble metal-based anodic catalysts in fuel cells.

•An N,S-rich conductive polymer PAMT prepared for electrocatalysis of H2 evolution.•Imine nitrogen atom pairs as free active sites for H+ reduction.•313 mV positive shift in open circuit potential due to PAMT catalyst.•PAMT facilitating electron transfer kinetics of H+ reduction.•PAMT facilitating desorption of hydrogen from active sites.A conductive polymer, poly(2-amino-5-mercapto-1,3,4-thiadiazole) (PAMT), was electrodeposited on a glassy carbon substrate for electrocatalyzing hydrogen evolution reaction (HER) in H2SO4 electrolytes. The prepared material was characterized by scanning electron microscope and X-ray photoelectron spectroscopy. The surface of the PAMT film was uniform, crack-free, and was full of curly short filaments (<100 nm long). The free active sites of the PAMT for HER could be represented as –N, which exist in pairs meeting the dual-site requirements for H–H combination. The Tafel analysis revealed that the open circuit potential was positively shifted by 313 mV due to the PAMT catalyst, with a prominent decrease in activation energy. Both the electrochemical impedance spectroscopy and chronopotentiometry suggested that the PAMT can not only significantly reduce the charge transfer resistance of the HER, but also facilitate the desorption of the generated hydrogen from the active sites. These results indicate that the N- and S-rich conductive polymers deserve further investigation as potential electrocatalyst candidates for hydrogen energy production.

•In situ monitoring of radical intermediate in redox of chlorpromazine.•High anodic current responses of chlorpromazine at physiological pH.•A positive and a negative voltabsorptometric peak due to formation and conversion of intermediate.•A pH-dependent mechanism proposed for oxidation of chlorpromazine.Chlorpromazine has been used widely as an antipsychotic agent and its major metabolic pathways include 7-hydroxylation, N-dealkylation, N-oxidation and S-oxidation. The present work focuses on in situ monitoring of the radical intermediate, for clarifying the mechanism diversity in anodic oxidation of chlorpromazine at acidic and physiological pH’s. Only in acidic media the colored cation radical was detected by UV–vis spectroelectrochemistry and cyclic voltabsorptometry. The radical was formed at the first anodic peak, followed by a further one-electron oxidation at the second peak to generate the sulfoxide. At physiological pH, the sulfoxide was generated at the first anodic peak through a one-step two-electron oxidation, resulting in much higher current response than at acidic pH’s. This work provides an example of a positive and a negative voltabsorptometric peak indicating the formation and conversion of a radical intermediate.

Acquisition of data from both in situ spectroscopy detection and online chromatography-like separation is important for studying complex electrochemical reactions. The present work provides an example of combination of thin-layer spectral and electrophoretic electrochemistry, both based on thin-layer electrolysis. Two thin-layer electrochemical cells were used to investigate the electro-oxidation of solid ellagic acid at different potentials, in acidic, physiological, and alkaline buffer media. UV–vis spectra and cyclic voltabsorptograms of the oxidation products were recorded in situ without interference from the solid reactant. Four oxidation products, depending upon the buffer pH and the applied potential, were separated and detected by electrophoretic electrochemistry. The major products possess redox stability, possibly with a diquinonemethide structure. The minor product is considered as an o-quinone derivative with a lactone-ring-opening, which can be reduced or further oxidized at appropriate potentials. A consecutive-parallel reaction mechanism is proposed for the formation of four products of ellagic acid in different pH media, which enriches the knowledge about the oxidation pathway and antioxidant property of this biologically active polyphenol compound.

Raloxifene is a selective estrogen receptor modulator that may produce toxic oxidative species in metabolism. The oxidation mechanism of raloxifene with different pH values was studied by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), in situ UV–vis spectral analysis and cyclic voltabsorptometry based on a long optical-path thin-layer electrochemical cell. Time-derivative cyclic voltabsorptograms were obtained for comparative discussion with the corresponding cyclic voltammograms. Raloxifene was initially oxidized to reactive phenoxyl radicals, followed by a series of transformation steps leading to different final products in different pH media. A parallel-consecutive reaction mechanism was proposed for the pH-dependent formation of 7-hydroxyraloxifene, raloxifene 6,7-o-quinone and two raloxifene dimers, each pathway following a complex electrochemical-chemical mechanism. Both raloxifene diquinone methide and its N-oxides were not detected by in situ UV–vis spectroscopy and XPS analysis. This work provides an electrochemical viewpoint and comparable information for better understanding of the oxidative metabolism and chemical toxicology of raloxifene under physiological conditions in vivo or in vitro.Highlights► Application and analysis of in situ thin-layer spectroelectrochemistry. ► Cyclic voltabsorptometry used for a drug study. ► Highly pH-dependent oxidative metabolism of raloxifene. ► A complex parallel-consecutive mechanism proposed for oxidation of raloxifene.

•A new coupling of thin-layer electrolysis with capillary electrophoresis (CE).•Rapid electrolysis, direct sampling followed by online CE separation.•At least 13 products of quercetin oxidation were separated.•Thermodynamic and kinetic parameters were determined from CE peak areas.A coupling technique of thin-layer electrolysis with high-performance capillary electrophoresis/UV–vis technique(EC/HPCE/UV–vis) is developed for online separation and determination of electrode reaction products. A chip-type thin-layer electrolytic (CTE) cell was designed and fabricated, which contains a capillary channel and a background electrolyte reservoir, allowing rapid electrolysis, direct sampling and online electrophoretic separation. This chip-type setup was characterized based on an electrophoresis expression of Nernst equation that was applied to the redox equilibrium of o-tolidine at different potentials. The utility of the method was demonstrated by separating and determining the electro-oxidation products of quercetin in different pH media. Two main products were always found in the studied time, potential and pH ranges. The variety of products increased not only with increasing potential but also with increasing pH value, and in total, at least 13 products were observed in the electropherograms. This work illustrates a novel example of capillary electrophoresis used online with thin-layer electrolysis to separate and detect electrode reaction products.

The phytic acid-coated titanium (IP6/Ti) electrode was prepared through a simple drop-drying process, with an aim of improving electrocatalytic activity toward the hydrogen evolution reaction (HER). Scanning electron microscope and X-ray photoelectron spectroscopy showed that the IP6 coated the substrate surface uniformly and completely. Evaluation of the electrode activity was carried out in 1.0 M NaOH by linear polarization, electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The kinetic parameters obtained from Tafel curves reveal that the IP6 coating can enhance the exchange current density of the HER by 489 times compared to the bare Ti, and reduce the HER activation energy by nearly 50%. The EIS data prove that the charge transfer resistance of the HER was considerably reduced due to the IP6 coating, with a decrease in real surface area of the electrode. The catalytic effect of IP6 is due to an improvement in the charge transfer kinetics of the HER. This work indicates that IP6 may be a potent candidate as a catalyst for hydrogen energy production.Highlights► A naturally occurring phosphorus compound was used as electrocatalyst of HER. ► Phytic acid coated Ti electrode was prepared through a simple drop-drying process. ► The prepared electrode shows excellent electrocatalytic activity toward HER. ► The catalytic effect is due to intrinsic electrocatalytic properties of phytic acid.

Electrochemical oxidation of two isomeric coumarins, umbelliferone (UF, 7-hydroxycoumarin) and benzotertonic acid (BA, 4-hydroxycoumarin), were comparatively studied in aqueous buffer solutions by cyclic voltammetry, in situ long-path-length thin-layer UV–vis spectroelectrochemistry and ex situ ATR-FTIR spectrometry. Both the coumarins undergo the completely irreversible oxidation but following totally different oxidation mechanisms. The 7-OH but not the 4-OH group can contribute to antioxidative activity of coumarin via an electron transfer mechanism. Electro-oxidation of UF occurs at the 7-OH position and produces an insulating polymer film at the electrode surface, which probably consists of a poly(ethylene oxide) backbone with coumarin side groups. The toxicity-related coumarin 3,4-epoxide is a possible intermediate in the UF oxidation. Electro-oxidation of BA occurs at the C3C4 double bond, also yielding a non-conductive film at the electrode surface. In this process salicylaldehyde as the possible intermediate undergoes further oxidation to form the poly(aryl ether) film. The knowledge of the mechanisms of UF and BA oxidation should be helpful in understanding the roles and conversion of coumarins in their biological and chemical processes.

Electrochemical processes of solid indirubin and its sulfonated form were comparatively studied in aqueous buffers by cyclic voltammetry and long-path-length thin-layer UV–vis spectroelectrochemistry, using a carbon paste working electrode with or without indirubin particles attached. The two forms of indirubin gave similar voltammetric features as well as main reaction products. Alkaline pH of electrolyte generally had a negative effect on both the reaction systems, compared with the acidic pH. Electro-reduction of both indirubins produced their leuco forms, which can be oxidized back to the initial reactants by oxygen. In the alkaline buffers the leuco-indirubin (not sulfonated) may form aggregates with poor solubility and poor electrochemical reactivity. Electro-oxidation of both indirubins led to the irreversible formation of isatin (sulfonated or not). An EC and ECE mechanisms are proposed for the reduction and the oxidation, respectively, of indirubin in two forms. The combination of solid state and solution phase spectroelectrochemistry shows the advantage of providing multidimensional information for reaction mechanism determination.

Cyclic voltammetry, square wave voltammetry and double potential step UV–vis spectroelectrochemistry were used to study the redox processes of indigo microparticles dispersed in a solid carbon paste electrode. The spectroelectrochemical measurements were performed in a home-made long-path-length thin-layer electrolysis cell. Both the indigo/leucoindigo and indigo/dehydroindigo redox couples underwent reversible 2e−/2H+ surface-confined reactions. Alkaline electrolytes showed more negative effect on the reduction of indigo to leucoindigo than on its oxidation to dehydroindigo. A new species (probably indolone) was monitored in the re-oxidation of leucoindigo, while isatin was found in the oxidation of indigo at enough positive potential. More detailed electrochemical mechanisms were proposed for the two redox systems, respectively. The present work shows that the microparticle-dispersed carbon paste is an attractive electrode material not only for solid state voltammetry but also for stripping spectroelectrochemistry.

A nonionic poly(2-amino-5-mercapto-thiadiazole) film was electrodeposited on a solid carbon paste electrode via a potential scanning procedure, and used for amperometric sensing of ascorbic acid (AA), dopamine (DA) and serotonin (ST). The highly electrocatalytic activity of the sensor to the three analytes was demonstrated from the sensitive and well separated voltammetric signals. The polymer film did not show significant accumulation effect on all the three species, reducing the fouling and deactivation of the electrode surface as well as the mutual interference among the analytes. The sensor achieved amperometric sensitivities of 1.92 nA (nmol L−1)−1 cm−2 to AA in the linear range of 0.025–1.95 μmol L−1, 3.76 nA (nmol L−1)−1 cm−2 to DA and 7.00 nA (nmol L−1)−1 cm−2 to ST both in the linear range of 0.02–1.56 μmol L−1. The lowest detection limits were found to be 1.5, 0.7 and 0.4 nmol L−1 for AA, DA and ST, respectively. This sensor was successfully employed for the successive determination of AA, DA and ST in pharmaceutical samples. The good antifouling property and reproducibility of the proposed sensor can be attributed to the nonionic polymer film without electrostatic attraction to the ionized species in the solutions.

An amperometric sensor with attractive sensing behavior was prepared by using solid carbon paste as substrate for electropolymerization of 2-amino-5-mercapto-1,3,4-thiadiazole (AMT). Morphology, polymerization mechanism and charge-transfer performance of the poly-AMT (PAMT) film on the composite substrate surface were investigated by using SEM, in situ UV–vis spectroelectrochemistry and electrochemical impedance spectrum methods. The results indicate that the PAMT film covered the wax portion prior to the graphite grains on the substrate surface, and the resulting sensor showed remarkable decrease in charge-transfer resistance with the increase in the amount of deposited PAMT. At the physiological pH the sensor gave sensitive and well separated voltammetric signals for two bioflavonoid isomers with the same electro-active moiety, luteolin and fisetin. The sensor achieved amperometric sensitivities of 1.41 nA nM−1 cm−2 to luteolin and 0.648 nA nM−1 cm−2 to fisetin in a linear range of 10–700 nM, with the lowest detection limits of 3 nM luteolin and 5 nM fisetin, respectively. The study demonstrates that the solid carbon paste is a superior substrate material for PAMT film sensor.

Using (+)-catechin electrodeposited on a carbon-paste electrode as a model system, we have demonstrated the usefulness of the time-derivative cyclic voltabsorptometry for voltammetric characterization of the deposited films, in the case when not only the deposited species but also the same ones in free solution participated in redox processes. A long-optical-path thin-layer cell was used for the voltabsorptometric measurements. The potential-dependent absorption signals were monitored for catechin at 252 and 279 nm in B-R buffer electrolytes with pH = 1.8. The combination of voltabsorptometry with voltammetry enabled one measured cyclic voltammogram to become four, which were attributed to catechin and its oxidized state, in free solution or in deposited state, respectively. The surface coverage of the electrode was evaluated from the cyclic voltammograms obtained for the deposited catechin, which decreased with the increasing scan rate. Also, the deposited species was found to make a major contribution to the total voltammetric current, especially at higher scan rates.

The present work reports a quercetin-modified wax-impregnated graphite electrode (Qu/WGE) prepared through an electrochemical oxidation procedure in quercetin-containing phosphate buffer solution (PBS), for the purpose of detecting uric acid (UA) in the presence of ascorbic acid (AA). During modification quercetin was oxidized to the corresponding quinonic structure, and in the blank buffer solution the electrodeposited film exhibits a voltammetric response anticipated for the surface-immobilized quercetin. Retarding effect of the film towards the reaction of anionic species was found; therefore the pH of sample solutions was selected to ensure the analyte in molecular form. At suitable pHs the Qu/WGE shows excellent electrocatalytic effect towards the oxidation of both AA and UA, and separates the voltammetric signal of UA from AA by about 280 mV, allowing simultaneous detection of these two species. A linear relation between the peak current and concentration was obtained for UA in the range of 1–50 μM in the presence of 0.5 mM AA, with a detection limit 1.0 μM (S/N = 3). This sensor was stable, reproducible and outstanding for long-term use.

This paper describes a novel method of obtaining cyclic voltammograms (CVs) from optical signals. The obtained CVs correspond to the various specific species involved in the electrode process, which give more detailed information on the system under investigation than the common CV. For this purpose cyclic voltabsorptometry was used to investigate the successive oxidation processes of rutin on a graphite-wax electrode by using a long optical-path thin-layer electrochemical cell. The dynamic UV spectra of rutin showed the information on the structures of the oxidation products at different potentials. Cyclic voltabsorptiograms (CVAs) were measured in three potential ranges at the characteristic absorption wavelengths of rutin, 346, 254 and 296 nm, respectively. The CVs of three species in solution (rutin and its two products) were obtained from the derivative cyclic voltabsorptiograms (DCVAs). Based on this the redox mechanisms of rutin in different CV peaks were discussed.

The adsorption and oxidation behavior of rutin was studied by in-situ UV spectroelectrochemistry in a long optical-path thin-layer electrochemical cell with a graphite-wax electrode. The dynamic UV spectra of rutin under potentiostatic oxidation were recorded, which indicated the formation of o-quinonic structure. During the repetitive cyclic potential scans, cyclic voltabsorptomogram was recorded at the three characteristic wavelengths 346, 254 and 296 nm, respectively. The profiles obtained showed two types of concentration fluctuation of species in solution, resulting from adsorption/desorption and redox reaction, respectively. Using derivative cyclic voltabsorptometry the contribution of the species in solution to the total current was estimated, and then the current of every step involved in the proposed redox mechanism was obtained for the first time. The result shows that rutin underwent a nearly reversible redox reaction in which the total current is mostly due to the contribution of adsorbed species. The present work developed cyclic voltabsorptometry into a useful tool for investigating redox process involving coupled adsorption/desorption steps.

A general and effective method for the synthesis of amides through decarboxylative amidation of α-keto acids with amines has been developed. The reaction proceeded smoothly to afford the corresponding amide products in good yield under air and shows excellent functional group tolerance. In addition, the protocol can be further applied in the synthesis of heterocyclic compounds like benzimidazoles.