Electrochemical Utilization Research Articles

Composite Material–Based Conducting Polymers for Electrochemical Sensor Applications: a Mini Review

Conducting polymers (CPs) represent a sizeable range of useful organic substances. Their unique electrical, chemical, and physical properties; reasonable price; simple preparation; small dimensions; and large surface area have enabled researchers to discover a wide variety of uses, including sensors, supercapacitors, solar cells, batteries, biochemical applications, and electrochromic devices. To promote the success of CPs, unique composite materials have been prepared with metals or steel oxides. The goal of this overview is to characterize the electrochemical sensor utility of CPs and their composites and to categorize future components of electrochemical sensing materials. CPs have been comprehensively applied over a wide range of variable industrial fields; such CPs have used many new materials with diverse compositions that are utilized as electrochemical sensors and biosensors. These materials have been fabricated inside numerous analytical instruments that are applied in healthcare settings and clinical, environmental, food, and pharmaceutical laboratories. Electrochemical sensors are essentially based on CPs and/or their corresponding composite materials. Therefore, the present work provides a brief illustration of electrochemical sensor and biosensor applications for the most important conducting polymer composite materials. Polyaniline (PANI), poly(o-toluidine) (PoT), and poly(o-anisidine) (PoAN) were considered in this study. Moreover, the most important electrochemical properties of these conducting polymer composite materials are discussed.

Oxygen vacancies enhance the lithium ion intercalation pseudocapacitive properties of orthorhombic niobium pentoxide

While orthorhombic niobium pentoxide (T-Nb2O5) is one of the most promising energy storage material with rapid lithium ion (Li+) intercalation pseudocapacitive response, a key challenge remains the achievement of high-rate charge-transfer reaction when fabricated into thick electrodes. Herein, we report a facile method to create intrinsic defects in T-Nb2O5 through a hydrogen (H2) reduction, which is effective to overcome the limitations of electrochemical utilization and rate capability. Due to the high number of active sites introduced, the specific capacity of hydrogenated (H-) Nb2O5 with oxygen vacancies reaches 649 C g−1 at 0.5 A g−1, greatly exceeding that of T-Nb2O5 which is 580 C g−1. In addition, theformation of oxygen vacancies leads to increased donor density and enhanced electrical conductivity, which accelerates charge storage kinetics and enables excellent long-term cycling stability (86% retention after 2000 cycles). The analysis of electrochemical impedance spectroscopy (EIS) plots and the calculation of Li+ diffusion coefficients (DLi) further explains the high rate-performance of H-Nb2O5. When the electrode thickness increased to 150 μm, the H-Nb2O5 still delivers excellent electrochemical properties. Therefore, the introduction of oxygen vacancies provides a new method towards the improvement of the electrochemical properties of various transition metal oxides.

Facile synthesis of nickel cobalt selenide hollow nanospheres as efficient bifunctional electrocatalyst for rechargeable Zn-air battery

Designing high active, low cost and bifunctional electrocatalysts is urgent for developing clean energy storage and conversion systems. Transition metal selenides exhibit optimal electronic conductivity and tunable physicochemical properties, which endow them with potential for efficient electrocatalysts to facilitate the oxygen reduction and oxygen evolution reactions (ORR and OER). Herein, hollow NixCo0.85-xSe nanospheres were synthesized using a facile polyol based solution chemical method. The NixCo0.85-xSe exhibits an onset overpotential of 0.89 V for ORR, and an overpotential of 305 mV to achieve 10 mA cm−2 for OER. Moreover, the NixCo0.85-xSe based Zn-air battery displays remarkable specific capacity and durability. Such superior catalytic performances can be attributed to the synergistic effect, large specific surface area and enhanced electron transfer rate. This approach provides a new way to design highly efficient bifunctional electrocatalysts for electrochemical energy storage and utilization.

3D graphene-encapsulated nearly monodisperse Fe3O4 nanoparticles as high-performance lithium-ion battery anodes

A facile self-assembly strategy is developed to construct three-dimensional graphene-encapsulated nearly monodisperse Fe3O4 (3D Fe3O4@rGO) composite. The nearly monodisperse Fe3O4 nanoparticles (NPs) are intimately wrapped by the flexible 3D rGO network that can remarkable increase the electrochemical utilization of Fe3O4. 3D Fe3O4@rGO composite indicates excellent electrochemical properties as lithium-ion battery anodes. The void holes between flexible rGO nanosheets and Fe3O4 NPs can not only act as the electrolyte reservoir to tremendously shorten the Li+ diffusion distance, but also provide extra accommodating space for fast Li+ insertion and extraction. A reversible discharge specific capacity of 1139 mA h g−1 and 85% capacity retention can achieve at a current density of 400 mA g−1 after 100 cycles. More impressively, 3D graphene-encapsulated Fe3O4 exhibits a high lithium storage capacity of 665 mA h g−1 at 1000 mA g−1 after 200 cycles, demonstrating the excellent lithium storage performance.

High-Energy Lithium/SeS2 Batteries with Hierarchical Nanocomposite Cathode and Ionic Liquid-Based Electrolytes

Selenium disulfide (SeS2)-based composites have received much attention as a promising cathode material for high-energy lithium metal batteries due to their higher theoretical capacity (1342 mAh g−1) than that (675 mAh g−1) of selenium and a less significant shuttle effect than that of sulfur.[1] However, the biggest challenges faced by Li/SeS2 batteries are (1) the development of high-performance cathodes that can maintain the functional interface during cycling and (2) the identification of suitable electrolytes that can allow good cyclability without the degradation of lithium metal anodes. It has been reported that an ionic liquid, N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, i.e. Py14TFSI, can be used as co-solvent in ether-based electrolytes to form high-quality protection layer on lithium metal.[2] In this study, an effective strategy for developing high-performance Li/SeS2 batteries is introduced via a synergistic enhancement comprising the effective cathode design and the lithium anode protection in Py14TFSI containing electrolytes. We found that the hierarchical SeS2@carbon nanocomposites, confirmed by X-ray nano-computed tomography, facilitated Li-ion transportation and enhanced the electrochemical utilization of SeS2, enabling the improved rate capability of Li/SeS2 battery. Furthermore, the systematic studies on the morphological change as well as the electrochemical stability of lithium anode indicate that Py14TFSI-LiNO3­-containing electrolytes could effectively suppress the shuttle effect and protect lithium metal by forming a stable and LiF-dominated solid-electrolyte-interphase layer, leading to much improved Coulombic efficiency and cyclability of Li/SeS2 batteries. These results underscore a synergistic approach toward the development of high-energy Li/SeS2 batteries with long cycle life. [1] a) A. Abouimrane, D. Dambournet, K. W. Chapman, P. J. Chupas, W. Weng, K. Amine, J. Am. Chem. Soc. 2012, 134, 4505-4508; b) Y. Cui, A. Abouimrane, J. Lu, T. Bolin, Y. Ren, W. Weng, C. Sun, V. A. Maroni, S. M. Heald, K. Amine, J. Am. Chem. Soc. 2013, 135, 8047-8056. [2] J. Zheng, M. Gu, H. Chen, P. Meduri, M. H. Engelhard, J.-G. Zhang, J. Liu, J. Xiao, Journal of Materials Chemistry A 2013, 1, 8464. Figure 1

Synthesis of monodispersed CoMoO4 nanoclusters on the ordered mesoporous carbons for environment-friendly supercapacitors

Binary metal oxides with superior charge capacity and electrochemical activity have gained great interests. In this work, monodispersed CoMoO4 nanoclusters on the ordered mesoporous carbons were fabricated by a facile self-developed impregnation method. The synthesized hybrids possess improved wettability, high specific surface area (>700 m2/g) and regular mesoporous channels (∼4 nm), resulting in improved electrochemical performance for supercapacitors. These well-dispersed CoMoO4 nanoclusters exhibit a significant specific capacitance up to 367 F/g in the aqueous KNO3 electrolyte and good reversibility with a cycling efficiency of 99.8%. It is proposed that the mesoporous structure can facilitate the diffusion of electrolyte ions and then accelerate the electrochemical utilization of CoMoO4 nanoclusters. The results demonstrate that the produced binary metal oxide nanoclusters with excellent capacitance and good retention can be used as promising electrodes for the environment-friendly supercapacitors.

A Li2S‐TiS2‐Electrolyte Composite for Stable Li2S‐Based Lithium–Sulfur Batteries

AbstractLi2S is a fully lithiated sulfur‐based cathode with a high theoretical capacity of 1166 mAh g−1 that can be coupled with lithium‐free anodes to develop high‐energy‐density lithium–sulfur batteries. Although various approaches have been pursued to obtain a high‐performance Li2S cathode, there are still formidable challenges with it (e.g., low conductivity, high overpotential, and irreversible polysulfide diffusion) and associated fabrication processes (e.g., insufficient Li2S, excess electrolyte, and low reversible capacity), which have prevented the realization of high electrochemical utilization and stability. Here, a new cathode design composed of a homogeneous Li2S‐TiS2‐electrolyte composite that is prepared by a simple two‐step dry/wet‐mixing process is demonstrated, allowing the liquid electrolyte to wet the powder mixture consisting of insulating Li2S and conductive TiS2. The close‐contact, three‐phase boundary of this system improves the Li2S‐activation efficiency and provides fast redox‐reaction kinetics, enabling the Li2S‐TiS2‐electrolyte cathode to attain stable cyclability at C/7 to C/3 rates, superior long‐term cyclability over 500 cycles, and promising high‐rate performance up to 1C rate. More importantly, this improved performance results from a cell design attaining a high Li2S loading of 6 mg cm−2, a high Li2S content of 75 wt%, and a low electrolyte/Li2S ratio of 6.

One-Pot hydrothermal approach to graphene/Poly(3,4-ethylenedioxythiophene) composites for high-capacitance supercapacitors

Here the oxidative polymerization of 3, 4-ethylenedioxythiophene and hydrothermal reduction of graphene oxide were proceeded simultaneously, which enables the in-situ coating of the bridged poly(3, 4-ethylenedioxythiophene) (PEDOT) nanodendrites onto the highly-conductive graphene skeletons. The as-produced homogeneous graphene/PEDOT composites possess open channels, large accessible surface, fast charge transport, and high electrochemical utilization. The as-fabricated supercapacitor based such nanocomposites therefore exhibits larger specific capacitance (364 F g−1 at 10 mV s−1; 174 F g−1 at 1.0 A g−1), better rate capability (154 F g−1 at 20.0 A g−1), lower interface resistance, and higher cycling stability (retaining 91.4% over 10,000 cycles at 10 A g−1) compared to the PEDOT counterpart (328 F g−1 at 10 mV s−1; 147 F g−1 at 1.0 A g−1; 87% retention after 10,000 cycles). Moreover, its energy density is as high as 24.2 Wh kg−1 at the power density of 1 kW kg−1, and can remain 21.4 Wh kg−1 at the high power density of 20 kW kg−1. The proposed method is facile, novel, and scalable in developing high-performance supercapacitors by combining graphene and pseudocapacitive PEDOT nanodendrites.

Dimerization of 9,10-anthraquinone-2,7-Disulfonic acid (AQDS)

The redox flow battery is a leading candidate to supply society with large-scale energy storage. In the category of aqueous organic redox flow batteries, 9,10-anthraquinone-2,7-disulfonic acid (AQDS) is considered a top-performing molecule and shows ideal electrochemical behavior at low concentrations around 1 mM. However, its behavior at higher concentrations was rarely studied. In this work, the correlation between concentration and electrochemical capacity is evaluated for AQDS in 1 M H2SO4 and 1 M alkaline sodium carbonate buffer, employing cyclic voltammetry, rotating disk electrode voltammetry and diffusion NMR. It was found that an electrochemically inactive dimer was formed by the unreduced AQDS molecules, with dimerization equilibrium constants determined to be K = 5 M−1 for the acidic electrolyte and 8 M−1 for the alkaline one, significantly inhibiting full electrochemical utilization of the system. However, upon reduction of the bulk material, the dimerization will shift in favor of monomer or quinhydrone formation, possibly alleviating the impediment to reduction. Finally, it was found that the dimerization could be decreased slightly by increasing the temperature of the system.

Effect of functionalized graphene on performance of magnesium/air battery

In order to decrease the corrosion rate of magnesium anode and increase the discharge property of magnesium/air battery, functionalized graphene, poly (sodium 4-styrenesulfonate)/reduced graphene oxide, and reduced graphene oxide/Mn3O4 nanometer composite are prepared successfully. The magnesium/air batteries are researched by addition of poly (sodium 4-styrenesulfonate)/reduced graphene oxide in NaCl electrolyte and reduced graphene oxide/Mn3O4 nanometer composite as cathode catalyst. The structure and morphology of the poly (sodium 4-styrenesulfonate)/reduced graphene oxide and the reduced graphene oxide/Mn3O4 nanometer composite are tested by x-ray diffraction, scanning electron microscope and fourier transform infrared spectrophotometer. The magnesium/air battery with the functionalized graphene is found to get high energy density of 1279 Wh kg−1 and anode utilization of 83%, which is higher than that of in NaCl solution and commercial air cathode (935 Wh kg−1 and 47%). The magnesium/air battery based on functionalized graphene shows higher electrochemical activity and anode utilization.

Investigating the Effect of Lewis Acid Base Interaction in Non-Stoichiometric Vanadium Oxide Based CNFs in Lithium – Sulfur Battery

Despite the potential of lithium-sulfur batteries to reach practical energy densities of 500–600 Wh/kg, there are still many problems plaguing their development. The sulfur cathode suffers from low electrochemical utilization and poor cycle life owing to the insulating nature of S and the Li2S discharge product, and the shuttling of lithium polysulfide species especially at high areal mass loadings. Enormous progress has been achieved in terms of capacity and static life during the past few years to overcome these issues, by employing various carbon and inorganic materials (metal oxides, metal-organic frameworks). Recently, non-stoichiometric metal oxides have been employed as sulfur host materials due to their high conductivity and ability to bind polysulfides chemically through unsaturated metal ion sites. However, the formation of metal-suboxides involves a high-temperature calcination process, which usually results in dense host materials with lower accessible surface area, and a reduced host/polysulfide interaction. Moreover, these studies are often reported with low sulfur loadings (1–2 mg/cm2) resulting in lower areal capacity. Herein, we report the use of electrospun vanadium monoxide/carbon nanofibers (VCNFs) as cathode host in Li-S batteries, which provide high electrical conductivity for better sulfur utilization and strong Lewis acid-base interaction with polysulfides to suppress shuttle. The developed cathode possesses hierarchical micro-mesoporous architecture with inter-fiber spacing and a high surface area of 181 m2/g. The unique architecture overcomes the challenges associated with dense, low active surface area based powered oxide materials. The developed VCNFs-S cathodes with a high sulfur loading of 3 mg/cm2 exhibit initial discharge capacities of ∼1350, ∼1247, and ∼1113 mAh g–1 at 0.1, 0.2, and 0.5 C rates, respectively, with long-term cycling (Fig 1a) over 200 cycles at 0.5 C rate. Moreover, at ~7mg/cm2, Li-S cells exhibit high areal capacity of 8 mAh/cm2 at a current density of C/10 up to 50 cycles (Fig 1b). The high surface area results in active reaction sites spread throughout the VCNFs, improving the accessible reaction area for better binding of polysulfides, enabling the smooth operation at high-loading in Li-S system. Using postmortem X-ray photoelectron spectroscopy analysis, this study reveals the presence of strong Lewis acid–base interaction between VO (3d3) and Sx 2– through the coordinate covalent V–S bond formation. Additionally, freestanding nature of the cathodes eradicate the need of additional inactive elements, viz., binders, additional current collectors (Al-foil), and additives. Our results highlight the importance of developing high surface area metal-suboxides/monoxide-based conducting polar host materials for next-generation Li–S batteries. Figure 1

Polyaniline-manganese dioxide-carbon nanofiber ternary composites with enhanced electrochemical performance for supercapacitors

A new ternary composite of polyaniline/manganese dioxide/nitrogen, phosphorus co-doped carbon nanofiber (PANI/MnO2/NPCNF) has been synthesized by electrospinning technology and chemical oxidative reactions. The results show that MnO2 and PANI layers are uniformly coated on the surface of N, P co-doped CNF. The optimal ternary nanocomposite shows excellent electrochemical performance (587.3Fg−1 at 0.5Ag−1, ~63% capacitance retention from 0.5Ag−1 to 20Ag−1 and capacitance retention is ~84% after 1000cycles at 10Ag−1). That could be attributed to the synergistic effects contributed from individual components. The N, P co-doped CNF acts as carbon scaffold to facilitate the electron transform and improves the poor electrical conductivity of MnO2. The outer PANI layer could achieve high pseudo-capacitance, protect the MnO2 layer from reductive-dissolving and further heighten its electrochemical utilization.

Hierarchical nanocomposite of hollow carbon spheres encapsulating nano-MoO2 for high-rate and durable Li-ion storage

As a new class of promising anode materials for Li-ion batteries, the application of transition metal oxides is hindered by poor electrochemical utilization and stability. In this work, we report a unique hollow structure of carbon-encapsulated MoO2 nanospheres as a high-rate and durable anode material for Li-ion batteries. Three-dimensional hierarchically porous carbon hollow nanospheres provide a universal and open framework for encapsulating MoO2 nanodeposits by a facile impregnation-calcination approach. The hybrid architecture is of great benefit in fast and stable Li-ion storage by shortening Li-ion and electron transport pathways, increasing electronic conductivity, and buffering volume fluctuation upon cycling. The electrochemical advantages bestow the MoO2/C nanocomposite with an outstanding specific capacity of 1051 mAh g−1 at 0.1 A g−1, superb rate capability (701 mAh g−1 at 2 A g−1) and excellent cycling stability. The present study demonstrates the promising use of hollow-structured MoO2/C nanocomposites for high performance Li-ion batteries.

Electrochemical nitrogen fixation and utilization: theories, advanced catalyst materials and system design.

Nitrogen is a fundamental constituent for all living creatures on the Earth and modern industrial society. The current nitrogen industry is largely powered by fossil fuels with huge energy consumption and carbon dioxide emission, and nitrogen pollution in surface water bodies induced by the indiscriminate discharge of industrial and domestic wastewater has become a worldwide environmental concern. Electrochemical techniques for nitrogen fixation and transformation under mild conditions are promising approaches to meet the challenge of efficiently managing and balancing the nitrogen cycle, where the rational design of advanced electrocatalysts from both structural and compositional aspects down to the nanoscale plays the most essential role. Herein, important nitrogen species including dinitrogen (N2), ammonia (NH3) and hydrazine (N2H4), their transformation processes between each other including the nitrogen reduction reaction (NRR), ammonia oxidation reaction (AOR) and hydrazine oxidation reaction (HzOR), and research progress on the development of related electrocatalysts are systematically summarized, aiming at establishing a general picture of the whole nitrogen cycle instead of a certain single reaction. Strategies combining theoretical computations and experimental optimizations are proposed to improve the catalytic performance including activity, efficiency, selectivity and stability, finally contributing to a self-sufficient and carbon-free "green" nitrogen economy.

Spatially Self-Confined Formation of Ultrafine NiCoO2 Nanoparticles@Ultralong Amorphous N-Doped Carbon Nanofibers as an Anode towards Efficient Capacitive Li+ Storage.

The exploration of anode materials with a high degree of electrochemical utilization for Li-ion batteries (LIBs) still remains a huge challenge despite pioneering breakthroughs. Rational engineering of electrode structures/components by facile strategies would offer infinite possibilities for the development of LIBs. In this study, one-dimensional ultralong nanohybrids of ultrafine NiCoO2 nanoparticles dispersed in situ in and/or on the surface of amorphous N-doped carbon nanofibers (NCO@ANCNFs) were fabricated by a bottom-up electrospinning protocol. By virtue of synergistic structural/component features, the obtained ultralong NCO@ANCNFs with low NCO loading (≈33.6 wt %) show highly efficient Li+ storage performance with high reversible capacity, high rate capability, and long cycle life. The unusual reversible crystalline transformation during cycling was analyzed. Quantitative analysis revealed that the pseudocapacitive contribution mainly accounts for the superior lithium storage of the NCO@ANCNFs. Besides, the ability of the hybrid anode to deliver competitive Li-storage properties even without conductive carbon greatly enhances its commercial applicability. An NCO@ANCNFs//LiNi0.8 Co0.15 Al0.05 O2 full battery was assembled and exhibited striking electrochemical properties. This contribution offers a scalable methodology to fabricate highly efficient hybrid anodes for advanced next-generation LIBs.

Boosted electrochemical performance of TiO2 decorated RGO/CNT hybrid nanocomposite by UV irradiation

Titania decorated RGO/CNT hybrid electrode material was prepared for supercapacitor (SCs) application through facile hydrothermal approach. Combined act of 1D CNTs and 2D RGO assists the uniform in-situ growth of titania towards high surface area mesoporous self-assembled interconnected morphology which is confirmed from FESEM and BET surface area analysis. This interconnected porous network like structure can efficiently allow the better mass transport and decreased contact resistance towards high electrochemical utilization. Electrochemical measurements revealed the superior performance of hybrid nanocomposite comprising of titania/RGO/CNT (TG1C1) which exhibited maximum specific capacitance around 477 F/g at current density 1 A/g. However, this capacitance value was further enhanced up to 537 F/g after 1 hour of UV light irradiation. This enhancement could be ascribed to photo-catalytic effect of TiO2 and corresponding genesis of photo-excited electrons on TiO2 surface which further channelized through interconnected RGO and CNT morphology and hence results in improved specific capacitance. These observations were in good agreement with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis. The hybrid nanocomposite also showed satisfying energy density and power density. It was also found that prepared hybrid nanocomposites were stable up to 2000 cycles with maximum specific capacitance retention of 92% of initial value.

Comparative study on the effects of different structural Ti substrates on the properties of SnO2 electrodes

To investigate the influence of substrate structure on the properties of titanium-based tin oxide electrodes, Ti/SnO2 with porous titanium, planar titanium, titanium mesh, and titanium-copper alloy as substrates was prepared by a thermal decomposition method. Electrodes were characterized and tested by scanning electron microscopy (SEM), X-ray diffraction (XRD), electrochemical techniques, and accelerated life tests. The surface morphology, phase composition, electrochemical performance, and service life of the electrodes were studied and analysed. With phenol as the target pollutant, the effects of different substrates on the electrocatalytic degradation performance of the electrodes were investigated. The porous Ti/SnO2-Sb-La electrode prepared by the porous titanium substrate has finer surface grains and a larger specific surface area. The phenomena of “cracking” and “agglomeration” are obviously improved. At the same time, the oxygen evolution potential increased to 2.09 V, and the membrane impedance decreased to 5.722 Ω. The degradation experiment of simulated phenol wastewater reveals that the porous Ti/SnO2-Sb-La electrode has the highest phenol degradation rate of 90.8% and a removal rate of COD of 79.9% within 120 min. Additionally, the porous Ti/SnO2-Sb-La electrode has the longest accelerated service lifetime of 105 min, which is 5 times that of the planar Ti/SnO2-Sb-La electrode. It can be seen from the experimental results that the electrode prepared by using the novel porous titanium as the matrix has obvious electrochemical performance, service life and current utilization efficiency advantages in the degradation process. At the same time, it is further proved that the electrode substrate has a significant effect on the electrode performance in the electrochemical treatment of phenol-containing water.

Long-Life Lithium-Sulfur Batteries with a Bifunctional Cathode Substrate Configured with Boron Carbide Nanowires.

Developing high-energy-density lithium-sulfur (Li-S) batteries relies on the design of electrode substrates that can host a high sulfur loading and still attain high electrochemical utilization. Herein, a new bifunctional cathode substrate configured with boron-carbide nanowires in situ grown on carbon nanofibers (B4 C@CNF) is established through a facile catalyst-assisted process. The B4 C nanowires acting as chemical-anchoring centers provide strong polysulfide adsorptivity, as validated by experimental data and first-principle calculations. Meanwhile, the catalytic effect of B4 C also accelerates the redox kinetics of polysulfide conversion, contributing to enhanced rate capability. As a result, a remarkable capacity retention of 80% after 500 cycles as well as stable cyclability at 4C rate is accomplished with the cells employing B4 C@CNF as a cathode substrate for sulfur. Moreover, the B4 C@CNF substrate enables the cathode to achieve both high sulfur content (70 wt%) and sulfur loading (10.3 mg cm-2 ), delivering a superb areal capacity of 9 mAh cm-2 . Additionally, Li-S pouch cells fabricated with the B4 C@CNF substrate are able to host a high sulfur mass of 200 mg per cathode and deliver a high discharge capacity of 125 mAh after 50 cycles.

Enabling Enhanced Capacity in Lithium-Sulfur Batteries Via an Intermediate Radical

Lithium-sulfur batteries offer a tremendous increase in specific capacity compared to the current state-of-the-art lithium-ion batteries, with the sulfur cathode having a large theoretical capacity of 1672 mA h g-1.1 In practice, however, this large theoretical capacity is rarely achieved, particularly at high loadings of sulfur within the cathode, at low electrolyte amount, or at kinetically limiting high rates and low temperatures.2, 3 A primary reason for this is the highly insulating nature of sulfur and its lithium polysulfide and lithium sulfide (Li2Sx, 1 < x <8) discharge products. During charge and discharge, reduction and oxidation of sulfur species are confined to a primarily surface-based reaction pathway, which limits the total utilizable capacity as insulating discharge products precipitate and drive the voltage down to the cell’s lower voltage limits.4 While many efforts have focused on limiting the solubility of intermediate polysulfide species, it has been reported that higher donor number solvents can offer a pathway to greater electrochemical utilization of sulfur through a solution-based reaction pathway, primarily driven by the S3 .- radical.4 Here, we demonstrate that some higher donor number solvents, such as Dimethyl Sulfoxide (DMSO) and N,N-Dimethylacetamide (DMA), exhibit greater utilization and relative disproportionation to the S3 ·- radical, while others, like 1-methylimidazole (MeIm), show little presence of the S3 ·- radical and less first cycle discharge capacity. We investigate this unique phenomenon via ultraviolet-visible spectrophotometry at a high concentration of Li2S6 in each solvent. Relative concentration of each polysulfide species varies with overall concentration, and we choose to study relative speciation at high concentrations as this approximates the typical polysulfide concentrations found in electrolyte while cycling. Favorable disproportionation to the S3 ·- radical can be strategically used as a framework to optimize electrolytes for increased capacity in lithium-sulfur cells through a solution-based reaction pathway. Intelligent use of these radical-inducing solvents in conjunction with the predominantly used Dioxolane (DOL) and Dimethoxyethane (DME) electrolytes can enable greater sulfur utilization in traditionally capacity limiting scenarios, such as low temperature, high rates, and large sulfur loadings.

Electrochemical Synthesis of Tungsten Carbide in Molten Salts, Its Properties and Applications

Currently, the most used material for electrocatalysis is platinum black. The use of this metal leads to the high costs of electrocatalytic devices. It is necessary to reduce Pt amount in the catalysts or to replace it by less expensive materials. Tungsten carbides (WC and W2C) are very attractive materials for catalysis due to unique properties: high electronic conductivity, corrosion resistance and ability to withstand the action of catalytic poisons. Because of the relative cheapness of the production methods, these materials can be an alternative to expensive platinum. For application as catalyst WC need to be produced in the form of nanodispersed powders with high surface area. In industry, WC is obtained by carbonizing of tungsten metal powders (or its oxide) with carbon black or carbon-containing gases. Commercial WC has micron sizes with a specific surface area of up to 2-5 m2/g. There are a number of non-traditional methods for production of nanosized WC. Among these methods, the high-temperature electrochemical synthesis (HTES) in molten salt is very promising one. Depending on the conditions of HTES, the ultradispersed powders and coatings of WC of high purity can be obtained; doping and modification of the product during the synthesis can be implemented. This method also allows to solve very important problem - electrochemical utilization of carbon dioxide with production a new chemical product. The purpose of this work was to determine the systems and conditions for the electrolytic production of tungsten carbide in molten salts, to establish its chemical and phase composition, to study the morphological and structural features. Also the evaluation of possibility of WC application as an electrocatalyst in the reaction of hydrogen evolution was carried out. The peculiarities of the partial and joint electroreduction of the synthesis components (tungsten and carbon from their oxygen-containing compounds) on platinum and glassy carbon cathodes were studied by cyclic voltammetry. Two compositions of electrolytic baths were used: (1) Na,К|Cl–Na2WO4–NaPO3; (2) Na,К|Cl–Na2W2O7. Carbon dioxide dissolved in the melt under excess pressure up to 17 atm was used as the carbon component of the synthesis. Mechanisms and kinetic features of electrode reactions were determined. The region of potentials and currents was chosen for the realization of electrochemical synthesis of carbides in the studied systems. The synthesis temperature was 700-750 °C. The cathode current density (ik) = 0.05 – 0.2 A/cm2. The highly dispersed powders were produced by electrolysis. The physico-chemical properties of the products were studied by XRD, SEM, TEM, Raman spectroscopy, BET, DTG methods. Electrocatalytic testing was carried out in a standard three-electrode cell. A graphite rod with a layer of catalytic inks was used as a working electrode. The catalytic ink was a suspension of carbide product in a 5 mas. % solution of Nafion. Non-working side part of an electrode was isolated. A platinum counter electrode and a silver chloride reference electrode were used. It has been established by XRD method that depending on conditions and parameters of electrolysis, the composite mixtures of WC and W2C carbides with nano-dimensional carbon structures and platinum can be obtained at the cathode. The parameters of hexagonal α-WC crystal lattice (a = 2.9081 ± 0.005, c = 2.8211 ± 0.01) were calculated and the average particle size (5 – 10 nm) was established. The specific surface area of the powders that was 25-30 m2/g was determined by the BET method. According to the results of TEM, DTG and Raman spectroscopy, the produced WC powders contain up to 4 wt. % of free carbon, which has an amorphous structure and covers the WC nanocrystallites as a "coat". Different morphologies of WC powders were found by SEM, TEM methods: fibers and rods, layered particles, “curdled” conglomerates (Fig.1). Electrocatalytic testing was carried out on different samples of composite mixtures: (1) WC: W2C; (2) WC: xPt; (3) WC: nC. Studies have shown that the overvoltage of hydrogen evolution (η) from 0.1 M solution of H2SO4 is in the range of 0.14 ÷ 0.5 V depending on the composition, morphology and conditions of synthesis. So for WC powders containing platinum (up to 60 mass %), the value of η is the smallest – 0.14 V. The electrodes consisting of a mixture of WC and W2C have the largest value of η. It has been established that it is possible to produce the composite mixtures based on ultradispersed tungsten monocarbide of different composition by the HTES method. These products have the required properties for the application in electrocatalysis. Fig 1. SEM images of electrolytic WC. Figure 1