Rate Of Electrochemical Reduction Research Articles

Effect of the anionic composition of sulfolane based electrolytes on the performances of lithium-sulfur batteries

In lithium-sulfur batteries, cell design, specifically electrolyte design, has a key impact on the battery performance. The effect of lithium salt anion donor number (DN) (DN[PF6]− = 2.5, DN[N(SO2CF3)2]− = 5.4, DN[ClO4]− = 8.4, DN[SO3CF3]− = 16.9, and DN[NO3]− = 21.1) on the patterns of lithium-sulfur batteries and lithium metal electrode performances with sulfolane-based electrolytes is investigated. An increase in DN of lithium salt anions leads to an increase in the depth and rate of electrochemical reduction of sulfur and long-chain lithium polysulfides and to a decrease in those for medium- and short-chain lithium polysulfides. DN of lithium salt anions has weak effect on the discharge capacity of lithium-sulfur batteries and the Coulomb efficiency during cycling, with the exception of LiSO3CF3 and LiNO3. An increase in DN of lithium salt anions leads to an increase in the cycling duration of lithium metal anodes and to a decrease in the presence of lithium polysulfides. In sulfolane solutions of LiNO3 and LiSO3CF3, lithium polysulfides do not affect the cycling duration of lithium metal anodes.

Study on the Influence of CaO on the Electrochemical Reduction of Fe2O3 in NaCl-CaCl2 Molten Salt.

The presence of calcium-containing molten salts in the electrolysis of oxides for metal production can lead to the formation of CaO and, subsequently, the generation of intermediate products, affecting the reduction of metals. To investigate the impact of CaO on the reduction process, experiments were conducted using a Fe2O3-CaO cathode and a graphite anode in a NaCl-CaCl2 molten salt electrolyte at 800 °C. The electrochemical reduction kinetics of the intermediate product Ca2Fe2O5 were studied using cyclic voltammetry and I-t curve analysis. The phase composition and morphology of the electrolysis products were analyzed using XRD, SEM-EDS, and XPS. The experimental results demonstrate that upon addition of CaO to the Fe2O3 cathode, Ca2Fe2O5 is formed instantly in the molten salt upon the application of an electrical current. Research conducted at different voltages, combined with electrochemical analysis, indicates that the reduction steps of Ca2Fe2O5 in the NaCl-CaCl2 molten salt are as follows: Ca2Fe2O5 ⟶ Fe3O4 ⟶ FeO ⟶ Fe. The presence of CaO accelerates the electrochemical reduction rate, promoting the formation of Fe. At 0.6 V and after 600 min of electrolysis, all of the Ca2Fe2O5 is converted into Fe, coexisting with CaCO3. With an increase in the electrolysis voltage, the electrolysis product Fe particles visibly grow larger, exhibiting pronounced agglomeration effects. Under the conditions of a 1 V voltage, a study was conducted to investigate the influence of time on the reduction process of Ca2Fe2O5. Gradually, it resulted in the formation of CaFe3O5, CaFe5O7, FeO, and metallic Fe. With an increased driving force, one gram of Fe2O3-CaO mixed oxide can completely turn into metal Fe by electrolysis for 300 min.

Electrochemical Reduction and Preparation of Cu-Se Thermoelectric Thin Film in Solutions with PEG.

Investigation of Cu(II) and Se(IV) electrochemical reduction processes in solutions with poly(ethylene glycol) (PEG) provides important theoretical guidance for the preparation of Cu-Se alloy films with stronger thermoelectric properties. The results reveal that PEG adsorbing on the electrode surface does not affect the electrochemical reduction mechanism of Cu(II), Se(IV), and Cu(II)-Se(IV), but inhibits the electrochemical reduction rates. The surface morphology and composition change with a negative shift in the deposition potentials. The Cu-Se alloy film, which was prepared at 0.04 V, was α-Cu2Se as-deposited and P-type thermoelectric material after annealing. The highest thermoelectric properties were as follows: Seebeck coefficient (α) was +106 μV·K−1 and 1.89 times of Cu-Se alloy film electrodeposited in Cu(II)-Se(IV) binary solution without PEG; resistivity (ρ) was 2.12 × 10−3 Ω·cm, and the calculated power factor (PF) was 5.3 μW·cm−1K−2 and 4.07 times that without PEG.

Efficient removal of uranium (VI) with a phytic acid-doped polypyrrole/ carbon felt electrode using double potential step technique

In order to efficiently extract uranium from uranium-containing wastewater, a novel acid doped polypyrrole/carbon felt (PA-PPy/CF) electrode was prepared via a facile electrodeposition method. For this material, PA and PPy combined to form a stable chemical structure by a charge compensation mechanism. The electrochemical characterization results showed that PA-PPy can significantly accelerate the electrochemical reduction rate of uranium ions. Moreover, a double potential step technique (DPST) was applied to prevent water splitting and maintained the electrocatalytic reduction activity of the surface groups during the electrochemical adsorption process. The removal efficiency obtained by the DPST method was six times higher than that obtained by the conventional chemical adsorption. When the concentrations of uranyl nitrate were 10, 20, 50, and 100 mg/L, the removal efficiencies of uranium were 98.8%, 98.1%, 94.6%, and 93.7%, and the adsorption capacities of uranium were 164.7, 326.9, 788.5, and 1562.0 mg/g, respectively. This material also showed an excellent recycling performance and remarkable selectivity for uranium ions. This work may shed light on the development of removal system for uranium (VI).

Electrochemical deposition of uranium oxide with an electrocatalytically active electrode using double potential step technique

The development of effective uranium-removal techniques is of great significance to the environment and human health. In this work, a double potential step technique (DPST) was applied to remove U(VI) from uranium-containing wastewater using a carbon felt electrode modified by graphene oxide/phytic acid composite (GO-PA@CF). The application of DPST can inhibit water splitting and prevent GO-PA from adsorbing other interfering ions in wastewater. The GO-PA composite can effectively accelerate the electrochemical reduction rate of U(VI), which significantly improved the electrochemical deposition rate of uranium oxide. As a result, the maximum removal efficiency and maximum removal capacity of GO-PA@CF electrode reached 98.7% and 1149.3 mg/g, respectively. The removal efficiency remained 97.2% after five cycles of reuse. Moreover, the removal efficiency of GO-PA@CF electrode can reach more than 70% in simulated wastewater.

Effect of stabilizing and brightening agents and some operated conditions on electroplating kinetics of NiCu alloys from citrate‐sulfate solutions

AbstractThe effect of stabilizing and brightening additives, temperature and stirring on electrodeposited kinetics of NiCu alloy has been studied by the linear polarization method. The additives almost do not affect on the electrochemical reduction rate of Cu2+ ions, however it has a much affect on the reduction rate of Ni2+ ions. The presence of boric acids, saccharin or 1,4‐butynediol have increased cathode polarization, inhibited precipitation alloy and shifted discharge potential of Ni2+ about 100 to 150 mV towards more negative. The solution temperature strongly affected on the polarization curve over the full range of investigated electrode potential, the cathode current increased about 1.5 times when the temperature increases from 30 to 50 oC. The stirring did not increase the discharge current of Ni2+ ions, on the contrary, it significantly increased the discharge current of Cu2+ ions, increasing from 2 to 3 times compared to when did not stir.

Modeling the Performance of A Flow-Through Gas Diffusion Electrode for Electrochemical Reduction of CO or CO2

A flow-through gas diffusion electrode (GDE) consisting of agglomerate catalysts for CO or CO2 reduction, gas channels for reactants, aqueous electrolytes for ionic transport, and metallic current collectors was simulated and evaluated using a numerical model. The geometric partial current densities and Faradaic Efficiencies (FE) for CH4, C2H4 and H2 generation in GDEs were calculated and compared to the behavior of analogous aqueous-based planar electrodes. The pH-dependent kinetics for CH4 and C2H4 generation were used to represent the intrinsic catalytic characteristics for the agglomerate catalyst. The modeling indicated that relative to planar electrodes for either CO reduction (COR) or CO2 reduction (CO2R), substantial increases in electrochemical reduction rates and Faradaic efficiencies are expected when flow-through GDEs are used. The spatially resolved pH and reaction rates within the flow-through GDEs were also simulated for two different operating pHs, and the resulting transport losses were analyzed quantitatively. For CO2 reduction, substantial loss of CO2 via chemical reaction with the locally alkaline electrolyte was observed due to the increased pH in operating GDEs.

Polypyrrole – Gold nanocomposites. Templateless synthesis and electrochemical properties

A novel approach is used for templateless synthesis of polypyrrole – gold nanocomposite material, taking advantage of polypyrrole nanoparticles synthesis method resulting in nanostructures of unblocked surface. Our approach leads to unsupported, nanocomposite system, where presence of gold nanostructures enables hydrogen ions reduction. Electrochemical methods: cyclic voltammetry, chronoamperometry or rotating disc electrode voltammetry allow to monitor the high rate of hydrogen ions transfer through the conductive polymer and their electrochemical reduction rate. These measurements confirm high active surface area of gold nanostructures immobilized in polypyrrole. On the other hand, electrochemical impedance spectroscopy reveals pseudocapacitive properties of the material, typical for polypyrrole nanostructures. Thus, the obtained nanocomposite material shows joint and unhindered properties typical for gold nanoparticles and polypyrrole nanospheres.

Electrochemical behaviors and influence factors of copper and copper alloys cathode for electrocatalytic nitrate removal.

The electrocatalytic activities of a series of copper alloys, Cu/Ni/Zn (Cu60 Ni15 Zn25 ) and Cu/Zn (Cu62 Zn38 ), toward the reduction of nitrate were investigated, in comparison with that of pure copper. Electrochemical analysis showed that the copper alloy electrode exhibited higher electrochemical reduction rate of nitrate. The extreme difference (R) between the orthogonal experiments revealed that the concentration was the main determinant of the removal percentage, followed by the current density and electrolyte concentration, while the impact of the initial pH was minimal. The conditions of the electrolysis experiments with Cu/Ni/Zn and Cu/Zn cathodes were optimized as follows: a current density of 8mA/cm2 , a NaCl concentration of 2.0g/L, and an initial pH of 3.0. The nitrate reduction reaction process with copper alloy cathodes was confirmed by electrochemical analysis and electrolysis experiments. Therefore, copper alloyed with Zn and Ni is a feasible option for practical application to the electrocatalytic reduction of nitrate. PRACTITIONER POINTS: Alloy of Cu, Zn, and Ni improved electrochemical nitrate reduction reaction. Electrochemical reduction of nitrate was confirmed in the presence of NaCl. The optimized current density with copper alloy cathodes was 6mA/cm2 . A feasible strategy was provided for the improving nitrate removal and minimizing by-products.

Electrochemical reduction of graphene oxide on biomedical grade CoCr alloy

In the present work, the characterization of Electrochemically Reduced Graphene Oxide (ErGO) films on a biomedical grade CoCr alloy has been performed. The direct electrodeposition process was carried out by means of two electrochemical techniques: chronoamperometry and cyclic voltammetry experiments, both taking into account the influence of oxygen in the solution. Characterization of the resulting ErGO films on CoCr alloys was carried out by SEM, AFM, Raman spectroscopy and XPS. Both electrochemical reduction procedures led to the deposition of graphene-based films on the CoCr surfaces after the partial removal of the oxygenated functional groups present in the graphene network of the starting material. The deconvolution of the XPS high-resolution C1s and O1s spectra mainly showed Csp2 bonding and the presence of CO and CO residual groups covering the CoCr surface. Moreover, the ErGO films that were stacked on the CoCr surfaces exhibited non-uniform thickness reaching values corresponding to the presence of 80 layers of the graphene material. In addition, Raman spectroscopy revealed a certain structural disorder in the ErGO films deposited by means of the different electrochemical techniques investigated. This fact was more evident on the film from the chronoamperometric experiments, as the fastest electrochemical reduction rate provided by the application of a high reduction potential (−2.1 Vvs. Ag/AgCl) could induce the development of defects in the resulting film, so the peaks corresponding to the CoCr substrate could be observed.

Effect of Si Addition on the Electrochemical Reduction Rate of SiO2 Granules in Molten CaCl2

The effect of adding Si powder/granules on the electrochemical reduction rate of SiO2 granules in molten CaCl2 was investigated. Various starting materials were prepared by mixing different amounts (up to 80 mass pct) of Si powder/granules (four different sizes: < 5, 20 to 100, 100 to 250 µm, 0.5 to 1.0 mm) with SiO2 granules, with these materials then being electrochemically reduced for 20 minutes in molten CaCl2 at 0.6 V vs Ca2+/Ca and a temperature of 1123 K (850 °C). The effects of the size and amount of Si added on the reaction rates were then studied. When the size of the Si powder added is less than 5 µm, the reaction rate decreases as more Si is added. When Si with a size of 20 to 100 or 100 to 250 µm is used, the reaction rate does not change substantially with the amount added. When 0.5 to 1.0 mm Si is used, the reduction rate is the largest when 40 mass pct Si is added. The reduction rate is influenced by the reaction surface and the diffusion of O2− ions in the mixture.

Enhancement of the direct electron transfer to encapsulated cytochrome c by electrochemical functionalization with a conducting polymer

The present work deals with the direct electrochemistry of cytochrome c (cyt c) encapsulated within a methyl-modified silica film prepared by the sol-gel method. It was observed that the voltammetric currents of the redox processes grow in proportion to the protein amount inserted within the gel up to a limiting value. From that point, recorded currents remain independent of the amount of protein loaded. Such a behavior indicates that a portion of cytochrome c molecules is located in non-accessible sites of the silica matrix. The electrochemical insertion of poly(3,4-ethylenedioxythiophene) (PEDOT) through silica pores was carried out to connect those electrically isolated protein molecules and in situ UV–vis spectroscopy was used to gain information on the redox process of encapsulated cyt c. It was observed that the presence of conducting polymer gives rise to a 3-fold enhancement in cyt c electrochemical reduction rate at the initial stages of the treatment, irrespective of the amount of inserted polymer. The faster electrochemical reduction rate was obtained after the insertion of 6.45μg PEDOT within a silica gel containing 0.5mg protein. The presence of low conductivity domains in the conducting polymer or the hindrance to the protein free movement could explain that higher amounts of inserted PEDOT cannot improve the electrochemical reduction rate.

Carbon monoxide/carbon dioxide electrochemical conversion on patterned nickel electrodes operating in fuel cell and electrolysis cell modes

Patterned Ni electrodes on single-crystal YSZ are applied as the fuel electrode for solid oxide cells (SOC) to investigate the electrochemical performance in CO/CO2 atmospheres in this paper. Patterned Ni maintains stable and intact during the whole test at high temperature (600 °C–700 °C). The polarization curves and EIS are measured with different operating voltages, temperatures, partial pressure of CO and CO2. The experimental results indicate that the patterned Ni electrode in SOEC mode performed a positive correlation with temperature, partial pressure of CO and CO2. The influence of partial pressure of CO2 is very weak, mainly due to the limited adsorption sites of CO2 on patterned Ni electrode. Besides, the rate of electrochemical reduction of CO2 is obviously slower than that of electrochemical oxidation of CO at the same temperature and gas component. Surface diffusion has a much greater effect on SOEC than on SOFC, especially when polarization voltage is small. Finally, a simplified reaction mechanism of CO2 electrolysis is proposed. The rate determining step of CO2 reduction on patterned Ni electrode could be the charge transfer reaction O(Ni) + (YSZ) + 2e− → O2−(YSZ) + (Ni). When the polarization voltage of electrolysis is small, the surface diffusion of O(Ni) also could become rate co-determining.

Faraday efficiency and mechanism of electrochemical surface reactions: CO2 reduction and H2 formation on Pt(111).

An atomic scale model of the electrical double layer is used to calculate the mechanism and rate of electrochemical reduction of CO2 as well as H2 formation at a Pt(111) electrode. The water layer contains solvated protons and the electrode has excess electrons at the surface. Density functional theory within the generalized gradient approximation is used to describe the electronic structure while the mechanism and activation energy of the various elementary reactions is obtained by calculating minimum energy paths using the nudged elastic band method. The applied electrical potential is deduced from the calculated work function. The optimal reaction mechanism for CO2 reduction to either methane or methanol is found and the estimated rate compared with that of the competing reaction, H2 formation. When the free energy of only the intermediates and reactants is taken into account, not the activation energy, Pt(111) would seem to be a good electrocatalyst for CO2 reduction, significantly better than Cu(111). This, however, contradicts experimental findings. Detailed calculations reported here show that the activation energy for CO2 reduction is high for both Heyrovsky and Tafel mechanisms on Pt(111) in the relevant range of applied potential. The rate-limiting step of the Heyrovsky mechanism, *COOH + H+ + e- → *CO + H2O, is estimated to have an activation energy of 0.95 eV at -0.9 V vs. standard hydrogen electrode. Under the same conditions, the activation energy for H2 formation is estimated to be only 0.5 eV. This explains why attempts to reduce CO2 using platinum electrodes have produced only H2. A comparison is made with analogous results for Cu(111) [J. Hussain et al., Procedia Comput. Sci., 2015, 51, 1865] where a reaction mechanism with low activation energy for CO2 electroreduction to methane was identified. The difference between the two electrocatalysts is discussed.

Hybrid electrode materials based on ion-exchange matrix containing copper nanoparticles and carbon fibers for the electroreduction of nitrate ions

Copper-containing composite electrodes have been obtained by the chemical deposition of copper into the film on an electrode composed of an ion-exchange matrix and carbon fibers. The size of copper particles determined by X-ray diffraction analysis amounts to 23–29 nm and is not practically changed with a growth in the copper concentration in the electrode material. The active area of the surface of copper particles is determined electrochemically using the underpotential electrodeposition of lead ions. It is shown that the rate of electrochemical reduction of nitrate ions increases with the concentration of copper particles and reaches its maximum at 90 μg/cm2. The electroreduction of nitrate ions proceeds in the mode of mixed kinetics and is an irreversible process.

Production of nano-sized chromium carbide powders from Cr2O3/C precursors by direct electrochemical reduction in molten calcium chloride

Chromium carbides were synthesized by direct electrolysis of non-sintering Cr2O3/C precursors in CaCl2 molten at 1123K. A series of electrolysis experiments were conducted with different Cr2O3/C molar ratios, cell voltages and electrolysis time. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) techniques, respectively. The results showed that the single-phase Cr7C3 powders with a mean diameter of <200nm were prepared after electrolysis at 2.8V for 5h in 1123K CaCl2 melt. By prolonging the electrolysis time to 10h, the product was converted into a Cr7C3 and Cr3C2 composite. Carbon was not only used as a raw material, but could also accelerate the rate of electrochemical reduction of Cr2O3. The reaction procedure forming the chromium carbides involved the electro-deoxidation of Cr2O3 and combined with carbon simultaneously.

Effect of the Anode-to-Cathode Distance on the Electrochemical Reduction in a LiCl-Li2O Molten Salt

Electrochemical reductions of UO2 at various anode-to-cathode distances (1.3, 2.3, 3.2, 3.7 and 5.8 cm) were carried out to investigate the effect of the anode-to-cathode distance on the electrochemical reduction rate. The geometry of the electrolysis cell in this study, apart from the anode-to-cathode distance, was identical for all of the electrolysis runs. Porous UO2 pellets were electrolyzed by controlling a constant cell voltage in molten Li2O-LiCl at 650 o C. A steel basket containing the porous UO2 pellets and a platinum plate were used as the cathode and anode, respectively. The metallic products were characterized by means of a thermogravimetric analyzer, an X-ray diffractometer and a scanning electron microscope. The electrolysis runs conducted dur- ing this study revealed that a short anode-to-cathode distance is advantageous to achieve a high current density and accelerate the electrochemical reduction process.

Thin film nonstoichiometric chromium oxide-based cathode material for rechargeable and primary lithium batteries

The application of nonstoichiometric chromium oxide-based thin film cathodes in lithium rechargeable and primary batteries operating at high rates has been demonstrated. Films of varying composition have been obtained by anionic Cr (VI) species electrodeposition on a 1X18N10Т grade stainless steel cathode from fluoride-containing electrolytes. The effect of film doping by Li+ ions during its electrosynthesis has been сonsidered. As-prepared films were studied by scanning and transmission electron microscopies, 3D optical profiler, thermogravimetric analysis, chemical analysis, and X-ray diffraction (XRD). The main phase components of the electrodeposited films regardless of Li+ in an electrolyte are Cr2O3, α-CrOOH, β-CrOOH, and metallic chromium as shown by XRD pattern refinement. The electrochemical reduction rate in a non-aqueous electrolyte (1 M LiClO4 in PC/DME) correlates with the chromium oxide-hydroxide component content of film. Primary CrO x -Li CR2325 mock-up cathode coating can be discharged in a pulsed mode at 10 Ω external resistance with 80–84 mA cm−2 current densities for 10–100 ms. Thin film cathodes electrodeposited in the presence of lithium ions become rechargeable when the lithium content of the film reaches 0.02 wt.%. Mock-ups of CR2325 coin battery with a thin film cathode doped with lithium ions can be discharged more than 40 times with 136 mAh g−1 specific capacity, 461 Wh kg−1 specific energy and 154 W kg−1 power density at 30 kΩ external resistance. The simplicity of thin film preparation makes this technology promising for thin film lithium batteries.

Effect of the UO2 form on the electrochemical reduction rate in a LiCl–Li2O molten salt

Electrochemical reductions of various UO2 forms were investigated in a molten LiCl–Li2O electrolyte. The study focused on the influence of their sizes and densities on the reduction rate by running experiments with eight UO2 forms. They can be classified into porous and dense forms. The porous forms are one granule and four porous pellets with different densities (55%, 60%, 70% and 80%), all of which were fabricated in our laboratory. The dense forms were prepared by crushing dense pellets, and these were similar in size to the porous pellets and the granules. Systematic comparisons of the reduction rate among the tested UO2 forms revealed that a lower density and smaller size of UO2 led to a faster reduction rate. Particularly, attention was focused on the observation that the size of the UO2 provides more dominant effect on the reduction rate than the density.

Application of SECM in tracing of hydrogen peroxide at multicomponent non-noble electrocatalyst films for the oxygen reduction reaction

The redox competition mode of scanning electrochemical microscopy (RC-SECM) was used to study the electrocatalytic activity of three different non-noble metal O2 reduction catalysts at a pH value of 7.4, namely; multi-walled carbon nanotubes (MWCNTs), cobalt protoporphyrin (CoP) and a composite of MWCNTs/CoP. The collection efficiency of a scanning electrochemical microscopy (SECM) tip for the H2O2 generated by the reduction of O2 at the catalyst layer was almost 100%. Consequently, SECM experiments in a combined redox competition and generator/collector mode could be applied for the determination of the number of electrons exchanged during O2 reduction, leading to improved understanding of the intrinsic features of catalyst activity. This approach avoids the typical limitations encountered with rotating ring disk electrode (RRDE) voltammetry, notably, the variation of the quantity of H2O2 in the proximity of the electrode with the speed of electrode rotation or the chemical decomposition of reaction intermediates on the Pt ring, which often introduce inconsistencies and errors in the measured values of the number of exchanged electrons. It is commonly assumed that the O2 reduction reaction on most non-noble metal catalysts proceeds via formation of H2O2 as an intermediate. The follow-up reaction of H2O2, typically chemical decomposition or electrochemical reduction, influences the overall number of electrons exchanged during O2 reduction. In this study, we have confirmed by comparing the rate of electrochemical reduction of H2O2 using rotating disk electrode (RDE) measurements with its rate of chemical decomposition studied using a positioned SECM tip, that for the MWCNTs/CoP catalyst, chemical decomposition is predominantly determining the overall number of exchanged electrons per O2 molecule.