Modelling of three-dimensional bipolar electrodes with irreversible reactions

The behaviour of an electrochemical reactor with three-dimensional bipolar electrodes for irreversible reactions is analysed. Copper deposition at the cathodic side and oxygen evolution at the anodic one were adopted as test reactions at the bipolar electrode, from an electrolyte solution with a copper concentration lower than 1000 mg dm⁻³, pH 2 and 1 M Na₂SO₄ as supporting electrolyte. A mathematical model considering the leakage current is proposed, which can represent the tendency observed in the experimental data related to cathodic thickness and potential at both ends of the bipolar electrode. High values of leakage current were determined, which restricts the faradaic processes to small thicknesses at both ends of the bipolar electrode. Likewise, the performance of the bipolar electrochemical reactor for the treatment of effluents is experimentally and theoretically examined. In this case, the conversion for copper removal was 90.1% after 480 min of operation with one bipolar electrode and 94.8% after 300 min of operation with two bipolar electrodes at a total current of 3 A.     

Electrochemical investigation of Li–Al anodes in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight 500 g mol⁻¹ was investigated as a new electrolyte (OEGDME500, 1 M LiPF₆) for metal deposition and battery applications. At 25 °C a conductivity of 0.48 × 10⁻³ S cm⁻¹ was obtained and at 85 °C, 3.78 × 10⁻³ S cm⁻¹. The apparent activation barrier for ionic transport was evaluated to be 30.7 kJ mol⁻¹. OEGDME500, 1 M LiPF₆ allows operating temperature above 100 °C with very attractive conductivity. The electrolyte shows excellent performance at negative and positive potentials. With this investigation, we report experimental results obtained with aluminum electrodes using this electrolyte. At low current densities lithium ion reduction and re-oxidation can be achieved on aluminum electrodes at potentials about 280 mV more positive than on lithium electrodes. In situ X-ray diffraction measurements collected during electrochemical lithium deposition on aluminum electrodes show that the shift to positive potentials is due to the negative Gibbs free energy change of the Li–Al alloy formation reaction.     

Composite electrodes for electrochemical supercapacitors

Manganese dioxide and Ag-doped manganese dioxide powders were prepared by a chemical precipitation method using KBH₄ as a reducing agent. The powders were studied by X-ray analysis, thermogravimetry, and electron microscopy. Composite electrodes for electrochemical supercapacitors (ES) were fabricated by impregnation of slurries of the precipitated powders and carbon black into porous nickel foam current collectors. In the composite electrodes, carbon black nanoparticles formed a secondary conductivity network within the nickel foam cells. Obtained composite electrodes, containing manganese dioxide and 20 wt% carbon black with total mass loading of 50 mg cm⁻², showed a capacitive behavior in the 0.5 M Na₂SO₄ solutions. The capacitive behavior of the composite electrodes can be improved by mixing of manganese dioxide and carbon black in solutions or using Ag-doped manganese dioxide powders. The highest specific capacitance (SC) of 150 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹. The electrodes showed good cycling behavior with no loss in SC during 1,000 cycles.     

An understanding of the electrodeposition process of Al–Mg alloys using an organometallic-based electrolyte

Al–Mg alloys were deposited using a base-electrolyte with the composition Na[AlEt₄] + 2Na[Et₃Al–H–AlEt₃] + 2.5AlEt₃ + 6toluene (where Et = −C₂H₅). Mg was introduced into this electrolyte by employing a pure Mg anode. It was found that initially the amount of Mg in the electrolyte increased with the deposition time but eventually a steady state was reached such that the amount of Mg dissolved at the anode became equal to that deposited at the cathode. Compositional and phase analyses indicated that this state is achieved at a critical Mg/Al ratio that resulted in the formation of the hcp Mg-rich phase. By devising various component electrolytes we have attempted to understand the roles of different compounds in the base-electrolyte and have proposed a scheme for the Al–Mg alloy deposition.     

Improved capacitive behavior of electrochemically synthesized Mn oxide/PEDOT electrodes utilized as electrochemical capacitors

A sequential synthetic approach and a one-step method were adopted to synthesize Mn oxide/PEDOT electrodes through anodic deposition on Au coated Si substrates from aqueous solutions. In the former case, free standing Mn oxide rods (about 10 μm long and less than 1.5 μm in diameter) were first synthesized without a template through anodic deposition from a dilute solution of Mn acetate, then coated by electro-polymerization of a conducting polymer (PEDOT) giving coaxial rods. The one-step, co-electrodeposition method produced agglomerated Mn oxide/PEDOT particles. The electrochemical behavior of the deposits depended on the morphology and crystal structure of the fabricated electrodes, which were affected by the pH of electrolyte, deposition potential, current density and polymer deposition time. Structural characterization of as-deposited and cycled electrodes was conducted using XPS, SEM, TEM and AES.The Mn oxide/PEDOT coaxial core/shell electrodes prepared by the sequential method showed significantly better specific capacity and redox performance properties relative to both uncoated Mn oxide rods and co-electrodeposited Mn oxide/PEDOT electrodes. The best specific capacitance for Mn oxide/PEDOT rods produced sequentially was ∼285 F g⁻¹ with ∼92% retention after 250 cycles in 0.5 M Na₂SO₄ at 20 mV s⁻¹.     

Nano-crystalline diamond electrodes with cap layer decorated by gold particles

We describe the electrochemical characteristics of a nano-crystalline diamond (NCD) electrode with a thin (～ 60 nm) low-doped cap layer on a highly boron-doped diamond contact layer, where the grain boundary areas of the surface are decorated by gold particles by electroplating. The presence of the surface cap layer leads to a reduced background current compared to the highly doped NCD electrodes. At the same time, the electrical resistance to the gold particles via grain boundary defects of the cap layer does not limit the activity of the particles in 0.1 M H₂SO₄ electrolyte. The equivalent circuit of the decorated cap-layer electrodes is discussed using the results of electrochemical impedance spectroscopy and data on gold–diamond Schottky diodes fabricated on identical NCD layers. This equivalent circuit can be reduced to a low number of key elements, which could be useful for optimizing such electrodes for high sensitivity/low noise applications.     

Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

Electrodeposition of AuCo alloys and multilayers

AuCo alloys and Au/Co multilayers have been fabricated by electrodeposition from non cyanide-containing electrolytes. The effect of pH and citric acid concentration on the deposit composition and current efficiency was investigated. Results showed that a lower citric acid concentration (0.47 m) at pH 6.15 was favored for multilayered Au/Co deposits with disparate compositions in each layer. An increase in the citric acid concentration or pH requires a larger applied current density to achieve the same cobalt concentration, and results in a drop of current efficiency.     

Hollow Sodium Tungsten Bronze (Na0.15WO3) Nanospheres: Preparation, Characterization, and Their Adsorption Properties

Abstract  

We report herein a facile method for the preparation of sodium tungsten bronzes hollow nanospheres using hydrogen gas bubbles as reactant for chemical reduction of tungstate to tungsten and as template for the formation of hollow nanospheres at the same time. The chemical composition and the crystalline state of the as-prepared hollow Na_0.15WO₃ nanospheres were characterized complementarily, and the hollow structure formation mechanism was proposed. The hollow Na_0.15WO₃ nanospheres showed large Brunauer–Emment–Teller specific area (33.8 m² g⁻¹), strong resistance to acids, and excellent ability to remove organic molecules such as dye and proteins from aqueous solutions. These illustrate that the hollow nanospheres of Na_0.15WO₃ should be a useful adsorbent.     

Electrodeposition and Capacitive Behavior of Films for Electrodes of Electrochemical Supercapacitors

Polypyrrole films were deposited by anodic electropolymerization on stainless steel substrates from aqueous pyrrole solutions containing sodium salicylate and tiron additives. The deposition yield was studied under galvanostatic conditions. The amount of the deposited material was varied by the variation of deposition time at a constant current density. SEM studies showed the formation of porous films with thicknesses in the range of 0–3 μm. Cyclic voltammetry data for the films tested in 0.5 M Na₂SO₄ solutions showed capacitive behavior and high specific capacitance (SC) in a voltage window of 0.9 V. The films prepared from pyrrole solutions containing tiron showed better capacitive behavior compared to the films prepared from the solutions containing sodium salicylate. A highest SC of 254 F g⁻¹ was observed for the sample with a specific mass of 89 μg cm⁻² at a scan rate of 2 mV s⁻¹. The SC decreased with an increasing film thickness and scan rate. The results indicated that the polypyrrole films deposited on the stainless steel substrates by anodic electropolymerization can be used as electrodes for electrochemical supercapacitors (ES).     

Electrochemical production of Ti-Al alloys using TiCl4-AlCl3-1-butyl-3-methyl imidazolium chloride (BmimCl) electrolytes

Electrochemical production of Ti-Al alloys was investigated using TiCl₄-AlCl₃-1-butyl-3-methyl imidazolium chloride (BmimCl) electrolytes (molar ratio 0.019:2:1). The experiments were conducted at different temperatures between 70 and 125 ± 3 °C and at various cell voltages between 1.5 and 3.0 V. Morphology and composition of deposited Ti-Al alloys were characterized using scanning electron microscope (SEM), along with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The Ti-Al alloys containing about 15-27 at% Ti were produced with a current efficiency of about 25-38%. TiCl₃ passivation on electrodes hinders the deposition kinetics and hence very low cathodic current density and cathodic current efficiency was obtained. This study also focused to determine the effect of process variables such as applied voltage, electrolyte composition and temperature on cathode current density, current efficiency, composition and morphology of Ti-Al alloys. The optimized condition for producing finer particle size and high Ti content in Ti-Al alloys was obtained between the applied voltages of 1.5-2.0 V and temperature ranges from 70 to 100 °C.     

Determination of hydrogenation ability and exchange current of H<Subscript>2</Subscript>O/H<Subscript>2</Subscript> system on hydrogen-absorbing metal alloys

A detailed analysis of potential versus time measurements at galvanostatic charge/discharge conditions (external current change from −1 to +1 mA cm⁻²) for two La–Ni alloys in Ar-saturated 0.1 M KOH solution is presented. It is shown that passivation of the electrodes does not affect the potential jump as a result of current switching over. The value of potential jump allows to calculate the exchange current density for H₂O/H₂ system on the tested material. Anodic potential of the hydrogenated electrode (at i _a = const) linearly increases with logarithm of time which allows to evaluate precisely time necessary for oxidation of hydrogen absorbed during cathodic charging. The method described enables to determine effectiveness of hydrogen absorption by materials applied for negative electrodes of NiMH batteries.     

A high-throughput study of PtNiZr catalysts for application in PEM fuel cells

The effects of adding Zr to PtNi oxygen reduction reaction (ORR) electrocatalyst alloys were examined in a study aimed at probing the possibility of creating catalysts with enhanced resistance to corrosion in a PEM fuel cell environment. Samples consisting of pure Pt or PtNiZr alloys with a range of compositions (not exceeding 11 at.% Zr) were fabricated using co-sputter deposition. A high-throughput fabrication approach was used wherein 18 distinct thin film catalyst alloy samples with varying compositions were deposited onto a large-area substrate with individual Au current collector structures. A multi-channel pseudo-potentiostat allowed for the simultaneous quantitative study of catalytic activity for all 18 electrodes in a single test bath, a first for the study of ORR electrocatalysts. A properly stirred oxygenated 1 M H₂SO₄ electrolyte solution was used to provide each electrode with a steady-state flow of reactants during electrochemical evaluation. The onset potentials, absolute current density values, and Tafel analysis data obtained using this technique were compared with literature reports. The analyses showed that most PtNiZr alloys tested offered improvements over pure Pt, however those surfaces with a high mole fraction (>4 at.%) of Zr exhibited reduced activity that was roughly inversely correlated to the amount of Zr present. Film composition, morphology, and crystallographic properties were examined using X-ray energy dispersive spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS), SEM, and synchrotron X-ray diffraction. These data were then correlated with electrochemical data to elucidate the relationships between composition, structure, and relative performance for this ternary system.     

Electrochemical investigation of Al–Li/Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FePO<Subscript>4</Subscript> cells in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight, 500 g mol⁻¹ (OEGDME500, 1 M LiPF₆), was investigated as an electrolyte in experimental Al–Li/LiFePO₄ cells. More than 60 cycles were achieved using this electrolyte in a Li-ion cell with an Al–Li alloy as an anode sandwiched between two Li _x FePO₄ electrodes (cathodes). Charging efficiencies of 96–100% and energy efficiencies of 86–89% were maintained during 60 cycles at low current densities. A theoretical investigation revealed that the specific energy can be increased up to 15% if conventional LiC₆ anodes are replaced by Al–Li alloy electrodes. The specific energy and the energy density were calculated as a function of the active mass per electrode surface (charge density). The results reveal that for a charge density of 4 mAh cm⁻² about 160 mWh g⁻¹ can be reached with Al–Li/LiFePO₄ batteries. Power limiting diffusion processes are discussed, and the power capability of Al–Li/LiFePO₄ cells was experimentally evaluated using conventional electrolytes.     

Incorporation of zirconia nanoparticles into coatings formed on aluminium by AC plasma electrolytic oxidation

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na₆P₆O₁₈ or Na₂SiO₃ · 5H₂O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na₂SiO₃ · 5H₂O/KOH electrolyte revealed γ-Al₂O₃ and amorphous phase; α-Al₂O₃ and AlPO₄ were additionally produced with the Na₆P₆O₁₈ electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting ∼70–90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating.     

Fe3O4 spinel-Mn3O4 spinel supercapacitor prepared using Celestine blue as a dispersant,capping agent and charge transfer mediator

An asymmetric spinel-spinel supercapacitor is fabricated with negative and positive electrodes respectively consisting of Fe₃O₄ and Mn₃O₄ nanoparticles, where carbon nanotubes (CNT) serve as conductive additives. High performance of the individual electrodes and devices is achieved at a high active mass (AM) loading of 40 mg cm⁻² of the individual electrodes. We implement a conceptually new strategy using multifunctional Celestine blue (CB) dye, which is strongly adsorbed on the spinel phases and CNT, facilitates dispersion, acts as a capping agent and allows for the fabrication of spinel decorated CNT. CB is an efficient charge transfer mediator, which allows for significant improvement of capacitive behavior. The use of CB as a charge transfer mediator allows for good utilization of capacitive properties of spinels at high AM. Mechanisms of spinel-CB-CNT interactions and charge transfer mediation are discussed. The capacitive properties of electrodes with different spinel/CNT mass ratios are tested by cyclic voltammetry, chronopotentiometry and impedance spectroscopy. The areal capacitances of 6.17 and 5.15 F cm⁻² are obtained for Fe₃O₄ and Mn₃O₄ based electrodes, respectively in 0.5 M Na₂SO₄ electrolyte. The high capacitances are achieved for the electrodes that have low resistance. Using these electrodes, an asymmetric device is fabricated that has a capacitance of 2.41 F cm⁻² in a voltage window of 1.6 V.     

Composite Electrodes for Electrochemical Supercapacitors

Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 4–6 nm were prepared by a chemical precipitation method. Composite electrodes for electrochemical supercapacitors were fabricated by impregnation of the manganese dioxide nanofibers and multiwalled carbon nanotubes (MWCNT) into porous Ni plaque current collectors. Obtained composite electrodes, containing 85% of manganese dioxide and 15 mass% of MWCNT, as a conductive additive, with total mass loading of 7–15 mg cm⁻², showed a capacitive behavior in 0.5-M Na₂SO₄ solutions. The decrease in stirring time during precipitation of the nanofibers resulted in reduced agglomeration and higher specific capacitance (SC). The highest SC of 185 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ for mass loading of 7 mg cm⁻². The SC decreased with increasing scan rate and increasing electrode mass.     

Study of the influence of a boric–sorbitol complex on Zn–Mn electrodeposition and on the morphology, chemical composition, and structure of the deposits

Zn–Mn electrodeposition onto Pt from an electrolyte containing boric–sorbitol complex (BSC) or boric acid alone (BA) was studied. The influence of BA or BSC content on the deposition process was investigated by cyclic voltammetry and electrodeposits, produced potentiostatically, were analysed by SEM, EDX, and XRD. The voltammetric studies indicated that an increase in the BSC concentration led to a decrease in the deposition current density. EDX analysis of deposits obtained at −1.60 V showed that increasing the BA or BSC concentration in the bath induced a fall in the Mn content of the electrodeposit and that for BSC this decrease was more significant. SEM images showed that the Zn–Mn electrodeposit obtained in the presence of 0.24 M BSC were smoother than other deposits; hence, BSC acted as a grain refiner at this concentration. XRD analysis of this deposit indicated that it was composed of Zn, Mn, MnZn₁₃, and MnH_0.8.     

ZnNi alloy electrodeposition from acid baths containing sorbitol or glycerol and characterization of ZnNi deposits

The influence of sorbitol or glycerol on the electrodeposition of ZnNi alloys and on the morphology, composition and structure of the ZnNi deposits was investigated. The highest current efficiency (CE), around 90%, was obtained in the presence of glycerol in the potential range from approximately −1.30 V to −1.40 V, while in the presence of sorbitol or absence of either polyalcohol the CE was 82–85%, for the same potential range. Scanning electron microscopy (SEM) analysis showed that ZnNi deposition at −1.26 V or −1.40 V from a bath with sorbitol led to the formation of more compact deposits than with glycerol. Energy dispersive X-ray spectroscopy (EDS) analysis showed that the Ni content in the deposit obtained in the presence of sorbitol remained in the range of 7–9.5 wt% Ni, over a large range of deposition conditions. On the other hand, ZnNi deposits with variable Ni content (5.5–19.5 wt% Ni) were obtained from baths with glycerol or without either polyalcohol, by shifting the deposition potential. All ZnNi deposits showed uniform distribution of the elements Zn and Ni. X-ray analysis of ZnNi deposits obtained from plating baths with and without polyalcohol’s at −1.26 and −1.40 V presented the γ, γ¹ and Pt₃–Zn phases.     

Synthesis and characterization of La0.75Sr0.25Cr0.9M0.1O3 perovskites as anodes for CO-fuelled solid oxide fuel cells

A series of La_0.75Sr_0.25Cr_0.9M_0.1O₃ (M = Mn, Fe, Co, Ni) perovskite compounds was synthesized by a modified citrate sol-gel route and employed as anode electrodes on YSZ electrolyte supported SOFC cells. Materials and anode electrodes were characterized for their chemical composition, crystal structure and film morphology. The electrochemical performance of the prepared anodes was evaluated in button cells under SOFC operation with CO/CO₂ mixtures in the temperature range of 900–1000 °C. It was shown that the performance of the perovskite materials in terms of maximum power density follows the sequence Fe > Ni > Co > Mn, based on the substitution cation into the B-site. No carbon deposition was observed under the operating conditions examined, even for prolonged (120 h) exposure to the reaction mixture.