Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br₂/Br⁻ in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge-discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge-discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm⁻². The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.     

Nanocomposites of manganese oxides and carbon nanotubes for aqueous supercapacitor stacks

Symmetrical supercapacitors and their serially connected two-cell stacks via a bipolar electrode were constructed with nanocomposites of manganese oxides and carbon nanotubes (MnO_x/CNTs) as the electrode materials. Nanocomposites with different contents of MnO_x were synthesised through the redox reaction between KMnO₄ and CNTs in aqueous solutions. The nanocomposites were characterised by scanning and transmission electron microscopy, BET nitrogen adsorption and X-ray diffraction before being examined in a three-electrode cell with a novel trenched graphite disc electrode by electrochemical means, including cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The nanocomposites demonstrated capacitive behaviour in the potential range of 0-0.85 V (vs Ag/AgCl) in aqueous KCl electrolytes with less than 9% capacitance decrease after 9000 charging-discharging cycles. Symmetrical supercapacitors of identical positive and negative MnO_x/CNTs electrodes showed capacitive performance in good agreement with the individual electrodes (e.g. 0.90 V, 0.53 F, 1.3 cm²). The bipolarly connected two-cell stacks of the symmetrical cells exhibited characteristics in accordance with expectation, including a doubled stack voltage and reduced internal resistance per cell.     

A laccase-glucose oxidase biofuel cell prototype operating in a physiological buffer

Here we report on the design and study of a biofuel cell consisting of a glucose oxidase-based anode (Aspergillus niger) and a laccase-based cathode (Trametes versicolor) using osmium-based redox polymers as mediators of the biocatalysts’ electron transfer at graphite electrode surfaces. The graphite electrodes of the device are modified with the deposition and immobilization of the appropriate enzyme and the osmium redox polymer mediator. A redox polymer [Os(4,4′-diamino-2,2′bipyridine)₂(poly{N-vinylimidazole})-(poly{N-vinylimidazole})₉Cl]Cl (E⁰′ = −0.110 V versus Ag/AgCl) of moderately low redox potential is used for the glucose oxidizing anode and a redox polymer [Os(phenanthroline)₂(poly{N-vinylimidazole})₂-(poly{N-vinylimidazole})₈]Cl₂ (E⁰′ = 0.49 V versus Ag/AgCl) of moderately high redox potential is used at the dioxygen reducing cathode. The enzyme and redox polymer are cross-linked with polyoxyethylene bis(glycidyl ether). The working biofuel cell was studied under air at 37 °C in a 0.1 M phosphate buffer solution of pH range 4.4-7.4, containing 0.1 M sodium chloride and 10 mM glucose. Under physiological conditions (pH 7.4) maximum power density, evaluated from the geometric area of the electrode, reached 16 μW/cm² at a cell voltage of 0.25 V. At lower pH values maximum power density was 40 μW/cm² at 0.4 V (pH 5.5) and 10 μW/cm² at 0.3 V (pH 4.4).     

Nickel foam and carbon felt applications for sodium polysulfide/bromine redox flow battery electrodes

The first use of nickel foam (NF) as electrocatalytic negative electrode in a polysulfide/bromine battery (PSB) is described. The performance of a PSB employing NF and polyacrylonitrile (PAN)-based carbon felt (CF) as negative and positive electrode materials, respectively, was evaluated by constant current charge-discharge tests in a single cell. Charge/discharge curves of the cell, positive and negative electrodes show that the rapid fall in cell voltage is due to the drop of positive potential caused by depletion of Br₂ dissolved in the catholyte at the end of discharge. Cell voltage efficiency was limited by the relatively high internal ohmic resistance drop (iR drop). Polarization curves indicated that both NF and CF have excellent catalytic activity for the positive and negative redox reactions of PSB. The average energy efficiency of the single cell designed in this work could be as high as 77.2% at 40 mA cm⁻² during 48 charge-discharge cycles.     

Improved performance of iron-chromium flow batteries using SnO2-coated graphite felt electrodes

Polyacrylonitrile-based graphite felt has the properties of high temperature resistance, corrosion resistance, low thermal conductivity, large surface area and excellent electrical conductivity. It has become the preferred material for flow battery electrodes, but its chemical activity is poor. In order to improve the electrochemical activity of graphite felt electrodes, the electrodes were prepared by SnO₂-coated graphite felt. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to analyze the microscopic morphology of SnO₂-coated graphite felt electrodes. Electrochemical impedance spectroscopy, cyclic voltammetry and charge-discharge tests were performed using an electrochemical workstation to investigate the electrocatalytic activity of SnO₂-coated graphite felt electrodes and their cell performance. The results show that the SnO₂ coating on the graphite felt surface forms a convex and concave microstructure, which further increases the specific surface area of the electrode, and at the same time successfully introduces oxygen-containing functional groups to the electrode surface, increasing the electrochemically active spots on the surface. In addition, the presence of oxygen defects in the SnO₂ crystal structure provides more electrochemically active sites and improves the electrochemical performance of the graphite felt electrode. At a current density of 142 mA cm^?2, the charge-discharge capacity of the battery assembled with the SnO₂-coated graphite felt electrode was significantly improved; when the current density was 250 mAcm^?2, the Coulombic efficiency of the electrode (TGF-2) coated with a concentration of 0.1 M could reach 84%.     

Hydrophilic carbon nanoparticle-laccase thin film electrode for mediatorless dioxygen reduction: SECM activity mapping and application in zinc-dioxygen battery

Laccase from Cerrena unicolor was adsorbed on hydrophilic carbon nanoparticles (diameter = ca. 7.8 nm) modified with phenyl sulfonate groups and immobilized on an ITO electrode surface in a sol-gel processed silicate film. As shown by scanning electron and atomic force microscopies, the nanoparticles are evenly distributed on the electrode surface forming small aggregates of tens of nanometers in size. The mediator-free electrode exhibits significant and pH-dependent electrocatalytic activity towards dioxygen reduction. The maximum catalytic current density (95 μA cm⁻²) is obtained at pH 4.8 corresponding to maximum activity of the enzyme. Under these conditions dioxygen electroreduction commences at 0.575 V vs. Ag|AgCl_sat, a value close to the formal potential of the T1 redox centre of the laccase. The scanning electrochemical microscopy images obtained in redox competition mode exploiting mediatorless electrocatalysis show that the laccase is evenly distributed in the composite film. The obtained electrode was applied as biocathode in a zinc-dioxygen battery operating in 0.1 M McIlvaine buffer (pH 4.8). It provides 1.48 V at open circuit and a maximum power density 17.4 μW cm⁻² at 0.7 V.     

Surface modification of graphitized carbonaceous materials by electropolymerization of thiophene and their effects on electrochemical properties

Highly oriented pyrolytic graphite (HOPG) and graphitized carbonaceous thin films prepared by plasma-assisted chemical vapor deposition (PACVD) were surface-modified by electropolymerization of thiophene. The electrochemical properties of the carbonaceous materials were studied by cyclic voltammetry and ac impedance spectroscopy. Irreversible cathodic current of the carbonaceous materials above 0.5 V (vs. Li/Li⁺) in the cyclic voltammograms significantly decreased by electropolymerization of thiophene, indicating that electropolymerization of thiophene suppress the decomposition of electrolytes on the carbonaceous materials. On the Nyquist plots, a semi-circle due to surface film resistance was observed, and the value significantly decreased at around 1.5 V. At potentials below 0.9 V, another semi-circle appeared in the middle to lower frequency region, which was assigned to the charge transfer resistance due to lithium-ion transfer at the surface-modified carbon electrode/electrolyte interface. The charge-transfer resistances were dependent on electrode potentials. The activation energy for lithium-ion transfer through interface between the surface-modified HOPG electrode and electrolyte was evaluated, and the value was almost identical to that obtained for an untreated HOPG electrode. Based on these results, it is concluded that electropolymerization of thiophene played an important role not in the phase transfer kinetics of lithium-ion but in reduction of the electrolyte decomposition at a graphite electrode.     

A stability comparison of redox-active layers produced by chemical coupling of an osmium redox complex to pre-functionalized gold and carbon electrodes

The production of stable redox active layers on electrode surfaces is a key factor for the development of practical electronic and electrochemical devices. Here, we report on a comparison of the stability of redox layers formed by covalently coupling an osmium redox complex to pre-functionalized gold and graphite electrode surfaces. Pre-treatment of gold and graphite electrodes to provide surface carboxylic acid groups is achieved via classical thiolate self-assembled monolayer formation on gold surfaces and the electro-reduction of an in situ generated aryldiazonium salt from 4-aminobenzoic acid on gold, glassy carbon and graphite surfaces. These surfaces have been characterized by AFM and electrochemical blocking studies. The surface carboxylate is then used to tether an osmium complex, [Os(2,2′-bipyridyl)₂(4-aminomethylpyridine)Cl]PF₆, to provide a covalently bound redox active layer, E^0′ of 0.29 V (vs. Ag/AgCl in phosphate buffer, pH 7.4), on the pre-treated electrodes. The aryldiazonium salt-treated carbon-based surfaces showed the greatest stability, represented by a decrease of <5% in the peak current for the Os(II/III) redox transition of the immobilized complex over a 3-day period, compared to a decrease of 19% and 14% for the aryldiazonium salt treated and thiolate treated gold surfaces, respectively, over the same period.     

Novel bipolar electrodes for battery applications

A novel bipolar graphite felt electrode for use in redox flow batteries and other electrochemical systems is described. The new electrode features a unique approach in the design of bipolar electrodes, employing carbon black free, nonconductive polymer materials as substrates. This innovation allows a dramatic reduction of processing time and cost compared to conventional carbon polymer composite electrodes used in bipolar battery systems. The conductivity of the new electrode assembly is similar to that of conventional bipolar electrodes, however, it shows significant improvements in mechanical properties. The functionality of these novel electrodes has been evaluated in the vanadium redox battery application and the results show comparable performance with conventional composite materials. An important operational advantage, however, is that side reactions leading to the deterioration of conductive filler in the electrode substrate material (i.e., electrode delamination due to CO₂-evolution) during cell overcharging are eliminated, making these electrodes more durable than the conventional designs. To date, these bipolar electrodes have been applied in vanadium redox cells but their design and properties promise further applications in a range of other redox flow batteries and bipolar electrochemical cell systems.     

Novel graphite/TiO2 electrochemical cells as a safe electric energy storage system

A graphite/TiO₂ full cell has been developed as a new safety energy storage system using a highly safety process. The crystal structures of the anatase TiO₂ electrode have been investigated with respect to the performance of the electrodes. Due to the large anion intercalation into the graphite positive electrode, the possible charging potential can be raised to around 5.3 V against the Li/Li⁺ electrode, which is a higher charging voltage than lithium-ion batteries (maximum voltage is around 4.3 V vs. Li/Li⁺). In situ XRD measurements have been carried out on both the cathode and anode electrodes of the graphite/TiO₂ cell during the charge process to elucidate the intercalation mechanism.     

Corrosion behavior of a positive graphite electrode in vanadium redox flow battery

The graphite plate is easily suffered from corosion because of CO₂ evolution when it acts as the positive electrode for vanadium redox flow battery. The aim is to obtain the initial potential for gas evolution on a positive graphite electrode in 2 mol dm⁻³ H₂SO₄ + 2 mol dm⁻³ VOSO₄ solution. The effects of polarization potential, operating temperature and polarization time on extent of graphite corrosion are investigated by potentiodynamic and potentiostatic techniques. The surface characteristics of graphite electrode before and after corrosion are examined by scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The results show that the gas begins to evolve on the graphite electrode when the anodic polarization potential is higher than 1.60 V vs saturated calomel electrode at 20 °C. The CO₂ evolution on the graphite electrode can lead to intergranular corrosion of the graphite when the polarization potential reaches 1.75 V. In addition, the functional groups of COOH and CO introduced on the surface of graphite electrode during corrosion can catalyze the formation of CO₂, therefore, accelerates the corrosion rate of graphite electrode.     

On the electrochemical performance of anthracite-based graphite materials as anodes in lithium-ion batteries

The electrochemical performance as potential negative electrode in lithium-ion batteries of graphite materials that were prepared from two Spanish anthracites of different characteristics by heat treatment in the temperature interval 2400-2800 °C are investigated by galvanostatic cycling. The interlayer spacing, d₀₀₂, and crystallite sizes along the c axis, L_c, and the a axis, L_a, calculated from X-ray diffractometry (XRD) as well as the relative intensity of the Raman D-band, I_D/I_t, are used to assess the degree of structural order of the graphite materials. The galvanostatic cycling are carried out in the 2.1-0.003 V potential range at a constant current and C/10 rate during 50 cycles versus Li/Li⁺. Larger reversible lithium storage capacities are obtained from those anthracite-based graphite materials with higher structural order and crystal orientation. Reasonably good linear correlations were attained between the electrode reversible charge and the materials XRD and Raman crystal parameters. The graphite materials prepared show excellent cyclability as well as low irreversible charge; the reversible capacity being up to ∼250 mA h g⁻¹. From this study, the utilization of anthracite-based graphite materials as negative electrode in lithium-ion batteries appears feasible. Nevertheless, additional work should be done to improve the structural order of the graphite materials prepared and therefore, the reversible capacity.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.     

Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte

Single-walled carbon nanotubes (SWCNTs) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in a non-aqueous electrolyte, 1 M Et₄NBF₄ in acetonitrile, suitable for supercapacitors. Further, in situ dilatometry and in situ conductance measurements were performed on single electrodes and the results compared to an activated carbon, YP17. Both materials show capacitive behavior characteristic of high surface area electrodes for supercapacitors, with the maximum full cell gravimetric capacitance being 34 F/g for YP17 and 20 F/g for SWCNTs at 2.5 V with respect to the total active electrode mass. The electronic resistance of SWCNTs and activated carbon decreases significantly during charging, showing similarities of the two materials during electrochemical doping. The SWCNT electrode expands irreversibly during the first electrochemical potential sweep as verified by in situ dilatometry, indicative of at least partial debundling of the SWCNTs. A reversible periodic swelling and shrinking during cycling is observed for both materials, with the magnitude of expansion depending on the type of ions forming the double layer.     

Characterization of a carbon composite electrode for an electrochemical immunosensor

A bioactive platform with a carbon composite electrode was developed for rapid detection of Escherichia coli O157:H7. The porous carbon composite electrode was prepared by a sol-gel method with a mixture of graphite powder and tetraethyl orthosilicate/ethanol. Escherichia coli O157:H7 antibodies were physically adsorbed onto the carbon composite electrode. Direct measurements by cyclic voltammetry and electrochemical impedance spectroscopy in the presence of [Fe(CN)₆]^3−/4− as a redox probe showed that the immobilization of antibodies onto the carbon composite electrode surface and the binding of Escherichia coli O157:H7 cells with antibodies systematically increased the electron-transfer resistance. Those results suggest that a sol-gel derived graphite composite electrode might be utilized as a label-free electrochemical immunosensor for diagnosis, biochemical research, food industry, and so on.     

Redox poly[Ni(saldMp)] modified activated carbon electrode in electrochemical supercapacitors

The complex (2,2-dimethyl-1,3-propanediaminebis(salicylideneaminato))-nickel(II), [Ni(saldMp)], was oxidatively electropolymerized on activated carbon (AC) electrode in acetonitrile solution. The poly[Ni(saldMp)] presented an incomplete coated film on the surface of carbon particles of AC electrode by field emission scanning electron microscopy. The electrochemical behaviors of poly[Ni(saldMp)] modified activated carbon (PAC) electrode were evaluated in different potential ranges by cyclic voltammetry. Counterions and solvent swelling mainly occurred up to 0.6 V for PAC electrode by the comparison of D^1/2C values calculated from chronoamperometry experiments. Both the Ohmic resistance and Faraday resistance of PAC electrode gradually approached to those of AC electrode when its potential was ranging from 1.2 V to 0.0 V. Galvanostatic charge/discharge experiments indicated that both the specific capacitance and energy density were effectively improved by the reversible redox reaction of poly[Ni(saldMp)] film under the high current density up to 10 mA cm⁻² for AC electrode. The specific capacitance of PAC electrode decreased during the first 50 cycles but thereafter it remained constant for the next 200 cycles. This study showed the redox polymer may be an attractive material in supercapacitors.     

Polyfurfuryl alcohol derived activated carbons for high power electrical double layer capacitors

Polyfurfuryl alcohol (PFA) derived activated carbons were prepared by the acid catalysed polymerization of furfuryl alcohol, followed by potassium hydroxide activation. Activated carbons with apparent BET surface areas ranging from 1070 to 2600 m² g⁻¹, and corresponding average micropore sizes between 0.6 and 1.6 nm were obtained. The porosity of these carbons can be carefully controlled during activation and their performance as electrode materials in electric double layer capacitors (EDLCs) in a non-aqueous electrolyte (1 M Et₄NBF₄/ACN) is investigated.Carbon materials with a low average pore size (<∼0.6 nm) exhibited electrolyte accessibility issues and an associated decrease in capacitance at high charging rates. PFA carbons with larger average pore sizes exhibited greatly improved performance, with specific electrode capacitances of 150 F g⁻¹ at an operating voltage window of 0-2.5 V; which corresponds to 32 Wh kg⁻¹ and 38 kW kg⁻¹ on an active material basis. These carbons also displayed an outstanding performance at high current densities delivering up to 100 F g⁻¹ at current densities as high as 250 A g⁻¹. The exceptionally high capacitance and power of this electrode material is attributed to its good electronic conductivity and a highly effective combination of micro- and fine mesoporosity.     

Graphite–graphite oxide composite electrode for vanadium redox flow battery

A graphite/graphite oxide (GO) composite electrode for vanadium redox battery (VRB) was prepared successfully in this paper. The materials were characterized with X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The specific surface area was measured by the Brunauer–Emmett–Teller method. The redox reactions of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ were studied with cyclic voltammetry and electrochemical impedance spectroscopy. The results indicated that the electrochemical performances of the electrode were improved greatly when 3 wt% GO was added into graphite electrode. The redox peak currents of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ couples on the composite electrode were increased nearly twice as large as that on the graphite electrode, and the charge transfer resistances of the redox pairs on the composite electrode are also reduced. The enhanced electrochemical activity could be ascribed to the presence of plentiful oxygen functional groups on the basal planes and sheet edges of the GO and large specific surface areas introduced by the GO.     

Enhancement of the Li ion transfer reaction at the LiCoO2 interface by 1,3,5-trifluorobenzene

This work presents a method of enhancing the kinetics of the interfacial reaction using 1,3,5-trifluorobenzene (TFB) which is used as an electron acceptor due to its locally biased polarity and as a source of rearranging the layer of the electrolyte around LiCoO₂ electrode, not a SEI layer source. The full cells with TFB show a decrease in irreversible capacity loss during the first charge-to-discharge process, regardless of the SEI layer formation, and also show better discharge properties even at high rate conditions. The charge transfer resistance (R_ct) of the LiCoO₂ half cell with TFB shows the smaller resistance than that of the TFB free half cell, and the activation energy calculated from the R_ct was 24.7 kJ/mol for the TFB free half cell and 19.3 kJ/mol for the half cell with TFB. In addition, the film resistance of the half cell with TFB shows higher value when the temperature is below 283 K. Since R_ct is related to the transfer resistance of the solvated Li⁺ ions on the surface of the LiCoO₂ electrode, it will help design the electrolyte to improve the transfer velocity of Li⁺ ions around the cathode electrode for high power Li ion battery.     

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70°C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W/cm² for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W/cm² for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance.