Exchange Current Density Values Research Articles

Exchange Current Density of SOFC Electrodes: Theoretical Relations and Partial Pressure Dependencies Rate-Determined by Electrochemical Reactions

As a theoretical consideration on electrode defect chemistry, general relations of exchange current density quantitatively representing Solid Oxide Fuel Cell (SOFC) electrode performance are systematically derived as a function of gas partial pressures, equilibrium constants of adsorption and dissociation reactions on electrode surfaces, and electrochemical reaction rate constants for possible elemental reactions at the cathode and the anode, in the case that an electrochemical reaction is the rate-determining electrode reaction. Simplified expressions are also derived, under the condition that one kind of neutral or charged adsorbed species is predominant at the electrode, to derive gas partial pressure dependence of exchange current density for given rate-determining electrochemical reactions. Importance of considering elementary steps is highlighted to derive rate equations and to clarify various dependencies. Partial pressure dependencies of the exchange current density are compiled and discussed by simulating normalized exchange current density values for given partial pressures. The applicability and limitation of the Butler-Volmer type expressions of exchange current density for SOFC electrodes are carefully discussed.

Ni3S2/carbon nanotube nanocomposite as electrode material for hydrogen evolution reaction in alkaline electrolyte and enzyme-free glucose detection

In this study, the nanocomposite of Ni3S2 and multi-walled carbon nanotubes (MWCNTs) with the high catalytic activities toward hydrogen evolution reaction (HER) and glucose oxidation was synthesized using glucose-assisted hydrothermal method. Ni3S2 nanoparticles with the diameters ranging from 10 to 80nm were highly dispersed over conductive MWCNT surface. A series of linear polarization measurements suggested that the HER activity of nanocomposite of Ni3S2 and MWCNTs was increased with decreasing the loading amount of Ni3S2 on MWCNTs and the optimal Ni3S2 loading on MWCNTs was 55wt%. Furthermore, the immersion of the composite catalyst in a concentrated KOH solution induced the morphological change of the Ni3S2 nanoparticles on MWCNTs, which increases the active surface area of the composite electrode. As a result, the KOH-treated composite electrode showed a higher HER activity than other electrodes. For example, the value of exchange current density of the KOH-treated composite electrode was ca. 395 times and 1.6 times larger than that of Ni3S2 electrode and as-synthesized composite, respectively. Furthermore, the impedance measurements showed the KOH-treated composite electrode had the smaller charge transfer resistance of the HER than Ni3S2 electrode. Based on the slopes obtained from Arrhenius curves of the electrodes, the estimated HER activation energy (71.8kJ/mol) of KOH-treated composite electrode was only one-third of that of the pure Ni3S2 electrode. The high catalytic activity of the KOH-treated composite electrode was stemmed from the synergistic effect of the large active surface area of Ni3S2 nanoparticles and the excellent electrical coupling to the conductive MWCNT network. More importantly, the current density of KOH-treated composite electrode showed no sign of degradation after the continuous 1000 cycling in a 1M KOH solution at the temperature of 323K. On the other hand, the nanocomposite of Ni3S2 and MWCNTs was proposed for the first time as an enzyme-free sensor for glucose. The Ni3S2 nanoparticles on MWCNTs exhibited high electrocatalytic activity toward glucose oxidation and were insensitive to uric acid and ascorbic acid. Furthermore, the composite electrode exhibited that its catalytic current was linearly dependent on the concentration of glucose in the range from 30 to 500μM and its sensitivity was as high as 3345μA/mM. The present work suggested that the nanocomposite of Ni3S2 and MWCNTs not only served as an inexpensive, highly active and stable electrode material for alkaline water electrolysis, but also showed a great potential application as a highly sensitive and selective biosensor for glucose.

Equiatomic ternary chalcogenide: PdPS and its reduced graphene oxide composite for efficient electrocatalytic hydrogen evolution.

The layered ternary chalcogenide, palladium phosphorous sulphide (PdPS), and its composite with reduced graphene oxide are shown to be efficient hydrogen evolution electrocatalysts. The Tafel slope and the exchange current density values associated with hydrogen evolution reaction are determined to be 46 mV dec(-1) and 1.4 × 10(-4) A cm(-2) respectively.

In situ activation with Mo of Ni–Co alloys for hydrogen evolution reaction

In this study Ni–Co alloys have been activated during hydrogen electrochemical production by adding Mo ions into the alkaline electrolyte. After dissolving different amounts of sodium molybdate in the Na(OH) electrolyte, Ni–Co alloys were used as cathodes for hydrogen evolution reaction. Afterwards a comparison between hydrogen overvoltage measured on Ni–Co alloys with and without in situ activation has been made. The in situ activation clearly shows an improvement of electrocatalytic properties of Ni–Co alloys for hydrogen evolution reaction. Depending on the alloy the best conditions are reached with different amounts of sodium molybdate in the electrolyte. The values of exchange current density for Ni–Co alloys without Mo, are an average of about 4.1 10−6 A/cm2, while by using in situ activation, these values are about 3.5·10−4 A/cm2. Therefore, exchange current density presents a value nearly one hundred-fold higher when molybdate ions are present in solution. Moreover, two Tafel slope values have been determined for HER on Ni–Co alloys with and without Mo in situ activation. Those Tafel slope values are different, so as their range of both overvoltage and current density, probably highlighting a different kinetic mechanism.

Electrodeposition of Ni–Co–Sn alloy from choline chloride-based deep eutectic solvent and characterization as cathode for hydrogen evolution in alkaline solution

The electrodeposition of Ni–Co–Sn alloy were carried out at room temperature from the chlorine chloride (ChCl)–ethylene glycol (EG) deep eutectic solvent (DES). For comparison of properties, Ni–Sn and Co–Sn alloys were also deposited using the same solvent. Deposition mechanism, microstructure, and electrochemical properties of the deposits in 1 M KOH solution were investigated. The deposition of Ni, Co, Sn, Ni–Sn, Co–Sn and Ni–Co–Sn on platinum electrode were also studied using cyclic voltammetry. Interestingly, the electrochemical stability of DES is observed to be increased in the presence of Sn2+ ions. The X-ray diffraction (XRD) patterns showed only Ni phases indicating that the other elements get incorporated inside the nickel matrix and the lattice constant have linear relation with Sn content in alloy. The morphologies of Ni–Sn and Ni–Co–Sn alloys were observed to be almost same with fine grains, the XRD studies confirm this. The potentiodynamic polarization measurements showed that the Ni–Co–Sn alloy exhibits the lowest corrosion current density (jcorr), noblest corrosion potential (Ecorr) and highest exchange current density (jc) value than the other two binary alloys, indicating that the ternary alloy is a good candidate for Hydrogen Evolution Reaction (HER).

Preparation and electrochemical characterization of Ruddlesden–Popper oxide La4Ni3O10 cathode for IT-SOFCs by sol–gel method

La4Ni3O10 oxide was synthesized as a cathode material for intermediate-temperature solid oxide fuel cells by a facile sol–gel method using a nonionic surfactant (EO)106(PO)70(EO)106 tri-block copolymer (F127) as the chelating agent. The crystal structure, electrical conductivity, and electrochemical properties of La4Ni3O10 were investigated by X-ray diffraction, DC four-probe method, electrochemical impedance spectra, and I–V measurements. The La4Ni3O10 cathode showed a significantly low polarization resistance (0.26 Ω cm2) and cathodic overpotential value (0.037 V at the current density of 0.1 A cm−2) at 750 °C. The results measured suggest that the diffusion process was the rate-limiting step for the oxygen reduction reaction. The La4Ni3O10 cathode revealed a high exchange current density value of 62.4 mA cm−2 at 750 °C. Furthermore, an anode-supported single cell with La4Ni3O10 cathode was fabricated and tested from 650 to 800 °C with humidified hydrogen (∼3 vol% H2O) as the fuel and the static air as the oxidant. The maximum power density of 900 mW cm−2 was achieved at 750 °C.

High capacity lithium-ion battery cathode using LiV3O8 nanorods

Nanorods of LiV3O8 are synthesized using citrate assisted sol–gel method followed by room temperate quenching technique. The physical parameters are identified by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), high resolution transmission electron spectroscopy (HR-TEM). The detailed kinetic studies of these electrodes comprising of different weight percentage of conductive carbon and fixed binder concentration are pursued by cyclic voltammetry (CV), in situ electrochemical impedance spectroscopy (EIS) experiments. The in situ EIS results during lithium-ion intercalation and deintercalation processes are discussed on the basis of equivalent electrical circuit model and analyzed by non linear least square fit method. The estimated kinetic parameters are analyzed after obtaining good agreement between fitted EIS spectra and experimental data. The change in the kinetic parameters with varying offset potentials is also discussed within the experimental limit. Due to high apparent lithium-ion diffusion in solid matrix as well as low charge transfer resistance and relatively high exchange current density value, made this nanorod materials attractive for high rate lithium-ion battery applications. Herein, LiV3O8 nanorods exhibit excellent electrochemical properties and demonstrate superior cycling behavior with first discharge capacity of 338mAhg−1, 283mAhg−1, 237mAhg−1 at 101mAg−1 for 30%, 20%, and 10% carbon additive, respectively.

DFT study of platinum and palladium overlayers on tungsten carbide: Structure and electrocatalytic activity toward hydrogen oxidation/evolution reaction

Platinum and palladium overlayers on W- and C-terminated WC(0001) surface, at coverage ranging from 0.25 ML to 2 ML, have been studied using DFT approach. Strong adhesion of metal monolayers to the WC support, accompanied by a modification of electronic structure, was evidenced. Calculated values of hydrogen binding energy on studied metal overlayers were correlated to available experimentally determined values of exchange current densities of hydrogen oxidation/evolution reaction (HOR/HER), resulting with the volcano curve with both explanatory and predictive power. None of the investigated metal/WC surfaces were found to exceed the Pt(111) surface in terms of catalytic activity toward HER/HOR. The obtained results indicate that WC may efficiently replace the precious metal support in the Pt (Pd) core–shell electrocatalyst for HER/HOR, but no synergism arising due to electronic effects of WC support was evidenced.

Numerical simulation and experiment in the systems of labile complexes of metals. Silver reduction from the thiourea electrolyte

Voltammograms of silver reduction from complex thiourea electrolytes with the ligand to metal concentration ratio varied from 1 to 50 were measured on the freshly prepared surface of silver electrode. The results were analyzed using the numerical model of this electrode process which takes into account homogeneous chemical reactions proceeding in solution, diffusion–migration transfer of the species present in solution, and electrode reactions participated by electroactive components of the electrolyte. We estimated the values of standard exchange current density for partial electrode reactions in which silver complexes with different number of ligands can take part. The highest electrochemical activity is exhibited in solutions with the low concentration of complex-forming agent – by silver aqua complexes, while in solutions with high thiourea concentration – by silver complexes with one ligand. It was found that kinetic hindrances to discharge of the thiourea silver complexes unlikely to be due to the adsorption of thiourea molecules blocking the growth sites or to the adsorption of complex silver ions. The results admit interpretation of the mechanism of electrode process either from the viewpoint of simultaneous discharge of all the complex silver ions present or from the viewpoint of discharge of silver complex with one ligand in combination with rapid chemical reactions of dissociation of other, non-discharging silver complexes.

Kinetics and mechanism of electrochemical oxygen reduction using Platinum/clay/Nafion catalyst layer for polymer electrolyte membrane fuel cells

This work demonstrates the use of amino functionalized Mg-phyllosilicate clay/Nafion nanocomposite film embedded with Pt nanoparticles (Pt/AC/N) for catalyzing oxygen reduction reaction (ORR) in sulphuric acid medium. Pt/AC/N nanocomposite films were surface characterized using transmission electron microscope. Cyclic and linear scan voltammetry studies were carried out under hydrodynamic conditions taking rotating-ring disc electrode (RRDE) as the working electrode. The effects of clay content, Pt mass loading, electrode rotation rate, and temperature on the ORR kinetics were studied. The Tafel slopes were found to vary between 118 and 126mVdec−1 indicating a good ORR kinetics. The exchange current density values calculated after mass transfer correction ranged from 5.8×10−7 to 2.4×10−6Acm−2. From the RRDE disc currents, Koutecky–Levich plots were constructed and the ORR mechanism was found to follow a four electron path with minimum H2O2 formation of ∼1.6%. The effect of temperature on ORR kinetics was found at 25, 40, and 50°C. The energy of activation calculated to be 7.68kJmol−1 and comparable to the standard Pt/C catalyzed ORR systems.

Investigation of the Effects of Catalyst Loading and Gas Flow Rate on Polymer Electrolyte Membrane (PEM) Fuel Cell Performance and Degradation

In this paper, the effects of gas flow rates and catalyst loading on polymer electrolyte membrane fuel cell (PEMFC) performance was investigated using a 50 cm2active area fuel cell fixture with serpentine flow field channels machined into poco graphite blocks. Membrane electrode assemblies (MEAs) with catalyst and gas flow rates at two levels each (0.5 mg/cm2, 1 mg/cm2; 0.3 l/min, 0.5 l/min, respectively) were tested at 60 °C without humidification. The cell performance was analyzed by taking ac impedance, Tafel plot, open circuit voltage, and area specific resistance measurements. It was observed that MEAs with lower gas flow rate had lesser cell resistance compared to MEAs with a higher gas flow rate. Tafel plot shows the highest exchange current density value of 10−2.05 mA cm2 for MEA with 0.5 mg/cm2 catalyst loading tested at reactant gas flow rate of 0.3 l/min signifying it had the least activation loss and fastest reaction rate. Open circuit voltage-time curve shows a higher output voltage and lesser voltage decay rate for MEAs tested at higher gas flow rates.

Solid oxide fuel cell with NiCo–YSZ cermet anode for oxidation of CO/H 2 fuel mixtures

The suggestion has been made in the literature that solid oxide fuel cells (SOFCs) operated with syngas as fuel may be viable in certain gas ratio regimes. We have explored this hypothesis with a promising bimetallic anode material. SOFCs with Ni 0.7Co 0.3–YSZ cermet anodes were operated with CO/H 2 mixtures in the full concentration range. Electrochemical impedance spectroscopy and voltammetry measurements were employed to measure the exchange current density ( i 0) values of each fuel mixture. The fuel mixtures of CO/H 2 ratios corresponding to the range 20/80 and 30/70 were found to have i 0 values larger than that of pure H 2 with the same cell. For these two fuel ratios, an improvement of 5–8 times, respectively, in the exchange current density has been observed. Higher CO/H 2 fuel ratios in the range of 60/40–80/20 produced i 0 values lower than H 2, as carbon poisoning is operational in this region. Continuous running of a cell with fuel ratio 25/75 CO/H 2 for 7 days produced i 0 values above the values for pure H 2 as has been recently suggested.

Characterization of nanocrystalline Co–W coatings on Cu substrate, electrodeposited from a citrate-ammonia bath

Cobalt–tungsten nanocrystalline coatings were electrodeposited on copper substrate using different current densities. The deposited coatings were single phase solid solution with an average grain size of about 18nm, showing a nodular type of surface morphology. By increasing the deposition current density, the density of nodules was increased, with no obvious variation in grain size. Electrochemical impedance spectroscopy (EIS) confirmed the codeposition of tungsten through reduction of tungsten oxide film formed during the electrodeposition process. However, the role of ternary complexes in the bath cannot be ruled out, especially at lower cathodic potentials. The Co–W coating deposited at lower current densities showed higher tungsten content, microhardness, wear resistance and friction coefficient. However, this coating showed an inferior corrosion resistance. By increasing the deposition current density, a low tungsten coating with high corrosion resistant was obtained. This is attributed to the lower value of exchange current density of water reduction in the present of oxygen (i0H2O) achieved on the coating with lower tungsten content.

Electrochemical properties of nitrogen-doped carbon nanotube anode in Li-ion batteries

Multi-walled nitrogen-doped carbon (CN x ) nanotubes, synthesized by an aerosol-assistant catalytic chemical vapor deposition technique, were investigated for electrochemical intercalation with lithium. Nanotube morphologies and nitrogen contents in the samples produced from toluene and acetonitrile taken in various proportions were determined by transmission electron microscopy and X-ray photoelectron spectroscopy. It was found that the first discharge capacity of CN x electrodes increases with nitrogen content. The irreversible capacity was partially attributed to pyridinic-like nitrogen atoms, which can strongly bind with Li ions confirmed by result of quantum-chemical calculation. The CN x electrode with the lowest nitrogen content (∼1 at.%) in the considered set of the samples showed the highest reversible capacity 270 mAh g − 1 at the current density of 0.2 mA cm − 2 and the highest value of exchange current density, suggesting that the electrochemical activity of carbon nanotubes is enhanced by light incorporation of nitrogen atoms.

Electrochemical performance of La 0.9Sr 0.1Co 0.8Ni 0.2O 3−δ–Ce 0.8Sm 0.2O 1.9 composite cathode for solid oxide fuel cells

The mixed ionic and electronic conductors (MIEC) of La 0.9Sr 0.1Co 0.8Ni 0.2O 3−δ (LSCN)–Ce 0.8Sm 0.2O 1.9 (SDC) were investigated for potential application as a cathode material for solid oxide fuel cells (SOFCs) based on a SDC electrolyte. Electrochemical impedance spectroscopy (EIS) technique was performed over the temperature range of 600–850 °C to determine the cathode polarization resistance, which is represented by area specific resistance (ASR). This study systematically investigated the exchange current densities ( i 0) for oxygen reduction reaction (ORR), determined from the EIS data and high-field cyclic voltammetry. The 70LSCN–30SDC composite cathode revealed a high exchange current density ( i 0) value of 297.6 mA/cm 2 at 800 °C determined by high-field technique. This suggested that the triple phase boundary (TPB) may spread over more surface of this composite cathode and revealing a high catalytically active surface area. The activation energies ( E a) of ORR determined from the slope of Arrhenius plots for EIS and high-field techniques are 96.9 kJ mol −1 and 90.4 kJ mol −1, respectively.

Electrocatalytic Activity Analysis of PEFC Cathode by 1-D Macrohomogeneous Model of Catalyst Layer

A simple macrohomogeneous 1-D model of the polymer electrolyte fuel cell (PEFC) cathode catalyst layer incorporating ORR kinetics and including the effects of Pt surface oxide coverage, proton migration, and oxygen diffusion is introduced. In the model, both the site-blocking and the energetic effects of the Pt surface oxide are incorporated and applied to actual fuel cell data. Measured polarization curves are fitted in both the cathodic and anodic directions using a single value of the intrinsic exchange current density (that is independent of the Pt oxide coverage) representing the entire potential range of operation. By fitting with this model, the catalytic activity is broken down into the intrinsic exchange current density, the oxide coverage, and the Temkin parameter. The fitting parameters used in the model are the exchange current density, the Temkin parameter, and the transmissibility of oxygen within catalyst layer. The electrochemically active surface area and the proton conduction resistance within catalyst layer were determined using experimental diagnostic techniques. The surface charge representing oxide species was also measured experimentally and converted into a surface coverage based on an assumed species. The analysis of polarization curves using this modeling approach may help to provide a deeper understanding that could accelerates the development and characterization of new catalysts.

Nickel–cobalt electrodeposited alloys for hydrogen evolution in alkaline media

Abstract A study on the electrocatalytic performances of nickel–cobalt alloys for hydrogen evolution in alkaline media has been carried out. After preparing, by electrodeposition on aluminium net supports, Ni–Co alloys of various compositions ranging from 0 to 100% Ni, the alloys were examined by polarization measurements, to evaluate their discharge potential and Tafel parameters. Moreover a durability test has been done on the alloys showing, among all, the best performance. The hydrogen over-potential appears to be lower in the case of Ni concentrations ranging between 35 and 59 weight percent. The synergism among the nickel catalytic properties, having low hydrogen over-potential and high hydrogen adsorption of cobalt, is best realized in this conditions and allow obtaining a largest value of exchange current density.

Cathodic polarization behavior of aluminum electrode containing iron in a substitute sea water

Attempt has been made to clarify an effect of iron element on cathodic behavior of aluminum in a substitute sea water. It is quantitatively shown that iron in aluminum shifts the cathodic polarization curve, which is the water reduction reaction, to noble direction. The values of exchange current density of water reduction reaction on FeAl3 intermetallic compound and 99.999%Al for the Tafel equation are experimentally estimated to 10−3.5 A·cm−2 and 10−7.0 A·cm−2, respectivery.

Oxygen Evolution Behavior of PTFE-F-PbO 2 Electrode in H 2SO 4 Solution

F-PbO 2 electrode and polytetrafluoroethylene (PTFE) doped F-PbO 2 electrode (PTFE-F-PbO 2) were prepared on a plexiglas sheet substrate by a series of procedure including chemical and electrochemical depositions. The electrochemical activities of these two electrodes for oxygen evolution (OE) reaction were examined by electrochemical tests. In comparison with F-PbO 2, PTFE-F-PbO 2 electrode exhibited larger active surface area and higher oxygen vacancy deficiency, which resulted in its higher electrocatalytic activity for OE. In addition, both exchange current density and activation energy of the electrodes for OE were calculated in terms of active surface area. The values of exchange current density and activation energy in 0.5 mol·L −1 H 2SO 4 aqueous solution were 1.125×10 −3 mA·cm −2 and 18.62 kJ·mol −1 for PTFE-F-PbO 2, and 8.384×10 −4 mA·cm −2 and 28.98 kJ·mol −1 for F-PbO 2, respectively. Because these values are calculated on the basis of the active surface areas of the electrodes, the enhanced activity of PTFE-F-PbO 2 can be attributed to an increase in oxygen vacancy deficiency of PbO 2 due to doping by PTFE. The influence of PTFE adulteration on the activity of PbO 2 film electrode for OE was investigated in detail in this study.

Production and electrochemical characterization of Ni-based composite coatings containing titanium, vanadium or molybdenum powders

Composite Ni + Ti, Ni + V and Ni + Mo coatings were prepared by codeposition of Ti, V or Mo particles in an Ni matrix on a carbon steel substrate from the nickel bath in which metallic powder was held in suspension. The influence of the metal powder amount in the bath, as well as the deposition current density on chemical composition of obtained coatings has been investigated. It was stated that the content of incorporated Ti, V and Mo increases with the increase in the particle concentration in the electrolyte, and diminishes with the increase in the deposition current density. The mechanism of metallic particles embedding was explained on the base of Ni 2+ ions adsorption process. Deposits exhibited a presence of Ti, V or Mo microsize particles embedded into the nanocrystalline nickel matrix. Incorporation of metallic powder into electrolytic nickel matrix results in the significant increase in the real surface area of the deposits. Electrodeposited composite coatings were tested as electrode materials for hydrogen evolution reaction (HER) in an alkaline environment. Electrochemical characterization was carried out by steady-state polarization method. All composite materials showed enhanced electrochemical activity for HER compared to the nickel electrode. Based on determined values of exchange current density– j 0 and the values of hydrogen evolution overpotential at 100 mA cm − 2 – η 100 the variation in the electrochemical activity of Ni + Ti, Ni + V and Ni + Mo composites in dependence on their chemical composition and the kind of incorporated component was evaluated. Comparison of the investigated materials leads to the statement that the highest activity towards the HER exhibit Ni + Mo deposits.