Overvoltage Of Hydrogen Evolution Research Articles

Kinetics of Ni–Mo electrodeposition from Ni-rich citrate baths

The kinetics of Ni–Mo alloy electrodeposition on steel substrates from an aqueous citrate–ammonia complex baths has been investigated by means of steady-state polarisation measurements in a system with a rotating disc electrode (RDE). Partial current densities for discharge of Ni(II) and Mo(VI) ions and hydrogen evolution as a function of molybdate concentration in the bath, cathode potentials and the rate of mass transport were determined. It has been shown that – under all investigated conditions – Ni–Mo alloy deposition is more favourable than pure nickel and the cathodic process is strongly influenced by the Mo(VI) content in the solution. The Ni(II) electroreduction rate initially increases, as the cathode potential shifts towards more negative values and the concentration of molybdate grows in the solution. However, for the highest examined MoO 4 2 − content, a considerable decrease in the rate of the process is subsequently observed at certain limit potentials, the values of which depend on molybdate concentration and hydrodynamic conditions. This effect, related to the formation of intermediate molybdenum oxides (characterised by very low overvoltage for hydrogen evolution), becomes less pronounced when the RDE rotation speed is increased. Hydrogen evolution is strongly associated with molybdenum deposition. An increase of the molybdate ions concentration in the bath, as well as an increase in the rate of mass transport, leads to an increase in Mo content in deposits and to the reduction of current efficiency. The Ni–Mo coatings electrodeposited from the designed bath (with the current efficiency of about 70%) containing about 30 wt.% Mo, are characterised by a shiny-grey appearance and good adhesion to the steel substrate. They are characterised by column growth and amorphous microstructure with randomly distributed nanocrystallites of the MoNi 4 intermetallic phase.

Electrochemical properties of porous bismuth electrodes

The properties of Bi surfaces with different roughnesses were characterized by electron microscopy, cyclic voltammetry, and impedance spectroscopy. Two different strategies were used for preparation of porous bismuth layers onto Bi microelectrode surface in aqueous 0.1 M LiClO 4 solution. Firstly, treatment at potential E < −2 V (vs. Ag|AgCl in sat. KCl) has been applied, resulting in bismuth hydride formation and decomposition into Bi nanoparticles which deposit at the electrode surface. Secondly, porous Bi layer was prepared by anodic dissolution ( E = 1 V) of bismuth electrode followed by fast electroreduction of formed Bi 3+ ions at cathodic potentials E = −2 V. The nanostructured porous bismuth electrode, with surface roughness factor up to 220, has negligible frequency dispersion of capacitance and higher hydrogen evolution overvoltage than observed for smooth Bi electrodes.

Effects of pretreatment of LM-Ni 3.9Co 0.6Mn 0.3Al 0.2 alloy powders in a KOH/NaBH 4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries

The discharge capacities of lanthanum-rich mischmetal (LM)-Ni 3.9Co 0.6Mn 0.3Al 0.2 alloy electrodes are significantly degraded by an increase in the C rate. Nevertheless, the discharge capacity of alloy electrodes pretreated with KOH/NaBH 4 is maintained higher than that of raw alloy electrodes, with the difference in discharge capacities between the raw and pretreated alloy electrodes more prominent at higher C rates. The charge retention of the electrodes decreases with increasing rest time. In particular, the charge retention of the pretreated alloy electrode is lower than that of the raw alloy electrode due to the higher self-discharge rate. The overvoltage for hydrogen evolution of the pretreated alloy electrode is superior to that of the raw alloy electrode, particularly at higher temperatures. This phenomenon indicates that the charge efficiency of the electrode was significantly improved by the surface pretreatment, resulting from its high surface catalytic activity. Repeated charge-discharge increases the inner pressure of the battery. Nevertheless, due to its higher charge efficiency and faster recombination rate, the inner pressure of the battery made using the pretreated alloy electrode is much smaller than that of the battery made using a raw alloy electrode.

Effect of Thiourea on the Kinetics of the Hydrogen Evolution Reaction at Iron and the Transport of Hydrogen through a Steel Membrane in Solutions of C2H4(OH)2-H2O-HCl

The effect of thiourea (0.5–10 mM) on the kinetics of the hydrogen evolution reaction (HER) at iron and the hydrogen transport through a steel membrane out of ethylene glycol (containing 2 and 10 wt % H2O) and aqueous solutions containing HCl (0.1–0.99 M) with a constant ionic strength equal to unity is studied in parallel experiments. The presence of 0.5 mM of thiourea in the solutions raises the overvoltage of hydrogen evolution, while a subsequent increase in its concentration does not effect the HER kinetics. The dependence of the flux of hydrogen diffusion through the membrane on the thiourea content passes through a maximum.

Peculiarities of the electrochemical behavior of modified electrodes containing single-wall carbon nanotubes

Impedance characteristics and voltammograms are measured for modified electrodes containing single-wall carbon nanotubes in neutral aqueous electrolyte solutions. Modifying nanotube electrodes by their exposure to a flow of heavy ions and highly active radicals is shown to substantially increase their low-frequency capacitance values and decrease the hydrogen evolution overvoltage. It was found that functional groups are not formed at nanotube surface under action of active radicals (OH and H). The phenomenon of autoelectron emission current from nanotubes to electrolyte solution at negative electrode potentials is experimentally evidenced.

Influence of sputtering parameters on hydrogen evolution overvoltage in sputter-deposited Co–Mo alloy electrode

Sputtering method was used to prepare the Co–Mo alloy electrode for the hydrogen evolution reaction in water electrolysis. We studied the influence of the sputtering parameters (Ar pressure during deposition, RF power, and substrate temperature) on the hydrogen evolution overvoltage. It was found that the overvoltage was significantly influenced by the Ar pressure which affected critically the Co–Mo surface geometrical structure. We referred to the Thornton model for the microstructure of sputter-deposited films to explain the observed structural change.

Electroreduction of Pentafluorobenzoic Acid

The literature information that the reduction of pentafluorobenzoic acid on metals with a high hydrogen-evolution overvoltage occurs via two parallel reactions, which yield penta- and tetrafluorobenzyl alcohol, is confirmed. In optimum conditions, the process selectivity by pentafluorobenzyl alcohol is 82–85%.

The effect of ultrasound on the electrochemical loading of hydrogen in palladium

Although the application of ultrasound for the control and modulation of various chemical processes is a subject of continued interest [l], there is surprisingly little discus- sion in the recent literature of the effects of ultrasound on fundamental electrochemical processes. In early systematic studies of the hydrogen evolution reaction under acidic conditions [2,3], it was observed that, on a platinum elec- trode, the overvoltage for hydrogen evolution decreased by up to 30mV on the application of ultrasound, and that this decrease was largely independent of current density in the approximate range l-100 mA cm-‘, i.e. the Tafel slope was largely unaffected under these conditions. On removal of the ultrasound, the original polarization was regained only on continued electrolysis for up to 1 h. These results refer to electrolytes which had been pre-electrolysed for 10 to 20h. With more extensive pre-electrolysis (36h), the results were generally similar; however, the polarization decrease on the application of ultrasound was smaller (by a factor of two) under comparable conditions, and the origi- nal polarization was recovered rapidly on cessation of irradiation. On the application of ultrasound to a nickel electrode evolving hydrogen, the same general phe- nomenon was observed, i.e. a decrease in overvoltage without a significant change in Tafel slope [3]. For these experiments, the acoustic frequency was 3OOkHz and the ;icoustic intensity was estimated to be approximately I Wcmw2 of electrode area.

Investigation of electrosorption of a quaternary ammonium salt using an Electrochemical Quartz Crystal Microbalance

Electrochemical Quartz Crystal Microbalance (EQCM) was applied to a study of electrosorption of Benzyldimethyldodecylammonium bromide (BDDB) on a gold coated quartz sensor from deaerated 0.1 M Na2SO4. The discrete voltammetry and mass curves obtained simultaneously made it possible to compute the effective equivalent molar mass (EMM) of the reaction species (the ratio of effective molecular weight of the species to the number of electrons exchanged) as a function of the potential applied.It is found that a cathodic mass increase of gold electrode in blank 0.1 M Na2SO4 solutions can be accounted for hydrogen evolution and the apparent quartz sensor response due to the change of the electrode-solution mechanical coupling. This effect completely disappears in the presence of a cationic surfactant as BDDB which increases the hydrogen evolution overvoltage. The low concentrations of BDDB as 10−5 M are found to inhibit anodic oxide formation on gold. At the concentrations higher than 10−4 M oxidation and reduction processes appear at 1.04 V and 0.69 M nhe respectively. The oxidation is accompanied by intensive electrosorption of organic species which are to a high extent desorbed upon the reduction. Although at higher concentrations, ca. 10% of the adsorbate remains on the surface upon the reduction.The EMM values obtained on the basis of the voltammetric cycles and for BDDB electrosorption at a constant potential give a clue to the analysis of the chemical transformations in the system. The process of electrosorption of dimethylbenzalkonium ion appears to be connected with the adsorption of the Br anions from the solution and the formation of the anodically adsorbed oxygen layer. The EMM value obtained for the electrosorption in stationary conditions was 260 g/mole. The ir and XPS spectroscopy data indicate that oxidation of BDDB results in a shrinkage of the alkyl chain and formation of amide and quinonic C=O bonds, which conforms with the obtained EMM value, assuming one electron exchange.

Electrochemical behavior of 2‐ and 8‐quinolinethiol complexes with cobalt and nickel and their catalytic hydrogen evolution

Abstract2‐ and 8‐quinolinethiols catalyze hydrogen evolution at −1.500 V and −1.380 V vs. Ag/AgCl, respectively. The more positive potential for hydrogen evolution in the case of the latter is a result of its more acidic character. The addition of cobalt shifts the potential overvoltage for hydrogen evolution to more positive potentials in the presence of 2‐quinolinethiol, i.e., 1.400 V vs. Ag/AgCl; while in the presence of 8‐ quinolinethiol a significant increase in height of the hydrogen wave at −1.380V vs. Ag/AgCl is observed. In the former case, hydrogen evolution follows reduction of the CoII‐2‐quinolinethio1 complex, and for higher cobalt concentrations the wave splits into two Brdicka's peaks. In the latter case, reduction of the CoII‐2‐quinolinethiol complex takes place at the same potential as the hydrogen wave and, as a result, only one peak is observed; this peak can be used for the determination of cobalt with a detection limit of 0.50 nM and a relative standard deviation (RSD) of 3.8%.

The effect of hydrophobic centres on the electrode surface on overvoltage in electrochemical gas evolution

Abstract Modification of the electrode surface of platinized platinum electrodes by a pattern consisting of a regular arrangement of PTFE sites causes a decline in the overvoltage of hydrogen evolution corresponding to a five—ten-fold increase in current density at constant overvoltage. The drop in total overvoltage equals the decrease in concentration polarization which is determined by gas supersaturation.

The oxidation of reduced glyoxal on gold polycrystalline electrodes in acidic solution

The oxidation of adsorbed glyoxal molecules reduced by hydrogen on gold electrodes in acidic solution has been studied. The adsorption of glyoxal on gold decreases the overvoltage for hydrogen evolution and provokes the adsorption and subsequent reaction of hydrogen on and with the glyoxal coverage. The oxidation mechanism of the reduced glyoxal is studied by means of pseudocapacitance- potential curves.

Kinetic correlations in codeposition of coatings of molybdenum-iron group metal alloys

Electrodeposition of Ni-Mo and Fe-Mo alloy coatings from acid citrate electrolyte was investigated over a wide range of cathode potentials and under various hydrodynamic conditions. Such alloys had been found to be deposited with substantial depolarization against iron group metals. With the same electrodeposition conditions, the molybdenum percentage in Fe-Mo coatings is much higher than in Ni-Mo. The molybdenum presence results in a lower overvoltage of hydrogen evolution, so that these alloys are deposited with a current efficiency much lower than that in the separate reduction of iron group metals. For a Ni-Mo system, the Mo percentage in the coating is increased together with increase of the transport rate of ions towards the cathode and with decrease of the cathode potential value being reduced; while for Fe-Mo alloy an inverse relation is observed. When passing from the Ni-Mo to the Fe-Mo system, the control of the molybdenum reduction reaction is changed. In codeposition with nickel it shows a diffusion control, while in discharging with iron its control is of the activation type. The Ni-Mo coatings obtained contained from 1.5–6% molybdenum whereas Fe-Mo deposits contained from 12–40% depending on electrolysis conditions. The quality of the coatings containing more than 20% Mo deteriorated and in these deposits some nonmetallic inclusions were present.

Electrochemical and surface characterization of the passive film on Fe-Cr-Pd alloys

Ferritic Fe-40Cr-0.1Pd, Fe-40Cr-0.2Pd and palladium-ion-implanted Fe-40Cr alloys were exposed to 0.5 M HCl solution at various temperatures. Electrochemical measurements show that the Fe-40Cr alloy containing a low concentration of palladium (0.13 wt.%) exhibits spontaneous passivation in 0.5 M HCl solution from 25 to 60 °C, and unstable passivation at 85 °C. However, the Fe-40Cr alloy with 0.23 wt.% Pd content undergoes spontaneous passivation in 0.5 M HCl solution from 25 to 85 °C. Surface implantation of palladium ions at a dose of 2.1 × 10 15 ions cm -2 leads to unstable passivation at 85 °C. Thus palladium doses higher than 2.1 × 10 15 ions cm -2 are needed for the maintenance of spontaneous passivation. Auger electron spectroscopy depth profiling and X-ray photoelectron spectroscopy (XPS) have been used for the characterization of the surface film formed on the Fe-40Cr-0.2Pd alloy in 0.5 M HCl solution at 25 °C. The AES depth profile shows that the surface film formed spontaneously on the Fe-40Cr-0.2Pd alloy in 0.5 M HCl solution is enriched in palladium relative to chromium. Furthermore, XPS measurements indicate that the palladium is incorporated in the spontaneous passive film as the Pd 2+ species. The enrichment of palladium in the spontaneous passive film permits a low overvoltage for hydrogen evolution and thereby promotes passivation of the Fe-40Cr-0.2Pd alloy. However, the AES depth profile reveals that there is an enrichment of chromium relative to iron in the passive film formed anodically on Fe-40Cr-0.2Pd alloy at 220 mV. The average chromium content [Cr]/ ([Cr] + [Fe]) reaches a value of about 0.765 within the passive film and decreases towards the film-substrate interface.

Amperometric oxygen sensors: problems with cathodes and anodes of metals other than silver

The ideal diffusion geometry of the metallised membrane electrode is used to show that silver is the ideal material for oxygen measurement with an amperometric sensor. Gold by itself has no particular advantages over silver, while the commonly used platinum suffers both from a low overvoltage for hydrogen evolution and usually from low catalytic activity for oxygen reduction to hydroxide ion. Problems arising from the use of a two-electrode “galvanic” system are discussed.

Lanthanum-molybdenum coating for hydrogen evolution on titanium

There is practical interest in the use of titanium as a cathode material in brine electrolysis processes. The overvoltage of hydrogen evolution on titanium is, however, very high and, at the same time, hydrogen penetrates the bulk of the metal. Here, a new type of catalytically active coating has been applied on the surface of titanium in order to overcome these problems. The coating consists of a mixture of the thermal decomposition products of lanthanum chloride and sodium molybdate. The coating has been tested for 18 months of continuous operation. No change in the overvoltage gain (of 0.4 V at 3 kA m−2, as compared with pure titanium) nor change in the properties of the bulk metal have been noticed during this period.

Hydrogen Overvoltage on Tin Self‐Cleaning Rotating Electrode (SRE) in Absence and Presence of Surface Active Ions

An investigation of hydrogen overvoltage on tin self‐cleaning rotating electrode (SRE) in sulfuric acid solution in the presence and absence of some halogen ions was carried out. It was established that the rate of the mechanical renewal exerts significant influence on the hydrogen evolution overvoltage decreasing it about 0.120V. It was found that the halogen ion adsorption practically does not change the magnitude of the overvoltage decrease which remains of the same order as it is in pure sulfuric acid. It was shown that the effective energy of activation of the hydrogen evolution process upon a continuously renewed surface decreases about two times in comparison with an unchanged surface.

Low overvoltage electrocatalysts for hydrogen evolving electrodes

An electrocatalyst for hydrogen evolving cathodes has been developed for use in alkaline media. The materials employed in manufacturing the coating are relatively cheap transition metals. The results of a variety of performance tests are reported. The electrodes exhibit a low overvoltage for hydrogen evolution (70–90 mV at 70°C and 1A cm −2 in 5 N KOH), stability to abuse (e.g. current interruption or reversal) and a long cathode life under operating conditions.

Determination of trace impurities in high-purity reagents by mercury thin-film anodic-stripping voltammetry

The use of an amalgamated silver-wire electrode, providing a relatively large rate of film area to volume of mercury, seems to present several advantages over the use of the hanging mercury drop electrode (HMDE) for the determination of trace impurities by anodic-stripping voltammetry (ASV). Peak resolution and sensitivity have both been improved by use of the mercury thin-film electrode (MTFE). Preparation, conservation and treatment of the electrode are simple, requiring brief etching of a silver wire with dilute nitric acid, followed by rinsing, drying and direct amalgamation with mercury. The MTFE is stored in mercury, the excess of which is wiped off before use; it is cleaned by anodic-stripping electrolysis at positive potentials. Under these conditions, ASV produces reproducible peak-heights, with excellent dependence on electrolysis time and concentration of impurities. Samples containing nanogram quantities or less of the impurities of interest, are analysed by a four-step procedure: (1) prolonged pre-electrolysis of the supporting electrolyte,in situ in the voltammetric cell, on an auxiliary MTFE; (2) ASV of the supporting electrolyte on a cleaned MTFE; (3) ASV after the dissolution of the sample; (4) ASV after the addition of standards. The method is suitable for the estimation of traces of Zn(II), Cd(II), Tl(I), Pb(II), Sb(III) and Cu(II) in various high-purity reagents. Most impurities have been adequately determined at the 10−9–10−8M level by the regular procedure, and at the 10−10–10−9M level may be determined if the preconcentration (electrolysis) time and rates of potential sweep are increased. The only disadvantage of the method is that the overvoltage of hydrogen evolution is lower for the MTFE than the HMDE.

Electrodeposition of Nickel from Ni(CF3COO)2, -HCONH2, Bath Containing Various Additives

Bright and adherent nickel depesits were obtained in a wider range of current densities from Ni(CF3COO)2-HCONH2 baths containing H3BO3, saccharin, sulfamic acid, urea, AlCl3 or NH4Cl, compared to Ni(CF3COO)2-HCONH2, bath without any additive.The addition of H3BO3 in Ni(CF3COO)2-HCONH3 bath gave adherent nickel deposits in a wider range ef current densities.The appearance of nickei deposits obtained from Ni(CF3COO)2-HCONH2 bath containing thiourea or saccharin was dull.For tre electrodeposition ef nickel-from Ni(CF3COO)2-HCONH2-H3BO3-thiourea bath, the apparent current efficiencies on both cathode and anode were about 120% at 60 C and 70 C.For the bath containing AlCl3 or NH4Cl the anode current efficiencies were high, and in the letter case the cathode current efficiency also high.The rate determining step of the electrodeposition of nickel from Ni(CF3COO)2-HCONH2bath containing additives, except thiourea, was the charge transfer reaction at current densities ranging between O.9 and 4 A/dm2.The activation overvoltage for hydrogen evolution on nickel was lewered by addition of H3BO3, sulfamic acid, chloride or thiourea into the CF3COOH-HCONH2 bath.