Hydrogen Electrode Reaction Research Articles

Electrochemical investigations of activation and degradation of hydrogen storage alloy electrodes in sealed Ni/MH battery

The MlNi 4.0Co 0.6Al 0.4 alloy was treated with hot alkaline solution containing a small amount of KBH 4 and its effect on the activation and degradation behaviors of the hydrogen storage alloy electrodes in sealed Ni/MH batteries was investigated. It was found that the treated alloy electrode exhibited a better activation property than the untreated one in the sealed battery as well as in open cell. For the treated alloy electrode activating, the polarization resistance in the sealed battery was almost equal to that in the open cell. But in the case of the untreated alloy electrode activating, the polarization resistance in the sealed battery was larger than that in the open cell. The reason is that the oxide film on the untreated alloy surface suppressed the combination of the oxygen evolved on the positive electrode with hydrogen on the negative alloy surface. In addition, the decaying of capacity of the untreated alloy electrode was much faster than that of the treated one. The reasons were, that after surface treatment, the Ni-rich and Al-poor layer on the alloy surface not only had a high electrocatalytic activity for hydrogen electrode reaction, but also facilitated the combination of the oxygen with hydrogen and hydrogen adsorption on the alloy surface.

Hydrogen adsorption on hydrogen storage alloy surface and electrochemical performances of the MlNi 4.0Co 0.6Al 0.4 alloy electrodes before and after surface treatment

The surfaces of the hydrogen storage alloy powders (MlNi 4.0Co 0.6Al 0.4) before and after surface treating were analyzed by X-ray photoelectron spectroscopy (XPS), electron probe micro-analysis (EPMA), inductively coupled plasma spectroscopy (ICP). It was found that a Ni-rich surface layer was produced and the specific surface area was augmented by the surface treatment with a hot 6 M KOH or 6 M KOH +0.02 M KBH 4 solution due to the preferential dissolution of Al and the etching of the hot alkaline solution. The effect of the surface treatments on the hydrogen adsorption on the alloy surface was for the first time evaluated by means of the thermal desorption spectroscopy (TDS). It was found that the surface treatments enhanced the hydrogen adsorption on the alloy surfaces and produced new adsorption states. The untreated alloy had only one hydrogen adsorption peak at about 400 K . There were two hydrogen adsorption states at about 540 and 630 K on the surface of the alloy treated with 6 M KOH . In the case of the alloy treated with 6 M KOH +0.02 M KBH 4 three hydrogen adsorption peaks were observed at about 390, 540 and 630 K , respectively. The results of electrochemical measurements showed that the treated alloy electrodes exhibited higher exchange current density for the hydrogen electrode reaction than the untreated hydrogen alloy. After the surface treatment, the hydrogen adsorption enhancement on the treated alloy surface facilitated the hydrogen electrode reaction process on the alloy electrodes and results in good activation and low polarization resistance.

Hypo–hyper-d-electronic interactive nature of interionic synergism in catalysis and electrocatalysis for hydrogen reactions

The nature, causes and consequences of the existence of volcano plots along transition series with resulting theoretical conclusions have been extended on the Brewer hypo–hyper-d-electronic combinations of both intermetallic phases and interionic composites. It has been pointed out that every such phase diagram behaves as the part of the Periodic Table between initial constituents, and thereby features the Gschneidner type of volcano curve for the hydrogen electrode reactions (helr), in common with the well-known electrocatalytic volcano plots of H. Kita and M. H. Miles for the hydrogen evolution reaction (her) upon individual transition metals. Several typical Brewer-type intermetallic systems (Ti–Ni, Zr–Ni, Mo–Ni, Mo–Pt) were investigated and their comparable electrocatalytic and hydridic properties were revealed from volcanic plots along the corresponding phase diagrams. In the light of such conclusions, it has been pointed out that catalytic volcano plots represent in their essence the main criterion and guidance leading to both optimal and synergetic effects in broader (electro)catalytic sense for the helr. Electrocatalysis and its synergetic effect appear as the hypo–hyper-d -electronic interactive effect of corresponding transition element composites either within metallic lattice, or their ions upon the substrate surface. The d-band both as the bonding and adsorptive orbital for intermediates in the rate determining step (rds) has been inferred to be decisive in (electro)catalytic hydrogen reactions, while electronic density, in addition, defines the overall kinetics and reaction rate. Such statements have been supported by the novel self-consistent calculated functional density of one-electron states (DOS) of adsorbed H-adatom, wherefrom the Fermi level relative to the antibonding peak plays decisive role in electrocatalysis for the helr. The surface state of a given interionic bulk structure reflects the latter and appears decisive in its mutual electronic configuration for the overall electrocatalytic effect and resulting synergism in the helr.

Activation mechanism of Ti 0.5Zr 0.5Ni 1.3V 0.7Mn 0.1Cr 0.1 electrode in nickel-hydride batteries

Nickel-hydride batteries using hydrogen storage alloys as a negative electrode material have attracted much attention for their cleanliness and high-energy density. In order to improve the performance of a negative electrode, electrochemical hydrogen electrode reaction on the surface and hydrogen diffusion in the bulk of alloy should proceed with negligible overvoltage in charging and discharging process. In this study, the discharge capacity and high-rate dischargeability were evaluated as a function of the pretreatment of the alloy surface and also molding pressure of the negative electrode. The results were discussed on the basis of the charge transfer reaction on the surface and the hydrogen diffusion behavior in the alloy.

Effects of addition of rare-earth element on electrochemical characteristics of ZrNi 1.1Mn 0.5V 0.3Cr 0.1 hydrogen storage alloy electrodes

The effects of the addition of cerium or Ce-rich mischmetal (Mm) on the electrochemical characteristics of the ZrNi 1.1Mn 0.5V 0.3Cr 0.1 hydrogen storage alloy electrodes were investigated. The results showed that a small addition of Ce or Mm improve greatly the activation behavior of ZrNi 1.1Mn 0.5V 0.3Cr 0.1 alloy electrodes. It was found that Zr 0.9 M 0.1Ni 1.1Mn 0.5V 0.3Cr 0.1 ( M=Ce, Mm) alloy electrodes can be activated before 3∼4 charge/discharge cycles in comparison with about 27 cycles needed to activate ZrNi 1.1Mn 0.5V 0.3Cr 0.1 alloy electrode. In addition, the maximum discharge capacity was increased from 300 mAh/g to 316∼340 mAh/g owing to the addition of Ce or Mm. However, the addition of Ce or Mm has a negative effect on the cyclic durability of Zr-based alloy electrodes. The dispersive CeNi or MmNi phases in the parent alloy act as active sites for hydrogen electrode reaction to improve markedly the electrocatalytic activity of the Zr-based alloy electrodes. Therefore, the Zr 0.9 M 0.1Ni 1.1Mn 0.5V 0.3Cr 0.1 alloy electrodes exhibited higher exchange current density and lower apparent activation energy for the hydrogen electrode reaction than ZrNi 1.1Mn 0.5V 0.3Cr 0.1 alloy electrode.

Interionic nature of synergism in catalysis and electrocatalysis

Fermi dynamics and the Brewer structural bonding factors have been employed to define synergism and electrocatalytic properties for hydrogen electrode reactions upon individual transition metals and their hypo-hyper- d-electronic combinations of intermetallic phases. It has clearly been shown and pointed out that electrocatalysis and its synergetic effect appear as the interionic interaction of hypo-hyper- d-electronic components either within metallic lattice, or upon the substrate surface. The induced external polarization or non-Faradaic (NEMCA) effect upon the electronic density of states and thereby advanced overall (electro)catalytic activity has been correlated with the basic catalytic properties and emphasized, too.

Hypo–hyper- d-electronic interactive nature of synergism in catalysis and electrocatalysis for hydrogen reactions

The nature, causes and consequences of the existence of volcano plots along transition series with resulting theoretical conclusions have been extended on the Brewer hypo–hyper- d-electronic combinations of both intermetallic phases and interionic composites. It has been pointed out that every such phase diagram behaves in electrocatalytic properties as the part of Periodic Table between initial constituents or their periods, and thereby features the Gschneidner type of volcano curve for the hydrogen electrode reactions (helr), in common with the well known electrocatalytic volcano plots for the hydrogen evolution reaction (her) upon individual transition metals. Several characteristic Brewer type intermetallic systems (Ti–Ni, Zr–Ni, Mo–Ni, Mo–Pt) were investigated and their comparable electrocatalytic and hydridic properties were revealed from volcanic plots along the corresponding phase diagrams. Electrocatalysis appears as the hypo–hyper- d-electronic interactive effect of transition element composites either within metallic lattice, or their ions upon the substrate surface.

Relationship Between Equilibrium Hydrogen Pressure and Exchange Current for the Hydrogen Electrode Reaction at MmNi3.9 − xMn0.4AlxCo0.7 Alloy Electrodes

We present a theoretical relationship between equilibrium hydrogen pressure and exchange current for the hydrogen electrode reaction which considers the degree of hydrogen coverage at the electrode surface. Electrochemical measurements at electrodes were performed to prove the theoretical model. The equilibrium hydrogen pressures were analyzed from electrochemical pressure‐composition isotherms, and the exchange currents were determined by linear polarization measurements. Fitting the experimental data to the theoretical model indicated that the rate constants for the chargetransfer reaction as well as the charge‐transfer coefficient were influenced by the partial substitution of nickel by aluminum. Also, the exchange current passed through a maximum with decreasing equilibrium hydrogen pressure, i.e., with increasing aluminum content, indicating that it is possible to design new materials which combine high electrocatalytic activity with an appropriate equilibrium hydrogen pressure. Because of its high energy density and high power density, was found to be the most appropriate composition. © 2000 The Electrochemical Society. All rights reserved.

Existence of Two Sets of Kinetic Parameters in the Correlation of the Hydrogen Electrode Reaction

It has been demonstrated that the dependence of the current density on overpotential for the hydrogen electrode reaction described through the Volmer‐Heyrovský‐Tafel mechanism can be correlated by two different sets of the kinetic parameters. Such sets lead to two different dependences of the surface coverage on overpotential, while a unique dependence of the corresponding pseudocapacitance of adsorption is obtained. The expressions that relate both sets of kinetic parameters have been established and analyzed for the simultaneous occurrence of the three steps and for the different kinetic routes. © 2000 The Electrochemical Society. All rights reserved.

Structural specificity of the kinetics of the hydrogen evolution reaction on the low-index surfaces of Pt single-crystal electrodes in 0.5 M dm−3 NaOH

The kinetics of the underpotential deposition (upd) of H and the hydrogen evolution reaction (her) have been found to be extremely sensitive to the surface geometry of Pt single-crystal surfaces. The order of electrocatalytic activity at the low-index planes was found in our previous studies to be (100)<(111)<(110). The study of the kinetics of the her at Pt(hkl) electrodes in 0.5 M H2SO4 is found to be complicated by the influence of H2 diffusion away from the surface. However, increase of the pH decreases the kinetic facility of the hydrogen electrode reaction, i.e. abstraction of H to form H2 is more difficult from H2O than from H3O+; at such lower rates, diffusive effects can be totally eliminated by electrode rotation. In the present paper we show that in 0.5 M NaOH the order of reactivity is identical with that found in acidic media, viz. (100)<(111)<(110). The slower rates arising in alkaline media allow a larger potential range to be probed and, unlike the situation in acid, quantitative rate information for each of the Volmer, Heyrovsky and Tafel steps of the her was obtained by using electrochemical impedance spectroscopy (EIS) and steady-state dc methods. As well as rate information, the charge, q1, corresponding to maximum fractional coverage by the overpotentially deposited H species (opd H), can be obtained and will be discussed in terms of the results previously found in acid, related to surface geometry and coverage by upd H. The results of the present work will also be compared with those of other recent studies conducted in alkaline media.

Electrocatalysis for hydrogen electrode reactions in the light of fermi dynamics and structural bonding FACTORS—I. individual electrocatalytic properties of transition metals

The Brewer valence-bond theory for bonding in metals and intermetallic phases has been employed, together with Fermi dynamics, to correlate with the electrocatalytic properties of both individual and composite transition metal catalysts for the hydrogen electrode reactions (HELR). It has been inferred that the electrocatalytic activity of both individual transition metals and their intermetallic phases and alloys for both hydrogen evolution (HER) and its oxidation (HOR), primarily correlates with the electronic density of states and obeys typical laws of catalysis reflected in the first place in the existence of volcano plots along the Periodic .Table. Since the bonding effectiveness of both individual and intermetallic hypo–hyper- d-electronic transition metal composite electrocatalysts correlates in a straightforward manner with their electrocatalytic activity, such evidence strongly suggests Fermi energy, as a typical elementary binding energy, which otherwise stays in the linear relation with cohesive energy, this forms the basis in investigation and correlation of electrocatalytic activity. Due to the fact that the Fermi wave-vector represents the individual and collective (alloys and intermetallic phases) bulk property of the available electronic number density (or its concentration, n, i.e., k F = (3 π fn2 fn2 The electronic density of states at the Fermi level, by definition [4, 26], is G( E F) = 3 n/2 E F, and thereby represents ( Eq. (2)), a function of n 1 3, which interrelates electronic concentration (or electronic number density) with electronic density of states on a straightforward manner. On the other hand, there exists a straightforward relationship between the electronic density of states and paramagnetic susceptibility ( χ), G( E F)= 3 nμ 2/2 E F=3 χ/2 μ 2 B, where μ is magnetic moment per unit volume, and μ B is Bohr magneton, and this is the reason why both atomic and intermetallic magnetic properties follow the volcano plot along transition series. n) 1 fn3 fn3 In fact, atomic mass increases all along the transition series, while specific atomic mass—(atomic mass per unit, volume or mass density), and vice versa, specific volume (volume per unit atomic mass)—obey the typical volcanic plot. ), and in a straightforward manner correlates with the electronic density of states at the Fermi level, and thereby defines all metallic properties of a metal (and intermetallics) as a solid with a Fermi surface, including electrocatalytic features, it has been taken as the main parameter to correlate with the exchange current density in the hydrogen electrode reactions.

Magnetic Field Effects on Nickel Electrodeposition: II. A Steady‐State and Dynamic Electrochemical Study

A very recent paper, referred to below as Paper I, had been devoted to the description of magnetic field effects on the structural features of nickel electrodeposits grown from a Watts solution, either pure or in the presence of small amounts of a primary brightener and leveling agent such as 2‐butyne‐1,4‐diol (BD). The conclusions of Paper I stressed the major effect of the magnetic field which was supposed to come down to a magnetohydrodynamic forced convection close to the cathodic interface. This convection enhances those among the cathodic reactions which are likely to be mass‐transport controlled, which is the case for the hydrogen electrode reaction as well as for the catalytic hydrogenation undergone by BD on the cathodic surface. The aim of this paper is to describe the results of electrochemical studies which have been carried out with the same system so as to check the validity of our assumption.

The effect of palladium coating on hydrogen storage alloy electrodes for nickel/metal hydride batteries

Charge / discharge studies carried out on Pd - coated misch metal - based alloys for use in nickel-metal hydride batteries are presented. The effect of Pd coating on the voltammetric characteristics, life cycle behavior, and rate capability of the alloy electrodes was determined. The number of cycles required to activate the alloy electrodes decreases with an increase in the Pd content. The results also show that Pd - coated alloys exhibit higher storage capacities and better performance than bare alloys.This improved performance can be attributed to the catalytic effect of Pd on the hydrogen electrode reaction.

Components of overvoltage of hydrogen electrode reaction on nickel

For the case of hydrogen evolution reaction (HER) in alkaline solutions on nickel, the overvoltage of the electron transfer step (the first step of HER) was separated from that of the overall reaction on the basis of the results obtained by the galvanostatic transient method, which has been developed by us earlier. The difference of the overall-reaction overvoltage and electron-transfer overvoltage, which is considered as the overvoltage of the step of an adsorbed hydrogen atom recombination (the last step of HER), was also determined. It is known that on nickel, during cathodic polarization in alkaline solutions, the intermetallic compound M(I) of an alkali metal with nickel is formed as practically the main intermediate. In this case, the Tafel line, corresponding to the last step, shows a slope of 70–80 mV, and the observed overvoltage can be explained in terms of the decrease of the work function of the surface component, caused by the M(I) formation. On the other hand, on a cobalt-deposit-modified nickel, the Tafel line shows a slope of 30 mV, even in alkaline solutions. The nickel surface may be completely covered with the cobalt deposit. Therefore, no intermetallic compound seems to be formed on the surface, and the main intermediate is likely to be an adsorbed hydrogen atom, as in the case of HER on platinum in acid solutions.

Electrochemical properties of the non-stoichiometric hydrogen storage alloys for nickel-hydrogen batteries

Behavior of the negative electrodes consisted of alleys with the composition of Mm(Ni3.6Mn0.4Al0.3Co0.7)x (Mm = misch metal, 0.88 ⩽ x ⩽ 1.12) was investigated to elucidate the effect of stoichiometric ratio on the electrode properties. The maximum discharge capacity at around x = 0.96, which was measured at a current density of 200 mA g−1, was found to be correlated with the unit-cell volume of the alloys. The discharge efficiency increased with increasing x value and reached about 85.2% even at a high current density of 2000 mA g−1, for the alloy with x = 1.12. The improvement in high-rate dischargeability was explained on the basis of high diffusibility of hydrogen in bulk of alloys as the result of the relatively low stability of the hydride and the high electrocatalytic activity for the hydrogen electrode reactions of the negative electrode.

Modification of Ti 0.35Zr 0.65Ni 1.2V 0.6Mn 0.2 alloy powder by electroless nickel coating and its influence on discharge performance

The electrochemical performance of an AB 2-type hydrogen storage alloy (Ti 0.35Zr 0.65Ni 1.2V 0.6Mn 0.2) modified by electroless Ni-P and Ni coatings in acid hypophosphite and hydrazine baths, respectively, is investigated. It is found that electrochemical properties of the modified electrodes (i.e., high-rate dischargeability, cycle life, ability to retain discharge capacity after overdischarge) and the electrocatalytic activity for the hydrogen electrode reaction are improved by this surface modification. The high-rate dischargeability of the alloy coated with Ni from the hydrazine bath is better than that coated with NiP from the acid hypophosphite bath, but the activation cycles required for the Ni-coated electrode to reach maximum capacity are four times as many as those required by the electrode coated with NiP. A duplex nickel coating method has therefore been developed. The duplex nickel-coated alloy (i.e., alloy first coated with NiP and then coated with Ni) retains the characteristics of better high-rate dischargeability but requires fewer activation cycles in comparison with the Ni-coated alloy.

Effect of the stoichiometric ratio on electrochemical properties of hydrogen storage alloys for nickel-metal hydride batteries

Effect of stoichiometric ratio on the electrochemical properties of negative electrodes was investigated for alloys with composition Mm(Ni 3.6Mn 0.4Al 0.3Co 0.7) x (Mm = misch metal, 0.88 ⩽ x ⩽ 1.12). The discharge capacity at a current density of 0.2 A g −1 increased with an increase in unit cell volume of the alloys in the range of x = 0.96−1.12. The discharge efficiency increased with increasing x value, and was about 85.2% even at a high current density of ca 2 A g −1 in the case of x = 1.12. The improved high-rate dischargeability was explained on the basis of high diffusibility of hydrogen in the bulk of alloys as the result of the relatively low stability of the hydride, in addition to the high electrocatalytic activity for the hydrogen electrode reactions of the negative electrode.

Electrocatalytic activity prediction for hydrogen electrode reaction: intuition, art, science

The factors which determine the electrocatalytic activity of metals in the hydrogen reaction are described. The problems are formulated which arise when plotting the volcano relationship between the current density logarithm and the metal—hydrogen bond energy. Experimental data on exchange currents and electron work function W e were selected. It was shown that the scatter in electrochemical data and the absence of reliable results for metals with low W e are the deciding factors for plotting the correlation relationships. It was concluded that, to date, one can only suggest that there is a tendency for the exchange-current to increase with the increase in W e, without giving any quantitative relationships.

The hydrogen pressure equivalent to the overpotential: observation on Pd hydrogen electrode and interpretation on the basis of a mixed-controlled mechanism

A quantitative analysis is presented of the hydrogen pressure that is in equilibrium with adsorbed hydrogen at a Pd cathode and discussed on the basis of a mixed-controlled Volmer-Tafel mechanism of the hydrogen electrode reaction. Experimental evidence which supports this, but not the Volmer-Heyrovsky mechanism on a smooth Pd electrode, are discussed. The effective pressure can be evaluated from the slowly decaying overpotential component appearing in overpotential decay transients. Evidence is given which indicates that the overpotential component is indeed caused by adsorbed hydrogen on the electrode surface. The effects of some catalytic poisons, which augment the equivalent pressure under otherwise the same conditions, are investigated.

Effect of gelling on the electrode kinetics of the Pb/PbSO 4 and hydrogen-electrode reactions in maintenance-free lead/acid batteries

The electrode kinetics of the Pb/PbSO 4 and the hydrogen-evolution reactions on porous lead electrodes in sulfuric acid has been studied using a quantitative analysis of galvanostatic transients in the linear polarization domain. Significant changes are observed in the kinetics with the addition of thixotropic agents (e.g., sodium silicate) that are often employed in the manufacture of maintenance-free lead/acid batteries. The effects of these thixotropic agents on the charge-transfer and ohmic components of the Pb/PbSO 4 reaction and charge-transfer components of hydrogen-electrode reaction are analysed with the help of a ‘pore-plugging’ model. This study allows a non-destructive determination of the electrode kinetic parameters and the internal resistance contribution from pore-plugging. This is useful when designing maintenance-free lead/acid batteries.