Steven J. Thorpe

Electrocatalytic behaviour of Ni-base amorphous alloys

Abstract Amorphous metals may be important electrode materials for use in alkaline fuel cells and electrolyser applications. The electrocatalytic activity of amorphous and crystalline Ni-base alloys with respect to the Hydrogen Evolution Reaction (HER) has been studied in the present work. The Tafel parameters have been measured, and the effect of Ni/Co ratio, metalloids and crystallinity on the catalytic activity of both amorphous and crystalline alloys has been examined and compared. The amorphous alloys exhibit lower activities than that of their crystalline counterparts in the as-polished state. However, an enhanced electrocatalytic activity was observed on amorphous alloys after chemical treatments, which was not found on crystalline alloys after similar chemical treatments. Amorphous alloys are more electrocatalytically active than crystalline alloys after chemical treatment, especially in the high overpotential regime.

Electrochemical and surface characterization of electrocatalytically active amorphous NiCo alloys

The electrochemical behaviour of electrocatalytically active amorphous NiCo alloys together with their crystalline counterparts have been studied by means of cyclic voltammetry (cv) in conjunction with X-ray photoelectron spectroscopy (XPS). The cv profiles of these alloys are quite different when tested after steady state polarization compared with potential cycling. The electrochemical behaviour of NiCo alloys shifted substantially from characteristics of Co to those of Ni. XPS analysis was used to identify the changes in surface oxidation species caused by electrochemical treatments. The oxides formed on amorphous alloys are different from those on crystalline alloys, in terms of their oxidation states, the amount of hydroxyl ions and solvent, film thickness, and ratio of components in the film. Furthermore, a hydrous oxide film, consisting of a micro-porous structure filled with solvent and hydroxyl ions, which is electrocatalytically active for the oxygen evolution reaction, was formed on potential cycled amorphous NiCo alloys.

The electrocatalytic activity of amorphous and crystalline NiCo alloys on the oxygen evolution reaction

Abstract The electrocatalytic behaviour of amorphous and crystalline NiCo alloys on the oxygen evolution reaction (OER) in 1 M KOH solution has been assessed in this work. The peroxidized amorphous NiCo alloys showed Tafel behaviour characteristic of Co oxides. However, after potential cycling, the electrocatalytic activity of amorphous and crystalline NiCo alloys increased drastically and their Tafel behaviour showed characteristics similar to pure Ni electrodes. Nickel-rich amorphous alloys showed particularly high activity towards the OER in comparison with their crystalline counterparts and the Co-rich amorphous alloy. In addition, Ni and the amorphous NiCo alloys showed good stability as electrocatalysis while crystalline NiCo alloys were not stable upon potential cycling.

Determination of the Avrami exponent for solid state transformations from non-isothermal differential scanning calorimetry

Abstract Many solid state transformations follow the Johnson-Mehl-Avrami kinetic equation, particularly crystallization of polymers, glasses and amorphous metals. The value of the Avrami exponent in this relationship, important for understanding the reaction mechanism, is normally determined by isothermal differential scanning calorimetry. A computer model has been developed to generate non-isothermal differential scanning calorimeter curves. This model was used to construct a function that consists of parameters easily derived from non-isothermal experiments and which is strongly dependent on the values of the Avrami exponent and is nearly independent of all other variables. Thus, from the values of peak temperature, peak height, activation energy and enthalpy, the value of the function may be calculated and the Avrami exponent read from a correlation curve. The validity of this procedure has been demonstrated using an experimental example of the crystallization of an amorphous metal alloy. The Avrami exponent determined by this procedure was in good agreement with the value of the exponent determined by the conventional isothermal method and resulted in model differential scanning calorimeter curves which closely agreed with those measured experimentally.

Interpretation of activation energies calculated from non-isothermal transformations of amorphous metals

Abstract Non-isothermal differential scanniong calorimetry is often used to determine the activation energy of transformation in amorphous metals by the way in which the transformation peaks shift with changes in heating rate. The two relationships used for this purpose were developed by Kissinger and Ozawa. There have been attemptsto compare the activation energie obtained from each of these methods and therey to infer the order of the underlying kinetic process. In fact, the differences found between the activation arise from different approximations used in developing the two relationships. When a correction is applied, both techniques give identification energies for th transformation and yield no information about the reaction order nor even about the type of kinetics involved.

Ni-base amorphous alloys as electrocatalysts for alkaline water electrolysis

Abstract The development of efficient and cost effective electrodes for hydrogen generation from water electrolysis and for hydrogen oxidation in power generation from fuel cells is very important in developing the hydrogen economy. This paper focuses on the role of the structure and chemistry of amorphous metal alloys (AMA) on the electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline water electrolysis. The test AMA electrodes, Ni-base alloys with different metal additive ratios, were prepared by a planar flow casting technique. The electrochemical behaviour of the AMA was investigated in 1 M KOH at 30 °C by cyclic voltammetry and steady-state polarization techniques. The AMA displayed voltammetric responses similar to that of crystalline Ni, however, differences were found in the oxide formation mechanism. The electrocatalytic activities of the AMA were dependent on the AMA composition. The prepared alloys displayed good stability relative to that of crystalline Ni. Even after extensive potential cycling which roughened and attacked the crystalline Ni electrodes, the surfaces of the amorphous alloys remained smooth. As a result of the roughening of the crystalline Ni electrodes, the amorphous alloys appeared to be less active than crystalline Ni for the HER on the basis of geometric surface area.

A tritium tracer technique for the measurement of hydrogen permeation in polymeric materials

Hydrogen permeation data are of interest in the selection of new materials in hydrogen fuel cell systems and water electrolyzers. Arrhenius type relations for the permeation and diffusion of hydrogen in polymers are determined using tritium as a radiotracer. Hydrated tritium, HT, in an H2/HT gas stream is measured by an ionization chamber tritium detector and an HTO water trap. Permeation and diffusion coefficients and, their respective activation energies are determined using the time lag method for Viton, Teflon, EPDM, Santoprene and Noryl. The measurements are consistent with earlier investigations, where polymers with higher glass transition temperatures exhibit lower permeation and diffusion coefficients. This study provides insight for choosing fuel cell materials for cell operating temperatures within 25 < T < 150°C.

Characterization of vanadium deposit formation at a hydrogen evolving electrode in alkaline media

Although nickel exhibits a high electrocatalytic activity toward the hydrogen evolution reaction, it undergoes extensive deactivation as a cathode during alkaline water electrolysis. The addition of dissolved V 2 O 5 to the electrolyte results in partial reactivation of nickel cathodes in 8 mol/L KOH at 70°C by formation of a vanadium-rich deposit. Various analytical techniques were employed to characterize the deposit in terms of surface morphology, phase analysis, and chemical composition. The deposit on nickel had a smooth and compact surface with a thickness of 1-2 μm. X-ray diffraction results indicated an amorphous structure with a Scherrers length less than 2 nm. Chemical and thermal analyses led to an empirical compound formula of K 2 H 2 V 10 O 26 .4H 2 O. That formula implied a mixed-valence (+4/+5) vanadium compound, later confirmed by X-ray photoelectron spectroscopy. The mechanism of deposit formation was then investigated by cyclic voltammetry on a mercury electrode. A cathodic peak representing the reduction of vanadium (+5) species was apparent near the onset of hydrogen evolution on mercury. The behavior of the cathodic peak with concentration and sweep rate revealed key steps in the mechanisms of deposit formation. Deposit formation was modeled as an irreversible precipitation/polymerization reaction following a charge transfer step, to produce a large, mixed-valence vanadium compound.

Investigating the atomic level influencing factors of glass forming ability in NiAl and CuZr metallic glasses

Bulk metallic glasses are a relatively new class of amorphous metal alloy which possess unique mechanical and magnetic properties. The specific concentrations and combinations of alloy elements needed to prevent crystallization during melt quenching remains poorly understood. A correlation between atomic properties that can explain some of the previously identified glass forming ability (GFA) anomalies of the NiAl and CuZr systems has been identified, with these findings likely extensible to other transition metal-transition metal and transition metal-metalloid (TM-M) alloy classes as a whole. In this work, molecular dynamics simulation methods are utilized to study thermodynamic, kinetic, and structural properties of equiatomic CuZr and NiAl metallic glasses in an attempt to further understand the underlying connections between glass forming ability, nature of atomic level bonding, short and medium range ordering, and the evolution of structure and relaxation properties in the disordered phase. The anomalous breakdown of the fragility parameter as a useful GFA indicator in TM-M alloy systems is addressed through an in-depth investigation of bulk stiffness properties and the evolution of (pseudo)Gruneisen parameters over the quench domain, with the efficacy of other common glass forming ability indicators similarly being analyzed through direct computation in respective CuZr and NiAl systems. Comparison of fractional liquid-crystal density differences in the two systems revealed 2-3 times higher values for the NiAl system, providing further support for its efficacy as a general purpose GFA indicator.

Effect of composition on the formation and thermal stability of Ni72(Mo, Co)8B20 metallic glass

Abstract The effect of the addition of Co and Mo on the formation and stability of a Ni-B amorphous alloy has been investigated. Metal ribbons were prepared by melt-spinning in vacuum and characterised by X-ray diffraction and isochronal differential scanning calorimetry. When no Mo is present, only crystalline material containing the phases Co 3 B and Ni is produced. As the content of Mo is increased, the distance between metal-metal nearest neighbours increases, the glass transition and the crystallisation temperatures increase, the enthalpy of crystallisation decreases, and there is a marked increase in the activation energy associated with crystallisation. The explanation for this pattern of glass formation and the increase in thermal stability induced by Mo has been explored by reference to similar systems and to a semi-empirical kinetic model of the thermal stability of amorphous alloys. This model predicts the dependence of crystallisation temperature on composition in this quaternary alloy from the enthalpies of hole formation of its constituent elements with good success.