Ti–Ni alloys as MH electrodes in Ni–MH accumulators

In hydrogen solid–gas reaction at 300 K and 1 bar, the hydrogen content for Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ × ≤0.8) alloys was in range 1.93–0.05 (Cwt.H,%), and discharge capacity of 360–235 A h/kg was achieved accordingly. The ΔHH₂$\Delta H_{{\text{H}}_2}$ and ΔSH₂$\Delta S_{{\text{H}}_2}$ values of −32.29 kJ mol⁻¹ and −111.04 J mol⁻¹ K⁻¹, respectively, for Ti_3.87Ni_1.73Fe_0.7O_0.5 alloy were obtained using experimental PCT relations, where hysteresis effect was only slightly visible. The half-cell potentials (vs. Hg/HgO) of metal hydride (MH) electrodes based on Ti_3.87Ni_1.73Fe_0.7O_x (0.2≤ ×≤ 0.8) alloys were calculated.     

Hydrogen absorption characteristics of mechanically alloyed Ti–Zr–Ni and Ti–V–Ni powders

A first investigation into the production of amorphous and nanostructured Ti-based alloys with nominal compositions Ti_41.5Zr_41.5Ni₁₇, Ti₆₁Zr₂₂Ni₁₇, Ti_41.5V_41.5Ni₁₇ and Ti₆₁V₂₂Ni₁₇ by mechanical alloying (MA) technique is presented. This technique was adopted to produce alloys' powders with high fresh surface area that were active for hydrogen storage. Hydrogen absorption characteristics and structure changes in the alloys after hydrogenation were investigated. Gas phase hydrogenation of the Ti–Zr–Ni alloys, at 573 K and an initial hydrogen pressure of 2 MPa, exhibited good hydriding properties and started at a maximal rate without induction period with a hydrogenation capacity up to 1.2 wt%. However, hydriding of Ti–V–Ni alloys at the same conditions exhibited slower rates. The Ti₆₁V₂₂Ni₁₇ composition showed high hydrogen absorption capacity of 1.8 wt% and exceeded 4 wt% at 345 K. In addition, the Ti–V–Ni alloys showed structure stability after hydrogenation and retained the amorphous structure.     

Microstructure and electrochemical properties of Zr–Ti–V–Ni–Mn–LM alloys for application in Ni–MH secondary battery

A series of multi-component Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4; y=0.0,0.02,0.05,0.1,0.2,0.3, LM; lantanum-rich-mischmetal) alloys are prepared and their crystal structure and P–C–T curves are analyzed. The alloys have been modified by adding LM and their gaseous and electrochemical hydrogenation properties are studied to find out the effect of LM elements. Also, the second phase and initial activation performance are investigated. The Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3,0.4; y=0.0,0.02,0.05,0.1,0.2,0.3) alloys have C14 Laves phase hexagonal structure, so the volume expansion ratio of lattice parameters with LM has increased. As the amount of LM in alloy has increased, correspondingly the second phase is also increased. The second phase is LM, Ti and V-rich. The second phase improve the activation of La-rich misch-metal, and also the concentration of elements Ti, V〉LM〉 matrix in alloys.The addition of LM in Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4) alloys have increased the activation rate and hydrogen storage capacity significantly, but the plateau pressure and the discharge capacity have been decreased due to the formation of second phase. For more Zr in electrode alloys, the activation of rate becomes slow.     

Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution

The electrocatalytic oxidation of methanol was studied on Ni–P and Ni–Cu–P supported over commercial carbon electrodes in 0.1 M KOH solution. Cyclic voltammetry and chronoamperometry techniques were employed. Electroless deposition technique was adopted for the preparation of these catalysts. The effect of the electroless deposition parameters on the catalytic activity of the formed samples was examined. They involve the variation of the deposition time, pH and temperature. The scanning electron micrography showed a compact Ni–P surface with a smooth and low porous structure. A decreased amount of nickel and phosphorus was detected by EDX analysis in the formed catalyst after adding copper to the deposition solution. However, an improvement in the catalytic performance of Ni–Cu–P/C samples was noticed. This is attributed to the presence of copper hydroxide/nickel oxyhydroxide species. It suppresses the formation of γ-NiOOH phase and stabilizes β-NiOOH form. Linear dependence of the oxidation current density on the square root of the scan rate reveals the diffusion controlled behaviour.     

Evaluation of Pt–Ru–Ni and Pt–Sn–Ni catalysts as anodes in direct ethanol fuel cells

In this study, the electrooxidation of ethanol on carbon supported Pt–Ru–Ni and Pt–Sn–Ni catalysts is electrochemically studied through cyclic voltammetry at 50 °C in direct ethanol fuel cells. All electrocatalysts are prepared using the ethylene glycol-reduction process and are chemically characterized by energy-dispersive X-ray analysis (EDX). For fuel cell evaluation, electrodes are prepared by the transfer-decal method. Nickel addition to the anode improves DEFC performance. When Pt₇₅Ru₁₅Ni₁₀/C is used as an anode catalyst, the current density obtained in the fuel cell is greater than that of all other investigated catalysts. Tri-metallic catalytic mixtures have a higher performance relative to bi-metallic catalysts. These results are in agreement with CV results that display greater activity for PtRuNi at higher potentials.     

Influence of Pd addition on the electrochemical performance of Mg–Ni–Ti–Al-based metal hydride for Ni–MH batteries

Amorphous Mg_0.9Ti_0.1NiAl_0.05 and Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 alloys were prepared by high energy ball milling and evaluated as metal hydride electrodes for Ni–MH batteries. The Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 alloy showed a much higher cycle life with a capacity retention of 72% after 100 cycles (C_100th = 288 mAh g⁻¹) compared to 26% for the Pd-free alloy (C_100th = 117 mAh g⁻¹). This was mainly attributed to the improvement of the alloy oxidation resistance in KOH electrolyte with Pd addition, as confirmed by cyclic voltammetry experiments and X-ray diffraction analyses on cycled electrodes. In addition, in situ acoustic emission (AE) measurements revealed that the energy of the AE signals related to the particle cracking is lower for the Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 electrode, suggesting that the cracks are smaller in size than with the Pd-free alloy. The Mg_0.9Ti_0.1NiAl_0.05Pd_0.1 electrode also displayed a higher discharge rate capability than the Mg_0.9Ti_0.1NiAl_0.05 electrode. On the basis of their respective electrochemical pressure–composition isotherm, it was shown that the presence of Pd in the alloy decreases the thermodynamic stability of the metal hydride. Through a comparative analysis of discharge polarization curves, it was also shown that Pd addition decreases substantially the H-diffusion resistance in the alloy whereas its positive effect on the charge-transfer resistance is limited.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Hydrogen storage properties of Nb–Hf–Ni ternary alloys

The hydrogen storage properties of Nb_xHf_(1−x)/2Ni_(1−x)/2 (x = 15.6, 40) alloys were investigated with respect to their hydrogen absorption/desorption, thermodynamic, and dynamic characteristics. The PCT curves show that all the specimens can absorb hydrogen at 303 K, 373 K, 423 K, 473 K, 523 K, 573 K, and 673 K, but they couldn't desorb hydrogen below 373 K. The maximum hydrogen absorption capacity reaches 1.23 wt.% for Nb_15.6Hf_42.2Ni_42.2 and 1.48 wt.% for Nb₄₀Hf₃₀Ni₃₀ at 303 K at a pressure of 3 MPa. When the temperature was increased, the hydrogen absorption capacities significantly decreased. However, the hydrogen equilibrium pressure increased. When the temperature exceeded 523 K, the hydrogen equilibrium pressure disappeared. When niobium content was increased, the kinetic properties of hydrogen absorption/desorption improved. The results from the microstructure analysis show that both alloys consist of the BCC Nb-based solid solution phase, the B_f-HfNi intermetallic phase, and the eutectic phase {B_f-HfNi + BCC Nb-based solid solution}. When the Nb content was increased, the volume fraction and Nb content in the Nb-based solid solution phase increased. Thus, the improved kinetics is related to the increase in the primary BCC Nb-based solid solution in the Nb₄₀Hf₃₀Ni₃₀ alloy. The kinetic mechanisms of hydrogen absorption/desorption in these two alloys are found to obey the chemical reaction mechanism at all temperatures tested.     

Hydrogenation and electrochemical studies of La–Mg–Ni alloys

La–Mg–Ni alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying with subsequent annealing under an argon atmosphere at 973 K for 0.5 h, were used to produce La_2-xMg_xNi₇ alloys (x = 0, 0.25, 0.5, 0.75, 1). Shaker type ball mill was used. An objective of the present study was to investigate an influence of amount of Mg in alloy on electrochemical, hydrogenation and dehydrogenation properties of La–Mg–Ni materials. X-ray diffraction analyses revealed formation of material with multi-phase structure. Obtained materials were studied by a conventional Sievert's type device at 303 K. It was observed that electrochemical discharge capacity and gaseous hydrogen storage capacity of La–Mg–Ni alloys increases with Mg content to reach maximum for La_1.5Mg_0.5Ni₇ alloy. Moreover, all of La–Mg–Ni alloys were characterized by improved hydrogen sorption kinetics in comparison to La–Ni alloy.     

Polymer Ni–MH battery based on PEO–PVA–KOH polymer electrolyte

An alkaline polymer electrolyte film has been prepared by a solvent-casting method. Poly(vinyl alcohol), PVA is added to improve the ionic conductivity of the electrolyte. The ionic conductivity increases from 10⁻⁷ to 10⁻² S cm⁻¹ at room temperature when the weight percent ratio of poly(ethylene oxide), PEO to PVA is increased from 10:0 to 5:5. The activation energy of the ionic conductivity for the PEO–PVA–KOH polymer electrolyte is 3–8 kJ mol⁻¹. The properties of the electrolyte film are characterized by a wide variety of techniques and it is found that the film exhibits good mechanical stability and high ionic conductivity at room temperature. The application of such electrolyte films to nickel–metal-hydride (Ni–MH) batteries is examined and the electrochemical characteristics of a polymer Ni–MH battery are obtained.     

Synthesis and electrochemical properties of layered Li–Ni–Mn–O compounds

An attempt to prepare solid solutions in the system of LiNiO₂, LiMnO₂ and Li₂MnO₃ was performed by heating metal acetates. The solid solutions between end members LiNiO₂ and Li₂MnO₃ can be successfully prepared in the overall compositional ranges. Both the structure and capacity were compared based on Rietveld analysis and electrochemical investigation on solid solutions between LiNiO₂ and Li₂MnO₃. The result showed that the cationic disorder as well as capacity was closely related to the ratio of Li, Mn and Ni in formula. The investigation of chronopotentiogram and ex situ XRD on the solid solutions indicated that the complex phase transitions in LiNiO₂ during delithiation were strongly suppressed with low Mn content (Mn/(Mn+Ni) ratio was 0.1 or 0.2) and completely suppressed with the ratio more than 0.5.     

Crystallographic and electrochemical characteristics of Ti–Zr–Ni–Pd quasicrystalline alloys

Ti₄₅Zr₃₅Ni_20−xPd_x (x = 0, 1, 3, 5 and 7, at%) alloys were prepared by melt-spinning. The phase structure and electrochemical hydrogen storage performances of melt-spun alloys were investigated. The melt-spun alloys were icosahedral quasicrystalline phase, and the quasi-lattice constant increased with increasing x value. The maximum discharge capacity of alloy electrodes increased from 79 mAh/g (x = 0) to 148 mAh/g (x = 7). High-rate dischargeability and cycling stability were also enhanced with the increase of Pd content. The improvement in the electrochemical hydrogen storage characteristics may be ascribed to better electrochemical activity and oxidation resistance of Pd than that of Ni.     

Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–Y alloys

Ti₄₅Zr₃₀Ni₂₅Y_x (x = 1, 3, 5 and 7) alloys were prepared by melt-spinning at wheel velocity of 20 m s⁻¹. The effect of additive Y on phase structure and electrochemical performance of melt-spun alloys was investigated. Ti₄₅Zr₃₀Ni₂₅Y_x melt-spun alloys were composed of I-phase and amorphous phase. The amorphous phase increased with increasing x value, indicating amorphous forming ability improved with increasing Y content. The maximum discharge capacity and high-rate dischargeability decreased with increasing x value, which may be ascribed to the decrease of nickel content. Cycling stability first increased with increasing x from 1 to 3, and then decreased when x increased to 7, which was resulted from the combined effect of the decrease of nickel content and the increase of amorphous phase.     

Ni–P and Ni–Co–P coated aluminum alloy 5251 substrates as metallic bipolar plates for PEM fuel cell applications

This study is aimed to replace graphite bipolar plates in PEM fuel cells with surface modified aluminum alloy. To improve the surface characteristics of aluminum alloy 5251 (AA5251) substrate, Ni–P and Ni–Co–P coatings were deposited using electroless and electroplating deposition techniques [power supply and chronoamperometry]. Surface morphology and chemical composition of prepared coatings have been investigated using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. The corrosion behaviour of Ni–P and Ni–Co–P coated AA5251 was studied in (0.5 M H₂SO₄ + 2 ppm HF) solution by potentiodynamic polarization technique. Lower corrosion current densities and more positive corrosion potentials were gained after coating AA5251 with Ni–P and Ni–Co–P deposits. Much better corrosion resistance was shown by coatings containing cobalt. Potentiostatic tests were carried out at +160 mV (MMS) in air-saturated solution to simulate cathode environment in PEM fuel cells. The current density of Ni–Co–P (1:1)/AA5251 was stabilized at a value lowered by 4 times relative to that at bare AA5251 substrate. Interfacial contact resistance values between coated substrates and carbon paper were measured. Ni–P and Ni–Co–P coatings prepared by electroless method showed ICR values, twice that at ones prepared by electroplating power supply technique.     

Methanol oxidation on carbon-supported Pt–Ru–Ni ternary nanoparticle electrocatalysts

Methanol oxidation on carbon-supported Pt–Ru–Ni ternary alloy nanoparticles was investigated based on the porous thin-film electrode technique and compared with that on Johnson–Matthey Pt–Ru alloy catalyst. Emphasis is placed on the effect of alloying degree on the electrocatalytic activity and stability of the ternary catalysts. The as-prepared Pt–Ru–Ni nanoparticles exhibited a single phase fcc disordered structure, and a typical TEM image indicates that the mean diameter is ca. 2.2 nm, with a narrow particle size distribution. Also, the as prepared Pt–Ru–Ni catalysts exhibited significantly enhanced electrocatalytic activity and good stability for methanol oxidation in comparison to commercial Pt–Ru catalyst available from Johnson–Matthey. The highest activity of methanol oxidation on Pt–Ru–Ni catalysts was found with a Pt–Ru–Ni atomic ratio of 60:30:10 and at a heat-treatment temperature of ca. 175 °C. The significantly enhanced catalytic activity for methanol oxidation is attributed to the high dispersion of the ternary catalyst, to the role of Ni as a promotion agent, and especially to the presence of hydroxyl Ru oxide. Moreover, the stability of the ternary nanocatalytic system was found to be greatly improved at heat-treatment temperatures higher than ca. 250 °C, likely due to a higher alloying degree at such temperatures for the ternary catalysts.     

Hydrogen absorption and desorption kinetics of Ag–Mg–Ni alloys

The hydrogen absorption/desorption (A/D) kinetics of hydrogen storage alloys Mg_2−xAg_xNi (x=0.05, 0.1) prepared by hydriding combustion synthesis in two-phase (α–β) region in the temperature range of 523–$\text{573}\text{K}$ have been investigated. The hydriding/dehydriding (H/D) reaction rate constants were extracted from the time-dependent A/D curves. The obtained hydrogen A/D kinetic curves were fitted using various rate equations to reveal the mechanism of the H/D processes. The relationships of rate constant with temperature were established. It was found that the three-dimensional diffusion process dominates the hydrogen A/D. The apparent activation energies of 63±5 and $\text{61±7}\text{kJ/mol}\text{H}{}_2$ in Mg_1.95Ag_0.05Ni alloy and $\text{52±2}\text{kJ/mol}\text{H}{}_2$ and of $\text{50±2}\text{kJ/mol}\text{H}{}_2$ in Mg_1.9Ag_0.1Ni alloy were found for the H/D processes in two-phase (α–β) coexistence region from 523 to $\text{573}\text{K}$, respectively. With the increasing content of Ag in Ag–Mg–Ni alloys, the apparent energy was decreased and the reaction rate was faster. It is reasonable to explain that the hydriding kinetics of Mg₂Ni was improved by adding Ag.     

Pd,Pd–Ni and Pd–Ni–Fe nanoparticles anchored on MnO2/Vulcan as efficient ethanol electro-oxidation anode catalysts

Pd–Ni–Fe nanoparticles supported on MnO₂/Vulcan XC-72 R (carbon black powder) as the electrocatalyst for the anodic oxidation of ethanol in a direct ethanol alkaline fuel cell (DEAFC) has been conducted. Electrocatalyst structures and morphologies are investigated by XRD, FE-SEM, EDX and elemental mapping techniques and subsequently electrochemical performance of electrocatalysts for ethanol oxidation reaction (EOR) are studied by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Pd/MnO₂/Vulcan, Pd–Ni/MnO₂/Vulcan and Pd–Ni–Fe/MnO₂/Vulcan efficiently advanced ethanol electro-oxidation reaction under alkaline conditions. Pd/MnO₂/Vulcan revealed best potential window and low charge transfer resistance (R_ct) for EOR. Pd–Ni/MnO₂/Vulcan and Pd–Ni–Fe/MnO₂/Vulcan electrocatalysts have a good anti CO-poisoning capability. Pd–Ni–Fe/MnO₂/Vulcan has significantly high current density, excellent catalyst durability and cyclic stability for ethanol oxidation which encourage researchers for application of such exceptional materials as anode electrocatalysts in DEFC.