A Depth‐Resolved In‐Situ Study of the Reduction and Oxidation of Ni‐Based Anodes in Solid Oxide Fuel Cells

The stability of Ni‐YSZ anodes as part of solid oxide fuel cells (SOFCs) towards redox cycling is an important issue for successfully introducing the technology. Detailed knowledge of the NiO‐Ni transitions and their impact on the mechanical integrity of the whole system is necessary to improve the overall stability. In the present paper, a unique in‐situ X‐ray diffraction setup is presented which allows monitoring of the local structural changes during processing of SOFCs. With this setup technological SOFCs – a half cell and a full cell – were studied with respect to NiO‐Ni transitions in repeated reduction‐oxidation cycles, under conditions relevant for SOFC application. It was found that the redox kinetics is a function of the sample depth. Ni particles further away from the surface were reduced/oxidized at a slower rate than particles close to the surface.     

Anodically grown films on copper and copper–nickel alloys in slightly alkaline solutions

Surface oxides grown on copper and copper–nickel alloys (UNS 72000 and UNS 70600) in aerated borax-borate buffer solution of pH 7.7 were characterized using various experimental techniques. Their influence on the mechanism of oxygen reduction was also investigated. The composition of the passive films formed in situ on the different materials was studied using differential reflectance spectroscopy. Electrochemical techniques such as voltammetry and potentiodynamic reductions, as well as hydrodynamic tools such as the rotating disk electrode were used to characterize the kinetic parameters of the oxygen reduction reaction. The results show that the addition of Ni to Cu to form copper–nickel alloys changes the composition of the surface film. As the amount of Ni in the alloy increases, the proportion of Cu(I) compounds decreases, and Ni(II) compounds are incorporated into the film structure. The films anodically grown at 0.5 V on Cu70Ni30 tend to be thinner but more resistive. This is supported by results from reflectance and impedance spectroscopy. The kinetics of oxygen reduction follows a four-electron path on surface-free films, independently of the Ni-content in the alloy.     

The production of a homogeneous and well-attached layer of carbon nanofibers on metal foils

Carbon nanofibers (CNFs) were deposited on metal foils including nickel (Ni), iron (Fe), cobalt (Co), stainless steel (Fe:Ni; 70:11 wt.%) and mumetal (Ni:Fe; 77:14 wt.%) by the decomposition of C₂H₄ at 600 °C. The effect of pretreatment and the addition of H₂ on the rate of carbon formation, as well the morphology and attachment of the resulting carbon layer were explored. Ni and mumetal show higher carbon deposition rates than the other metals, with stainless steel and Fe the least active. Pretreatment including an oxidation step normally leads to higher deposition rates, especially for Ni and mumetal. Enhanced formation of small Ni particles by in situ reduction of NiO, compared to formation using a Ni carbide, is probably responsible for higher carbon deposition rates after oxidation pretreatment. The addition of H₂ during the CNF growth leads to higher carbon deposition rates, especially for oxidized Ni and mumetal, thus enhancing the effect of the reduction of NiO. The diameters of CNFs grown on metal alloys are generally larger compared to those grown on pure metals. Homogenously deposited and well-attached layers of nanotubes are formed when the carbon deposition rate is as low as 0.1-1 mg cm⁻² h⁻¹, as mainly occurs on stainless steel.     

Active surface area in oxide electrodes by overpotential deposited oxygen species for the oxygen evolution reaction

The electrochemical active surface area at oxide electrodes of Pt and electrodeposited Ni, Co and Ni₂₀Co₈₀ alloys was evaluated in 5m KOH solutions based on the charge for electrochemical desorption of a monolayer of overpotential deposited oxygen (OPD O) species. Thein situ technique empllyed for the charge measurement involves galvanostatic charging (OPD O) adsorption), followed by simple discharging (OPD O desorption) experiments. It is observed that surface area estimated by this new technique for the oxidized surfaces of the metals studied here are consistent with those from a.c. impedance spectroscopy. The activity of the metal towards the oxygen evolution reaction (OER) is also discussed in terms of their active surface area estimated in this study.     

Effect of Iron Coating on Thermal Properties of Aluminum‐Lithium Alloy Powder

Aluminum‐lithium alloys are widely studied to improve performance in energetic materials. However, their high reactivity causes severe surface oxidation over micro‐Al particles in storage, resulting in significant deterioration in the overall performance. This study deals with the effect of iron coating on thermal properties and aging stability of aluminum‐lithium alloy powder. Gas atomized Al‐3Li (3 wt. %) alloy powder was prepared and then successfully coated with nano‐sized iron film by modified chemical liquid deposition method. The morphology, thermal properties and combustion enthalpies were characterized by SEM/EDX, TG/DTA and oxygen bomb calorimeter. The results showed that Fe/Al‐3Li composite powder presented obvious core‐shell structure and the outer iron film was very uniform and compact. Significantly enhanced thermal reactivities of Al‐3Li alloy powder and Fe/Al‐3Li composite powder were achieved compared with pure Al. In addition, aging studies indicated that, after coating, the reactivity decay rates of alloy powder decreased significantly, and the mass combustion enthalpies remained basically stable, which demonstrated that iron coating indeed prevented pre‐oxidation of the alloy powder and improved its aging stability.     

Amorphous nickel-valve metal-platinum group metal alloy electrodes for hydrogen-oxygen sulphuric acid fuel cells

Powder catalysts were prepared by immersion of amorphous Ni-40Zr and Ni-40Ti alloys containing a few at % of platinum group elements in HF solution. This treatment led to preferential dissolution of the valve metal and nickel with a consequent formation of microcrystalline alloy powders consisting of concentrated platinum group elements and some nickel and valve metal. Porous gas-diffusion electrodes prepared by using these alloy catalyst powders were employed for electrochemical reduction of oxygen and oxidation of hydrogen in 1 M H₂SO₄ at 25°C. The activity of the electrodes prepared from the amorphous alloys containing Pt–Ru, Pt–Rh, Pt and Pd for oxygen reduction was considerably higher than that of the platinum black electrode. Oxidation of hydrogen occurred readily close to the equilibrium potential. Amorphous alloy electrodes containing Pt–Ru, Pt–Rh and Pt were more active than the platinum black electrode for the hydrogen oxidation.     

Nickel–Iron Anode Substrate for Smart Solid Oxide Fuel Cells with a Self‐Protecting Function Against Reoxidation

Development of highly reliable solid oxide fuel cells (SOFCs) is strongly requested, and the introduction of a self‐protecting function is an ideal approach to increase the reliability of SOFCs. A highly porous (>33%) Ni–Fe metal substrate, which has well‐developed nanopores, is prepared by reduction of NiO–Fe₂O₃. In an oxidizing atmosphere, a thin layer of Fe₂O₃ forms on the surface of the substrate. As a result, the porous morphology changes at the surface and becomes denser. This morphological change occurs only at the surface and prevents oxidation of Ni in the bulk of the substrate. Furthermore, the surface morphology returns to its original state following reduction. Therefore, despite the fact that Ni is readily oxidized, Ni metal phase is sustained in the Ni–Fe bimetallic alloy substrate even after 480 h oxidation in air. The cell power density is also stably sustained after a few reduction–reoxidation cycles. Here, we report that Ni–Fe bimetal alloy substrate exhibits a self‐protecting function against reoxidation of the substrate, which would otherwise lead to a permanent failure of the cell.     

Anodic oxidation of o-toluenesulphonamide to saccharine on a NiO(OH)-coated nickel anode

o-Toluenesulphonamide has been electrolytically oxidized at low current density to saccharine in aqueous solutions of alkali carbonates on anodes coated with NiO(OH). This electrolytic oxidation led to a 40% yield of saccharine. The application of carbon and glassy-carbon counter electrodes or of various supports for the working electrodes did not result in improved saccharine yield. Moreover, the choice of a different potential and a different current density or the use of organic co-solvents did not substantially affect the course of the electrolytic oxidation.In the electrolytic oxidation ofo-toluenesulphonamide a parasitic evolution of oxygen occurred which caused a partial degradation of the starting material. In strongly alkaline media, i.e. in aqueous solutions of alkali hydroxides, a fission of the NH₂ group with formation ofo-toluenesulphonate occurred during the electrolysis.     

Oxidation of methanol on nickel—zinc catalysts

The activity of nickel—zinc and smooth nickel electrodes for methanol oxidation is investigated in KOH 1 M + CH₃OH 1 M at 60°C. Nickel—zinc alloys of different composition are easily obtained from electrodeposition. After their attack in KOH 1 M, the electrodes contain about 50% of zinc and they have the βNiZn alloy structure. The I(V) curves are compared. On smooth nickel the oxidation of methanol is strongly inhibited by the superficial oxides whereas on nickel—zinc electrodes superficial oxides are easier to reduce. The effect of adsorbed hydrogen and of mixed oxides is discussed.     

Abnormal infrared effects and electrocatalytic properties of nanometer scale thin film of PtRu alloys for CO adsorption and oxidation

Nanometer scale thin films of PtRu alloy supported on glassy carbon (nm-PtRu/GC) were prepared using electrochemical codeposition under cyclic voltammetric conditions. The composition of the PtRu alloy was altered by varying the concentration of Pt⁴⁺ and Ru³⁺ ions in the deposition solution. STM results demonstrated that the nm-PtRu film was composed of crystallites appearing in a layered hexagonal form. The nm-PtRu/GC exhibited a high catalytic activity for CO oxidation, consisting mainly in a negative shift of potential for CO_ad oxidation. In situ FTIR spectroscopic studies revealed that the nm-PtRu/GC alloy electrodes of different surface composition exhibit abnormal infrared effects (AIREs) as observed on electrodes of nanometer scale thin films of transition metals. The AIREs observed on nm-PtRu/GC electrodes consist in the inversion of CO_ad bands, the significant enhancement of IR absorption and an increase in the FWHM. Following the increase in Ru component in the nm-PtRu thin film the FWHM was increased progressively from 20 cm^–1 on a nm-Pt/GC to 55 cm^–1 on a nm-Ru/GC electrode, and the enhancement factor of IR absorption by CO_L was between 10.5 to 13.1. The present study has provided new understanding of the structure and properties of nanometer scale thin film PtRu alloy material, and highlights its potential for fuel cell applications.     

硼对非晶态LaMg11Zr＋200%Ni储氢合金电化学性能的影响

以熔炼LaMg₁₁Zr为母体合金,采用机械合金化法制备了非晶态LaMg₁₁Zr＋200%Ni+xB(x=0,2%,5%,10%)系列储氢合金,研究了B含量对合金结构和电化学性能的影响。结果表明：球磨20 h后,各合金均达到非晶态,B促进了非晶态合金的形成,提高了非晶态合金的热稳定性。合金电极均有良好的活化性能;随B含量的增加,合金电极的放电容量先增加后减少,LaMg₁₁Zr＋200%Ni+2%B合金电极达到最大放电容量614.2 mAh.g^-1,比不含B合金电极提高了67%;添加B提高了合金电极的循环稳定性,充放电30周循环后合金电极的容量保持率由44.4%(x=0)增加到70.4%(x=10%);且添加B在一定程度上改善了合金电极的高倍率放电性能。     

Enhanced electrochemical properties of nonstoichiometric amorphous Mg2Ni1.3 electrodes

Nonstoichiometric amorphous Mg₂Ni_1.3 alloys were synthesized by mechanically milling crystalline Mg₂Ni alloy with Ni powders. In comparison with the stoichiometric material, the nonstoichiometric Mg₂Ni_1.3 phase showed a higher discharge capacity because of the amorphization of the alloy. Surface modification with graphite was also carried out for further improvement of its electrode performance. The coated powders showed a better cyclic stability because the graphite protected the Mg from oxidation. The rate capability (RC) and discharge capacity of electrodes was also markedly improved with graphite coating due to the excellent electrical conductivity and electrocatalytic activity of graphite.     

Redox of Ni/YSZ anodes and oscillatory behavior in single-chamber SOFC under methane oxidation conditions

A single-chamber solid oxide fuel cell made of Ni/YSZ anodes, YSZ electrolytes and SDC-impregnated LSM cathodes was tested in methane–oxygen mixture at furnace temperature equal to 700 °C. Two Ag wires were arranged on the anode surface to in situ measure the changes of the local anode resistance (R_s) during testing. Oscillations of the R_s, the cell voltage and the actual temperature of the cell were observed and attributed to Ni/NiO redox cycles. Steady-state tests of the cell showed the corresponding oscillation patterns mainly depended on methane-to-oxygen ratio (M = 1, 2). Higher current density (J) to a certain extent could suppress NiO reduction and promote Ni oxidation. Scanning electron micrographs confirmed that Ni/NiO redox cycles had occurred mainly near the anode surface. The obtained results imply that gradual reoxidation of the Ni-anodes accompanying the various oscillation behaviors plays an important role in the degradation of the SC-SOFCs, especially under oxidation conditions.     

Nanocrystalline NiO thin film anode with MgO coating for Li-ion batteries

The nanocrystalline NiO thin films with the mean size of 30 nm are prepared by pulsed laser reactive ablation in an oxygen ambient and subsequent coated by MgO on the NiO film surface. As compared with bare NiO, coated NiO film electrode heat-treated at 500 °C exhibits excellent structural stability and electrochemical performance. Excellent electrochemical performance, a reversible capacity as high as 650 mAh/g in the range 0.01–3.0 V at high discharge rate of 2C with a high capacity retention up to 150 cycles, could be achieved with MgO-coated NiO films. Preliminary electrochemical cycling measurements show that capacity retention with capacity fading for bare NiO and MgO-coated NiO film electrodes are 0.43 and 0.28% per cycle, respectively, at the discharge rate of 2C after 150 cycles. This result is related to good structural stability of the MgO-coated NiO film as verified by cyclic voltammetric (CV) measurement and scanning electron microscopy (SEM) analysis.     

Preparation and electrical properties of ZnO‐Bi2O3‐based multilayer varistors with base metal nickel inner electrodes

In this study, ZnO‐Bi₂O₃‐based multilayer varistors (MLVs) cofired with nickel (Ni) inner electrodes were prepared by tape‐casting method. Samples were sintered in pure nitrogen (N₂) to keep Ni from being oxidized, and then reoxidized in air to obtain the nonlinear properties (reduction‐reoxidation method). The EDAX results showed that Ni inner electrodes are stable and have no evident migration into ZnO‐Bi₂O₃‐based ceramics when sintered in N₂. The influence of reoxidation temperature on microstructures and nonlinear properties of samples were studied. Samples reoxidized at the temperatures lower than 650°C showed poor nonlinear properties. After reoxidized in air at 700°C for 2 hours, samples exhibited nonlinear properties of V_1 mA=16.3 V, α=26.5, I_L=0.68 μA. At the reoxidation temperature higher than 750°C, the oxidation of Ni inner electrodes deteriorated the nonlinear properties of samples. It demonstrated that ZnO‐Bi₂O₃‐based MLVs with base metal Ni inner electrodes proposed in this work are suitable for reduction‐reoxidation method. The replacement of noble metals Pt or Ag/Pd alloys by base metal Ni is expected to lower the cost of ZnO‐Bi₂O₃‐based MLVs.     

Calciothermic reduction of NiO by molten salt electrolysis of CaO in CaCl2 melt

Metallic nickel powders with low and uniform residual oxygen content were produced from NiO using the molten salt electrolysis of CaO in CaCl₂ melt. Suitable amount of CaO for the reduction was in the range of 0.5–3.0 mol% CaO.The electrical isolation of NiO from both electrodes could produce metallic Ni in CaCl₂ melt. Separating the metal oxides from the cathode confirmed the mechanism of calciothermic reduction that the electrolysis of dissolved CaO in CaCl₂ melt produces Ca, and that the dissolved Ca in molten CaCl₂ successfully reduces NiO to metallic Ni. An average of about 600 ppm oxygen in Ni sample was achieved directly from oxide, when NiO was detached from the cathode.     

Corrosion performance of NiAl intermetallic with Mo,Ga and Fe in neutral and alkaline media

The corrosion resistance of mechanically alloyed NiAl intermetallics with additions of Mo, Ga and Fe, and their combinations, has been studied using potentiodynamic polarization curves and linear polarization resistance tests at room temperature. Solutions included 0.5 m NaCl and 0.5 m NaOH. The Al–42Ni+6Fe and Al–40Ni+6Fe+2Mo alloys exhibited the best corrosion resistance in NaCl whereas the highest corrosion rate was observed on Al–39Ni+6Fe+6Mo alloy, above the NiAl-base alloy. In NaOH, the highest corrosion rate was exhibited by Al–41Ni+6Mo alloy, whereas the best corrosion performance was obtained by alloys with 6Ga, followed by alloys containing 6Fe+2Ga and/or 2Mo+2Ga, and NiAl-base alloy had an intermediate corrosion rate. The alloys with the lowest corrosion rate also had the lowest pitting potential values. So, generally speaking, additions of 6Mo decreased the corrosion resistance of NiAl-base alloys in these environments. The results were supplemented by detailed scanning electronic microscopy studies and chemical microanalysis of the corroded specimens.     

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: Characterization and catalytic performance

This work demonstrates the high potential of a new class of catalytic materials based on nickel for the oxidative dehydrogenation of ethane to ethylene. The developed bulk Ni–Nb–O mixed oxides exhibit high activity in ethane ODH and very high selectivity (∼90% ethene selectivity) at low reaction temperature, resulting in an overall ethene yield of 46% at 400 °C. Varying the Nb/Ni atomic ratio led to an optimum catalytic performance for catalysts with Nb/Ni ratio in the range 0.11–0.18. Detailed characterization of the materials with several techniques (XRD, SEM, TPR, TPD-NH₃, TPD-O₂, Raman, XPS, electrical conductivity) showed that the key component for the excellent catalytic behavior is the Ni–Nb solid solution formed upon the introduction of niobium in NiO, evidenced by the contraction of the NiO lattice constant, since even small amounts of Nb effectively converted NiO from a total oxidation catalyst (80% selectivity to CO₂) to a very efficient ethane ODH material. An upper maximum dissolution of Nb⁵⁺ cations in the NiO lattice was attained for Nb/Ni ratios ⩽0.18, with higher Nb contents leading to inhomogeneity and segregation of the NiO and Nb₂O₅ phases. A correlation between the specific surface activity of the catalysts and the surface exposed nickel content led to the conclusion that nickel sites constitute the active centers for the alkane activation, with niobium affecting mainly the selectivity to the olefin. The incorporation of Nb in the NiO lattice by either substitution of nickel atoms and/or filling of the cationic vacancies in the defective nonstoichiometric NiO surface led to a reduction of the materials nonstoichiometry, as indicated by TPD-O₂ and electrical conductivity measurements, and, consequently, of the electrophilic oxygen species (O⁻), which are abundant on NiO and are responsible for the total oxidation of ethane to carbon dioxide.     

Performance of a novel Ni/Nb cathode material for molten carbonate fuel cells (MCFC)

In this work the performance of NiO and a novel cathode material preoxidized nickel–niobium alloy were investigated. It is found that under a cathode atmosphere of p(CO₂)/p(O₂) = 0.67 atm/0.33 atm, the equilibrium solubility of nickel ions in (Li_0.62, K_0.38)₂CO₃ melt at 650 °C is about 17 ppm for the nickel oxide electrode and 8 ppm for the preoxidized nickel–niobium alloy electrode. The improvement in the stability of material in the melt may be attributed to the formation of a more dense nodular structure for the nickel–niobium alloy electrode when compared with a Ni electrode during preoxidation. The formation of a dense nodular structure for the nickel–niobium alloy electrode depresses the dissolution of NiO from the electrode into the carbonate melt and, accordingly, enhances the stability of the electrode material in the melt. The polarization performance of the NiO cathode was improved by electrodeposition of niobium. As far as the thermal stability and the polarization performance are concerned, the preoxidized nickel–niobium alloy can be considered as a candidate for the cathode material of MCFCs.     

Effect of cathodic reduction on catalytic activity of amorphous alloy electrodes for electrooxidation of sulfite

Active electrode materials for a new zinc electrowinning process, in which the thermodynamic cell voltage is about half that of the conventional process by replacing oxygen evolution by anodic oxidation of SO₂ produced in the zinc smelting process have been studied. Immersion in HF solution and subsequent cyclic voltammetry (CV) in sulfuric acid are known to be effective surface activation treatments of the amorphous alloy electrodes. The galvanostatic cathodic reduction (CR) treatment was applied to obtain further activation for sulfite oxidation for HF- and CV-treated electrodes prepared from amorphous nickel-valve metal-platinum group metal alloys. This treatment has been found to be effective in enhancing the activity. Among the amorphous Ni-40Nb alloys containing platinum group elements, the platinum-containing electrode showed the highest catalytic activity, which was higher than that of platinized platinum. Furthermore, the electrocatalytic activities of CR-treated electrodes prepared from amorphous alloys containing platinum and rhodium, and platinum and ruthenium were higher than that of the electrode containing only platinum. According to XPS analysis of the amorphous Ni-40Nb-1Pt-1Ru alloy specimen the enrichment of platinum and ruthenium occurred by CV treatment, and a small amount of oxidized platinum and ruthenium species remained on the electrode surfaces, but most of them were cathodically reduced to the metallic state by CR treatment. High catalytic activities for sulfite oxidation can be attributed to the metallic state of platinum and ruthenium contained in the alloy electrodes, even though the activity of these electrocatalysts is higher than that of pure Pt or Ru.