Electrooxidation of formic acid on carbon supported PtxPd1−x (x = 0-1) nanocatalysts

In this work, carbon supported Pt_xPd_1−x (x = 0-1) nanocatalysts were investigated for formic acid oxidation. These catalysts were synthesized by a surfactant-stabilized method with 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12) as the stabilizer. They show better Pt/Pd dispersion and higher catalytic performance than the corresponding commercial catalysts. Furthermore, the electrocatalytic properties of Pt_xPd_1−x/C were found to depend strongly on the Pt/Pd deposition sequence and on the Pt/Pd atomic ratio. At a lower potential, formic acid oxidation current on co-deposited Pt_xPd_1−x/C catalysts increase with increasing Pd surface concentration. Nanoscale Pd/C is a promising formic acid oxidation catalyst candidate for the direct formic acid fuel cell.     

Properties of Ni1−xFex (0.1 < x < 0.9) and Invar (x = 0.64) alloys obtained by electrodeposition

Electrodeposition of Ni_1−xFe_x (x = 0.1-0.9) films was carried out from a chloride plating solution containing saccharin as an organic additive at a constant current density (5 mA/cm²) and a controlled pH of 2.5. X-ray diffraction studies revealed the existence of an fcc, or γ phase, in the range of 10-58 wt.% Fe, a mixed fcc/bcc phase in the range of 59-60 wt.% Fe, and a bcc, or α phase in the range of 64-90 wt.% Fe. The saturation magnetization, B_s, of electrodeposited Ni_1−xFe_x alloys at the room temperature was found to increase with the increase of Fe-content and follows the Slater-Pauling curve, but deviates from as-cast bulk NiFe alloys. The coefficient of thermal expansion, CTE, of electrodeposited alloys at room temperature also deviates from as-cast bulk NiFe alloys. Annealing of α-Ni₃₆Fe₆₄ alloy results in a martensitic α → γ phase transformation, which takes place between 300 and 400 °C. It was demonstrated that thermal treatment above 400 °C was necessary to obtain magnetic and mechanical properties similar to those to conventional Invar alloy. Annealing of α-Ni₃₆Fe₆₄ alloy at 700 °C brings about a decrease of B_s from 1.75 to 0.45 T. By controlling the annealing conditions of α → γ martensitic transformation, it is possible to adjust the CTE of Ni₃₆Fe₆₄ alloy over the broad limits from 2.7 to 8.7 × 10⁻⁶/°C.     

Nanostructured CNx (0 < x < 0.2) films grown by supersonic cluster beam deposition

Nanostructured CN_x thin films were prepared by supersonic cluster beam deposition (SCBD) and systematically characterized by transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The incorporation of nitrogen in the films (0 < x < 0.2) and the nanostructure were controlled by using different synthesis routes. Films containing bundles of well-ordered graphene multilayers, onions and nanotubes embedded in an amorphous matrix were grown alongside purely amorphous films by changing the deposition parameters. Graphitic nanostructures were synthesized without using metallic catalysts. The structural and electronic properties of the films have been studied by EELS. The role played by N in the carbon nanostructures has been deduced from XPS line-shape analysis.     

Stress evolution in CoxFe1−x (x = 0.33-0.87) electrodeposited films

Electrodeposition of 0.5 μm thick Co_xFe_1−x (x = 0.33-0.87) films was carried out from a sulfate/chloride plating solution containing saccharin as an organic additive at constant current density and a controlled pH 2.3. The increase of Fe²⁺ concentrations in plating solution resulted in an increase of Fe-content and tensile stress in Co_xFe_1−x films, which is accompanied by a decrease of plating rate. Several possible origins for generation of tensile stress include the following: interfacial stress between CoFe films and Cu-substrate, crystal texture and grain size, coalescence and stress evolution during film growth, and hydrogen adsorption/desorption. The adsorption/desorption mechanism of hydrogen seems to be the most likely dominant stress mechanism. The relationship between increase of the tensile stress and decrease of plating rate was discussed.     

Structural and magnetic characterization of MnxZn1 − xFe2O4 (x = 0.2; 0.35; 0.65; 0.8; 1.0) ferrites obtained by the citrate precursor method

In this work the microstructure and magnetic properties of Mn-Zn ferrites powders were investigated. Mn_xZn1 _− xFe₂O₄ powders where x = 0.2; 0.35; 0.5; 0.65; 0.8 and 1.0 were obtained by citrate precursor method. Citrate resin precursor was burned on air atmosphere at 400 °C for 3 h. Mn-Zn powders were calcined at 950 °C during 150 min under inert atmospheres: N₂ and rarefied atmosphere. Thermal analysis of precursor resin, phase evolution and microstructure of Mn-Zn ferrites powders were investigated by TG, DTA, XRD and SEM techniques. The powders calcined under rarefied atmosphere show spinel cubic structure and contamination of α-Fe₂O₃, while powders calcined under N₂ presents only the spinel cubic structure. Particle size was observed by SEM ranging from 80 to 150 nm. The magnetic properties were measured employing a vibrating sample magnetometer (VSM). It was observed that the saturation magnetization M_s increased with the increase of Mn content. The M_s of Mn_0.8Zn_0.2Fe₂O₄ calcined on rarefied atmosphere and Mn_0.8Zn_0.2Fe₂O₄ calcined on N₂ was 23.31 emu g⁻¹ and 56.23 emu g⁻¹, respectively.     

Electrochemical hydrogen storage in (Ti1−xVx)2Ni (x = 0.05-0.3) alloys comprising icosahedral quasicrystalline phase

For (Ti_1−xV_x)₂Ni (x = 0.05, 0.1, 0.15, 0.2 and 0.3) ribbons, synthesized by arc-melting and subsequent melt-spinning techniques, an icosahedral quasicrystalline phase was present, either in the amorphous matrix or together with the stable Ti₂Ni-type phase. With increasing x values, the maximum discharge capacity of the alloy electrodes increased until reached 271.3 mAh/g when x = 0.3. The cycling capacity retention rates for these electrodes were approximately 80% after a preliminary test of 30 consecutive cycles of charging and discharging. Ti_1.7V_0.3Ni alloy electrode displayed the best high-rate discharge ability of 82.7% at the discharge current density of 240 mA/g.     

Microstructures and electrochemical properties of Mg0.9Ti0.1Ni1−xMx (M = Co, Mn; x = 0, 0.1, 0.2) hydrogen storage alloys

Mg-Ni-Ti-based hydrogen storage alloys Mg_0.9Ti_0.1Ni_1−xM_x (M = Co, Mn; x = 0, 0.1, 0.2) were prepared by means of mechanical alloying (MA). The effects of partial substitution of Ni with Co or Mn on the microstructures and electrochemical performance of the alloys were investigated. The result of X-ray diffraction (XRD) shows that the alloys exhibit dominatingly amorphous structures. The electrochemical measurements indicate that the substitution of Ni can dramatically enhance the cycle stability of Mg-Ni-Ti-based alloys. After 50 charge/discharge cycles, the capacity retention rate of the alloy electrodes increases from 30% (Mg_0.9Ti_0.1Ni) to 59% (Mg_0.9Ti_0.1Ni_0.9Co_0.1), 58% (Mg_0.9Ti_0.1Ni_0.9Mn_0.1), 46% (Mg_0.9Ti_0.1Ni_0.8Co_0.2) and 53% (Mg_0.9Ti_0.1Ni_0.8Mn_0.2), respectively. Among these alloys, the Mg_0.9Ti_0.1Ni_0.9Mn_0.1 alloy presents better overall electrochemical performance. The cyclic voltammograms (CV) and anti-corruption test reveal that the electrochemical cycle stability of these alloys is improved by substituting Ni with Co or Mn.     

Structure and electrochemical performances of Mg2Ni1−xMnx (x = 0-0.4) electrode alloys prepared by melt spinning

The melt-spinning technique is applied to the preparation of the nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4). The as-spun alloy ribbons possessing a continuous length, a thickness of about 30 μm and a width of about 25 mm were prepared. The structures of the as-spun alloy ribbons are characterized by XRD and TEM. The electrochemical performances of the as-spun alloy ribbons are measured by an automatic galvanostatic system. The results show that no amorphous structure is detected in the as-spun Mg₂Ni alloy, whereas the as-spun Mg₂Ni_0.6Mn_0.4 alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni notably intensifies the amorphous forming ability of the Mg₂Ni-type alloy. The amorphization degree of the as-spun alloys containing Mn increases with increasing spinning rate. The melt spinning also significantly enhances the electrochemical performances such as the discharge capacity and the electrochemical cycle stability of the Mn-containing alloys. Furthermore, the high rate dischargeability (HRD) of the (x ≤ 0.1) alloys increases with an increase in the spinning rate, while for the (x ≥ 0.2) alloys, the HRD exhibits a maximum value at a particular spinning rate, and it varies with the change in Mn contents of the alloys.     

Quantitative study of pH change during LaNi5−xAlx (x = 0, 0.3) discharge process by SECM

Oxygen reduction reaction (ORR) on Pt microelectrode was used for developing a micro pH sensor for scanning electrochemical microscopy (SECM) study in this work. When the potential of Pt microelectrode was held constant in ORR region, the ORR current (cathodic current) increased with decreasing solution pH and vice versa. The response time of the ORR current to pH changes was measured to be ca. 30 ms which implies that the pH response is fast enough for monitoring the temporal pH changes. Furthermore, a fine linear relationship was found to exist between the half wave potential of ORR (E_1/2) and the solution pH value, and the slope is −46 mV/pH. The Pt micro pH sensor was located 1 μm above the LaNi_5−xAl_x (x = 0, 0.3) substrate electrode surface in pH = 9 KOH solution to perform the tip-substrate voltammetry of SECM. In tip voltammogram, the ORR tip current qualitatively reflects the transit solution pH changes during LaNi_5−xAl_x discharge reaction. Also, the minimum values of the solution pH near LaNi₅ and LaNi_4.7Al_0.3 surface during the discharge reaction were quantitatively detected; they were 7.17 and 7.57, respectively. The result indicates that Al partial substitution for Ni degrades the maximum discharge ability of the alloy and decreases the hydrogen diffusion coefficient in alloy bulk.     

Formic acid electro-oxidation on carbon supported PdxPt1−x (0 ≥ x ≥ 1) nanoparticles synthesized via modified polyol method

Carbon supported nanoparticle catalysts of Pd_xPt_1−x (0 ≥ x ≥ 1) were synthesized using a modified polyol method and poly(N-vinyl-2-pyrrolidone) (PVP) as a stabilizer. Resulting nanoparticles were characterized by X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperommetry (CA) study for formic acid electro-oxidation. Surface composition of the synthesized nanoparticles found from XPS revealed the Pt surface segregation even for the Pd-rich compositions. It is suggested that the surface segregation behavior in PdPt nanoparticles supported on carbon may be influenced, in addition to the difference in Pd and Pt surface energies, by particle size and particle interaction with the support. According to CA, the carbon supported Pd nanoparticles show the highest initial activity towards formic acid electro-oxidation at the potential of 0.3 V (RHE), due to the promotion of the direct dehydrogenation mechanism. However its stability is quite poor resulting in the fast deactivation of the Pd surface. Addition of Pt considerably improves the steady-state activity of Pd in 12 h CA experiment. CA measurements show that the most active catalyst is Pd_0.5Pt_0.5 of 4 nm size, which displays narrow size distribution and Pd to Pt surface atomic ratio of 27-73.     

Synthesis and characterization of abundant Ni-doped LiNixMn2−xO4 (x = 0.1-0.5) powders by spray-drying method

The structure and electrochemical properties of LiNi_xMn_2−xO₄ cathode materials for lithium ion batteries were studied by the means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), cyclic voltammetry, and galvanostatic charge-discharge tests. The cathodes with different Ni contents (LiNi_xMn_2−xO₄, x = 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by a spray-drying method and showed a single-phase spinel structure without any impurity. The amount of Ni has a large effect on the electrochemical characteristics. Capacity values of different voltage ranges (4- and 5-V ranges) change obviously with amount of Ni-doped. Also, the total discharge capacities increase with the Ni content, and all of them have good cycle stability.     

Structure and electrochemical characteristics of TiV1.1Mn0.9Nix (x = 0.1-0.7) alloys

The structure and electrochemical properties of TiV_1.1Mn_0.9Ni_x (x = 0.1-0.7) solid solution electrode alloys have been investigated. It is found that these alloys mainly consist of a solid solution phase with body centered cubic (bcc) structure and a C14 Laves secondary phase. The solid solution alloys show easy activation behavior, high temperature dischargeability, high discharge capacity and favorable high-rate dischargeability as a negative electrode material in Ni-MH battery. The maximum discharge capacity is 502 mAh g⁻¹ at 303 K when x = 0.4. Electrochemical impedance spectroscopy (EIS) test shows that the charge-transfer resistance at the surface of the alloy electrodes decreases obviously with increasing Ni content.     

Zr-doped Li[Ni0.5−xMn0.5−xZr2x]O2 (x = 0, 0.025) as cathode material for lithium ion batteries

Layered Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) have been prepared by the mixed hydroxide and molten-salt synthesis method. The individual particles of synthesized materials have a sub-microsize range of 200-500 nm, and LiNi_0.475Mn_0.475Zr_0.05O₂ has a rougher surface than that of LiNi_0.5Mn_0.5O₂. The Li/Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) electrodes were cycled between 4.5 and 2.0 V at a current density of 15 mA/g, the discharge capacity of both cells increased during the first ten cycles. The discharge capacity of the Li/LiNi_0.475Mn_0.475Zr_0.05O₂ cell increased from 150 to 220 mAh/g, which is 50 mAh/g larger than that of the Li/LiNi_0.5Mn_0.5O₂ cell. We found that the oxidation of oxygen and the Mn³⁺ ion concerned this phenomenon from the cyclic voltammetry (CV). Thermal stability of the charged Li[Ni_0.5−xMn_0.5−xZr_2x]O₂ (x = 0, 0.025) cathode was improved by Zr doping.     

Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni-MH batteries

La_(1−x)Sr_xFeO₃ (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La_(1−x)Sr_xFeO₃ perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La_(1−x)Sr_xFeO₃ oxide electrodes are reversible. The discharge capacities of La_(1−x)Sr_xFeO₃ oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g⁻¹ to 543.0 mA h g⁻¹ (when x = 0.2) and from 147.0 mA h g⁻¹ to 501.5 mA h g⁻¹ (when x = 0.4) at the current density of 31.25 mA g⁻¹, respectively. After 20 charge-discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La_(1−x)Sr_xFeO₃ oxide electrodes increase with the increase of the temperature. Compared with La_0.8Sr_0.2FeO₃, La_0.6Sr_0.4FeO₃ electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures.     

A TDS study of D adsorption on terraces and terrace edges of graphite (0 0 0 1) surfaces

Adsorption of thermal (2000 K) D atoms on (0 0 0 1) surfaces of various highly oriented pyrolytic graphite (HOPG) and natural graphite substrates was studied under ultra high vacuum (UHV) conditions with thermal desorption spectroscopy (TDS). D chemisorption on terrace and terrace edge sites of graphite (0 0 0 1) surfaces was identified. Recombinative desorption of D adsorbed on terraces was observed between 400 and 600 K. The analysis of TD spectra from various graphite surfaces reveals the existence of three desorption states intrinsic to graphite (0 0 0 1), and proposed as being due to adsorbate structures composed of one (monomer) and two neighbouring (dimer) chemisorbed D atoms, and aggregates thereof (mixed). The dimer structure is supposed to exhibit higher stability than the monomer. Reaction of D with terrace edges leads to the formation of CD, CD₂ and CD₃-groups at edge C atoms. These groups decompose during heating between 790 and 1300 K via release of gaseous D₂ and CD_x, C₂D_x and C₃D_x-hydrocarbons.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

Effect of Mn content on the structure and electrochemical characteristics of La0.7Mg0.3Ni2.975−xCo0.525Mnx (x = 0-0.4) hydrogen storage alloys

The effect of Mn content on the crystal structure and electrochemical characteristics of La_0.7Mg_0.3Ni_2.975−xCo_0.525Mn_x (x = 0, 0.1, 0.2, 0.3, 0.4) alloys has been studied systematically. The results of the Rietveld analyses show that all these alloys mainly consist of two phases: the La(La,Mg)₂Ni₉ phase with the rhombohedral PuNi₃-type structure and the LaNi₅ phase with the hexagonal CaCu₅-type structure. The pressure-composition isotherms shows that the partial substitution of Mn for Ni results in lower desorption plateau pressure and steeper pressure plateau of the alloy electrodes. For a Mn content of x = 0.3, the electrochemical performances, including specific discharge capacity, high rate chargeability (HRC) and high rate dischargeability (HRD), of the alloy are preferable. Moreover, the data of the polarization resistance R_p and the exchange current density I₀ of the alloy electrodes is consistent with the results of HRC and HRD. The hydrogen diffusion coefficient D increases with increasing Mn content, and thereafter increases the low temperature dischargeability (LTD) of the alloy electrodes.     

Uncommon potential hysteresis in the Li/Li2xVO(H2−xPO4)2 (0 ≤ x ≤ 2) system

Physical and electrochemical investigations of vanadium phosphates, Li_2xVO(H_2−xPO₄)₂ (0 < x < 2), have been undertaken. H⁺/Li⁺ ionic exchange from VO(H₂PO₄)₂ to Li₂VO(HPO₄)₂ leads to grain decrepitation. Further ionic exchange toward formation of Li₄VO(PO₄)₂ lowers the symmetry. As inferred from potentiodynamic cycling correlated to ex situ and in situ X-ray diffraction (XRD), the system Li/Li₄VO(PO₄)₂ shows several phase transformations that are associated with thermodynamical potential hysteresis that span from roughly 15 mV to more than 1.8 V. Small hysteresis are associated with topotactic reactions and with V^V/V^IV and V^III/V^II redox couples. Large potential hysteresis values (>1 V) were observed when oxidation of V^III to V^IV is involved.     

Microstructural and electrical behavior of Bi4V2−xCuxO11−δ (0 ≤ x ≤ 0.4)

Cation substituted bismuth vanadate possesses high oxygen ion conductivity at lower temperatures. The ionic conductivity of this material at 300 °C is 50–100 times more than any other solid electrolyte. Three phases (α, β, γ) are observed in the substituted compound; α and γ are low and high conducting phase, respectively. Samples of Bi₄V_2−xCu_xO_11−δ (x = 0–0.4) were prepared by solid-state reaction technique. Impedance spectroscopy measurements were carried out in the frequency range of 100 Hz to 100 kHz using gold sputtered cylindrical shaped pellets to obtain bulk ionic conductivities as a function of the substitution and temperature. The change of slopes observed in the Arrhenius plots is in agreement with the phase transitions for all the compositions. The highest ionic conductivity of the Cu-substituted compound was observed in Bi₄V_1.8Cu_0.2O_11−δ which is attributed to its lower activation energy. Microstructural studies indicated the stabilization of high temperature γ-phase at low temperature in those samples whose ionic conductivity observed was higher.