Enhancing photoelectrochemical activity of nanocrystalline WO3 electrodes by surface tuning with Fe(Ⅲ)

Tungsten oxide (WO₃) photoelectrodes with the surface tuned by Fe(Ⅲ) for photoelectrochemical water splitting were successfully synthesized. Nanostructured WO₃ films were prepared using doctor blade method, then a facile and economical deposition-annealing process was employed to fabricate Fe(Ⅲ) modified WO₃ films. The resulting composite's structural and optical properties were analyzed by SEM, EDX, XRD, UV–Vis spectrometry and XPS. The photoelectrochemical properties were evaluated by photocurrent density under 500 W Xe lamp with an intensity of 100 mW/cm². The Fe(Ⅲ) modified WO₃ electrode exhibited a larger photocurrent than the pure WO₃ electrode. Significantly, the optimized Fe(Ⅲ) modified WO₃ film achieved the maximum photocurrent density of 1.18 mA/cm² at 0.8 V vs. Ag/AgCl in the 0.2 M Na₂SO₄. The enhanced photocurrent was attributed to the extension of the light response and the electron hole separation at the interface Fe(Ⅲ)/WO₃ which was confirmed by Mott–Schottky and electrochemical impedance spectroscopy.     

Comparison of electrocatalytic activity of carbon-supported Au–M (M = Fe,Co, Ni,Cu and Zn) bimetallic nanoparticles for direct borohydride fuel cells

The Au–M (M = Fe, Co, Ni, Cu and Zn) bimetallic nanoparticles supported on the Vulcan XC-72R (Au–M/C) were synthesized by a reverse micelle method. The structures and compositions of the carbon supported Au–M catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS). The electrocatalytic activity of the Au–M bimetallic nanoparticles with respect to borohydride electro-oxidation for the application of fuel cell was investigated by voltammetry, chronoamperometry and chronopotentiometry. The results showed that alloying Au with 3d transition metals Fe, Co, Ni, Cu or Zn, a metal that leads to the maximum eight-electron oxidation of BH₄⁻, not only improved the electrode kinetics of BH₄⁻ oxidation but also reduced catalyst cost. Among the various investigated Au–M/C electrocatalysts, the Au–Zn, Au–Fe and Au–Cu catalysts showed no activity of NaBH₄ hydrolysis, and Au–Zn presented an attractive catalytic activity for borohydride oxidation.     

Effect of transition metal on stability and activity of La-Ca-M-(Al)-O (M = Co,Cr, Fe and Mn) perovskite oxides during partial oxidation of methane

Perovskite oxides of the type of La_xCa_1-xM_yAl_1-yO_3-δ (M = Co, Cr, Fe, Mn; x = 0.5; y = 0.7–1.0) were prepared using the polymerization methods and evaluated via N₂ adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX) spectroscopy, temperature-programmed reduction by hydrogen (TPR-H₂) and temperature-programmed oxidation by oxygen (TPO-O₂). Catalytic behaviour of the perovskite oxides during methane oxidation was studied using a tubular fixed-bed reactor. In a partial oxidation, which proceeded in two steps, there was total oxidation in the first step and CO₂ and H₂O were formed; in the second step, the total oxidation products oxidized methane by (dry and wet) reforming reactions to yield CO and H₂. Total oxidation and the two reforming reactions proceeded on two types of an active centre formed by transition metal ions, oxygen vacancies and oxide ions. The catalytic system La-Ca-Co-Al-O which contained aluminium, decomposed in partial oxidation of methane (POM) into a composite that contained firmly bonded cobalt nanoparticles in the surface of a substrate made up of La₂O₃, CaO and Al₂O₃ which catalysed POM with a high methane conversion and hydrogen selectivity.     

Synthesis and characterization of macroporous Ni,Co and Ni–Co electrocatalytic deposits for hydrogen evolution reaction in alkaline media

In this work, macroporous Ni, Co and Ni–Co electrodes have been developed by co-deposition at high current density on stainless steel (AISI 304) substrates. The obtained materials were characterized both morphologically and chemically by confocal laser scanning microscopy, and SEM coupled with EDX analysis. The activity for hydrogen evolution reaction (HER) on the obtained layers was assessed by using pseudo-steady-state polarization curves and electrochemical impedance spectroscopy (EIS) in alkaline solution (30 wt.% KOH). The electrochemical results show that HER on these electrodes takes place by the Volmer–Heyrovsky mechanism. The synthesized coatings present higher catalytic activity for HER than commercial smooth Ni electrode. As the Co content increases in the electrodeposition bath the obtained structures show lower surface roughness factors. Ni–Co deposit with a Co content of 43 at.% manifests the highest intrinsic activity for HER as a consequence of the synergetic combination of Ni and Co.     

Fabrication of bimetallic Pt–M (M = Fe,Co, and Ni) nanoparticle/carbon nanotube electrocatalysts for direct methanol fuel cells

The electrochemical activities of three bimetallic Pt–M (M = Fe, Co, and Ni) catalysts in methanol oxidation have been investigated. An efficient approach including chemical oxidation of carbon nanotubes (CNTs), two-step refluxing, and subsequent hydrogen reduction was used to thoroughly disperse bimetallic nanopartilces on the oxidized CNTs. Three catalysts with a similar Pt:M atomic ratio, Pt–Fe (75:25), Pt–Co (75:25), and Pt–Ni (72:28), were prepared for the investigation of methanol oxidation. The Pt–M nanoparticles with an average size of 5–10 nm are uniform and cover the surface of CNTs. Cyclic voltammetry showed that the three pairs of catalysts were electrochemically active in the methanol oxidation. On the basis of the experimental results, the Pt–Co/CNT catalyst has better electrochemical activity, antipoisoning ability, and long-term cycleability than the other electrocatalysts, which can be justified by the bifunctional mechanism of bimetallic catalysts. The satisfactory results shed some light on how the use of Pt–Co/CNT composite could be a promising electrocatalyst for high-performance direct methanol fuel cell applications.     

Mutual effect of extrinsic defects and electronic carbon traps of M-TiO2 (M = V,Co, Ni) nanorod arrays on photoexcited charge extraction of CdS for superior photoelectrochemical activity of hydrogen production

Understanding the photoexcited charge carrier dynamics such as separation, transportation and extraction in smart hybrid nanocomposites is the key to high performance solar cells. Nanocomposites possess advantage of broader solar absorption with their fast photoexcited charge separation and transportation but their use as photocorrosion-stable material is yet to be explored. Also, bulk and surface defects in individual components of the nanocomposites boost the efficiency of the solar cells, despite of the fact the recombination of the photoexcited charges at the interfaces lead to a substantial loss of charges and realizing a big challenge. Herein, the extrinsic defects like bulk and surface defects are induced by transition metal (M = V, Co, Ni) doping of M ? TiO₂ nanorod arrays. Consequently, the hydrothermal synthesis method offers the tuning of the carbon trapping states depending upon the type of the metal doped in M ? TiO₂ that decelerates the charge carrier dynamics in the M-TiO₂/CdS (M = V, Co, Ni) nanocomposites with the increase in the amount of carbon. Excellent charge extraction is observed in VTiO₂ (4% carbon) from its CdS sensitizer with photocurrent density of 2.06 mA/cm² than NiTiO₂ (14.6% carbon), TiO₂ (18.94% carbon) and CoTiO₂ (39.2% carbon) with photocurrent densities of 1.83, 1.46 and 1.34 mA/cm² at 0 V versus Ag/AgCl under 100 mW/cm² light intensity, respectively. This shows primary dependence of photoexcited charge dynamics upon the density of the carbon trapping states to be least while secondary dependence upon the density of the extrinsic defects in M ? TiO₂ to be maximum. This work creates a paradigm for future studies to have a broader insight of the photocatalyst's overall functioning to boost the efficiencies in solar cells by controlling the amount of electronic carbon traps during the synthesis of a large class of inorganic semiconductor photocatalysts.     

Evaluation of discharge and cycling properties of skutterudite-type Co1−2yFeyNiySb3 compounds in lithium cells

The preparation, structural characterization and study of the electrochemical behavior in lithium cells of Co_1−2yFe_yNi_ySb₃ (0.125<y≤0.5) compounds is described. The refinement of X-ray diffraction (XRD) patterns evidenced the skutterudite-type structure for all compositions, with an increasing partial filling of 2a sites. The first discharge of lithium anode cells shows two plateaus. The plateau at higher potential, which occurs at ca. 0.8 V and extends from ca. 150 to 250 Ah/kg depending on “y”, is mainly assigned to side reactions of lithium with the electrolyte, as confirmed by ⁷Li NMR. The second plateau occurs at 0.5–0.6 V and is assigned to Li–Sb alloys formation. The charge of the cell shows that only the second step is reversible. Further discharge/charge cycles present a plateau at 0.8 V in the discharge and at 1.05 V in the charge, which agree well with the Li–Sb alloying/de-alloying process. Extended cycling results on a loss of capacity, to stabilize over 100 Ah/kg after 20 cycles.     

The effect of the absence of Ni,Co, and Ni–Co catalyst pretreatment on catalytic activity for hydrogen production via oxidative steam reforming of ethanol

This article presents a study of the catalytic performance of Ni, Co, and Ni–Co–Mg–Al mixed oxides obtained from hydrotalcite precursors for the oxidative steam reforming of ethanol (OSRE) when no pretreatment (pre-reduction) is accomplished. Two catalysts (a Ni-based monometallic and an equimolar Ni–Co-based catalyst) achieve in situ reduction over shorter time periods compared with the other bimetallic catalysts and also, exhibit the best catalytic activity. On the contrary, the monometallic Co catalyst did not exhibit good catalytic performance, likely because of the existence of resistant spinel phases to soft reduction processes and/or to the re-oxidation of Co. The equimolar presence of Ni and Co generates a synergistic effect evidenced by the increase in the reducibility, basicity, and mobility of electrophilic oxygen species of the solid. The results yield important information for better understanding the catalytic system under study.     

Electrocatalytic activity for the hydrogen evolution of the electrodeposited Co–Ni–Mo,Co–Ni and Co–Mo alloy coatings

The electrocatalytic activity for the HER of the ternary Co–Ni–Mo and the binary Co–Ni and Co–Mo alloy coatings is investigated in 1 M KOH solution. The surface morphology and the structure of the studied coatings is characterized by SEM and XRD analysis. The electrocatalytic activity for the HER is evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, cathodic polarization and chronopotentiometry techniques. XRD analysis reveals that all studied coatings are composed of the Co hcp structure. However, alloy deposits with Mo is characterized by more nanocrystalline structure. Electrochemical experiments reveal superior electrocatalytic activity of coatings with Mo in comparison to Co–Ni alloy. This is the results of larger real surface area of Co–Mo and Co–Ni–Mo alloys, which is confirmed by the higher surface roughness factors (R_f) calculated based on the EIS results. The ternary alloy coating is characterized by the highest R_f parameter and the highest catalytic activity for the HER.     

Fast synthesis and electrocatalytic activity of M@Pt (M = Ru,Fe3O4, Pd) core–shell nanostructures for the oxidation of ethanol and methanol

We report the fast synthesis and electrochemical evaluation of M@Pt core–shell nanostructures (M = Ru, Fe₃O₄, Pd) as well as Pt-alone nanoparticles for the ethanol and methanol oxidation reactions (EOR and MOR, respectively) in H₂SO₄ solution. The cores were obtained in 60 s. The Pt shells were deposited afterward on the cores also in 60 s. The reducing agent was NaBH₄. XRD results showed that crystalline Pd, Fe₃O₄ and Pt materials were obtained from this rapid process, while Ru was formed as a quasi-amorphous structure. The average particle sizes of the core–shell nanostructures determined with the Scherrer equation, ranged from 7.35 to 9.29 nm. The electrochemical characterization revealed a catalytic activity of the novel Fe₃O₄@Pt anode as high as that of Ru@Pt for the EOR, and higher than the activity of Pt-alone. However, the Fe₃O₄@Pt catalysts showed a lower activity for the MOR than Ru@Pt and Pt-alone. Durability tests indicated a high electrochemical stability of the Fe₃O₄@Pt and Ru@Pt nanostructures.     

Hydriding properties of Zr(FexCr1−x)2 intermetallic compounds

One group of potential hydrogen storage compounds, i.e. Zr(Fe_xCr_1−x)₂ intermetallics, are investigated. These intermetallics are predominantly single-phased, identified as the hexagonal Laves phase. Hydrogen is absorbed quite readily, with no special activation treatment. Hydrogen absorption results in the formation of a distinct hydride phase, with the same crystal structure as the parent compound. Of these intermetallics, Zr(Fe_0.75Cr_0.25)₂ demonstrates the best overall energy storage characteristics, i.e. a relatively high sorption capacity (H/M = 1) and low hydride stability (ΔH = −25 kJ mol⁻¹ H₂). This composition compares quite favorably to LaNi₅ in terms of hydrogen capacity, hydride stability and reaction kinetics.     

A comparative study of catalytic behaviors of Mn,Fe, Co,Ni, Cu and Zn–Based catalysts in steam reforming of methanol,acetic acid and acetone

This study investigated the distinct catalytic behaviors of mono Mn, Fe, Co, Ni, Cu and Zn catalysts in the reforming of the small organics including methanol, acetic acid and acetone. The results showed that Mn, Fe or Zn-based catalysts showed almost no activity for steam reforming of either methanol, acetic acid or acetone, due to their low capacity to break the chemical bonds of the organics or to activate steam. Co and Cu-based catalysts were generally active for steam reforming of methanol. Nevertheless, Co-based catalysts promoted methanol decomposition to form a substantial amount of CO. Alumina as a support remarkably influenced catalytic stability of the catalyst. The unsupported Cu catalyst showed a much lower stability than Cu/Al₂O₃. Nevertheless, the unsupported Ni was more stable than Ni/Al₂O₃ catalyst, due to its high resistivity towards coking. The unsupported Co, however, was prone to coking. The C/H ratios in the coke formed over the unsupported and alumina-supported Ni or Co catalysts were distinct, indicating the involvement of alumina in the coking process. In addition, Ni and Co catalysts behaved differently. Ni/Al₂O₃ showed a superior stability than Co/Al₂O₃ in steam reforming of acetone. The coke formed on Ni/Al₂O₃ was more aromatic than that over Co/Al₂O₃ catalysts while morphologies of coke (nanotubes over Ni/Al₂O₃ versus fibrous coke over Co/Al₂O₃) were also different.     

Correlation between microstructural defects and photoelectrochemical performance of 1D Ti–Nb composite oxide photoanodes for solar water splitting

We report on the optimized fabrication of ultrathin wall nanotubes grown on Ti–Nb alloy via anodization in organic-based electrolytes. The nanotubes are vertically aligned with wall thicknesses of 5–8 nm, diameters of 180–200 nm, and lengths of 2–2.8 μm. Raman spectroscopy and glancing angle x-ray diffraction (GAXRD) measurements indicated the formation of composite oxides of anatase TiO₂ and monoclinic Nb₂O₅. The composite oxides showed better stability at elevated temperatures up to 650 ᵒC and with much smaller induced microstrain compared to the TiO₂ counterpart. X-ray photoelectron spectroscopy (XPS) analysis confirmed the composition of the fabricated nanotubes as a mixed TiO₂–Nb₂O₅ composite. Upon their use as photoanodes to split water, the composite TiO₂–Nb₂O₅ NTs showed almost 2-fold increase in the obtained photocurrent compared to that of bare TiO₂ NTs prepared under the same conditions. Moreover, the incident photon to current efficiency (IPCE) of the mixed oxide nanotubes was higher than that of bare TiO₂ with a positive shift towards longer wavelengths, indicating improved electron mobility and charge collection. Hence, the addition of Nb resulted in the formation of thermally stable and photoactive photoanodes for solar fuel production.     

Novel synthesis of Ti–Fe2O3/MIL-53(Fe) via the in situ process for efficient photoelectrochemical water oxidation

The slow kinetics of water oxidation has become a challenge for photoelectrochemical hydrogen production. Here, a novel organic-inorganic integrated photoanode system was constructed by using MIL-53(Fe) formed during the in-situ etching process as a cocatalyst to modify Ti–Fe₂O₃. The photocurrent density of Ti–Fe₂O₃/MIL-53(Fe) reaches 2.5 mA/cm², 10 times that of bare Ti–Fe₂O₃ at 1.23 V vs. RHE, and the water oxidation photocurrent onset potential shifts 105 mV negatively. Ti–Fe₂O₃/MIL-53(Fe) reaches 52% at 390 nm for IPCE. The excellent photoelectrochemical performance is due to iron oxide clusters boost charge separation and transfer, in-situ etching exposes more reactive sites, and the tight connection reduces interfacial resistance, which greatly accelerates the surface kinetics of Ti–Fe₂O₃. The in-depth understanding is provided for in-situ modification of photoanodes by metal organic frameworks in this work.     

Studies on the thermal characteristics of hydrides of Mg,Mg2Ni,Mg2Cu and Mg2Ni1−xMx (M = Fe,Co, Cu or Zn; 0 < × < 1) alloys

X-ray results on the alloys of the composition Mg₂Ni_1−xM_x (M = Fe, Co, Cu or Zn; 0 < × < 1) suggest a Mg₂Ni-type structure. The alloys, Mg₂Ni_0.75M_0.25(M = Fe, Co, Cu or Zn) upon hydriding lead to the formation of quarternary hydrides while on dehydriding yield the starting ternary alloys except for copper containing alloys which show multi-phase regions and follow a different path way for the hydriding-dehydriding process. Thermal studies (TG-DTA) on the hydrides indicate the amount of hydrogen evolved as well as the desorption temperatures. The thermodynamic quantities, namely the enthalpies and entropies of formation of the hydrides were deduced from the DTA peak maximum temperature data. The kinetic parameters such as activation energies, reaction rates and orders of the reaction for the decomposition of the hydrides formed from Mg, Mg₂Ni and Mg₂Cu alloys were evaluated from the DTA data. As to the modified Mg₂Ni system, the copper substituted alloys show lower thermal stability and also presents some interesting properties. Hence, it is considered as one of the promising ternary combinations (Mg-Ni-Cu) for hydrogen storage purposes.     

Structural and electrochemical characterization of YBa(Fe,Co,Cu)2O5+δ layered perovskites as cathode materials for solid oxide fuel cells

Single- and double-doped YBa(Fe,Co,Cu)₂O_5+δ layered perovskites are prepared by solid state reaction method and their structural characteristics, thermal expansion coefficient, oxygen nonstoichiometry, electrical conductivity, and electrochemical performance are comparatively studied. The substitution of Co by Fe or/and Cu significantly improves thermal expansion properties as compared to undoped YBaCo₂O_5+δ. Electrochemical tests demonstrate the promising performance of synthesized materials as cathode materials at intermediate temperatures. Single doped YBaCuCoO_5+δ cathodes reveal the lowest polarization resistance equal to 0.24 and 0.78 Ω cm² at P(O₂) P^?1 = 0.2 at temperature of 800 and 700 °C, respectively.     

Influence of hydrogen in magnetic properties of Ce(Fe1 − xAlx)2

Results on the effect in magnetic and structural properties of hydrogen in Ce(Fe_{1 − x}Al_x)₂ samples, dissolved up to the terminal solid solubility, are presented. A comparison of our data with previously reported studies on the magnetic properties of the system under pressure allow us to determine that hydrogen affects magnetic properties of this system further than structural effects. In addition, magnetic and structural preliminary results on the hydride phase of this system are presented.     

Investigation on synthesis,characterization and hydrogenation behaviour of hydrogen storage material: Fe1−xZrxTi1.3 (x = 0.2)

The present study is aimed at investigations on the structural and microstructural characterization and hydrogenation behaviour of Zr substituted Fe_1−xZr_xTi_1.3 (x = 0.2) alloys. This storage alloy has been synthesized using R.F.induction melting under an argon atmosphere in a previously outgassed graphite crucible. The structural characterization (XRD) has revealed that the as-synthesized sample is multiphasic in nature and embodies the phases FeTi, Fe₂Ti, FeTi₂ and Ti. Microstructural evaluations (SEM) exhibited the presence of interfaces and cracking. The P–C isotherm was determined and it showed the storage capacity of ∼1.20 wt.% at 200°C. The activation as well as desorption kinetics (75% desorption of H₂ in 6 min) were found to be better for Zr-substituted alloy (Fe_0.8Zr_0.2Ti_1.3) as compared to FeTi_1.3. The improved activation and desorption kinetics are thought to be due to presence of several interfaces e.g. Fe(Zr)Ti⧸ Fe(Zr)Ti₂, Fe(Zr)Ti⧸{Fe(Zr)}₂Ti and volume expansion induced cracking of Ti.