Effect of thermal treatment on phase composition and ethanol oxidation activity of a carbon supported Pt50Sn50 alloy catalyst

A carbon supported Pt–Sn electrocatalyst in the Pt/Sn atomic ratio 50:50 was prepared by the reduction of Pt and Sn precursors with formic acid and thermally treated at 200 °C (i.e., in the presence of solid tin) and 500 °C (in the presence of molten tin) in flowing hydrogen. In the absence of thermal treatment, X-ray diffraction (XRD) analysis showed a solid solution of Sn in the face centered cubic (fcc) Pt and SnO₂. After thermal treatment, the formation of a main phase of hexagonal PtSn (niggliite) and a secondary phase of cubic Pt₃Sn was observed in the Pt50Sn50 catalyst. The relative amount of the PtSn phase increased with increasing thermal treatment temperature. The presence of molten tin gave rise to the formation of some big particles during annealing at 500 °C. The activity for the ethanol oxidation reaction (EOR) of the as-prepared catalyst was higher than that of both thermally treated catalysts and Pt75Sn25/C and Pt50Ru50/C by E-TEK. The higher activity for the EOR of the as-prepared Pt–Sn catalysts was ascribed to the presence of a large amount of SnO₂. Dedicated to Teresa Iwasita’s 65th birthday.     

Synthesis,characterization and electrocatalytic activity for ethanol oxidation of carbon supported Pt,Pt–Rh,Pt–SnO2 and Pt–Rh–SnO2 nanoclusters

Nanoclusters of Pt, Pt–Rh, Pt–SnO₂ and Pt–Rh–SnO₂ were successfully synthesized by polyol method and deposited on high-area carbon. HRTEM and XRD analysis revealed two phases in the ternary Pt–Rh–SnO₂/C catalyst: solid solution of Rh in Pt and SnO₂. The activity of Pt–Rh–SnO₂/C for ethanol oxidation was found to be much higher than Pt/C and Pt–Rh/C and also superior to Pt–SnO₂/C. Quasi steady-state measurements at various temperatures (30–60 °C), ethanol concentrations (0.01–1 M) and H₂SO₄ concentrations (0.02–0.5 M) showed that Pt–Rh–SnO₂/C is about 20 times more active than Pt/C in the potential range of interest for the fuel cell application.     

WO3 decorated carbon nanotube supported PtSn nanoparticles with enhanced activity towards electrochemical oxidation of ethylene glycol in direct alcohol fuel cells

This paper describes the concept of the utilization of metal oxide (WO₃) modified multi-walled carbon nanotubes (MWCNT) for supporting and activating PtSn nanoparticles (PtSn/WO₃-MWCNT and PtSn/MWCNT) for ethylene glycol oxidation. The resulting nanocomposite was developed and characterized using electrochemical and microscopic (TEM, SEM−EDS) techniques, as well as XRD analysis. The electrocatalytic currents measured under voltammetric and chronoamperometric conditions were greater than those found with the commercially available Vulcan-supported Pt₃Sn nanoparticles, which were used as reference catalysts. In situ FTIR spectroscopy was used to detect the formation of oxidation intermediates or products during the ethylene glycol oxidation. Combining the transition metal oxide species with Pt-based nanoparticles can generate OH groups at low potentials. These groups participate in the oxidation of passivating CO adsorbates on the Pt surface, and can also potentially break CH bonds. Further, the effectiveness of synthesized catalyst has been assessed through testing both catalysts in the single fuel cell. A single fuel cell with a PtSn/WO₃-MWCNT anode gave a better performance than one with a pristine PtSn/Vulcan anode, with a current density of around 79.8 mA cm⁻² and an output power density of 20.5 mW cm⁻².     

Highly active PtRuFe/C catalyst for methanol electro-oxidation

High methanol electro-oxidation activity was obtained on novel PtRuFe/C (2:1:1 at.%) catalyst. Mass and specific activities were 5.67 A  g⁻¹ catal. and 177 mA m⁻² for the PtRuFe/C catalyst while those of the commercial PtRu/C catalyst were 2.28 A g⁻¹ catal. and 87.7 mA m⁻², respectively. CO stripping results showed that on-set voltage for CO electro-oxidation was lowered by incorporation of Fe. XRD and XPS results revealed that Fe₂O₃ was formed instead of Fe(0), which resulted in large electron deficiency in Pt and easy CO electro-oxidation. The electron deficiency of Pt was proved by XPS results of Pt4f peaks, which moved to higher binding energies in PtRuFe/C than PtRu/C.     

Platinum-Tin/Tin Oxide/CNT Catalysts for High-Performance Electrocatalytic Ethanol Oxidation

Ethanol is a promising liquid clean energy source in the energy conversion field. However, the self-poisoning caused by the strongly adsorbed reaction intermediates (typically, CO) is a critical problem in ethanol oxidation reaction. To address this issue, we proposed a joint use of two strategies, alloying of Pt with other metals and building Pt/metal-oxide interfaces, to achieve high-performance electrocatalytic ethanol oxidation. For this, a well-designed synthetic route combining wet impregnation with a two-step thermal treatment process was established to construct PtSn/SnO_x interfaces on carbon nanotubes. Using this route, the alloying of Pt−Sn and formation of PtSn−SnO_x interfaces can simultaneously be achieved, and the coverage of SnO_x thin films on PtSn alloy nanoparticles can be facilely tuned by the strong interaction between Pt and SnO_x. The results revealed that the partial coverage of SnO_x species not only retained the active sites, but also enhanced the CO anti-poisoning ability of the catalyst. Consequently, the H−PtSn/SnO_x/CNT-2 catalyst with an optimized PtSn−SnO_x interface showed significantly improved performances toward the ethanol oxidation reaction (825 mA mg_Pt⁻¹). This study provides deep insights into the structure-performance relationship of PtSn/metal oxide composite catalysts, which would be helpful for the future design and fabrication of high-performance Pt-based ethanol oxidation reaction catalysts.     

Catalysts of ethanol anodic oxidation for ethanol-air fuel cell with a proton-conducting polymer electrolyte

Synthesis techniques for binary PtSn, PdM (M = Sn, V, Mo) and ternary PtSnNi, PtRuSn catalysts of ethanol electrooxidation on highly dispersed carbon materials are suggested. The highest activity in the 0.5 M H₂SO₄ solution containing 1 M C₂H₅OH corresponds to the system of PtSn (3: 1, 40 wt % Pt) with the particle size of 2–4 nm and tin content in the alloy with platinum of about 6%. It was shown that the catalyst efficiency as regards ethanol oxidation depth decreases in the series of Pt > PtRu ≈ PtSn, and the catalyst activity by current forms the series of PtSn > PtRu > Pt. The membrane-electrode assembly (MEA) with the anodes on the basis of the PtSn (3: 1, 40 wt % Pt) catalyst had stable characteristics for 220 h at the current density of ∼50 mA/cm².     

Partially reduced SnO2 nanoparticles anchored on carbon nanofibers for high performance sodium-ion batteries

One-dimensional (1-D) carbon nanofibers anchored with partially reduced SnO₂ nanoparticles (SnO₂/Sn@C) were successfully synthesized through a simple electrospinning method followed by carbon coating and thermal reduction processes. The partially reduced Sn frameworks, combined with the carbon fibers, provide a more favorable mechanism for sodiation/desodiation than SnO₂. As a result, SnO₂/Sn@C exhibits a high reversible capacity (536 mAh g^− 1 after 50 cycles) and an excellent rate capability (396 mAh g^− 1 even at 2 C rate) when evaluated as an anode material for sodium-ion batteries (SIBs).     

ZIF-8 derived nano-SnO2@ZnO as anode for Zn/Ni secondary batteries

SnO₂@ZnO was synthesized by a new method involving the immobilization of Sn onto zeolitic imidazolate framework-8 (ZIF-8) followed by calcination. The synthesized nanoparticles were characterized as 20–30 nm spherical ZnO particles uniformly dotted with SnO₂. When SnO₂@ZnO were used as anode material for Zn/Ni batteries, the average specific capacity was approximately 600 mAh g^− 1 and remained stable after 150 cycles at a rate of 1 C.     

Carbon-supported IrSn catalysts for a direct ethanol fuel cell

Carbon-supported Ir₃Sn/C and Ir/C catalysts were simply prepared with NaBH₄ as a reducing agent under the protection of ethylene glycol at room temperature. TEM and X-ray diffraction (XRD) data showed that the catalysts with small particle size exhibited the typical characteristic of a crystalline Ir fcc structure. Their electro-catalytic activities in comparison with Pt/C and Pt₃Sn/C catalysts also prepared by the NaBH₄ reduction process were characterized by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) techniques. The results indicated that Ir-based catalysts showed superior electro-catalytic activity towards ethanol oxidation to Pt/C and Pt₃Sn/C catalysts, mainly at low potential region. During single-cell tests at 90 °C, better performances of Ir-based catalysts as anodes were obtained compared to that of Pt/C catalyst. The comparable overall performance of Ir₃Sn/C to Pt₃Sn/C makes it a promising alternative choice of anode catalyst for direct ethanol fuel cells.     

Theoretical study on the stability and electronic property of Ag2SnO3

Bulk crystal properties of Ag₂SnO₃ were investigated with the advantage of density functional theory. The whole structure has layered feature: hexagonal metallic planes formed by Ag atoms and distorted octahedrons of SnO₆ clusters are configured alternatively along c axis of hexagonal cell. The cohesive energy is about ?2.792 eV/atom, which is less than SnO₂. The Debye temperature of Ag₂SnO₃ is about 231.6 K, and the bulk and shear moduli are 62.13 and 20.63 GPa, respectively. Band structure and DOS show the compound has a small pseudo-band gap value of 1.0 eV and so may be a semiconductor. When checking the PDOS intensity at the Fermi surface of Ag atoms, a weak metallic character can be seen. The distortion mechanism becomes less effective to reduce the total orbital energy both in SnO₂ and in Ag₂SnO₃ and as a result the bond lengths of Sn–O are intended to be isotropy.     

XPS/EXAFS study of cycleability improved LiMn2O4 thin film cathodes prepared by solution deposition

The influence of Sn substitution in LiMn₂O₄ thin films as a cathode has been studied via solution deposition to improve the electrochemical performance of thin film lithium batteries. LiSn_0.025Mn_1.95O₄ thin films showed the most promising performance, i.e. a high capacity retention of 77% at 10 C after the 500th cycle, due to the increased average Mn valence state. The thin films of LiSn_x/2Mn_2?xO₄ (x ? 0.10) showed significant precipitation of SnO₂ and SnO after the cycling evaluation.     

In situ time-resolved XAFS study on the structural transformation and phase separation of Pt3Sn and PtSn alloy nanoparticles on carbon in the oxidation process

The dynamic behavior and kinetics of the structural transformation of supported bimetallic nanoparticle catalysts with synergistic functions in the oxidation process are fundamental issues to understand their unique catalytic properties as well as to regulate the catalytic capability of alloy nanoparticles. The phase separation and structural transformation of Pt(3)Sn/C and PtSn/C catalysts during the oxidation process were characterized by in situ time-resolved energy-dispersive XAFS (DXAFS) and quick XAFS (QXAFS) techniques, which are element-selective spectroscopies, at the Pt L(III)-edge and the Sn K-edge. The time-resolved XAFS techniques provided the kinetics of the change in structures and oxidation states of the bimetallic nanoparticles on carbon surfaces. The kinetic parameters and mechanisms for the oxidation of the Pt(3)Sn/C and PtSn/C catalysts were determined by time-resolved XAFS techniques. The oxidation of Pt to PtO in Pt(3)Sn/C proceeded via two successive processes, while the oxidation of Sn to SnO(2) in Pt(3)Sn/C proceeded as a one step process. The rate constant for the fast Pt oxidation, which was completed in 3 s at 573 K, was the same as that for the Sn oxidation, and the following slow Pt oxidation rate was one fifth of that for the first Pt oxidation process. The rate constant and activation energy for the Sn oxidation in PtSn/C were similar to those for the Sn oxidation in Pt(3)Sn/C. In the PtSn/C, however, it was hard for Pt oxidation to PtO to proceed at 573 K, where Pt oxidation was strongly affected by the quantity of Sn in the alloy nanoparticles due to swift segregation of SnO(2) nanoparticles/layers on the Pt nanoparticles. The mechanisms for the phase separation and structure transformation in the Pt(3)Sn/C and PtSn/C catalysts are also discussed on the basis of the structural kinetics of the catalysts themselves determined by the in situ time-resolved DXAFS and QXAFS.     

Influence of Pt and Au nanophases on electrochromism of WO3 in nanostructure thin-film electrodes

We report electrochromic properties of WO₃ in Au–WO₃ and Pt–WO₃ nanostructure thin-film electrodes prepared by co-sputtering deposition method. The nanostructure electrodes consisted of Au or Pt metallic nanophase and a tungsten oxidative phase, indicating the formation of crystalline metallic nanophases in the amorphous oxide matrix. In particular, due to metallic nanophases, the modified electrochromic properties of WO₃ were observed in the Au–WO₃ and Pt–WO₃. The nanostructure electrodes showed a reverse optical modulation with respect to applied potentials in H₂SO₄ solution compared to that of pure WO₃ electrode. However, due to an excellent electrocatalytic activity of platinum for methanol electrooxidation at 25 °C, the electrochromism of the Pt–WO₃ in contrast with that of the Au–WO₃ was affected by the potentials for methanol electrooxidation in 2 M CH₃OH and 0.5 M H₂SO₄.     

Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: Thermoelectric and p-type conductivity properties

In this paper, we report structural, electrical, optical, and especially thermoelectrical characterization of iron (Fe) doped tin oxide films, which have been deposited by spray pyrolysis technique. The doping level has changed from 0 to 10 wt% in solution ([Fe]/[Sn] = 0–40 at% in solution). The thermoelectric response versus temperature difference has exhibited a nonlinear behavior, and the Seebeck coefficient has been calculated from its slope in temperature range of 300–500 K. The Hall effect and thermoelectric measurements have shown p-type conductivity in SnO₂:Fe films with [Fe]/[Sn] ≥ 7.8 at%. In doping levels lower than 7.8 at%, SnO₂:Fe films have been n-type with a negative thermoelectric coefficient. The Seebeck coefficient for SnO₂:Fe films with 7.8 at% doping level has been obtained to be as high as +1850 μV/K. The analysis of as-deposited samples with thicknesses ～350 nm by X-ray diffraction (XRD) and scanning electron microscopy (SEM) has shown polycrystalline structure with clear characteristic peak of SnO₂ cassiterite phase in all films. The optical transparency (T%) of SnO₂:Fe films in visible spectra decreases from 90% to 75% and electrical resistivity (ρ) increases from 1.2 × 10^?2 to 3 × 10³ Ω cm for Fe-doping in the range 0–40 at%.     

A novel route for preparation of PtRuMe (Me = Fe,Co, Ni) and their catalytic performance for methanol electrooxidation

PtRuMe (Me = Fe, Co, Ni) catalysts dispersed on multi-wall carbon nanotubes (MWCNTs) were prepared by ultrasonic-assisted chemical reduction. X-ray diffraction (XRD) showed that Pt existed as face-centered cubic structure, while Ru and Me alloyed with Pt. The calculated particle sizes from XRD data are of 3.40, 3.40, 2.61 and 3.06 nm for PtRu, PtRuFe, PtRuCo and PtRuNi, respectively, and are consistent with TEM results. The electrochemical measurements showed that the addition of Me to PtRu enhances the electrocatalytic properties for methanol oxidation and PtRuNi has the best catalytic activity and stability.     

Synthesis and characterization of porous WO3–SnO2 nanomaterials: An infrared study of adsorbed pyridine and dimethyl methylphosphonate

High surface area porous W/Sn oxide nanomaterials were prepared via water/oil based (W/O) emulsion. Tungstic acid solution was generated by cation exchange of sodium tungstate in acidic Dowex resin. The acid was then mixed with a clear homogeneous aqueous N-cetyl trimethyl ammonium bromide (CTAB) solution followed by a slow addition of 0.2 M SnCl₄ solution. The mixture was stirred for 24 h and then subjected to slow calcination at 500 °C. The prepared materials were characterized using SEM-EDX, BET surface area, and sorption of nitrogen and water. Fourier transform infrared spectroscopy (FTIR) was used to characterize the surface acidic properties using pyridine vapor as a probe. The materials were then tested toward the Dimethyl methylphosphonate (DMMP) adsorption at various temperatures using infrared spectroscopy. At elevated temperatures, the desorption of DMMP from WO₃ and SnO₂ surfaces results in forming methyl phosphonate that strongly bounds on the metal oxide surfaces. In contrast, the FTIR spectra showed that the adsorbed dimethyl methylphosphonate (DMMP) on the mixed W/Sn oxide powders can be molecularly desorbed without any decomposition.     

Pd/Pt core–shell nanowire arrays as highly effective electrocatalysts for methanol electrooxidation in direct methanol fuel cells

Highly ordered Pd/Pt–core–shell nanowire arrays (Pd/Pt NWAs) have been prepared by anodized aluminum oxide (AAO) template-electrodeposition and magnetron sputtering methods. Pd/Pt NWA electrode shows a very high electrochemical active surface area and high electrocatalytic activity for the methanol electrooxidation in acid medium for direct methanol fuel cells (DMFCs). The mass specific anodic peak current density is 756.7 mA mg⁻¹ Pt for the methanol oxidation on the Pd/Pt NWA electrode, an increase by a factor of four as compared to conventional E-TEK PtRu/C electrocatalysts. The mechanism of the significant enhancement of the Pd/Pt core/shell NWA nanostructure in the efficiency and electrocatalytic activity of Pt for the methanol electrooxidation in acid medium is discussed.     

Synthesis of highly active nanostructured PtRu electrocatalyst with three-dimensional mesoporous silica template

Nanostructured PtRu material has been successively synthesized via chemical co-reduction of hexachloroplatinic acid and ruthenium trichloride using three-dimensional (3D) hexagonal mesoporous SBA-12 silica as a solid template, and has been studied as an electrocatalyst toward methanol electro-oxidation. The ordered nanostructure of the PtRu particles has been disclosed by transmission electron micrographs and is characterized by regular pores of ca. 3.0 ± 0.3 nm in diameter separated by walls of ca. 3.0 ± 0.3 nm thick. X-ray diffraction and energy dispersive X-ray spectroscope studies indicate that the PtRu material comprises of complicated phases rather than a single alloy phase of Pt and Ru. The specific electrochemical surface area of the nanostructured powder measured using both CO and underpotential deposited Cu stripping techniques is 74–78 m² g^–1, higher than that of unsupported precious metal catalysts prepared using standard techniques. The combination of high surface area and periodic nanostructure of the templated PtRu makes it an interesting promising fuel cell electrocatalyst. This has been demonstrated by the high activity of the templated PtRu towards the methanol electrooxidation. Therefore the solid template route based on 3D mesoporous silica with controlled pore size and high pore interconnectivity provides an interesting alternative to produce promising high-surface-area electrode materials.     

Unique CO-tolerance of Pt–WOx materials

Well-defined tungsten-oxide-supported platinum nanoparticles (Pt/WO_x) were elaborated by impregnation-reduction of a platinum salt onto commercial monoclinic WO₃. Field-emission gun scanning electron microscopy (FEG-SEM) and transmission electron microscopy (TEM) revealed that the Pt particles are well-distributed on the oxide support, present a narrow particle size distribution centered on ca. 2–3 nm and a low degree of agglomeration. Carbon black was added to ensure electronic percolation in the electrodes during the electrochemical measurements. CO_ads electrooxidation currents were monitored at potentials as low as 0.1 V vs. RHE on Pt/WO_x, demonstrating high CO-tolerance compared to carbon-supported Pt or PtRu catalysts.     

Sn-0.4BPO4 composite as a promising negative electrode for rechargeable lithium batteries

The structural and textural properties of a Sn-0.4BPO₄ composite material synthesized by ex situ dispersion of β-Sn in a BPO₄ matrix were investigated by using several complementary techniques to study the global order (XRD, TGA-DSC, SEM-XEDS) and the local order (FT-IR, ¹¹⁹Sn Mössbauer spectroscopy and X-ray absorption spectroscopy). The results reveal that the composite material consists of three main components: an electrochemically active species “Sn”, an inactive matrix “BPO₄”, and an amorphous Sn(II) borophosphate which acts as a link between the two former and which improves the cohesion of the composite. The electrochemical performances of the composite material were tested in Swagelok-type cells with metallic Li as counter-electrode. It shows a high reversible capacity of about 500 mAh g^?1 at a C/20 rate, and a very good stability under cycling even at very fast rates of C or C/1.3.