Enhancement of the electrooxidation of ethanol on Pt–Sn–P/C catalysts prepared by chemical deposition process

In this paper, five Pt₃Sn₁/C catalysts have been prepared using three different methods. It was found that phosphorus deposited on the surface of carbon with Pt and Sn when sodium hypophosphite was used as reducing agent by optimization of synthetic conditions such as pH in the synthetic solution and temperature. The deposition of phosphorus should be effective on the size reduction and markedly reduces PtSn nanoparticle size, and raise electrochemical active surface (EAS) area of catalyst and improve the catalytic performance. TEM images show PtSnP nanoparticles are highly dispersed on the carbon surface with average diameters of 2 nm. The optimum composition is Pt₃Sn₁P₂/C (note PtSn/C-3) catalyst in my work. With this composition, it shows very high activity for the electrooxidation of ethanol and exhibit enhanced performance compared with other two Pt₃Sn₁/C catalysts that prepared using ethylene glycol reduction method (note PtSn/C-EG) and borohydride reduction method (note PtSn/-B). The maximum power densities of direct ethanol fuel cell (DEFC) were 61 mW cm⁻² that is 150 and 170% higher than that of the PtSn/C-EG and PtSn/C-B catalyst.     

Ethanol electro-oxidation on Pt/C and PtSn/C catalysts in alkaline and acid solutions

Carbon supported Pt and PtSn were prepared by a modified polyol method. The electrocatalytic activities and stabilities of the Pt/C and PtSn/C catalysts towards ethanol electro-oxidation reactions (EORs) were investigated by potentiodynamic and potentiostatic methods in a 0.1 M NaOH solution (or 0.5 M H₂SO₄) containing 0.01 M ethanol. On both catalysts, the EOR currents in the alkaline solutions were much higher than those in the acid solutions, and the onset potentials of the EOR in alkaline solutions were less positive than those in acid solutions, indicating that the kinetics of the EOR improve in alkaline solutions. Even though a significant improvement was observed in acid media on PtSn/C, compared with Pt/C, only negligible improvement was observed in alkaline media. The apparent activation energies of the EOR on the PtSn/C catalyst varies from 21 to 33 kJ mol⁻¹, depending on the potentials, which are slightly lower than the corresponding values on the Pt/C catalyst (25∼42 kJ mol⁻¹) under the same conditions. The Tafel slopes are divided into two parts–at low overpotentials, Tafel slopes on both catalysts are close to 120 mV dec⁻¹, which is in agreement with the proposed mechanism–Temkin-type adsorption for both OH_ad and ethoxi at low overpotentials; in contrast, at high overpotentials, Tafel slopes on both catalysts are over 300 mV dec⁻¹ due to the oxide formation on the surface.     

Electrodeposition and pseudocapacitive properties of tungsten oxide/polyaniline composite

Composite films of tungsten oxide (WO₃) and polyaniline (PANI) have been electrodeposited by cyclic voltammetry in a mixed solution of aniline and precursor of tungsten oxide. Surface morphology and chemical composition of WO₃/PANI composite are characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of H₂O₂ on the electrodeposition of WO₃/PANI composite film is also investigated. Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) results show that WO₃/PANI composite film exhibit good pseudocapacitive performance over a wide potential range of −0.5 to 0.7 V vs. SCE with the specific capacitance of 168 F g⁻¹ at current density of 1.28 mA cm⁻² and energy density of 33.6 Wh kg⁻¹, which is 91% higher than that of similarly prepared PANI (17.6 Wh kg⁻¹). An asymmetric model capacitor using WO₃/PANI as negative and PANI as positive electrodes over voltage range of 1.2 V displays a specific capacitance of 48.6 F g⁻¹ and energy density of 9.72 Wh kg⁻¹ at the power density of 53 W kg⁻¹, which is two times higher than that of a symmetric capacitor modeled by using two PANI films as both positive and negative electrodes.     

Preparation of Pt supported on WO3-C with enhanced catalytic activity by microwave-pyrolysis method

The WO₃-C hybrid materials are prepared by intermittently microwave-pyrolysis using ammonium tungstate as the precursor, and then Pt nano-particles are deposited by microwave-assited polyol process on WO₃-C. The TEM images show the dispersion of ∼10 nm WO₃ particles size supported on carbon and ∼3 nm Pt metal crystallites supported on WO₃-C. XRD results illustrate that WO₃ presented as monoclinic phase and the content of WO₃ in WO₃/C and Pt/WO₃-C catalysts is further characterized by EDAX. Furthermore, XPS characterizations indicate that the interaction between Pt and WO₃ is dramatically enhanced after heat treatment at 200 °C. The activities of Pt/WO₃-C for the electrochemical oxidation of methanol are compared with Pt/C in acid solution by cyclic voltammetry, CO-stripping and chronoaperometry. Pt/WO₃-C catalyst calcined at 200 °C exhibits the highest activity per electrochemical active surface area for methanol oxidation and is 60 mV more negative for CO electro-oxidation than that of Pt/C and Pt/WO₃-C without heat treatment. The great enhancement of electrochemical performance may be due to the improvement of the synergistic effect between Pt and WO₃ in Pt/WO₃-C catalyst after heat treatment.     

Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation

Hexagonal structured WO₃ films with tree-like morphology were synthesized on tungsten foils by a hydrothermal method. Each nanotree was composed of several (typically six) nanosheet-shaped “branches”. TEM examination revealed that the nanosheet was a single crystal and its long axis was oriented toward 〈0 0 1〉 direction. The WO₃ nanotree films retain the hexagonal structure after being annealed up to 400 °C for 2 h, while they have a phase transition to monoclinic structure after being annealed at 500 °C for 2 h. Electrochromic (EC) performance of the films was examined in a propylene carbonate solution of 1 M LiClO₄ using an electrochemical workstation and an UV-Vis spectrometer. Due to the large tunnels of hexagonal structure and highly porous surface morphology, a large modulation of optical reflectance of WO₃ nanotree films up to 30% and coloration efficiency of 43.6 cm² C⁻¹ at 500 nm were achieved by annealing the WO₃ nanotree films at 400 °C for 2 h.     

SnO2-Au hybrid nanoparticles as effective catalysts for oxygen electroreduction in alkaline media

SnO₂ nanoparticles were synthesized by a facile electrochemical method based on anodic oxidation of a tin metal sheet. The resulting particles, with an average diameter of about 20 nm, were then loaded with varied amounts of HAuCl₄, forming SnO₂-Au(x) (x = 0, 0.095, 0.38, 0.76, 1.9, 3.0, and 3.8 at.%) hybrid nanoparticles after calcination at elevated temperatures (700 °C). X-ray diffraction (XRD) and transmission electron microscopic (TEM) measurements showed that the SnO₂ particles exhibited high crystallinity with a rutile structure, and spherical Au nanoparticles were dispersed on the surface of the SnO₂ support. Based on the TEM images and the width of the Au(2 0 0) XRD diffraction peak, the size of the Au nanoparticles was found to be between 15 and 35 nm in diameter and decrease with increasing loading of the original HAuCl₄ precursor. The electrocatalytic activity of the resulting SnO₂-Au(x) composite nanoparticles toward oxygen reduction reactions (ORR) was then evaluated by cyclic and rotating disk voltammetric measurements in alkaline solutions. It was found that the incorporation of gold nanoparticles led to apparent improvement of the catalytic activity of SnO₂ nanoparticles. Moreover, the ORR electrocatalytic activity exhibited a strong dependence on the gold loading in the hybrid nanoparticles, and the most active catalyst was found with a gold loading of 1.9 at.%, based on the reduction current density and onset potential of ORR. Furthermore, at this gold loading, oxygen reduction was found to follow the efficient four-electron reaction pathway, whereas at other Au loadings, the number of electron transfer involved in oxygen reduction varied between 1 and 3. Additionally, Tafel analysis suggested that at low overpotentials, the first electron transfer might be the limiting step in oxygen reduction, whereas at high potentials, oxygen adsorption appeared to play the determining role at the SnO₂-Au hybrid electrodes. These results indicate that the SnO₂-Au composite nanoparticles might serve as effective catalysts for oxygen electroreduction in alkaline media.     

High activity of Pd-WO3/C catalyst as anodic catalyst for direct formic acid fuel cell

Pd nanoparticles supported on the WO₃/C hybrid are prepared by a two-step procedure and the catalysts are studied for the electrooxidation of formic acid. For the purpose of comparison, phosphotungstic acid (PWA) and sodium tungstate are used as the precursor of WO₃. Both the Pd-WO₃/C catalysts have much higher catalytic activity for the electrooxidation of formic acid than the Pd/C catalyst. The Pd-WO₃/C catalyst prepared from PWA shows the best catalytic activity and stability for formic acid oxidation; it also shows the maximum power density of approximately 7.6 mW cm⁻² when tested with a small single passive fuel cell. The increase of electrocatalytic activity and stability is ascribed to the interaction between the Pd and WO₃, which promotes the oxidation of formic acid in the direct pathway. The precursors used for the preparation of the WO₃/C hybrid support have a great effect on the performance of the Pd-WO₃/C catalyst. The WO₃/C hybrid support prepared from PWA is beneficial to the dispersion of Pd nanoparticles, and the catalyst has potential application for direct formic acid fuel cell.     

The electrochromic and photocatalytic properties of electron beam evaporated vanadium-doped tungsten oxide thin films

The electrochromic and photocatalytic properties of vanadium-doped tungsten trioxide thin films prepared at room temperature (300 K) by the electron beam evaporation technique are reported in this paper. The vanadium to tungsten ratio (V/W) in these films are 0.003, 0.019, 0.029 and 0.047. The optical band gap of the vanadium-doped tungsten oxide (WO₃) thin film initially increases from 3.16 to 3.28 eV for V/W ratio 0.003 then decreases to 3.15 eV for V/W ratio 0.047. These vanadium-doped films switch between neutral gray and transparent states. The coloration efficiency (CE) decreases from 82 cm² C⁻¹ (pure WO₃) to 27 cm² C⁻¹ for the film containing V/W ratio 0.047. The photocatalytic activity has enhanced with vanadium doping and maximum activity of 15% (percentage change in optical density of methylene blue due to photo degradation) has been observed for the film containing V/W ratio of 0.019. The Kelvin probe measurements show that the work function of pure WO₃ films is 4.07 eV and vanadium doping initially increases the work function to 4.19 eV for V/W ratio 0.019 and then decreases it to 3.97 eV for film with V/W ratio 0.047.     

Enhancement of catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol through interfacial modification with heteropolymolybdates

As evidenced from the increase of electrocatalytic currents measured under voltammetric and chronoamperometric conditions, the activity of bimetallic Pt-Ru and Pt-Sn nanoparticles towards oxidation of ethanol is increased by modification of their surfaces with ultra-thin films of phosphododecamolybdic acid (H₃PMo₁₂O₄₀). The enhancement effect has been most pronounced in a case of heteropolymolybdate-modified carbon-supported Pt-Sn catalysts. Independent high-resolution XPS measurements indicate the ability of heteropolymolybdates to stabilize tin (in bimetallic Pt-Sn particles) at higher oxidation states (presumably as tin oxo species). The overall activation effect may also be ascribed to changes in the morphology of catalytic films following modification with heteropolymolybdates. Presence of the polyoxometallate is also likely to increase of the interfacial population of reactive oxo groups in the vicinity of platinum centers.     

Microwave-assisted hydrothermal synthesis of crystalline WO3-WO3·0.5H2O mixtures for pseudocapacitors of the asymmetric type

Crystalline tungsten oxide mixtures, WO₃-WO₃·0.5H₂O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180 °C for various periods, show capacitive-like behavior at 200 mV s⁻¹ and C_S ≈ 290 F g⁻¹ at 25 mV s⁻¹ in 0.5 M H₂SO₄ between −0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO₃·0.5H₂O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO₃ and WO₃·0.5H₂O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400 °C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO₃-WO₃·0.5H₂O anode and a RuO₂·xH₂O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg⁻¹ and 5.2 kW kg⁻¹, respectively.     

Effect of Ni on Pt/C and PtSn/C prepared by the Pechini method

Different compositions of Pt, PtNi, PtSn, and PtSnNi electrocatalysts supported on carbon Vulcan XC-72 were prepared through thermal decomposition of polymeric precursors. The nanoparticles were characterized by morphological and structural analyses (XRD, TEM, and EDX). XRD results revealed a face-centered cubic structure for platinum, and there was evidence that Ni and Sn atoms are incorporated into the Pt structure. The electrochemical investigation was carried out in slightly acidic medium (H₂SO₄ 0.05 mol L⁻¹), in the absence and in the presence of ethanol. Addition of Ni to Pt/C and PtSn/C catalysts significantly shifted the onset of ethanol and CO oxidations toward lower potentials, thus enhancing the catalytic activity, especially in the case of the ternary PtSnNi/C composition. Electrolysis of ethanol solutions at 0.4 V vs. RHE allowed for determination of acetaldehyde and acetic acid as the reaction products, as detected by HPLC analysis. Due to the high concentration of ethanol employed in the electrolysis experiments (1.0 mol L⁻¹), no formation of CO₂ was observed.     

Electrodeposition preparation of Pt–HxWO3 composite and its catalytic activity toward oxygen reduction reaction

Platinum–hydrogen tungsten bronze (Pt–H_xWO₃) was prepared on glass carbon electrode by potentiostat in 0.1 mM H₂PtCl₆ + 4 mM Na₂WO₄ + 2 M H₂SO₄. Its surface morphology, structure and activity toward oxygen reduction reaction were studied with scan electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, and linear sweeping voltammetry. It is found that platinum and hydrogen tungsten bronze can be co-deposited together on glassy carbon and the activity of platinum toward oxygen reduction can be improved significantly by H_xWO₃. Furthermore, the activity of Pt–H_xWO₃ toward oxygen reduction is hardly influenced by methanol.     

Hydrogen evolution reaction on single crystal WO3/C nanoparticles supported on carbon in acid and alkaline solution

Single crystal tungsten oxide (WO₃) nanoparticles were prepared via a microwave-assisted method. Electrochemical activity for hydrogen evolution reaction (HER) on WO₃ supported on carbon black (WO₃/C) electrocatalyst was first studied in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH) at room temperature. The overall experimental results revealed that the electrocatalytic activity for HER on WO₃/C is one order magnitude higher than those obtained with carbon black in 0.5 M H₂SO₄ and is six times than in the case of carbon black in 1.0 M KOH. These results demonstrated that WO₃ could enhance the electrocatalytic activity for hydrogen evolution reaction in acid solution (0.5 M H₂SO₄) and alkaline solution (1.0 M KOH). On the other hand, the kinetic reaction mechanisms were discussed on WO₃/C electrocatalysts and carbon black in acid solution and alkaline solution for HER. Consequently, the rate-determining step changed from Tafel to Volmer due to the incorporation of WO₃ in acid solution. However, the rate-determining step carries through Tafel reaction on both electrocatalysts in alkaline solution though WO₃ was introduced.     

Study of ethanol electro-oxidation in acid environment on Pt3Sn/C anode catalysts prepared by a modified polymeric precursor method under controlled synthesis conditions

A carbon-supported binary Pt₃Sn catalyst has been prepared using a modified polymeric precursor method under controlled synthesis conditions. This material was characterized using X-ray diffraction (XRD), and the results indicate that 23% (of a possible 25%) of Sn is alloyed with Pt, forming a dominant Pt₃Sn phase. Transmission electron microscopy (TEM) shows good dispersion of the electrocatalyst and small particle sizes (3.6 nm ± 1 nm). The polarization curves for a direct ethanol fuel cell using Pt₃Sn/C as the anode demonstrated improved performance compared to that of a PtSn/C E-TEK, especially in the intrinsic resistance-controlled and mass transfer regions. This behavior is probably associated with the Pt₃Sn phase. The maximum power density for the Pt₃Sn/C electrocatalyst (58 mW cm⁻²) is nearly twice that of a PtSn/C E-TEK electrocatalyst (33 mW cm⁻²). This behavior is attributed to the presence of a mixed Pt₉Sn and Pt₃Sn alloy phase in the commercial catalysts.     

Catalytic activity of platinum/tungsten oxide nanorod electrodes towards electro-oxidation of methanol

Tungsten oxide (WO₃) nanorods are synthesized using an Anodisc alumina membrane as a template and platinum nanoparticles are supported on the nanorods. The nanorods, serving as platinum catalyst supports, are characterized by electron microscopy and by electrochemical analysis. Methanol oxidation on the prepared electrodes is studied by means of cyclic voltammetry and chronopotentiometry. A film of Pt/WO₃ nanorods on a glassy carbon electrode exhibits good electrocatalytic activity towards the oxidation of methanol. High electrocatalytic activities and good stabilities are attributed to a synergistic effect between Pt and WO₃ that avoids poisoning of the electrodes.     

Tungsten(VI) oxide as a versatile support for Pd nanocatalysts for alkaline direct alcohol fuel cells

The instability of carbon support materials has motivated the development of metal oxides supports which are stable under the fuel cell environment. In this study, tungsten (VI) oxide (WO₃) is utilized as a secondary support and cocatalyst for the electrooxidation of methanol and ethanol. Functionalized carbon nanodots employed as primary supports were blended with WO₃ nanoparticles to form a composite support onto which Pd nanoparticles were deposited by a borohydride reduction method. The synthesized Pd/fCNDs-WO₃ electrocatalysts were characterized by Transmission Electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy. XRD results proved that incorporating WO₃ into Pd/fCNDs electrocatalyst shifts the Pd diffraction peaks to lower 2Ɵ value due to lattice relaxation. XPS results revealed that W exist in oxidised form and confirmed the strong interaction between the support material and the catalyst. The Pd/fCNDs-WO₃ electrocatalysts exhibited a remarkable catalytic activity towards methanol and ethanol oxidation. High current densities of 87.24 mA cm⁻² and 44.23 mA cm⁻² were obtained for ethanol and methanol oxidation, respectively, using a catalyst with 2.5% Pd loading. EIS, CA and stability tests revealed that the presence of WO₃ in Pd/fCNDs electrocatalyst improves the kinetics, tolerance to poisoning and long-term durability in alkaline conditions. This superior performance is attributed to the electronic coupling between Pd and WO₃ nanoparticles.     

Semitransparent inverted polymer solar cells using MoO3/Ag/WO3 as highly transparent anodes

We demonstrate semitransparent inverted polymer solar cells with highly transparent anodes. The structure of the anode is made up of molybdenum trioxide (MoO₃)/silver (Ag)/tungsten oxide (WO₃). The inner MoO₃ layer is introduced as a buffer layer to improve hole collection, while the outer WO₃ layer serves as a light coupling layer to enhance optical transmittance of the photovoltaic device. The dependence of device performances on thickness of the outer WO₃ layer was investigated, and the transmittance and reflectance of MoO₃ (1 nm)/Ag(10 nm)/WO₃(x=0, 20, 40, 60, and 80 nm) electrode are compared. A high transmission of 90% was achieved for semitransparent inverted polymer solar cells with a 40 nm thick outer WO₃ layer.     

Electrooxidation of CO and methanol on well-characterized carbon supported PtxSn electrodes. Effect of crystal structure

The role of two intermetallic phases of PtSn, namely Pt₃Sn (fcc phase) and PtSn (hcp phase) for the electrooxidation of CO and methanol has been evaluated. Carbon supported Pt₃Sn and PtSn nanosized particles have been prepared by controlled surface reactions. The actual structure of the PtSn alloys has been evaluated and confirmed by means of XRD and HR-TEM studies which reveal the predominance of either the hcp or the fcc phase in each catalyst. The catalysts have been further characterized to identify the actual metal loading and Pt/Sn atomic ratio in order to eliminate particle size or metal loading effects on their electrocatalytic performance. The performance of the catalysts for the electrooxidation of CO and methanol has been evaluated by electrochemical techniques along with in situ techniques such as electrochemical coupled Infrared Reflection Absorption Spectroscopy (EC-IRAS) and differential electrochemical mass spectrometry (DEMS). Altogether, the results presented in this work reveal that Pt₃Sn fcc is more active than PtSn hcp for the electrooxidation of CO and methanol and that the contribution of the hcp phase in those electrocatalytic processes is negligible.     

A comparative study of tungsten-modified PtRu electrocatalysts for methanol oxidation

The carbon supported PtRu nanocatalyst is modified by two kinds of tungsten compounds, i.e., tungsten oxide (WO_x) and phosphotungstic acid (H₃PW₁₂O₄₀, PW₁₂), respectively, and the catalytic performances of the modified catalysts for methanol oxidation are evaluated. The results show that tungsten oxide and phosphotungstic acid exhibit different promoting effects on the catalytic performance of the PtRu nanocatalyst for methanol oxidation. The WO_x-modified PtRu nanocatalyst has a considerably high catalytic activity, which is attributed to the uniform distribution of PtRu nanoparticles on the carbon support and the strong metal-support interaction (SMSI) between the hypo-d-tungsten and the hyper-d-platinum. The PW₁₂-modified PtRu nanocatalyst has a good poison resistance, which is ascribed to the protective effect of the self-assembled PW₁₂ layer on the catalyst surface.