Pt-Ru-TiO2 photoelectrocatalysts for methanol oxidation

Novel photoelectrocatalysts composed of PtRuTiO₂/C are prepared by the polymeric precursor method and are characterized by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy and cyclic voltammetry. The onset potential for methanol oxidation is similar (0.3 V vs. RHE) for all of the photoelectrocatalyst layers investigated, although the peak current density is dependent on the layer composition. Irradiation of UV light on the photoelectrocatalyst surfaces enhances the chronoamperometric responses up to 18%, which clearly demonstrates a synergistic effect between the photo- and electrocatalysts. The comparison between all the layers prepared indicates that there is an appropriate ratio of metallic nanoparticles and TiO₂ to obtain the best performance of these photoelectroactive layers. These results demonstrate that methanol oxidation is achieved by electro- and photocatalysis using a simple and affordable method. This procedure can be conveniently exploited to enhance the response of direct methanol fuel cell electrodes.     

Electrochemically synthesized polypyrrole/MWCNTs-Al2O3 ternary nanocomposites supported Pt nanoparticles toward methanol oxidation

As known, a good support enhances the activity and durability of any catalyst. In the current study, polypyrrole (PPY)/nanocomposite (MWCNTs and Al₂O₃) films were fabricated by electrochemical polymerization of pyrrole solution with a certain amount of nanoparticles on titanium substrates and were used as new support materials for Pt catalyst. The modified electrodes were characterized by Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX) techniques. High catalytic activity and long-time stability toward methanol oxidation of Pt/PPY–MWNTs-αAl₂O₃ catalyst have also been verified by cyclic voltammetry results and chronoamperometric response measurements. This catalyst exhibits a vehemently high current density (345.03 mA cm^?2) and low peak potential (0.74 v) for methanol oxidation. Other electrochemical measurements (electrochemical impedance spectroscopy (EIS), CO stripping voltammetry and Tafel test) clearly confirmed that Pt/PPY–MWNTs-αAl₂O₃/Ti electrode has a better performance toward methanol oxidation compared to the other electrodes and that can be used as a promising electrode material for application in direct methanol fuel cells (DMFCs).     

Pt/C doped by MoOx as the electrocatalyst for oxygen reduction and methanol oxidation

The oxidation of methanol and reduction of oxygen were studied on MoO_x–Pt/C nano-catalysts prepared by the polyole method combined by MoO_x post-deposition. The catalysts were characterized by TEM and EDX. The presented composition of the electrode is very similar to the nominal ones and post-deposited MoO_x species block only a small fraction of the active Pt particle surface area. MoO_x deposition on the carbon support can be ruled out from the EDX results and the low mobility of these oxides at corresponding conditions. The electrode catalytic activity in the electrooxidation of methanol and the reduction of oxygen was studied by steady-state voltammetry and cyclic voltammetry. MoO_x–Pt/C catalyst exhibits higher catalytic activity than Pt/C for the oxygen reduction. The catalytic effect in oxidation of methanol is achieved only under potentiodynamic conditions, when poisoning species have no enough time to develop fully.     

Carbon nanotube-supported Pt-HxMoO3 as electrocatalyst for methanol oxidation

Carbon nanotube (CNT)-supported platinum modified with H_xMoO₃ (Pt-H_xMoO₃/CNT) was prepared and used as an electrocatalyst for methanol oxidation. In the preparation of this electrocatalyst, a platinum precursor was loaded on CNTs and reduced by sodium borohydride in ethylene glycol, resulting in CNT-supported platinum without modification (Pt/CNT), and then the Pt/CNT was modified with H_xMoO₃ that was formed by hydrolysis and subsequent reduction of ammonium molybdate. The surface morphology, structure and composition of Pt-H_xMoO₃/CNT and Pt/CNT as well as their activity toward methanol oxidation were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), chronoamperometry (CA), chronopentiometry (CP), and electrochemical impedance spectroscopy (EIS). The results, obtained from TEM, XRD, EDS, and FTIR, indicate that the platinum loaded on CNTs has a face-centered cubic structure with particle sizes of 2–5 nm, and the modification of H_xMoO₃ on platinum with an atom ratio of Pt:Mo = 2:1 has little effect on the particle size, distribution and structure of the platinum. The results, obtained from CV, CA, CP, and EIS, show that the Pt-H_xMoO₃/CNT exhibits higher electrocatalytic activity toward methanol oxidation and better carbon monoxide tolerance than Pt/CNT.     

Building three-dimensional Pt catalysts on TiO2 nanorod arrays for effective ethanol electrooxidation

Pt nanoparticles on TiO₂ nanorod arrays have been prepared and used for the electrooxidation of ethanol in acidic and alkaline media. The vertically aligned TiO₂ nanorod arrays provide a three-dimensional frame for supporting Pt nanoparticles. FESEM and TEM show that the Pt nanoparticles about 3 nm in diameter are evenly distributed on the surface of TiO₂ nanorods. Cyclic voltammetry and chronoamperometry are performed to investigate electrochemical properties of Pt/TiO₂/Ti, Pt/C, and Pt/Ti electrocatalysts. The Pt/TiO₂/Ti electrocatalyst shows better catalytic activity and higher catalytic stability compared with other electrocatalysts.     

Enhanced electrocatalytic performance for methanol oxidation on Pt–TiO2/ITO electrode under UV illumination

Pt is one of the most important electrode materials employed in direct methanol fuel cell, and many efforts have been directed to improving its electrocatalytic performance. In this work, Pt–TiO₂ nanocomposites are successfully prepared by a sol–gel method. As revealed by TEM, Pt nanoparticles with an average size of 2.6 nm are well uniformly dispersed on porous TiO₂. XRD structural characterization indicates that Pt possesses a face centered cubic crystal structure while TiO₂ is in the format of both rutile and anatase phases. The electrochemical performance of as-prepared nanocomposite electrode (Pt–TiO₂/ITO) is evaluated by studying the electrocatalytic oxidation of methanol in an alkaline medium with or without UV illumination. Comparative experiments evince that the electrochemical performance of Pt–TiO₂/ITO for methanol electrooxidation is markedly improved under UV illumination. Under UV illumination, moreover, the poisoning resistance of Pt–TiO₂/ITO for methanol electrooxidation is significantly improved, as supported by the results of time-coursed current measurements.     

Electrodeposited Ni-Cr2O3 nanocomposite anodes for ethanol electrooxidation

Nanocomposite coatings of Ni-Cr₂O₃ supported on carbon electrodes have been prepared by electrodeposition technique from nickel Watts bath in presence of Cr₂O₃ nanoparticles. Their electrochemical catalytic activities have been evaluated towards electrooxidation of ethanol in 1.0 M NaOH solution by using cyclic voltammetry, chronoamperometry and Tafel plots. The performance of the prepared anodes towards electrooxidation of ethanol as a function of co-deposited Cr₂O₃ content was studied. The catalytic activity of fabricated electrodes increases with increasing the volume fraction percent (V_f%) of Cr₂O₃ in the deposited film up to 7V_f%. The Ni-Cr₂O₃/C (7V_f%) electrode displayed significantly enhanced catalytic activity and stability towards electrooxidation of ethanol compared with Ni/C electrode. The kinetic parameters of Ni(OH)₂/NiOOH and ethanol oxidation at Ni/C and Ni-Cr₂O₃/C electrodes have been evaluated.     

Platinum nanoparticles embedded in layer-by-layer films from SnO2/polyallylamine for ethanol electrooxidation

Self-assembled films from SnO₂ and polyallylamine (PAH) were deposited on gold via ionic attraction by the layer-by-layer (LbL) method. The modified electrodes were immersed into a H₂PtCl₆ solution, a current of 100 μA was applied, and different electrodeposition times were used. The SnO₂/PAH layers served as templates to yield metallic platinum with different particle sizes. The scanning tunnel microscopy images show that the particle size increases as a function of electrodeposition time. The potentiodynamic profile of the electrodes changes as a function of the electrodeposition time in 0.5 mol L⁻¹ H₂SO₄, at a sweeping rate of 50 mV s⁻¹. Oxygen-like species are formed at less positive potentials for the Pt–SnO₂/PAH film in the case of the smallest platinum particles. Electrochemical impedance spectroscopy measurements in acid medium at 0.7 V show that the charge transfer resistance normalized by the exposed platinum area is 750 times greater for platinum electrode (300 kΩ cm²) compared with the Pt–SnO₂/PAH film with 1 min of electrodeposition (0.4 kΩ cm²). According to the Langmuir–Hinshelwood bifunctional mechanism, the high degree of coverage with oxygen-like species on the platinum nanoparticles is responsible for the electrocatalytic activity of the Pt–SnO₂/PAH concerning ethanol electrooxidation. With these features, this Pt–SnO₂/PAH film may be grown on a proton exchange membrane (PEM) in direct ethanol fuel cells (DEFC).     

Coverage-dependent electro-catalytic activity of Pt sub-monolayer/Au bi-metallic catalyst toward methanol oxidation

The coverage-dependent catalytic properties of a Pt nanofilm formed on a Au substrate were investigated for the electro-oxidation of methanol. The coverage of Pt nanofilm was precisely fabricated by the formation of coverage-controlled under potential deposition of Cu adlayer and followed by the surface limited redox replacement reaction with different Pt complex ions. The STM images exhibit the formation of Pt nano-film on Au substrate with different coverage. It was clearly shown that the activity of Pt nanofilm deposited on Au substrate toward the methanol electro-oxidation was highly sensitive to its surface coverage. Pt–Au bimetallic catalyst was found to become active at the Pt surface coverage near 0.2 and reached its maximum around 0.6. The electro-catalytic activity as well as CO tolerance on Pt–Au bimetallic system was found higher than those on a Pt electrode.     

nPt (Hx−2nMoO3) as a promising catalyst for the oxidation of methanol. Synthesis and electrocatalytic properties

A new method for the synthesis of the catalyst systems Pt–Mo was suggested. nPt⁰(H_x−2nMoO₃)/GC electrodes were prepared by a redox reaction between the hydrogen-containing molybdenum bronzes and potassium tetrachloroplatinate (II) in acid solutions at open circuit potential. The electrodes were characterized by CVA, SEM, X-ray microanalysis, XRD, XPS and ICP-AES. Pt⁰conglomerates formation with nonuniform distribution over the molybdenum bronzes surface has been revealed. nPt⁰(H_x−2nMoO₃)/GC electrodes showed high catalytic activity (not inferior to Pt–Ru-catalyst) in the oxidation of carbon monoxide and methanol as compared with Pt/GC-electrodes. Catalytic effect is apparently achieved by effective oxidation of strongly chemisorbed species (CO_ads, HCO_ads), which occurs at boundaries platinum – molybdenum oxide. Therefore nPt⁰(H_x−2nMoO₃) can be considered as one of perspective catalysts for DMFC.     

Sulfated SnO2 modified multi-walled carbon nanotubes - A mixed proton-electron conducting support for Pt catalysts in direct ethanol fuel cells

We report on the synthesis of sulfated SnO₂ modified multi-walled carbon nanotubes (MWCNTs) composites as new supports of Pt catalyst (Pt-S-SnO₂/MWCNTs) with the aims to enhance electron and proton conductivity and also catalytic activity for ethanol oxidation. The Pt-S-SnO₂/MWCNTs catalyst is synthesized by a combination of improved sol-gel and pulse-microwave assisted polyol methods. The surface presence, morphology and structure of the Pt-S-SnO₂/MWCNTs catalyst are characterized by Fourier transform infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), respectively. The electrocatalytic properties of the Pt-S-SnO₂/MWCNTs catalyst for ethanol oxidation reactions are investigated by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The results show that Pt-S-SnO₂/MWCNTs catalyst exhibits higher catalytic activity for ethanol oxidation than Pt supported on non-sulfated SnO₂/MWCNTs composites.     

Pt electrodeposited on CeZrO4/MCNT as a new alternative catalyst for enhancement of ethanol oxidation

Electrocatalytic preparation of Pt-based nanocomposites has been investigated for improvement of direct ethanol fuel cells (DEFCs). In this study, new alternative catalysts of Pt-decorated cerium zirconium oxide-modified multiwalled carbon nanotubes (Pt/CeZrO₄/MCNT) were successively prepared to improve the activity of the ethanol oxidation reaction (EOR). The prepared CeZrO₄ with a face-centered cubic (fcc) structure compatibly dispersed onto MCNT provides abundant active Pt sites for highly active catalysts. The fcc-structured Pt was also satisfactorily decorated onto CeZrO₄/MCNT, resulting in highly active Pt. The Ce⁴⁺/Ce³⁺ redox property can promote oxygen vacancies to improve the electrochemical activity for oxidation of carbonaceous species. An increase in roughness and a stabilized catalyst structure can also be produced by inserting Zr⁴⁺ into the ceria metal oxide. The prepared Pt/20%CeZrO₄/MCNT catalysts present excellent electrochemical active surface area, mass activity, CO tolerance and high electron kinetic transfer with low resistance and high stability over commercial PtRu/C toward EOR. This promising catalyst material could be introduced to enhance the anodic oxidation reaction in DEFCs.     

Pt modified TiO2 nanotubes electrode: Preparation and electrocatalytic application for methanol oxidation

Pt nanoparticles decorated TiO₂ nanotubes (Pt/TiO₂NTs) modified electrode has been successfully synthesized by depositing Pt in TiO₂NTs, which were prepared by anodization of the Ti foil. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize their structures and properties. The Pt/TiO₂NTs electrode shows excellent electrocatalytic activity toward methanol oxidation reaction (MOR) in alkaline electrolyte without UV irradiation.     

Ethanol oxidation on carbon supported (PtSn)alloy/SnO2 and (PtSnPd)alloy/SnO2 catalysts with a fixed Pt/SnO2 atomic ratio: Effect of the alloy phase characteristics

To evaluate the effect of the alloy phase characteristics on the ethanol oxidation activity, carbon supported (PtSnPd)_alloy/SnO₂ catalysts were prepared and their electrocatalytic activity compared with that of carbon supported (PtSn)_alloy/SnO₂. Pt-Sn-Pd/C samples in the atomic ratio (1:1:0.3) and (1:1:1) were characterized by energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD analysis shows the presence of fcc Pt reflexions, shifted to lower angles, and SnO₂ reflexions. By comparison with the XRD patterns of carbon supported Pt-Sn (1:1) and Pt-Pd (3:1) samples, prepared by the same method, the formation of ternary PtSnPd alloys is postulated. The crystallite size of the ternary catalysts is smaller than that of both binary Pt-Sn/C (1:1) and Pt-Pd/C (3:1) catalysts. Chronoamperometry experiments and tests in direct ethanol fuel cells of the as-prepared catalysts shows that the activity for ethanol oxidation of (PtSn)_alloy/SnO₂ is higher than that of (PtSnPd)_alloy/SnO₂. This result, obtained with the same Pt/SnO₂ atomic ratio in all the samples, indicates the critical role of the alloy phase characteristics of these catalysts on their activity for ethanol oxidation.     

Hydrogen production over highly active Pt based catalyst coatings by steam reforming of methanol: Effect of support and co-support

The effect of metal oxide (CeO₂, Al₂O₃ and ZrO₂) support and In₂O₃ co-supported Pt catalysts has been investigated on steam reforming of methanol in microreactors. CeO₂, Al₂O₃ and ZrO₂ were prepared by the sol-gel method and they were used as a support, which was impregnated with In₂O₃ as co-support followed by the introduction of Pt species via the wet impregnation method. The size and dispersion of the Pt nanoparticles on In₂O₃/support have been examined by transmission electron microscopy. From these TEM and XPS results, it was found that the addition of In₂O₃ supports the formation of a high concentration of metallic Pt nanoparticles with enhanced dispersion and controlled particle size on the surface. The activity and stability of all the developed catalysts were tested for the steam reforming of methanol in microreactors at different temperatures. Under reforming conditions without prior reduction, a Pt/CeO₂ catalyst containing 15 wt % of Pt exhibited complete methanol conversion and high selectivity towards hydrogen at 350 °C. However, the CO formation was found to be very high (7.0 vol %) for this catalyst. Upon addition of In₂O₃ as a co-support to this formulation the formation of CO decreased considerably. Pt/In₂O₃/CeO₂ catalyst containing 15 wt % of Pt and 15 wt % of In₂O₃ showed excellent catalytic performance at much lower concentration of CO. This change could be closely associated with the formation of metallic Pt nanoparticles with smaller size, higher dispersion with strong interaction between Pt, In₂O₃ and support, which creates more oxygen vacancies to activate the water molecule which then react with methanol to produce H₂ and CO₂ suppressing the CO formation. Moreover, CeO₂ supported Pt/In₂O₃ catalyst displayed higher stability with lowest CO formation under continuous steam reforming operation of 100 h. The superior performance of this catalyst is thought to be due to the relative abundance of redox sites on the CeO₂ surface, which is able to create an oxygen vacancy as it possesses higher oxygen storage capacity and oxygen exchange capacity. This work demonstrates that the nature of support plays a crucial role for the continuous activation of reactants and determines the catalytic stability during methanol steam reforming.     

The electrochemical oxidation of methanol on a Pt/TNTs/Ti electrode enhanced by illumination

A Pt/TNTs/Ti electrode is prepared by electrochemically depositing Pt using the modulated pulse current method onto high density, well ordered and uniformly distributed TiO₂ nanotubes (TNTs) on a Ti substrate. The results show that the performance and anti-poison ability of the Pt/TNTs/Ti electrode for methanol electro-oxidation under illumination is remarkably enhanced and is even better than the best bi-metallic Pt-Ru catalysts. CO poisoning is no longer a problem during methanol electro-oxidation with the Pt/TNTs/Ti electrode under illumination.     

Hydrogen production by auto-thermal reforming of ethanol over Ni catalysts supported on ZrO2: Effect of preparation method of ZrO2 support

Zirconia supports were prepared by a sol–gel method (S-ZrO₂) and by a templating sol–gel method (M-ZrO₂). Nickel catalysts supported on zirconia were then prepared by an incipient wetness impregnation method for use in hydrogen production by auto-thermal reforming of ethanol. For comparison, a commercial zirconia (C-ZrO₂) was also employed as a support for nickel catalyst. The effect of preparation method of zirconia on the catalytic property and catalytic performance of supported nickel catalysts (Ni/C-ZrO₂, Ni/S-ZrO₂, and Ni/M-ZrO₂) was investigated. The crystalline and physical property of zirconia supports and the catalytic performance of supported nickel catalysts were strongly affected by the preparation method of zirconia. BET surface area and pore volume were decreased in the order of M-ZrO₂ > S-ZrO₂ > C-ZrO₂. Both M-ZrO₂ and S-ZrO₂ supports showed only tetragonal phase of ZrO₂, while C-ZrO₂ support exhibited tetragonal and monoclinic phases of ZrO₂. Crystalline size of nickel species in the Ni/ZrO₂ catalysts decreased with increasing surface area and pore volume of ZrO₂ supports. All the Ni/ZrO₂ catalysts exhibited 100% conversion of ethanol at 500 °C, while product distributions over the Ni/ZrO₂ catalysts were different depending on the preparation method of zirconia. Among the catalysts tested, the Ni/M-ZrO₂ catalyst showed the best catalytic performance in hydrogen production by auto-thermal reforming of ethanol. Well developed mesopore, high surface area, and pure tetragonal phase of ZrO₂ were responsible for fine nickel dispersion and high catalytic performance of Ni/M-ZrO₂. C–C bond cleavage reaction and methane steam reforming reaction were also accelerated over the Ni/M-ZrO₂ catalyst.     

Ethanol oxidation reaction activity of highly dispersed Pt/SnO2 double nanoparticles on carbon black

Highly dispersed Pt and SnO₂ double nanoparticles containing different Pt/Sn ratios (denoted as Pt/SnO₂/CB) were prepared on carbon black (CB) by the modified Bönnemann method. The average size of Pt and SnO₂ nanoparticles was 3.1 ± 0.5 nm and 2.5 ± 0.3 nm, respectively, in Pt/SnO₂(3:1)/CB, 3.0 ± 0.5 nm and 2.6 ± 0.3 nm, respectively, in Pt/SnO₂(1:1)/CB, and 2.8 ± 0.5 nm and 2.5 ± 0.3 nm, respectively, in Pt/SnO₂(1:3)/CB. The Pt/SnO₂(3:1)/CB electrode showed the highest specific activity and lowest overpotential for ethanol oxidation reaction (EOR), and was superior to a Pt/CB electrode. Current density for EOR at 0.40 and 0.60 V vs. reversible hydrogen electrode for the Pt/SnO₂(3:1)/CB electrode decayed more slowly than that for the Pt/CB electrode because of a synergistic effect between Pt and SnO₂ nanoparticles. The predominant reaction product was acetic acid, and its current efficiency was about 70%, while that for CO₂ production was about 30%.     

n-Si/SnO2 junctions based on macroporous silicon for photoconversion

For the first time it is demonstrated that a photovoltaic junction can be formed by spraying a thin, transparent, conductive SnO₂ layer onto macroporous n-type silicon produced by photoelectrochemical etching. With a macropore density of 10⁷ cm⁻², and an average pore diameter in the range 1–2 μm, the cell reflectivity spectrum is flat and drops to a few percent in the visible and near infrared spectral range. EDX analysis and impedance measurements show that charge separation and current collection occur in the upper part of the pores and in the interpore region, whereas the bottom of the pores only acts as a photon absorber. The active junction area is found to be four times larger than with a mirror-polished substrate. A solar cell equipped with just a front ring contact was realized, which attains a solar conversion efficiency of 10% under AM 1.5 conditions.     

Electrooxidation study of pure ethanol/methanol and their mixture for the application in direct alcohol alkaline fuel cells (DAAFCs)

Aliphatic alcohol mainly, ethanol, methanol and their mixture were subjected to electrooxidation study using cyclic voltammetry (CV) technique in a three electrodes half cell assembly (PGSTAT204, Autolab Netherlands). A single cell set up of direct alcohol alkaline fuel cell (DAAFC) was fabricated using laboratory synthesized alkaline membrane to validate the CV results. The DAAFC conditions were kept similar as that of CV experiments. The anode and cathode electrocatalysts were Pt-Ru (30%:15% by wt.)/Carbon black (C) (Alfa Aesar, USA) and Pt (40% by wt.)/High Surface Area Carbon (C_HSA) (Alfa Aesar, USA) respectively. The CV and single cell experiments were performed at a temperature of 30 °C. The anode electrocatalyst was in the range of 0.5 mg/cm² to 1.5 mg/cm² for half cell CV analysis. The cell voltage and current density data were recorded for different concentrations of fuel (ethanol or methanol) and their mixture mixed with different concentration of KOH as electrolyte. The optimum electrocatalyst loading in half cell study was found to be 1 mg/cm² of Pt-Ru/C irrespective of fuel used. The single cell was tested using optimum anode loading of 1 mg/cm² of Pt-Ru/C which was found in CV experiment. Cathode loading was kept similar, in the order of 1 mg/cm² Pt/C_HSA. In single cell experiment, the maximum open circuit voltage (OCV) of 0.75 V and power density of 3.57 mW/cm² at a current density of 17.76 mA/cm² were obtained for the fuel of 2 M ethanol mixed with 1 M KOH. Whereas, maximum OCV of 0.62 V and power density of 7.10 mW/cm² at a current density of 23.53 mA/cm² were obtained for the fuel of 3 M methanol mixed with 6 M KOH. The mixture of methanol and ethanol (1:3) mixed with 0.5 M KOH produced the maximum OCV of 0.66 V and power density of 1.98 mW/cm² at a current density of 11.54 mA/cm².