PtxSn/C electrocatalysts synthesized by improved microemulsion method and their catalytic activity for ethanol oxidation

The Pt_xSn/C (x = 1, 2, 2.5, 3, 4) anodic catalysts for direct ethanol fuel cell (DEFC) have been prepared by an improved microemulsion method. Ethylene glycol is used as cosurfactant, and metal precursors are dissolved in it beforehand to prevent the hydrolysis of metal precursors. The composition, particle size and structure of these catalysts are characterized by energy dispersive X-ray spectrum (EDX), transmission electron microscope (TEM) and X-ray diffraction (XRD). The results show that the synthesized Pt₃Sn/C catalyst has part of Pt and Sn alloying. The average diameter is about 2.9 nm, and has a narrow size distribution and a good dispersivity. The electrochemical experiments indicate that the Pt₃Sn/C catalyst prepared in the neutral microemulsion has superior catalytic activity for ethanol oxidation. The Pt_xSn/C nanoparticle formation in the improved microemulsion is also discussed.     

A novel binary Pt3Tex/C nanocatalyst for ethanol electro-oxidation

The Pt₃Te_x/C nanocatalyst was prepared and its catalytic performance for ethanol oxidation was investigated for the first time. The Pt₃Te/C nanoparticles were characterized by an X-ray diffractometer (XRD), transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy equipped with TEM (TEM-EDX). The Pt₃Te/C catalyst has a typical fcc structure of platinum alloys with the presence of Te. Its particle size is about 2.8 nm. Among the synthesized catalysts with different atomic ratios, the Pt₃Te/C catalyst has the highest anodic peak current density. The cyclic voltammograms (CV) show that the anodic peak current density for the Pt₃Te/C, commercial PtRu/C and Pt/C catalysts reaches 1002, 832 and 533 A g⁻¹, respectively. On the current–time curve, the anodic current on the Pt₃Te/C catalyst was higher than those for the catalysts reported. So, these findings show that the Pt₃Te/C catalyst has uniform nanoparticles and the best activity among the synthesized catalysts, and it is better than commercial PtRu/C and Pt/C catalysts for ethanol oxidation at room temperature.     

High temperature polymer electrolyte membrane fuel cell performance of PtxCoy/C cathodes

Carbon-supported Pt-Co alloy nanoparticles of varying Pt:Co atomic ratios of 1:1, 2:1, 3:1 and 4:1 are prepared, characterized and tested in high temperature PEM fuel cell intend to reduce the Pt loading. These electrocatalysts are prepared by borohydride reduction method in the presence of citric acid as stabilizing agent. Face-centered cubic structure of Pt is evident from XRD. The positive shift of Pt diffraction peaks with increasing cobalt content in the Pt_xCo_y/C catalysts indicated the solubility of Co in Pt lattice. The average crystallite size is found to be 6 nm in all the prepared catalysts. The electrochemical active surface area (EAS) of the catalysts from CO-stripping voltammetry is calculated to be 65.2, 51.4, 47.7, 41.5 and 38.3 m² g⁻¹ Pt for Pt/C, Pt-Co(4:1)/C, Pt-Co(3:1)/C, Pt-Co(2:1)/C and Pt-Co(1:1)/C, respectively. These catalysts are used as cathode in the fabrication of polybenzimidazole-based membrane electrode assembly (MEA) and the polarization curves are recorded at 160 and 180 °C. The results indicate the good performance of Pt-Co alloys than that of Pt under the PEM fuel cell conditions. Among the investigated electrocatalysts, Pt-Co(1:1)/C and Pt-Co(2:1)/C exhibited good fuel cell performance. Durability tests also indicated the good stability of Pt-Co(1:1)/C and Pt-Co(2:1)/C compared to Pt/C.     

Understanding the electrocatalytic activity of PtxSny in direct ethanol fuel cells

In the present work, the activity of Pt_xSn_y/C catalysts towards ethanol, acetaldehyde and acetic acid electrooxidation reactions is investigated for each one separately by means of cyclic voltammetry. To this purpose, a series of Pt_xSn_y/C catalysts with different atomic ratio (x:y = 2:1, 3:2, 1:1) and small particle size (∼3 nm) are fast synthesized by using the pulse microwave assisted polyol method. The catalysts are well dispersed over the carbon support based on the physicochemical characterization by means of XRD and TEM. Concerning the ethanol electrooxidation, it is found that the Sn addition strongly enhances Pt's electrocatalytic activity and the contributing effect of Sn depends on: (i) the Sn content and (ii) the operating temperature. More precisely, at lower temperatures, Sn-rich catalysts exhibit better ethanol electrooxidation performance while at higher temperatures Sn-poor catalysts give better performance. In the case of acetaldehyde electrooxidation, Pt₁Sn₁/C catalyst exhibits the highest activity at all the investigated temperatures; due to the role of Sn, which could effectively remove C₂ species and inhibit the poison formation by supplying oxygen-containing species. Finally, it is found that the Pt_xSn_y/C catalysts are almost inactive (little current was measured) towards the acetic acid electrooxidation. The above findings indicate that Sn cannot substantially promote the electrooxidation of acetic acid to C₁ species.     

Carbon-supported Ni1−x@Ptx (x = 0.32, 0.43, 0.60, 0.67, and 0.80) core-shell nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on the carbon supported Ni core-Pt shell Ni_1−x@Pt_x/C (x = 0.32, 0.43, 0.60, 0.67, and 0.80) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The catalysts are prepared through a polyol synthesis process with oleic acid as the surfactant. The structure, morphology, and chemical composition of the obtained samples are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), inductively coupled plasma emission spectroscopy (ICP), and nuclear magnetic resonance (NMR). The results show that the Ni core-Pt shell nanoparticles are uniformly dispersed on the carbon surface with the diameters of 2-4 nm, and furthermore, the catalysts show favorable performance toward the hydrolysis of NH₃BH₃. Among the nanoparticles, Ni_0.33@Pt_0.67/C displays the highest catalytic activity, delivering a high hydrogen release rate of 5469 mL min⁻¹ g⁻¹ and a low activation energy of 33.0 kJ mol⁻¹.     

A facile method to synthesize well-dispersed PtRuMoOx and PtRuWOx nanoparticles and their electrocatalytic activities for methanol oxidation

PtRuMoO_x and PtRuWO_x catalysts supported on multi-wall carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction method. XRD measurements indicate that Pt exists as face-centered cubic structure, Ru is alloyed with platinum, and the metal oxides exist as an amorphous structure. TEM pictures show that PtRuMoO_x and PtRuWO_x catalysts are well dispersed on the surface of MWCNTs with the particle size of about 3 nm and a narrow particle size distribution. The electrochemical properties of the catalysts for methanol electrooxidation are studied by cyclic voltammetry (CV), chronoamperometry (CA) and chronopotentiometry (CP). The onset potentials for methanol oxidation on PtRuMoO_x and PtRuWO_x are more negative than that of pure Pt catalyst, shifting negatively by about 0.20 V and have better electrocatalytic activities than PtRu/MWCNTs.     

Highly efficient Zr-substituted catalysts CuZnAl1−xZrxO used for hydrogen production via dimethyl ether steam reforming

A series of CuZnAl_1−xZr_xO catalysts with different weight ratios of ZrO₂/(Al₂O₃ + ZrO₂) were prepared by co-precipitation and used for catalytic production of hydrogen via the route of dimethyl ether steam reforming (DME SR). Multiple techniques such as N₂ physisorption, X-ray diffraction (XRD), temperature-programmed reduction by hydrogen (H₂-TPR), N₂O chemisorption and X-ray absorption fine structure (XAFS, including XANES and EXAFS) were employed for catalyst characterization. It is found that the relative contents of Al and Zr greatly influence the catalytic performance of the catalysts including DME conversion, H₂ yield and CO/CO₂ selectivity. The catalyst CuZnAl_0.8Zr_0.2O shows not only the highest DME conversion but also the highest H₂ yield in the whole reaction temperature region of 300–425 °C. Poorly crystallized CuO and ZnO phases were identified by XRD for CuZnAl_1−xZr_xO catalysts. The crystallinity of them increases with the decrease of Al content. The partial substitution of Al by Zr improves both the reducibility and the dispersion of copper species as revealed by H₂-TPR results. The N₂O chemisorption and Cu K-edge XAFS results conformably indicate that the Cu species in CuZnAl_0.8Zr_0.2O possesses the highest dispersion. In addition, after used in DME SR reaction, the catalyst CuZnAl_0.8Zr_0.2O possesses the highest Cu⁺/Cu⁰ ratio, as calculated by Cu K-edge XANES fitting. The lowest CO selectivity during DME SR over this catalyst is highly related to the highest Cu⁺/Cu⁰ ratio.     

Facile synthesis of PdSbx/C nanocatalyst with high performance for ethanol electro-oxidation in alkaline medium

The PdSb_x/C (x = 0, 0.5, 0.1, 0.15 and 0.2) nanocatalysts were synthesized by the microwave treatment. The structure and morphology of the as-prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It is found that the addition of antimony into the Pd/C catalyst leads to formation of PdSb alloy and reduction in particle size. The electrochemical measurements indicate that the PdSb_0.15/C catalyst has high electrocatalytic activity and excellent anti-poisoning ability towards ethanol oxidation in alkaline medium. The onset potential of ethanol oxidation on PdSb_0.15/C shifts in negative direction as compared with Pd/C. The mass activity of PdSb_0.15/C for ethanol oxidation reaches 3690 mA mg⁻¹ Pd, which is ca. 1.7 times higher than that of Pd/C. The enhanced performance of PdSb_0.15/C is mainly ascribed to the bifunctional mechanism and electronic effect.     

Biogas reforming for hydrogen production over mesoporous Ni2xCe1−xO2 catalysts

Biogas reforming for hydrogen production over mesoporous Ni_2xCe_1−xO₂ catalysts were proposed in this study. Mesoporous Ni_2xCe_1−xO₂ (x = 0.05, 0.13, 0.2) was prepared by a reverse precipitation method. The effects of nickel content were investigated in physicochemical properties and catalytic activities. All of the catalysts were reduced with 10% H₂/Ar at 600 °C before reactions, the reduced catalysts were found to be active for both dry and steam reforming of methane (CH₄:CO₂:H₂O = 3:1:2) to produce hydrogen and syngas. The studies were firstly carried out by temperature program reaction from 400 °C to 900 °C to verify the activity of temperature dependency. The long-term stability analysis was also studied at 700 °C for 24 h. Commercial catalyst (R67) was also employed for a comparative purpose.     

LaFeyNi1−yO3 supported nickel catalysts used for steam reforming of ethanol

LaFe_yNi_1−yO₃ perovskite-type oxide supported highly dispersed NiO catalysts were prepared by one-step citric-complexing method, and applied to the steam reforming of ethanol for hydrogen production. NiO/LaFeO₃ prepared by impregnation was also presented for comparison. The XRD and TEM results indicate that one-step citric-complexing method is a simple as well as an effective way for producing well-dispersed NiO particles supported on perovskite oxides. The dispersive NiO particles tend to interact with the perovskite oxide and partially incorporate into the perovskite structure, leading to the formation of LaFe_yNi_1−yO₃ and some resultantly separated Fe ions onto the perovskite surface. The smaller the NiO particles are, the easier the incorporation is. The catalystic performance tests showed that the high activities of NiO/LaFe_yNi_1−yO₃ were attributed to the metallic Ni with high dispersion. The CH₄ selectivity was sensitive to the particle sizes of supported Ni, and the smaller nickel particles favor the lower amount of methane formed. Characterizations of used catalysts indicated that the sintering of nickel particles was not significant even at the high reaction temperature. The LaFe_yNi_1−yO₃ supported nickel catalysts exhibited very good carbon deposition resistance, which could be ascribed to the highly dispersed Ni particles and the formation of oxygen vacancies in LaFe_yNi_1−yO₃ due to the partial substitution of Ni ions for Fe ions.     

Synthesis and characterization of H5PMo10V2O40 deposited Pt/C nanocatalysts for methanol electrooxidation

Pt/C(a) catalysts are firstly prepared by modified impregnation method. In order to enhance the ability of Pt/C catalysts for methanol electrooxidation, H₅PMo₁₀V₂O₄₀ (PMV) is adsorbed on Pt/C catalysts to obtain the PMV-Pt/C catalysts. The Pt/C(a) and PMV-Pt/C are characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It is shown that Pt particles with small average size are uniformly disperesed on carbon. Cyclic voltammetry and chronoamperometry show that the PMV-Pt/C catalysts exhibit excellent catalytic activity and stability for methanol electrooxidation.     

Enhanced ethanol oxidation on PbOx-containing electrode materials for fuel cell applications

The synthesis, characterization and utilization of lead oxide-based catalysts, deposited by the sol–gel method on carbon powder to be used as anode in direct ethanol fuel cells (DEFC) is described. For comparison, other materials, based on Ru and Ir (and mixtures of Ru, Ir or Pb) were tested in the same experimental conditions. X-ray diffraction analysis showed that the Pb was deposited on carbon powder as a mixture of PbO and PbO₂ molecular structures. The catalysts Pt-(RuO₂-PbO_x) and Pt-(RuO₂-IrO₂) exhibited significantly enhanced catalytic activity for the ethanol oxidation as compared to Pt/C commercial powder. Quasi-steady-state polarization curves showed that the composites Pt-(RuO₂-PbO_x)/C and Pt-(RuO₂-IrO₂)/C started the oxidation process in very low potentials (155 and 178 mV, respectively). So, the addition of metallic oxides by the sol–gel route to Pt is presented as a very interesting way to prepare materials with high catalytic activity for direct ethanol fuel cell systems. Current–time studies also showed the good performance of the Pt-(RuO₂-PbO_x) catalyst due to smaller poisoning of the material as the process advances.     

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.     

Preparation,characterization of ZrOxNy/C and its application in PEMFC as an electrocatalyst for oxygen reduction

In this paper, a noble-metal-free electrocatalyst based on carbon-supported zirconium oxynitride (ZrO_xN_y/C) was prepared by ammonolysis of carbon-supported zirconia (ZrO₂/C) at 950 °C and investigated as cathode electrocatalyst towards oxygen reduction reaction (ORR) in PEMFCs. The electrocatalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The catalytic activity of the catalyst towards ORR was investigated by using the rotating disk electrode (RDE) technique in an O₂-saturated 0.5 M H₂SO₄ solution. The ZrO_xN_y/C electrocatalyst presented attractive catalytic activity for ORR. The onset potential of ZrO_xN_y/C electrocatalyst for oxygen reduction was 0.7 V versus RHE and the four-electron pathway for the ORR was achieved on the surface of ZrO_xN_y/C electrocatalyst. The ZrO_xN_y/C electrocatalyst showed a comparatively good cell performance to ORR in PEMFCs, especially when operated at a comparatively high temperature.     

Effect of the preparation method on the performance of CuO-MnOx-CeO2 catalysts for selective oxidation of CO in H2-rich streams

Selective oxidation of CO in H₂-rich streams is performed over a series of CuO-MnO_x-CeO₂ catalysts prepared by hydrothermal (CuMC-HY), co-precipitation (CuMC-CP), impregnation (CuMC-IM) and citrate sol-gel (CuMC-SG) methods. The catalysts are characterized by N₂ adsorption/desorption, XRD, SEM, HR-TEM, TPR and XPS techniques. The results show that the catalyst prepared by a hydrothermal method exhibits the best catalytic activity, especially at low temperatures. The temperature of 50% CO conversion (T₅₀) is only 74 °C and the temperature window of CO conversions up to 99.0% is about 40 °C wide, from 110 to 140 °C. Moreover, the temperature window is still maintained 20 °C wide even at lower temperatures when there are 15% CO₂ and 7.5% H₂O in the reaction gas. The superior catalytic performance of CuMC-HY is attributed to the formation of Mn-Cu-Ce-O solid solution, the unique pore structure and the existence of more Cu⁺ and Mn⁴⁺ species as well as oxygen vacancies. The sequence of catalytic activity is as follows: CuMC-HY > CuMC-SG > CuMC-IM > CuMC-CP. The worst catalytic activity, obtained from the catalyst prepared by the co-precipitation method, is possibly related to the existence of independent CuO_x and MnO_x oxides, which weakly interact with ceria in the catalyst.     

Platinum decorated Ru/C: Effects of decorated platinum on catalyst structure and performance for the methanol oxidation reaction

Platinum decorated Ru/C catalysts are prepared by successive reduction of a platinum precursor on pre-formed Ru/C. Pt:Ru atomic ratios are varied from 0.13:1 to 0.81:1 to investigate the platinum decoration effects on the catalyst's structure and electrochemical performance towards the methanol oxidation reaction (MOR) at room temperature. The catalysts are extensively characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Ru@Pt/C catalysts show enhanced mass-normalized activity and specific activity for the MOR relative to Pt/C. For the anodic oxidation of methanol, the ratio of forward to reverse oxidation peak current R (I_f/I_b) varies considerably: R decreases from 5.8 to 0.8 when the Pt:Ru ratio increases from 0.13:1 to 0.81:1. When the ratio of Pt:Ru is 0.42:1, R reaches 0.99 (close to that of Pt/C), and further increase of the Pt:Ru ratio leads to almost no decrease in R. Coincidentally, maximum mass-normalized activity is also obtained when Pt:Ru is 0.42:1.     

Enhancing the electrocatalytic water splitting efficiency for amorphous MoSx

Amorphous molybdenum sulfide (MoS_x) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoS_x catalysts prepared by the low temperature thermolysis of the (NH₄)₂MoS₄ precursors on carbon clothes (catalyst loading: 3.2 mg/cm²) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 × 10⁻³ mA/cm² in 0.5 M H₂SO₄ solutions. Spectroscopic studies of the amorphous MoS_x catalysts show that the increase of HER efficiency is positively correlated to the concentration of S₂²⁻ species, providing strong evidence to support the argument that S₂²⁻ is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production.     

Formic acid oxidation reaction on a PdxNiy bimetallic nanoparticle catalyst prepared by a thermal decomposition process using ionic liquids as the solvent

The nano-catalysts of Pd_xNi_y bimetallic nanoparticles (NPs, the nominal atomic ratios of Pd to Ni are 2:1, 3:2 and 1:1) supported on multi-walled carbon nanotubes (MWCNTs) (denoted as Pd_xNi_y/MWCNTs) have been synthesized by a thermal decomposition process using room temperature ionic liquids (RTILs) of N-butylpyridinium tetrafluoroborate (BPyBF₄) as the solvent. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were employed to characterize the morphology of the samples, revealing that the prepared Pd_xNi_y NPs were quite uniformly dispersed on the surface of MWCNTs with an average particle size of ∼8.0 nm. Formic acid oxidation reaction (FAOR) was investigated on the as-prepared catalysts by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), demonstrating that the peak current on the Pd₃Ni₂/MWCNTs catalyst was about three times higher than that on the Pd/MWCNTs. The lower electrode potential and easier hydrogen evolution, based on the results obtained from chronopotentiometry and CV, respectively, were thought as the main reasons for the excellent electrocatalysis of the Pd₃Ni₂/MWCNTs toward formic acid oxidation reaction (FAOR) when compared to other samples.     

Modulated crystalline Ag-Ci oxygen-evolving catalysts for electrocatalytic water oxidation

In this paper we synthesized a crystalline Ag-C_i oxygen-evolution catalyst with an entirely new composition and structure in situ in a carbonate solution (pH = 8.3) with Ag⁺ at 70 °C. We studied the factors that influenced the synthesis of the oxygen-evolution catalyst (electrolyte environment temperature, silver ion concentration, concentration of carbonates) under mild conditions. The structure, morphology, composition, and catalytic activity of Ag-C_i catalyst were analyzed using techniques such as X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and electrochemical measurements. Our results showed that the diffraction peaks of the Ag-C_i catalysts synthesized at 70 °C were consistent with the AgO phase and that the catalytic composition of the Ag-C_i catalyst was the Ag^ⅠO/Ag^ⅡO redox couple. The oxygen-evolution rate of the catalyst synthesized in a carbonate solution (pH = 8.3) at 70 °C was 50 times higher than that synthesized at 5 °C (18.3 μmol h⁻¹); additionally, its oxygen evolution overpotential decreased to 246.7 mV. The electrolyte solution and Ag⁺ ion concentration greatly impacted the catalytic activity of the Ag-C_i catalyst for water splitting.     

Perovskite-type oxides La1−xSrxMnO3 for cathode catalysts in direct ethylene glycol alkaline fuel cells

Carbon-supported La_1−xSr_xMnO₃ (LSM/C) was prepared by reversible homogeneous precipitation method, and its catalytic activities for oxygen reduction under the existence of ethylene glycol (EG) were investigated by using rotating disk electrode. LSM/C exhibited the high activity for oxygen reduction irrespective with the presence of EG, indicating that EG is not oxidized by LSM/C at the cathode side in the present system. Consequently, LSM/C can serve as a cathode catalyst in alkaline direct alcohol fuel cells with no crossover problem. Performance test for fuel cells operation also supported these results and showed cathodic polarization curves were not affected by the concentration of EG supplied to anode even at 5 mol dm⁻³.