Preparation and influence of performance of anodic catalysts for direct methanol fuel cell

This research aims at increasing the utilization of platinum-ruthenium alloy (Pt-Ru) catalysts and thus lowering the catalyst loading in anodes for methanol electrooxidation. The direct methanol fuel cell’s (DMFC) anodic catalysts, Pt-Ru/C, were prepared by chemical reduction with a reducing agent added in two kinds of solutions under different circumstances. The reducing agent was added in hot solution with the protection of inert gases or just air, and in cold solution with inert gases. The catalysts were treated at different temperatures. Their performance was tested by cyclic voltammetry and potentiostatic polarization by utilizing their inherent powder microelectrode in 0.5 mol/L CH₃OH and 0.5 mol/LH₂SO₄ solution. The structures and micro-surface images of the catalysts were determined and observed by X-ray diffraction and transmission electron microscopy, respectively. The catalyst prepared in inert gases showed a better catalytic performance for methanol electrooxidation than that prepared in air. It resulted in a more homogeneous distribution of the Pt-Ru alloy in carbon. Its size is small, only about 4.5 nm. The catalytic performance is affected by the order of the reducing agent added. The performance of the catalyst prepared by adding the reductant at constant temperature of the solution is better than that prepared by adding it in the solution at 0°C and then heating it up to the reducing temperature. The structure of the catalyst was modified, and there was an increase in the conversion of ruthenium into the alloyed state and an increase in particle size with the ascension of heat treatment temperature. In addition, the stability of the catalyst was improved after heat treatment. Translated from Journal of Harbin Institute of Technology, 2006, 38 (4): 541-545 [译自: 哈尔滨工业大学学报]     

炭载体前处理对Pt-Ru/C催化剂性能影响

为研究水蒸气处理后热处理对炭黑表面特性的影响,提高DMFC阳极催化剂的催化活性,利用先水蒸气处理后热处理的Vnlcan XC-72炭黑为载体制备Pt-Ru/C催化剂,与水蒸气处理的和未经处理的炭载体制备Pt-Ru/C催化剂的性能进行比较.采用XPS和BET测试了处理后的炭粉表面的含氧浓度和比表面,结果表明：水蒸气处理后,炭载体比表面积增大,含氧浓度降低;水蒸气处理后热处理,炭载体比表面积进一步减小,含氧浓度增加.用XRD对催化剂的结构进行了表征,结果表明：水蒸气处理后热处理的炭黑为载体制备Pt-Ru/C催化剂结晶状态良好,催化剂颗粒较小.在0.5mol/L CH3OH和0.5mol/L H2SO4混合溶液中,利用玻炭电极测试了循环伏安曲线和阶跃电位曲线,结果表明：用先水蒸气处理后热处理的炭粉为载体制备的催化剂比仅水蒸气处理和未经处理的炭粉为载体制备的催化剂的活性最高.     

Effects of ozone treatment of carbon support on Pt-Ru/C catalysts performance for direct methanol fuel cell

This research is aimed to increase the activity and utilization of Pt-Ru alloy catalysts and thus to lower the catalyst loading in anodes for methanol electrooxidation. The Pt-Ru/C catalysts were prepared by chemical reduction. The support of Vulcan XC-72 carbon black was pretreated by ozone at different temperatures for different times. The specific surface area of the samples was evaluated by the standard BET method. The surface concentrations of oxygen were determined by XPS. The results showed that the surface concentrations of oxygen on the carbon were first decreased and then increased with pretreating times, and the specific surface area of the carbon was decreased with pretreating times at the same temperature. The specific surface area was increased with increasing temperature, and the surface concentration of oxygen was first decreased and then increased with increasing temperature for the same pretreating time. Pt-Ru/C catalysts supported by untreated and O₃ treated carbon black were characterized and tested for methanol electrooxidation. X-ray diffraction (XRD) was used to characterize the influence of carbon treated with ozone on Pt-Ru/C catalysts. It was found that the catalysts were composed only of f.c.c. Pt-Ru alloy particles without metallic Ru or Ru oxide. Cyclic voltammetry (CV) and Tafel curves were used for methanol electrooxidation on Pt-Ru/C catalysts in a solution of 0.5 mol/L CH₃OH and 0.5 mol/L H₂SO₄, showing that the catalytic activity of Pt-Ru/C catalysts supported by ozone treated carbon was higher than that by the untreated one. The ozone treatment time and temperature, which affect the performance of Pt-Ru/C catalysts, were discussed. Electrochemical measurements showed that the catalysts supported by the carbon after ozone treatment for 6 min at 140 °C had the best performance.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

PtRu colloidal catalysts: Characterisation and determination of kinetics for methanol oxidation

The activities of three 30 wt.% PtRu/C catalysts of the same nominal composition (Pt:Ru, 1:1 a/o) were investigated with regard to methanol oxidation. The catalysts were synthesized by Bönnemann's colloidal precursor method using different metal salt precursors and reducing agents. The catalysts were characterized ex situ by energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) and in situ CO stripping and cyclic voltammetry. The activity towards methanol electro-oxidation was checked in steady-state experiments at 22 and 60 °C. The experimental rate data can be described well by a kinetic model, which includes methanol adsorption on the Pt-sites, formation of C-containing adsorbed species, OH_ads formation on the Ru sites and heterogeneous surface reaction between C-adsorbate and OH_ads. The kinetic model parameters were identified from the experimental data and were used to explain the differences in catalytic activity.     

Novel methanol electro-oxidation catalyst assisting with functional phthalocyanine supports

Novel electro-catalyst based on phthalocyanine stabilized Pt colloids has been developed for methanol electro-oxidation. Water soluble Cu²⁺ phthalocyanine functioned with sulfonic groups were selected as catalyst supports because of the relatively high catalytic activity of Pt catalyst and nearly the same catalytic selectivity complex with Cu-phthalocyanine, compared to others that chelated with Fe, Co and Ni ions. The as-resulting Pt-CuTsPc catalysts have average particle size of 2 nm and narrow size distribution. With the assistance of CuTsPc supports, the methanol electro-oxidation activity and poison tolerance of Pt catalyst have a significant increase. I_f/I_b ratio (anodic peak current density, forward to backward) of the Pt-CuTsPc/C catalysts also has obvious increase to 2.5, from value of 0.8 for pure Pt/C catalyst. The reaction Tafel slope of Pt-CuTsPc/C catalysts is 56.6 mV dec⁻¹, much smaller than that of the Pt/C catalyst. The transient current density on Pt-CuTsPc/C at 0.60 V is enhanced to 650% of that on the Pt/C catalyst while the enhancement factor R for comparison of steady-state current obtained on Pt-CuTsPc/C and Pt/C catalyst varies between 111% and 534% in the potential region of 0.3-0.75 V.     

乙二醇稳定的Pt/C催化剂的制备与表征

以乙二醇(EG)兼作溶剂和稳定剂,分别通过NaBH4和EG还原法制备了高度细化与分散的Pt/C催化剂,对其形貌、组成、结构和电化学活性比表面等进行了表征比较,并测试了它们对甲醇与乙醇电催化氧化的活性. 结果表明,2种催化剂中,Pt均为面心立方结构,粒径小且分布窄,在炭黑载体上分散均匀,单位质量Pt对甲醇与乙醇电催化氧化的活性相当;NaBH4还原法所制Pt/C催化剂中Pt0和Pt(220)晶面含量更高,Pt对甲醇与乙醇电催化氧化的峰电流密度分别为0.68与0.67 mA/cm2,分别是EG还原法所制Pt/C催化剂的1.2倍;2种催化剂对甲醇与乙醇电催化氧化的活性均与商品E-TEK催化剂相当.     

Highly active PtRu catalysts supported on carbon nanotubes prepared by modified impregnation method for methanol electro-oxidation

A simple and rapid synthesis method (denoted as modified impregnation method, MI) for PtRu/CNTs (MI) and PtRu/C (MI) was presented. PtRu/CNTs (MI) and PtRu/C (MI) catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffractometry. It was shown that Pt-Ru particles with small average size (2.7 nm) were uniformly dispersed on carbon supports (carbon nanotubes and carbon black) and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. Cyclic voltammetry and chronoamperometry showed that the Pt-Ru/CNTs (MI) catalyst exhibited better methanol oxidation activities than Pt-Ru/C (MI) catalyst and commercial Pt-Ru/C (E-TEK) catalyst. The single cells with Pt-Ru/CNTs (MI) catalyst exhibited a power density of 61 mW/cm², about 27% higher than those single cells with commercial Pt-Ru/C (E-TEK) catalyst.     

Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation

To determine the influence of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation, this work investigated methanol electrooxidation on as received and different electrochemically polarized PtRu/C catalysts. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to characterize the redox state of PtRu/C after different electrochemical polarization. The methanol electrooxidation activity was measured by cyclic voltammetry (CV), Tafel steady state plot and electrochemical impedance spectroscopy (EIS). The results indicate that the metallic state Pt⁰Ru⁰ can be formed during cathodic polarization and contribute to electrooxidation of methanol, while the formation of inactive ruthenium oxides during anodic polarization cause the negative effect on methanol electrooxidation. Different Tafel slopes and impedance behaviors in different potential regions also reveal a change of the mechanism and rate-determining step in methanol electrooxidation with increasing potentials. The kinetic analysis from Tafel plots and EIS reveal that at low potentials indicate the splitting of the first CH bond of CH₃OH molecule with the first electron transfer is rate-determining step. However, at higher potentials, the oxidation reaction of adsorbed intermediate CO_ads becomes rate-determining step.     

Graphitized mesoporous carbon supported Pt-SnO2 nanoparticles as a catalyst for methanol oxidation

A mesostructured composite catalyst, Pt-SnO₂ supported on graphitized mesoporous carbon (GMC), has been prepared and its electrochemical activity for methanol oxidation has been investigated. The materials were characterized by XRD, FESEM, TEM, EDX spectrum and N₂ sorption techniques. Cyclic voltammetry, chronoamperometry and steady-state polarization tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that, the overall methanol electro-oxidation catalytic activity of the mesostructured composite catalyst, 20 wt.% PtSnO₂ (1:1, mass ratio)/GMC, is obviously higher than that of 20 wt.% PtSnO₂/C with commercial carbon black as support under the same loading amount of Pt-SnO₂ catalysts, and is also much higher than that of commercial catalyst 20 wt.% Pt/C at half Pt using amount.     

Ethylene diamine-grafted carbon nanotubes: A promising catalyst support for methanol electro-oxidation

In this paper, carbon nanotubes (CNTs) were modified by ethylene diamine (ED) and then used as the support of the Pt-Ru catalyst. The cyclic voltammetry (CV) and Fourier transform infrared spectroscopy were employed to study the interaction between ED and CNTs. The morphology and elemental composition of the as-prepared Pt-Ru/ED/CNT electrode were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic properties of the Pt-Ru/ED/CNT electrode for methanol electro-oxidation were investigated by CV, polarization method and electrochemical impedance spectroscopy. The long-term stability of the Pt-Ru/ED/CNT electrode was also evaluated. Compared with the Pt-Ru/CNT electrode, the Pt-Ru/ED/CNT electrode exhibits excellent electrocatalytic properties and long-term stability. These results show that the ED-grafted CNTs are the promising catalyst support for methanol electro-oxidation.     

Investigations of Compositions and Performance of PtRuMo/C Ternary Catalysts for Methanol Electrooxidation

PtRuMo/C catalyst was prepared by impregnation reduction method and characterised. Comparison is made between a home‐made PtRu/C prepared by similar method and Pt/C (E‐Tek Co., Pt/C‐ET) catalysts. One glassy carbon disc electrode for ternary alloy catalyst was used to evaluate the catalytic performances by cyclic voltammetric, chronoamperometric, amperometric i–t curves, and electrochemical impedance spectra (EIS). The electrochemical measurement results indicated that the performance of PtRuMo/C with a molar ratio of 6:3:1 was the highest among 15 Pt_xRu_yMo_10–x–y/C catalysts with different molar ratios. The composition, particle size, lattice parameter and morphology of the PtRuMo(6:3:1)/C catalyst were determined by means of X‐ray energy dispersive analysis, X‐ray diffraction (XRD) and transmission electron micrographs (TEM). The result of XRD analysis exhibits that PtRuMo(6:3:1)/C has the fcc structure with the smaller lattice parameter than the home‐made PtRu/C and Pt/C‐ET. Its typical particle sizes is only about 5 nm. With respect to the catalytic activity and stability, the PtRuMo(6:3:1)/C catalyst is superior to PtRu/C despite their comparable active areas. Though the electrochemically active surface area of Pt/C‐ET is the biggest, its performance is the lowest. EIS results also indicate that the reaction resistances for methanol electrooxidation on the PtRuMo(6:3:1)/C catalyst are smaller than those of PtRu/C at different polarisation potentials.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

A highly efficient synthesis approach of supported Pt-Ru catalyst for direct methanol fuel cell

A highly efficient synthesis approach of urea-assisted homogeneous deposition (HD) coupled with H₂ reduction has been employed to synthesize carbon-supported Pt-Ru catalyst with high metal loading of 60 wt%. The urea-assisted HD method permits in situ gradual and homogeneous generation of hydroxide ions, resulting in smaller Pt-Ru nanaoparticles deposited on the carbon support along with better particle size distribution compared with the catalyst prepared by the conventional impregnation-NaBH₄ reduction. The Pt-Ru catalyst prepared by the HD-H₂ method has demonstrated greatly enhanced catalytic activity towards methanol oxidation and much improved fuel cell performance compared with the one prepared by impregnation-NaBH₄ reduction. HD-H₂ method is simple with mild synthesis condition, but provides very efficient approach for the preparation of Pt-based catalysts for fuel cells.     

Electrodeposition of mesoscopic Pt-Ru on reticulated vitreous carbon from reverse emulsions and microemulsions: Application to methanol electro-oxidation

High surface area Pt-Ru (between 120 and 400 cm² mg⁻¹) meso-sized particles and mesoporous coatings were electrodeposited on reticulated vitreous carbon (RVC) three-dimensional electrodes using reverse emulsions and microemulsions. The organic phase of the colloidal media was composed of cyclohexane, Triton X-100 non-ionic surfactant and tetrabutylammonium perchlorate (for ionic conductivity) while the aqueous phase contained H₂PtCl₆ and RuCl₃ (or (NH₄)₂RuCl₆). For microemulsification to occur isopropanol was also added as co-surfactant. The catalytic activity for the electro-oxidation of methanol was assessed by cyclic voltammetry and chronopotentiometry in conjunction with surface area measurement by Cu underpotential deposition. The composition and morphology of the Pt-Ru deposit was analyzed by inductively coupled plasma atomic emission spectroscopy and scanning electron microscopy, respectively. The effects on the catalytic activity of the deposition current density, temperature, RVC pretreatment and plating bath composition are presented. It was found that the electrodeposition of Pt-Ru in reverse microemulsion yielded the highest specific surface area (400 cm² mg⁻¹) and catalytic activity toward CH₃OH electro-oxidation as shown, for example, by a 50-200 mV more negative anode potential determined by chronopotentiometry compared to a catalyst obtained by pure aqueous and emulsion electroplating.     

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO₃)₃ and [Pt(H₂NCH₂CH₂NH₂)₂]Cl₂ as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40-20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40-20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.     

Preparation of Pt/CeO2/HCSs anode electrocatalysts for direct methanol fuel cells

Hollow carbon spheres (HCSs) were prepared through a simple hydrothermal method using silica particles and glucose as the template and carbon precursor, respectively. HCSs used as supports for platinum catalysts deposited with cerium oxide (CeO₂) were prepared for application as anode catalysts in direct methanol fuel cells. The composition and structure of the samples were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic properties of the as-prepared catalysts for methanol oxidation were investigated by cyclic voltammetry (CV). The Pt/CeO₂/HCSs catalyst heated at 550 °C for 1 h exhibited the best catalytic activity for methanol oxidation.     

Effect of heat treatment on the performance of TiO2-Pt/CNT catalysts for methanol electro-oxidation

TiO₂-Pt/CNT catalysts before and after heat treatment were prepared. Their catalytic activities for methanol and CO electro-oxidation were studied in detail. The results showed that the proper amount of hydrous TiO₂ in TiO₂-Pt/CNTs (e.g. heated at 200 °C for 2 h) was favorable for enhancing the catalytic activity of Pt/CNTs, which provided evidence for bi-functional mechanism. The studies on the catalysts with different TiO₂/Pt molar ratio displayed that the optimum molar ratio varied with the increase of heat treatment temperature. It was found that the optimum molar ratio of TiO₂/Pt was at 1:2 for the catalysts without heat treatment and was at 1:1 for the catalysts by heat treatment at 500 °C. This fact was ascribed to the difference in compact degree between TiO₂ and Pt/CNTs before and after heat treatment. Considering the influence of heating temperature, it was found that TiO₂-Pt/CNT catalyst heated at 200 °C for 2 h had better catalytic activity for methanol oxidation.     

Catalytic dehydration of methanol to dimethyl ether over mordenite catalysts

The conversion of methanol to dimethyl ether was carried out over various commercial mordenite and ion-exchanged catalysts to evaluate the catalytic performance of mordenite catalysts with different pore structures and acidities. These catalysts were compared for their catalytic properties in a fixed-bed reactor at 1 atm, 573 K and LHSV of 2.84 h^− 1. The catalysts were characterized by BET, ICP, NH₃-TPD, XRD, TGA and FT-IR techniques. The ion-exchanged mordenite showed higher activity, selectivity and good stability in dehydration of methanol due to the addition of medium acid sites. Also, the effect of water on catalyst deactivation was investigated over two selected catalysts in order to develop a suitable catalyst for synthesis of dimethyl ether. It was found that the H-mordenite catalyst supplied by Süd-chemie Co., (MCDH-1) was more active and less deactivated than another one in a feed containing 20 wt.% water.