Size-controlled synthesis and impedance-based mechanistic understanding of Pd/C nanoparticles for formic acid oxidation

This work provides a detailed electrochemical impedance study for formic acid electro-oxidation on size-controlled Pd/C nanoparticles, the synthesis of which was done by a simple protocol using ethylene glycol as a reducing agent. By controlling KOH concentration, this strategy provides a synthesis method for Pd nanoparticles with a selective size range of 3.9–7.5 nm. The as-prepared Pd nanoparticles exhibited size-dependent electrochemical property and electrochemical characterizations of four different Pd/C nanocatalysts (3.9, 5.2, 6.1, and 7.5 nm) showed that Pd particle with average size of 6.1 nm has the highest formic acid oxidation activity. Electrochemical impedance-based characterizations of formic acid oxidation on Pd/C suggested that at high potentials the adsorbed oxygen species could block the catalyst surface and inhibit the oxidation reaction, as reflected by the negative polarization resistance. Unlike Pd/C, the intermediate adsorbed CO species (CO_ads) plays a critical role for formic oxidation on Pt/C and thus the impedance spectra of Pd/C and Pt/C appear different potential-dependent patterns in the second quadrant. The issue of CO was investigated by an impedance investigation of Pd/C in a mixture of formic acid containing dissolved CO.     

Electrochemical Characteristics and Performance of Platinum Nanoparticles Supported by Vulcan/Polyaniline for Oxygen Reduction in PEMFC

We report a Pt/Vulcan carbon–polyaniline (VC–PANI) catalyst for the oxygen reduction reaction (ORR). This electrocatalyst was prepared from Pt nanoparticles supported by a VC–PANI composite substrate. Electrochemical performance was measured using potentiostat/galvanostats technique and a proton exchange membrane fuel cell (PEMFC) test station. The electrochemical properties of the electrodes were characterized using linear sweep voltammetry, AC impedance spectroscopy and chronoamperometry. Electrochemical characterization by hydrogen adsorption/desorption cyclic voltammetry and CO stripping voltammetry indicates that the electrochemical active surface areas of the Pt/VC–PANI are comparable to the commercial catalyst. The performance of the Pt/VC–PANI and Pt/C(E‐TEK) + PANI electrocatalysts were found to be 1.82 and 1.33 times higher than of the Pt/C(E‐TEK) electrode. The surface morphologies of the electrodes were characterized by using scanning electron microscopy (SEM). PANI has a fibrous structure and the improved performance was attributed to the PANI effect and synergistic effects between the carbon Vulcan and the PANI fiber. These results indicate that Pt/VC–PANI is a promising catalyst for the ORR in PEMFCs using an H₂/O₂ feed.     

Morphology controllable growth of Pt nanoparticles/nanowires on carbon powders and its application as novel electro-catalyst for methanol oxidation

Pt nanowires (PtNWs) have been controllably synthesized on carbon powders by the reduction of H(2)PtCl(6) with HCOOH. By adjusting the pH value of the solution, PtCl(6)(2-) can be controllable reduced into particles or nanowires. The Pt nanowires are single crystals growing along the <111> direction with a diameter of 3 nm and a length of 10 nm. The dispersion of Pt nanowires on the surface of carbon powders can be controlled by changing the loading of Pt. The PtNWs/C is evaluated as the catalyst for methanol oxidation. The PtNWs/C with 20 wt% Pt has a larger electrochemical active surface area and much higher mass activity for methanol oxidation than that of commercial Pt/C catalyst. The PtNWs/C catalyst shows significant improvement in the kinetics for methanol oxidation and mass transfer property due to the single crystal structure of the Pt nanowires. The PtNWs/C catalyst holds promising potential applications in energy converting devices and environmental protection.     

炭载体对Pt-C复合物非碱性条件下催化甘油氧化性能的影响

采用等量浸渍法制备了具有相似平均粒径的活性炭（AC）和碳纳米管（CNTs）负载的Pt催化剂,并比较研究了非碱性条件下两种催化剂催化甘油氧化反应的性能。结果表明,炭载体对Pt-C复合物催化甘油氧化反应的活性、选择性和稳定性有重要影响。相对于Pt/CNTs催化剂,Pt/AC催化剂中Pt 4f结合能较低,导致其表面氧的覆盖度相对较高,因而抑制了甘油的吸附,降低了甘油氧化反应的初始活性;Pt/AC催化剂会促进甘油醛进一步氧化成甘油酸以及C₃产物的氧化断键;Pt/AC催化剂失活的主要原因是氧中毒和中间产物的吸附,而Pt/CNTs催化剂的失活主要是由于甘油酸的吸附堵塞Pt表面的活性位造成的。     

Diffusion study in a novel three-dimensional electrode for direct methanol fuel cells

A three-dimensional electrode formed by depositing Pt on polypyrrole treated polystyrene spheres (denoted as Pt/Ppy/PS) is prepared and characterized by different methods. The Pt/Ppy/PS prepared by using mixed polystyrene spheres of 200 nm-2 μm as support shows best performance for methanol oxidation due to the big and small holes or channels coexistent structure, which causes the difference in pressure inside the electrode and results in the reduction of the liquid sealing effect. The three-dimensional structure makes it easier for the liquid reactant to diffuse into the catalyst layer and the gaseous products evolve out from the catalyst layer. The diffusion behaviours of Pt/Ppy/PS and Pt/C electrodes are characterized by cyclic voltammetry. It is shown that the methanol oxidation on Pt/Ppy/PS electrode is not controlled by concentration polarization at slow scan rates, while the reaction on the traditional Pt/C electrode is diffusion controlled at all scan rates. The electrochemical impedance spectroscopic study (EIS) reveals that the three-dimensional electrode has higher active surface area.     

Flame synthesis of carbon nanofibers on carbon paper: Physicochemical characterization and application as catalyst support for methanol oxidation

We developed a simple, rapid and highly efficient flame synthesis method for direct growing carbon nanofibers (CNFs) on carbon paper (CP) using a common laboratory ethanol flame as both heat and carbon sources. High density CNFs with tangled solid-cored structure were uniformly formed over the Ni-plated CP surface in ∼20 s. The morphologies of the CNFs were characterized by scanning electron microscopy and transmission electron microscopy. X-ray diffraction study revealed the graphitic nature of the CNFs. Raman spectroscopy analysis confirmed that the CNFs are disordered graphitic nanocrystallites with high degree of exposed edges. Electrochemical impedance spectroscopy and cyclic voltammetry were used to show that growing CNFs directly on CP facilitates electron transfer with concomitant increase in double-layer capacitance. The CNF/CP was used as support for Pt nanoparticles to study their supporting effect on the catalyst performance. The as prepared Pt/CNF electrocatalyst exhibited much improved electrocatalytic activity for methanol oxidation compared to Pt/CP and commercial Pt/C on CP. High electronic conductivity and improved electrochemical behavior of the CNF/CPs, resulted from direct contact of the nanofibers with CP, combined with unique properties of CNFs, make the synthesized CNF/CPs promising for fuel cell applications.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

Improved kinetics of methanol oxidation on Pt/hollow carbon sphere catalysts

Hollow carbon spheres (HCSs) have been prepared by combining the hydrothermal method and intermittent microwave heating (IMH) technique. The preparation factors affecting the performance of the HCSs are studied. The results show that Pt nanoparticles supported on HCSs (Pt/HCS), which were heated for 3 min in a microwave oven, give the best performance for methanol oxidation. The higher electrochemical active surface area of the Pt/HCS catalysts results in higher catalytic activity for methanol oxidation compared to that of the commercial Pt/C catalyst at the same Pt loadings. Higher exchange current density and lower reaction activation free energy are observed on Pt/HCS catalysts, indicating improved kinetics. It is recognized that the hollow structure of the Pt/HCS with open microspores and nanochannels is responsible for this higher catalytic activity for methanol oxidation.     

Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis

A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells.     

Platinum nanoparticles supported on epitaxial TiC/nanodiamond as an electrocatalyst with enhanced durability for fuel cells

An epitaxial TiC/nanodiamond (ND) was used as novel support for Pt electrocatalysts to improve its durability in fuel cells. The TiC/ND was fabricated by a simple one-pot synthesis method. TiC/ND-supported Pt electrocatalysts were synthesized using a microwave-assisted ethylene glycol method. Pt nanoparticles (NPs) with a mean size of 4.4 nm were highly dispersed on the TiC/ND’s surface. The Pt/TiC/ND catalyst exhibited much higher electrocatalytic activity and stability in methanol oxidation reactions and oxygen reduction reactions than the Pt/ND catalyst. The electrochemical stability of the Pt/TiC/ND catalyst is more outstanding compared with the conventional carbon black supported Pt catalysts. The superior durability can be attributed to the chemical stability of ND core and the anchoring effect of the TiC layer to Pt NPs.     

High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell

This research aims to enhance the activity of Pt catalysts, thus to lower the loading of Pt metal in fuel cell. Highly dispersed platinum supported on single-walled carbon nanotubes (SWNTs) as catalyst was prepared by ion exchange method. The homemade Pt/SWNTs underwent a repetition of ion exchange and reduction process in order to achieve an increase of the metal loading. For comparison, the similar loading of Pt catalyst supported on carbon nanotubes was prepared by borohydride reduction method. The catalysts were characterized by using energy dispersive analysis of X-ray (EDAX), transmission electron micrograph (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectrum (XPS). Compared with the Pt/SWNTs catalyst prepared by borohydride method, higher Pt utilization was achieved on the SWNTs by ion exchange method. Furthermore, in comparison to the E-TEK 20 wt.% Pt/C catalyst with the support of carbon black, the results from electrochemical measurement indicated that the Pt/SWNTs prepared by ion exchange method displayed a higher catalytic activity for methanol oxidation and higher Pt utilization, while no significant increasing in the catalytic activity of the Pt/SWNTs catalyst obtained by borohydride method.     

Effect of pre-treatment approach of a carbon support on activity of PtSn/C electrocatalysts for direct ethanol fuel cells

Vulcan XC-72R carbon was pretreated using acid and thermal activation methods, and the carbons obtained were used as supports for a PtSn/C catalyst synthesized by a successive reduction process. Surface characteristics of the supports, including BET surface area, pH_PZC and functional group, were analyzed using physical N₂ adsorption, mass titration, acid–base titration, and Fourier transform infrared (FTIR) spectrometer technique, respectively. The prepared PtSn/C catalysts were characterized by X-ray diffractometer (XRD), energy dispersive X-ray spectrometer (EDX), inductively coupled plasma–atomic emission spectrometry (ICP–AES), and transmission electron microscope (TEM) techniques, and then were examined for their behavior under ethanol oxidation as well as for their performance in a direct ethanol fuel cell (DEFC). The results showed that pretreatment by HNO₃ produced various oxygenated functional groups on the support surface and increased its acidic property. The strong acidity of the acid-treated support led to an unfavorable condition for the Pt reduction reaction and resulted in low Pt content but high Pt:Sn ratio in the PtSn/C catalyst. On the other hand, thermal activation increased the base functional groups on the carbon surface, which enhanced reduction of Pt precursor, and consequently, provided a small average metal particle size of 2.2 nm. The results from cyclic voltammetry, chronoamperometry and cell performance testing confirmed that the catalytic activity for ethanol oxidation and the performance in the direct ethanol fuel cell of the heat-treated carbon-supported PtSn catalyst was superior to the fresh PtSn/C catalyst and the acid-treated carbon-supported PtSn catalyst.     

Carbon-supported Pt(Cu) electrocatalysts for methanol oxidation prepared by Cu electroless deposition and its galvanic replacement by Pt

Bimetallic Pt–Cu carbon-supported catalysts (Pt(Cu)/C) were prepared by electroless deposition of Cu on a high surface area carbon powder support, followed by its partial exchange for Pt; the latter was achieved by a galvanic replacement process involving treatment of the Cu/C precursor with a chloroplatinate solution. X-ray diffraction characterization of the Pt(Cu)/C material showed the formation of Pt-rich Pt–Cu alloys. X-ray photoelectron spectroscopy revealed that the outer layers are mainly composed of Pt and residual Cu oxides, while metallic Cu is recessed into the core of the particles. Repetitive cyclic voltammetry in deaerated acid solutions in the potential range between hydrogen and oxygen evolution resulted in steady-state characteristics similar to those of pure Pt, indicating the removal of residual Cu compounds from the surface (due to electrochemical treatment) and the formation of a compact Pt outer shell. The electrocatalytic activity of the thus prepared Pt(Cu)/C material toward methanol oxidation was compared to that of a commercial Pt/C catalyst as well as of similar Pt(Cu)/C catalysts formed by simple Cu chemical reduction. The Pt(Cu)/C catalyst prepared using Cu electroless plating showed more pronounced intrinsic catalytic activity toward methanol oxidation than its counterparts and a similar mass activity when compared to the commercial catalyst. The observed trends were interpreted by interplay between mere surface area effects and modification of Pt electrocatalytic performance in the presence of Cu, both with respect to methanol oxidation and poisonous CO removal.     

Microwave-assisted synthesis and pulse electrodeposition of palladium nanocatalysts on carbon nanotube-based electrodes

The deposition of Pd nanoparticles prepared by microwave-assisted synthesis (MS) and pulse electrodeposition (PE) on networks of multiwall carbon nanotubes (CNTs) was investigated. The CNTs were grown directly on microscaled carbon paper using catalytic chemical vapor deposition. Both MS and PE methods enabled the quick formation of nanosized Pd particles over a CNT surface without any additional thermal reduction. Cyclic voltammetry and electrochemical impedance spectroscopy were used to examine the electrochemical behavior of the Pd catalysts. The Pd catalyst prepared with the MS method not only offers a higher active coverage for adsorption/desorption of hydrogen but also a more stable durability toward acid electrolytes when compared with that of the catalyst prepared with the PE method. The electrochemical surface area of the Pd catalyst was approximately 1.38 times than that of the Pt catalyst, which was also prepared with MS method. The equivalent series resistance for all the catalyst electrodes was kept between 2.07 and 2.25 Ω after potential cycling. Based on the results, the Pd catalyst is found to be a feasible alternative to the Pt catalyst because of its low cost, durability, and high catalytic activity.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Enhancement of electrocatalytic O<Subscript>2</Subscript> reduction on carbon nanotube-supported Pt alloys nanoparticles in gas diffusion electrodes

The use of Pt binary and ternary alloys prepared by alloying of Pt with transition metals, as catalysts for fabricating of gas diffusion electrodes (GDEs) is reported. Electrocatalytic properties of oxygen reduction reaction (ORR) were evaluated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. The morphology of the GDEs and elemental compositions of the Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The results indicate that the introduction of Pd and Cd as transition metals in Pt alloys provides fast ORR kinetics. The performance of GDEs with Pt–Pd alloy surfaces for ORR was also studied as a function of overall composition and surface atomic distribution of Pt alloys. The results also show that alloying of Pt with transition metals and various amounts of Pt and Pd in the binary catalysts has a large effect on the performance of GDEs for ORR.     

PtAu插层类水滑石对甲醇的电催化氧化

为提高直接甲醇燃料电池阳极催化剂抗CO中毒性能,降低成本,实现其大规模商业化,通过浸渍法和电化学还原法制备了一种新型PtAu纳米双金属催化剂,利用循环伏安扫描,交流阻抗测及计时电流测试研究不同摩尔比的PtAu插层水滑石材料对甲醇氧化的电催化性能。采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)、BET比表面积对催化剂的晶体结构,形貌等进行表征。结果表明,4种不同摩尔比的PtxAuy/MgAl-LDH催化剂均具有良好的层状结构特征,且BET比表面积均比商业型镁铝水滑石大。其中,Pt1Au0.5/MgAl-LDH催化剂对甲醇的电催化氧化具有最高的电流密度和最小的电荷转移阻力,且表现出了良好的抗CO中毒性能及稳定性。     

Low-temperature complete oxidation of BTX on Pt/activated carbon catalysts

The catalytic destruction of volatile organic compound (VOC) benefits from a low oxidation temperature due to less energy consumption. In this study, activated carbon-supported Pt catalysts were prepared for benzene, toluene and xylene (BTX) deep oxidation at below 200°C. Activated carbon can serve as a media for concentrating VOC. The carbon supports were heated to 400 or 800°C under N₂ flow and washed with HF acid to remove surface impurities and/or minerals. The 0.3 wt.% Pt/activated carbon catalysts were prepared by the incipient wetness method, followed by H₂ reduction at 300°C for 2 h. The catalytic oxidation was conducted with a BTX concentration ranging from 640 to 2000 ppmv in air at volume hour space velocity (VHSV) of approximately 21 000 h⁻¹. The light-off curves were very steep and the light-off temperatures ranged between 130 and 150°C, well below those of the Pt/Al₂O₃ catalyst. The oxidation activity was promoted because of a higher surface BTX concentration due to the adsorption capability of activated carbons. Moisture reduces the activity only slightly due to the hydrophobicity of activated carbon. Generally, the Pt catalysts with thermally-treated activated carbon had lower ignition temperatures. Experimental results indicated that high-temperature pretreatment of activated carbon could effectively increase the catalyst activity. Meanwhile, X-ray photoelectron spectroscopy (XPS)/secondary ion mass spectroscopy (SIMS) investigation revealed that the graphitized surface might play a role in catalytic activity. Finally, this work suggested a reaction mechanism based on the adsorption-migration of hydrocarbons to reveal the enhanced activity of activated carbon support.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

Variations in interfacial properties during cell conditioning and influence of heat-treatment of ionomer on the characteristics of direct methanol fuel cells

Variations in interfacial properties in the anode catalyst layer during cell conditioning were characterized, and influence of the heat-treatment of ionomer on the characteristics of direct methanol fuel cells was investigated in this work. The anode catalyst layer was made by mixing a solvent-substituted Nafion solution with unsupported Pt/Ru black and curing the mixture in an oven with an inert environment. Materials characterization (SEM and optical microscopy) and electrochemical characterization (cell polarization, anode polarization, electrochemical impedance spectroscopy, and CO-stripping cyclic voltammetry) were performed. During cell conditioning, the enhanced kinetics of MeOH electrochemical oxidation and severe limiting current phenomenon are due to the combination of variations in interfacial properties and swelling of ionomer in the anode catalyst layer over time. Ru oxides at the catalyst surface are reduced continuously during cell conditioning. The nearly constant integrated areas under the CO-stripping CV peaks and broadened peak shapes indicate a stable number of Pt/Ru bimetallic alloy surface sites, yet the surface composition distribution is broadened. Heat-treatment influences ionomer crystallinity, altering its swelling behavior and hence affecting the characteristics of the direct methanol fuel cell (DMFC) anode.