Improved electrocatalytic performance of Pd nanoparticles with size-controlled Nafion aggregates for formic acid oxidation

This work focuses on the effect of Nafion ionomer aggregation within the Pd catalytic electrode on electrocatalytic oxidation of formic acid. By a simple heat-treatment, the particle sizes of both Nafion ionomers in Nafion solution and congeries formed between Pd nanoparticles and Nafion ionomers in the catalyst ink decrease and their size distribution becomes narrow. Heat treatment of the catalyst ink leads to a significantly enhanced catalytic activity for formic acid oxidation on the Pd catalytic electrode. Such an enhancement is ascribed to the improvement in catalyst utilization verified by CO stripping voltammograms and to the decrease in charge-transfer resistance of oxidation reaction confirmed by impedance analysis. Typical XPS analysis shows that there are at least two kinds of Pd and S surface states within the catalytic electrode with the ink pre-heated at 25 °C and only one kind of Pd and S surface state at 80 °C, indicative of a better dispersion between Pd nanoparticles and smaller Nafion ionomers at a higher heat treatment temperature. Furthermore, the decrease in congeries size within the anode catalyst ink leads to a significant decrease in Nafion loading within the catalytic layer and a remarkable improvement in direct formic acid fuel cell's performance.     

Au/Pd core-shell nanoparticles with varied hollow Au cores for enhanced formic acid oxidation

A facile method has been developed to synthesize Au/Pd core-shell nanoparticles via galvanic replacement of Cu by Pd on hollow Au nanospheres. The unique nanoparticles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, and electrochemical measurements. When the concentration of the Au solution was decreased, grain size of the polycrystalline hollow Au nanospheres was reduced, and the structures became highly porous. After the Pd shell formed on these Au nanospheres, the morphology and structure of the Au/Pd nanoparticles varied and hence significantly affected the catalytic properties. The Au/Pd nanoparticles synthesized with reduced Au concentrations showed higher formic acid oxidation activity (0.93 mA cm^-2 at 0.3 V) than the commercial Pd black (0.85 mA cm^-2 at 0.3 V), suggesting a promising candidate as fuel cell catalysts. In addition, the Au/Pd nanoparticles displayed lower CO-stripping potential, improved stability, and higher durability compared to the Pd black due to their unique core-shell structures tuned by Au core morphologies.     

钛基纳米多孔钯电极的制备及对甲酸氧化的电活性

采用水热法,通过改变溶液组成制备了5种不同的钛基纳米多孔钯电极(Pd/PEG、Pd-EDTA/PEG、Pd-EDTA/HCHO、Pd/EG和Pd/HCHO)。扫描电镜图(SEM)分析表明,加入络合剂乙二胺四乙酸(EDTA)后,钯颗粒均匀,粒径明显减小,仅有60 nm左右。利用循环伏安法研究了甲酸在这些电极上的电催化氧化,发现在1.0 mol/L NaOH+0.5 mol/L HCOOH溶液中,加入EDTA且以甲醛作还原剂的电极(Pd-EDTA/HCHO)对甲酸氧化电流密度达132.00 mA/cm2,甲酸氧化的起始电位为-0.85 V,表明电催化活性优于其他电极。同时研究了Pd-EDTA/HCHO电极对不同浓度甲酸电催化氧化,结果表明,在一定甲酸浓度范围内,甲酸氧化的阳极电流密度随浓度的增加而增大。     

Sonochemical synthesis of tungsten carbide-palladium nanocomposites and their electrocatalytic activity for hydrogen oxidation reaction

Nanocomposites between β-WC and Pd nanoparticles supported on carbon are synthesized and their electrocatalytic properties for the hydrogen oxidation reaction have been investigated. The Pd nanoparticles are obtained by a chemical reduction reaction of PdCl₂ and the β-WC nanoparticles by a sonochemical decomposition of W(CO)₆ on Pd-loaded carbon followed by heat-treatment. Depending on the relative amounts of W to Pd, the Pd nanoparticles can be reacted with W to form Pd-W alloy nanoparticles. The Pd-W alloy, whose composition is estimated to have W less than 18 at.% based on its lattice parameter, lost most of the catalytic activity of Pd. On the other hand, the nanocomposite between β-WC and pure Pd shows an enhanced activity compared with that of Pd nanoparticles alone. This enhancement can be explained with the H⁺-spill-over to β-WC.     

Time-efficient microwave synthesis of Pd nanoparticles and their electrocatalytic property in oxidation of formic acid and alcohols in alkaline media

In this study time-efficient size controlled synthesis of stable Pd nanoparticles was carried out using microwave heating method by the decomposition of palladium chloride with glucose in aqueous medium using poly(ethyleneglycol) as capping agent. The benefit of the synthesis is that it was achieved in only 20 s. The synthesized Pd nanoparticles were characterized by UV–Visible spectroscopy, transmission electron microscopy, X-ray diffraction, and particle size analysis. The relative rates of electro-oxidation of formic acid, methanol and ethanol measured by cyclic voltammetry showed that efficiency of Pd nanoparticles as catalyst followed the order: formic acid < methanol < ethanol. The current–voltage characteristics improved with increase in either electrolyte (NaOH) or fuel concentrations but decreased with further increase in NaOH concentration.     

Electrosynthesis of Pd/Au hollow cone-like microstructures for electrocatalytic formic acid oxidation

Pd/Au hollow cone-like microstructures (HCMs) have been electrodeposited on indium tin oxide (ITO) using a two-step protocol, which involves the nucleation pulse and succedent constant potential reduction in the presence of metal precursors and polyvinylpyrrolidone (PVP). Scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscope (XPS) were used to characterize the Pd/Au HCMs. The electrochemical results (cyclic voltammetry and chronoamperometry) testify that the Pd/Au HCMs exhibit significantly higher electrocatalytic activity and stability for the oxidation of formic acid than that of Pd/Au solid microhemispheres (SMHs). These attractive features are attributable to the unique hollow structures of Pd/Au with much higher electrochemical active surface areas and the exposure of favorable planes. Our work points to a new path for the preparation of Pd/Au HCMs, which are promising as electrocatalysts in direct formic acid fuel cell (DFAFC).     

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.     

硼掺杂碳纳米管负载纳米Pd的甲酸电氧化性能

制备了高分散和窄尺寸分布的低载量(3.1%)钯纳米颗粒,负载在硼掺杂的碳纳米管上,与Pd/OCNTs(3.2%)催化剂相比,表现出了较好的甲酸电氧化性能。表面分析表明,Pd上的电子转移给B掺杂的碳纳米管,引起Pd的3d电子密度的提高,增强了其在甲酸氧化中的性能。     

硼掺杂碳纳米管负载纳米Pd的甲酸电氧化性能

制备了高分散和窄尺寸分布的低载量(3.1%)钯纳米颗粒,负载在硼掺杂的碳纳米管上,与Pd/OCNTs(3.2%)催化剂相比,表现出了较好的甲酸电氧化性能。表面分析表明,Pd上的电子转移给B掺杂的碳纳米管,引起Pd的3d电子密度的提高,增强了其在甲酸氧化中的性能。     

Glycerol-mediated synthesis of Pd nanostructures with dominant {111} facets for enhanced electrocatalytic activity

We report shape-controlled Pd nanoparticles (G-Pd NPs) synthesized by means of polyol process using glycerol as a reducing agent with poly(vinyl pyrrolidone) as a capping agent in aqueous solution. The G-Pd NPs consist of triangular and hexagonal plates and decahedron with dominant {111} facets confirmed by X-ray diffraction analysis and field-emission transmission electron microscopy. In cyclic voltammograms, the G-Pd NPs exhibit such an improved formic acid electrooxidation due to the metallic nanophases with the dominant {111} facets without any serious agglomeration in comparison with commercial Pd/C.     

Pt修饰的Pd/石墨烯纳米复合材料的制备及对乙二醇氧化反应的电催化活性

以氧化石墨(GO)和Pd(NO₃)₂为原料,通过化学还原法制备Pd纳米粒子-石墨烯(Pd/G)纳米复合材料,然后以H₂PtCl₆作为Pt前体,在Pd纳米粒子的表面恒电位沉积Pt,制备不同Pt负载量的Pd/G(Pt-Pd/G)电极.利用场发射扫描电镜(FE-SEM)、透射电镜(TEM)和X射线能谱仪(EDX)对材料的微观结构进行了表征和分析.结果显示石墨烯上的金属粒子分散均匀,平均粒径约7.2nm.电化学测试结果显示Pt-Pd/G电极对乙二醇电化学氧化反应具有良好的催化性能.当纳米粒子的Pt:Pd原子百分比为1:42时,其反应峰电流密度分别为Pd/G和Pt/G电极的3.0倍和2.7倍.少量的Pt沉淀可显著改进Pd/G电极的催化活性.本研究采用的修饰方法简单,修饰效果明显,可应用于其他金属纳米复合材料的异金属修饰.     

Synthesis of bimetallic PdAu nanoparticles for formic acid oxidation

In this work, a simple co-deposition strategy for the synthesis of carbon-supported Pd–Au alloy was reported. Our approach involves the co-reduction of Au and Pd ions using ethylene glycol and sodium citrate as the reducing and stabilizing reagents. Both alloy and non-alloy bimetallic Pd–Au nanoparticles are produced using a right rate-limiting strategy. For example, when ethylene glycol and sodium citrate are the limiting reagent with Au and Pd ions in excess, the synthesis environment favors preferential nucleation and growth of Au nanoparticles followed by deposition of Pd either as the shell of Au core or as separate Pd clusters. On the other hand, if the supply of metal ions (not the reducing reagents) limits the reaction, it creates a synthesis condition for Pd–Au alloy particles. The as-prepared Pd–Au alloys exhibit higher Pd-specific activities towards formic acid oxidation compared with the non-alloy counterpart or individual Pd catalyst and an easier removal of adsorbed oxygen species (e.g., O_ads or OH_ads) was observed from the surface of Pd–Au alloy with a higher content of Au.     

Carbon nanotube supported platinum-palladium nanoparticles for formic acid oxidation

Pt, Pd and Pt_xPd_y alloy nanoparticles (Pt₁Pd₁, Pt₁Pd₃, atomic ratio of Pt to Pd is 1:1, 1:3, respectively) supported on carbon nanotube (CNT) with high and uniform dispersion were prepared by a modified ethylene glycol method. Transmission electron microscopy images show that small Pt and Pt_xPd_y nanoparticles are homogeneously dispersed on the outer walls of CNT, while Pd nanoparticles have some aggregations and comparatively larger particle size. The average particle sizes of Pt/CNT, Pt₁Pd₁/CNT, Pt₁Pd₃/CNT and Pd/CNT obtained from the Pt/Pd (2 2 0) diffraction peaks in the X-ray diffraction patterns are 2.0, 2.4, 3.1 and 5.4 nm, respectively. With increasing Pd amount of the catalysts, the mass activity of formic acid oxidation reaction (FAOR) on the CNT supported catalysts increases in both cyclic voltammetry (CV) and chronoamperometry (CA) tests, although the particle size gets larger (thus, the relative surface area gets smaller). The CV study indicates a ‘direct oxidation pathway’ of FAOR occurred on the Pd surface, while on the Pt surface, the FAOR goes through ‘CO_ads intermediate pathway’. Pd/CNT demonstrates 7 times better FAOR mass activity than Pt/CNT (2.3 mA/mgPd vs. 0.33 mA/mgPt) at an applied potential of 0.27 V (vs. RHE) in the CA test.     

SnO2-C载体对Pd催化剂电催化性能的影响研究

以SnO₂和Cabot Vulcan XC-72为原料合成了SnO₂-C材料,并将其用作担载Pd纳米颗粒的载体,从而制备出Pd/SnO₂-C催化剂,并研究其在甲酸环境下的电催化氧化性能,在电化学测试中,催化剂的抗CO中毒能力以及活性有明显的提高,通过进一步优化SnO₂在载体材料中所占比例,当SnO₂占载体质量20%时,催化剂在甲酸中抗CO中毒能力以及活性达到最高。     

Vesicle-templating poly (3,4-ethylenedioxythiophene) hollow microspheres with enhanced electrocatalytic activity toward ascorbic acid oxidation

Poly(3,4-ethylenedioxythiophene) (PEDOT) hollow microspheres ranging from 50 to 950?nm are synthesized by chemically oxidative polymerization of 3,4-ethylenedioxythiophene using ammonium persulfate in the aqueous solution of cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfate (SDBS). Vesicles formed by CTAB and SDBS serve as templates for the formation of PEDOT hollow microspheres. The obtained PEDOT hollow microspheres were characterized by Fourier transform infrared spectroscopy, elemental analysis, X-ray photoelectron spectroscopy, and conductivity measurement. Compared to PEDOT granular particles, PEDOT hollow microspheres showed a more effective electrocatalytic activity in lowering the ascorbic acid oxidation potential.     

High efficient electrocatalytic oxidation of formic acid on Pt/polyindoles composite catalysts

Four novel composite catalysts have been developed by the electrodeposition of Pt onto glassy carbon electrode (GCE) modified with polyindoles: polyindole, poly(5-methoxyindole), poly(5-nitroindole) and poly(5-cyanoindole). As-formed composite catalysts are characterized by SEM, XRD and electrochemical analysis. Compared with Pt nanoparticles, respectively, deposited on the bare GCE and on the GCE modified with polypyrrole, the four newly developed composite catalysts exhibit higher catalytic activity towards formic acid electrooxidation by improving selectivity of the reaction via dehydrogenation pathway and thus mostly suppressing the generation of poisonous CO_ads species. The enhanced performance is proposed to come from the synergetic effect between Pt and polyindoles and the increase of electrochemical active surface area (EASA) of Pt on polyindoles.     

Electrocatalytic properties of Pd clusters on Au nanoparticles in formic acid electro-oxidation

Pd clusters were formed on highly dispersed Au nanoparticles (∼3.5 nm in diameter) using a seed-mediated growth process. The structural information and electrocatalytic activities of these Pd clusters on Au nanoparticles were confirmed by high-resolution-transmission-electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The resulting nanoparticles, which had a uniform size (<5 nm in diameter), were highly dispersed on carbon particles, and Pd clusters (<0.44 nm in size, <2 atomic layers) were formed selectively on Au nanoparticles. XPS results show that the Pd 3d_5/2 peak shifted to lower binding energies and that the amount of surface oxide decreased as the Pd content was decreased on the Au nanoparticles. In formic acid electro-oxidation, these Pd clusters exhibit enhanced electrocatalytic activity relative to that of carbon-supported Pd nanoparticles. These results may be due to the modified electronic and geometric structure of the Pd clusters on the Au nanoparticle substrate.     

Enhanced electrocatalytic activity for the oxidation of liquid fuels on PtSn nanoparticles

PtSn nanoparticles with different Pt/Sn ratio have been prepared by a chemical reduction method. XRD data indicate that Sn atom is introduced into the Pt lattice. Their electrocatalytic activity is evaluated using cyclic voltammetry (CV), differential electrochemistry mass spectrometry (DEMS), and rotating disk electrode (RDE) experiments. The Pt₉Sn₁ nanoparticles exhibit higher electrocatalytic activity than commercial Pt nanoparticles (E-TEK) for the oxidation of ethanol. The rate constants for the oxidation of formic acid, formaldehyde, methanol, ethanol, glycol, and glycerol on Pt₉Sn₁ electrocatalyst are much higher than that on Pt. The results indicate that Pt₉Sn₁ is an excellent electrocatalyst for the oxidation of liquid fuels. The activation energy studies show that the higher electrocatalytic activity of PtSn catalyst can be ascribed to the bifunctional mechanism, instead of the dilation of Pt crystal structure.     

Graphene-supported platinum and platinum-ruthenium nanoparticles with high electrocatalytic activity for methanol and ethanol oxidation

In this study, Pt and Pt-Ru nanoparticles were synthesized on graphene sheets and their electrocatalytic activity for methanol and ethanol oxidation was investigated. Experimental results demonstrate that, in comparison to the widely-used Vulcan XC-72R carbon black catalyst supports, graphene-supported Pt and Pt-Ru nanoparticles demonstrate enhanced efficiency for both methanol and ethanol electro-oxidations with regard to diffusion efficiency, oxidation potential, forward oxidation peak current density, and the ratio of the forward peak current density to the reverse peak current density. For instance, the forward peak current density of methanol oxidation for graphene- and carbon black-supported Pt nanoparticles is 19.1 and 9.76 mA/cm², respectively; and the ratios are 6.52 and 1.39, respectively; the forward peak current density of ethanol oxidation for graphene- and carbon black-supported Pt nanoparticles is 16.2 and 13.8 mA/cm², respectively; and the ratios are 3.66 and 0.90, respectively. These findings favor the use of graphene sheets as catalyst supports for both direct methanol and ethanol fuel cells.