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.     

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.     

Preparation of high performance Pd catalysts supported on untreated multi-walled carbon nanotubes for formic acid oxidation

Due to the inherent inertness of carbon nanotubes (CNTs), one of the most significant challenges in the preparation of CNT-supported catalysts is achieving a uniform deposition of nanoparticles on the surface of the nanotubes. In this paper, we report on the preparation and characterization of Pd nanoparticles supported on untreated multi-walled carbon nanotubes (MWCNTs), synthesized in the presence of glutamate. The results of Raman spectroscopy revealed that this synthetic procedure does not have a detrimental effect on the surface structure of MWCNTs. Transmission electron microscopy (TEM) measurements indicated that the dispersion of Pd nanoparticles on untreated-MWCNTs in the presence of glutamate were uniform, and a narrow particle size was observed. X-ray diffraction (XRD) patterns indicated that the Pd/MWCNT catalyst possessed a face-centered cubic crystal structure. Cyclic voltammetry and chronoamperometry tests demonstrated that the obtained Pd/MWCNT catalyst displayed superior electrocatalytic activity and stability in formic acid oxidation, as compared to both a Pd/MWCNT catalyst synthesized without glutamate and a Pd catalyst on acid-oxidized MWCNTs, under otherwise identical experimental conditions. These results indicate that the catalyst developed in this study is a superior candidate for direct formic acid fuel cells (DFAFCs).     

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.     

络合还原法制备的Pd/C催化剂对甲酸氧化的电催化性能

分别以硼氢化钠和乙二醇为还原剂,经络合还原法制备了炭载钯（Pd/C）催化剂。透射电镜（TEM）和X射线粉末衍射谱（XRD）结果表明,以乙二醇为还原剂制备的Pd/C催化剂中Pd粒子具有较小的粒径、均匀的粒径分布和较大的相对结晶度,Pd粒子的平均粒径和相对结晶度分别为4.2±2 nm和1.88。电化学测试结果显示,以乙二醇为还原剂制备的Pd/C催化剂具有较大的电化学活性面积,对甲酸氧化表现出较高的电催化活性和稳定性。     

Activity of a novel titanium-supported bimetallic PtSn/Ti electrode for electrocatalytic oxidation of formic acid and methanol

Bimetallic platinum–tin nanoparticles were co-deposited on a titanium surface using a simple one step hydrothermal method process. The electrochemical catalytic activity of this titanium-supported nanoPtSn/Ti electrode towards the oxidation of formic acid and methanol in 0.5 M H₂SO₄ was evaluated by voltammetric techniques, chronoamperometric responses and electrochemical impedance spectra (EIS). According to the cyclic voltammograms of the oxidation of both formic acid and methanol, the nanoPtSn/Ti presents high anodic current densities and low onset potentials. Potential-time transient measurements show that the nanoPtSn/Ti exhibits high steady-state current densities for the oxidation of both formic acid and methanol. The EIS data indicate that the nanoPtSn/Ti presents very low electrochemical impedance values, showing that for the oxidation of both formic acid and methanol, low charge transfer resistances are present on the nanoPtSn/Ti catalyst. This confirms the high electrocatalytic activity of the nanoPtSn/Ti for the formic acid and methanol oxidation.     

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.     

Enhanced electrocatalytic performance of cyano groups containing conducting polymer supported catalyst for oxidation of formic acid

A new palladium (Pd) based catalyst was developed using poly(diphenylamine-co-3-aminobenzonitrile) (P(DPA-co-3ABN)) as the new catalyst support. The sizes, distribution and stability of Pd nanoparticles (Pd NPs) are strongly influenced by the cyano group (–CN) present in P(DPA-co-3ABN). Field emission scanning electron microscopy image and energy dispersive x-ray analysis revealed good dispersion of Pd NP onto P(DPA-co-3ABN) matrix. The electrocatalytic activity of P(DPA-co-3ABN)/Pd catalyst electrode (CE) was investigated in terms of formic acid (FA) electro oxidation. The onset potential and catalytic current for the electro oxidation of FA are higher at P(DPA-co-3ABN)/Pd-CE as compared to PdNPs loaded pristine PDPA catalyst electrode (PDPA/Pd-CE). P(DPA-co-3ABN)/Pd-CE exhibited 18 time higher electrocatalytic current than PDPA/Pd-CE for oxidation of FA.     

A catalytic study of formic acid oxidation on preferentially oriented platinum electrodes

The oxidation of formic acid was examined by cyclic voltammetry and chronoamperometry in order to determine the rate of catalytic activity (reaction turnover) as a function of surface crystallography on preferentially oriented (electrochemically modified) platinum electrodes. The resulting turnover rates indicated a maximum fourfold current enhancement for an approximately 60% (111)-oriented surface versus a polycrystalline surface, suggesting that preferentially oriented electrodes are of potential practical significance.     

Electrochemical preparation of epinephrine/Nafion chemically modified electrodes and their electrocatalytic oxidation of ascorbic acid and dopamine

Two types of epinephrine and cyclized epinephrine quinone films have been prepared using cyclic voltammetry from the epinephrine in the strong acidic solutions and neutral aqueous solutions over different scanning potential ranges. The cyclic voltammogram of the epinephrine film is characterized by one redox couple at about +0.5 V (versus Ag|AgCl) and cyclized epinephrine quinone film exhibits one redox couples at about −0.15 V (versus Ag|AgCl) .In addition to cyclic voltammetry and an electrochemical quartz crystal microbalance (EQCM) were used to study the growth mechanism of the epinephrine and cyclized epinephrine quinone molecules. The electrocatalytic oxidation of catecholamines (dopamine and norepinephrine) and also ascorbic acid were investigated in acidic aqueous solutions using epinephrine films. The rotating ring-disk electrode technique was used to investigate the mechanism of electrochemical oxidation of dopamine and ascorbic acid.     

Sonochemical synthesis of Pt-doped Pd nanoparticles with enhanced electrocatalytic activity for formic acid oxidation reaction

Pt-doped Pd nanoparticle catalysts (Pd _n Pt, n is 12, 15 and 19) supported on carbon were synthesized by an ultrasound assisted polyol method. The catalysts were characterized by X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The electrochemical activity of the electrocatalysts was investigated in terms of formic acid oxidation reaction (FAOR) at low concentration of formic acid in 0.1 M perchloric acid at room temperature. Formic acid oxidation on the Pd _n Pt/C commences at lower potential than a commercial Pt/C. Pd₁₉Pt/C catalyst showed the highest catalytic activity in FAOR compared to that of other catalysts. The obtained electrochemical results from voltammograms indicate that Pt-doped Pd catalysts can be a promising candidate for the anode material in direct formic acid fuel cells. The synthesis procedure is not only a very facile route but also a mass producible method for preparing carbon supported alloy nanoparticles.     

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.     

Boosting the performance of Pt electro-catalysts toward formic acid electro-oxidation by depositing sub-monolayer Au clusters

CO poisoning is the main obstacle to the application of Pt nanoparticles as anode catalysts in direct formic acid fuel cells (DFAFCs). Significant types of Pt alloys have been investigated, which often demonstrate evidently improved catalytic performance governed by difference mechanisms. By using a well-known electrochemical technique of under potential deposition and in situ redox replacement, sub-monolayer Au clusters are deposited onto Pt nanoparticle surfaces in a highly controlled manner, generating a unique surface alloy structure. Under optimum conditions, the modified Pt nanoparticles can exhibit greatly enhanced specific activity (up to 23-fold increase) at potential of −0.2 V vs. MSE toward formic acid electro-oxidation (FAEO). Interestingly, the mass specific activity can also be improved by a factor of 2.3 at potential of −0.35 V vs. MSE although significant amount of surface Pt atoms are covered by the overlayer Au clusters. The much enhanced catalytic activity can be ascribed to a Pt surface ensemble effect, which induces change of the reaction path. Moreover, the sub-monolayer Au coating on the surface also contributes to the enhanced catalyst durability by inhibiting the Pt oxidation. These results show great potential to rationally design more active and stable nanocatalysts by modifying the Pt surface with otherwise inactive materials.     

Highly dispersed carbon-supported Pd nanoparticles catalyst synthesized by novel precipitation–reduction method for formic acid electrooxidation

The highly dispersed and ultrafine carbon-supported Pd nanoparticles (Pd/C) catalyst is synthesized by using an improved precipitation–reduction method, which involves in Pd^II → PdO·H₂O → Pd⁰ reaction path. In the method, palladium oxide hydrate (PdO·H₂O) nanoparticles (NPs) with high dispersion is obtained easily by adjusting solution pH in the presence of 1,4-butylenediphosphonic acid (H₂O₃P-(CH₂)₄-PO₃H₂, BDPA). After NaBH₄ reduction, the resulting Pd/C catalyst possesses high dispersion and small particle size. As a result, the electrochemical measurements indicate that the resulting Pd/C catalyst exhibits significantly high electrochemical active surface area and high electrocatalytic performance for formic acid electrooxidation compared with that prepared by general NaBH₄ reduction method.     

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).     

不同体系Pd/C催化剂的制备及对甲酸的电催化氧化

采用不同体系制备了碳载Pd催化剂(Pd/C),发现在乙二醇体系中制备的Pd/C催化剂对甲酸氧化具有最负的峰电位和最低的起始氧化电位,Tafel斜率最小为155mV,并且在1h的计时电流曲线测试表明,用乙二醇体系制备的Pd/C-3催化剂具有较高的稳定电流。TEM结果可以看出,用乙二醇体系制备的Pd/C催化剂Pd粒子在活性碳表面分散得最好,Pd粒径的大小约为4～5nm。     

Electrooxidation of methanol and formic acid on PtCu nanoparticles

Improving the catalytic activity of the anode catalyst is an important task in direct methanol and formic acid fuel cell development. In the present work, catalytic activity of shape-controlled PtCu nanoparticles toward methanol and formic acid oxidation was investigated. The results show that the addition of Cu to Pt increases the catalytic activity of both reactions. In addition, the shape of PtCu nanoparticles plays an important role on improving the reactivity of both reactions. Cubic PtCu nanoparticles are more active for methanol oxidation while spheres are better for formic acid oxidation. The present study demonstrates controlling shape of Pt alloy catalysts is an effective way of improving catalytic activity. Likely mechanisms of the activity enhancement are briefly discussed.     

Modification of Pt nanoelectrodes dispersed on carbon support using irreversible adsorption of Bi to enhance formic acid oxidation

In this work, formic acid oxidation on Pt nanoelectrodes modified by irreversible adsorption of Bi is presented. The coverage of Bi, as measured by voltammetry and X-ray photoelectron spectroscopy, was controlled from 0.05 to 0.25. Chronoamperometric measurement of the catalytic activities of the Bi-modified Pt nanoelectrodes revealed that the catalytic enhancement depended on oxidation potential and Bi coverage: elemental Bi in the potential range from −0.1 V to 0.6 V enhanced formic acid oxidation by factor of 4, while partially oxidized Bi in the potential range from 0.3 V to 0.7 V increased by factor of 8. The enhancement in the latter potential range was effective only when the Bi coverage was more than 0.18. Single cell performance of the Pt nanoelectrodes modified by Bi increased by factor of 2-3, depending on operation conditions such as formic acid concentration, temperature, and humidity in the feeding gas into cathode. When the Bi coverage was more than 0.18, the single cell performances were nearly identical. Based on measurement of adsorbed CO and catalytic poison from formic acid, an oxidation path not involving catalytic poison in the potential range from 0.5 V to 0.7 V is discussed in detail.     

Pd nanoparticles supported on carbon-modified rutile TiO2 as a highly efficient catalyst for formic acid electrooxidation

In this article, Pd nanoparticles supported on carbon-modified rutile TiO₂ (CMRT) as a highly efficient catalyst for formic acid electrooxidation were investigated. Pd/CMRT catalyst was synthesized by using liquid phase reduction method in which Pd nanoparticles was loaded on the surface of CMRT obtained through a chemical vapor deposition (CVD) process. Pd/CMRT shows three times the catalytic activity of Pd/C, as well as better catalytic stability towards formic acid electrooxidation. The enhanced catalytic property of Pd/CMRT mainly arises from the improved electronic conductivity of carbon-modified rutile TiO₂, the dilated lattice constant of Pd nanoparticles, an increasing of surface steps and kinks in the microstructure of Pd nanoparticles and slightly better tolerance to the adsorption of poisonous intermediates.     

The enhancement effect of MoOx on Pd/C catalyst for the electrooxidation of formic acid

Molybdenum oxide (MoO_x) was added to a Pd/C catalyst using a novel two-step procedure. The enhancement effect of MoO_x on Pd/C catalyst for the electrooxidation of formic acid was verified by electrochemical experiments. Compared to the Pd/C catalyst, the experimental results showed that the addition of MoO_x could significantly enhance the electrocatalytic performances for the electrooxidation of formic acid. Significant improvements in electrocatalytic activity and stability were primarily ascribed to the effect of MoO_x on the Pd catalyst. In addition to the large specific surface area, the hydrogen spillover effect is speculated to have accelerated the electrooxidation rate of formic acid in the direct pathway.