Microstructure effect of carbon nanofiber on electrocatalytic oxygen reduction reaction

Carbon nanofibers (CNFs) with controlled microstructures, i.e. platelet CNF (p-CNF), fish-bone CNF (f-CNF) and tube CNF (t-CNF), are synthesized, and their behaviors in electrocatalytic oxygen reduction reaction (ORR) in acid media are investigated in this paper. The physico-chemical properties of the CNFs are characterized by high resolution transmission electron microscope (HRTEM), N₂ adsorption–desorption and Raman spectrum. Cyclic voltammetry experiments show that the CNFs have higher ORR activities than graphite. The p-CNF, which has the highest ratio of edge atoms to basal atoms, demonstrates the most positive ORR onset potential and ORR peak potential. The f-CNF, which has the largest amounts of ORR active sites, exhibits the highest ORR peak current. The t-CNF demonstrates the most negative ORR onset potential, negative ORR peak potential, and the least ORR peak current, which is a result of the fewest catalytic active sites. Furthermore, the microstructures of CNFs can impact the reaction process. The ORR on p-CNF or f-CNF is controlled by diffusion, while the ORR on t-CNF is jointly controlled by surface reaction and diffusion.     

Stability and electrocatalytic activity for oxygen reduction in WC + Ta catalyst

Tantalum (Ta)-added tungsten carbide (WC) (WC+Ta) was examined in order to obtain surperior characteristics in stability and electrocatalytic activity for the oxygen reduction reaction (ORR) in acid electrolyte. The stability and the electrocatalytic activity of the WC+Ta catalyst were electrochemically investigated and compared to the pure WC. It was proved that the stability of the tungsten carbide was significantly increased by the addition of tantalum compared to the pure WC. The enhanced stability might be due to the formation of the W-Ta alloy in the WC+Ta catalyst. The reduction current of the WC+Ta catalyst for the ORR was observed at a potential of 0.8 V (versus dynamin hydrogen eletrode (DHE)) or less noble potential. This value was about 0.35 V higher than that of the pure WC. The enhanced electrocatalytic activity for the ORR might be caused by the presence of tungsten carbide, which exists on the surface and/or sub-surface.     

The electrochemical reduction of oxygen on zinc corrosion films in alkaline solutions

Kinetics of the O₂ reduction has been characterized on Zn corrosion films by Pt/Zn rotating ring-disc electrode (RRDE) and EIS methods. On zinc-oxide films a two-step reduction was identified in various buffer solutions of pH 10.5, while a small quantity of H₂O₂ intermediate could be detected. On the basis of results obtained from Pt/Zn and Pt/Pt RRDE experiments in solutions containing H₂O₂, it was further confirmed that the HO₂⁻ was reduced to OH⁻ through the zinc-oxide corrosion layer. Capacitance data of the zinc-oxide/electrolyte interface calculated from steady-state impedance diagrams measured at various cathodic potentials indicate the presence of a space charge layer of the semi-metallic ZnO. The solid-state reaction mechanism of HO₂⁻ disproportion with participation of Zn_i⁺ interstitials, oxygen ion vacancies of the non-stoichiometric Zn-oxide, and chemisorbed HO₂⁻ is discussed.     

氮掺杂科琴黑碳材料的制备及 电催化氧还原性能研究

氮掺杂碳材料是一种有应用前景的电催化氧还原催化剂。以尿素和三聚氰胺作为氮源,在氮气气氛下高温焙烧,制得两种氮掺杂科琴黑碳材料并将其用于电催化氧还原反应。使用X射线衍射仪（XRD）、X射线光电子能谱仪（XPS）、场发射扫描电子显微镜（FESEM）、比表面物理吸附分析仪等对氮掺杂前后的科琴黑的结构和形貌进行了分析。结果表明：氮掺杂之后科琴黑仍保持石墨结构,其形貌和比表面积均无明显改变。在XPS谱图上,氮掺杂后科琴黑上存在氮元素,其中以三聚氰胺为氮源比以尿素为氮源更容易得到吡啶氮。通过循环伏安法和线性扫描伏安法研究了3个样品的电催化氧还原性能。结果表明：氮掺杂能明显提高科琴黑的电催化氧还原性能,未掺杂的 科琴黑（AC）的半波电位为0.746 V,而以尿素和三聚氰胺为氮源掺杂后的科琴黑碳材料的半波电位分别提高到了 0.756 V（尿素-N/AC）和0.786 V（三聚氰胺-N/AC）。     

Electrochemical reduction of oxygen on palladium nanocubes in acid and alkaline solutions

The electrocatalytic properties of cubic palladium nanoparticles towards the electrochemical reduction of oxygen were studied in acid and alkaline solutions and compared with those of spherical nanoparticles and bulk Pd. The synthesised Pd nanoparticles were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrooxidation of pre-adsorbed CO was employed for cleaning the palladium catalyst surface. Oxygen reduction was studied using the rotating disk electrode (RDE) method and enhanced electrocatalytic activity of Pd nanocubes was revealed both in acid and alkaline solutions, which was attributed to the prevalence of Pd(1 0 0) facets. The mechanism of oxygen reduction on Pd nanoparticles was similar to that on bulk Pd, the first electron transfer being the rate-limiting step, and the reaction predominantly followed a four-electron pathway in both solutions.     

Enhancement of the rate of the hydrogen evolution reaction on polycrystalline silver in alkaline solutions

The changes produced by electrochemical treatments on the voltammograms, mainly those related to the enhancement of the rate of the hydrogen evolution reaction on silver in alkaline solutions are described.Voltammograms do not yield a steady profile, at least up to 7 h of cycling. The current enhancements are analysed in terms of a minor contribution due to a roughness factor increase and of a major contribution involving highly reactive silver centres.The apparent +1 reaction order with respect to OH⁻ ions for the hydrogen evolution reaction is interpreted through the participation of (OH⁻)_ad in the reaction pathway.     

SECM imaging of electrocatalytic activity for oxygen reduction reaction on thin film materials

The oxygen reduction reaction (ORR) on sputtered Pt thin films in acidic solution was successfully studied by scanning electrochemical microscopy (SECM) in a modified tip generation-substrate collection (TG-SC) mode. SECM images of ORR activity in different sample areas were obtained and it is shown that this TG-SC SECM technique can be used to screen electrocatalytic activity of continuous thin film samples efficiently and quickly for the ORR in an acidic medium. It is observed that this technique is not very sensitive to the tip-substrate separation within a certain range. The SECM images obtained are strongly dependent on the substrate potential. The advantages of this technique for studying ORR electrocatalysts are discussed.     

Electrodeposition of silver particles from alkaline aqueous solutions and their electrocatalytic activity for the reduction of nitrate, bromate and chlorite ions

A pulsed electrochemical method was used for the preparation of silver deposit onto glassy carbon surfaces from an alkaline medium containing ammonia ligand. The morphology of the silver deposit, obtained under pulsed and cyclic conditions was examined and compared using the scanning electron microscopy (SEM) technique. From the experimental evidences, it can be inferred that pulsed method induces a favourable dendritic growth of silver particles, while deposited films obtained under cyclic voltammetry appear formed of globular particles with well regular and defined surface geometry. The electroanalytical performance of the deposited silver, towards the electroreduction of NO₃⁻, BrO₃⁻ and ClO₂⁻ ions, using cyclic voltammetry and chronoamperometry technique, was evaluated. Interesting analytical results, in terms of sensitivity and temporal reproducibility, in alkaline medium and under constant applied potentials (i.e., −1.45 and −0.6 V vs. SCE), were obtained.     

Environmental remediation of aqueous nitrate solutions by photocatalytic reduction using Pd/NaTaO3 nanoparticles

NaTaO₃ nanoparticles were prepared by an ultrasonic method, and Pd was deposited onto the surface of NaTaO₃ via photoassisted deposition. The resulting samples were characterised by X-ray diffraction, ultraviolet and visible spectroscopy, photoluminescence emission spectroscopy, transmission electron microscopy, extended X-ray absorption fine structure analysis, and surface area measurements. The catalytic performance of the samples in the photoreduction of nitrate under visible light was determined. The UV–vis analysis indicated that a red shift occurred after Pd was loaded onto NaTaO₃. The maximum reduction efficiency was 100%, which was obtained using 0.6 wt% Pd/NaTaO₃ after a reaction time of 50 min.     

Nb–TiO2 supported platinum nanocatalyst for oxygen reduction reaction in alkaline solutions

Platinum based nanocatalyst at home made Nb–TiO₂ support was synthesized and characterized as the catalyst for oxygen reduction reaction in 0.1 mol dm⁻³ NaOH, at 25 °C. Nb doped TiO₂ catalyst support, containing 5% of Nb, has been synthesized by modified acid-catalyzed sol–gel procedure in non-aqueous medium. BET and X-ray diffraction (XRD) techniques were applied for characterization of synthesized supporting material. XRD analysis revealed only presence of anatase TiO₂ phase in synthesized support powder. Existence of any peaks belonging to Nb compounds has not been observed, indicating Nb incorporated into the lattice.Nb–TiO₂ supported Pt nanocatalyst synthesized, using borohydride reduction method, was characterized by TEM and HRTEM techniques. Platinum nanoparticles distribution, over Nb doped TiO₂ support, was quite homogenous. Mean particle size of about 4 nm was found with no pronounced particle agglomeration. Electrochemical techniques: cyclic voltammetry and linear sweep voltammetry at rotating disc electrode were applied in order to study kinetics and estimate catalytic activity of this new catalyst for the oxygen reduction reaction in alkaline solution. Two different Tafel slopes were found: one close to −90 mV dec⁻¹ in low current density region and other approximately −200 mV dec⁻¹ in high current density region, which is in good accordance with literature results for oxygen reduction at Pt single crystals, as well as Pt nanocatalysts in alkaline solutions. Similar specific catalytic activity (expressed in term of kinetic current density per real surface area) of Nb(5%)–TiO₂/Pt catalyst for oxygen reduction reaction in comparison with the carbon supported platinum (Vulcan/Pt) nanocatalyst, was found.     

Kinetics and electrocatalytic activity of nanostructured Ir-V/C for oxygen reduction reaction

Active, carbon-supported Ir-V nanoparticle catalysts have been synthesized by an ethylene glycol reduction method under controlled conditions at pH 10-13 and 120 °C, then further reduced at elevated temperature from 150 to 500 °C using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts have been characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). Ir nanoparticles, after modification with V, show a narrow particle size distribution in the range 0.5-4.5 nm, centered at 1.8 nm, and are uniformly dispersed on Vulcan XC-72. No particle agglomeration was observed, not even at high V loadings (V:Ir = 4:1 in atomic ratio). Investigation of the catalytic activity of the Ir-V/C by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV) employing a rotating disk electrode (RDE) has revealed that the presence of V may suppress the electrochemical oxidation of Ir and stabilize the Ir active centers. About six times higher kinetic current density was obtained for Ir-V/C compared to that of the pure Ir/C catalyst at 0.8 V versus RHE for the oxygen reduction reaction (ORR). The ORR in acid solution proceeds by an approximately four-electron pathway, through which molecular oxygen is directly reduced to water. The performance of a membrane electrode assembly (MEA) prepared with the most active 40% Ir-10% V/C as the cathode catalyst in a single proton-exchange membrane fuel cell (PEMFC) generated a maximum power density of 517 mW cm⁻² at 0.431 V and 70 °C, and 100 h of stable cell operation due to no loss of catalyst sites on the cathode.     

The kinetics of dissolved oxygen reaction in aqueous sodium dithionite solutions

The kinetics of the reaction between dissolved oxygen and sodium dithionite in alkaline aqueous solution was investigated in a stirred cell. For dithionite concentrations below 0.08 kmol/m³ and sodium hydroxide concentrations from 0.044 to 0.27 kmol/m³, the results show the reaction to be first order in dithionite and zero order in oxygen. The Arrhenius activation energy was determined to be 76.2 MJ/kmol in the temperature range 15 to 34°C. Addition of other electrolytes (sodium sulphate or potassium chloride) had no effect on the kinetics.     

Enhancement of oxygen reduction activity by sequential impregnation of Pt and Pd on carbon support

Two Pd-based PtPd bimetallic catalysts (mole ratio of Pt to Pd=1: 18) were prepared by co-impregnation (Pt-Pd/C) and sequential impregnation of Pt on Pd/C [Pt(Pd/C)] for the application to oxygen reduction reaction (ORR). The prepared bimetallic catalysts had lower ORR activities than Pt/C, while they showed largely enhanced activity compared to Pd/C. In particular, the extent of enhancement was found to be dependent on the surface composition. The observed mass and specific activities of Pt(Pd/C) were more than two times higher than those of Pt-Pd/C. The superior activity of Pt(Pd/C) observed from the performed studies was attributed to its Pt-rich surface.     

A highly active Pd coated Ag electrocatalyst for oxygen reduction reactions in alkaline media

We synthesized and characterized a highly active electrocatalyst for oxygen reduction reactions (ORRs) in alkaline media by coating carbon-supported silver nanoparticles with Pd (Pd@Ag/C) via a galvanic displacement method. The electrochemical measurements were carried out using an ultrathin film rotating disk electrode. Compared to the Pt/C electrocatalyst, the specific and mass activities of the Pd@Ag/C were enhanced by a factor of 3 and 2.5, respectively. The potentiostatic measurements showed that the Pd@Ag/C is less stable than the Pt/C at the potential of −0.1 V vs. Hg/HgO/OH⁻ in alkaline media. The Pd@Ag/C is insensitive to alcohol, and, as a cathode electrocatalyst of a direct alcohol fuel cell, can resist poisoning by the possible alcohol crossover from the anode.     

载体的碱处理对Pd/C催化剂电催化性能的影响

将活性炭放入质量分数为10%的NaOH溶液中进行预处理,然后将其与未处理的活性炭分别作为载体制备Pd/C催化剂。对比两种催化剂的电化学性能发现,预处理的活性炭所制备的Pd/C催化剂,在甲酸电催化氧化活性和稳定性方面好于未处理的活性炭所制备的Pd/C催化剂。     

Comparison of the reaction mechanisms of electrocatalytic oxygen reduction using transition metal thiospinels and chelates

The cathode process of oxygen reduction on thiospinels and other sulphides was examined by potentiodynamic, galvanostatic and rotating ring-disc electrode measurements. The highest activity was found with the sulphur compounds of cobalt and iron. The results are compared with those obtained with iron and cobalt melt chelates of the macrocyclic N₄ type (phthalocyanines, dibenzotetraazaannulenes) because knowledge is required about the reaction itself and the mechanism of the reduction of activity of the non-precious metal catalysts. The reaction scheme for the oxygen reduction is discussed and values of the rate constants of the electrochemical reactions are given.     

Ethylene glycol, 2-propanol electrooxidation in alkaline medium on the ordered intermetallic PtPb surface

The ethylene glycol and 2-propanol electrooxidation reaction was studied on carbon dispersed ordered intermetallic PtPb nanocatalysts in KOH solution. X-ray diffraction and X-ray photoelectron spectroscopy were used to characterize ordered intermetallic PtPb/C catalysts. The electrochemical behaviors for the ethylene glycol and 2-propanol electrooxidation reaction were measured in a thin film electrode by cyclic voltammetry, Tafel curves and electrochemical impedance spectroscopy. The results showed that in contrast with PtRu/C and Pt/C catalyst, ordered intermetallic PtPb/C had better electroactivity, and kinetic mechanism of PtPb/C is complex. Although the activity of electrocatalysts depends on many factors, such as modification of geometric and electronic structure by Pt-Pb interaction, crystalline size and so on. But the key factor for each electrooxidation reaction was different. For ethylene glycol electrooxidation, the effect of formation and desorption of poisonous species on activity of catalyst was very significant. For 2-propanol electrooxidation, the modification of geometric and electronic structures may be play a decisive role in the enhance activity of electrocatalyst.     

Enhancement of oxygen reduction by incorporation of heteropolytungstate into the electrocatalytic ink of carbon supported platinum nanoparticles

Nafion stabilized inks of Vulcan XC-72 supported platinum (20 wt.%) nanoparticles (Pt/XC-72) were utilized to produce electrocatalytic films on glassy carbon. The catalysts were modified (activated) with phosphododecatungstic acid H₃PW₁₂O₄₀ (PW₁₂). Comparison was made to bare (PW₁₂-free) electrocatalytic films. Electroreduction of dioxygen was studied at 25 °C in 0.5 mol dm⁻³ H₂SO₄ electrolyte using rotating disk voltammetry. For the same loading of platinum (≈95 μg cm⁻²) and for the approximately identical distribution of the catalyst, the reduction of oxygen at a glassy carbon electrode modified with the ink containing PW₁₂ proceeded at ca. 30-60 mV more positive potential (depending on the PW₁₂ content), and the system was characterized by a higher kinetic parameter (rate of heterogeneous electron transfer), when compared to the PW₁₂-free electrocatalyst. Gas diffusion electrodes with Pt/XC-72 supported on carbon paper (Pt loading 1 mg cm⁻²) were also tested. Under the same experimental conditions, while the exchange current density and the total resistance contribution to polarization components, computed from the galvanostatic polarization curves were found to be clearly higher and lower, respectively, for the ink modified with PW₁₂ relative to the unmodified system. The results demonstrate that addition of heteropolytungstatic acid (together with Nafion) enhances the electrocatalytic activity of platinum towards reduction of oxygen.