Platinum-alloy nanostructured thin film catalysts for the oxygen reduction reaction

In an effort to study advanced catalytic materials for the oxygen reduction reaction (ORR), a number of metallic alloy nanostructured thin film (NSTF) catalysts have been characterized by rotating disk electrode (RDE). Optimal loadings for the ORR and activity enhancement compared to conventional carbon supported nanoparticles (Pt/C) were established. The most efficient catalyst was found to be PtNi alloy with 55 wt% of Pt. The enhancement in specific activity is more than one order of magnitude, while the improvement factor in mass activity is 2.5 compared to Pt/C. Further lowering of the platinum to nickel ratio in NSTF catalysts did not lead to increased mass activity values.     

In situ synthesis of Pt nanoparticles in hyperbranched thin film for electrocatalytic reduction of dioxygen

Hyperbranched thin film (HTF) with amino and imino groups, which can accommodate PtCl₆²⁻, was synthesized with small organic molecules on gold substrate based on SN₂ displacement reaction. Platinum nanoparticles (Pt_NPs) were in situ synthesized by electrochemically reduction of precursors, PtCl₆²⁻, within HTF. The prepared films were characterized by X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). It was confirmed that densely packed Pt_NPs were prepared with a narrow size distribution. EIS indicated that HTF inhibits electron transfer slightly while Pt nanoparticles in the HTF enhanced the electron-transfer ability greatly. Cyclic voltammetry (CV) indicated that HTF containing Pt_NPs exhibited a remarkable electrocatalytic activity for the electrochemical reduction of dioxygen. The quantity of Pt_NPs could be expediently controlled by the thickness of HTF. So the catalytic ability can be tailored correspondingly.     

Heat treatment effect of Pt-V/C and Pt/C on the kinetics of the oxygen reduction reaction in acid media

This work studies the heat treatment effect of carbon-dispersed platinum and platinum-vanadium alloys on the kinetics of the oxygen reduction reaction (ORR) in acid medium. The catalyst powders were subjected to heat treatments at three temperatures for 1 h. The electronic and structural features of the materials were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The XANES results for the oxidized state composites showed an increase of the Pt 5d band occupancy with increased heat treatment temperature for the Pt/C catalyst, while no changes were noted for Pt-V/C for the same treatments. The electrochemical characteristics for the ORR were investigated by cyclic voltammetry and state-state polarization measurements. The results showed that the ORR takes place by the multi-electronic charge transfer process, following a four electron mechanism. The kinetics of the ORR was evaluated using Tafel diagrams. It was observed that the ORR activity of the Pt/C and Pt-V/C is enhanced with the increase of the heat treatment temperature. The catalytic activity of the materials was analyzed in terms of the electronic and structural properties of Pt in the metallic particles.     

Pt decorating of PdNi/C as electrocatalysts for oxygen reduction

A low-cost and high performance catalyst consisting of Pt decorating PdNi/C (Pt-PdNi/C) for oxygen reduction is prepared by a two-stage route. The characterization techniques considered are X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX) technique. The results show that the Pt-PdNi/C catalyst has an average diameter of ca. 5 nm. The electrochemical activity for the ORR is evaluated from steady state polarization measurements, which are carried out in an ultra-thin layer rotating disk electrode (RDE). The RDE tests show that the Pt-PdNi/C catalyst has the highest ORR activity compared to pure Pt/C, Pd/C and PdNi/C catalysts. High electrocatalytic activities could be attributed to the synergistic effect between Pt and PdNi.     

Reconsideration of the quantitative characterization of the reaction intermediate on electrocatalysts by a rotating ring-disk electrode: The intrinsic yield of H2O2 on Pt/C

To solve the problem of the catalyst-loading-effect on quantifying the reaction intermediates on the surface of electrocatalysts with a rotating ring-disk electrode, we studied the formation of hydrogen peroxide in the oxygen reduction reaction on Pt/C with various sample loadings and then proposed an extrapolation model for measuring the intrinsic yield of H₂O₂, which can quantitatively reflect the characteristics of the surface of a given catalyst. In the extrapolation model, the catalyst loading effect can be compensated by taking the catalyst loading-dependent probability of the re-adsorption + further reaction of the desorbed H₂O₂ into consideration. The core concept in this extrapolation model is that the probability of the re-adsorption + reaction of the desorbed H₂O₂ becomes zero if there is no other active site available (i.e., at the extrapolated hypothetical point of zero catalyst loading) for re-adsorption of the desorbed H₂O₂. The intrinsic yield of H₂O₂ by extrapolation was much higher than that measured by the conventional model, in which the re-adsorption + reaction of the desorbed H₂O₂ is not considered, and thus the catalyst loading-dependent apparent yield of H₂O₂ does not properly reflect the intrinsic characteristics of the surface of a given catalyst.     

Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: Scale-up synthesis, structure and activity of Pt shells on Pd cores

We have established a scale-up synthesis method to produce gram-quantities of Pt monolayer electrocatalysts. The core-shell structure of the Pt/Pd/C electrocatalyst has been verified using the HAADF-STEM Z-contrast images, STEM/EELS, and STEM/EDS line profile analysis. The atomic structure of this electrocatalyst and formation of a Pt monolayer on Pd nanoparticle surfaces were examined using in situ EXAFS. The Pt mass activity of the Pt/Pd/C electrocatalyst for ORR is considerably higher than that of commercial Pt/C electrocatalysts. The results with Pt monolayer electrocatalysts may significantly impact science of electrocatalysis and fuel-cell technology, as they have demonstrated an exceptionally effective way of using Pt that can resolve problems of other approaches, including electrocatalysts’ inadequate activity and high Pt content.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Electrocatalytic behavior of thin Co-Te-O films in oxygen evolution and reduction reactions

Co-Te-O catalytic films, obtain by vacuum co-evaporation of Co and TeO₂ are investigated as electrocatalysts for oxygen reactions in alkaline media. Bifunctional gas-diffusion oxygen electrodes (gde) are prepared by direct deposition of catalyst films on gas-diffusion membranes (gdm) consisting of hydrophobized carbon blacks or hydrophobized “Ebonex” (suboxides of titanium dioxide). Thus obtained electrodes with different atomic ratio R_Co/Te of the catalyst, treated at different temperatures were electrochemically tested by means of cyclic voltammetry and steady-state voltammetry. It is shown that the electrodes exhibit high catalytic activity toward oxygen evolution and reduction reaction despite very low catalyst loading of about 0.05-0.5 mg cm⁻².     

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.     

Homogeneous Co3O4 film electrode with enhanced oxygen evolution electrocatalysis via surface reduction

Homogeneous NaBH_4-reduced Co_O_4 thin film electrodes with enhanced oxygen evolution electrocatalysis were obtained via a controlled-synthesis route.Firstly CoOx colloids were synthesized via ethylene glycol solvothermal method and cast on conductive glass substrates.The oxygen evolution reaction(OER) electrocatalysis of these asprepared Co_3O_4 thin films were then significantly enhanced via a simple surface reduction by NaBH_4 solution.The OER catalytic performance of the NaBH_4-reduced thin films was strongly dependent on the NaBH_4 concentration.The use of NaBH_4-reduced thin film electrodes for OER in alkaline solution supported higher current density and consequently negative shifts of the onset potential compared to that of the pristine.The optimal B_(12.5,20)-Co_3O_4 thin films exhibited excellent OER catalytic performances:At the current density of 10 mA·cm~(-2),a low overpotential of 365 mV and a small Tafel slope of 59.0 mV·dec~(-1) were observed.In addition,these B_(12.5,20)-Co_3O_4 thin film electrodes possessed good stability that can well recover its OER performance in a 24-h chronoamperometric stability test.     

电沉积Co-Mo合金薄膜的结构和热稳定性

通过电沉积,在铜基体上制备了Co-Mo合金薄膜.讨论了薄膜组成与结构以及非晶合金的晶体结构与热处理温度的关系.测定了薄膜磁性能(饱和磁化强度和矫顽力)随热处理温度变化的关系曲线.结果表明,薄膜中钼含量(质量分数)为6.05%～30.03%时,镀态Co-Mo合金薄膜具有非晶态结构;经连续升温到400℃并热处理1.5 h后,Co-Mo非晶态合金发生晶化,且随着薄膜中钼含量的增加,薄膜的晶化温度提高,热稳定性增强;在较高温度(高于500 ℃)下热处理后,Co-Mo非晶态合金晶化,并析出单一的hcp-Co相;热处理后,Co-Mo合金薄膜的软磁性变差.     

游离TDI和薄膜蒸发

降低聚氨酯涂料中游离TDI最有效的方法是采用薄膜蒸发器，文章对其结构、原理及其国内生产厂均作了简要叙述。     

Oxygen reduction reaction on carbon-supported CoSe2 nanoparticles in an acidic medium

We investigated the effect of CoSe₂/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H₂O₂ production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe₂ nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H₂O₂ production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe₂ loading on the disk electrode. H₂O₂ production also decreased with increasing catalytic site density, a phenomenon related to the CoSe₂ loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe₂/C nanoparticles at 22 μg cm⁻², we determined that the transfer process involves about 3.5 electrons.     

Preparation of Pt/mesoporous carbon (MC) electrode catalyst and its reactivity toward oxygen reduction

Highly structurally controllable mesoporous carbon (MC) support was synthesized through the self-organization of surfactants and carbon precursors, followed by carbonization. Then, Pt catalysts were successfully deposited on MC, in order to create a model of an ideal triple phase boundary in such a nano-space. The resulting Pt/MC catalysts showed better oxygen reduction reactivity with the existence of Nafion^® than without that. Handling of ionomers is a key to develop an ideal triple phase boundary within the pores of MC. Depending on the solvent where Nafion^® was diluted, the reactivity toward oxygen reduction was different. Due to its hydrophobic pores of MC, Nafion^® diluted by more hydrophobic solvent was able to access to the pores easily. As a result, Pt inside the mesopores was efficiently used. Furthermore, by changing a Pt precursor, oxygen reduction current started to increase at more positive potential, indicating the enhanced activity.     

光诱导纳米二氧化钛薄膜亲水性的研究

用溶胶-凝胶法制备了锐钛矿型纳米二氧化钛薄膜,并用XRD分析了样品的晶相和热处理温度的关系,用接触角仪测定了其亲水性。纳米二氧化钛薄膜的接触角经紫外光照射后减小,通过120min的紫外光照,薄膜的水接触角几乎降为0°,表现出超亲水性。     

Influence of carbon support on the performance of platinum based oxygen reduction catalysts in a polymer electrolyte fuel cell

Novel carbons from the Sibunit family prepared via pyrolysis of hydrocarbons [Yermakov YI, Surovikin VF, Plaksin GV, Semikolenov VA, Likholobov VA, Chuvilin AL, Bogdanov SV (1987) React Kinet Catal Lett 33:435] possess a number of attractive properties for fuel cell applications. In this work Sibunit carbons with BET surface areas ranging from ca. 20 to 420 m² g⁻¹ were used as supports for platinum and the obtained catalysts were tested as cathodes in a polymer electrolyte fuel cell. The metal loading per unit surface area of carbon support was kept constant in order to maintain similar metal dispersions (∼0.3). Full cell tests revealed a strong influence of the carbon support texture on cell performance. The highest mass specific activities at 0.85 V were achieved for the 40 and 30 wt.% Pt catalysts prepared on the basis of Sibunit carbons with BET surface areas of 415 and 292 m² g⁻¹. These exceeded the mass specific activities of conventional 20 wt.% Pt/Vulcan XC-72 catalyst by a factor of ca. 4 in oxygen and 6 in air feed. Analysis of the I–U curves revealed that the improved cell performance was related to the improved mass transport in the cathode layers. The mass transport overvoltages were found to depend strongly on the specific surface area and the texture of the support.     

Additive treatment effect of TiO2 as supports for Pt-based electrocatalysts on oxygen reduction reaction activity

In this study, we investigated the additive treatment effect of TiO₂ as alternative support materials to common carbon black for Pt-based electrocatalysts on electrocatalytic activity for oxygen reduction reaction (ORR). The shape of TiO₂ was varied by hydrothermal treatment with various additives, such as urea, thiourea, and hydrofluoric acid. From the results of transmission electron microscopy (TEM) images and ultraviolet-visible spectroscopy (UV-vis) spectra, it was identified that the morphology of hydrofluoric acid (HF)-treated TiO₂ was changed into a round shape having lower aspect ratio than other samples, and its band gap was decreased. Notably, the electronic state of HF-treated TiO₂ support was changed into highly reduced (electron rich) state which led to the increase of ORR activity, compared to other samples treated with different additives or before treatment. The electrocatalytic characteristics changes after treatment with various additives were investigated by using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), cyclic voltammograms (CV), and rotating disk electrode (RDE) techniques.     

Enhancing the temperature coefficient of resistance of Pt thin film resistance-temperature-detector by short-time annealing

In this work, we report the influence of annealing time on the electrical properties of Pt thin film resistance-temperature-detectors (RTD) with Ti adhesion layers. It is found that the temperature coefficient of resistance (TCR) of the Pt thin film RTD strongly depends on the air annealing time. Increasing the annealing durations from 1, 3, 10 to 60 min at 900 °C, the TCR tends to rise up firstly, and then drops down. A maximum TCR of 3.2 × 10⁻³/°C at 25 °C is achieved in the RTD annealed for 3 min, which is larger than most of other reported values. It is believed that the annealing time of 3 min may be sufficient to enlarge the grain size and to reduce the lattice defects, giving rise to the maximum TCR by decreasing the resistance at 25 °C. On the contrary, prolonging the annealing duration causes the interdiffusion and oxidation Ti significantly, which has been clearly evidenced by the depth analyses of X-ray photoelectron spectroscopy. Such interdiffusion and oxidation of Ti reduces the TCR by increasing the resistance.     

Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles

In this work pyrolyzed porphyrins were investigated for oxygen reduction electrocatalysis. Pyrolysis of non-supported cobalt and iron tetraphenylporphyrins in the temperature range of 500-800 °C generates high surface area catalysts with high degree of exposure of active sites to the reacting species. This is achieved through templating porphyrins on fumed amorphous silica that is removed after pyrolysis by etching with concentrated KOH.Detailed material characterization of the pyrolyzed materials is presented here. X-ray photoelectron spectroscopy (XPS) analysis of cobalt and iron porphyrins was used to elucidate the transformations of nitrogen, carbon, cobalt and iron species resulting from the heat treatment. Partial decomposition of the precursor material and formation of polymer-like network decorated by metal oxide particles are identified. Differences in the chemical composition of products of pyrolysis of FeTPP, CoTPP and Co/FeTPP are discussed. Transmission electron microscopy (TEM) imaging revealed the structure of the pyrolyzed porphyrins and was used to gain insight into the size of the metal crystals formed in the bulk. X-ray diffraction spectra (XRD) provided information about the type of crystals formed in the different formulations of the precursor porphyrins. Further, steady state polarization curves were obtained utilizing gas diffusion type electrodes in 0.5 M sulfuric acid and membrane electrode assembly (MEA) configurations under working PEM fuel cell conditions. This work revealed the necessity of the metal phases for the oxygen reduction process.