Electrocatalytic activity of nitrogen doped carbon nanotubes with different morphologies for oxygen reduction reaction

Nitrogen doped carbon nanotubes (NCNTs) were synthesized by a single step chemical vapor deposition technique using either ferrocene or iron(II) phthalocyanine as catalyst and pyridine as the carbon and nitrogen precursor. Variations in surface morphology and electrocatalytic activity for oxygen reduction reaction (ORR) were observed between the NCNTs synthesized using different catalysts. The structural and chemical characterizations were carried out using transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical activity of NCNTs was evaluated with rotating ring disc electrode (RRDE) voltammetry. Structural characterization suggested more defects formed on the NCNTs synthesized from ferrocene (Fc-NCNTs) which led to a rugged surface morphology compared to the NCNTs synthesized from iron(II) phthalocyanine (FePc-NCNTs). Based on the RRDE voltammetry study, Fc-NCNTs demonstrated much higher activity for ORR than FePc-NCNT. Evidences from the structural and chemical characterizations illustrate the potential impact of catalyst structure in shaping the surface structure of NCNTs and the positive effect of surface defects on ORR activity. These results showed that potential improvements on ORR activity of NCNTs could be achieved by tailoring the surface structure of NCNTs by using catalysts with different structures.     

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.     

Nitrogen doped carbon nanotubes synthesized from aliphatic diamines for oxygen reduction reaction

In the present study, nitrogen doped carbon nanotubes (N-CNTs) were synthesized using three different aliphatic diamines as nitrogen–carbon precursor solutions with varying carbon chain lengths, in order to elucidate the effect of precursor solution on the overall nitrogen content and ORR activity of the synthesized materials. Increasing the nitrogen to carbon ratio in the precursor solution resulted in higher nitrogen contents in the synthesized N-CNTs, along with enhanced ORR activity for all three samples tested. The increase in activity was attributed to the enhanced properties and edge plane defects of N-CNTs resulting from higher nitrogen contents, illustrating the importance of using a nitrogen rich precursor solution.     

Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM)

The redox competition mode of scanning electrochemical microscopy (RC-SECM) has been utilized to visualize the local electrocatalytic activity of metalloporphyrin spots towards oxygen reduction in 0.1 M phosphate buffer as electrolyte solution. The metalloporphyrin spots were obtained by electrochemically induced deposition using a droplet cell. Tetratolyl porphyrins (TTPs) of Mn, Fe and Co have been investigated, with that containing Mn as central metal atom showing highest catalytic activity. The multiple stable oxidation states of Mn were seen as a key factor in the influence of the metal ion on the catalytic activity. From the RC-SECM results, it is shown that oxygen reduction at a manganese TTP (MnTTP) modified electrode surface yielded the least amount of H₂O₂ when compared to iron TTP (FeTTP) and cobalt TTP (CoTTP). As further confirmed by means of rotating disc electrode (RDE) measurements this was attributed to the high activity of MnTTP for H₂O₂ reduction.     

Electrocatalytic activity of electrodeposited composite films of polypyrrole and CoFe2O4 nanoparticles towards oxygen reduction reaction

Electrochemical behaviour of sandwich-type composite electrodes of polypyrrole (PPy) and CoFe₂O₄ nanoparticles (Ox) were investigated in an aqueous solution of 0.5 M K₂SO₄ and 5mM KOH at 25 °C using electrochemical impedance (EI), cyclic voltammetry (CV) and Tafel polarization techniques. EI and CV studies indicated that the incorporation of oxide nanoparticles influenced the charge transfer and transport behaviours of the polymer matrix greatly. The bulk electrical resistances of pure polymer (4.5 ± 1.7 Ω) as well as composite (2.7 ± 0.8 Ω) electrodes were practically constant in the potential region, +0.1 to −0.7 V. The latter electrode showed a good electrocatalytic activity towards the oxygen reduction reaction (ORR).     

Oxygen reduction at Fe-N-modified multi-walled carbon nanotubes in acidic electrolyte

Multi-walled carbon nanotubes (MWCNTs) modified with iron tetramethoyxphenyl-porphyrin chloride (FeTMPP-Cl) and heat treated are active towards electrocatalytic oxygen reduction in acidic media. The activity slightly depends on the heat treatment temperature (850 < 550 °C) and the amount of porphyrin deposited onto the nanotubes before the heat treatment step. In comparison with as-received MWCNTs no increase in activity has been found with iron phenanthroline or iron acetate impregnated and heat treated MWCNTs. When MWCNTs are pretreated in an oxidation step using HNO₃, there is only a slight increase in activity after FeTMPP-Cl modification and heat treatment compared to the not pretreated MWCNTs. The HNO₃ treatment itself, however, leads to an increase in activity of the unmodified MWCNTs. TEM-measurements revealed an amorphous layer surrounding the MWCNTs after HNO₃ treatment, while XPS showed an increased amount of oxygen functional groups. It is suggested that there are different kinds of active sites at the catalyst surface, the first ones consisting of oxygen functionalities or other entities introduced by the HNO₃ treatment, and the second ones containing nitrogen (and probably iron) introduced via the porphyrin. Pyridine-type nitrogen has been found by XPS after heat treatment at both temperatures, indicating that the active sites are already formed at 550 °C.     

高分子聚合物担载手性金属卟啉作为不对称催化剂的研究进展

手性金属卟啉经常用作催化合成对映体纯的化合物,但在实际应用中却有很多不利因素,如稳定性差、催化剂回收困难等。使用聚合物作为载体可以很好地解决这些问题。现常用的载体聚合物有电引发聚合物膜、大分子树脂和大孔分子筛,这些极大地提高了手性金属卟啉的催化性能。     

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.     

Vacuum evaporated thin films of mixed cobalt and nickel oxides as electrocatalyst for oxygen evolution and reduction

Mixed cobalt and nickel oxides, obtained by vacuum coevaporation of Co, Ni and TeO₂ are investigated as electrocatalysts for oxygen reduction and evolution reaction. Gas-diffusion bifunctional oxygen electrodes (GDE) are prepared by direct deposition of catalyst on gas-supplying membrane. Thus obtained GDE with different atomic ratio R_Co/Ni and R_(Co+Ni)/Te of the catalyst are electrochemically tested by means of steady-state voltammetry. It is shown that the films exhibit high catalytic activity toward both oxygen reduction and evolution reactions despite very small catalyst loading of about 0.07 mg cm⁻².     

Electrocatalytic reduction of oxygen at glassy carbon electrode modified by polypyrrole/anthraquinones composite film in various pH media

The electrocatalytic reduction of dioxygen by one mono and four dihydroxy derivatives of 9,10-anthraquinone (AQ) incorporated in polypyrrole (PPy) matrix on glassy carbon electrode has been investigated. The electrochemical behaviour of the modified electrodes was examined in various pH media and both the formal potential of anthraquinones and reduction potential of dioxygen exhibited pH dependence. AQ and PPy composite film showed excellent electrocatalytic performance for the reduction of O₂ to H₂O₂. pH 6.0 was chosen as the most suitable medium to study the electrocatalysis by comparing the peak potential of oxygen reduction and enhancement in peak current for oxygen reduction. The diffusion coefficient values of AQ at the modified electrodes and the number of electrons involved in AQ reduction were evaluated by chronoamperometric and chronocoulometric techniques, respectively. In addition, hydrodynamic voltammetric studies showed the involvement of two electrons in O₂ reduction. The mass specific activity of AQ used, the diffusion coefficient of oxygen and the heterogeneous rate constants for the oxygen reduction at the surface of modified electrodes were also determined by rotating disk voltammetry.     

Immobilization of laccase onto 1-aminopyrene functionalized carbon nanotubes and their electrocatalytic activity for oxygen reduction

Carbon nanotubes (CNTs) were non-covalent-functionalized with 1-aminopyrene (1-AP) and used for the first time to immobilize laccase (Lac) with the aid of glutaraldehyde (GA). The results of Fourier transform infrared (FTIR) spectra confirmed the successful modification of CNTs with 1-AP. The dispersibility of CNTs in aqueous solution was improved by the functionalization of 1-AP. The electrocatalytic properties of the Lac immobilized on the 1-AP functionalized CNTs (Lac/AP-CNTs) for oxygen reduction have been investigated by cyclic voltammetry in the presence of 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS) in the Britton-Robinson (B-R) buffer solution (pH 3.0). Under the same experimental condition, the Lac/AP-CNTs catalyst shows higher electrocatalytic activity and better stability than the Lac immobilized on the pristine CNTs (Lac/CNTs). Additionally, effects of the mass ratio of 1-AP to CNTs in the AP-CNTs composites, the loading mass of the Lac/AP-CNTs catalyst and the pH value of the electrolyte on the electrocatalytic activity of the Lac/AP-CNTs/glassy carbon electrode for oxygen reduction were also optimized.     

Nitrogen-doped carbon coated palygorskite as an efficient electrocatalyst support for oxygen reduction reaction

An electrocatalyst support, nitrogen-doped graphitic layer (CN_x) coated palygorskite (PLS) (donated as PLS@CN_x), is synthesized by carbonizing the polypyrrole (PPy) coated PLS and is explored for the first time as a cathode electrocatalyst support in proton exchange membrane fuel cell. The structural and chemical properties of the PLS@CN_x are investigated by Fourier-Transform infrared spectrometer, thermogravimetric analysis, X-ray diffraction and transmission electron microscopy. The electrocatalytic activity and stability of Pt/PLS@CN_x toward oxygen reduction reaction (ORR) are studied by cyclic voltammetry (CV) and steady state polarization measurements. Upon loading Pt (20 wt%), the catalysts exhibit superior catalytic performance during ORR, surpassing the conventional Pt/C (Vulcan XC-72) catalysts. High electrocatalytic activity and good stability can be attributed to the nitrogen atom incorporation and SiO₂ component in PLS.     

Electrocatalytic oxygen reduction on single-walled carbon nanotubes supported Pt alloys nanoparticles in acidic and alkaline conditions

The objective of this study is to improve the catalytic activity of platinum by alloying with transition metal (Pd) in gas diffusion electrodes (GDEs) by oxygen reduction reaction (ORR) at cathode site and comparison of the acidic and alkaline electrolytes. The high porosity of single-walled carbon nanotubes (SWCNTs) facilitates diffusion of the reactant and facilitates interaction with the Pt surface. It is also evident that SWCNTs enhance the stability of the electrocatalyst. Functionalized SWCNTs are used as a means to facilitate the uniform deposition of Pt on the SWCNT surface. The structure of SWCNTs is nearly perfect, even after functionalization, while other types of CNTs contain a significant concentration of structural defects in their walls. So catalysts supported on SWCNTs are studied in this research. The electrocatalytic properties of ORR were evaluated by cyclic voltammetry, polarization experiments, and chronoamperometry. The morphology and elemental composition of Pt alloys were characterized by X-ray diffraction (XRD) analysis and inductively coupled plasma atomic emission spectroscopy (ICP-AES) system. The catalytic activities of the bimetallic catalysts in GDEs have been shown to be not only dependent on the composition, but also on the nature of the electrolytes. The GDEs have shown a transition from the slow ORR kinetics in alkaline electrolyte to the fast ORR kinetics in the acidic electrolyte. The results also show that introduction of Pd as transition metal in the Pt alloys provides fast ORR kinetics in both acidic and alkaline electrolytes. The performance of GDEs with Pt–Pd alloy surfaces towards the ORR as a function of the alloy’s overall composition and their behavior in acidic electrolyte was also studied. These results show that the alloy’s overall composition and also the nature of the electrolytes have a large effect on the performance of GDEs for ORR.     

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.     

Electrocatalytic reduction of dioxygen at a modified glassy carbon electrode based on Nafion-dispersed single-walled carbon nanotubes and cobalt–porphyrin with palladium nanoparticles in acidic media

Cathodic dioxygen (O₂) reduction was performed at a modified glassy carbon electrode (GCE) by single-walled carbon nanotubes (SWCNT)/Nafion^® (NF) film with cobalt (II) tetra (2-amino-phenyl) porphyrin (CoTAPP) and palladium (Pd) nanoparticles incorporated and employed as doping agents. Both the electrochemical behavior of SWCNT with a P(CoTAPP)–Pd nanoparticle matrix and the electrocatalytic reduction of O₂ were investigated using transmission electron microscopy (TEM), cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) techniques in 0.1 mol l⁻¹ H₂SO₄ aqueous solutions. The electrocatalytic reduction of O₂ at the SWCNT/NF/P(CoTAPP)–Pd composite film established a pathway of four-electron transfer reductions into H₂O. Hydrodynamic voltammetry revealed that the modified electrode was catalyzed effectively by the four-electron transferred reduction of dioxygen into H₂O with minimal generation of H₂O₂. The SWCNT/NF/P(CoTAPP)–Pd composite film showed a highly efficient electrocatalytic performance. P(CoTAPP)–Pd was an effective mediator for the reduction of dioxygen and was responsible for the enhanced catalytic activity.     

Electrodeposition of macroporous silver films from ionic liquids and assessment of these films in the electrocatalytic reduction of nitrate

Macroporous silver films, ordered or fragmented, were fabricated by electrodeposition of silver into the interstitial spaces of templates formed by polystyrene (PS) latex spheres that had been self-assembled onto bare indium tin oxide (ITO) electrodes or onto gold-coated ITO (ITO/Au) electrodes (in which the electrode had been coated by gold sputtering deposition) from two room-temperature ionic liquids (ILs): N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and N-butyl-N-methylpyrrolidinium dicyanamide (BMP-DCA), respectively, under normal atmospheric conditions. After electrodeposition of silver, the PS spheres were removed by dissolution in tetrahydrofuran (THF) to leave a macroporous silver structure. The higher wettability of ILs onto PS spheres leads to improved penetration of the ILs into the cavities of the PS templates. Electrodeposition is easier if an electrolyte that has a good penetration into the interstitial spaces between the PS spheres. The macroporous silver electrode exhibited much better electrocatalytic performance with respect to nitrate reduction than a regular silver wire electrode. Quantitative determination of nitrate was also examined.     

Electrocatalytic reduction of dioxygen at glassy carbon electrodes modified with polypyrrole/anthraquinonedisulphonate composite film in various pH solutions

Functionalized polypyrrole (PPy) film with anthraquinonedisulphonate (AQDS) incorporated as dopant was prepared by anodic polymerization of pyrrole (Py) at a glassy carbon electrode from aqueous solution. The electrochemical behavior of AQDS in PPy matrix and the electrocatalytic reduction of dioxygen on the resulting composite film were investigated in various pH solutions. The formal potential of AQDS and the reduction potential of dioxygen both exhibit pH dependence. In all pH solutions employed, the electrocatalytic reduction of dioxygen at the PPy/AQDS composite film establishes a pathway of irreversible two-electron reduction to form hydrogen peroxide. The pH 6.0 buffer solution is a more suitable medium for the reduction of dioxygen, where the PPy/AQDS composite film showed a more efficient electrocatalytic performance. It was found that AQDS is an effective mediator for the reduction of dioxygen and the reduced AQ is responsible for the enhanced catalytic activity. The catalytic current is under mixed kinetic-diffusion control. The number of electrons transferred and kinetic parameters of dioxygen reduction were determined using cyclic voltammetry, rotating disk voltammetry and Tafel polarization technique.     

Study of the oxygen reduction reaction (ORR) at Pt interfaced with phosphoric acid doped polybenzimidazole at elevated temperature and low relative humidity

The oxygen reduction reaction on platinum interfaced with phosphoric acid doped PBI at elevated temperature and low relative humidities has been investigated by using a micro band electrode technique. Both the kinetic and the mass transport parameters in the Pt/PBI-H₃PO₄ system are comparable to those of the Pt/H₃PO₄ system under similar conditions. The study suggests that it is the amorphous H₃PO₄ phase that functions as the electrolyte. The oxygen reduction reaction is first order with respect to both proton concentration and oxygen saturation concentration in the electrolyte, which indicates that the proton transfer is the rate-determining step in oxygen reduction. The H₃PO₄ doping level and the water content of the electrolyte affect the ORR exchange current density, oxygen diffusion, and the oxygen solubility in PBI-H₃PO₄ membranes. The dissolved O₂ molecules permeate mainly through the amorphous H₃PO₄. However, the oxygen solubility in PBI-H₃PO₄ is higher than its solubility in H₃PO₄, which is explained by the presence of the crystalline PBI region formed during electrolyte preparation.     

Synthesis and characterization of Pd-Ni nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Carbon-supported Pd-Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol method, followed by heat treatment in a reducing atmosphere at 500-900 °C. The samples have been characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), rotating disk electrode (RDE) measurements, and single-cell proton exchange membrane fuel cell (PEMFC) tests for oxygen reduction reaction (ORR). XRD and TEM data reveal an increase in the degree of alloying and particle size with increasing heat-treatment temperature. XPS data indicate surface segregation with Pd enrichment on the surface of Pd₈₀Ni₂₀ after heat treatment at ≥500 °C, suggesting possible lattice strains in the outermost layers. Electrochemical data based on CV, RDE, and single-cell PEMFC measurement show that Pd₈₀Ni₂₀ heated at 500 °C has the highest mass catalytic activity for ORR among the Pd-Ni samples investigated, with stability and catalytic activity significantly higher than that found with Pd. With a lower cost, the Pd-Ni catalysts exhibit higher tolerance to methanol than Pt, offering an added advantage in direct methanol fuel cells (DMFC).     

Novel electrocatalyst for the oxygen reduction reaction in acidic media using electrochemically activated iron 2,6-bis(imino)-pyridyl complexes

Electrochemically activated materials produced from iron 2,6-bis(imino)-pyridyl complexes were deposited onto a glassy carbon electrode from an acetonitrile solution containing 0.1 mol dm⁻³ tetrabutylammonium perchlorate and 0.002 mol dm⁻³ monomeric iron chelate by successively scanning the potential between −0.6 and 0.8 V vs. the Ag/Ag⁺ reference electrode (RE). The electrocatalytic activity of the resultant material to the reduction of dioxygen molecules in aqueous sulfuric acid solution was studied by hydrodynamic voltammetry. It is found that although the material can dissolve in sulfuric acid solution, it is re-deposited on the electrode surface during cathodic polarization. The re-deposited material can efficiently catalyse the electrochemical reduction of molecular dioxygen through different pathways depending upon the structure of the ligands. The material produced from the iron chelate with 2,4,6-trimethylphenyl substituents allows only a two-electron reduction of dioxygen molecules, while the reduction of dioxygen on the material produced from the iron chelate with 2,6-biisopropylphenyl substituents follows the four-electron pathway to produce water. The latter material shows good stability and unusually high mass activity towards the oxygen reduction reaction in the acidic medium. Although the onset potential is quite low (−0.2 V vs. SCE), the material is a prospective candidate in power sources, oxygen sensors and some chemical processes. It is suggested that the active center for oxygen reduction is determined by the structure of the activated material.