Carbon nanocapsules as an electrocatalyst support for the oxygen reduction reaction in alkaline electrolyte

Carbon Nanocapsules (CNCs) were investigated for their electrocatalytic performances for the oxygen reduction reaction in alkaline electrolyte. With an average diameter of 10–30 nm, the CNCs are composed of graphene layers encapsulating a hollow core. A gas diffusion electrode (GDE) made of CNCs revealed a much enhanced i–V polarization response than that of Vulcan XC72. However, its performance was moderately lower than that of Black Pearls 2000. In addition, the CNCs were impregnated with nanoparticles of Ag, MnO _x and CoO _x . The i–V and galvanostatic results of the catalyzed CNCs indicated significant improvements over that of noncatalyzed CNCs. For example, a Ag–CNC derived GDE was capable of delivering 1.03 and 0.88 V at current densities of 100 and 200 mA cm⁻², respectively. Our study offers direct evidence that the CNCs not only exhibit unique electrocatalytic abilities but also function superbly as an electrocatalyst support.     

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.     

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.     

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.     

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).     

Effects of cobalt precursor on pyrolyzed carbon-supported cobalt-polypyrrole as electrocatalyst toward oxygen reduction reaction

A series of non-precious metal electrocatalysts, namely pyrolyzed carbon-supported cobalt-polypyrrole, Co-PPy-TsOH/C, are synthesized with various cobalt precursors, including cobalt acetate, cobalt nitrate, cobalt oxalate, and cobalt chloride. The catalytic performance towards oxygen reduction reaction (ORR) is comparatively investigated with electrochemical techniques of cyclic voltammogram, rotating disk electrode and rotating ring-disk electrode. The results are analyzed and discussed employing physiochemical techniques of X-ray diffraction, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma, elemental analysis, and extended X-ray absorption fine structure. It shows that the cobalt precursor plays an essential role on the synthesis process as well as microstructure and performance of the Co-PPy-TsOH/C catalysts towards ORR. Among the studied Co-PPy-TsOH/C catalysts, that prepared with cobalt acetate exhibits the best ORR performance. The crystallite/particle size of cobalt and its distribution as well as the graphitization degree of carbon in the catalyst greatly affects the catalytic performance of Co-PPy-TsOH/C towards ORR. Metallic cobalt is the main component in the active site in Co-PPy-TsOH/C for catalyzing ORR, but some other elements such as nitrogen are probably involved, too.     

Electrodepositing Pt on a Nafion-bonded carbon electrode as a catalyzed electrode for oxygen reduction reaction

The research aims to increase the utilization of platinum (Pt) catalysts and thus to lower the catalyst loadings in the electrode for oxygen reduction reaction (ORR). The electrodeposition of Pt was performed on a rotation disk electrode (RDE) of glass carbon (GC), on which a layer of Nafion-bonded carbon of Vulcan XC 72R was dispersed in advance. The behaviors of Pt RDE and GC RDE in an aqueous solution containing HCl and H₂PtCl₆ were firstly studied. It was found that Pt deposition could be achieved if the electrode potential is controlled below −0.20 V versus (saturated-potassium-chloride silver chloride electrode) SSCE. However, quite a high overpotential is necessary if a quick and apparent deposition were required. Unfortunately, at a high overpotential, the hydrogen evolution would be unavoidable and even accelerated by the formation of nanometer size of Pt particles on the RDE. It was found that it is futile to increase platinum deposits just through extending the deposition time. It was also found that too large deposition current is not helpful for increase of platinum deposition because most of the current was consumed on hydrogen evolution in this case. It has been confirmed that it is conducive to richen Pt ions, present in the form of anionic complex in solution, onto the working electrode to be deposited. It is also helpful to eliminate the hydrogen bubbles formed on the working electrode, i.e., uncatalyzed carbon electrode (UCE), by imposing a positive current on the UCE for a length of time in advance of each cathodic deposition. The potential changes during deposition were recorded. Cyclic voltammograms (CV) of electrodes in 0.5 M H₂SO₄ before and after the deposition were used to assess loading of metal catalysts in a wide range of potential from −0.20 to 1.1 V versus SSCE. The results have shown that the performance of such an electrode with loadings estimated to be 50 μg Pt/cm² is much better than those of a conventional electrode with loadings of 100 μg Pt/cm².     

Kinetics of oxygen reduction reaction at electrochemically fabricated tin-palladium bimetallic electrocatalyst in acidic media

In the present article, oxygen reduction reaction (ORR) at electrochemically fabricated tin-palladium (Sn-Pd) bimetallic electrocatalyst-modified glassy carbon (GC) electrode (Sn-Pd/GC electrode) in acidic media is addressed. Hydrodynamic voltammetric measurements were employed with a view to evaluating various kinetic parameters of the ORR at the Sn-Pd/GC electrode. The obtained results obviously demonstrated that the Sn-Pd bimetallic electrocatalyt substantially promoted the activity of the GC electrode and drove the ORR through an exclusive one-step four-electron pathway forming H₂O as the final product.     

Pt/WC-CNTs催化剂的制备及其对氧还原的电催化性能

通过表面修饰和还原碳化技术制备了以WC为主相的碳化钨/碳纳米管材料（WC-CNTs）,并进一步采用微波多元醇法载铂制备复合催化剂Pt/WC-CNTs。该催化剂相比于Pt/CNTs催化剂,具有更低的过电位、更大的电流密度和交换电流密度,且具有更小的电荷转移电阻和更好的氧还原选择性,显示了优异的氧还原电催化性能。XRD结果表明催化剂由多晶面的WC、Pt晶粒和CNTs组成,TEM和HRTEM显示细小的Pt颗粒均匀地分布在WC-CNTs表面。Pt颗粒和WC颗粒紧密接触,这有利于它们之间的催化协同效应,从而大大增强了Pt的氧还原催化活性。旋转圆盘电极研究结果表明Pt/WC-CNTs催化剂对氧还原反应为直接四电子过程。碳化钨/碳纳米管载铂催化剂性能优异、成本较低,在燃料电池阴极催化剂的研究应用中具有良好的发展前景。     

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.     

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.     

Phase-pure ditungsten carbide nanoparticles covered by carbon as efficient electrocatalysts for hydrogen evolution reaction

Development of economical noble-metal-free electrocatalysts is important for hydrogen evolution reaction (HER). Herein, phase-pure ditungsten carbide (W₂C) nanocrystallines covered with thin carbon layer were fabricated via simple pyrolysis of urea and WO₃ wires under catalyst-free condition at 700 °C. Phase composition of as-prepared samples, including W₂N, W₂C or WC, could be modulated by changing the amount of urea. Moreover, urea not only regulated their phase composition and carbon content, but also acted as a reducing agent for facilitating the formation of phase-pure W₂C. Under optimized phase composition, the sample displayed excellent HER activity and remarkable stability in acid-solution. These prominent electrocatalytic performances could be attributed to phase-pure W₂C dispersed in thin carbon layer with high conductivity, leading to enhanced the activity area. This study exhibits simple route for the fabrication of economical electrocatalysts under mild reaction conditions.     

Studies of oxygen reduction reaction active sites and stability of nitrogen-modified carbon composite catalysts for PEM fuel cells

A non-precious nitrogen-modified carbon composite (NMCC) catalyst is synthesized by the pyrolysis of cobalt, iron-ethylenediamine-chelate complexes on silica followed by chemical and pyrolysis treatments. Pyrolysis temperature and time have a remarkable impact on the content and the type of the nitrogen-containing functional groups in the NMCC catalysts, which affect their catalytic activity and stability. Based on the analysis of the nitrogen functional groups before and after the stability tests, the ORR active sites of the NMCC catalysts are proposed to be pyridinic-N and quaternary-N functional groups. However the pyridinic-N group is not stable in the acidic environment due to the protonation reaction.     

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.     

Porous TiO2/rGO nanocomposites prepared by cold sintering as efficient electrocatalyst for nitrogen reduction reaction under ambient conditions

Haber–Bosch process as the current dominant artificial NH₃ production process in industry, requires relatively high temperature (350–550 °C) and pressure (150–350 atm). Electrocatalytic nitrogen reduction reaction (NRR) as a green and sustainable strategy for ammonia production has raised intensive research interest in recent years but still remains a significant challenge because of the lack of high performance electrocatalysts. In this work, porous TiO₂-reduced graphene oxide (TiO₂/rGO) nanocomposite as self-supporting efficient electrocatalyst for NRR under ambient conditions were prepared by cold sintering associated with sacrificial template method. The porous TiO₂/rGO nanocomposite with grain size of ～40 nm were prepared by cold sintering process at 220 °C and 147 MPa. Given the 220 °C as cold sintering temperature, anatase TiO₂ were preserved as the final phase which exhibit much better NRR electrocatalytic performance than the rutile phase. The oxygen vacancy densities in the nanocomposites were also tuned by heat treatment at 450 °C under different atmosphere, while samples heat treated under H₂/Ar atmosphere gave the best electrocatalytic NRR performance with a FE of 8.88 % and an NH₃ yield of 7.75 μg h^?1 cm^?2 at ambient conditions. Experiments also shows that the addition of rGO significantly improved the electrocatalytic NRR performance especially the conductivity. This work not only designed a framework of ceramic nanocomposites based self-supporting and durable electrocatalysts system but also paves a feasible way towards preparing electrocatalysts that are sensitive to high temperature fabrication process.     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

Hexagonal boron nitride nanosheets as metal-free electrochemical catalysts for oxygen reduction reactions

A high-performance ball milling technique was developed for synthesizing hexagonal boron nitride (h-BN) carbon paper (CP) electrodes as metal-free catalyst for the oxygen reduction reaction (ORR) and hydrogen storage (electrochemically) in acidic media. The h-BN nanosheets were functionalized with glycine to enhance the number of active sites and to boost the catalytic activity. The ball-milled h-BN catalytic electrodes demonstrated ultra-high catalytic activity toward electrochemical hydrogen adsorption/desorption (～3.5 times higher than pristine electrodes) as well as ORR in acidic electrolytes. Furthermore, in-situ durability analysis of the h-BN electrodes was performed via conducting a long-duration cycling experiment (>200 cycles). A mechanistic reaction pathway (sequential) including chemisorption and charge transfer reactions (four-electron and two-electron pathways) was also proposed for the ORR. Considering superior catalytic activity of as-prepared h-BN/CP electrodes, this class of metal-free nanostructured materials can be employed as inexpensive catalysts for the electrochemical H-storage and ORR within various energy storage/conversion devices (e.g., batteries, electrolyzers, and fuel cells).     

The effect of boron doping into Co-C-N and Fe-C-N electrocatalysts on the oxygen reduction reaction

Co-C-N and Fe-C-N thin film catalysts have been modified by controlled doping with boron. Corresponding novel thin film catalysts Co-C-N-B and Fe-C-N-B were synthesized by combinatorial magnetron sputter deposition in an Ar/N₂ gas mixture followed by subsequent heat-treatment between 700 and 1000 °C in an argon atmosphere. The nitrogen content of the as-prepared thin film catalysts could be increased by the addition of boron. Furthermore, the amount of remaining nitrogen in heat-treated catalyst samples was significantly higher in case of boron containing samples. The thin film catalysts were characterized by means of X-ray diffraction (XRD) analysis, electron microprobe and electrochemical measurements. For electrochemical studies the activity as oxygen reduction reaction (ORR) catalyst was investigated using the rotating ring-disk electrode (RRDE) technique in 0.1 M HClO₄ solution at room temperature. The catalytic activity was found to decrease with the boron content in the thin film catalysts even though the N-content increased.     

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.