Oxalate supported pyrolysis of CoTMPP as electrocatalysts for the oxygen reduction reaction

The utilisation of different metal oxalates in the pyrolysis of cobalt-tetramethoxyphenylporphyrin (CoTMPP) has been investigated as a structure forming agent to obtain highly active electrocatalysts for the oxygen reduction reaction (ORR). Decomposition products of the metal oxalates provide a nano-scaled template for the carbonisation of CoTMPP. After the pyrolysis this template is removed by an etching step so that highly porous carbon-based particles with different morphologies are attained. Thermogravimetric measurements, gas sorption isotherms (BET and pore size distribution), neutron activation analysis (NAA), SEM and XRD analysis examine the pyrolysis process of CoTMPP in the presence of the metal oxalates and the subsequent conditioning step. Thereby, the degree of graphitisation and the morphology of the formed carbon matrix are influenced by the decomposition products of the metal oxalates. Furthermore, the solubility of the decomposition products in the etching step is a crucial factor for the porosity of the final obtained product. Electrochemical analysis (CV and RDE) shows that the catalysts exhibit high kinetic current densities towards the ORR in acidic electrolyte, which is correlated with the contribution of mesopores within the catalyst. Among the investigated metal oxalates, the utilisation of tin oxalate reveals the most beneficial characteristics for the preparation.     

Synthesis and characterizations of Ni-NiO nanoparticles on PDDA-modified graphene for oxygen reduction reaction

We are presenting our recent research results about the Ni-NiO nanoparticles on poly-(diallyldimethylammonium chloride)-modified graphene sheet (Ni-NiO/PDDA-G) nanocomposites prepared by the hydrothermal method at 90°C for 24 h. The Ni-NiO nanoparticles on PDDA-modified graphene sheets are measured by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and selected area electron diffraction (SAED) pattern for exploring the structural evidence to apply in the electrochemical catalysts. The size of Ni-NiO nanoparticles is around 5 nm based on TEM observations. The X-ray diffraction (XRD) results show the Ni in the (012), (110), (110), (200), and (220) crystalline orientations, respectively. Moreover, the crystalline peaks of NiO are found in (111) and (220). The thermal gravimetric analysis (TGA) result represents the loading content of the Ni metal which is about 34.82 wt%. The electron spectroscopy for chemical analysis/X-ray photoelectron spectroscopy (ESCA/XPS) reveals the Ni⁰ to Ni^II ratio in metal phase. The electrochemical studies with Ni-NiO/PDDA-G in 0.5 M aqueous H₂SO₄ were studied for oxygen reduction reaction (ORR).     

Three-dimensional carbon-based endogenous-exogenous MoO2 composites as high-performance negative electrode in asymmetric supercapacitors and efficient electrocatalyst for oxygen evolution reaction

It is not an easy way to design composite electrodes with a high concentration of the constituent. This study cleverly exploited the phase transformation of molybdenum oxide to synthesize three-dimensional carbon-based endogenous-exogenous MoO₂ composites (EEC) by a two-step process. MC-15 exhibited the most outstanding electrochemical performance among EEC, with a specific capacitance up to 411.1 F g^?1 in Na₂SO₄, due to the design of MoO₂, which could be highly loaded with three-dimensional carbon. In addition, the electrode capacitance remains up to 94.1% after 5000 cycles, attributed to the synergy effect of three-dimensional carbon and molybdenum dioxide by providing an abundance of active sites for MoO₂ and overcoming its stacking. In this way, the electrochemical properties of the EEC electrode are not compromised by the volume expansion during the electrochemical process. The energy density of the asymmetric supercapacitor using this material as the negative electrode and MnO₂@CC is 14 W h kg^?1 at a power density of 802 W kg^?1, showing a significant increase in energy density over the asymmetric supercapacitor with a conventional negative electrode (activated carbon, energy density of 3.36 W h kg^?1 and power density of 700 W kg^?1). Its specific capacitance remained 84.9% after 2500 cycles. In addition, an overpotential of only 348 mV was required to drive oxygen evolution in alkaline electrolytes with a Tafel slope as low as 88.7 mV dec^?1; the 20 h stability test retains almost 100%. The results show that the design optimization of the negative electrode material provides a simple and effective strategy to increase the energy density of supercapacitors, and EEC electrode materials are a great candidate to be utilized in supercapacitors with excellent performance as well as electrolytic water.     

Platinum monolayer electrocatalysts for oxygen reduction

We have synthesized a new class of electrocatalysts for the oxygen reduction reaction, consisting of a monolayer of Pt or mixed monolayer of Pt and another late transition metal (Au, Pd, Ir, Ru, Rh, Re or Os) deposited on a Pd(1 1 1) single crystal or on carbon-supported Pd nanoparticles. Several of these electrocatalysts exhibited very high activity, amounting to 20-fold increase in a Pt mass activity, compared with conventional all-Pt electrocatalysts. Their superior activity reflects a low OH coverage on Pt, caused by the lateral repulsion between the OH adsorbed on Pt and the OH or O adsorbed on neighboring, other than Pt, late transition metal atoms. The origin of this effect was identified through a combination of experimental and theoretical methods, employing electrochemical techniques, X-ray absorption spectroscopy, and periodic, self-consistent density functional theory calculations. This new class of electrocatalysts promises to alleviate some major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance.     

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).     

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.     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

掺杂炭黑电催化剂氧还原性能研究

碳纳米材料由于其良好的结构和功能特性,常做为氧还原反应中电催化剂的载体或直接做为氧还原反应的电催化剂.本文从具有良好导电性和高比表面积的炭黑(BP2000)出发,在对其进行表面氧化的基础上分别通过高温氨化和Fe(NO3)3溶液浸渍/高温煅烧进行了掺氮(N-BP2000)和掺铁(Fe-BP2000),对比考察了掺氮和掺铁...     

Preparation and electrochemical characteristics of CoTMPP-TiO2NT/BP composite electrocatalyst for oxygen reduction reaction

CoTMPP-TiO₂NT/BP composites have been synthesized by preparing CoTMPP and depositing CoTMPP on carbon-supported titania nanotube (TiO₂NT/BP) using microwave irradiation method at the same time, followed by heat-treatment from 300 to 900 °C in N₂ atmosphere. The catalytic activity for oxygen reduction was evaluated by rotating disc electrode technique in half cells with 0.5 M H₂SO₄. The number of electrons exchanged during ORR and the percentage of peroxide (%H₂O₂) produced by the reaction were evaluated for catalysts by rotating ring disk electrode (RRDE) measurements. The influences of TiO₂NT doping, the heat-treating temperature and the different ratios of BP:TiO₂NT on the activity of electrocatalysts for oxygen reduction were investigated. The stability of the CoTMPP-TiO₂NT/BP electrocatalysts was studied with potentialstatic-polarization measurements in 0.5 M H₂SO₄ + 0.5 M CH₃OH. CoTMPP-TiO₂NT/BP composites show higher catalytic activity and better stability than CoTMPP/BP. The mechanism for the enhanced catalytic activity of CoTMPP-TiO₂NT/BP is discussed.     

Electrophoretic deposition of ZnCo2O4 spinel and its electrocatalytic properties for oxygen evolution reaction

In this paper, ZnCo₂O₄ was deposited on nickel substrate by electrophoretic deposition (EPD) method as electrocatalyst for the oxygen evolution reaction. The effect of electrophoresis variables including the deposition time, the applied voltages was discussed. XRD, SEM, and electrochemical measurement techniques were used to characterize the deposit and ZnCo₂O₄/Ni electrode. Compared with the ZnCo₂O₄ electrode prepared by nitrate decomposition method, the electrophoretic ZnCo₂O₄ electrodes exhibit better electrocatalytic properties and higher mechanical stability. And the electrode prepared at 10 V for 5 min has the best electrocatalytic properties with the overpotential of only 0.203 V at current density of 100 mA cm⁻².     

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.     

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.     

Fe-based electrocatalysts made with microporous pristine carbon black supports for the reduction of oxygen in PEM fuel cells

Fe-based catalysts for the oxygen reduction reaction at the cathode of polymer electrolyte membrane (PEM) fuel cells have been prepared using several highly microporous (defined as pores having a size <2 nm) carbon supports. The aim is to produce better performing catalysts as it is known that catalytic sites are hosted in the micropores of the carbon supports. All catalysts were loaded with a nominal Fe content of 0.2 wt% and were obtained by heat-treatment at 950 °C in pure NH₃ atmosphere. It is demonstrated, however, that the use of highly microporous carbon supports does not lead to improved catalytic activity, as originally expected, since the surface of these micropores is devoid of the nitrogen functionalities necessary to build the catalytic sites. Also, it is shown that for these microporous carbon supports, it is only the new micropores, i.e. those created during NH₃ etching at high temperature, that are capable of hosting catalytic sites.     

Study of electrochemical stability and physical characteristics of ball milled tantalum carbide as a support for oxygen evolution reaction electrocatalysts

Tantalum carbide (TaC) is investigated as a potential support for oxygen evolution reaction electrocatalysts. It is ball milled with zirconia balls for 7 and 14 days. The particle size and surface area of the ball milled TaC are analyzed using scanning electron microscopy and nitrogen physisorption methods, respectively. After 14 days of ball milling, the TaC shows smaller particle size and larger surface area compared to after 7 days of ball milling and unmilled TaC. The long-term electrochemical stability of the ball milled TaC is evaluated under oxygen evolution reaction conditions using cyclic voltammetry and chronoamperometry. The ball milled support at a fixed voltage of 1.6 V is shown to be stable up to 5000 cycles. Chronoamperometric measurement also shows the ball milled TaC remains stable over the 70 h of the test.     

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.     

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.     

Structural evolution of multi-walled carbon nanotube/MnO2 composites as supercapacitor electrodes

Multi-walled carbon nanotube (MWCNT)/MnO₂ supercapacitor electrodes containing MnO₂ nanoflakes in the MWCNT network are fabricated through the oxidation of manganese acetate with poly(4-styrenesulfonic acid) (PSS) dispersed MWCNTs. The structural evolution of the electrodes under charge/discharge (reduction/oxidation) cycles and its impact on the electrodes’ electrochemical properties are evaluated. Structural evolution involves the dissolution of MnO₂ upon reduction, the diffusion of the reduced Mn species from the MWCNT network toward the electrolyte solution, and the deposition of MnO₂ on the electrode surface upon oxidation. Electrode structural changes, including the electrode dissolution and the growth of the MnO₂ crystals, are scan rate dependent and have deteriorating effect on the electrode's electrochemical properties including the specific capacitance and cyclic stability.     

Methanol-tolerant PdPt/C alloy catalyst for oxygen electro-reduction reaction

A carbon-supported Pd-based PdPt catalyst (PdPt/C) with a small amount of Pt was prepared by borohydride reduction method and its activity in the oxygen electro-reduction reaction (ORR) was investigated in acidic conditions both with and without methanol. For comparison, carbon-supported Pt (Pt/C) and Pd (Pd/C) catalysts were prepared and the ORR activities were compared. Results revealed that the PdPt/C catalyst showed slightly lower ORR activity in terms of onset potential of oxygen reduction than Pt/C catalyst in 0.1M HClO₄. However, PdPt/C catalyst exhibited enhanced activity toward selective ORR with methanol-tolerant characteristics in 0.1M HClO₄ in the presence of methanol. The PdPt/C catalyst prepared here is suitable for use as a cathodic electrocatalyst in direct alcohol fuel cells after addition of small amount of expensive Pt metal.