Yongjun Feng

Size-controlled hydrothermal synthesis and high electrocatalytic performance of CoS2 nanocatalysts as non-precious metal cathode materials for fuel cells

Non-precious metal chalcogenides are considered as a potential alternative to Pt-based cathode catalysts in polymer electrolyte membrane fuel cells because of their promising electrocatalytic performance and low cost. However, size-controlled synthesis of this class of materials still remains a big challenge. In this paper, we directly prepared CoS2 nanocatalysts by a hydrothermal route without any post treatment, developed a facile way to tune the particle size by adjusting the initial Co2+ concentration in the reaction system in the presence of a surfactant, and investigated the corresponding electrocatalytic performance for the oxygen reduction reaction (ORR) in alkaline medium in detail. The results show that the ORR activity mainly depends on the CoS2 mass loading on the electrode disk surface and the average particle size of the CoS2 nanocatalysts. The CoS2 catalyst with an average particle size of 30.7 nm exhibits excellent electrocatalytic performance with an OCP (open circuit potential) of 0.94 V vs. RHE, a half-wave potential (E1/2) of ca. 0.71 V vs. RHE, and complete methanol tolerance for the ORR in 0.1 M KOH. This OCP value is the largest among non-precious metal chalcogenides to date, much close to that of 0.99 V vs. RHE for commercial Pt/C catalyst (E-TEK). In addition, the CoS2 nanocatalyst has comparable durability to the Pt/C catalyst in 0.1 M KOH. The CoS2 nanocatalyst is a promising candidate for alkaline membraneless fuel cell systems.

Electrocatalytic Cobalt Nanoparticles Interacting with Nitrogen-Doped Carbon Nanotube in Situ Generated from a Metal–Organic Framework for the Oxygen Reduction Reaction

A metal organic framework (MOF), synthesized from cobalt salt, melamine (mela), and 1,4-dicarboxybezene (BDC), was used as precursor to prepare Co/CoNx/N-CNT/C electrocatalyst via heat treatment at different temperature (700-900 °C) under nitrogen atmosphere. Crystallites size and microstrain in the 800 °C heat-treated sample (MOFs-800) were the lowest, whereas the stacking fault value was the highest among the rest of the homemade samples, as attested to by the Williamson-Hall analysis, hence assessing that the structural or/and surface modification of Co nanoparticles (NPs), found in MOFs-800, was different from that in other samples. CNTs in MOFs-800, interacting with Co NPs, were formed on the surface of the support, keeping the hexagonal shape of the initial MOF. Among the three homemade samples, the MOF-800 sample, with the best electrocatalytic performance toward oxygen reduction reaction (ORR) in 0.1 M KOH solution, showed the highest density of CNTs skin on the support, the lowest ID/IG ratio, and the largest N atomic content in form of pyridinic-N, CoNx, pyrrolic-N, graphitic-N, and oxidized-N species. Based on the binding energy shift toward lower energies, a strong interaction between the active site and the support was identified for MOFs-800 sample. The number of electron transfer was 3.8 on MOFs-800, close to the value of 4.0 determined on the Pt/C benchmark, thus implying a fast and efficient multielectron reduction of molecular oxygen on CoNx active sites. In addition, the chronoamperometric response within 24 000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with that of Pt/C.

Tolerant Chalcogenide Cathodes of Membraneless Micro Fuel Cells

The most critical issues to overcome in micro direct methanol fuel cells (μDMFCs) are the lack of tolerance of the platinum cathode and fuel crossover through the polymer membrane. Thus, two novel tolerant cathodes of a membraneless microlaminar-flow fuel cell (μLFFC), Pt(x)S(y) and CoSe(2), were developed. The multichannel structure of the system was microfabricated in SU-8 polymer. A commercial platinum cathode served for comparison. When using 5 M CH(3)OH as the fuel, maximum power densities of 6.5, 4, and 0.23 mW cm(-2) were achieved for the μLFFC with Pt, Pt(x)S(y), and CoSe(2) cathodes, respectively. The Pt(x)S(y) cathode outperformed Pt in the same fuel cell when using CH(3)OH at concentrations above 10 M. In a situation where fuel crossover is 100 %, that is, mixing the fuel with the reactant, the maximum power density of the micro fuel cell with Pt decreased by 80 %. However, for Pt(x)S(y) this decrease corresponded to 35 % and for CoSe(2) there was no change in performance. This result is the consequence of the high tolerance of the chalcogenide-based cathodes. When using 10 M HCOOH and a palladium-based anode, the μLFFC with a CoSe(2) cathode achieved a maxiumum power density of 1.04 mW cm(-2). This micro fuel cell does not contain either Nafion membrane or platinum. We report, for the first time, the evaluation of Pt(x)S(y)- and CoSe(2)-based cathodes in membraneless micro fuel cells. The results suggest the development of a novel system that is not size restricted and its operation is mainly based on the selectivity of its electrodes.

SYNTHESIS OF Cu-CONTAINING LAYERED DOUBLE HYDROXIDES WITH A NARROW CRYSTALLITE-SIZE DISTRIBUTION

Hydrotalcite-like layered double hydroxides (LDHs) containing different ratios of Ni2+, Cu2+, Mg2+ and Al3+ in the layers have been prepared by a new method, the key features of which are a very rapid mixing and nucleation process in a colloid mill followed by a separate ageing process. The compositions and structural parameters of the materials synthesized using the two routes are very similar, although the degree of crystallinity is slightly higher for the LDHs produced using the new method. The major advantage of the new method is that it produces smaller crystallites, having a very narrow range of distribution of crystallite size. In the conventional coprecipitation process at constant pH, the mixing process takes a considerable time during which nuclei formed at the beginning of the process have a much longer time to undergo crystal growth than those formed at the end of the process. The consequence is that a wide dispersion of crystallite sizes is obtained. In the colloid mill process, however, the mixing and nucleation is complete in a very short time and is followed by a separate ageing process.

Co-intercalation of Acid Red 337 and a UV absorbent into layered double hydroxides: enhancement of photostability.

Organic-inorganic hybrid pigments with enhanced thermo- and photostability have been prepared by co-intercalating C.I. Acid Red 337 (AR337) and a UV absorbent (BP-4) into the interlayer of ZnAl layered double hydroxides through a coprecipitation method. The obtained compounds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric-differential thermogravimetric-differential thermal analysis, UV-visible spectroscopy, and the International Commission on Illumination (CIE) 1976 L*a*b* color scales. The results show the successful co-intercalation of AR337 and BP-4 into the interlayer region of layered double hydroxides (LDHs) and reveal the presence of host-guest interactions between LDH host layers and guest anions of AR337 and BP-4 and guest-guest interactions between AR337 and BP-4. The intercalation can improve the thermostability of AR337 due to the protection of LDH layers. Moreover, the co-intercalation of AR337 and BP-4 not only markedly enhances the photostability of AR337 but also significantly influences the color of the hybrid pigment.

Facile synthesis of mesoporous hierarchical Co3O4–TiO2 p–n heterojunctions with greatly enhanced gas sensing performance

The development of highly active, sensitive and durable gas sensing materials for the detection of volatile organic compounds (VOCs) is extremely desirable for gas sensors. Herein, a series of mesoporous hierarchical Co3O4–TiO2 p–n heterojunctions have been prepared for the first time via the facile thermal conversion of hierarchical CoTi layered double hydroxides (CoTi-LDHs) precursors at 300–400 °C. The resulting Co3O4–TiO2 nanocomposites showed superior sensing performance towards toluene and xylene in comparison with Co3O4 and TiO2 at low temperature, and the sample with a Co/Ti molar ratio of 4 shows an optimal response (Rg/Ra = 113, Rg and Ra denote the sensor resistance in a target gas and in air, respectively) to 50 ppm xylene at 115 °C. The ultrahigh sensing activity of these Co3O4–TiO2 p–n heterojunctions originates from their hierarchical structure, high specific surface area (>120 m2 g−1), and the formation of numerous p–n heterojunctions, which results in full exposure of active sites, easy adsorption of oxygen and target gases, and large modulation of resistance. Importantly, hierarchical Co3O4–TiO2 heterojunctions possess advantages of simple preparation, structural stability, good selectivity and long-term durability. Therefore, this work provides a facile approach for the preparation of hierarchical Co3O4–TiO2 p–n heterojunctions with excellent activity, sensitivity and durability, which can be used as a promising material for the development of high-performance gas sensors.

Synthesis and applications of layered double hydroxides based pigments.

Layered double hydroxides (LDHs) have attracted a great deal of attention owing to their structural anisotropy, anion-exchange capability and compositional flexibility and have been widely investigated as catalysts, adsorbents, anion-exchangers, polymer additives, optical materials, and so on. The intercalation of chromophores into the interlayer galleries of LDHs has drawn considerable interest since it can result in a kind of functional pigments showing different photophysical and photochemical properties from the pristine chromophores due to the host-guest and guest-guest interactions. This paper reviews recent patents progress made for the synthesis and applications of LDHs based pigments. The potentional applications and the future development are also discussed.

Layered Double Hydroxides as Flame Retardant and Thermal Stabilizer for Polymers

Layered double hydroxides (LDH) has wide applications as non-toxic and halogen-free flame retardant for various resins and highly efficient thermal stabilizer for halogen-containing polymers. This review will discuss some public patents and relevant papers on the flame retardancy and the thermal stability of LDH/polymer composites when the LDHs with different chemical compositions are used as the additive in the polymer matrix. We have summarized these related LDHs in two tables: one for flame retardant and the other for thermal stabilizer.

Antioxidant intercalated Zn-containing layered double hydroxides: preparation, performance and migration properties

A straightforward preparation of the antioxidant anion 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (DBHP), intercalated into layered double hydroxides (DBHP-LDH) via a co-precipitation method, and adjusting the (Mg, Zn, Al) metal ratio was reported. The influence of the Zn-containing LDH composition was studied by measuring the thermal stability and the DBHP anti-migration ability when dispersed into polypropylene (PP). The overall crystallinity of α-PP is found to remain similar with the dispersion of DBHP-LDH particles, but shaper diffraction peaks indicate the presence of larger crystallized domains, most probably arising from an anisotropic connection of smaller coherent PP domains with the help of PP chains diffusing inside the layered inorganic structure. The latter is acting as a coalescent agent yielding an intercalated PP nanocomposite structure with extended interfaces inducing a shift in the glass transition temperature to a higher temperature. The radical-scavenging activity of DBHP when interleaved between LDH layers is found to be conserved while an optimized cation composition for MgZnAl–DBHP is found for the thermo-oxidative stability in association with a lower DBHP migration among the PP nanocomposite series, making the resulting PP nanocomposite a highly promising candidate for possible applications.

Structure Phase Transition and Oxygen Reduction Activity in Acidic Medium of Carbon-Supported Cobalt Selenide Nanoparticles

Polymer Electrolyte Membrane Fuel Cells (PEMFCs), as an environmentally friendly, and high energy efficiency electricity-generating system, have been widely studied in recent years. Two major barriers: high cost and short durability hinder the commercialization of PEMFCs. High cost results from Pt-based cathodic catalysts with the highest price and the lowest abundance metals on the earth (1). To date, Pt-based catalysts are still the best and the most used catalyst for oxygen reduction reaction (ORR) in acidic medium. It is urgently necessary and of great benefit to develop non-Pt, especially nonprecious metal electrocatalysts for the ORR (2). Two potential alternatives have been developed: Ru-based chalcogenides (3) and Co/Fe-macrocycles (4). Ru, however, is one of platinum group metals and is also expensive. The ORR activity of transition metal chalcogenides, such as Co3S4 and Co9S8, were investigated thirty-five years ago (5). Recently, these type of materials have attracted new attention due to the development of nanomaterial and nanotechnology. For example, (M1M2)S2 (M1, M2 = Co, Fe or Ni) and Co1-xSe thin films have some promising ORR activity (6). Among these materials, (Co, Ni)S2 thin film has an OCP value of 0.89 V vs. RHE in O2-saturated 0.1 M HClO4. More recently, we have developed carbon-supported cobalt selenide nanoparticles, as one of the novel non-precious metal catalysts for ORR, with an OCP value of 0.82 V vs. RHE in O2-saturated 0.5 M H2SO4 (7). About 3.5 electrons were evaluated during the ORR procedure per oxygen molecule. Furthermore, we have found that the structure transition of CoSe2 nanoparticles from the orthorhombic to the cubic structure is beneficial for the ORR (8). In the present work, we further investigated the structure conversion with increase of temperature from 250 C to 900 C under a high purity nitrogen atmosphere, using in situ high temperature powder X-ray diffraction (HT-PXRD). In this case, the measured temperature was kept for 30 min before measurement. Figure 1 shows that the conversion from the orthorhombic (ICDD-PDF card No. 00-053-0449) to the cubic (03-065-3327) begins from 350 C as the occurrence of Bragg reflection peaks, e.g., 211 (37.6 /2θ), 311 (51.7 /2θ), 023 (56.6 /2θ) and 321 (58.9 /2θ). The orthorhombic structure, however, remains until the temperature reaches 500 C according to 101 (30.8 /2θ) and 211 (47.7 /2θ) reflection peaks. We only observed some traces of the orthorhombic structure after heat treatment at 400 C for 3 hours, as described in Ref. 7 (c). Possibly, heating time of 30 min is insufficient for the structure conversion. The cubic CoSe2 nanoparticles are stable at the temperature lower than 550 C. Another phase CoO (01-071-1178) was observed from 400 C to 900 C, resulting from oxygen (trace amount) in the nitrogen gas or in the HT-PXRD equipment. This phase was not observed for ex situ heat treatment by ex situ PXRD. Properties of materials mainly depend on the material structure. The cubic CoSe2/C nanoparticles have shown higher activity towards ORR and hydrogen evolution reaction (HER) in 0.5 M H2SO4 at 25 C, related to the orthorhombic (8). Here, we will gain a further comparison in the electrochemical stability and the H2O2 production between both the structures of CoSe2/C in acidic medium at 25 C, using rotating disk electrode and rotating ring-disk electrode techniques.