Download This Paper Open PDF in Browser Add Paper to My Library Share: Permalink Using these links will ensure access to this page indefinitely Copy URL Copy DOI

This paper presents a study of the platinum activity in the ORR in a hydrogen polymer electrolyte membrane fuel cell with electrodes containing multi-walled CNTs in a wide range of compositions and conditions. The data of the comparative analysis of the platinum activity on a fraction of Nafion in the electrode, the composition of the oxidizing agent (oxygen, air), pressure, and temperature are provided. The reasons for the dependence of the platinum surface activity on the component composition of the electrode are considered. Specific mass activity and surface activity of platinum in the ORR in MEA with the electrodes with CNTs depend on the ionomer/platinum ratio. Both dependences have a maximum at the level of the 25% Nafion fraction. The maximum appears as a result of an optimal structure formation, which ensures the fullest use of the platinum surface and minimal concentration overvoltages. Specific mass activity and surface activity of platinum for the sample with 34% CNTs at T = 60 °C and excessive pressure of p = 2 atm amount to 0.46 A/mg and 0.72 mA/cm2, respectively.

The stability of Nafion–carbon composites is important for the efficient functioning of fuel cells. The thermal decomposition of Nafion, nanostructured carbon materials, such as multi-walled carbon nanotubes, graphene-like materials, and their composites, have been studied using constant heating rate thermogravimetry in air. Materials were characterized by quantitative and qualitative analysis methods, such as thermogravimetry, X-ray photoelectron spectroscopy, scanning, and transmission electron microscopy with field emission. In Nafion–carbon composites, an increase in the thermal stability of the Nafion polymer is observed due to the formation of surface compounds at the Nafion–carbon interface. In this case, the degree of stabilization is affected by both the component composition of the composite and the structure of the nanocarbon material. The greatest effect was obtained in the case of using thermally expanded graphite (few-layer graphene). Nafion is distributed to a greater extent over the surface of the carbon material due to its high structural accessibility. The most thermally stable composite is Nafion–graphene in a mass ratio of components 1:4 with one stage Nafion degradation at 422 °C, whereas the degradation of pristine Nafion occurs in three stages at 341, 413, and 430 °C. The dependences of thermal stability and features of thermal degradation on the composition and structure of composites are discussed.

Compositional proton-conducting membranes based on perfluorinated Aquivion®-type copolymers modified by detonation nanodiamonds (DND) with positively charged surfaces were prepared to improve the performance of hydrogen fuel cells. Small-angle neutron scattering (SANS) experiments demonstrated the fine structure in such membranes filled with DND (0–5 wt.%), where the conducting channels typical for Aquivion® membranes are mostly preserved while DND particles (4–5 nm in size) decorated the polymer domains on a submicron scale, according to scanning electron microscopy (SEM) data. With the increase in DND content (0, 0.5, and 2.6 wt.%) the thermogravimetric analysis, potentiometry, potentiodynamic, and potentiotatic curves showed a stabilizing effect of the DNDs on the operational characteristics of the membranes. Membrane–electrode assemblies (MEA), working in the O2/H2 system with the membranes of different compositions, demonstrated improved functional properties of the modified membranes, such as larger operational stability, lower proton resistance, and higher current densities at elevated temperatures in the extended temperature range (22–120 °C) compared to pure membranes without additives.

Behavior of supported alloyed and de-alloyed platinum-copper catalysts, which contained 14% - 27% wt. of Pt, was studied in the reactions of methanol electrooxidation (MOR) and oxygen electroreduction (ORR) in 0.1 M HClO4 solutions. Alloyed PtCux/C catalysts were prepared by a multistage sequential deposition of copper and platinum onto a Vulcan XC72 dispersed carbon support. De-alloyed PtCux-y/C catalysts were prepared by PtCux/C materials pretreatment in acid solutions. The effects of the catalysts initial composition and the acid treatment condition on their composition, structure, and catalytic activity in MOR and ORR were studied. Functional characteristics of platinum-copper catalysts were compared with those of commercial Pt/C catalysts when tested, both in an electrochemical cell and in H2/Air membrane-electrode assembly (MEA). It was shown that the acid pretreatment of platinum-copper catalysts practically does not have negative effect on their catalytic activity, but it reduces the amount of copper passing into the solution during the subsequent electrochemical study. The activity of platinum-copper catalysts in the MOR and the current-voltage characteristics of the H2/Air PEMFC MEAs measured in the process of their life tests were much higher than those of the Pt/C catalysts.

The behavior of supported alloyed and de-alloyed platinum-copper catalysts, which contained 14–27% wt. of Pt, was studied in the reactions of methanol electrooxidation (MOR) and oxygen electroreduction (ORR) in 0.1 M HClO4 solutions. Alloyed PtCux/C catalysts were prepared by a multistage sequential deposition of copper and platinum onto a Vulcan XC72 dispersed carbon support. De-alloyed PtCux−y/C catalysts were prepared by PtCux/C materials pretreatment in acid solutions. The effects of the catalysts initial composition and the acid treatment condition on their composition, structure, and catalytic activity in MOR and ORR were studied. Functional characteristics of platinum-copper catalysts were compared with those of commercial Pt/C catalysts when tested, both in an electrochemical cell and in H2/Air membrane-electrode assembly (MEA). It was shown that the acid pretreatment of platinum-copper catalysts practically does not have negative effect on their catalytic activity, but it reduces the amount of copper passing into the solution during the subsequent electrochemical study. The activity of platinum-copper catalysts in the MOR and the current-voltage characteristics of the H2/Air proton-exchange membrane fuel cell MEAs measured in the process of their life tests were much higher than those of the Pt/C catalysts.

A simple, low-cost and environmentally friendly method has been used to obtain highly porous biomorphic carbon monoliths with a good combination of interconnected macro-, mesoand microporosity, and good electrical conductivity and mechanical strength, making these biocarbon materials interesting for electrochemical applications as binder-free electrodes. Highly porous monolithic biocarbons were obtained from beech wood precursors through pyrolysis and subsequent surface modification in a steam heated to 970°C with different activation times. The obtained biocarbons demonstrated good electrical conductivity and mechanical strength. They were studied as electrodes for supercapacitors in half cell experiments, demonstrating maximum gravimetric capacitance of 200 F g-1 in a basic media at scan rate 1 mV s-1. Galvanostatic charge-discharge experiments showed maximum capacitance of 185 F g-1 at current density of 0.15 A g-1 and ~100 F g-1 at current density of 0.75 A g-1. It has been shown that in addition to the developed porous surface, the micropores with diameters exceeding 1 nm play a key role for the enhanced electrochemical capacity. Long-cycling experiments demonstrated excellent stability of the monolithic biocarbon electrodes with no reduction of the initial capacitance values after 600 cycles in voltammetry.

The polymerization of isoprene with catalytic systems efficient for the cis polymerization of butadiene, i.e., cobalt 2-ethyl hexanoate-diethylaluminum chloride and cobalt 2-ethyl hexanoate-ethylaluminum sesquichloride, is studied. The catalysts are obtained via in situ mixing of the components or at a reduced temperature (0–5°C) in the presence of a small amount of the monomer. In situ polymerization proceeds with an induction period, whose duration depends on the polymerization conditions, or without this period if a preformed catalyst is used. In each case, the resulting polymer is characterized by a mixed microstructure with a predominant content of cis-1,4 units and 3,4 units.

The catalytic properties of neodymium tris-phosphates with various diarylphosphate ligands in the stereoregular 1,4-cis-polymerisation of butadiene and isoprene were studied. The considerable variability of the diaryl phosphate structure allowed for the systematic investigation of the dependence of the catalytic properties of neodymium tris-diarylphosphates on the electronic and steric properties of the ligand. Electron-withdrawing substituents (F, Cl, Br) in the aryl moiety increased the catalyst activity of tris-diarylphosphate. Neodymium aryl phosphates containing lipophilic bulky ligands provided the synthesis of polydienes with a monomodal molecular-weight distribution. The optimal catalytic properties demonstrated that the neodymium aryl phosphate prepared from bis(2,6-dimethy1-4-tert-butylpheny1)phosphoric acid showed high activity and ensured a monomodal MWD of polydienes (M-w/M-n similar to 2 for polybutadiene and M-w/M-n similar to 3 for polyisoprene) in various conditions. (C) 2014 Elsevier B.V. All rights reserved.

The polymerization of butadiene catalyzed by the preformed Co(2-ethylhexanoate)2-Et3Al2Cl3-isoprene (a molar ratio of 1: 20: 20) catalytic system in toluene, cyclohexane, and hexane has been studied. In toluene, the catalyst demonstrates high activity and stereospecificity and yields a high-molecular-mass polymer. In cyclohexane and hexane, the catalyst has a high activity but affords polymers with lower contents of 1,4-cis-units and reduced intrinsic viscosities. The addition of EtAlCl2 and/or a decrease in the polymerization temperature make it possible to obtain polybutadiene containing more than 96% 1,4-cis-units and an acceptable intrinsic viscosity (2.0–2.8 dl/g) in the nonaromatic solvents as well.

Butadiene polymerization in aliphatic and aromatic solvents mediated by a preliminarily formed catalyst consisting of cobalt 2-ethylhexanoate, methylaluminoxane, and diene was studied. It was shown that the catalyst was active at a molar ratio of methylaluminoxane: cobalt: diene = 75−200:1:20, yielding high-molecular-mass polymers (intrinsic viscosity, 1.8–2.7 dl/g) containing at least 98% cis -1,4-units. Preliminary formation of the catalyst was found to increase its activity in butadiene polymerization at a low Al:Co ratio. In this case, the catalyst retains its high stereospecificity (the amount of cis -1,4-units in the polymer reaches 98% and more) independently of the nature of the solvent used for polymerization.

The kinetics of butadiene polymerization in hexane initiated by a catalyst prepared through the interaction of cobalt 2-ethylhexanoate with ethylaluminum sesquichloride and isoprene in the absence of water at Co: Al: isoprene = 1: 20: 20 (mol/mol) has been studied. The reaction orders with respect to the monomer and cobalt ethylhexanoate are estimated as 1.45 ± 0.04 and 1.5 ± 0.3, respectively. The concentration of active centers of the catalyst turns out to be 60 mol % of the cobalt concentration, as determined by the fractional inhibition of polymerization with cyclopentadiene. The rate constant of chain propagation, as calculated from the kinetic data, appears to be 1700 l/(mol min). The effective activation energy is 25.1 ± 7.5 kJ/mol.

The published data on some peculiarities of cobalt catalysts for cis-polymerization of butadiene are analyzed. According to the data of the authors, consumption of alkylaluminum chloride can be markedly decreased with efficiency of the cobalt system remaining high. On the basis of data on the use of organocobalt compounds in butadiene polymerization, the nature of cis-stereogulating active centers is discussed.

Kinetics of cia polymerization of butadiene initiated by the catalyst (dicresyldithiophosphate)(2)Co-Et(3)Al(2)Cl(3) was studied. The rate orders of polymerization with respect to monomer and catalyst, temperature dependence of the rate of polymerization, concentration of active centers, and the rate constant for chain propagation were determined.

The relative reactivities of the double bonds in some unsaturated compounds towards dimethylcyanomethyl radicals have been determined.