V. E. Sosenkin

The standard contact porosimetry

Abstract A new porosimetric method — the Method of Standard Contact Porosimetry (MSP) is described which allows the investigation of the structure and the properties of all kinds of porous materials including soft, frail, amalgamated materials and also of powders. This method is relatively simple, nondestructive, is not connected with the use of mercury and can be applied for measurements in a wide range of pore sizes from about 1 to 3×105 nm. This method is very informative. It has been used for measuring the pore volume and the pore surface area distribution in terms of the pore radii and the pore shapes, the distribution of liquids in porous materials in terms of the liquid-sample free energy and the capillary pressure, and also for the measuring of adsorption isotherms, of structural changes during contraction and swelling of the samples in different liquids, of different properties of multicomponent hydrophilic-hydrophobic systems etc. The results obtained by applying the MSP for investigating different processes in porous systems are discussed. The following processes were investigated; swelling and ion exchange polymeric materials (membranes, conducting polymers); pressing of powdered materials (PVC, Raney silver); the influence of temperature on the porous structure; the influence of pore-forming agents; chemical and electrochemical sintering of catalysts; deposition of solid products in the pore volume of the cathode during reduction of SOCl2 in lithium batteries; structural changes during formation and cycling of lead and silver oxide electrode, etc. The MSP includes different manual operations. In order to avoid them on the base of MSP, an automated standard porosimeter (ASP) was developed which includes a computer, an electronic balance, an automatic manipulator, and a device for a controlled drying of the porous samples.

Porous structure and hydrophilic-hydrophobic properties of gas diffusion layers of the electrodes in proton-exchange membrane fuel cells

The porous structure and hydrophilic-hydrophobic properties of the gas diffusion layers (GDL) of electrodes on a substrate of carbon paper in proton-exchange membrane fuel cells have been investigated with the methods of standard porosimetry and of pycnometry. GDL containing various fluoroplast concentrations were impregnated with water, and this process has been investigated at 20 and 80°C. The impregnation rate is significantly higher for untreated carbon paper than for teflonated GDL and also increases significantly with increasing temperature. With teflonization of the carbon paper, hydrophilic porosity decreases, while hydrophobic porosity increases. This increase, however, ceases at high fluoroplast concentrations. The concept of hydrophobization effectiveness of the porous carbon substrate of GDL is introduced. It has been established that hydrophobization effectiveness decreases with increasing fluoroplast concentration and depends on the type of suspension. Curves of the angle of wetting of GDL by water versus the pore radius exhibit a minimum. Different values of the angle of wetting of GDL by water in different pores are explained by nonuniform distributions of both fluoroplast particles and hydrophilic surface groups in pores of different dimensions.

Polymer Ion-Exchangers Modified with Zirconium Hydrophosphate for Removal of Cd2+ Ions from Diluted Solutions

Hybrid ion-exchangers have been synthesized by modification of strongly acidic cation-exchange resin with zirconium hydrophosphate. The samples were investigated with scanning and transmission electron microscopy and standard contact porosimetry. Single nanoparticles and their aggregates have been found in the polymer. The nanoparticles in transport pores increase the electrical conductivity of ion-exchanger from 0.21 to 0.65 Ohm−1 m−1. The diffusion coefficient of Cd2+ ions reaches 2.43 × 10−11 m2 s−1 for initial resin and (2.50–4.34) × 10−11 m2 s−1 for nanocomposites. The inorganic constituent improves Cd2+ recovery from a solution, which contains also Ca2+ and Mg2+. The degree of Cd2+ removal is 18% for non-modified resin and 99% for the sample containing 38 mass% zirconium phosphate.

Hydrophilic-hydrophobic and sorption properties of the catalyst layers of electrodes in a proton-exchange membrane fuel cell: A stage-by-stage study

In the electrodes of a proton-exchange membrane fuel cell, the hydrophilic-hydrophobic properties of the catalyst layers (CL), which contain a carbon substrate (CS), an ionomer in the form of Nafion resin, and a platinum catalyst, are investigated with standard contact porosimetry. Regularities in the influence of ionomer on the hydrophilic-hydrophobic properties of ten different CS, including Vulcan XC-72 carbon black, are investigated. The following plots are obtained: pore distribution curves with respect to radii, water desorption isotherms, moisture content distribution curves with respect to capillary pressure and to the free energy of binding water to material, and the wetting angle of water for the samples under investigation as a function of pore radius. It is established that both a hydrophobic effect and a hydrophilic effect occur in CL as a result of ionomer application to the CS under investigation. It is concluded that these different effects are determined by the orientation of the sulfonate groups (inside and outside) in the resin particles. This orientation depends on the extent of the binding of sulfonate groups to the CS surface by adsorption. CS surface properties are determined by the type and concentration of surface groups. Thus, the phenomenon of ionogenic-group inversion is established. When platinum is applied to CS, the CL become partially hydrophilic.

Conducting properties of a gel ionite modified with zirconium hydrophosphate nanoparticles

The organo-inorganic composites based on a strongly acid gel resin including zirconium hydro-phosphate nanoparticles and their aggregates were studied by impedance spectroscopy, electron microscopy, and standard contact porosimetry. The porous structure of the polymer was transformed under the action of the inorganic filler. The nanoparticles in the transport pores provided a three- to fivefold increase in the electric conductivity of the nanocomposites compared with the conductivity of the nonmodified ionite and a decrease in the percolation threshold. The nanocomposite ionites demonstrated stability against the accumulation of organic substances during electrodeionization to extract Na+ from low-concentrated solutions.

Carbon nanotubes as a support for Pt-and Pt-Ru-catalysts of reactions proceeding in fuel cells

Comparative study of two types of single-wall carbon nanotubes and standard carbon black Vulcan XC-72 as supports for catalysts of reactions proceeding in fuel cells is carried out. The nanotubes were prepared by arc method; they differed in the degree of their purifying from amorphous carbon and metal impurities. The structure and hydrophobic-hydrophilic properties of these carbon supports are studied by etalon porosimetry. The effect of the supports’ specific surface area on the deposited catalyst particles size and specific surface area is studied. The catalysts (Pt-Ru and Pt) were deposited from aqueous solutions of their salts. Platinum was also deposited by thermal decomposition of ethoxy clusters. It is shown that in methanol oxidation reaction at the Pt-Ru catalysts the current values per unit true surface area do not depend on the support nature, provided the catalyst loading is equal and the particle size is similar. When oxygen is reduced at platinum deposited onto purified nanotubes and the carbon black Vulcan XC-72, specific kinetic currents also are close to each other. It is shown that the degree of nanotubes purification and their structure affect the kinetics of this reaction significantly.

Effect of the porous structure of polymer on the kinetics of Ni2+ exchange on hybrid inorganic-organic ionites

Hybrid organic-inorganic ion exchangers are obtained by incorporating amorphous zirconium hydrophosphate into the gel of strong acidic cation exchange resin. Hybrid organic-inorganic ion exchangers are obtained by modifying strong acidic cation exchange resin with amorphous zirconium hydrophosphate. The synthesized materials are studied by standard porometry contact. It is found that raising the inorganic component content to 34 wt % diminishes the microporosity of the samples and simultaneously enhances the of meso- and macropore volume. Experiments establish that modification of a polymer matrix lowers the self-diffusion coefficient of Ni2+ from 8.1 × 10−12 to 2.4 × 10−12–4.1 × 10−12 m2 s−1; nevertheless, an inorganic ion exchanger minimizes the inhibitory effect of co-ions on the Ni2+ → H+ exchange rate. One possible mechanism for of filling of the matrix by with particles of zirconium hydrophosphate is discussed.

Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance

During the last decades different new capacitor types were developed based on electrochemical processes. According to Conway [1] an electrochemical capacitor is a device in which different quasi-reversible electrochemical charging/discharging processes take place and for which the shape of the charging and discharging curves is almost linear, similarily to those in common electrostatic capacitors [1-13]. Electrochemical capacitors can be classified as film-type (dielectric), electrolytic and supercapacitors.

Effect of the functionalizing of carbon nanotubes on the electrodeposited catalysts’ structure and catalytic properties

The structure and hydrophilic-hydrophobic properties of functionalized single-wall carbon nanotubes are studied by the standard porosimetry method. It is shown that the functionalized nanotubes have highly hydrophilic surface; at that the summary surface area measured “by octane” decreased, as a result of the functionalizing, due to the blocking of the nanotubes’ inner channels by the functional groups located at the nanotubes’ ends. The nanotubes’ capacitive properties are studied; their charging-discharging curves appeared being highly reversible, unlike those of other carbonaceous materials. Catalytic properties of the functionalized nanotubes are studied, with particular tendency toward their using as a carrier of platinum catalysts for the methanol oxidation and oxygen electroreduction reactions. When minor amounts (5–10 µg cm−2) of platinum or platinum-ruthenium alloy are deposited onto the nanotubes’ hydrophilic surface, uniform layer of the catalyst is formed, with specific surface area up to 150–300 m2 g−1; high current of the methanol oxidation or oxygen electroreduction is observed at these catalysts. When the catalyst deposit mass increased, its specific surface area decreased, as well as the specific current of the reactions occurring thereon. When the current is related to the electrochemically active unit surface, the catalytic activity is nearly the same both for different catalyst mass deposited onto the nanotubes and the same catalyst mass at different carbonaceous carriers.

Structural and Electrochemical Properties of Carbon Nanotubes and Nanofibers

We studied electrochemical and structural properties of electrodes prepared from new carbonaceous nanomaterials (CNM), specifically, one-walled nanotubes (ONT) and graphite nanofibers (GNF). The ONT were synthesized by an electric arc method using 3Co/Ni as a catalyst [1]. The ONT yield after purification was more than 90%. The ONT diameters, determined by Raman spectroscopy, were 1.2‐1.5 nm. The GNF were synthesized from a 4 : 1 mixture of CO and H 2 at 600ie with Fe : Cu = 7 : 3 as a catalyst. GNF diameters were 100‐300 nm. According to standard porosimetry [2], the specific volume of ONT micropores with radii r < 1.5 nm was 0.29 cm 3 /g, and for GNF, 0.065 cm 3 /g. The hydrogen-sorption capacity of ONT was 3.5 wt % at a hydrogen pressure of 10 MPa. Assuming that the micropore volume is equal to the inner volume of ONT and that the adsorbed hydrogen stays mainly inside ONT, it was found that the hydrogen density was 0.12 g/cm 3 (this value is very close to that obtained theoretically in [3]), i.e. was 1.7 times the liquid hydrogen density, which is caused by a strong interaction between hydrogen molecules and the inner surface of ONT. The electrochemical studies of CNM were performed in a 0.5 M H 2 SO 4 aqueous solution. The hydrogen evolution was observed at E < ‐0.4 V (RHE). Chronovoltammograms for ONT feature a sharp reversible current maximum at E = ‐0.3 V, caused probably by the generation, adsorption/desorption, and ionization of hydrogen, which confirms a strong interaction between hydrogen molecules and the inner surface of ONT. Unlike ONT, GNF possess no reversible mechanism of hydrogen oxidation‐reduction, which is probably explained by different hydrogen sorption mechanisms. On the CNM-based electrodes, the EDL charging occurs much faster and is more reversible as compared to activated carbons (AC). For example, at the EDL charging time τ of 20 s, for AC TSA capacitance Q = 100 F/g, and for GNF it is 20 F/g, while at τ = 2 s for AC TSA Q = 6 F/g, and for GNF it is 12 F/g. This effect is probably due to the absence of pore tortuosity in CNM.