V. E. Guterman

Microstructure and electrochemically active surface area of PtM/C electrocatalysts

The results of the study of microstructural parameters and the data on the electrochemically active surface area of Pt/C and Pt50M50/C (M = Ni, Cu, Ag) catalysts in 1 M H2SO4 solutions are compared. The metal-carbon nanomaterials were prepared by the method of chemical reduction of metals from the organoaqueous solutions of their compounds. The loading of metal component in them was 30–33 wt %. It is found that actual composition of metal component in the synthesized binary systems fits best the theoretically expected one (1: 1) for the PtAg/C catalyst whereas in the PtNi/C and PtCu/C systems, a considerable fraction of alloying component is present in the form of the corresponding oxides. A decrease in the average size of crystallites of metal component from 3.8 to 1.6 nm in the series of studied materials PtAg/C > Pt/C ≥ PtCu/C s> PtNi/C does not correspond to the character of the variation of electrochemically active surface area of the catalysts: PtNi/C ≈ PtCu/C < Pt/C ≪ PtAg/C increasing from 16–20 to 62–69 m2/g(Pt). The contradiction can be caused by the preferential segregation of platinum on the surface of nanoparticles of PtAg alloy, a higher degree of agglomeration of smaller nanoparticles, and, in the case of PtNi/C and PtCu/C materials, also by the insulation of a fraction of nanoparticle surface area by the corresponding oxides.

Influence of water-organic solvent composition on composition and structure of Pt/C and PtxNi/C electrocatalysts in borohydride synthesis

Nanostructured Pt/C and PtxNi/C materials containing from 4 to 30 wt % Pt were prepared using the method of chemical reduction of platinum and nickel compounds in the liquid phase by sodium borohydride solution. The change in the nature of the organic component (ethylene glycol, dimethyl sulfoxide, dimethyl formamide, tetrahydrofuran, acetone, ethyl alcohol, and glycerin) of binary water-organic solvent influences considerably the reaction product yield, structural characteristics (average nanoparticle size) of the material, mass fraction of metals in the nanocomposite, and the PtNi alloy composition. The fundamental possibility of controlling the PtNi alloy composition and structural characteristics of Pt/C and Ptx-Ni/C nanocomposites by variation of the water-organic solvent composition was demonstrated. The minimum average size of nanoparticles in synthesized metal-carbon materials promising as electrocatalysts in low-temperature fuel cells was 2 nm for Pt/C (for about 20 wt % Pt) and 1.8 nm for Pt2Ni (for about 30 wt % Pt).

Atomic structure of PtCu nanoparticles in PtCu/C catalysts from EXAFS spectroscopy data

Deposited electrocatalysts with different distributions of components in PtCu bimetallic nanoparticles involved in their composition were synthesized by simultaneous and sequential reduction of Cu(2+) and Pt(IV) in a carbon suspension. The dependence of the atomic structure of PtCu nanoparticles on the synthesis conditions and the degree of influence of post-treatment was established from analysis of the changes in Fourier transforms of the experimental Pt and Cu EXAFS spectra, as well as the structural parameters obtained by their fitting before and after the treatment of the materials in an acid solution. A technique was proposed for visualizing the atomic structure of synthesized bimetallic nanoparticles. This technique made it possible to determine the character of the distribution of the components over the nanoparticle volume in accordance with the component composition and local atomic structure parameters determined from EXAFS spectroscopy and to obtain the visualization of the distribution of the components in PtCu nanoparticles synthesized by the aforementioned methods.

Effect of CO atmosphere on morphology and electrochemically active surface area in the synthesis of Pt/C and PtAg/C electrocatalysts

An effect of CO atmosphere on the microstructure of Pt/C, PtAg/C, and Ag@Pt/C electrocatalysts formed in the synthesis and on the electrochemically active surface area (ECAS) has been studied. Synthesis is carried out via the joint or sequential chemical reduction of silver and platinum precursors in a suspension of Vulcan XC-72 disperse carbon carrier. Adsorption of CO molecules on a surface of platinum and Pt-Ag is shown to hamper their growth and aggregation, leading to a considerable increase in ECAS of platinum, as well as Pt/C and PtAg/C materials to be synthesized. The impact of CO on the morphological characteristics of the Ag@Pt/C materials containing a significant proportion of bimetallic nanoparticles (NPs) with an Ag-core/Pt-shell structure is less because of the weak adsorption of CO on the silver surface. ECAS values of platinum in the materials synthesized in the CO atmosphere were 152, 88, and 75 m2/g for Pt/C, PtAg/C, and Ag@Pt/C materials, respectively.

Effect of Wet Synthesis Conditions on the Microstructure and Active Surface Area of Pt/C Catalysts

Nanostructured Pt/C electrocatalysts containing about 20 wt % Pt have been produced by chemical reduction in Pt(IV) solutions. The nature of the reductant (sodium borohydride, ethylene glycol, formaldehyde, or formic acid) and the associated changes in synthesis conditions have a significant effect on the structural characteristics of the materials obtained. In particular, the average size of Pt nanoparticles (crystallites) ranges from 1.8 to 5.5 nm. The largest electrochemically active surface area of the Pt in the catalysts obtained in this study (128 m2/g Pt) considerably exceeds that of E-TEK, a commercially available Pt/C catalyst similar in composition ( 110 m2/g Pt).

The relationship between activity and stability of deposited platinum-carbon electrocatalysts

The operation-mode stability and the catalytic activity in electrode reactions are the most important properties of electrocatalysts that determine the possibility of using them in fuel cells. The negative linear correlations between stability and catalytic activity of a series of Pt/C and Pt–Cu/C materials in the oxygen electroreduction reaction are revealed and studied. A method of selecting electrocatalysts with the optimal combination of activity and stability is proposed. The Cu@Pt/C catalysts containing bimetallic nanoparticles with the core–shell architecture which demonstrate the anomalously high combination of activity and stability are synthesized.

Studying the initial stage of nucleation and growth of a new phase during cathodic intercalation of lithium into aluminum

Kinetics of cathodic intercalation of lithium into aluminum from a 0.5 M LiCl solution in dimethylformamide at the stage of nucleation and growth of intermetallic compound β-LiAl is studied by one- and two-pulse potentiostatic methods. If the length of the first potential pulse is short, the current at the beginning of the second pulse is proportional to the overvoltage squared. The experimental data point to a lamellar-spiral growth of β-LiAl crystals at the initial stage of their development and to a change in the balance between different growth mechanisms as a function of the overvoltage and surface coverage by β-LiAl.

Phase behavior of Pt–Cu nanoparticles with different architecture upon their thermal treatment

Using the methods of powder X-ray diffraction, transmission electron microscopy, and EXAFS spectroscopy, the phase behavior of bimetallic Pt–Cu nanoparticles with different architecture that are deposited on a highly disperse carbon carrier has been investigated during their thermal treatment in inert atmosphere. It is established that Pt–Cu nanoparticles with a Cu-core–Pt-shell structure rearrange into nanoparticles with a Pt–Cu solid-solution structure in the temperature range from 280 to 300°C. This transformation is accompanied by a sharp change in the unit-cell parameter. Such a change in the crystal lattice parameter does not occur during the thermal treatment of material with similar composition containing Pt–Cu nanoparticles with a solid-solution structure. The results can be used in elucidating the structure of Pt–M/C materials with different nanoparticle architectures.

Synthesis of nanostructured Pt/C electrocatalysts and effects of ambient atmosphere composition and an intermediate support on their microstructure

Pt/C catalysts containing 10 to 20 wt % Pt have been prepared by chemical reduction of platinum from Pt(IV) solutions. The use of an intermediate hydroxide support (Fe(OH)2 or SiO2 · nH2O) in Pt/C synthesis has been shown to have a significant effect on the weight percentage, crystallite size, and electrochemically active surface area of Pt. We have established how the composition of the liquid-phase synthesis atmosphere (air, Ar, or CO) influences the structural characteristics of the Pt/C materials. The electrochemically active surface area of Pt in the synthesized catalysts ranges from 32 to 152 m2/g Pt.

Borohydride synthesis of the Pt x -Ni/C electrocatalysts and investigation of their activity in the oxygen electroreduction reaction

Nanosized Pt-Ni//C electrocatalysts are prepared by methods of liquid-phase synthesis. For the factors that have a direct bearing on the composition of the synthesized materials, the pH, temperature, and composition of a water-organic solvent are studied. The weight percentage of metals in the electrocatalyst, the average size of the formed nanoparticles, and the composition of the Pt-Ni alloy are determined by methods of X-ray diffraction and elemental analyses. The electrocatalytic materials that are characterized by a high platinum content of 25–35 wt % and by a small average diameter of their nanoparticles (3.2–4.5 nm) are produced when using water-ethylene glycol mixtures as solvents. The electrocatalytic activity of the obtained Ptx-Ni/C materials in the oxygen electroreduction reaction in a 0.5 M solution of orthophosphoric acid is studied by the potentiodynamic method. The potentiodynamic study makes it possible to single out electrocatalysts whose specific characteristics are superior to those of commercial Pt/C electrocatalyst TEC10V50E.