V. A. Volochaev

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.

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.

The effect of thermal treatment on the atomic structure of core–shell PtCu nanoparticles in PtCu/C electrocatalysts

PtCu/C electrocatalysts with bimetallic PtCu nanoparticles were synthesized by successive chemical reduction of Cu2+ and Pt(IV) in a carbon suspension prepared based on an aqueous ethylene glycol solution. The atomic structure of as-prepared PtCu nanoparticles and nanoparticles subjected to thermal treatment at 350°C was examined using PtL3 and CuK EXAFS spectra, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). The results of joint analysis of TEM microphotographs, XRD profiles, and EXAFS spectra suggest that the synthesized electrocatalysts contain PtCu nanoparticles with a Cu core–Pt shell structure and copper oxides Cu2O and CuO. Thermal treatment of electrocatalysts at 350°C results in partial reduction of copper oxides and fusion of bimetallic nanoparticles with the formation of both homogeneous and ordered PtCu solid solutions.

Microstructure Optimization of Pt/C Catalysts for PEMFC

Development of optimal methods for the synthesis and for the control of multi-level microstructure of Pt/C materials is essential for the preparation of electrocatalysts , which have high activity and durability. Nucleation/growth of metallic nanoparticles could take place in the liquid phase, on the surface and in the pores of the carbon microparticles during the process of chemical reduction of Pt precursor in a liquid phase. These processes are sensitive to the composition of the medium, temperature, pH, mass transfer conditions, and other factors. On the one hand, that creates more opportunities to search the optimal conditions of synthesis. On the other hand, it makes difficult to control the reproducibility of the composition/structure of Pt/C . Pt/C materials with metal fraction from 10 to 20 wt% were obtained in this work. An average size of Pt crystallites was from 1.0 to 5.5 nm, depending on the method and conditions of synthesis. Electrochemically active surface area of catalysts measured by hydrogen electrodesorption method, was from 32 to 152 m2/gPt.

Cu@Pt/C Catalysts: Synthesis, Structure, Activity in Oxygen Reduction Reaction

Nanostructured Cu@Ptx/C catalysts with low platinum content (x = 0.8), comprising nanoparticles of core-shell architecture, were obtained by method of synthesis, which combines galvanic substitution of Cu at Pt and chemical reduction of Pt(IV). The obtained catalysts show high values of the electrochemically active surface area of platinum 80–100 m2/g (Pt) and higher both activity in the oxygen electroreduction reaction (ORR) and stability compared with commercial Pt/C catalyst HiSPEC 3000 (Johnson Matthey).

On the possibilities of recognizing the architecture of binary Pt–M nanoparticles

The identification of subtle structural effects in bimetallic nanoparticles via conventional methods is discussed using the example of PtCu/C nanostructured electrocatalysts. The divergence factors between experimental X-ray diffraction profiles and patterns simulated in the single-phase approximation for pure (or solid solution) and bi-phase metals in a suspected core–shell structure are found to be different. The catalysts containing the core–shell nanoparticles may be chosen from some materials with nanoparticles of different architectures based on X-ray diffraction and cyclic voltammetry data if the nanoparticles consist of a relatively massive Cu core and a thick Pt shell.