S. V. Belenov

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.

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 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.

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.

Pt(Cu)/C Electrocatalysts with Low Platinum Content

The structure characteristics and the electrochemical behavior of Pt(Cu)/C electrocatalysts synthesized by consecutive deposition of copper and platinum on carbon-support microparticles is studied. The stability and catalytic activity of Pt(Cu)/C materials in reactions of oxygen electroreduction and methanol electrooxidation are assessed and compared with analogous characteristics of a commercial Pt/C material. It is shown that combining the method of galvanic displacement of Cu by Pt with the additional chemical deposition of Pt favors optimization of the structure and functional characteristics of Pt(Cu)/C electrocatalysts. The effect of thermal treatment on the characteristics and properties of electrocatalysts is studied and the optimal conditions of such pretreatment are revealed.

Activity and Stability of Pt/C and Pt-Cu/C Electrocatalysts

AbstractStability in the course of exploitation and catalytic activity in reactions, taking place on the electrodes, are the most important characteristics of electrocatalysts that determine their application in fuel cells. Relationship of the electrochemical behavior of Pt/C catalysts with their morphology is studied in this article. Negative linear correlation between stability and catalytic activity of Pt/C in the reaction of oxygen electroreduction (ORR) has been established. The procedure for choosing electrocatalysts with the optimal ratio of activity and stability has been proposed. Having used CuxPt/C catalysts as an example, we have shown that bimetallic electrocatalysts, prepared by sequential deposition of copper and platinum, can demonstrate significantly higher activity and stability, compared to Pt/C electrocatalysts. Graphical Abstractᅟ

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.