УДК 541.49
COORDINATION COMPOUNDS AS THE PRECURSORS FOR PREPARATION OF NANOSIZED PLATINUM OR PLATINUM-CONTAINING MIXED-METAL CATALYSTS OF OXYGEN REDUCTION REACTION
© 2015 V. A. Grinberg1, * , V. V. Emets1, N. A. Mayorova1, A. A. Pasynskii2, A. A. Shiryaev1, V. V. Vysotskii1, V. K. Gerasimov1, V. V. Matveev1, E. A. Nizhnikovskiy1, and V. N. Andreev1
1 Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow 2 Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow
*E-mail: vgrinberg@phyche.ac.ru Received February 26, 2015
Nanosized mixed-metal platinum—iron, platinum—manganese and platinum—nickel catalysts supported on highly dispersed carbon black are synthesized by using of the corresponding metal complexes with the atomic metal ratio of1 : 1. The obtained catalysts are characterized by X-ray phase diffraction and scattering analysis, electron dispersion analysis, scanning and transmission electron microscopy. Thin film rotating disk electrode technique was used to study kinetic parameters of the oxygen reduction reaction at these catalysts. It has been demonstrated that electrochemical activity of the prepared catalysts is comparable to that of a commercial Е-Tek platinum catalyst. The membrane electrode assembly (MEA) with the synthesized platinum-iron catalyst was tested in a laboratory hydrogen-air fuel cell setup at room temperature. It was shown that the power performance of MEA was twice better than that of of MEA on the base of a commercial Pt/C E-Tek catalyst.
DOI: 10.7868/S0132344X1511002X
INTRODUCTION
Hydrogen-oxygen fuel cells (FCs) belong to most promising self-contained electric power sources, particularly for electromobiles, some stationary and portable devices. Among diverse FC types the most suitable devices for these purposes are low temperature hydrogen-oxygen (air) FCs with ion-exchange membranes fueled by pure hydrogen or hydrogen obtained by conversion of liquid organic fuels. Such devices are characterized by high performance, enhanced power density (W cm-2), sufficient lifetime; they are environmentally friendly and compact. So far, the technology of production of such elements is generally developed, but serious problems hampering large-scale FC application still exist. The most important problems include not only hydrogen production, purification, and storage, but also development of more efficient elec-trocatalysts that would provide longer FC operation without deterioration of their characteristics, would contain minimum amounts of noble metals, would be inexpensive and processable.
It is well known that characteristics of hydrogen-oxygen FCs depend mainly on the rate of the cathodic oxygen reduction reaction. To enhance the efficiency of catalysts in this reaction, multicomponent, particularly binary, Pt-based metallic nanosized systems (Pt-Co, Pt-Ni, Pt-Cr, Pt-Fe etc.), that are also tolerant towards organic admixtures, are studied extensively [110]. However the results obtained are often controver-
sial. Thus, in some works the authors observed true catalytic effects after introduction of the second metal, i.e., an increase in kinetic current per true surface area or per single surface platinum atom [7]. In other cases an increase in current was related to a partial etching of the basic metal component and therefore to an increase in the alloy true surface area [1]. In some cases an advantage of binary catalysts was manifested largely in their tolerance towards methanol [10]. Different mechanisms of the catalytic effect are also discussed: a change in the length of the Pt-Pt interatomic bond as a result of alloy formation [5], inhibition of the water decomposition reaction due to formation of the surface Pt-OH groups hindering oxygen adsorption [8], the Fermi level effect [10, 11] or combined influence of structural factors and variation in the electronic state of metals in the alloy [6, 12]. It should be noted that the phase composition and structure of binary systems are varied from mixed deposits (Pt-Cr [10]) to solid solutions (Pt-Co, Pt-Ni [7]) and intermetal-lides with the composition Pt3M (Pt3Co, Pt3Fe [8]). Analysis of literature data shows that the observed catalytic effects and their mechanism are determined not only by the chemical nature of a binary catalyst, but also by its structure and phase composition that, in turn, depend on the method of synthesis.
In the case of co-reduction of the salts of different metals, one often fails to obtain a catalyst of a given composition and structure due to difference in the
rates of metallization processes. In this work binary catalysts were synthesized from precursors of coordination complexes of platinum and other metals. Impregnation of a suitable support by a solution of such compounds and removal of organic shell of the metal core by thermodestruction allows obtaining a homogeneous heterometallic coating on the support surface [13]. Herewith, the ratio of Pt : Me in the metal core of the precursor can be varied in a sufficiently wide range. The processes of thermal decomposition can be controlled using differential thermal analysis, thermo-gravimetry, and differential scanning calorimetry.
This work reports results of investigation of binary catalysts of the oxygen reduction reaction, namely: PtFe/C, PtMn/C, and PtNi/C. For comparison, measurements were also carried out on samples of a catalyst obtained with clustered Pt only (specifically, from ethoxy-dicyclopentadienyl-platinum-ethoxide (C10H12OC2H5)2Pt3(OC2H5)4), and on a commercial E-Tek platinum catalyst (20 wt % Pt on Vulcan XC-72).
EXPERIMENTAL
Preparation ofbinary catalysts. Ethoxy dicyclopentadi-ene—platinum—ethoxide (C10H12OC2H5)2Pt3(OC2H5)4 was chosen as the initial platinum compound for synthesis of heteroorganometallic precursors. Commercial coordination complexes of iron, manganese, and nickel were used as its partners, namely: [CpFe(CO)2]2, HOOCC5H4Mn(CO)3, (a-Pic)2Ni(OOCCMe3)2. For the synthesis a multistep procedure was used: 1) sonication of highly dispersed Ketjen Black (the specific surface area 600 m2 g-1) in absolute tetrahydrofuran (THF); 2) dropwise addition of the corresponding mixed precursors solution in THF; 3) sonication, 4) drying at 100°C under vacuum; 5) annealing at 500°C in a hydrogen atmosphere for 45 min; 6) cooling in the atmosphere of a high-purity argon. The obtained catalysts contained 30 wt % of the metals and 70 wt % of the carbon black. The atomic ratio of the metals in binary systems was close to 1 : 1.
Structural studies. Studies of morphology and structure in the initial state and after electrochemical polarization were carried out using a Quanta 650 FEG scanning electron microscope (SEM) equipped with a field emission cathode (FEI, Netherlands) and an energy-dispersive detector.
X-ray phase analysis was carried out using an Empyrean diffractometer (Panalytical) with filtered Cu^a radiation in the standard Bragg-Brentano geometry ("reflection"). Samples were studied in the absence of binders.
Small-angle X-ray scattering measurements were performed using a specialized SAXSess diffractometer (Anton Paar). Samples in an envelope of non-scattering polymer were analyzed at room temperature in the transmission geometry; the sample chamber was evacuated; imaging plates were used as detectors. Experi-
mental curves were normalized for sample absorption; standard desmearing procedures were applied. Particle size distribution was calculated after subtraction of initial carbon support scattering according to the Tikhonov regularization method using a GNOM software [14]. The average particle size and size distribution were also determined using a Philips EM-301 transmission electron microscope at the accelerating voltage 80 kV.
Electrochemical measurements. Electrochemical studies were carried out using PI-50.1 or PAR 273A potentiostats. Measurements on a rotating disk electrode were carried out using an EL-02.06 potentiostat. The true surface area of supported metallic catalysts was determined by anodic stripping of the carbon monoxide monolayer [15].
Thin film rotating disk electrode (TFRDE) technique was used to estimate kinetic currents of the oxygen reduction at the studied catalysts [7-10, 15]. The electrochemical cell and TFRDE manufacturing procedure are described in detail in [16]. The aqueous catalyst suspension stabilized by sonication was transferred to the surface of a glassy carbon disk electrode (the surface area 0.07 cm2) in the amount 21 ^g cm-2 (per platinum). After drying in air at 60° C the catalyst layer was fixed on the electrode surface using a Nafion (Aldrich) 0.05% aqueous solution. The calculated thickness of the Nafion film after drying was 0.15 ^m. All TFRDE measurements were performed in a 0.5 M H2SO4 solution prepared from extra-pure grade sulfuric acid and deionized water; the solution was oxygen saturated under atmospheric pressure. A platinum gauze (~10 cm2) served as the counter electrode and a Hg/Hg2SO4/0.5 M H2SO4 was used as the reference electrode. Compressed gases were used to argon purge the solution or to saturate it with oxygen. All measurements were performed at room temperature.
Thin film RDE was cleaned and activated electro-chemically by cycling its potential in the range 0.0-1.2 V in case of monoplatinum and in the range 0.0-1.0 V in case of bimetallic catalysts.
The study of the ORR kinetics was carried out by cyclic voltammetry over the potential range 1.1-0.2 V at a scan rate of 5 mV s-1 and the electrode rotation speed ~2000 rpm. Reproducibility of the current measurements was ±5-6%. In order to estimate stability of the synthesized catalysts under working conditions, accelerated tests in a continuous electrode cycling mode were carried out in the potential range 0.2-1.0 V at a scan rate of 100 mV s-1 under continuous bubbling of the solution by oxygen (500 cycles followed by analysis of t
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