Pyrolysis of nitrogen-containing complexes of iron and cobalt on the surface of disperse carbon materials was used for synthesis of cathode catalysts for oxyhydrogen fuel cells (FC) with proton-conducting (acidic) and anion-conducting (alkaline) electrolytes. The catalysts were characterized by XPS and tested using a thin-film disc electrode and in oxyhydrogen FC under model conditions. Properties of the CoFe/C system prepared by pyrolysis of macroheterocyclic compounds of iron and cobalt on carbon materials (soot HS-72 and multilayer nanotubes (CNT)) were described for the first time. From XPS data, the surface of the catalytic CoFe/C systems is rich in carbon (95,5 at.%), contains nitrogen (2 at.%), oxygen (2 at.%) and metals (0,5 at.%). The data obtained by electrochemical measurements under model conditions revealed that the catalytic systems CoFe/CNT are close to the commercial platinum catalyst 60%Pt/C (HiSPEC9100) in their activity to oxygen reduction in an alkali medium (0,5 M KOH). Half-wave potentials are 0,85 and 0,88 V for catalysts CoFe/CNT and 60%Pt/C (HiSPEC9100), respectively. The maximal specific capacity of the oxyhydrogen FC with an anion-conducting electrolyte is 210 mW/cm2 (a 60%Pt/C (HiSPEC9100) based cathode) and 180 mW/cm2 (CoFe/CNT based cathode). In its characteristics, MEA with the non-platinum cathode compete well with the best analogues described in literature. The results obtained demonstrated the necessity of the further studies on scaling-up the technology for synthesis of the developed non-platinum cathode catalysts and on optimization of the MEA FC architecture based thereon.

Рассмотрены современные задачи в области электрокатализа токогенерирующих реакций низкотемпературных водородо-воздушных топливных элементов (ТЭ). Эти задачи сформулированы в рамках основных закономерностей электрокатализа и его непосредственной связи с электрохимической кинетикой и адсорбционными процессами на межфазной границе. Анализ опубликованных данных и материалов, представленных на специализированных конференциях по ТЭ в США, Канаде и Англии, показывает, что целью мировой электрохимической науки является снижение расхода платины вплоть до ее полного устранения и решение вопроса об использовании дешевого водорода с примесями СО и СО 2 из различных альтернативных источников. На примере исследований в ИФХЭ РАН продемонстрированы результаты работ в этих направлениях.

Results on the development of new cathodic catalysts (monoplatinum and cobalt-modified platinum) applied on carbon nanotubes are shown. By means of a complex of electrochemical and structural techniques, it is shown that as regards their activity under model conditions and within membrane-electrode assembles, the catalysts synthesized by the polyol method are close to commercial monoplatinum systems with the same mass content of platinum (20 wt %) and their corrosion stability is double that of commercial catalysts. Platinum modified with cobalt is characterized by still higher stability, which allows considering these catalytic systems as the candidates to be used in fuel cells after the corresponding optimization.

Multiwalled carbon nanotubes (CNTs) were synthesized by catalytic pyrolysis of methane on iron-cobalt or cobalt-molybdenum catalyst and investigated by electrochemical and physico-chemical methods before and after chemical or electrochemical corrosion treatment. It is shown that CNTs have a higher corrosion resistance than does turbostratic carbon (carbon black) in corrosion testing under the same conditions. This is expressed in a smaller change in the amount of oxygen on the surface of the carbon material, the values of the electrochemically active surface area (EAS), and in significant differences of these quantities for the CNTs compared to carbon black. Quantitative comparison of the results of chemical and electrochemical treatment of CNT and carbon black, which was performed in this paper for the first time, leads to the conclusion regarding the advantages of corrosion testing by chemical method. Chemical testing simulates to a greater extent the long-term testing conditions of the supported catalysts composed of membrane-electrode assemblies of fuel cells in terms of evaluating the stability of the carbon material as a support of the catalytically active centers.

Cathode catalysts for a hydrogen–oxygen fuel cell (FC) with proton-conducting (acidic) and anion-conducting (alkaline) electrolytes are synthesized via the pyrolysis of nitrogen-containing iron and cobalt complexes on the surfaces of highly disperse carbon materials. The catalysts are characterized by X-ray photoelectron spectroscopy (XPS) and tested under model conditions on a thin-layer disk electrode and as a part of a membrane electrode assembly of hydrogen–oxygen FCs. The properties of the CoFe/C system formed via the pyrolysis of macroheterocyclic cobalt and iron compounds on carbon materials (XC-72 soot and multiwall nanotubes (MNTs)) are described for the first time. According to XPS data, the surface of the CoFe/C catalytic systems is enriched with carbon (95.5 at %) and contains nitrogen (2 at %), oxygen (2 at %), and metals (0.5 at %). According to the results from electrochemical measurements under model conditions, the CoFe/MNT catalytic systems approaches 60% Pt/C (HiSPEC9100) commercial platinum catalyst according to their activity in the oxygen reduction reaction in an alkaline medium (0.5 M KOH). The half-wave potentials are 0.85 and 0.88 V for CoFe/MNT and 60% Pt/C (HiSPEC9100) catalysts, respectively. The maximum specific powers of hydrogen–oxygen FCs with anion-conducting electrolytes are 210 mW/cm 2 (60% Pt/C (HiSPEC9100) based cathode) and 180 mW/cm 2 (CoFe/MNT based cathode). The characteristics of a membrane electrode assembly with a non-platinum cathode correspond to the best analogs described in the literature. The results of this work show the prospects for further studies on scaling this technology for the synthesis of the proposed non-platinum cathode catalysts and optimizing the architecture of the membrane electrode assembly of FCs based on them.

Pyrolysis of nitrogen-containing complexes of iron and cobalt on the surface of disperse carbon materials was used for synthesis of cathode catalysts for oxyhydrogen fuel cells (FC) with proton-conducting (acidic) and anion-conducting (alkaline) electrolytes. The catalysts were characterized by XPS and tested using a thin-film disc electrode and in oxyhydrogen FC under model conditions. Properties of the CoFe/C system prepared by pyrolysis of macroheterocyclic compounds of iron and cobalt on carbon materials (soot HS-72 and multilayer nanotubes (CNT)) were described for the first time. From XPS data, the surface of the catalytic CoFe/C systems is rich in carbon (95,5 at.%), contains nitrogen (2 at.%), oxygen (2 at.%) and metals (0,5 at.%). The data obtained by electrochemical measurements under model conditions revealed that the catalytic systems CoFe/CNT are close to the commercial platinum catalyst 60%Pt/C (HiSPEC9100) in their activity to oxygen reduction in an alkali medium (0,5 M KOH). Half-wave potentials are 0,85 and 0,88 V for catalysts CoFe/CNT and 60%Pt/C (HiSPEC9100), respectively. The maximal specific capacity of the oxyhydrogen FC with an anion-conducting electrolyte is 210 mW/cm2 (a 60%Pt/C (HiSPEC9100) based cathode) and 180 mW/cm2 (CoFe/CNT based cathode). In its characteristics, MEA with the non-platinum cathode compete well with the best analogues described in literature. The results obtained demonstrated the necessity of the further studies on scaling-up the technology for synthesis of the developed non-platinum cathode catalysts and on optimization of the MEA FC architecture based thereon.

The corrosion stability (under the chemical effect of acid environment and cycling of electrode potential) of 50 wt % PtCoCr/C trimetallic cathode catalyst has been studied. It is proposed that the dominant mode of degradation is dissolution of platinum nanoparticles and their redeposition on a surface. The values of activation energy of electrolytic reduction of molecular oxygen on 50 wt % PtCoCr/C catalysts, as well as the activation energy of its corrosion in 0.5 M H2SO4 solution, have been determined. The high corrosion stability of PtCoCr/C catalyst is attributed to the lower extent of filling of platinum surface with oxygen-containing particles (Q O/2Q H), which constitute the initial stage of platinum dissolution. It has been shown that the decrease in mass activity in electrolytic reduction of O2 during cycling of potential at 20°C up to 4000 cycles is 15%, which is significantly higher than for the monoplatinum system. The above-proposed mechanism of catalyst degradation in model experiments can lead to permanent loss of Pt ions in electrolyte bulk. In the case of measurements in a fuel cell (FC), a degradation mechanism described as dissolution of platinum nanoparticles and their redeposition on surface can be accompanied (as a consequence of a low amount of electrolyte in the FC) with redeposition of platinum particles in polymeric electrolyte.

The kinetic parameters of oxygen reduction reaction on Vulcan XC72R carbon black, carbon black modified by pyrolysis products of cobalt 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrine named by MXC72R, and commercial Pt/C platinum catalyst have been considered. Comparison of their electrochemical properties including pH influence in a broad interval from 0.3 to 14.6 has shown platinum-similar behavior of MXC72R in the reaction under study. An increasing order of electrochemical activity for oxygen reduction reaction is obtained, namely XC72R < MXC72R < Pt/C. The mechanism of oxygen reduction reaction is discussed emphasizing the important accelerating role of protonation of adsorbed oxygen molecule and inhibiting role of oxygen-containing species (formed due to water molecule discharge) and strongly adsorbed foreign species. The importance of the research on oxygen reduction reaction in a broad pH interval to evaluate the efficiency of various precious metal-free catalytic systems in comparison to platinum is emphasized.

Creation of multicomponent catalytic systems is the main way to decrease the content of or completely replace Pt in fuel cell cathodes. Compared to the conventional catalytic systems, production of PtCoCr catalysts on different substrates (XC-72 carbon nanotubes, TiO 2 ) differs in high-temperature conditions and the use of nitrogen-containing transient-metal precursors. According to electrochemical and structural studies, during synthesis and subsequent treatment, alloy nanoparticles with a core-shell structure enriched in platinum are formed on a carbon material doped with nitrogen. The ligand effect of the alloy core results in an increase in the electron density of the platinum d-level, acceleration of oxygen reduction, and deceleration of water molecule discharge and platinum corrosion. A architecture of membrane electrode assembly involving PtCoCr-based active layers of varying composition is developed for fuel cells operating at a temperature of 65°C in hydrogen-air and hydrogen-oxygen environments. In both cases, the use of PtCoCr instead of monoplatinum catalysts enabled us to halve the platinum consumption at the same discharge current density and specific power. The results of life testing and potential cycling of membrane electrode assemblies under severe conditions showed that the resistance of PtCoCr systems is not inferior to platinum.

Electrocatalytic activity in reactions of oxygen reduction and corrosion stability (chemical in the sulfuric acid solution saturated by air) and electrochemical stability (under catalyst potential cycling in different ranges) were studied on the commercial monoplatinum catalyst and synthesized PtCoCr trimetallic system. The trimetallic system characterized by alloy formation exceeds the monoplatinum catalyst in all three parameters (specific surface area of the platinum component, electrocatalytic activity, and stability). The dissolution of the trimetallic system components changes the composition of the system and leads to the formation of a new surface platinum-enriched structure of nanoparticles with different properties caused by the strong ligand effect of the subsurface alloy layer.

The effect of blocking species, namely, oxygen-containing species adsorbed from water and blank electrolyte anions, on the kinetics of oxygen electroreduction on 40% Pt ETEK (denoted as 40Pt/C) and 20% Pd ETEK (20Pd/C) commercial catalysts, as well as on 20% Pd ETEK 6% Co (denoted as 20Pd6Co/C) and 20% Pd ETEK 5% Pt 2.9% Co (20Pd5Pt2.9Co/C) catalysts synthesized at the laboratory, is studied in sulfuric and perchloric acid solutions. It is shown that on a monopalladium catalyst, the first inhibiting factor prevails, whereas, on a monoplatinum catalyst, a substantial role in decelerating the reaction is played by the adsorption of HS anions. In the case of a trimetal catalyst, the adsorption curve of oxygen-containing species is noticeably shifted toward the curve typical of platinum. When perchloric acid is replaced with sulfuric acid, the potential of the kink point of the Tafel dependence of the oxygen electroreduction (on going from the low to high polarization) is shifted toward the less positive potentials. This shift of the kink potential is most clearly pronounced in the case of a monoplatinum catalyst.

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mgPt) and specific activity (mA/c Pt 2 ) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N4-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt3Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O2 adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy

A PdCoPt/C catalytic system is synthesized by modifying a commercial 20% Pd/C (ETEK) catalyst, and its morphology and composition are determined. The stability of the catalyst is estimated by means of accelerated durability testing in a 0.5 M H 2 SO 4 solution either using potential cycling or in the absence of polarization. The partial dissolution of Pd and Co components is shown to result in a restructuring of the catalyst surface and the formation of a core-shell Pt/PdCoPt/C structure with higher catalytic activity and stability compared to a Pt/C system. The results of experiments with a rotating ring disc electrode indicate the change in the reaction path of oxygen reduction after the chemical and electrochemical treatment of the catalyst and confirm the formation of a core-shell structure.

An express method for determining the corrosion stability of palladium-based cathodic catalytic systems was developed. The method consists of cycling the electrode potential with a thin catalyst layer in 0.5 M H2SO4 in the range of potentials of the fuel cell operation and a subsequent comparison of the characteristics of catalytic systems (specific surface area and activity in the oxygen reduction reaction) before and after the corrosion action. The suggested method was used to characterize the corrosion behavior of the Pd/C commercial catalyst, as well as the PdCo/C and PdCoCr/C catalysts synthesized at the Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences. According to the XRD data, they correspond to systems with a high level of alloy formation. It is shown that the PdCo/C and PdCoCr/C catalysts are considerably more stable towards corrosion action than 20% Pd/C (E-TEK). The results were compared with the stability data obtained using a chronoamperometric method.

The electrochemical behavior of composite materials based on phenyl substituted cobalt porphyrins and Nafion is studied. Several cobalt porphyrins with presumably predictable variation of their hydrophilic/hydrophobic properties due to different donor and acceptor substituents in the para position of phenyl rings are synthesized and studied. It is shown that introduction of Nafion into a system with acceptor substituents results in a significant acceleration of the model oxygen reduction reaction. This allows assuming that a bond between a proton of the Nafion sulfogroup with the porphyrin active center is most probable in this very group of porphyrins, which facilitates the protonation step required for activation of the oxygen molecule. A certain correlation is found between the model reaction of oxygen electroreduction (halfwave potential, reaction rate constant) and Hammett constant varying as dependent on the nature of peripheric substituents.

Binary nanodispersed carbon XC72 supported PdFe catalysts with different atomic palladium-to-iron ratios are synthesized and studied in oxygen reduction reaction in acid solution at 60 degrees C. The Pd:Fe ratio was well controlled by the initial concentrations of Pd and Fe in the precursor solutions. The nanoparticles were characterized by transmission electron microscopy, X-ray diffractometry and X-ray photoelectron spectroscopy. The optimum Pd:Fe ratio for this reaction was determined to be 3:1.The comparison of activities of the catalysts with component ratios equaled 3:1 and 10:1 is shown that the activities are differed from each other by 10-15 times in advantage of catalyst with lesser content of palladium. This phenomenon can be related to the different particle size of both catalysts and different distribution of particles by size discovered by TEM method. The achievement of maximum activity near the ratio of Pd:Fe = 3:1 is due to as effect of alloy-forming and the influence of binary system component ratio and synthesis conditions on dispersity degree of metallic phase nanoparticles. Under optimal conditions of precursor mixture high-temperature pyrolysis, iron produces the stabilizing effect palladium. It gives rise to obtaining the uniform and finely divided (7-8 nm) metallic particles. (c) 2007 Elsevier Ltd. All rights reserved.