L. P. Kazanskii

Study of degradation of membrane-electrode assemblies of hydrogen-oxygen (air) fuel cell under the conditions of life tests and voltage cycling

The degradation processes of HiSPEC 9100 (60% Pt/C) and 13100 (70% Pt/C) cathodic monoplatinum catalysts, which were tested under the model conditions and in the composition of membrane-electrode assemblies (MEA) of hydrogen-air and hydrogen-oxygen fuel cells, are studied. It is shown that, in all cases, the main reason for a decrease in the catalyst activity was a decrease in its surface area, which was caused by the coarsening of platinum nanoparticles, irreversible oxidation of a fraction of active centers, and the destruction of the catalyst due to the carbon support oxidation. The results of electrochemical measurements are supplemented with the structural investigations by the methods of transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and X-ray photoelectron spectroscopy (XPS). It is found that the degradation processes of MEA in the accelerated stress tests (AST) are similar to those in the long-term life tests. With respect to a decrease in the catalyst active surface area, the application of 2500 cycles in the voltage range of 0.6 to 1.2 V in the AST is equivalent to the life tests for 1010 h. During the fuel cell operation, the destruction of polymer electrolyte proceeds along with the catalyst degradation. This leads to a decrease in the ion-exchange capacity of the membrane and ionomer in the composition of cathode active layer.

Corrosion of uranium and its low content Zr, Nb, and Ru alloys in aqueous solutions

Corrosion of uranium and its alloys with low content (0.5–5.0 at %) of Zr, Nb, and Ru in water and bicarbonate aqueous solutions is studied; the effect of hydrogen peroxide, the main product of radiation processes, on the corrosion rate is elucidated. The rate of the primary corrosion process U +(2 +n)H2O=UO2·nH2O+ 2H2↑ is measured by electrochemical methods in anaerobic and aerobic conditions for uranium metal and its alloys containing 0.5 to 5.0 at % of Zr, Nb, and Ru. It is shown that the corrosion rates for the alloys are lower than that of reactor-grade uranium; however, the difference is rather close to the measurement error. The corrosion mechanism is studied; U(III) is shown to be rather unstable in neutral solutions when uranium(III) hydroxide is precipitated and no significant amount of U(III) and UH3 is present among the products of the metallic uranium corrosion in water. The kinetics of the second corrosion stage U(IV) + O2→U(VI) is studied by spectrophotometric method. It is shown that the reaction of U(IV) oxidation by atmospheric oxygen is similar in weakly acid solutions (pH 1.5–4.0) and in bicarbonate media: in particular, it has an induction period for uranium (IV) accumulation, after which the reaction accelerates; it is formally a first-order reaction with respect to uranium. The reaction mechanisms differ in the two media: in weakly acid solutions, after the appearance of U(VI), the reproportionation reaction proceeds; thus formed U(V) interacts with O2 faster than U(IV). In the bicarbonate medium, the acceleration of the reaction is due to the formation of a [U(IV)ΣU(VI)] complex whose reactivity is higher than that of uranium (IV). In the absence of bicarbonate, of great importance is the formation of a copolymer of U(IV) and U(VI), which at pH≥4 prevents formation of U(V). It is shown that on the introduction of hydrogen peroxide to aqueous solutions, the metallic uranium surface becomes transpassive, which increases the rate of corrosion process by at least an order of magnitude,. The introducing of oxidants and platinum mesh lowers the hydrogen accumulation at 120–150°C and, hence, the hydrogen-explosion danger of the uranium-water-corrosion-products system. Methods of deposition of metal oxide (Tc, Ru, Mo, W) films onto uranium surfaces by immersing uranium metal into Tc(VII), Ru(VI), or Mo and W heteropoly compound solutions are studied.

PtCoCr catalysts for fuel cell cathodes: Electrochemical activity, Pt content, substrate nature, structure and corrosion properties

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, TiO2) 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.

Physico-chemical properties of carbon nanotubes as supports for cathode catalysts of fuel cells. Surface structure and corrosion resistance

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.

VARIATIONS IN THE STRUCTURE AND ELECTROCHEMICAL CHARACTERISTICS OF MEMBRANE ELECTRODE ASSEMBLIES DURING THE ENDURANCE TESTING OF HYDROGEN-AIR FUEL CELLS

Variations in the characteristics of a membrane-electrode assembly (MEA) are studied during the endurance testing of a hydrogen-air fuel cell (FC) based on a Nafion 212 proton conducting membrane and platinum catalysts. It is shown that the voltage drop observed during MEA testing was mainly due to physicochemical transformations of the cathode catalyst, i.e., the oxidation of platinum and its subsequent recrystallization with nanoparticle coarsening. It is established that the rate of degradation increases along with temperature and loading, and with periodic FC depressurization. It is concluded that the enhancing effects of additional factors of degradation, e.g., platinum ion transport to the proton-conducting membrane and corrosion of the carbon carrier, were responsible for these processes.

On the passivation of iron in aqueous solutions of zinc 1-hydroxyethane-1,1-diphosphonate

This work is devoted to studying the passivating ability of the zinc complex of the 1-hydroxyethane-1,1-diphosphonic acid (HEDP) in a borate buffer solution. For the first time, we used the in situ ellipsometric method to study the mechanism of formation of a protective film on iron in the presence of HEDP, HEDPZn, and ZnSO4 in the course of the cathodic polarization of the electrode. The investigations of adsorption of HEDPZn on iron (at E = −0.65 V) in combination with X-ray photoelectron spectroscopy (XPS) have shown that on the metal surface there is formed a multilayer protective film consisting of an internal layer of Zn(OH)2 and an outer layer consisting of HEDP complexes with Fe2+ and/or Zn2+. It has been found that the thickness of the passivating film does not exceed 60 Å, of which 7–10 Å correspond to the low-soluble zinc hydroxide.

Chemical structures of benzoimidazoles and their protective effects on zinc and copper in phosphate electrolytes

The effects of the concentration and chemical structure of 2-substituted benzoimidazoles (BIs) on copper and zinc dissolution in phosphate electrolytes were studied. X-ray photoelectron study of copper or zinc electrodes revealed that their surface coverages with an inhibitor depend on its concentration in solution; metal complexes with the inhibitor form a protective surface film. The protective effects of substituted BIs on copper and zinc in phosphate electrolytes vary with the chemical structure of an inhibitor and the metal nature. Electron-withdrawing groups enhance the inhibitive effects of BIs on copper dissolution, while electron-donating groups are favorable for zinc protection; this is associated with the different σ- and π-bonding abilities of these metals. The formation of protective copper complexes is facilitated by π-bonding between the metal and an inhibitor, while zinc is protected best in the presence of strong σ-bonds.

Formation of protective layers by 5-chlorobenzotriazole and its mixture with sodium fluphenaminate on low-carbon steel from aqueous solutions

It is shown that introduction of R = Cl substituent into benzotriazole (BTA) structure imparts elevated acidity and hydrophobicity to the molecule and improves the compound’s protective and adsorption properties for low-carbon steel in aqueous solution. The surface layer composition of 5-chloro-BTA and a blend thereof with sodium fluphenaminate is revealed by XPES. It is shown that the inhibitors under investigation are located in a position close to vertical on the surface of oxidized St3. The layer thicknesses of the inhibitors under investigation are determined and compared by XPES and ellipsometry.

XPES of 1,2,3-benzotriazole nanolayers formed on iron surface

The effect of 1,2,3-benzotriazole (BTA) on the electrochemical behavior of St3 steel in different environments, as well as the adsorption of BTA on the steel surface, is studied using the XPES technique. Based on the XPES data using the MultiQuant program, the thickness of the layers formed on the St3 steel surface are calculated. In neutral borate solutions, a thin protective layer of iron-BTA complex with a thickness of 0.7–1.5 nm, which completely suppresses the anodic process, but does not prevent the local depassivation in the presence of chlorides, is found to form on the surface of St3 steel. In alkaline solutions, BTA hardly chemisorbs at all, though adding BTA to the solutions and increasing its concentration results in the increase in the pit-formation potential. In sulfuric-acid solutions, a monomolecular layer is formed on the steel surface. Independently of the pH value, the interaction between BTA and iron cations is shown to result in the proton detachment from BTA molecules.

Electrochemical synthesis of iron and platinum nanoparticles in deionized water

The processes of formation of disperse phase in water in a two-electrode electrochemical cell with platinum or steel electrodes in the constant electric potential application regime are studied. It is established by the method of dynamic light scattering (DLS) that the charge transfer process in the system is accompanied by formation of nanoparticles (NPs) made of electrode material in the aqueous medium. It is shown that the dynamics of changes in the electric current conforms to the dynamics of NP formation. Simultaneous registration of absorbance spectra of the aqueous medium at various moments of time has shown the presence of plasmon resonance bands corresponding to a platinum NP. In case of platinum electrode, a residue of platinum-containing NPs was found on the cathode using energy-dispersive X-ray microanalysis (EDXMA) and electronic spectroscopy for chemical analysis (ESCA) methods. The possibility of cathophoresis of platinum NPs forming on the anode to the cathode under effect of the applied voltage in the cell with water.