O. V. Korchagin

Electrocatalysis and pH (a review)

The factors determining pH effects on principal catalytic reactions in low-temperature fuel cells (oxygen reduction, hydrogen oxidation, and primary alcohols oxidation) are analyzed. The decreasing of hydrogen oxidation rate when passing from acidic electrolytes to basic ones was shown to be due to the electrode surface blocking by oxygen-containing species and changes in the adsorbed hydrogen energy state. In the case of oxygen reduction, the key factors determining the process’ kinetics and mechanism are: the O2 adsorption energy, the adsorbed molecule protonation, and the oxygen reaction thermodynamics. The process’ high selectivity in acidic electrolytes at platinum electrodes is caused by rather high Pt-O2 bond energy and its protonation. The passing from acidic electrolytes to basic ones involves a decrease in the oxygen adsorption energy, both at platinum and nonplatinum catalysts, hence, in the selectivity of the oxygen-to-water reduction reaction. The increase in the methanol and ethanol oxidation rate in basic media, as compared with acidic ones, is due to changes in the reacting species’ structure (because of the alcohol molecules dissociation) on the one hand, and active OHads species inflow to the reaction zone, on the other hand. In the case of ethanol, the above-listed factors determine the process’ increased selectivity with respect to CO2 at higher pHs. Based on the survey and valuation, priority guidelines in the electrocatalysis of commercially important reactions are formulated, in particular, concepts of electrocatalysis at nonplatinum electrode materials that are stable in basic electrolytes, and approaches to the practical control of the rate and selectivity of oxygen reduction and primary alcohols oxidation over wide pH range.

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.

Rapid diagnostics of characteristics and stability of fuel cells with proton-conducting electrolyte

Processes underlying the degrading of membrane-electrode assemblies of hydrogen-air fuel cells with Nafion 212 and MF-4SK membranes under the conditions of their accelerated stress testing and long-term life tests are analyzed. The cathode platinum catalyst corrosion was shown to be the main cause of the degrading of the fuel cell’s kinetically controlled current-voltage characteristics; the corrosion is accompanied by the platinum nanoparticles’ growth and the platinum ion partial transfer into the membrane. The overvoltage components of the membrane-electrode assembly and their changing during accelerated stress testing are determined. The voltage decrease at currents >0.5 A/cm2 is shown to be mainly caused by the transport and ohmic resistance growth. The transport resistance components are calculated; the dependence of the cathode active layer resistance on the platinum catalyst surface area is revealed.

Catalysis of oxygen reaction on positive electrode of a lithium–oxygen cell in the presence of metallic nanosystems

Electrocatalytic characteristics of a series of carbon materials (carbon blacks XC-72 and Super P and also multiwall nanotubes) and binary metallic nanosystems formed on carbon black XC-72 (PtRu/C and PdRu/C) are studied in the cathodic and anodic reactions of the positive electrode of a lithium–oxygen cell with nonaqueous electrolyte in the first discharge/charging cycles. It is found that a significant decrease in the cell charging overpotential is observed at a transition from carbon supports to binary systems. Overvoltage of the cathodic process also decreases when DMSO-based electrolyte is used in the case of binary systems. The obtained results are due to acceleration of oxygen reduction (cell discharge stage) and facilitation of lithium peroxide oxidation (cell charging stage) on the PtRu/C and PdRu/C systems.

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.

Lifetime prediction for the hydrogen–air fuel cells

Degrading of membrane–electrode assemblies of hydrogen–air fuel cells during their long-term and accelerated stress testing is analyzed by complex of electrochemical and structural methods. In both testing types, the principal degrading factor is the cathode catalyst destruction accompanied by the active surface loss due to the platinum oxidation, its particles’ coarsening, and platinum ion transfer to polymer electrolyte. The approach to membrane–electrode-assembly state evaluation during the long-term testing on basis of the accelerated stress testing is validated. A method of the lifetime prediction for fuel cells operating in various conditions, differing in temperature, current load and regime, as well, as the membrane–electrode assembly architecture, is suggested.

Carbon nanotubes as efficient catalyst supports for fuel cells with direct ethanol oxidation

Several carbon materials, namely, single-walled nanotubes (CNT1), two-walled nanotubes (CNT2), multiwalled nanotubes (CNT3), and nanofibers (CNF) are synthesized by methane pyrolysis. The resulting nanomaterials are characterized by physical (BET) and electrochemical (charging curves) methods. A catalyst of ethanol electrooxidation PtSn (3: 1, 40 wt % Pt) that involves the mentioned nanomaterials as the supports is synthesized. The catalyst formed on two-walled nanotubes demonstrates the highest activity in ethanol oxidation under model conditions. X-ray diffraction analysis is used in studying the PtSn (3: 1, 40 wt % Pt)/CNT2 catalyst structure. The attained depth of ethanol oxidation is determined by the gas-liquid chromatography. Tests of an ethanol-oxygen fuel cell (FC) with the anodic active layer (AL) based on this catalyst are carried out.

Specific features of the oxygen reaction on catalytic systems in acetonitrile-based electrolytes

The oxygen reaction is studied in acetonitrile solutions on various nanosystems: ХС72, 20Au/C, 20Pt/C, 15Ru/C, 20Pd/C, 20Pt10Ru/C, 20PdRu/C. It is shown that as regards their activity in the oxygen electroreduction reaction, the studied materials form the following series: Pd/C > PtRu/C > PdRu>Pt/C> Ru/C ≈ Au/C ≈ ХС72, whereas in the reaction of Li2O2 electrooxidation the activity series is different: Ru/C > PtRu/C > Pd/C > PdRu/C> ХС72 > Pt/C > Au/C. Assumptions are drawn on the nature of passivation for systems with the highest activity. The prospects of bimetallic catalysts (PtRu/C and PdRu/С) that combine the high activity in reactions of oxygen electroreduction and Li2O2 electrooxidation and also retain a considerable part of their activity on cycling are discussed. These results make it possible to judge on the possible applications of bimetallic nanosystems with bifunctional catalytic properties in lithiumoxygen fuel cells.

Characteristics of non-platinum cathode catalysts for a hydrogen–oxygen fuel cell with proton- and anion-conducting electrolytes

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/cm2 (60% Pt/C (HiSPEC9100) based cathode) and 180 mW/cm2 (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.

Lithium–oxygen (air) batteries (state-of-the-art and perspectives)

Functional bases of power sources of Li–O2 type have been considered. Particular attention has been devoted to the Li–O2 system with liquid aprotic electrolyte as the most promising version of a rechargeable Li–O2 cell. The current status of research on the design of the principal components of Li–O2 battery represented by catalytically active and patterned materials, as well as binders for the formation of positive electrode, solvents and electrolytes, and separation membranes has been characterized. Insights into the mechanisms of the reactions that occur during discharge and recharge have been challenged and the factors that restrict cycling and discharge capacity of Li–O2 cell have been considered. Top-priority scientific and technological problems of the design of Li–O2 battery, which is competitive with respect to lithium-ion batteries, have been stated.