M. V. Radina

Composites based on phenyl substituted cobalt porphyrins with Nafion as catalysts for oxygen electroreduction

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.

Characteristics of membrane-electrode assemblies of hydrogen-air fuel cells with PtCoCr/C catalyst

A cathodic catalyst, which can replace monoplatinum commercial catalyst, is developed and investigated. New catalyst combines a smaller consumption of platinum with a higher mass activity and corrosion resistance. A method of fabrication of ternary (PtCoCr/C) catalyst is improved in order to obtain the catalytic system containing 45–50 wt % platinum. This is necessary to form thinner active layers of cathodes of membrane-electrode assemblies of hydrogen-air fuel cells. The activity of the synthesized PtCoCr/C catalyst is by 1.2–1.5 times higher than that of the monoplatinum catalyst containing 60–70 wt % Pt. According to the accelerated-test data, the corrosion resistance of PtCoCr/C catalyst is also higher than that of Pt/C system.

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.

Electrochemical and structure characteristics of PtCoCr/C-Catalyst with platinum content 50 wt % and cathode on its basis for fuel cell with proton-conducting polymer electrolyte

Electrochemical and structure characteristics of PtCoCr/C-catalyst with platinum content 50 wt % under model conditions and in cathode of membrane-electrode assembly (MEA) of hydrogen-oxygen fuel cell are studied. The metal-phase nanoparticles are shown to represent a Pt3Co alloy with partial inclusion of chromium, the nanoparticles surface being enriched with platinum. The platinum mass activity and the PtCoCr/C-catalyst high corrosion resistance do not depend on the amount of platinum deposited onto XC72 carbon black (20 or 50 wt %). Herewith the platinum surface area decreases in the same way as the carbon black specific surface area (measured with the BET method); the latter is 227, 169, and 105 m2/g for pure carbon black and that containing 20 and 50 wt % Pt, respectively. Testing of the 50PtCoCr/C-catalyst in the MEA cathode active layer in hydrogen-oxygen fuel cell showed performance not inferior to MEAs with pure-platinum systems, the catalyst being more stable.

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.

The structure and characteristics of a PtCoCr nanosized polymetallic cathode catalyst on a carbon carrier

The paper presents X-ray and transmission electron microscopy data characterizing the structure of trimetallic PtCoCr catalysts synthesized on a disperse carbon carrier (carbon black KhS 72) and the influence of the structure on electrocatalytic activity in the reduction of oxygen in 0.5 M H2SO4. The mechanisms of oxygen reduction on platinum and trimetallic catalysts were shown to be similar. A higher activity of platinum contained in the trimetallic catalyst was caused by smaller PtCoCr/C catalyst surface coverage by oxygen-containing particles formed from water and interfering with the adsorption of molecular oxygen, which was, in turn, determined by the electronic structure of trimetallic system nanoparticles.

Effect of Carbonaceous Carrier and Promoter on Electrocatalytical Properties of Composite Cathodes in Fuel Cells with Alkaline Electrolyte

The activity of composite catalysts, Pt and Co-porphyrin- or Fe-phthalocyanine-based pyropolymers on low-disperse carbonaceous carriers (graphite, carbon black), in the oxygen and H2O2electroreduction in 1 M KOH is studied. Kinetic parameters of oxygen electroreduction are determined from experiments with rotating disk and model floating electrodes. Possible mechanism of the oxygen electroreduction reaction is discussed; it includes a slow stage of attachment of the second electron on the pyropolymer/carbonaceous carrier or joining the first electron (under the conditions of Temkin adsorption) on the platinum/graphite catalysts.

Характеристики неплатиновых катодных катализаторов для водородо-кислородного топливного элемента с протонпроводящим и анионпроводящим электролитами

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.

Mesoporous Nanostructured Materials for the Positive Electrode of a Lithium–Oxygen Battery

Nanostructured carbon materials (CMs), the structure can vary widely, are promising materials for the positive electrode of a lithium–oxygen battery (LOB). The electrochemical characteristics of CMs studied in model conditions and their porous structure, as well as testing them as an active material for the positive electrode in an LOB sample, show that nanotubes (CNTs) and Super P carbon black possess the highest charge–discharge characteristics in an aprotic solvent (DMSO). Mono- and bimetallic systems containing Pt, Pd, and Ru and synthesized on CNT and Super P allow one to reduce discharge and charge overvoltage. In the presence of catalytic systems, an improvement in the energy-conversion efficiency of up to 73–76.7% is achieved for the LOB positive electrode. The possibility of achieving a stable cycling process in an LOB with a positive electrode on the basis of developed catalysts and with a LiClO4/DMSO electrolyte is shown. For the first time, the positive influence of iodine (reducing the charge voltage to about 0.8–1.0 V as compared to the characteristics of an LOB using an electrolyte without additives) on the electrode characteristics of a Li–O2 cell with the highly electron-donating solvent DMSO is demonstrated.