We have tested platinum–carbon electrodes with mixed conductivity as parts of membrane electrode assemblies of fuel cells containing structure-modifying additives with different structural elements (carbon nanotubes with extended structural elements and graphene-like materials with nearly 2D planes). Mass-transport losses of molecular oxygen transfer in these electrodes have been investigated on the basis of data on the limiting current density obtained in the potentiodynamic and potentiostatic regimes. The pressure dependences of the current density have been plotted using various conditions of measurements. Limiting factors and oxygen transport mechanisms in the tested structures and the role of introduced modifiers have been determined.

The variation of surface area of platinum–carbon material of hydrogen fuel cell electrodes in the presence of structure-modifying carbon nanotubes under long-term electrochemical action is studied. The method of cyclic voltammetric curves is used to study the specific features of the variation of platinum nanoparticle surface area by the hydrogen desorption in the hydrogen region and the charging current of electrical double layer in the double-layer potential region. The results are obtained separately for the electrodes on the cathodic and anodic sides.

The article discusses platinum-carbon electrodes with mixed conductivity as part of membrane-electrode assemblies of fuel cells containing structural-modifying additives with structural elements of various types: carbon nanotubes with elongated structural elements and graphene-like materials with almost two-dimensional planes. Based on the data on the limiting current density obtained in potentiodynamic and potentiostatic modes, the mass transport losses of molecular oxygen transfer in these electrodes are investigated. Using different measurement conditions, the baric dependences of the current density were constructed, the limiting factors and mechanisms of oxygen transfer in the studied structures and the role of the introduced modifiers were clarified.

The paper shows the possibility of describing the charge-discharge characteristics of supercapacitors with porous carbon electrodes using a simple equivalent circuit with constant capacitances and resistances. Using numerical simulation, it is shown that there is a relationship between the parameters of the equivalent circuit and the pore structure of the electrode material. The simulation results also indicate the inhomogeneous nature of the charging of various groups of pores of the electrode material in the structure of the supercapacitor.

Behavior of supported alloyed and de-alloyed platinum-copper catalysts, which contained 14% - 27% wt. of Pt, was studied in the reactions of methanol electrooxidation (MOR) and oxygen electroreduction (ORR) in 0.1 M HClO4 solutions. Alloyed PtCux/C catalysts were prepared by a multistage sequential deposition of copper and platinum onto a Vulcan XC72 dispersed carbon support. De-alloyed PtCux-y/C catalysts were prepared by PtCux/C materials pretreatment in acid solutions. The effects of the catalysts initial composition and the acid treatment condition on their composition, structure, and catalytic activity in MOR and ORR were studied. Functional characteristics of platinum-copper catalysts were compared with those of commercial Pt/C catalysts when tested, both in an electrochemical cell and in H2/Air membrane-electrode assembly (MEA). It was shown that the acid pretreatment of platinum-copper catalysts practically does not have negative effect on their catalytic activity, but it reduces the amount of copper passing into the solution during the subsequent electrochemical study. The activity of platinum-copper catalysts in the MOR and the current-voltage characteristics of the H2/Air PEMFC MEAs measured in the process of their life tests were much higher than those of the Pt/C catalysts.

It is demonstrated that charge/discharge characteristics of supercapacitors with electrodes made of porous carbon can be described using a simple equivalent scheme with fixed values of capacities and resistances. Results of numerical modeling show that there is a relationship between the parameters of equivalent scheme and structure of pores in the electrode material. The results also indicate that there is an inhomogeneous character of charging for various groups of pores in electrode material of the supercapacitor structure.

The behavior of supported alloyed and de-alloyed platinum-copper catalysts, which contained 14–27% wt. of Pt, was studied in the reactions of methanol electrooxidation (MOR) and oxygen electroreduction (ORR) in 0.1 M HClO4 solutions. Alloyed PtCux/C catalysts were prepared by a multistage sequential deposition of copper and platinum onto a Vulcan XC72 dispersed carbon support. De-alloyed PtCux−y/C catalysts were prepared by PtCux/C materials pretreatment in acid solutions. The effects of the catalysts initial composition and the acid treatment condition on their composition, structure, and catalytic activity in MOR and ORR were studied. Functional characteristics of platinum-copper catalysts were compared with those of commercial Pt/C catalysts when tested, both in an electrochemical cell and in H2/Air membrane-electrode assembly (MEA). It was shown that the acid pretreatment of platinum-copper catalysts practically does not have negative effect on their catalytic activity, but it reduces the amount of copper passing into the solution during the subsequent electrochemical study. The activity of platinum-copper catalysts in the MOR and the current-voltage characteristics of the H2/Air proton-exchange membrane fuel cell MEAs measured in the process of their life tests were much higher than those of the Pt/C catalysts.

We have investigated the structural and electric characteristics of composite electrodes structure-modified by the introduction of carbon nanofibers with a high porosity and a fraction of transport pores due to the formation of Nafion proton-conducting polymer predominantly in island form. Energy losses for various compositions have been analyzed and the cathode composition has been optimized. The efficiency of the cathode process has been raised and cathode characteristics have been improved. The following characteristics of the membrane–electrode assembly were achieved: for electric efficiency of about 0.5, the specific load on the platinum was 0.1 g/kW and the maximal power density was 1 W/cm 2 .

AbstractThe ionic resistance of membrane–electrode assemblies of oxygen–hydrogen fuel cell electrodes containing platinum nanoparticles on carbon black, carbon nanofibers, and proton-conducting Nafion polymer in a wide range of compositions (10–80 wt %) are studied in situ by the methods of current–voltage characteristics, electrochemical impedance spectroscopy, and simulation of the impedance hodograph. Conditions that make it possible to correctly determine the ionic resistance of the electrode on the basis of an analysis of the linear approximation of the high-frequency region of the impedance hodograph are found. It is shown that the occurrence of inhomogeneities and an anomalous increase in the ionic resistance with an increase in the content of Nafion in the electrode are associated with a decrease in the volume fraction of water-generation centers (particles of electrochemically active platinum), which leads to incomplete wetting of Nafion.

The structural and electrical characteristics of composite electrodes structurally modified due to the introduction of carbon nanofibers with high porosity and a fraction of transport pores and the formation of the Nafion proton-conducting polymer mainly in island form are studied. The analysis of energy losses for various compositions and optimization of the cathode composition are carried out. The efficiency of the cathode process was increased and the cathode characteristics were improved, the following characteristics of the membrane-electrode assemblies were achieved: at an electrical efficiency of ~ 0.5, the specific loading of platinum was 0.1 g / kW, and the maximum power density was 1 W / cm2.

Despite such technical advantages as high energy conversion efficiency, low noise, autonomy, etc., hydrogen fuel cells have not yet been widely used due to insufficiently high economic competitiveness. It is known that a significant fraction of the hydrogen fuel cell cost is the cost of the electrode materials and electrodes. In this regard, the paper studies the electrode materials and electrodes of hydrogen fuel cell. The performance of porous electrochemical electrodes is determined by electrode activity, substance transfer efficiency, and charge transfer efficiency. Since these factors act, as a rule, in the opposite direction, the task of selecting the component composition of the electrode often comes down to obtaining optimization dependencies. It is important to note that transport losses in a running fuel cell are usually dominant. In connection with this, our work focuses on the structure and transport characteristics. It is believed that the determining factors of the diffusion component of the functioning of the fuel cell are the characteristics of the porous structure of the electrode affecting the conditions of mass exchange and the processes of water condensation. A significant phenomenon is the inhomogeneity of the ionic resistance associated with inhomogeneities of humidity and temperature, since the ionic resistance of proton-conducting component depends on humidity and temperature. In order to control the porous structure and transport properties, we used the technique of introducing into the electrode material a highly porous functional additive with a large proportion of transport pores and creating the island structure of the proton-conducting polymer Nafion. Two materials were investigated as functional additives: carbon nanofibers and thermally expanded graphite. The fabricated electrode materials and membrane-electrode assemblies were investigated by electron microscopy, voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy. The result is the dependences connecting the composition of the electrode with its porosity, specific ion and electronic resistance, specific surface area of platinum. The study gives the results of diffusion resistance to mass transport depending on the composition. We have developed the technology of electrode material with increased efficiency of mass and charge transport. The results allow us to predict the electrical characteristics of the cathode, to produce electrodes with desired properties.

The ionic resistance of membrane–electrode assemblies of oxygen–hydrogen fuel cell electrodes containing platinum nanoparticles on carbon black, carbon nanofibers, and proton-conducting Nafion polymer in a wide range of compositions (10–80 wt %) are studied in situ by the methods of current–voltage characteristics, electrochemical impedance spectroscopy, and simulation of the impedance hodograph. Conditions that make it possible to correctly determine the ionic resistance of the electrode on the basis of an analysis of the linear approximation of the high-frequency region of the impedance hodograph are found. It is shown that the occurrence of inhomogeneities and an anomalous increase in the ionic resistance with an increase in the content of Nafion in the electrode are associated with a decrease in the volume fraction of water-generation centers (particles of electrochemically active platinum), which leads to incomplete wetting of Nafion.

Despite such technical advantages as high energy conversion efficiency, low noise, autonomy, etc., hydrogen fuel cells have not yet been widely used due to insufficiently high economic competitiveness. It is known that a significant fraction of the hydrogen fuel cell cost is the cost of the electrode materials and electrodes. In this regard, the paper studies the electrode materials and electrodes of hydrogen fuel cell. The performance of porous electrochemical electrodes is determined by electrode activity, substance transfer efficiency, and charge transfer efficiency. Since these factors act, as a rule, in the opposite direction, the task of selecting the component composition of the electrode often comes down to obtaining optimization dependencies. It is important to note that transport losses in a running fuel cell are usually dominant. In connection with this, our work focuses on the structure and transport characteristics. It is believed that the determining factors of the diffusion component of the functioning of the fuel cell are the characteristics of the porous structure of the electrode affecting the conditions of mass exchange and the processes of water condensation. A significant phenomenon is the inhomogeneity of the ionic resistance associated with inhomogeneities of humidity and temperature, since the ionic resistance of proton-conducting component depends on humidity and temperature. In order to control the porous structure and transport properties, we used the technique of introducing into the electrode material a highly porous functional additive with a large proportion of transport pores and creating the island structure of the proton-conducting polymer Nafion. Two materials were investigated as functional additives: carbon nanofibers and thermally expanded graphite. The fabricated electrode materials and membrane-electrode assemblies were investigated by electron microscopy, voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy. The result is the dependences connecting the composition of the electrode with its porosity, specific ion and electronic resistance, specific surface area of platinum. The study gives the results of diffusion resistance to mass transport depending on the composition. We have developed the technology of electrode material with increased efficiency of mass and charge transport. The results allow us to predict the electrical characteristics of the cathode, to produce electrodes with desired properties.

AbstractThe ionic resistance of the membrane electrode assembly (MEA) of O_2/H_2 fuel cells is studied in situ in a broad range of MEA compositions by using electrochemical impedance spectroscopy and polarization measurements. The MEAs are composed of platinum on carbon, carbon nanotubes, and Nafion, a proton- conducting polymer. We investigate atypical growth of the MEA resistance with the increasing Nafion content and spatial nonuniformity in the MEA ionic resistivity. A mechanism underlying this phenomenon is proposed.

Using the methods of current-voltage characteristics and spectroscopy of electrochemical impedance, the paper studies the features of proton and electron transfer in high-porous electrodes of the hydrogen fuel cell containing Nafion mainly in insular form in a wide range of concentrations. Electrode structures were manufactured in two steps: 1. Mechanical mixing of platinized carbon black of the E-TEK type, carbon nanofibers of the Taunit MD type (manufactured in Tambov) and the water -i-propanol dispersion of Nafion; 2. Ultrasonic homogenization to obtain a homogeneous dispersion of electrode material. The resulting dispersion was applied directly to a proton-conductive membrane of the Nafion-212 type. The quantities of the initial components were measured gravimetrically, the component composition of the electrode material was monitored by thermogravimetric analysis. The structure of the obtained materials was studied by the methods of scanning and transmission electron microscopy. The given electrodes in the membrane-electrode assembly were activated by repeatedly cycling the potential difference from the potential of the open circuit to ~ 0 until the voltage-current characteristics stabilized. The ion resistance determined by the resistance to proton transfer was measured by the method of electrochemical impedance in the region of high frequencies of the hodograph and by the method of current-voltage characteristics in a wet (activated) electrode in the membrane-electrode assembly. The electronic resistance was measured in an air-dry electrode in which the ionic resistance of a dry Nafion was several orders of magnitude larger than the electron resistance and practically did not contribute to the value of the measured resistance. The dependence of the ion resistance on the Nafion content is shown to have a minimum at 40% mass. The electronic resistance increases linearly with the increase in the Nafion content. The extreme nature of the dependence of the ion resistance on the Nafion content is explained by the decrease in the concentration of the water generation centers (platinum nanoparticles) with the increase in the Nafion content to some practical value at which the water produced is not sufficient to completely moisten Nafion.

The ionic resistance of the membrane electrode assembly (MEA) of O2/H2 fuel cells is studied in situ in a broad range of MEA compositions by using electrochemical impedance spectroscopy and polarization measurements. The MEAs are composed of platinum on carbon, carbon nanotubes, and Nafion, a proton- conducting polymer. We investigate atypical growth of the MEA resistance with the increasing Nafion content and spatial nonuniformity in the MEA ionic resistivity. A mechanism underlying this phenomenon is proposed.

Electrochemical experiments with a rotating disk electrode are used to measure specific catalytic activity of Pt/C structures in the oxygen reduction reaction at the density of Pt nanoparticles on the glassy carbon support surface below one monolayer. The specific activity maximum is found at the coverage of about 0.4 monolayer. An explanation of the observed dependence is suggested that is based on consideration of the relationship between the surface density and charge state of the system of metallic catalyst particles. A numeric model is developed that describes charge transfer in the catalyst structure due to the difference in the work functions between the metal nanoparticles and support with account for the discrete nature of the nanoparticle charging and their mutual polarization. Calculations show that the carbon support coverage by Pt particles of about 0.4 monolayer corresponds to the largest amount of charged particles with the maximum energy of electrons, which provides the maximum catalyst activity and explains the dependence observed in the experiment.

The effect of the water-repellent agent and carbon nanotubes introduced in the porous cathode of an oxygen–hydrogen fuel cell on the porous structure and electrochemical characteristics of the cell was studied by chronoamperometry, adsorption structural analysis, electron microscopy, and gravimetric analysis.