Agglomerates Of Carbon Particles Research Articles

Multi-Scale Modeling of Hydrogen Transport in a Porous Fuel Cell Anode

Proton-exchange-membrane fuel cells (PEMFC) are a clean energy conversion alternative to traditional fossil-fuel combustion; however, transport resistances in the electrode pose a lower-limit to catalyst loading and commercialization of PEMFCs. PEMFCs consist of simultaneous hydrogen (H2) oxidation and oxygen (O2) reduction at the anode and cathode, respectively. While oxygen transport resistances in PEMFCs have been widely studied both experimentally and analytically, hydrogen transport resistances are less understood. Herein, we present a physics-based model that encompasses multi-scale transport within the anode side of the PEMFC. The O2 in the cathode here is omitted and replaced with H2 to deconvolute O2 transport resistance contributions, similar to that of a hydrogen pump. Replication of the hydrogen pump setup allows for comparison of the model against experimental analysis of H2 gas-transport resistance in H2 limiting-current experiments, which can also inform gas transport (including oxygen) in general. Herein, we present a multi-scale analytical model of the porous anode catalyst layers and individual catalyst agglomerates that enables determination of the effects of electrode morphology such as agglomerate size, catalyst loading, etc. on H2 transport resistance through the porous electrode to complement and better understand H2 limiting current experiments and deconvolute local H2 transport resistances.Electrodes within PEMFCs are heterogeneous porous structures formed by agglomerates of carbon particles containing Pt nanoparticles, held together by an ion-conducting polymer (ionomer). The gas transport hence involves transport through multiple phases and length-scales: (i) gas-phase transport through electrode pores at the length scale of electrode thickness (micrometer) and (ii) diffusion into the agglomerate through an ionomer film at the length scale of agglomerate radius (nanometer). Based on previous modeling efforts of varying length-scales of the catalyst layer, our analytical model adequately and simultaneously captures reaction-diffusion at all of the aforementioned scales (i.e., the electrode thickness and agglomerate radius) to better describe porous anode performance. A sensitivity analysis is subsequently performed to compare current generated against transport parameters for both scales. This work ultimately provides insight that will guide the design and engineering of future porous anodes for applications in hydrogen pumping and PEMFCs.

Физико-технические свойства пленки, полученной из анилина плазменным осаждением при атмосферном давлении

The polyaniline (PANI) films filled by carbon particles can demonstrate the conductivity increasing. The carbon particles uniform distribution in the PANI films can be received by their contemporaneously formation in the atmospheric pressure plasma. The films properties produced from aniline contemporaneously with carbon particles in the atmospheric pressure plasma are considered in that paper. The energy density threshold (~ 30 mJ/m) of gas discharge starting from which it is possible to obtain a continuous film filled with agglomerates of carbon particles was defined. It was showing that the such films has the amorphous morphology. It was defined that the carbon particles agglomerates in the PANI films. The carbon particles agglomerates concentration in the PANI films is about ~(4 – 7)·104 1/cm2 and slowly depends on the energy density. So the conductivity of that films cant be increasing by this particles. It was shown that the films moisture saturation is the main reason of them conductivity increasing. It was shown that the films creep are decreases and the hardness increases when the gas discharge energy density increases. Its because the crosslinks number is growth in the samples. The molecular weight of PANI films is low, so the Martens hardness value has low-level.

Measurement and Analysis of Gas Transport Properties in Catalyst Layers of Polymer Electrolyte Fuel Cells with Different Ionomer to Carbon Ratio

Gas transport properties in catalyst layers of polymer electrolyte fuel cells are key factors for improving cell performance. It is necessary to clarify the relationship between the gas transport properties and the microstructure in catalyst layers. In this study, the oxygen diffusivities in catalyst layers with different ionomer-to-carbon ratio (I/C) were evaluated using a microfluidic device, and tortuosity factors were obtained. The porous structures of catalyst layers were evaluated by the nitrogen physisorption method and observation using a field emission scanning electron microscope. It was revealed that the tortuosity factor of the catalyst layers with high and low I/C was larger than that of catalyst layers with middle I/C. This can be attributed to the segregation of ionomers in high-I/C catalyst layers and the agglomeration of carbon particles in low-I/C catalyst layers.

Computer Simulation of the Cathode Active Layer in Hydrogen–Oxygen Fuel Cell with Polymer Electrolyte: The Nature of the Overall Current Variations

Full computer simulation of the cathode structure in hydrogen–oxygen fuel cell with polymer electrolyte is performed. Both transport, support grains (agglomerates of carbon particles onto whose surface Pt-catalyst is deposited), and the current generation in active layer are simulated. The active layer operation in potentiostatic mode is studied. The effect of variations of the active layer and the fuel cell temperature (Ts and Т, respectively) on the cathode overall current I and the support grain flooding with water is calculated. The changes in the temperature difference Ts–Т was shown for the first time, experimentally and by the simulation, to generate variations of I and the degree of the support grain flooding with water. In particular, with the increasing of Ts–Т the current I increased, whereas the support grain flooding with water decreased; and vice versa, with the decreasing of Ts–Т the current I drops down, while, the support grain flooding with water grows. An explanation of the phenomena is presented, which takes account of structure of the support grains in which О2 reduction and Н2О generation occur. There exist intrinsic channels for protons and О2 molecules transportation to the catalyst. Water releasing in the support grains is able to fill partially or even entirely the gas pores through which oxygen is supplied to the platinum. As a result, the current generated in the support grains can drop down significantly; at the same time, the value of I also drops down. The degree of the support grainfilling with water is determined by two processes, namely, the flooding and draining. The source of flooding is the current generation; that of draining, the water saturated vapor diffusion and water filtration in nanopores. The lower cathode potential, the higher the flooding rate, whereas the water removal rate grows or drops down with the increasing of decreasing of the temperature difference Тs–Т, respectively. Thus, the temperature difference variations naturally lead to those of the quantity I.

Hierarchical Hybrids Integrated by Dual Polypyrrole-Based Porous Carbons for Enhanced Capacitive Performance.

Architectural design of nitrogenous polymer-based carbons represents a facile and efficient strategy to improve performance because of their morphological diversity, tailorability, and N-containing structure. In this research, 0D and 1D forms of polypyrrole-derived porous carbons (A-PCS and A-PCT, respectively) are first integrated into nitrogen-doped hierarchically porous A-PCS/PCT hybrids by applying an easy multistep method. This integration, along with chemical activation, prevents serious agglomeration of carbon particles or tubes and creates a connected porous network structure, leading to improved textural properties (high surface area of 1684 m2 g-1 , high pore volume of 1.57 cm3 g-1 , and hierarchical porosity). Thus, A-PCS/PCT hybrids in a three-electrode setup reach high specific capacitances of 224 and 206 F g-1 at 1 and 20 A g-1 , respectively, with high rate capability (92 % capacitance retention). A symmetrical supercapacitor with A-PCS/PCT electrodes presents the highest power and energy densities of 12.6 kW kg-1 and 8.58 Wh kg-1 , respectively, and exceptional cycling life and stability with 92.4 % retention for up to 20 000 cycles. This study on conductive polymer-based hybrid materials may guide the design of architectures with new structures for applications in energy storage and conversion technologies.

COMPUTER SIMULATION OF THE CATHODE ACTIVE LAYER IN FUEL CELLS WITH SOLID POLYMER ELECTROLYTE: OVERALL CATHODE CURRENT TRANSIENT AND CALCULATION OF CATHODE OVERALL CHARACTERISTICS

A computer simulation of the structure of the active layer cathode of a fuel cell with solid polymer electrolyte is carried out. The calculation process of the overall transient current for several values of the potential of the cathode was carried out. It was assumed that in the processes of wet interface in the substrate grains (agglomerates of carbon particles on the surface of which is covered with platinum) there are two factors: flooding of the pores in the grains of the substrate by water released during the electrochemical recovery of platinum, and draining of the grains as a result of water filtration. The paper calculates the current-voltage curves. When the temperature Ts of the active layer exceeds the temperature T at which the fuel cell operates, there has been a significant increase in overall value of the currents of the cathode with increase Ts . This is due to the fact that at the transient overall value of the current, the substrate grains in the active layer are only partially submerged by water. It is also shown that the higher the flow rate of filtration is (it is proportional to the constant k in Darcy's law), the greater the release of the grains of the substrate from the water and a corresponding increase in the overall value of the current of the cathode is. Thus, to increase the amount of overall current of the cathode for a given value of capacity is possible by varying (increasing) the temperature of the active layer T and the Darcy constant k.

ACTIVE LAYER OF FUEL CELL CATHODE WITH POLYMER ELECTROLYTE: COMPUTER SIMULATION PROCESS OF MOISTURE EXCHANGE IN SUPPORT GRAINS

In the cathode active layer of a fuel cell with a solid polymer electrolyte process, the current generation takes place in support grains (agglomerates of carbon particles with supported platinum). The speed of this process essentially depends on a degree of support grains pores filling with water. Calculations show if support grains pores are completely filled with water the overall current density in the cathode active layer is much less than in the case of the pores partially or even completely free of water. The last variant of a cathode active layer functioning is realized when the release rate of pores from moisture due to evaporation and filtration of water exceeds the rate of flooding process of ones with water in the process of current generation. This study by method of computer simulation presents a specific example of determining the overall current density in the cathode active layer with varying the speed of emptiness process in pores from moisture due to filtration.

Computer-aided simulation of the cathodic active layer in fuel cells with solid polymer electrolyte: the nature of overall current transient

Total computer-aided simulation of the structure and current-generation processes in the cathodic active layer of a fuel cell with solid polymer electrolyte is carried out. Not only the transport structure of the active layer but also the structure of support grains (agglomerates of carbon particles with platinum-covered surface) are modeled. The process of active layer functioning under potentiostatic conditions is studied. It is demonstrated for the first time how the moisture exchange in the pores of support grains affects the cathode overall characteristics. The time variations of the overall current, the average temperature of the active layer, and the total degree of water-flooding of support-grain pores within the active layer are calculated by numerical methods. It is shown that for the fuel cell voltage of 0.6 V and its working temperature of 80°C, the flooding process dominates over the process of drying of pores in support grains. In 10–15 s, all support-grain pores turn out to be entirely filled with water. Then they begin functioning not in the kinetic mode (in the moment of switching-on the current, the Knudsen diffusion of oxygen in the support grains is observed) but in the inner-diffusion mode. As a result, the overall cathodic current decreases from its initial value of 4.323 A/cm2 to its final value of 0.526 A/cm2 and the active layer temperature decreases from the initial value of 102°C to the final value of 82.5°C. The overall current transient is studied also experimentally, the qualitative coincidence of theoretical and experimental data is demonstrated.

A cathode for solid polymer electrolyte fuel cells: Designing the optimal structure of the active layer

The complete computer simulation of the cathodic active layer with solid polymer electrolyte (Nafion) is carried out. The active layer structure can be described by 8 parameters. In designing the optimal structure, it is shown that to provide the high overall characteristics of the cathode and save the catalyst, 0.5 of the active layer volume should be set aside for the support grains (agglomerates of carbon particles covered with platinum and containing Nafion incorporations and microvoids). Protons and oxygen molecules must be supplied to the active layer by means of peculiar combined percolation clusters. The latter consist of a combination of support grains with either Nafion grains (to produce “protonic” clusters) or grains-voids (to afford “gas” clusters). The volume fractions of Nafion grains and grain-voids are assumed to be 0.25 and 0.25. The computer simulation of the support grain structure is also carried out. Their composition, i.e., the volume fractions of the carbon component (g e), Nafion (g ii), and microvoids (g gg), is varied. The support grains play the key role in the active layer functioning. It is impossible to organize three full-value percolation clusters (electronic, protonic, and gas); hence, one has to have one or two combined clusters in the active layer. Thus the double load fells on the support grains. Their optimal structure should not only sustain the transport of protons and electrons in the active layer but also create the best conditions for the electrochemical process in each grain. The maximum current I max (realized upon reaching the optimal active layer thicknesses Δ*) is calculated. The dependences of I max and Δ* on the main parameters characterizing the support grains (g e and g ii) are analyzed. Here, two goals are sought: (1) to obtain the high currents, (2) to provide the low consumption of platinum per power unit. To solve the first problem, one has to work with high values of g e. The second problem requires the opposite: the values of g e must be minimal possible.

How important is oxygen transport in agglomerates in a PEM fuel cell catalyst layer?

We derive analytical solution for the polarization curve of the cathode catalyst layer (CCL), taking into account oxygen transport in agglomerates of carbon particles. The solution shows that in modern catalyst layers with agglomerates of the radius ≲100 nm, the oxygen transport in agglomerates manifests itself at cell potentials below 100 mV only. Thus, the account of this transport in CCL modeling is redundant, i.e., the standard macro–homogeneous model is adequate for CCL performance simulations.

Active layer of fuel cell electrode with polymer electrolyte: Nature of proton and oxygen supply channels

The nature of proton and oxygen supply channels in the active layer of a cathode of fuel cell with polymer electrolyte is discussed. There are three types of electron, proton, and oxygen carriers in the active layer: agglomerates of carbon particles with supported platinum (support grains), agglomerates of Nafion molecules (Nafion grains), and void grains. In computer simulation of the active layer structure, the three types of grains were assumed equal-sized, cube-shaped and arranged into a cubic node lattice (in the terms of the percolation theory). Impossibility of forming on the basis solely the above three grain types of three percolation clusters (“electron”, “proton”, and “gas”) that could supply all that is required for the electrochemical process is proved. But in this, the following question arises: how can satisfactory operation of the cathode with polymer electrolyte be provided? The required supply of protons and oxygen can be provided only if the support grains can feature not only electronic conductivity, but can also participate in transport of both protons and oxygen. As a result, the transport of protons and oxygen is carried out via special combined percolation clusters that must include apart from the support grains either Nafion grains (combined “proton” cluster) or void grains (combined “gas” cluster). The paper describes the technique of calculation of effective specific conductivity of a combined “proton” cluster. The effective specific diffusion coefficient of a combined “gas” cluster can also be calculated in a similar way.

Cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte: The effect of the flooding of pores by water on the characteristics of the active layer

A computer model of the active layer of the cathode of a hydrogen-oxygen fuel cell with a solid polymer electrolyte is studied. The active mass of the electrode consists of equidimensional grains of the substrate (agglomerates of carbon particles with platinum particles embedded in them) and a solid polymer electrolyte (Nafion). The flooding by water can be experienced by both the pores in the substrate grains, which facilitate the oxygen penetration into the active layer of the electrode, and the voids between the grains. All possible versions of the flooding of these pores by water are considered. A calculation of the optimum, at a given polarization of the electrode, value of electrochemical activity, the thickness of the active layer, and the weight of platinum is performed. The major parameters of the system are the concentrations of grains of the substrate and solid polymer electrolyte, the size of these grains, the platinum concentration in the substrate grains, the average diameter of pores in the substrate grains, and the polarization of electrodes. The ultimate aim of the work is to estimate how the flooding of pores of the active layer of the cathode by water affects the magnitude of the optimum current, the effective thickness of the active layer, and the weight of platinum.

Optimizing Weights of Platinum in the Active Layer of the Cathode of a Hydrogen–Oxygen Fuel Cell with a Solid Polymer Electrolyte

The active layer of the cathode of a hydrogen–oxygen fuel cell with a solid polymer electrolyte is computer simulated. The active mass of the electrode consists of substrate grains (agglomerates of carbon particles with Pt particles embedded into them) and grains of a solid polymer electrolyte (Nafion). The substrate grains presumably contain hydrophobic pores, which facilitate the oxygen penetration into the active mass. A calculation of characteristics of such an electrode focuses on the optimization of platinum weights. The principal parameters of the system are concentration and size of grains of substrate and Nafion, Pt concentration in substrate grains, average diameter of hydrophobic pores in substrate grains, and the electrode polarization. The optimum, at a given electrode polarization, electrochemical activity of the active layer, its thickness, and the platinum weight are calculated. A link between these quantities and principal parameters of the active layer is revealed.

Active Layer of an Oxygen Electrode Based on Nanocomposite Disperse Carbon Carrier + Laccase Material

Effect of the carbon material dispersivity on the efficiency of oxygen electroreduction by laccase immobilized on finely divided colloidal graphite (FCG) and carbon black AD-100 is studied. A highly active composite material based on FCG with laccase immobilized on it is proposed and investigated. This creates optimum conditions for direct bioelectrocatalysis by enzyme molecules. The specific oxygen reduction current calculated per enzyme molecule for nanocomposite FCG + laccase is five times that on an AD-100-based composite. Increasing the active-layer thickness, which is of importance for creating a gas-diffusion oxygen electrode, reduces specific activity of composite and only the activity of ultrathin layers is thickness-independent. This is explained by percolation restrictions on the electron transport, which reduce the number of catalytically active centers in the electrode's active layer that take part in reaction. The FCG particles are presumed to form agglomerates in the active layer. The size of the agglomerates is determined on the basis of computer-aided modeling of percolation processes and experimental data on the dependence of the specific capacitance of the active mass on the active-layer thickness. Hypotheses on the origin of percolation phenomena are put forth. One such hypothesis is that agglomerates of carbon particles are fractal clusters.

Effects of agglomeration of carbon particles in the semiconducting material on the dielectric strength of XLPE insulation

The relationship between the breakdown strength and the material structure of the order of a few micrometers near the interface between the polyethylene insulation and the semiconducting material is discussed. It was found that the agglomeration of the carbon particles in the semiconducting material ranges from about 100-300 nm. The autocorrelation function A(D) of the transmission electron microscope (TEM) photographs can represent some aspects of the agglomeration of the carbon particles. The characteristic distance D/sub 0 /in the autocorrelation function ranges from about 100-300 nm, and corresponds closely to the size of the agglomeration of the carbon particles. It was found that the highest breakdown strength in terms of the Weibull 10% breakdown strength may be obtained when D/sub 0/ is around 200 nm. The thermal breakdown may explain the relationship between the breakdown strength and the agglomeration of the carbon particles. >