Application of conventional activated carbon loaded with dispersed Pt to PEFC catalyst layer

Improvement of the polymer electrolyte fuel cell (PEFC) requires development of highly active electrodes of low cost to facilitate its widespread use. In the present study, the possibility of applying conventional activated carbon particles loaded with Pt to the electrode catalyst layer was tested because the particles were promising in dispersion of Pt and preparation cost. The catalyst layer was formed from the particles and Nafion® and was supported as a thin film on a rotating glassy carbon disk electrode (GC RDE). Activity for oxygen reduction was evaluated by the hydrodynamic voltammetry in perchloric acid to give a current free of the influence of mass transfer in the solution. Compared with a conventional catalyst layer formed from carbon black loaded with Pt, the new catalyst layer exhibited a significant, approximately 6-fold increase in current in the high potential region corresponding to a 100 mV increase in electrode potential. Activity, however, was retarded in the low potential region. This disadvantage was overcome by mixing a conductive agent into the layer and covering it with another layer containing carbon black loaded with Pt. This double catalyst layer exhibited increased activity across all potential regions, indicating the availability of the activated carbon in the electrodes.     

Fabrication and Electrocatalytic Application of Nanoporous Carbon Material with Different Pore Diameters

Here we report on the preparation of nanoporous carbons with different pore diameters loaded with different amounts of platinum and their application as anodic electrodes for the polymer electrolyte membrane fuel cells (PEMFCs). The materials were characterized by nitrogen adsorption, XRD and HRTEM. The role of the amount of the platinum and the pore diameter of the nanoporous carbon supports on the anodic performance in the PEMFC has been investigated. It has been found that the anodic activity of the platinum supported nanoporous carbon materials significantly increases with increasing the amount of platinum on the surface of the supports. Interestingly, nanoporous carbon material with a larger pore diameter shows excellent performance as an anodic electrode support. The electrode activity of the platinum loaded nanoporous carbon was also compared with that of the commercially available carbon black support for the PEMFCs. Nanoporous carbon supports are found to be the best, showing much higher performance as compared to that of carbon black.     

Silica-sol-templated mesoporous carbon as catalyst support for polymer electrolyte membrane fuel cell applications

Mesoporous carbon (MC) samples having especially high specific surface area, pore size, and pore volume (e.g. pore volume in excess of 4 cm³ g⁻¹) were prepared and their suitability as Pt catalyst supports in polymer electrolyte membrane fuel cells was examined. Pt particles on the MC support were slightly larger than those on commercial samples of Pt on carbon black, and they showed a greater tendency to agglomerate on the MC support than on carbon black. Ex situ cyclic voltammetry gave values for electrochemically active surface area that were about half that for a commercial Pt-on-carbon black sample. Preliminary attempts to prepare thin-film electrodes from Pt/MC samples with a Nafion binder using conventional ink formulations failed, probably because much of the Nafion electrolyte was taken up inside support pores and was not available to bind the support particles together. An alternate approach involving painting of catalyst inks directly onto gas diffusion layers was used to prepare membrane electrode assemblies (MEAs) from Pt/MC samples, which were tested using single-cell test hardware. Performance of these Pt/MC sample MEAs was compared with that prepared by decal transfer method with commercially obtained Vulcan XC-72R supported Pt catalyst. The reasons for the lower performance of Pt/MC were discussed.     

Performance enhancement of PEMFC through temperature control in catalyst layer fabrication

Pore size distribution and specific pore volume in the catalyst layer of polymer electrolyte membrane fuel cells were modified by controlling the temperature during the catalyst layer fabrication. Raising the temperature of the gas diffusion layer where the platinum catalyst is coated facilitated evaporation of the solvent in the catalyst ink and induced a large pore volume especially in the secondary pore. Fuel cell electrodes with large amounts of pores exhibit 30% improved single cell performance. The microstructure and electrochemical properties of electrodes were investigated by field emission scanning electron microscopy, mercury intrusion porosimetry, electrochemical impedance spectroscopy, and current-voltage polarization measurement. The results indicate that increased volume of the secondary pore reduces the mass transfer resistance and improves the performance.     

Scanning electron microscopic studies of porous carbon electrodes used in alkaline fuel cells

Multilayer, polytetrafluoroethylene (PTFE)-bonded gas diffusion-type electrodes were prepared by the rolling method. Changing the electrode structure and manufacturing method improved alkaline fuel cell performance. Activated carbon or carbon black was used as the support material, with platinum as a catalyst and nickel screen as the backing material. Double-layer electrodes possessed both active and diffusion layers on the backing layer. However, the single-layer electrodes had only the active layer on the backing layer. The electrodes were prepared by using different PTFE contents and platinum loadings. In this study the surface photographs of the electrodes were taken with a scanning electron microscope. Elemental analyses of the surface elements were performed by energy dispersive X-ray spectroscopy (EDXS). Electrodes having activated carbon on their surfaces were observed to possess a nonuniform and porous structure. These electrodes showed better performance than electrodes having carbon black, which presented a uniform and nonporous structure.     

Effect of diffusion-layer porosity on the performance of polymer electrolyte fuel cells

The gas-diffusion layer (GDL) influences the performance of electrodes employed with polymer electrolyte fuel cells (PEFCs). A simple and effective method for incorporating a porous structure in the electrode GDL using sucrose as the pore former is reported. Optimal (50 w/o) incorporation of a pore former in the electrode GDL facilitates the access of the gaseous reactants to the catalyst sites and improves the fuel cell performance. Data obtained from permeability and porosity measurements, single-cell performance, and impedance spectroscopy suggest that an optimal porosity helps mitigating mass-polarization losses in the fuel cell resulting in a substantially enhanced performance.     

The effect of vapor-grown carbon fiber as an additive to the catalyst layer on the performance of a direct methanol fuel cell

Vapor-grown carbon fibers (VGCFs) were added to the anode catalyst layer of a direct methanol fuel cell to improve the cell performance through structural modification of the catalyst layer. The amount of VGCF varied up to 6 wt.% with respect to the weight of the PtRu black catalyst that was used. A catalyst layer with 2 wt.% VGCF loading showed the best cell performance. The electrodes that included the catalyst, VGCF, and gas diffusion layer, were directly examined by electron microscopic analyses. Electrochemical methods, such as cyclic voltammometry and impedence analysis, were applied to investigate the actual role of VGCF in the electrode. The porosity of the catalyst layer was increased by the addition of the fibers. This was clearly observed in pore diameters less than 1 μm. Sub-micron pore diameters are significant as they relate to micro-diffusive transport, compared to the macro-diffusion experienced by the large pores in the GDL. However, improved mass transport was only observed for 2 wt.% VGCF loading, probably due to insufficient optimization of the cell design. Microstructural and electrochemical analyses indicated that the improved performance was mainly ascribed to an increased electrochemically active surface area of the catalyst.     

The performance of electric double layer capacitors using particulate porous carbons derived from PAN fiber and phenol-formaldehyde resin

Activated carbon fibers are known to contain pores with a small resistance for electrolyte migration while possessing a large electrical resistance between the fibers. A carbon powder derived from pulverization of PAN-based carbon fibers was examined as an electrode for electric double layer capacitors using H₂SO₄ as the electrolyte solution. The performance of conventional-type activated carbon powders derived from phenol-formaldehyde resin char was also measured for comparison. The fiber-derived carbon exhibited an electrical resistance comparable to that of the conventional carbons while showed a larger specific capacitance as well as a lesser extent of capacitance decrease at high currents due to a smaller pore resistance. An ultimate capacitance as high as 290 F g⁻¹ can be reached for this fiber-derived carbon powder (with a BET surface area of ≈1300 m² g⁻¹). This large capacitance value was suggested to be associated with the high activity feature of the pore wall.     

Morphological Analyses of Polymer Electrolyte Fuel Cell Electrodes with Nano‐Scale Computed Tomography Imaging

We report a three‐dimensional (3D), pore‐scale analysis of morphological and transport properties for a polymer electrolyte fuel cell (PEFC) catalyst layer. The 3D structure of the platinum/carbon/Nafion electrode was obtained using nano‐scale resolution X‐ray computed tomography (nano‐CT). The 3D nano‐CT data was analyzed according to several morphological characteristics, with particular focus on various effective pore diameters used in modeling gas diffusion in the Knudsen transition regime, which is prevalent in PEFC catalyst layers. The pore diameter metrics include those based on chord length distributions, inscribed spheres, and surface area. Those pore diameter statistics are evaluated against computational pore‐scale diffusion simulations with local gas diffusion coefficients determined from the local pore size according to the Bosanquet formulation. According to our comparison, simulations that use local pore diameters defined by inscribed spheres provide effective diffusion coefficients that are consistent with chord‐length based estimations for an effective Knudsen length scale. By evaluating transport rates in regions of varying porosity within the nano‐CT data, we identified a Bruggeman correction scaling factor for the effective diffusivity.     

Simulation studies on the fuel electrode of a H2---O2 polymer electrolyte fuel cell

The macro-homogeneous porous electrode theory is used to develop a model which describes the catalyst layer of the hydrogen electrode formed by catalyst particles that are bonded to the membrane. The water transport in the catalyst layer and polymer electrolyte membrane is considered. The effects of catalyst layer structure parameters such as polymer volume fraction, catalyst layer thickness, platinum loading and reactant gas humidity as well as CO poison on the hydrogen electrode behavior are examined. The results show that the catalyst layer thickness has a significant effect on the electrode performance. A thicker catalyst layer will result in a larger ohmic voltage loss and higher catalyst cost. The optimal polymer volume fraction and catalyst layer thickness are 0.5 and 1.5–4 μm, respectively, for this electrode. An optimal platinum surface coverage on carbon need not exceed 20% (20 wt% Pt/C). Larger platinum coverage will increase the cost, but only slightly enhance the electrode performance.     

Gas diffusion electrodes for polymer electrolyte fuel cells using novel organic/inorganic hybrid electrolytes: effect of carbon black addition in the catalyst layer

We have prepared novel gas diffusion electrodes for polymer electrolyte fuel cells (PEFC) using new organic/inorganic hybrid electrolytes. The catalyst layers were prepared by mixing 3-(trihydroxysilyl)-1-propanesulfonic acid [(THS)Pro-SO₃H], 1,8-bis(triethoxysilyl) octane (TES-Oct), Pt loaded carbon black (Pt-CB) and water, followed by a sol-gel reaction. It was found that addition of uncatalyzed carbon black (u-CB) into the cathode catalyst layer enhanced the performance at high current density region, due to an increase in the gas diffusion rate. The optimum volume ratio of u-CB/Pt-CB was found to be 0.1, at which the gas diffusivity and the catalyst utilization are well balanced.     

A method to increase the energy density of supercapacitor cells by the addition of multiwall carbon nanotubes into activated carbon electrodes

The performance of supercapacitor cells with activated carbon (AC) electrodes was improved by adding a small amount of multiwall carbon nanotubes (MWCNTs). The electrode structure investigated comprised AC, four different types of MWCNTs and two polymer binders, polyvinylidene fluoride or polyvinyl alcohol. All fabricated devices were of the electrochemical double layer capacitor type. The organic electrolyte used was tetraethyl ammonium tetrafluoroborate (TEABF₄) in two different solvents: propylene carbonate or acetonitrile (AN). The electrodes were characterised with scanning electron microscopy and tested for their specific surface area and pore size distribution. The electrode fabrication process was fine-tuned by investigating the effect of the coating thickness on the supercapacitor cell performance. It was established that an AC/MWCNT-based supercapacitor with 30 μm thick roll-coated, composite electrodes of just 0.15%w/w MWCNT content provided superior tested power and energy densities of 38 kW/kg and 28 W h/kg, respectively, compared to 18 kW/kg and 17 W h/kg for AC only–based cells in a 1.5 TEABF₄/AN electrolyte. The increased energy density was attributed to a fine lace of MWCNTs covering the AC microparticles with visible 20–30 nm lace pores and to the high specific area of micropores.     

Water Flow in,Through, and Around the Gas Diffusion Layer

Liquid water produced in polymer electrolyte membrane fuel cells is transported from the cathode catalyst/membrane interface through the gas diffusion layer (GDL) to the gas flow channel. Liquid water travels both laterally (in the plane of GDL) and transversely through the largest pores of the porous GDL structure. Narrow apertures in the largest pores are the primary resistance to liquid water penetration. Carbon paper has limiting apertures ∼20 μm in diameter and ∼1 μm in length whereas carbon cloth has apertures ∼100 μm in diameter and ∼200 μm in length. After sufficient hydrostatic pressure is applied, water penetrates the limiting aperture and flows through the pore. The pressure required for water to flow through the pores is less than the pressure to penetrate the limiting aperture of the pores. Water moved laterally and directed through a small number of transverse pores. There is less resistance to lateral liquid water flow at the interface between the GDL and a solid surface than through the GDL. The results from these experiments suggest that water flow through the GDL is dominated by a small number of pores and most pores remain free of liquid water.     

Functionalized carbon support with sulfonated polymer for direct methanol fuel cells

Recently electrodes for direct methanol fuel cell (DMFC) have been developed for getting high fuel cell performances by controlling composition of catalysts and sulfonated polymers, developing catalyst particles, modifying carbon supports, etc. The electrodes in DMFCs are porous layers, which are composed of catalyst, which is black or carbon supported, and sulfonated polymers in a blended form. In the present study, carbon support for catalysts was functionalized to play dual roles of a mass transport and a catalyst support. The functionalized carbon support was characterized and compared with pristine one by thermal and spectroscopic analysis, and loading of platinum (Pt) catalysts on modified support was performed by gas reduction. The electrodes with Pt on functionalized carbon support were fabricated, though the conventional electrodes were prepared with sulfonated polymer and Pt catalysts. Membrane electrode assembly with Pt catalyst on functionalized support showed a higher DMFC performance of 30 mW cm⁻² at 50 °C without using additional sulfonated polymer. Integration of electrode components in one body has another advantage of easier and simpler process in preparing electrodes for DMFCs. Improved DMFC performance of the electrode containing functionalized carbon was be attributed to a better mass transport which maximize the catalytic activities.     

The pore texture of zinc electrodes characterized by impedance measurements

Impedance measurements have been applied to the characterization of the pore texture of zinc electrodes used in electrochemical batteries. It is shown that these electrodes are equivalent to cylindrical pore electrodes. The electrode parameters (radius of pore, pore depth and surface density of pores) have been determined from the values of the electrode capacitance, the electrolyte resistance inside the pores and the electrode porosity.     

Iodine adsorption and structure of activated carbons

Studies were conducted on the adsorption of iodine from saturated aqueous solutions and from saturated vapor by eight activated carbons of greatly diverse pore structures. Water adsorption data were used to determine the pore size distribution curves which provided both the distributions of the pore constriction (desorption) and cavity (adsorption) diameters. Adsorption from aqueous phase formed a unimolecular layer on the carbon surface while adsorption from saturated vapor produced pore-filling of micropores (pores less than 30 Å diameter) and surface coverage of the macropores. A great deal of steric interference was present because of the small difference in the diameter of iodine molecule (4.94 Å) relative to the 10–30 Å diameter pores. Good correlation was attained between adsorption and pore structure when corrections were made for the steric effect and the mean diameter distributions of the constrictions and cavities were used. The model for the iodine-on-carbon adsorption resembles packing of spheres into cylinders.     

A Study on Process Parameters of Direct Ethanol Fuel Cell

Ethanol is one of the promising future fuels of Direct Alcohol Fuel Cells (DAFC). The electro‐oxidation of ethanol fuel on anode made of carbon‐supported Pt‐Ru electrode catalysts was carried out in a lab scale direct ethanol fuel cell (DEFC). Cathode used was Pt‐black high surface area. The membrane electrode assembly (MEA) was prepared by sandwiching the solid polymer electrolyte membrane, prepared from Nafion^® (SE‐5112, DuPont USA) dispersion, between the anode and cathode. The DEFC was fabricated using the MEA and tested at different catalyst loadings at the electrodes, temperatures and ethanol concentrations. The maximum power density of DEFC for optimized value of ethanol concentration, catalyst loading and temperature were determined. The maximum open circuit voltage (OCV) of 0.815 V, short circuit current density (SCCD) of 27.90 mA/cm² and power density of 10.30 mW/cm² were obtained for anode (Pt‐Ru/C) and cathode (Pt‐black) loading of 1 mg/cm² at a temperature of 90°C anode and 60°C cathode for 2M ethanol.     

Characterization of gas diffusion layers for PEMFC

A carbon-filled gas diffusion layer (CFGDL), which is in the configuration similar to conventional carbon cloth gas diffusion layer (GDL) coated with carbon layer on both faces, was investigated and compared with conventional carbon paper-based single-layer and dual-layer GDLs. Like the carbon cloth GDL, CFGDL has presented superior performances over the single-layer or dual-layer GDL in all three polarization (activation, ohmic and concentration) controlled regions under electrochemical characterizations (steady-state polarization and electrochemical impedance spectra). The results from SEM showed that CFGDL has the same thickness of 0.11 mm as that of single-layer GDL, while dual-layer GDL has a thickness of 0.18 mm. The fully filled carbon paper with carbon/PTFE filler, as seen in the SEM image, displayed good support for the catalyst layer and electrolyte phase, allowing good electrical contact between the GDL/catalyst/membrane and GDL/flow field plate to be achieved. From porosimetry analysis, CFGDL presented a lower porosity of 67% and a much smaller average pore diameter of 4.7 μm compared to the single-layer GDL (porosity of 77% and pore diameter of 35.8 μm) and dual-layer GDL (porosity of 73% and pore diameter of 25.5 μm); however, it also gave the largest limiting current density, which reflects the improvement in mass transportation. This phenomenon is likely attributed to the fast removal of micro-water droplets formed in the CFGDL structure of the electrode.     

Sputtered cathodes for polymer electrolyte fuel cells: insights into potentials, challenges and limitations

The level of Pt loadings in polymer electrolyte fuel cells (PEFC) is still one of the main hindrances for implementation of PEFCs into the market. Therefore, new catalyst and electrode preparation methods such as sputtering are of current interest, because they allow thin film production and have many cost saving advantages for electrode preparation. This paper summarises some of the most important studies done for sputtered PEFCs, including non carbon supported electrodes. Furthermore, it will be shown that an understanding of the main morphological differences between sputtered and ink-based electrodes is crucial for a better understanding of the resulting fuel cell performance. Especially, the electrochemical surface area (ECSA) plays a key role for a further increase in PEFC performance of sputtered electrodes. The higher surface specific activities i(k,spec) of sputtered compared to ink-based electrodes will be discussed as advantage of the thin film formation. The so- called particle size effect, known in literature for several years, will be discussed as reason for the higher i(k,spec) of sputtered electrodes. Therefore, a model system on a rotating disc electrode (RDE) was studied. For sputtered PEFC cathodes Pt loadings were lowered to 100 μg(Pt)/cm(2), yet with severe performance losses compared to ink-based electrodes. Still, for Pt sputtered electrodes on a carbon support structure remarkably high current densities of 0.46 A/cm(2) at 0.6 V could be achieved.