Highly-dispersed Ta-oxide catalysts prepared by electrodeposition in a non-aqueous plating bath for polymer electrolyte fuel cell cathodes

The Ta-oxide cathode catalysts were prepared by electrodeposition in a non-aqueous solution. These catalysts showed excellent catalytic activity and have an onset potential of 0.92 V(RHE) for the oxygen reduction reaction (ORR). The highly-dispersed Ta species at the nanometer scale on the carbon black was an important contributor to the high activity.     

Modification of palladium-based catalysts by chalcogenes for direct methanol fuel cells

Palladium-based catalysts, such as PdS_x/C and PdSe_x/C, have been developed as oxygen reduction catalysts for direct methanol fuel cells. Pd/C catalysts containing chalcogens have been synthesized and tested for oxygen reduction and the results have been analyzed based on changes in the palladium phase. Selenium addition to the catalyst promotes the oxygen reduction due to the modification of the palladium surface. However, sulfur reduces the oxygen reduction activity to a great extent as a result of semi-amorphous palladium phase formation. Both PdS_x/C and PdSe_x/C are highly methanol tolerant.     

Nanostructured catalysts for cathodes of oxygen-hydrogen fuel cells

Bimetallic catalysts platinum-cobalt, platinum-chromium, and platinum-tungsten, deposited onto highly dispersed carbon black from complex cluster-type compounds of corresponding metals with a 1: 1 atomic ratio of metals are developed. The catalysts are characterized by methods of x-ray diffraction analysis and energy dispersive analysis of x-rays. The procedure involving use of a thin-film rotating disk electrode is employed to probe kinetic parameters of the oxygen reduction reaction on the catalysts developed. The investigated binary catalysts exhibit specific electrochemical characteristics that are not inferior and, in some cases, are superior to the characteristics intrinsic to the commercial platinum catalyst E-TEK, when tested in the composition of a gas-diffusion electrode under conditions that are close to real conditions in which cathodes of oxygen-hydrogen fuel cells operate.     

Polymer electrolyte membranes for the direct methanol fuel cell: A review

The direct methanol fuel cell (DMFC) has the potential to replace lithium‐ion rechargeable batteries in portable electronic devices, but currently experiences significant power density and efficiency losses due to high methanol crossover through polymer electrolyte membranes (PEMs). Numerous publications document the synthesis and characterization of new PEMs for the DMFC. This article reviews this research, transport phenomena in PEMs, and experimental techniques used to evaluate new PEMs for the DMFC. Although many PEMs do not show significant improvements over Nafion®, the benchmark PEM in DMFCs, experimental results show that several new PEMs exhibit lower methanol crossover at similar proton conductivities and/or higher DMFC power densities. These results and recommendations for future research are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Parts B: Polym Phys 44: 2201–2225, 2006     

Synthesis and test of palladium-based nanostructured anodic electrocatalysts for hydrogen fuel cells with solid polymer electrolyte

Palladium-based nanostructured electrocatalysts on the Vulcan XC-72 carbon support for fuel cells with solid polymer electrolyte are synthesized and studied. In particular, electrochemical studies of the synthesized catalysts are carried out and membrane-electrode assemblies are assembled on their basis and tested. The test results indicate that platinum can be replaced with palladium in the hydrogen electrode of the fuel cells.     

Microfluidic hydrogen fuel cell with a liquid electrolyte

We report the design and characterization of a microfluidic hydrogen fuel cell with a flowing sulfuric acid solution instead of a Nafion membrane as the electrolyte. We studied the effect of cell resistance, hydrogen and oxygen flow rates, and electrolyte flow rate on fuel cell performance to obtain a maximum power density of 191 mW/cm2. This flowing electrolyte design avoids water management issues, including cathode flooding and anode dry out. Placing a reference electrode in the outlet stream allows for independent analysis of the polarization losses on the anode and the cathode, thereby creating an elegant catalyst characterization and optimization tool.     

Direct ethanol oxidation fuel cell with anionite membrane and alkaline electrolyte

A number of cathode catalysts were synthesized from nitrogen-containing organic complexes on XC-72R carbon black for an alkaline electrolyte. The catalysts were studied by the rotating disc electrode (RDE) technique. The polyacrilonitrile (PAN), phthalocyanine (Pc), and cobalt tetra(methoxyphenyl)porphyrin (CoTMPP) systems showed the highest activity. The slope of the oxygen polarization curves in the first region in 1 M KOH was 35–40 mV; this corresponds to concentration polarization in an alkaline solution in the O₂-HO₂⁻ system. A cyclic voltammetry study demonstrated that the catalytic systems with the highest corrosion stability were Pc + Co + Fe/XC-72R and CoTMPP/XC-72R pyropolymer. The activity of the catalysts decreased by 20–25% compared with the initial current densities on average. An ethanol-oxygen fuel cell with a Fumasep FAA anionite membrane and nonplatinum catalysts was tested. The maximum power density was 32 mW/cm² at 40°C. The stability test of the fuel cell showed that the materials used for the membrane-electrode assembly allowed more than 100 h of continuous operation with constant working characteristics.     

Electrocatalysts and membrane for direct ethanol-oxygen fuel cell with alkaline electrolyte

Results of studies of anodic (RuNi/C) and cathodic (PtCo/C; CoN₄/C) catalysts, polybenzimidazole membrane, and membrane-electrode assemblies on their basis for alkaline ethanol-oxygen fuel cell are presented. It is shown that the anodic catalyst RuNi/C optimized in its composition (Ru: Ni = 68: 32 in atomic percent) and the metal mass on carbonaceous support (15–20%) is sufficiently effective with respect to ethanol oxidation; it is well superior to commercial Pt/C- and RuPt/C-catalysts when calculated per unit mass of the precious metal. The effect of electrolyte composition, electrode potential, and temperature on the CO₂ yield is studied by chromatographic analysis of the ethanol oxidation products. It is shown that the highest CO₂ yield (the process involves the C-C bond break) is achieved at low electrolysis overvoltage and elevated temperature. The mean number of electrons given up by C₂H₅OH molecule approaches 10 at temperatures over 60°C. The studied cathodic catalysts form the following series of their specific activity in the oxygen reduction reaction: (20 wt % Pt) E-TEK ≥ (7.3 wt % Pt) PtCo/C > CoN₄/C; however, in the presence of alcohol the activity series is reversed. On this reason fuel cell cathodes were prepared by using synthesized CoN₄/C-catalyst. For the alkali-doped polybenzimidazole membrane the conductivity and ethanol crossover were determined. A membrane-electrode assembly for platinum-free alkaline ethanol-oxygen fuel cell is designed. It comprised anodic (RuNi/C) and cathodic (CoN₄/C) catalysts and polybenzimidazole membrane. The period of service of the fuel cell exceeded 100 h at a voltage of 0.5 V and current of 100 mA/cm².     

Structure-activity relationship of palladium phosphanesulfonates: toward highly active palladium-based polymerization catalysts

Palladium phosphanesulfonate [R(2)P(C(6)H(4)-o-SO(3))PdMeL] catalysts permit the copolymerization of an exceptional large number of functional olefins with ethylene. However, these catalysts usually have reduced activity. We here have conducted a systematic study on the influence of the phosphane substituent, R, on activity and molecular weight. Phosphanes with strong σ-donating character are shown to lead to the most active catalysts. Thus, the catalyst based on phosphane bis-tert-butyl-phosphanyl-benzenesulfonic acid (R=tBu) exhibits unprecedented high activity, rapidly polymerizing ethylene at room temperature to yield a linear polymer of high molecular weight (M(w)=116,000 g mol(-1)). The influence of the R group on the catalyst ability to incorporate methyl acrylate is also investigated.     

Electrochemical characteristics of platinum-based binary catalysts for middle-temperature hydrogen-air fuel cells with phosphoric acid electrolyte

The high-temperature synthesis based on commercial catalyst E-TEK (40% Pt) using cobalt, chromium, and iron organic precursors as well as d-metal salts yielded PtM (1:1) catalysts (PtCo, PtCr, PtMn, PtNi, PtFe, and PtV) for electroreduction of molecular oxygen in concentrated H₃PO₄ at the temperature of 160°C. The phase composition of the synthesized catalysts was studied by powder diffraction. The electrochemical measurements were carried out in 15 M H₃PO₄ at 20 and 160°C using a model gas diffusion electrode. An assumption was made that close charging curves recorded for synthesized PtM catalysts in both hydrogen and oxygen adsorption ranges were due to formation of the core-shell structure: alloy core and surface layers enriched with platinum. The Tafel curves of molecular oxygen reduction in 15 M H₃PO₄ at 160°C were characterized with the sole slope of 0.10 to 0.11 V. The catalytic activity in the range of potentials from 0.8 to 0.9 V (RHE) was shown approximately twice as that of pure platinum catalyst. The highest activity was recorded for PtCo and PtCr binary catalysts. Their use in middle-temperature hydrogen-air fuel cells with solid polymeric electrolyte based on polybenzimidazole doped with phosphoric acid enabled 2- to 3-fold decrease of the platinum share in the cathode.     

Performance of a polymer electrolyte fuel cell with long oxygen channel

Along-the-channel analytical model of a polymer electrolyte fuel cell is developed. The model takes into account oxygen diffusion in backing layer, diffusion and electroosmotic transport of water in membrane and oxygen depletion in a feed channel. Voltage current curve of a cell, which takes into account all these processes is obtained and expression for limiting current density is derived. The latter shows, that cell performance is described by design parameters, which are combinations of geometrical and working parameters. The region of optimal cell performance on the plane of the design parameters is determined.     

First principles calculations of oxygen reduction reaction at fuel cell cathodes

The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, in particular for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO₃-type materials are used for this purpose. The dominating surface terminations are (001) AO and BO₂, with the relative fractions depending on materials composition and ambient conditions.Here, results of recent large-scale first principles (ab initio) calculations for the two alternative polar (La,Sr)O and MnO₂ (001) terminations of (La,Sr)MnO₃ cathode materials are discussed. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller compared to MnO₂, which leads to drastically decreased estimated ORR rates. Thus, it is predicted for prototypical SOFC cathode materials that the BO₂ termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases, which slows down cathode kinetics.     

Synthesis of nitrogen-containing carbon materials for solid polymer fuel cell cathodes

The following nitrogen-containing supports with various nitrogen contents and structure and texture properties were synthesized: carbon nanofibers (N-CNFs) and amorphous microporous carbon materials (N-AMCMs). It was found that the above characteristics can be regulated by varying synthesis conditions: precursor compositions and reaction temperature and time. Mesoporous nitrogen-containing CNFs with a specific surface area of 30–350 m²/g and a pore volume of 0.10–0.83 cm³/g were formed by the catalytic decomposition of a mixture of ethylene with ammonia at 450–675°C. Microporous materials (N-AMCMs) with a specific surface area of 472–3436 m²/g and a micropore volume of 0.22–1.88 cm³/g were prepared by the carbonization of nitrogen-containing organic compounds at 700–900°C. An increase in the carbonization temperature and reaction time resulted in an increase in the specific surface area and microporosity of N-AMCMs, whereas lower temperatures of 450–550°C and reaction times of 1–3 h were optimal for the preparation of N-CNFs with a developed texture. It was found that milder synthesis conditions and higher nitrogen contents of precursors were required for obtaining high nitrogen concentrations in both N-CNFs and N-AMCMs. The synthetic method developed allowed us to prepare carbon supports with nitrogen contents to 8 wt %.     

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.     

Mechanical endurance of polymer electrolyte membrane and PEM fuel cell durability

The life of proton exchange membrane fuel cells (PEMFC) is currently limited by the mechanical endurance of polymer electrolyte membranes and membrane electrode assemblies (MEAs). In this paper, the authors report recent experimental and modeling work toward understanding the mechanisms of delayed mechanical failures of polymer electrolyte membranes and MEAs under relevant PEMFC operating conditions. Mechanical breach of membranes/MEAs in the form of pinholes and tears has been frequently observed after long‐term or accelerated testing of PEMFC cells/stacks. Catastrophic failure of cell/stack due to rapid gas crossover shortly follows the mechanical breach. Ex situ mechanical characterizations were performed on MEAs after being subjected to the accelerated chemical aging and relative humidity (RH) cycling tests. The results showed significant reduction of MEA ductility manifested as drastically reduced strain‐to‐failure of the chemically aged and RH‐cycled MEAs. Postmortem analysis revealed the formation and growth of mechanical defects such as cracks and crazing in the membranes and MEAs. A finite element model was used to estimate stress/strain states of an edge‐constrained MEA under rapid RH variations. Damage metrics for accelerated testing and life prediction of PEMFCs are discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2346–2357, 2006     

Effect of water electrolysis catalysts on carbon corrosion in polymer electrolyte membrane fuel cells

A new approach to preventing electrochemical carbon corrosion in the cathode of polymer electrolyte membrane fuel cells (PEMFCs) was developed. The addition of 2 wt % IrO(2) (0.016 mg cm(-2)) to the catalyst layer of the cathode was demonstrated to reduce the electrochemical corrosion of carbon by 76% at 1.6 V(NHE) and 70 °C compared with a commercial Pt/C catalyst of the same Pt loading of 0.4 mg cm(-2) and under the same test conditions. The IrO(2) was shown to behave as a catalyst for water electrolysis, thereby removing water from the catalyst layer, which promoted electrochemical carbon corrosion.     

Potential application of anion-exchange membrane for hydrazine fuel cell electrolyte

A platinum electrocatalyst layer was directly bound to a perfluorinated anion-exchange membrane (AEM) by the electroless plating method and used for the characterization of AEM as a polymer electrolyte membrane for a direct hydrazine fuel cell. The crossover amount of hydrazine through AEM was much lower than that through the cation-exchange membrane (CEM) that did not depend on the applied current density. The fuel cell performance was far superior when using AEM than when using CEM.     

Calculation of optimum thickness of active layer of oxygen and air cathodes of fuel cell with Nafion and platinum

It is shown that, for the electrodes of fuel cells with solid polymer electrolyte, the dependence of overall current on the active layer thickness contains an extremum. There is an optimum thickness of active layer, at which the overall current reaches its maximum possible value. The nature of this dependence is explained. The character of the distribution of electrochemical process intensity over the depth of active layer of cathode with solid polymer electrolyte is analyzed. The optimum thicknesses of active layers of oxygen and air cathodes of fuel cells with Nafion and platinum and the corresponding overall currents and contents of catalyst in the active layer are calculated. In the calculations, the temperature of fuel cell, the pressure in the cathode gas chamber, and the cathodic potential were varied. The optimization of active layer thickness of cathode with solid polymer electrolyte can reduce the platinum consumption, i.e. its amount per 1 kW of power produced in a membrane-electrode assembly.     

Effects of polymer electrolyte membrane's property on fuel cell performances

Platinum and/or metal‐oxide nanocrystals (d = 1 ‐ 2 nm) were highly dispersed in membranes such as a Nation® commercially available (denoted as Pt‐PEM or Pt‐oxide‐PEM) attempting to self‐humidify the PEMs and/or to suppress the short‐circuit reaction by a catalytic oxidation of the crossover hydrogen or methanol with oxygen on the Pt catalyst. High and stable performances under the suppressed crossover and lowered internal resistance are demonstrated at the H₂/O₂ fuel cells applied Pt‐PEM or Pt‐oxide‐PEM without any external humidification. An appreciable increase of the cathode potential due to the reduced methanol crossover was clearly demonstrated at a direct methanol fuel cell (DMFC) with Pt‐PEM. It also becomes clear that the development of new PEMs with lowered permeability against methanol is essential for DMFCs.