Palladium-based electrodes: A way to reduce platinum content in polymer electrolyte membrane fuel cells

To decrease the Pt content, a polymer electrolyte membrane fuel cell (PEMFC) was formed using a carbon supported Pd₉₆Pt₄ catalyst as the anode material, and a carbon supported Pd₄₉Pt₄₇Co₄ catalyst as the cathode material. The as-obtained Pd-based PEMFC with an overall Pd:Pt:Co atomic composition of electrodes (anode + cathode) = 72:26:2 exhibited a performance not too far from that of the fuel cell with the conventional 100% Pt electrodes. With a Pt content of 35 wt% of that of the cell with full Pt electrodes, at a current density of 1 A cm⁻² the performance loss of the cell with the Pd-based catalysts was only 11%, with 6% ascribed to the anode catalyst and 5% to the cathode catalyst. The maximum power density of the Pd-based cell was 76% of that of the cell with Pt catalysts.     

Membrane electrode assembly with Pt/SiO2/C anode catalyst for proton exchange membrane fuel cell operation under low humidity conditions

In this work, a novel self-humidifying membrane electrode assembly (MEA) with Pt/SiO₂/C as anode catalyst was developed to improve the performance of proton exchange membrane fuel cell (PEMFC) operating at low humidity conditions. The characteristics of the composite catalysts were investigated by XRD, TEM and water uptake measurement. The optimal performance of the MEA was obtained with the 10 wt.% of silica in the composite catalyst by single cell tests under both high and low humidity conditions. The low humidity performance of the novel self-humidifying MEA was evaluated in a H₂/air PEMFC at ambient pressure under different relative humidity (RH) and cell temperature conditions. The results show that the MEA performance was hardly changed even if the RHs of both the anode and cathode decreased from 100% to 28%. However, the low humidity performance of the MEA was quite susceptible to the cell temperature, which decreased steeply as the cell temperature increased. At a cell temperature of 50 °C, the MEA shows good stability for low humidity operating: the current density remained at 0.65 A cm⁻² at a usual work voltage of 0.6 V without any degradation after 120 h operation under 28% RH for both the anode and cathode.     

The improved methanol tolerance using Pt/C in cathode of direct methanol fuel cell

Membrane electrode assemblies (MEA) were prepared using PtRu black and 60 wt.% carbon-supported platinum (Pt/C) as their anode and cathode catalysts, respectively. The cathode catalyst layers were fabricated using various amounts of Pt (0.5 mg cm⁻², 1.0 mg cm⁻², 2.0 mg cm⁻², and 3.0 mg cm⁻²). To study the effect of carbon support on performance, a MEA in which Pt black was used as the cathode catalyst was fabricated. In addition, the effect of methanol crossover on the Pt/C on the cathode side of a direct methanol fuel cell (DMFC) was investigated. The performance of the single cell that used Pt/C as the cathode catalyst was higher than single cell that used Pt black and this result was pronounced when highly concentrated methanol (above 2.0 M) was used as the fuel.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

A parametric study of a platinum ruthenium anode in a direct borohydride fuel cell

Data on the performance of a direct borohydride fuel cell (DBFC) equipped with an anion exchange membrane, a Pt–Ru/C anode and a Pt/C cathode are reported. The effect of oxidant (air or oxygen), borohydride and electrolyte concentrations, temperature and anode solution flow rate is described. The DBFC gives power densities of 200 and 145 mW cm⁻² using ambient oxygen and air cathodes respectively at medium temperatures (60 °C). The performance of the DBFC is very good at low temperatures (ca. 30 °C) using modest catalyst loadings of 1 mg cm⁻² for anode and cathode. Preliminary data indicate that the cell will be stable over significant operating times.     

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.     

Distribution profile of water and suppression of methanol crossover in sulfonated polyimide electrolyte membrane for direct methanol fuel cells

We have developed novel cross-linked sulfonated polyimide (c-SPI) membrane as an electrolyte for direct methanol fuel cells (DMFCs). When the DMFC using the c-SPI membrane (thickness = 155 μm), Pt-Ru dispersed on carbon black (Pt-Ru/CB) anode and Pt/CB cathode with a Nafion^® ionomer was operated at 80 °C and 0.1 A cm⁻² with 1 M CH₃OH and oxygen (oxidant), the methanol crossover rate, j(CH₃OH), was suppressed to about 1/2 compared with that of the Nafion^® 117 membrane (thickness = 180 μm) with the same electrodes. It was found for both cells that the j(CH₃OH) was not so small as expected from the membrane thickness. In order to obtain a clue for the suppression of j(CH₃OH), the distribution profiles of water (containing CH₃OH) in thickness direction were investigated by measuring the specific resistances (ρ) between Pt probes inserted into the electrolyte membrane. Values of ρ at the anode side were low irrespective of the discharge current density, because such a part of the membrane was humidified thoroughly by liquid water (1 M CH₃OH) allowing free penetration of CH₃OH into the swollen polymer. In contrast, the values of ρ at the cathode side were high at the low current density due to drying of the membrane contacting with oxidant gas (O₂ or air) in low humidity. We have succeeded to suppress the j(CH₃OH) further (about 1/2 at 0.2 A cm⁻²) by using bilayer c-SPI, having a low ion exchanging (low swelling) barrier layer at the anode side without increasing the ohmic resistance, compared with that of the single c-SPI.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

The effect of two-layer cathode on the performance of the direct methanol fuel cell

To reduce the effect of methanol permeated from the anode, the structure of the cathode was modified from a single layer with Pt black catalyst to two-layer with PtRh black and Pt black catalysts, respectively. The current density of the direct methanol fuel cell (DMFC) using the two-layer cathode was improved to 228 mA/cm^-2 compared to that (180 mA/cm^-2) of the DMFC using the single layer cathode at 0.3 V and 303 K. From the cyclic voltammograms (CVs), it is indicated that the amount of adsorbates on the metal catalyst in the two-layer cathode is less than that of adsorbates in the single layer cathode after methanol test. In addition, the adsorbates were removed very rapidly by electrochemical oxidation from the two-layer cathode. It is suggested fromex situ X-ray absorption near edge structure analysis that the d-electron vacancy of Pt atom in the two-layer cathode is not changed by the methanol test. Thus, Pt is not covered with the adsorbates, which agrees well with the results of CV.     

Stability characteristics of Pt1Ni1/C as cathode catalysts in membrane electrode assembly of polymer electrolyte membrane fuel cell

To understand the difference in degradation characteristics between carbon-supported platinum (Pt/C) and platinum–nickel alloy (Pt₁Ni₁/C) cathode catalysts in membrane electrode assemblies (MEAs) of a polymer electrolyte membrane fuel cell (PEMFC), constant current operation of MEA in a single cell was conducted for 1100 h. A significant change in cell potential for the Pt₁Ni₁/C MEA was observed throughout the test. High-resolution transmission electron microscopy showed that sintering and detachment of metal particles in the Pt₁Ni₁/C catalyst occurred more sparingly than in the Pt/C catalyst. Instead, X-ray photoelectron spectroscopy element mapping revealed dissolution of Ni atoms in the Pt₁Ni₁ catalysts even when the Pt₁Ni₁/C catalyst used in the MEA was well synthesized.     

Effects of membrane electrode assembly preparation on the polymer electrolyte membrane fuel cell performance

This paper presents results of recent investigations to develop an optimized in-house membrane electrode assembly (MEA) preparation technique combining catalyst ink spraying and assembly hot pressing. Only easy steps were chosen in this preparation technique in order to simplify the method, aiming at cost reduction. The influence of MEA fabrication parameters like electrode pressing or annealing on the performance of hydrogen fuel cells was studied by single cell measurements with H₂/O₂ operation. Toray paper and carbon cloth as gas diffusion layer (GDL) materials were compared and the composition of electrode inks was optimized with regard to most favorable fuel cell performance. Commercial E-TEK catalyst was used on the anode and cathode with Pt loadings of 0.4 and 0.6 mg/cm², respectively. The MEA with best performance delivered approximately 0.58 W/cm², at 65 °C cell temperature, 80 °C anode humidification, dry cathode and ambient pressure on both electrodes. The results show, that changing electrode compositions or the use of different materials with same functionality (e.g. different GDLs), have a larger effect on fuel cell performance than changing preparation parameters like hot pressing or spraying conditions, studied in previous work.     

Gas crossover and membrane degradation in polymer electrolyte fuel cells

Hydrogen gas crossover measurements and durability tests of a single cell under open-circuit conditions were carried out to investigate membrane degradation in polymer electrolyte fuel cells (PEFCs). The limiting current density for hydrogen crossover was approximately 0.8 mA cm⁻² at 80 °C under atmospheric pressure, and gas crossover increased with an increase in cell temperature, humidity and hydrogen gas pressure. Under open-circuit conditions, the perfluorinated ionomer electrolyte membrane deteriorated significantly although no net electrochemical reactions took place at the cathode and anode. The mechanism for membrane degradation was discussed in terms of heat generation and hydrogen peroxide formation upon gas crossover and the resulting catalytic combustion, and it was concluded that the latter is the primary reason, in which hydrogen peroxide is most probably formed by gas crossover of oxygen and the resulting catalytic combustion at the anode side. In addition, it was inferred that reactive oxygen radicals (HO and HO₂) were formed in the presence of minor impurities such as Fe²⁺ and Cu²⁺ ions, which could accelerate the membrane degradation.     

Calculation of reversible electrode heats in the proton exchange membrane fuel cell from calorimetric measurements

A calorimeter was used to measure the heat production in polymer electrolyte membrane (PEM) fuel cells operated on hydrogen and oxygen at 50 °C and 1 bar. Two cells were examined, one using a 35 μm thick Nafion membrane and a catalyst loading of 0.6/0.4 mg Pt cm⁻², for the cathode and anode layer, respectively, the other using a 180 μm thick Nafion membrane and loading of 0.4/0.4 mg Pt cm⁻². The cells investigated thus had different membranes and catalyst layers, but identical porous transport layers and micro-porous layers. The calorimeter is unique in that it provides the heat fluxes out of the cell, separately for the anode and the cathode sides. The corresponding cell potential differences, ohmic cell resistance and current densities are also reported. The heat fluxes through the current collector plates were decomposed by a thermal model to give the contributions from the ohmic and the Tafel heats to the total heat fluxes. Thus, the contributions from the reversible heat (the Peltier heats) to the current collectors were determined. The analysis suggests that the Peltier heat of the anode of these fuel cell materials is small, and that it is the cathode reaction which generates the main fraction of the total heat in a PEM fuel cell. The entropy change of the anode reaction appears to be close to zero, while the corresponding value for the cathode is near −80 J K⁻¹ mol⁻¹.     

Importance of catalyst stability vis-à-vis hydrogen peroxide formation rates in PEM fuel cell electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H₂O₂) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts supported on ketjen black (KB) carbon. The selectivity of these catalysts towards H₂O₂ formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H₂O₂ formation rates in a PEMFC anode and cathode. The effect of H₂O₂ formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H₂O₂ formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.     

The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs

In the present work, the cross-over rates of methanol and ethanol, respectively, through Nafion^®-115 membranes at different temperatures and different concentrations have been measured and compared. The changes of Nafion^®-115 membrane porosity in the presence of methanol or ethanol aqueous solutions were also determined by weighing vacuum-dried and alcohol solution-equilibrated membranes. The techniques of anode polarization and adsorption stripping voltammetry were applied to compare the electrochemical activity and adsorption ability, respectively. To investigate the consequences of methanol and ethanol permeation from the anode to the cathode on the performance of direct alcohol fuel cells (DAFCs), single DAFC tests, with methanol or ethanol as the fuel, have been carried out and the corresponding anode and cathode polarizations versus dynamic hydrogen electrode (DHE) were also performed. The effect of alcohol concentration on the performance of PtRu/C anode-based DAFCs was investigated.It was found that ethanol shows lower cross-over rates than methanol through the Nafion^® membrane in spite of the higher membrane porosity resulted in presence of ethanol aqueous solutions. Furthermore, it was found that ethanol presents less negative effect on the cathode performance due to both its smaller permeability through Nafion^® membrane and its slower electrochemical oxidation kinetics over Pt/C cathode.     

DESIGN OF AN ELECTROLYTIC CELL FOR A MONOLITHIC PHOTOVOLTAIC-ELECTROLYTIC HYDROGEN GENERATION SYSTEM: THE ELECTRODE ASPECTS

The combination of a photovoltaic (PV) and an electrolytic cell into one single system, a monolithic PV-electrolytic cell, has been suggested as an efficient solar hydrogen generation system. In this study, we demonstrate an efficient prototype electrolysis system applied to a monolithic PV-electrolytic cell. Specifically, the relatively large unit cell (active area of 36 cm²) used in the study made it possible to directly measure in volume the amount of hydrogen and oxygen generated. To enhance the activity of the electrodes we introduced channels, Co₃O₄ film, and Pt particles on stainless steel (SUS) substrates. The highest hydrogen production rate was obtained in the system in which Co₃O₄ electrocatalytic film on a SUS foil and Pt particles on a SUS plate with channels were used as an anode and a cathode, respectively.     

Degradation of the beta-blocker propranolol by electrochemical advanced oxidation processes based on Fenton's reaction chemistry using a boron-doped diamond anode

The electro-Fenton (EF) and photoelectro-Fenton (PEF) degradation of solutions of the beta-blocker propranolol hydrochloride with 0.5 mmol dm⁻³ Fe²⁺ at pH 3.0 has been studied using a single cell with a boron-doped diamond (BDD) anode and an air diffusion cathode (ADE) for H₂O₂ electrogeneration and a combined cell containing the above BDD/ADE pair coupled in parallel to a Pt/carbon felt (CF) cell. This naphthalene derivative can be mineralized by both methods with a BDD anode. Almost overall mineralization is attained for the PEF treatments, more rapidly with the combined system due to the generation of higher amounts of hydroxyl radical from Fenton's reaction by the continuous Fe²⁺ regeneration at the CF cathode, accelerating the oxidation of organics to Fe(III)-carboxylate complexes that are more quickly photolyzed by UVA light. The homologous EF processes are less potent giving partial mineralization. The effect of current density, pH and Fe²⁺ and drug concentrations on the oxidation power of PEF process in combined cell is examined. Propranolol decay follows a pseudo first-order reaction in most cases. Aromatic intermediates such as 1-naphthol and phthalic acid and generated carboxylic acids such as maleic, formic, oxalic and oxamic are detected and quantified by high-performance liquid chromatography. The chloride ions present in the starting solution are slowly oxidized at the BDD anode. In PEF treatments, all initial N of propranolol is completely transformed into inorganic ions, with predominance of NH₄⁺ over NO₃⁻ ion.     

Studies of some operating parameters and cyclic voltammetry for a direct ethanol fuel cell

A direct ethanol fuel cell (DEFC) of 5 cm² membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm⁻² of Pt–Ru/C (40%:20%) as the anode and 1 mg cm⁻² of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm⁻² and a power density of 10.3 mW cm⁻². The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm⁻² catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.     

The electrochemical hydrogenation of edible oils in a solid polymer electrolyte reactor. I. Reactor design and operation

A new electrochemical method has been devised and tested for the moderate temperature/atmospheric pressure hydrogenation of edible oils, fatty acids, and fatty acid methyl esters. The method employed a solid polymer electrolyte (SPE) reactor, similar to that used in H₂/O₂ fuel cells, with water as the source of hydrogen. The key component of the reactor was a membrane-electrode-assembly, composed of a RuO₂ powder anode and either a Pt-black or Pd-black powder cathode that were hot-pressed as thin films onto the opposing surfaces of a Nafion cation-exchange membrane. During reactor operation at a constant applied current, water was back-fed to the RuO₂ anode, where it was oxidized electrochemically to O₂ and H⁺. Protons migrated through the Nafion membrane under the influence of the applied electric field and contacted the Pt or Pd cathode, where they were reduced to atomic and molecular hydrogen. Oil was circulated past the back side of the cathode and unsaturated triglycerides reacted with the electrogenerated hydrogen species. The SPE reactor was operated successfully at a constant applied current density of 0.10 A/cm² and a temperature between 50 and 80°C with soybean, canola, and cottonseed oils and with mixtures of fatty acids and fatty acid methyl esters. Reaction products with iodine values in the range of 60–105 were characterized by a higher stearic acid content and a lower percentage of trans isomers than those produced in a traditional hydrogenation process.     

Oxygen and water vapor gas barrier poly(ethylene naphthalate) films by deposition of SiOx plasma polymers from mixture of tetramethoxysilane and oxygen

SiO_x films were deposited from a mixture of tetramethoxysilane (TMOS) and oxygen on poly(ethylene 2,6‐naphthalate) film using ion‐assisted plasma polymerization technique (Method II) and conventional plasma polymerization technique (Method I), and were compared in chemical composition and gas barrier properties. Methods I and II were different in electrical circuit between electrodes (anode and cathode) and electric power supply. In Method I, the anode electrode was grounded, and the cathode electrode was coupled to the discharge power supply. In Method II, the anode electrode was connected with the discharge power supply, and the cathode electrode was grounded. There was not large difference in SiO_x deposition rate between the plasma polymerizations by Methods I and II. Plasma polymers deposited from TMOS/O₂ mixtures by Method II possessed smaller C/Si and O/Si atomic ratios than those deposited by Method I and showed advantage in gas barrier properties. The oxygen and water vapor permeation rates were 0.08–0.13 cm³ m^?2 day^?1 atm^?1 at 30°C at 90% RH and 0.244–0.276 g m^?2 day^?1 at 40°C at 90% RH, respectively. From these results, it can be concluded that the ion‐assisted plasma polymerization is a useful technique for deposition of gas barrier SiO_x thin films. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 915–925, 2007