We have clarified for Pt nanoparticles supported on carbon black that the oxygen reduction reaction (ORR) activities are not affected by differences in the particle size. Enhanced activities in the ORR have been demonstrated at Pt alloyed with non-precious metals such as Fe, Co, and Ni. By use of multiple analysis methods, we found that the dissolution of the surface alloy layer was followed by a rearrangement of the remaining Pt layer, resulting in a Pt skin layer, which can protect the underlying bulk alloy. It was shown that a modified electronic structure at the Pt skin layer induced enhanced ORR activity due to an increased coverage of oxygen atoms. These results contribute to establishing a clear strategy for the design of highly active electrocatalysts for fuel cells.
A long-term operation test of PEFCs was conducted for single cells having an effective area of 25 cm2. The decay rate of the cell voltage was relatively large in the beginning, decreased to about 4 µV h−1 from 3000 to 6000 h, and about 7 µV h−1 in the later stage from 9000 to 12000 h. In order to clarify the cause of the voltage decay, materials composing membrane electrode assembly (MEA) were analyzed by disassembling a cell every 3000 h. After operation, changes of materials, such as particle size change of Pt-Co cathode catalyst, loss of ionomer layers on Pt-Co/C, decrease in thickness of the polymer electrolyte membrane, pore size changes of the cathode, were observed. Electrochemical active surface area of the electrode showed a decreasing tendency. Based on these results, we focused on the improvement of cathode, and developed a “four-layer electrode catalyst” which consisted of Pt-Co alloy particle, carbon support, ionomer and fluorocarbon resin. A test cell using this new type cathode exhibited better performances than a cell using conventional cathode in the durability test under low humidity or low temperature conditions.
A carbon-based noble-metal-free fuel cell cathode catalyst was formed by heat treatment of a mixture of glucose, Fe gluconate, and 1,4-diamino-2,3-dicyano-9,10-anthraquinone as a nitrogen source. The factors for the formation of the active site and the pore development were the heat-treatment temperature and the content of the nitrogen source in the starting mixture. The activity for oxygen reduction was dependent on the active site and the pore development, which were optimized to form the catalyst with the improved activity compared to those formed from various glucose/nitrogen source/Fe salts mixtures
The effects of CO2 on the performance of an anion-exchange membrane fuel cell were investigated using a three-electrode single cell equipped with a reference electrode. Though the membrane resistance decreased at high current densities in the presence of CO2via the self-purging mechanism, the cell voltage was significantly lower than that for pure O2 especially at low current densities even at a very low CO2 concentration of 100 ppm. The overpotential at the cathode was hardly changed by the presence of CO2, while that at the anode significantly increased in the presence of CO2. It was concluded that carbonate/bicarbonate ions accumulated at the anode during operation, and reduces the ionic conductivity and the pH in the anode catalyst layer, which results in a high overpotential at the anode.
A simple one-dimensional numerical simulation for cathode catalyst layers was conducted to elucidate the signifi-cance of a structure with a gradient in the ionomer content in the cross-plane direction. First, under an assumption of uniform (one-layer) catalysts, equivalent pairs of the effective ionic conductivity and effective gas diffusion coefficient were identified. Next, results with two-layer catalysts were calculated with two pairs of parameters among the equivalent pairs obtained earlier. Comparison of the performance of the two-layer catalysts revealed that ionic conductivity is more important near the PEM, and gas diffusivity is more important near the GDL.
In this study, we succeeded in measuring the impedance spectra at the anode and cathode during the direct methanol fuel cell power generation using a single cell having a Ag/Ag2SO4 reference electrode. When the anode impedance is measured at 20 mA cm−2, two semicircles are observed in the high and middle frequency ranges, and the magnitude of the latter one increases with the increasing methanol concentration. At the cathode, the impedance spectra at 20 mA cm−2 show three semicircles, of which the magnitudes decrease with the increasing methanol concentration from 1 mol dm−3 to 5 mol dm−3, and increase again at 10 mol dm−3. These results demonstrate that the increased methanol concentration magnifies the methanol oxidation reaction resistance at the anode and affects the total resistance at the cathode.
A novel durable electrode catalyst of Pt/Ketjen black decorated with SnO2 nanoparticles (Pt/SnO2-KB) was prepared for polymer electrolyte fuel cells (PEFCs). The particle size of SnO2 and Pt in the catalyst is ca. 10 nm and 2 nm, respectively. The activity of Pt/SnO2-KB for oxygen reduction reaction is almost same as that without SnO2; on the other hand, the durability as the cathode is excellent.
Graphite oxide nanosheets with three different sheet sizes were prepared from different graphite materials. The graphite oxide nanosheets were used as precursors to prepare three 20 mass% Pt/graphene composite electrocatalysts composed of different sizes of graphene. The structure and electrochemical properties in 0.1 M HClO4 at 60°C of the three Pt/graphene electrocatalysts were studied. Well dispersed Pt nanoparticles with 2.2 nm average diameter and electrochemically active surface area of ∼50 m2 (g-Pt)−1 were obtained irrespective of the size of graphene. Despite the similarity in Pt nanoparticle state, the oxygen reduction reaction was strongly affected by the size of graphene. Smaller sized graphene afforded higher mass and specific activity towards the oxygen reduction reaction.
Zirconium oxide-based compounds without partial oxidation of zirconium carbonitride were investigated as nonplatinum cathode for polymer electrolyte fuel cell. Zirconium oxide-based compounds were prepared by solutionprecipitation method and heat-treated at different temperature. In X-ray diffraction patterns tetragonal ZrO2 peaks decreased whereas monoclinic ZrO2 peaks increased with increasing temperature. Onset potential for oxygen reduction reaction (ORR) of the specimen heat-treated at 1000°C reached 0.90 V vs. RHE, indicating that the specimen had definite ORR activity. The increase of ORR activity may be explained by the increase of Zr3+ and/or oxygen vacancies due to the transformation of ZrO2 from tetragonal to monoclinic structure from the XRD results.
Hydrogen for fuel cell vehicles is likely to contain impurities from its production process, and such impurities may poison the fuel cell catalyst, thus degrading the power generation performance of the polymer electrolyte fuel cell (PEFC) especially when the Platinum(Pt) loading is reduced. The effects of anode Pt loading on power generation performance degradation by H2S and NH3 in hydrogen fuel were investigated. The test results indicated that the lower the anode Pt loading, the more rapid the decrease in cell voltage under the influence of H2S in the hydrogen. The amount of H2S supplied to the cell until the cell voltage decreased by the same level (30 mV) was proportional to the anode Pt loading regardless of the hydrogen sulfide concentration. The cell voltage decrease by NH3 in the hydrogen was not dependent on the anode Pt loading, but was influenced by the cathode Pt loading.
Hydrogen gas reformed from the biogas has recently been paid attention particularly from the point of view of environmental protection. In order to investigate the effect of the biogas on a polymer electrolyte membrane fuel cell, the cell performances using CO2 containing gas as anode gas were measured by fuel cell experiments and cyclic voltammetry. The membrane-electrolyte assemblies were manufactured with a Pt or PtRu catalyst and Nafion 117 or Nafion 212, and some electrodes were loaded with a Nafion solution. As a result, it was found that the presence of coexisting CO2 gas decreased the cell performance, while PtRu suppressed the negative effect. In the case of Nafion 212, which was a thinner membrane than Nafion 117 used as the electrolyte, it was more sensitive to the presence of CO2. It was found that the cathode catalyst in the cell was also poisoned by the crossovered CO2.
The reaction selectivity of Pt loading carbon electrocatalysts (Pt/C) having different Pt particle sizes was electrochemically evaluated in the presence of methanol and O2. The electrochemical measurement of these electrocatalysts was performed using a porous microelectrode technique without any binder resin. In this study, we found an electrocatalyst which exhibits a reaction selectivity during the competitive reactions of methanol oxidation and O2 reduction. For the Pt/C of O2 reduction, the desired electrocatalyst is required to retain a low Pt loading amount irrespective of Pt particle size. For the methanol oxidation, the electrocatalyst having a Pt particle size around 4 nm is effective.
Pt-carbonyl cluster complexes were prepared just by bubbling CO in various acetonitrile- or ethanol-water mixed solutions containing PtCl62−. They had compositions of [Pt3(CO)3(μ-CO)3]n2− (n=3–6) complexes in which a Pt3(CO)3(μ-CO)3 unit composed of triangular Pt atoms and two types of CO ligands was stacked. The number of Pt3(CO)3(μ-CO)3 units tended to decrease as water content in the mixed solutions increased. Pt nanoparticles prepared from the Pt-carbonyl cluster complexes had much smaller mean particle size (1.8–2.0 nm) and narrower size distribution (ca. 0.3 nm) than commercial ones. The Pt nanoparticles-loaded carbon black (Pt/CB) catalysts improved mass activity at 0.9 V for oxygen reduction reaction compared to the commercial catalyst (Pt/CBTKK). In particular, the mass activity for Pt/CB prepared from the [Pt3(CO)3(μ-CO)3]52− complex produced in a water-ethanol mixed solution (water content: 50 vol.%) was ca. 1.7 times higher than that of the Pt/CB-TKK.
Pure Pt, Pure Ru, and Pt-Ru alloy thin films of different compositions (Ru:Pt=0.5–2.0) were prepared by magnetron sputtering. These thin films were characterized by Cu stripping voltammetry at different deposition potentials. Furthermore, we developed a novel technique to determine the surface Ru/Pt ratio of Pt-Ru alloys using Cu stripping voltammetry. The Ru/Pt ratios calculated from Cu stripping data agreed well with those measured by EDX in the Ru/Pt range of 0.6 to 2.0.
To improve the catalyst utilization and reduce the catalyst amount, we designed the anode structure. The superior catalyst utilization of direct methanol fuel cell (DMFC) was obtained by localized catalyst loading on the reaction sites. Hence the multi-layer anode with localized catalyst loading was prepared by the sputtering method. The designed multi-layer electrode showed a high cell performance and mass activity because of the good contact at the interface between the catalyst and the electrolyte membrane and low mass transfer resistance. The ultra-low catalyst loading electrode preparing by the sputtering method was a very effective method for reducing the catalyst amount and enhancing the mass activity.
Metal-sulfides with high chemical stability in acidic condition were tried to develop as an alternative none-noble metal electrocatalyst for polymer electrolyte fuel cell (PEFC). In particular, various types of Ni-based sulfides have synthesized by wet-chemical method and analyzed for the cathodic performances. Among the NiS, NiS2, and Ni3S4, nickel-mono sulfide (NiS) system showed high oxygen reduction performance in 2N-H2SO4 at 70°C. In addition, Ni0.9Cu0.1S was found to show the highest oxygen reduction performance in the Ni-mono sulfide based catalysts.
The electrochemical oxidation of ethanol on novel Pt-natural zeolite electrocatalyst has been studied. It was found that Pt/Zeolite (Pt mean size, 4.14 nm) had higher electrocatalytic activity and CO-tolerance suggested by the increase of oxidation current density and the onset potential of Pt/Zeolite, 0.06 V, which is much lower than that of Pt/MWCNTs.
The electrochemical behavior of anhydrous rutile-type RuO2, hydrous rutile-type RuO2 (RuO2·0.6H2O), and ruthenate nanosheets derived from layered ruthenium oxide (H0.2RuO2.1) in CH3OH containing 0.5 M H2SO4 was studied. The pseudo-capacitance of rutile-type RuO2 nanoparticles decreased under the presence of methanol when cycled between 0.05–1.2 V vs. RHE (60°C), which is attributed to the adsorption of methanol onto the oxide surface. In contrast, the pseudo-capacitance for ruthenate nanosheets was not influenced by the presence of methanol. The results suggest that methanol adsorbs on the surface of RuO2 nanoparticles while no adsorption occurred on ruthenate nanosheets in potential regions of interest for the anodic reaction in direct methanol fuel cells. Thus, ruthenate nanosheets should act as a more efficient co-catalyst for methanol electro-oxidation.
The Fuel Cell Electrode (FCE), consisting of platinum particles or platinum alloy particles supported on carbon substrates, is one of the key components in the polymer electrolyte fuel cells (PEFCs). In order for the detailed characterization, it is crucial to determine three-dimensional (3D) morphologies of Pt nanoparticles on carbon substrates (Pt/C). In this communication, 3D structures of Pt/C with two different kinds of substrates have been examined using a nano-scale 3D imaging technique, transmission electron microtomography. It was found that the TEC10V50E had Pt catalytic nanoparticles located only at the substrate surface, while the TEC10E50E showed Pt nanoparticles both inside and at the surface of the carbon substrate.
In order to improve the fuel cell performance using a diethylmethylammonium trifluoromethanesulfate ([dema] [TfO]) absorbed sulfonated polyimide membrane, the effects of H3PO4 addition to the [dema] [TfO] and the loading amount of [dema] [TfO] in the gas diffusion electrode have been investigated. The ohmic resistance decreased with an increase in the [dema] [TfO] loading, which would correspond to the ohmic resistance between the electrode and the electrolyte membrane. The 5 mol% of H3PO4 addition enhanced the cell performance by accelerating the mass transfer, and the reaction polarization was a minimum at a 2.1 mg cm−2 the [dema] [TfO] loading.
The objective of this research was to assess the feasibility of the use of highly corrosion resistant graphitized carbon (GC) as a support for Pt nanoparticles in polymer electrolyte fuel cells and to assess the role of the state of Pt dispersion in the maintenance of performance. Three types of 50 wt% Pt-loaded catalysts (commercial Pt/CB, Pt/GC and an in-house-prepared nanocapsule Pt/GC) were subjected to durability testing by means of a standard voltage step protocol (0.9 V↔1.3 V vs. RHE, holding 30 s at each voltage, 1 min for one cycle) at 65°C with H2 (anode) and N2 (cathode), and ambient pressure (0.1 MPa). The durability was estimated on the basis of either 3000 potential cycles (commercial Pt/CB) or 10000 cycles (commercial Pt/GC and nanocapsule Pt/GC). The current-voltage curves were measured initially and after certain numbers of cycles N at 65°C, 100% RH with H2 and air. The electrochemically active surface area (ECA) decreased with increasing N, particularly the commercial Pt/CB, which underwent severe degradation in the cathode. In contrast, commercial Pt/GC and nanocapsule Pt/GC showed slow ECA degradation, due to the high corrosion resistance of GC. Furthermore, it was found that the decrease in cell performance was smaller for the nanocapsule Pt/GC compared to that for the commercial Pt/GC by 10 to 50 mV, because the Pt nanoparticles of the nanocapsule Pt/GC were well dispersed over the whole GC surface. We also examined the changes in the state of dispersion of the Pt nanoparticles by use of transmission electron microscopy (TEM).
Gas diffusion layer (GDL) is an important component for the performance of polymer electrolyte fuel cells controlling the gas flow and the water transport. In this study, the real-time formation process of water droplets on two GDLs with different pore sizes was imaged using a CCD camera during the cell operation. A porphyrin luminescent dye was coated on the GDLs to clarify the shape of the water droplets. On the GDL with a smaller pore size, the droplets were distributed uniformly on the GDL in the flow channel at the cathode. On the GDL with a larger pore size, the water droplets were larger and formed mainly near the ribs of the flow channel, and the droplets were observed to move slowly towards the gas outlet. The difference in the formation of water droplets were explained by the difference in the partial air flow inside the GDLs along the gas channel.
We carried out quantitative transmission electron microscopy (TEM) analysis for the Pt morphology in the cathode catalyst layer of polymer electrolyte fuel cells (PEFCs) for investigating the transportation of Pt species during the cell operation. The specimens for the TEM observation were cut offfrom the catalyst layer with approximately 100nm thickness without embedding it in a resin. The size and number of the Pt particles contained in the same volume of the catalyst layer were accumulated to obtain their size distributions. The distributions of Pt surface areas and volumes were also estimated from the size distributions, assuming that the Pt particles are sphere. The total volumes of the Pt particles estimated by the analysis corresponded to 65–86% of those calculated from the Pt loadings at the MEA preparations. The change in the Pt morphology before and after a potential cycling test without power generation was investigated. For the cycled MEA, the Pt surface area per weight (the specific Pt surface area) calculated from the TEM observation was nearly identical to the electrochemically active surface area (ECSA) by cyclic voltammetry(CV). This novel method for the TEM analysis provides the distributions of the Pt concentration in the whole catalyst layer as well as the Pt surface distribution and the Pt volume distribution in a given area of the catalyst layer without chemical analysis or spectroscopy. Those data can be used to understand the dependence of the microstructures of the catalyst layer on the cell performances.
We synthesized a Pt catalyst supported on nanometer-size titanium nitride particles (Pt/TiN) by the nanocapsule method. The titanium nitride (TiN) support, which was synthesized by the radio-frequency (RF) plasma method, had high electrical conductivity, up to 850 S cm −1 at room temperature, with a surface area of 40 m2 g−1. The Pt loading on the catalyst was 19.5 wt%. The electrochemically active surface area (ECA) was 72 m2 g(Pt)−1. During the potential step cycling test (0.9∼1.3 V), the ECA values for Pt/TiN remained high and exceeded that of a commercial Pt catalyst supported on carbon black (Pt/CB) for potential step cycle numbers above 300. From the results of linear sweep voltammetry using a rotating disk electrode, we observed that the oxygen reduction reaction activity of the Pt/TiN exceeded that of Pt/CB. We conclude that the nanometer-size TiN might be a good candidate support material for the cathode of the polymer electrolyte fuel cell (PEFC).
A simple macrohomogeneous 1-D model of the polymer electrolyte fuel cell (PEFC) cathode catalyst layer incorporating ORR kinetics and including the effects of Pt surface oxide coverage, proton migration, and oxygen diffusion is introduced. In the model, both the site-blocking and the energetic effects of the Pt surface oxide are incorporated and applied to actual fuel cell data. Measured polarization curves are fitted in both the cathodic and anodic directions using a single value of the intrinsic exchange current density (that is independent of the Pt oxide coverage) representing the entire potential range of operation. By fitting with this model, the catalytic activity is broken down into the intrinsic exchange current density, the oxide coverage, and the Temkin parameter. The fitting parameters used in the model are the exchange current density, the Temkin parameter, and the transmissibility of oxygen within catalyst layer. The electrochemically active surface area and the proton conduction resistance within catalyst layer were determined using experimental diagnostic techniques. The surface charge representing oxide species was also measured experimentally and converted into a surface coverage based on an assumed species. The analysis of polarization curves using this modeling approach may help to provide a deeper understanding that could accelerates the development and characterization of new catalysts.
To improve the proton conductivity of an aromatic polymer electrolyte membrane (PEM), the relationship between the size of a micro-phase-separated structure and the proton conductivity of the membrane were investigated. Micro-phase-structures, which are comprised of hydrophilic and hydrophobic domains, were measured by using the stained small-angle X-ray scattering (SAXS) method in wet conditions. Periodic sizes calculated from SAXS profiles in a dry state were as large as those measured by scanning transmission electron microscopy (STEM), which indicates that micro-phase-separated structures can be effectively evaluated by using SAXS. The membranes with the larger periodic size showed higher proton conductivity; however, water content was not simply increased with the increasing in the periodic size. Conceivably, the periodic size of the phase-separated-structure had an effect on both the proton conductivity at the same water content and on the water content itself, and the proton conductivity was increased by these two factors. The results indicated that the optimum periodic size of micro-phase-separated structures between proton conductivity and water content was 35 nm in this study, and it may be possible to develop a membrane with high proton conductivity and low water content by controlling the size of the phase-separated-structure.
In order to develop a principle of the materials design for the high CO2 generation efficiency during the C2H5OH electrooxidation, the reaction pathways during the electrooxidation of C2H5OH and CH3CHO which is a by-product during the electrooxidation of C2H5OH have been quantitatively analyzed. As a result, a direct pathway from C2H5OH to CO2 and a pathway via CH3CHO toward CO2 coexisted at 0.4 V, 70°C. At 0.6 V, 70°C, the both pathways from C2H5OH to directly CO2 and from C2H5OH via CH3COOH to CO2 existed. Especially in the latter pathway, the activity of Pt was found to be deteriorated by the adsorbed species which would be formed as an intermediate of CH3COOH formation.
To develop better and less expensive electrocatalysts for the oxidation of ethanol in direct ethanol fuel cells, several combinations of a conductive polymer polyaniline (PANI), dispersed Pt particles and pre-dispersed metal particles, such as Sn and Fe, were examined. The anodic current for the ethanol oxidation (iEtOH) was gained in larger quantities using the glassy carbon electrode covered with the Pt-dispersed PANI film than using the naked glassy carbon electrode. The enhancement of iEtOH strongly depended on the morphology and the electrical conductivity of the five PANI films with different dopant anions: SO42−, NO3− and Cl−. The highest activity was achieved for the SO42−-doped PANI film. To reduce the amount of the expensive Pt particles, inexpensive base metal particles were pre-dispersed on the PANI film, and the Pt particles were then dispersed on the film. Among the investigated pre-dispersed metal particles (Sn, Cu, Zn and Fe), the highest activity was obtained with Sn particles for the ethanol oxidation. When the ratio of the dispersed Pt to Sn particles ranged from 10:90 to 100:0, iEtOH was higher than that measured with the dispersed Pt particle PANI films without the Sn particles. This meant that utilizing dispersed Sn particles could reduce the amount of the dispersed Pt particles needed.