Pt−Ru is the favored anode catalyst for the oxidation of methanol in direct methanol fuel cells (DMFCs). The nanoscale Pt−Ru blacks are accepted to be bimetallic alloys as based on their X-ray diffraction patterns. Our bulk and surface analyses show that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal. Thermogravimetric analysis and X-ray photoelectron spectroscopy of as-received Pt−Ru electrocatalysts indicate that DMFC materials contain substantial amounts of hydrous ruthenium oxide (RuO x H y ). A potential misidentification of nanoscale Pt−Ru blacks arises because RuO x H y is amorphous and cannot be discerned by X-ray diffraction. Hydrous ruthenium oxide is a mixed proton and electron conductor and innately expresses Ru−OH speciation. These properties are of key importance in the mechanism of methanol oxidation, in particular, Ru−OH is a critical component of the bifunctional mechanism proposed for direct methanol oxidation in that it is the oxygen-transfer species that oxidatively dissociates −C⋮O fragments from the Pt surface. The catalysts and membrane-electrode assemblies of DMFCs should not be processed at or exposed to temperatures >150 °C, as such conditions deleteriously lower the proton conductivity of hydrous ruthenium oxide and thus affect the ability of the Ru component of the electrocatalyst to dissociate water. With this analytical understanding of the true nature of practical nanoscale Pt−Ru electrocatalysts, we can now recommend that hydrous ruthenium oxide, rather than Ru metal or anhydrous RuO2, is the preferred Ru speciation in these catalysts.
Hydrous ruthenium oxide (RuO2·xH2O or RuO x H y ) is a mixed electron−proton conductor with a specific capacitance as high as 720 F/g/proton, making it a candidate material for energy storage. The correlation between the structure and properties of RuO2·xH2O materials is not well understood due to their amorphous nature and compositional variability. In this study, ruthenium oxides with the compositions RuO2·2.32H2O, RuO2·0.29H2O, and anhydrous RuO2 are characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses. XANES cannot be used to distinguish between Ru(III) and Ru(IV) in the hydrous oxides, but the EXAFS analyses show large differences in the short-range structures of the materials. Whereas anhydrous RuO2 has the rutile structure comprising chains of RuO6 octahedra linked in three dimensions, the structure of RuO2·0.29H2O is rutile-like at the RuO6 core, but less connected and progressively disordered beyond the RuO6 core. The structure of RuO2·2.32H2O is composed of chains of disordered RuO6 octahedra that exhibit no chain-to-chain linking or three-dimensional order. Although the local structures of RuO2·0.29H2O and RuO2·2.32H2O markedly differ, their specific capacitances are large and essentially equivalent, so nonunique local structures can balance effective electron transport (along dioxo bridges) with the effective proton transport (through structural water) necessary for charge storage.
Several important aspects relating to the mechanism of formation of protective Cr-based oxide films on aluminum alloy 2024T3 generated from CrO, + NaF-containing solutions were observed with electrochemical, AES and XPS measurements. Although the film deposition rate and surface composition were very much influenced by the presence of both Fe-and Cu-containing intermetallic phases, a uniform composition and thickness was eventually reached with increased coating time. This behavior is believed to be responsible for obtaining corrosion-resistant films on such heterogeneous surfaces. Surface Cr was consistently found to be in both the 3+ and 6+ oxidation states in an approximately 40 : 60 ratio, respectively, if x-ray beam reduction of Cr(V1) was accounted for. The enriched Cu layer found on the surface of polished 2024-T3 was found to remain intact during Cr-0 film formation. The presence of Cr(V1) provides a 'self-healing' aspect to the film's protective mechanism by remaining available for reduction to Cr(1II) during oxidative attack. Addition of potassium ferre or ferricyanide to the bath resulted in a film which obtained Fe, C and N enriched at the surface on both the matrix and intermetallic regions. These constituents were found, however, to be enriched and distributed throughout the entire depth of the film on the high Cu-bearing intermetallic regions, suggesting that the formation of a Cwferrocyanide complex is responsible for the benefit derived from these compounds.
The expansion of battery material during lithium intercalation is a concern for the cycle life and performance of lithium ion batteries. In this paper, electrode expansion is quantified from in situ neutron images taken during cycling of pouch cells with lithium iron phosphate positive and graphite negative electrodes. Apart from confirming the overall expansion as a function of state of charge and the correlation with graphite transitions that have been observed in previous dilatometer experiments we show the spatial distribution of the expansion along the individual electrodes of the pouch cell. The experiments were performed on two cells with different electrode areas during low and high c-rate operation. The measurements show how charging straightened the cell layers that were slightly curved by handling of the pouch cell during setup of the experiment. Subsequent high charging rate, that exceeded the suggested operating voltage limits, was shown to have a strong influence on the observed expansion. Specifically, during high-rate cycling, the battery showed a much larger and irreversible expansion of around 1.5% which was correlated with a 4% loss in capacity over 21 cycles.Expansion and contraction of battery material during charging and discharging can lead to fracture of the electrode and eventually capacity loss as particles are no longer electrically connected to the current collector, each other, or the carbon matrix in which they are suspended. The carbon anode material is known to expand upon intercalation of lithium into the host structure that occurs during charging of the battery. Expansion of the graphite can cause deformations as large as 10% of the anode volume 1,2 depending upon the type of carbon. The stress that develops inside the battery is also related to the rate of charging. 3 Lee et al. 4 measured the dimensional changes in lithium cobalt oxide pouch cells during cycling using a specialized dilatometer setup. They found that the expansion of the battery consists of two components: an irreversible thickness increase, corresponding to initial formation of the solid electrolyte interface (SEI), and one which is reversible and follows the battery state of charge, expanding upon charging. 4 They attributed the volume change during cycling, approximately 2% of the total battery initial thickness, to the anode active material since the cobalt oxide does not show significant volume change upon lithium intercalation.In this paper we document the expansion of Lithium Iron Phosphate (LiFePO 4 or LFP) pouch cells upon charging. The measurements are taken using Neutron Imaging (NI), an in situ technique similar to X-ray imaging that is sensitive to lighter elements such as hydrogen and lithium. We also provide a method for quantifying the expansion from the NI data. We observed a 0.5% total cell expansion (after SEI formation), which corresponds to a 1.7% expansion of the graphite material if attributed entirely to the negative electrode active material and ignoring the potential contraction of the ...
Pt/Ru catalysts with two very different structures were examined with X-ray absorption near edge structure (XANES). One sample was an industrial methanol fuel cell Pt/Ru alloy catalyst black, and the second was a carbon-supported Pt/Ru catalyst. In both cases the as-prepared Pt/Ru catalysts were found, with XANES, to be predominately in the form of Pt and Ru oxides. When these catalysts were placed in an electrochemical cell and held in the potential region where methanol oxidation occurs, XANES data indicate that the metal oxides were reduced to the metallic form. These results also demonstrate that conclusions about the electrocatalytic activity of Pt/Ru materials for methanol oxidation drawn from the characterization of as-prepared samples have little relevance.
Boron-doped diamond electrodes prepared by chemical vapor deposition were used to determine if phenol could be oxidized to CO 2 . Cyclic voltammetry showed that phenol was oxidized by the diamond electrodes and remained electroactive after multiple cycles. Experiments were also run with a flow cell in which 1 L of 10 mM phenol in 0.1 M H 2 SO 4 was circulated through the cell and the total organic carbon ͑TOC͒ was monitored as a function of time and cell current. The TOC in solution was reduced from ϳ1% to Ͻ0.1% with no observable decrease in decomposition rate. This means that the reacted phenol was converted completely to CO 2 .In recent years, there has been increasing interest in the electrochemical properties of boron-doped diamond ͑BDD͒ coated substrates. 1-13 For the most part BDD has been deposited on silicon substrates. However, the use of metal wires or meshes as substrate materials is highly desirable for electrochemical applications because of the enhanced electrical conductivity and the increased surface area. Previous work has shown that diamond can be deposited on a variety of metals. 1,6,14,15 Recently, Glesener et al. discussed the use of a high surface area BDD mesh for electrochemical applications. 8 The cleanup of liquids or slurries that contain Ͼ1% total organic carbon ͑TOC͒, defined as nonwastewater hazardous waste, 16 offer particular difficulties for on-site remediation efforts. At the present time the preferred method of remediation for nonwastewater hazardous waste is by incineration. The electrolytic destruction of organic wastes has shown promise for remediation of a wide variety of organic materials in aqueous waste streams. 17 However, the use of electrochemical oxidation for organic waste decomposition has been limited by traditional electrode materials, e.g., platinum, ruthenium dioxide, graphite, lead dioxide, and tin dioxide. The limitations arise from low reaction rates and efficiencies, corrosion of the electrodes, and fouling and poisoning of the active electrode surfaces. Recent work has shown that BDD is not susceptible to these limitations and has been used to oxidize a number of organic compounds. [9][10][11][12][13] The work reported here demonstrates that BDD electrodes prepared by chemical vapor deposition ͑CVD͒ can be used to oxidize phenol to CO 2 . Phenol was chosen as a test compound because it is one of the most difficult organic molecules to oxidize electrochemically. Phenol is well known for its rapid fouling of the electrode surface due to formation of a blocking polymer layer produced by the polymerization of the phenoxy radicals generated in the initial stages of the reaction. 18,19 This results in a decrease in the active surface area and a termination of the reaction within minutes. ExperimentalElectrodes were prepared by coating titanium ͑Ti͒ mesh substrates (1 ϫ 2 cm) with approximately 10 m of BDD via microwave plasma-enhanced CVD ͑PECVD͒. A two-step deposition process was utilized to achieve uniform nucleation and good film adhesion. The films were char...
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