Ru Dissolution Research Articles

Impacts of Ru Dissolution and Crossover on PEMFC Performance and Anode Functionality

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Evaluation of Solubility of Ru in Acidic Solution

The Pt–Ru alloy is used as an electrocatalyst for stationary polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells because of its anti –CO poisoning property. Degradation of the Pt–Ru anode is one of the important problems for the durability of PEFC systems. In this study, the dissolution of Ru and RuO2 has been investigated in sulfuric acid as a fundamental study of the stability of the Pt–Ru anode. The stability of Ru was very sensitive to the history of the surface treatment. The steady-state concentration of Ru ions from RuO2/Ti was less than 2 μmol dm−3 and slightly increased with potential in the range from 0.2 to 0.5 V vs. RHE. On the other hand, the concentration of Ru ions from Ru-black/Au was a several times greater than that from RuO2/Ti up to 0.3 V. vs. RHE and significantly increased above 0.5 V vs. RHE. The maximum concentration was 16 μmol dm−3 at 0.6 V vs. RHE. The surface of Ru might be irreversible with a potential change, and the steady-state concentration would be affected by the surface property.

The effects of ruthenium-oxidation states on Ru dissolution in PtRu thin-film electrodes

The effects of ruthenium (Ru)-oxidation states were investigated on Ru dissolution from PtRu thin-film electrodes, with the 200 cycles between 0.4 and 1.05 V (versus normal hydrogen electrode). The Ru-oxidation states of the PtRu thin films were systematically modified by an anodic (oxidation) treatment. The anodic-treated PtRu electrodes, whose methanol-oxidation activity was similar to untreated electrodes before the 200 cycles, showed a remarkable decrease in methanol oxidation after the cycles, because of the Ru dissolution from the PtRu surface. The results suggest that the Ru-oxide species are the origin of Ru dissolution in the PtRu alloy.

Effects of iron-phosphate coating on Ru dissolution in the PtRu thin-film electrodes

The effects of FePO4 nanoscale coating on PtRu thin films were investigated on the block of Ru crossover. Ru dissolution was examined by the accelerated-potential cycles between 0.4 and 1.05 V. The results showed that Ru dissolution from FePO4-coated PtRu surface was inevitable due to the direct contact between the PtRu surface and aqueous electrolyte. However, the FePO4 coating layer on PtRu thin-film electrodes effectively retained the dissolved Ru species, thus preventing the dissolved Ru species from diffusing into the electrolyte. Moreover, the retained Ru species within the FePO4-coating layer were redeposited onto the PtRu surface during the cycling in the fresh electrolyte.

H2O2 Formation Depending on Ru Content of Pt-Ru/C Catalyst

H2O2 formation on Pt/C catalyst and Pt1Run/C (n = 1/3, 1/2, 1, 3/2, 2) catalysts was studied with rotating ring-disk electrode (RRDE) in 0.5 M H2SO4. H2O2 formation on Pt/C catalyst and Pt1Run/C catalysts with smaller Ru content increased sharply below 0.1 V vs. RHE. On the other hand, for larger Ru content catalysts, Pt1Run/C (n = 1, 3/2, 2), their H2O2 formation was lower than that of smaller Ru content catalysts below 0.1 V vs. RHE. Pt1Run/C catalysts with larger Ru content prevented H2O2 formation in anode catalyst layer and mitigated membrane degradation in PEFC. Potential cycles between 0.1 V and 1.0 V and between 0.1 V and 1.2 V vs. RHE were applied for Pt1Ru2/C catalyst. H2O2 formation below 0.1 V vs. RHE increased after potential cycles. Ru dissolution from Pt1Ru2/C catalyst was considered to promote H2O2 formation.

The Effect of Concentration on the Galvanostatic Pulse Deposition of PtRu

Fabrication of PtRu nanoparticles was conducted by a galvanostatic pulse electrodeposition technique. We studied various deposition parameters including current on-time and off-time, current density, as well as concentrations for the Pt and Ru precursors in the plating bath in order to determine the optimized process condition for the highest methanol oxidation ability. Cyclic voltammetric profiles were obtained in electrolytes of 0.5 M H2SO4, and mixture of 0.5 M H2SO4 and 1.0 M CH3OH to determine characteristics in the hydrogen absorption/desorption and peak currents in the anodic/cathodic scans. We observed that a shorter current off-time resulted in better catalytic abilities when the Pt precursor concentration was increased. Partial dissolution of Ru in the as-deposited PtRu nanoparticles during the current off-time might be responsible.

Methanol electro-oxidation by a ternary Pt–Ru–Cu catalyst identified by a combinatorial approach

Composition optimization of the ternary Pt–Ru–Cu system for the methanol electro-oxidation reaction (MOR) was performed by combinatorial synthesis and high-throughput screening methods. A thin film library of the Pt–Ru–Cu system was prepared by a sputtering system to generate 63 different compositions. The compositions were characterized in parallel by a multichannel multielectrode analyzer. The highest MOR activity was observed for the Pt 66Ru 17Cu 17 composition. The Pt 66Ru 17Cu 17/C composition was synthesized and characterized as a powder catalyst to verify the performance of this new composition. During cyclic voltammetry tests, the Pt 66Ru 17Cu 17/C catalyst showed less dissolution and irreversible oxidation of Ru than a Pt 50Ru 50/C catalyst, with increasing number of cycles. In MOR activity measurement experiments, the Pt 66Ru 17Cu 17/C catalyst exhibited 26 and 86% higher activities in cyclic and chronoamperometric tests, respectively, than those of the Pt 50Ru 50/C catalyst.

EQCM Study on Dissolution of Ruthenium in Sulfuric Acid

This study reports on the nanolevel dissolution and polarization behavior of electrodeposited Ru in solution with the help of an electrochemical quartz crystal microbalance (EQCM) and inductively coupled plasma-mass spectrometry (ICP-MS) analysis. Results of the cyclic voltammetry showed that during anodic polarization a mass gain at Ru electrode was observed from due to water adsorption and the significant mass gain due to Ru oxide formation was observed from 0.5 V; the start of these processes depended on the potential. ICP-MS analysis of the solution after potential cycling of the Ru electrode showed that only a slight amount of the dissolution occurred when the potential was less than 0.5 V, but the amount of dissolution significantly increased from 0.5 V onward. The potential cycle accelerated the dissolution of Ru. A simple immersion test revealed that the potential of Ru increased until 0.8 V from during the immersion. This shows that the on–off cycle of a polymer electrolyte fuel cell (PEFC) can induce the potential cycle; this will lead to accelerate the dissolution of Ru in the anode catalyst of the PEFC.

On the Accelerating Degradation of DMFC at Highly Anodic Potential

A convenient method of investigating the degradation of a direct methanol fuel cell (DMFC) was carried out at highly anodic potential (i.e., ). Accelerating degradation of a DMFC was investigated by electrochemical methods, X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), and X-ray photoelectron spectroscopy (XPS). The degradation was preliminarily diagnosed and predicted with electrochemical impedance spectroscopy and verified with microscopic examination. The EPMA and XPS results showed that the sulfonic acid vanished in the broken anodic layer compared to the original one, which results in an increase of internal resistance (Rs). The increase of interfacial and electrochemical reaction resistance in the degraded cell might be a result from catalytic degradation. Ru dissolves from the anodic catalyst to decrease the catalytic activity then reduced near the cathode. The evidence for Ru dissolution was determined by stripping combined with EPMA and XPS analyses. Its reduction near the cathode was observed through TEM and EPMA. The catalysts in both the anode and cathode aggregated to decrease the catalytic activity in the degradation process by XRD and TEM. On the basis of the electrochemical study and related analysis, we proposed a mechanism for this accelerating degradation of DMFC.

Stabilization of the platinum–ruthenium electrocatalyst against the dissolution of ruthenium with the incorporation of gold

The dissolution of Ru from the PtRu electrocatalyst has been identified as one of the most critical factors in degrading the performance of polymer electrolyte membrane fuel cells (PEMFC). In this work, we prepared an Au-modified PtRu catalyst (Au/PtRu) and found that the incorporation of Au could significantly stabilize the PtRu electrocatalyst against the dissolution of Ru. The X-ray photoelectron spectroscopy (XPS) characterization of the Au/PtRu catalysts revealed that the incorporation of Au increased the oxidation potential of Ru, which is the mechanism that leads to a reduction in the dissolution of Ru from the alloyed catalyst. The XPS characterization of the cathode catalyst also showed that with the PtRu as the anode catalyst Ru appeared at the cathode, but the crossover of Ru could be reduced when the anode catalyst was changed to Au/PtRu.

Study on Degradation of the Pt-Ru Anode of PEFC for Residential Application (2): Analyses of the MEAs Degraded in the Anode Performance

Significant decrease in the voltage and increase in H2 gain were observed after the low current density operation. However, recovery in voltage and decrease in H2 gain were also observed by the dry treatment. The impedance of degraded MEA just after the low current density operation increased at the low frequency region in CO(10ppm)/H2(balance). On the other hand, the impedance of MEA that the H2 gain was increased by long term operation for more than 20,000 h was increased at higher frequency region in CO(10ppm)/H2(balance). It was concluded that the increase of impedance was caused by the flooding in the anode in the first case, and by the essential degradation by Ru dissolution from anode catalyst in the later case.

Understanding the Anodic Dissolution of Ru from Select PtRu Electrocatalysts During DMFC Operating Environment

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Anodic Dissolution of Ru and its effect on the Electrocatalysis of ORR in a DMFC

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Dissolution of Ru from PtRu Electrocatalysts and its Consequences in DMFCs

Rapid and irreversible performance loss could be observed for a single cell DMFC using a PtRu anode electrocatalyst when it was operated under non-optimized conditions. Post-mortem cyclic voltammetry and EDAX data reveal that the anode electrode became depleted in Ru, and the dissolved Ru crossed through the PE membrane and then deposited onto the Pt cathode electrode. Comparative elemental compositional as well as CO and MeOH electrooxidation performance analysis was performed with commercial PtRu black samples having the same 50:50 atomic ratio but different degree of alloying. The data suggests that the latter physical property is crucial in maintaining the anodic activity for prolonged time. On the other hand, dissolved Ru species present in the micromolar concentration range in liquid electrolyte are shown to have severe detrimental impacts on ORR kinetics on a Pt thin-film RDE (having a surface roughness factor of ca. 6).

Pt-Ru electrodeposited on gold from chloride electrolytes

Voltammetric behavior of submicron-thick electrodeposited Pt-Ru on gold support is studied in sulfuric acid solution as a function of deposition potential and Pt:Ru ratio in chloride bath. In contrast to Pt-Ru, deposition of pure Ru is observed only at potentials of hydrogen evolution. The reason is found to be of kinetic nature, namely an inhibition of Ru deposition in presence of chloride. Chloride ions remain adsorbed on Ru at more negative potentials than on Pt and Au because of more negative ruthenium potential of zero free charge. Cu-UPD is applied to test the surface content of the oxidized Ru on pure Ru and various Pt-Ru surfaces. An enhancement of Ru oxohydroxides reduction in presence of Pt is observed. The electrocatalytic activity of Pt-Ru in respect to methanol oxidation correlates with the content of rechargeable surface Ru oxide. Ageing and ‘training’ of Pt-Ru electrodeposits under various modes is studied in order to determine the conditions of irreversible Ru oxidation. No manifestations of Ru dissolution from Pt-Ru electrodeposits in 0.5 M H 2SO 4 are found for anodic potential limits up to 1.1 V (RHE), in agreement with thermodynamic predictions. Electrodeposited Pt-Ru can be considered as a convenient model system for the study of Ru dissolution and crossover, as well as for determining the nature of the active surface species in the real composite catalysts for methanol oxidation.

Dissolution of Ru from PtRu Electrocatalysts and its Mobility in DMFCs

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Carbon Monoxide Adsorption on Ru-Modified Pt Surfaces: Time-Resolved Infrared Reflection Absorption Studies in Ultrahigh Vacuum

The vibrational spectra of CO adsorbed on Ru-modified Pt(100) surfaces prepared by chemical vapor deposition (condensation of Ru(3)(CO)(12) at 105 K followed by X-ray irradiation and thermal decomposition at 650 K in ultrahigh vacuum, UHV) was investigated by time-resolved infrared reflection absorption spectroscopy (IRAS) in UHV. Spectra were recorded while Ru/Pt(100) bimetallic surfaces (theta(Ru) = 0.24 and 0.52 by X-ray photoelectron spectroscopy, XPS) were dosed with gas-phase CO. Analysis of the data revealed that for a wide range of calibrated CO exposures, the linear CO-stretching region displays two features: a higher energy peak (2085-2100 cm(-1)), attributed to CO adsorbed on pristine Pt(100) sites, and a lower energy peak (2066-2092 cm(-1)), ascribed to adsorption of CO on sites on the surface induced by the presence of Ru. Similar experiments were performed on bimetallic specimens annealed repeatedly in UHV to 650 K to promote partial Ru dissolution into the lattice and thus render surfaces gradually enriched in Pt. For all surfaces and CO exposures examined, the total integrated area under the two CO spectral features remained fairly constant and equal in value to the corresponding areas found for bare Pt(100). If it is assumed that a fixed exposure leads to a fixed coverage on both bare and Ru-modified Pt(100)surfaces, and the thermal treatment leads to an exchange of Ru by Pt sites without altering significantly the total number of metal sites on the surface, the absorption cross sections for both of these peaks are virtually the same.

In-Situ X-ray Absorption Spectroscopy Study of Pt and Ru Chemistry during Methanol Electrooxidation

Methanol electrooxidation in a 0.5 M sulfuric acid electrolyte containing 1.0 M CH3OH was studied on 30% Pt/carbon and 30% PtRu/carbon (Pt/Ru = 1:1) catalysts using X-ray absorption spectroscopy (XAS). Absorption by Pt and Ru was measured at constant photon energy in the near edge region during linear potential sweeps of 10-50 mV/s between 0.01 and 1.36 V vs rhe. The absorption results were used to follow Pt and Ru oxidation and reduction under transient conditions as well as to monitor Ru dissolution. Both catalysts exhibited higher activity for methanol oxidation at high potential following multiple potential cycles. Correlation of XAS data with the potential sweeps indicates that Pt catalysts lose activity at high potentials due to Pt oxidation. The addition of Ru to Pt accelerates the rate of methanol oxidation at all potentials. Ru is more readily oxidized than Pt, but unlike Pt, its oxidation does not result in a decrease in catalytic activity. PtRu/carbon catalysts underwent significant changes during potential cycling due to Ru loss. Similar current density vs potential results were obtained using the same PtRu/carbon catalyst at the same loading in a membrane electrode assembly half cell with only a Nafion (DuPont) solid electrolyte. The results are interpreted in terms of a bifunctional catalyst mechanism in which Pt surface sites serve to chemisorb and dissociate methanol to protons and carbon monoxide, while Ru surface sites activate water and accelerate the oxidation of the chemisorbed CO intermediate. PtRu/carbon catalysts maintain their activity at very high potentials, which is attributed to the ability of the added Ru to keep Pt present in a reduced state, a necessary requirement for methanol chemisorption and dissociation.

XPS surface study of nanocrystalline Ti–Ru–Fe materials

The surface properties of Ti:Ru:Fe (2−x:1+x/2:1+x/2) (with x=0, 0.25, 0.5, 0.75, and 1) and Ti:Ru:Fe:O (2:1:1:w) (with w=0.0, 0.5, 1.0, 1.5, and 2.0) have been determined by X-ray photoelectron spectroscopy (XPS) in both their as-milled state and after being in contact with a chlorate oxidizing solution. The O surface concentrations of both sets of samples are almost identical, indicating that the O-free samples readily react with air. All samples in their as-milled state have an elemental Ti, Ru and Fe surface contents that closely follow that expected from their bulk composition, indicating that there is no surface enrichment in any of the elements. In the as-milled state, more than 90% of Ti and Fe atoms are in the highest possible oxidation state, while Ru is in the metallic state. Following immersion of the samples in an oxidizing chlorate electrolyte, the Ru surface concentration decreases by a factor of two. This is also accompanied by an increase in the oxidation state of the Ru atoms left at the surface from 0 to +4. From a comparison between the Ru 3d5/2,3/2 core level spectra of the electrodes with those of crystalline and hydrated RuO2, it is postulated that dissolution and re-deposition of Ru in the form of hydrated RuO2 occurs at open circuit potential in the chlorate electrolyte. The consequences of these findings for the electrocatalytic activity of the electrodes in chlorate electrolyte are finally discussed.

The electrooxidation of carbon monoxide on ruthenium modified Pt(110)

The effect of adsorbed and incorporated (alloyed) sub-monolayers of Ru on Pt(110) on the electrooxidation of CO has been investigated using UHV-electrochemical transfer techniques. Overlayers of metal vapour deposited (MVD) Ru were characterised using LEED, XPS and LEISS. Both adsorbed and incorporated Ru overlayers were prepared, the alloy layers formed by annealing in the temperature range 400–1100 K. The reversible hydrogen adsorption characteristics of Pt(110) were suppressed in the cyclic voltammetry when Ru was adsorbed, incorporated in the top layer, or when absent from the top layer where there was a significant concentration of Ru in the second/third layers. Surfaces exhibited significant redox behaviour in the double layer region (0.3–0.6 V) of Pt(110) also when ruthenium was present in the adsorbed or top surface phases, but to a much lesser extent when incorporated in the second/third layer. CO electrooxidation, however, was promoted primarily by Ru incorporated in the top layer. Cycling to potentials of 1.2 V resulted in facile oxidative dissolution of adsorbed Ru, but only slow dissolution of Ru when incorporated in the top layer. The effect of cycling to 1.2 V on surfaces with negligible Ru in the top layer, but Ru in the second and third layers, was to modify the CO oxidation behaviour substantially, producing a significant coverage of a Pt(110)–Ru phase which promoted CO oxidation in a peak at 0.5 V.