Kinetics and PEMFC performance of RuxMoySez nanoparticles as a cathode catalyst

Kinetics of Ru_xMo_ySe_z nanoparticles dispersed on carbon powder was studied in 0.5 M H₂SO₄ electrolyte towards the oxygen reduction reaction (ORR) and as cathode catalysts for a proton exchange membrane fuel cell (PEMFC). Ru_xMo_ySe_z catalyst was synthesized by decarbonylation of transition-metal carbonyl compounds for 3 h in organic solvent. The powder was characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Catalyst is composed of uniform agglomerates of nanocrystalline particles with an estimated composition of Ru₆Mo₁Se₃, embedded in an amorphous phase. The electrochemical activity was studied by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Tafel slopes for the ORR remain invariant with temperature at −0.116 V dec⁻¹ with an increase of the charge transfer coefficient in dα/dT = 1.6 × 10⁻³, attributed to an entropy turnover contribution to the electrocatalytic reaction. The effect of temperature on the ORR kinetics was analyzed resulting in an apparent activation energy of 45.6 ± 0.5 kJ mol⁻¹. The catalyst generates less than 2.5% hydrogen peroxide during oxygen reduction. The Ru_xMo_ySe_z nanoparticles dispersed on a carbon powder were tested as cathode electrocatalyst in a single fuel cell. The membrane-electrode assembly (MEA), included Nafion^® 112 as polymer electrolyte membrane and commercial carbon supported Pt (10 wt%Pt/C-Etek) as anode catalyst. It was found that the maximum performance achieved for the electro-reduction of oxygen was with a loading of 1.0 mg cm⁻² Ru_xMo_ySe_z 20 wt%/C, arriving to a power density of 240 mW cm⁻² at 0.3 V and 80 °C.     

Evaluation of RuxWySez Catalyst as a Cathode Electrode in a Polymer Electrolyte Membrane Fuel Cell

The oxygen reduction reaction (ORR) on Ru_xW_ySe_z is of great importance in the development of a novel cathode electrode in a polymer electrolyte membrane fuel cell (PEMFC) technology. The Ru_xW_ySe_z electrocatalyst was synthesised in an organic solvent for 3 h. The powder was characterised by transmission electron microscopy (TEM), and powder X‐ray diffraction (XRD). The electrocatalyst consisted of agglomerates of nanometric size (∼50–150 nm) particles. In the electrochemical studies, rotating disc electrode (RDE) and rotating ring‐disc electrode (RRDE) techniques were used to determine the oxygen reduction kinetics in 0.5 M H₂SO₄. The kinetic studies include the determination of Tafel slope (112 mV dec^–1), exchange current density at 25 °C (1.48 × 10^–4 mA cm^–2) and the apparent activation energy of the oxygen reaction (52.1 � 0.4 kJ mol^–1). Analysis of the data shows a multi‐electron charge transfer process to water formation, with 2% H₂O₂ production. A single PEMFC with the Ru_xW_ySe_z cathode catalysts generated a power density of 180 mW cm^–2. Performance achieved with a loading of 1.4 mg cm^–2 of a 40 wt% Ru_xW_ySe_z and 60 wt% carbon Vulcan (i.e. 0.56 mg cm^–2 of pure Ru_xW_ySe_z). Single PEMFC working was obtained with hydrogen and oxygen at 80 °C with 30 psi.     

Chalcogenide metal centers for oxygen reduction reaction: Activity and tolerance

This mini-review summarizes materials design methods, oxygen reduction kinetics, tolerance to small organic molecules and fuel cell performance of chalcogenide metal catalysts, particularly, ruthenium (Ru_xSe_y) and non-precious transition metals (M_xX_y: M = Co, Fe and Ni; X = Se and S). These non-platinum catalysts are potential alternatives to Pt-based catalysts because of their comparable catalytic activity (Ru_xSe_y), low cost, high abundance and, in particular, a high tolerance to small organic molecules. Developing trends of synthesis methods, mechanism of oxygen reduction reaction and applications in direct alcohol fuel cells as well as the substrate effect are highlighted.     

Effect of support type and synthesis conditions on the oxygen reduction activity of RuxSey catalyst prepared by the microwave polyol method

Ru_xSe_y nanoparticles supported on different carbon substrates were synthesized by microwave heating of ethylene glycol solutions of Ru(III) chloride and sodium selenite at different pH and Ru/Se mole ratios. The resulting catalysts were used for the electrochemical oxygen reduction reaction (ORR) in acidic solution. The electrochemical activity was highest for the supported catalyst synthesized at pH 8. Increasing the Se concentration of the catalyst up to 15 mol% increased the catalytic activity for the ORR; at this Se concentration, the activity of the catalyst was considerably higher than that observed for pure Ru catalyst synthesized at exactly the same conditions. The influence of the type of carbon support on the activity of the electrocatalyst was also investigated. Among the different supports, including carbon black (Vulcan XC-72R) (C1), and nanoporous carbons synthesized from resorcinol- (C2) and phloroglucinol-formaldehyde (C3) resins, the Ru_xSe_y catalyst supported on C3 exhibited highest activity for ORR.     

Performance of the Direct Methanol Carbonate Fuel Cell Using Anion Exchange Materials and Non‐Noble Metal Cathode Catalyst

A direct methanol fuel cell using a mixture of O₂ and CO₂ at the cathode was evaluated using anion exchange materials and cathode catalysts of Pt and a non‐Pt catalyst. The MEA based on non‐noble metal catalyst Acta 4020 showed superior performance than Pt/C based MEA in terms of open circuit potential and power density in carbonate environment. The fuel cell performance was improved by applying anion exchange ionomer in the catalyst layer. A maximum power density of 4.5 mW cm^–2 was achieved at 50 °C using 6.0 M methanol and 2.0 M K₂CO₃.     

Investigations of Compositions and Performance of PtRuMo/C Ternary Catalysts for Methanol Electrooxidation

PtRuMo/C catalyst was prepared by impregnation reduction method and characterised. Comparison is made between a home‐made PtRu/C prepared by similar method and Pt/C (E‐Tek Co., Pt/C‐ET) catalysts. One glassy carbon disc electrode for ternary alloy catalyst was used to evaluate the catalytic performances by cyclic voltammetric, chronoamperometric, amperometric i–t curves, and electrochemical impedance spectra (EIS). The electrochemical measurement results indicated that the performance of PtRuMo/C with a molar ratio of 6:3:1 was the highest among 15 Pt_xRu_yMo_10–x–y/C catalysts with different molar ratios. The composition, particle size, lattice parameter and morphology of the PtRuMo(6:3:1)/C catalyst were determined by means of X‐ray energy dispersive analysis, X‐ray diffraction (XRD) and transmission electron micrographs (TEM). The result of XRD analysis exhibits that PtRuMo(6:3:1)/C has the fcc structure with the smaller lattice parameter than the home‐made PtRu/C and Pt/C‐ET. Its typical particle sizes is only about 5 nm. With respect to the catalytic activity and stability, the PtRuMo(6:3:1)/C catalyst is superior to PtRu/C despite their comparable active areas. Though the electrochemically active surface area of Pt/C‐ET is the biggest, its performance is the lowest. EIS results also indicate that the reaction resistances for methanol electrooxidation on the PtRuMo(6:3:1)/C catalyst are smaller than those of PtRu/C at different polarisation potentials.     

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.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Carbon supported Ru–Se as methanol tolerant catalysts for DMFC cathodes. Part I: preparation and characterization of catalysts

Carbon supported Ru _x Se _y O _z catalysts were prepared from Ru₃(CO)₁₂ and RuCl₃ · xH₂O as ruthenium precursors and H₂SeO₃ and SeCl₄ as the selenium sources. Highly active catalysts for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFC) were obtained via a multi-step preparation procedure consisting of a CO₂-activation of the carbon support prior to the preparation of a highly disperse Ru particles catalyst powder that is subsequently modified by Se. Ultimately, an excess of Se was removed during a final thermal annealing step at 800 °C under forming gas atmosphere. The morphology of the catalysts was analyzed by transmission electron microscopy (TEM) and X-ray diffraction (XRD), which shows that the catalysts consist of crystalline Ru-particles with sizes ranging from 2 to 4 nm exhibiting a good dispersion over the carbonaceous support. The corresponding catalytic activity in the process of oxygen reduction was analyzed by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. The nature of the carbon support used for the preparation of RuSe cathode catalysts is of significant importance for the activity of the final materials. Catalysts supported on CO₂-activated Black Pearls 2000 gave the highest ORR-activity. Se stabilizes the Ru-particles against bulk oxidation and actively contributes to the catalytic activity. An exceptional property of the carbon supported Ru-particles modified with Se is their resistance to coalescence up to temperatures of 800 °C under inert or reducing conditions. Additional effects of Se-modification are the enhanced stability towards electrochemical oxidation of Ru and a lowering of the H₂O₂ formation in the ORR.     

Electrochemical kinetics and X-ray absorption spectroscopy investigations of select chalcogenide electrocatalysts for oxygen reduction reaction applications

Transition metal-based chalcogenide electrocatalysts exhibit a promising level of performance for oxygen reduction reaction applications while offering significant economic benefits over the state of the art Pt/C systems. The most active materials are based on Ru_xSe_y clusters, but the toxicity of selenium will most likely limit their embrace by the marketplace. Sulfur-based analogues do not suffer from toxicity issues, but suffer from substantially less activity and stability than their selenium brethren. The structure/property relationships that result in these properties are not understood due to ambiguities regarding the specific morphologies of Ru_xS_y-based chalcogenides. To clarify these properties, an electrochemical kinetics study was interpreted in light of extensive X-ray diffraction, scanning electron microscopy, and in situ X-ray absorption spectroscopy evaluations. The performance characteristics of ternary M_xRu_yS_z/C (M = Mo, Rh, or Re) chalcogenide electrocatalysts synthesized by the now-standard low-temperature nonaqueous (NA) route are compared to commercially available (De Nora) Rh- and Ru-based systems. Interpretation of performance differences is made in regards to bulk and surface properties of these systems. In particular, the overall trends of the measured activation energies in respect to increasing overpotential and the gross energy values can be explained in regards to these differences.     

The oxygen reduction reaction on Pt/TiO x N y -based electrocatalyst for PEM fuel cell applications

Titanium oxy-nitride was developed for the first time as Pt electrocatalyst support for the ORR in PEM fuel applications. The conditions of the support preparation and the Pt/TiO _x N _y -based electrodes’ elaboration by chemical reduction method were determined. Comparison of the polarization curves of the carbon and the TiO _x N _y supported how clearly TiO _x N _y was more stable than the Vulcan XC-72R. It was found that the 40 wt% Pt/TiO _x N _y -based electrocatalyst is active for the ORR in acid medium, but the activity was less than that of Pt/C. The normalized electrochemical surface area degradation of Pt/TiO _x N _y was significantly less than that of Pt/C. The kinetics of the ORR on Pt/TiO _x N _y proceeded through a four-electron transfer process. The single-cell hydrogen/oxygen PEM fuel cell performances based on Pt/TiO _x N _y cathode electrocatalyst exhibited the same range of characteristics as those based on Pt/C.     

Chalcogenide electrocatalysts for oxygen-depolarized aqueous hydrochloric acid electrolysis

Several Rh- and Ru-based carbon-supported chalcogenide electrocatalysts were evaluated as oxygen-depolarized cathodes for HCl electrolysis applications. The roles of both crystallinity and morphology of the electrocatalysts were explored by investigating several synthetic processes for materials, specifically patented E-TEK methods and the non-aqueous method. The activity of the electrocatalysts for ORR was evaluated via RDE studies in 0.5 M HCl, and compared to state of the art Pt/C and Rh/C systems. Rh_xS_y/C, Co_xRu_yS_z/C, and Ru_xS_y/C materials synthesized from the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The amorphous non-aqueous moieties, while exhibiting little depolarization due to the presence of high concentrations of Cl⁻ in the RDE studies, were unsuitable for operation in a true ODC HCl electrolyzer cell because of irreversible dissolution resulting from the high concentration (∼5 M) of HCl. In contrast, the Ru-based materials from the E-TEK methods were unaffected by the depolarizing conditions of an uncontrolled shutdown. These Ru-based electrocatalysts, being on the order of seven times less expensive than the state of the art Rh_xS_y material, may prove to be of economic benefit to the HCl electrolysis industry.     

The effect of diluting ruthenium by iron in RuxSey catalyst for oxygen reduction

This study has focused on the synthesis of novel oxygen reduction reaction (ORR) chalcogenide catalysts, with Ru partially replaced by Fe in a cluster-type Ru_xSe_y. The catalysts were obtained by thermal decomposition of Ru₃(CO)₁₂ and Fe(CO)₅ in the presence of Se. As indicated by the XPS data, the composition of catalyst nanoparticles depends on the solvent used (either p-xylene or dichlorobenzene). The presence of iron in synthesized catalysts has been confirmed by both EDAX and XPS. Voltammetric activation of the catalysts results in a partial removal of iron and unreacted selenium from the surface. The ORR performance of electrochemically pre-treated catalysts was evaluated using rotating disk and ring-disk electrodes in a sulfuric acid solution. No major change in the ORR mechanism relative to the Se/Ru catalyst has been observed with Fe-containing catalysts.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Effect of Porosity in Catalyst Layers on Direct Methanol Fuel Cell Performances

Effects of porosity of catalyst layers (CLs) on direct methanol fuel cell (DMFC) performances are investigated using silicon dioxide (SiO₂) particles as a pore former. The pore size and volume of CLs are controlled by changing the size and content of SiO₂. As the size of pore formed by removal of SiO₂ increases, DMFC performances are enhanced. The augmentation in performances can be explained by facilitation of fuel transport to catalyst particles, increase of utilization efficiency of catalysts, diminishment in methanol crossover, reduction in activation loss and facilitation of water discharging out of CLs of cathode due to the controlled porosity in CLs. The enhanced fuel transport, accessibility of fuels to Pt catalyst surface, is proved by the active areas of Pt catalyst. In addition to the active area of Pt catalyst, porous CLs exhibit a decline in methanol crossover, leading to increase of open circuit voltage (OCV). The porous CLs also show improvements in activation loss due to high porosity, causing enhancement in DMFC performances. In aspect of pore volume contribution to cathode performance, the SiO₂ content is optimized. Based on the DMFC performances, it can be suggested that the optimum conditions of SiO₂ are 500 nm in size and 20 wt.% in content. The porosity effect on both electrodes appears as follows: the pores in cathode are more effective on DMFC performances (55.5%) than those of anodes (44.5%) based on the maximum power of DMFC, indicating that the pores in CLs facilitate removal of water from electrodes.     

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 well-dispersed PtRuSnOx by ultrasonic-assisted chemical reduction and its property for methanol electrooxidation

PtRuSnO_x supported on multi-wall carbon nanotubes (MWCNTs) was prepared by ultrasonic-assisted chemical reduction method. The as-prepared catalyst was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns indicate that Pt exists as the face-centered cubic structure, Ru is alloyed with platinum, while non-noble metal oxide SnO_x exists as an amorphous state. From TEM observation, PtRuSnO_x is well dispersed on the surface of MWCNTs with the particle size of several nanometers. The electrochemical properties of the as-prepared catalyst for methanol electrooxidation were studied by cyclic voltammetry (CV) and chronoamperometry (CA). The onset potential of methanol oxidation on PtRuSnO_x and PtRu catalysts is much more negative than that on Pt catalyst, shifting negatively by about 0.20 V, while the peak current density of methanol oxidation on PtRuSnO_x is higher than that on PtRu. Electrochemical impedance spectroscopy (EIS) studies also show that the reaction kinetics of methanol oxidation is improved with the presence of SnO_x. The addition of non-noble metal oxide SnO_x to PtRu promotes the catalytic activity for methanol electrooxidation and the possible reaction mechanism is proposed.     

LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell

LaNi_0.9Ru_0.1O₃ as cathode catalyst for a direct borohydride fuel cell (DBFC) was synthesized and investigated for the first time. The electrochemical experiments indicated that perovskite-type oxide LaNi_0.9Ru_0.1O₃ exhibited higher electrochemical performance compared with LaNiO₃, which suggested incorporation of element Ru into LaNiO₃ could further improve the catalytic ability for oxygen reduction reaction (ORR) in alkaline solution. LaNi_0.9Ru_0.1O₃ catalyst was found to have good tolerance of BH₄⁻. Meanwhile the maximum power density of 171 mW cm⁻² was obtained at 65 °C without using any precious ion exchange membrane. A life test indicated that the DBFC displayed no significant degradation for about 70 h testing. The electrochemical data suggested that LaNi_0.9Ru_0.1O₃, which provided a simple way to construct DBFCs without using any ion exchange membrane, might be promising cathode catalyst with high performance and low cost for DBFCs.     

A nonelectrochemical reductive deposition of ruthenium adatoms onto nanoparticle platinum: anode catalysts for a series of direct methanol fuel cells

The surface of Pt nanoparticles was cleaned and saturated with hydrogen by treatment first with a 3% aqueous solution of H₂O₂ and then with hydrogen gas under water at room temperature. Reaction between the surface hydrogen and aqueous RuCl₃ deposited 0.18 surface equivalents of Ru_ad onto the Pt nanoparticles. The deposition was repeated several times, with each reaction depositing ∼0.18 surface equivalents more Ru_ad onto the Pt-Ru_ad nanoparticles. The resulting Pt-Ru_ad nanoparticles were analysed using cyclic voltammetry, CO stripping voltammetry, and as catalysts for electrooxidation of MeOH in three-electrode experiments and in prototype direct methanol fuel cells. The optimum surface coverage (θ_Ru) for electrooxidation of MeOH was ∼0.33 under these conditions.     

PtCo/Polypyrrole‐Multiwalled Carbon Nanotube Complex Cathode Catalyst Containing Two Types of Oxygen Reduction Active Sites Used in Direct Methanol Fuel Cells

Low oxygen reduction reaction (ORR) activity and high cost of noble metal catalysts are two major challenges in direct methanol fuel cells (DMFCs). Pt‐based catalysts are considered as an ideal alternative to deal with these two problems. While the second component metals play only the role of synergy effect with Pt, they themselves are inert towards activity towards ORR. It is necessary to design a new route to ultilize the second component metal by forming CoN_x ORR active site on the base of PtM catalyst. In this paper, PtCo/polypyrrole‐multiwalled carbon nanotubes (PtCo/PPy‐MWCNTs) catalyst containing two types of ORR active site (Pt and CoN_x) was synthesized by one pot synthesis route. The effect and dynamic mechanism of the named CoN_x active site towards ORR was discussed by X‐ray photoelectron sprectroscopy and linear sweep voltammetry. PtCo/PPy‐MWCNTs cathode catalyst showed improved activity towards ORR and great potential in DMFCs.