Pt–Ru electrocatalysts for fuel cells: a representative review

Three periods of Pt–Ru research are considered step-by-step: the initial period after discovery (1963–1970); observation and classification of basic tendencies (like the effects of composition, segregation, structural features on the activity; up to 1990); nanostructural studies and molecular level consideration of electrocatalytic phenomena in combination with advanced applied studies of materials, mechanistic, and applied aspects (after 1990). The main idea of this review is to balance various aspects of Pt–Ru electrochemistry related to material science and electrocatalysis as well as to remember the early basic results being of importance for future understanding of Pt–Ru functional properties. Dedicated to Professor Teresa Iwasita, as a token of her remarkable contribution to electrocatalysis.

---

Synthesis,characterization and electrocatalytic activity for ethanol oxidation of carbon supported Pt,Pt–Rh,Pt–SnO2 and Pt–Rh–SnO2 nanoclusters

Nanoclusters of Pt, Pt–Rh, Pt–SnO₂ and Pt–Rh–SnO₂ were successfully synthesized by polyol method and deposited on high-area carbon. HRTEM and XRD analysis revealed two phases in the ternary Pt–Rh–SnO₂/C catalyst: solid solution of Rh in Pt and SnO₂. The activity of Pt–Rh–SnO₂/C for ethanol oxidation was found to be much higher than Pt/C and Pt–Rh/C and also superior to Pt–SnO₂/C. Quasi steady-state measurements at various temperatures (30–60 °C), ethanol concentrations (0.01–1 M) and H₂SO₄ concentrations (0.02–0.5 M) showed that Pt–Rh–SnO₂/C is about 20 times more active than Pt/C in the potential range of interest for the fuel cell application.     

Investigation of unsupported Pt–Ru catalysts for high temperature methanol electro-oxidation

Two unsupported Pt–Ru catalysts, varying in the nature and content of RuOx species, were investigated for methanol electro-oxidation in a solid polymer electrolyte fuel cell at high temperature. The catalyst containing a lower amount of ruthenium oxide showed higher catalytic activity and lower mass transport limitations. A physicochemical investigation was carried out to support the interpretation of electrochemical results. Pt–Ru alloy appears more active than Pt–RuOx in the chemisorption of labile-bonded oxygenated species, which promote the oxidation of the methanolic residues adsorbed on the catalyst surface.     

The problem of Ru dissolution from Pt–Ru catalysts during fuel cell operation: analysis and solutions

Platinum–ruthenium catalysts are widely used as anode materials in polymer electrolyte fuel cells (PEMFCs) operating with reformate gas and in direct methanol fuel cells (DMFCs). Ruthenium dissolution from the Pt–Ru anode catalyst at potentials higher than 0.5?V vs. DHE, followed by migration and deposition to the Pt cathode can give rise to a decrease of the activity of both anode and cathode catalysts and to a worsening of cell performance. A major challenge for a suitable application of Pt–Ru catalysts in PEMFC and DMFC is to improve their stability against Ru dissolution. The purpose of this paper is to provide a better knowledge of the problem of Ru dissolution from Pt–Ru catalysts and its effect on fuel cell performance. The different ways to resolve this problem are discussed.     

Highly stable Pt–Ru/C as an anode catalyst for use in polymer electrolyte fuel cells

A new method for preparing highly stable Pt–Ru/C catalysts at low temperature is reported. Pt–Ru supported on high surface carbon was prepared from Pt(NH₃)₄Cl₂, RuNO(NO₃) _x (OH) _y and borohydride as a reducing agent. Simultaneous reduction of both metals was done by heat treatment and small and homogeneously dispersed catalyst particles were obtained with increased stability, as observed from solubility tests. Catalysis, XRD and TG data gave clear evidence of the different chemical states between the material produced and the commercially available sample. The electrochemical measurements showed that the novel catalysts have a performance similar to that of E-Tek samples.     

Quantum-chemical study of the potential anti-cancer drug Ru–NAMI-A in complex with estrogen and the simulated VA and VCD spectra of estrogen, the Ru–NAMI-A drug, and possible/proposed estrogen–Ru–NAMI-A complexes

Following up on an earlier theoretical report by Knapp-Mohammady (Phys Lett A 372:1881–1884, 2008) on the ground state of the neutral Ru complex NAMI-A (trans-imidazoledimethylsulfoxide-tetrachlororuthenate), we first report here a quantum-chemical study of the effect of both oxidation and reduction of the parent molecule to form the anionic and cationic species. The new structures are compared with the equilibrium nuclear structure reported earlier for the neutral complex. We anticipate that one such Ru cluster, with potential as an anti-cancer drug, will interact via an appropriate receptor, rather than directly with DNA. A receptor for NAMI-A binding in here proposed to be the steroid hormone, estrogen, C₁₈H₂₄O₂. The biomolecular structure of the dicomplex is predicted from restricted Hartree–Fock theory and density functional theory (DFT) calculations. The vibrational frequencies of NAMI-A and the dicomplex with estrogen are also reported. Some maps of the ground-state electron-density for the three neutral biomolecular species are finally presented. The use of vibrational spectroscopy, vibrational absorption (VA) and vibrational circular dichorism (VCD) are advocated to be measured, simulated and be used to understand the nature of the interaction of the Ru complex NAMI-A in complex with estrogen. Our aim in presenting these spectral simulations is to motivate the measurement of the VA and VCD spectra of estrogen, the Ru complex NAMI-A and finally of the estrogen–Ru NAMI-A complex. It should also be instructive to measure the VA and VCD spectra of estrogen and the estrogen receptor, both alone, together and finally together in the presence of the Ru NAMI-A complex to substantiate our claim that the Ru complex NAMI-A ties up estrogen, and hence prevents estrogen binding to the estrogen receptor.     

New intermetallic compounds in the Ce–Pt system

The Ce-rich part of Ce–Pt phase diagram has been investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), optical and electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Two new intermediate phases, Ce₅Pt₃ and Ce₅Pt₄, have been discovered and found to crystallize with tetragonal Pu₅Pd₃-type structure and orthorhombic Ge₄Sm₅-type structure, respectively. All the Ce-rich compounds of the system were found to crystallize with a peritectic type formation. We have determined the temperatures of the peritectic plateaus and established the liquidus curves up to 1500 °C.     

Synthesis,spectroscopic, luminescent and electrochemical studies of new Ru–Ni–Ru and Pd–Ni–Pd trinuclear complexes with an azolo-linked tetraaza macrocycle

Ru–Ni–Ru and Pd–Ni–Pd trinuclear complexes containing azolo [thiazolo (L¹) and triazolo (L²)] linked 6,8,15,17-tetramethyldibenzo-5,9,14,18-tetraazacyclotetradecene nickel(II) diacetate as a bridging ligand were synthesised and characterized by elemental analysis, conductance and i.r., u.v.–vis., ¹H-n.m.r. spectra and FAB-mass data. The luminescent and electrochemical properties of the complexes were also studied.     

Hybrid materials based on polymethylsilsesquioxanes containing Fe,Pt, and Fe–Pt metallic nanoparticles

New hybrid materials based on Pt, Fe, and Pt–Fe nanoparticles stabilized in a matrix of polymethylsilsesquioxane nanogel and ultrahigh molecular weight polyethylene (UHMWPE) were prepared. Metal vapor synthesis was used to produce mono- and bimetallic nanoparticles. It was shown that organosilicon nanogel effectively stabilizes Pt nanoparticles with an average size of 0.9 nm. Using the nanogel results in the formation of superparamagnetic Fe particles 3–5 nm in size that consist of ferromagnetic Fe⁰ core and antiferromagnetic shells of Fe oxides. It is established that using an organosilicon matrix in the formation of Pt-Fe/UHMWPE systems helps reduce the average particle size of Fe in the material from 6.5 to 4.5 nm and narrow their particle size distribution. The composition, magnetic and electronic characteristics of the nanocomposites are studied via transmission electron microscopy, X-ray photoelectron spectroscopy, Mössbauer spectroscopy, XANES, and EXAFS.     

Effect of activated graphite nanofibers on electrochemical activities of Pt–Ru nanoparticles for fuel cells

In this work, graphite nanofibers (GNFs) were chemically activated for high specific surface area, small pore diameter, and high oxygen-containing groups with different KOH/GNFs ratios and used as carbon supports of Pt–Ru nanoparticles for fuel cells. As a result, the oxygen functional groups and specific surface area of carbon supports were increased with increasing the ratios of KOH/GNFs up to 4:1, while the average of Pt–Ru nanoparticle size was decreased owing to the improvement of dispersibility of the Pt–Ru/K–GNFs catalysts. The electrochemical activity of the Pt–Ru/K–GNFs catalysts was improved by the larger available active surface area due to the increase of oxygen functional groups and specific surface area. Therefore, it was found that chemical activation using KOH could influence the surface characteristic of carbon supports, resulting in enhanced electrochemical activity of the Pt–Ru/K–GNFs catalysts of fuel cells.     

Some recent advances in transition-metal-catalyzed ortho SP~2 C–H functionalization using Ru,Rh, and Pd

In the past decade,transition-metal-catalyzed C–H functionalization by weak coordination has emerged as a practical and powerful tool to access many valuable chemicals.Two classes of weakly coordinating directing groups,commonly occurring functional groups,and easily removable auxiliaries,have been found to be efficient and practical for C–H activation reactions.This mini-review contains examples of recent research advances on transition-metal-catalyzed SP2 C–H functionalization via weak coordination,using Ru,Rh,and Pd.A number of weakly coordinating functional groups(e.g.,ketone,ester,carbamate,tertiary amide,ether,thioether,alcohol,and some others)are covered.As the field of transition-metal-catalyzed C–H functionalization continues to develop and more synthetically useful chemo-,regio-,and enantioselective reactions catalyzed by transition metal via weak coordination are discovered,this promising and attractive strategy will play a more important role in modern organic synthesis.     

Electron-deficient adduct site in the ring opening of methylcyclopentane (MCP) on tungsten-oxide-supported Pt,Ir and Pt–Ir catalysts

The activities of Pt/WO₂, Ir/WO₂ and Pt–Ir/WO₂ toward the conversion of methylcyclopentane (MCP) were investigated. The catalysts were prepared using impregnation and co-impregnation methods and were characterized by SEM, XRD, N₂-sorption and TEM investigations. The most active catalyst toward the conversion of MCP, irrespective of the temperature, was Ir/WO₂. The order of the reactivity was Ir/WO₂ > Pt–Ir/WO₂ > Pt/WO₂. Strong metal–support interactions (SMSI) were observed for all the catalysts over the entire investigated temperature range. At 400 °C, the Pt and Pt–Ir showed 100% selectivity toward ring-enlargement reactions associated with the presence of electron-deficient adduct sites on the reducible acidic WO₂ support. Ring opening occurred over all the catalysts in three positions, resulting in the formation of 2-methylpentane (2-MP), 3-methylpentane (3-MP), and n-hexane (n-H). Difficulty in breaking the secondary – tertiary carbon bonds was observed predominantly on the Ir catalyst, which opens the MCP ring via a selective mechanism.     

Mechanism of amorphisation in Cu–Ru,a binary alloy with a positive heat of mixing

Cu–Ru has a positive heat of formation and does not form equilibrium alloys. Nevertheless, amorphous alloys have been obtained by He (Phys. Rev. B 75, 045431 (2007)) by ion mixing of multilayers. Analysis of the free energies of the competing phases (the glass and the crystalline solid solutions based on Cu and Ru) leads us to propose that formation of glasses occurs as a result of kinetic frustration between the hcp and fcc solid solutions. These two have lower free energies than the glass, but those free energies are very similar, so a strong driving force for the formation of a particular crystalline phase does not exist. In addition, formation and growth of hcp and the fcc phases appears equally difficult from a kinetic point of view. Very small embryos can form but their growth will be frustrated by the presence of embryos of the other phase.     

Underpinning energetics of lithium bonding and stability in the Li–Pt–Sn system

Within the Li–Pt–Sn system, we examine the electronic structures and Li-binding of LiPtSn₂, Li₂PtSn and Li₃Pt₂Sn₃ with fluorite-related crystal structures. The structures with totally de-intercalated lithium keep the characteristics of the pristine ternary compound with a reduction of the volume. In Li₃Pt₂Sn₃ the binding energies of lithium belonging to three crystallographically inequivalent Wyckoff sites are different and point to distinct activities of de-intercalation concomitant with site-selective bonding magnitudes. The derived potentials are within the range of non-oxide binary and ternary lithium based compounds and indicate the possibility of at least partial delithiation.     

Pt–Ru nanoparticles supported PAMAM dendrimer functionalized carbon nanofiber composite catalysts and their application to methanol oxidation

Polyamidoamine (PAMAM) dendrimers has been anchored on functionalized carbon nanofibers (CNF) and supported Pt–Ru nanoparticles have been prepared with NaBH₄ as a reducing agent. The samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM) analysis. It was shown that Pt–Ru particles with small average size (2.6 nm) were uniformly dispersed on PAMAM/CNF composite support and displayed the characteristic diffraction peaks of Pt face-centered cubic structure. The electrocatalytic activities of the prepared-composites (20% Pt–Ru/PAMAM-CNF) were examined by using cyclic voltammetry for oxidation of methanol. The electrocatalytic activity of the CNF-based composite (20% Pt–Ru/PAMAM-CNF) electrode for methanol oxidation showed better performance than that of commercially available Johnson Mathey 20% Pt–Ru/C catalyst. The results imply that CNF-based PAMAM composite electrodes are excellent potential candidates for application in direct methanol fuel cells.     

Recent developments in Pt–Co catalysts for proton-exchange membrane fuel cells

Development of Pt-based oxygen reduction reaction catalysts with high efficiency and high durability is central to the application of proton-exchange membrane fuel cell systems. Pt–Co bimetallic catalysts have drawn extensive attention owing to their capability of delivering high performance and long lifetime for fuel cell applications including light-duty and heavy-duty vehicles. However, further improvements in durability and performance are needed to meet market requirements. To fully exploit the potential of Pt–Co catalysts, new insights into the relationship between catalyst properties and fuel cell performance and durability are needed, and more effective methods to tailor the features of Pt–Co catalysts need to be developed. This review provides a summary and perspective on recent efforts, including work on customizing the Pt shell and Pt:Co ratio, tailoring the crystal structure, and improving carbon support properties, with a particular emphasis on mechanisms leading to enhancement of mass activity, power density, and durability in membrane electrode assembly testing.     

Low-temperature sol–gel synthesis of anatase nanoparticles modified by Au, Pd and Pt and activity of TiO2/Au, Pd, Pt photocatalysts in water splitting

Base catalyzed hydrolysis and condensation of Ti isopropoxide with water at ambient temperature and molar ratio H₂O/Ti(i-PrOH)₄ varied from 1 to 100 was studied. It was found that molar ratio H₂O/Ti(i-PrOH)₄ = 100 originated amorphous TiO₂-precursor of summary composition TiO_1.9(OH)_0.2 with a specific surface area of 354 m²/g. Low-temperature crystallization of amorphous TiO₂-precursor to nanostructured anatase at 80 °C in a slightly reducing environment of d-glucose was studied. It was found that the low-temperature nucleation and crystallization of anatase was initiated (activated) by combined effects of d-glucose and Au⁰, Pd⁰ and Pt⁰ nanoparticles, generated in situ by slow reduction of Au³⁺, Pd²⁺ and Pt⁴⁺ ions using d-glucose. Considerable photocatalytic activity of Ti-hydroxide-oxide/Au, Pd, Pt catalysts prepared at low-temperature was associated with high content of nanostructured anatase and low content of Au, Pd and Pt nanoparticles (0.02 wt.%) effectively deposited on the surface of titania particles. The maximum hydrogen evolution rates 3.4 μmol/min g at Ti-hydroxide-oxide/Au, 4.0 μmol/min g at Ti-hydroxide-oxide/Pd and 4.1 μmol/min g at Ti-hydroxide-oxide/Pt were found. The activity of all TiO₂/Au, Pd and Pt catalysts increased by calcination at 600 °C from 50 to 100 %.     

Ru,Sn和Co促进的Pt/C催化剂电催化氧化甲醇的性能

 采用溶胶法制备了用于阴离子膜直接甲醇燃料电池的Pt-M/C(M=Ru, Sn和Co)阳极电催化剂,并用X射线衍射和X射线能谱技术对催化剂进行了表征. 结果表明,制备的Pt-M合金颗粒分布均匀,粒径为2～6 nm, 其组成与前驱体中相应金属的原子比基本吻合. 用循环伏安法测定了催化剂在不同碱性条件下的活性. 结果显示,随着碱性的增加,甲醇的起始氧化电位降低,峰电流和催化剂的活性增大; 相同碱强度下催化剂活性顺序为Pt50Ru50/C>Pt50Sn50/C>Pt75Co25/C, 添加Ru可明显提高Pt/C催化剂的活性. Pt50Ru50/C在1.0 mol/L NaOH+1.0 mol/L CH3OH溶液中的峰电流密度可达634.7 mA/mg.     

Chemically dealloyed Pt–Au–Cu ternary electrocatalysts with enhanced stability in electrochemical oxygen reduction

We report simple synthesis of ternary Pt–Au–Cu catalysts consisting of active Pt-rich shell and Pt transition-metal alloy core for use as highly active and durable electrocatalysts in oxygen reduction reactions. The ternary Pt–Au–Cu catalysts were synthesized by chemical coreduction followed by thermal treatment and chemical dealloying. During synthesis, thermal treatment formed metal particles into high-degree alloys, and chemical dealloying led to selective dissolution of soluble Cu species from the outer surface layer of the thermally treated alloy particles, resulting in Pt-based alloys@Pt-rich surface core–shell configuration. Compared with a commercial Pt/C catalyst, our Pt_1?xAu _x Cu₃/C-AT catalysts exhibited approximately 2.4-fold enhanced performance in oxygen reduction reactions. Among the catalysts employed in this work, Pt_0.97Au_0.3Cu₃/C-AT showed the highest performance in terms of mass activity, specific activity, and electrochemically active surface area loss with negligible change during 10,000 potential cycles. The synthesis details, electrochemical characteristics, oxygen reduction reaction performance, and durability of the chemically dealloyed ternary Pt–Au–Cu catalysts are presented and discussed.