Binary Pt–Pd and ternary Pt–Pd–Ru nanoelectrocatalysts for direct methanol fuel cells

Nano-sized binary and ternary alloys are synthesized by polyol process on Vulcan XC72-R support. Nanostructured binary Pt–Pd/C catalysts are prepared either by co-deposition or by depositing on each other. Ternary Pt–Pd–Ru/C catalysts are prepared by co-deposition. The structural characteristics of the nanocatalysts are examined by TEM and XRD. Their electrocatalytic activity toward methanol oxidation and CO stripping curves were measured by electrochemical measurements and compared with that of commercial Pt/C catalyst. The results show that the binary nanocatalyst prepared by depositing the Pt precursor colloids on Pd-Vulcan XC-72R are more active toward methanol oxidation than that of the co-deposited binary alloy nanocatalyst. The co-deposited ternary Pt–Pd–Ru/C nanocatalyst based membrane electrodes assembly shows higher power density compared to the binary nanocatalysts as well as commercial Pt/C catalyst in direct methanol fuel cell. Significantly higher catalytic activity of the nanocatalysts toward methanol oxidation compared to that of the commercial Pt/C is believed to be due to lower level of catalyst poisoning.     

Methanol oxidation on carbon-supported Pt–Ru–Ni ternary nanoparticle electrocatalysts

Methanol oxidation on carbon-supported Pt–Ru–Ni ternary alloy nanoparticles was investigated based on the porous thin-film electrode technique and compared with that on Johnson–Matthey Pt–Ru alloy catalyst. Emphasis is placed on the effect of alloying degree on the electrocatalytic activity and stability of the ternary catalysts. The as-prepared Pt–Ru–Ni nanoparticles exhibited a single phase fcc disordered structure, and a typical TEM image indicates that the mean diameter is ca. 2.2 nm, with a narrow particle size distribution. Also, the as prepared Pt–Ru–Ni catalysts exhibited significantly enhanced electrocatalytic activity and good stability for methanol oxidation in comparison to commercial Pt–Ru catalyst available from Johnson–Matthey. The highest activity of methanol oxidation on Pt–Ru–Ni catalysts was found with a Pt–Ru–Ni atomic ratio of 60:30:10 and at a heat-treatment temperature of ca. 175 °C. The significantly enhanced catalytic activity for methanol oxidation is attributed to the high dispersion of the ternary catalyst, to the role of Ni as a promotion agent, and especially to the presence of hydroxyl Ru oxide. Moreover, the stability of the ternary nanocatalytic system was found to be greatly improved at heat-treatment temperatures higher than ca. 250 °C, likely due to a higher alloying degree at such temperatures for the ternary catalysts.     

Tailoring a Pt–Ru catalyst for enhanced methanol electro-oxidation

A carbon-supported (1:1) Pt–Ru (Pt–Ru/C) alloy catalyst has been prepared in-house by the sulfito-complex route, and has been tailored to achieve enhanced activity towards methanol electro-oxidation by annealing it at varying temperatures in air. The catalyst samples annealed between 250 and 300 °C in air for 30 min exhibit superior catalytic activity towards methanol electro-oxidation in a solid-polymer-electrolyte direct methanol fuel cell (SPE-DMFCs) operating at 90 °C. Both the as-prepared and annealed Pt–Ru/C catalysts have been characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and cyclic voltammetry. It is conjectured that while annealing the Pt–Ru/C catalysts, both PtPt and PtRu bonds increase whereas the PtO bond shrinks. This is accompanied with a positive variation in Ru/Pt metal ratio suggesting the diffusion of Ru metal from the bulk catalyst to surface with an increase in oxidic ruthenium content. Such a treatment appears seminal for enhancing the electrochemical activity of Pt–Ru catalysts towards methanol oxidation.     

Evaluation of Pt–Ru–Ni and Pt–Sn–Ni catalysts as anodes in direct ethanol fuel cells

In this study, the electrooxidation of ethanol on carbon supported Pt–Ru–Ni and Pt–Sn–Ni catalysts is electrochemically studied through cyclic voltammetry at 50 °C in direct ethanol fuel cells. All electrocatalysts are prepared using the ethylene glycol-reduction process and are chemically characterized by energy-dispersive X-ray analysis (EDX). For fuel cell evaluation, electrodes are prepared by the transfer-decal method. Nickel addition to the anode improves DEFC performance. When Pt₇₅Ru₁₅Ni₁₀/C is used as an anode catalyst, the current density obtained in the fuel cell is greater than that of all other investigated catalysts. Tri-metallic catalytic mixtures have a higher performance relative to bi-metallic catalysts. These results are in agreement with CV results that display greater activity for PtRuNi at higher potentials.     

Methanol electrooxidation at Pt–Ru–W sputter deposited on Au substrate

This work reports an improved electrocatalytic activity for methanol oxidation at Pt–Ru–W electrode sputter deposited on Au substrate. The performance of Pt–Ru–W was compared with that of Pt–W and of Pt–Ru alloy electrodes. All the alloys tested exhibited catalytic activity higher than Pt. Among the alloys tested, the Pt–Ru–W demonstrated a significant cathodic shift in the onset potential and a remarkable enhancement in the current density for methanol oxidation reaction (MOR). The onset potentials for the MOR matched well the anodic peak potentials recoded in the base electrolyte (H₂SO₄), i.e., 0.15 V versus Ag/AgCl for Pt–Ru–W and 0.35 V versus Ag/AgCl for Pt–W and Pt–Ru electrodes. From these findings, it was postulated that the background peak current generates oxide species necessary to complete the methanol oxidation to CO₂. Next, it was observed that the current density at Pt–Ru–W electrode decreased when the Au substrate was changed to Pt, C, or Si, although, the onset potential for MOR remained almost unaffected by the nature of the substrate. Afterwards, the effect of Au substrate on methanol oxidation at Au-based alloy electrodes was investigated.     

Alcohol electrooxidation at Pt and Pt–Ru sputtered electrodes under elevated temperature and pressurized conditions

The electrooxidation properties of methanol and 2-propanol, which are both promising candidates for direct alcohol fuel cells (DAFCs), have been studied under elevated temperature and pressurized conditions. Sputter-deposited Pt and Pt–Ru electrodes were well-characterized and utilized for the electrochemical measurement of the alcohol oxidation at 25–100 °C. The Pt electrode prepared at 600 °C had a flat surface, and the Pt–Ru formed an alloy. The electrochemical measurements were carried out in a gas-tight cell under elevated temperature, which accompanies the pressurized condition. This is a representative example of the DAFC rising temperature operation. As a result, at 25 °C, the onset potential of the 2-propanol oxidation is about 400 mV more negative than that of the methanol oxidation, and current density of the 2-propanol oxidation exceeds that of the methanol oxidation. Conversely, at 100 °C, the methanol oxidation current density overcomes that of 2-propanol, and the onset potentials of the two are almost the same. The highest current density for the methanol oxidation is obtained at the Pt:Ru = 50:50 electrode, whereas at the Pt:Ru = 35:65 for the 2-propanol oxidation. A Tafel plot analysis was employed to investigate the reaction mechanism. For the methanol oxidation, the number of electrons transferred during the rate-determining process is estimated to be 1 at 25 °C and 2 at 100 °C. This suggests that the methanol reaction mechanism differs at 25 and 100 °C. In contrast, the rate-determining process of the 2-propanol oxidation at 25 and 100 °C was expected to be 1-electron transfer which accompanies the proton-elimination reaction to produce acetone. Consequently, it is deduced that methanol and 2-propanol have an advantage under the rising temperature and room temperature operation, respectively.     

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 Pt₆₆Ru₁₇Cu₁₇ composition. The Pt₆₆Ru₁₇Cu₁₇/C composition was synthesized and characterized as a powder catalyst to verify the performance of this new composition. During cyclic voltammetry tests, the Pt₆₆Ru₁₇Cu₁₇/C catalyst showed less dissolution and irreversible oxidation of Ru than a Pt₅₀Ru₅₀/C catalyst, with increasing number of cycles. In MOR activity measurement experiments, the Pt₆₆Ru₁₇Cu₁₇/C catalyst exhibited 26 and 86% higher activities in cyclic and chronoamperometric tests, respectively, than those of the Pt₅₀Ru₅₀/C catalyst.     

A comparative study of electrochemical methods on Pt–Ru DMFC anode catalysts: The effect of Ru addition

In the present study comparative electrochemical study of methanol electro-oxidation reaction, the effect of ruthenium addition and experimental parameters on methanol electro-oxidation reaction at high performance carbon supported Pt and Pt–Ru catalysts have been studied by cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) in H₂SO₄ (0.05–2.00 M) + CH₃OH (0.01–4.00 M) at 20–70 °C. Tafel plots for the methanol oxidation reaction on Pt and Pt–Ru catalysts show reasonably well-defined linear region with the slopes of 128–174 mV dec⁻¹(α = 0.34–0.46). The activation energies from Arrhenius plots have been found as 39.06–50.65 kJ mol⁻¹. As a result, methanol oxidation is enhanced by the addition of ruthenium. Furthermore, Pt–Ru (25:1) catalyst shows best electro–catalytic activity, higher resistance to CO, and better long term stability compared to Pt–Ru (3:1), Pt–Ru (1:1), and Pt. Finally, the EIS measurements on Pt–Ru (25:1) catalyst reveals that methanol electro-oxidation reaction consists of two process: methanol dehydrogenation step at low potentials (<700 mV) and the oxidation removal of CO_ads by OH_ads at higher potentials (>700 mV).     

Synthesis and characterization of Cu core Pt–Ru shell nanoparticles for the electro-oxidation of alcohols

Cu@Pt–Ru core–shell supported electrocatalysts have been synthesized by a two-step process via a galvanic displacement reaction. XRD diffraction and EDX analysis, and cyclic voltammetry measurements revealed the presence of nanoparticles composed by a Cu-rich Pt–Cu core surrounded by a Pt-rich Pt–Ru shell. Cyclic voltammetry and chronoamperometric measurements showed that as-synthesized core–shell materials exhibit superior catalytic activity towards methanol and ethanol electro-oxidation compared to a commercial Pt–Ru/C catalyst with higher Pt loading. This behavior can be associated with the lattice mismatch between the Pt-rich shell and the Cu rich core, which in turn produces lattice-strain, surface ligand effects and a large amount of surface defect sites. In addition, the core–shell electrodes displayed a better catalytic activity and lower onset potentials for ethanol oxidation than for methanol oxidation.     

A DMFC stack operating with hydrocarbon blend membranes and Pt–Ru–Sn–Ce/C and Pd–Co/C electrocatalysts

Direct methanol fuel cell (DMFC) stacks consisting of 5 cells and 20 cells were assembled with low-cost hydrocarbon blend membranes and new electrocatalysts with better methanol tolerance and stability. The hydrocarbon blend membranes consisting of an acidic polymer (sulfonated poly (ether ether ketone), SPEEK) and a basic polymer (polysulfone-2-amide-benzimidazole, PSf-ABIm) exhibited low methanol crossover, high conductivity, and good mechanical stability. The Pt–Ru–Sn–Ce/C anode catalyst exhibited better stability than the commercial PtRu/C catalyst, while the cathode catalyst Pd–Co/C showed better methanol tolerance than the commercial Pt/C catalyst. A maximum power of around 20 W was achieved with a DMFC stack consisting of 20 membrane-electrode assemblies (MEAs) fabricated with the above membranes and electrocatalysts. The results demonstrate the feasibility of utilizing these acid-base blend membranes and novel catalysts for DMFC applications.     

Enhanced activity of Pt(HY) and Pt–Ru(HY) zeolite catalysts for electrooxidation of methanol in fuel cells

The investigation describes the synthesis of Pt and Pt–Ru catalysts by a new method using a HY zeolite support. The catalysts are used to study the anodic oxidation of methanol in an acidic medium to investigate their suitability for use in direct methanol fuel cells (DMFCs). The catalysts prepared in a HY zeolite support display significantly enhanced electrocatalytic activity in the order: HY<Pt/C<Pt(HY)<Pt–Ru/C<Pt–Ru(HY). The enhanced electrocatalytic activity is explained on the basis of the formation of specific CO clusters in zeolite cages.     

Electroactivity of high performance unsupported Pt–Ru nanoparticles in the presence of hydrogen and carbon monoxide

The electrochemical activity of high performance unsupported (1:1) Pt–Ru electrocatalyst in the presence of hydrogen and carbon monoxide has been studied using the thin-film rotating disk electrode (RDE) technique. The kinetic parameters of these reactions were determined in H₂- and CO-saturated 0.5 M H₂SO₄ solutions by means of cyclic voltammetry, including CO stripping, and RDE voltammetry. Pt–Ru/Nafion inks were prepared in one step with different Nafion mass fractions, allowing determining the ionomer influence in electrocatalytic response and obtaining the kinetic current density in absence of mass-transfer effects, being 41 and 12 mA cm² (geometrical area), for H₂ and CO oxidation, respectively. These values correspond to mass activities of 1.37 and 0.40 A mg_Pt¹ and to specific activities of 1.52 and 0.44 mA cm_Pt². The Tafel analysis confirmed that hydrogen oxidation was a two-electron reversible reaction, while CO oxidation exhibited an irreversible behavior with a charge-transfer coefficient of 0.42. The kinetic results for CO oxidation are in agreement with the bifunctional theory, in which the reaction between Pt–CO and Ru–OH is the rate-determining step. The exchange current density for hydrogen reaction was 0.28 mA cm² (active surface area), thus showing similar kinetics to those found for carbon-supported Pt and Pt–Ru electrocatalyst nanoparticles.     

Electrochemical synthesis and characterization of carbon-supported Pt and Pt–Ru nanoparticles with Cu cores for CO and methanol oxidation in polymer electrolyte fuel cells

Pt and Pt–Ru shells on Cu cores supported on Vulcan carbon XC72R have been synthesized and tested as possible anode electrocatalysts for polymer electrolyte fuel cells. Pt(Cu)/C was prepared by Cu electrodeposition on the black carbon support at constant potential followed by Pt deposition on Cu by galvanic exchange, whereas Pt–Ru(Cu)/C was prepared by spontaneous deposition of Ru species on Pt(Cu)/C. The corresponding cyclic voltammograms in 0.5 M H₂SO₄ solution showed the hydrogen adsorption/desorption peaks and no Cu oxidation. The respective CO stripping peak potentials of Pt(Cu)/C and Pt–Ru(Cu)/C were about 0.1 and 0.2 V more negative than those corresponding to Pt/C and Ru-decorated Pt/C. The best conditions for CO oxidation were found for Cu deposition potentials between −0.2 and −0.4 V vs. Ag/AgCl/KCl(sat). The Pt economy of the Pt–Ru(Cu)/C system was proved for the methanol oxidation, with specific currents more than twice those obtained on the Ru-decorated commercial Pt/C catalysts.     

Non-stoichiometric tungsten-carbide-oxide-supported Pt–Ru anode catalysts for PEM fuel cells – From basic electrochemistry to fuel cell performance

Durability and cost of Proton Exchange Membrane fuel cells (PEMFCs) are two major factors delaying their commercialization. Cost is associated with the price of the catalysts, while durability is associated with degradation and poisoning of the catalysts, primarily by CO. This motivated us to develop tungsten-carbide-oxide (W_xC_yO_z) as a new non-carbon based catalyst support for Pt–Ru–based anode PEMFC catalyst. The aim was to improve performance and obtain higher CO tolerance compared to commercial catalysts. The performance of obtained PtRu/W_xC_yO_z catalysts was investigated using cyclic voltammetry, linear scan voltammetry and rotating disk electrode voltammetry. Particular attention was given to the analysis of CO poisoning, to better understand how W_xC_yO_z species can contribute to the CO tolerance of PtRu/W_xC_yO_z. Improved oxidation of CO_ads at low potentials (E < 0.5 V vs. RHE) was ascribed to OH provided by the oxide phase at the interfacial region between the support and the PtRu particles. On the other hand, at high potentials (E > 0.5 V vs. RHE) CO removal proceeds dominantly via OH provided from the oxidized metal sites. The obtained catalyst with the best performance (30% PtRu/W_xC_yO_z) was tested as an anode catalyst in PEM fuel cell. When using synthetic reformate as a fuel in PEMFC, there is a significant power drop of 35.3 % for the commercial 30% PtRu/C catalyst, while for the PtRu/W_xC_yO_z anode catalyst this drop is around 16 %.     

Mechanically activated Pt–Ni and Pt–Co alloys as electrocatalysts in the oxygen reduction reaction

Mixtures of powders of platinum with nickel or cobalt to obtain Ni_0.75Pt_0.25 or Co_0.75Pt_0.25 were mechanical alloyed by high energy ball milling. The results of crystal structure, morphology and electrocatalytic performance are presented for mechanically activated powders after 3 and 9 h of ball milling. Total solid solutions of Ni and Co with platinum were analyzed by X-ray diffraction after 3 h of ball milling. After 9 h of ball milling, in both cases, the total solid solution was accompanied by the appearance of NiO or CoO and ZrO associated with a redox reaction with the milling media. The presence of zirconium monoxide was confirmed by energy dispersive spectroscopy analysis. In both cases, an amorphization was detected. X ray absorption spectroscopy measurements showed changes in atomic and electronic environment of platinum, a reduction of the distance to the first coordination sphere and increased d-band vacancy vs pure Pt and Pt nanoparticles were observed for both studied systems. The electrocatalytic activity was determined using cyclic and linear voltammetry. The Co_0.75Pt_0.25 alloy milled for 9 h showed a higher electrochemical activity for the oxygen reduction reaction (ORR) compared with the other samples, including Pt-Etek. The degree of the ORR electrochemical activity was correlated with the presence of ZrO, which could affect the oxygen adsorption and improve the catalytic activity for the oxygen reduction reaction.     

Carbon supported nano-sized Pt–Pd and Pt–Co electrocatalysts for proton exchange membrane fuel cells

Nano-sized Pt–Pd/C and Pt–Co/C electrocatalysts have been synthesized and characterized by an alcohol-reduction process using ethylene glycol as the solvent and Vulcan XC-72R as the supporting material. While the Pt–Pd/C electrodes were compared with Pt/C (20 wt.% E-TEK) in terms of electrocatalytic activity towards oxidation of H₂, CO and H₂–CO mixtures, the Pt–Co/C electrodes were evaluated towards oxygen reduction reaction (ORR) and compared with Pt/C (20 wt.% E-TEK) and Pt–Co/C (20 wt.% E-TEK) and Pt/C (46 wt.% TKK) in a single cell. In addition, the Pt–Pd/C and Pt–Co/C electrocatalyst samples were characterized by XRD, XPS, TEM and electroanalytical methods. The TEM images of the carbon supported platinum alloy electrocatalysts show homogenous catalyst distribution with a particle size of about 3–4 nm. It was found that while the Pt–Pd/C electrocatalyst has superior CO tolerance compared to commercial catalyst, Pt–Co/C synthesized by polyol method has shown better activity and stability up to 60 °C compared to commercial catalysts. Single cell tests using the alloy catalysts coated on Nafion-212 membranes with H₂ and O₂ gases showed that the fuel cell performance in the activation and the ohmic regions are almost similar comparing conventional electrodes to Pt–Pd anode electrodes. However, conventional electrodes give a better performance in the ohmic region comparing to Pt–Co cathode. It is worth mentioning that these catalysts are less expensive compared to the commercial catalysts if only the platinum contents were considered.     

Effect of carbon substrate materials as a Pt–Ru catalyst support on the performance of direct methanol fuel cells

The support effect of carbon nanotubes (CNTs) for direct methanol fuel cell (DMFC) was studied using CNTs with and without defect preparation, carbon black, and fishbone-type CNTs. The Pt–Ru/defect-free CNTs afforded the highest catalytic activity of methanol oxidation reaction (MOR) in rotating disk electrode experiments and the highest performance as the anode catalysts in DMFC single cell tests with the one-half platinum loading compared to Pt–Ru/VulcanXC-72R. CO stripping voltammograms with Pt–Ru/defect-free CNTs also revealed the lowest CO oxidation potential among other Pt–Ru catalysts using different carbon support. It is thus considered that the carbon substrates significantly affect the CO oxidation activity of anode electrocatalysts in DMFC. This is ascribed to the geometrical effect that the flat interface between CNTs and metal catalysts has a unique feature, at which the electron transfer occurs, and this interface would modify the catalytic properties of Pt–Ru particles.     

Performance studies of Pd–Pt and Pt–Pd–Au catalyst for electro-oxidation of glucose in direct glucose fuel cell

Platinum is employed as anode catalyst for low temperature electro-oxidation of glucose in direct glucose fuel cell (DGFC), but it suffers from poisoning by intermediate oxidation products. In the present investigation, palladium and gold precursors are added with platinum precursor to form low metal loading (∼15–20% by wt.) carbon supported catalyst by NaBH₄ reduction technique. The prepared PtPdAu/C (metal ratio 1:1:1) and PdPt/C (metal ratio 4:1) catalysts are tested in DGFC. The Physical characterization of electro-catalysts by scanning electron microscope, transmission electron microscope, energy dispersive X-ray, X-ray diffraction and thermo-gravimetric analysis confirms the formation of nano-sized metal particles on carbon substrate with two prominent homogeneous bi- or tri-metallic crystal phases for PtPdAu/C. The cyclic voltammetry studies carried out for glucose (0.05 M) oxidation in (0.5 M KOH) alkaline medium shows the metal catalysts can efficiently electro-oxidize glucose. The catalysts tested as anode in a batch type DGFC using commercial activated charcoal as cathode produced peak power density of 0.52 mW cm⁻² for both PdPt/C and PtPdAu/C in 0.3 M glucose in 1 M KOH solution.