Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Electrochemical and physicochemical characterizations of methanol-tolerant platinum-macrocycle cocatalyst for oxygen reduction

Carbon supported iron tetraphenylporphyrin and platinum (FeTPP-Pt/C) cocatalysts were prepared by supporting FeTPP on Pt/C followed by a heat-treatment at temperatures ranging from 300 to 900 °C. The relationship of the electrocatalytic properties of cocatalysts and the heat-treatment temperature was studied in detail. Compared to the Pt/C counterpart, it was found that heat-treated cocatalysts exhibit a comparable catalytic activity for oxygen reduction reaction (ORR) and lower reactivity for methanol oxidation reaction (MOR). The cocatalysts treated at ca. 500-700 °C are, in our experimental conditions, the best ORR catalysts with high methanol tolerance. The physicochemical characterizations of the cocatalysts, Pt/C and FeTPP/C were performed by ICP-AES, TEM (EDX), XPS, and XRD techniques. Combined with the electrochemical results, it was revealed that the presence of Fe-N-based species surrounding the Pt sites could greatly hinder the MOR.     

Novel system of electro-catalysts for methanol oxidation based on platinum and organic metal complexes

Novel system of electro-catalysts was developed for methanol oxidation reaction (MOR) in direct methanol fuel cells. Mixture of platinum tetraammine complex with cobalt quinolyldiamine complex with various mixing ratio was supported on graphite powder, and heat treated at 400-1000 °C in argon atmosphere. Powder thus obtained was put on a graphite disk electrode, and tested for electrochemical MOR in acid media. Although the cobalt complex itself showed almost no catalytic ability for MOR, it enhanced the activity of Pt more than several 10-fold when it was mixed with Pt. MOR performance was best exploited at about equal mixing ratio of platinum and cobalt complexes. Compared with platinum-ruthenium alloy catalysts, the new catalysts exhibited promising catalytic ability. The present investigation revealed good potentiality of organic complex catalysts in combination with metal catalysts for MOR, which opens the way to synthesize and develop a new class of electro-catalysts of low cost through wide range of molecular designing.     

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.     

Alternative cathodes based on iron phthalocyanine catalysts for mini- or micro-DMFC working at room temperature

Iron phthalocyanine based cathodes were prepared either by dispersion of FePc on carbon or by electropolymerization of aniline in presence of FeTsPc. The macrocycles based cathodes were compared to a classical commercial Pt/C cathode in a standard three-electrode electrochemical cell and under DMFC conditions at room temperature. It was shown that the molecular dispersion of FeTsPc into a PAni film greatly enhances the activity of the macrocycle catalyst towards oxygen reduction reaction (ORR). But, in the same time, the stability under DMFC conditions is drastically decreased compared to the stability obtained with a FePc/C electrode. It was suggested that this instability of the catalytic film was rather due to the release of the FeTsPc from the polymer than to the destruction of the macrocycle active centre. Even if iron phthalocyanine catalysts display total tolerance to methanol when the anode is fed with a 5 M methanol solution, the comparison between a PAni-FeTsPc/C cathode and a Pt/C cathode in DMFC working conditions is in favor of the Pt/C cathode, in term of maximum achieved power density. However, the ratio (platinum atoms per cm²/number of FeTsPc molecules per cm²) is close to 100, which allows to be optimistic for further enhancement of activity of polymer-FeTsPc electrodes. It was suggested that researches to develop new electron conductive polymers stable under oxidative environment and with a high doping capacity could be a direction to use platinum alternative cathode catalysts in DMFC technology.     

Electrodeposited PtCo and PtMn electrocatalysts for methanol and ethanol electrooxidation of direct alcohol fuel cells

PtCo and PtMn electrocatalyst particles were successfully synthesized on Ti substrate by the electrodepostion method. PtCo particles deposited are star-shaped particles with size of 100–200 nm and very porous with many slices of 10 nm. On the other hand, PtMn particles are spherical and have no obvious conglomeration, and the particle is in the range of 100–200 nm. The results reveal that the effect of the incorporation of Co and Mn on the electrochemical active surface area of Pt nanoaprticles is very small. However, incorporation of trace Co and Mn in Pt (e.g., Pt₁₀₀₀Co and Pt₁₀₀₀Mn) has dramatic effect on the electrochemical oxidation reaction of alcohol. The mass specific peak current for the methanol oxidation in alkaline media is 49 mA cm⁻² and 39 mA cm⁻² on Pt₁₀₀₀₀Mn and Pt₁₀₀₀Co, which is three and two times higher, respectively, than that on pure Pt electrocatalyst nanoparticles. PtMn and PtCo electrocatalysts also show significant enhanced stability for methanol oxidation. However, the electrocatalytic enhancement of Co or Mn to Pt is relatively small for the electrooxidation reactions of ethanol in alkaline media.     

Synthesis and characterization of PDDA-stabilized Pt nanoparticles for direct methanol fuel cells

Pt nanoparticles are synthesized by the alcoholic reduction of H₂PtCl₆ in the presence of a polycation, poly(diallyldimethylammonium chloride) (PDDA). The size of the PDDA-Pt nanoparticle colloids is in the range of 2-4 nm, depending on the PDDA to Pt ratio in the solution. The PDDA-Pt nonoparticles can be self-assembled to the sulfonic acid group, SO₃⁻, at the Nafion membrane surface by the electrostatic interaction, forming a self-assembled monolayer (SAM). The study shows that such SAM reduced the methanol crossover and enhanced the power output of direct methanol fuel cells (DMFC) by as much as 34% as compared to the cell based on an un-modified Nafion membrane. In addition, PDDA-Pt nanoparticles synthesized with low PDDA/Pt ratios show considerable catalytic activity for the methanol oxidation reaction (MOR) in comparison to a commercial Pt/C catalyst. However, the electrocatalytic activity of PDDA-Pt nanoparticles decreased significantly with the increase in the PDDA/Pt molar ratio, indicating that the excess PDDA inhibits the MOR.     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Preparation of Pt/CeO2/HCSs anode electrocatalysts for direct methanol fuel cells

Hollow carbon spheres (HCSs) were prepared through a simple hydrothermal method using silica particles and glucose as the template and carbon precursor, respectively. HCSs used as supports for platinum catalysts deposited with cerium oxide (CeO₂) were prepared for application as anode catalysts in direct methanol fuel cells. The composition and structure of the samples were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic properties of the as-prepared catalysts for methanol oxidation were investigated by cyclic voltammetry (CV). The Pt/CeO₂/HCSs catalyst heated at 550 °C for 1 h exhibited the best catalytic activity for methanol oxidation.     

Methanol tolerant electrocatalysts for the oxygen reduction reaction

Direct methanol fuel cells (DMFCs) represent an interesting alternative in obtaining electricity in a clean and efficient way. Portable power sources are one of the most promising applications of passive DMFCs. One of the requirements in these devices is to use high alcohol concentration, which due to methanol crossover causes a considerable loss of fuel cell efficiency. In order to develop methanol tolerant cathodes with suitable activity, different supported catalysts namely PtCo/C and PtCoRu/C, were prepared either via ethylene glycol reduction (EG) with or without microwave heating assistance (MW) or via the alloy method, the latter followed by a thermal treatment in a reducing atmosphere (N₂/H₂). All cathode-catalysts were tested to determine the role of the components in simultaneously enhancing the oxygen reduction reaction (ORR) and discouraging the methanol oxidation reaction. According to the synthesis methodology, X-ray photoelectron spectra showed that the amount of metal oxides on the surface varies, being higher on the PtCo/C EG and PtCoRu/C EG catalysts. The electrochemical characterization of the catalysts was accomplished in a three electrodes electrochemical cell with a glassy carbon rotating disk electrode covered with a thin catalytic film as working electrode. To study the ORR and the influence of different methanol concentrations, linear sweep voltammetry and cyclic voltammetry were employed. The PtCo/C EG, with an important metal oxide amount on the surface, and the PtCoRu/C MW and EG electrodes, both with RuO₂ on their surfaces, were the most tolerant to methanol presence.     

Ultrasonic synthesis of highly dispersed Pt nanoparticles supported on MWCNTs and their electrocatalytic activity towards methanol oxidation

A new method of synthesis of highly dispersed Pt nanoparticles with large catalytic surface area on multi-walled carbon nanotubes (MWCNTs) under high-intensity ultrasonic field was developed. The method, with low processing temperature at 25 °C, took only about 5 min. The surface characterization of MWCNTs was carried out by fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy methods. The electrochemical surface area and pore volume of MWCNTs were also examined. The result showed that functional groups of the MWCNTs which favored the high loading and high dispersion of particles and electrochemical surface area of MWCNTs were reinforced in the case of high-intensity ultrasonic field. The Pt/MWCNT catalysts were characterized by energy dispersion X-ray spectra analysis (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements. The prepared Pt nanoparticles were uniformly dispersed on the MWCNT surface. The mean size of Pt particles was 3.4 ± 0.2 nm. The electrocatalytic properties of Pt/MWCNT composites and kinetic characterization for methanol electro-oxidation were investigated by cyclic voltammetry. The Pt/MWCNT catalysts prepared for 5 min in ultrasonic field present excellent electrochemical activities. The schematic of the reaction was also introduced.     

Novel methanol electro-oxidation catalyst assisting with functional phthalocyanine supports

Novel electro-catalyst based on phthalocyanine stabilized Pt colloids has been developed for methanol electro-oxidation. Water soluble Cu²⁺ phthalocyanine functioned with sulfonic groups were selected as catalyst supports because of the relatively high catalytic activity of Pt catalyst and nearly the same catalytic selectivity complex with Cu-phthalocyanine, compared to others that chelated with Fe, Co and Ni ions. The as-resulting Pt-CuTsPc catalysts have average particle size of 2 nm and narrow size distribution. With the assistance of CuTsPc supports, the methanol electro-oxidation activity and poison tolerance of Pt catalyst have a significant increase. I_f/I_b ratio (anodic peak current density, forward to backward) of the Pt-CuTsPc/C catalysts also has obvious increase to 2.5, from value of 0.8 for pure Pt/C catalyst. The reaction Tafel slope of Pt-CuTsPc/C catalysts is 56.6 mV dec⁻¹, much smaller than that of the Pt/C catalyst. The transient current density on Pt-CuTsPc/C at 0.60 V is enhanced to 650% of that on the Pt/C catalyst while the enhancement factor R for comparison of steady-state current obtained on Pt-CuTsPc/C and Pt/C catalyst varies between 111% and 534% in the potential region of 0.3-0.75 V.     

Enhanced activity and stability of Pt/C fuel cell anodes by the modification with ruthenium-oxide nanosheets

Ruthenium-oxide nanosheet (RuO₂ns) crystallites with thickness less than 1 nm were prepared via chemical exfoliation of a layered potassium ruthenate and deposited onto carbon supported platinum (Pt/C) as a potential co-catalyst for fuel cell anode catalysts. The electrocatalytic activity towards carbon monoxide and methanol oxidation was studied at various temperatures for different RuO₂ns loadings. An increase in electrocatalytic activity was evidenced at temperatures above 40 °C, while little enhancement in activity was observed at room temperature. The RuO₂ns modified Pt/C catalyst with composition of RuO₂:Pt = 0.5:1 (molar ratio) exhibited the highest methanol oxidation activity. CO-stripping voltammetry revealed that RuO₂ns promotes oxidation of adsorbed CO on Pt. In addition to the enhanced initial activity, the RuO₂ns modified Pt/C catalyst exhibited improved stability compared to pristine Pt/C against consecutive potential cycling tests.     

PtCo supported on ordered mesoporous carbon as an electrode catalyst for methanol oxidation

A catalyst for methanol oxidation, PtCo supported on graphitized mesoporous carbon, has been synthesized and its electrochemical activity for methanol oxidation has been investigated. The graphitized mesoporous carbon support with ordered pore structure and high surface area of 585 m² g⁻¹ was prepared by one-step melt casting method using Al doped hexagonal mesoporous silica as hard templates and mineral pitches as carbon precursors followed by carbonization at 800 °C. The materials were characterized by X-ray diffraction, Raman spectra, field emission scanning electron microscopy, transmission electron microscopy and nitrogen sorption techniques. Cyclic voltammetry and amperometric i-t tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that the graphitized mesoporous carbon exhibits large electrochemical capacitance and good electric property. After supported with 20 wt%Pt or 20 wt%PtCo nanoparticles, the resultant mesostructured composites show 26-97% higher electrochemical catalytic activity for methanol oxidation than commercial catalyst 20 wt%Pt/C in mass activity (mA mg Pt⁻¹).     

Performance and stability of Pt-based ternary alloy catalysts for PEMFC

Carbon-supported Pt-based ternary alloy electrocatalysts were prepared by incipient wetness method in order to elucidate the origin of the enhanced activity of oxygen reduction reaction in PEMFC. To measure the catalytic activity and stability of the cathode alloy catalysts (electrodes containing Pt loading of 0.3 mg/cm², 20 wt.% Pt/C, E-TEK), the I-V polarization curves were obtained. All alloy catalysts showed higher performances than Pt/C. It can be concluded that as platinum formed alloys with transition metals, the electronic state of Pt and the nearest neighbor Pt-Pt distance changes, which significantly influence the electrocatalytic activity for oxygen reduction.Long-term stability test was performed with the Pt₆Co₁Cr₁/C alloy catalyst for 500 h. According to XPS analysis, the lower oxide component with Pt₆Co₁Cr₁/C electrocatalyst provides a large portion of platinum in metallic species in the electrocatalyst and it seems to be mainly responsible for its enhanced activity towards oxygen reduction.     

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

A porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N₂, O₂, CO and under mixed reactant O₂/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.     

Synthesis and characterization of Pd-Ni nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Carbon-supported Pd-Ni nanoalloy electrocatalysts with different Pd/Ni atomic ratios have been synthesized by a modified polyol method, followed by heat treatment in a reducing atmosphere at 500-900 °C. The samples have been characterized by X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), rotating disk electrode (RDE) measurements, and single-cell proton exchange membrane fuel cell (PEMFC) tests for oxygen reduction reaction (ORR). XRD and TEM data reveal an increase in the degree of alloying and particle size with increasing heat-treatment temperature. XPS data indicate surface segregation with Pd enrichment on the surface of Pd₈₀Ni₂₀ after heat treatment at ≥500 °C, suggesting possible lattice strains in the outermost layers. Electrochemical data based on CV, RDE, and single-cell PEMFC measurement show that Pd₈₀Ni₂₀ heated at 500 °C has the highest mass catalytic activity for ORR among the Pd-Ni samples investigated, with stability and catalytic activity significantly higher than that found with Pd. With a lower cost, the Pd-Ni catalysts exhibit higher tolerance to methanol than Pt, offering an added advantage in direct methanol fuel cells (DMFC).     

Preparation and the physical/electrochemical properties of a Pt/C nanocatalyst stabilized by citric acid for polymer electrolyte fuel cells

Pt/C nanocatalysts were prepared by the reduction of chloroplatinic acid with sodium borohydride, with citric acid as a stabilizing agent in ammonium hydroxide solution. These nanocatalysts were obtained by altering the molar ratio of citric acid to chloroplatinic acid (CA/Pt) from 1:1, 2:1, 3:1 to 4:1. Transmission electron microscopy and X-ray diffraction analyses indicated that the well-dispersed Pt nanoparticles of around 3.82 nm in size were obtained when the CA/Pt ratio was maintained at 2:1. X-ray photoelectron spectroscopy measurements revealed that the 2:1, 3:1 and 4:1 molar ratio catalysts had a relatively higher amount of Pt in their metallic state than did the 1:1 molar ratio catalyst. Cyclic voltammetry results demonstrated that the Pt/C nanocatalysts annealed at 400 °C in an N₂ atm provided higher electrocatalytic activity. Among all the molar ratio catalysts, the 2:1 molar ratio catalyst exhibited the largest electrochemical active surface (EAS) area, and its methanol oxidation reaction current was superior to the E-TEK catalyst. The oxygen reduction reaction of the catalysts studied by linear sweep voltammetry and tested in a fuel cell indicated that the catalytic activity of the 2:1 molar ratio catalyst was comparable to that of an E-TEK catalyst.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.