Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part I: Tuneable low platinum loadings, specific H upd feature and evidence for oxygen reduction

A bottom-up approach is used here to combine carbon nanotubes synthesized by CVD and organically capped platinum nanoparticles electrocatalyst exhibiting a direct electrochemical activity towards oxygen reduction. Both nano-objects are handled in liquid suspension and are associated together in a controlled way. The nanocomposite liquid dispersions can be precisely controlled in terms of platinum nanoparticles to carbon nanotubes weight ratios (NP/NT) which correspond to different coverages of nanotubes by nanoparticles. Electrodes with low to ultra-low platinum loadings can then be prepared on porous fuel cell carbon supports by filtration. The direct electrochemical activity towards aqueous oxygen reduction reaction (ORR) of electrodes with platinum loadings ranging from about 1 to 60 μg/cm² is reported without any activation step in order to keep the features of the nanoparticles intact. Before that, we studied the responses obtained when impregnating our hydrophobic electrodes by a voltamperometric gas consumption procedure. These responses are also dependent of the composition of our electrodes. Whereas our results are of particular interest with respect to the optimization of platinum loading in fuel cell electrodes, the specific behaviour of these capped platinum nanoparticles towards proton adsorption-desorption reveals the difficulty to determine reliable active surface area with related regard to the platinum loading and point to the necessity to determine other characteristic parameters for the electrodes.     

Nanocomposite electrodes based on pre-synthesized organically capped platinum nanoparticles and carbon nanotubes. Part II: Determination of diffusion area for oxygen reduction reflects platinum accessibility

In this paper we report the determination of the diffusion area for oxygen reduction in porous electrode structure having a controlled platinum loading and based on capped platinum electrocatalysts and carbon nanotubes. Such a parameter is expected to be higher than the macroscopic geometrical area of the active porous layer. The oxygen diffusion area is determined by cyclic voltammetry after impregnation of the electrode structure by the electrolyte, and using the equations available for peak potential and peak current as a function of scan speed for irreversible redox couple. First it is found first that the oxygen diffusion area is dependent on the total amount of platinum in the electrode. Second, for a given platinum loading, the diffusion area is higher when the mass ratio of platinum to carbon nanotube decreases. This point indicates that the accessibility of platinum capped electrocatalyst is better in such cases. It is thus concluded that the oxygen diffusion area determination in porous electrode structures may be used to characterize the accessibility of the capped electrocatalysts for oxygen reduction. Even if this area is different in nature from the one calculated by Hydrogen Underpotential Deposition, we believe that its determination might be of interest for the characterization of porous electrodes structures in which the electrocatalyst is combined with a finely divided carbon support.     

Polyoxometallate-stabilized platinum catalysts on multi-walled carbon nanotubes for fuel cell applications

A novel catalyst, Pt-PMo₁₂-CNT, with well-dispersed Pt nanoparticles and monolayer PMo₁₂ on multi-walled carbon nanotubes (CNTs) is reported. A polyoxometallate (PMo₁₂) was chemically adsorbed on the surface of Pt nanoparticles and on outer walls of CNTs. These effectively prevented the agglomeration of Pt nanoparticles and CNTs due to the electrostatic repulsive interactions between negatively charged PMo₁₂ monolayers. The as-prepared Pt-PMo₁₂-CNT materials show higher electrocatalytic activity, higher cycle stability, and better tolerance to poisoning species in methanol oxidation than do Pt-CNT catalysts prepared by the same method. The reasons for the improved catalytic performance of the Pt-PMo₁₂-CNT catalysts are discussed.     

Recent advances in platinum monolayer electrocatalysts for oxygen reduction reaction: Scale-up synthesis, structure and activity of Pt shells on Pd cores

We have established a scale-up synthesis method to produce gram-quantities of Pt monolayer electrocatalysts. The core-shell structure of the Pt/Pd/C electrocatalyst has been verified using the HAADF-STEM Z-contrast images, STEM/EELS, and STEM/EDS line profile analysis. The atomic structure of this electrocatalyst and formation of a Pt monolayer on Pd nanoparticle surfaces were examined using in situ EXAFS. The Pt mass activity of the Pt/Pd/C electrocatalyst for ORR is considerably higher than that of commercial Pt/C electrocatalysts. The results with Pt monolayer electrocatalysts may significantly impact science of electrocatalysis and fuel-cell technology, as they have demonstrated an exceptionally effective way of using Pt that can resolve problems of other approaches, including electrocatalysts’ inadequate activity and high Pt content.     

Hollow graphitic carbon spheres for Pt electrocatalyst support in direct methanol fuel cell

Hollow graphitic carbon spheres (HGCSs) with a high surface area are produced by the carbonization of hollow polymer spheres obtained by the polymerization of core/shell-structured pyrrole micelles. HGCSs are employed as a carbon support material in a direct methanol fuel cell catalyst, and their effect on the electrocatalytic activity toward methanol oxidation is investigated. Pt catalyst supported on HGCSs shows a better electrocatalytic activity compared to that on Vulcan XC-72, which has been commonly used in fuel cell catalysts. The observed enhancement in the electrocatalytic activity is attributed to the improved electronic conductivity and high surface area of HGCSs.     

Synthesis of OMC supported Pt catalysts and the effect of the metal loading technique on their PEM fuel cell performances

Abstract

Ordered mesoporous carbon (OMC) supported Pt catalysts were prepared by different loading techniques, in order to be used in the catalysis of oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. OMC was synthesized by an organic-inorganic self assembly route using Pluronic F127 as surface directing agent, resorcinol-formaldehyde as carbon source and tetraethyl ortosilicate (TEOS) as silica source. Pt loading was achieved by three different approaches; one pot in situ synthesis, wet impregnation and surface-modified wet impregnation. Nitrogen adsorption studies showed slight reductions in surface areas, which can be attributed to partial losses of micropore volumes. The XRD and TEM analysis revealed a better metal distribution and smaller particle size in the surface-modified sample with a mean Pt particle size of 3.83?nm. The modified sample also gave the most promising performance among the catalysts with a maximum power density of 73?mW cm^?2, which was very close to the commercial Pt/C catalyst.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

Synthesis of Pt and Pd nanosheaths on multi-walled carbon nanotubes as potential electrocatalysts of low temperature fuel cells

Pt and Pd nanosheaths are successfully synthesized on multi-walled carbon nanotubes (MWCNTs) using the non-covalent poly(diallyldimethylammonium chloride) (PDDA) functionalization and seed-mediated growth methods. In this method, negatively charged Pt or Pd metal precursors are self-assembled with positively charged PDDA-functionalized MWCNTs, forming uniformly distributed Pt or Pd nanoseeds on MWCNTs supports. The contiguous and highly porous Pt and Pd nanosheath structured catalysts are then formed by the seed-mediated growth in corresponding metal precursors using ascorbic acid as the reducing agent. The essential role of uniformly dispersed Pt and Pd nanoseeds on PDDA-MWCNTs is demonstrated. The results indicate that both Pt and Pd nanosheaths show an enhanced catalytic activity for the methanol and formic acid oxidation reaction in acid solution, respectively, as compared with conventional Pt/C and Pd/C catalysts. The enhanced activities are most likely due to the reduced oxophilicity, which results in a weakened chemisorption energy with oxygen-containing species such as CO_ad, and the increased reactive sites due to the large number of grain boundaries of the Pt and Pd nanosheath structured electrocatalysts.     

Improved electrochemical activity of Bi0.5Sr0.5FeO3-δ–Ce0.9Gd0.1O1.95composite cathode electrocatalyst for solid oxide fuel cells

Developing MIEC materials with high electrocatalytic performance for the ORR and good thermal/chemical/structural stability is of paramount importance to the success of solid oxide fuel cells (SOFCs). In this work, high-activity Bi_0.5Sr_0.5FeO_3-δ-xCe_0.9Gd_0.1O_1.95 (BSFO-xGDC, x = 10, 20, 30 and 40 wt%) oxygen electrodes are synthesized, and confirmed by XRD, SEM and EIS, respectively. The crystal structure, microstructure, electrochemical property and performance stability of the promising BSFO-xGDC composite cathodes are systematically evaluated. It is found that introducing GDC nanoparticles can obviously improve the electrochemical property of the porous composite electrode. Among all these composite cathodes, BSFO-30GDC composite cathode shows the best ORR activity. The peak power density of anode supported single cells employing BSFO-30GDC composite cathode reaches 709 mW cm^?2 and the electrode polarization resistance (R_p) of the BSFO-30GDC is about 0.14 Ω cm² at 700 °C. The analysis of the oxygen reduction kinetic indicates that the major electrochemical process of the GDC-decorated composite cathode is oxygen adsorption-dissociation. These preliminary results demonstrated that BSFO-30GDC is a prospective composite cathode catalyst for SOFCs because of its outstanding ORR activity.     

Tetrahydrofuran-functionalized multi-walled carbon nanotubes as effective support for Pt and PtSn electrocatalysts of fuel cells

A novel and simple method to functionalize multi-walled carbon nanotubes (MWCNTs) is developed using tetrahydrofuran (THF) solvent as the functionalization and anchoring agent. The effectiveness of the method is demonstrated by the synthesis of uniformly distributed Pt and PtSn nanoparticles on THF-functionalized MWCNTs. Transmission electron microscopy and X-ray diffraction results indicate that Pt and PtSn nanoparticles with a narrow particle size distribution and an average particle size of ∼4 nm are synthesized on THF-functionalized MWCNTs. The lattice parameter of PtSn/MWCNTs increases with the Sn content, indicating the successful formation of PtSn binary nanoparticles. The results demonstrate the applicability and effectiveness of the THF-functionalized MWCNTs as effective catalyst supports in the development of highly dispersed and active Pt and Pt-based electrocatalysts for low temperature fuel cells. The successful functionalization of MWCNTs by THF also indicates that there could be a strong σ-π interaction between the MWCNTs and the THF.     

Influence of carbon support on the performance of platinum based oxygen reduction catalysts in a polymer electrolyte fuel cell

Novel carbons from the Sibunit family prepared via pyrolysis of hydrocarbons [Yermakov YI, Surovikin VF, Plaksin GV, Semikolenov VA, Likholobov VA, Chuvilin AL, Bogdanov SV (1987) React Kinet Catal Lett 33:435] possess a number of attractive properties for fuel cell applications. In this work Sibunit carbons with BET surface areas ranging from ca. 20 to 420 m² g⁻¹ were used as supports for platinum and the obtained catalysts were tested as cathodes in a polymer electrolyte fuel cell. The metal loading per unit surface area of carbon support was kept constant in order to maintain similar metal dispersions (∼0.3). Full cell tests revealed a strong influence of the carbon support texture on cell performance. The highest mass specific activities at 0.85 V were achieved for the 40 and 30 wt.% Pt catalysts prepared on the basis of Sibunit carbons with BET surface areas of 415 and 292 m² g⁻¹. These exceeded the mass specific activities of conventional 20 wt.% Pt/Vulcan XC-72 catalyst by a factor of ca. 4 in oxygen and 6 in air feed. Analysis of the I–U curves revealed that the improved cell performance was related to the improved mass transport in the cathode layers. The mass transport overvoltages were found to depend strongly on the specific surface area and the texture of the support.     

Nitrogen doped carbon nanotubes synthesized from aliphatic diamines for oxygen reduction reaction

In the present study, nitrogen doped carbon nanotubes (N-CNTs) were synthesized using three different aliphatic diamines as nitrogen–carbon precursor solutions with varying carbon chain lengths, in order to elucidate the effect of precursor solution on the overall nitrogen content and ORR activity of the synthesized materials. Increasing the nitrogen to carbon ratio in the precursor solution resulted in higher nitrogen contents in the synthesized N-CNTs, along with enhanced ORR activity for all three samples tested. The increase in activity was attributed to the enhanced properties and edge plane defects of N-CNTs resulting from higher nitrogen contents, illustrating the importance of using a nitrogen rich precursor solution.     

The study of Pt@Au electrocatalyst based on Cu underpotential deposition and Pt redox replacement

Electrochemical study of the decorated Pt@Au catalyst synthesized by Cu underpotential deposition (UPD)-Pt redox replacement technique has been conducted in this work. The parameters affecting the Cu UPD on Au/C nanoparticles in sulfuric acid electrolyte, including the UPD potential, deposition time and potential sweep rate, were investigated in detail. Anode stripping method was used to calculate the charge of the deposited Cu adlayers. Results showed that Pt@Au catalyst prepared by this UPD-redox replacement approach is not a core-shell structure but a decorated structure. A series of decorated Pt@Au/C catalysts with various Pt coverages were synthesized and examined for formic acid oxidation (FAO). It is found that the specific activity of Pt atoms increases with the decrease of Pt surface coverage on Au. Life test showed that better stability was pertained for this decorated Pt@Au/C catalyst compared to Pt/C towards FAO.     

Progress in the synthesis of carbon nanotube- and nanofiber-supported Pt electrocatalysts for PEM fuel cell catalysis

This paper reviews the literature on the synthesis of carbon nanotube- and nanofiber-supported Pt electrocatalysts for proton exchange membrane (PEM) fuel cell catalyst loading reduction through the improvement of catalyst utilization and activity, especially focusing on cathode nano-electrocatalyst preparation methods. The features of each synthetic method were also discussed based on the morphology of the synthesized catalysts. It is clear that synthesis methods play an important role in catalyst morphology, Pt utilization and catalytic activity. Though some remarkable progress has been made in nanotube- and nanofiber-supported Pt catalyst preparation techniques, the real breakthroughs have not yet been made in terms of cost-effectiveness, catalytic activity, durability and chemical/electrochemical stability. In order to make such electrocatalysts commercially feasible, cost-effective and innovative, catalyst synthesis methods are needed for Pt loading reduction and performance optimization.     

Electrocatalytic activity of nitrogen doped carbon nanotubes with different morphologies for oxygen reduction reaction

Nitrogen doped carbon nanotubes (NCNTs) were synthesized by a single step chemical vapor deposition technique using either ferrocene or iron(II) phthalocyanine as catalyst and pyridine as the carbon and nitrogen precursor. Variations in surface morphology and electrocatalytic activity for oxygen reduction reaction (ORR) were observed between the NCNTs synthesized using different catalysts. The structural and chemical characterizations were carried out using transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical activity of NCNTs was evaluated with rotating ring disc electrode (RRDE) voltammetry. Structural characterization suggested more defects formed on the NCNTs synthesized from ferrocene (Fc-NCNTs) which led to a rugged surface morphology compared to the NCNTs synthesized from iron(II) phthalocyanine (FePc-NCNTs). Based on the RRDE voltammetry study, Fc-NCNTs demonstrated much higher activity for ORR than FePc-NCNT. Evidences from the structural and chemical characterizations illustrate the potential impact of catalyst structure in shaping the surface structure of NCNTs and the positive effect of surface defects on ORR activity. These results showed that potential improvements on ORR activity of NCNTs could be achieved by tailoring the surface structure of NCNTs by using catalysts with different structures.     

Electrodeposited Pd-Co catalyst for direct methanol fuel cell electrodes: Preparation and characterization

Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current density (−10 or −200 mA cm⁻²); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co deposited at −10 mA cm⁻² was smooth and possessed smaller particles (ca. 10 nm), while that at −200 mA cm⁻² was dendritic (or rough) and possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd₇Co₃ with Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 μA cm⁻² was 0.05 V). As for methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm⁻³ methanol to the oxidant solution. Our approach provides fundamental technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.     

Functionalization of carbon nanotubes by an effective intermittent microwave heating-assisted HF/H2O2 treatment for electrocatalyst support of fuel cells

A method is developed to effectively functionalize carbon nanotubes (CNTs) by intermittent microwave heating (IMH)-assisted HF/H₂O₂ solution treatment. CNTs functionalized by IMH-assisted HF/H₂O₂ solution treatment (CNTs-HF/H₂O₂) are characterized by high oxygen-containing groups and high graphitization degree, as compared with CNTs treated by HF (CNTs-HF) or by IMH-assisted H₂O₂ solution treatment without prior HF treatment (CNTs-H₂O₂). Pt supported on CNTs-HF/H₂O₂ (Pt/CNTs-HF/H₂O₂) has an average particle size of 2.8 nm, smaller than 2.9 nm for Pt supported on CNTs-HF, 3.3 nm for Pt supported on CNTs-H₂O₂ and 4.0 nm for Pt supported on pristine CNTs. Pt/CNTs-HF/H₂O₂ electrocatalysts display a high electrochemical surface area, high Pt utilization efficiency, a superior electrocatalytic and mass activity for the O₂ reduction reaction (ORR) with respect to other catalyst samples in the present study. The results demonstrate the efficiency and effectiveness of the IMH-assisted HF/H₂O₂ solution methods for the functionalization of CNTs, and the method could be easily scaled-up to treat CNTs in large quantities.     

Development and activity enhancement of zirconium/vanadium oxides as micro-heterogeneous ceramic electrocatalyst for ORR in low temperature fuel cell

A porous nano-metal oxide composite consisting of zirconium oxide and vanadium oxide (ZrVNPs) was prepared using precipitation hydrothermal technique, followed by microwave sintering process in order to enhance the catalytic activity toward oxygen reduction reaction (ORR) which is a key reaction in the development of low-temperature fuel cell technology. Microwave sintering process conditions were optimized using response surface methodology models (RSM) and the optimized produced material was called ZrV-T. The physiochemical and electrochemical activities of the fabricated materials were investigated and the results elucidated that, the catalytic efficiency of prepared samples is influenced by morphology, lattice structure, and porosity, as well as metal–support interactions and interfacial structural changes that affect oxygen molecules adsorption and activation. The thermally treated composite (ZrV-T) exhibits higher oxygen reduction performance than the non-sintered material (ZrVNPs). Both materials have different half-wave potentials (E1/2 = ?0.36 V) and (E1/2 = ?0.29 V) for ZrVNPs and ZrV-T respectively. In addition, ZrV-T has a higher stability than the Pt/C catalyst in 0.1 M KOH solution by 4% after 120 min. Using ZrV-T is likely to reduce costs in FCs and promote the use of electrochemical energy devices and it may also be beneficial in other systems that promote catalytic activity.     

Boosting the electrocatalytic performance of Fe-based perovskite cathode electrocatalyst for solid oxide fuel cells

The development of cathode materials with excellent electrocatalytic activity and CO₂ tolerance is an important direction for the wide application of solid oxide fuel cells. Herein, the cobalt-free perovskite oxides Bi_0.5Sr_0.5Fe_1-xV_xO_3-δ (BSFVx, x = 0.025, 0.05 and 0.075) are developed as the efficient cathode electrocatalysts for SOFCs. The V-doping strategy is beneficial to improve the thermal stability, CO₂ tolerance and electrochemical performance of undoped Bi_0.5Sr_0.5FeO_3-δ. Among all samples, Bi_0.5Si_0.5Fe_0.95V_0.05O_3-δ (BSFV0.05) cathode presents excellent oxygen reduction reaction activity, achieving a lower polarization resistance of 0.076 Ω cm² and the peak power density of the single cell with the BSFV0.05 cathode reaches to 1.16 W cm⁻² at 700 °C, which can be comparable to those of the representative cobalt-based cathodes. Furthermore, the improved CO₂ tolerance of the BSFV0.05 cathode can be ascribed to the high acidity of the V⁵⁺ and the larger average bonding energy in the oxide.     

Design of an assembly of pyridine-containing polybenzimidazole, carbon nanotubes and Pt nanoparticles for a fuel cell electrocatalyst with a high electrochemically active surface area

We describe the fabrication of a carbon nanotube (CNT) hybrid wrapped by pyridine-containing polybenzimidazole (PyPBI). PyPBI acts as an efficient dispersant for CNT wrapping and produces a stable complex after removal of the unbound PyPBI. We found that the wrapped PyPBI serve as a glue for immobilizing Pt nanoparticles onto the surface of multi-walled carbon nanotubes (MWCNTs) without any strong oxidation process for the MWCNTs, which is often used to produce sites where metallic nanoparticles are immobilized. Based on this method, a highly homogeneous and remarkably efficient Pt loading onto the surface of MWCNTs through a coordination reaction between Pt and PyPBI has been achieved. Cyclic voltammogram measurements have revealed that the Pt nanoparticles deposited on the PyPBI-wrapped MWCNTs have a high electrochemically active surface area. Present results provide useful information for the design and fabrication of triple-phase interface structures of fuel cell electrode catalysts with high efficient performance.