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Biomimetic design of monolithic fuel cell electrodes with hierarchical structures
Abstract
Despite the significant improvement of polymer electrolyte membrane fuel cell catalyst activities, a cost-effective and stable membrane electrode assembly is still lacking, which greatly inhibits the commercialization of this efficient and environmental friendly technology in stationary and transportation applications. The main reason is that the engineering of different components of an electrode, such as catalytically active metals, electron transport and reactant diffusion paths in a compatible way is very challenging. Here we show the design and preparation of a monolithic fuel cell electrode with a compatible wire on wire structure that mimics the configuration of a pine tree. We developed a procedure to make a flexible carbon thin film composed of porous nanofibers with a thickness of ~100 nm and centimeter scale lengths. Platinum nanowires (ca. 3 nm diameter) were deposited on these microscale carbon nanofiber films, resulting in a hierarchical structure. The platinum nanowires were then decorated with a porous bismuth coating to modulate the atomic structure and induce catalytic activity toward formic acid electrooxidation. The end result is a monolithic structure used as a fuel cell electrode that combines microscale diffusive pathways and nanoscale catalyst structures. Prepared by a process that is readily scalable, this design strategy offers a new way to tailor catalytic functions at a system level.
... The functionalized SiC webs (by introducing the hydroxyl group and phosphoric acid) showed a 70% better ion-exchange capability than that of the conventional membrane. The Pt NWs deposited on electrospun carbon film with a hierarchical structure were also used as MEA for fuel cells [171], which can induce the catalytic reaction toward formic acid electrooxidation. oxidation [167]. ...
... The functionalized SiC webs (by introducing the hydroxyl group and phosphoric acid) showed a 70% better ion-exchange capability than that of the conventional membrane. The Pt NWs deposited on electrospun carbon film with a hierarchical structure were also used as MEA for fuel cells [171], which can induce the catalytic reaction toward formic acid electrooxidation. ...
With global concerns about the shortage of fossil fuels and environmental issues, the development of efficient and clean energy storage devices has been drastically accelerated. Nanofibers are used widely for energy storage devices due to their high surface areas and porosities. Electrospinning is a versatile and efficient fabrication method for nanofibers. In this review, we mainly focus on the application of electrospun nanofibers on energy storage, such as lithium batteries, fuel cells, dye-sensitized solar cells and supercapacitors. The structure and properties of nanofibers are also summarized systematically. The special morphology of nanofibers prepared by electrospinning is significant to the functional materials for energy storage.
... Work by Wang et al. 172 shows a hierarchical structure design of a PEMFC catalyst layer, mimicking the configuration of a pine tree. A carbon nanofiber mat with Pt nanowires is fabricated through annealing electrospun polyvinylpyrrolidone titanium dioxide (PVP-TiO2) film. ...
With the climate crisis gathering recognition, there has been a significant interest in the clean energy field. As a reliable and clean energy solution, markets transitioning towards sustainable energy have identified polymer electrolyte membrane fuel cells (PEMFCs) as a critical technology. The increasing interest in PEMFCs is primarily due to their high energy density, high efficiency, zero greenhouse gas emission, and the ability to be used for combined heat and power (CHP) applications. However, additional development is required to overcome the current barriers associated with fuel cell component design, high manufacturing cost, and insufficient stability. Bio‐inspired designs have the potential to provide new and valuable insights into the continual improvements necessary to replace the non‐renewable technologies by using novel techniques for extracting design inspirations from scaled biological hierarchical structures. Applying these new design concepts to develop alternative materials and structures for various fuel cell components can be transformative. The well‐adapted structures and functional features of biological systems have been proven to greatly enhance the performance of current designs through a novel and optimized pathways. As manifested and reinforced by substantial data in the specified area of study, bio‐inspired designs have indeed lived up to the performance expectations and, in some cases, exceeded them. Here, we discuss the potential and value of using these bio‐inspired designs for improving PEMFCs. We discuss the recent studies and advancements based on nature‐inspired structures for multiple components of PEMFCs and illustrate just how these design considerations substantiate their claim as a more attractive alternative to conventional designs.
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... In order to reach performance levels which can compete with traditional internal combustion technologies, the inherently sluggish oxygen reduction reaction (ORR) at the cathode must be overcome by using electrocatalysts [3]. Platinum-based electrocatalysts are currently the main ORR catalysts, suffering from the drawbacks of prohibitive cost and monopolized supply factors [4][5][6][7][8]. Therefore, development of economically viable platinum group metal-free (PGM-free) catalysts with high activity and durability toward the ORR is of importance and will perpetuate commercialization efforts for PEMFCs [9,10]. ...
... Polymer electrolyte membrane fuel cells (PEMFCs) are highly promising clean energy devices considered as ideal alternatives to the conventional fossil fuel based technologies used in the automotive industry, telecommunications backup and materials handling [1][2][3]. Although the target markets exist, technical challenges relating to cost and durability must be addressed [4]. ...
Heat treated iron-polyaniline-carbon – based non-precious metal catalysts represent a promising class of material to replace the platinum based ORR catalysts for PEMFC technologies. In the present research, we apply an ammonia treatment to tune the structure and activity of electrocatalysts derived from iron, polyaniline and carbon nanotubes (CNTs). By controlling the NH3 reaction conditions, we were able to tune the chemistry of nitrogen incorporation, including concentration and dopant type. The final catalyst had a robust morphology consisting of highly porous 2-D in-situ formed graphene-like structures that, along with the intermixed 1-D CNTs, were decorated with an abundance of nitrogen and iron species. The resultant surface chemistry led to impressive catalyst activity, with a half-wave potential of 0.81 V observed through half-cell testing and under H2-air fuel cell testing, a current density of 77 mA cm−2 at 0.8 V was achieved, along with a maximum power density of 335 mW cm−2.
The growing energy demand leads to the excessive combustion of fossil fuels and contributes to CO2-induced global warming. On the other hand, new sustainable energy solutions require the development of advanced functional materials. Carbon composite nanofibers are attractive functional materials for a wide range of applications, including energy storage and conversion, sensing, catalysis, water filtration, drug delivery, gas separation, and tissue engineering, owing to their unique properties, such as a high surface area to volume ratio, excellent chemical resistance, superior electrical and thermal conductivity, and mechanical stability. Electrospinning coupled with heat treatment is an inexpensive and straightforward technique that allows the homogeneous mixing of precursor salts at the molecular level, easy functionalization of fibers, a combination of materials, and deposition of fibers onto other substrates. This review focuses on polyvinylpyrrolidone (PVP) as a sole carbon source without any additional polymers for composite nanofibers prepared by electrospinning with subsequent heat treatments. We discuss the main production parameters affecting the morphology and structure of prepared composites and summarize them by applications. The advantages and challenges of using PVP as a carbon source are highlighted by classifying the designed materials as electrodes for lithium-ion batteries and other energy storage systems, semiconductors, and beyond.
Over the past few decades, considerable advances have been achieved in polymer electrolyte membrane fuel cells (PEMFCs) based on the development of material technology. Recently, an emerging multiscale architecturing technology covering nanometer, micrometer, and millimeter scales has been regarded as an alternative strategy to overcome the hindrance to achieving high-performance and reliable PEMFCs. This review summarizes the recent progress in the key components of PEMFCs based on a novel architecture strategy. In the first section, diverse architectural methods for patterning the membrane surface with random, single-scale, and multiscale structures as well as their efficacy for improving catalyst utilization, charge transport, and water management are discussed. In the subsequent section, the electrode structures designed with one-dimensional and three-dimensional (3D) multiscale structures to enable low Pt usage, improve oxygen transport, and achieve high electrode durability are elucidated. Finally, recent advances in the architectured transport layer for improving mass transportation including pore gradient, perforation, and patterned wettability for gas diffusion layer and 3D structured/engineered flow fields are described. This article is protected by copyright. All rights reserved.
Carbon-based nanofibers decorated with metallic nanoparticles as hierarchically-structured electrodes offer significant opportunities for use in low-temperature fuel cells, electrolyzers, flow and air batteries, and electrochemical sensors. We present a facile and scalable method for preparing nanostructured electrodes composed of Pt nanoparticles on graphitized carbon nanofibers. The electrospinning directly addresses the issues related to large-scale production of Pt-based fuel cell electrocatalysts. Through precursor containing polyacrylonitrile (PAN) and Pt salt electrospinning along with an annealing protocol, we obtain approximately 180 nm thick graphitized nanofibers decorated with approximately 5 nm Pt nanoparticles. By in-situ annealing scanning transmission electron microscopy, we qualitatively resolve and quantitatively analyze the unique dynamics of Pt nanoparticle formation and movement. Interestingly, by very efficient thermal-induced segregation of all the Pt from the inside to the surface of the nanofibers, we increase overall Pt utilization as electrocatalysis is a surface phenomenon. The obtained nanomaterials are also investigated by spatially-resolved Raman spectroscopy, highlighting higher structural order in nanofibers upon doping with Pt-precursors. The rationalization of the observed phenomena of segregation and ordering mechanisms in complex carbon-based nanostructured systems is critically important for the effective utilization of all metal-containing catalysts like electrochemical oxygen reduction reaction; among many other applications.
L’objectif de cette thèse est le développement d’une méthodologie d’aide à l’écoconception permettant l’optimisation multi-objectifs du cycle de vie de produits. Le premier axe de travail est focalisé sur la phase d’utilisation. Il s’intéresse à la manière dont différents segments d’utilisateurs vont utiliser un même produit et aux répercussions que vont avoir ces différentes utilisations sur les performances de son cycle de vie. Le deuxième axe vise à élaborer une méthodologie de modélisation réaliste et pertinente du cycle de vie de produits. Il est centré sur une problématique cruciale des problèmes d’optimisation : l’élaboration de l’objectif de résolution. Il propose l’utilisation de technique d’identification et de quantification des besoins clients afin de définir un objectif de résolution orientant vers les solutions de conception respectant au mieux les attentes des clients. Le troisième axe a pour but d’obtenir, par l’optimisation, une sélection d’alternatives de conception permettant de maximiser les performances du système. Une modélisation “gros grain” du produit incluant ses composants et leurs alternatives y est réalisée.
A highly selective and durable electrocatalyst for carbon dioxide (CO2) conversion to formate is developed, consisting of tin (Sn) nanosheets decorated with bismuth (Bi) nanoparticles. Owing to the formation of active sites through favorable orbital interactions at the Sn‐Bi interface, the Bi‐Sn bimetallic catalyst converts CO2 to formate with a remarkably high Faradaic efficiency (96%) and production rate (0.74 mmol h−1 cm−2) at −1.1 V versus reversible hydrogen electrode. Additionally, the catalyst maintains its initial efficiency over an unprecedented 100 h of operation. Density functional theory reveals that the addition of Bi nanoparticles upshifts the electron states of Sn away from the Fermi level, allowing the HCOO* intermediate to favorably adsorb onto the Bi‐Sn interface compared to a pure Sn surface. This effectively facilitates the flow of electrons to promote selective and durable conversion of CO2 to formate. This study provides sub‐atomic level insights and a general methodology for bimetallic catalyst developments and surface engineering for highly selective CO2 electroreduction. Orbital interactions of Bi‐Sn lead to the electrocatalytic conversion of CO2 to formate with high selectivity, activity, and durability. This is attributed to the electronic states of Sn upshifting away from the Fermi level due to the coupling with Bi, making the HCOO* intermediate adsorb more favorably on Bi‐Sn than on pure Sn surfaces.
Rational design of bifunctional electrode that efficiently pair oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in a single system is critical for electrocatalytic pure oxygen and hydrogen generation and overall water splitting. Herein, we report a novel biomimetic architectural design and development of 3D flexible electrodes with high electrocatalytic activity and durability in mild alkaline system. Benefiting from the ultralong single-crystal Co-doped ZnO nanowires densely and in situ grown on highly conductive carbon fabric, the novel material and unique composite electrode architecture favor the direct electrochemical water splitting process over the conventionally fabricated electrodes by providing abundant active sites, efficient electron transfers pathways, and sufficient gas exchange channels. A low overpotential of 360 and 275 mV is required to reach a current density of 10 mA cm⁻² for OER and HER, respectively. When these thick electrodes are used as electrocatalysts for both anode and cathode in an overall water electrolyzer, a low cell voltage of 1.90 V and 2.06 V is required to reach a current density of 4.0 and 10.0 mA cm⁻², respectively, with an extremely long durability. Such single-cell prototype exhibiting superior performance to noble metal-based precious catalyst counterpart holds great promise in future practical applications for pure hydrogen generation.
Structures and morphologies of Fe-N-C catalysts have been believed crucial for the number of active sites and local bonding structures, therefore governing overall catalyst performance for the oxygen reduction reaction (ORR). The relevant knowledge is still lacking for the rational catalyst design. Through combining different nitrogen/carbon precursors, including polyaniline (PANI), dicyandiamide (DCDA), and melamine (MLMN), we aim to tune catalyst morphology and structure for facilitating the ORR. Instead of commonly studied single precursors, multiple precursors during the synthesis provide a new opportunity to promote catalyst activity and stability via a likely synergistic effect. The best performing Fe-N-C catalyst derived from PANI+DCDA is superior to individual PANI or DCDA-derived one. In particular, when compared to extensively explored PANI-derived catalysts, the binary precursors are able to achieve improved half-wave potential of 0.83 V and enhanced electrochemical stability in challenging acidic media, indicating significantly increased active site number and strengthened local bonding structures. Multiple key factors associated with the observed promotion are elucidated including the optimal pore size distribution, highest electrochemically active surface area, presence of dominant amorphous carbon, and thick graphitic carbon layers with more pyridinic nitrogen edge sites likely bonded with atomic iron.
Direct formic acid fuel cells (DFAFCs) are promising portable energy conversion devices for supplying our off-grid energy demands. However, traditional Pt-based catalysts suffer from poor performance; consequently the precious metal loading in an actual fuel cell has to be maintained at a very high value, typically orders of magnitude higher than the acceptable level. Through a molecular self-assembly/electro-deposition process, Pt atoms are effectively dispersed onto the surface of a nanoporous gold substrate, and the resulting nanocomposites demonstrate superior electrocatalytic performance toward formic acid electro-oxidation, which can be attributed to a nearly ideal catalyst configuration where all the Pt atoms are involved in a highly desired direct reaction path. In both half-cell electrochemical testing and actual DFAFCs, these rationally designed electrodes show over two orders of magnitude improvement in Pt efficiency, as compared with the state-of-the-art Pt/C catalyst. This design strategy allows customized development of new generation electrocatalysts for high performance energy saving technologies.
Direct formic acid fuel cells (DFAFCs) allow highly efficient low temperature conversion of chemical energy into electricity and are expected to play a vital role in our future sustainable society. However, the massive precious metal usage in current membrane electrode assembly (MEA) technology greatly inhibits their actual applications. Here we demonstrate a new type of anode constructed by confining highly active nanoengineered catalysts into an ultra-thin catalyst layer with thickness around 100 nm. Specifically, an atomic layer of platinum is first deposited onto nanoporous gold (NPG) leaf to achieve high utilization of Pt and easy accessibility of both reactants and electrons to active sites. These NPG-Pt core/shell nanostructures are further decorated by a sub-monolayer of Bi to create highly active reaction sites for formic acid electro-oxidation. Thus obtained layer-structured NPG-Pt-Bi thin films allow a dramatic decrease in Pt usage down to 3 μg·cm−2, while maintaining very high electrode activity and power performance at sufficiently low overall precious metal loading. Moreover, these electrode materials show superior durability during half-year test in actual DFAFCs, with remarkable resistance to common impurities in formic acid, which together imply their great potential in applications in actual devices.
Graphene-based hybrid nanostructures have attracted increasing interest worldwide. Benefiting from their remarkable electrochemical catalytic properties derived from chemical compositions and synergetic effects of multi-functionalities, these graphene-based hybrid nanomaterials will play a significant role in cutting-edge innovation for novel electrocatalysts. In this review, we summarize recent progress in the design and synthesis of graphene-based hybrid nanomaterials with controlled shape, size, composition and structure, and their application as efficient electrocatalysts for energy related systems. We conclude this article with some future trends and prospects which are highlighted for further investigations on graphene-based nanomaterials as advanced electrocatalysts.
Three-dimensional, ordered macroporous materials such as inverse opal structures are attractive materials for various applications in electrochemical devices because of the benefits derived from their periodic structures: relatively large surface areas, large voidage, low tortuosity and interconnected macropores. However, a direct application of an inverse opal structure in membrane electrode assemblies has been considered impractical because of the limitations in fabrication routes including an unsuitable substrate. Here we report the demonstration of a single cell that maintains an inverse opal structure entirely within a membrane electrode assembly. Compared with the conventional catalyst slurry, an ink-based assembly, this modified assembly has a robust and integrated configuration of catalyst layers; therefore, the loss of catalyst particles can be minimized. Furthermore, the inverse-opal-structure electrode maintains an effective porosity, an enhanced performance, as well as an improved mass transfer and more effective water management, owing to its morphological advantages.
Graphene supported Pt nanostructures have great potential to be used as catalysts in electrochemical energy conversion and storage technologies; however the simultaneous control of Pt morphology and dispersion, along with ideally tailoring the physical properties of the catalyst support properties has proven very challenging. Using sulfur doped graphene (SG) as a support material, the heterogeneous dopant atoms could serve as nucleation sites allowing for the preparation of SG supported Pt nanowire arrays with ultra-thin diameters (2-5 nm) and dense surface coverage. Detailed investigation of the preparation technique reveals that the structure of the resulting composite could be readily controlled by fine tuning the Pt nanowire nucleation and growth reaction kinetics and the Pt-support interactions, whereby a mechanistic platinum nanowire array growth model is proposed. Electrochemical characterization demonstrates that the composite materials have 2-3 times higher catalytic activities toward the oxygen reduction and methanol oxidation reaction compared with commercial Pt/C catalyst.
A novel membraneelectrodeassembly (MEA) structure for direct formic acidfuelcell (DFAFC) has been designed. The novelty is that Pd nanothorns are directly electrodeposited onto the carbon paper to form the anode catalyst layer. The Pd nanothorns are formed by a two step square wave electrodeposition. The dispersion and morphology of the Pd nanothorns on the carbon paper are observed by scanning electron microscopy (SEM). The crystal characteristics of the Pd nanothorns are studied by high resolution transmission electron microscopy (HRTEM) and the metallic property of the deposited Pd nanothorns is investigated by X-ray photoelectron spectroscopy (XPS). MEA is prepared with the Pd nanothorn covered carbon paper. The novel MEA provides 2.4 times higher peak power density than the conventional MEA. This increase in the performance is due to the improved mass transport of formic acid in the catalyst and diffusion layers, better Pd utilization and higher electroactivity of the Pd single crystal nanothorns.
The high cost of low temperature fuel cells is to a large part dictated by the high loading of Pt required to catalyse the oxygen reduction reaction (ORR). Arguably the most viable route to decrease the Pt loading, and to hence commercialise these devices, is to improve the ORR activity of Pt by alloying it with other metals. In this perspective paper we provide an overview of the fundamentals underlying the reduction of oxygen on platinum and its alloys. We also report the ORR activity of Pt5La for the first time, which shows a 3.5- to 4.5-fold improvement in activity over Pt in the range 0.9 to 0.87 V, respectively. We employ angle resolved X-ray photoelectron spectroscopy and density functional theory calculations to understand the activity of Pt5La.
Transparent conducting electrodes are essential components for numerous flexible optoelectronic devices, including touch screens and interactive electronics. Thin films of indium tin oxide-the prototypical transparent electrode material-demonstrate excellent electronic performances, but film brittleness, low infrared transmittance and low abundance limit suitability for certain industrial applications. Alternatives to indium tin oxide have recently been reported and include conducting polymers, carbon nanotubes and graphene. However, although flexibility is greatly improved, the optoelectronic performance of these carbon-based materials is limited by low conductivity. Other examples include metal nanowire-based electrodes, which can achieve sheet resistances of less than 10Ω □(-1) at 90% transmission because of the high conductivity of the metals. To achieve these performances, however, metal nanowires must be defect-free, have conductivities close to their values in bulk, be as long as possible to minimize the number of wire-to-wire junctions, and exhibit small junction resistance. Here, we present a facile fabrication process that allows us to satisfy all these requirements and fabricate a new kind of transparent conducting electrode that exhibits both superior optoelectronic performances (sheet resistance of ∼2Ω □(-1) at 90% transmission) and remarkable mechanical flexibility under both stretching and bending stresses. The electrode is composed of a free-standing metallic nanotrough network and is produced with a process involving electrospinning and metal deposition. We demonstrate the practical suitability of our transparent conducting electrode by fabricating a flexible touch-screen device and a transparent conducting tape.
Photopatterned resists pyrolyzed at different temperatures and different ambient atmospheres can be used as a carbonaceous mate - rial for microelectromechanical systems. Carbon films were prepared by pyrolysis of photoresists at temperatures ranging from 6 00 to 11008C. The carbon films were characterized by several analytical techniques, viz., profilometry, thermogravimetric analysis, four-point probe measurements, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In addition, cyclic voltammetry was performed on the carbon film electrodes, and the carbon films were compared to glassy carbon (GC) for their electrochemical behavior. Electron-transfer rate constants for the benchmark Fe(CN)6 32/42 and Ru(NH3)6 31/21 redox systems increased with increasing heat-treatment temperature, and approached those observed on GC following treatment at 11008C. The pyrolyzed films have low capacitance and background current, approximately one-fourth of that observed on GC. The oxygen/carbon atomic ratio determined from XPS was low ( ,1% for 11008C pretreatment), and increased more slowly upon exposure to air than that for GC treated under identical conditions. Pyrolysis of photoresist films permits photolithographic fabrication of carbon electrode devices, and also appears to yield a ca rbon film with a smooth surface and unusual surface chemistry.
A three-dimensional interconnected graphene monolith was used as an electrode support for pulsed electrochemical deposition of platinum (Pt) nanoparticles. Ptnanoparticles with well-defined morphology and small size can be obtained by controlling electrodeposition potential and time. Electrochemical characterization was carried out to examine the electrocatalytic activity of this monolithic electrode towards methanol oxidation in acidic media. The results show that the carbon material surface and structure have a strong influence on the Pt particle size and morphology. Compared with the three-dimensional scaffold of carbon fibers, the three-dimensional graphene when used as a free-standing electrode support resulted in much improved catalytic activity for methanol oxidation in fuelcells due to its three-dimensionally interconnected seamless porous structure, high surface area and high conductivity.
Among the most challenging issues in technologies for electrochemical energy conversion are the insufficient activity of the catalysts for the oxygen reduction reaction, catalyst degradation and carbon-support corrosion. In an effort to address these barriers, we aimed towards carbon-free multi/bimetallic materials in the form of mesostructured thin films with tailored physical properties. We present here a new class of metallic materials with tunable near-surface composition, morphology and structure that have led to greatly improved affinity for the electrochemical reduction of oxygen. The level of activity for the oxygen reduction reaction established on mesostructured thin-film catalysts exceeds the highest value reported for bulk polycrystalline Pt bimetallic alloys, and is 20-fold more active than the present state-of-the-art Pt/C nanoscale catalyst.
Fuel cells powered by hydrogen from secure and renewable sources are the ideal solution for non-polluting vehicles, and extensive research and development on all aspects of this technology over the past fifteen years has delivered prototype cars with impressive performances. But taking the step towards successful commercialization requires oxygen reduction electrocatalysts--crucial components at the heart of fuel cells--that meet exacting performance targets. In addition, these catalyst systems will need to be highly durable, fault-tolerant and amenable to high-volume production with high yields and exceptional quality. Not all the catalyst approaches currently being pursued will meet those demands.
Converting all U.S. onroad vehicles to hydrogen fuel-cell vehicles (HFCVs) may improve air quality, health, and climate significantly, whether the hydrogen is produced by steam reforming of natural gas, wind electrolysis, or coal gasification. Most benefits would result from eliminating current vehicle exhaust. Wind and natural gas HFCVs offer the greatest potential health benefits and could save 3700 to 6400 U.S. lives annually. Wind HFCVs should benefit climate most. An all-HFCV fleet would hardly affect tropospheric water vapor concentrations. Conversion to coal HFCVs may improve health but would damage climate more than fossil/electric hybrids. The real cost of hydrogen from wind electrolysis may be below that of U.S. gasoline.
Unique tin oxide-mesoporous carbon (SnO2-CMK-3) composites have been synthesized as platinum nanoparticle electrocatalyst supports for low temperature fuel cell applications. In comparison with state-of-the-art commercial carbon-supported platinum (Pt/C) and pure CMK-3-supported platinum (Pt/CMK-3), Pt/SnO2-CMK3 demonstrated improved Pt-mass and surface area based ethanol oxidation reaction (EOR) activity through half-cell electrochemical investigations, providing a 64.7 and 97.6 mV reduction in overpotential at 100 mA mg(Pt)(-1) upon comparison to Pt/CMK-3 and commercial Pt/C. Furthermore, improvements to the oxygen reduction reaction (ORR) kinetics were observed, with Pt/SnO2-CMK3 providing a kinetic current density of 3.40 mAcm(-2) at an electrode potential of 0.9 V vs RHE. The improved performance of Pt/SnO2-CMK-3 for EOR and ORR was attributed to the beneficial impact of the support properties, along with potential interactions occurring between the support and catalyst particles. Complemented by extensive physicochemical characterization, these unique materials show high promise for application in low temperature fuel cells.
Direct formic acid fuel cells (DFAFCs) have attracted much attention in the last few years for portable electronic devices, due to their potential of being high efficiency power sources. They have the potential to replace the state-of-the-art batteries in cell phones, PDAs, and laptop computers if their power density and durability can be improved. In the present investigation, the influence of increased anode catalyst layer porosity on DFAFC power density performance is studied. Lithium carbonate (Li2CO3) was used as a pore-former in this study because of its facile and complete removal after catalyst layer fabrication. The anode catalyst layers presented herein contained unsupported Pt/Ru catalyst and Li2CO3 (in the range of 0–50 wt%) bound with proton conducting ionomer. Higher DFAFC performance is obtained because of the increased porosity within the anode catalyst layer through enhanced reactant and product mass transport. The maximum power density of DFAFC increased by 25% when pore-former was added to the anode catalyst ink. The formic acid onset potential for the anode catalyst layer with 17.5 wt% pore-former was reduced by 30 mV. A constant phase element based equivalent-circuit model was used to investigate anode impedance spectra. Fitted values for the anode impedance spectra confirm the improvement in performance due to an increase in formic acid concentration inside the anode catalyst layer pores along with efficient transport of reactants and products.
Potential step experiments permit a more detailed investigation of the oxidation of formic acid in aqueous perchloric acid and at a platinum electrode covered by submonolayer amounts of lead. It is confirmed that lead atoms are very effective catalysts allowing high rates of oxidation over long periods of time. Indeed, at low concentrations of formic acid (<1 mM) the rate of oxidation can be determined by diffusion of formic acid in solution. At higher concentrations (10–500 mM); the current is kinetically controlled at short times, but later becomes diffusion controlled; it is suggested that the rate-determining step at short times is dissociative adsorption of formic acid at two platinum atoms adjacent to a lead ad-atom. The mechanism of catalysis is discussed.
PtPb/C and PtSb/C bi-metallic catalysts were synthesized by chemical deposition of Pb or Sb on a commercial 40% Pt/C catalyst. The performances of catalysts with a range of compositions were compared in a multi-anode direct formic acid fuel cell in order to optimize compositions and evaluate the statistical significance of differences between catalysts. The catalytic activity for formic acid oxidation increased approximately linearly with adatom coverage for both PtPb/C and PtSb/C, to maxima at fractional coverages of ca. 0.7. At a cell voltage of 0.5V, the currents at the optimum Pb or Sb coverages were ca. 8 times higher than at unmodified Pt/C. CO-stripping results indicate that the presence of Pb or Sb facilitates the oxidation of adsorbed CO. In addition, both metals appear to produce electronic effects that inhibit poison formation on the modified Pt surface.
The effect of controlled amounts of irreversibly adsorbed bismuth on a Pt (111) oriented electrode surface on the electrocatalytic oxidation of formic acid has been studied in the whole range of coverage. The experimental method used in this work enables us to maintain a constant surface coverage in heteroatoms while the electrode is cycled in the whole range of potential of oxidation of the organics. For a coverage range from 0 up to 0.8 the electroactivity of the surface for the direct oxidation of formic acid is enhanced by a factor of 40, while in the whole range of coverage the accumulation of the blocking intermediate is lowered to an undetectable level.
Carbon nanofibers (CNFs) have been synthesized from thermoplastic polyvinylpyrrolidone (PVP) using electrospinning in combination with a novel three-step heat treatment process, which successfully stabilizes the fibrous morphology before carbonization that was proven to be difficult for thermoplastic polymers other than polyacrylonitrile (PAN). These CNFs are both mesoporous and microporous with high surface areas without subsequent activation, and thus overcome the limitations of PAN based CNFs, and are processed in an environmentally friendly and more cost effective manner. The effects of heat treatment parameters and precursor concentration on the morphologies and porous properties of CNFs have been investigated, and their application as anodes for lithium ion batteries has also been demonstrated.
An integrated surface science and electrochemistry approach has been used to prepare and characterize SnO{sub x}/Pt(111) model catalysts and evaluate their electrochemical activity for the ethanol oxidation reaction (EOR). Nanoislands of SnO{sub x} are deposited onto the Pt(111) by reactive layer assisted deposition in which Sn metal is vapor deposited onto a Pt(111) surface precovered by NO{sub 2}. X-ray photoelectron spectroscopy (XPS) shows that the SnO{sub x} islands are highly reduced with Sn{sup 2+} being the dominant chemical species. After exposing the SnO{sub x}/Pt(111) surface to H{sub 2}O or an electrolyte solution, XPS provides evidence for a significant amount of H{sub 2}O/OH adsorbed on the reduced SnO{sub x} surfaces. Electrochemical testing reveals that the catalytic performance of Pt(111) toward ethanol electrooxidation is significantly enhanced with SnO{sub x} islands added onto the surface. The enhanced EOR activity is tentatively attributed to the efficient removal of CO{sub ads}-like poisoning species at Pt sites by oxygen-containing species that are readily formed on the SnO{sub x} nanoislands. Moreover, the strong dependence of the EOR activity on SnO{sub x} coverage provides experimental evidence for the importance of SnO{sub x}-Pt interface sites in the EOR.
The use of multiwalled carbon nanotubes as a platinum support for proton exchange membrane fuel cells has been investigated as a way to reduce the cost of fuel cells through an increased utilization of platinum. Carbon nanotubes were selectively grown directly on the carbon paper by chemical vapor deposition with electrodeposited cobalt catalyzing the growth of the carbon nanotubes. The as-prepared carbon nanotubes were employed as the support for the subsequent platinum catalyst, which is electrodeposited on the carbon nanotubes. Physicochemical and electrochemical characterizations were conducted to identify the morphologies of the cobalt, the carbon nanotubes, and the electrodeposited platinum on the carbon nanotubes. The feasibility of a fuel cell using the carbon nanotube-based electrodes was demonstrated.
Electrification of the automobile provides a means of sustaining personal mobility in the face of petroleum resource limitations and environmental imperatives. Lithium ion (Li ion) batteries and hydrogen fuel cells provide pure-electrification solutions for different mass and usage segments of automotive application. Battery electric vehicles based on current and targeted Li ion battery technology will be limited to small-vehicle low-mileage-per-day applications; this is due to relatively low specific energy (kWh/kg) and long recharge time constraints. We briefly discuss new generations of Li ion positive and negative electrode intercalation compounds that are needed and under development to achieve energy storage density, durability, and cost targets. Lithium−air batteries give promise of extending the range, but scientists and engineers must surmount a plethora of challenges if growing research investments in this area are to prove effective. Hydrogen fuel cell vehicles have demonstrated the required 300 mile range and the ability to operate in all climates, but the cost of Pt-based catalysts, a low efficiency of utilization of presently cost-effective renewable sources of primary energy (e.g., electricity from wind), and the development of hydrogen infrastructure present significant challenges. Dramatic decreases in the amount of Pt used are required and are being brought to fruition along several lines of development that are described in some detail.
Nitrogen-doped carbon nanotubes (N-CNTs) were utilized as platinum nanoparticle support materials, with the significant effect of the nitrogen precursor solution utilized N-CNT growth elucidated. N-CNTs synthesized from a nitrogen-rich ethylenediamine (ED) precursor solution (ED-CNTs) were found to have superior catalytic activity toward the oxygen reduction reaction (ORR) compared with N-CNTs grown from a precursor solution with relatively low nitrogen content pyridine (Py-CNTs). Significant increase in the nitrogen incorporation and edge plane exposure was observed for ED-CNTs. When utilized as platinum nanoparticle supports, Pt/ED-CNTs displayed significantly enhanced electrocatalytic activity toward the ORR when compared with Pt/Py-CNTs and nitrogen free Pt/CNTs, with the increase in performance being attributed to the distinct structural and electronic enhancements resulting from heterogeneous nitrogen doping. The performance of Pt/ED-CNTs as a cathodic catalyst for proton exchange membrane fuel cell operation was found to be significantly higher than that of Pt/CNT.
Large-scale synthesis and characterization of platinum nanowire (NW)-carbon nanotube (CNT) heterostructures between single-crystalline Pt NWs and multiwalled CNT (MWCNT) through their contact in the absence of prior CNT functionalization was described. The photomicrographs of the Nanostructures show NWs growth on the CNT stem while x-ray diffraction (XRD) pattern shows Pt NWs crystallized in a face-centers cubic (fcc) structure similar to bulk Pt. The results also show that the closely packed NW arrays contain single-crystal atomic structures growing along a fixed direction with a lattice spacing of 0.23 nm. The intact MWCNT structure and the crystalline Pt NW fringes are observed and the spacing between adjacent MWCNT is determined to be 0.34 nm. The entire surfaces of NTs are found to be covered with a high density Pt NWs and the self-assembled Pt NWs form NW arrays with multiple junctions to the CNT.
A one-dimensional model for a porous fuel cell electrode using a liquid electrolyte with dissolved reactant is presented. The model consists of a Poisson, second-order ordinary differential equation, describing the effect of the electric field and a one-dimensional; Fickian diffusion, second-order ordinary differential equation describing the concentration variation associated with diffusion. The model accounts for mass transport and heterogeneous electrochemical reaction. The solution of this model is by the approximate Adomian polynomial method and is used to determine lateral distributions of concentration, overpotential and current density and overall cell polarisation. The model is used to simulate the effects of important system and operating parameters, i.e. local diffusion rates, and mass transport coefficients and electrode polarisation behaviour.
The facile, efficient, and economical route for the large-scale synthesis of 3D flower-like platinum nanostructures through a simple chemical reduction of hexachloroplatinic acid was investigated. The structure and morphology of the Pt nanoflowers were investigated by scanning electron microscopy (SEM). The enlarged SEM image revealed that numerous nanowires, with lengths of 100-200 nm, assemble into 3D flower-like superstructures. Platinum nanoflowers can further be assembled, forming more complex hierarchical nanostructures. The nanoflowers were further characterized by transmission electron microscopy (TEM). It was observed that the nanoflowers adhere to carbon paper, exhibiting an enlarged electroactive surface area comparable to that of a commercial Pt/C electrode.
A study was conducted to demonstrate a simple room temperature aqueous phase synthesis of single-crystal nanowires of Pt on the nanosheres of a carbon black and investigate their catalytic activity for the oxygen reduction reaction (ORR) in PEM fuel cells. The study demonstrated the development of a facile wet-chemical procedure to synthesize unsupported single-crystal Pt nanowires and their flower-like assembly through the reduction of hexachloroplatinic acid by formic acid at room temperature without surfactant or template. Syntheses also revealed that by adding a certain amount of carbon black in the aqueous solution of hexachloroplatinic acid and formic acid, large quantities of Pt nanowires can be directly grown within 72 hours at room temperature on the nanosphere of the carbon support.
A study was conducted to demonstrate a new method that used electroconductive templates to fabricate electrospun mats, with controllable architectures and patterns. The perimeters that affect the formation of the patterns of the fibrous materials were also studied. It was demonstrated that the protrusions in the collectors were essential parameters that are expected to affect the structures of the electrospun mats. It was also demonstrated that woven structures can be generated by a time-dependent control of the arrangement of the electroconductive protrusions in the collector. The study used D,L-poly(lactic acid (PDLLA), to demonstrate the new method. PDLLA was dissolved in a mixture of dimethyl-formamide (DMF) and tetrahydrofuran (THF) and stirred for several minutes, to obtain a homogeneous and stable solution. The solution was placed in a syringe and supplied, using a syringe pump.
Electrospinning provides a simple and versatile method for generating ultrathin fibers from a rich variety of materials that include polymers, composites, and ceramics. This article presents an overview of this technique, with focus on progress achieved in the last three years. After a brief description of the setups for electrospinning, we choose to concentrate on the mechanisms and theoretical models that have been developed for electrospinning, as well as the ability to control the diameter, morphology, composition, secondary structure, and spatial alignment of electrospun nanofibers. In addition, we highlight some potential applications associated with the remarkable features of electrospun nanofibers. Our discussion is concluded with some personal perspectives on the future directions in which this wonderful technique could be pursued.
The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.
Polymer electrolyte membrane-based direct formic acid fuel cells (DFAFC) have been investigated for about a decade, and are now becoming an important area of portable power system research. DFAFCs have the advantages of high electromotive force (theoretical open circuit potential 1.48 V), limited fuel crossover, and reasonable power densities at low temperatures. This paper provides a review of recent advances in DFAFCs, mainly focussing on the anodic catalysts for the electro-oxidation of formic acid. The fundamental DFAFC chemistry, formic acid crossover through Nafion® membranes, and DFAFC configuration development are also presented.
This paper provides a comparative evaluation of electrocatalyst surface area stability in PEM fuel cells under accelerated durability testing. The two basic electrocatalyst types are conventional carbon-supported dispersed Pt catalysts (Pt/C), and nanostructured thin film (NSTF) catalysts. Both types of fuel cell electrocatalysts were exposed to continuous cycling between 0.6 and 1.2 V, at various temperatures between 65 and 95 °C, with H2/N2 on the anode and cathode, while periodic measurements of electrochemical surface area were recorded as a function of the number of cycles. The NSTF electrocatalyst surface areas were observed to be significantly more stable than the Pt/C electrocatalysts. A first order rate kinetic model was applied to the normalized surface area changes as a function of number of cycles and temperature, and two parameters extracted, viz. the minimum stable surface area, Smin, and the activation energy, Ea, for surface area loss in this voltage range. Smin was found to be 10% versus 66%, and Ea 23 kJ mole−1 versus 52 kJ mole−1, for Pt/C versus NSTF-Pt, respectively. The loss of surface area in both cases is primarily the result of Pt grain size increases, but the Pt/C XRD grain sizes increase significantly more than the NSTF grain sizes. In addition, substantial peak shifts occur in the Pt/C CVs, which ultimately end up aligning with the NSTF peak positions, which do not change substantially due to the voltage cycling. NSTF catalysts should be more robust against shut down/start-up, operation near OCV and local H2 starvation effects.
In situ electrochemical surface-enhanced infrared absorption spectroscopy (EC-SEIRAS) together with a periodic density functional theory (DFT) Calculation has been initially applied to investigate the mechanism of formic acid electro-oxidation oil Sb-modified Pt (Sb/Pt) electrode. EC-SEIRAS measurement reveals that the main formic acid oxidation Current oil Sb/Pt electrode is ca. 10-fold enhanced as compared to that oil clean Pt electrode, mirrored by nearly synchronous decrease of the CO and formate surface species, Suggesting a "non-formate" oxidation as the main pathway on the Sb/Pt electrode. Oil the basis of the calculations from periodic DFT, the catalytic role of Sb adatoms can be rationalized as a promoter for the adsorption of the CH-down configuration but an inhibitor for the adsorption of the O-down configuration Of formic acid, kinetically facilitating the complete oxidation of HCOOH into CO(2). In addition, Sb modification lowers the CO adsorption energy oil Pt, helps to mitigate the CO poisoning effect on Pt.
True n-type doping of titanium oxide without formation of midgap states would expand the use of metal oxides for charge-based devices. We demonstrate that plasma-assisted fluorine insertion passivates defect states and that fluorine acts as an n-type donor in titanium oxide. This enabled us to modify the Fermi level and transport properties of titanium oxide outside the limits of O vacancy doping. The origin of the electronic structure modification is explained by ab initio calculation.
The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O(2) + 4H(+) + 4e(-) → 2H(2)O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt-C; refs 1-7). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt(3)Ni(111) is one of the most active surfaces for the ORR (ref. 8), owing to a shift in the d-band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O(2) adsorption. However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization, especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni-Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.
Decorating platinum nanoparticles having preferential (100) orientation with palladium adatoms in the submonolayer range enhances their catalytic performance in the electrooxidation of formic acid (see picture). Moreover, as the palladium adatoms are deposited in a similar way to that used in analogous single-crystal studies, proper relationships between single-crystal and nanoparticle catalysis or electrocatalysis can be established.
A robust new electrocatalyst with ultralow Pt loading, great poisoning resistance, and high stability (see figure) shows an over 100-fold increase in the efficiency of formic acid electro-oxidation, compared with the commercial Pt/C catalyst. In situ IR spectroscopy proves that the greatly enhanced performance is mainly achieved by changing reaction pathways using Au clusters, which simultaneously improve the stability.
High activity, carbon supported Pt electrocatalysts were synthesized using a supercritical fluid method and a selective heterogeneous nucleation reaction to disperse Pt onto single walled carbon nanotube and carbon fiber supports. These nanocomposite materials were then incorporated into catalyst and gas diffusion layers consisting of polyelectrolytes, i.e., Nafion, polyaniline, and polyethyleneimine using layer-by-layer (LBL) assembly techniques. Due to the ultrathin nature and excellent homogeneity characteristics of LBL materials, the LBL nanocomposite catalyst layers (LNCLs) yielded much higher Pt utilizations, 3,198 mW mg Pt −1 , than membrane electrode assemblies produced using conventional methods (∼800 mW mg Pt −1 ). Thinner membranes (100 bilayers) can further improve the performance of the LNCLs and these layers can function as catalyzed gas diffusion layers for the anode and cathode of a polymer electrolyte membrane fuel cell. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/61240/1/3003_ftp.pdf
Advances in catalyst development offer hope for commercially viable hydrogen fuel cells.
Formic acid electrooxidation was studied on Bi modified polyoriented and preferential (111) Pt nanoparticles. For both types of nanoparticles, Bi coverage was progressively increased and its effect on formic acid electrooxidation was evaluated using cyclic voltammetry and chronoamperometric measurements. In both experiments, significant and progressive enhancements on the electrooxidation current densities were obtained in comparison to the bare Pt nanoparticles. In voltammetry, at maximum Bi coverage, higher current densities at peak potential were obtained with the preferential (111) Pt nanoparticles (approximately 42 mA cm(-2)) as compared to the polyoriented Pt nanoparticles (approximately 32 mA cm(-2)) in agreement with previous single crystal studies. Nevertheless, this tendency was not observed in chronoamperometry at 0.4 V where currents obtained at maximum Bi coverage were similar. On the other hand, CO poison formation was also evaluated at open circuit potential. The resulting electrochemical activity has been rationalized using different parameters, such as surface structure, size domains, particle size and Bi coverage.
(Chemical Equation Presented) Cheap and stable: A PtBi catalyst was fabricated in three consecutive electrochemical steps (see picture): electrochemical oxidation of carbon paper to form an adequate catalyst support (1), Pt electrodeposition (2), and underpotential deposition of Bi onto the as-prepared Pt (3). This process resulted in a well-dispersed and thin catalyst layer as well as a significantly enhanced power performance with a Pt loading of only 0.5 mg cm-2.
Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.
Micro fuel cells stack up well against batteries on paper. But the devices still face engineering, financial, and even political hurdles.
We describe the fabrication, characterization, and applications of ultrathin, free-standing mesoporous metal membranes uniformly decorated with catalytically active nanoparticles. Platinum-plated nanoporous gold leaf (Pt-NPG) made by confining a plating reaction to occur within the pores of dealloyed silver/gold leaf is 100 nm thick and contains an extremely high, uniform dispersion of 3 nm diameter catalytic particles. This nanostructured composite holds promise as a prototypical member of a new class of fuel cell electrodes, showing good electrocatalytic performance at low platinum loading (less than 0.05 mg cm-2), while also maintaining long-term stability against coarsening and aggregation of catalytic nanoparticles.
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