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DOI: 10.1002/adma.
Nanostructured Materials for
Electrochemical Energy Conversion and
Storage Devices**
By Yu-Guo Guo, Jin-Song Hu, and Li-Jun Wan*
1. Introduction
One of the great challenges for today’s information-rich, mobile society is providing high-efficient, low-cost, and environmentally friendly electrochemical energy conversion and storage devices for powering an increasingly diverse range of applications, ranging from portable electronics to electric vehicles (EVs) or hybrid EVs (HEVs). [1,2]^ As the performance of these devices depends intimately on the properties of their materials, considerable attention has been paid to the research and development of key materials. [1–12] Micrometer-sized bulk materials are reaching their inherent
limits in performance and cannot fully satisfy the increasing needs of consumer devices. Therefore, rapid development of new materials with high performance is essential. Nano- structured materials are becoming increasingly important in the field and hence have attracted great interest in recent years. A variety of nanometer size effects have been found in the materials used in electrochemical energy conversion and storage devices, which can be divided into two types: i) ‘trivial size effects’, which rely solely on the increased surface-to- volume ratio and ii) ‘true size effects’, which also involve changes of local materials properties. As the coming of ‘nano- ionics’ [2]^ has demonstrated an important position in the field, similar to that of nanoelectronics in semiconductor physics, its development may lead to breakthroughs in this field, which holds the key to new generations of clean-energy devices. However, it is beyond the scope of this progress report to give an exhaustive summary of those energy devices that may benefit now or in the future from the use of nanoparticles; rather, we shall limit ourselves to the fields of lithium-based batteries and fuel cells. In particular, we focus on nano- structured electrode materials for rechargeable lithium-ion batteries and nanostructured Pt-based electrocatalysts for direct methanol fuel cells (DMFCs).
PROGRESS REPORT
One of the greatest challenges for our society is providing powerful electrochemical energy conversion and storage devices. Rechargeable lithium-ion batteries and fuel cells are amongst the most promising candidates in terms of energy densities and power densities. Nanostruc- tured materials are currently of interest for such devices because of their high surface area, novel size effects, significantly enhanced kinetics, and so on. This Progress Report describes some recent developments in nanostructured anode and cathode materials for lithium-ion batteries, addressing the benefits of nanometer-size effects, the disadvantages of ‘nano’, and strategies to solve these issues such as nano/micro hierarchical structures and surface coatings, as well as developments in the discovery of nanostructured Pt-based electrocatalysts for direct methanol fuel cells (DMFCs). Approaches to lowering the cost of Pt catalysts include the use of i) novel nanostructures of Pt, ii) new cost-effective synthesis routes, iii) binary or multiple catalysts, and iv) new catalyst supports.
[] Prof. L.-J. Wan, Prof. Y.-G. Guo, Dr. J.-S. Hu Beijing National Laboratory for Molecular Sciences (BNLMS) Institute of Chemistry, Chinese Academy of Sciences (CAS) Beijing 100190 (China) E-mail: [email protected] [*] This work is supported by the National Natural Science Foundation of China (Grant Nos. 20673121, 20603041, 50730005, and 20701038), National Key Project on Basic Research (Grant Nos. 2006CB and 2006CB932100), and the Chinese Academy of Sciences. The authors thank the scientific community in the field of energy conversion and storage for laying foundations, Professor Chun-Li Bai for valuable advices, and Professor Hong Li for discussion.
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2. Nanostructured Electrode Materials for
Lithium-Ion Batteries
Lithium-ion battery currently represents the state-of-the-art technology in small rechargeable batteries because of its many merits (e.g., higher voltage, higher energy density, and longer cycle life) compared with traditional rechargeable batteries such as lead acid and Ni-Cd batteries. Typically, a lithium-ion battery consists of a negative electrode (anode, e.g., graphite), a positive electrode (cathode, e.g., LiCoO 2 ), and a lithium-ion- conducting electrolyte (Fig. 1a). When the cell is charged, Li ions are extracted from the cathode and inserted into the anode. On discharge, the Li ions are released by the anode and
taken up again by the cathode (Fig. 1a). Although such lithium- ion batteries are commercially successful, especially in small- scale devices, these cells are still objects of intense research to enhance their properties and characteristics, which is largely promoted by the increasingly diverse range of applications they need to power, such as next-generation wireless communica- tion devices (e.g., 3G mobile phones, MP4), EVs, HEVs, power tools, uninterrupted power sources (UPS), stationary storage batteries (SSBs), and microchips. Since no single lithium-ion battery type can meet all the demands of such a large variety of applications, different types of batteries with specific proper- ties for certain applications should be considered, including: i) high-energy lithium-ion batteries for modern communication
Prof. Li-Jun Wan received his Ph.D. in Materials Chemistry from Tohoku University of Japan, joined the Institute of Chemistry of Chinese Academy of Sciences (ICCAS) as a professor in 1999, and is currently director of the institute and director of CAS Center for Molecular Science. His research has centered on physical chemistry, with an emphasis on molecular self-assembly, functional nanomaterial, electrochemistry, and scanning probe microscopy.
Yu-Guo Guo received his Ph.D. in Chemistry from ICCAS under the supervision of Prof. Chun-Li Bai and Prof. Li-Jun Wan. From 2004 to 2007 he worked with Prof. Joachim Maier at the Max Planck Institute for Solid State Research at Stuttgart (Germany) first as a Guest Scientist and then a Staff Scientist. He joined ICCAS as a professor in 2007. His current research interests are centered on the nanostructured materials for advanced energy conversion and storage devices, the size- dependent properties of energy materials, as well as ion/electron transport in nanoscaled systems.
Jin-Song Hu received his Ph.D. in Chemistry (2005) from ICCAS with Prof. Chun-Li Bai and Prof. Li-Jun Wan as his supervisors. He joined ICCAS as an assistant professor in 2005 and was prompted as an associate professor two years later. His current scientific interests are focused on functional nanomaterials for environmental remediation, energy system and electronics.
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from the larger surface area. The former makes full Li diffusion possible within a short storage time, i.e., at high charge/ discharge rates, and the latter greatly reduces the specific current density of the active material. For example, the above- mentioned nanometer-sized rutile TiO 2 with a specific surface area of ca. 110 m^2 g^1 also exhibits an excellent high rate performance (100 mA h g^1 at 10 C and 70 mA h g^1 at 30 C, where 1 C ¼ 336 mA g^1 ), which makes it a promising anode material for high-power lithium-ion batteries.[13]^ These findings have encouraged people to reinvestigate materials that were thought to be electrochemically inactive in bulk form due to poor electronic and Liþ^ conductivity, but that could present improved electrochemical performance at the nanoscale. More examples should present themselves in due course.
2.1.2. Enhanced Structural Stability
Since structural transition to thermodynamically undesir- able structures can only occur if the particle radius r (^) p is larger than the critical nucleation radius r (^) c for that phase, it is possible to eliminate such transitions by using nanoparticles with rp > rc. Thus, small particles would more easily accommodate the structural changes occurring during the cycling process where Li is inserted and extracted. For example, layered LiMnO (^2) suffers from structural instability during cycling and as a result, exhibits significant capacity fade. As a way to overcome such difficulties, nanocrystalline structures have attracted increasing attention, since the lattice stress caused by Jahn–Teller distortion can be accommodated more easily, and hence they exhibit much higher Li-intercalation capacity than their conventional crystalline counterparts. [4] In nanoparticles the charge accommodation occurs largely at or very near the surface and the smaller the particles are, the larger the portion of these constituent atoms at the surface. This fact reduces the need for diffusion of Liþ^ in the solid phase, greatly increasing the charge and discharge rate of the cathode as well as reducing the volumetric changes and lattice stresses caused by repeated Li insertion and expulsion.
2.1.3. New Lithium-Storage Mechanisms
Another benefit of nanostructured electrode materials is that they can lead to new Li-storage mechanisms, affording high capacities, rechargeability, and general applicability to a range of battery systems. One such new mechanism is referred to as a ‘conversion’ mechanism,[14]^ first found in transition metal oxides, followed by fluorides, sulfides, and nitrides.[5–7]^ The mechanisms are mainly related to reversible in situ formation and decom- position of LiyX (where X ¼ O, S, F, or N) upon Li uptake and release, which can be described by the following equation:
MX þ yLiþ^ þ ye^ $ LiyX þ M (2)
where M ¼ Fe, Co, Ni, Cr, Mn, Cu, and so on. Usually, reversible capacities in these systems, which have been demonstrated as innovative high energy anode materials for lithium-ion batteries, are in the range of 400–1100 mA h g^1. It is reported that electrodes made of CoO nanoparticles can
deliver a specific capacity of 700 mA h g^1 with 100% capacity retention for up to 100 charge/discharge cycles and high recharging current rates.[14] In addition to the ‘conversion’ mechanism, the extra Li- storage capacity in nanometer-sized transition metal oxides at low potential has recently been explained by an interfacial Li storage mechanism. [2,8]^ According to this model, Liþ^ ions are stored on the ionic conducting side of the interface (LiyX), while electrons (e) are localized on the metallic side (M), resulting in a charge separation. In view of the fact that the interface area of LiyX/M can be extremely large, this mechanism forms the bridge between a supercapacitor and a battery electrode and may offer a reasonable compromise between rate and capacity. In nanostructured systems, Li surface storage may play an important role in the overall capacity. This mechanism can be energetically more favorable for nanometer-sized particles than for bulk insertion. In the case of rutile TiO 2 , it has been primarily demonstrated that, in addition to the bulk storage of Li (Cbulk ) there is a large part of capacity can be attributed to Li surface storage (Csurface), which is located at the beginning (viz., the sloped region) of its voltage profiles. The former is kinetically favored, as discussed above, while the latter is not a kinetic phenomenon, but appears to be thermodynamically preferable.[13]^ The Csurface of nanometer-sized rutile exhibits no significant changes with varied rates, which may show its potential in novel asymmetric hybrid energy storage cells (which charge and discharge in a few minutes, like a supercapacitor, but exhibits an order-of-magnitude higher energy density than a supercapacitor). A successful example of this is the nanometer-sized Li 4 Ti 5 O 12 used as effective electrodes in activated carbon/Li 4 Ti 5 O 12 cells based on nonaqueous electrolytes. [9] Besides the lattices, surface and interface storage, Li can also be stored in nanopores. Wang et al.[15]^ reported that nanopores (ca. 0.4 nm) in monodisperse hard carbon spherules can store a large quantity of Li. This discovery widens the systems towards mass storage, which may also useful for hydrogen storage. We should also mention the development of novel Li storage materials based on tin alloys, silicon, and inter-metallic compounds based on an alloy mechanism toward Li storage. [10] Work in the area of nanoparticles and nano-composites has been significant. By accommodating the strains associated with the Li-Sn alloying-dealloying reactions, as the volume can expand or contract several-fold, these nano-systems have made it possible to use them.
2.2. Disadvantages of Nano
For material (MX) at nanoscale, an extra contribution form surface free energy should be taken into consideration for the chemical potential, which can be approximately given by
m^0 ðrÞ ¼ m^0 ðr ¼ 1Þ þ 2 ðg=rÞV (3)
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where g is the effective surface tension, V is the partial molar volume, and r is the effective grain radius. Although the excess surface free energy (i.e., 2(g/r)V) contributes to the high electroactivity toward Li storage, it also results in several disadvantages. 2.2.1. Low Thermodynamic Stability One should note that as nanometer-sized particles have very high specific surface area and high surface energy, and tend to form agglomerates, they are therefore difficult to disperse and mix them with carbon black and binder to produce electrodes. Thus, the contact resistance of an electrode using nanometer- sized particles is much higher than that of commercial one, which explains the failing of performance (e.g., capacity fading) frequently happens in some cases. Another drawback of nanometer-sized electrode materials is the pronounced electrochemical agglomeration during electrochemical cycling. For example, nano-SnSb undergoes successive agglomeration during Li insertion and extraction, which consequently leads to quick capacity fading.[16] 2.2.2. High Surface Reactions The high surface area of nanoparticles also raises the risk of secondary reactions involving electrolyte decomposition between electrode and electrolyte, which causes a high level of irreversibility (low columbic efficiency) and poor cycle life. For nanostructured electrode materials, the stabilized electro- chemical windows become narrow compared with bulk materials. In nanometer-sized anodes, thick solid electrolyte interphase (SEI) films commonly form, which consume lots of the Li ions supplied by cathodes. Furthermore, it has been found in many nanometer-sized transition metal oxides, the thick SEI films formed during Li uptake may disappear completely catalyzed by transition metal upon Li extraction, which may lead to capacity fade and safety problem. Residual species such as organic surfactants may also exist on the surface of nanomater- ials. The high level impurities means pronounced secondary reactions that may also lead to serious safety problem. So far, most of the nano-systems studied exhibit a low coulombic efficiency ca. 60–80% during the first cycle, the remaining 20– 40% capacity loss is mainly due to the thick SEI film formation and other Li-consuming surface secondary reactions. It should be noted that nanomaterials usually exhibit a poor packing density of electrodes, which limits the volumetric energy density because there is a larger proportion of ‘inert’ components such as binder or carbon black.
2.3. Strategies for Materials Research
Nanometer-sized electrode materials are favorable in terms of kinetics and capacity, while their practical applications suffer from low thermodynamic stability and high activity towards surface reactions besides handle problems, all of which link to the small size and the high surface area (i.e., excess surface free energy). Therefore, ‘kinetically stabilized’ nano-
materials should be considered, which, in fact, have been developed by using nano/micro hierarchical structures and proper surface coatings. 2.3.1. Nano/Micro Hierarchical Electrode Materials Electrode materials with nano/micro hierarchical structures are the best systems of choice because they can take both the advantages of nanometer-sized building blocks and micro- or submicrometer-sized assemblies. While the former provides negligible diffusion times and possible new Li storage mechanisms and hence is the key to the favorable kinetics and high capacities, the latter guarantees good stability and easy of fabrication. Many strategies have been proposed.
i) Self-assembled nano/micro materials: Self-assembled
nano/micro structure, i.e., a higher level structure
assembled from nanometer-sized building blocks includ-
ing nanoparticles (0D), nanorods (1D), nano films (2D),
is one of the hierarchical structure with great interest in
the field of lithium-ion batteries promoted by the rapid
advancements in synthetic strategies. Many facile
solution-based methods have been reported. For
example, we have developed a mediated polyol process
to synthesize V 2 O 5 with highly ordered superstruc-
tures,[17]^ in which nanoparticles interconnect to form
nanorods, and these rods circle around to form hollow
microspheres (Fig. 2a and b). The self-assembled V 2 O 5
hollow microspheres with uniform hedgehog-like
morphology show desirable electrochemical properties
such as high capacity and remarkable reversibility when
being used as cathode materials in lithium-ion batteries.
ii) Nanostructured composites: Lithium alloys are promis-
ing anode materials due to their higher Li storage
capacity, which provides higher energy density than
commercial Li-intercalated carbons. For example, Sn
can react with Li to form Li 4.4 Sn, giving a specific
capacity of 992 mA h g^1 , which is much higher than
that of conventional graphite (LiC 6 , 372 mA h g^1 ).
Unfortunately the biggest challenge for employing alloy
systems is that they are suffering form huge volume
variation during Li insertion/extraction cycle, which
leads to pulverization of the electrodes and very rapid
capacity decay.
One solution to solve the problem is the active-
inactive nanocomposite strategy, which consist of
‘active’ nanoparticles that can alloy with Li and ‘inac-
tive’ matrix that not only acts as a buffer to relieve the
strain associated with the volume variations of the active
one but also prevents the aggregation of the nanoparti-
cles upon cycling. Particularly relevant in this area is the
work of Derrien et al. [20]^ who have demonstrated that
nanostructured Sn-C composite, i.e., Sn nano particles
dispersed in carbon matrix, can effectively alleviate the
electrode pulverization problem and lead to an advanced
anode material with large capacity of about
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550 mA h g^1 even after 100 cycles, as well as good
cycling performance. The results demonstrate the power
of the strategy of using elastic hollow carbon spheres as
buffer and container and could be extended to other
anode and cathode materials.
iii) Mesoporous materials: There has always been a need for
developing highly porous electrode materials with large
surface areas readily accessible to electrolyte, which, as
a result, reduce the transport lengths for both electronic
and ionic transport and improve the rate capability. In
this context, more emphasis should be laid on synthesis
of optimized pore sizes and connectivity rather than
striving for micropores ordered arrangements. Mesopor-
ous electrodes with pore sizes ranging from 2 nm to
50 nm are currently of interest based on the above
consideration. Even with micrometer-sized particle size
like bulk materials, due to the presence of nanometer-
sized sub-grains inside individual large particle, meso-
porous electrodes usually exhibit better electrochemical
performance than bulk materials. [22]^ Furthermore, the
relatively large particle size enables easy operation in
terms of separation or film-formation and better nano-
particle contact compared with nanometer-sized
particles. However, the electronic conductivity of the
mesoporous electrode remains unimproved.
iv) Hierarchical 3D mixed conducting networks: Though
mesoporous electrode materials have shown enhanced
electrochemical performance compared with bulk
materials due to the ‘ionic wiring’, the rate performance
enhancement of such materials is still limited especially
if the pore-walls themselves are poor electronic con-
ducting. An example of this is the mesoporous anatase
TiO 2 sub-micrometer spheres (particle size ca. 300 nm,
grain size ca. 7 nm, pore size ca. 3–30 nm, BET specific
surface area ca. 131 m 2 g^1 , porosity ca. 48%) [19]^ which
exhibit comparable performance to nanometer-sized
anatase (5 nm TiO 2 ) at low current rates, while at high
current rates above 10 C, the performance becomes
worse. This indicates that for semiconductors like
TiO 2 , the electronic conductivity becomes insufficient
at very high rates, and ‘electronic wiring’ is also needed.
An optimized nanostructure design of electrode materials for high energy and high power lithium-ion batteries is shown to be the introduction of hierarchical 3D mixed conducting networks on both nanoscale and microscale levels through which the effective diffusion length is reduced to only a few nanometers (Fig. 2e). [19]^ The nanoscopic network structure is composed of a dense net of metalized mesopores that allow both Liþ^ and e^ to migrate. This network with mesh size of about 10 nm is superimposed by a similar net on the microscale formed by the composite of the mesoporous particles and the conductive admixture (Fig. 2e). While the nanometer-sized network provides negligible diffusion times, enhanced local conductivities and possibly faster phase transfer reactions, and
hence is the key to the extremely good power performance, the microscopic network guarantees high absolute capacities, easy of fabrication and quick infiltration. [19]^ The hierarchical 3D mixed conducting networks put new insight for large-sized lithium-ion batteries to be used for EVs, HEVs, and SSBs. The power of this concept is demonstrated by the synthesis of mesoporous TiO 2 :RuO 2 nanocomposite which shows supe- rior high rate capability when used as anode materials for lithium-ion batteries. [19]^ It is noteworthy that in addition to the electronic function, RuO 2 or Li (^) x RuO 2 formed during Li insertion also allows for quick Li permeation. It was found that, at the very high rate of 30 C (Discharge/charge of all the TiO (^2) within 2 min!), the specific charge capacity of the mesoporous TiO 2 :RuO 2 nanocomposite is still 91 mA h g^1 , which is about two times larger than that of 5 nm anatase (48 mA h g^1 ) and nine times larger than that of mesoporous anatase spheres without interior electronic wiring (10 mA h g^1 ). [19] The concept is simple, yet very effective, and owing to its versatility, is also successfully extended to other cathode materials such as LiFePO 4. [23]^ A key to its success is, besides the preparation of mesopores, the use of a suitable electronic conductor – here the oxide RuO 2 – that enables favorable surface-surface interactions. RuO 2 is most beneficial as it is, owing to similar bonding properties, expected to spread much better on TiO 2 and LiFePO 4 than carbon would, and thus can efficiently coating the tiny channels in porous TiO 2 and even ‘repairing’ incomplete carbon networks in porous LiFePO (^4) due to the ionic characteristic of both oxides (RuO 2 and TiO 2 ; RuO 2 and LiFePO 4 ). However, the challenge is how to further make the ‘ionic’ and ‘electronic’ wiring down to 10 nm scale especially for cathode materials, and the finding of low cost ionic coating materials to replace RuO 2. Recently reported optimization procedure of Fe 3 O 4 -based Cu nano-architectured electrodes intend for high power performance may be mentioned in the context of conducting networks. [24]^ The use of a 3D current collector network of Cu nanorods is the key to the high rate capabilities, but is naturally not meant for achieving high energy demands due to the limitation of electrode thickness, which is limited by the thickness of the porous alumina template used. It should be noted that, in addition to electrochemical properties, packing density of anode and cathode is also important for higher volumetric energy density. An electrode consisting of homogeneous particle is expected to have a regular network which can maintain a uniform intercalation reversibility of each particle through repeated cycles. So, the morphology uniformity of assembled nanoparticles in terms of shape and size should be paid more attention for the practical application. 2.3.2. Surface Coatings The surface structures of electrode materials are of great importance to their electrochemical performance. In the case of nanostructured electrode materials, the effect becomes more remarkable due to the high surface area as discussed above. Though it has been demonstrated in many cases that by using proper surface coatings remarkable improvements in the
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electrochemical performance including reversible capacity, coulombic efficiency in the first cycle, cycling behavior, and high rate capability can be achieved, intensive efforts are underway to understand all the fine details regarding the surface structures of these materials, their surface chemistry and the correlation between these details and the electro- chemical behavior of resulted electrode materials. The research in this field may be promoted by the novel in situ techniques (e.g., Raman, FTIR, SPM, XRD, and MS) and novel methods such as solid state NMR, neutron diffraction, XANES, EXAFS. With an improved understanding of the natures related, the surface properties of nanostructured electrode materials may be modified by effective coatings, leading to ‘kinetically stabilized’ nanostructured electrode materials in the near future. Carbon coating is one of the most widely used coating techniques for anodes (especially in the cases of transition metal oxides) and cathodes (e.g., LiFePO 4 ). The carbon coating layers can not only significantly enhance the electronic conductivity of electrode materials, but also lead to stabilized SEI films especially on anodes, which result in improved rate and cycling performance. For cathode materials such as LiCoO 2 , LiNiO 2 , and LiMnO 4 , the surface coating with oxides such as MgO and Al 2 O 3 can prevent direct contact of the electrolytes with the cathode materials, improve the structural stability and suppress phase transitions. [12]
3. Nanostructured Electrocatalysts for Direct
Methanol Fuel Cells
Direct methanol fuel cells (DMFCs), as one of the important polymer electrolyte membrane fuel cells (PEMFCs), have been recognized as a potential future power sources for portable electronic devices. However, to become commercially viable, DMFCs have to overcome the barrier of high electrocatalyst cost caused by the exclusive use of Pt catalysts (generally Pt-carbon) for oxygen reduction and Pt-based catalysts (generally Pt/Ru-carbon) for methanol oxidation in the cathodes and anodes, respectively (Fig. 1b). When such a situation arises, it is important to open up new avenues for making low cost and effective catalysts. Until now many avenues that are already opening up are that of i) novel nanostructures of Pt, ii) new cost-effective synthesis routes, iii) binary or multiple catalysts, and iv) new catalyst supports to replace the generally used activated carbons, besides ideally the Pt-based catalysts should be replaced with abundant, non- precious materials. Here, we address these issues mainly by using work of the authors for illustration.
3.1. Novel Nanostructures of Pt
To lower the cost of Pt catalysts, great efforts have focused on the development of novel nanostructures of Pt catalysts with a high surface area to achieve high catalytic performance and utilization efficiency. Various Pt nanostructures such as
nanoparticles, nanowires, and nanotubes have been proposed. Recently we have shown that enhanced electrocatalysts of Pt can be obtained as hollow nanospheres without changing catalyst loading.[25]^ The individual Pt nanosphere (diameter ca. 24 nm) is composed of a porous shell consisting of 2 nm-Pt nanoparticles (Fig. 3a). These features endow the Pt hollow nanospheres with a high surface area which contributes to the high catalytic activity towards methanol electrooxidation. The result suggests a simple route to enhance the catalytic efficiency of Pt catalysts by a simple improvement of the morphology. In the search for novel nanostructures of Pt, high-index facets such as {730}, {210}, and {520} surfaces of tetrahexahedral Pt nanocrystals have been pointed out having high catalytic activity for electrooxidation of small organic fuels such as formic acid and ethanol due to the large density of atomic steps and dangling bonds. [28]^ The large-scale synthesis method of them still remains a challenge.
3.2. New Cost-Effective Synthesis Routes
Another avenue to lower the cost of Pt catalyst is to develop cost-effective routes for making more efficient Pt catalysts. Recently nanoporous catalysts have attracted great interest in the field of catalysis. Most syntheses of nanoporous materials reported so far have focused on template-assisted bottom-up processes, including soft templating and hard templating methods, which are relatively complicated. Recently, the conversion Li storage mechanism occurring in transition metal compounds has been developed into a template-free up-down method of wide applicability for the synthesis of well-crystal- lized materials with favorable nanoporous structures.[29]^ Based on this strategy, nanoporous Pt can be obtained from submicrometer PtO 2 by electrochemical lithiation followed by dissolving the Li 2 O in acidic aqueous solution or even water (Fig. 3b). The synthesis is relatively simple (starting from micrometre-sized transition metal oxides), yet very effective. Owing to the high surface area (142 m^2 g^1 ), the presence of various pore sizes (2–20 nm) and the pronounced stability of the nanoporous Pt, the so-prepared Pt shows outstanding proper- ties when used as an electrocatalyst for methanol oxidation.[29]
3.3. Binary and Multiple Catalysts
Considerable attention has been paid to Pt-based binary catalysts (generally Pt alloys, Pt-M), ternary catalysts (gen- erally Pt-M 1 -M 2 ), and multiple catalysts (generally Pt-M 1 -M 2 - M 3 O) because the systems can not only reduce the cost but also improve the catalytic performance, such as mitigating CO poisoning, lowing overpotential, and suppressing Pt dissolu- tion. For example, we have demonstrated that the electro- deposition of Pt nanoparticles into Sn nanotubes provides a simple and convenient method to design effective Pt-Sn catalyst for DMFC. [26]^ The so-prepared composite nanostructures
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Another approach is by in situ growth methods. Zheng et al.[27]^ reported a one-step in situ method to disperse Pt nanoparticles on CNTs, using H 2 PtCl 6 as a Pt source, ethylene glycol as a reducing agent, and dimethyl formamide (DMF) as a solvent. By this method, well-dispersed Pt nanoparticles can be directly loaded onto the CNT walls (Fig. 3d). So far, many in situ methods have been used, but preparing well-dispersed Pt on CNTs still remains a challenge.
4. Concluding Remarks
It is believed that nanostructured materials will play a more and more important role in improving the performance of electrochemical energy conversion and storage devices, such as Li-ion batteries and DMFCs. On one hand, nanomaterials show favorable properties such as enhanced kinetics and activity, which may lead to low-cost and/or high performance energy devices. On the other hand, ‘nano’ also has disadvan- tages such as low thermodynamic stability, high side reactions as well as handle problems. The remaining challenges include i) well understanding various nano-size effects and developing new theories, ii) investigating fine details regarding the surface features of ‘nano’, iii) designing optimized nano/micro structures and surface modifications, and iv) searching for new synthetic routes and new material systems. Solving these challenges will require researchers from a range of disciplines, and their success will promote the development of next generation green and sustainable energy devices.
Received: March 4, 2008 Published online: July 9, 2008
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