Encapsulation Strategies in Energy Conversion ... - ACS Publications

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Encapsulation Strategies in Energy Conversion Materials Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim, Germany ABSTRACT: Many energy conversion materials show increased performance, if the materials are used in nanostructured form. However, this could be detrimental for stability of the materials, since during cycling the nanostructuring tends to be lost because of particle growth. This problem may be solved by encapsulation of the active material in different types of matrices or coatings, which beyond the stabilization may also provide additional functionality, such as conductivity or mechanical reinforcement. This Perspective covers the general features of encapsulation strategies, and desribes selected examples for different types of energy conversion materials. At the end, promising development lines will be discussed, together with the need for a more systematic study of the effects of encapsulation. KEYWORDS: encapsulation, stability, carbon, battery, catalyst, supercapacitor, hydrogen storage, thermoelectric

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INTRODUCTION With an expanding basis of our energy infrastructure, which has heavily relied, and still is relying, on coal, oil, gas, and nuclear energy, conversion between different types of energy is becoming ever more important. For instance, light is converted to electrical energy in photovoltaic devices and back to light in LEDs, electrical energy is converted to chemical energy and vice versa in batteries or fuel cells, light is converted to chemical energy in water splitting catalysts or related systems, or one form of chemical energy is converted to another form over various types of catalysts. Key to the overall performance of such conversion systems is often a material that effects the primary conversion process, such as the electrode materials in batteries, the donor and acceptor materials in photovoltaic cells, or the catalytic material, which facilitates the transfer of electrons or atoms in (electro)catalysts. While appropriate material parameters themselves are a necessary requirement for good performance, nanostructuring has been identified as an important tool for further improving the performance of many materials used in energy conversion devices.1 This is because nanostructuring leads to increased surface or interface area, relevant in catalytic applications or in systems where charge is transferred across interfaces, and short length scales within one material, which is highly important for charge transport. Moreover, in many energy conversion processes, such as intercalation or deintercalation in batteries, the process leads to volume changes which can be much better accommodated, if the grains are very small so that the stress does not build up to high values. Also thermoelectric materials can profit from nanostructuring, since in such systems thermal conductivity can be reduced which improves the performance of the materials. The beneficial effect of nanostructuring has been covered in excellent reviews for various different energy conversion materials. In the field of battery materials, especially lithium © XXXX American Chemical Society

ion batteries, the challenges and the progress, including the progress in nanostructuring is, for instance, described by Tarascon and Armand,2 by Ellis et al.,3 and specifically for nanomaterials by Bruce et al.4 Kanatzidis has recently discussed the state of the art in nanostructuring thermoelectric materials.5 Also for organic solar cells, nanostructuring is of high importance. Since charge carrier separation is optimal at the boundary between a donor and acceptor material, and the mean exciton diffusion distance in organic semiconductors before recombination is typically only on the order of a few tens of nanometers, it is crucial for high efficiency, that the photogenerated charge carrier pairs have only short distances to travel before they reach an interface to allow efficient separation and thus reduce the probability for recombination. This is, for instance, achieved by nanophase-separated structures, as reviewed by Koch.6 In catalysis, nanostructures have been used for more than a century, but only in the last two decades we have seen a level of control over the structures that one could rightfully label it to some extent “nano-engineering”. Such nanoengineered catalysts are highly relevant in a number of energy related transformations, such as, for instance, in Fischer−Tropsch catalysis where it has been shown that there is a particular cobalt particle size of around 7 nm which is optimal for performance of supported cobalt catalysts.7,8 Solid state hydrogen storage materials also profit from nanosizing of the materials, since this reduces diffusion pathways and possibly also changes thermodynamics of the systems due to the increased contribution of surface energy.9 Special Issue: Celebrating Twenty-Five Years of Chemistry of Materials Received: August 19, 2013 Revised: October 1, 2013

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dx.doi.org/10.1021/cm402791v | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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Figure 1. Major functions of encapsulation for energy conversion materials.

inorganic films.11 While this is an extremely important topic for the practical applicability of organic photovoltaic cells, the materials chemistry questions are rather different compared to encapsulation on the nanoscale, and thus, this type of encapsulation on the macroscale shall not be considered further in this Perspectives article. In the following, first the general approaches to encapsulation are discussed, together with the most important functions which the encapsulation provides and of the most suitable materials. Then selected examples for successful encapsulation will be introduced, starting with encapsulated catalysts for syngas conversion and for fuel cells, followed by encapsulated hydrogen storage materials, electrode materials for lithium ion batteries, and pseudocapacitors, and finally encapsulated thermoelectrics. The paper will close with a perspective on future developments and open questions.

As advantageous as nanostructuring may be in energy conversion materials, it is also associated with problems. Due to the higher surface energy contribution in nanosized materials, they have a tendency to agglomerate to reduce the surface energy, which typically leads to deterioration of performance. Since pronounced aging effects are always undesirable in technical devices and systems, one often does not fully exploit the performance gain which could be achieved with a fully developed nanostructure, but compromises between good performance and long lifetime. One way to counteract this effect is the encapsulation of the nanostructures by a coating or a matrix, which is ideally inert or even functional, and which prevents agglomeration of the nanomaterials. For this, the nanomaterials are either formed directly in a confining matrix, or the prefabricated nanomaterials are postsynthetically covered with protective shells. Such encapsulation strategies have proven to be effective in a number of different energy conversion materials, and selected examples will be highlighted in the following. Moreover, the general features of encapsulation strategies will be discussed, and it will be attempted to highlight future potential and development pathways of such approaches. Encapsulation of energy conversion materials is not only important on the nanoscale, but also on the macroscale. For instance, organic photovoltaic cells suffer from degradation processes brought about by reaction of the materials with oxygen or moisture, 10 and this can be prevented by encapsulation of the active layer in polymeric, inorganic or hybrid barrier coatings. Often the simplest way is covering the active layer with glass, and in this case, edge sealing is the most important problem. There are, however, more sophisticated approaches in which barrier coatings made of polymers, inorganic materials, or hybrids, directly applied onto the active layer, are used to prevent access of oxygen or moisture to the active layer. Hybrid layers are often most efficient, since polymers alone do not have sufficiently low permeability, while inorganic layers are often defective, with the defects extending through the whole layer. In a composite film, the inorganic/ organic sandwich structure leads to interruption of defects in the inorganic layer so that they do not channel all the way through the film structure, while the permeability of the organic layer is less of a problem due to the tortuous pathway the gas molecules have to take to diffuse through the defects in the

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GENERAL ASPECTS OF NANOSCALE ENCAPSULATION The encapsulations should fulfill a number of different functions (Figure 1), i.e. maintaining nanoscale structuration under harsh conditions and/or high temperatures, while still providing access of electrons and/or ions or molecules to the active phase. In addition, for many applications, such as in batteries and supercapacitors, gravimetric and volumetric specific performance is key. Since the encapsulation does not contribute directly to the energy conversion functionality, it adds weight and volume, thus reducing normalized performance. The encapsulation therefore needs to be lightweight and should typically be thin so that not too much material is added. Because of these reasons, the choice of materials is somewhat restricted. However, for each system the requirements are different so that there is some variability in the encapsulating materials which have been used. Knowing the desired functions of the encapsulating materials, one can derive the required properties: (i) chemical inertness is important, both with respect to reactions with the functional materials to be coated, and with the environment in order to guarantee integrity of the coating over extended periods of use. (ii) The bulk density should be as low as possible to allow high weight specific performance, but (iii) the encapsulation should also fulfill its function with as little materials volume as possible, since for many energy conversion systems good volumetric B



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Figure 2. Generic encapsulation pathways. 1. Active phase is coated directly with encapsulating species (direct pathway), possibly using a primer layer, and/or via a precursor for encapsulation, for instance a polymer for encapsulation in carbon. 2. Filling of a preformed porous matrix either directly or via a precursor with the active species. 3. Melt solidification of a multicomponent melt which solidifies under inclusion of encapsulated nanoparticles. In most cases, complete filling of the encapsulated space is desired in order to maximize volume-normalized performnce, but in some cases, as for catalysts, this is not required.

performance data are important. (iv) For many applications, the melting point must be high. (v) Since electrons are transferred to or from many energy conversion materials, some electronic conductivity is important. However, because the encapsulation is normally rather thin, typical values of several hundred S/cm, as found in amorphous carbons, are more than sufficient. (vi) For some applications, transport of ions and/or molecules is required, and such materials thus provide pathways to allow this. (vii) The material is ideally highly elastic to accommodate possible volume changes of the encapsulated solid. Encapsulation is possible via different pathways (Figure 2). The most perfect encapsulation is probably achieved, if the nanomaterial is prepared first and then a coating of the encapsulating material is applied (pathway 1). This allows complete enclosure of the nanomaterial − although it might hinder exchange of molecules. Alternatively, an encapsulating matrix is created first, and then the nanomaterial is formed within this matrix or introduced into it. While in some cases this can also lead to almost perfect encapsulation, for instance, if the pore system is only accessible via few and narrow openings, the encapsulating matrix is mostly porous, which means that enclosure of the nanomaterial is imperfect (pathway 2). However, for many applications this type of encapsulation is sufficient to allow sufficiently long lifetime of an energy conversion device, sometimes such imperfect encapsulation is even required, if matter − and not only charge − has to be exchanged across the encapsulating matrix. Pathway three is a special case. It has been found useful for the creation of thermoelectrics with low thermal conductivity, where the

encapsulated nanoparticles lead to strong phonon scattering, see below. The most versatile encapsulating matrix seems to be carbon in its various forms. The use of carbon and carbon nanocomposites has recently been reviewed, and also encapsulation approaches are covered in this publication.12 Carbon has many advantages which make it suitable for application in energy conversion devices. It is relatively lightweight (helium density around 2 g/cm3, depending on carbonization conditions),13 chemically resistant except under harsh oxidizing conditions, it can withstand very high temperatures in nonoxidative atmosphere, has a moderate electronic conductivity, is predominantly elastically deformed so that it can accommodate mechanical forces without damage up to a certain threshold,14 can be made porous, is obtained from a variety of different precursors, and the precursors are easily shaped, because they are mostly organic polymers. This, however, is also one major disadvantage: Since the precursors are polymers, they normally need to be converted to carbonaceous material by a pyrolysis process, which requires temperatures typically exceeding 500 °C; often temperatures for carbonization are even substantially higher. If the material to be encapsulated does not withstand such high processing temperatures, the encapsulating carbon matrix has to be prefabricated and the active species is afterwards filled in the pores of the carbon matrix. However, in this case the encapsulation is not perfect, since the pores need to be accessible from the outside; otherwise the loading would not be possible. In fortunate cases, the carbon can even contribute to the function of the energy conversion material itself. Microporous carbon is, for instance, a good material for electrical C



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feed than if methanol were first synthesized in a separate step, requiring the 1:2 ratio. This is advantageous, if dimethylether (DME) shall be synthesized from biomass, since biomass gasification results in a syngas feed with a CO:H2 ration close to 1.

double layer capacitors.15,16 In combination with a pseudocapacitor, for instance by encapsulating a redox-capacitor in carbon, both materials properties could fulfill their capacitor function, while the carbon encapsulation in addition would provide a stabilizing matrix. Beyond carbon, there are also other interesting encapsulating materials. If thermal stability is not a crucial problem, such as is often the case with supercapacitors and also batteries, polymeric shells or matrices are attractive. They have a low density, can be brought into arbitrary shapes, and can be electronically or ionically conductive, depending on the nature of the polymers. Often a polymeric coating is the precursor for a carbon coating, or polymeric coatings, such as poly(vinylalcohol), PVA, or poly(vinylpyrrolidon), PVP, are used as buffer or primer layers to induce or allow growth of another layer on the original material. Silica is the most often used oxidic encapsulation material. This is due to the easy processability of silica for which the sol− gel chemistry is very well-known and easily controlled; however, due its insulating nature, silica may not be optimal in applications where conductivity is required. Silicon with its relatively low partial charge and the lack of participation of dstates in hydrolysis/condensation reactions17 reacts with moderate rates from many of its compounds, so that the systems have time to form well developed layers. In contrast, many of the transition metal oxide precursors react much more rapidly, so that often oxide formation occurs also in the solution and not only on the surface of the materials to be coated. Use of strongly coordinating ligands is a remedy to this problem, but protocols mostly need to be carefully adapted to each system. It is frequently advantageous, if the initial nanoparticles are coated first with a primer layer to induce nucleation exclusively on the surface of the particles to be covered. Similar surfactants as in the case of metal particles are used, but also fatty acids were found to be useful for this purpose.

2CO + 4H 2 ⇄ 2CH3OH

(1)

2CH3OH ⇄ CH3OCH3 + H 2O

(2)

CO + H 2O ⇄ CO2 + H 2

(3)

overall: 3CO + 3H 2 ⇄ CH3OCH3 + CO2

(4)

The one-step DME synthesis can be catalyzed by a mechanical mixture of a methanol synthesis catalyst and an acidic etherification catalyst. It has, however, been argued, that encapsulation of the methanol synthesis catalyst in an acidic shell would both stabilize the catalyst and be beneficial for the reaction, due to the close proximity of the different sites and the sequential nature of the process on a microscopic scale. Such a structure has not been realized on the nanoscale, yet. Yang et al.18 succeeded in coating of millimeter-sized pellets of a CuZnO-Al2O3 methanol synthesis catalyst with a dense layer of a ZSM-5 zeolite (the metallic copper is only formed during a formation step in the reactor and thus not present during the coating with the zeolite). This catalyst showed superior selectivity compared to the mechanically mixed system. We have incorporated Cu-nanoparticles in the pore system of a mesostructured γ-alumina matrix.19 Encapsulation is not perfect in this system, and the copper loading is relatively low, so that the performance was good, but not exceptional. It would be very interesting to see the performance of a catalyst composed of copper nanoparticles as a core material, covered by a thin zeolitic layer, in this reaction. In principle, synthesis of such a system should be possible by embedding copper-nanoparticles in a thin silica shell which could then be converted to a zeolite by impregnation with a template and hydrothermal treatment, or by growing zeolites around copper nanoparticles which would need to be protected by carbon or another coating against dissolution by the zeolite synthesis mixture. While DME is an interesting, potentially renewable fuel, the Fischer−Tropsch reaction, invented approximately 90 years ago in our institute, appears to be the more versatile reaction for the production of synthetic fuels from any carbon source. Since this reaction is essentially a chain growth process, the chain length distribution is the result of a statistical process, governed by the ratio of chain growth and termination probabilities, resulting in the so-called Anderson−Schulz−Flory (ASF) distribution. In many cases, one is interested in only a certain fraction of this product slate, and thus, methods to shift this distribution are highly desirable. A functional encapsulation of the Fischer− Tropsch-active metal (mostly cobalt or iron) could provide such a selectivation effect, if cracking/isomerization/sieving activity were integrated in such a shell. Zeolites could provide this function, and thus, core−shell systems with a COhydrogenation core and a zeolite shell have been investigated to this effect.20,21 Bao et al.21 have coated a Co/Al2O3 pellet with a thin zeolite beta layer. This shifts the selectivity of the Fischer−Tropsch reaction completely away from the ASFdistribution and results in formation of a high fraction of isoparaffins in the region up to C12. Similar results were obtained in the group of Kapteijn, who states that close proximity

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SELECTED EXAMPLES FOR NANOSCALE ENCAPSULATION OF ENERGY CONVERSION MATERIALS In the following, several examples for encapsulation in order to improve the performance of energy conversion materials will be given. It is not attempted to comprehensively review the literature, since this would not be possible in such a vast field. Instead, typical application fields will be addressed, and prototypical cases will be highlighted, stating the problems that are encountered in these fields, and how encapsulation of the active materials can help to solve them. Encapsulated Catalysts for Synthesis Gas Conversion. Encapsulation is an interesting strategy to improve the performance of different types of catalysts for syngas conversion, that is, the Fischer−Tropsch reaction and the direct synthesis of dimethylether. Here, the function of the encapsulating layer goes beyond a stabilization of small particles. Instead, additional functionality is induced by a suitably tailored shell. In dimethylether synthesis, a bifunctional catalyst is required, since in a first step the hydrogenation of CO needs to be catalyzed, in a second step, the acid-catalyzed etherification reaction is carried out (reactions 1 and 2; the water gas shift, reaction 3, easily proceeds on the methanol synthesis function). Overall, a one step process is advantageous, since it can overcome the equilibrium limitation of the methanol synthesis and can make use of lower CO:H2 ratios in the synthesis gas D



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between the Fischer−Tropsch catalyst and the zeolite functionality improves performance.22 However, as in the case of DME-synthesis, these coatings are rather on a macroscopic than on the nanoscopic level. Whether direct incorpation of Fischer−Tropsch active metal particles within zeolite crystals or a thin zeolite coating on cobalt or iron nanoparticles would improve the systems further is an interesting question which can hopefully be answered in the coming years. One should also bear in mind that the pore system of the zeolite could be blocked by waxes or coke, which could be a severe disadvantage. Encapsulated Fuel Cell Catalysts. Fuel cells are one of the most promising energy conversion devices, since they are intrinsically more efficient than internal combustion engines, although real efficiencies are still somewhat away from the theoretical limits, that is, about 50% on a systems level. Nevertheless, many car companies are planning market introduction of proton exchange membrane (PEM) fuel cell cars for the next years. One of the problems in PEM fuel cells is the high consumption of platinum. Platinum loadings of the fuel cell catalysts are typically in the range of several ten percent. This is partly necessary to guarantee high activity, partly because deactivation is one problem which reduces activity over time. In order to provide sufficient residual activity also after deactivation, the initial loading needs to be high. There are three major deactivation pathways of platinum-based fuel cell catalysts:23 platinum particles can grow and thus loose surface area, they can dissolve, and they can detach from the carbon support. Encapsulation can help to reduce the effect of all three deactivation processes, even if they can not be fully suppressed. One of the early publications on a catalyst which provided some encapsulation of the platinum particles reported the use of CMK-5 ordered mesoporous carbon, a hexagonal array of amorphous carbon tubes prepared by a hard templating pathway.24 However, while in that publication high mass activity was reported, the effect of this matrix on stability of the electrocatalyst, which is as important as activity for practical applications, does not seem to have been investigated for this catalyst class. Carbon nanotubes, which are a very frequently used fuel cell catalyst support, could in principle provide an encapsulating matrix. However, in most cases the noble metal particles are deposited on the outside of the nanotubes,25 which function primarily as a relatively stable, high surface area support. Encapsulation of platinum within the tubes could reduce accessibility of the catalyst particles, and the onedimensional structure of the nanotubes is unfavorable for good mass transfer. This negative effect is to some extent reduced by encapsulation of the noble metal catalyst particles in nanotubes of other materials, which provide additional mesoporosity. Metal particles are deposited on titanate sheets, then curling of the sheets is induced by hydrothermal treatment in the presence of base (Figure 3). This process leads to encapsulation of the metal particles previously deposited on the surface of the titania sheets. Stabilization of the activity in methanol electrooxidation is claimed for palladium or gold based catalysts, although only 25 potential cycles were investigated in this study.26 It is, however, questionable, whether such materials would have sufficiently high electronic conductivity for fuel cells under practical operating conditions. We have recently worked on a strategy which has proven to be very useful and flexible for the encapsulation of noble metal fuel cell catalysts. The strategy relies on the synthesis of a

Figure 3. Schematic process for encapsulation metal cluster in titania nanotubes. Reproduced from ref 26 with kind permission. Copyright 2009 Elsevier.

hollow shell mesoporous graphitized carbon support by a hard templating process, followed by incorporation of noble metal nanoparticles in the strongly confining pores of the support.27 The general synthetic principle is sketched in Figure 4. First dense silica spheres are synthesized by a Stöber-type process. These dense spheres are coated by a mesoporous silica layer by condensating a mixture of octadecyltrimethoxysilane and tetraethoxysilane onto the initial spheres. Calcination removes the octadecyl-groups, creating pores in the small mesopore size range. This pore system is impregnated with an iron-compound as graphitization catalyst and then filled with a carbon precursor, which is polymerized in the pore system. After high-temperature treatment and removal of the silica with NaOH solution, hollow graphitic spheres are obtained which have a high mesoporosity with a narrow mesopore size distribution. Impregnation with platinum-salt solution, reduction and a final high-temperature treatment at 800−900 °C results in a very stable electrocatalyst with platinum particle sizes in the range of 4 nm. Under electrochemical cycling conditions, this catalyst outperforms standard materials by a wide margin with respect to stability, since the encapsulation of the particles in the mesopores largely prevents their detachment, one of the primary deactivation pathways. The encapsulation strategy can be extended further to the formation of alloy nanoparticles via confined space alloying. If not a single metal precursor is introduced in the support pore system, but instead two different metals, they can form an alloy at high temperatures without excessive particle growth, since, like in the case described above, the particles are protected against each other by the confining matrix. Following this pathway, PtNi alloys of different composition have been produced in the pore system of the hollow graphitic spheres. The resulting catalysts are very stable and have extraordinary mass activities of around 1 A/mgPt which exceeds the 2020 DOE targets by more than a factor of 2.28 The synthetic protocol to arrive at these catalysts is rather complicated, as is often the case with encapsulation strategies, because these are usually multistep procedures. However, continuous synthesis is a development line which could simplify the production of the catalysts and make it economically viable, after all, the platinum content in fuel cell catalysts is on the order of several ten percent which means that relative to production costs material costs are the most relevant factor. If one could only slightly reduce the required amount of noble E



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Figure 4. Synthetic principle for the synthesis of platinum-loaded hollow graphitic spheres.

state, nanosizing could have a stabilizing or destabilizing effect on the hydride. Since hydrogen uptake and release is typically associated with substantial changes of the materials, involving mass transfer and formation of new phases (for instance, MgH2 ⇆ Mg + H2, or: 3NaAlH4 ⇆ Na3AlH6 + 2 Al + 3 H2 as a first step and Na3AlH6 ⇆ 3 NaH + Al + 1.5 H2), it would be extremely difficult to retain the hydrogen storage material in a nanosized state without additional precautions. Here encapsulation is a strategy which could help to improve cycle life of such nanostructured systems.9,34 This beneficial effect had been claimed by us about 10 years ago in a patent application,35 and subsequently quite a number of papers has appeared in the open literature exploiting this principle (for a recent survey, see ref 33). For the encapsulation of hydrides, the choice of the host matrix is highly important. Metal hydrides are highly reactive species, and silanol groups in silica typcially react with the hydrides, which can substantially reduce the storage capacity, which is lower compared to the pure hydrides to begin with, due to the weight and volume of the matrix. This is the reason why by now basically all encapsulated hydrides rely on carbon as matrix material. However, also oxygen-containing surface functionalities of high surface area carbon can lead to undesired reactions with the hydrides. It is thus important to anneal the host matrix prior to use in order to passivate the surface as much as possible, or remove the oxygen functionalities by other methods. In addition to chemical inertness, the encapsulating material should not completely enclose the hydrogen storage material, since efficient transfer of hydrogen to and from the hydride is necessary. Porosity is therefore required to some extent. This, however, counters the stabilizing effect, since the hydrogen storage compound may leave the matrix through the pores. A suitable balance between the confining effect and sufficient mass transfer thus has to be found. The improvement of de- and rehydrogenation kinetics has consistently been reported in publications on encapsulated hydrides for hydrogen storage. This is no surprise, since the small dimensions, typically on the order of 10 nm, leads to short diffusion pathways. Proving experimentally a thermodynamic effect of the nanostructure encapsulation has proven to be much more difficult. However, by now this has also been achieved: For encapsulated NaAlH4, which as a bulk material decomposes in two steps via the hexahydride, the decom-

metal by effective encapsulation, even rather complex synthetic protocols could be justified. The possible benefit of encapsulation does not need to stop at the stabilization effect of the catalyst particles. Very critical for fuel-cell operation is proper design of the three-phase boundary at which the electron transfer reaction occurs. Judicious choice of the design of this boundary can help to optimize it. Partial encapsulation of Pt/C with poly(acrylic acid) and Nafion by electrospinning is such a method, by which at the same time stabilization and increase of electronic and proton conductivity was achieved.29 So far the confining matrix in electrocatalysts has been restricted to carbon, with the exception of the case discussed above, where titania-based nanotubes have been used. However, in principle, the hard templating pathway is rather versatile for the production of a wide range of materials,30,31 and thus extension to other interesting compositions,32 which are sufficiently conductive for electrocatalytic applications and sufficiently stable under the harsh conditions of a working fuel cell seems feasible. Encapsulated Hydrides for Hydrogen Storage. While fuel cells are highly interesting systems for mobile applications beyond the internal combustion engine, storage of hydrogen on board of cars remains a challenge which is probably as big as that associated with the fuel cell itself. Many different options have been discussed,33 but the 700 bar high pressure tank so far remains the benchmark. In addition to the various types of high surface area porous materials, which form the basis for sorptive storage systems, different hydrides have been discussed as potential hydrogen storage materials. Unfortunately, there are numerous boundary conditions the hydrides have to meet, among them sufficiently high gravimetric and volumetric storage densities, favorable thermodynamic properties and suitable kinetic performance.32 While gravimetric and volumetric storage densities are to a first approximation an intrinsic property of the hydrides, both the thermodynamic and kinetic properties can be influenced by nanostructuring.9,1 The shortened diffusion pathways associated with nanostructured materials enhance the rate of de- and rehydrogenation, and the thermodynamics could be altered, since in nanoparticles the surface energy contributes significantly to the overall energy of the system. Depending on the sign of the difference of the surface energies of the hydrogenated and the dehydrogenated F



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Most of the commercial cathode materials rely on oxide compositions, with LiCoO2 as the prototypical example. However, also metal compounds with complex anions, such as LiFePO4, have attracted substantial interest.3 LiFePO4 is attractive, because it is composed of abundant and cheap elements, intercalates Li at high potential of 3.5 V, has a high gravimetric storage capacity with 170 mAh/g, and is stable under battery operation conditions.3 However, LiFePO4 is a bad electronic conductor, with a conductivity of only 10−7 S/ cm at room temperature in the bc-plane (extrapolated from Arrhenius plots) and by about 1 order of magnitude lower in adirection.42 Also lithium diffusion in the material is relatively slow, with diffusion coefficients on the order of 10−11 cm2/s extrapolated to room temperature.40 Thus, in order to achieve good performance of this compound as electrode material, nanosizing (typically on the order of 100 nm) of the material is highly beneficial.43 However, for practically relevant rate capabilities, it seems necessary to coat the phosphate nanoparticles by a very thin conducting layer. Carbon has often been reported as a highly suitable encapsulating layer for LiFePO4.44−46 It is typically created in a firing step by carbonization of either carbon-containing anions of the precursor salts, or deliberately added organic compounds. A high degree of graphitization is helpful for creating well performing cathodes.44 Typcially, the processing temperatures for LiFePO4 powders are too low to achieve substantial graphitization. However, here the nature of the material may come conveniently: iron compounds are known to be good graphitization catalysts, and thus, with proper choice of precursors, the conductivity of the carbon coating can be increased to sufficiently high levels. The thin carbon layer, only a few nanometers thick, also seems to have additional functions beyond improving the electronic conductivity: it is thought to remove impurities by a carbothermal reduction, and it improves the ordering of the normally disordered surface layer.41 Together, these effects are responsible for the improved performance of LiFePO4 cathode particles encapsulated in a thin carbon layer. Fortuitously, the typical synthetic protocols result in a sufficiently thin and conductive layer, since thicker carbon layers would reduce both the gravimetric and volumetric capacity of the cathode materials. An interesting approach to reduce the carbon layer thickness is a “self-controlling” oxidative polymerization of poly(aniline) on the surface of forming FePO4. The polyaniline in turn restricts the growth of the FePO4 particles.47 After carbonization, a 1−2 nm thick, partly graphitic shell is formed. The resulting composite has excellent capacity and cyclability. With respect to processing, electrospinning is a promising technique, since it allows continuous synthesis. The group of J. Maier48 has succeeded in encapsulating single crystalline LiFePO4 in a carbon layer by such an approach, and the resulting material had very good capacity and rate performance. Also surface layers with a different nature were reported to improve performance of LiFePO4. However, these layers are typically created by, sometimes adventitious, surface chemical reactions, which convert part of the material to another compound, and it is thus a question of terminology, whether one would consider this as an encapsulation. Under the reducing conditions used in the processing of LiPO4, FeP, and Fe2P have been observed as products of such a surface reaction, along with Li3PO4.49 The presence of the conducting Fe2P significantly improved the performance of the material. This study also revealed another important aspect of such

position proceeds in only one step which is smeared out over a broad temperature range, probably due to the presence of different particle sizes of the encapsulated hydride.36−38 Also for MgH2 which has a high gravimetric storage density of 7.6 wt. %, but unfavorable thermodynamic properties (hydrogen pressure of 1 atm around 280 °C), has been encapsulated as nanosized particles in carbon matrices. While again the improved desorption rates are undisputed, the effect of the encapsulation/nanosizing on the thermodynamic properties is still a matter of discussion.9 Beyond carbons, recently metal organic frameworks (MOFs) and polymers have been used for the incorporation of nanosized metal hydrides and complex hydrides (reviewed in ref 33). Enhanced kinetics are again undisputed, but whether the nanosizing/confinement has an additional thermodynamic effect is still an open question. The major intended effect of the encapsulation, however, is not the change in properties; this could be achieved by nanosizing alone. Maintaining a stable high dispersion is the major challenge. This does not seem to have been studied in much detail, though. From the data available, however, it appears that the stabilization is not perfect − not fully surprising considering that the requirement of accessibility to hydrogen does not allow complete encapsulation. After decomposition of NaAlH4, often large aluminum particles are detected by XRD, the size of which exceeds the size of the pore system, and which therefore have to be located outside of the matrix.35 Reintroducing this aluminum would be difficult, and its formation thus leads to a permanent loss of capacity. The capacity of NaAlH4 confined in the pores of a MOF was reduced to 3.6% after four cycles, from an initial capacity of 4.1%.39 Considering the demanding boundary conditions for hydrogen storage materials with respect to storage capacity and cyclability, and the fact that any encapsulation would decrease gravimetric and volumetric storage capacity further, because of the “dead “ weight and volume of the encapsulating material, such approaches will probably not find practical applications. This might be different, though, if the encapsulating material would be a hydrogen storage material with substantial capacity itself, such as a MOF. Then the capacities of both components could add up. A first such material has been reported, although hydrogen could not be released reversibly; instead, the other framework constituent, pyrazine, was hydrogenated to piperazine.40 Nevertheless, even if hydrogen could be reversibly released, this imposes an additional challenge, since storage in hydrides and in MOFs occurs under rather different conditions (elevated temperatures typically for the hydrids versus 77 K for MOFs). Whether these can be made to match remains to be seen. Encapsulation of Electrode Materials in Li-Ion Batteries. The performance of Li-batteries with respect to power and energy density relies to a large extent on the electrodes, since they account for a major fraction of the weight and the volume of the battery, with the positive electrode material typically comprising around half of the total weight of the battery.41 Nanostructuring is beneficial for both the anode and the cathode, and there are different ways to create electrode nanostructures.4 However, to further improve performance, additional encapsulation strategies have been employed with the goal to improve charge and ion transfer rates as well as to prevent mechanical degradation of the electrode materials. G



Perspective

Figure 5. Principle for encapsulation of Li2S for use as cathode material in lithium−sulfur battery. Reproduced with kind permission from ref 51. Copyright 2013 Elsevier.

standard cathode materials, nevertheless, the overall energy density of lithium batteries could be improved with higher capacity anodes. In principle, there are interesting lithium-alloys which could achieve much higher capacities, such as Li4.4Sn (993 mAh/g), Li4.4Ge (1600 mAh/g), or Li4.4Si (4200 mAh/ g).4,52 However, the alloy formation is typically accompanied by large volume expansion, which upon cycling eventually leads to electrode failure, because of cracking, and finally pulverization. Nanostructuring helps to better accommodate the stresses caused by the dramatic volume changes, and an additional encapsulation of the silicon particles keeps the nanostructures intact and also helps to accommodate the stress. Moreover, since carbon also intercalates lithium, composite electrodes combine the storage effect of the alloy component and the carbon, as early on realized by Wilson and Dahn.53 However, their CVD method with SiCl4 and benzene as precursors is not viable for large scale, cost-effective production. Solution-based methods provide better perspectives, and there is a number of different approaches published. One of the simplest seems to be a hydrothermal carbonization processes in the presence of silicon nanoparticles.54 For this, silicon particles with sizes in the range of 20−50 nm are dispersed in glucose-containing water, and the mixture is hydrothermally treated at 200 °C for 12 h. A final carbonization step results in formation of a carboncoating on the silicon nanoparticles; between silicon and carbon, an SiOx layer is present at the interface which provides good bonding between the core and the shell (Figure 6). However, cycling in a vinylene carbonate containing electrolyte is required to obtain an electrode with good cyclability and reversible capacity exceeding 1000 mAh/g after 60 cycles. This is a substantial improvement over previous approaches. Inspecting the micrographs, though, the encapsulating carbon layer appears to be rather thick, so that the volumetric capacity of this anode material may be less favorable. In a recent publication, similar approaches relying on silicon nanoparticle coating by carbonization of sugars have been described, in which even higher reversible capacities of 1700 mAh/g were reported.55 Also in that publication the necessity of vinylene carbonate was reported which seems to be an essential ingredient for good encapsulation and passivation of the carbon layer.49 A good compromise between encapsulation and packing density was developed for Sn encapsulated in carbon microfibers. Here an electrospinning approach was used, and the resulting material had excellent performance with respect to reversible capacity, cycling stability and rate performance.56,57 An interesting approach relies on the conversion of the stabilizing ligand layer of Ni2P nanoparticles to carbon by a

encapsulating layers: the cathode showed a pronounced conditioning behavior, which was attributed to the formation of microcracks in the phosphide layer, required for efficient lithium migration. The conventional Li-insertion electrodes, in which not much change of the host structure occurs, have a limited capacity due to the high molecular mass of the host material as compared to the low-weight lithium which essentially carries the energy. A much higher theoretical capacity can be achieved, if a compound reacts with the lithium to form a different material altogether. Li-sulfur electrodes are a prominent example for such a reaction, where the lithium reacts with sulfur to form, at the theoretical end point, Li2S. Also this type of cathodes benefits from efficient encapsulation strategies, since the lithium−sulfur batteries suffer from a number of disadvantages: both the sulfur itself and the resulting lithium sulfide species are poor electronic conductors; in addition, the polysulfides, which are intermediates in the reaction, are soluble in the electrolyte. They can then directly react with the anode and eventually lead to reduction in capacity. Both problems are, at least partly, solved by encapsulation of the sulfur into a nanostructured carbon.50 In the reference, sulfur was melt-impregnated into CMK-3, an ordered mesostructured carbon related to the CMK-5 mentioned in the fuel cell catalyst section. The carbon matrix provides both sufficient conductivity and minimizes the migration of polysulfide species out of the matrix. While the retention of the active mass in the carbon matrix is not perfect, it is far superior to acetylene black used as a standard additive to the sulfur. While after 30 cycles 96% of the active mass was lost to the electrolyte with the carbon black additive, it was only 25% after 30 cycles for the CMK-3 matrix. The CMK-3 matrix does not provide full encapsulation, but this can be achieved, if individual lithium particles were carbon coated. This was found to be not possible by a direct coating process, but success was achieved by coating in effect the discharge product of the battery, Li2S, with polyacrylonitrile as carbon precursor. On charging, volume shrinkage instead of volume expansion occurs, which can be better accommodated (Figure 5).51 However, also this encapsulated system was far from perfect, still, since over 50 cycles substantial loss of capacity was observed. Nevertheless, encapsulation seems to be a promising method for the improvement of the cathode for lithium−sulfur batteries. For the anode of lithium batteries, graphite is the standard host material, into which Li is intercalated. However, in the ideal case graphite intercalates lithium up to a stoichiometry of LiC6, which limits the theoretical capacity to 372 mAh/g; in practice, the capacities are lower. This is still more than for the H



Perspective

Indeed, for EDLCs encapsulation does not seem to provide advantages. High surface areas and suitable pore size for electrical double layer formation are key.60 For pseudocapacitors, on the other hand, encapsulation may be a useful tool for improving performance under certain circumstances. Since the redox reaction is confined to the surface region of the oxide, capacity scales with surface area to a first approximation, and nanosizing the pseudocapacitor material is thus key to enhance capacity.61 However, the reactions occurring on the surface can also lead to dissolution62 or coalescence of the nanoparticles, especially during further processing.55 In addition, the oxides (except RuO2) have low electronic conductivities which results in inferior performance at high charge−discharge rates.63 Encapsulation in a suitable coating, which would provide a conducting framework and stabilize small particles, could alleviate these problems. In spite of these possible benefits, such approaches do not seem to have been used much so far. Small pseudocapacitor particles are mostly supported on carbon materials without pronounced encapsulation.64,65 This can improve system performance significantly. However, it is unclear whether such composites are as stable in cycling as capacitors relying only on charging and discharging of the electrical double layer. There are some approaches to encapsulate pseudocapacitors in carbon matrices. MnO2 in finely dispersed form has been embedded in the pores of CMK-3 ordered mesoporous carbon.66 The composite has a specific capacitance of 200 F/ g, but some fading (8% capacity loss) is observed over 1000 cycles, the reason for which is unclear. It would be interesting to see the stability behavior of a similar MnO2 deposited on the external surface of a reference material. While in this case the nanoparticles were not fully encapsulated, but only partly embedded in the walls of a nanoporous carbon framework, there are few reports on the beneficial effect of true encapsulation. Polymers appear to be the materials of choice for such applications. Balan et al.67 have deposited 2−3 nm sized RuO2 nanoparticles via a polyol process on the external and internal surface of hollow carbon nanofibers. While this is interesting, it corresponds to the state of the art for many pseudocapacitor materials. However, a second step went beyond this by encapsulating the carbon fiber/RuO2 composite in the proton conducting phosphoric acid doped polybenzimidazole. TEM analysis suggests encapsulation of the RuO2 particles both on the outside and the inside of the carbon fibers. This encapsulation improved the capacitance up to a maximum of 1262 F/g at a medium thickness of the coating; with more polybenzimidazole, the capacity decreased again. This was attributed to the beneficial effect of the polymer coating on proton/ion conduction, while at too high loading, the diffusion resistance gained influence. The encapsulation also significantly improved the stability over 1500 cycles, but the exact reason for this stabilization remained unclear. A similar stabilization was also achieved with a layered double hydroxide (LDH) encapsulated in poly(3,4-ethylenedioxythiophene) (PEDOT) which is arranged on a nickel substrate as current collector (Figure 7).68 In this case, the coating is claimed to have two functions: First it provides the electronic conductivity required for the function of the pseudocapacitor, which is based on the insulating LDH, second, it also stabilizes the material, with 92.5% capacity retained after 5000 cycles, substantially better than a nonencapsulated reference sample, which only retained 68% of the capacity.

Figure 6. TEM images of the Si@SiOx/C nanocomposite produced by hydrothermal carbonization and further carbonization at 750 °C under N2. (a) Overview of the Si@SiOx/C nanocomposites and a TEM image at higher magnification (in the inset) showing uniform spherelike particles; (b) HRTEM image clearly showing the core/ shell structure; (c, d) HRTEM image displaying details of the silicon nanoparticles coated with SiOx and carbon. Reproduced with kind permission from ref 54. Copyright 2008 Wiley.

mild thermal treatment at 400 °C.58 The capacity was not exceedingly high at 200 mAh/g, but the carbon coating remarkably prevented particle growth in spite of the reaction resulting in the reversible formation of LixP and metallic nickel; in addition, it provided sufficient electronic conductivity. Carbon is not the only material that can be used for the encapsulation of alloy anodes for lithium batteries. Titanium silicide has recently been reported as an interesting alternative.59 This coating was produced by first depositing a thin TiO2 layer onto the silicon particles from titanium tetrabutoxide as precursor. Heating to 1000 °C led to silicothermal reduction of the TiO2 with corresponding formation of a TixSiy layer. This electrode had a capacity of 1430 mAh/g after 90 cycles, corresponding to almost no capacity fading during the cycling experiment. This suggests that there is ample room for the exploration of other coatings for Li battery anode materials. Encapsulated Pseudocapacitors. Energy storage in electrochemical capacitors addresses a different regime than batteries. Compared to batteries, supercapacitors are characterized by very high power densities, that is, high charge and discharge currents but low energy densities. There are two major mechanisms of energy storage in supercapacitors, electrical double layer charging and discharging (electrical double layer capacitors, EDLCs), which is typically realized on high surface area carbon materials, and surface redox reactions on suitable oxides (for instance, hydrated RuO2 and MnO2), which are often labeled as pseudocapacitors. Since very high charge and discharge currents are key for the performance of supercapacitors, encapsulation is not one of the strategies which would first come to mind, if the performance of such systems should be improved, since an additional barrier, which is needed for encapsulation, would hinder easy transfer of ions and electrons to and from the surface and thus limit rates. I



Perspective

small nanoparticles of Sb, Bi, or InSb are formed.73 By proper choice of the components, the nanoparticle forming phase should be insoluble in the majority phase, and the majority phase should have a slightly higher melting point of the nanoparticle forming phase, upon rapid cooling a matrix of PbTe with encapsulated nanoparticles forms. The composites show reduced thermal conductivity at low concentrations of the nanoparticles at around 2%, which is attributed to phonon scattering on the encapsulated nanoparticles. Overall, grain boundary engineering is of highest importance in thermoelectric materials. Bulk thermoelectrics can gain from nanostructuring, if phonon scattering is strong on the grain boundaries and electronic conductivity can be retained. This is, for instance, achieved by encapsulation of a lower density phase with coherent interfaces in a majority phase. The coherent interfaces guarantee electronic conductivity, which is little affected, while the large density difference causes phonon scattering at the interfaces. An example for such a situation was given by the group of Kanatzidis.74 Ag(Pb1−ySny)mSbTe2+m was found to be in fact a bulk nanocomposite, in which Pb/Sn poor regions are formed endotaxially within the surrounding matrix, thus not disturbing electron flow, but restricting heat flow. Up to now, such nanostructures encapsulated in a matrix were spontaneously formed. However, with better understanding and control of the systems, it might be possible to create similar structures at will, possibly by using preformed core−shell precursor particles, as has been alluded to in a recent publication.75

Figure 7. Schematic of supercapacitor assembled from PEDOTencapsulated LDH arranged on nickel-substrate (top). Low magnicifaction SEM of LDH array on nickel substrate (bottom left) and high magnification view of LDH platelets coated with PEDOT after 180 s electrodeposition. Figure compiled from different figures from ref 68 with kind permission. Copyright 2013 Wiley.

These recent results suggest, that encapsulation of pseudocapacitors with suitably thin coatings could improve performance, even if such approaches appear to be counterintuitive. Coatings should be thin to not inhibit mass transfer, and superior performance is achieved if the coatings are ionically or electronically conductive. Whether there are suitable materials beyond conducting polymers remains to be seen, it seems certainly worthwhile to explore such approaches further and to elucidate the physicochemical origin of the beneficial effect of the coatings in detail. Encapsulation for Thermoelectric Materials. Encapsulation can also be beneficial in the design of thermoelectric materials, although here the encapsulation is typically embodied in a different manner than in the examples discussed above. Thermoelectric materials are an interesting class of energy conversion materials that convert thermal gradients directly to electricity. A thermoelectric material is characterized by the figure-of-merit ZT, which is defined as

ZT = S2σT /κ

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FUTURE POTENTIAL The preceding discussion has shown that encapsulation strategies are important for improving the performance of energy conversion devices in different, often quite unrelated fields, and there are scattered examples also beyond the areas addressed above. Many of the methods and problems, however, are of a generic nature. This suggests that cross-fertilization between the different fields should be possible, and inspiration across borders could lead to new discoveries. There is a number of generic questions which are important in the different application fields. For almost all encapsulation strategies, the right balance between efficient encapsulation and sufficient mass transfer is crucial. Efficient encapsulation can best be achieved, if the openings in the capsule are smaller than the particles which are encapsulated. This prevents coalescence of whole particles, which would lead to loss in surface area. However, dissolution or detachment of single surface atoms can not be avoided, if there are openings in the embedding material. This requires complete enclosure of the encapsulated particles. For transfer across such encapsulating layers, selective transport of the crucial species is required, since openings, across which unselective mass transfer could occur, are missing. Ion-and electron conducting capsules could provide such selective transport functionality, for instance for charging and discharging pseudocapacitors by the incorporation of protons in the surface layer of the oxide. For satisfactory rate capabilities, such encapsulating layers should be extremely thin, because otherwise the current densities would become too small. It has already been stated that carbon has many advantageous properties as a coating. It is relatively inert, lightweight, and provides protection already as very thin layers. We have, for instance, found that a coating corresponding of roughly three graphene layers on cobalt nanoparticles protects these particles so well, that the majority of them is not affected by exposure to

(5)

with S the Seebeck coefficient, σ the electrical conductivity, T the absolute temperature, and κ the thermal conductivity. For an efficient thermoelectric material, this figure of merit should be as high as possible. Typical values for commercially used thermoelectrics are around 1 for good materials, but also higher values have been reported.69,70 If one inspects eq 5, one realizes that one way to improve the figure-of-merit is reduction of the thermal conductivity while not affecting the other parameters. Here nanostructuring/encapsulation approaches gain importance, since this could lead to reduction of the thermal conductivity while not affecting the electronic conductivity. For instance, it has been calculated71 and experimentally confirmed72 that Ge-core/Si-shell nanowires have very strongly reduced thermal conductivity compared to bulk material or single-component nanowires, which is attributed to the increased phonon scattering. Increased phonon scattering was also found to be key for the good performance of PbTe thermoelectric materials in which J



Perspective

HF for 72 h.76 The ultimate limit for an encapsulating shell would be a single graphene layer, through which access could be provided by small defects. Whether such encapsulating shells could be generated and whether their existence could be proven remains to be seen. With respect to synthetic pathways, there seems to be ample room for improvement. The easiest method for encapsulation is the impregnation of a porous carbon material with the active material (or with precursors of the active material). However, this often leaves the active material rather exposed so that stabilization is less than perfect. Moreover, the achievable loadings are often not very high, so that overall gravimetric and volumetric normalized performance, a key property of many energy conversion materials, suffers. Generally, it seems perferable to prefabricate nanoparticles of the active material and coat them with a thin layer of the encapsulating material. For high volume-normalized performance, a compaction step will probably be required, which is almost unexplored in the scientific literature, although this may be a crucial factor for practical application. In an ideal situation, the encapsulating material would also provide additional functionality, such as additional capacitance of a carbon matrix encapsulating pseudocapacitor materials. However, this will only be possible in fortunate cases and probably, one would have to compromise performance in other dimensions. Many encapsulation strategies are rather complex and require multistep procedures. Aerosol processing could provide an interesting and simple alternative, if the process is properly designed. It has been shown to be a suitable technique for the encapsulation of active sites for catalytic materials (see, for instance, Debecker et al.77), and transfer of the concepts to energy materials could provide a cheap and scalable process option. There are some fields, where encapsulation strategies are already well used, such as in the design of electrode materials for batteries, although also here there seems to be still room for improvement. In other domains these approaches are rather underdeveloped or not explored at all. For instance, there is substantial work on thermochemical cycles for the production of hydrogen or syngas, which relies on thermal reduction and reoxidation of oxides with water or CO2.78 To reach sufficient reaction rates, the reactive solids should be sufficiently well dispersed, which, however, is not easily compatible with the high temperature operation. Encapsulation in suitable matrices may help here. For pseudocapacitor systems, encapsulation has only tentatively been explored, although especially conducting polymers or carefully tuned carbon capsules could improve overall performance of the systems. Also in fuel cell systems encapsulation strategies seem to have substantial potential. Here, however, as in most energy conversion systems, it is more than the material itself which needs to be optimized: The decisive unit is the device, and the materials properties need to match the demands of the overall device. Thus, even if encapsulation is highly beneficial for the energy conversion material, one needs to leverage such an advantage to the device level which is not necessarily straightforward. A final deficit in the field of encapsulation of energy conversion materials is the lack of systematic studies. A substantial fraction of the published work is rather empirical, and the body of published literature is more a compilation of scattered observations, which does not yet give a coherent picture. Encapsulating matrices are often rather used more like an additive which enhances the performance of the system, but how this improvement is achieved remains unclear. Sometimes

it is not even clear whether the improvement is really due to encapsulation, or rather to other, unknown factors. Often the exact role of an encapsulating matrix has not been explored at all, and which factors contribute most to improved materials performance is an open question. This impedes systematic improvement of both encapsulation pathways and encapsulation materials. More systematic studies in this field to explore physicochemical relations and also in the direction of integrating encapsulated materials in devices could strongly improve systems performance in a number of energy conversion technologies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biography Prof. Dr. Ferdi Schüth studied Chemistry and Law at Münster, where he received the Ph.D. in Chemistry in 1988 and the State Examination in Law in 1989. 1988/89 he was Post-Doc in the group of L.D. Schmidt at the Department of Chemical Engineering at the University of Minnesota. From 1989 to 1995, he worked as Habilitand with K. Unger in Mainz and for five months in 1993 with G. Stucky at Santa Barbara. In 1995, he became full professor at Frankfurt University. In 1998, he was appointed Director at the Max-Planck-Institut für Kohlenforschung, Mülheim. He is vice president of the German Science Foundation (DFG), chairman of the scientific council of the Max-Planck-Society, editor of Chemistry of Materials, and serves on the editorial boards of several other international journals. He has received many awards, among them the Leibniz-Prize, the highest German science award. He is also cofounder and member of the board of hte AG in Heidelberg, which has 250 employees and which was recently acquired by BASF SE. His research interests include catalysis, porous solids, biomass utilization, hydrogen storage, and solids formation from solution.

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REFERENCES

(1) Zhang, Q. F.; Uchaker, E.; Candelaria, S. L.; Cao, G. H. Chem. Soc. Rev. 2013, 43, 3127. (2) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359. (3) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mater. 2010, 22, 691. (4) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Angew. Chem. Int. Ed. 2008, 47, 2930. (5) Kanatzidis, M. G. Chem. Mater. 2010, 22, 648. (6) Koch, N. ChemPhysChem 2007, 8, 1438. (7) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Osterbeek, H.; Holewijn, J. E.; Xu, X. D.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956. (8) den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Froseth, V.; Holmen, A.; de Jong, K. P. J. Am. Chem. Soc. 2009, 131, 7197. (9) Vajo, J. J. Curr. Opin. Solid State Mater. Sci. 2011, 15, 52. (10) Joergensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008, 92, 686. (11) Kim, N.; Potscavage, W. J., Jr.; Sundaramoothi, A.; Henderson, C.; Kippelen, B.; Graham, S. Sol. Energy Mater. Sol. Cells 2012, 101, 140. (12) Su, D. S.; Schlögl, R. ChemSusChem 2010, 3, 136. (13) Kercher, A. K.; Nagle, D. C. Carbon 2003, 41, 50. (14) Charitidis, C. A. Int. J. Refract. Met. Hard Mater. 2010, 28, 51. (15) Frackowiak, E.; Beguin, B. Carbon 2001, 39, 937. (16) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. K



Perspective

(17) Brinker, J. C.; Scherer, G. W. Sol−Gel ChemistryThe Physics and Chemistry of Sol−Gel Processing; Academic Press: New York, 1990. (18) Yang, G.; Tsubaki, N.; Shamoto, J.; Yoneyama, Y.; Zhang, Y. J. Am. Chem. Soc. 2010, 132, 8129. (19) Jiang, H.; Bongard, H.; Schmidt, W.; Schüth, F. Microporous Mesoporous Mater. 2012, 164, 3. (20) He, J. J.; Liu, Z. L.; Yoneyama, Y.; Nishiyama, N.; Tsubaki, N. Chem.Eur. J. 2006, 12, 8296. (21) Bao, J.; He, J. J.; Zhang, Y.; Yoneyama, Y.; Tsubaki, N. Angew. Chem., Int. Ed. 2008, 47, 353. (22) Sartipi, S.; van Dijk, J. E.; Gascon, J.; Kapteijn, F. Appl. Catal., A 2013, 456, 11. (23) Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Kostka, A.; Schüth, F.; Mayrhofer, K. J. J. ACS Catal. 2012, 2, 832. (24) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169. (25) Candelaria, S. L.; Shao, Y.; Zhou, W.; Li, X.; Xiao, J.; Zhang, J. G.; Wang, Y.; Liu, J.; Li, J.; Cao, G. Nano Energy 2012, 1, 195. (26) Xue, X.; Gu, L.; Cao, X.; Song, Y.; Zhu, L.; Chen, P. J. Solid State Chem. 2009, 182, 2912. (27) Galeano, C.; Meier, J. C.; Peinecke, V.; Bongard, H.; Katsounaros, I.; Topalov, A. A.; Lu, A. H.; Mayrhofer, K.; Schüth, F. J. Am. Chem. Soc. 2012, 134, 20457. (28) Baldizzone, C.; Mezzavilla, S.; Meier, J. C.; Heggen, M.; Galeano, C.; Schüth, F.; Mayrhofer, K. K. J. DOI: 10.1002/ adfm.201302239. (29) Zhang, W.; Brodt, M. W.; Pintauro, P. N. ECS Trans. 2011, 41 (1), 891. (30) Gu, D.; Schüth, F. Chem.Soc.Rev. 2013, DOI: 10.1039/ C3CS60155B. (31) Lu, A. H.; Zhao, D. Y.; Wang, Y. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials; Royal Society of Chemistry: Cambridge, U.K., 2010. (32) Shao, Y. Y.; Liu, J.; Wang, Y.; Lin, Y. H. J. Mater. Chem. 2009, 19, 46. (33) Eberle, U.; Felderhoff, M.; Schüth, F. Angew. Chem., Int. Ed. 2009, 48, 6608. (34) De Jongh, P. E.; Allendorf, M.; Vajo, J. J.; Zlotea, C. MRS Bull. 2013, 38, 488. (35) Schüth, F.; Bogdanovic, B.; Taguchi, A. Int. Patent WO2005/ 014469, 2003. (36) Gao, J.; Adelhelm, P.; Verkuijlen, M. H. W.; Rongeat, C.; Herrich, M.; van Bentum, P. J. M.; Gutfleisch, O.; Kentgens, A. P. M.; de Jong, K. P.; de Jongh, P. E. J. Phys. Chem. C 2010, 114, 4674. (37) Lohstroh, W.; Roth, A.; Hahn, H.; Fichtner, M. ChemPhysChem 2010, 11, 789. (38) Adelhelm, P.; Gao, J.; Verkuijlen, M. H. W.; Rongeat, C.; Herrich, M.; van Bentum, P. J. M.; Gutfleisch, O.; Kentgens, A. P. M.; de Jong, K. P.; de Jongh, P. E. Chem. Mater. 2010, 22, 2233. (39) Bhakta, R. K.; Maharray, S.; Stavila, V.; Highley, A.; Alam, T.; Majzoub, E.; Allendorf, M. Phys. Chem. Chem. Phys. 2012, 14, 8160. (40) Ingleson, M. J.; Barrio, J. P.; Basca, J.; Steiner, A.; Darling, G. R.; Jones, J. T. A.; Khimyak, Y. Z.; Rosseinsky, M. J. Angew. Chem., Int. Ed. 2009, 48, 2012. (41) Broussely, M.; Archdale, G. J. Power Sources 2004, 136, 386. (42) Amin, R.; Maier, J.; Bayala, P.; Chen, D. P.; Lin, C. T. Solid State Ionics 2008, 179, 1683. (43) Zaghib, K.; Guerfi, A.; Hovington, P.; Vijh, A.; Trudeau, M.; Goddenough, J. B.; Julien, C. M. J. Power Sources 2013, 232, 357. (44) Ravet, N.; Goodenough, J. B.; Besner, S.; Simoneau, M.; Hovington, P.; Armand, M. Proc.Electrochem. Soc., 196th 1999, 99e102, No. 127. (45) Huang, H.; Yin, S.-C.; Nazar, L. F. Electrochem. Solid State Lett. 2001, 4, A170. (46) Hu, Y. Q.; Doeff, M. M.; Kostecki, R.; Finones, R. J. Electrochem. Soc. 2004, 151, A1279. (47) Wang, Y. G.; Wang, Y. R.; Hosono, E.; Wang, K.; Zhou, H. S. Angew. Chem., Int. Ed. 2008, 47, 7461.

(48) Zhu, C.; Yu, Y.; Gu, L.; Weichert, K.; Maier, J. Angew. Chem., Int. Ed. 2011, 50, 6278. (49) Rho, Y. H.; Nazar, L. F.; Perry, L.; Ryan, D. J. Electrochem. Soc. 2007, 154, A283. (50) Ji, X. L.; Lee, K. T.; Nazar, L. F. Nat. Mater. 2009, 8, 500. (51) Jeong, S.; Bresser, D.; Buchholz, D.; Winter, M.; Passerini, S. J. Power Sources 2013, 235, 220. (52) Cui, G.; Gu, L.; Zhi, L. J.; Kaskhedikar, N.; van Aken, P. A.; Müllen, K.; Maier, J. Adv. Mater. 2008, 20, 3079. (53) Wilson, A. M.; Dahn, J. R. J. Electrochem. Soc. 1995, 142, 326. (54) Hu, J. S.; Demir-Cakan, R.; Titirici, M. M.; Müller, J. O.; Schlögl, R.; Antonietti, M.; Maier, J. Angew. Chem., Int. Ed. 2008, 47, 1645. (55) Zhao, G.; Zhang, L.; Meng, Y.; Zhang, N.; Sun, K. Mater. Lett. 2013, 96, 170. (56) Yu, Y.; Gu, L.; Zhu, C.; van Aken, P. A.; Maier, J. J. Am. Chem. Soc. 2009, 131, 15984. (57) Yu, Y.; Gu, L.; Wang, C.; Dhanabalan, A.; van Aken, P. A.; Maier, J. Angew. Chem., Int. Ed. 2009, 48, 6485. (58) Carenco, S.; Surcin, C.; Morcrette, M.; Larcher, D.; Mezailles, M.; Boissiere, C.; Sanchez, C. Chem. Mater. 2012, 24, 688. (59) Park, O.; Lee, J.-I.; Chun, M.-J.; Yeon, J.-T.; Yoo, S.; Choi, S.; Choi, N.-S.; Park, S. RSC Adv. 2013, 3, 2538. (60) Frackowiak, E. Phys. Chem. Chem. Phys. 2007, 9, 1774. (61) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845. (62) Yan, W.; Kim, J. Y.; Xing, W. D.; Donovan, K. C.; Ayvazian, T.; Penner, R. M. Chem. Mater. 2012, 24, 2382. (63) Han, J. B.; Dou, Y.; Zhao, J. W.; Wei, M.; Evans, D. G.; Duan, X. Small 2013, 9, 98. (64) Miller, J. M.; Dunn, B.; Tran, T. D.; Pekala, R. W. Electrochem. Soc. Lett. 1998, 144, No. L309. (65) Sassin, M. B.; Chervin, C. N.; Rolison, D. R.; Long, J. W. Acc. Chem. Res. 2013, 46, 1062. (66) Dong, X. P.; Shen, W. H.; Gu, J. L.; Xiong, L. M.; Zhu, Y. F.; Li, H.; Shi, J. L. J. Phys. Chem. B 2006, 110, 6015. (67) Balan, K. B.; Chaudhari, H. D.; Kharul, U. K.; Kurungot, S. RSC Adv. 2013, 3, 2428. (68) Han, J. B.; Dou, Y. B.; Zhao, J. W.; Wei, M.; Evans, D. G.; Duan, X. Small 2013, 9, 98. (69) Kanatzidis, M. G. Chem. Mater. 2010, 22, 648. (70) Lan, Y. C.; Minnich, A. J.; Chen, G.; Ren, Z. F. Adv. Funct. Mater. 2010, 20, 357. (71) Wingert, M. C.; Chen, Z. C. Y.; Dechaumphai, E.; Moon, J.; Kim, J. H.; Xiang, J.; Chen, R. Nano Lett. 2011, 11, 5507. (72) Hu, M.; Zhang, X. L.; Giapis, K. P.; Poulikakos, D. Phys. Rev. B 2011, 84, No. 085442. (73) Sootsman, J. R.; Pcionek, R. J.; Kong, H. J.; Uher, C.; Kanatzidis, M. G. Chem. Mater. 2006, 18, 4993. (74) Androulakis, J.; Hsu, K. F.; Pcionek, R.; Kong, H. J.; Uher, C.; D’Angelo, J. J.; Downey, A.; Hogan, T.; Kanatzidis, M. G. Adv. Mater. 2006, 18, 1170. (75) Kovalenko, M. V.; Spokoyny, B.; Lee, J.-S.; Scheele, M.; Weber, A.; Perera, S.; Landry, D.; Talapin, D. V. J. Am. Chem. Soc. 2010, 132, 6686. (76) Lu, A. H.; Schmidt, W.; Matoussevitch, N.; Bönnemann, H.; Spliethoff, B.; Tesche, B.; Bill, E.; Kiefer, W.; Schüth, F. Angew. Chem., Int. Ed. 2004, 43, 4303. (77) Debecker, D. P.; Stoyanova, M.; Colbeau-Justin, F.; Rodemerck, U.; Boissiere, C.; Gaigneaux, E. M.; Sanchez, C. Angew. Chem., Int. Ed. 2012, 51, 2129. (78) Stamatiou, A.; Loutzenhiser, P. G.; Steinfeld, A. Chem. Mater. 2010, 22, 851.

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