Metal-semiconductor core–shell nanomaterials for energy applications

Chapter

5

Metal-semiconductor core–shell nanomaterials for energy applications Rupali Nagar* and Bhaghavathi P. Vinayan**,† *Symbiosis Institute of Technology, Symbiosis International University, Pune, Maharashtra, India; **Helmholtz Institute Ulm (HIU) for Electrochemical Energy Storage, Ulm, Germany; †Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany

5.1  ENERGY AND ENVIRONMENT The subject of Energy and Environment together has more relevance in today’s world than ever before. After the industrial revolution the dependence on energy increased manifolds as it is one of the crucial sectors in any nation’s economy and influences the growth to a large extent. Fig. 5.1A depicts the sector-wise world energy consumption in industries, transportation, residential and commercial sectors [1]. Transportation is the second highest sector in this index and the energy demands are met largely by fossil fuels (almost 98%) as shown in Fig. 5.1B, while a very small fraction of demand is met by bio-fuel or electricity [1]. Thus, it is clear that humankind is heavily dependent on fossil fuels. The adverse effects of using fossil fuels have now become evident. The green-house gases emitted due to combustion of fossil fuels trap heat in the outer atmosphere of Earth. As more and more heat is trapped, the Earth’s surface temperature increases. A steady rise in the Earth’s temperature after 1980s as depicted in Fig. 5.2 has now perhaps culminated in changed weather patterns and global warming; the oceans and climate are getting warmer [2,3]. One may ask as to why the issue of global warming is becoming increasingly important to environmentalists? The reason is that environment directly or indirectly affects the sectors of agriculture, water, energy, and health to name a few. Favorable environmental conditions have made the survival of life on Earth possible. In the long run an imbalance in the environment is certain to pose a danger to the survival of life on our planet. From health perspective, wet and warm climates are conducive for bacteria or viruses that may lead to outbreak of old or new diseases. So far, the Metal Semiconductor Core–Shell Nanostructures for Energy and Environmental Applications. http://dx.doi.org/10.1016/B978-0-323-44922-9.00005-3 Copyright © 2017 Elsevier Inc. All rights reserved.

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100 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

■■FIGURE 5.1  (A) World-wide energy consumption till 2012 in various sectors. (B) Share of different fuels contributing to global transportation consumption till 2012. (Figures created by author from data in ref. [1] Earthzine IEEE Oceanic Engineering Society. Vermont Law School: The Ethical Dimensions of Energy Policy.)

human activities have resulted in global environmental changes but Earth has managed to be in a stable environmental state. As depicted in Fig. 5.3, nine earth system processes have been identified, limits of which if violated, could result in intolerable environmental changes leading to threat to civilization and life on Earth [4]. Environmentalists believe that we have now started exploiting and abusing our own environment and that, boundaries of three of the nine earth systems have already been crossed. These are (1) climate change (includes atmospheric carbon dioxide, radiations), (2) biodiversity loss (includes extinction rate of species), and (3) nitrogen cycle (amount of nitrogen removed from atmosphere). Awareness about environment has encouraged nations across the world to pledge reducing their carbon print and contributing to a healthier, greener, and safer environment. In 2010, 193 nations came together to work toward limiting the global temperature rise to 2°C. Steps taken thereafter did not show much improvement as the global rise in temperature in the year 2015 was recorded as 1°C. This fact was highlighted in the recently concluded 2015 United Nations Climate Change Conference organized in Paris, where it was accepted that resolutions passed in the Kyoto protocol to arrest the estimated 2°C temperature rise above the preindustrial level globally are likely to be missed. An urgent reexamination of the accepted resolutions demanding stricter actions has become more of a compulsion than an option. With time, the responsibilities towards the environment are increasing, thereby, putting more and more pressure on the developed as well as developing nations to seize the global temperature rise. Realizing that a significant harm to environment is caused by fossil fuels and their effects are manifold in terms of air pollution, respiratory hazards, and so on, only corrective and preventive measures together can

5.1 Energy and environment 101

help in restoring a safer and cleaner environment. By using nonconventional and renewable sources of energy, the environment can be shielded by adverse effects of greenhouse gases, which are the most worrying factors of all earth system process boundaries. In the words of Dr. Ernest Moniz (United States Secretary of Energy), “Clean Energy Innovation is the Solution to Climate Change”. His call for clean energy innovation during the 21st Conference of Parties (COP21) is expected to pave the path of future innovations [5]. Similar vision is echoed in Fig. 5.4 in which the efforts to achieve the target of reducing emission to 95 gCO2 equivalent per kilometer fleet by 2020 by European Union will be made [6,7].

5.1.1  Alternate energy options An alternate fuel should be efficient, easily available and scalable, environment friendly and economically viable. All sources of energy that occur naturally, that is sun, wind, ocean, or energy from bio-mass are abundantly available in nature already. Their efficient conversion to useful form of energy is, however, to be ensured. Some of these energy forms are already into use in most developed nations. Hydrogen energy has also shown promise and vehicles running on hydrogen or hydrogen-based fuels can be considered successful in terms of their clean energy output. New energy storage

■■FIGURE 5.2  Annual global land and ocean temperature anomalies from 1880–2016 where temperature anomalies have been calculated from 20th Century average temperature. (Reprinted by permission from National Oceanic and Atmospheric Administration (NOAA). Source of data: National Centers for Environmental Information.)

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■■FIGURE 5.3  Beyond the boundary. The inner green shading represents the proposed safe operating space for nine planetary systems. The red wedges represent an estimate of the current position for each variable. The boundaries in three systems (rate of biodiversity loss, climate change and human interference with the nitrogen cycle), have already been exceeded. (Reprinted by permission from Macmillan Publishers Ltd: [Nature] [4] J. Rockstrom, W. Steffen, K. Noone, A. Persson, F.S. Chapin, E.F. Lambin, T.M. Lenton, M. Scheffer, C. Folke, H.J. Schellnhuber, B. Nykvist, C.A. de Wit, T. Hughes, S. van der Leeuw, H. Rodhe, S. Sorlin, P.K. Snyder, R. Costanza, U. Svedin, M. Falkenmark, L. Karlberg, R.W. Corell, V.J. Fabry, J. Hansen, B. Walker, D. Liverman, K. Richardson, P. Crutzen, J.A. Foley, A safe operating space for humanity. Nature 461 (2009) 472–475, copyright (2009). Available at http://www.nature.com/nature/journal/v461/n7263/full/461472a.html)

technologies like lithium/sodium/magnesium-ion rechargeable batteries, lithium-sulfur, lithium-air batteries, supercapacitors, and energy conversion technologies like solar cells and fuel cells, and so on are promising [8–12]. France is one of the leading examples for promoting renewable energy based technology in energy sector. A major share of electricity generated in France by 2012 came from zero emission fuels [13]. This was achieved by increasing the capacity of electricity generation by renewable sources and at the same time reducing use of fossil fuels. Electricity consumption hit the lowest levels in France in 2014. This was attributed to factors like warmer temperatures (which resulted in lesser energy consumption), economic crisis,

5.1 Energy and environment 103

■■FIGURE 5.4  Well-to-wheels greenhouse gas emissions for various propulsion types and fuel sources. (Reproduced from ref. [7] U. Eberle, B. Muller, R. von Helmolt, Fuel cell electric vehicles and hydrogen infrastructure: status 2012. Energy Environ. Sci. 5 (2012) 8780–8798, with permission of the Royal Society of Chemistry. http://dx.doi.org/10.1039/C2EE22596D)

increased energy efficiency and popular use of photovoltaic technology [14]. Fig. 5.5 shows that hydropower and wind power are two leading naturally available sources of energy utilized for electricity generation in France. Therefore, the problem can be addressed by either utilizing naturally available energy sources or using technological innovations for efficient energy storage and utilization. Fig. 5.6 depicts the potential of lithium-ion battery technology to provide high power to energy ratio for a broad range of present and future transport and/or mobile applications [15]. This chapter will discuss innovations in energy sector for alternate energy and portable consumer electronics.

■■FIGURE 5.5  (A) Breakdown of renewable energy generation as on December 31, 2014 in France. (B–C) show trends of wind energy generation and photovoltaic power generation. (Adapted from RTE-Réseau de transport d’éléctricité published from the source France Electricity Report for 2014 available at http:// www.rte-france.com/sites/default/files/bilan_ electrique_2014_en.pdf)

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■■FIGURE 5.6  Specific power and energy roadmap for battery pack for hybrid (HEV), plug-in hybrid (PHEV) and full electric (BEV) vehicles. (Reproduced from ref. [15] D. Andre, S.-J. Kim, P. Lamp, S.F. Lux, F. Maglia, O. Paschos, B. Stiaszny, Future generations of cathode materials: an automotive industry perspective. J. Mater. Chem. A 3 (2015) 6709–6732, with permission of the Royal Society of Chemistry. http://dx.doi.org/10.1039/C5TA00361J)

Around the year 2000, fuel cell based electric buses were introduced in California in order to introduce transportation based on technology with lower emissions. With the initial success met with these hybrid vehicles, many such vehicles were pushed into service by the next decade. This paved the path for commercialization of fuel cell buses. Australia and Europe too introduced fuel cell based buses around the same time. Karl Friesenbichler in his working paper on Innovations in the energy sector rightly points to the fact that the current environmental related challenges cannot be curbed simply by waiting for new technological innovations [16]. This goal can and would be achieved only if the restructuring of energy sector is done by diffusing the existing environmental technologies and increasing the efforts in the direction of innovation [16]. Lowering of emissions and no or very less consumption of petroleum fuels established the feasibility of this new technology. In the market of portable electronics, the energy storage and conversion devices like rechargeable batteries, fuel cells, and supercapacitors can be regarded as innovations of last century, which have today gradually diffused into consumer portable electronics. The next stage of innovation is awaited wherein materials will have better performances. To improve the efficiency of existing materials employed in alternative energy solutions, constant efforts are being made by material scientists to find newer materials or tailor properties of existing materials to possess desirable properties.

5.2 Electrochemical energy storage and conversion devices 105

5.2  ELECTROCHEMICAL ENERGY STORAGE AND CONVERSION DEVICES The alternative energy conversion and storage devices like fuel cells, lithium ion batteries, and supercapacitors will be discussed in this section. Portable devices like watches, laptops, mobile phones, and automobiles (hybrid and electric vehicles) need power from either rechargeable batteries or fuel cells or supercapacitors. Generally speaking, in all these three energy devices, a redox couple reaction takes place during which chemical energy of materials is extracted in a manner to produce a charge flow. They are all similar in their basic structure, and comprise of an anode, cathode, and separator. In all these electrochemical devices, separator is soaked in a suitable electrolyte. Fuel cells can deliver high energy densities but not high power densities, while supercapacitors can deliver high power densities but not high energy densities. Batteries lie between supercapacitors and fuel cells on the Ragone plot as depicted in Fig. 5.7 [17]. The primary target in these electrochemical devices is how to extract high power and energy densities with longer stability from the materials used in these devices. A catalyst can expedite the rate of redox reactions at the electrode surface. The miniaturization of electrode materials/catalyst from bulk to nanoscale can influence their electrochemical performances in an efficient way. Electrode surfaces containing nanostructured materials offer a very large surface-to-volume ratio and shorter diffusion path lengths for ions. Larger surface areas aid in exposing more number of catalytically active sites and thus promote faster rate of redox reactions. Additionally, the electrolyte or fuel can easily reach successive layers of

■■FIGURE 5.7  Ragone plot for various energy-storing devices. (Adapted with permission from M. Winter, R.J. Brodd, What are batteries, fuel cells, and supercapacitors? Chem. Rev., 104, (2004) 4245–4270. Copyright (2004) American Chemical Society.)

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■■FIGURE 5.8  Schematic of reported MEA mass activities (full figures) of Pt-based ORR catalysts appearing viable for mass manufacturing (filled symbols) and interesting PGM (platinum-group metal)-based concept catalyst showing promising RDE and in some cases MEA based mass activities (empty symbols). Standard Pt/C, PtCo/C alloys (gen. 1), process-optimized PtCo/C (gen. 2), dealloyed PtCu3, 3 M NSTF, dealloyed Pt/NiC and catalysts derived from dealloyed shapecontrolled Pt-alloy particles. At the bottom of the figure are the estimated material design freeze periods for fuel cell electric vehicles (FCEVs) which have been introduced today (2010–2012), for the fast-follower’s FCEV introduction in 2017–2020 (2014–2016) and for the first wave during market expansion in 2020+ (around 2018+). (Reproduced from http://jes.ecsdl.org/ content/162/14/A2605 licensed under. CC BY, http://creativecommons.org/licenses/by/4.0/), DOI: 10.1149/2.0211514jes)

electrode materials through the pores of nanostructures and form multiple surface-electrolyte interfaces. All these properties aid in increasing the electrochemical activity of electrode materials/ catalysts. For example, Fig. 5.8 shows the progress in mass activities of various Pt based alloy nanocatalysts toward oxygen reduction in fuel cell applications [6]. Nanostructured materials have attracted a great deal of attention in the past few decades and today invariably are used in different electrode materials. Carbon nanomaterials like graphene (two-dimensional carbon allotrope) and carbon nanotubes (one-dimensional carbon allotrope) have been applied successfully as catalyst support materials in various electrochemical devices. Besides, different morphologies of catalyst nanostructures (spherical, core–shell (CS), conical, pyramidal, rod, tube, dendrimer, tetrahedral, or octahedral) can also influence the catalytic performance in redox reactions.

5.3 Role of nanomaterials in supercapacitors, fuel cells and lithium ion batteries applications 107

5.3  ROLE OF NANOMATERIALS IN SUPERCAPACITORS, FUEL CELLS AND LITHIUM ION BATTERIES APPLICATIONS Material science helps in making the right choice of materials appropriate for a given application. Nanomaterials offer different properties as compared to their bulk counterparts, have very high surface areas for a given volume and predominantly exhibit quantum effects. Nanoparticles have shown great promise in terms of their performance in various fields [18–24]. The general factors which can affect the performance of nanomaterials can be classified as (1) size, (2) shape, (3) composition (4) synthesis methodology, and (5) postsynthesis treatments. Some parameters that particularly influence properties and performance of CSNs are size of the core and shell, type of materials making up the core and shell, void space between the core and shell and thickness of shell. These fundamentally affect the way core and/ or shell interacts with their environment. Research on the electrochemical devices like supercapacitors, fuel cells and lithium ion battery can be categorized by anode materials, cathode materials, and types of electrolytes or design aspect of devices. The following sections briefly describe the basics of these devices.

5.3.1 Supercapacitors Supercapacitors are capable of providing capacitances of the order of few farads, have faster charge-discharge characteristics and therefore offer high power densities. The charge dq stored by a capacitor when a potential difference of dV is applied across it is given as:

dq = CdV,

(5.1)

where, C is the capacitance and is directly proportional to the area of the electrodes (A) and inversely proportional to the separation distance between them (d) separated by a dielectric with permittivity ε. The characteristic large surface area of nanomaterials therefore makes them desirable for increasing capacitance. The mechanisms of charge storage in capacitors are either purely due to formation of electric double layer or pseudo capacitance in conjunction with formation of electric double layer. Due to the coulombic attraction of unlike charges, electric charges appear on the electrode–­electrolyte interfacial surface. These charges give rise to a virtual parallel plate capacitor whose plates are at a distance of intermolecular scale (i.e. ∼few nanometers). As capacitance of a parallel plate capacitor varies inversely as the interplate distance, the formation of electric double layer e­nhances the ­capacitance

108 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

manifold. Two electric layers are formed near each electrode. Thus, the mechanism is termed as electric double layer. The pseudocapacitance is the predominant mechanism when polymer or metal oxide based electrode materials are used and electric double layer when carbon nanomaterials are employed as electrode materials. The former mechanism involves charge transfer through the electrode surface and is termed as faradaic charge transfer. The faradaic charge transfer resembles the mechanism which works in case of batteries. In case of electric double layer capacitors carbon nanomaterials are used extensively as they involve ion adsorption. Carbon nanomaterials like nanotubes (single and multiwalled carbon nanotubes), sheets (graphene or graphene nanoplatelets) and hybrid nanomaterials (composites of nanotubes and sheets) are employed as electrode materials [25–30]. Polymers exhibit conducting states and can promote charge exchange by either formation or removal of radical cation or radical anion centers. This mechanism involves flow of charges that has a faradic origin, suggesting that the capacitance is pseudo capacitance. The challenges that have to be met include synthesizing materials economically while having higher energy densities. Most of the efforts are directed towards designing pseudocapacitive electrodes that have fast redox reactions at surface regions or in nanodomains formed on the electrodes [31–35].

5.3.2  Fuel cells Fuel cells can work as long as a continuous supply of fuel is available. When the fuel supply is cut off, they stop and can resume their operation if the supply of fuel is restored. These systems do not work on the combustion of fuel but on conversion of chemical energy of the fuel to electric energy via a chemical reaction. If hydrogen is used as a fuel at anode and oxygen/air as oxidizing agent at cathode, energy conversion results in heat and water as byproducts and so fuel cells are regarded as an environmentally benign energy conversion method. Of the various types of fuel cells, the proton-exchange membrane fuel cells (PEMFCs) are popular as they have moderate operating temperatures (∼60–80°C), have shorter start-up times, yield high power densities and make use of a solid electrolyte. In PEMFC, the catalysts at the anode and cathode expedite the oxidation/reductions reactions, while the electrolyte (hydrated Nafion membrane) not only provides a physical separation of the two electrodes avoiding an internal short, but also controls the flow of positive ions (H+) through it and at the same time inhibits the electron flow. The electrons are thus forced to flow through the external circuit from anode to cathode. One of the main challenges being faced by fuel cells is to minimize the precious Pt catalyst at the electrodes. Some success has been met in

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 109

r­ educing the Pt content by PtM (M = Fe, Co, Ni, Cu, Au, etc.) alloy catalysts or non Pt catalysts, while maintaining high electrochemical activity as shown in Fig. 5.8 [21,23,24,36,37]. CS structures help in minimizing the Pt content while maintaining the high level of performance (catalytic activity and stability). Carbon nanomaterial supports have shown great promise by aiding in the dispersion of catalyst nanoparticles, and hence enhancing the electrochemical surface area and much faster electronic charge transport.

5.3.3  Lithium ion batteries Li ion batteries are characterized with high energy densities (∼200 Wh/kg) and long cyclic stability. They are much suitable for portable/home electronics and electric vehicle applications. In LIB, Li+ ions move from anode to cathode through the electrolyte during discharge and vice versa during charge. The electrolytes are usually a combination of lithium salts (LiClO4, LiPF6, LiTFSI, or LiBF4), in a suitable organic solvent. Electrode materials with high specific capacity are required for high energy density applications. Present commercial LIBs use graphite electrodes at the anode side with a theoretical specific capacity of ∼372 mAh/g. Different carbon-nanomaterials and their composites with Si, Sn, and 3D-metal oxides also have been investigated as anode materials in LIB and these materials show higher specific capacity as compared to graphite [22,3–43]. The positive electrode is usually made from lithium-cobalt oxide (LiCoO2), and new LIBs use lithium iron phosphate (LiFePO4), lithium ion manganese oxide battery (LMnO or LMO), or lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC) as cathode materials. The process of lithiation/delithiation in high capacity anode electrodes like oxides of transition metals (Fe, Co, etc.) Sn, SnO2, and Si give rise to large strains within the material and ultimately lead to fracture and pulverization of the electrode [44–46]. The exposed surfaces after pulverization serve as fresh sites where electrolyte decomposition can take place and increase charge transfer resistance. The main challenges include the search for new electrode materials that can deliver maximum energy density, have faster charge-discharge rates, long cyclic stability, are less toxic and mechanically robust.

5.4  METAL-SEMICONDUCTOR CORE–SHELL NANOMATERIALS FOR ENERGY STORAGE AND CONVERSION The choice of materials is governed by their end-use and properties required to attain satisfactory material performance. Thus, it is imperative to understand which material properties are important while considering

110 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

materials for energy applications. Broadly, the properties can be classified as chemical, architectural, electronic and morphological. These properties will now be discussed individually in some detail. Among the various chemical properties, high specific charge capacity of a material is desirable for battery operation. Additionally, good electronic and ionic conductivity of electrode materials are also essential to attain high charge/discharge current rate. In case of supercapacitors, one requires high capacitance, a wider electrolyte stability window along with a thermodynamically stable window. If materials are to be used as cathode catalyst in fuel cells, then they must possess higher oxygen reduction catalytic activity and should minimize the use of precious Pt. In this regard, chemically modified high surface area conductive carbon nanomaterials can be used as supports for dispersing such electrode/catalyst particles [21,45,47]. That helps them for the uniform dispersion at the nanosize and in turn prevents their agglomeration with a strong bonding between support and particle. The architecture of nanoparticles also influences their performance. For instance, CSNs help in strain relaxation by offering internal void space. Layered materials that have good ionic conductivity are considered to be useful intercalating materials. Porosity is another factor that influences the reaction kinetics to a great extent as the pore size and pore volume, particularly in fuel cells, can act as triple phase boundaries, where reaction kinetics are fast. The size and shape of nanoparticles are also crucial. The surface-to-volume ratio is higher in such particles and quantum effects dominate at this length scale. Shape of nanoparticles may influence the surface energies and influence the interaction of the particle with its surroundings. Nature of nanoparticles, that is, single or alloyed have different physical and chemical properties due to change in their electronic properties and offer more ways to engineer them. It is possible to design CS particles for a specific core and shell combination and to modify their dimensions. Metal-semiconductor nanoparticles are interesting systems to study due to changes in their electronic, chemical and optical properties at smaller scales. When the dimension of the nanoparticles is comparable to the de Broglie wavelength, quantum effects set in which manifest in the form of “remarkable properties”. These effects become more remarkable when two or more such systems are brought together and form a new nanosystem. The nanosystems thus formed have multiple functionalities and carry the desirable properties of their constituents (Table 5.1).

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 111

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications Electrode Material

Remarks

References

Lithium ion cells/batteries Yolk-shell Al core 30 nm dia, TiO2 shell 3 nm thick at anode

TiO2 shell enhances charge capacity, electrode loading ∼3 mg/cm2 on Cu foil, reversible capacity >650 mAh/g after 500 cycles at 10C

[48]

Synthesis of Al@TiO2 (a) in-situ water shift synthesis of Al@TiO2. Colors represent chemical contents in the solution: the equiibrated mixture of H2SO4 and TiOSO4 is light yellow, H2O is blue, and H2SO4 is green. (b) SEM image of Al@TiO2 with a broken shell. Reproduced from reference [48] Li, S. et al., High-rate aluminium yolk–shell nanoparticle anode for Li-ion battery with long cycle life and ultrahigh capacity. Nat. Commun. 6 (2015) 7872 doi: 10.1038/ncomms8872, http://www.nature.com/articles/ncomms8872 licensed under. CC BY, http://creativecommons.org/licenses/by/4.0/), DOI: 10.1038/ncomms8872 Si/SiO CSNs of size 50 nm dia used at anode

Original capacity of 827 mAh/g faded to 538 mAh/g after 20 cycles

[49]

Fe3O4@Fe3C–C yolk–shell nanospindles Reversible capacity of 1,128 mAh/g at 500 mA/g, high rate used at anode capacity of 604 mAh/g at 2000 mA/g. Good stability, cyclability by maintaining 1,120 mAh/g at 500 mA/g for 100 cycles.

[50]

Si nanoparticles coated with 5-sulfoisophthalic acid (SPA) doped polyaniline

A high capacity of 925 mAh/g and high coulombic efficiency of 99.6% after long-term cycling 1,000 cycles was achieved.

[51]

Fe@Fe2O3 CSNs on graphene (Fe@ Fe2O3/graphene) hybrid material used at anode

Reversible charge capacity of 959.3 mAh/g up to 90 cycles @ current density of 100 mA/g, about 86.4% retention of first charge capacity. The electrode material exhibited long-life cycling performance at high currents.

[52]

(Continued )

112 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Commercially available Si wrapped by carbon shell CSNs

Used at anode, high capacity of 1384 mAh/g achieved. Capacity retention to 721 mAh/g and cycle life of 300 cycles with almost no capacity loss

[53]

Fe3O4@C CS structures were prepared such that core was about 30 nm diameter and shell of 3–7 nm used at anode

An initial discharge capacity of 982 mAh/g was achieved. At 0.1C a reversible capacity of 718 mAh/g was observed after 100 cycles. At 2C, the reversible capacity of 302 mAh/g was obtained.

[54]

Cu3Si@Si core–shell nanoparticles used at anode

Cu3Si@Si core–shell nanoparticles exhibited capacity ∼903.6 mAh/g at current density of 2 A/g over 400 cycles

[55]

Nanoparticle-nanorod core–shell LiNi0.5Mn1.5O4 spinel structures used at cathode

CS spinel exhibited discharge capacities of 121 and 100 mAh/g at 0.1C and 7C rates, respectively. Energy density of 1.6 Wh/cm3 observed

[56]

Dual yolk-shell comprising of Si/void/ SiO2/void/carbon structures used at anode

A stable and high capacity of 956 mAh/g after 430 cycles with capacity retention of 3% was observed

[57]

(a) Cyclic voltammograms (CVs) from the first 5 cycles for Si/void/SiO2/void/C from 0.01 V tp 3 V (with only 0.01–1.2 V shown) at a scan rate of 0.05 mV/s. (b) Charge and discharge profiles of Si/void/SiO2/void/C composite for the 1st, 2nd, and 100th cycles tested between 0.01 V and 3 V at a rate of 0.46 A/g. EIS (electrochemical impedance spectroscopy) results for Si/void/SiO2/void/C and Si/C composites. Reproduced from reference [57] Yang, L. Y. et al., Dual yolk–shell structure of carbon and silica-coated silicon for high-performance lithium-ion batteries. Sci. Rep. 5, (2015) 10908; doi: 10.1038/srep10908, http://www.nature.com/articles/srep10908, licensed under. CC BY, http://creativecommons.org/licenses/by/4.0/), DOI: 10.1038/srep10908.

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 113

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

(a) Schematic illustration of the fabrication process for the dual yolk-shell structure, (b), (c), (d), and (e) corresponding TEM images of Si, Si/SiO2, Si/SiO2/C, and Si/void/C spheres. Reproduced from reference [57] Yang, L. Y. et al., Dual yolk–shell structure of carbon and silica-coated silicon for high-performance lithiumion batteries. Sci. Rep. 5, (2015) 10908; doi: 10.1038/srep10908, http://www.nature.com/articles/srep10908, licensed under. CC BY, http://creativecommons.org/licenses/by/4.0/), DOI: 10.1038/srep10908. TiO2@graphitic-like C CS (TiO2@C) at anode

A current rate of 0.2 C resulted in better cycling performance of [58] nanostructures up to 40 cycles with a reversible capacity of 111 mAh/g.

Relationship between interface capacitance and electron density characteristics of (a) TiO2-700 and (b) TiO2@C-700 nanostructures according to Stern model. Reprinted from Kim et al., Improved Lithium Ion Behavior Properties of TiO2@Graphitic-like Carbon Core@Shell Nanostructure, Electrochem. Acta 147, 241–249, Copyright (2014), with permission from Elsevier.

(Continued )

114 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

SnO2@carbon spheres

When the SnO2/C ratio is 78.6/21.4 (w/w), surface area of SnO2@ mesoporpus hollow C spheres is 183 m2/g, specific capacity value is 450 mAh/g at 1/5 C after 50 cycles.

[59]

(a, b) TEM image of mesoporous hollow carbon spheres. Reprinted from Chen et al., New easy way preparation of core/shell structured SnO2@carbon spheres and application for lithium-ion batteries, J. Power Sources, 216, 475–481, Copyright (2012), with permission from Elsevier.

CS comprising of SiO2 (core) nanoparticles and poly(lithium acrylate) (in the shell) were used as functional fillers

Graphite electrode, LiNi0.6Co0.2Mn0.2O2 as cathode and composite polymer as electrolyte were studied. Lithium polymer cells were observed to have high ionic conductivity and good thermal stability.

Reaction scheme for synthesis of CS-structured SiO2(Li+) particles containing poly(lithium acrylate) in the shell. Reproduced from reference [57] Park, S. M. et al., High-performance lithium-ion polymer cells assembled with composite electrolytes based on CS structures SiO2 particles containing poly(lithium acrylate) in the shell, J. Electrochem. Soc. 162 (2) (2015) A3071, http://jes.ecsdl.org/content/162/2/A3071/F6.expansion.html, licensed under. CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), DOI: 10.1149/2.0081502jes.

[60]

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 115

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Carbon encapsulated tin (Sn@C) embedded graphene nanosheet (GN) composites (Sn@C–GNs) used at anode

Excellent cycle stability and high specific capacity of 1069 mAh/g was observed which faded to 566 mAh/g after 100 cycles.

[20]

SEM images of (a) Sn@C composites and (b) Sn@C-GN nanocomposites (insets show the relevant low magnification images). (c) and (d) TEM images. The schematic representation of chemical bonding and lattice compression in Sn@C-GN composites. Reproduced in part from [20] with permission of The Royal Society of Chemistry. Supercapacitors Nanostructured carbon onions

Micro-supercapacitors designed with power densities comparable to electrolytic capacitors, specific capacitance of 0.9 mF/cm2 at 100 V/s indicates high instantaneous power along with high specific capacitance.

[61]

Functionalized MWCNTs packed densely using layer-by-layer technique used at cathode

Reversible gravimetric capacity of ∼200 mAh/g at 100 kW/kg, improved lifetimes. Nanotube electrode as positive electrode and lithium titanium oxide as negative electrode yielded gravimetric energy ∼5 times higher than conventional electrochemical capacitors and power delivery ∼10 times higher than conventional lithium-ion batteries.

[62]

Vertically aligned CNTs, ionic liquid as electrolyte

Energy density of 148 Wh/kg, power density of 315 kW/kg at the voltage of 4 V

[63]

Hybrid nanocomposite of Zn2SO4 nanowires grown radially on carbon microfibers coated with MnO2 shells

Crystalline Zn2SO4 nanowires grown radially on carbon microfibers. Maximum specific capacitance of 621.6 F/g pristine MnO2 at a scan rate of 2 mV/s and 642.4 F/g at current density of 1 A/g, hybrid nanocomposite, specific energy density of 36.8 Wh/ kg at current density of 40 A/g.

[64]

(Continued )

116 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Polyaniline-MWCNT CS nanocomposite

Specific capacitance of 322 F/g and specific energy density of 22 Wh/kg, high retention upto 87% of initial capacity @ current density of 5 mA/cm2.

[65]

Manganese oxide/PEDOT coaxial CS

The nanocomposite exhibited a specific capacitance of 285 F/g and 92% retention after 250 cycles in 0.5 M Na2SO4 at 20 mV/s.

[66]

Highly graphitic carbon shells enclosed nitrogen doped carbon core.

Zeolite imidazole framework (ZIF) are sub-family of metalorganic frameworks were prepared with ZIF-8 crystals as core and ZIF-67 crystals as shells. Their thermal treatment results in nitrogen-doped carbon as core and graphitic carbon as shell. High N-doping of 16 wt. % and high surface area (∼1499 m2/g) were achieved. Graphitic carbon from ZIF-67 possessed highly graphitic walls with good conductivity. Material exhibited a specific capacitance value of 270 F/g at a current density of 2 A/g.

[67]

Synthetic scheme for the preparation of (a) ZIF-8 crystals and NC, (b) ZIF-67 crystals and GC, and (c) core– sell (CS) ZIF-8@ZIF-67 crystals and NC@GC. Reprinted with permission from (Tang et al., J. Am. Chem. Soc., 2015, 137, 1572–1580). Copyright (2015) American Chemical Society. Etched TiC to obtain carbon-derived carbon CS structures electrodes

Higher capacitance retention achieved. CS supercapacitor electrodes exhibit 27 Wh/kg energy densities and 40 kW/kg power densities. Core is microporous, amorphous, and shell has mesoporous graphitic structure.

[68]

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 117

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Schematic of the CS porous particle. Reprinted from Microporous and Mesoporous Materials, 218, Ariyanto et al., Synthesis of carbon core–shell pore structures and their performance as supercapacitors, 130–136, Copyright (2015), with permission from Elsevier.

Carbon nanocapsules@MnO2 CS particles

Specific capacitance of 163 F/g observed at a scan rate of 2 mV/s. Specific capacity retained to 97.8% after 5,000 cycles.

[69]

Hydrogenated single-crystal ZnO@ amorphous ZnO-doped MnO2 CS nanocables on carbon cloth

Specific capacitance of 1260.9 F/g. Capacitance per unit area of 26 mF/cm2, retention ∼87.5% obtained after 10,000 charge/ discharge cycles.

[70]

(a, b) SEM images of HZnO (hydrogenated ZnO nanowires grown on carbon cloth). (c) SEM image of HZC (HZnO coated with a layer of carbon) (d) SEM image of HZM (HZC deposited with a layer of MnO2). Reprinted with permission from (Yang et al., ACS Nano, 2013, 7, 2617–2626). Copyright (2013) American Chemical Society. (Continued )

118 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

0-D nanostructures: • • •

Porous Au/MnO2 Multishelled NiO PAN coated C-spheres

References [71–81]

• •

•

Form: hybrid nanoparticles, 1,145 F/g at 50 mV/s, 80% retention Form hollow nanospheres, 612.5 F/g at 0.5 A/g, 83.1% rate capability from 0.5A–0.3 A/g, 90.1% capacitance retention after 1000 cycles Form: hollow nanospheres, 525 F/g at 0.1 A/g; 50% rate capability from 0.1 to 10 A/g; 73% capacitance retention after 1,000 cycles

1-D nanostructures: • • • • •

O-deficient α-Fe2O3 and MnO2 AuPd@MnO2 CS CuO@AuPd@MnO2 CS CNT@PPy–MnO2 CS Co3O4@MnO2 core–shell nanowires

• • •

• •

Form: nanorods, Maximum energy density of 0.41 mW h/cm3​ achieved Form: nanopillars, 603 F/g at 5 mV/s; 52% rate capability from 5 to 100 mV/s; 93% capacitance Form: nanowhiskers, 1376 F/g at 5 mV/s; 58% rate capability from 5 to 100 mV/s; 99% capacitance retention after 5,000 cycles For: nanotubes, 268 F/g; 93% rate capability from 5 to 100 mV/s; 90% capacitance retention after 5,000 cycles 480 F/g at 2.67 A/g; 56% rate capability from 4 to 44.7 mA/cm2; 97.3% capacitance retention after 5,000 cycles

2-D nanostructures: MnO2-graphene

310 F/g at 2 mV/s; 73.5% rate capability from 2 to 500 mV/s; 95% capacitance retention after 15,000 cycles

3-D nanostructures: Mn/MnO2 CS porous structure

∼1200 F/g at 5 mV/s; 83% rate capability: 5–500 mV/s; 96% capacitance retention after 2,000 cycles

Self-supporting TiO2@Ni(OH)2CS nanowire arrays on carbon fiber paper

Specific capacity of 264 mAh/g at 1 A/g and 178 mAh/g at 10 A/g.

[82]

5.4 Metal-semiconductor core–shell nanomaterials for energy storage and conversion 119

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Schematic illustration for the fabrication of TiO2@Ni(OH)2 CS nanowire arrays on CFP. (b) SEM image of the TiO2 nanowire arrays, and (c) TiO2@Ni(OH)2 core/shell nanowire arrays on the CFP. Reproduced from reference [57] Ke, Q. et al. 3D TiO2@Ni(OH)2 CS arrays with tunable nanostructure for hybrid supercapacitor application. Sci. Rep. 5, (2015) 13940; doi: 10.1038/srep13940, http://www.nature. com/articles/srep13940, licensed under CC BY, http://creativecommons.org/licenses/by/4.0/), DOI: 10.1038/ srep13940. CS from mesocarbon microbead

Graphitic shell/amorphous core, used as cathode delivering 55 Wh/kg, power density of 6474 W/kg.

[83]

Crystalline core@amorphous shell (Ni3S4@MoS2)

A high specific capacitance of 1440.9 F/g at 2 A/g was obtained and good capacitance retention of 90.7% after 3000 cycles at 10 A/g.

[84]

Hydrogented-TiO2@MnO2 core–shell nanowires (CSNW) as cathode, hydrogenated-TiO2@C CSNW as anode

High specific capacitance of 139.6 F/g and maximum volumetric energy density of 0.30 mWh/cm3. Very good cycling performance.

[85]

Laser ablation for tantalum core and carbon shell

TaC-C CS nanostructures exhibited large specific capacitance and excellent rate capability. The cycling ability was also found to be remarkable.

[86]

Fuel cells Pt−Cu alloy nanoparticle in PEMFC as ORR catalyst.

The electrocatalytic Pt mass activity of the dealloyed CS particles (0.413 A/mg) for the ORR exceeds that of state-of-the art Pt electrocatalyst (0.104 A/mg) by 4 times

[87]

(Continued )

120 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Pd3Cu1/C cores and the selective Pt shell formation

Catalysts exhibited high activity (mass activity of ∼33 mA/mg of Pt, about 2.6 times higher than commercial Pt/C catalyst), high selectivity, and 4,000 h of long-term durability at the single-cell level

[88]

DFT predictions on high electrocatalytic activity of Pt-Cu CSNs.

Pt shell forms on amorphous core with smaller lattice parameter, compressive stresses appear in shell; d-band structure of Pt atoms changes, weak adsorption energy of reactive intermediates, increased ORR reactivity

[89]

Ru nanoparticles encapsulated by Pt shell

Alloy shells yielded higher steady state currents for methanol oxidation reaction (MOR) as compared to shells containing pure Pt.

[90]

Pt-based CS nanoparticles

Pt shell thickness influences the electronic and structural properties.

[91]

Pd/PdCo CS particles

Electrocatalysts were used to oxidize formic acid.

[92]

Single crystalline Ru core with Pt bilayer shell

Electrocatalyst applied successfully for alleviating carbon monoxide poisoning of the catalyst.

[93]

Ru nanoparticles coated with Pt layers

The effect of Pt packing density was studied and it was found that a packing density of 0.31 yielded 150% higher peak activity in MOR

[94]

Pt3Co as cathode electrocatalyst in PEMFCs

The study shows that Pt–Co yield trivalently oxidized cobalt in the cathode layer with fuel cell operation.

[95]

Method to synthesize Pt shell on nonPt core

Cu-under potential deposition forms thin Pt shell on non-Pt core. Pt/Au/C catalysts showed mass activity of 306 A/g, that is, ∼1.5 times more than Pt/C.

[96]

Scheme for oxygen reduction reaction at Pt catalyst. Reproduced with permission from Inaba, M. and Daimon, H., Journal of the Japan Petroleum Institute, Development of Highly Active and Durable Platinum CS Catalysts for Polymer Electrolyte Fuel Cells, 58 (2), (2015) 55–63, The Japan Petroleum Institute. CSNs made from titanium dioxide (TiO2) and carbon derived from egg white protein

The catalyst was used as a capacitive layer in microbial fuel cells and a power density of 2.59 W/m2 was obtained. This was higher than the conventional graphite electrodes by 201%.

[97]

5.5 Correlation between electronic structure and electrochemical activity of core–shell nanomaterials 121

Table 5.1  Summarizes Some of the Important Results of the Core–Shell Nanoparticles Applied for Supercapacitor, Battery and Fuel Cell Applications (cont.) Electrode Material

Remarks

References

Pd core and FePt shell nanoparticles

1 nm thin FePt shell on Pd core exhibited 15 times more ORR activity with a 140 mV gain in onset potential as compared to 3 nm FePt thick shell.

[98]

Rh5@Ptx/C CS particles

2-step H reduction method to synthesize Rh5@Ptx/C CSNs. Strong interaction between Rh and Pt but alloy formation between Rh and Pt was not found. Rh core was found to enhance catalytic activity of Pt shell.

[99]

Au@Pt, Pt@Au, Fe3O4@Au@Pt

CSNs show catalytic activity towards ORR and MOR

[100]

Pt shell over a copper core applied for ORR

Electrocatalyst performs better than commercial Pt catalyst. It was [101] observed that specific activities of the Pt@Cu samples increase linearly with increasing initial nominal Cu content.

Pt monolayers deposited on C-supported nonnoble metal-noble metal forming CSNs

Three Pt monolayer electrocatalysts investigated namely, Pt shell on Au/Ni, Pd/Co and Pt/Co core. High activity of electrocatalysts achieved by using very less amounts of noble metals. Total noble metal mass activity of Pt/AuNi10/C, Pt/PdCo5/C and Pt/ PtCo5/C w.r.t. Pt/C were approximately 3.7 mA/µg, 2.5 mA/µg and 5.1 mA/µg, respectively at 0.8 V.

[102]

Pt-based icosahedral nanocages

Nanocages enclosed by 111 facets and twin boundaries. Specific activity of 3.5 mA/cm2 observed towards ORR. After about 5,000 cycles, mass activity dropped from 1.28 to 0.76 A/mgPt.

[103]

Bimetallic Pt-Cu CS particles

Catalyst studied nonaqueous O evolution. Pt core stabilizes Cu(I) surface sites, exhibits better performance through reduced charging overpotential.

[104]

Pt-Ru bimetallic nanoparticles

Nanoparticles have Pt-rich core and Ru-rich shell. Annealing at 500°C, more Pt atoms diffuse to surface thereby increasing the extent of alloying.

[105]

5.5  CORRELATION BETWEEN ELECTRONIC STRUCTURE AND ELECTROCHEMICAL ACTIVITY OF CORE–SHELL NANOMATERIALS As discussed previously, amalgamation of more than one material is known to give rise to different properties. The properties of hybrid materials are also influenced by their arrangement at nanoscale. The CSNs are no different in this regard. The combination of core and its shell, their order (material A as core and B as shell or vice versa), dimensions and diameters, and so on all influence their properties. Application of CSNs as alternative energy

122 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

■■FIGURE 5.9  Change in (A) O and (B) OH adsorption energy upon a strain being imposed on the CSNP due to the foreign core metal. The inset of (A) shows the same data on the Pt(100) slab models as a control experiment. The strain was evaluated relative to the size of the Pt55 NP and thus it was termed as ‘relative strain’. Likewise, the adsorption energy was evaluated relative to that of the Pt55 NP. Several CSNPs are not very consistent with the regression lines; CSNPs with a deviation larger than 0.1 eV are highlighted with arrows. (Reproduced from ref. [107] J. Shin, J.-H. Choi, P.-R. Cha, S.K. Kim, I. Kim, S.-C. Lee, D.S. Jeong, Catalytic activity for oxygen reduction reaction on platinumbased core–shell nanoparticles: all-electron density functional theory, Nanoscale 7 (2015) 15830–15839, with permission of The Royal Society of Chemistry.)

options is broadly included to exploit their electronic properties. The electron distribution in such systems has been tailored from their nascent form influencing the mechanism of charge flow or charge interaction. Theoretical studies carried out on CS particles shed some more light on the origin of the “interesting” properties of these particles. The core and the shell comprise of different materials and therefore, naturally there would be a mismatch between their lattice parameters. This gives rise to strain between core and shell that results in a changed electronic structure of the shell. [106] Density functional theory computations showed that for elements in group 8 to group 11, the adsorption energy of O and OH on Pt shell decreased with decrease in strain [107]. Fig. 5.9 depicts the variation in adsorption energy with strain. Theoretically, elements of lower period and lower group appear to be more catalytically active owing to their larger compressive strains and associated larger adsorption energies. Another effect termed as ligand effect also influences the adsorption energies when different core metals are used. The effect of the core metals can shift the energy of the d-band. If the presence of a core metal in core shell particles, results in a negative shift of d-band towards the Fermi level, then antibonding orbitals can also be filled by electrons thereby resulting in a loose binding of adsorbates to the surface [107]. These changes result in interesting properties of CS particles and govern their field of application. For instance, CS particles employed for fuel cell applications are mostly developed with the intention to catalyze oxygen reduction or hydrogen oxidation. Before embarking on understanding the catalytic activity of CSNs, let us first understand the advantage of using two metals over one as catalyst. Among bimetallic catalysts, Pt–Ru catalysts have been particularly employed for methanol reduction (MOR) in methanol fuel cells (MFCs). The catalysts used in these fuel cells suffer

5.5 Correlation between electronic structure and electrochemical activity of core–shell nanomaterials 123

from the problem of catalyst poisoning by carbon monoxide that may enter from the fuel gas or CO released due incomplete oxidation of fuel (methanol or ethanol). If only Pt is present, the CO formed as a result of methanol oxidation gets adsorbed on Pt sites causing catalyst poisoning and eventually resulting in the cell to stop working. This can be alleviated by incorporating oxophilic metals along with Pt to oxidize carbon monoxide. Oxophilic species like Fe and Ru have been used to oxidize CO to CO2 [108]. Thus, bimetallic catalysts were investigated and gradually gained popularity owing to better performances than single metal catalysts. The performance of Pt-Ru bulk alloys has also been studied towards hydrogen oxidation in the presence of CO [109,110]. The presence of Ru near Pt sites results in preferential adsorption of hydroxyl ions (OH) onto Ru sites and oxidize CO at a nearby Pt site in the process. Oxidation of CO from a Pt site saves Pt from being poisoned and later becoming inactive. It was found that the structural sensitivity of both spectator (Pt) and reactive species (Ru) is important in reactions [110]. Such effects have also been investigated in the case of nanoparticles or CSNs. It has been reported that the CS particles follow bifunctional mechanism in which the core and shell materials have different electronic distribution, especially at sub-micrometer scales [111]. These differences in charge distribution within the particles influences the way in which adsorption of fuel gas or reactant gas takes place on catalyst particle. The use of CSNs has also been investigated from this aspect. In Pt-Ru core shell nanoparticles, Pt and Ru exist in close proximity at the nanometer scale as compared to bulk in such a way that their orbitals can overlap. Hence, Ru can more efficiently convert the CO to CO2 using the adsorbed hydroxyl ions and results in longer and enhanced catalytic activity of Pt [106]. Similarly, gold nanoparticle decorated PtFe catalysts have been employed for anti-CO poisoning and mass activity and specific activity of 1324 mA/mg and 3.01 mA/cm2 were observed respectively for the ­as-synthesized ternary PtFeAu catalyst [112]. The amorphous Fe2O3 core was reported to have more probability of adsorbing oxygen employed for MOR [108]. The Fe2O3 CSNs cause the Pt 4 f7 2 orbital to shift up in energy causing the Pt-CO bond to weaken and promote dehydrogenation during MOR [108]. In another example, monocrystalline silver shell coated over platinum core nanoparticles exhibited increased resistance towards oxidation [113]. It was found that platinum core transfers electron density to silver shell. This transfer is more for thin silver shells and decreases as shell thickness increases. Due to platinum core, silver shell acquires more electron density and therefore exhibits resistance to oxidation. Further, the electron transfer from platinum to silver and back-donation of electron density to platinum

124 CHAPTER 5  Metal-semiconductor core–shell nanomaterials for energy applications

plays a crucial role in altering the binding energy of the adsorbates. The advantage of CS architecture is not limited to electronic properties alone. The flexibility of tailoring the electronic properties of CS particles has also been extended to bio-molecular diagnostics. The use of silver shell onto gold core nanoparticles for bio-molecular diagnostics makes use of the electronrich silver, which act as efficient probes [114]. Reversible optical properties of such nanoparticles have been investigated by employing electrochemical modifications in nanoparticle pairs or in individual nanoparticles [115]. The optical and electronic properties could be modified by controlling the morphology, chemical composition, fundamental and electronic coupling strength of nanostructures. Besides the celebrated spherical CSNs, CS nanocubes and CS concave decahedra have also been designed [116,117]. The Ag-Pt-Ag and Ag-Pt nanocubes were subjected to rotating ring disk electrode and exhibited better mass activities towards ORR. Both the catalysts displayed four-electron pathway mechanism for ORR [116]. Excellent durability was exhibited by Pd@Pt CS concave octahedral [117]. Thus, the family of CSNs gives flexibility to material scientists in designing them and attaining desirable properties.

5.6  FUTURE OUTLOOK AND CHALLENGES The advantages of using core shell nanomaterials range from exploiting their architecture to using the synergistic effects of materials comprising the core and the shell. The proximity of a few nanometer thin shell material coated onto a different core material has been found to induce strain in these hybrid nanoparticles, which in turn influences their electronic structure. Ligand effect in which hybridization between orbitals of metals takes place influences the properties of metals [118]. A hybrid nanoparticle system emerges that carries interesting physical, chemical, optical, magnetic and mechanical properties. The size, shape, flexibility of choosing materials for core and shell combinations provides a very good tool to synthesize new combinations with newer properties. The CSNs help in reducing the amount of precious catalysts (e.g., Pt, Au) in the fuel cells electrode, while maintaining high oxygen reduction or hydrogen oxidation activity and hence the cost of these devices. The future studies and efforts would be directed in controlling the sizes and architecture of such catalyst/electrode particles. The control in designing such CSNs would serve as a tool to tune the strain as well as the related electronic structure in the shell also. To meet these challenges, new synthesis methods or better control on synthesis techniques is required. These structures have a wide applicability as they can easily be diffused with the current technology of energy storage and conversion devices. Nexus of awareness towards environment and fundamentals of material science

References 125

holds the key for designing future energy solutions. Dedicated and accelerated efforts are required in this direction to come up with alternate green energy solutions that help Earth to remain in its stable environmental state.

ACKNOWLEDGEMENTS RN acknowledges Symbiosis International University for research seed grant. B. P. Vinayan acknowledges the Alexander von Humboldt Foundation for research funding.

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