Themed issue: nanomaterials for energy conversion and storage

One of the main challenges of the twenty-first century is adequately meeting ever-growing energy needs. Global energy consumption up-to-date is about 15 TW per year. Most of the electric power for our needs comes from burning fossil fuels (coal, oil and natural gas). There are two main problems with using fossil fuels. Fossil fuels found underneath the earth's surface take millions of years to form, but they are being consumed rapidly at a prohibitively high rate. There are many projections that oil will run out of stock in the next 30–50 years, leaving coal as the only option. The second point of concern with power generation using fossil fuels is the damage to the environment caused by unusually large amounts of CO₂ released in the process. One-megawatt coal-powered reactor releases 2.58 million lbs of CO₂ per year. Shocked by the visible damage occurring to the natural environment around us, all nations of the world have agreed through the Kyoto Protocol to take new measures so as to limit the amount of greenhouse gas emissions released. For emerging power-houses like China and India, this would mean generating all new power essentially via renewable energy pathways involving non-C fuel resources such as wind, sun, geothermal energy, biogas and hydroelectric power.

Sun and wind are abundant natural fuel reserves that can be harvested using novel power conversion devices, even to the extent that they could fully replace power production by conventional routes. The amount of solar energy reaching the earth's surface in one hour (18 TW) compares well to the annual global energy consumption as of now. Serious limitations of sun and wind is that they are intermittent, available only for limited hours per day (sun) or with unreliable frequency and intensity (wind). Hence for these two energy routes to be viable, suitable cost-effective storage systems have to be identified. Using suitable storage systems, the captured energy can be put to use 24/7 all 365 days of the year. All current research efforts worldwide thus focus on two directions: the capture and storage of sun and wind energy.

There are many promising approaches to solar and wind energy harvesting and storage. Solar radiation covers a very broad spectral range, from near-UV to infrared. Photovoltaic solar cells permit conversion of sunlight in the visible and near-IR region directly to electricity. The energy content of infrared photons is low and a preferable mode of harvesting them is through a thermal route. Using planar, parabolic and trough collectors it is possible to efficiently capture sun energy as heat. The most popular and efficient method of storage is via storage batteries that can be repeatedly charged and discharged (secondary type). Portable electronic devices such as laptop computers and mobile phones are used by billions worldwide and they depend largely on portable power packs. There is an urgent need to find methods of making cost-effective, energy efficient and light-weight portable batteries. Another key area is the generation of energy-fuels such as H₂ gas and methanol from the photodecomposition of water and biomass respectively. These fuels can subsequently be burnt in fuel cells to have power on demand. Finely divided colloidal particles of Pt have been shown to be effective photocatalysts for use in various photochemical redox systems that lead to H₂ evolution from water. In a contributed paper to this themed issue, Fukuzumi and Yamada (DOI: 10.1039/c2jm32926c) have discussed the possible use of non-Pt metal nanoparticles for the photooxidation and photoreduction of water. For example efficient photocatalytic H₂ evolution was made possible by using nanoparticles of Ru.

The design of new energy conversion devices can follow one of the following two approaches. In a top-down approach, one can start with macroscopic/large size materials and slice them to the small dimensions that are required for device fabrication. An alternate, more elegant route is the bottom-up approach where we start with simple atoms and molecules (at a nanometric scale) and build up the device in several steps. There are several valid reasons to explore this second route. During the last two decades there has been tremendous progress in the synthesis of a wide variety of nanomaterials (zero, one and two dimensional materials) by various physico-chemical techniques. Other progress has been made in the experimental techniques for the characterization and even manipulation of such nano-sized materials at the molecular level. Tailored design of photonic and opto-electronic devices is feasible today. The advantages are full control of the chemical composition, structural, electronic and morphological properties of the device components and the device structure as well. In this topical issue of Journal of Materials Chemistry, an attempt has been made to collect a number of research papers that demonstrate the rich and diverse aspects of this exciting field of research: nanomaterials for energy conversion and storage.

Photovoltaic solar cells for the direct conversion of sunlight to electricity can be grouped under three generations, partly on history and partly on the device fabrication methods. First generation solar cells are well represented by crystalline Si, or GaAs wafer-based solar cells. This is a fairly well developed (mature) technology. Theoretical calculations indicate that for solar cells based on single band gap semiconductors, the maximum possible light-to-electricity conversion can be 32% (known as the Schokley–Queisser limit). The efficiency of small size single-crystal Si solar cells has reached over 24% and commercial large area solar cells (modules) are available at modest prices with 16–18% conversion. Thin film versions of semiconductor-based solar cells of Si, CdTe and Cu–In–Ga–Se (CIGS) form the second generation of photovoltaic solar cells. Significant advances and breakthroughs are taking place currently in the second generation solar cells, with new world record conversion efficiencies being reported every few months. Commercial thin film solar cells (including flexible light weight ones) have been made with impressive conversion efficiencies of 14–18%. For CIGS, Tiwari et al. have reported efficiencies up to 18.8% on glass substrates and flexible thin film solar cells on polymer films with a new record efficiency of 18.7%, which has been independently certified by the Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg, Germany.¹ For CdTe, the same group has reported the fabrication of thin film solar cells with efficiencies of up to 15.6% on glass substrates and up to 13.5% on polyimide foil.

Third generation solar cells

Third generation solar cells involve conceptually novel structures where the role of nanoscience and nanotechnology have an increasingly major role at the design and device fabrication stages. One approach targets solar cells that can deliver a solar-to-electrical conversion efficiency well beyond the single-junction Schokley–Queisser limit of 32% using multi-layers of a graded series of light absorbers (multi-junctions), Devices that permit conversion of low energy photons to ones of a higher energy content (up-conversion) and the inverse process (down-conversion) are being tested. Up-conversion studies for conversion of infrared light to visible wavelength invariably use doped rare-earth fluorides as the main luminophore. Faulkner et al. report in this issue key quantitative measurements (absolute quantum yields) in NaYF₄:Er, Yb upconverters (DOI: 10.1039/c2jm33457G). The authors show that formation of the hexagonal phase of this material is not in itself sufficient to guarantee high upconversion efficiency.

One exciting possibility is a multiple exciton generation (MEG) solar cell. MEG refers to the process where a high energy photon can produce more than one electron–hole pair per absorbed photon. Experimental confirmation of MEG (also referred to as carrier multiplication) was reported last year (Dec 2011) using PbSe quantum dots in Science by Nozik et al.² The external quantum efficiency (specially resolved ratio of collected charge carriers to incident photons) peaked at 114% for the best device they could fabricate.

Sol–gel chemistry permits the synthesis of colloidal particles at a nanometric scale with good control of particle size and uniform size control. These nanosized colloids can be deposited on surfaces to form elegant mesoscopic layers with an enormous surface area with good control of pore size and pore volume. Numerous studies reported during the last two decades have shown that the mesoporous structures formed from colloidal suspensions of semiconducting oxides are excellent substrates for various photonic and optoelectronic devices.

Dye sensitized solar cells (DSC)

A major breakthrough in the application of such mesoscopic films occurred in the early nineties with the report of a solar cell based on the concept of dye-sensitization. In dye-sensitized solar cells (DSC), efficient light energy conversion is achieved through light induced electron transfer from adsorbed dye molecules to the semiconducting oxide substrate and subsequent collection of the injected charge carriers at the back/collector electrode. Tremendous research efforts on dye-sensitized solar cells during the last two decades (over 5000 research publications) have led to the identification of several designs of DSC in terms of color, transparency and portability.³

The DSC in the standard design examined extensively uses mesoscopic layers of TiO₂ as the electron-acceptor and key substrate and a mixture of iodine and iodide as the redox electrolyte. The solar conversion efficiency for this design with Ru-dyes as the light absorber has reached an "independently certified" efficiency of 11.4%.⁴ The highest solar conversion efficiency of 12.4% was obtained using a Zn-porphyrin dye, YD2, with good light absorption properties and a polypyridine complex of Co(III) as the redox mediator.⁵ An attractive solid state design with potential for a portable version has also been studied extensively and this uses a solid state hole transport material, such as Spiro-OMeTAD, spin-coated over the dye layer to shuttle the charges between the two electrodes. In a very recent report, Miyasaka, Snaith and coworkers have reported a novel meso super-structured solar cell based on a highly crystalline Peroskite light absorber and spiro-OMeTAD as the electron couple.⁶ The solar cell delivering an open-circuit voltage of 1.1 V showed a solar conversion efficiency of 10.9% in a single junction device.

The choice of dyes for use in DSC has expanded significantly. Tailoring of absorption and redox properties is achieved with the introduction of various donor, acceptor and π-spacer units to the basic chromophoric unit (family of dyes referred to as D–π–A dyes). In a contributed paper of this issue, Palomares et al. have discussed recent progress in the design of photosensitizers using various porphyrins, phthalocyanines and organic dyes such as squarines, indolenes and perylenes as examples (DOI: 10.1039/c2jm34289h). In another paper Yang et al. described the design of a DSC with 7.7% conversion efficiency using organic dyes with quinacridone and furan moieties as planar π-spacers (DOI: 10.1039/c2jm31929b).

In addition to the electronic properties, the morphological properties of the oxide anode affects significantly the performance of a dye-sensitized solar cell. Mesoporous films made out of very small (<20 nm diameter) particles are optically translucent and they provide an enormous surface area for dye adsorption and limited pore diffusion due to smaller pore sizes. Films made out of larger (>50 nm) particles have their own advantages. They are opaque and exhibit much stronger light scattering properties. It turns out, a judicious combination of these two types of films gives the best overall conditions for dye loading and light harvesting. There have been a number of studies in recent years on maximizing light harvesting using different approaches including plasmonics. Zhu et al., in a contributed paper describe results of their studies on electrospun nest-shaped TiO₂ structures as a scattering layer for DSCs (DOI: 10.1039/c2jm33219a). Though TiO₂ has been the dominant anode of choice for dye sensitization studies, promising results have also been obtained with other semiconducting oxides, ZnO in particular. As in the case of titania, ZnO can be grown easily in various morphologies such as wires, nanoparticles and hierarchical nanostructures. In a contributed paper, Etgar et al. have reported on a simple method to produce highly stable and crystalline ZnO nanowire-based films that exhibit low charge recombination (DOI: 10.1039/c2jm34904c).

Quantum dot solar cells

A major application of the sol–gel chemistry has been in the synthesis of nanosized particles of controllable dimensions of various inorganic semiconducting materials. At large (macroscopic) dimensions, all semiconductors exhibit bulk behaviour with a single bandgap. However when the particle size becomes very small (on a nanometric scale), quantum size effects kick in. Nanosized Quantum Dot particles exhibit a tunable bandgap across a wide range of energy levels. Thus they are attractive candidates for fundamental studies and for application in various photonic devices. In one line of research, quantum dot particles of different sizes (bandgap absorption) have been used as sensitizers deposited on top of mesoscopic layers of TiO₂. A number of studies have used quantum sized particles of CdTe and PbSe. To date these cells have reached solar conversion efficiency in the range of 7%. High surface area-to-volume ratios of these particulate systems results in bare surfaces that can become traps. Sargent and coworkers have reported an elegant solution to this surface traps problem by passivating the surface of spin-coated QDs using a chlorine solution. Solar cells fabricated using hybrid passivated QD have shown a certified record efficiency of 7.0%.⁷

Carbon nanodots (C-dots) are a new class of carbon nanomaterials with sizes below 10 nm, first obtained during the purification of single-walled carbon nanotubes through preparative electrophoresis in 2004. Compared to traditional semiconductor quantum dots (QDs) and organic dyes, photoluminescent C-dots are superior in terms of high aqueous solubility, robust chemical inertness, easy functionalization, high resistance to photobleaching, low toxicity and good biocompatibility. In a review paper, Li et al. have provided an overview of this exciting area of research: synthesis, properties and applications of carbon nanodots (DOI: 10.1039/c2jm34690g).

Organic photovoltaic cells (OPV)

The efficiency of light energy conversion in organic photovoltaic solar cells has also been raised significantly in recent years. Bulk heterojunctions of interpenetrating polymer networks carrying donor and acceptor groups have been found to be efficient routes for light energy conversion. Earlier this year (April 2012) the German firm Heliatek announced that it had reached a new world record for organic photovoltaic cells (OPV) with a conversion efficiency of 10.7% on a 1.1 cm² cell, confirmed independently by the Swiss firm SGS.⁸ The cells use specially synthesized organic polymeric oligomers. In 2010 a firm, Konarka, based in Boston, USA reported on the design of an OPV with a NREL-certified record conversion efficiency of 8.3% for a 1 cm² cell. In June 2011, Konarka reached a major milestone when it completed the installation of the largest OPV system of its kind in the world at its large-scale manufacturing plant in New Bedford, Mass., USA.⁹ In further improvements, in a press release in Feb 2012, Konarka reported achieving a 9.0% efficiency for their “power plastic” solar cell (certified by the Newport Corporation).¹⁰ Konarka's organic solar modules have passed through aging tests following the type approval standard for photovoltaic thin-film modules IEC 61646. Unfortunately Konarka declared bankruptcy this summer (May 2012), putting further developments in this area under question.

In another key development, the design of inverted polymer solar cells with high solar-conversion efficiency has been reported in recent years. In bulk heterojunction solar cells (BHJ), a transparent conducting oxide ITO and a low-working functional metals such as Al or Ca are used as the anode and cathode, respectively. The performance of the device is influenced strongly by the stability of these electrodes. The cathode in particular is susceptible to degradation by oxygen and water vapor. In inverted solar cells, the material choices for the anode and cathode are reversed, e.g., an ITO-electrode modified with a conducting polymer such as PFN is used as the cathode. Air-stable high working functional metals such as Au or Ag are used as anodes. Recently there have been several reports on inverted polymer solar cells exhibiting record conversion efficiencies of 9.3%.^11,12

Li-ion batteries

Li-ion batteries work on the “insertion” of Li-ions into various host materials. Changes in chemical potential of the host that occurs upon insertion are responsible for the electric storage mechanism.¹³ Charge and discharge reactions correspond to the insertion and leaching out of the Li-ions of the host lattice. In the very early studies, Li-ions were inserted into metal halides such as AgCl, CuCl₂, CuF₂ and NiF₂. A major breakthrough occurred with reports of Scrosati using two insertion compounds based on metal oxides or sulfides (the rocking chair concept).¹⁴ A Li_xWO₂–Li_yTiS₂ cell was described with an average voltage of 1.8 V. Work of Goodenough and Mizushima et al. extended this to include the possible use of Li_xCoO₂ or Li_xNiO₂ (spinel oxides) as potential candidates.¹⁵ Since these early reports, tremendous progress has been obtained in the design of high efficiency Li-insertion batteries using various rock salt spinel structure-type oxides. Ma et al. report in this issue the possible use of anatase TiO₂ microspheres with adjustable mesoporosity for the reversible storage of Li-ions (DOI: 10.1039/c2jm33724j). In a related work, Liu et al. describe a facile synthesis of nanostructured vanadium oxide V₂O₅ as cathode materials for efficient Li-ion batteries (DOI: 10.1039/c2jm34078j). Radha et al. report on the thermochemical properties of two recently introduced cathode materials in the fluorosulfate family with the composition LiFe_1−xMn_xSO₄F (0 ≤ x ≤ 1) assuming the triplite and tavorite structures (DOI: 10.1039/c2jm34071b). Cathodes based on fluorosulphates show storage capacity in excess of 125 mA h gm⁻¹.

Supercapacitors

Like batteries, “supercapacitors” are a type of electrochemical energy storage applied in “power” industries. While batteries store charges chemically, super/ultracapacitors store them electrostatically. Ultracapacitors are true capacitors in that energy is stored via charge separation at the electrode–electrolyte interface, and they can withstand hundreds of thousands of charge–discharge cycles without degrading. Supercapacitors are ideal choices where there is the need for a quick surge of power. Supercapacitors based on two metal plates coated with a sponge-like porous, activated C are being incorporated to solar cells and hydrogen fuel cell car batteries. Graphene based electrodes show great promise due to their excellent electronic and electrical properties, high surface area, good chemical stability and easy processability. In a contributed paper Khanra et al. report on possible improvements of the graphene performance through surface modification using 9-anthracene carboxylic acid (DOI: 10.1039/c2jm34838a). Fic et al. describe the possible use of protic aqueous electrolytes for better control of the charge transfer processes taking place at the electrical double layer (DOI: 10.1039/c2jm35711a).

Thermoelectric energy conversion

Thermoelectric materials provide a means of the direct conversion of thermal energy to electrical energy through the Seebeck effect. The Seebeck effect refers to the flow of electric current through the junctions of two different conductors kept at different temperatures. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (σ), while reducing the thermal conductivity (κ). Nanostructures make it possible to modify the fundamental trade-offs between the bulk material properties through the changes in the density of states and interface effects on the electron and phonon transport. Since zT (≡ S²σT/κ) is composed of material-dependent quantities, materials engineering approaches are desired in order to achieve high thermoelectric efficiencies. Bi₂Te₃ is a well characterized material used for such thermal to electrical conversion processes. In a contributed paper, Ikeda et al. describe the results of their studies on the microstructures and the Seebeck coefficients of thermoelectric alloys made out of PbTe–Ag₂TE-Sbb₂Te₃ systems (DOI: 10.1039/c2jm32677a).

A related approach is “latent thermal energy storage” through the use of solid–liquid organic and inorganic phase change materials such as paraffin wax and salt eutectics. Latent heat storage is attractive due to its ability to provide a high energy storage density and its characteristics to store heat at a constant temperature corresponding to the phase transition temperature of the heat storage substance. Phase Change Materials (PCMs) for use in latent thermal energy storage can be organic or inorganic compounds. Organic compounds are paraffins and non-paraffinic materials, such as fatty acids. Inorganic compounds are salt hydrates, salts, metals and alloys. On this topic, Mallow et al. report in this issue their investigation of the stability of paraffin-exfoliated graphite nanoplatelet composites for latent thermal energy storage systems (DOI: 10.1039/c2jm35112a).

We would like to thank all the authors who have contributed papers to this themed issue. We sincerely hope that this small collection of papers will give a cross-sectional view of the kind of exciting research area and inspire many more to enter this field. Our thanks also go to the editorial staff of Journal of Materials Chemistry for all their support.

M. Grätzel
K. Kalyanasundaram

References

1. A. Chirila, S. Buecheler, F. Planezzi, P. Bloesch, C. Gretener, A. R. Uni, C. Felia, L. Kranz, J. Perrenoud, S. Seyrling, R. Varma, S. Nishiwaki, Y. E. Romanyuk, G. Bilger and A. N. Tiwari, Nat. Mater., 2011, 10, 857 , and references cited therein.
2. O. E. Semonin, J. M. Luther, S. Choi, H.-Y. Chen, J. Gao, A. J. Nozik and M. C. Baird, Science, 2011, 334, 1530.
3. Dye Sensitized Solar Cells, ed. K. Kalyanasundaram, EPFL Press, CRC Press, Lausanne, 2010, ISBN-13: 978-1-4398-0866-5, 978-2-940222-36-0.
4. L. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S. Zhang, A. Yang and M. Yanagida, Energy Environ. Sci., 2012, 5, 6057; Y. Chiba, A. Islam, Y. Watanabe, R. Komiya, N. Koide and L. Han, Jpn. J. Appl. Phys., 2006, 45, L638.
5. A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, Md. K. Nazeeruddin, E. W.-G. Diao, C.-Y. Yeh, S. M. Zakeeruddin and M. Graetzel, Science, 2012, 334, 629.
6. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012 DOI:10.1126/science.1228604 , in press.
7. M. Shalom, S. Buhbut, S. Tirosh and A. Zaban, J. Phys. Chem. Lett., 2012, 3, 2436; A. H. Ip, S. M. Thon, S. Hoogland, O. Voznyy, D. Zhitomirsky, R. Debnath, L. Levina, L. R. Rollny, G. H. Carey, A. Fischer, K. W. Kemp and E. H. Sargent, Nat. Nanotechnol., 2012, 7, 877.
8. Heliatek, Dresden, Germany, Press release, 8 May 2012.
9. Konarka, Boston, Mass, USA, Press release, 30 November 2010.
10. Konarka, Boston, Mass, USA, Press Release, 28 February 2012.
11. Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591; J. Sun, Y. Zhu, X. Xu, L. Lan, L. Zhang, P. Cai, J. Chen, J. Peng and Y. Cao, J. Phys. Chem. C, 2012, 116, 14188.
12. Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter and A. J. Heeger, Adv. Mater., 2011, 23, 1679; L. Dou, J. You, J. Yang, C.-C. Chen, Y. He, S. Murase, T. Moriarty, K. Emery, G. Li and Y. Yang, Nat. Photonics, 2012, 6, 180.
13. R. Marom, S. F. Amalraj, N. Leifer, D. Jacob and D. Aurbach, J. Mater. Chem., 2011, 21, 9938–9954; J.-M. Tarascon, Philos. Trans. R. Soc. London, Ser. A, 2011, 368, 3227–3241.
14. F. Croce, E. Appetecchi, L. Pensi and B. Scrosati, Nature, 1998, 394, 456; B. Scrosati and J. Garche, J. Power Sources, 2010, 195, 2419.
15. M. M. Thackery, W. I. F. David, P. G. Bruce and J . B. Goodenough, Mater. Res. Bull., 1983, 18, 461; J. B. Goodenough, J. Solid State Electrochem., 2012, 16, 2019; J. B. Goodenough and Y. Kim, Chem. Mater., 2010, 22, 587.

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