Showing papers in "Advanced Energy Materials in 2011"

Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air

Abstract: In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li-air and Zn-air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of the new electrochemical systems.

Graphene–Cellulose Paper Flexible Supercapacitors

Abstract: [Weng, Zhe; Su, Yang; Li, Feng; Du, Jinhong; Cheng, Hui-Ming] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. [Wang, Da-Wei] Univ Queensland, ARC Ctr Excellence Funct Nanomat, Australian Inst Bioengn & Nanotechnol, Brisbane, Qld 4072, Australia.;Cheng, HM (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;cheng@imr.ac.cn

A Stable Vanadium Redox-Flow Battery with High Energy Density for Large-scale Energy Storage

Abstract: The all-vanadium redox flow battery is a promising technology for large-scale renewable and grid energy storage, but is limited by the low energy density and poor stability of the vanadium electrolyte solutions. A new vanadium redox flow battery with a significant improvement over the current technology is reported in this paper. This battery uses sulfate-chloride mixed electrolytes, which are capable of dissolving 2.5 M vanadium, representing about a 70% increase in energy capacity over the current sulfate system. More importantly, the new electrolyte remains stable over a wide temperature range of −5 to 50 °C, potentially eliminating the need for electrolyte temperature control in practical applications. This development would lead to a significant reduction in the cost of energy storage, thus accelerating its market penetration.

All-Solid-State Lithium-Ion Microbatteries: A Review of Various Three-Dimensional Concepts

Abstract: With the increasing importance of wireless microelectronic devices the need for on-board power supplies is evidently also increasing. Possible candidates for microenergy storage devices are planar all-solid-state Li-ion microbatteries, which are currently under development by several start-up companies. However, to increase the energy density of these microbatteries further and to ensure a high power delivery, three-dimensional (3D) designs are essential. Therefore, several concepts have been proposed for the design of 3D microbatteries and these are reviewed. In addition, an overview is given of the various electrode and electrolyte materials that are suitable for 3D all-solid-state microbatteries. Furthermore, methods are presented to produce films of these materials on a nano- and microscale.

Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend

Abstract: Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10% In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these two components

Hydrothermal Carbonization of Abundant Renewable Natural Organic Chemicals for High-Performance Supercapacitor Electrodes

Abstract: This is the peer reviewed version of the following article: Wei, L., Sevilla, M., Fuertes, A. B., Mokaya, R. and Yushin, G. (2011), Hydrothermal Carbonization of Abundant Renewable Natural Organic Chemicals for High‐Performance Supercapacitor Electrodes. Adv. Energy Mater., 1: 356-361. doi:10.1002/aenm.201100019, which has been published in final form athttps://doi.org/10.1002/aenm.201100019 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions

Semi‐Solid Lithium Rechargeable Flow Battery

Abstract: Global energy and climate-change concerns have accelerated the electrifi cation of vehicles, aided by advances in battery technology. It is now recognized that low-cost, scalable energy storage will also be key to continued growth of renewable energy technologies (wind and solar) and improved effi ciency of the electric grid. While electrochemical energy storage remains attractive for its high energy density, simplicity, and reliability, existing battery technologies remain limited in their ability to meet many future storage needs. Here we propose and demonstrate a new storage concept, the semi-solid fl ow cell (SSFC), which combines the high energy density of rechargeable batteries with the fl exible and scalable architecture of fuel cells and fl ow batteries. In contrast to previous fl ow batteries, energy is stored in suspensions of solid storage compounds to and from which charge transfer is accomplished via dilute yet percolating networks of nanoscale conductors. These novel electrochemical composites constitute fl owable semi-solid ‘fuels’ that are here charged and discharged in prototype fl ow cells. Potential advantages of the SSFC approach include projected system-level energy densities that are more than ten times those of aqueous fl ow batteries, and the simplifi ed low-cost manufacturing of large-scale storage systems compared to conventional lithiumion batteries. Demand for batteries of higher energy and power has driven several decades of research in electrochemical storage materials, resulting recently in signifi cant improvements in the stored energy of cathodes and anodes. [ 1 , 2 ] However, most batteries have designs that have not departed substantially from Volta’s galvanic cell of 1800, and which accept an inherently poor utilization of the active materials. [ 3 ] Even the highest energy density lithium ion cells currently available, e.g., 2.8‐2.9 Ah 18650 cells having > 600 Wh L − 1 , have less than 50 vol% active material. The reduced energy density, along with higher cost, result because the high-energy-storage compounds are diluted by inactive and costly components necessary to extract power (e.g., currentcollector foils, tabs, separator fi lm, liquid electrolyte, electrode binders and conductive additives, and external packaging). Further dilution of energy density, by about a factor of two, occurs between the cell and system level. [ 4 ] Electrode designs that minimize inactive material, bio- and self-assembly, and 3D architectures are new approaches that promise improved design effi ciency but have yet to be fully realized. [ 5 ‐ 9 ] Decoupling power components from energy-storage components so that stored energy can be scaled independently of power is a strategy for improving system-level energy density. Redox fl ow batteries have such a design, in which active materials are stored within external reservoirs and pumped into an ion-exchange/electron-extraction power stack. [ 10 ] As the system increases in capacity, its energy density may asymptotically approach that of the redox active solutions. Aqueous-chemistry fl ow batteries are of much current interest for stationary applications due to their scalability, relative safety, and potentially low cost. However, they currently use low energy density chemistries limited by electrolysis to ≈ 1.5 V cell voltage and have low ion concentrations (typically 1‐2 M ), yielding ≈ 40 Wh L − 1 energy density for the fl uids alone. [ 10 ] Furthermore, the large fl uid volumes that must be pumped produce parasitic mechanical losses that detract signifi cantly from round-trip effi ciency. The fl ow-cell’s design advantages are therefore offset by the use of low-energy-density active materials. In a new system we call a semi-solid fl ow cell (SSFC, Figure 1 ), the inherent advantages of a fl ow architecture are retained while dramatically increasing energy density by using suspensions of energy-dense active materials in a liquid electrolyte. [ 11 ] This approach to fl owable electrodes can produce more than 10 times the charge storage density of typical fl ow-battery solutions, due to the much greater energy density inherent to solidstorage compounds. For example, in molarity units the concentration of reversibly-stored lithium in lithium-ion cathodes such as LiCoO 2 , LiFePO 4 , LiNi 0.5 Mn 1.5 O 4 , and 0.3 Li 2 MnO 3 ‐0.7 Li M O 2 ( M = Mn, Co, Ni), and anodes such as Li 4 Ti 5 O 12 , graphite, and Si (assuming ≈ 1000 mAh g − 1 reversible capacity), is 51.2, 22.8, 24.1, 39.2, 22.6, 21.4, and 87 M , respectively. [ 2 ] Assuming a solids content of 50% (up to 70 vol% solids have been achieved in fl suspensions of other materials), the volumetric capacity of the semi-solid suspensions is 5‐20 times greater (e.g., 10 to 40 M ) than that of aqueous redox solutions ( ≈ 2 M). The semi-solid approach may be applied to aqueous chemistries, in which case the volumetric energy density is also 5‐20 times greater since cell voltages remain limited by electrolyte hydrolysis to ≈ 1.5 V. [ 12 , 13 ] When applied to nonaqueous Li-ion chemistries, however, energy density is further increased by another factor of 1.5‐3, in direct proportion to cell voltage. (Energy density is the product of volumetric charge capacity, e.g., in molarity or Ah l − 1 units, and the cell voltage. Specifi c values for the systems studied are given later.) In this work, we demonstrate working prototype SSFCs using fl owable suspensions having up to ≈ 12 M concentration. We show that in addition to energy density advantages, the SSFCs can operate at low fl ow rates with very low mechanical energy dissipation. The design fl exibility inherent in the SSFC approach may enable new use-models for electrical storage, such as rapid refueling

Thermoelectric Property Studies on Cu-Doped n-type CuxBi2Te2.7Se0.3 Nanocomposites

Abstract: Combining high energy ball-milling and hot-pressing, significant enhancements of the thermoelectric figure-of-merit (ZT) have been reported for p-type Bi0.4Sb1.6Te3 nanocomposites. However, applying the same technique to n-type Bi2Te2.7Se0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n-type Bi2Te2.7Se0.3. Even though texturing was effective in improving peak ZT of Bi2Te2.7Se0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi2Te2.7Se0.3 to make Cu0.01Bi2Te2.7Se0.3 samples. A peak ZT value of 0.99 was achieved in Cu0.01Bi2Te2.7Se0.3 samples without texturing. With texturing by re-pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.

High Efficiency Polymer Solar Cells with Long Operating Lifetimes

Abstract: To date there has been considerable work done in understanding and quantifying the lifetime and degradation of bulk heterojunction solar cells (BHJs) based on poly( para -phenylene vinylene) (PPV) [ 8–11 ] and poly(3-hexylthiophene) (P3HT) polymers. [ 12–15 ] A comparison of OPV lifetime experimental results across different research groups has posed challenges due to the lack of standardized testing and reporting procedures; however, great strides were made in this regard during the most recent International Summit on OPV Stability (ISOS-3). Modules based on P3HT/fullerene BHJs have shown lifetimes of 5000 h when state-of-the-art encapsulation with a glass-on-glass architecture is used. [ 16 ] Assuming negligible degradation in the dark and 5.5 h of one-sun intensity per day, 365 days per year, this translates into an operating lifetime approaching three years. More recently P3HT/PCBM devices utilizing an inverted architecture have been shown to retain more than 50% of their initial effi ciency after 4700 h of continuous exposure to one-sun intensity at elevated temperatures [ 17 ] and have exhibited a long shelf-life when stored in the dark in ambient conditions. [ 18 , 19 ]

Atomic Layer Deposition of Tin Monosulfide Thin Films

Abstract: Thin film solar cells made from earth-abundant, non-toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se)2 and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single-phase SnS films were obtained by atomic layer deposition (ALD) using the reaction of bis(N,N′-diisopropylacetamidinato)tin(II) [Sn(MeC(N-iPr)2)2] and hydrogen sulfide (H2S) at low temperatures (100 to 200 °C). The direct optical band gap of SnS is around 1.3 eV and strong optical absorption (α > 104 cm−1) is observed throughout the visible and near-infrared spectral regions. The films are p-type semiconductors with carrier concentration on the order of 1016 cm−3 and hole mobility 0.82–15.3 cm2V−1s−1 in the plane of the films. The electrical properties are anisotropic, with three times higher mobility in the direction through the film, compared to the in-plane direction.

Superior CO2 Adsorption Capacity on N‐doped, High‐Surface‐Area, Microporous Carbons Templated from Zeolite

Abstract: Zeolite-templated, high-surface-area, microporous, N-doped carbons exhibit the highest CO2 uptake capacity recorded to date for any carbon material and one of the highest for any inorganic or organic porous material of up to 6.9 mmol g−1 at 273 K and ambient pressure and 4.4 mmol g−1 at ambient temperature and pressure, along with an initial CO2 adsorption energy of 36 kJ mol−1 at lower coverage and 20 kJ mol−1 at higher CO2 coverage. Combined with their ease of preparation, excellent recyclability and regeneration stability, and high selectivity for CO2, the N-doped zeolite-templated carbons are amongst the most promising solid-state absorbents reported so far for CO2 capture and storage.

Recent progress in the development of anode materials for solid oxide fuel cells

Abstract: The field of research into solid oxide fuel cell (SOFC) anode materials has been rapidly moving forward. In the four years since the last in-depth review significant advancements have been made in the reduction of the operating temperature and improvement of the performance of SOFCs. This progress report examines the developments in the field and looks to draw conclusions and inspiration from this research. A brief introduction is given to the field, followed by an overview of the principal previous materials. A detailed analysis of the developments of the last 4 years is given using a selection of the available literature, concentrating on metal-fluorite cermets and perovskite-based materials. This is followed by a consideration of alternate fuels for use in SOFCs and their associated problems and a short discussion on the effect of synthesis method on anode performance. The concluding remarks compile the significant developments in the field along with a consideration of the promise of future research. The recent progress in the development of anode materials for SOFCs based on oxygen ion conducting electrolytes is reviewed.

Vertically Aligned Carbon Nanotubes Grown on Graphene Paper as Electrodes in Lithium-Ion Batteries and Dye-Sensitized Solar Cells

Abstract: Ministry of Science and Technology of China [2011CB932601, 2011CB932604, 2009AA03Z337, 2008DFA51400]; National Natural Science Foundation of China [90606008, 50872137, 50921004, 20725311, 50972101]

Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers

Abstract: We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work-function of the hole collecting indium-tin oxide (ITO) contact, modified with a solution-processed nickel oxide (NiOx) hole-transport layer (HTL), is matched to the ionization potential of the donor material in a bulk-heterojunction solar cell. Addition of the NiOx HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL on ITO. The impact of these NiOx films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiOx HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiOx HTL is shown to provide superior contact properties by increasing the ITO/NiOx contact work-function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiOx interface with the poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothidiazole) (PCDTBT):[6,6]-phenyl-C61 butyric acid methyl ester PCBM and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) active layer. A high work-function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.

Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

Abstract: An enhancement in the dimensionless thermoelectric fi gure-of-merit ( ZT ) of an n-type half-Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 ° C‐700 ° C, which is about 25% higher than the previously reported highest value. The samples were made by ball-milling ingots of composition Hf 0.75 Zr 0.25 NiSn 0.99 Sb 0.01 into nanopowders and hot-pressing the powders into dense bulk samples. The ingots were formed by arc-melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.

Vanadium Oxide Nanowire–Carbon Nanotube Binder‐Free Flexible Electrodes for Supercapacitors

Abstract: Vanadium pentoxide (V2O5) layered nanostructures are known to have very stable crystal structures and high faradaic activity. The low electronic conductivity of V2O5 greatly limits the application of vanadium oxide as electrode materials and requires combining with conducting materials using binders. It is well known that the organic binders can degrade the overall performance of electrode materials and need carefully controlled compositions. In this study, we develop a simple method for preparing freestanding carbon nanotube (CNT)-V2O5 nanowire (VNW) composite paper electrodes without using binders. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode (Spectracarb 2225) as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibits a power density of 5.26 kW Kg−1 and an energy density of 46.3 Wh Kg−1. (Li)VNWs and CNT composite paper electrodes can be fabricated in similar manner and show improved overall performance with a power density of 8.32 kW Kg−1 and an energy density of 65.9 Wh Kg−1. The power and energy density values suggest that such flexible hybrid nanocomposite paper electrodes may be useful for high performance electrochemical supercapacitors.

Template‐Free Synthesis of Interconnected Hollow Carbon Nanospheres for High‐Performance Anode Material in Lithium‐Ion Batteries

Abstract: The ever-increasing demand for rechargeable batteries in some newly emerging portable electronic devices, advanced medical devices, and in particular, electric vehicles and hybrid electric vehicles has sparked research efforts in developing lithium ion batteries (LIBs) with high storage capacity and excellent rate performance. [ 1 ] Graphite, the mainstay of anode materials for commercialized LIBs, could hardly meet the demand because of its low theoretical specifi c capacity (372 mAh g − 1 ), and thus provides ample opportunity for exploring alternative highperformance electrode materials. Si, Sn, SnO 2 and some transition metal oxides with ultra high theoretical capacities have been considered due to their special mechanism for Li storage. Unfortunately, huge volume change (in alloying electrodes) and large voltage hysteresis (in conversion electrodes) occurring in these materials during Li insertion and extraction handicap their extensive applications. [ 2 , 3 ] Therefore, continual research

Amorphous Carbon Coated High Grain Boundary Density Dual Phase Li4Ti5O12‐TiO2: A Nanocomposite Anode Material for Li‐Ion Batteries

Abstract: This work introduces an effective, inexpensive, and large-scale production approach to the synthesis of a carbon coated, high grain boundary density, dual phase Li4Ti5O12-TiO2 nanocomposite anode material for use in rechargeable lithium-ion batteries. The microstructure and morphology of the Li4Ti5O12-TiO2-C product were characterized systematically. The Li4Ti5O12-TiO2-C nanocomposite electrode yielded good electrochemical performance in terms of high capacity (166 mAh g−1 at a current density of 0.5 C), good cycling stability, and excellent rate capability (110 mAh g−1 at a current density of 10 C up to 100 cycles). The likely contributing factors to the excellent electrochemical performance of the Li4Ti5O12-TiO2-C nanocomposite could be related to the improved morphology, including the presence of high grain boundary density among the nanoparticles, carbon layering on each nanocrystal, and grain boundary interface areas embedded in a carbon matrix, where electronic transport properties were tuned by interfacial design and by varying the spacing of interfaces down to the nanoscale regime, in which the grain boundary interface embedded carbon matrix can store electrolyte and allows more channels for the Li+ ion insertion/extraction reaction. This research suggests that carbon-coated dual phase Li4Ti5O12-TiO2 nanocomposites could be suitable for use as a high rate performance anode material for lithium-ion batteries.

Solvent‐Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells

Abstract: It is demonstrated that squaraine:PC70BM blends can result in solar cells with high efficiency and fill factor when the nano­structure scale is increased to lead to conduction of photogenerated carriers to the electrodes. Morphological control occurs by a combination of thermal and solvent annealing of these small molecule, solution-processed bulk heterojunction cells. Peak efficiencies of a population of devices is 5.2% at fill factors of 0.5 are achieved under optimized conditions.

Quantum Dot–Sensitized Solar Cells Featuring CuS/CoS Electrodes Provide 4.1% Efficiency

Abstract: CuS, CoS, and CuS/CoS onto fluorine-doped tin oxide glass substrates were deposited to function as counter electrodes for polysulfide redox reactions in CdS/CdSe quantum dot–sensitized solar cells (QDSSCs). Relative to a Pt electrode, the CuS, CoS, and CuS/CoS electrodes provide greater electrocatalytic activity, higher reflectivity, and lower charge-transfer resistance. Measurements of fill factor and short-current density reveal that the electrocatalytic activities, reflectivity, and internal resistance of counter electrodes play strong roles in determining the energy-conversion efficiency (η) of the QDSSCs. Because the CuS/CoS electrode has a smaller internal resistance and higher reflectivity relative to those of the CuS and CoS electrodes, it exhibits a higher fill factor and short-circuit current density. As a result, the QDSSC featuring a CuS/CoS electrode provides a higher value of η. Under illumination of one sun (100 mW cm−2), the QDSSCs incorporating Pt, CuS, CoS, and CuS/CoS counter electrodes provide values of η of 3.0 ± 0.1, 3.3 ± 0.3, 3.8 ± 0.2, and 4.1 ± 0.2%, respectively.

Solution Processed Vanadium Pentoxide as Charge Extraction Layer for Organic Solar Cells

Abstract: Organic solar cells (OSCs) based on polymers and small molecules have seen a tremendous increase in interest during the past few years. Signifi cant progress in this fi eld seeded the prospect for a cost-effective and easy-to-fabricate photovoltaic technology—typical advantages claimed for organic (opto-)electronic devices. Very recently, certifi ed cell effi ciencies in excess of 7% have been reported for polymer based cells. [ 1 ] For large-scale and high-throughput production of OSCs, liquid processing of the functional layers is desirable. Aside from the active organic layers, inter-layers are typically required to facilitate the extraction of the photo-generated charges. Specifi cally, on the anode side, polyethylene dioxythiophene:polystyrenesulfonate (PEDOT:PSS) is regularly used. [ 2 ] However, PEDOT:PSS is burdened with structural and electrical inhomogeneity [ 3,4 ] and has been demonstrated to be an origin of limited device lifetime. [ 5 ] Particularly, the aqueous PEDOT:PSS dispersion and the acidic nature can cause substantial degradation. [6,7 ] Very recently, transition metal-oxides (TMOs) such as molybdenum-, vanadium-, or tungsten-oxide (MoO 3 , V 2 O 5 , and WO 3 ) with high work functions (WFs) of up to 6.9 eV have been shown to be promising alternatives to PEDOT:PSS. [ 8‐11 ] TMOs have also been used as constituents of the connecting architecture in stacked organic light-emitting diodes and organic tandem solar cells. [ 12‐15 ] The unique energetics of these TMOs has so far been predominantly