Carbon Nanotube‐Silicon Solar Cells

Due to the high cost of silicon photovoltaics there is currently great interest in finding alternative semiconductor materials for light harvesting devices. Single‐walled carbon nanotubes are an allotrope of carbon with unique electrical and optical properties and are promising as future photovoltaic materials. It is thus important to investigate the methods of exploiting their properties in photovoltaic devices. In addition to already extensive research using carbon nanotubes in organic photovoltaics and photoelectrochemical cells, another way to do this is to combine them with a relatively well understood model semiconductor such as silicon. Nanotube‐silicon heterojunction solar cells are a recent photovoltaic architecture with demonstrated power conversion efficiencies of up to ～14% that may in part exploit the photoactivity of carbon nanotubes.     

Nitrogen‐Doped Graphene Nanoribbons with Surface Enriched Active Sites and Enhanced Performance for Dye‐Sensitized Solar Cells

Superior electrocatalytic activities and excellent electrochemical stabilities of inexpensive counter electrodes (CEs) are crucial to the large‐scale practical application of dye‐sensitized solar cells (DSSCs). Herein, an efficient strategy for fabricating nitrogen‐doped graphene nanoribbons (N‐GNRs) via chemical unzipping of carbon nanotubes coupled with nitrogen doping process is reported, where abundant edge sites are produced and fully exposed basal planes of GNRs are activated by the N atoms within GNRs backbone. Benefiting from such unique characteristics, when first applied as CEs for DSSCs with triiodide/iodide electrolyte, a power conversion efficiency of 8.57% is delivered, outperforming GNRs (8.01%) and being superb to that of Pt (7.84%), and outstanding electrochemical stabilities of N‐GNRs are also demonstrated. Density functional theory calculations reveal that the N species within GNRs matrix, especially the predominant quaternary ones, could remarkably decrease the ionization energy of GNRs, which is instrumental to transfer electrons rapidly from external circuit to triiodide, and reduce charge‐transfer resistance, thus contributing to the enhanced photovoltaic performance. The present work has an insight into the unique role of N species on GNRs to the triiodide reduction, and provides an efficient strategy for design of high‐efficiency carbon electrodes with fully exposed active sites in energy conversion/storage devices.     

Rational Design of Metal Oxide–Based Cathodes for Efficient Dye‐Sensitized Solar Cells

Recently, there is an urgent need for alternative energy resources due to the nonrenewable nature of fossil fuels and increasing CO₂ greenhouse gas emissions. The photovoltaic technologies which directly utilize the abundant and sustainable solar energy are critical. Among various photovoltaic devices (solar cells), dye‐sensitized solar cells (DSSCs) have gained increasing attention due to their high efficiency and easy fabrication process in the past decade. The cathode is a critical part in DSSCs while the benchmark Pt cathode suffers from high cost and scarcity. Thus, the development of alternative Pt‐free cathodes has attracted significant attention with the aim to heighten the cost competitiveness of DSSCs. Among various cathodes, metal oxides are of growing interest due to their superior activity, robust stability, and low cost. Simple oxides such as WO₃ and SnO₂ are used as cathodes for DSSCs. Considering the fixed atomic environment in simple oxides, complex oxides are more attractive as cathodes because of their more flexible physical and chemical properties. This review attempts to present the rational design of simple/complex metal oxide–based cathodes in DSSCs and then to provide useful guidance for the future design of Pt‐free cathodes. The demonstrated design strategies can be extended to other electrocatalysis‐based applications.     

Graphene Based Materials: Enhancing Solar Energy Harvesting

Due to excellent electronic and optical properties as well as tunable work functions, graphene and graphene‐based materials are highly attractive for applications in enhancement of harvesting solar energy. In particular, they can be used as electron and hole transport materials, buffer layers, and window or/and counter electrodes in solar cells. This research news surveys very recent advances in this emerging field, with emphasis on fundamental understanding of their performance enhancement mechanisms for photovoltaic devices, and discusses future challenges.     

Life Cycle Energy and Climate Change Implications of Nanotechnologies

The potential environmental and health impacts of nanotechnologies triggered a recent surge of life cycle assessment (LCA) studies on nanotechnologies. Focusing on the energy use and greenhouse gas emissions impacts, we reviewed 22 LCA‐based studies on nanomaterials, coatings, photovoltaic devices, and fabrication technologies that were published until 2011. The reviewed LCA studies indicate that nanomaterials have higher cradle‐to‐gate energy demand per functional unit, and thus higher global warming impact, than their conventional counterparts. Depending on the synthesis method, carbon‐based nanoparticles (i.e., carbon nanofibers, carbon nanotubes, and fullerenes) require 1 to 900 gigajoules per kilogram (GJ/kg) of primary energy to produce, compared with ～200 megajoules per kilogram (MJ/kg) for aluminum. This is mainly attributed to the fact that nanomaterials involve an energy‐intensive synthesis process or an additional mechanical process to reduce particle size. Most reviewed studies ascertain, however, that the cradle‐to‐grave energy demand and global warming impact from nanotechnologies at a device level are lower than from conventional technologies because nanomaterials are typically used in a small amount to improve functionality and the upgraded functionality offers more energy‐efficient operation of the device. Because of the immature status of most nanotechnologies, the studies reviewed here often rely on inventory data estimated from literature values and parametric analyses based on laboratory or prototype production, warranting future analyses to confirm the current findings.     

Emerging 2D Layered Materials for Perovskite Solar Cells

Perovskite solar cells (PSCs) are now at the forefront of the state‐of‐the‐art photovoltaic technologies due to their high efficiency and low fabrication costs. To further realize the potential of this fascinating class of solar cells, nanostructured functional materials have been playing important roles. 2D layered materials have attracted a great deal of interest due to their fascinating properties and unique structure. Recently, the exploration of a wide range of novel 2D materials for use in PSCs has seen considerable progress, but still a lot remains to be done in this field. In this progress report, the advancements that have recently been made in the application of these emerging 2D materials, beyond graphene, for PSCs are presented. Both the advantages and challenges of these 2D materials for PSCs are highlighted. Finally, important directions for the future advancements toward efficient, low‐cost, and stable PSCs are outlined.     

Recent Progress in Flexible Electrochemical Capacitors: Electrode Materials,Device Configuration,and Functions

With increasing demand for portable, flexible, and even wearable electronic devices, flexible energy storage systems have received increasing attention as a key component in this emerging field. Among the options, supercapacitors, commonly referred to as ultracapacitors or electrochemical capacitors, are widely recognized as a potential energy storage system due to their high power, fast charge/discharge rate, long cycling life‐time, and low cost. To date, considerable effort has been dedicated to developing high‐performance flexible supercapacitors based on various electrode materials; including carbon nanomaterials (e.g., carbon nanotubes, graphene, porous carbon materials, carbon paper, and textile), conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), and hybrid materials. A brief introduction to the field is provided and the state‐of‐the‐art is reviewed with special emphasis on electrode materials and device configurations.     

The Development of Pseudocapacitor Electrodes and Devices with High Active Mass Loading

Pseudocapacitive materials are used for supercapacitor applications due to their exceptionally high capacitance and low cost. Good capacitive performance of the pseudocapacitive materials at high active mass loadings is vital for the development of the next generation of supercapacitor devices. This review describes recent advances in materials and nanotechnologies, which allows the development of advanced pseudocapacitive devices with high active mass. An important breakthrough is the discovery of novel dispersing and capping agents for the colloidal processing of nanoparticles. Particularly important are novel co‐dispersants that exhibit enhanced adsorption on materials of different types, such as inorganic nanoparticles, carbon nanotubes, and graphene. Conceptually new strategies are designed to fabricate coated particles. Recent innovations pave the way for the development of multifunctional redox‐active dopants‐dispersants and dopants‐oxidants to manufacture conductive polymer composites. Among the most important advances in nanotechnology is the development of template methods and heterocoagulation techniques for composite manufacturing. The progress in the design of novel surface modification techniques and materials, discovery of advanced anchoring groups, and development of liquid–liquid extraction allows agglomerate‐free processing of nanomaterials and composites. This review describes fundamental aspects of novel technologies and their applications in the manufacturing of pseudocapacitive devices for energy storage.     

Comparing the Fundamental Physics and Device Performance of Transparent,Conductive Nanostructured Networks with Conventional Transparent Conducting Oxides

Networks made of single‐walled carbon nanotubes (SWNTs) and metallic nanowire networks, graphene, and ultra‐thin metal films have all been proposed as replacements for transparent conducting oxides (TCOs) in photovoltaic and other applications. However, only limited comparisons of nanostructured networks and TCOs are available. Several common figures of merit that are often used to compare the electrical and optical performance of the transparent contacts are evaluated here, and the merits of each method of comparison are discussed. Calculating the current loss due to absorption in the TCO is the most useful metric for evaluating new materials for use in solar cells with well‐defined sheet resistance requirements and known quantum efficiencies. The ‘Haacke’ figure of merit, Φ_H, correlates fairly well with current loss and is a good metric for evaluating electro‐optical performance for more general applications. The analyses presented here demonstrate that silver nanowire networks are much closer to achieving optimal electrical and optical properties than carbon‐based networks.     

Redox‐enzymes,cells and micro‐organisms acting on carbon nanostructures transformation: A mini‐review

Carbon nanotubes, graphene and fullerenes are actual nanomaterials with many applications in different industrial areas, with increasing potentialities in the field of nanomedicine. Recently, different proactive approaches on toxicology and safety management have become the focus of intense interest once the industrial production of these materials had a significant growth in the last years, even though their short‐ and long‐term behaviors are not yet fully understood. The most important concerns involving these carbon‐based nanomaterials are their stability and potential effects of their life cycles on animals, humans, and environment. In this context, this mini review discuss the biodegradability of these materials, particularly through redox‐enzymes, micro‐organisms and cells, to contribute toward the design of biocompatible and biodegradable functionalized carbon nanostructures, in order to use these materials safely and with minimum impact on the environment. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2013     

Wind farm and solar park effects on plant–soil carbon cycling: uncertain impacts of changes in ground‐level microclimate

Global energy demand is increasing as greenhouse gas driven climate change progresses, making renewable energy sources critical to future sustainable power provision. Land‐based wind and solar electricity generation technologies are rapidly expanding, yet our understanding of their operational effects on biological carbon cycling in hosting ecosystems is limited. Wind turbines and photovoltaic panels can significantly change local ground‐level climate by a magnitude that could affect the fundamental plant–soil processes that govern carbon dynamics. We believe that understanding the possible effects of changes in ground‐level microclimates on these phenomena is crucial to reducing uncertainty of the true renewable energy carbon cost and to maximize beneficial effects. In this Opinions article, we examine the potential for the microclimatic effects of these land‐based renewable energy sources to alter plant–soil carbon cycling, hypothesize likely effects and identify critical knowledge gaps for future carbon research.     

Photo‐Rechargeable Electric Energy Storage Systems

The global energy demand is increasing at the same time as fossil fuel resources are dwindling. Consequently, the search for alternative energy sources is a major topic worldwide. Solar energy is one of the most promising, effective and emission‐free energy sources. However, the energy has to be stored to compensate the fluctuating availability of the sun and the actual energy demand. Photo‐rechargeable electric energy storage systems may solve this problem by immediately storing the generated electricity. Different combinations of solar cells and storage devices are possible. High efficiencies can be achieved by the combination of dye‐sensitized solar cells (DSSC) and capacitors. However, other hybrid devices including DSSCs or organic photovoltaic systems and redox flow batteries, lithium ion batteries and metal air batteries are playing an increasing role in this research field. This Progress Report reviews the state of the art research of photo‐rechargeable batteries based on organic solar cells, as well as storage modules.     

High‐Performance Solution‐Processed Double‐Walled Carbon Nanotube Transparent Electrode for Perovskite Solar Cells

Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.     

Emergent Pseudocapacitance of 2D Nanomaterials

Two dimensional (2D) nanomaterials are very attractive due to their unique structural and surface features for energy storage applications. Motivated by the recent pioneering works demonstrating “the emergent pseudocapacitance of 2D nanomaterials,” the energy storage and nanoscience communities could revisit bulk layered materials though state‐of‐the‐art nanotechnology such as nanostructuring, nanoarchitecturing, and compositional control. However, no review has focused on the fundamentals, recent progress, and outlook on this new mechanism of 2D nanomaterials yet. In this study, the key aspects of emergent pseudocapacitors based on 2D nanomaterials are comprehensively reviewed, which covers the history, classification, thermodynamic and kinetic aspects, electrochemical characteristics, and design guidelines of materials for extrinsically surface redox and intercalation pseudocapacitors. The structural and compositional controls of graphene and other carbon nanosheets, transition metal oxides and hydroxides, transition metal dichalcogenides, and metal carbide/nitride on both microscopic and macroscopic levels will be particularly addressed, emphasizing the important results published since 2010. Finally, perspectives on the current impediments and future directions of this field are offered. Unlimited combinations and modifications of 2D nanomaterials can provide a rational strategy to overcome intrinsic limitations of existing materials, offering a new‐generation energy storage materials toward a high and new position in the Ragone plot.     

Graphene‐Containing Nanomaterials for Lithium‐Ion Batteries

Graphene‐containing nanomaterials have emerged as important candidates for electrode materials in lithium‐ion batteries (LIBs) due to their unique physical properties. In this review, a brief introduction to recent developments in graphene‐containing nanocomposite electrodes and their derivatives is provided. Subsequently, synthetic routes to nanoparticle/graphene composites and their electrochemical performance in LIBs are highlighted, and the current state‐of‐the‐art and most recent advances in the area of graphene‐containing nanocomposite electrode materials are summarized. The limitations of graphene‐containing materials for energy storage applications are also discussed, with an emphasis on anode and cathode materials. Potential research directions for the future development of graphene‐containing nanocomposites are also presented, with an emphasis placed on practicality and scale‐up considerations for taking such materials from benchtop curiosities to commercial products.     

Iron‐Oxide‐Based Advanced Anode Materials for Lithium‐Ion Batteries

Iron oxides, such as Fe₂O₃ and Fe₃O₄, have recently received increased attention as very promising anode materials for rechargeable lithium‐ion batteries (LIBs) because of their high theoretical capacity, non‐toxicity, low cost, and improved safety. Nanostructure engineering has been demonstrated as an effective approach to improve the electrochemical performance of electrode materials. Here, recent research progress in the rational design and synthesis of diverse iron oxide‐based nanomaterials and their lithium storage performance for LIBs, including 1D nanowires/rods, 2D nanosheets/flakes, 3D porous/hierarchical architectures, various hollow structures, and hybrid nanostructures of iron oxides and carbon (including amorphous carbon, carbon nanotubes, and graphene). By focusing on synthesis strategies for various iron‐oxide‐based nanostructures and the impacts of nanostructuring on their electrochemical performance, novel approaches to the construction of iron‐oxide‐based nanostructures are highlighted and the importance of proper structural and compositional engineering that leads to improved physical/chemical properties of iron oxides for efficient electrochemical energy storage is stressed. Iron‐oxide‐based nanomaterials stand a good chance as negative electrodes for next generation LIBs.     

Structure Design and Composition Engineering of Carbon‐Based Nanomaterials for Lithium Energy Storage

Carbon‐based nanomaterials have significantly pushed the boundary of electrochemical performance of lithium‐based batteries (LBs) thanks to their excellent conductivity, high specific surface area, controllable morphology, and intrinsic stability. Complementary to these inherent properties, various synthetic techniques have been adopted to prepare carbon‐based nanomaterials with diverse structures and different dimensionalities including 1D nanotubes and nanorods, 2D nanosheets and films, and 3D hierarchical architectures, which have been extensively applied as high‐performance electrode materials for energy storage and conversion. The present review aims to outline the structural design and composition engineering of carbon‐based nanomaterials as high‐performance electrodes of LBs including lithium‐ion batteries, lithium–sulfur batteries, and lithium–oxygen batteries. This review mainly focuses on the boosting of electrochemical performance of LBs by rational dimensional design and porous tailoring of advanced carbon‐based nanomaterials. Particular attention is also paid to integrating active materials into the carbon‐based nanomaterials, and the structure–performance relationship is also systematically discussed. The developmental trends and critical challenges in related fields are summarized, which may inspire more ideas for the design of advanced carbon‐based nanostructures with superior properties.     

Single‐Walled Carbon Nanotubes in Emerging Solar Cells: Synthesis and Electrode Applications

Emerging solar cells, namely, organic solar cells and perovskite solar cells, are the thin‐film photovoltaics that have light to electricity conversion efficiencies close to that of silicon solar cells while possessing advantages in having additional functionalities, facile‐processability, and low fabrication cost. To maximize these advantages, the electrode components must be replaced by materials that are more flexible and cost‐effective. Researchers around the globe have been looking for the new electrodes that meet these requirements. Among many candidates, single‐walled carbon nanotubes have demonstrated their feasibility as the new alternative to conventional electrodes, such as indium tin oxide and metals. This review discusses various growth methods of single‐walled carbon nanotubes and their electrode applications in thin‐film photovoltaics.     

Conductive Hole‐Selective Passivating Contacts for Crystalline Silicon Solar Cells

Defect state passivation and conductivity of materials are always in opposition; thus, it is unlikely for one material to possess both excellent carrier transport and defect state passivation simultaneously. As a result, the use of partial passivation and local contact strategies are required for silicon solar cells, which leads to fabrication processes with technical complexities. Thus, one material that possesses both a good passivation and conductivity is highly desirable in silicon photovoltaic (PV) cells. In this work, a passivation‐conductivity phase‐like diagram is presented and a conductive‐passivating‐carrier‐selective contact is achieved using PEDOT:Nafion composite thin films. A power conversion efficiency of 18.8% is reported for an industrial multicrystalline silicon solar cell with a back PEDOT:Nafion contact, demonstrating a solution‐processed organic passivating contact concept. This concept has the potential advantages of omitting the use of conventional dielectric passivation materials deposited by costly high‐vacuum equipment, energy‐intensive high‐temperature processes, and complex laser opening steps. This work also contributes an effective back‐surface field scheme and a new hole‐selective contact for p‐type and n‐type silicon solar cells, respectively, both for research purposes and as a low‐cost surface engineering strategy for future Si‐based PV technologies.     

Holey 2D Nanomaterials for Electrochemical Energy Storage

2D nanomaterials provide numerous fascinating properties, such as abundant active surfaces and open ion diffusion channels, which enable fast transport and storage of lithium ions and beyond. However, decreased active surfaces, prolonged ion transport pathway, and sluggish ion transport kinetics caused by self‐restacking of 2D nanomaterials during electrode assembly remain a major challenge to build high‐performance energy storage devices with simultaneously maximized energy and power density as well as long cycle life. To address the above challenge, porosity (or hole) engineering in 2D nanomaterials has become a promising strategy to enable porous 2D nanomaterials with synergetic features combining both 2D nanomaterials and porous architectures. Herein, recent important progress on porous/holey 2D nanomaterials for electrochemical energy storage is reviewed, starting with the introduction of synthetic strategies of porous/holey 2D nanomaterials, followed by critical discussion of design rule and their advantageous features. Thereafter, representative work on porous/holey 2D nanomaterials for electrochemical capacitors, lithium‐ion and sodium‐ion batteries, and other emerging battery technologies (lithium‐sulfur and metal‐air batteries) are presented. The article concludes with perspectives on the future directions for porous/holey 2D nanomaterial in energy storage and conversion applications.