High‐Efficiency Large‐Area Carbon Nanotube‐Silicon Solar Cells

Currently studied carbon nanotube‐silicon (CNT‐Si) solar cells are based on relatively small active areas (typically <0.15 cm²); increasing the active area generally leads to reduced power conversion efficiencies. This study reports CNT‐Si solar cells with active areas of more than 2 cm² for single cells, yet still achieving cell efficiencies of about 10%, which is the first time for CNT‐Si solar cells with an active area more than 1 cm² to reach the level for real applications. In this work, a controlled number of flattened highly conductive CNT strips is added, in simple arrangement, to form a CNT‐Si solar cell with CNT strips in which the middle film makes heterojunctions with Si while the top strips act as self‐similar top electrodes, like conventional metal grids. The CNT strips, directly condensed from as‐grown CNT films, not only improve the CNT‐Si junctions, but also enhance the conductivity of top electrodes without introducing contact barrier when the CNT strips are added onto the film. This property may facilitate the development of large‐area high‐performance CNT or graphene‐Si solar cells.     

Nitrogen‐Doping‐Induced Defects of a Carbon Coating Layer Facilitate Na‐Storage in Electrode Materials

A nitrogen‐doped, carbon‐coated Na₃V₂(PO₄)₃ cathode material is synthesized and the formation of doping type of nitrogen‐doped in carbon coating layer is systemically investigated. Three different carbon‐nitrogen species: pyridinic N, pyrrolic N, and quaternary N are identified. The most important finding is that different carbon‐nitrogen species in the carbon layer have different impacts on the improvement of the electrochemical properties of Na₃V₂(PO₄)₃. Pyridinic N and pyrrolic N significantly increase the electronic conductivity and create numerous extrinsic defects and active sites. Quaternary N only increases the electronic conductivity without creating extrinsic defects. Therefore, it is unexpectedly demonstrated that the Na₃V₂(PO₄)₃/C+N, in which with minimize content of quaternary N or exist most extrinsic defects, exhibits the best electrochemical performance, particularly the rate performance and cycling stability. For example, when the discharging rate increased from 0.2 C to 5 C, its capacity of 101.9 mAh g^?1 decays to 84.3 mAh g^?1 and an amazing capacity retention of 83% is achieved. Moreover, even at higher current density of 5 C, an excellent capacity retention of 93% is maintained even after 100 cycles.     

Hierarchically Porous N‐Doped Carbon Fibers as a Free‐Standing Anode for High‐Capacity Potassium‐Based Dual‐Ion Battery

Potassium‐based dual‐ion batteries (KDIBs) have emerged as a new generation of rechargeable batteries, due to their high cell voltage, low cost, and the natural abundance of potassium resources. However, the low capacity and poor cycling stability largely hinder the further development of KDIBs. Herein, the fabrication of hierarchically porous N‐doped carbon fibers (HPNCFs) as a free‐standing anode for high‐performance KDIBs is reported. With a free‐standing hierarchical structure (micro/meso/macropores and nanochannels) and high‐content of nitrogen doping, the HPNCFs not only provide intrinsic electron pathways and efficient ion transport channels, but also afford sufficient free space to tolerate the volume change during cycling. Consequently, the KDIBs made from a graphite cathode and an optimized HPNCFs anode deliver a high reversible capacity of 197 mAh g^?1 at a specific current of 50 mA g^?1, and excellent cycling stability (65 mAh g^?1 after 346 cycles at a specific current of 100 mA g^?1, the capacity calculation of the KDIBs is based on the mass of the anode). These results indicate that the properly designed HPNCFs can effectively improve the capacity and cycling stability of the KDIBs, indicating a great potential for applications in the field of high‐performance energy‐storage devices.     

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.     

Lithium‐Pretreated Hard Carbon as High‐Performance Sodium‐Ion Battery Anodes

Hard carbon (HC) is the state‐of‐the‐art anode material for sodium‐ion batteries (SIBs). However, its performance has been plagued by the limited initial Coulombic efficiency (ICE) and mediocre rate performance. Here, experimental and theoretical studies are combined to demonstrate the application of lithium‐pretreated HC (LPHC) as high‐performance anode materials for SIBs by manipulating the solid electrolyte interphase in tetraglyme (TEGDME)‐based electrolyte. The LPHC in TEGDME can 1) deliver > 92% ICE and ≈220 mAh g^?1 specific capacity, twice of the capacity (≈100 mAh g^?1) in carbonate electrolyte; 2) achieve > 85% capacity retention over 1000 cycles at 1000 mA g^?1 current density (4 C rate, 1 C = 250 mA g^?1) with a specific capacity of ≈150 mAh g^?1, ≈15 times of the capacity (10 mAh g^?1) in carbonate. The full cell of Na₃V₂(PO₄)₃‐LPHC in TEGDME demonstrated close to theoretical specific capacity of ≈98 mAh g^?1 based on Na₃V₂(PO₄)₃ cathode, ≈2.5 times of the value (≈40 mAh g^?1) with nontreated HC. This work provides new perception on the anode development for SIBs.     

A Dual‐Carbon Battery Based on Potassium‐Ion Electrolyte

Although potassium‐ion batteries (KIBs) have been considered to be promising alternatives to conventional lithium‐ion batteries due to large abundance and low cost of potassium resources, their development still stays at the infancy stage due to the lack of appropriate cathode and anode materials with reversible potassium insertion/extraction as well as good rate and cycling performance. Herein, a novel dual‐carbon battery based on a potassium‐ion electrolyte (named as K‐DCB), utilizing expanded graphite as cathode material and mesocarbon microbead as anode material is developed. The working mechanism of the K‐DCB is investigated, which is further demonstrated to deliver a high reversible capacity of 61 mA h g^‐1 at a current density of 1C over a voltage window of 3.0–5.2 V, as well as good cycling performance with negligible capacity decay after 100 cycles. Moreover, the high working voltage with medium discharge voltage of 4.5 V also enables the K‐DCB to meet the requirement of some high‐voltage devices. With the merits of environmental friendliness, low cost and high energy density, the K‐DCB shows attractive potential for future energy storage application.     

Metal‐Free Artificial Photosynthesis of Carbon Monoxide Using N‐Doped ZnTe Nanorod Photocathode Decorated with N‐Doped Carbon Electrocatalyst Layer

An artificial photosynthesis system based on N‐doped ZnTe nanorods decorated with an N‐doped carbon electrocatalyst layer is fabricated via an all‐solution process for the selective conversion of CO₂ to CO. Substitutional N‐doping into the ZnTe lattice decreases the bandgap slightly and improves the charge transfer characteristics, leading to enhanced photoelectrochemical activity. Remarkable N‐doping effects are also demonstrated by the N‐doped carbon layer that promotes selective CO₂‐to‐CO conversion instead of undesired water‐to‐H₂ reduction by providing active sites for CO₂ adsorption and activation, even in the absence of metallic redox centers. The photocathode shows promising performance in photocurrent generation (?1.21 mA cm^?2 at ?0.11 V_RHE), CO selectivity (dominant CO production of ≈72%), minor H₂ reduction (≈20%), and stability (corrosion suppression). The metal‐free electrocatalyst/photocatalyst combination prepared via a cost‐effective solution process exhibits high performance due to synergistic effects between them, and thus may find application in practical solar fuel production.     

Hierarchical Carbide‐Derived Carbon Foams with Advanced Mesostructure as a Versatile Electrochemical Energy‐Storage Material

Highly porous carbide‐derived carbon (CDC) mesofoams (DUT‐70) are prepared by nanocasting of mesocellular silica foams with a polycarbosilane precursor. Ceramic conversion followed by silica removal and high‐temperature chlorine treatment yields CDCs with a hierarchical micro‐mesopore arrangement. This new type of polymer‐based CDC is characterized by specific surface areas as high as 2700 m² g^?1, coupled with ultrahigh micro‐ and mesopore volumes up to 2.6 cm³ g^?1. The relationship between synthesis conditions and the properties of the resulting carbon materials is described in detail, allowing precise control of the properties of DUT‐70. Since the hierarchical pore system ensures both efficient mass transfer and high capacities, the novel CDC shows outstanding performance as an electrode material in electrochemical double‐layer capacitors (EDLCs) with specific capacities above 240 F g^?1 when measured in a symmetrical two‐electrode configuration. Remarkable capacities of 175 F g^?1 can be retained even at high current densities of 20 A g^?1 as a result of the enhanced ion‐transport pathways provided by the cellular mesostructure. Moreover, DUT‐70 can be infiltrated with sulfur and host the active material in lithium–sulfur battery cathodes. Reversible capacities of 790 mAh g^?1 are achieved at a current rate of C/10 after 100 cycles, which renders DUT‐70 an ideal support material for electrochemical energy‐storage applications.     

Paper‐Based Energy‐Storage Devices Comprising Carbon Fiber‐Reinforced Polypyrrole‐Cladophora Nanocellulose Composite Electrodes

Composites of polypyrrole (PPy) and Cladophora nanocellulose, reinforced with 8 μm‐thick chopped carbon filaments, can be used as electrode materials to obtain paper‐based energy‐storage devices with unprecedented performance at high charge and discharge rates. Charge capacities of more than 200 C g^?1 (PPy) are obtained for paper‐based electrodes at potential scan rates as high as 500 mV s^?1, whereas cell capacitances of ～60–70 F g^?1 (PPy) are reached for symmetric supercapacitor cells with capacitances up to 3.0 F (i.e.,0.48 F cm^?2) when charged to 0.6 V using current densities as high as 31 A g^?1 based on the PPy weight (i.e., 99 mA cm^?2). Energy and power densities of 1.75 Wh kg^?1 and 2.7 kW kg^?1, respectively, are obtained when normalized with respect to twice the PPy weight of the smaller electrode. No loss in cell capacitance is seen during charging/discharging at 7.7 A g^?1 (PPy) over 1500 cycles. It is proposed that the nonelectroactive carbon filaments decrease the contact resistances and the resistance of the reduced PPy composite. The present straightforward approach represents significant progress in the development of low‐cost and environmentally friendly paper‐based energy‐storage devices for high‐power applications.     

Carbon sources and trophic structure in a macrophyte‐dominated polyculture pond assessed by stable‐isotope analysis

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Ultra‐Lightweight 3D Carbon Current Collectors: Constructing All‐Carbon Electrodes for Stable and High Energy Density Dual‐Ion Batteries

Dual‐ion batteries (DIBs) attract great interest because they allow two types of ions for reversibly intercalating into electrodes, resulting in various advantages. However, there are three critical problems using graphite‐based cathodes, namely, low active material proportion in the electrodes, current collector corrosion, and massive cathode variation. For addressing these problems, an ultra‐lightweight 3D carbon current collector (CCC) is developed to fabricate all‐carbon electrodes as both cathodes and anodes. Compared with the conventional DIBs using Al and Cu foils as current collectors, the DIBs with 3D CCC of electrically conductive pathways and sufficient ionic diffusion channels deliver enhanced specific capacity stabilized around 140 and 120 mAh g^?1 at 0.5 and 1C, respectively. The electrochemically inert 3D CCC could essentially promote the energy density when calculating the entire electrode mass, along with long‐life cycle stability of 1000 cycles at 5C and no electrochemical corrosion on either anodes or cathodes. With an in situ optical microscope, the cathode expansion is found to massively reduce because the porous 3D CCC could effectively alleviate the huge volume. The results suggest a novel strategy for achieving low‐cost and high energy density DIBs with both mechanically and electrochemically stable features.     

Flexible Nano‐felts of Carbide‐Derived Carbon with Ultra‐high Power Handling Capability

Nano‐fibrous felts (nano‐felts) of carbide‐derived carbon (CDC) have been developed from the precursor of electrospun titanium carbide (TiC) nano‐felts. Conformal transformation of TiC into CDC conserves main features of the precursor including the high interconnectivity and structural integrity; the developed TiC‐CDC nano‐felts are mechanically flexible/resilient, and can be used as electrode material for supercapacitor application without the addition of any binder. After synthesis through chlorination of the precursor at 600 °C, the TiC‐CDC nano‐fibers show an average pore size of ～1nm, a high specific surface area of 1390 m²/g; and the nano‐fibers have graphitic carbon ribbons embedded in a highly disordered carbon matrix. Graphitic carbon is preserved from the precursor nano‐fibers where a few graphene layers surround TiC nanocrystallites. Electrochemical measurements show a high gravimetric capacitance of 110 F/g in aqueous electrolyte (1 M H₂SO₄) and 65 F/g in organic electrolyte (1.5 M TEA‐BF₄ in acetonitrile). Because of the unique microstructure of TiC‐CDC nano‐felts, a fade of the capacitance of merely 50% at a high scan rate of 5 V/s is observed. A fade of just 15% is observed for nano‐felt film electrodes tested in 1 M H₂ SO₄ at 1 V/s, resulting in a high gravimetric capacitance of 94 F/g. Such a high rate performance is only known for graphene or carbon‐onion based supercapacitors, whereas binders have to be used for the fabrication of those supercapacitors.     

High‐Efficiency Polymer Homo‐Tandem Solar Cells with Carbon Quantum‐Dot‐Doped Tunnel Junction Intermediate Layer

The tunnel junction (TJ) intermediate connection layer (ICL), which is the most critical component for high‐efficient tandem solar cell, generally consists of hole conducting layer and polyethyleneimine (PEI) polyelectrolyte. However, because of the nonconducting feature of pristine PEI, photocurrent is open‐restricted in ICL even with a little thick PEI layer. Here, high‐efficiency homo‐tandem solar cells are demonstrated with enhanced efficiency by introducing carbon quantum dot (CQD)‐doped PEI on TJ–ICL. The CQD‐doped PEI provides substantial dynamic advantages in the operation of both single‐junction solar cells and homo‐tandem solar cells. The inclusion of CQDs in the PEI layer leads to improved electron extraction property in single‐junction solar cells and better series connection in tandem solar cells. The highest efficient solar cell with CQD‐doped PEI layer in between indium tin oxide (ITO) and photoactive layer exhibits a maximum power conversion efficiency (PCE) of 9.49%, which represents a value nearly 10% higher than those of solar cells with pristine PEI layer. In the case of tandem solar cells, the highest performing tandem solar cell fabricated with C‐dot‐doped PEI layer in ICL yields a PCE of 12.13%; this value represents an ≈15% increase in the efficiency compared with tandem solar cells with a pristine PEI layer.     

Rational Construction of Nitrogen‐Doped Hierarchical Dual‐Carbon for Advanced Potassium‐Ion Hybrid Capacitors

Potassium‐ion hybrid capacitors (PIHCs), elaborately integrate the advantages of high output power as well as long lifespan of supercapacitors and the high energy density of batteries, and exhibit great possibilities for the future generations of energy storage devices. The critical next step for future implementation lies in exploring a high‐rate battery‐type anode with an ultra‐stable structure to match the capacitor‐type cathode. Herein, a “dual‐carbon” is constructed, in which a three‐dimensional nitrogen‐doped microporous carbon polyhedron (NMCP) derived from metal‐organic frameworks is tightly wrapped by two‐dimensional reduced graphene oxide (NMCP@rGO). Benefiting from the synergistic effect of the inner NMCP and outer rGO, the NMCP@rGO exhibits a superior K‐ion storage capability with a high reversible capacity of 386 mAh g^?1 at 0.05 A g^?1 and ultra‐long cycle stability with a capacity of 151.4 mAh g^?1 after 6000 cycles at 5.0 A g^?1. As expected, the as‐assembled PIHCs with a working voltage as high as 4.2 V present a high energy/power density (63.6 Wh kg^?1 at 19 091 W kg^?1) and excellent capacity retention of 84.7% after 12 000 cycles. This rational construction of advanced PIHCs with excellent performance opens a new avenue for further application and development.     

Two‐Step Activated Carbon Cloth with Oxygen‐Rich Functional Groups as a High‐Performance Additive‐Free Air Electrode for Flexible Zinc–Air Batteries

A flexible air electrode (FAE) with both high oxygen electrocatalytic activity and excellent flexibility is the key to the performance of various flexible devices, such as Zn–air batteries. A facile two‐step method, mild acid oxidation followed by air calcination that directly activates commercial carbon cloth (CC) to generate uniform nanoporous and super hydrophilic surface structures with optimized oxygen‐rich functional groups and an enhanced surface area, is presented here. Impressively, this two‐step activated CC (CC‐AC) exhibits superior oxygen electrocatalytic activity and durability, outperforming the oxygen‐doped carbon materials reported to date. Especially, CC‐AC delivers an oxygen evolution reaction (OER) overpotential of 360 mV at 10 mA cm^?2 in 1 m KOH, which is among the best performances of metal‐free OER electrocatalysts. The practical application of CC‐AC is presented via its use as an FAE in a flexible rechargeable Zn–air battery. The bendable battery achieves a high open circuit voltage of 1.37 V, a remarkable peak power density of 52.3 mW cm^?3 at 77.5 mA cm^?3, good cycling performance with a small charge–discharge voltage gap of 0.98 V and high flexibility. This study provides a new approach to the design and construction of high‐performance self‐supported metal‐free electrodes.     

Short‐Range Order in Mesoporous Carbon Boosts Potassium‐Ion Battery Performance

The adequate potassium resource on the earth has driven the researchers to explore new‐concept potassium‐ion batteries (KIBs) with high energy density. Graphite is a common anode for KIBs; however, the main challenge faced by KIBs is that K ions have the larger size than Li and Na ions, hindering the intercalation of K ions into electrodes and thus leading to poor rate performance, low capacity, and cycle stability during the potassiation and depotassiation process. Herein, an amorphous ordered mesoporous carbon (OMC) is reported as a new anode material for high‐performance KIBs. Unlike the well‐crystallized graphite, in which the K ions are squeezed into the restricted interlayer spacing, it is found that the amorphous OMC possesses larger interlayer spacing in short range and fewer carbon atoms in one carbon‐layers cluster, making it more flexible to the deformation of carbon layers. The larger interlayer spacing and the unique layered structure in short range can intercalate more K ions into the carbon layer, accommodate the increase of the interlayer spacing, and tolerate the volume expansion, resulting in a battery behavior with high capacity, high rate capability, and long cycle life.