Improving Fresh and End-Used Carbon Surface by Sunlight: A Step Forward in Sustainable Carbon Processing

Carbon is at the forefront of sustainable materials; the modification of its surface is pivotal to many traditional and advanced applications. Conventional high-temperature activation or chemical etching for carbon surface modification is time- and energy-intensive as well as requiring a high volume of toxic chemicals; therefore, a cheaper, quicker, and eco-friendly technique is a step forward toward its sustainable processing. Herein, modification of fresh and end-used carbon surface through focusing the sunlight is demonstrated as a clean, sustainable, and instantaneous surface modification technique for electrochemical charge storage application. Temporal evolution of the carbon surface is monitored using field-emission scanning electron microscopy, gas adsorption measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Results demonstrate that solar irradiation led to the rapid release of moisture, which in turn generated newer pores. Electrochemical analyses showed that treating the porous carbon for 20 s boosted its electrical double layer capacitance by 56%. The usefulness of the solar treatment in recovering degraded electrochemical capacitor electrodes was also investigated, where 95% of the electrochemical performance was restored. This work demonstrated the feasibility of utilizing focused sunlight for surface treatment, suggesting utilizing sunlight for a sustainable, activation agent-free, and rapid surface treatment technique

Charge Transport through Electrospun SnO₂ Nanoflowers and Nanofibers: Role of Surface Trap Density on Electron Transport Dynamics

A larger amount of tin precursor was dispersed in electrospun polyvinyl acetate fibers than that required for SnO₂ fiber formation upon annealing, thereby creating a constraint such that all nuclei formed during annealing could not be accommodated within the fiber, which leads to enhanced reaction kinetics and formation of highly crystalline–cum–higher surface area SnO₂ flowers. The flowers are shown to have a lower density of surface trap states than fibers by combining absorption spectra and open circuit voltage decay (OCVD) measurements. Charge transport through the SnO₂ flowers in the presence of the iodide/triiodide electrolyte was studied by OCVD, electrochemical impedance spectroscopy, and transient photodecay techniques. The study shows that the flowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with fibers. Photocurrent transients were used to extract the effective electron diffusion coefficient and mobility which were an order of magnitude higher for the flowers than that for the fibers. The flowers are also shown to have an enhanced Fermi energy, on account of which as well as higher electron mobility, dye-sensitized solar cells fabricated using the SnO₂ flowers gave <i>V</i>_OC ∼700 mV and one of the highest photoelectric conversion efficiencies achieved using pure SnO₂

Environment-Modulated Crystallization of Cu₂O and CuO Nanowires by Electrospinning and Their Charge Storage Properties

This article reports the synthesis of cuprous oxide (Cu₂O) and cupric oxide (CuO) nanowires by controlling the calcination environment of electrospun polymeric nanowires and their charge storage properties. The Cu₂O nanowires showed higher surface area (86 m² g^–1) and pore size than the CuO nanowires (36 m² g^–1). Electrochemical analysis was carried out in 6 M KOH, and both the electrodes showed battery-type charge storage mechanism. The electrospun Cu₂O electrodes delivered high discharge capacity (126 mA h g^–1) than CuO (72 mA h g^–1) at a current density of 2.4 mA cm^–2. Electrochemical impedance spectroscopy measurements show almost similar charge-transfer resistance in Cu₂O (1.2 Ω) and CuO (1.6 Ω); however, Cu₂O showed an order of magnitude higher ion diffusion. The difference in charge storage between these electrodes is attributed to the difference in surface properties and charge kinetics at the electrode. The electrode also shows superior cyclic stability (98%) and Coulombic efficiency (98%) after 5000 cycles. Therefore, these materials could be acceptable choices as a battery-type or pseudocapacitive electrode in asymmetric supercapacitors

Pseudocapacitive Charge Storage in Single-Step-Synthesized CoO–MnO₂–MnCo₂O₄ Hybrid Nanowires in Aqueous Alkaline Electrolytes

A new pseudocapacitive combination, viz. CoO-MnO₂–MnCo₂O₄ hybrid nanowires (HNWs), is synthesized using a facile single-step hydrothermal process, and its properties are benchmarked with conventional battery-type flower-shaped MnCo₂O₄ obtained by similar processing. The HNWs showed high electrical conductivity and specific capacitance (<i>C</i> _s) (1650 F g^–1 or 184 mA h g^–1 at 1 A g^–1) with high capacity retention, whereas MnCo₂O₄ nanoflower electrode showed only one-third conductivity and one-half of its capacitance (872 F g^–1 or 96 mA h g^–1 at 1 A g^–1) when used as a supercapacitor electrode in 6 M KOH electrolyte. The structure–property relationship of the materials is deeply investigated and reported herein. Using the HNWs as a pseudocapacitive electrode and commercial activated carbon as a supercapacitive electrode we achieved battery-like specific energy (<i>E</i> _s) and supercapacitor-like specific power (<i>P</i> _s) in aqueous alkaline asymmetric supercapacitors (ASCs). The HNWs ASCs have shown high <i>E</i> _s (90 Wh kg^–1) (volumetric energy density <i>E</i> _v ≈ 0.52 Wh cm^–3) with <i>P</i> _s up to ∼10⁴ W kg^–1 (volumetric power density <i>P</i> _v ≈ 5 W cm^–3) in 6 M KOH electrolyte, allowing the device to store an order of magnitude more energy than conventional supercapacitors

One-Dimensional Assembly of Conductive and Capacitive Metal Oxide Electrodes for High-Performance Asymmetric Supercapacitors

A one-dimensional morphology comprising nanograins of two metal oxides, one with higher electrical conductivity (CuO) and the other with higher charge storability (Co₃O₄), is developed by electrospinning technique. The CuO–Co₃O₄ nanocomposite nanowires thus formed show high specific capacitance, high rate capability, and high cycling stability compared to their single-component nanowire counterparts when used as a supercapacitor electrode. Practical symmetric (SSCs) and asymmetric (ASCs) supercapacitors are fabricated using commercial activated carbon, CuO, Co₃O₄, and CuO–Co₃O₄ composite nanowires, and their properties are compared. A high energy density of ∼44 Wh kg^–1 at a power density of 14 kW kg^–1 is achieved in CuO–Co₃O₄ ASCs employing aqueous alkaline electrolytes, enabling them to store high energy at a faster rate. The current methodology of hybrid nanowires of various functional materials could be applied to extend the performance limit of diverse electrical and electrochemical devices

Electrospun ZnO Nanowire Plantations in the Electron Transport Layer for High-Efficiency Inverted Organic Solar Cells

Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly­(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO₃/Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol–gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate

Pseudocapacitive Charge Storage in Single-Step-Synthesized CoO–MnO₂–MnCo₂O₄ Hybrid Nanowires in Aqueous Alkaline Electrolytes

A new pseudocapacitive combination, viz. CoO-MnO₂–MnCo₂O₄ hybrid nanowires (HNWs), is synthesized using a facile single-step hydrothermal process, and its properties are benchmarked with conventional battery-type flower-shaped MnCo₂O₄ obtained by similar processing. The HNWs showed high electrical conductivity and specific capacitance (<i>C</i> _s) (1650 F g^–1 or 184 mA h g^–1 at 1 A g^–1) with high capacity retention, whereas MnCo₂O₄ nanoflower electrode showed only one-third conductivity and one-half of its capacitance (872 F g^–1 or 96 mA h g^–1 at 1 A g^–1) when used as a supercapacitor electrode in 6 M KOH electrolyte. The structure–property relationship of the materials is deeply investigated and reported herein. Using the HNWs as a pseudocapacitive electrode and commercial activated carbon as a supercapacitive electrode we achieved battery-like specific energy (<i>E</i> _s) and supercapacitor-like specific power (<i>P</i> _s) in aqueous alkaline asymmetric supercapacitors (ASCs). The HNWs ASCs have shown high <i>E</i> _s (90 Wh kg^–1) (volumetric energy density <i>E</i> _v ≈ 0.52 Wh cm^–3) with <i>P</i> _s up to ∼10⁴ W kg^–1 (volumetric power density <i>P</i> _v ≈ 5 W cm^–3) in 6 M KOH electrolyte, allowing the device to store an order of magnitude more energy than conventional supercapacitors

Suppressed Volume Change of a Spray-Dried 3D Spherical-like Si/Graphite Composite Anode for High-Rate and Long-Term Lithium-Ion Batteries

Morphology plays a vital role in controlling the volume variation in Si-based anode materials and enhances lithium-ion battery performances. Here, we demonstrated advanced techniques that combine electrostatic self-assembly and spray-drying methods to form 3D spherical-like silicon/graphite (denoted “Si/G”) composite anode materials. This spherical morphology alleviates issues relating to silicon volume changes that occur in high-rate lithium-ion batteries. Commercial graphite (G) flakes were initially mixed with silicon nanoparticles (ca. 50 nm) to form a bare-Si/G composite through electrostatic interaction; spherical-like composite particles were then obtained through single and double spray-drying processes, giving samples SD1-Si/G and SD2-Si/G, respectively. We examined the charge/discharge characteristics of the fabricated electrodes (CR2032-type coin cells) in the voltage range 0.02–1.5 V (vs Li/Li+). The as-fabricated bare-Si/G, SD1-Si/G, and SD2-Si/G half-cells provided initial discharge specific capacities of 897, 866, and 1020 mA h g–1, respectively. The SD2-Si/G half-cell shows better cycling stability at a high current rate of 400 mA g–1 than the SD1-Si/G and bare-Si/G half-cells due to effective inhibition of the volume change in the more stable spherical structure of the SD2-Si/G composite, as evidenced through in situ dilatometry. Thus, the spherical Si/G composite material produced through this simple spray-drying process had structural characteristics that could effectively resist silicon’s high expansion rate, lower the production rate of broken silicon particles, and improve the electrochemical performance of the anode

Vertical TiO₂ Nanorods as a Medium for Stable and High-Efficiency Perovskite Solar Modules

Perovskite solar cells employing CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> active layers show power conversion efficiency (PCE) as high as 20% in single cells and 13% in large area modules. However, their operational stability has often been limited due to degradation of the CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> active layer. Here, we report a perovskite solar module (PSM, best and av. PCE 10.5 and 8.1%), employing solution-grown TiO₂ nanorods (NRs) as the electron transport layer, which showed an increase in performance (∼5%) even after shelf-life investigation for 2500 h. A crucial issue on the module fabrication was the patterning of the TiO₂ NRs, which was solved by interfacial engineering during the growth process and using an optimized laser pulse for patterning. A shelf-life comparison with PSMs built on TiO₂ nanoparticles (NPs, best and av. PCE 7.9 and 5.5%) of similar thickness and on a compact TiO₂ layer (CL, best and av. PCE 5.8 and 4.9%) shows, in contrast to that observed for NR PSMs, that PCE in NPs and CL PSMs dropped by ∼50 and ∼90%, respectively. This is due to the fact that the CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> active layer shows superior phase stability when incorporated in devices with TiO₂ NR scaffolds