Excellent Electrical Conductivity Research Articles

Recent Advances in Constructing Three-Dimensional Graphitic Carbon Nitride Based Materials and Their Applications in Environmental Photocatalysis, Photo-Electrochemistry, and Electrochemistry

Recently, graphitic carbon nitride (g-C3N4), a promising visible-light-driven semiconductor material, has received enormous attention for photocatalytic water splitting, organic pollutant degradation, and CO2 reduction. However, the photocatalytic activity of bulk g-C3N4 is restricted due to the insufficient light adsorption, ineffective utilization of photogenerated charge carriers, and low specific surface area. Compared with bulk g-C3N4, the three-dimensional graphitic carbon nitride based materials (3D CNBMs) have outstanding physical and chemical characteristics, such as large specific area, plentiful active sites, and excellent electrical conductivity. This article reviews the latest achievements in 3D CNBMs, and presents the state-of-the-art advances in the synthetic methods of 3D CNBMs. Meanwhile, various applications of 3D CNBMs in photocatalysis, photo-electrochemistry, and electrochemistry are systematically reviewed and discussed. In addition, possible improvements and perspectives of 3D CNBMs are proposed. This review aims to summarize a panorama of the up-to-date processes of 3D CNBMs in environmental and energy applications and provide some innovative thoughts to accelerate the ground-breaking research and development of 3D CNBMs for a sustainable future.

Advances and challenges in MXene-based electrocatalysts: unlocking the potential for sustainable energy conversion.

MXenes, a novel class of two-dimensional materials, have garnered significant attention for their promising electrocatalytic properties in various energy conversion applications such as water splitting, fuel cells, metal-air batteries, and nitrogen reduction reactions. Their excellent electrical conductivity, high specific surface area, and versatile surface chemistry enable exceptional catalytic performance. This review highlights recent advancements in the design and application strategies of MXenes as electrocatalysts, focusing on key reactions including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and nitrogen reduction reaction (NRR). We discuss the tunability of MXenes' layered structures and surface properties through surface modification, MXene lattice substitution, defect and morphology engineering, and heterostructure construction. Despite the considerable progress, MXenes face challenges such as restacking during catalysis, stability issues, and difficulties in large-scale production. Addressing these challenges through innovative engineering approaches and advancing industrial synthesis techniques is crucial for the broader application of MXene-based materials. Our review underscores the potential of MXenes in transforming electrocatalytic processes and highlights future research directions to optimize their catalytic efficiency and stability.

Synthesis of gallic acid-grafted epoxidized natural rubber and its role in self-healable flexible temperature sensors.

Developing a flexible temperature sensor with appreciable sensitivity is critical for advancing research related to flexible electronics. Although various flexible sensors are available commercially, most such temperature sensors are made from polymeric materials obtained from petrochemical resources. Such sensors will contribute to electronic waste and increase the carbon footprint after usage. While there are reports on various sensors made from sustainable polymers, research related to developing self-healable flexible temperature sensors made from sustainable polymers is significantly less. Herein, we report on developing a flexible temperature sensor made of gallic acid-grafted epoxidized natural rubber and multi-walled carbon nanotubes. Various spectroscopic and thermal techniques vetted the modification of the epoxidized natural rubber. The highest grafting of 20.9% was achieved in the selected window of stoichiometry. A self-healing behavior was achieved by leveraging the FeCl3 based metal-ligand crosslinking of the composite. The healing efficiency was noted to be 31.2% for the composite material. The fabricated sensor demonstrated an electrical resistance of 4.46 × 103 Ω, thereby warranting the composite to demonstrate an Ohmic behavior in the I-V plot. Appropriate data fitting suggested a variable range hopping mechanism as causation towards excellent electrical conduction. The temperature sensitivity and the thermal index of the developed sensor were noted to be -0.17% °C-1 and 781.2 K, respectively, in the temperature range of 30 °C to 50 °C. The proposed method of fabricating sustainable, high-strength, self-healable, and robust temperature sensors and conductors is a unique and value-added approach for next-generation flexible electronics.

Effect of substrate temperature and oxygen plasma treatment on the properties of magnetron-sputtered CdS for solar cell applications

Cadmium sulfide (CdS) is an n-type semiconductor with excellent electrical conductivity that is widely used as an electron transport material (ETM) in solar cells. At present, numerous methods for preparing CdS thin films have emerged, among which magnetron sputtering (MS) is one of the most commonly used vacuum techniques. For this type of technique, the substrate temperature is one of the key deposition parameters that affects the interfacial properties between the target film and substrate, determining the specific growth habits of the films. Herein, the effect of substrate temperature on the microstructure and electrical properties of magnetron-sputtered CdS (MS-CdS) films was studied and applied for the first time in hydrothermally deposited antimony selenosulfide (Sb<sub>2</sub>(S,Se)<sub>3</sub>) solar cells. Adjusting the substrate temperature not only results in the design of the flat and dense film with enhanced crystallinity but also leads to the formation of an energy level arrangement with a Sb<sub>2</sub>(S,Se)<sub>3</sub> layer that is more favorable for electron transfer. In addition, we developed an oxygen plasma treatment for CdS, reducing the parasitic absorption of the device and resulting in an increase in the short-circuit current density of the solar cell. This study demonstrates the feasibility of MS-CdS in the fabrication of hydrothermal Sb<sub>2</sub>(S,Se)<sub>3</sub> solar cells and provides interface optimization strategies to improve device performance.

MXene-based composites in smart wound healing and dressings.

MXenes, a class of two-dimensional materials, exhibit considerable potential in wound healing and dressing applications due to their distinctive attributes, including biocompatibility, expansive specific surface area, hydrophilicity, excellent electrical conductivity, unique mechanical properties, facile surface functionalization, and tunable band gaps. These materials serve as a foundation for the development of advanced wound healing materials, offering multifunctional nanoplatforms with theranostic capabilities. Key advantages of MXene-based materials in wound healing and dressings encompass potent antibacterial properties, hemostatic potential, pro-proliferative attributes, photothermal effects, and facilitation of cell growth. So far, different types of MXene-based materials have been introduced with improved features for wound healing and dressing applications. This review covers the recent advancements in MXene-based wound healing and dressings, with a focus on their contributions to tissue regeneration, infection control, anti-inflammation, photothermal effects, and targeted therapeutic delivery. We also discussed the constraints and prospects for the future application of these nanocomposites in the context of wound healing/dressings.

Ultrafine VN quantum nanodots anchored in three-dimensional porous N-doped carbon with excellent pseudocapacitive behaviour for high rate lithium-ion batteries

In situ formation of VN quantum nanodots (VNQDs) anchored on a three-dimensional porous N-doped carbon (PNC) skeleton was achieved by a simple strategy to build porous VNQDs@PNC composites as pseudocapacitive anode materials for LIBs.

High performance thermoelectrics from low-cost and abundant CuS/CuI composites

Electrically highly conducting CuS particles were encapsulated within a compound with a high Seebeck coefficient, i.e. CuI, to form core–shell type composites. As a result, an improved zT of 0.46 at room temperature has been achieved.

Electrochemical aptamer sensor based on AgNPs@PDANSs and "sandwich" structure guidance for the detection of tobramycin in water samples.

In this study, an ultrasensitive detection platform for tobramycin (TOB) was developed, featuring a "sandwich" structure guided by AgNCs@PDANSs and Thi-AuNCs@ZnONSs. To address the issue of large background current peak signals in tagless sensors, Thi-AuNCs@ZnONSs composites were synthesized as signal tags. Zinc oxide nanosheets (ZnONSs) served as the loading agent, and AuNCs with the electroactive molecule Thi acted as carriers. Furthermore, AgNPs@PDANSs nanocomposites, possessing excellent electrical conductivity and large specific surface areas, were prepared as substrate materials for the modified electrodes. A "sandwich" structure strategy was also introduced to enhance the accuracy of the electrochemical aptasensor. This strategy, utilizing a dual sequence for target labeling and capture, yielded higher sensitivity and simplified the sensor construction compared to methods employing a single sequence. Under optimal conditions, the detection limit for TOB was established at 1.41 pM, with a detection range of 0.05-5000 nM. The aptasensor was effectively applied in the detection of TOB in tap and lake water, demonstrating outstanding reproducibility, selectivity, and stability. These results may serve as a reference for environmental TOB detection.

One step synthesis of 3D BaTiO3@C for novel electromagnetic shielding of epoxy composites

The ever-increasing electromagnetic interference (EMI) pollution becomes an important issue in current environmental protection and human society. In this work, 3D BaTiO3(3DBT)@C porous ceramic framework was one-step fabricated by sol-gel method using environmentally friendly cleanroom wipers as template and calcined under inert atmosphere. Subsequently, epoxy (EP) resin was reversely incorporated into the above 3DBT@C network, resulting in the 3DBT@C/EP composite. Results showed that the cleanroom wiper was calcined and transformed into amorphous carbon within the 3DBT, leading to the excellent electrical conductivity of 3DBT@C. The reverse infiltration of EP maintained the successive 3DBT framework and excellent conductivity, thus endowing the 3DBT@C/EP composite with a good electromagnetic interference shielding effectiveness (EMI SE) as high as 36.6 dBꞏmm-1 at 10.2 GHz. The absorption mechanism of EMI prevailed the EMI SE due to the large Ohmic loss and porous 3D network, implying potential applications in flexile shielding materials.

Boosting oxygen evolution reaction rates with mesoporous Fe-doped MoCo-phosphide nanosheets.

Transition metal-based catalysts are commonly used for water electrolysis and cost-effective hydrogen fuel production due to their exceptional electrochemical performance, particularly in enhancing the efficiency of the oxygen evolution reaction (OER) at the anode. In this study, a novel approach was developed for the preparation of catalysts with abundant active sites and defects. The MoCoFe-phosphide catalyst nanosheets were synthesized using a simple one-step hydrothermal reaction and chemical vapor deposition-based phosphorization. The resulting MoCoFe-phosphide catalyst nanosheets displayed excellent electrical conductivity and a high number of electrochemically active sites, leading to high electrocatalytic activities and efficient kinetics for the OER. The MoCoFe-phosphide catalyst nanosheets demonstrated remarkable catalytic activity, achieving a low overpotential of only 250 mV to achieve the OER at a current density of 10 mA cm-2. The catalyst also exhibited a low Tafel slope of 43.38 mV dec-1 and maintained high stability for OER in alkaline media, surpassing the performance of most other transition metal-based electrocatalysts. The outstanding OER performance can be attributed to the effects of Mo and Fe, which modulate the electronic properties and structures of CoP. The results showed a surface with abundant defects and active sites with a higher proportion of Co2+ active sites, a larger specific surface area, and improved interfacial charge transfer. X-ray photoelectron spectroscopy (XPS) analysis revealed that the catalyst's high activity originates from the presence of Mo6+/Mo4+ and Co2+/Co3+ redox couples, as well as the formation of active metal (oxy)hydroxide species on its surface.

Graphene’s Role in the Clean-Up Act: Progress in Industrial Wastewater Treatment

Water scarcity and environmental pollution are the two pressing global challenges that demand innovative solutions. In recent years, graphene-based nanomaterials have emerged as promising candidates for revolutionizing the field of water purification, particularly in the treatment of industrial wastewater. This review article, the first part of a two-part series, comprehensively explores the applications, synthesis methods and mechanisms of graphene-based nanomaterials for industrial wastewater treatment. Industrial activities generate vast quantities of wastewater contaminated with a myriad of organic and inorganic pollutants, heavy metals and dyes. Traditional water treatment methods often fall short in efficiently removing these contaminants, leading to significant environmental repercussions. In contrast, graphene-based nanomaterials exhibit exceptional properties, including a high specific surface area, excellent electrical conductivity and superior mechanical strength, making them highly attractive for wastewater treatment applications. This article begins by providing an overview of the properties of graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), which form the foundation of graphene-based nanomaterials. Our discussion focuses on the many approaches to the synthesis of these nanomaterials, with particular attention paid to their scalability, cost-effectiveness, and influence on the environment. This review focuses on the various applications of graphene-based nanomaterials in the in the removal of organic pollutants and dyes through adsorption, coagulation and photocatalysis. The unique structural and chemical properties of graphene-based nanomaterials enable them to selectively adsorb pollutants, even in the presence of competing ions, making them highly efficient in complex wastewater matrices. Furthermore, we explore the mechanistic aspects of pollutant removal by graphene-based nanomaterials, elucidating the role of surface functionalization, intermolecular interactions and photocatalytic processes. In addition to their pollutant removal capabilities, graphene-based nanomaterials also exhibit remarkable regeneration potential, making them economically sustainable for long-term industrial wastewater treatment applications. The challenges associated with large-scale implementation, including cost considerations, environmental impact assessments and regulatory compliance are also discussed. This review aims to provide a comprehensive understanding of the current state of research in graphene-based nanomaterials for industrial wastewater treatment. By critically evaluating the existing iterature, this review serves as a valuable resource for researchers and practitioners seeking to advance sustainable and effective strategies for the industrial wastewater treatment through the integration of graphene-based nanomaterials.

MOFs-derived Cu/Carbon membrane as bi-functional catalyst: Improving catalytic activity in detection and degradation of PFOA via regulation of reactive Cu species

Developing bi-functional materials to simultaneously detect and degrade persistent pollutant perfluorooctanoic acid (PFOA) is of significant importance for environmental and human health. In this study, a copper-based mimic enzyme colorimetric detecting sensor coupling with solar photo-electro-Fenton degradation (SPEF) system was built based on Cu/carbon nanofiber (CNF-Cu/C) membrane. Self-supporting flexible CNF-Cu/C membrane was successfully prepared by solvothermal method with further secondary seed growth and in-situ thermal reduction. Based on calcination temperature regulation, the obtained CNF-Cu/C membrane derived from MOFs/polyacrylonitrile (HKUST/PAN) membrane was characterized by 3D carbon fiber network dispersed with porous carbon skeleton embedded copper species, which exhibited good dispersibility, cycling stability and excellent electrical conductivity. With excellent peroxidase-like activity, CNF-Cu/C can rapidly detect PFOA based on the inhibition of color reaction of tetramethylbenzidine (TMB) with detection limit of 0.133 μM. Furthermore, CNF-Cu/C also exhibited outstanding optical properties and oxygen redox reaction (ORR) activity, and the removal efficiency of PFOA in Cu-SPEF system reached 98.22 % within 180 min under optimal conditions. Based on the results of free radical quenching experiment, EPR experiment and HPLC-MS experiments, the degradation mechanism and degradation path of PFOA by SPEF system were further speculated.

Electrospun carbon nanofibers decorated with NiS2-FeS2 used as efficient counter electrode for quantum dot sensitized solar cells

NiS2-FeS2 composite material is uniformly coated on CNFs by a one-step hydrothermal method with carbon nanofibers (CNFs) prepared by the electrostatic spinning method. A composite NiS2-FeS2/CNFs with Ti-mesh substrate counter electrode (NiS2-FeS2/CNFs CE) is prepared by scraping and coating method on a metal titanium mesh and applied to quantum dot sensitized solar cells (QDSSCs). The photoelectric power conversion efficiency of QDSSCs assembled on NiS2-FeS2/CNFs electrode substrate is 9.85%, which is much higher than that of traditional Cu2S/brass (7.85%). The exceptional achievement can be attributed to the superb amalgamation of the Ti-mesh's excellent electrical conductivity and the NiS2-FeS2/CNFs nanostructure's high and stable catalytic activity. The co-loading of NiS2 and FeS2 reduces the particle size of the composite, enhancing the specific surface area of the counter electrode. It provides abundant catalytic active sites for QDSSCs. The experimental results show that NiS2-FeS2 is expected to be a competitive and efficient QDSSCs CE material.

Intercross-linked aramid nanofibers/graphene hybrid films toward high mechanical strength and electrical conductivity

Although macro graphene films exhibit excellent electrical conductivity, the low tensile strength severely hampers the application of graphene films. Combined with polymers can improve the mechanical properties to a degree, but it usually dramatically decreases the electrical conductivities of graphene films. It is thus a great challenge to make a trade-off between high electrical conductivity and excellent mechanical properties of graphene films. Herein, we designed a homogeneous layer-by-layer microstructure combining reduced graphene oxide (rGO) nanosheets with aramid nanofiber (ANF) to obtain composite films referred to as rGO/ANF. By tuning the concentration of ANF, a balanced electrical and mechanical performance can be achieved. It is revealed that the tensile strength of the optimized composite film with 10 wt% ANF was dramatically improved by 194% comparing to the pure rGO film, and the electrical conductivity still maintained at a relatively high level (8495 S m−1). The remarkable overall properties can be attributed to the unique structure with abundant hydrogen bonds and π-π interactions formed between rGO and ANF. The results demonstrate that this is indeed an efficient strategy to balance the electrical conductivity and tensile strength of the graphene-polymer composites, making it promising in the application of multi-functional electronic devices.

Enhancing the Toughness of PAA/LCNF/SA Hydrogel through Double-Network Crosslinking for Strain Sensor Application.

Lignin-containing nanocellulose fibers (LCNF) have been considered as a valuable enhancer for polyacrylic acid (PAA)-based hydrogels that can form rigid porous network structures and provide abundant polar groups. However, the PAA-LCNF hydrogel is dominated by a single-network (SN) structure, which shows certain limitations when encountering external environments with high loads and large deformations. In this paper, sodium alginate (SA) was introduced into the PAA-LCNF hydrogel network to prepare a double-network (DN) hydrogel structure of the SA-Ca2+ and PAA-LCNF through a two-step process. The covalent network of PAA-LCNF acts as the resilient framework of the hydrogel, while the calcium bridging networks of SA, along with the robust hydrogen bonding network within the system, function as sacrificial bonds that dissipate energy and facilitate stress transfer. The resulting hydrogel has porous morphologies. Results show that SA can effectively improve the mechanical properties of DN hydrogels and endow them with excellent thermal stability and electrical conductivity. Compared with pure PAA-LCNF hydrogel, the elongation at break of DN hydrogel increased from 3466% to 5607%. The good electrical conductivity makes it possible to use the flexible sensors based on DN hydrogel to measure electrophysiological signals. Our results can provide a reference for developing multifunctional hydrogels that can withstand ultra large deformation.

Facile synthesis of ultrathin carbon nanosheets through NaCl-KCl templates coupled with ice-induced assembly strategy from carbon quantum dots as lithium-ion batteries anodes

Carbon nanosheets (CNSs) have drawn numerous attention as promising anodes in energy storage devices due to their large specific surface area and excellent electrical conductivity close to graphene materials. Restricted by the expensive raw materials, harsh conditions and cumbersome post-processing procedures, the large-scale production of CNSs is a major challenge. Herein, ultrathin CNSs were constructed through NaCl-KCl templates coupled with ice-induced assembly strategy from coal tar pitch-based carbon quantum dots (CQDs). The ice can manipulate the salt crystal growth direction and dominantly guide the distribution of CQDs on the surface of NaCl-KCl crystal templates, whereas the liquid salts not only act as the solvent to load and disperse CQDs but as molecular templates to facilitate the formation of ultrathin CNSs during the carbonization process. The prepared CNSs exhibit twisted and ultrathin lamellar structures with a thickness of ∼1.70 nm, abundant hierarchical nanopores, and a moderate specific surface area of 302.00 m2·g−1. Benefiting from these unique microstructure features, the optimized CNS-3 adopted as anode for lithium-ion batteries displays prominent electrochemical features with high reversible capacity (453 mAh·g−1 at 50 mA·g−1), outstanding rate capability (225 mAh·g−1 at 1000 mA·g−1), and superior cycle stability (94.3% capacitance retention after 500 cycles at 500 mA·g−1). Our work provides a novel and effective approach to fabricate ultrathin carbon nanosheets by employing low-cost carbon quantum dots derived from coal tar pitch as potential candidates applied for lithium-ion batteries and next-generation energy storage devices.

Highly sensitive detection of kaempferol using electrochemical sensors based on 3D-ordered mesh interconnect C60-GO, Ni-MOF, and β-cyclodextrin

Kaempferol (KA), which is a biologically active natural polyphenol compound with considerable medicinal value, can be used to fight various diseases. Herein, a β-cyclodextrin/fullerene-graphene oxide/Nickel-metal–organic framework (β-CD/C60-GO/Ni-MOF) composite-based electrochemical sensor was developed to detect KA; the composite was characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron (XPS) spectroscopy. The biocompatibility and host–guest recognition performance of β-cyclodextrin (β-CD); the porosity and the catalytic performance of the nickel-metal–organic framework (Ni-MOF); and the excellent electrical conductivity of fullerene-graphene oxide (C60-GO) work together to considerably improve the detection performance of the sensor for KA. The sensor has an extensive linear range (0.5–50 μM) and a low detection limit (58 nM) with an electrochemically active specific surface area of 0.0971 cm2; furthermore, the redox mechanism of KA in the sensor is explained. For the detection of actual natural samples of broccoli containing KA, satisfactory recovery rates and accuracy were observed; the recovery rate was 96.4 %–101.1 %.

Doping induced mixed polytypic interfaces of MoS2 for superior electrocatalytic hydrogen evolution

Metallic 1Tphase of MoS2 is desirable for accelerating electrochemical hydrogen generation from water due to its excellent electrical conductivity. However, it is challenging to synthesize stable 1T-MoS2 on a large scale using conventional chemical or physical routes. Here in, we report highly durable Mn-doping assisted in-plane 1T/2H mixed phases of MoS2 synthesized by a simple one-step hydrothermal method. The Mn-doping not only results in a favourable electronic modification in the host lattice but also in a structural evolution from 2H (trigonal prismatic) to 1T (octahedral) phase through sulfur-plane gliding with additional defects. The 1T/2H interfaces exhibit more active sites for hydrogen evolution reaction (HER) than its bulk 2H-counterpart. The developed electrode shows relatively low overpotential of 95 mV at a current density of 10 mA/cm2 and a low Tafel slope of 72 mV/dec in 0.5 M H2SO4. In addition, Density Functional Theory based calculations also support that Mn-doping in pristine 2H-MoS2 encourages a structural phase separation. Locally hybridized d-orbitals of Mn induce the formation of 2H/1T interfaces. It results in the delocalization of electrons, and thus facilitates the hydrogen adsorption and desorption process, better than the 2H phase.

(Invited) New Synthesis Strategies to Develop High Performance Sulfur-Carbon Composites Cathodes for Lithium/Sulfur Batteries

Lithium/sulfur cells (Li/S) are recognized as a highly promising, inexpensive, and high-specific energy storage system due to the low cost of elemental sulfur and high theoretical specific capacity (1,675 mAh/g) of a sulfur cathode when paired with a lithium anode.1 However, practical applications of Li/S cells have been impeded by challenges related to sulfur cathodes, such as low sulfur conductivity of sulfur, the polysulfides shuttle, and changes in electrode microstructure during cycling.2-4 Sulfur-carbon (S-C) composites have been extensively studied as an effective approach to overcome these issues and improve the electrochemical performance of Li/S cells. Carbon materials, including graphene oxide, carbon nanotubes or nanofibers, and porous carbon, have been used in sulfur cathodes due to their high surface area, excellent electrical conductivity, and sulfur-immobilizing ability from their porous structure or surface functional groups.4 Designing high-performance S-C composites involves carefully considering various factors, such as the type of carbon materials, surface area or pore volume of carbon, sulfur-carbon weight ratio, and synthesis method, to achieve optimal performance. Key design strategies for high-performance S-C composites include: (1) confining nano-sized sulfur in porous carbon, (2) coating nano-sized sulfur onto sulfur-philic functional carbon surface, (3) engineering sulfur structures that form an adequate sulfur-electrolyte interface, and (4) preserving the electronically conductive carbon network in sulfur-carbon composite to minimize the electronic resistance of the sulfur cathode.4 S-C composites consisting of sulfur nanostructures confined in carbon porous structures or coated onto large sulfur-philic surface areas of carbon (e.g., graphene oxide) are promising composite designs as they can promote the kinetics and reversibility of the electrochemical process of the sulfur cathode. In this presentation, important recent findings about how to design and synthesize high performance S-C composite design will be introduced, and their effects on the electrochemical behavior of the sulfur cathode for Li/S cells will be discussed. References Bruce, P.G.; Freunberger, S.A.; Hardwick, L.J.; Tarascon, J.-M., Nat. Mater. 2012, 11, 19–29.Fotouhi, A.; Auger, D.J.; O’Neill, L.; Cleaver, T.; Walus, S., Energies 2017, 10, 1937.Peng, H.-J.; Huang, J.-Q.; Cheng, X.-B.; Zhang, Q., Adv. Energy Mater. 2017, 7, 1700260.Hwa, Y.; Cairn, E. J., ChemElectroChem 2020, 7, 3927-3942.

Boosting the Performance of on-Chip Vertically-Aligned Carbon Nanotube Supercapacitors Via Solvent-Free Radiofrequency Nitrogen Plasma Treatment

Supercapacitors are essential components for energy storage devices in many fields, including renewable energy systems, electric vehicles, and portable electronics [1]. Carbon nanotube arrays have shown great promise for use in supercapacitors due to their high surface area and excellent electrical conductivity. However, the specific capacitance of these arrays needs to be increased to improve the overall energy storage capacitance of the supercapacitor.This study presents an innovative approach to enhance the specific capacitance of on-chip vertically-aligned carbon nanotube (VACNT) structures for the development of high-performance supercapacitors. The proposed technique involves employing a solvent-free radiofrequency (RF) nitrogen plasma treatment to dope nitrogen onto VACNT surfaces. The plasma doping process utilizes reactive ion etching-induced coupled plasma (RIE-ICP) to optimize the parameters for nitrogen incorporation [2]. The nitrogen plasma treatment introduces nitrogen-containing functional groups, thereby improving their specific capacitance. The dry process is compatible with semiconductor fabrication and can be integrated into existing manufacturing techniques [1]. This study provides a promising avenue for tailoring the nitrogen doping process to achieve desired performance enhancements for on-chip VACNT supercapacitors.The plasma doping process involves the generation of reactive nitrogen species in the plasma, which subsequently interact with the VACNT surfaces. Under the influence of the RF electric field, nitrogen gas molecules are ionized and can react with the carbon atoms of the VACNTs, resulting in the formation of nitrogen-containing functional groups and the substitution of carbon atoms with nitrogen atoms within the carbon lattice [3].The improved capacitance can be attributed to the incorporation of nitrogen functional groups and to the modification of the electronic properties of the carbon nanotubes, including additional charge carriers, leading to an increase in electrical conductivity [4] and introduction of the pseudocapacitance effect [5]. There are various types of nitrogen species in the carbon lattice, such as pyridinic, pyrrolic, and graphitic nitrogen. Among these, pyridinic and pyrrolic nitrogen are known to be more electrochemically active [5], contributing to the enhanced specific capacitance. The increased electrical conductivity stems from the introduction of nitrogen dopants, which alter the electronic structure of the VACNTs, creating localized states near the Fermi level and thus facilitating charge transport [4].The level of nitrogen doping depends on several process factors, including the plasma power, gas pressure, and treatment duration. Higher plasma power generates a larger number of reactive nitrogen, while lower gas pressure results in a higher ionization efficiency. Furthermore, a longer treatment duration increases the chance of nitrogen incorporation into the VACNT structures [3]. By carefully controlling these parameters during the nitrogen plasma treatment, it is possible to tailor the nitrogen doping process to achieve the desired performance enhancement for on-chip VACNT supercapacitors.The treatment is performed in a chamber with a pressure of 10 Pa, a nitrogen gas flow rate of 10 sccm, an ICP power of 200W, and a bias voltage of -120 V. We report a specific capacitance of modified VACNTs of 18 mF/cm2(Figure 1), nearly twice that of pristine samples (Figure1(B)), for dense arrays of 1012 CNTs/ cm2. XPS result confirms 11.76% nitrogen content and 13% oxygen content in treated samples (N-VACNTs), aligning with previous studies on wet process nitrogen-doped carbon nanotubes [5]. Oxygen content also plays a crucial role in VACNT-based supercapacitor performance, improving surface wettability which will reduce the resistance between electrode and electrolyte [6, 7]. However, excessive oxygen content can negatively affect VACNT supercapacitor lifetime. By finding the trade-off between nitrogen and oxygen content, we optimized the doping process, enhancing specific capacitance, stability (Figure 2), and lifetime for on-chip VACNT structures.In conclusion, our study demonstrates that the solvent-free RF nitrogen plasma treatment can effectively enhance the specific capacitance of on-chip VACNT structures by up to 100%, making them suitable for integration into high-performance on-chip supercapacitor.