Hierarchically Mesostructured Aluminum Current Collector for Enhancing the Performance of Supercapacitors

Ag(e)ing and Degradation of Supercapacitors: Causes, Mechanisms, Models and Countermeasures

The most prominent and highly visible advantage attributed to supercapacitors of any type and application, beyond their most notable feature of high current capability, is their high stability in terms of lifetime, number of possible charge/discharge cycles or other stability-related properties. Unfortunately, actual devices show more or less pronounced deterioration of performance parameters during time and use. Causes for this in the material and component levels, as well as on the device level, have only been addressed and discussed infrequently in published reports. The present review attempts a complete coverage on these levels; it adds in modelling approaches and provides suggestions for slowing down ag(e)ing and degradation.

Experimental and computational investigation on the charge storage performance of a novel Al2O3-reduced graphene oxide hybrid electrode

The advancements in electrochemical capacitors have noticed a remarkable enhancement in the performance for smart electronic device applications, which has led to the invention of novel and low-cost electroactive materials. Herein, we synthesized nanostructured Al₂O₃ and Al₂O₃-reduced graphene oxide (Al₂O₃-rGO) hybrid through hydrothermal and post-hydrothermal calcination processes. The synthesized materials were subject to standard characterisation processes to verify their morphological and structural details. The electrochemical performances of nanostructured Al₂O₃ and Al₂O₃- rGO hybrid were evaluated through computational and experimental analyses. Due to the superior electrical conductivity of reduced graphene oxide and the synergistic effect of both EDLC and pseudocapacitive behaviour, the Al₂O₃- rGO hybrid shows much improved electrochemical performance (~ 15-fold) as compared to bare Al₂O₃. Further, a symmetric supercapacitor device (SSD) was designed using the Al₂O₃- rGO hybrid electrodes, and detailed electrochemical performance was evaluated. The fabricated Al₂O₃- rGO hybrid-based SSD showed 98.56% capacity retention when subjected to ~ 10,000 charge-discharge cycles. Both the systems (Al₂O₃ and its rGO hybrid) have been analysed extensively with the help of Density Functional Theory simulation technique to provide detailed structural and electronic properties. With the introduction of reduced graphene oxide, the available electronic states near the Fermi level are greatly enhanced, imparting a significant increment in the conductivity of the hybrid system. The lower diffusion energy barrier for electrolyte ions and higher quantum capacitance for the hybrid structure compared to pristine Al₂O₃ justify improvement in charge storage performance for the hybrid structure, supporting our experimental findings.

The electrochemical behaviour of butyltrimethylammonium bis(trifluoromethylsulfonyl)imide at negatively polarised aluminium electrode studied by in situ soft X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry techniques

The in situ X-ray photoelectron spectroscopy data indicate that butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N4111(TFSI)) adsorbs strongly within the potential range −3.25 V < E < −2.25 V and specifically at E < −3.25 V (vs. Ag-QRE) at the Al electrode. Strong adsorption of the intermediates of N4111(TFSI) electrochemical decomposition was observed in electrochemical impedance spectroscopy and cyclic voltammetry measurements. At E < −4.25 V (vs. Ag-QRE), very intensive electrochemical reduction of N4111(TFSI) took place at the Al electrode giving gaseous products. In the potential range from − 2.25 to 0.00 V (vs. Ag-QRE), non-specific adsorption of N4111(TFSI) exists et al. surface.

Preparation and Characterization of Electric Double-Layer Capacitors Having a 3D Stainless-Steel Fiber Sheet as the Current Collector

One strategy to improve the performance of electric double-layer capacitors (EDLCs) is changing the current collector material. In this study, a three-dimensional porous current collector comprising stainless-steel fibers is fabricated using a relatively simple method. Capacitor properties of the EDLC using this unique current collector are characterized by cyclic voltammetry and charge-discharge tests. The voltammograms of the EDLC develop a more butterfly shape and an increased specific capacity at higher electrolyte concentrations. It shows reversible charge-discharge potential profiles, little capacity degradation (∼98% of the initial capacity at 1000th cycle), and a good rate performance at higher electrolyte concentrations (90% capacity retention for 2.5 times increase in discharge current). Its capacitance values (95-99 F g^-1) are roughly twice the specific capacitance of an EDLC using the flat stainless-steel plate current collector (51 F g^-1) without any performance degradation even at a higher loading of electrode active materials. Based on the AC impedance analysis, these good properties are attributed to the reduction in several resistances compared to the case of a flat stainless-steel plate: (i) the contact resistance between the electrode active material and the current collector, (ii) the resistance of the electrolyte in the finely branched space formed by the fibers and the active material, and (iii) the resistance in the diffusion layer. Increasing the electrolyte concentration further reduces the latter two resistances and the bulk electrolyte resistance, resulting in higher performance of the EDLC using the stainless-steel fiber sheet current collector.

Fabrication and Characterization of Superhydrophobic Al-Based Surface Used for Finned-Tube Heat Exchangers

Enhancing the heat transfer performance of heat exchangers is one of the main methods to reduce energy consumption and carbon emissions in heating, ventilation, air-conditioning and refrigeration (HVAC&R) systems. Wettability modified surfaces developed gradually may help. This study aims to improve the performance of heat exchangers from the perspective of component materials. The facile and cost-effective fabrication method of superhydrophobic Al-based finned-tube heat exchangers with acid etching and stearic acid self-assembly was proposed and optimized in this study, so that the modified Al fins could achieve stronger wettability and durability. The effect of process parameters on the wettability of the Al fins was by response surface methodology (RSM) and variance analysis. Then, the modified fins were characterized by field-emission scanning electron microscopy (FE-SEM), 3D topography profiler, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), respectively. The durability of the superhydrophobic fins was investigated by air exposure, corrosion resistance, and mechanical robustness experiments. The RSM and variance analysis demonstrated that a water contact angle (WCA) of 166.9° can be obtained with the etching time in 2 mol/L HCl solution of 10.5 min, the self-assembly time in the stearic acid ethanol solution of 48 h, and drying under 73.0 °C. The surface morphology showed suitable micro-nano structures with a mean roughness (Ra) of 467.58 nm and a maximum peak-to-valley vertical distance (Rt) of 4.095 μm. The chemical component demonstrated the self-assembly of an alkyl chain. The WCAs declined slightly in durability experiments, which showed the feasibility of the superhydrophobic heat exchangers under actual conditions.

Compact 3D Metal Collectors Enabled by Roll-to-Roll Nanoimprinting for Improving Capacitive Energy Storage

Reducing the contact resistance between active materials and current collectors is of engineering importance for improving capacitive energy storage. 3D current collectors have shown extraordinary promise for reducing the contact resistance, however, there is a major obstacle of being bulky or inefficient fabrication before they become viable in practice. Here a roll-to-roll nanoimprinting method is demonstrated to deform flat aluminum foils into 3D current collectors with hierarchical microstructures by combining soft matter-enhanced plastic deformation and template-confined local surface nanocracks. The generated 3D current collectors are inserted by and interlocked with active electrode materials such as activated carbon, decreasing the contact resistance by at least one order of magnitude and quadrupling the specific capacitance at high current density of 30 A g^-1 for commercial-level mass loading of 5 mg cm^-2 . The 3D current collectors are so compact that they have a low volume percentage of 7.8% in the entire electrode film, resulting in energy and power density of 29.1 Wh L^-1 and 12.8 kW L^-1 , respectively, for stack cells in organic electrolyte. Furthermore, roll-to-roll nanoimprinting of metal microstructures is low-cost, high-throughput, and can be extended to other systems that involve the microstructured metal interface, such as batteries and thermal management.

Degeneration of Key Structural Components Resulting in Ageing of Supercapacitors and the Related Chemical Ageing Mechanism

The research on supercapacitors (SCs) is one of the hot topics in the field of energy storage, and the intrinsic ageing mechanism of SCs is significant from both the economic and the scientific point of view. In this paper, the negative effects of decay of the key structural components on ageing of SCs were investigated by factorial design and analysis of variance (ANOVA). The ANOVA results showed that the degree of the negative influence on ageing of SCs could be ranked in descending order as anode > separator > cathode. The ageing would be accelerated due to the interaction between the electrode and separator, especially at a high charge-discharge current density. Further, the intrinsic chemical ageing mechanism of SCs was revealed by the morphology, microstructure, and chemical composition analyses of the fresh and aged key components (the electrode carbon materials, current collectors, and separators) with scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectra (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared (FTIR), etc. Moreover, the minimum pore width of electrode carbon materials suitable for electrolyte ion diffusion was obtained by density functional theory (DFT) calculations, which corroborated the assumption that the pore structure deterioration was one of the direct causes of capacitance loss for aged SCs. Generally, the ageing mechanism of key components of SCs could be a reference to develop advanced electrode materials and separators for SCs.

Nano-porous Al/Au skeleton to support MnO2 with enhanced performance and electrodeposition adhesion for flexible supercapacitors

A nano-porous Al/Au skeleton is constructed to effectively improve the utilization rate of the active MnO₂ and the overall adhesion between the current collector and MnO₂ in an electrodeposition system. The Al/Au current collector is prepared by first forming a nano-porous structure on the surface of Al foil through etching modification, and subsequently coating an ultra-thin Au layer onto the Al foil. The active MnO₂ is electrodeposited on the Al/Au current collector to fabricate a novel Al/Au/MnO₂ electrode. The nano-porous skeleton supports MnO₂ to grow autonomously inside-out. The ultra-thin Au layer acts as a transition layer to improve the overall conductivity of the current collector (0.35 Ω m⁻¹) and to improve the adhesion with MnO₂ as well. Owing to the highly porous structure, the electrochemical properties of the electrode are greatly improved, as evidenced by a remarkable specific capacitance of 222.13 mF cm⁻² at 0.2 mA cm⁻² and excellent rate capability of 63% capacitance retention at 6.0 mA cm⁻². Furthermore, the assembled solid-state symmetric supercapacitor exhibits a high energy density of 0.68 mW h cm⁻³, excellent cyclic stability (86.3% capacitance retention after 2000 cycles), and prominent flexibility.

A nano-porous Al/Au skeleton is constructed to effectively improve the utilization rate of the active MnO₂ and the overall adhesion between the current collector and MnO₂ in an electrodeposition system.

A novel and cost-effective method for fabrication of a durable superhydrophobic aluminum surface with self-cleaning properties

A hierarchical superhydrophobic surface is prepared via a two-step boiling water immersion process and anodization of the treated aluminum substrate in a novel hydrophobic electrolyte of aluminum nitrate and stearic acid mixture at room temperature. The immersion time in boiling water had a significant influence on the morphology and durability of the sample. A pseudoboehmite coating is created on the aluminum surface during the boiling process, as revealed by the field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared (FTIR) spectrophotometer results. The energy-dispersive x-ray spectroscopy analysis confirmed the formation of hydrophobic coating surface after anodization. Also, the FE-SEM images and the atomic force microscopy (AFM) investigation proved the hierarchical nano-and microstructure stem from boiling and anodizing procedures, respectively. The successively boiled and anodized surface exhibited contact angle of about 155˚, sliding and hysteresis contact angles of <5˚ and 2˚, respectively. It also demonstrated a self-cleaning property and remarkable durability.

Progress in supercapacitors: roles of two dimensional nanotubular materials

Overcoming the global energy crisis due to vast economic expansion with the advent of human reliance on energy-consuming labor-saving devices necessitates the demand for next-generation technologies in the form of cleaner energy storage devices. The technology accelerates with the pace of developing energy storage devices to meet the requirements wherever an unanticipated burst of power is indeed needed in a very short time. Supercapacitors are predicted to be future power vehicles because they promise faster charging times and do not rely on rare elements such as lithium. At the same time, they are key nanoscale device elements for high-frequency noise filtering with the capability of storing and releasing energy by electrostatic interactions between the ions in the electrolyte and the charge accumulated at the active electrode during the charge/discharge process. There have been several developments to increase the functionality of electrodes or finding a new electrolyte for higher energy density, but this field is still open to witness the developments in reliable materials-based energy technologies. Nanoscale materials have emerged as promising candidates for the electrode choice, especially in 2D sheet and folded tubular network forms. Due to their unique hierarchical architecture, excellent electrical and mechanical properties, and high specific surface area, nanotubular networks have been widely investigated as efficient electrode materials in supercapacitors, while maintaining their inherent characteristics of high power and long cycling life. In this review, we briefly present the evolution, classification, functionality, and application of supercapacitors from the viewpoint of nanostructured materials to apprehend the mechanism and construction of advanced supercapacitors for next-generation storage devices.

Experimental and Theoretical Investigation of Laser Pretreatment on Strengthening the Heterojunction between Carbon Fiber-Reinforced Plastic and Aluminum Alloy

Besides aluminum alloys, lightweight carbon fiber-reinforced plastics (CFRPs) have been adopted progressively in automobiles to save energy and reduce emission, so constructing a reliable heterojunction between aluminum alloys and CFRPs has come to be the key issue. In this study, ultrafast picosecond infrared (IR) and excimer ultraviolet (UV) lasers were introduced to pretreat the joint surface to enhance the adhesive strength. Scanning electron microscopy, white light interferometry, and X-ray photoelectron spectroscopy examinations indicated that because the energy absorptivities for the two lasers were different, the variation of the roughness, wettability, and chemical composition were a little different for the patterned surface. Correspondingly, the shear strengths of the adhesive joints were increased from 5.6 to 24.8 and 21.9 MPa for IR and UV laser-pretreated samples, respectively. Furthermore, finite element analysis was adopted to evaluate the effects of strengthened mechanical interlocking and fortified chemical bonding force on the enhancement of joint strength. It was shown that chemical bonding, instead of mechanical interlocking, played the dominant role in reinforcing the heterogeneous joints. As a whole, the picosecond IR laser was more preferable for surface pretreatment in adhesive heterojunctions due to its higher processing and enhancing efficiency.