Polydopamine-coated graphene for supercapacitors with improved electrochemical performances and reduced self-discharge

Laser-ablated graphene electrodes modified with redox melanin-like film for redox capacitive sensing via the scavenging of nitrite ions

Improper use and harmful effects of nitrite ions pose a significant risk to human health. To address this concern, the use of carbon-based materials for electrochemical sensing is regarded as one of the most promising detection tools for ensuring the quality of drinking water and food products. In this context, we developed laser-ablated graphene electrodes (LAGEs) by direct laser scribing on a polyimide substrate, which were subsequently modified by electrochemical deposition of a redox-active melanin-like film (MeLF/LAGEs). Electrochemical investigations showed that the polymeric film had a beneficial effect on the heterogeneous electron transfer rate and induced an increase in the electrochemically active surface area and the charge capacitance of the modified electrodes owing to the newly added catechol and o-quinone moieties. Taking advantage of the redox activity of MeLF films, in-solution probe-free redox capacitance spectroscopy was used as a sensitive and highly adaptable method for sensing nitrite ions. Upon the interaction between the nitrite ions and the MeLF/LAGE redox interface, the charge distribution and its inherent redox capacitance were altered, which allowed the successful detection of nitrite ions with a detection limit of 2.45 μM (S/N = 3) and a wide dynamic range (10 μM to 10 mM). This sensor demonstrated high recovery rates when applied to tap and mineral water samples and five different processed meat samples, highlighting its potential for the routine detection of nitrite ions through scavenging.

Exceptional Suppression of the Self-Discharge Behavior of Supercapacitors by Precisely Tuning the Surface Assets of MXene by a Spontaneous Single-Atom Doping Strategy

MXene-based supercapacitors (SCs) are widely regarded as promising energy storage devices. However, the inevitable and ignored self-discharge behavior of MXene-based SCs causes an unavoidable voltage decay and energy loss. Herein, the Ru single-atom doping strategy is used to fabricate a Ru-MXene film to modulate the surface properties of MXene, and the assembled Ru-MXene film-based SC showed a suppressed self-discharge behavior due to the simultaneously reduced activation-controlled and diffusion-controlled reactions. Regarding the self-discharge mechanism, three positive synergistic effects including increased adsorption energy, increased work function, and oxidation of the Ti atom of Ru-MXene simultaneously led to suppressing the self-discharge behavior. Benefiting from abundant electroactive sites and higher adsorption energy, the assembled Ru-MXene film-based SC exhibited an excellent electrochemical performance. This work provides a glimpse into the single-atom doping strategy to suppress the self-discharge behavior of MXene-based SCs and a prospective guide to promote their practical applications.

Theoretical insights into dopamine photochemistry adsorbed on graphene-type nanostructures

The equilibrium geometry structures and light absorption properties of the dopamine (DA) and dopamine-o-quinone (DAQ) adsorbed on the graphene surface have been investigated using the ground state and linear-response time-dependent density functional theories. Two types of graphene systems were considered, a rectangular form of hexagonal lattice with optimized C-C bond length as the model system for graphene nanoparticles (GrNP) and a similar system but with fixed C-C bond length (1.42 Å) as the model system for graphene 2D sheet (GrS). The analysis of the vertical excitations showed that three types of electronic transitions are possible, namely, localized on graphene, localized on the DA or DAQ, and charge transfer (CT). In the case of the graphene-DA complex, the charge transfer excitations were characterized by the molecule-to-surface (MSCT) character, whereas the graphene-DAQ was characterized by the reverse, i.e. surface-to-molecule (SMCT). The difference between the two cases is given by the presence of an energetically low-lying unoccupied orbital (LUMO+1) that allows charge transfer from the surface to the molecule in the case of DAQ. However, it was also shown that the fingerprints of excited electronic states associated with the adsorbed molecules cannot be seen in the spectrum, as they are mostly suppressed by the characteristic spectral shape of graphene.

Faradaic and Non-Faradaic Self-Discharge Mechanisms in Carbon-Based Electrochemical Capacitors

Electrochemical capacitors (ECs) play a crucial role in electrical energy storage, offering great potential for efficient energy storage and power management. However, they face challenges such as moderate energy densities and rapid self-discharge. Addressing self-discharge necessitates a fundamental understanding of the underlying processes. This review sets itself apart from other reviews by focusing on the basic principles of self-discharge processes in carbon-based ECs, particularly examining the nature of the process and the involvement of redox reactions. This study delineates the potential conditions for various self-discharge processes and proposes plausible criteria for differentiation, complemented by mathematical modeling. Additionally, the model selection, curve fitting, and effective tuning methods are explored to control self-discharge processes.

Nitrogen-Doped Cellulose-Derived Porous Carbon Fibers for High Mass-Loading Aqueous Supercapacitors

Biomass-based porous carbon with renewability and flexible structure tunability is a promising electrode material for supercapacitors. However, there is a huge gap between experimental research and practical applications. How to maintain good electrochemical performance of high mass-loading electrodes and suppress the self-discharge of supercapacitors is a key issue that urgently needs to be addressed. The structure regulation of electrode materials such as heteroatom doping is a promising optimization strategy for high mass-loading electrodes. In this work, nitrogen-doped cellulose-derived porous carbon fibers (N-CHPCs) were prepared by a facile bio-template method using cotton cellulose as raw material and urea as dopant. The prepared N-CHPCs have high specific surface area, excellent hierarchical porous structure, partial graphitization properties and suitable heteroatom content. The assembled high mass-loading (12.8 mg cm-2 ; 245 μm) aqueous supercapacitor has excellent electrochemical performance, i. e., low open-circuit voltage attenuation rate (21.39 mV h-1 ), high voltage retention rate (78.81 %), high specific capacitance (295.8 F g-1 at 0.1 A g-1 ), excellent area capacitance (3.79 F cm-2 at 0.1 A g-1 ), excellent cycling stability (97.28 % over 20,000 cycles at 1.0 A g-1 ). The excellent performance of high mass-loading N-CHPCs is of great significance for their practical applications in advanced aqueous supercapacitors.

Advances in terahertz metasurface graphene for biosensing and application

Based on the extraordinary electromagnetic properties of terahertz waves, such as broadband, low energy, high permeability, and biometric fingerprint spectra, terahertz sensors show great application prospects in the biochemical field. However, the sensitivity of terahertz sensing technology is increasingly required by modern sensing demands. With the development of terahertz technology and functional materials, graphene-based terahertz metasurface sensors with the advantages of high sensitivity, fingerprint identification, nondestructive and anti-interference are gradually gaining attention. In addition to providing ideas for terahertz biosensors, these devices have attracted in-depth research and development by scientists. An overview of graphene-based terahertz metasurfaces and their applications in the detection of biochemical molecules is presented. This includes sensor mechanism research, graphene metasurface index evaluation, protein and nucleic acid sensors, and other chemical molecule sensing. A comparative analysis of graphene, nanomaterials, silicon, and metals to develop material-integrated metasurfaces. Furthermore, a brief summary of the main performance results of this class of devices is presented, along with suggestions for improvements to the existing shortcoming.