Graphene Electrodes Research Articles

Bipolar Electrochemistry Enabled Synthesis and Application of 2D Materials

Graphene is a promising 2D material for biosensing and energy storage applications due to its unique properties such as high surface area and high charge mobility. However, there is a significant variation in the high quality graphene contents in commercial products. The conventional wet chemical and electrochemical exfoliation approaches rely on harsh chemicals and can introduce relatively high level of defects in the 2D materials. In our recent research, we have demonstrated a simple and cost-effective bipolar electrochemical (BPE) approach which has the advantages of producing high quality 2D materials such as graphene and phosphorene. Moreover the BPE can simultaneously achieve exfoliation, reduction and deposition which enables the production of high quality microelectrodes based on 2D materials. In one exemplary application, the BPE based graphene electrodes show enhanced areal capacitance and outstanding frequency behavior as supercapacitor electrodes. In another exemplary application, the BPE enabled reduced graphene oxide microelectrodes demonstrated high sensitivity and low detection limit as aptasensors. Furthermore, the mechanistic studies including the temperature effect were conducted in order to further tune the functionality and enhance the exfoliation rate of the 2D materials.

Efficient Fabrication of Disordered Graphene with Improved Ion Accessibility, Ion Conductivity, and Density for High-Performance Compact Capacitive Energy Storage.

High-performance compact capacitive energy storage is vital for many modern application fields, including grid power buffers, electric vehicles, and portable electronics. However, achieving exceptional volumetric performance in supercapacitors is still challenging and requires effective fabrication of electrode films with high ion-accessible surface area and fast ion diffusion capability while simultaneously maintaining high density. Herein, a facile, efficient, and scalable method is developed for the fabrication of dense, porous, and disordered graphene through spark-induced disorderly opening of graphene stacks combined with mechanical compression. The obtained disordered graphene achieves a high density of 1.18gcm-3, sixfold enhanced ion conductivity compared to common laminar graphene, and an ultrahigh volumetric capacitance of 297Fcm-3 in ionic liquid electrolyte. The fabricated stack cells deliver a volumetric energy density of 94.2WhL-1 and a power density of 13.7kWL-1, representing a critical breakthrough in capacitive energy storage. Moreover, the proposed disordered graphene electrodes are assembled into ionogel-based all-solid-state pouch cells with high mechanical stability and multiple optional outputs, demonstrating great potential for flexible energy storage in practical applications.

(Invited) Laser-Induced Graphene for Improved Electrochemical Biosensors

Laser-induced graphene (LIG) has emerged as a promising technique for fabricating porous graphene electrodes for many applications, including biosensors and medical devices. However, improving electrochemical properties of LIG hinges on not only understanding the kinetics and energetics governing the physicochemical process of lasing, but also the tunability of surface chemistry and morphology of the produced LIG. This talk reveals the spatiotemporal laser control for achieving different LIG morphology and atomic structures with demonstrations of seamless transitions between conductive networked LIG structures and insulating fibrous LIG morphologies. The effect of laser fluence and multi-pass lasing on LIG morphology and properties will be shown. In addition, molecular engineering of the polymer substrate is explored for controlling the chemical and physical properties of the produced LIG. Finally, the electrochemical detection of neurotransmitters in the nanomolar range will be demonstrated. Our results show that heteroatom doping of graphene is possible by including sulfur or fluorine atoms into the backbone of the polyimide synthesized by two-step polycondensation. Accordingly, both the surface hydrophobicity and sensing sensitivity are shown to depend on the heteroatom-doping content. Taken together, our work paves the way for scalable fabrication of functional graphene-based electrodes and coatings on polymers for flexible and wearable electronics, as well as implantable devices, such as neural probes.

A wearable nanozyme–enzyme electrochemical biosensor for sweat lactate monitoring

In this study, we developed a wearable nanozyme–enzyme electrochemical biosensor that enablies sweat lactate monitoring. The biosensor comprises a flexible electrode system prepared on a polyimide (PI) film and the Janus textile for unidirectional sweat transport. We obtained favorable electrochemical activities for hydrogen peroxide reduction by modifying the laser-scribed graphene (LSG) electrode with cerium dioxide (CeO2)–molybdenum disulphide (MoS2) nanozyme and gold nanoparticles (AuNPs). By further immobilisation of lactate oxidase (LOx), the proposed biosensor achieves chronoamperometric lactate detection in artificial sweat within a range of 0.1–50.0 mM, a high sensitivity of 25.58 μA mM−1cm−2 and a limit of detection (LoD) down to 0.135 mM, which fully meets the requirements of clinical diagnostics. We demonstrated accurate lactate measurements in spiked artificial sweat, which is consistent with standard ELISA results. To monitor the sweat produced by volunteers while exercising, we conducted on-body tests, showcasing the wearable biosensor's ability to provide clinical sweat lactate diagnosis for medical treatment and sports management.

Facile assembly of flexible, stretchable and attachable symmetric microsupercapacitors with wide working voltage windows and favorable durability

With the increasing development of intelligent robots and wearable electronics, the demand for high-performance flexible energy storage devices is drastically increasing. In this study, flexible symmetric microsupercapacitors (MSCs) that could operate in a wide working voltage window were developed by combining laser-direct-writing graphene (LG) electrodes with a phosphoric acid-nonionic surfactant liquid crystal (PA-NI LC) gel electrolyte. To increase the flexibility and enhance the conformal ability of the MSC devices to anisotropic surfaces, after the interdigitated LG formed on the polyimide (PI) film surface, the devices were further transferred onto a flexible, stretchable and transparent polydimethylsiloxane (PDMS) substrate; this substrate displayed favorable flexibility and mechanical characteristics in the bending test. Furthermore, the electrochemical performances of the symmetric MSCs with various electrode widths (300, 400, 500 and 600 μm) were evaluated. The findings revealed that symmetric MSC devices could operate in a large voltage range (0–1.5 V); additionally, the device with a 300 μm electrode width (MSC-300) exhibited the largest areal capacitance of 2.3 mF cm−2 at 0.07 mA cm−2 and an areal (volumetric) energy density of 0.72 μWh cm−2 (0.36 mWh cm−3) at 55.07 μW cm−2 (27.54 mW cm−3), along with favorable mechanical and cycling stability. After charging for ~20 s, two MSC-300 devices connected in series could supply energy to a calculator to operate for ~130 s, showing its practical application potential as an energy storage device. Moreover, the device displayed favorable reversibility, stability and durability. After 12 months of aging in air at room temperature, its electrochemical performance was not altered, and after charging-discharging measurements for 5000 cycles at 0.07 mA cm−2, ~93.6% of the areal capacitance was still retained; these results demonstrated its practical long-term application potential as an energy storage device.

Frequency Response of the Diffuse-Charge Dynamics in Electrochemical Systems with a Graphene Electrode

We investigate the frequency response of diffuse-charge dynamics related to a 1:1 symmetric electrolyte containing a graphene electrode by solving the governing Poisson-Nernst-Planck equations subjected to appropriate boundary conditions in the asymptotic limit ε = λ D /H → 0, where λ D is the Debye screening length and H is the half-thickness of the electrolyte. Using the method of matched asymptotic expansion, we first solve the leading order non-linear problem for equilibrium state at a nonzero applied DC voltage in the presence of Stern layer(s). Then, we extend the leading order asymptotic analysis to derive an analytic expression for the impedance of the graphene-based electrochemical cell when a small AC voltage perturbation is added to the applied DC voltage. Finally, we use a suitable scaling of the impedance parameters to expose the impacts of the ion concentration and the DC bias voltage on the frequency response for possible applications involving the graphene-electrolyte interface.

High-performance humidity sensors based on SnO2/Ti3C2Tx nanocomposites coated on porous graphene electrodes

This study proposes the synthesis of SnO2/Ti3C2Tx composites on porous graphene electrodes (PGEs) for high-performance humidity sensors. In the heterojunction structure of SnO2/Ti3C2Tx composites, SnO2 nanoparticles prevented the layered structure of Ti3C2Tx to collapse and restack. Moreover, the high specific surface area of the SnO2/Ti3C2Tx composites provided more adsorption sites that improved their response. Porous graphene was fabricated by a fiber laser-induced process on the polyimide film as an electrode layer for flexible humidity sensors. The characteristics of the SnO2/Ti3C2Tx composites and PGEs were examined and investigated. SnO2/Ti3C2Tx composites were drop-cast on PGEs to fabricate the humidity sensor. Furthermore, the response of humidity sensors was tested in various RHs. The performance of the proposed humidity sensor demonstrated a wide detection range of 11–97 % RH, low hysteresis of 2.8 % RH, low response/recovery times of 14/49 s, high sensitivity of 862.19 kΩ/%RH, excellent long-time stability and repeatability, and good flexibility. In the future, the proposed sensor can be widely applied in human respiration monitoring, non-contact switch, and detection of volatile organic compounds.

Resistive Switching in Silver Iodide with Multi-Layer Graphene Electrodes

Resistive Switching in Silver Iodide with Multi-Layer Graphene Electrodes

Supercapacitors on hairs with quantum capacitance-dominant extraordinary capacitance

Graphene-based planar microsupercapacitors are promising energy storage devices for microelectromechanical systems. However, the capacitance and energy density of such graphene-based microsupercapacitors cannot achieve further promotion because of the lack of strategies that permit tailoring both chemical and geometric structures of graphene at the nanoscale. In this study, supercapacitors on hairs with quantum capacitance-dominant extraordinary capacitance are found. Simulations reveal that the record high capacitance and energy density are attributed to the enhanced quantum capacitance effect. The graphene electrodes are prepared by femtosecond laser direct writing-assisted reduction and N-doping of graphene oxides on the human hair surface. Due to the distinct quantum capacitance dominance, the microsupercapacitors exhibit extremely high capacitance (∼3000 mF/cm2) and energy density (∼417 μWh/cm2). We hope that this work will promote the development of microsupercapacitors with high capacitance and energy density.

Cross-linked and pillared graphene with nonconventional redox benzidine dimethyl ether for high-performance supercapacitors

To hinder Van der Waals-induced agglomeration between graphene sheets to increase specific surface area, and to supplement pseudocapacitance, graphene processing with active molecules is highly effective. Redox polymers and quinones, instead of aromatic ethers, were popular candidates. They were universally blended with graphene sheets through noncovalent modes, leading to issues of electrochemical stability and rate capability, etc. Herein, we broke through stereotype to develop aromatic ethers–benzidine with two remote methoxyls (i.e. 3,3′–dimethoxybenzidine, DMeOBD)–to covalently functionalize graphene, obtaining the cross-linked and pillared graphene electrode material (DMeOBD-rGO). It exhibited redox activity and featured a combination of electrochemical double layer supercapacitor and pseudocapacitor, thus yielded an enhanced specific capacitance of 363 F g−1. When assembled into two-electrode solid-state supercapacitors, the whole devices also delivered an impressive specific capacitance of 263 F g−1, and a high energy density of 13.2 Wh kg−1, along with excellent rate capability and cycling stability.

Smart wearable flexible sensor based on laser-induced graphene/gold nanoparticles/black phosphorus nanosheets for in situ quercetin detection

In situ detection of metabolic substances in plants is necessary for the development of smart agriculture. However, there is a mechanical mismatch between the traditional rigid sensing interfaces and the softness surface of plant leaves, which can reduce the reliability and accuracy of sensor. Stretchable and flexible sensors provide new solutions to this problem. Herein, using quercetin (Que) as a model, we developed a flexible electrochemical sensor for in situ monitoring of metabolite in plants. A light and thin laser-induced graphene (LIG) electrodes with good flexibility were obtained after polydimethylsiloxane (PDMS) transfer. Gold nanoparticles (AuNPs) and two-dimensional black phosphorus (BP) nanosheets were fabricated on the electrode to further improve the electrochemical performance. The sensor can detect Que concentrations in the range of 1–100 μM with a detection limit of 0.65 μM (S/N=3). The sensor can adapt well to the soft plant leaf surfaces, thus realizing the application of in situ and in vivo detection in the field. This innovative plant wearable biosensor is expected to be a next-generation electronic candidate for intelligent agriculture, paving the way for in-situ and in vivo detection, and facilitating the intelligentsias of modern agriculture.

Stick-and-sensing microneedle patch for personalized nutrition management

The focus of the next-generation healthcare system has moved forward from single disease detection to comprehensive health interpretation, namely, from disease diagnosis to prophylaxis via the early management of sub-health status. Among the technologies, microneedles have emerged as an efficient and powerful tool to realize painless and real-time sensing in the interstitial fluid (ISF), offering promising solutions for dynamic and direct reflection of nutritional profiles. Herein, we present a film-like, multifunctional microneedle patch, that is composed of three laser-induced graphene (LIG) based electrodes for minimally invasive and timely detection of glucose and vitamin C in the ISF. The microneedle patch is simply fabricated with a thickness of ∼0.25 mm polyimide (PI) film by a commercial laser platform, where laser-engraving is operated to form graphene electrodes, and laser-cutting accounts for the needle-tip shaping. Such fast, low-cost, and scalable manufactured microneedle patches showed excellent performance for electrochemical sensing and therefore allowed reliable on-the-tip monitoring of changes in glucose and vitamin C levels. Long-term studies in vivo and demonstrations in planta indicated the great promise of our microneedle patch to revolutionize dietary guidelines based on individualized status for effective nutrition management.

Seeing Through Muddy Water: Laser-Induced Graphene for Portable Tomography Imaging.

Due to its outstanding physical and chemical properties, graphene synthesized by laser scribing on polyimide (PI) offers excellent opportunities for photothermal applications, antiviral and antibacterial surfaces, and electrochemical storage and sensing. However, the utilization of such graphene for imaging is yet to be explored. Herein, using chemically durable and electrically conductive laser-induced graphene (LIG) for tomography imaging in aqueous suspensions is proposed. These graphene electrodes are designed as impedance imaging units for four-terminal electrical measurements. Using the real-time portable imaging prototypes, the conductive and dielectric objects can be seen in clear and muddy water with equivalent impedance modeling. This low-cost graphene tomography measurement system offers significant advantages over traditional visual cameras, in which the suspended muddy particles hinder the imaging resolution. This research shows the potential of applying graphene nanomaterials in emerging marine technologies, such as underwater robotics and automatic fisheries.

A smartphone-based electrochemical sensing platform for the portable and simultaneous determination of flavonoids in Citri Reticulatae Pericarpium

BackgroundThe efficient and timely determination of polymethoxylated flavones (PMFs, primarily nobiletin and tangeretin) and flavanone glycosides (primarily hesperidin) in Citri Reticulatae Pericarpium (CRP) is of paramount importance for the production of CRP and the evaluation of its efficacy. Conventional analytical methods including chromatography-based approaches commonly provide high sensitivity and selectivity, but require bulky equipment and complicated procedures performed by professional technicians and are thus inconvenient in practical applications. Therefore, there is a clear need for portable and miniaturized sensing platforms that can rapidly and simultaneously detect PMFs and hesperidin in CRP product. ResultsA state-of-the-art three-dimensional porous graphene electrode was first fabricated by direct laser scribing of a poly(ether-ether-ketone) (PEEK) film for electrocatalysis of nobiletin, tangeretin and hesperidin. Kinetic analysis was conducted to investigate the reaction mechanisms of these three flavonoids at such prepared PEEK-laser induced graphene (PEEK-LIG) electrodes. Since the as-prepared PEEK-LIG electrodes exhibited high electrocatalytic efficiency towards these three flavonoids, a portable electrochemical sensing platform assembled with a smartphone, a miniatured electrochemical workstation, and an integrated PEEK-LIG microchip was developed. Consequently, the developed portable electrochemical sensing platforms exhibited great sensitivity and low detection limits for both PMFs and hesperidin. More importantly, tests conducted on real CRP extract samples demonstrated that the developed portable electrochemical sensing platform exhibited high validity, high reliability, as well as excellent reproducibility. SignificanceThis is the inaugural report on the portable and simultaneous determination of PMFs and hesperidin in the pericarp of Citrus Reticulata, which may be utilized for differentiating CRP products. Furthermore, the portable and powerful electrochemical sensing platforms developed could also potentially be applied for a wide range of analytes, thanks to their simple and rapid fabrication and determination processes.

Heterogeneous polymer of aniline and benzoquinone enabled high energy density of graphene-based supercapacitors

Polyaniline and benzoquinone with powerful electrochemical activity and reversibility were widely used to boost specific capacitance of graphene electrode material and energy density of the corresponding supercapacitor devices. Inspired by these previous reports, herein, a novel heterogeneous polymer (polyAHQDME) containing both aniline units and benzoquinone ones were designed and synthesized, and finally grafted on reduced graphene oxide (rGO) framework. As expected, redox-active polyAHQDME modifier enabled rGO with an ultrahigh specific capacitance of 685.4 F g−1, five folds bare rGO counterpart. When assembled into symmetric two-electrode devices in quasi-solid-state aqueous and organic electrolytes, respectively, the key energy density parameter could reach as high as 21.6 and 100.6 Wh kg−1, outperforming most recently-reported modified carbon electrode materials.

Performance enhancement of hydrogen PEM fuel cell using graphene coated graphite electrodes

The experimental tests were carried out on a hydrogen proton exchange membrane fuel cell (PEMFC) to analyze its performance with graphene (25 % by weight) coated on the surface of the graphite electrodes. The results are compared with the neat graphite electrodes. A single cell with 25 cm2 active area, 63.63 % porosity for graphite, 40 % platinum mixed with carbon by weight, Nafion 117 membrane, graphite monopolar plate, and current collector with copper was developed. The crystalline structure of the graphene is confirmed using Raman Spectroscopy which indicates the graphene coated electrodes has higher electrical conductance. The improved connected network of graphene with catalyst and electrodes leads to better charge transfer. The cell voltage increased significantly from 0.85 with graphite electrodes to 1.0 V with graphene-coated electrodes due to mainly a decrease in ohmic losses. Peak power density with graphene-coated electrodes (79.2 mW/cm2) is significantly improved compared with base graphite (53 mW/cm2) due to higher intrinsic mobility and lesser cracks. The contact angle with the graphene-coated is 143.7° indicating the hydrophobic nature of the electrode. The ohmic losses were reduced by 20.93 %. The results indicate the efficiency and power density of a hydrogen PEM fuel cell could significantly be improved using partially coated graphene electrodes.

A disposable electrochemical magnetic immunosensor for the rapid and sensitive detection of 5-formylcytosine and 5-carboxylcytosine in DNA

5-formylcytosine (5 fC) and 5-carboxylcytosine (5caC) serve as key intermediates in DNA demethylation process with significant implications for gene regulation and disease progression. In this study, we introduce a novel electrochemical sensing platform specifically designed for the sensitive and selective detection of 5 fC and 5caC in DNA. Protein A-modified magnetic beads (ProtA-MBs) coupled with specific antibodies facilitate the immunorecognition and enrichment of these modified bases. Signal amplification is achieved through several chemical reactions involving the interaction between N3-kethonaxl and guanine, copper-free click chemistry for the attachment of dibenzocyclooctyne (DBCO)-Biotin, and the subsequent recognition by streptavidin-conjugated horseradish peroxidase (SA-HRP). The assay's readout is performed on a disposable laser-induced graphene (LIG) electrode, modified with the bead-antibody-DNA complex in a magnetic field, and analyzed using differential pulse voltammetry in a system employing hydroquinone (HQ) as the redox mediator and H2O2 as the substrate. This immunosensor displayed excellent sensitivity, with detection limits of 14.8 fM for 5 fC across a 0.1–1000 pM linear range and 87.4 fM for 5caC across a 0.5–5000 pM linear range, and maintained high selectivity even in the presence of interferences from other DNA modifications. Successful application in quantifying 5 fC and 5caC in genomic DNA from cell extracts, with recovery rates between 97.7% to 102.9%, underscores its potential for clinical diagnostics. N3-kethoxal was used for the first time in an electrochemical sensor. This work not only broadens the toolkit for detecting DNA modifications but also provides a fresh impetus for the development of point-of-care testing (POCT) technologies.

Theoretical study on the energy storage performance of Electrical double layer supercapacitors with choline amino acid ionic liquids electrolyte

Choline amino acid ionic liquids have attracted much attention in recent years due to their simple extraction process, rich sources, and easy degradation. Due to the high viscosity of such ionic liquids, the addition of organic solvents can adjust the transport performance of ionic liquids. Here, we explore the effects of different solvents (methanol, ethanol) and different temperatures on the diffusion properties and conductivity of ionic liquids through molecular dynamics simulations. The results showed that with the increase of temperature, the conductivity linearly increased. The addition of organic solvents can improve the conductivity of ionic liquids, and the addition of methanol solvent has a higher compatibility with choline amino acid ionic liquids. The conductivity after mixing methanol and choline glycine in a certain proportion is 5 times higher than before. In addition, this article investigated the performance of choline glycine ionic liquid electrolytes in Electrical Double Layers (EDLs) near planar graphene electrodes. The results indicate that similar double layer structures and double layer capacitors can be observed regardless of whether solvents are added.

Detection of Urea in Human Blood using Graphene-Based Amperometric Biosensor

Urea being an essential clinical analyte, its early and easy detection would be helpful in preventing various diseases. In this study, a graphene-based screen-printed enzymatic biosensor was fabricated for the blood urea detection by amperometric method. The electrode surface was modified with urease and glutamate dehydrogenase (Urs-GLDH) enzymes for sensitive and selective detection of urea with a wide detection range. The diffusion coefficient and the electron transfer rate constant of the screen-printed graphene electrode (SPGE) was found to be 8.53 × 10−7 cm2 s−1 and 7.58 × 10−5 cm s−1, respectively. The proposed graphene-based enzymatic urea biosensor can detect urea ranging from 2.5 to 30 mM with a sensitivity of 10.59 μA·mM−1·cm−2. The reliability of this enzymatic biosensor was confirmed by measuring urea in human blood and the results showed excellent agreement with the clinical testing method. Hence, the graphene-based biosensor described herein could provide rapid, sensitive, and selective detection of urea with low-cost and simple fabrication. In conclusion, this urea detection approach could be a promising method for developing feasible low-cost clinical diagnostic systems.

Batch-to-Batch Variation in Laser-Inscribed Graphene (LIG) Electrodes for Electrochemical Sensing.

Laser-inscribed graphene (LIG) is an emerging material for micro-electronic applications and is being used to develop supercapacitors, soft actuators, triboelectric generators, and sensors. The fabrication technique is simple, yet the batch-to-batch variation of LIG quality is not well documented in the literature. In this study, we conduct experiments to characterize batch-to-batch variation in the manufacturing of LIG electrodes for applications in electrochemical sensing. Numerous batches of 36 LIG electrodes were synthesized using a CO2 laser system on polyimide film. The LIG material was characterized using goniometry, stereomicroscopy, open circuit potentiometry, and cyclic voltammetry. Hydrophobicity and electrochemical screening (cyclic voltammetry) indicate that LIG electrode batch-to-batch variation is less than 5% when using a commercial reference and counter electrode. Metallization of LIG led to a significant increase in peak current and specific capacitance (area between anodic/cathodic curve). However, batch-to-batch variation increased to approximately 30%. Two different platinum electrodeposition techniques were studied, including galvanostatic and frequency-modulated electrodeposition. The study shows that formation of metallized LIG electrodes with high specific capacitance and peak current may come at the expense of high batch variability. This design tradeoff has not been discussed in the literature and is an important consideration if scaling sensor designs for mass use is desired. This study provides important insight into the variation of LIG material properties for scalable development of LIG sensors. Additional studies are needed to understand the underlying mechanism(s) of this variability so that strategies to improve the repeatability may be developed for improving quality control. The dataset from this study is available via an open access repository.