Nanoporous Carbon Electrodes Research Articles

Electrosorption Mechanism of Zwitterionic Amino Acid Neurotransmitters in Iontronic Devices

AbstractNeurotransmitters carrying specific chemical information play a key role in the nervous system, which inspires ion‐based devices to mimic biological functions of nervous system, including sensing, transduction, and computing. In this work, the interaction mechanism of zwitterionic amino acids, glycine (Gly), and gamma‐aminobutyric acid (GABA) (two common inhibitory neurotransmitters) are analyzed with porous carbon electrodes. The charge balance of these zwitterionic neurotransmitters relies on zwitterion protonation in the vicinity of the negative electrodes (produced cations adsorbed by the negative electrode) and deprotonation near the positive electrode (produced anions to balance the positive charges) as demonstrated based on the theoretical dissociation analyses and local pH measurements. Additionally, electrosorption‐induced concentration changes of Gly and GABA in aqueous solutions and phosphate‐buffered saline (PBS) solutions are realized by electrosorption via nanoporous carbon electrodes. Based on this mechanistic understanding, the ionologic functions in printed ionic electrolyte double‐layer transistors are demonstrated. These neurotransmitter‐based iontronic devices hold potential for further organism‐machine interfacing and neuromorphic computing applications.

Improved Capacitive Performance of High‐Voltage Electric Double‐Layer Capacitors by Electrolyte Optimization Using Hierarchically Porous Carbon Electrodes

Advancement of the voltage window with stringent safety concerns is an imperative strategy to enhance the energy densities and output power delivery of electrical double‐layer capacitors (EDLCs). Solid‐state, water‐in‐salt (WIS), and ionic liquid‐based electrolytes are attractive and ideal choices for high‐voltage EDLCs due to their superior voltage window, electrochemical performance, and predominant safety aspects. Mindful of this, the current report demonstrates the electrochemical performance of new nanoporous carbon electrodes derived from waste dry leaves of Swietenia macrophylla with a high specific surface area (SSA) (1834 m2 g−1) in polymer–gel, WIS, and ionic liquids‐based electrolytes. Symmetric EDLC devices are fabricated using polymer gel solid‐state (PVA–Na2SO4), ionic liquids (EMIMBF4), and WIS (21 m LiTFSI) electrolytes with the electrochemical stability windows of (ESW) 2, 2.5, and 2 V, respectively. The assembled devices with Na2SO4/PVA, EMIMBF4, and 21 m LiTFSI (WIS) electrolytes‐based devices exhibit high energy densities of 40, 31, and 25 Wh kg−1 at 0.5 A g−1 and good output power delivery of 5, 6.23, and 3 kW kg−1 at 5 A g−1 with better cyclability features. Self‐discharging studies and systematic evaluation of high‐voltage EDLCs offer a sustainable method to develop high‐performance aqueous and diverse electrolyte‐based supercapacitors.

Structural disorder determines capacitance in nanoporous carbons.

The difficulty in characterizing the complex structures of nanoporous carbon electrodes has led to a lack of clear design principles with which to improve supercapacitors. Pore size has long been considered the main lever to improve capacitance. However, our evaluation of a large series of commercial nanoporous carbons finds a lack of correlation between pore size and capacitance. Instead, nuclear magnetic resonance spectroscopy measurements and simulations reveal a strong correlation between structural disorder in the electrodes and capacitance. More disordered carbons with smaller graphene-like domains show higher capacitances owing to the more efficient storage of ions in their nanopores. Our findings suggest ways to understand and exploit disorder to achieve highly energy-dense supercapacitors.

Bioactive Ion-Based Switchable Supercapacitors

Switchable supercapacitors enable a reversible electrically driven uptake and release of bioactive ions by polarizing nanoporous carbon electrodes. In this work we demonstrate the first example of a bioactive ion-based switchable supercapacitor. Based on choline chloride and nanoporous carbons with defined porosity we unravel the mechanism of physisorption vs. electrosorption by nuclear magnetic resonance, Raman, and impedance spectroscopy. Weak physisorption leads to pronounced electrically driven electrolyte depletion enabling the release and capture of electrolyte ions in highly controllable and reversible manner. A new 4-terminal device structure is proposed, with a main capacitor and a smaller inserted detective capacitor for monitoring bioactive ions electrosorption in situ. Controlling the ion concentration in a printed switchable supercapacitor based on bioactive ions realizes switching down to 8.3% residual capacity.The exploration of electrically driven adsorption mechanisms in printable microdevices will open an avenue of manipulating bioactive ions for the further application of drug delivery, neuromodulation, or neuromorphic devices. Figure 1

A nanoporous screen-printed carbon sensor for environmental and clinical monitoring of the antibiotic ciprofloxacin

A very sensitive, fast and simple electrochemical procedure using a nanoporous screen-printed carbon electrode (NPSPCE) was developed for the determination of the fluoroquinolone antibiotic ciprofloxacin (CFX). Changes in the surface morphology and electrochemical properties of the SPCE before and after pre-anodization were examined using scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). CV studies also demonstrated unequivocally adsorption controlled and irreversible oxidation of CFX. Using optimized conditions, the developed procedure is characterized by excellent selectivity and high sensitivity, allowing to achieve a very low limit detection (6.3 × 10−11 mol/L). The measurements taken allow us to consider the NPSPCE as a very effective tool for direct determination of CFX in samples with a complex matrix such as sewage and body fluids. The developed environment friendly sensor and procedure meet the requirements of green chemistry (reduction of toxic chemicals/reagents, and generation of minimal waste).

An artificial neural network model for capacitance prediction of porous carbon-based supercapacitor electrodes

Among energy storage devices, the last decades have witnessed the rapid spread of usage of carbon-based electrodes for electric double-layer capacitors (EDLCs) due to their large surface area, low cost, and high porosity. It is crucial to develop an accurate and efficient forecasting model for electrochemical performance to reduce the time needed for making suitable designs and choosing testing electrode materials. As a result, the use of machine learning (ML) approaches in creating a predicting model for the capacitance of carbon-based supercapacitors looks critical and provides the electrode characteristics' relative relevance. Data extracted from nearly a hundred published experimental research papers to select supercapacitors with certain electrode morphologies such as mesoporous, nanoporous, microporous, and hierarchical porous carbon electrode. The data was examined using machine learning techniques to predict the supercapacitor's specific capacitance (F/g). Electrode material structural qualities and various physicochemical test features such as electrolyte material, pore volume, and specific surface area. Electrochemical test features acquired via electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) test investigations for the same purpose include: cell configuration, current density, applied potential window, charge-transfer resistance (RCT), and equivalent series resistance (ESR) were used as input features to predict the corresponding capacitance performance. In the present study, Lasso, Support Vector Machine Regression (SVMR), and Artificial Neural Networks (ANN) with different structures were examined to predict the capacitance of the supercapacitor. The exhibition of the ML models measured concerning the root mean square error (RMSE), the correlation between expected yield and yield provided by the system. The developed ANN model with RMSE, MAE, and R values of 30.82, 46.5624 and 0.89537, respectively, provides outcomes for the prediction that are highly accurate compared to other models created for this purpose. According to the analysis of the input features done using the SHAP (SHapley Additive exPlanations) framework, the specific surface area had the biggest impact on the ANN model.

Numerical Modeling Unlocks Remarkable Ion Selectivity of Capacitive Deionization

Ion-selective water treatment is an important frontier in water research, as for many applications removing all ions indiscriminately leads to significant extra energy and downstream re-ionization costs. Capacitive deionization (CDI) is under intensive investigations for ion-selective treatment of polluted feedwaters [1]. CDI has the remarkable feature of being not only highly selective, but additionally dynamically tuneable to adjust, real-time, to varying feedwater composition and produced water targets [1]. However, we will show that to further develop CDI technologies to achieve desired separations, in feedwaters of several competing anions and cations, requires detailed numerical models. Such models couple ion transport theory to nanopore electrosorption, and often include pH dynamics. We here describe our recent work exploring the limits of ion-ion selectivity by capacitive deionization with inexpensive nanoporous carbon electrodes. We show how theory enabled us to achieve in the lab remarkable selectivity and a diverse set of separations, such as “perfect” divalent cation selectivity[2], monovalent ion selectivity[3], and removal of amphoteric pollutant species such as boric acid and nutrient species[4]. We show using strong-acid functionalized electrodes that the same two-electrode system can be used for either excellent divalent selectivity, or long-lasting monovalent ion selectivity, depending on cell operational parameters [2,3]. Our work furthers the argument that membraneless CDI, based on inexpensive and easily-scalable porous carbon electrodes, can address a wide variety of important applications in water treatment.

Oxygen-Enriched Hierarchical Nanoporous Carbon Electrodes for Supercapacitors

Enhancing the energy storage capacity of carbon electrodes is a key challenge to developing high-performance supercapacitors. In this study, we design an oxygen-enriched hierarchical nanoporous carbon electrode using laser fabrication and subsequent electrochemical activation, which demonstrates superior energy storage capacity. The carbon electrode is first obtained through the carbonization of phenolic resin under instantaneous high temperature induced by laser, resulting in multiple ion transport channels and a specific capacitance of 64.17 mF cm–2 at the current density of 0.2 mA cm–2. To further improve the performance, electrochemical activation is successfully conducted, leading to a 12.1-fold increase of the specific capacitance of the carbon electrodes. It is found that O–C═O-containing functional groups formed during the activation process significantly contribute to the high capacitance enhancement. In a 1 M H2SO4 electrolyte, the activated hierarchical nanoporous carbon electrode delivers a high specific capacitance of 778.3 mF cm–2 under the same test conditions. Additionally, the abundant porous structure and excellent reversibility of redox reaction of the oxygen-containing functional groups enable the assembled supercapacitor to obtain a specific capacitance of 45.3 mF cm–2 at the current density of 0.4 mA cm–2 and maintain a capacitance retention rate of 93.6% after 10,000 charge–discharge tests. This study presents a strategy to prepare carbon electrode materials with both high performance and excellent stability.

Impact of Iodine Electrodeposition on Nanoporous Carbon Electrode Determined by EQCM, XPS and In Situ Raman Spectroscopy.

The charging of nanoporous carbon via electrodeposition of solid iodine from iodide-based electrolyte is an efficient and ecofriendly method to produce battery cathodes. Here, the interactions at the carbon/iodine interface from first contact with the aqueous electrolyte to the electrochemical polarization conditions in a hybrid cell are investigated by a combination of in situ and ex situ methods. EQCM investigations confirm the flushing out of water from the pores during iodine formation at the positive electrode. XPS of the carbon surface shows irreversible oxidation at the initial electrolyte immersion and to a larger extent during the first few charge/discharge cycles. This leads to the creation of functional groups at the surface while further reactive sites are consumed by iodine, causing a kind of passivation during a stable cycling regime. Two sources of carbon electrode structural modifications during iodine formation in the nanopores have been revealed by in situ Raman spectroscopy, (i) charge transfer and (ii) mechanical strain, both causing reversible changes and thus preventing performance deterioration during the long-term cycling of energy storage devices that use iodine-charged carbon electrodes.

Coal-Based Nanoporous Carbon Electrode for High-Performance Supercapacitor: Surface and Diffusion Charge Kinetics in Aqueous and Non-Aqueous Electrolytes

South Africa generates around 60 million tonnes of coal discards each year, with over 1.2 billion tonnes landfilled in discard dumps and slurry ponds. The huge quantity of coal discards generated within the past decades, which is a significant liability to the mining industry and the environment, presents an opportunity to reuse this resource. This study investigated the potential of nanoporous carbon material from coal discard via KOH activation for use as electrode material in supercapacitors. Hierarchical morphology is seen via scanning electron microscopy (SEM). Raman spectroscopy reveals structural defects, X-ray Diffraction spectroscopy (XRD) confirms the material's amorphous nature, and N2 at 77k adsorption reveals a carbon material with high porosity and a surface area of 1424 m2/g.Electrochemical analysis reveals that the charge storage mechanism of the material is surface-controlled (non-faradaic) in the non-aqueous (TEABF4/ACN) electrolyte and diffusion-controlled (faradaic) mechanism in the aqueous (Na2SO4) electrolyte. The material has a higher specific capacitance (162 F/g) and cycling stability of 87.1% capacitance retention after 5000 cycles in Na2SO4. The material exhibits a better rate capability in TEABF4/ACN with 92.3% capacitance retention. A symmetrical two-electrode device using Swagelok T-cell assembly achieves energy densities of 15.5 and 10.4 Wh/kg in Na2SO4 and TEABF4/ACN, respectively, and a power density of 100 W/kg in both electrolytes. The variation in the capacitive performance of the material in both electrolytes is due to differences in electrolyte ion size, ionic conductivity, and pore structure. This study not only establishes a link between the structure of coal-based nanoporous carbon and its electrochemical performance in both electrolytes but also demonstrates the potential of coal-based functional materials for sustainable energy storage.

Numerical Modeling Unlocks Remarkable Ion Selectivity of Capacitive Deionization

Ion-selective water treatment is an important frontier in water research, as for many applications removing all ions indiscriminately leads to significant extra energy and downstream re-ionization costs. Capacitive deionization (CDI) is under intensive investigations for ion-selective treatment of polluted feedwaters [1]. CDI has the remarkable feature of being not only highly selective, but additionally dynamically tuneable to adjust, real-time, to varying feedwater composition and produced water targets [1]. However, we will show that to further develop CDI technologies to achieve desired separations, in feedwaters of several competing anions and cations, requires detailed numerical models. Such models couple ion transport theory to nanopore electrosorption, and often include pH dynamics. We here describe our recent work exploring the limits of ion-ion selectivity by capacitive deionization with inexpensive nanoporous carbon electrodes. We show how theory enabled us to achieve in the lab remarkable selectivity and a diverse set of separations, such as “perfect” divalent cation selectivity[2], monovalent ion selectivity[3], and removal of amphoteric pollutant species such as boric acid and nutrient species[4]. We show using strong-acid functionalized electrodes that the same two-electrode system can be used for either excellent divalent selectivity, or long-lasting monovalent ion selectivity, depending on cell operational parameters [2,3]. Our work furthers the argument that membraneless CDI, based on inexpensive and easily-scalable porous carbon electrodes, can address a wide variety of important applications in water treatment.

Nanoporous Carbon Electrodes Derived from Coffee Side Streams for Supercapacitors in Aqueous Electrolytes.

Coffee, as one of the most traded resources, generates a vast amount of biogenic by-products. Coffee silver skins (CSS), a side stream from the roasting process, account for about 4 wt.%. Despite the abundancy of CSS, possible routes to generate added value for broad applications are limited. Herein, we present an approach to use CSS as a precursor material for supercapacitor electrodes. KOH activated carbon (AC) was produced from CSS. The resulting AC—CSS was characterized by X-ray diffraction, gas sorption analysis, scanning electron microscopy, and Raman spectroscopy. The highly porous AC—CSS exposes a specific surface area of more than 2500 m2 g−1. Electrodes formed with AC—CSS were electrochemically characterized by performing cyclic voltammetry and galvanostatic cycling. The electrodes were further assembled into a supercapacitor device and operated using 1 M sulfuric acid as electrolyte. In addition, various quinones were added to the electrolyte and their impact on the capacitance of AC—CSS electrodes was analyzed. In this work, we were able to show that CSS are a valuable source for supercapacitor applications and that coffee-waste-derived quinones can act as capacitance enhancers. Thus, the findings of this research show a valuable path towards sustainable and green energy storage solutions.

Effect of partial oxidation and repolarization of TiC-derived nanoporous carbon electrodes on supercapacitor performance using a pH-neutral aqueous electrolyte

Effect of partial oxidation and repolarization of TiC-derived nanoporous carbon electrodes on supercapacitor performance using a pH-neutral aqueous electrolyte

Effects of Local Graphitization on the Charging Mechanisms of Capacitive Microporous Carbon Supercapacitor Electrodes Application

Electrochemical double layer capacitors (EDLCs) store the charge by reversible adsorption of ions at the electrode/electrolyte interface. The optimization of EDLC performance needs to push further our basic understanding of the ion fluxes that occur during the charging/discharging process. Microporous carbon has been widely used as supercapacitor electrode material thanks to their tunable porosity and high specific surface area. However, most of the porous carbon materials have complex carbon structures, so that the mechanism is still poorly understood.Using electrochemical quartz crystal microbalance technique (EQCM) technique, we studied the charge storage mechanisms of two nanoporous carbide-derived carbon (TiC-CDC) electrodes, namely CDC-800 synthesized at 800℃ and CDC-1100 synthesized at 1100℃. These two carbons have a similar porosity but the CDC-1100 sample shows more graphitic carbon microstructure. We found that a counter-ion adsorption process mainly controls the charging mechanism of CDC-800 (“V” shape m vs Q plot), while an expanded ion-exchange and counter-ion adsorption process is obtained for CDC-1100 (“U” shape). We show here that the local carbon structure affects the charging mechanism of electrical double layer capacitance, and a strong interaction between the anions and graphitic carbons explains these different behaviors. Our results provide more insights into the fundamental understanding of the carbon/electrolyte interface and the charge of the electrical double layer capacitance.Figure 1. Electrode mass change versus charge during the polarization of CDC-800 (a) and CDC-1100 (b) coated Au electrodes in 2M EMI-TFSI/ACN electrolyte at the scan rate of 10 mV·s−1. Figure 1

Nanoconfinement effect of nanoporous carbon electrodes for ionic liquid-based aluminum metal anode

Rechargeable aluminum batteries (RABs), which use earth-abundant and high-volumetric-capacity metal anodes (8040 mAh cm−3), have great potential as next-generation power sources because they use cheaper resources to deliver higher energies, compared to current lithium ion batteries. However, the mechanism of charge delivery in the newly developed, ionic liquid-based electrolytic system for RABs differs from that in conventional organic electrolytes. Thus, targeted research efforts are required to address the large overpotentials and cycling decay encountered in the ionic liquid-based electrolytic system. In this study, a nanoporous carbon (NPC) electrode with well-developed nanopores is used to develop a high-performance aluminum anode. The negatively charged nanopores can provide quenched dynamics of electrolyte molecules in the aluminum deposition process, resulting in an increased collision rate. The fast chemical equilibrium of anionic species induced by the facilitated anionic collisions leads to more favorable reduction reactions that form aluminum metals. The nanoconfinement effect causes separated nucleation and growth of aluminum nanoparticles in the multiple confined nanopores, leading to higher coulombic efficiencies and more stable cycling performance compared with macroporous carbon black and 2D stainless steel electrodes.

Microscopic Simulations of Electrochemical Double-Layer Capacitors.

Electrochemical double-layer capacitors (EDLCs) are devices allowing the storage or production of electricity. They function through the adsorption of ions from an electrolyte on high-surface-area electrodes and are characterized by short charging/discharging times and long cycle-life compared to batteries. Microscopic simulations are now widely used to characterize the structural, dynamical, and adsorption properties of these devices, complementing electrochemical experiments and in situ spectroscopic analyses. In this review, we discuss the main families of simulation methods that have been developed and their application to the main family of EDLCs, which include nanoporous carbon electrodes. We focus on the adsorption of organic ions for electricity storage applications as well as aqueous systems in the context of blue energy harvesting and desalination. We finally provide perspectives for further improvement of the predictive power of simulations, in particular for future devices with complex electrode compositions.

Origin of Enhanced Performance in Nanoporous Electrical Double Layer Capacitors: Insights on Micropore Structure and Electrolyte Composition from Molecular Simulations.

We explore the effect of solvation and micropore structure on the energy storage performance of electrical double layer capacitors using constant potential molecular dynamics simulations of realistically modeled nanoporous carbon electrodes and ionic liquid/organic solvent mixtures. We show that the time-dependent charging profiles of electrodes with larger pores reach the plateau regime faster, while the charging time has a nonmonotonic dependence on ion concentration, mirroring the composition dependence of bulk electrolyte conductivity. When the average pore size of the electrode is similar to or slightly larger than the size of a solvated ion, the solvation enhances ion electrosorption into nanopores by disrupting anion-cation coordination and decreasing the barrier to counterion penetration while blocking the co-ions. In these systems, areal capacitance exhibits a significant nonmonotonic dependence on ion concentration, in which capacitance increases with the introduction of solvent in the concentrated regime followed by a decrease with further dilution. This gives rise to a maximum in capacitance at intermediate dilution levels. When pores are significantly larger than solvated ions, capacitance maximum weakens and eventually disappears. These findings provide novel insights on the combined effect of electrolyte composition and electrode pore size on the charging kinetics and equilibrium behavior of realistically modeled electrical double layer capacitors. Generalization of the approach developed here can facilitate the rational optimization of material properties for electrical double layer capacitor applications.

Monolithic In‐Plane Integration of Gate‐Modulated Switchable Supercapacitors

Monolithic integration of iontronic devices is a key challenge for future miniaturization and system integration. The G‐Cap, a novel iontronic element, is a switchable supercapacitor with gating characteristics comparable to transistors in electronic circuits, but switching relies on ionic currents and ion electroadsorption. The first monolithic in‐plane G‐Cap integration through 3D‐inkjet printing of nanoporous carbon precursors is reported. The printed G‐Cap has a three‐electrode architecture integrating a symmetric “working” supercapacitor (W‐Cap) and a third “gate” electrode (G‐electrode) that reversibly depletes/injects electrolyte ions into the system, effectively controlling the “working” capacitance. The symmetric W‐Cap operates with a proton‐conducting hydrogel electrolyte PVA/H2SO4 and shows a high capacitance (1.6 mF cm−2) that can be switched “on” and “off” by applying a DC bias potential (−1.0 V) at the G‐electrode. This effectively suppresses AC electroadsorption in the nanoporous carbon electrodes of the W‐Cap, resulting in a high capacitance drop from an “on” to an “off” state. The new monolithic structures achieve high rate performance, reversible on‐off switching with an off‐value reaching 0.5 %, which even surpasses recently reported values. Establishing technologies and device architectures for functional ionic electroadsorption devices is crucial for diverse fields ranging from microelectronics and iontronics to biointerfacing and neuromodulation.

Capacitance of Carbon Nanotube/Graphene Composite Electrodes with [BMIM+][BF4–]/Acetonitrile: Fixed Voltage Molecular Dynamics Simulations

The capacitance of nanoporous carbon electrode materials is dictated by the physical interactions with electrolyte ions and molecules at the accessible interior electrode surface. While significant progress has been made in designing and synthesizing carbon materials with well-defined and relatively homogeneous nanoporosity, the majority of materials remain heterogeneous, and the resulting properties reflect an average over this distribution. In this regard, computer simulations can be a valuable tool to predict or validate structure/property relationships by systematically investigating well-defined, model electrode morphologies. We utilize fixed-voltage molecular dynamics simulations to predict structure/capacitance relationships for five different model morphologies of carbon nanotube/graphene (CNT/G) composite electrodes with 1-butyl-3-methylimidazolium tetrafluoroborate/acetonitrile electrolyte. The CNT/G electrode models are inspired by experimental “layer-by-layer” syntheses and strike a balance between realism and computational tractability. Comparison between different model electrode architectures elucidates important structure/property relationships. We find that CNT/graphene contact points serve as “hot spots” with significantly enhanced charge separation relative to the rest of the electrode. Furthermore, we demonstrate a specific nanoconfinement motif that provides substantial 3–4× enhancement of local capacitance, resulting in a ∼40% increase of the total electrode differential capacitance. Because the accessible surface area of the model CNT/G electrodes is precisely determined, a comparison of per-area capacitance across systems is unambiguous. Our results thus complement a prior computational demonstration of capacitance enhancement in nanoconfinement while elucidating additional interaction motifs at CNT/G electrochemical interfaces.

Simultaneous electrochemical investigation and detection of two glucocorticoids; interactions with human growth hormone, somatropin

In the present work, the simultaneous determination of dexamethasone (DXM) and hydrocortisone (HYD) is described by the use of a nano-porous glassy carbon electrode (GCE), prepared via an electrochemical treatment. Nano-porous/GCE provided the porosity which was increased the sensitivity of the surface of electrode to monitor the drugs. Differential pulse voltammetry (DPV) was used for the determination of DXM and HYD simultaneously at the surface of the electrode. Under the optimum pH 2 (0.1 M, phosphate buffer solution), it is found that the calibration plots for DXM and HYD are linear in the concentration ranges 0.02–22 and 0.1–42 µM with detection limits of 5 and 30 nM for DXM and HYD, respectively. Also, the interaction of human growth hormone (somatropin) with DXM and HYD has been studied by electrochemical and UV–Vis spectroscopy techniques. The binding of DXM and HYD to human growth hormone was also modeled by molecular docking methods. The glucocorticoids could inhibit the application of the human growth hormone.