Understanding performance limitation and suppression of leakage current or self-discharge in electrochemical capacitors: a review

Effect of Ion Identity on Capacitance and Ion-to-Electron Transduction in Ion-Selective Electrodes with Nanographite and Carbon Nanotube Solid Contacts

The use of large surface area carbon materials as transducers in solid-contact ion-selective electrodes (ISEs) has become widespread. Desirable qualities of ISEs, such as a small long-term drift, have been associated with a high capacitance that arises from the formation of an electrical double layer at the interface of the large surface area carbon material and the ion-selective membrane. The capacitive properties of these ISEs have been observed using a variety of techniques, but the effects of the ions present in the ion-selective membrane on the measured value of the capacitance have not been studied in detail. Here, it is shown that changes in the size and concentration of the ions in the ion-selective membrane as well as the polarity of the polymeric matrix result in capacitances that can vary by up to several hundred percent. These data illustrate that the interpretation of comparatively small differences in capacitance for different types of solid contacts is not meaningful unless the composition of the ion-selective membrane is taken into account.

Mini review about metal organic framework (MOF)-based wearable sensors: Challenges and prospects

Among many types of wearable sensors, MOFs-based wearable sensors have recently been explored in both commercialization and research. There has been much effort in various aspects of the development of MOF-based wearable sensors including but not limited to miniaturization, size control, safety, improvements in conformal and flexible features, improvements in the analytical performance and long-term storage of these devices. Recent progress in the design and deployment of MOFs-based wearable sensors are covered in this paper, as are the remaining obstacles and prospects. This work also highlights the enormous potential for synergistic effects of MOFs used in combination with other nanomaterials for healthcare applications and raise attention toward the economic aspect and market diffusion of MOFs-based wearable sensors.

Transforming the Electrochemical Behaviors of Cobalt Oxide from "Supercapacitator" to "Battery" by Atomic-Level Structure Engineering for Inspiring the Advance of Co-Based Batteries

Cobalt-based electrodes receive emerging attention for their high theoretical capacity and rich valence variation ability, but state-of-the-art cobalt-based electrodes present performance far below the theoretical value. Herein, the in-depth reaction mechanisms in the alkaline electrolyte are challenged and proven to be prone to the surface-redox pseudocapacitor behavior due to the low adsorption energy to OH. Using the atomic-level structure engineering strategy after substitution metal searching, the adsorption energy is effectively enhanced, and the peak of CoOOH can be observed from in situ characterization for the first time, leading to the successful transition of charge storage behavior from "supercapacitor" to "battery". When used in a Zn-Co battery as a proof of concept, it shows comprehensive electrochemical performance with a flat discharge voltage plateau of ≈1.7 V, an optimal energy density of 506 Wh kg^-1 , and a capacity retention ratio of 85.1% after 2000 cycles, shining among the reported batteries. As a practical demonstration, this battery also shows excellent self-discharge performance with the capacity retention of 90% after a 10 h delay. This work subtly tunes the intrinsic electrochemical properties of the cobalt-based material through atomic-level structure engineering, opening a new opportunity for the advance of energy storage systems.

A novel bilayer heterogeneous poly(ionic liquid) electrolyte for high-performance flexible supercapacitors with ultraslow self-discharge

Flexible supercapacitors with high power density and long cyclic stability represent a promising candidate to be used as power supplies for portable electronics, but often suffer from the disadvantages of a limited working voltage and rapid self-discharge (spontaneous drop of open-circuit voltage). Here, we design a bilayer heterogeneous poly(ionic liquid) electrolyte (BHPE) consisting of a polycation complex and a polyanion complex with different zeta potentials to suppress the self-discharge of flexible symmetric supercapacitors. The resultant BHPE-based supercapacitors using active carbon/carbon nanotube composite electrodes exhibit a high working potential of 3.0 V and an energy density of 33 W h kg^-1, which are comparable with those of devices obtained by using a homogeneous poly(ionic liquid) electrolyte (HPE). More significantly, the developed BHPE-based supercapacitor charged under forward bias exhibits a self-discharge time of 23.2 h, which is at least twice that of the device charged under reverse bias and is also much superior to those of HPE-based supercapacitors. The BHPE-based supercapacitors also possess excellent mechanical flexibility and stability, due to the stabilized interface contact between two layers of poly(ionic liquid)s.

High-strength and machinable load-bearing integrated electrochemical capacitors based on polymeric solid electrolyte

Load bearing/energy storage integrated devices (LEIDs) allow using structural parts to store energy, and thus become a promising solution to boost the overall energy density of mobile energy storage systems, such as electric cars and drones. Herein, with a new high-strength solid electrolyte, we prepare a practical high-performance load-bearing/energy storage integrated electrochemical capacitors with excellent mechanical strength (flexural modulus: 18.1 GPa, flexural strength: 160.0 MPa) and high energy storage ability (specific capacitance: 32.4 mF cm^-2, energy density: 0.13 Wh m^-2, maximum power density: 1.3 W m^-2). We design and compare two basic types of multilayered structures for LEID, which significantly enhance the practical bearing ability and working flexibility of the device. Besides, we also demonstrate the excellent processability of the LEID, by forming them into curved shapes, and secondarily machining and assembling them into complex structures without affecting their energy storage ability.

In situ synthesis of advantageously united copper stannate nanoparticles for a new high powered supercapacitor electrode

A novel CuNi2O4@SnS@rGO/NF multicomponent hybrid material leads to fast ion/electron transfers at the electrode/electrolyte interface.

Effects of Varied Stimulation Parameters on Adipose-Derived Stem Cell Response to Low-Level Electrical Fields

Exogenous electrical fields have been explored in regenerative medicine to increase cellular expression of pro-regenerative growth factors. Adipose-derived stem cells (ASCs) are attractive for regenerative applications, specifically for neural repair. Little is known about the relationship between low-level electrical stimulation (ES) and ASC regenerative potentiation. In this work, patterns of ASC expression and secretion of growth factors (i.e., secretome) were explored across a range of ES parameters. ASCs were stimulated with low-level stimulation (20 mV/mm) at varied pulse frequencies, durations, and with alternating versus direct current. Frequency and duration had the most significant effects on growth factor expression. While a range of stimulation frequencies (1, 20, 1000 Hz) applied intermittently (1 h × 3 days) induced upregulation of general wound healing factors, neural-specific factors were only increased at 1 Hz. Moreover, the most optimal expression of neural growth factors was achieved when ASCs were exposed to 1 Hz pulses continuously for 24 h. In evaluation of secretome, apparent inconsistencies were observed across biological replications. Nonetheless, ASC secretome (from 1 Hz, 24 h ES) caused significant increase in neurite extension compared to non-stimulated control. Overall, ASCs are sensitive to ES parameters at low field strengths, notably pulse frequency and stimulation duration.

Chain-Elongated Ionic Liquid Electrolytes for Low Self-Discharge All-Solid-State Supercapacitors at High Temperature

High power and good stability enable supercapacitors to work efficiently at high temperatures. However, the high-temperature-induced excessive ion transfer of the electrolyte would lead to severe self-discharge behavior, which has often been overlooked but can be highly detrimental. In this study, solid electrolytes consisting of poly(ethylene oxide) (PEO), bentonite clay, and ionic liquids (IL)-PEO-clay@[EMIM][BF₄ ] (PCE), PEO-clay@[BMIM][BF₄ ] (PCB), and PEO-clay@[HMIM][BF₄ ] (PCH) lead to dramatic decreases in self-discharge when used in all-solid-state supercapacitors at high temperature of 70 °C, which correlate with chain elongation (i. e., [EMIM⁺ ]<[BMIM⁺ ]<[HMIM⁺ ]). Benefiting from both cation adsorption and high-temperature stabilization by bentonite clay, PCH-based supercapacitors (IL=[HMIM][BF₄ ]) deliver an extremely low self-discharge rate, with only a 30.7 % voltage drop over 10 h at 70 °C (44.5 % for 38 h), which is much lower than that of traditional liquid supercapacitors (63.7 % drop over 10 h at 70 °C). This improvement in high-temperature self-discharge behavior is found to be from the decrease in diffusion-controlled faradaic process. Based on the longer-chain [HMIM⁺ ], soft-packaged supercapacitors exhibit a low self-discharge rate and work consistently at 70 °C. This chain-elongation strategy provides a new possibility for the suppression of self-discharge behavior in supercapacitors and further aids long-term energy storage by supercapacitors at high temperatures.

Proton-assisted calcium-ion storage in aromatic organic molecular crystal with coplanar stacked structure

Rechargeable calcium-ion batteries are intriguing alternatives for use as post-lithium-ion batteries. However, the high charge density of divalent Ca²⁺ establishes a strong electrostatic interaction with the hosting lattice, which results in low-capacity Ca-ion storage. The ionic radius of Ca²⁺ further leads to sluggish ionic diffusion, hindering high-rate capability performances. Here, we report 5,7,12,14-pentacenetetrone (PT) as an organic crystal electrode active material for aqueous Ca-ion storage. The weak π-π stacked layers of the PT molecules render a flexible and robust structure suitable for Ca-ion storage. In addition, the channels within the PT crystal provide efficient pathways for fast ionic diffusion. The PT anode exhibits large specific capacity (150.5 mAh g^-1 at 5 A g^-1), high-rate capability (86.1 mAh g^-1 at 100 A g^-1) and favorable low-temperature performances. A mechanistic study identifies proton-assisted uptake/removal of Ca²⁺ in PT during cycling. First principle calculations suggest that the Ca ions tend to stay in the interstitial space of the PT channels and are stabilized by carbonyls from adjacent PT molecules. Finally, pairing with a high-voltage positive electrode, a full aqueous Ca-ion cell is assembled and tested.

Development of negative electrode active materials alternative to Ca metal is essential for the progress of Ca-ion battery technology. Here, the authors disclose the proton-assisted Ca-ion storage behavior of a pentacenetetrone organic crystal reporting high-power cell performances.

Highly conductive colloidal carbon based suspension for flow-assisted electrochemical systems

Carbon suspension electrodes are promising for flow-assisted electrochemical energy storage systems. They serve as flowable electrodes in electrolyte solutions of flow batteries, or flow capacitors. They can also be used for other applications such as capacitive deionization of water. However, developments of such suspensions remain challenging. The suspensions should combine low viscosity and high electronic conductivity for optimized performances. In this work, we report a flowable aqueous carbon dispersion which exhibits a viscosity of only 2 Pa.s at a shear rate of 5 s^-1 for a concentration of particles of 7 wt%. This suspension displays an electronic conductivity of 65 mS/cm, nearly two orders of magnitude greater than previously investigated related materials. The investigated suspensions are stabilized by sodium alginate and arabic gum in the presence of ammonium sulfate. Their use in flowable systems for the storage and discharge of electrical charges is demonstrated.

Effects of Fe Impurities on Self-Discharge Performance of Carbon-Based Supercapacitors

Activated carbon is widely used as an electrode material in supercapacitors due to its superior electrochemical stability, excellent electrical conductivity, and environmental friendliness. In this study, the self-discharge mechanisms of activated carbon electrodes loaded with different contents of Fe impurities (Fe and Fe₃O₄) were analyzed by multi-stage fitting to explore the tunability of self-discharge. It is was found that a small quantity of Fe impurities on carbon materials improves the self-discharge performance dominated by redox reaction, by adjusting the surface state and pore structure of carbon materials. As the content of Fe impurities increases, the voltage loss of activated carbon with the Fe impurity concentrations of 1.12 wt.% (AF-1.12) decreases by 37.9% of the original, which is attributable to the reduce of ohmic leakage and diffusion, and the increase in Faradic redox at the electrode/electrolyte interface. In summary, self-discharge performance of carbon-based supercapacitors can be adjusted via the surface state and pour structure, which provides insights for the future design of energy storage.

Outstanding Electrode-Dependent Seebeck Coefficients in Ionic Hydrogels for Thermally Chargeable Supercapacitor near Room Temperature

Thermoelectric power generation from waste heat is an important component of future sustainable development. Ion-conducting materials are promising candidates because of their high Seebeck coefficients. This study demonstrates that ionic hydrogels based on imidazolium chloride salts exhibit outstanding Seebeck coefficients of up to 10 mV K^-1. Along with their relatively high ionic conductivities (1.6 mS cm^-1) and extremely low thermal conductivities (∼0.2 W m^-1 K^-1), these hydrogels have good potential for use in heat recovery systems. The voltage behavior in response to temperature difference (stable or transient) differs significantly depending on the metal electrode material. We evaluated the electrode-dependent temperature sensitivity of the double layer capacitance of these hydrogels, which revealed that the thermally induced polarization of ions at the interface is one of the main contributors to the thermovoltage. Our results demonstrate the potential capability for ion and metal interactions to be used as an effective baseline for exploring ionic thermoelectric materials and devices. The developed thermoelectric supercapacitor exhibits reversible charging-discharging behavior under repeated disconnecting-connecting of an external load with a constant temperature difference, which offers a novel strategy for heat-to-electricity energy conversion from steady-temperature heat sources.

Manipulation of Nitrogen-Heteroatom Configuration for Enhanced Charge-Storage Performance and Reliability of Nanoporous Carbon Electrodes

In this study, various nitrogen-containing functional groups, namely, pyridine (N-6), pyrrole (N-5), oxidized N (N-O), and quaternary N (N-Q), are created on activated carbon (AC) surface via melamine, ammonia, and nitric oxide doping methods. N-5 and N-6 groups markedly alter the specific surface area and pore size of AC. N-O is found to affect electrolyte wettability, and the N-Q content is closely associated with AC electronic conductivity. The nitrogen-containing groups do not contribute to pseudocapacitance in propylene carbonate and acetonitrile electrolytes. However, the nitric-oxide-treated carbon (AC-NO) exhibits the best high-rate charge-discharge performance among the investigated materials. The N-Q-enriched and N-5/N-6-depleted AC-NO most effectively suppresses the leakage current and gas evolution of supercapacitors. Online gas chromatography is used to analyze the gaseous species produced from AC electrodes. With an appropriate surface functionality on carbon, the cell voltage can be increased to ∼3 V, increasing the energy and power densities. The aging behavior of the carbon electrodes with and without nitrogen modification after being floated at 2.5 V and 70 °C for 3 days is investigated. An effective strategy for enhancing supercapacitor performance and reliability is proposed.

Unraveling and Regulating Self-Discharge Behavior of Ti3C2Tx MXene-Based Supercapacitors

Rich chemistry and surface functionalization provide MXenes enhanced electrochemical activity yet severely exacerbate their self-discharge behavior in supercapacitors. However, this self-discharge behavior and its related mechanism are still remaining issues. Herein, we propose a chemically interface-tailored regulation strategy to successfully unravel and efficiently alleviate the self-discharge behavior of Ti₃C₂T_x MXene-based supercapacitors. As a result, Ti₃C₂T_x MXenes with fewer F elements (∼0.65 atom %) show a positive self-discharge rate decline of ∼20% in comparison with MXenes with higher F elements (∼8.09 atom %). Such decline of the F elements can highly increase tight-bonding ions corresponding to an individual self-discharge process, naturally resulting in a dramatic 50% increase of the transition potential (V_T). Therefore, the mixed self-discharge rate from both tight-bonding (contain fewer F elements) and loose-bonding ions (contain more F elements) is accordingly lowered. Through chemically interface-tailored engineering, the significantly changed average oxidation state and local coordination information on MXene affected the interaction of ion counterparts, which was evidently revealed by X-ray absorption fine structures. Theoretically, this greatly improved self-discharge performance was proven to be from higher adsorption energy between the interface of the electrode and the electrolyte by density functional theory. Therefore, this chemically interface-tailored regulation strategy can guide the design of high-performance MXene-based supercapacitors with low self-discharge behavior and will promote its wider commercial applications.

Controlling Electrode Spacing by Polystyrene Microsphere Spacers for Highly Stable and Flexible Transparent Supercapacitors

Transparent polymer electrolytes such as poly(vinyl alcohol)-based H⁺, Li⁺, K⁺, and Na⁺ gels have been widely used as both an electrolyte and a separator for flexible transparent supercapacitors (FTSCs). However, these gels sandwiched between the electrodes in FTSCs are easily compressed under bending and compression due to their viscous flow behavior, resulting in the deformation of electrode spacing and the unstable capacitance performance. To resolve this issue, herein, we introduce monodispersed polystyrene (PS) microspheres into PVA-LiCl polymer gel electrolytes as spacers to precisely control the electrode spacing during the assembly of FTSCs using single-walled carbon nanotubes/indium tin oxide-polyethylene terephthalate (ITO-PET) or MnO₂/multiwalled carbon nanotubes/ITO-PET as transparent electrodes. The electrode spacing could be tuned by varying the diameter of PS microspheres, for example, 20, 40, and 80 μm. More importantly, the PS microsphere spacers protect the gel electrolyte from the squeeze when bending takes place, allowing the stable performance output by FTSCs under a bending state. After repeating bending tests, the capacitance remains 95.6%, indicating the high stability and flexibility of the devices with the assistance of PS microsphere spacers.

Exploring the Capacitive Behavior of Carbon Functionalized with Cyclic Ethers: A Rational Strategy To Exploit Oxygen Functional Groups for Enhanced Capacitive Performance

The presence of oxygen functional groups (OFGs) on a carbon surface is a double-edged sword in electric double-layer capacitors (EDLCs) because of their mixed influences on capacitance. Critical problems of common OFGs are greatly decreased electrical conductivity, steric hindrance limiting the migration of ions, and promoted self-discharge via faradaic reactions. To explore a new breakthrough to these long-standing problems, carbon electrodes selectively functionalized with cyclic ether groups (CEGs) are investigated with in-depth spectroscopic and electrochemical analyses. The in-plane CEGs embedded in the graphene matrix are greatly advantageous over conventional out-of-plane OFGs for EDLC performance because they can boost capacitance via pseudocapacitance while substantially minimizing all of the negative effects of traditional OFGs. This study also reveals that preserving the original sp² carbon network during surface functionalization is crucial to maximizing the benefits of OFGs. These new insights call for the development of elaborate surface engineering strategies that can introduce functionalities with no significant damage to π-conjugation.

Structural Evolution of Phosphorus Species on Graphene with a Stabilized Electrochemical Interface

Phosphorus doping is an effective approach to tailor the surface chemistry of carbon materials. In this work, two-dimensional graphene, as a simplified model for all sp² hybrid carbon allotropes, is employed to explore the surface chemistry of P-doped carbon materials. Thermally reduced graphene oxide, with abundant residual oxygen functionalities, is doped by phosphorus heteroatoms through H₃PO₄ activation, followed by passivation in an inert atmosphere. The structural evolution of the phosphorus species in the carbon lattice during the thermal treatment is systematically studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy with the assistance of first-principles calculations. The C₃-P═O configuration is identified as the most stable structure in the graphene lattice and plays a key role in stabilizing the electrochemical interface between the electrode and electrolyte. These features enable an electrode based on P-doped graphene to exhibit an enlarged potential window of 1.5 V in an aqueous electrolyte, a remarkable improved cycling stability, and an ultralow leak current. Therefore, this contribution provides insights for designing phosphorus-doped carbon materials toward electrocatalysis, energy-related applications, and so forth.

Suppression of self-discharge in solid-state supercapacitors using a zwitterionic gel electrolyte

The zwitterionic gel electrolyte developed here can be applied to minimize self-discharge whilst maintaining the closed circuit electrochemical performance of solid-state supercapacitors.

Towards establishing standard performance metrics for batteries, supercapacitors and beyond

Electrochemical energy storage (EES) materials and devices should be evaluated against clear and rigorous metrics to realize the true promises as well as the limitations of these fast-moving technologies.

o-Benzenediol-Functionalized Carbon Nanosheets as Low Self-Discharge Aqueous Supercapacitors

Widening the voltage window is often proposed as a way to increase the energy density of aqueous supercapacitors. However, attempting to operate beyond the aqueous supercapacitor stability region can undermine the supercapacitor reliability due to pronounced electrolyte decomposition, which can lead to a significant self-discharge process. To minimize this challenge, charge injection by grafting o-benzenediol onto the carbon electrode is proposed through a simple electrochemical cycling technique. Due to charge injection from o-benzenediol into the carbon electrode, the equilibrium potential of the individual electrode can be reduced. In addition, due to its small molecular size, charge distribution, which is commonly faced by bulk pseudocapacitive materials, is also avoided. The assembled supercapacitor based on the o-benzenediol-grafted carbon demonstrated a maximum energy density of 24 Wh kg^-1 and a maximum power density of 69 kW kg^-1 , with a retention of 89 % after 10 000 cycles at 10 A g^-1 . A low self-discharge of about 4 h was recorded; this could be attributed to the low driving force arising from the lower equilibrium potential. Thus, the proposed technique may provide insight towards the tuning of the equilibrium potential to attain reliable, high-performing supercapacitors with a low self-discharge process.

Energy breakdown in capacitive deionization

We explored the energy loss mechanisms in capacitive deionization (CDI). We hypothesize that resistive and parasitic losses are two main sources of energy losses. We measured contribution from each loss mechanism in water desalination with constant current (CC) charge/discharge cycling. Resistive energy loss is expected to dominate in high current charging cases, as it increases approximately linearly with current for fixed charge transfer (resistive power loss scales as square of current and charging time scales as inverse of current). On the other hand, parasitic loss is dominant in low current cases, as the electrodes spend more time at higher voltages. We built a CDI cell with five electrode pairs and standard flow between architecture. We performed a series of experiments with various cycling currents and cut-off voltages (voltage at which current is reversed) and studied these energy losses. To this end, we measured series resistance of the cell (contact resistances, resistance of wires, and resistance of solution in spacers) during charging and discharging from voltage response of a small amplitude AC current signal added to the underlying cycling current. We performed a separate set of experiments to quantify parasitic (or leakage) current of the cell versus cell voltage. We then used these data to estimate parasitic losses under the assumption that leakage current is primarily voltage (and not current) dependent. Our results confirmed that resistive and parasitic losses respectively dominate in the limit of high and low currents. We also measured salt adsorption and report energy-normalized adsorbed salt (ENAS, energy loss per ion removed) and average salt adsorption rate (ASAR). We show a clear tradeoff between ASAR and ENAS and show that balancing these losses leads to optimal energy efficiency.