Supercapattery: merit merge of capacitive and Nernstian charge storage mechanisms

Supercapattery is the generic name for hybrids of supercapacitor and rechargeable battery. Batteries store charge via Faradaic processes, involving reversible transfer of localised or zone-delocalised valence electrons. The former is governed by the Nernst equation. The latter leads to pseudocapacitance (or Faradaic capacitance) which may be differentiated from electric double layer capacitance with spectroscopic assistance such as electron spin resonance. Since capacitive storage is the basis of supercapacitors, the combination of capacitive and Nernstian mechanisms has dominated supercapattery research since 2018, covering nanostructured and compounded metal oxides and sulfides, water-in-salt and redox active electrolytes and bipolar stacks of multi-cells. The technical achievements so far, such as specific energy of 270 Wh/kg in aqueous electrolyte, and charging-discharging for over 5000 cycles, benchmark a challenging but promising future of supercapattery

New Precursors Derived Activated Carbon and Graphene for Aqueous Supercapacitors with Unequal Electrode Capacitances

Carbon materials can offer various micro-and nano-structures, and bulk and surface functionalities, and hence remain most popular for manufacturing supercapacitors. This article reviews critically recent development in preparation of carbon materials from new precursors for supercapacitors. Typical examples are activated carbon (AC) and graphene which can be prepared from various conventional and new precursors, such as biomass, polymers, graphite oxide, CH4 and even CO2, via innovative processes to achieve low cost and/or high specific capacitance. Specifically, when producing AC from natural biomasses or synthetic polymers, either new, spent, or waste, popular activation agents, such as KOH and ZnCl2, are often used to process the ACs derived from these new precursors while the respective activation mechanisms always attract interest. The traditional two-step calcination process at high temperatures is widely employed to achieve high performance, with or without retaining the morphology of the precursors. The three-step calcination, including a post-vacuum treatment, is also the preferred choice in many cases, but it can increase the cost per capacity (kWh·g-1). More recently, one-step molecular activation promises a better and more economical approach to the commercial application of AC, although further increase of the yield is necessary. In addition to activation, graphitization, N doping, and template control can further improve ACs in terms of the charging and discharging rates, or pseudocapacitance, or both. Considerations are also given to material structure design, and carbon regeneration during activation. Metal-organic frameworks, which were initially used as templates, have been found to be good direct carbon precursors. Various graphene structures, including powders, films, aerogels, foams, and fibers, can be produced from graphite oxide, CO2, and CH4. Similar to AC, graphene can possess micropores by activation. Self-propagating high-temperature synthesis and molten salt processing are newly-reported methods for fabrication of mesoporous graphene. Macroporous graphene hydrogels can be produced by hydrothermal treatment of graphite oxide suspension, which can also be transferred into films. Hierarchically porous structures can be achieved by H2O2 etching or ZnCl2 activation of the macroporous graphene precursor. Sponges as templates combined with KOH activation are applied to create both micro-and macropores in graphene foams. Graphene can grow on fibers and textiles by electrodeposition, dip-coating, or filtration, which can be woven into clothes with a large area or thick loading, illuminating the potential application in flexible and wearable supercapacitors. The key obstacles in AC and graphene production are high cost, low yield, low packing density, and low working potential range. Most Carbon materials derived from new precursors work very well with aqueous electrolytes. Charge storage occurs not only in the electric double layer (i.e., the "carbon|electrolyte" interface), but also via redox activity in association with the bulk and surface functionalities, and the resulting partial delocalization of valence electrons. The analysis of the capacitive electrode has shown a design defect that prevents the working voltage of a symmetrical supercapacitor from reaching the full potential window of the carbon material. This defect can be avoided in AC-based supercapacitors with unequal electrode capacitances, leading to higher cell voltages and hence higher specific energy than their symmetrical counterparts. There are also emerging ways to raise the energy capacity of AC supercapacitors, such as the use of redox electrolytes to enable the Nernstian charge storage mechanism, and of the three dimensional printing method for a desirable electrode structure. All these developments are promising carbon materials from various precursors of new and waste sources for a more affordable and sustainable supercapacitor technology

Supercapatteries as High-Performance Electrochemical Energy Storage Devices

Abstract: The development of novel electrochemical energy storage (EES) technologies to enhance the performance of EES devices in terms of energy capacity, power capability and cycling life is urgently needed. To address this need, supercapatteries are being developed as innovative hybrid EES devices that can combine the merits of rechargeable batteries with the merits of supercapacitors into one device. Based on these developments, this review will present various aspects of supercapatteries ranging from charge storage mechanisms to material selection including electrode and electrolyte materials. In addition, strategies to pair different types of electrode materials will be discussed and proposed, including the bipolar stacking of multiple supercapattery cells internally connected in series to enhance the energy density of stacks by reducing the number of bipolar plates. Furthermore, challenges for this stack design will also be discussed together with recent progress on bipolar plates. Graphic Abstract: Supercapattery is an innovated hybrid electrochemical energy storage (EES) device that combines the merit of rechargeable battery and supercapacitor characteristics into one device. This article reviews supercapatteries from the charge storage mechanisms to the selection of materials including the materials of electrodes and electrolytes. Strategies for pairing different kinds of electrode materials and device engineering are discussed.[Figure not available: see fulltext.

Linear and non-linear pseudocapacitances with or without diffusion control

Pseudocapacitance is an important reversible charge storage mechanism in many electrode materials. Although the concept was first proposed in early 1960s, it has been more widely studied following the observation of rectangular cyclic voltammograms (CVs) when testing some transition metal oxides and electronically conducting polymers, and the association with supercapacitor. However, interpretation of pseudocapacitance is inconsistent in the literature. Although all agree that materials are pseudocapacitive if they undergo Faradaic reactions and exhibit rectangular CVs, some have regarded any surface confined Faradaic reactions which may present non-rectangular or even peak-shaped CVs to be pseudocapacitive. In the case of rectangular CVs, the amount of charge stored in the electrode is a linear function of the electrode potential, whilst for non-rectangular or peak-shaped CVs, the relationship is non-linear. It is shown in this article that only linear pseudocapacitance is of relevance to supercapacitor, but non-linear pseudocapacitance may find applications in rechargeable battery and supercapattery. Further, it is clarified that the equation i ​= ​k1v ​+ ​k2v1/2 is useful in analysis of electrode kinetics in terms of surface confinement and diffusion control. However, this kinetic equation is blind to the thermodynamically determined charge storage mechanisms as shown by experimental evidence, and should not be used to differentiate non-capacitive Faradaic processes from pseudocapacitance, either linear or non-linear

Half-Electrolysis of Water with the Aid of a Supercapacitor Electrode

Half-electrolysis runs one desirable half-cell reaction with the aid of a counter supercapacitor electrode which replaces the other unwanted half-cell reaction occurred inevitably in conventional electrolysis. Herein, it is developed to complete the whole cell reaction of water electrolysis, in alternative steps, with a capacitive activated carbon (AC) electrode and an electrolysis Pt electrode. When positively charging the AC electrode, a hydrogen evolution reaction occurs at the Pt electrode. By reversing the current, the charge stored in the AC electrode is discharged to assist the oxygen evolution reaction on the same Pt electrode. Consecutive completion of the two processes realizes the overall reaction of water electrolysis. This strategy leads to stepwise production of H2 and O2 without the need of a diaphragm in the cell and hence results in a lower energy consumption compared with the practical conventional electrolysis

Alternate Water Electrolysis

Hydrogen gas is a net zero carbon emission clean fuel with an unmatched high specific energy. Water electrolysis is an important alternative method to produce hydrogen to the traditional fossil hydrocarbon reforming in industry. The main challenges of water electrolysis are the high energy consumption (ca. 5 kWh m−3 (H2) at 80 ℃) and, if accidentally formed, the explosive nature of any unintended mixing of the produced hydrogen and oxygen gases. In order to solve these problems, alternate water electrolysis has been developed by, for example, decoupling of the hydrogen evolution reaction (HER) from the oxygen evolution reaction (OER) in space or time. This critical review intends to introduce the concept and recent developments of alternate water electrolysis in different schemes, including the alternate thermolysis and electrolysis of water, the alternate water electrolysis by using a liquid or solid redox intermedium and the alternate half-electrolysis of water. All the alternate water electrolysis methods solve the gas mixing problem whilst half-electrolysis and those with a solid redox medium omit the membranes. Specifically, only the alternate half-electrolysis of water can save the energy consumption without compromising the operation life and production rate

Nano-Scale Engineering of Heterojunction for Alkaline Water Electrolysis

Alkaline water electrolysis is promising for low-cost and scalable hydrogen production. Renewable energy-driven alkaline water electrolysis requires highly effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the most active electrocatalysts show orders of magnitude lower performance in alkaline electrolytes than that in acidic ones. To improve such catalysts, heterojunction engineering has been exploited as the most efficient strategy to overcome the activity limitations of the single component in the catalyst. In this review, the basic knowledge of alkaline water electrolysis and the catalytic mechanisms of heterojunctions are introduced. In the HER mechanisms, the ensemble effect emphasizes the multi-sites of different components to accelerate the various intermedium reactions, while the electronic effect refers to the d-band center theory associated with the adsorption and desorption energies of the intermediate products and catalyst. For the OER with multi-electron transfer, a scaling relation was established: the free energy difference between HOO* and HO* is 3.2 eV, which can be overcome by electrocatalysts with heterojunctions. The development of electrocatalysts with heterojunctions are summarized. Typically, Ni(OH)2/Pt, Ni/NiN3 and MoP/MoS2 are HER electrocatalysts, while Ir/Co(OH)2, NiFe(OH)x/FeS and Co9S8/Ni3S2 are OER ones. Last but not the least, the trend of future research is discussed, from an industry perspective, in terms of decreasing the number of noble metals, achieving more stable heterojunctions for longer service, adopting new craft technologies such as 3D printing and exploring revolutionary alternate alkaline water electrolysis

SWAP: Exploiting Second-Ranked Logits for Adversarial Attacks on Time Series

Time series classification (TSC) has emerged as a critical task in various domains, and deep neural models have shown superior performance in TSC tasks. However, these models are vulnerable to adversarial attacks, where subtle perturbations can significantly impact the prediction results. Existing adversarial methods often suffer from over-parameterization or random logit perturbation, hindering their effectiveness. Additionally, increasing the attack success rate (ASR) typically involves generating more noise, making the attack more easily detectable. To address these limitations, we propose SWAP, a novel attacking method for TSC models. SWAP focuses on enhancing the confidence of the second-ranked logits while minimizing the manipulation of other logits. This is achieved by minimizing the Kullback-Leibler divergence between the target logit distribution and the predictive logit distribution. Experimental results demonstrate that SWAP achieves state-of-the-art performance, with an ASR exceeding 50% and an 18% increase compared to existing methods.Comment: 10 pages, 8 figure

The effect of variable operating parameters for hydrocarbon fuel formation from CO2 by molten salts electrolysis

The emission of CO2 has been increasing day by day by growing world population, which resulted in the atmospheric and environmental destruction. Conventionally different strategies; including nuclear power and geothermal energy have been adopted to convert atmospheric CO2 to hydrocarbon fuels. However, these methods are very complicated due to large amount of radioactive waste from the reprocessing plant. The present study investigated the effect of various parameters like temperature (200–500 oC), applied voltage (1.5–3.0 V), and feed gas (CO2/H2O) composition of 1, 9.2, and 15.6 in hydrocarbon fuel formation in molten carbonate (Li2CO3-Na2CO3-K2CO3; 43.5:31.5:25 mol%) and hydroxide (LiOH-NaOH; 27:73 and KOH-NaOH; 50:50 mol%) salts. The GC results reported that CH4 was the predominant hydrocarbon product with a lower CO2/H2O ratio (9.2) at 275 oC under 3 V in molten hydroxide (LiOH-NaOH). The results also showed that by increasing electrolysis temperature from 425 to 500 oC, the number of carbon atoms in hydrocarbon species rose to 7 (C7H16) with a production rate of 1.5 μmol/h cm2 at CO2/H2O ratio of 9.2. Moreover, the electrolysis to produce hydrocarbons in molten carbonates was more feasible at 1.5 V than 2 V due to the prospective carbon formation. While in molten hydroxide, the CH4 production rate (0.80–20.40 µmol/h cm2) increased by increasing the applied voltage from 2.0–3.0 V despite the reduced current efficiencies (2.30 to 0.05%). The maximum current efficiency (99.5%) was achieved for H2 as a by-product in molten hydroxide (LiOH-NaOH; 27:73 mol%) at 275 oC, under 2 V and CO2/H2O ratio of 1. Resultantly, the practice of molten salts could be a promising and encouraging technology for further fundamental investigation for hydrocarbon fuel formation due to its fast-electrolytic conversion rate and no utilization of catalyst

Effects of pore widening vs oxygenation on capacitance of activated carbon in aqueous sodium sulfate electrolyte

The commercial activated carbon has a relatively low specific capacitance in the Na2SO4 electrolyte, which hinder the development of asymmetrical supercapacitors with high voltage. Re-activation and oxidative etching methods were applied to change the pore structure of activated carbon, respectively, to study the capacitive behavior of carbon in the Na2SO4 electrolyte. The pore distributions combining with capacitive properties deduce that 0.85 nm is the threshold diameter of the ion-accessible micropores for hydrated Na+ and SO42−. The specific capacitances of both the carbon materials by re-activation and oxidative etching methods are increased by 40 %, in comparison with the commercial activated carbon. The enhanced capacitive performances of the carbon materials were mainly attributed to the increased ion-accessible specific surface area and pseudocapacitance, respectively. The oxidative etching is a more facile and economical method for practice application. Combining with MnO2 as the positive electrode, the asymmetrical supercapacitor with a high voltage of 1.8 V exhibits a maximum specific cell capacitance of 50 F g–1 and specific energy of 22.5 Wh kg–1