Achieving both high redox activity and rapid ion transport is a critical and pervasive challenge in electrochemical energy storage applications. This challenge is significantly magnified when using large-sized charge carriers, such as the sustainable ammonium ion (NH₄⁺). A self-assembled MXene/n-type conjugated polyelectrolyte (CPE) superlattice-like heterostructure that enables redox-active, fast, and reversible ammonium storage is reported. The superlattice-like structure persists as the CPE:MXene ratio increases, accompanied by a linear increase in the interlayer spacing of MXene flakes and a greater overlap of CPEs. Concurrently, the redox activity per unit of CPE unexpectedly intensifies, a phenomenon that can be explained by the enhanced de-solvation of ammonium due to the increased volume of 3 Å-sized pores, as indicated by molecular dynamic simulations. At the maximum CPE mass loading (MXene:CPE ratio = 2:1), the heterostructure demonstrates the strongest polymeric redox activity with a high ammonium storage capacity of 126.1 C g⁻¹ and a superior rate capability at 10 A g⁻¹. This work unveils an effective strategy for designing tunable superlattice-like heterostructures to enhance redox activity and achieve rapid charge transfer for ions beyond lithium.

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Titanium carbide MXene, Ti₃C₂T_x, exhibits ultrahigh capacitance in acidic electrolytes at negative potentials yet poor stability at positive potentials, resulting in low-energy densities for Ti₃C₂T_x-based symmetric supercapacitors. Utilizing “water-in-salt” electrolytes has successfully expanded the stable operation potential window of MXenes. However, this advancement comes at the cost of sacrificing their high capacitance in acidic electrolytes. In this work, we report an acidic “water-in-salt” (AWIS) electrolyte composed of sulfuric acid and saturated lithium halide, which effectively doubled the energy density of the Ti₃C₂T_x-based symmetric supercapacitor compared to those with bare acidic electrolytes. Specifically, the AWIS electrolyte successfully expanded the voltage window of the symmetric device to 1.1 V. A high specific capacitance of 112.34 F g^-1 (at 10 mV s^-1) was obtained due to the presence of proton redox. As a result, the symmetric device achieved a high-energy density of 19.1 Wh kg^-1 and a high capacitance retention of 96.3% after 10,000 cycles. This work demonstrates the importance of designing stable and redox-active electrolytes for high-energy MXene-based symmetric supercapacitors.

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Environmentally-benign materials play a pivotal role in advancing the scalability of energy storage devices. In particular, conjugated polymers constitute a potentially greener alternative to inorganic- and carbon-based materials. One challenge to wider implementation is the scarcity of n-doped conducting polymers to achieve full cells with high-rate performance. Herein, this work demonstrates the use of a self-doped n-doped conjugated polymer, namely poly(benzodifurandione) (PBDF), for fabricating aqueous supercapacitors. PBDF demonstrates a specific capacitance of 202 ± 3 F g−1, retaining 81% of the initial performance over 5000 cycles at 10 A g−1 in 2 m NaCl(aq). PBDF demonstrates rate performances of up to 100 and 50 A g−1 at 1 and 2 mg cm−2, respectively. Electrochemical impedance analysis reveals a surface-mediated charge storage mechanism. Improvements can be achieved by adding reduced graphene oxide (rGO), thereby obtaining a specific capacitance of 288 ± 8 F g−1 and high-rate operation (270 A g−1). The performance of PBDF is examined in symmetric and asymmetric membrane-less cells, demonstrating high-rate performance, while retaining 83% of the initial capacitance after 100 000 cycles at 10 A g−1. PBDF thus offers new prospects for energy storage applications, showcasing both desirable performance and stability without the need for additives or binders and relying on environmentally friendly solutions.@en

The growing demand for safe, cost-efficient, high-energy and high-power electrochemical energy storage devices has stimulated the development of aqueous-based supercapacitors with high capacitance, high rate capability, and high voltage. 2D titanium carbide MXene-based electrodes have shown excellent rate capability in various dilute aqueous electrolytes, yet their potential window is usually narrower than 1.2 V. In this study, we show that the potential window of Ti₃C₂T _x MXene can be efficiently widened to 1.5 V in a cost-effective and environmentally benign polyethylene glycol (PEG) containing molecular crowding electrolyte. Additionally, a pair of redox peaks at −0.25 V/−0.05 V vs. Ag (cathodic/anodic) emerged in cyclic voltammetry after the addition of PEG, yielding an additional 25% capacitance. Interestingly, we observed the co-insertion of the molecular crowding agent PEG-400 during the Li⁺ intercalation process based on in-situ x-ray diffraction analysis. As a result, Ti₃C₂T _x electrodes presented an interlayer space change of 4.7 Å during a complete charge/discharge cycle, which is the largest reversible interlayer space change reported so far for MXene-based electrodes. This work demonstrates the potential of adding molecular crowding agents to improve the performance of MXene electrodes in aqueous electrolytes and to enlarge the change of the interlayer spacing.

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In alignment with widespread interest in carbon neutralization and sustainable practices, we disclose that conjugated polyelectrolyte (CPE) hydrogels are a type of recyclable, electrochemically stable, and environmentally friendly pseudocapacitive material for energy storage applications. By leveraging ionic-electronic coupling in a relatively fluid medium, one finds that hydrogels prepared using a fresh batch of an anionic CPE, namely, Pris-CPE-K, exhibit a specific capacitance of 32.6 ± 6.6 F g^-1 in 2 M NaCl and are capable of 80% (26.1 ± 6.5 F g^-1) capacitance retention after 100,000 galvanostatic charge-discharge (GCD) cycles at a current density (J) of 10 A g^-1. We note that equilibration under a constant potential prior to GCD analysis leads to the K⁺ counterions in the CPE exchanging with Na⁺ and, thus, the relevant active material Pris-CPE-Na. It is possible to remove the CPE material from the electrochemical cell via extraction with water and to carry out a simple purification through dialysis to produce a recycled material, namely Re-CPE-Na. The recycling workup has no significant detrimental impact on the electrochemical performance. Specifically, Re-CPE-Na hydrogels display an initial specific capacitance of 26.3 ± 1.2 F g^-1 (at 10 A g^-1) and retain 77% of the capacitance after a subsequent 100,000 GCD cycles. Characterization by NMR, FTIR, and Raman spectroscopies, together with XPS and GPC measurements, revealed no change in the structure of the backbone or side chains. However, rheological measurements gave evidence of a slight loss in G′ and G′′. Overall, that CPE hydrogels display recyclability argues in favor of considering them as a novel materials platform for energy storage applications within an economically viable circular recycling strategy.

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Activated carbon has long been recognized as a promising electrode material for energy storage devices. The extraordinarily high specific area makes it challenging to replace in supercapacitors since electrical double-layer capacitors need such surfaces but also porous networks to enable electrolyte penetration. As a raw material for synthesizing activated carbon, sawdust offers key benefits, such as its renewability, abundance, favorable physical attributes for energy storage, and a more environmentally friendly synthesis process compared to mined alternative sources. In this work, electrochemical characterization is carried out which highlights the critical role of pelletization in enhancing the capacitive performance of sawdust-derived activated carbon, in addition to the implicit handling and logistical benefits. Subsequently, a Li-ion capacitor is assembled with an organic solvent-based electrolyte, sawdust-derived activated carbon serving as the positive electrode, and an Al-based foil negative electrode, potentially combining high energy and power density materials into a hybrid device. Despite commendable electrochemical performance and the use of a sustainable waste-derived positive electrode with a commoditized negative electrode, challenges remain regarding the ability to mitigate the role of surface functional groups that are stabilized by bio-carbon thermal treatments. Nevertheless, this distinctive architecture holds promise as an alternative high-power energy storage technology for a future filled with renewable energy, electric vehicles, and portable electronic devices.

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The 2D transition metal carbide and nitride MXenes, especially titanium carbide, are ideal electrode materials for pseudocapacitors due to their metallic conductivity, open interlayer space, and redox-active surface. This article summarizes the electrochemical ion intercalation processes in MXene-based pseudocapacitors with both aqueous and non-aqueous electrolytes. We discuss the impact of ion intercalation on the interfacial characteristics of MXenes, such as interlayer space, surface groups, and interfacial ion-solvent arrangement. Furthermore, we reveal the importance of ion accessibility on the overall pseudocapacitive performance of MXenes and summarize the general strategies to facilitate ion intercalation.

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The development of new materials for electromagnetic interference (EMI) shielding is an important area of research, as it allows for the creation of more effective and high-efficient shielding solutions. In this sense, MXenes, a class of 2D transition metal carbides and nitrides have exhibited promising performances as EMI shielding materials. Electric conductivity, low density, and flexibility are some of the properties given by MXene materials, which make them very attractive in the field. Different processing techniques have been employed to produce MXene-based materials with EMI shielding properties. This review summarizes processes and the role of key parameters like the content of fillers and thickness in the desired EMI shielding performance. It also discusses the determination of power coefficients in defining the EMI shielding mechanism and the concept of green shielding materials, as well as their influence on the real application of a produced material. The review concludes with a summary of current challenges and prospects in the production of MXene materials as EMI shields.

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A subclass of organic semiconductors known as conjugated polyelectrolytes (CPEs) is characterized by a conjugated backbone with ionic pendant groups. The water solubility of CPEs typically hinders applications of thin films in aqueous media. Herein, it is reported that films of an anionic CPE, namely CPE-K, drop cast from water produces single-component solid-state pseudocapacitive electrodes that are insoluble in aqueous electrolyte. That X-ray diffraction experiments reveal a more structurally ordered film, relative to the as-obtained powder from chemical synthesis, and dynamic light scattering measurements show an increase in aggregate particle size with increasing [KCl] indicate that CPE-K films are insoluble because of tight interchain contacts and electrostatic screening by the electrolyte. CPE-K film electrodes can maintain 85% of their original capacitance (84 F g⁻¹) at 500 A g⁻¹ and exhibit excellent cycling stability, where a capacitance retention of 93% after 100 000 cycles at a current density of 35 A g⁻¹. These findings demonstrate that it is possible to use initially water soluble ionic-organic materials in aqueous electrolytes, by increasing the electrolyte concentration. This strategy can be applied to the application of conjugated polyelectrolytes in batteries, organic electrochemical transistors, and electrochemical sensors, where fast electron and ion transport are required.

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Conjugated polymer hydrogels (CPHs) are emerging pseudocapacitive materials capable of forming redox-active hydrogels. Current efforts focus on increasing their areal capacitance (C_Areal) and cycling stabilities by using binders tolerant to H₂SO₄-based electrolytes, while alternatives in more environmentally friendly electrolytes underperform due to low-capacity values. Herein, we demonstrate that it is possible to use conjugated polyelectrolyte (CPE), namely CPE-K, to create a single-component binder-free pseudocapacitive gel in environmentally friendly electrolytes (2 M: NaCl, MgCl₂, and MgSO₄), with C_Areal 1.9 times larger than those reported for single-component binder-free CPHs. The resulting pseudocapacitive gel exhibited C_Areal (523 mF cm⁻² at 0.25 mA cm⁻²) scalable with its thickness in NaCl electrolytes, providing an attractive solution to improve the capacitance of devices while maintaining a minimal charge-collecting electrode surface footprint. In addition, the CPE-K gel demonstrates 86% capacitance retention after 100 000 cycles at 10 mA cm⁻², which is higher than those reported for conventional state-of-the-art conjugated polymers. Electrochemical characterization revealed that C_Areal at all cycling rates tested is proportional to d_Thk up to 750 μm, primarily due to facile ionic diffusion within the 3D conductive network of the gel. Thicker electrodes (d_Thk = 1250 μm) can be operated at a rate of 15 mA cm⁻² with minimal capacity loss. These results demonstrate the potential applications of self-doped CPE gels in designing the next generation of multi-functional electrochemical energy storage and conversion technologies for targeting high energy and power density applications.

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Understanding energy storage mechanisms in electrochemical energy storage devices lays the foundations for improving their energy and power density. Here we introduce in situ ultraviolet–visible (UV–Vis) spectroscopy method to distinguish battery-type, pseudocapacitive and electrical double-layer charge storage processes. On the basis of Ti₃C₂T_x MXene in aqueous acidic and neutral electrolytes, and lithium titanium oxide in an organic electrolyte, we found a correlation between the evolution of UV–Vis spectra and the charge storage mechanism. The electron transfer number for Ti₃C₂T_x in an acidic electrolyte was calculated using quantitative analysis, which was close to previous measurements using X-ray absorption spectroscopy. Further, we tested the methodology to distinguish the non-Faradaic process in Ti₃C₂T_x MXene in a water-in-salt electrolyte, despite well-defined peaks in cyclic voltammograms. In situ UV–Vis spectroscopy is a fast and cost-effective technique that effectively supplements electrochemical characterization to track changes in oxidation state and materials chemistry and determine the charge storage mechanism.

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Limited Li resources, high cost, and safety risks of using organic electrolytes have stimulated a strong motivation to develop non-Li aqueous batteries. Aqueous Zn-ion storage (ZIS) devices offer low-cost and high-safety solutions. However, their practical applications are at the moment restricted by their short cycle life arising mainly from irreversible electrochemical side reactions and processes at the interfaces. This review sums up the capability of using 2D MXenes to increase the reversibility at the interface, assist the charge transfer process, and thereby improve the performance of ZIS. First, they discuss the ZIS mechanism and irreversibility of typical electrode materials in mild aqueous electrolytes. Then, applications of MXenes in different ZIS components are highlighted, including as electrodes for Zn²⁺ intercalation, protective layers of Zn anode, hosts for Zn deposition, substrates, and separators. Finally, perspectives are put forward on further optimizing MXenes to improve the ZIS performance.

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The discovery of the Ti₃C₂T_x compounds (MXenes) a decade ago opened new research directions and valuable opportunities for high-rate energy storage applications. The unique ability of the MXenes to host various mono- and multivalent cations and their high stability in different electrolyte environments including aqueous, organic, and ionic liquid solutions, promoted the rapid development of advanced MXene-based electrodes for a large variety of applications. Unlike the vast majority of typical intercalation compounds, the electrochemical performance of MXene electrodes is strongly influenced by the presence of co-inserted solvent molecules, which cannot be detected by conventional current/potential electrochemical measurements. Furthermore, the electrochemical insertion of ions into MXene interspaces results in strong coupling with the intercalation-induced structural, dimensional, and viscoelastic changes in the polarized MXene electrodes. To shed light on the charging mechanisms of MXene systems and their associated phenomena, the use of a large variety of real-time monitoring techniques has been proposed in recent years. This review summarizes the most essential findings related to the charging mechanism of Ti₃C₂T_x electrodes and their potential induced structural and mechanical phenomena obtained by in situ investigations.

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Conjugated polyelectrolyte (CPE) hydrogels synergize the electrical properties of redox-active polymers with the physical properties of hydrogels. Of particular relevance is their implementation as pseudocapacitors due to their high ionic conductivity, strong ionic-electronic coupling, and large electroactive surface area. To date, efforts to improve the cycling stability of such hydrogels are predominated by the use of additives - optimization of the CPE's intrinsic properties remains underexplored. Herein, the systematic increase in the molecular weight (MW) of a self-doped CPE, namely CPE-K, has been demonstrated as an effective strategy to enhance the cycling stability of the resulting hydrogel. At high MW, mechanically stronger hydrogels were obtained with a specific capacitance as high as 88 ± 4 F g⁻¹ at 0.25 A g⁻¹ and a cycling stability of 76% capacitance retention after 100 000 cycles at 2.5 A g⁻¹. Furthermore, this strategy yields a wider working pseudocapacitive window, less internal resistance, and higher ionic conductivity within the 3D conductive network. We attribute the enhanced electrochemical performance to stronger inter-chain contacts for optimal morphological organization, as revealed by rheological measurements, resulting in stress-tolerant hydrogels with a higher degree of percolation within a 3D conductive network. These results position CPE-K hydrogels as a state-of-the-art organic material for long-term pseudocapacitive technologies and potentially for the next generation of multi-functional pseudocapacitive devices that go beyond high energy density and power density.

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Achieving pseudocapacitive intercalation in MXenes with neutral aqueous electrolytes and driving reversible redox reactions is scientifically appealing and practically useful. Here, we report that the partial oxidation of MXene intensifies pseudocapacitive Li+ intercalation into Ti3C2Tx MXene from neutral water-in-salt electrolytes. An in situ X-ray absorption near-edge structure analysis shows that the Ti oxidation state changes during the Li+ intercalation, indicating the presence of a surface redox reaction. The Ti oxidation/reduction is further confirmed by an in situ extended X-ray absorption fine structure analysis, which shows a reversible contraction/expansion of the Ti-C interatomic distance. The intensified Li+ pseudocapacitive intercalation can be explained by the higher oxidation state of Ti at the open circuit potential. This work demonstrates the possibility of tuning the pseudocapacitive intercalation by adjusting the initial oxidation state of the transition metal on the MXene and offers a facile way to enhance the pseudocapacitance of various MXenes.

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