Recent progress in sodium/potassium hybrid capacitors

Ultrafast Na-Ion Storage in Amorphization Engineered Hollow Vanadium Oxide/MXene Nanohybrids for High-Performance Sodium-Ion Hybrid Capacitors

Sodium ion hybrid capacitors (SIHCs) address the high power and energy requirements in energy storage devices but face significant challenges arising from the slow kinetics and cycling instability of the anode side. Introducing atomic disorder and employing structural engineering in anode materials proves to be effective strategies for achieving rapid charge storage. Here, it is demonstrated that N-doped MXene encapsulated amorphous vanadium oxide hollow spheres (VOx@N-MXene HSs) offer multidirectional open pathways and sufficient vacancies, enabling reversible and fast Na+ insertion/extraction. Machine learning potentials, coupled with molecular simulation techniques, confirm the presence of more abundant pores within the amorphous vanadium oxide (VOx) structure. The simulation of the charging/discharging process elucidates the authentic reaction path and structural evolutions of the VOx@N-MXene HSs, providing sufficient insight into the atomic-scale mechanisms associated with these structural superiorities. The full SIHCs devices demonstrate a high energy density of 198.3 Wh kg-1, along with a long-term cycling lifespan of 8000 cycles. This study offers valuable strategies into the intricate design and exploration of amorphous electrodes, contributing to the advancement of next-generation electrochemical energy devices.

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

3D Hierarchical Ti3C2TX@PANI-Reduced Graphene Oxide Heterostructure Hydrogel Anode and Defective Reduced Graphene Oxide Hydrogel Cathode for High-Performance Zinc Ion Capacitors

The practical application of supercapacitors (SCs) has been known to be restricted by low energy density, and zinc ion capacitors (ZICs) with a capacitive cathode and a battery-type anode have emerged as a unique technology that can effectively mitigate the issue. To this end, the design of electrodes with low electrochemical impedance, high specific capacitance, and outstanding reaction stability represents a critical first step. Herein, we report the synthesis of hierarchical Ti3C2TX@PANI heterostructures by uniform deposition of conductive polyaniline (PANI) polymer nanofibers on the exposed surface of the Ti3C2TX nanosheets, which are then assembled into a three-dimensional (3D) cross-linking framework by a graphene oxide (GO)-assisted self-convergence hydrothermal strategy. This resulting 3D Ti3C2TX@PANI-reduced graphene oxide (Ti3C2TX@PANI-RGO) heterostructure hydrogel shows a large surface area (488.75 F g-1 at 0.5 A g-1), outstanding electrical conductivity, and fast reaction kinetics, making it a promising electrode material. Separately, defective RGO (DRGO) hydrogels are prepared by a patterning process, and they exhibit a broad and uniform distribution of mesopores, which is conducive to ion transport with an excellent specific capacitance (223.52 F g-1 at 0.5 A g-1). A ZIC is subsequently constructed by utilizing Ti3C2TX@PANI-RGO as the anode and DRGO as the cathode, which displays an extensive operating voltage (0-3.0 V), prominent energy density (1060.96 Wh kg-1 at 761.32 W kg-1, 439.87 Wh kg-1 at 9786.86 W kg-1), and durable cycle stability (retaining 67.9% of the original capacitance after 4000 cycles at 6 A g-1). This study underscores the immense prospect of the Ti3C2TX-based heterostructure hydrogel and DRGO as a feasible anode and cathode for ZICs, respectively.

Synchronous embedded growth of Mo2C nanodisk arrays immobilized on porous carbon nanosheets for ultra-stable sodium storage

Sodium ion capacitors (SICs) that combine the merits of both rechargeable batteries and supercapacitors have gained widespread recognition for their high energy density and extended cycle life as new energy storage devices. However, the purposeful design of advanced battery-type anodes has become an urgent need to remedy the dynamics mismatch with the capacitive cathode. Herein, we propose a simple but efficient bottom-up approach to build three-dimensional Mo2C/C hybrid architectures in situ as anodes for SICs. By finely regulating the ratio of carbon and molybdenum sources, the optimized Mo2C/C, where even thinner subunit assembled Mo2C nanodisk (∼47.1 nm in thickness) arrays are immobilized on carbon nanosheet substrate via the synchronous embedded growth, rapid electron and ion diffusion/transport expressways, abundant active sites and robust structural stability were achieved for efficient sodium storage. Benefiting from the synergistic contributions of the components, the optimum Mo2C/C anode displays an outstanding rate and long-cycle properties as a competitive anode. Moreover, the constructed Mo2C/C-based SICs exhibited an energy density of ∼16.7 W h kg-1 at 10 kW h kg-1, along with ∼22.5% capacitance degradation over 4000 cycles at 1 A g-1. This contribution will guide the precise synthesis of other versatile Mo2C-based hybrids towards energy-related applications and beyond.

Metal-organic frameworks and their derivatives for metal-ion (Li, Na, K and Zn) hybrid capacitors

Metal-ion hybrid capacitors (MIHCs) hold particular promise for next-generation energy storage technologies, which bridge the gap between the high energy density of conventional batteries and the high power density and long lifespan of supercapacitors (SCs). However, the achieved electrochemical performance of available MIHCs is still far from practical requirements. This is primarily attributed to the mismatch in capacity and reaction kinetics between the cathode and anode. In this regard, metal-organic frameworks (MOFs) and their derivatives offer great opportunities for high-performance MIHCs due to their high specific surface area, high porosity, topological diversity, and designable functional sites. In this review, instead of simply enumerating, we critically summarize the recent progress of MOFs and their derivatives in MIHCs (Li, Na, K, and Zn), while emphasizing the relationship between the structure/composition and electrochemical performance. In addition, existing issues and some representative design strategies are highlighted to inspire breaking through existing limitations. Finally, a brief conclusion and outlook are presented, along with current challenges and future opportunities for MOFs and their derivatives in MIHCs.

Homologous Nitrogen-Doped Hierarchical Carbon Architectures Enabling Compatible Anode and Cathode for Potassium-Ion Hybrid Capacitors

Potassium-ion hybrid capacitors (PIHCs) have been considered as an emerging device to render grid-scale energy storage. Nevertheless, the sluggish kinetics at the anode side and limited capacity output at the cathode side remain daunting challenges for the overall performances of PIHCs. Herein, an exquisite "homologous strategy" to devise multi-dimensional N-doped carbon nanopolyhedron@nanosheet anode and activated N-doped hierarchical carbon cathode targeting high-performance PIHCs is reported. The anode material harnessing a dual-carbon structure and the cathode candidate affording a high specific surface area (2651 m² g^-1 ) act in concert with a concentrated ether-based electrolyte, resulting in an excellent half cell performance. The related storage mechanism is systematically revealed by in situ electrokinetic characterizations. More encouragingly, the thus-derived PIHC full cell demonstrates a favorable energy output (157 Wh kg^-1 ), showing distinct advantages over the state-of-the-art PIHC counterparts.

Cation-Dependent Hydrogel Template-Activation Strategy: Constructing 3D Anode and High Specific Surface Cathode for Dual-Carbon Potassium-Ion Hybrid Capacitor

Here, a universal template-activation strategy is proposed to prepare two different types of porous carbonaceous materials for potassium ion hybrid capacitor (PIHC) anode and cathode, which is realized by only changing the type of cation (Na⁺ and K⁺ ) in the polysilicic acid-organic mixed hydrogel precursor originating from adding organic acid into Na₂ SiO₃ and K₂ SiO₃ solution, respectively. TG-IR data demonstrate that K⁺ exhibits a stronger etching ability for activating carbonaceous materials during the annealing process. Accordingly, a 3D carbon anode obtained with the assistance of NaOH (NPC-500) exhibits abundant edge-N doping (8.14 at%), displaying a high K-storage capacity of 314.1 mA h g^-1 at 0.2 A g^-1 and a capacity of 219.3 mA h g^-1 at 10 A g^-1 after 10000 cycles. A porous carbon particles cathode prepared in the presence of KOH (KCP-800) shows a high specific surface area (1326.8 m² g^-1 ), contributing to high PF₆^- (de)adsorb capacity of 103.0 mA h g^-1 at 1 A g^-1 . Remarkably, the assembled NPC//KCP PIHC delivers a high energy density of 137.7 W h kg^-1 at a power density of 275.4 W kg^-1 even over 20 000 cycles.

Application of 2D Materials to Potassium-Ion Hybrid Capacitors

Metal-ion hybrid supercapacitors (MICs) are a new type of electrochemical energy storage (EES) device, consisting of a battery-type electrode and a supercapacitor (SC)-type electrode. Exhibiting the advantages of both batteries and SCs (e. g., good energy density, excellent power density and long cycle life), these advanced energy storage devices have considerable commercial application prospects. Among MICs, potassium-ion hybrid supercapacitors (PICs) have several further advantages, including abundancy of resources, low standard electrode potential, and low cost. PICs are regarded as potential substitutes for lithium- or sodium-ion hybrid supercapacitors. However, the practical applications of PICs remain limited, owing to the imbalance of kinetics and capacity between the electrodes, the slow ion/electron diffusion rate, and the poor electrode structural stability. Recently, 2D materials with distinct structures and fascinating features have elicited widespread attention for application in PICs, thus achieving significant enhancements, ranging from charge storage capacity to reaction kinetics. This Review discusses research progress in 2D materials for PICs. Firstly, the energy storage principle and development requirements of MICs are introduced. The pivotal advantages and significant roles of 2D materials in the fabrication of PICs are then discussed in detail. Lastly, the challenges and prospects of the application of 2D materials to high-performance PICs are presented.