Zinc (Zn) ion supercapacitors (ZISCs) have attracted considerable attention as a viable energy storage technology because they are cost-effective, safe, and environmentally friendly. However, cathode materials with suitable properties are rare and need to be explored. In this regard, metal carbides (MXenes) are a good choice for capacitive energy storage, but they exhibit low capacitance. The energy storage performance of MXenes can be bossed using functionalization with heteroatom doping, e.g., nitrogen (N), to simultaneously modify ZISCs’ fundamental characteristics and electrochemical properties. Herein, we present an in-situ N-functionalization of Ti₃C₂T_x-MXene via a hydrothermal reaction with urea (denoted as N-Ti₃C₂T_x-MXene). N-functionalization into Ti₃C₂T_x-MXene raised Ti₃C₂T_x-MXene’s interlayer spacing and boosted the Zn-ion storage in 1 M ZnSO₄ electrolyte. The N-Ti₃C₂T_x-MXene electrode delivered an excellent specific capacitance of 582.96 F/g at 1 A/g and retained an outstanding cycle stability of 94.62% after 5000 cycles at 10 A/g, which is 1.8 times higher than pristine Ti₃C₂T_x-MXene at identical conditions. Moreover, the N-Ti₃C₂T_x-MXene//Zn device demonstrated a maximum capacitance of 153.55 F/g at 1 A/g, retained 92% of its initial value after 5000 cycles, and its Coulombic efficiency was ~100%. This strategy considerably reduced Ti₃C₂T_x-MXene nanosheet restacking and aggregation and enhanced electrochemical performance. Further, this research elucidated N-Ti₃C₂T_x-MXene’s charge–storage process and offered a fresh approach to the rational design of novel electrode materials for ZISCs.

The interaction between cathode and anode materials is critical for developing a high-performance asymmetric supercapacitor (SC). Significant advances have been made for cathode materials, while the anode is comparatively less explored for SC applications. Herein, we proposed a high-performance binder-free anode material composed of two-dimensional ZnFe₂O₄ nanoflakes supported on carbon cloth (ZFO-NF@CC). The electrochemical performance of ZFO-NF@CC as an anode material for supercapacitor application was examined in a KOH solution via a three-electrode configuration. The ZFO-NF@CC electrode demonstrated a specific capacitance of 509 F g⁻¹ at 1.5 A g⁻¹ and was retained 94.2% after 10,000 GCD cycles. The ZFO-NF@CC electrode showed exceptional charge storage properties by attaining high pseudocapacitive-type storage. Furthermore, an asymmetric SC device was fabricated using ZFO-NF@CC as an anode and activated carbon on CC (AC@CC) as a cathode with a KOH-based aqueous electrolyte (ZFO-NF@CC||AC@CC). The ZFO-NF@CC||AC@CC yielded a high specific capacitance of 122.2 F g⁻¹ at a current density of 2 A g⁻¹, a high energy density of 55.044 Wh kg⁻¹ at a power density of 1801.44 W kg⁻¹, with a remarkable retention rate of 96.5% even after 4000 cycles was attained. Thus, our results showed that the enhanced electrochemical performance of ZFO-NF@CC used as an anode in high-performance SC applications can open new research directions for replacing carbon-based anode materials.

Symmetric pseudocapacitors (SPCs) have the ability to provide an extended operating life-span with high power density; however, their energy density is not sufficient for various critically vital applications. To reduce the overall cost and complexity of fabrication of SPCs while simultaneously increasing their energy density, we present a “two birds with one stone” approach. Herein, to prove this idea, low-cost silicon (Si) embedded in metal–organic framework (MOF)-derived cobalt nano-sized particles/nitrogen-doped carbon (Si/Co-NC) was prepared as an integrated electrode material which works at positive and negative potential windows simultaneously. The optimized electrode with 30% Si content (Si/Co-NC1) achieved a remarkable capacitance of 845.41 F g⁻¹ at 1 A g⁻¹ in a broader potential window range (−1.0–0.4 V) and 97% of the capacitance was retained after 10 000 cycles. According to the charge storage analysis, the Si/Co-NC1 electrode works on the basis of a pseudocapacitive charge storage mechanism and possesses 79.8% capacitive contribution at 1 mV s⁻¹. Moreover, a symmetric pseudocapacitor, Si/Co-NC1‖Si/Co-NC1-SPC, shows a broader operating potential window range (0.0–1.8 V), exhibits an excellent capacitance of 277 F g⁻¹ at 2 A g⁻¹, maintains a capacitance of 96.8% after 50 000 cycles and achieves ∼100% coulombic efficiency. Additionally, the Si/Co-NC1‖Si/Co-NC1-SPC provides an ultrahigh energy density of 125.43 W h kg⁻¹ at 1804.32 W kg⁻¹, which is the greatest energy density for symmetric SCs to date.

It is of great significance to design electrochemical energy conversion and storage materials with excellent performance to fulfill the growing energy demand. Bimetallic cobalt/nickel-based electrode materials exhibit excellent electrical conductivity compared to mono oxides. However, their potential as electrode materials for high-performance supercapacitors (SCs) is limited because of their poor cycling stability and high-capacity fading. This work demonstrates the synthesis of binder-free bimetallic NiCo₂O₄ nano-needles supported on CC (NCO@CC) via a facile and scalable hydrothermal process. Excellent electrical conductivity and interconnected nanostructure of NCO@CC nano-needles provide the fast transfer of electrons with numerous channels for ion diffusion. Owing to such features, the binder-free NCO@CC electrode for SC discloses excellent specific capacitance (1476 Fg⁻¹ at 1.5 Ag⁻¹) with 94.25% capacitance retention even after 5000 cycles. From these outstanding electrochemical performances, it can be inferred that NCO@CC nano-needle array-structured electrodes may be potential candidates for SC applications.

Scientific research is being compelled to develop highly efficient and cost-effective energy-storing devices such as supercapacitors (SCs). The practical use of SC devices is hindered by their low energy density and poor rate capability due to the binding agents in fabricating electrodes. Herein, we proposed flower-like highly open-structured binder-free ZnCo₂O₄ micro-flowers composed of nanosheets supported in nickel foam (ZnCoO@NF) with improved rate capability up to 91.8% when current varied from 2 to 20 A·g⁻¹. The ZnCoO@NF electrode exhibited a superior specific capacitance of 1132 F·g⁻¹ at 2 A·g⁻¹ and revealed 99% cycling stability after 7000 cycles at a high current density of 20 A·g⁻¹. The improved performance of the ZnCoO@NF electrode is attributed to the highly stable structure of the micro/nano-multiscale architecture, which provides both the high conduction of electrons and fast ionic transportation paths simultaneously.