Abstract:

With the escalating demand for reliable grid-scale energy storage systems， aqueous zinc-ion capacitors （ZICs） have garnered extensive research attention owing to their intrinsic safety， cost-effectiveness， and their ability to bridge the performance gap between high-energy batteries and high-power supercapacitors. Among myriads of electrode materials， two-dimensional （2D） layered Ti₃C₂T_xmaterials have emerged as highly promising candidates due to their metallic conductivity， excellent hydrophilicity， and tunable surface chemistry. However， the severe self-restacking of nanosheets， driven by strong van der Waals forces， remains the critical bottleneck restricting their practical application. This phenomenon drastically reduces the accessible electrochemically active surface area and severely hinders internal ion transport kinetics. To address this inherent challenge， this work rationally designed a novel composite electrode by integrating chlorella-derived boron and nitrogen （B， N） co-doped carbon with Ti₃C₂T_xnanosheets， aiming to enhance energy storage performance through synergistic effects.

In this work， a cost-effective and eco-friendly preparation process was developed using Chlorella microalgae and boric acid as precursors. Initially， B， N co-doped carbon （CC） was synthesized via a hydrothermal pretreatment， followed by high-temperature carbonization and potassium hydroxide （KOH） activation. Subsequently， the obtained CC was assembled via electrostatic self-assembly with etched Ti₃C₂T_xnanosheets to fabricate a series of CC/Ti₃C₂T_xcomposites （CTxy） with varying mass ratios. Comprehensive microstructural characterization indicate that the appropriate introduction of carbon materials effectively inhibits the self-restacking of Ti₃C₂T_xlayers， expands the interlayer spacing， and constructs a robust multidimensional interconnected architecture. Particularly， the composite with the optimal proportion， CT11 （with a CC to MXene mass ratio of 1∶1）， exhibits a significantly increased specific surface area of 626.54 m² g^-1， while concurrently forming a hierarchical porous network that provides abundant ion transport channels and high-density electrochemically active sites.

Electrochemical measurements reveal that the composite electrode exhibits outstanding performance in a three-electrode testing system： at a current density of 0.1 A g^-1， the specific capacity of the CT11 electrode reaches 243.53 mAh g^-1. Upon assembling CT11 with a zinc anode into a full aqueous zinc-ion capacitor， the device demonstrates immense potential for practical application. At a current density of 0.1 A g^-1， the device delivers a high reversible specific capacity of 166.5 mAh g^-1； even at an ultra-high current density of 20 A g^-1， it maintains an excellent rate capability of 81.1 mAh g^-1. Crucially， the CT11//Zn zinc-ion capacitor exhibits ultralong cycling stability， retaining 85.71% of its initial capacity after 16 000 continuous charge-discharge cycles at a high current density of 10 A g^-1， demonstrating its superior electrochemical reversibility. In summary， the superior electrochemical performance of the composite originates from the synergistic interaction between Ti₃C₂T_xand the B， N co-doped carbon. This highly efficient structural modulation strategy not only optimizes the ion and electron transport pathways but also enhances the interfacial pseudocapacitive storage mechanisms. This work proposes a facile， sustainable， and practical structural modification strategy for MXene-based composites， providing broad insights for the precise design of next-generation high-performance aqueous zinc-ion capacitors.

Key words: Ti₃C₂T_xMXene, chlorella, boron and nitrogen co-doped carbon, electrostatic self-assembly, zinc-ion capacitor, electrochemical performance