Synergistic Design of Flexible Nanopapers for High-Performance Proton Pseudocapacitors

Abstract

Two-dimensional materials for flexible energy storage commonly face huge challenges in limited active surface and hindered charge transport. Herein, we report an innovative asymmetric pseudocapacitor based on synergistic design of modified MXene and graphene, integrating gas-induced rapid expansion technology and precise surface chemical regulation methods. For graphene modification, rapid vaporization induces exfoliation and expansion of graphene oxide layers. Subsequently, pseudocapacitive oxygen-containing groups were selectively introduced through acid oxidation, yielding expanded-and-oxidized graphene (OEG) for positive porous-nanopaper electrode. For MXene modification, alkali-treated MXene underwent hydrazine assistance to facilitate gas expansion and –NH2 grafting, producing MXene-NH2 (NOM) for negative porous-nanopaper electrode. Density functional theory calculations show that –COOH more effectively modulate graphene’s electronic structure by inducing charge redistribution and creating active sites, thereby enhancing H+ adsorption and ion interactions compared to –OH. Meanwhile, –NH2 on MXene enable electron delocalization and dynamic Ti–N–H+ interactions, speeding up proton adsorption/desorption and boosting both pseudocapacitance and conductivity. Through collaborative optimized spatial architecture and surface properties, flexible OEGB and NOMB exhibited of 333.6 and 500.5 F g−1 at high mass loading, respectively. The assembled proton pseudocapacitor readily achieved energy and power densities of 58.9 Wh kg−1 and 3802 W kg−1, respectively, with excellent stability for potential applications.

1. Introduction

Flexible supercapacitors have garnered significant attention due to their broad applications in wearable and portable electronic devices [1–3]. Among various candidate materials, two-dimensional (2D) materials—such as graphene, MXenes, black phosphorus, and covalent organic frameworks (COFs)—stand out owing to their exceptional flexibility, high packing density, and strong interlayer interactions, demonstrating huge potential [4–8]. As an emerging 2D material, MXene has the booming development for high-performance flexible supercapacitors, particularly in proton-type supercapacitors [9]. Proton-type supercapacitors leverage smallest ionic radius and fast diffusion kinetics, enabling outstanding energy storage performance in solid-state systems, and represent a critical pathway toward achieving high-rate capability and high-energy density [10, 11]. Notably, MXene exhibits excellent specific capacitance and stability in acidic environments, effectively highlighting its tremendous potential as a proton-type negative electrode material [12].

However, 2D materials commonly suffer from severe interlayer restacking under high mass loading, resulting in insufficient exposure of active sites and increased ion diffusion path tortuosity [13]. Thus, materials optimization remains a critical issue that urgently needs to be addressed [14, 15]. Meanwhile, although MXene-based flexible supercapacitors have achieved relatively high energy density via asymmetric configurations, this approach is limited by the lack of efficient and performance-matched 2D positive electrode materials. To our knowledge, no one is more suitable as a positive electrode material than graphene to match with MXene, given that 2D carbon materials have better stability and voltage window at the same acid condition [16]. However, graphene’s energy storage is predominantly governed by electric double-layer capacitance, with limited pseudocapacitive contribution and relatively low capacitance, which hinders full utilization of asymmetric system potential [17–19]. Therefore, the development of a proton-type solid-state asymmetric supercapacitor employing a high-pseudocapacitance 2D MXene negative electrode and a well-