Applications of nuclear magnetic resonance in exploring structure and energy storage mechanism of supercapacitors

Supercapacitors, comprising electrical double-layer capacitors (EDLCs) and pseudocapacitors, are widely acknowledged as high-power energy storage devices. However, their local structures and fundamental mechanisms remain poorly understood, and suitable experimental techniques for investigation are also lacking. Recently, nuclear magnetic resonance (NMR) has emerged as a powerful tool for addressing these fundamental issues with high local sensitivity and non-invasiveness. In this paper, we first review the limitations of existing characterization methods and highlight the advantages of NMR in investigating mechanisms of supercapacitors. Subsequently, we introduce the basic principle of ring current effect, NMR-active nuclei, and various NMR techniques employed in exploring energy storage mechanisms including cross polarization (CP) magic angle spinning (MAS) NMR, multiple-quantum (MQ) MAS, two-dimensional exchange spectroscopy (2D-EXSY) NMR, magnetic resonance imaging (MRI) and pulsed-field gradient (PFG) NMR. Based on this, recent progress in investigating energy storage mechanisms in EDLCs and pseudocapacitors through various NMR techniques is discussed. Finally, an outlook on future directions for NMR research in supercapacitors is offered.