Understanding the relationship between the geometrical structure of interfacial water and operating voltage window in graphene and nitrogen-doped graphene-based supercapacitors

Abstract

Understanding of the electrode/electrolyte interface interaction is crucial to electrochemical energy storage devices. In aqueous electrochemical double-layer capacitors (supercapacitors), the energy density is limited by the narrow voltage window, which is dominated by water dissociation. However, molecular-level understandings of the operating voltage window are still ambiguous because of the lacks of accurate description of this electric double layer structure and the clear picture of the adsorption structure of interfacial water molecules during electrochemical process, which makes it difficult to improve electrochemical performance. We focus on the typical graphene/aqueous electrolyte interface and employ Ab inito molecular dynamics simulations to accurately describe the electrified interface in simulation cells. Thus, a fundamental relationship between the asymmetric response of interfacial water molecules to applied electrode potentials and operating voltage window has been revealed. It is found that the response for orientation of the interfacial water molecules to varied potentials affects their adsorption structure, and the adsorption behavior determines the practical EDL region. Moreover, our calculations show that nitrogen dopants effectively adjust adsorption behavior of interfacial water molecules and thus broaden the negative voltage window. Our work provides new insights into the operating voltage window in electrochemical double-layer capacitors and efficient strategies for broadening voltage window.