Unraveling the mechanisms of wider negative voltage window in single-layer graphene/DMSO-H2O hybrid electrolyte interface by the theoretical study of the sodium-ion solvation sheath interfacial model

Graphene has demonstrated to be a great candidate as electrode materials in applications of electrical double-layer (EDL) capacitors. However, for graphene/electrolyte EDL, the hydrogen evolution reaction (HER)-dominated negative voltage window has become the challenge to obtain devices with high energy density. The introduction of dimethyl sulfoxide (DMSO) in water solvent for co-solvent electrolyte has been demonstrated to be an efficient strategy to suppress HER because the O atom in DMSO acts as a hydrogen bond acceptor to anchor a water molecule. However, the role of DMSO in broadening the negative voltage window still needs further explanation. Here, we employ ab initio molecular dynamics simulations on a model system composed of singlelayer graphene/co-solvent NaCl electrolyte interface to describe the co-solvent based Na+ solvation sheath interfacial model. It is found that a passivation layer has been constructed on the electrode surface, where DMSO replaces part of water molecules and tunes the adsorption structure of the remaining interfacial water molecules in Na+ solvation sheath (Na center dot H2O). Our calculation elucidates that the interaction between DMSO and water molecules is stronger than that between interfacial water molecules and negatively-charged electrode. Thus, the interfacial water molecules in Na+ solvation sheath no longer evolve to 0-coordinated Na center dot H2O state and have less HER activity. Moreover, we relate the amount of Na center dot H2O with high HER activity to the operating negative voltage window. Our work provides new insights into the mechanism of the co-solvent electrolyte in broadening negative voltage window and the formation of the practical negative voltage window during electrochemical processes.

Automated summary

Graphene is a promising electrode material for electrical double-layer capacitors, but the dominant hydrogen evolution reaction (HER) in the negative voltage window limits the energy density. Introducing a co-solvent electrolyte of dimethyl sulfoxide (DMSO) in water has been shown to suppress HER, but the role of DMSO in expanding the negative voltage window needs further explanation. Using molecular dynamics simulations, we found that DMSO replaces part of water molecules on the electrode surface, leading to a passivation layer that reduces HER activity. The amount of Na center dot H2O, which exhibits high HER activity, is related to the operating negative voltage window. Our work provides new insights into the mechanism of co-solvent electrolyte in expanding the negative voltage window.