Proton Redox and Transport in MXene-Confined Water

Abstract

The redox reaction of intercalated protons is key to the pseudocapacitance of MXenes (two-dimensional (2D) carbides and nitrides) in H 2 SO 4 . However, an atomistic understanding of proton redox and transfer in water confined between MXene layers is still lacking. Here, we use firstprinciples molecular dynamics (FPMD) simulations to reveal the protontransfer mechanism in MXene-confined water layers of different thicknesses by using O-terminated Ti 3 C 2 as a prototypical MXene. We found that the proton redox process takes place reversibly between surface −O sites and interfacial water molecules, intermitted by the more frequent in-water proton-transfer events. The surface redox rate is much higher in the highly confined one-layer water than in the two or three layers of water. Proton mobility increases with the water-layer number and already approaches the bulk value in the threelayer water. The proton transfer still follows the Eigen−Zundel−Eigen mechanism in the 2D-like confined water (regardless of the thickness) as in the 3D bulk water via the special pair dance. Our model in the case of two layers of water is in excellent agreement with the experimental interlayer spacing after charging the Ti 3 C 2 O 2 electrode in H 2 SO 4 . Our finding from FPMD of fast surface redox and in-water transfer for the intercalated protons implies that other processes such as the intercalating step are likely the bottleneck for the ionic transport.

Achieving ultrahigh electrochemical performance by surface design and nanoconfined water manipulation

“…Nanoconfined water tends to reside in quasi-integer layers in between MXenes and shows layer-dependent properties as theoretically reported [ 22 ]. Increasing the layers of nanoconfined water, for example from two to three, is expected to significantly promote the water mobility and proton diffusion coefficients, and hence the electrochemical performance [ 22 ].…”

“…Nanoconfined water tends to reside in quasi-integer layers in between MXenes and shows layer-dependent properties as theoretically reported [ 22 ]. Increasing the layers of nanoconfined water, for example from two to three, is expected to significantly promote the water mobility and proton diffusion coefficients, and hence the electrochemical performance [ 22 ]. Though surface modification by adjusting the proportion of terminal -F, -OH and -O of Ti 3 C 2 T x MXene has affected the way nanoconfined water resides, most cases did not yield more than two layers of nanoconfined water, and the capacitance enhancement was also unsatisfactory [ 17 , 18 ].…”

“…On the other hand, experimental and computational approaches studied water confinement and revealed its role in capacitive performance in neutral electrolytes [ 20 , 21 ]. In acidic electrolytes, where superior capacitance was achieved, proton redox and transport in MXene-confined water were also theoretically investigated [ 22 ]. However, the literature did not often give a complete picture of how the interactions of surface chemistry, protons and nanoconfined water contribute to the high capacitance and high-rate performance.…”

Computational Insights into Charge Storage Mechanisms of Supercapacitors

“…The conversion of =O to -OH causes a change of the oxidation state of Ti from +2.33 to +2.43, confirmed by electrochemical in situ X-ray absorption spectroscopy measurements. [57] Considering the critical role of intercalated protons in the redox reaction, Sun et al [44] further investigated the pseudocapacitive charge storage mechanism of MXene in H 2 SO 4 electrolyte using FPMD simulations. They show that proton redox and transfer processes could reversibly occur between interfacial water molecules and =O surface sites, accompanied by a more frequent in-water proton transfer.…”

Morphology and crystal structure dependent pseudocapacitor performance of hydrated WO3 nanostructures

“…42 On the other hand, the surface redox chemistry via the terminal oxygen atom and diffusion of a proton within the hydrogen bond network of confined water layers in MXene hydrate has offered suitable faradaic reactions. 43 Accordingly, the electrochemical charge storage mechanisms of electrode materials are highly subjective to their morphology, electrical conductivity and crystalline nature. 44,45 The 2D layered crystalline MoO 3 , Nb 2 O 5 , V 2 O 5 , anatase TiO 2 , and d-MnO 2 with porous nature have demonstrated a highrate charge storage capability via the intercalation pseudocapacitor mechanism.…”