Proton Redox and Transport in MXene-Confined Water.

The redox reaction of intercalated protons is key to the pseudocapacitance of MXenes (two-dimensional (2D) carbides and nitrides) in H2SO4. However, an atomistic understanding of proton redox and transfer in water confined between MXene layers is still lacking. Here, we use first-principles molecular dynamics (FPMD) simulations to reveal the proton-transfer mechanism in MXene-confined water layers of different thicknesses by using O-terminated Ti3C2 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 three-layer 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 Ti3C2O2 electrode in H2SO4. 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.