posted on 2021-08-31, 20:04authored byThomas B. Sobyra, Tyler S. Mathis, Yury Gogotsi, Paul Fenter
MXenes are a large family of two-dimensional
materials that are
attractive for energy storage due to their high-rate charging capabilities
as well as for electrochemical actuators, water purification, and
many other technologies. Ion intercalation during electrochemically
driven charge and discharge processes is the fundamental process associated
with MXene functionality, which we have characterized using in situ and operando X-ray reflectivity
(XRR). Experiments performed at the Advanced Photon Source at Argonne
National Laboratory monitored the changes in the structure of a Ti3C2 MXene film on a platinum current collector as
a function of static applied potential between 0.3 and −0.7
V vs Ag/AgCl in an aqueous 0.1 M Li2SO4 electrolyte.
Negative potential sweeps lead to a contraction of 1.2 Å in the
interlayer spacing and a loss of electron density between the layers,
likely due to Li+ ion insertion and water removal. The
change in lattice spacing includes a continuous variation vs potential
as well as an additional discrete contraction that occurs near −0.35
V that has the characteristics of a first-order transition. The continuous
change in the MXene interlayer spacing is associated with the capacitive
charge, while the discrete change in structure correlated to the weak
feature in the cyclic voltammogram at −0.35 V can be interpreted
as either a pseudocapacitive charging process or a potential-dependent
change in capacity.