(<i>Invited</i>) Carbon as Proton Storage Site in Ti₃C₂T<i> _x </i> during Electrochemical Protonation

In electrochemical energy materials, the ability of mixed ion-electron conductors to host ions and electrons is fundamental to their success as electrodes for lithium-ion batteries and pseudocapacitors. While many transition metal ions can accommodate electrons during redox reactions, only limited anions are available to host cations. The most predominant ones are chalcogens like oxygen, sulfur, and selenium. Even in MXenes, oxygen surface termination groups are believed to enable redox activities and store protons as hydroxyl groups. However, recent surface termination quantification of Ti 3 C 2 T x MXenes shows insufficient surface oxygen groups to accommodate the electrochemically intercalated protons. In this work, we reveal that, in addition to the oxygen surface groups, electrochemically intercalated protons are stored in the interstitial sites of the Ti 3 C 2 T x MXene slab, forming C-H bonds using inelastic neutron scattering (INS) and atomic-resolution secondary ion mass spectroscopy (SIMS). We further showed the effect of C-H bond formation in Ti 3 C 2 T x MXene on the crystal structure evolution during electrochemical protonation using synchrotron-based X-ray diffraction. This work shows the possibility of carbon as proton storage sites, suggesting more carbides should be considered for proton-based energy storage.