The evolution of local electronic and atomic structures on self-assembly  Co/MXene composite for HER

Hydrogen is clean and efficient energy. However, sluggish kinetic for both Hydrogen evolution reaction (HER) should be overcome. Ti3C2Tx (MXene) with a 2-dimensional channel serves as conductive supporting material to improve HER of Co. To control these performance, we synthesized Co/MXene with different specific ratios of volume, 0.5, 1 and 2. Co/MXene-2 performs the best HER in 1M KOH. The Raman of Co at Co/MXene-2 indicates Co-OH at 525 cm-1.

In Co K-edge EXAFS and Co L3,2-edge XAS for further investigation of Cobalt formation in Co/MXene, Less magnitude of second coordination shell and spectrum dissimilar to that of CoO and Co₃O₄ indicates Co is atoms and adheres on MXene.

In Co K-edge and Ti K-edge XAS, when the ratio of volume is 2, rich electrons in Co 4p and Ti 4p, possibly the least coordination shell or the highest structural disorder of Ti-C/O, possibly the most coordination number or the least structural disorder of Ti-Ti, possibly the least coordination number or the highest structural disorder of Co-O in Co/MXene-2 compared to Co/MXene-0.5 and Co/MXene-2 probably provoke the well HER performance.

The in-situ Co K-edge XAS with applied voltage from 0V to -0.8V of Co/MXene-2 is conducted to investigate how connection of Cobalt and termination of MXene effect the electrochemical measurement. When -0.2V, which is near the potential at 10 mA/cm2 of LSV for HER of Co/MXene-2 is applied, Radial distance of Co-O is remain, then, possibly coordination number of Co-O start to decrease or structural disorder of Co-O start to increase as well suggesting disconnection of termination of MXene with Co.

Besides, The valence state of Co, unoccupied state of Co 4p start to decrease indicating electron transfer out of Co 4p which causes the decrease of the valence state of Co and absorb H2O. On the other hand, unoccupied state of Co 4p and valence state of Co start to decrease and increase at applied -0.4V possibly contributed by desorption and production of H2.