This research finding originates from the accidental finding in the laboratory work. Our initial goal is to test the electrochemical HER performance of unexfoliated Mo2TiC2Tx MXene. We anticipated that due to the stacking of MXene nanosheets, the performance should not be superior than most catalysts. However, we find that the electrochemical performance of MXene continuously increases during the electrochemical scans. In order to investigate the mechanism of how the HER process is affecting the structure of MXene, we performed a series of characterizations on the cycled MXene and surprisingly found that the MXene nanosheets have been exfoliated into small lateral sizes. Further evidence of scanning transmission electron microscopy showed that Mo vacancies were generated simultaneously. Combining with the XPS results during the process, we proposed that hydrogen cations were mostly responsible for the unexpected results. Hydrogen cations interacted with the active Mo-O terminals on MXenes, which could either lead to the hydrogen evolution or the formation of Mo-OH2 terminals. The former results in the exfoliation by hydrogen and the latter induces Mo vacancies. This finding provides an interesting route to prepare vacancy-rich ultrathin Mo2TiC2Tx MXene during the HER process.

The outcomes provide us with further inspiration that single atoms might be immobilized into the Mo vacancies. However, finding a suitable way to provide single atoms was the next challenge. We figured out that by replacing graphene rod with Pt foil as the counter electrode, the Pt atoms dissolve into the electrolyte and re-deposit in the Mo vacancies during HER. This process occurs simultaneously with the exfoliation and Mo-vacancy generation. This provided a controllable method to efficiently load single Pt atoms on the MXene nanosheets. Surprisingly, the single Pt atoms could be stabilized by forming three Pt-C bonds inside the MXene structure, which is further confirmed by synchrotron data. The loading of single Pt atoms reached the highest of 1.2 wt%, which leads to the exceptionally high HER performance. 

We are grateful for fruitful discussions with Professor Yury Gogotsi from Drexel University, Professor Chen Chen, and Professor Yadong Li from Tsinghua University during this research. Professor Chung-Li Dong from Tamkang University and Professor Ru-Shi Liu from National Taiwan University also provided great help for the Synchrotron characterisation. We also appreciate help from Professor Peng Li from Nanjing University of Aeronautics and Astronautics, Dr Yao Zheng from University of Adelaide, and Dr Yu-Cheng Huang from National Synchrotron Radiation Research Centre
 
This research has now been published in Nature Catalysis
https://www.nature.com/articles/s41929-018-0195-1