First-Principles Study Of The Ti₂C-based MXenes As Hydrogen Evolution Reaction Catalysts And Electrode Materials

With the proposal of carbon peak and neutrality targets,taking full advantage of clean energy to reduce the pollution and protect the environment has become an important issue today.Hydrogen,produced by electrolysis of water,is an ideal energy carrier.However,hydrogen storage requires such high pressure and special storage materials that its development is slow.Also,the noble metal catalysts for hydrogen evolution reaction（HER）limit the large-scale application of electrolytic water in industry.Therefore,seeking more efficient and cheaper HER catalysts has become a popular direction of scientific study.On the other hand,the rapid development of ion battery technologies has become an effective energy storage and transportation.For energy storage technologies,the most matured lithium-ion batteries（LIBs）have the disadvantages of lithium element shortage and low safety.Meanwhile,other metal-ion batteries are emerging,such as sodium ion batteries with good safety and abundant element.The choice of anode material is an important factor that affect the performance of battery.Two-dimensional transition metal carbon/nitrogen compound—MXene has performed well in these fields due to its advantages of high stability,controllable surface properties,and multiple layer thicknesses.In this paper,basing on the Ti₂CT₂ MXene,we employ first-principles calculations to first construct new MXene materials Sn2CO2 and Pb2CO2,and explore their stable configuration and electronic properties.As a result,Sn2CO2 is a semiconductor while Pb2CO2 is a metal.The calculated Gibbs free energy（as low as-0.03 eV）of hydrogen adsorption and reaction kinetics of HER（Volmer-Tafel reaction path）prove that Sn2CO2 is a potential catalyst for HER.Then,the structural and electronic properties of O,S,Se and Te terminated Ti2C monolayers and multilayers are systematically studied.The results show that Ti2CO2 and Ti2CTe2 multilayers share stacking model I,while Ti₂CS₂ and Ti2CSe2 multilayers share another stacking model Ⅱ.The Ti2CO2 monolayer to four-layer are semiconductors with decreasing indirect band gaps（from 1.28 eV to 1.14 eV）,while Ti₂CS₂,Ti2CSe2 and Ti2CTe2 are all metallic.Finally,starting with O,S,Se and Te terminated Ti2C monolayers,we studied the anode materials of Li,Na,K,Mg,Ca and Al ion batteries.All the results show that Ti₂CS₂ and Ti2CSe2 are possible anode materials for Na,K and Ca ion batteries.To be specific,the theoretical capacity of Ca-ion on Ti₂CS₂ can achieve 600 mAh/g,and the cases of Na,K,and Ca ions on Ti2CSe2 have relatively low average open circuit voltages and diffusion barriers.Additionally,we provide a high-throughput calculation of 600 MXenes—Mn+1XnT2（n=1,2,3;M=Sc,Ti,V,Cr,Zr,Nb,Mo,Hf,Ta,and W;X=C and N;T=bare,OH,O,S,Se,Te,F,Cl,Br,and I）,and the structural properties of these MXenes are listed in table with results from literature for reference.