First-principles Studies On Hydrogen Isotope Separation Via Two-dimensional Materials

Hydrogen isotope separation is a technology to measure national strength and to guarantee national defense security and often used for the production,conversion,extraction,monitoring and safety control of hydrogen isotopes.That traditional hydrogen isotope separation methods require high-temperature or low-temperature equipment during the separation process greatly increases the energy consumptions,and brings great economic burdens to the hydrogen isotope separation.The recently emerged separation method based on two-dimensional materials can realize the separation of hydrogen isotopes via an electrochemical pump driving hydrogen isotope ions through under normal temperature conditions.And it is a low-energy-consumption,high-efficiency and wide-applicability method.However,the separation mechanism that the hydrogen isotope ions transport through the two-dimensional materials greatly challenges the recognition that two-dimensional materials such as graphene are impermeable to ions and molecules including proton.A lot of theoretical efforts have been devoted to understand this phenomenon,but the calculated transport barrier values of protons through graphene are much higher than the experimental activation energy making the mechanism not clear enough and bringing difficulties to study the hydrogen isotope separation.Therefore,based on the first-principles,we studied the impacts of hydrogenation of graphene,the electrolyte in the environment,and the nuclear quantum effect during the process.And we found that only the hydrogenation of graphene could eliminate the difference between the experimental value and the calculated value.In addition,we found the evidence of the existence of fully hydrogenated graphene(graphane)under the experimental conditions through the electrochemical phase diagram.Finally,combining the thermodynamic and kinetic analysis,we proposed the most likely mechanism for protons to transport through graphene,that is,protons actually transport through graphane during the transportation.The transport of proton in graphane system is divided into three steps,the proton transfer step from the electrolyte molecule to the graphane,the proton penetration step from one side of the graphane to the other,and the proton escape step from the graphane to the electrolyte molecule on the other side,in which the decisive step is the transfer step in most cases.In the electrochemical pump,there are water,Nafion and other substances that may be mixed in the preparation process,which could affect the behaviors of proton transport.Thus,we constantly updated the electrolyte models of graphane system,from the molecular form model to the bulk form model,and to the hydration model of Nafion for gradually approaching the experimental electrolyte conditions to study the behaviors of proton transport.And we found that the effect of electrolyte on proton transport is mainly concentrated in the proton transfer step,i.e.the decisive step.The barrier decreases with the decrease of the stability of the solvated protons in the electrolyte,the increase in the local proton concentration,and the increase in temperature.After clarifying the proton transport mechanism in the graphene and graphane system,we could screen out the proton conductors and two-dimensional materials to obtain better performance based on the macro indicators of separation factor and the energy consumption(corresponding to the kinetic isotope effect and the potential barrier properties in a first-principles study)for evaluating hydrogen isotope separation method.We calculated the separation efficiencies of two types aqueous proton conductors of KAUST-7’ and HUP-1,and found that their separation performances are similar to Nafion.And we systematically studied the existence of the hydrogenation phases of the group-ⅣA and group-ⅤA two-dimensional materials,the proton transport behavior and the hydrogen isotope separation performances before and after hydrogenation,,and found that complete hydrides of group-ⅣA under certain pH and applied voltage conditions both can exist,and the separation performances after hydrogenation is better than that of origin ones;while hydrides of group-VA cannot exist except for the antimonene.Finally,based on the proton transport mechanism in the graphene and graphane system,we combined the three microscopic properties of electrochemical adaptability,kinetic isotope effect and proton transport barrier and screened out silicene as a hydrogen isotope separation material that is superior to graphene.