One important material for hydrogen storage is lithium imide (Li2NH) because it can reversibly absorb as much as 6.5wt% hydrogen, via the reaction Li2NH + H2 = LiNH2 + LiH. However, its hydrogenation and dehydrogenation temperatures are currently too high, about 200C versus 80C required for transportation applications. The long term goal of this project is to develop high hydrogen storage capacity materials via doping Li2NH by anions. The PI hypothesizes that anions doping can be used to tune structures and properties of ionic compounds (Li2NH and LiNH2), which are key factors controlling hydrogen storage reactions. The PI bases his hypothesis on the observations that 1) anion/cation interactions dramatically affect catalyst performance, reaction kinetics, and reaction selectivity, 2) anion can selectively modify the properties of solid materials, 3) different Cl= containing promoters exhibit the same promoting effect on the dehydrogenation of LiNH2/LiH, which is clearly an indication of anion effects, and 4) Li2NH with O= ion showed much better hydrogen storage performance than that without O= ion. The specific aims of this experimental work are to: 1) correlate the effects of doping-anions on the hydrogen storage reactions of Li2NH with their intrinsic properties; 2) evaluate how anions doping affects the structures and properties of Li2NH and LiNH2; and 3) examine the effects of anions doping on the intermediate species of hydrogen storage reactions of Li2NH.
This research has significant intellectual merit. Although metals and cations were widely used as promoters, anions have not yet been recognized as effective components to promote the hydrogen storage materials. Knowledge gained from this project on how anions doping affect the structures and properties of hydrogen storage materials could provide a new approach for developing new hydrogen storage materials. This project also has multiple broader impacts. The highly effective storage materials developed in this research can lead to low cost hydrogen storage materials which will impact the commercial feasibility of fuel cell vehicles, thus reducing the requirement of oil. This research has also strong impacts on the education of students. A "summer institute in hydrogen energy" program will be created. This program will promote the knowledge and skills of hydrogen energy science and engineering into the high school science classroom via training high school teachers. This project will train one graduate and one undergraduate student in this area. In addition, the PI is hoping to use this project to recruit under-represented (female) high school students as summer interns to his research group.
Research activities in this project produced important scientific findings and educational achievements. They are summarized as follows: Scientific finding highlights: The decomposition of LiNH2 is a critical step for hydrogen storage in Li-N based materials. This research revealed that the dehydrogenation of LiNH2 into Li2NH took place via two steps (LiNH2 to Li1.5NH1.5 and then to Li2NH). However, when Cl- anion was introduced into LiNH2, its dehydrogenation to Li2NH became one step. This indicates that Cl- can improve the decomposition of LiNH2 into Li2NH. Furthermore, it was demonstrated that the dehydrogenation of Li2NH first generated Li4NH, followed by decomposition to Li without Li3N formation. The dehydrogenation temperatures of LiNH2 and Li2NH can be decreased by doping anions (F-, Cl-, and O2-). In addition, interesting results were also obtained for the decomposition, hydrogenation, and stability of various lithium nitride halides (such as Li13N4Br, Li7N2I, and Li9N2Cl3). A reverse phase transformation of Li3N from β to α was revealed. Furthermore, it was found that β-Li3N can react with H2O in air faster than α-Li3N, indicating that α-Li3N is more stable than β-Li3N in air. In addition, a novel method was developed for the first experimental determination of β-Li3N energy gap (2.15eV). The high reactivity of Li3N to CO2 was revealed. Furthermore, this fast reaction provides a novel approach to convert CO2 to two novel solid materials: crystal lithium cyanamide (Li2CN2) and amorphous carbon nitrides (CxNy). The reaction between Li2O and CO, which was invented in this project, constitutes a simple and efficient approach to synthesize graphene with 3D honeycomb-like shape. Furthermore, the dye-sensitized solar cell (DSSC) with the honeycomb-structured graphene counter electrode exhibited energy conversion efficiency as high as 7.8%, which is even comparable to that of DSSCs with an expensive Pt counter electrode. Educational highlights: 7 graduate students and 5 undergraduate students were trained by this project. As a result, they obtained abilities and skills for (a) material synthesis and characterization and (b) data analysis. Furthermore, they obtained ability, skills, and experience to prepare draft versions of manuscripts for publication in peer-reviewed journals and presenting results in national professional conferences. The PI’s lab has trained SYP (Summer Youth Program) students with this project. This type of outreach would be the effective effort to create the interests of high school students in sustainable energy science and engineering. PI introduced the results obtained from this project into a graduate-level course "Materials for Energy Application" in MTU and presentations for middle school and high school science teachers in the ASM Materials Summer Camp.
