Developing Ideal Metalorganic Hydrides for Hydrogen Storage: From Theoretical Prediction to Rational Fabrication

Materials for hydrogen storage have been extensively explored for a few decades. Thousands of materials have been synthesized and tested; however, few systems could meet the practical requirements. Metalorganic hydrides discovered recently offer new opportunities. It is, however, extremely time-consuming and inefficient to experimentally screen potential materials from a large variety of metal cations and organic anions. In the present study, we performed wide-ranging theoretical predictions and screened more than 90 metalorganic hydrides; 20 of them were identified with both high hydrogen capacities (≥5 wt %) and suitable thermodynamics (heat of H2 desorption: 25–35 kJ/mol-H2) that allow hydrogen uptake and release near ambient condition. We picked up four of them, i.e., Li/Na indolides and 7-azaindolides, for experimental validation. All the four candidates can be easily synthesized via the reactions between the metal hydrides and the corresponding organic precursors. Among them the structures of lithium indolide (P21/c (no. 14)) and sodium 7-azaindolide (P4̅21c (no. 114) were successfully resolved. Photoelectron spectroscopy and quantum chemical calculations confirmed that the charge density of the organic ring increased with the introduction of an alkali metal, thus optimizing their ΔHd for hydrogen storage. Importantly, we demonstrated experimentally that lithium indolide with a theoretical hydrogen capacity of 6.1 wt % has a heat of hydrogen absorption of ca. 33.7 kJ/mol-H2 which is in excellent agreement with the value (33.6 kJ/mol-H2) predicted theoretically. The partially reversible hydrogen release and uptake can be can be achieved at the temperature as low as 100 °C. These results illustrate the great potential of metalorganic hydrides for tackling the grand challenge of hydrogen storage and manifest the effectiveness of theoretical prediction in guiding materials fabrication.