Role of charged defects and impurities in kinetics of hydrogen storage materials: A first-principles study

We identify hydrogen-related point defects as a dominant defect species involved in (de)hydrogenation of sodium alanate, a viable hydrogen storage material. These defects are positively or negatively charged, and hence their formation energies are Fermi-level dependent-an important feature that has not been recognized in past studies. This dependence enables us to explain why small amounts of transition-metal additives drastically alter the kinetics of dehydrogenation. The rate-limiting step for hydrogen release is the creation of charged hydrogen-related defects, while transition-metal additives act as electrically active impurities that lower the formation energy of these defects.