| Autor: |
White JL; Sandia National Laboratories , Livermore , California 94550 , United States., Rowberg AJE; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States.; University of California Santa Barbara , Santa Barbara , California 93106 , United States., Wan LF; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States.; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Kang S; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States., Ogitsu T; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States., Kolasinski RD; Sandia National Laboratories , Livermore , California 94550 , United States., Whaley JA; Sandia National Laboratories , Livermore , California 94550 , United States., Baker AA; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States., Lee JRI; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States., Liu YS; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Trotochaud L; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Guo J; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Stavila V; Sandia National Laboratories , Livermore , California 94550 , United States., Prendergast D; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Bluhm H; Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States., Allendorf MD; Sandia National Laboratories , Livermore , California 94550 , United States., Wood BC; Lawrence Livermore National Laboratory , Livermore , California 94550 , United States., El Gabaly F; Sandia National Laboratories , Livermore , California 94550 , United States. |
| Abstrakt: |
Solid-state metal hydrides are prime candidates to replace compressed hydrogen for fuel cell vehicles due to their high volumetric capacities. Sodium aluminum hydride has long been studied as an archetype for higher-capacity metal hydrides, with improved reversibility demonstrated through the addition of titanium catalysts; however, atomistic mechanisms for surface processes, including hydrogen desorption, are still uncertain. Here, operando and ex situ measurements from a suite of diagnostic tools probing multiple length scales are combined with ab initio simulations to provide a detailed and unbiased view of the evolution of the surface chemistry during hydrogen release. In contrast to some previously proposed mechanisms, the titanium dopant does not directly facilitate desorption at the surface. Instead, oxidized surface species, even on well-protected NaAlH 4 samples, evolve during dehydrogenation to form surface hydroxides with differing levels of hydrogen saturation. Additionally, the presence of these oxidized species leads to considerably lower computed barriers for H 2 formation compared to pristine hydride surfaces, suggesting that oxygen may actively participate in hydrogen release, rather than merely inhibiting diffusion as is commonly presumed. These results demonstrate how close experiment-theory feedback can elucidate mechanistic understanding of complex metal hydride chemistry and potentially impactful roles of unavoidable surface impurities. |
