Microstructure and hydrogen absorption/desorption behavior of nanoporous Pd thin films

Nanoporous Pd (np-Pd) prepared from Pd-Ni alloy films on Si substrates was studied to understand hydriding/dehydriding processes in nanoscale Pd. Porous structures of the np-Pd thin films can be changed under different dealloying conditions. Stress measurement of the np-Pd showed that the np-Pd thin films can survive hydrogen pressures from 0 to 1 atm with no blistering, although this problem often occurs in dense Pd films in actual, high-pressure hydrogen environments. These tests indicated that hydrogen atoms can be stored in the np-Pd for much longer times than in fully dense Pd films subjected to ambient conditions. It is proposed that the stress distribution in np-Pd and the small pore size (<10 nm) inhibit hydrogen diffusion to free surfaces and thus prevent hydrogen degassing from the nanoporous structure. Moreover, phase transformation of Pd hydrides and effect of hydrogen trapping are also considered as possible reasons for the slow release of hydrogen.