A study of Parylene coated Pd/Mg nanoblabes for reversible hydrogen storage

Abstract Catalyst Pd decorated Mg nanoblades grown by oblique angle vapor deposition has been shown to possess low desorption temperature and fast kinetics in the dehydrogenation process. Using chemical vapor deposition, we coated these Pd/Mg nanoblades with a shell of ultrathin Parylene N layer that allows high permeation of hydrogen but low permeation of non-hydrogen gases in an attempt to protect the Pd/Mg nanoblades. The temperature programmed desorption experiments demonstrated that hydrogen can permeate through the Parylene shelled Pd/Mg nanoblades and is capable for reversible hydrogen cycling under a low dehydrogenation temperature of 403 K. Comparing the Pd/Mg nanoblades without Parylene coating to the Pd/Mg nanoblades with Parylene coating exhibit a comparable hydrogen capacity but a slower kinetics of hydrogen desorption. To understand the kinetics of hydrogen desorption, a modified Johnson–Mehi–Avrami (JMA) model was employed. From data fitting, we found that the experimental observation of a slowing-down kinetics can be interpreted in terms of simultaneously increased activation energy and impingement factor in Parylene coated samples. Oxidation characteristics under thermal annealing in ambient air between Parylene coated and no Parylene coated Mg nanoblades were studied by X-ray photoelectron spectroscopy. It shows that the increased amount of contaminations and oxygen near sample surface in Parylene coated samples are responsible for the slower kinetics of desorption. Our experimental work shows the feasibility that conformal Parylene coating of nanoscale Pd/Mg nanoblades can be used for reversible hydrogen storage.