Hydrogen Permeability of TiN
x
Membrane Based on Bulk Mixed Hydride Ion and Electron Conductivity
Authors: Kura Chiharu , Aoki Yoshitaka , Zhu Chunyu , Tsuji Etsushi , Habazaki Hiroki 
Published: February 2020

Hydrogen is an ideal fuel resource, because it can be generated by water electrolysis by using renewable electricity from solar, window and water power generation. Hydrogen permeable metals such as Pd and group V meals (V, Nb, Ta) are promising as a hydrogen membrane to separate pure hydrogen from other process gases. Pd is most famous hydrogen membrane material, but it’s fiercely expensive and the resource is limited. Group V metals reveal excellent permeability, but they are ductile due to the fierce hydrogen embrittlement. Therefore, it is the motivation to develop the Pd-free hydrogen membranes which can work without hydrogen embrittlement. Recently, mixed proton and electron conductor (MPEC) oxides have attracted as new materials to use as hydrogen permeation membrane, and they have found applications in energy devices such as fuel cells and batteries. On the other hand, the conduction of hydride ions, H–, is also attractive. Recently, Verbraeken et al. reported barium hydride BaH2 exhibits fast H- ion conduction. Charge density on hydride ion is 105 times smaller than protons due to the difference of ionic radii so that bipolar diffusion of H- could be easier than H+. Unfortunately, the materials to show both hydride ion and electronic conduction are not reported.

In this work, we demonstrate the highly-nonstoichiometric TiN
x
membrane reveal efficient hydrogen permeability because of mixed H- ion electron conduction.
Highly-nonstoichiometric TiN
x
membranes (x = 0.71-1.02) were fabricated on a porous alumina support by RF reactive sputtering. The deposition was performed at the ubstrate temperatures of 500 oC by using Ti target (purity: 99.9%) under various nitrogen partial pressures. The morphologies and compositions of the membranes were analyzed by XRD, WDX and TEM. The hydrogen gas permeability was examined in the temperature range of 25 to 500 oC by using a homemade gas permeation test system equipped with a gas chromatography.

Highly-nonstoichiometric TiN
x
films (x = 0.71 -1.02) were successfully fabricated by reactive RF sputtering processes by adjusting the nitrogen partial pressure. From TEM image, densely-packed TiN
x
films with 600 nm thickness are uniformly formed over a wide area of the porous alumina support and apparent cracks and pinholes were not formed. TiN
x
membranes clearly revealed the selective permeation of hydrogen. Hydrogen fluxes of the membrane exhibit Arrhenius-type temperature dependence and these are at least two orders of magnitude higher than the nitrogen fluxes due to the leakage from gas sealing. Highly-N deficient TiN0.71 show larger hydrogen flux than TiN0.95. This result discloses that N-vacancy sites make an important role for hydrogen transport in TiN
x
.

In situ FT-IR spectra of TiN0.71 membranes exposed to hydrogen and deuterium atmosphere were measured in order to clarify the mobile hydrogen species. In H2 atmosphere, O-H, Ti-N and Ti-H stretching mode appeared. As switched from H2 to D2 atmosphere at 500°C a new sharp peak spontaneously develops at around 1200 cm-1, which is attributed to Ti-D stretching mode. These strongly suggest that H atoms bound to Ti atoms are diffusive in TiN
x
films so as to enable exchanging Ti-H bonding by Ti-D bonding while the H bound to oxygen are not mobile. Accordingly, it is concluded that the hydrogen permeability of TiN
x
film was caused by bulk conduction of mixed hydride ion and electron.