dual behaviour of hydrogen?

Answer 1

Hydrogen can act as a proton, or a hydride, an ion with a negative charge.

Hydrogen act as cations in acids. For example hydrochloric acid, etc. Or it can act as a hydride as in Lithium Hydride, etc.

Answer 2

Here's a website about the dual behavior of hydrogen at Si -- SiO2 interfaces. There's an introductory paragraph that tells about it.

http://www.physics.vanderbilt.edu/pantelides/APL81-1839.pdf