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### Topic: Max mass of hydrogen in Nickel Hydride  (Read 2624 times)

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#### BTE-Dan

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##### Max mass of hydrogen in Nickel Hydride
« on: May 22, 2017, 10:10:52 PM »
I'm trying to derive what the max mass of hydrogen that a Nickel Hydride can hold.

Wikipedia says: "Hydrogen's content in nickel hydride is up to 0.002% by weight."

But it also says: "Hydrogen to nickel atomic ratios are up to one, with hydrogen occupying an octahedral site."

So with an atomic ratio of one, you would expect:

Ni atomic mass = 58.6934
H atomic mass = 1.00794
Max % mass of H in NiH = 1.00794 / (1.00794+58.6934) x 100 = 1.69%.

But 1.69% is a long way from .002%. Anyone have an explanation?

#### Enthalpy

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##### Re: Max mass of hydrogen in Nickel Hydride
« Reply #1 on: May 23, 2017, 07:18:31 PM »
Welcome, BTE-Dan!

"Hydrogen to nickel atomic ratios are up to one"
occurs in the paragraph
"high pressure hydrogen gas at 600 MPa. Alternatively [a surface layer] can be produced electrolytically."

So under normal conditions, the answer is
"0.00005% at 25 °C"
which is the kind of figure expected from most normal materials. If 1 mol Ni could absorb 1 mol H, we would all have fuel cell cars already.

By the way, the Wiki article is written by a metallurgist, but he probably forgot to tell that hydrogen as a hardening agent is impractical because it outgasses too easily. At least steel (nickel should be similar) is freed from hydrogen (which embrittles it) by just 10min at 180°C in a kitchen oven.

#### BTE-Dan

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##### Re: Max mass of hydrogen in Nickel Hydride
« Reply #2 on: May 23, 2017, 10:00:36 PM »
Thanks Enthalpy, that all makes sense. Nice analogy about the cars.

Can you say how you arrived at "0.00005% at 25 °C"?

#### Enthalpy

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##### Re: Max mass of hydrogen in Nickel Hydride
« Reply #3 on: May 24, 2017, 11:12:50 AM »
Wiki.