At what pressure will hydrogen start to liquefy at room temperature?

Question

I want to increase a fixed-size object's internal gas pressure by generating hydrogen in it, but I could not find the proper phase diagram for it. So I am wondering how high pressures I can get.

Answer 1

$\ce{H2}$ cannot be liquified at room temperature, whatever the pressure. Generally speaking, all gases can only be liquified when the temperature is under its critical value.

Answer 2

The critical temperature of Hydrogen is $\pu{32.938 K, resp. -240.21 ^{\circ}C}$. Above this temperature, it cannot be liquified.

So to answer your question, you can get as high pressure as you can produce and the container can withstand, as there is no condensation reducing the pressure.

WARNING: An accidental explosive container rupture can easily cause severe injuries or even death.

The industrial liquifying process involves:

• Cooling down with liquid nitrogen. The purpose is to reach temperatures below about $\pu{200 K}$ where Joule--Thomson-(Kelvin)-coefficient becomes positive, which enables cooling by Joule-Thompson effect via throttled gas expansion.
• Liquifying hydrogen by further cooling down by the Linde process ( Hampson-Linde cycle. For gases other than hydrogen and helium, this is possible even at room temperature ( neon is at the edge ).

Answer 3

As others have said, hydrogen cannot be liquified above its critical temperature, which my source (Wolfram Alpha chemical database*) says is $\pu{32.97 K} = \pu{-240.18 ^\circ C}$

However, with sufficient pressure, the molecules can be squeezed together until they have a liquid-like density**, and are thus no longer considered a gas, but rather a supercritical fluid. The pressure required to reach this point is called the critical pressure which, for hydrogen, is $\pu{1.239 MPa} = \pu{179.7 psi}$.

Hence, while you can't change hydrogen into a liquid at room temperature, you can change it into a (supercritical) fluid.

Incidentally, there are two established storage technologies for pure hydrogen in vehicles. One involves cooling the hydrogen below its critical temperature and liquifying it. The other stores it at ambient temperature, at pressures of $5\,000$ to $10\,000\ \mathrm{psi}$. In the ambient temperature cases they are not storing compressed hydrogen gas, but rather compressed supercritical hydrogen.

*Their sources are: https://www.wolframalpha.com/input/sources.jsp?sources=ChemicalData&sources=ElementData

**Achieving a liquid-like density is not the formal definition of the transition from gas to supercritical fluid, but it does, I think, paint a helpful physical picture.

Answer 4

Hydrogen can be stored at relatively high density (in some cases actually higher density than pure liquid hydrogen) at ambient pressure, by reversible formation of electropositive metal hydrides. One of the simplest such proposed storage compounds is [magnesium hydride]. Various Synthesis methods are reported by Wikipedia:

In 1951 preparation from the elements was first reported involving direct hydrogenation of Mg metal at high pressure and temperature (200 atmospheres, 500 °C) with MgI2 catalyst:[1]

$\ce{Mg + H2 -> MgH2}$

Lower temperature production from Mg and H2 using nano crystalline Mg produced in ball mills has been investigated.[2] Other preparations include:

• the hydrogenation of magnesium anthracene under mild conditions:[3]

$\ce{Mg(anthracene) + H2 -> MgH2}$

• the reaction of diethylmagnesium with lithium aluminium hydride[4]

• product of complexed $\ce{MgH2}$ e.g. $\ce{MgH2.THF}$ by the reaction of phenylsilane and dibutyl magnesium in ether or hydrocarbon solvents in the presence of THF or TMEDA as ligand.[5]

The first method with magnesium anthracene offers espcially good reversibility, as the magnesium anthracene can be formed under mild conditions from the metal and the anthracene recovered after reaction with hydrogen to form the hydride.

Cited References

1. Egon Wiberg, Heinz Goeltzer, Richard Bauer (1951). "Synthese von Magnesiumhydrid aus den Elementen (Synthesis of Magnesium Hydride from the Elements)" (PDF). Zeitschrift für Naturforschung B. 6b: 394.

2. Zaluska, A; Zaluski, L; Ström–Olsen, J.O (1999). "Nanocrystalline magnesium for hydrogen storage". Journal of Alloys and Compounds. 288 (1–2): 217–225. https://doi.org/10.1016/S0925-8388(99)00073-0.

3. Bogdanović, Borislav; Liao, Shih-Tsien; Schwickardi, Manfred; Sikorsky, Peter; Spliethoff, Bernd (1980). "Catalytic Synthesis of Magnesium Hydride under Mild Conditions". Angewandte Chemie International Edition in English. 19 (10): 818. https://doi.org/10.1002/anie.198008181.

4. Barbaras, Glenn D; Dillard, Clyde; Finholt, A. E; Wartik, Thomas; Wilzbach, K. E; Schlesinger, H. I (1951). "The Preparation of the Hydrides of Zinc, Cadmium, Beryllium, Magnesium and Lithium by the Use of Lithium Aluminum Hydride1". Journal of the American Chemical Society. 73 (10): 4585. doi:10.1021/ja01154a025.

5. Michalczyk, Michael J (1992). "Synthesis of magnesium hydride by the reaction of phenylsilane and dibutylmagnesium". Organometallics. 11 (6): 2307–2309. https://doi.org/10.1021/om00042a055.