Chemistry - At what pressure will hydrogen start to liquefy at room temperature?
Solution 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.
Solution 2:
Hydrogen critical temperature 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.
Industrial liquifying process involves:
- Cooling down by liquid nitrogen. The purpose is to reach temperature below about $\pu{200 K}$ where Joule--Thomson-(Kelvin)-coefficient becomes positive, what 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 ).
Solution 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.