**5. Conclusions**

Hydrogen permeability of investigated epoxy resins are similar to the permeability of salt rock in certain pressure conditions (after the salt creep). However, storage of pressurized gas in salt caverns is causing that kind of constant pressure anyway, which will lead to drop of salt permeability in time. Obtained results of epoxies are very promising for the purpose of use as the sealing liners in Lined Rock Caverns (LRC) underground gas storage, particularly for the storage of hydrogen. Epoxy resin can be used as a substitution of stainless-steel, because of the satisfying mechanical properties and extraordinary adhesive properties, which will be reinforcing the general construction, and thanks to the low density, the construction will not be overloaded at the same time. Despite the orders of magnitude higher of hydrogen permeability of epoxies, the hydrogen loss through this material is still acceptable, compared to the steel. For example, using a 3 cm thick epoxy liner in storing the 20/80 hydrogen/methane blend at 1.0 MPa, the loss of hydrogen in the period of 30 days is only approximately 0.003–0.005% [28]. It is fully acceptable, taking the other properties and economy of epoxies. Only applying the liquid epoxy on the surface of cavern may cause some technical problems. Despite the better sealing properties of stainless-steel, the

hydrogen permeability coefficient of investigated epoxy resin is still satisfying. Epoxies are also less susceptible to hydrogen erosion, which is making them a suitable material for hydrogen storage and cost-effective substitution for steel.

**Author Contributions:** Conceptualization D.G. and M.L., methodology D.G., investigation D.G., data curation D.G., formal analysis D.G. and M.L., writing—original draft preparation D.G., writing review and editing M.L., supervision M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Own Scholarship Fund of the Silesian University of Technology in year 2019/2020, grant number: 19/FSW18/0003-03/2019.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All analyzed data in this study has been included in the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
