**1. Introduction**

Large scale energy storage is essential to increase the share of renewable energy in energy production. Power-to-X, and among them Power-to-gas technologies, could solve some of the problems related to the fluctuations in energy production from renewables, which is the main obstacle in their future implementation [1,2]. In Power-to-gas technologies, excess renewable energy is converted into hydrogen gas through PEM electrolysis or methane through methanation. This technology is already well developed and in connection with CCU technologies and can contribute to the reduction of CO<sup>2</sup> emissions [3–5]. Hydrogen can be injected into natural gas grids and existing infrastructure can be used for such purposes [6,7]. Nevertheless, hydrogen storage is much more challenging than storing natural gas. High mobility and lightness, as well as reactivity in the presence of microorganisms cause nontrivial hydrodynamic effects and cause safety concerns [6,8]. Large scale hydrogen storage was already proven in salt caverns. A vast amount of research and modelling of salt cavern behavior was made, including the salt rock permeability, sealing properties, and general geomechanics of the cavern. Salt rock remains impermeable for gas in the zones, where the rock properties are not affected by the extraction process. Micro fracturing processes during the salt rock formation are also responsible for the presence of damage zones in salt rock, where the gas impermeability may be limited [9]. An alternative to store hydrogen in salt caverns is Lined Rock Caverns (LRC) technology which was successfully proven in natural gas storage. This technology uses underground hard rocks to store natural gas. The reservoir is fully isolated from the outer environment. Hard rock is only a mechanical base, where the cavern is drilled. Base rock does not need to have

**Citation:** Gajda, D.; Luty ´nski, M. Hydrogen Permeability of Epoxy Composites as Liners in Lined Rock Caverns—Experimental Study. *Appl. Sci.* **2021**, *11*, 3885. https://doi.org/ 10.3390/app11093885

Received: 31 March 2021 Accepted: 23 April 2021 Published: 25 April 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

any isolating properties (like salt rock). Then, the shotcrete reinforcement, followed by the necessary installations, like drainage, are made. Final layer is the sealing lining. In order to ensure isolation and erosion, proven properties of sealing material are essential. A scheme of the LRC linings is shown in the Figure 1. In this technology, the most common lining material is steel [10,11]. It is justified to seek substitutional sealing materials beside stainless-steel, which will make the LRC storage more available and economical. One of the potential solutions is to use polymer-based concrete or epoxy resins, which are characterized by good sealing efficiencies. Research of gas permeability was done, including different methods, different polymer materials, and different gases. These studies include, inter alia, permeability of epoxy composites, using helium and CO<sup>2</sup> [12,13], permeability of N2, O2, CO2, and H2O through number of polymer materials (inter alia LDPE, HDPE, PVC, polypropylene) [14], and hydrogen permeability of high density polyethylene (HDPE) [15]. However, research related with hydrogen permeability through epoxy resins are very limited [16,17]. not need to have any isolating properties (like salt rock). Then, the shotcrete reinforcement, followed by the necessary installations, like drainage, are made. Final layer is the sealing lining. In order to ensure isolation and erosion, proven properties of sealing material are essential. A scheme of the LRC linings is shown in the Figure 1. In this technology, the most common lining material is steel [10,11]. It is justified to seek substitutional sealing materials beside stainless-steel, which will make the LRC storage more available and economical. One of the potential solutions is to use polymer-based concrete or epoxy resins, which are characterized by good sealing efficiencies. Research of gas permeability was done, including different methods, different polymer materials, and different gases. These studies include, inter alia, permeability of epoxy composites, using helium and CO<sup>2</sup> [12,13], permeability of N2, O2, CO2, and H2O through number of polymer materials (inter alia LDPE, HDPE, PVC, polypropylene) [14], and hydrogen permeability of high density polyethylene (HDPE) [15]. However, research related with hydrogen permeability through epoxy resins are very limited [16,17].

ogy which was successfully proven in natural gas storage. This technology uses underground hard rocks to store natural gas. The reservoir is fully isolated from the outer environment. Hard rock is only a mechanical base, where the cavern is drilled. Base rock does

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 2 of 12

**Figure 1.** Scheme of the LRC linings. **Figure 1.** Scheme of the LRC linings.

The purpose of this work is to compare hydrogen permeability of different materials, which could be used as the sealing liner for hydrogen storage in LRC with focus on epoxy resins modified with various additives. Additionally, permeability of typical rocks that can be found around LRC, and rock salt, were also measured in order to compare results with available data. In this work, a setup combining Steady-State Flow Method and Carrier Gas Method was used for experiments. Samples were also examined under SEM to compare their structural properties. The purpose of this work is to compare hydrogen permeability of different materials, which could be used as the sealing liner for hydrogen storage in LRC with focus on epoxy resins modified with various additives. Additionally, permeability of typical rocks that can be found around LRC, and rock salt, were also measured in order to compare results with available data. In this work, a setup combining Steady-State Flow Method and Carrier Gas Method was used for experiments. Samples were also examined under SEM to compare their structural properties.
