Citation: Grigorova, E.; Markov,P.; Tsyntsarski, B.; Tzvetkov,P.; Stoycheva, I. Hydrogen Storage Properties of Ball Milled MgH 2 with Additives- Ni, V and Activated Carbons Obtained from Different By-Products. Materials 2023, 16, 6823. https://doi.org/10.3390/ ma16206823 Academic Editor: Ting Zhang Received: 29 September 2023 Revised: 12 October 2023 Accepted: 20 October 2023 Published: 23 October 2023 Copyright: © 2023 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/). materials Article Hydrogen Storage Properties of Ball Milled MgH 2 with Additives- Ni, V and Activated Carbons Obtained from Different By-Products Eli Grigorova 1, *, Pavel Markov 1 , Boyko Tsyntsarski 2 , Peter Tzvetkov 1 and Ivanka Stoycheva 2 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bld. 11, 1113 Sofia, Bulgaria; pvlmarkov@svr.igic.bas.bg (P.M.); ptzvetkov@svr.igic.bas.bg (P.T.) 2 Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bld. 9, 1113 Sofia, Bulgaria; boiko_sf@yahoo.com (B.T.); ivanka.stoycheva@orgchm.bas.bg (I.S.) * Correspondence: egeorg@svr.igic.bas.bg Abstract: The hydrogen sorption of materials based on 80 wt.% MgH 2 with the addition of 15 wt.% Ni or V and 5 wt.% activated carbons synthesized from polyolefin wax, a waste product from polyethylene production (POW), walnut shells (CAN), and peach stones (CPS) prepared by milling under an inert Ar atmosphere for a period of 1 h, is investigated. All precursors are submitted to pyrolysis followed by steam activation in order to obtain the activated carbons. The hydrogen sorption evaluations are carried out for absorption at 473 and 573 K with pressure of 1 MPa and for desorption at 623 and 573 K with pressure of 0.15 MPa. The composition of the samples after milling and hydrogenation is monitored by X-ray diffraction analyses. The 80 wt.% MgH 2 –15 wt. %Ni–5 wt.% POW or CAN after absorption–desorption cycling and in a hydrogenated state at 573 K and 1 MPa are analyzed by TEM. Keywords: sorption; hydrogen storage; metal hydrides; carbon materials 1. Introduction Materials based on solid-state hydrogen storage include adsorbents, liquid organ- ics, complex and interstitial hydrides as well as chemical hydrogen. Each of them has advantages and disadvantages. Some of these materials have higher capacity but poor reversibility, and some have an elevated explosive risk such as, for example, in the case of complex hydrides. Other materials such as ammonia store hydrogen in an irreversible way, and their toxicity and recycling problems are very important. The highly porous adsorbents such as carbon nanotubes and MOFs adsorb hydrogen at a temperature below 100 K and high pressure [1]. Hydrogen storage in the form of metal hydrides provides important safety and high energy density advantages over the gas and liquid storage methods. Among a lot of metals and intermetallics that are capable to react with hydrogen in a reversible way magnesium–based materials as hydrogen storage media are more per- spective. There has been intensive research regarding their hydrogen storage application due to the high theoretical hydrogen absorption capacity of magnesium (7.6 wt.%), very good reversibility, abundance, and low cost. These materials have also some fallibilities, such as slow kinetics, necessity of activation, and increased sorption temperatures, es- pecially for the desorption process. Using a variety of dopants combined with milling in a planetary or vibratory mill leads to a decrease in hydrogen sorption temperatures and enhanced kinetics, and in general, to an improvement in the hydrogen absorption– desorption properties. Depending on the nature of the additives, their catalytic effect is different. Some of the additives can easily form hydrides; especially non-stoichiometric and others like iron, for example, act as active sites during the dissociative chemisorption of hydrogen. The amount of additives should be carefully balanced. A larger amount Materials 2023, 16, 6823. https://doi.org/10.3390/ma16206823 https://www.mdpi.com/journal/materials