Citation: Manna, J.; Huot, J. Effect of KCl Addition on First Hydrogenation Kinetics of TiFe. Compounds 2022, 2, 240–251. https://doi.org/10.3390/ compounds2040020 Academic Editor: Konda Gokuldoss Prashanth Received: 22 July 2022 Accepted: 28 September 2022 Published: 6 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). Article Effect of KCl Addition on First Hydrogenation Kinetics of TiFe Joydev Manna 1 and Jacques Huot 2, * 1 Hydrogen Energy Division, National Institute of Solar Energy, Gurugram 122003, India 2 Institut de Recherche sur L’hydrogène, Université du Québec à Trois-Rivières, 3351 des Forges, Trois-Rivieres, QC G9A 5H7, Canada * Correspondence: jacques.huot@irh.ca; Tel.: +1-819-376-5011 Abstract: In this paper, the effect of the addition of potassium chloride (KCl) by ball milling on the first hydrogenation kinetics of TiFe is reported. After milling, KCl was uniformly distributed on the TiFe’s surface. As-synthesized TiFe does not absorb hydrogen. However, after ball milling with KCl, it absorbed 1.5 wt.% of hydrogen on the first hydrogenation without any thermal treatment. The storage capacity of TiFe with KCl addition is higher than that of the ball milled pure TiFe. The effects of the amount of KCl additive in TiFe and ball milling time on first hydrogenation kinetics are reported. It is noted that, with an increase in KCl amount and ball milling time, hydrogenation kinetics are improved. However, hydrogen storage capacity decreased for both cases. Keywords: hydrogen storage; TiFe; KCl; ball milling; hydrogenation; kinetics 1. Introduction Intermetallic hydrides (IMHs), especially LaNi 5 and TiFe, are of great interest due to their reversible hydrogen absorption-desorption abilities [1]. In particular, titanium– iron (TiFe) intermetallic compound (IMC) is considered promising for hydrogen storage applications due to its abundance, low pyrophoricity, low costs and adequate reversible hydrogen storage capacity (~1.9 wt.%) at low pressures (1–2 MPa) and temperatures (30–70 ◦ C)[2–6]. TiFe’s crystal structure is CsCl-type and shows very fast reaction kinetics during hydrogen absorption–desorption cycles [7]. During the hydrogenation of TiFe, three phases, namely, TiFeH solid solution (α), FeTiH monohydride (β) and FeTiH 2 dihydride (γ), are formed [8]. However, TiFe IMC and its derivatives suffer from poor first hydrogenation (also called activation) performances and low poisoning tolerance in the presence of trace amounts of oxidative gases such as oxygen and water vapor. The first hydrogenation of TiFe samples prepared by conventional methods (e.g., arc-melting, induction melting, etc.) is a difficult and energy-intensive process [9–11]. This happens mainly due to the formation of a surface oxide layer of TiO 2 and/or Fe 2 O 3 during the synthesis process or air exposure of TiFe [12]. The surface oxide layer blocks the metal–hydrogen electron interactions and prevents the hydrogenation process. Mechanical processes such as ball milling, Equal Chanel Angular Pressing (ECAP), High-Pressure Torsion (HPT), and cold rolling (CR) are found to be effective on the activation of TiFe [13–19]. It is suggested that these processes can introduce non-equilibrium phase, a nanoscale structure, and active sites such as defects or grain boundaries that can facilitate the hydrogenation kinetics. On the other hand, partial substitution of the main components with other elements have also been considered as an alternative approach to improve the hydrogen storage properties of intermetallic compounds [20,21]. Presence of a third component such as Zr, V, Cr, Mn, Co, Ce, Nb or Y in TiFe could improve the first hydrogenation kinetics [22–28]. The effect of other additives such as Zr 7 Ni 10 , ZrMn 2, etc., on the activation of TiFe was also studied [29,30]. Room temperature oxidation of Mn-doped TiFe alloys by O 2 and H 2 O was studied by Shwartz et al. and they reported that TiO and TiO 2 formed after exposure [31]. Compounds 2022, 2, 240–251. https://doi.org/10.3390/compounds2040020 https://www.mdpi.com/journal/compounds