Notable hydrogen storage properties in nanocrystalline Al–Cr–Cu–Fe–Ni high entropy alloy

Abstract

The hydrogen storage mechanism of a single-phase nanocrystalline mechanically alloyed Al–Cr–Cu–Fe–Ni high-entropy alloy (HEA) was investigated in this study. The alloys were synthesized from the elemental powders using high-energy attritor ball mill with hexane as the process control agent. The material obtained after 40 h of milling was nanocrystalline and exhibited body-centered cubic (BCC) phase with a lattice parameter of 0.289 nm. The nanocrystalline Al–Cr–Cu–Fe–Ni HEA demonstrated remarkable hydrogen storage capacity at 300°C and 50 atm hydrogen pressure, absorbing 2.1wt% of hydrogen within 3 min and desorbing approximately 1.6wt% of hydrogen in 6 min. These rapid absorption and desorption processes highlighted the efficiency of the alloy for hydrogen uptake and release. Additionally, the alloy exhibited good cyclic stability, with a loss of only 0.2wt% of its hydrogen capacity across 25 cycles. The exceptional cycle stability and rapid kinetics of hydrogen storage and release make the nanocrystalline Al–Cr–Cu–Fe–Ni HEA a viable choice for hydrogen storage applications.

1. Introduction

The Sixth Assessment Report (AR6) from the Intergovernmental Panel on Climate Change highlights the remarkable shifts in the Earth’s climate caused by human-induced global warming, which has now reached 1.1°C above pre-industrial levels [1]. CO2 levels are at their highest point in two million years, glaciers are retreating at the fastest rate in two thousand years, and the past decade was the warmest in 125000 years. Further, sea levels are rising at the fastest rate in 3000 years, ocean acidity is at a 26000-year high, and the oceans are warming rapidly. Ecosystems and human populations are significantly affected by climate change, and the risk increases with rising temperatures [2]. The primary driver of global warming is the overuse of fossil fuels; CO2 emissions increased from 11 billion tonnes in 1960 to an estimated 36.6 billion tonnes last year [3–5]. Therefore, pollution-free renewable fuels are urgently needed. Although hydrogen is an abundant and renewable energy source, it is difficult to store and transport because of its flammability [6]. Solid-state storage, which is safer and less expensive, is a viable substitute for high-pressure cryogenic techniques [7–8].

Despite advances in hydrogen storage using metal hydrides, complex hydrides, metal-organic frameworks (MOFs), and carbon nanostructures, issues such as difficult synthesis, low reversibility, and poor cyclability persist [8]. Dai et al. [9] demonstrated enhanced hydrogen storage performance in MgH2 doped with 2.5wt% [email protected]. The composite absorbed 5.18wt % H2 at 423 K within 200 s and released hydrogen at a reduced temperature of 585 K, with a lower activation energy of 41.9 kJ/mol H2 due to in situ formed Mg2Ni/Mg2NiH4 and Zn/MgZn2 phases. Therefore, high-capacity, economical, safe, and reliable hydrogen storage may be made possible by combining metal hydrides with metals [10–11].

High-entropy alloys (HEAs), which consist of five or more principal elements in nearly equal proportions, represent a fascinating class of metallic materials that have recently garnered considerable attention owing to their potential for hydrogen storage applications [12–13]. The four primary factors associated with HEAs that determine efficient hydrogen storage are high entropy, sluggish diffusion, severe lattice distortion, and the cocktail effect [14]. A significant space is created for hydrogen storage because of the high-entropy effect, which enables the creation of a solid-solution phase [15–16]. Grain formation is facilitated by the slow diffusion effect, which improves the kinetics of hydrogen absorption and desorption [17]. Larger interstitial sites are produced by the extreme lattice distortion effect, which enhances the hydrogen storage capacity [18]. Finally, the physical and chemical characteristics of HEAs are influenced by the cocktail effect, which results from the combination of these three factors. Wu et al. [19] reported that HEAs are considerably superior to conventional metallic materials in terms of their mechanical and functional properties, particularly in extreme application environments. The idea of hydrogen storage in HEAs, Co–Fe–Mn–Ti–V–Zr alloy, was first presented in 2010 [20], which paved the way for further investigations into alloy composition optimization, especially for alloys that efficiently store hydrogen at ambient temperatures. The C14 Laves phase structure, which is advantageous for hydrogen storage because of its effective hydrogen absorption capacity, was demonstrated to be one of the first HEA hydrogen storage successes. At room temperature, the reversible hydrogen capacity of this alloy reached up to 1.6wt% [21]. The quick absorption and desorption kinetics, primarily...