Effects of Magnesium Content on Structure and Electrochemical Properties of La-Mg-Pr-Al-Mn-Co-Ni Hydrogen Storage Alloys

Received Oct 25, 2017; Accepted Dec 20, 2017

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1. Introduction

Over the past years, extensive research has been concentrated on the study of hydrogen storage alloys as a negative electrode of nickel-metal hydride (Ni/MH) secondary battery, as shown in recent reviews on the subject [1–5]. Commercial alloy systems for Ni/MH batteries are rare earth-based AB₅-type alloys, Ti- and Zr-based AB₂-type alloys, and, recently, A₂B₇- and AB₃-type RE-Mg-Ni-based superlattice alloys. Although representing the first generation of negative electrodes, the rare earth-based AB₅-type alloys (with discharge capacity limited to 320 mAh/g) are very popular in use on commercial Ni/MH batteries [1]. Nickel substitution in La-Ni-type electrodes of AB₅ system multicomponent metal hydride alloys has been widely reported, and the purpose of alloy modification is to improve the electrode performance. Aluminum, manganese, and/or cobalt are always present in Ni/MH electrodes. Lanthanum is frequently substituted for cerium, neodymium, and/or praseodymium. Magnesium has also been included in this type of hydrogen storage alloys as an element that can increase the number of hydrogen atoms stored per metal atom [6]. Mg additions vary from impurity levels to considerable atomic concentrations aiming to reduce costs (by reducing the amount of Co) and improving cyclic stability or durability of the Ni/MH batteries [7–11].

Reported microstructural investigations and chemical analyses for as-cast hydrogen storage alloys with Mg addition are very scarce. This paper addresses this aspect and reports the results of a study with partial and total substitution of La by Mg, on hydrogen storage La_0.7−xMg_xPr_0.3Al_0.3Mn_0.4Co_0.5Ni_3.8 as-cast alloys (x = 0.0, 0.1, 0.3, 0.5 and 0.7). A thorough investigation of the microstructures of these alloys and the phases present has been carried out using SEM/EDX and XRD. Moreover, it is well known that the Ni/MH batteries work in a strong oxidizing medium composed of high-concentration alkaline electrolyte. Therefore, among the desired properties of negative electrodes alloys, high corrosion resistance is essential for long cycle lifetime [6–13]. In the...