Hydrogen Storage Alloys

With RE-Mg-Ni Based Negative Electrodes

Shumin Han, Yuan Li, Baozhong Liu

• Publisher: De Gruyter
• Year: 2017
• Topic: Technology & Engineering
• eBook ISBN:
  9783110498387
• Subtopic: Chemistry
• Index: Chemistry

The book presents current research progress on hydrogen storage alloys, with a special focus on their applications in batteries. Background, formation mechanisms, electrochemical characteristics, and effects of elemental substitution are covered. Provides an up-to-date overview of the theme for experienced researchers, while including enough fundamentals to serve as a handy, practical introduction for newcomers to the field.

1Introduction

1.1Overview of RE–Mg–Ni-Based Hydrogen Storage Alloys

Requirements for clean energy have become increasingly urgent with the depletion of fossil energy resources and growing environmental issues. Investigations show that nearly half of the oil resources are consumed by transportation vehicles which subsequently release one-quarter of total global CO2 [1–3]. The transformation of power sources from fossil fuels to clean energy is expected to relieve this situation. Nickel metal hydride (Ni/MH) secondary batteries are considered to be an ideal clean energy source due to the advantages of high-power output, high-rate capability, long lifetime, good reliability, non-toxicity, no memory effect and low cost [4].

The essential activity of electrochemical charge/ discharge is the hydrogen absorption/desorption behaviour of hydrogen storage alloys which depends on the types of main phases, microstructure, the nature and amount of each element in the intermetallic compound, and the electrochemical process taking place. In general, the discharge pressure of metal hydride should be between 0.1 and 1 atm at room temperature to ensure the complete utilization of the stored hydrogen in the alloys [5]. The amounts of hydrogen that can be absorbed and desorbed under ambient conditions determine the electrochemical capacity of a hydride electrode, and consequently, the energy storage capacity of a battery. For applications in Ni/MH rechargeable batteries, both high hydrogen capacity and moderate plateau pressure are expected from hydrogen storage alloys. Information about these properties can be described using pressure–composition (P–C) isotherms [3] as shown in Figure 1.1. The P–C curves of each alloy sample for hydrogen absorption and desorption contain three steps: (1) a ramping up step that is typical with the α-phase at low capacity, (2) a flat plateau corresponding to a two-phase co-existing period (α to β transformation) and (3) a final increase due to the presence of metal hydride β phase. The value of hydrogen absorption/desorption equilibrium plateau pressure is used to predict the electrochemical potential of hydrogen storage alloys, and the length of plateau is used to estimate the theoretical electrochemical capacity of hydrogen storage alloys in Ni/MH batteries through the following equations [3]:

$E=-0.9324-0.0291\times \log P_{\text{H}2}(\text{vs}\text{.Hg/HgO},6\text{mol}\text{KOH})(1.1)$

$C(\text{mAh}{\text{g}}^{\text{-1}})=6\times 26800\times \frac{{\text{(H/M)}}_{\text{5}}{\text{(H/M)}}_{\text{0}\text{.1}}}{\text{MW}}\left(1.2\right)$

where PH2 represents the dehydriding equilibrium pressure, (H/M)5 and (H/M)0.1 represent the hydrogen contents of alloy according to P–C–T curves at the pressures of 5 and 0.1 atm, respectively, and MW is the molecular weight of the hydrogen storage alloys. The bond energy in this pressure range can be calculated by the following equation:

Figure 1.1: Typical pressure–composition isotherm of hydrogen storage alloys.

$\ln P_{\text{H}}=-\frac{\Delta H}{RT}+\frac{\Delta S}{R}(1.3)$

where ΔH is the change in enthalpy, ΔS is the change in entropy, R is the gas constant and T is the absolute temperature.

The electrochemical reactions of hydrogen storage alloys are very simple. Hydrogen atoms are transformed between positive and negative electrodes, as shown in Figure 1.2. During charging, hydrogen atoms from hydrolysis are absorbed into alloy bulk to form hydrides, and during discharging hydrogen atoms from the decomposed hydrides are oxidized into water.

Figure 1.2: Charge/discharge diagram in Ni/MH batteries [1].

The electrochemical reactions are as follows:

$\text{Negative}:\text{M}+x{\text{H}}_{\text{2}}O+x{e}^{-}\underset{\text{discharge}}{\overset{\text{charge}}{\rightleftarrows }}{\text{MH}}_x+x{\text{OH}}^{-}\left(1.4\right)$

$\text{Positive}:{\text{Ni(OH)}}_2+{\text{OH}}^{-}\underset{\text{discharge}}{\overset{\text{charge}}{\rightleftarrows }}\text{NiOOH}+{\text{H}}_{\text{2}}\text{O}+e^{-}\left(1.5\right)$

$\text{Overall}\text{reaction}:x{\text{Ni(OH)}}_2+\text{M}\underset{\text{discharge}}{\overset{\text{charge}}{\rightleftarrows }}x\text{NiOOH}+{\text{MH}}_x\left(1.6\right)$

During the exploitation of Ni/MH batteries, novel negative electrode materials have always been the focus. In general, the development of the negative electrode materials for Ni/MH batteries has experienced three generations. The representative for the first generation is rare-earth-based AB5-type alloys. This alloy system with the advantages of easy activation and fast hydrogen desorption has been ...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• 1 Introduction
• 2 Preparation, Electrochemical Properties and Gaseous Hydrogen Storage Characteristics of the Single-Phase Superlattice RE–Mg–Ni-Based Hydrogen Storage Alloys
• 3 Effect of Multiphase Structures on Electrochemical Properties of the Superlattice RE–Mg–Ni-Based Hydrogen Storage Alloys
• 4 Effect of Element Composition on Microstructure and Electrochemical Characteristics of RE–Mg–Ni-Based Hydrogen Storage Alloys
• 5 Effect of Surface Treatment on Electrochemical Characteristics of RE–Mg–Ni-Based Hydrogen Storage Alloys
• 6 Outlook and Challenges of RE–Mg–Ni-Based Alloys as Negative Electrode Materials for Ni/MH Batteries
• Index