Page 269 - Handbook of Battery Materials

239

9
Metal Hydride Electrodes

James J. Reilly

9.1
Introduction

Many metals and alloys reversibly absorb large quantities of hydrogen to form
metal hydride (MH) phases. In most cases the volumetric hydrogen density in
the hydride phase exceeds that of liquid hydrogen. While very few binary hydrides
(phases consisting of the elemental metal and hydrogen) are suitable for hydrogen
storage, many of them have properties that make them practical, convenient, and
safe energy storage media.
For many years the focus of MH research was on the storage of hydrogen for use
as a gaseous fuel, although the possibility of electrochemical applications was also
recognized. In about 1980 that focus dramatically shifted toward electrochemical
applications where a metal hydride (MH x ) electrode is used to replace the cadmium
electrode in Cd/Ni batteries. The driving force for such replacement was the
environmental problems associated with cadmium. An additional benefit was the
higher energy density of the MH x electrode. In the first edition this was concerned
primarily with the electrochemical, thermodynamic, and structural properties of
MHs that pertain to their use in Ni–MH x batteries.
In the late 1990s the research focus again shifted, this time toward lithium ion
batteries. In that same period the MH x –Ni battery became an article of commerce,
and consequently almost all government sponsored and academic research and
development shifted toward Li based batteries employing nonaqueous electrolytes.
Certainly research by private interests continues, but such efforts are considered
proprietary and any significant advances are not likely to be published. Thus the
original chapter appearing in the first edition has been little changed since, as
noted above, it was concerned with basic properties which are little changed.

9.2
Theory and Basic Principles
Many metals and alloys reversibly absorb large quantities of hydrogen to form
metal hydride (MH) phases, and for many years the focus of MH research was

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.