DOUBLE-PHASE HYDRIDE FORMING COMPOUNDS - A NEW CLASS OF HIGHLY ELECTROCATALYTIC MATERIALS

A new class of materials is proposed to improve the electrocatalytic activity of hydride forming intermetallic compounds of the AB5-type without making use of highly electrocatalytic precious metals like Pd or Pt. These materials, denoted as AB5.5, consist of two different crystallographic phases: the bulk phase, still responsible for hydrogen storage, is formed by the corrosion-resistant multicomponent "standard alloy" based on LaNi5; and a second phase, homogeneously decorating the surface of the bulk-phase particles, provides for the extremely fast electrochemical hydrogen reaction. The composition of the second-phase alloy is such that synergism in the electrocatalysis occurs. A simple metallurgical method of producing double-phase materials is described. Various analytical techniques such as EPMA and x-ray diffraction are employed to characterize the solids produced. It is shown that the kinetics of the charge-transfer reaction can be characterized electrochemically by the overall exchange current. In accordance with the Brewer-Engel theory, MoCo3 precipitates are found to be highly electrocatalytic, which is reflected in an increase of the overall exchange current from 190 mA.g-1 for the single-phase AB5 compound to 588 mA.g-1. As a consequence very high discharge efficiencies are accomplished with these MoCo3-based powder electrodes, even under extreme conditions: at 0-degrees-C the efficiency is improved from 34 to 90%.