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 compoundsof the AB5-type without making use of highly electrocatalytic precious metals like Pd or Pt. These materials, denotedas AB5.5, consist of two different crystallographic phases: the bulk phase, still responsible for hydrogen storage, isformed by the corrosion-resistant multicomponent "standard alloy" based on LaNi5; and a second phase, homogeneouslydecorating the surface of the bulk-phase particles, provides for the extremely fast electrochemical hydrogen reaction. Thecomposition of the second-phase alloy is such that synergism in the electrocatalysis occurs. A simple metallurgicalmethod of producing double-phase materials is described. Various analytical techniques such as EPMA and x-ray diffractionare employed to characterize the solids produced. It is shown that the kinetics of the charge-transfer reaction can becharacterized electrochemically by the overall exchange current. In accordance with the Brewer-Engel theory, MoCo3 precipitatesare found to be highly electrocatalytic, which is reflected in an increase of the overall exchange current from190 mA · g–1 for the single-phase AB5 compound to 588 mA · g–1. As a consequence very high discharge efficiencies are accomplishedwith these MoCo3-based powder electrodes, even under extreme conditions: at 0°C the efficiency is improvedfrom 34 to 90%