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 AB_5.5, 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 ; 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, 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 powder electrodes, even under extreme conditions: at 0°C the efficiency is improved from 34 to 90%.