Electrocatalysis of hydrogen evolution in the light of the brewer—engel theory for bonding in metals and intermetallic phases

A comprehensive survey of achievements in electrocatalysis for the hydrogen evolution reaction (h.e.r.) has been reviewed with the main emphasis on the composite d-metal catalysts. The basic concept of the Brewer—Engel valence-bond theory that relates the electronic state of highest multiplicity which corresponds to the electronic configuration of lowest energy level or the structure of atoms, and corresponding phase structure both in individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer—Engel model it has been pointed out that whenever metals of the left-half of the transition series that have empty or half filled vacant d-orbitals are alloyed with metals of the right-half of the transition series that have internally paired d-electrons not available for bonding in the pure metal, there arises well pronounced synergism in electrocatalysis for the h.e.r., which often exceeds individual effects of precious metals and each other (the synergism condition) and approaches the reversible behaviour within the wide range of current density. It has been inferred that the upmost electrocatalytic activity reach the composite d-metal catalysts of improved d-orbital overlap in intermetallic phases of highest symmetry and minimal entropy such as Cr3Si(A15) type like MoCo3, WNi3, MoNi3, LaNi5, that the Brewer—Engel theory predicts for the most stable systems.