Advances in electrocatalysis for hydrogen evolution in the light of the Brewer-Engel valence-bond theory

Abstract

The Brewer-Engel valence-bond theory for bonding in metals and intermetallic phases has been employed to correlate the electrocatalytic features of both individual and composite transition metal catalysts for the hydrogen evolution reaction (h.e.r.). The basic concept of the Brewer-Engel valence-bond theory, which relates the electronic state of highest multiplicity that corresponds to the electronic configuration of lowest energy level or the structure of atoms, with the corresponding phase structure in both individual metallic and multicomponent intermetallic systems, is also given. On the basis of the Brewer intermetallic bonding model as a generalized Lewis acid-base reaction, it is pointed out that whenever metals of the left half of the transition series, having empty or half-filled vacant d-orbitals, are alloyed with metals of the right half of the transition series having internally paired d-electrons not available for bonding in the pure metal, which proceeds with definite charge transfer, there arises a well-pronounced synergism in electrocatalysis for the h.e.r., which often exceeds the individual catalytic effects of precious metals by themselves or in combination (the synergism condition) and approaches reversible behaviour within a wide range of current density. It is inferred that the maximum electrocatalytic activity extends to the composite d-metal catalysts of improved d-orbital overlap in intermetallic phases of highest symmetry and minimal entropy, such as Laves phases and A 15 or Cr 3Si types such as MoCo 3, WNi 3, VNi 3, HfPd 3, ZrPt 3, LaNi 5, HfPt 3, that the Brewer theory for intermetallic bonding predicts for the most stable systems. The theoretical explanation for an optimal (d 8) electronic configuration in both individual and composite synergetic electrocatalytic systems is given and compared with similar catalytic processes. The induced hydridic feature of high activity synergetic transition metal composite electrocatalysts is also pointed out.