In Situ Activation of Cathodes for Hydrogen Evolution by Electrodeposition

In Situ Activation of Cathodes for Hydrogen Evolution by Electrodeposition 

The possibility of activation of the electrocatalysis for Hj evolution at various materials by introduction of depositable transition metal salts has been recognized for some time. Some practical applications refer to depolarization of amalgam electrodes in the old Hg cell chloralkali process. This procedure can be applied to various other substrates, for example, graphite, Fe, Ni steel, and Ti (164-167). 

In the case of codeposition of Co and Mo on Au and Fe electrodes, very strong synergistic effects of coplating of these two elements were found (168), with Tafel slopes of around 60 mV in the low current-density range being observed. With Ni and Mo coplating on Ni or Fe, slopes of 23-28 mV are observed (75). The Co plus Mo electrodeposits remain cathodically protected during the Hj evolution but on open circuit evolve H2 rapidly-from active Mo centers, as in decomposition of Raney nickels. However, this may be due as much to desorption of codeposited H as to evolution of H2 by corrosion of the base metal. Mo. 

Deactivation of the Co plus Mo cathodes occurred during the first 1000 s of cathodic electrolysis, an effect that was tentatively attributed to sorption of H into the composite lattice by diffusion. However (cf. Ref 75), because H is codeposited anyway during electroforming of these composite cathode materials, this may not be the critical reason for this effect other impurity effects may be involved. 

An interesting question arises concerning the nature of the electrolytic codeposition of Mo or Mo-containing species. It is well known that metallic Mo cannot be deposited by itself from aqueous solutions of Mo salts or molybdates since deposition is the preferred cathodic process at all potentials. However, evidently during the deposition of another transition metal such as Co or Ni, acting as a host lattice. Mo, and also W or V, can be codeposited. It is possible that the Mo species actually deposited is not free Mo metal but a lower oxide, MoO or MoOj, having quasi-metallic properties. However, this question has not yet been settled. 

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