Electrochemical hydrogen reactions on tungsten carbide

The kinetics of hydrogen ionisation and electrochemical evolution on a smooth rotating tungsten carbide electrode have been investigated. Polarisation curves in 1 N H₂SO₄, H₃PO₄ and HCl solutions, in the temperature range of 20?80°C, have been obtained. For hydrogen ionisation at ?_r=0·3 V the apparent activation energies (Kcal/moles) were 8·1 in H₂SO₄, 7·4 in H₃PO₄ and 12·8 in HCl. At potentials of ?_r>0·4V a decrease in the hydrogen ionisation rate was observed, explained as a decrease in the number of active surface sites due to surface oxidation and adsorption of anions. However, after a moderate anodic polarisation not leading to the formation of a thick tungsten carbide oxides layer, reiterated recording of the polarisation curve yields an increase in electrode activity.In all acids the first order of reaction with respect to molecular hydrogen was found. The hydrogen ionisation rate was independent of the pH-value of the solution. These results showed adsorption, accompanied by molecular hydrogen dissociation, to be the rate-determining step.Modifying the concentration of sulphuric acid in the presence of excess normal salt or without it, as well as passing from acidic to alkaline solutions caused no change in the course of the polarisation curves. This suggests that recombination of adsorbed hydrogen atoms into molecules on tungsten carbide is the rate determinating step, ie, the same step as in the reverse process of hydrogen ionisation.