EFFECT OF ELECTROLYTE-COMPOSITION ON THE DYNAMICS OF HYDROGEN GAS BUBBLE EVOLUTION AT COPPER MICROELECTRODES

The dynamics of hydrogen gas bubble evolution at copper microelectrode s in H2SO4 and HCl solutions of various compositions have been studied by means of galvanostatic polarization experiments and simultaneous v ideo taping. As long as the solution contains acid only, gas evolution is dominated by the growth of a single bubble at the electrode at any one time. The transients in H2SO4 solutions exhibit regular sustained relaxation oscillations that can be distinctly related to events occu rring at the cathode. The electrode response in HCl solutions is somew hat noisier and shows much larger cathodic polarization and oscillatio n frequency and amplitude, presumably due to an interaction between Cl - and the copper surface. Additions of Na2SO4 and NaCl to these soluti ons promote the nucleation and growth of numerous small bubbles at any one time and the diminishing of the oscillations of the electrode pot ential. The addition of thiourea to 2.0 M H2SO4 does not destroy the s ingle bubble growth characteristic of acid-only solutions, but increas es the cathodic overpotential at all organic levels with a maximum occ urring at about 0.13 mM thiourea. Oscillation frequency and amplitude, and bubble departure diameter all show a similar dependence on thiour ea concentration to that of overpotential.