An Optical Study of Cathodic Hydrogen Evolution in High-Rate Electrolysis

Abstract

Hydrogen bubbles evolved cathodically under conditions encountered in electrochemical machining have been studied by stop‐motion photography. Constant current densities up to 150 A/cm2 and flow rates up to 2500 cm/sec have been employed with an experimental flow channel of 0.5 mm gap width. The observed bubble size decreased strongly with increasing flow rate and increased with increasing current density. At flow rates above 800 cm/sec, the bubble size was always below 20µ, the smallest diameter resolved by the optical arrangement used. Less gas was evolved in nitrate than in chloride electrolytes under otherwise identical conditions. The hydrogen bubbles were usually confirmed to a region near the cathode. Voltage oscillations and electric breakdown coincided with the appearance of a new type of bubble.

Gas-Evolving Electrodes

“…(73) Our present knowledge is still unsatisfactory in predicting the size of departure diameters. At metal or graphite electrodes, faced horizontally upward or vertical, d is of the order of 50 ~m, (30,4S,53,74-79) depending on the polarity of the electrode (53,56,SO) and the nature of the electrolyte solution (SO,S1) and its concentration at the electrode (SO) which in turn is influenced by the current density (30,75) and the liquid bulk flow velocity,(S2) Poorly wettable surfaces as PTFE-coated electrodes (77) or electrodes facing downward (70) show much larger diameters.…”

The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes

“…Assuming a first order reaction, the rate of the chemical step is given by The hypothetical mass transfer coefficient K1 was calculated for three different rotation speeds (20,50 and 130r.p.m.) and different current densities using Equation 11.…”

“…The direction and magnitude of natural convection and its variation with current density is difficult to assess owing to the lack of information on the density change in relation to the transport properties of some of the ionic species involved in the reaction at the cathode. Apart from the enhancing effect of cylinder rotation on the mass transfer coefficient Kt at relatively low speeds, cylinder rotation increases the rate of Cr 6+ reduction through decreasing the degree of cathode coverage by hydrogen bubbles and the consequent increase in the effective cathode area [20,21]. However, it seems that this effect is modest; according to Leistra and Sides [22] who measured the degree of surface coverage by hydrogen bubbles in 1 M HzSO 4 at cylindrical electrodes the degree of surface coverage does not exceed 3%.…”

In situ observation of bubble traces in nickel electrodeposition

“…We constructed an in situ observation device by introducing a microscopic optical system into the setup used for electrodeposition. Ni electrodeposition was performed at room temperature of approximately 25 ℃, the temperature used in high precision electroforming 12) . As reported in this paper, we successfully observed the birth, growth and separation of bubbles in detail.…”