Effect of Tetraalkylammonium Cations on Gas Coalescence at a Hydrogen-Evolving Microelectrode

Abstract

Hydrogen gas evolution at the surface of a microelectrode may result in periodic release of single bubbles larger than the electrode diameter. Bubbles often grow by incorporating smaller bubbles that coalesce with them. To explore the coalescence, we investigate how a series of six tetralkylammonium cations (TXA(+)), where the number of carbons on the alkyl chain varies from 1 to 6, affects the oscillatory behavior of the gas-evolving microcathode. Different concentrations of TXA(+) bromide salts ranging from a few micromolar up to 1 M were added in the acid electrolyte. The frequency of bubble release and the transition from periodic to aperiodic release are related to the inhibition of bubble coalescence and gas streaming. The concentration range where this transition occurs depends strongly on the cation hydrophobicity and it ranges from very small values for the hydrophobic cations to over 1 M for the most hydrophilic one. For some of the TXA(+) cations, the transition shows a smooth increase in release frequency before switching completely to bubble-stream behavior, while for others the transition is abrupt. A smooth increase in the gas oscillator frequency with concentration indicates that the adsorption of TXA(+) cations on the bubble surface is mass transport-limited. The inhibition of bubble coalescence by the smallest cations is electrochemically driven, facilitated by specific interactions established between the ions and the electrode surface.

Gas Bubbles in Electrochemical Gas Evolution Reactions

“…The driving force for coalescence is the decrease in surface energy from the reduced total area of the gas–liquid interface. The coalescence process was found to depend on many factors, including the concentration of salts as well as the specific combination of cations and anions in the electrolyte. − With the development of advanced high-speed optical microscopy, the coalescence process has become more evident than before. As shown in Figure , the coalescence process starts by pushing away the liquid layer between the two bubbles, from which a thin liquid film is formed as a collapsing neck.…”

Solutal Marangoni effect determines bubble dynamics during electrocatalytic hydrogen evolution

“…40). A more gradual change in the gas bubble coalescence inhibition was reported with the addition of bulky tetraalkylammonium cations 41 . Although a number of explanations have been proposed concerning the origin of coalescence inhibition, there is still no definitive agreement on explaining the underlying mechanism of coalescence inhibition and the specific ion effects 42,43 .…”

Magnetically-Induced Flow during Electropolishing

“…These ions can inhibit bubble coalescence because they behave like surfactants, changing the surface tension at the gas/liquid interface depending on their concentration. 35 The magnitude of the charge, q, can be estimated from the circular paths that the bubbles follow in the 0.5 T field. The paths have radius R ≈ 3 mm and the bubble velocity, v ≈ 10 mm s −1 .…”