Electrochemistry of single nanobubbles. Estimating the critical size of bubble-forming nuclei for gas-evolving electrode reactions

Abstract

In this article, we address the fundamental question: "What is the critical size of a single cluster of gas molecules that grows and becomes a stable (or continuously growing) gas bubble during gas evolving reactions?" Electrochemical reactions that produce dissolved gas molecules are ubiquitous in electrochemical technologies, e.g., water electrolysis, photoelectrochemistry, chlorine production, corrosion, and often lead to the formation of gaseous bubbles. Herein, we demonstrate that electrochemical measurements of the dissolved gas concentration, at the instant prior to nucleation of an individual nanobubble of H, N, or O at a Pt nanodisk electrode, can be analyzed using classical thermodynamic relationships (Henry's law and the Young-Laplace equation - including non-ideal corrections) to provide an estimate of the size of the gas bubble nucleus that grows into a stable bubble. We further demonstrate that this critical nucleus size is independent of the radius of the Pt nanodisk employed (<100 nm radius), and weakly dependent on the nature of the gas. For example, the measured critical surface concentration of H of ∼0.23 M at the instant of bubble formation corresponds to a critical H nucleus that has a radius of ∼3.6 nm, an internal pressure of ∼350 atm, and contains ∼1700 H molecules. The data are consistent with stochastic fluctuations in the density of dissolved gas, at or near the Pt/solution interface, controlling the rate of bubble nucleation. We discuss the growth of the nucleus as a diffusion-limited process and how that process is affected by proximity to an electrode producing ∼10 gas molecules per second. Our study demonstrates the advantages of studying a single-entity, i.e., an individual nanobubble, in understanding and quantifying complex physicochemical phenomena.

The Nucleation Rate of Single O2 Nanobubbles at Pt Nanoelectrodes

“… 21 The electrode apparent radius a is also affected during the application of the conditioning cycles (section 1 in the Supporting Information ); consequently, i nb p may be affected. 21 , 22 , 27 Figure 3 shows several cyclic voltammograms recorded at the same electrode, displaying a different i nb p in every cycle. However, by reducing the scan range or applying a “conditioning cycle”, a reproducible electrode surface and a constant apparent radius, and therefore a consistent i nb p , can be achieved.…”

Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors

“…53 This implies that nucleation is defined as the event in which a critical nucleus size of the gas bubble, rcrit, is formed. 52 The sum of the energy required to form a new interface and the energy gained as dissolved gas is transferred to the newly created phase, is defined as the free energy of formation of a gas nucleus in solution, ΔGtot. Figure 2(right) shows ΔGtot as a function of the e's r s, rnb.…”

Intermittent photocatalytic activity of single CdS nanoparticles

“…Once the photocatalytic activity stopped, the nanobubble disappeared rapidly due to the high Laplace pressure (31,32), i.e., the hydrogen molecules dissolved in the solution and diffused away from the nanoparticles, leading to the recovery of the local RI. The stop of photocatalytic activity could not be attributed to the surface blockade by nanobubble.…”