Abstract
Gas bubbles grown on solids are more than simple vehicles for gas transport. They are charged particles with surfaces populated with exchangeable ions. We here unveil a gateway for alkali metal ion transport between oxygen bubbles and semi-conducting (iron oxide) and conducting (gold) surfaces. This gateway was identified by electrochemical impedance spectroscopy using an ultramicroelectrode in direct contact with bubbles pinned onto these solid surfaces. We show that this gateway is naturally present at open circuit potentials, and that negative electric potentials applied through the solid enhance ion transport. In contrast, positive potentials or contact with an insulator (polytetrafluoroethylene) attenuates transport. We propose that this gateway is generated by overlapping electric double layers of bubbles and surfaces of contrasting (electro)chemical potentials. Knowledge of this ion transfer phenomenon is essential for understanding electric shielding and reaction overpotential caused by bubbles on catalysts. This has especially important ramifications for predicting processes including mineral flotation, microfluidics, pore water geochemistry, and fuel cell technology.
Highlights
Gas bubbles grown on solids are more than simple vehicles for gas transport
We reveal a gateway through which ions can transfer between the overlapping electric double layer (EDL) of gas bubble and conducting solid surfaces
The impedance (Z) of submillimeter-sized (~144–460 μm; Supplementary Fig. 2) oxygen bubbles pinned on hematite were measured with a UME by Electrochemical Impedance Spectroscopy (EIS)[36] (Figs. 1c, d and 2a)
Summary
Gas bubbles grown on solids are more than simple vehicles for gas transport. They are charged particles with surfaces populated with exchangeable ions. We propose that this gateway is generated by overlapping electric double layers of bubbles and surfaces of contrasting (electro)chemical potentials Knowledge of this ion transfer phenomenon is essential for understanding electric shielding and reaction overpotential caused by bubbles on catalysts. We reveal a gateway through which ions can transfer between the overlapping EDLs of gas bubble and (semi) conducting solid surfaces Harnessing this gateway could even offer new avenues in the study of gas-evolving reactions of natural and technological importance. Our primary focus on hematite (α-Fe2O3), as an oxygen evolution reaction catalyst, directly relates to emerging water-splitting technologies[31,32], and for the important roles it plays in the biogeochemical cycling of elements on Earth, and even on the geochemistry of planet Mars[33,34,35]
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