The smallest electrochemical bubbles

Abstract

Many of the relevant electrochemical processes in the context of catalysis or energyconversion and storage, entail the production of gases. This often implicates thenucleation of bubbles at the interface, with the concomitant blockage of theelectroactive area leading to overpotentials and Ohmic drop. Nanoelectrodes have beenenvisioned as assets to revert this effect, by inhibiting bubble formation. Experimentsshow, however, that nanobubbles nucleate and attach to nanoscale electrodes, imposinga limit to the current, which turns out to be independent of size and applied potential ina wide range from 3 nm to tenths of microns. Here we investigate the potential-currentresponse for disk electrodes of diameters down to a single-atom, employing molecularsimulations including electrochemical generation of gas. Our analysis reveals thatnanoelectrodes of 1 nm can offer twice as much current as that delivered by electrodeswith areas four orders of magnitude larger at the same bias. This boost in the extractedcurrent is a consequence of the destabilization of the gas phase. The grand potentialof surface nanobubbles shows they can not reach a thermodynamically stable stateon supports below 2 nm. As a result, the electroactive area becomes accessible to thesolution and the current turns out to be sensitive to the electrode radius. In this way,our simulations establish that there is an optimal size for the nanoelectrodes, in betweenthe single-atom and ∼3 nm, that optimizes the gas production.