(Allen J. Bard Award) The Electrochemical Nucleation and Physical Behavior of Hydrogen Nanobubbles

Gas nanobubbles formed at solid/liquid interfaces have received significant attention during the past decade due to their remarkable properties.  For instance, interfacial nanobubbles may exist for hours or days, in contrast to their expected short lifetime (< 1 s, based on the theoretical rate of gas dissolution from a nanoscale bubble).  We have developed an electrochemical approach for investigating the formation and properties of a single nanobubble of H₂ with a radius between 5 and 50 nm.  To create a H₂ nanobubble, a Pt nanodisk electrode, shrouded in a glass sheath, is used to reduce H⁺ in a concentrated acid solution, creating a supersaturated solution of H₂ adjacent to the electrode surface.  As the electrode potential is scanned towards negative potentials, the current arising from H₂ generation increases exponentially and then suddenly decreases to near background levels, signaling a liquid-to-gas phase transformation associated with the formation of a single nanobubble at the electrode surface. The critical concentration of dissolved H₂ required for nanobubble nucleation, obtained from the electrochemical response, is measured to be ~0.25 M, corresponding to ~310-fold supersaturation of H₂.   We estimate the critical size of the smallest stable H₂ gas bubble nucleus to be ~4 nm; this nucleus grows and covers the Pt nanodisk surface until it becomes pinned at the Pt/glass interface.  Thermodynamic relations indicate that the internal pressure within these nanobubbles is of the order of 100 atm. 

The nanobubble experiments also provide insight into the structure and chemical dynamics of electrochemical three-phase solid/liquid/gas boundaries.  We demonstrate that a steady-state residual current of ~100 pA, measured after bubble formation, results from continuous electrogeneration of H₂ at the Pt/electrolyte/H₂ gas interface; steady-state reduction of H⁺ is required to balanced the rate of H₂ dissolution from the nanobubble into the solution.  These experiments allow unique measurements of H⁺ reduction rates at a three-phase boundary, as well as estimation of the width of the three-phase boundary.  We also report additional experiments that demonstrate that the stability and shape of H₂ nanobubbles are sensitive to the applied pressure and to the presence of surfactants.  Finally, we show that individual H₂ bubbles can be electrochemically nucleated within nanopores.