Is hydrogen gas in water present as bubbles or hydrated form?

Abstract

a b s t r a c tWhen hydrogen gas is bubbled into water, it may well be present as stabilized bubbles rather than hydrated hydrogen molecules, as in the spontaneous emulsification at oil|water interfaces without surfactant. On this prediction, we used dynamic light scattering (DLS) to find bubbles 0.4-0.5 lm in diameter, which were stable for more than 9 h. The intensity of the scattering light, which was proved to be proportional to concentrations of polystyrene latex suspensions, was also kept in solution in contact with hydrogen gas atmosphere. The bubbles were stable even at 50 g (gravity) by centrifugation. Voltammograms of the bubble-included solution had the oxidation peak, of which current was proportional to the intensity of DLS. Concentration of hydrogen in solution was evaluated accurately by comparing voltammetric currents at a regular electrode and a small electrode. The oxidation of hydrogen should be caused by the hydrated hydrogen which was supplied by dissolution of bubbles. Kinetic data of the dissolution were obtained at microelectrodes by using the advantage of extracting kinetics from diffusion currents. Voltammetric currents at microelectrodes were smaller by 10 times than those predicted from diffusion of hydrated hydrogen. Therefore, the oxidation is controlled by the dissolution rate at the high current density. The rate was estimated to be 2 Â 10 À8 mol s À1 cm À2 , which was converted to the linear transfer rate, 0.4 mm s À1 , at gas|water interface.

Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction

“…As already mentioned, this refinement of the model seems necessary for a full, accurate description, since most of the H 2 in the electrolyte is known to be present in the form of small gas bubbles, whereas only hydrated H 2 molecules in the liquid participate in the electrode reactions [14].…”

“…However, most of this is apparently in the form of submicron (diameter ≈ 440 nm) gas bubbles that also serve as the nucleation centers for bubble growth, when H 2 is produced, and less than 10% is in hydrated form that may react at the electrodes [14,15]. We assume that in equilibrium the effective density of the gas corresponds to a concentration of 0.77 mM and the concentration of hydrated H 2 molecules is one tenth of that, i.e., 0.077 mM.…”

“…where a is the bubble surface area per unit volume (m 2 /m 3 ), 2 the molar mass of H 2 and k diss the rate constant for the dissolution process (0.4 mm/s [14]). Negative mass exchange rates correspond to dissolved molecules forming gas and positive ones to gas dissolving into the liquid.…”

“…This is a worthwhile detail to pay attention to because experiments indicate that only the hydrated H 2 molecules in liquid can react at the electrode, whereas the gaseous H 2 is only a passive spectator of the electrode reactions, although most of the H 2 in water actually exists in the gaseous form (bubbles) [14]. When H 2 is generated, these nanobubbles (diameter ≈ 440 nm [14]) act as the nucleation centers for the growth of bubbles [15]. Moreover, it has been observed that increasing the amount of gas bubbles in liquid electrolyte may enhance mass transport, i.e., reduce the thickness of the diffusion layer [16].…”

Alkaline Water Electrolysis

“…Then, after gas/liquid separation, electrolytes coming from each compartment (O 2 and H 2 ) are usually mixed together and reinjected in the electrolyser. A recent work [58] showed that hydrogen bubbles in aqueous solutions, even at atmospheric pressure, are very stable: continuing to have a diameter of around 0.4 μm for an unexpectedly long time (after 9 h, only 50% of the number of bubbles had disappeared). 1) Consequences are that the purity of produced gases decreases rapidly when increasing the pressure.…”