Water-Splitting Photoelectrolysis Reaction Rate via Microscopic Imaging of Evolved Oxygen Bubbles

Abstract

Bubble formation and growth on a water-splitting semiconductor photoelectrode under illumination with above-bandgap radiation provide a direct measurement of the gas-evolving reaction rate. Optical microscopy was used to record the bubble growth on single-crystal strontium titanate immersed in basic aqueous electrolyte and illuminated with UV light at 351/364 nm from a focused argon laser. By analyzing the bubble size as a function of time, the water-splitting reaction rate was determined for varying light intensities and was compared to photocurrent measurements. Bubble nucleation was explored on an illuminated flat surface, as well as the subsequent light scattering and electrode shielding due to the bubble. This technique allows a quantitative examination of the actual gas evolution rate during photoelectrochemical water splitting, independent of current measurements. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3462997͔ All rights reserved. The production of hydrogen using solar energy to directly split water in a semiconductor photoelectrochemical cell is a promising source of carbon-free fuel, 1 but many issues with the semiconductor-liquid interface remain. Semiconductors with bandgaps ϳ2 eV useful for absorbing the solar flux and possessing the necessary potential to split water 2 often suffer stability issues due to photoexcited charge carriers corroding the semiconductor rather than transferring into the electrolyte.3 Adding heterogeneous catalysts or structuring the electrode surface can provide more active sites for charge transfer, lowering the overpotentials required and possibly increasing stability, but often the nature of the active sites is unknown.4-9 Traditional experimental techniques involve modifying or structuring the entire surface of a semiconductor photoelectrode and measuring the photocurrent under illumination; this method provides only the average reaction rate of all the various redox reactions occurring over the entire exposed surface. Localized methods that differentiate the activity on separate patterned areas on the same electrode could offer a wealth of information about the surface sites important for efficient charge transfer. Previous research has focused on utilizing a scanned electrochemical probe near the surface 10 that can even include local illumination through an optical fiber, 11 a very useful but rather perturbative and difficult method to implement.Examining the evolved gas bubbles could provide this type of local measurement by recording their growth rate at different surface features. Additionally, measuring the bubble and hence the actual gas produced differentiates between the gas-evolving reaction of interest and any parasitic corrosion reactions that can still contribute to the measured current. Optical microscopy of the photogenerated gas bubbles during their growth is a straightforward technique and provides a localized measurement of the reaction rate at various areas on a semiconductor surface.As a model system to explore the use of this technique...

New class of photocatalytic materials and a novel principle for efficient water splitting under infrared and visible light: MgB_2 as unexpected example

“…(It is worth stressing again that in case of VS-IR light illumination none of the relevant photoprocesses takes place and electron-hole pairs are not generated in the TiO 2 .) conversion efficiency has been calculated using bubble microscopy [15] (and following gas chromatography). These measurements confirmed 0.95% Faraday efficiency (each 2 electrons generated ~0.95 molecules H 2 ).…”

Analysis of hydrogen bubble flow between nanorods using lattice boltzmann method

“…However, the movement of the bubbles toward the top surface could have been improved by increasing the inlet velocity in the z-direction. The bubbles will remain more or less spherical during the simulation due to the low Re, which can be understood from a shape regime map presented in [8]. LBM can also handle the two-phase fl ow with the bubbles and the surrounding fl uid in a domain with nanorods.…”

“…The electrochemical reactions are implemented by source terms [7,8]. (7) (8) where i is the current density and F the Faraday constant.…”

Photoelectrochemical Oxidation of Ethanol under Visible Light Irradiation on TaON-Based Catalysts

“…In this context, Photoelectrochemical (PEC) cells are very attractive devices due to their capacity to convert light into electric power or chemical fuel (renewable hydrogen) by photo-electrochemical decomposition of a target substance using solar light. Several works have investigated PEC cells for the production of electricity and hydrogen from renewable resources, mainly water [1][2][3][4] and biomass [5][6][7][8][9][10][11][12][13] including ethanol, glycerol and ethylene glycol.…”