Surface wettability control of bubble flow guide for a thin aqueous electrolyte layer of solar photoelectrochemical reactors

Abstract

Hydrogen is a promising energy carrier as no carbon dioxide is emitted during its use in fuel cells or combustion. Solar photoelectrochemical water splitting is a potential process for producing renewable hydrogen. Herein, energy transport phenomena are addressed for the future design of large-scale reactors. First, we show that the thickness of the aqueous electrolyte layer is an essential factor for utilizing the full spectrum of solar radiation. The transport of solar irradiation through the aqueous electrolyte is theoretically analysed. Next, based on the measurement of light transmission through hydrogen bubbles generated from a hydrogen evolving electrode, the energy loss caused by the bubbles covering a photoelectrode is discussed. The bubble size distributions at practical current densities are also presented. Then, a bubble flow guide for controlling the stream of bubbles in a thin electrolyte layer is proposed. A design strategy and experimental results verifying the performance of the bubble flow guide are presented. We demonstrate that surface wettability and inclination angle are important for designing an effective bubble flow guide. We examine the surface wettability control using hydrophilic coatings in detail. Changes in the water contact angles as well as bubble adhesion forces on the coated surfaces are demonstrated. In addition, the current experimental method can be used to identify essential issues in photoelectrochemical processes. Because bubble trapping and growth in a flow guide are reflected in the electrode potential variation, the discussion of electrode potential variation would be useful for further developing bubble flow guides. Overall, this study demonstrates the potential for developing and designing solar photoelectrochemical reactors.