Effect of laser spot diameter on oxygen bubble behavior in photoelectrochemical water splitting

Abstract

The detrimental effect caused by the long-term adhesion of bubbles on the gas production efficiency has been the main bottleneck limiting the efficiency improvement of photoelectrochemical water splitting. The evolution of oxygen bubbles on the fixed photoelectrode surface of TiO2 films was observed in-situ using a combination of a synchronized high-speed microscopic camera and an electrochemical workstation. The effect of oxygen bubble evolution on the current during the photoelectrochemical reaction was investigated under different laser spot diameters. When the spot diameter increased 1.68 times from 700 μm to 1176 μm with a laser power of 5 mW, the bubble detachment diameter and growth period increased 2.7 times and 20.2 times, respectively, yet the gas production efficiency decreased by 34.7%. Meanwhile, the peak current caused by the bubble detachment gradually increased. However, the valley current at the nucleation waiting stage of the bubble did not have obvious changes. The model for the gas evolving based on supersaturation was adopted. It illustrated that the concentration gradient of dissolved gas perpendicular to the photoelectrode surface increased with the increase of spot diameter, leading to an increase in the bubble surface tension gradient, and causing the Marangoni force to inhibit the detachment of bubbles. A force balance model was developed to evaluate the Marangoni forces and the buoyancy forces. The growth rate of the Marangoni force was found to be proportional to the spot diameter. Therefore, reducing the laser spot diameter is a means to effectively remove the bubbles from the photoelectrode surface.