1. Introduction
Water electrolysis for hydrogen (H2) evolution has been considered to be one of the most promising strategies to facilitate “Carbon peak” and “Carbon neutrality”.1, 2 Currently, most researchers are focused on the development of state-of-the-art catalysts with high intrinsic activity to improve the water adsorption, activation, and splitting process.3-7 However, even for robust catalysts, the generated gas bubbles during the hydrogen evolution process will cover the catalyst surface, which creates a dead area, restricts the mass transfer between electrolyte and catalysts, and causes an overpotential, especially under higher current density (hundreds of mA cm-2).8-10 Besides, for the electrodes with three-dimensional porous structures, the bubbles formed in electrode pores will get trapped in the porous structure and the uplifting bubbles in the electrode external surface will form continuous bubble curtains.11-13 Thus, the contact between the electrolyte and the catalyst would be weakened, which inevitably gives rise to energy losses and decreases electrochemical conversion efficiencies. In order to overcome these problems, it is important to accelerate the detachment of the generated bubbles from the electrodes during water electrolysis for hydrogen evolution.
Generally, fabricating superaerophobic electrodes with nanostructure is an efficient strategy for gas bubble detachment.14, 15 For such a surface with nanostructure like nanosheets,16, 17nanoarrays,18 and nanocones,19 the gas-liquid-solid interface is discontinuous, which reduces the adhesive force of the bubbles.20, 21 As a result, the bubbles can detach from the electrode surface timely with a small size. For example, an underwater superaerophobic pine-shaped Pt nanoarray electrode was presented by Sun’s group.18 The adhesive force decreased by almost 13 times and the bubble size decreased by about 6 times can be obtained compared with the Pt plat electrode. Besides, in order to further accelerate the detachment of generated bubbles, introducing external fields to enhance the driving force of bubble detachment is an effective strategy. It has been reported that the performance of alkaline water electrolysis (AWE) by foam electrodes can be enhanced under the existence of a magnetic field.22 Besides, ultrasonic and supergravity fields have been also used to introduce convection for the rapid removal of gas bubbles to avoid bubbles covering the active site for a long time.23-25 However, the introduction of these external fields above inevitably increases the total equipment investment.
In order to avoid the introduction of additional equipment investment, flow-through electrodes would be a promising strategy for bubble detachment during hydrogen production by water electrolysis, since the directional movement of generated bubbles can be driven by the fluid flowing through the electrodes. Currently, there is a growing interest in the flow-through electrodes for performing electrochemical processes with enhanced mass transfer be achieved.26-28 Herein, a flow-through electrode with Co-based nanosheets immobilized on Ni foam is presented for promoting bubble detachment and enhancing mass transfer during water splitting. Firstly, the electrode’s internal/external surface was immobilized with the nanosheet-shaped catalysts for increasing the roughness of electrode surface and weakening the adhesion of the gas bubbles. Then, the electrolyte was flowing through the prepared electrodes with interconnected pores and low permeation resistance. The electrolyte flux was controlled by a peristaltic pump and recirculated in the corresponding compartment of the cell (Figure S1). The enhanced hydrogen evolution performance and mechanism under the condition of electrolyte flowing through electrode has been investigated. In addition, the energy consumption of the electrolyzer assembled with flow-through electrodes was also studied.