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.