Figure 3. Polarization curves of Ni and
Co(OH)2/Ni electrodes (A). Optical image of Ni foam (B),
Co(OH)2/Ni (C) during hydrogen evolution (current
density: 100 mA cm-2), underwater H2bubble contact angle of Ni foam and Co(OH)2/Ni (D),
H2 bubble adhesive force measurements of Ni foam (E) and
Co(OH)2/Ni (F) (insets: optical images of the
H2 bubble states in the corresponding position).
Gas evolution from the electrode surface usually brings disturbance to
the electrode, which is reflected in the electrochemical curves. As can
be seen in Figure 3A and S6, the curves show massive fluctuations as the
current density increases. As can be seen in Figure 3B-C, the pore
surface of the electrode can be covered by H2 bubbles,
severely reducing the catalytically active area of the electrode, and
posing to decrease in current density. The catalytic sites can be
re-exposed after generated bubble detaching, resulting in an instant
increase in the current density. The fluctuations of LSV curves for
Co(OH)2/Ni electrode are smaller than that of Ni
electrode, which can be attributed to the microstructure change in the
pore surface. It can be seen that, for Ni foam, the process of bubble
nucleation to detachment takes 10 s (Figure S11) with an average bubble
size of 47.76 μm (Figure S13A), at 10 mA cm-2.
However, after immobilizing nanosheet-shaped Co(OH)2catalysts, the residence time of H2 bubble can be
reduced to less than 1 s (Figure S12) with a smaller average bubble size
of 23.13 μm (Figure S13B). The obvious decrease in bubble size can be
attributed to the shortening of bubble growth time, which is satisfied
with the bubble growth
dynamic. In addition, the apparent difference in detachment
behavior can also be observed in Video S1-S2 at 100 mA
cm-2. For Ni electrode, there exists an apparent
bubble growth phase before it detaches, while for
Co(OH)2/Ni electrode, the evolved bubbles can be
directly detached from the electrode pores almost without any residence
time can be observed. This phenomenon can be attributed to the
difference in bubble adhesive force. It can be seen in Figure 3D, after
immobilizing Co(OH)2/Ni catalysts in Ni pores, the
underwater H2 bubble contact angle rises from 137.5° to
142.5°, suggesting that loading
Co(OH)2 nanosheets on the pore surface improve the
superaerophobicity which is beneficial to release H2bubbles. Meanwhile, the apparent bubble adhesive force of 14 μN and the
apparent bubble transformation at position 3 can be found for Ni foam
(Figure 3E), while no bubble adhesive force and bubble transformation
can be found for Ni foam loaded with
Co(OH)2nanosheets (Figure 3F). All those results prove that the adhesive force
between bubbles and electrode active surface becomes lower after loading
nanosheet-shaped catalysts. Hence, the generated H2bubbles can be timely detached from the electrode surface, and efficient
and stable HER performance can be achieved.
The generated H2 bubbles in electrode pores can be
trapped in the pores (Figure S14B), although the enhanced bubble
detachment behavior can be achieved by attenuating the adhesive force
between bubbles and catalysts. Thus, the generated H2bubbles are difficult to remove out of pores timely, limiting the mass
transfer of electrolytes, eventually resulting in a decrease in
electrochemical active sites. In addition, after the H2bubbles diffuse from pores to the electrode surface, a continuous
H2 bubble curtain can be formed on the electrode surface
due to bubbles uplifting (inset image of Figure 4A), which also limits
the mass transfer between the electrolyte and the catalytic sites in the
electrode pores, especially under hundreds of current densities.
For accelerating the bubble
detachment from the internal/external of the electrode, the prepared
electrode was operated under the flow-through mode. In other words, the
electrolytes were pumped through the electrode pores during the HER
process (Figure S15). Figure 4A and Figure S16 show the polarization
curves of the flow-through electrode under different fluxes. As can be
seen, when the electrolyte was flowing through the electrode pores, the
overpotential of HER became lower than that without electrolyte flows.
It can be calculated that the potential of HER can be reduced more as
the current densities increase (Figure 4B, Table S1). In this work,
almost 130 mV potential can be reduced at 400 mA cm-2,
if the electrolyte flux was kept at 339 m3m-2 h-1. The obvious enhancement of
HER performance can be attributed to the rapid removal of gas products
(the residence time of H2 bubbles was furtherly
shortened to less than 0.1 s, as shown in Figure S17-S18). In this case,
the bubbles formed at the pore surface and electrode external surface
can be removed quickly with the electrolyte flow. Thus, the blockage of
electrode pores (Figure S14D) and the formation of a bubble curtain
(inset of Figure 4A) can be avoided, which enhanced the contact between
electrolyte and electrode.
In addition, it can be found that the overpotential of HER can be
affected by the flux of electrolytes (Figure 4C). The decreases in
potential of HER were more obvious by increasing the flux of
electrolytes flowing through the electrode pores. This result can be
attributed to the shortening of the formed H2 bubble
residence time on the catalyst surface, which promotes the re-exposure
of catalyst active sites. During the flow-through process, the generated
bubbles are not only subject to adhesive fore, gravity (G ) and
buoyancy (Fb ), but also to the dragging force
(Fd ) caused by the electrolyte fluid flowing, as
shown in Figure 4D-E. Thus, the residence time for the bubbles to detach
from the pore surface mainly depends on the flux of the electrolyte.
Higher flux could lead to a shorter residence time, since
a greater dragging force can be
generated under the condition of higher flux. Therefore, bubbles can be
removed quickly from the micro-grade pores and re-exposing the catalyst
active site and attenuating the
negative effect of H2 bubbles covered on the catalytic
performance can be achieved.
Figure 4F, Figure S19 and S20 show the effects of electrolyte flux on
current density. As can be seen in the chronoamperograms, the potential
was set at -320, -400, -460, and -520 mV vs RHE, the initial values of
current densities were corresponding to 100, 200, 300, and 400 mA
cm-2 in the absence of electrolyte flowing. During the
test progress, the electrolyte flux was increased every 500 s. It can be
found that at each current density, the current density of HER can be
gradually increased by continuously increasing the flux of electrolytes.
Besides, a graph of plotting increases in current density versus current
density at a flux of 339 m3 h-1m-2 can be seen in Figure S21. The scattering point
shows a good positive linear correlation. The results show that around a
10% increase in current density can be achieved by introducing
electrolytes flowing in electrode pores at the same operating voltage.
Meanwhile, the H2 production also shows the same
increase amount, and a close to 100% faraday efficiency can be achieved
(Figure S22). In other words, more hydrogen production can be obtained
for the same energy consumption conditions under the condition of
electrolyte flows through the electrode pores.