3. Results and discussion
The morphologies of the flow-through electrode before and after HER/OER
activation were characterized by SEM (Figure 1B-E). And the catalysts
collected by treating flow-through electrodes under ultrasonic
conditions were detected by TEM (Figure S2-S3). The Ni foam substrate
shows an interconnected porous structure with a relatively smooth pore
surface, as can be seen in Figure 1B. After the flowing synthesis of CoB
nanocatalysts, CoB nanocatalysts can be uniformly immobilized in the
pore surface of Ni foam without any blockage (Figure 1C). After further
analysis by HAADF-STEM and elemental mapping, it can be seen that CoB
nanoparticles were loaded on the nanosheet (Figure S2A) and the
nanosheet was distributed with the element of Co, B, and O (Figure S3).
Among these, the nanosheets obtained are probably derived from side
reactions during the preparation of the catalyst. That is,
Co2+ is precipitated as cobalt hydroxide nanosheets
under alkaline conditions. Subsequently, after the CoB/Ni flow-through
electrodes have been subjected to the HER/OER activation, the
morphologies of nanocatalysts both changed. It can be seen in Figure
1D-F and Figure S2B-C, the nano CoB immobilized on the pore surface were
completely transformed into the shape of nanosheets, which implies the
active catalysts immobilized on the pore surface have beenin-situ restructured.
For further investigation of the in-situ restructuration, the
prepared flow-through electrodes before and after HER/OER activation
have been tested by XPS (Figure 2A-C) and XRD (Figure S4). The surface
chemical states of Co element for CoB/Ni were shown in Figure 2A, the Co
2p3/2 peak at 777.8 eV is assigned to
Co0 in CoB, and the peaks at 781.2 eV are assigned to
Coδ+ in CoOx , which is resulted
from the surface oxidation after the sample is exposed to air and
water.31 The peak intensity of Co0in CoB is relatively weak compared with that of
Coδ+ in
CoOx , because the CoB was covered by surface
oxidation species (Figure 2D). In addition, the same oxidation can also
be found in the high-resolution spectrum of B 1s (Figure 2C). The XRD
results (Figure S4) and the HRTEM image (Figure 2G) show that the
prepared CoB on the pore surface is an amorphous compound. After HER/OER
activation, the binding energy of Co 2p3/2 was shifted
to low binding energy with a binding energy
of 780.6 eV and 780.1 eV (Figure
2A), and the corresponding binding energy of O 1s for Co-OH (530.9 eV)
and Co-O (529.3 eV) can be found (Figure 2B), while no signal can be
found in high-resolution spectrum of B 1s (Figure 2C). Subsequently, the
ICP-OES was used to detect the leaching amounts of flow-through
electrode after HER/OER activation, as shown in Figure S5. The results
show that the B element was completely leached out from CoB during the
activation process. Eventually, the XRD results (Figure S4) and the
HRTEM images (Figure 2H-I) of CoB/Ni after HER/OER furtherly proves that
the active catalysts on the pore surface have been in-siturestructured into Co(OH)2 and CoOOH, respectively.
Hence, the prepared flow-through electrodes are named
Co(OH)2/Ni for HER and CoOOH/Ni for OER in the analysis
and discussion.