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.