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.