Figure 4. Electrochemical analysis of Ni and Co(OH)2/Ni flow-through electrodes with different pore sizes for HER electrocatalysis with flows. LSV curves (A) for Co(OH)2/Ni flow-through electrode with a pore size of 75 PPI under different flux, insets are optical images of the electrode surface with flows (right) and without flows (left). Decreases in potential under different current densities (B), different flux (C), and decreases in potential were calculated from the difference between the potential in the flowing and non-flowing cases. Schematic diagram of the force analysis of bubbles on the electrode surface without flows (D) and with flows (E). Chronoamperograms of Co(OH)2/Ni flow-through electrodes with a pore size of 75 PPI under different current densities and different flux, the flux is increased every 500 s (F).
As another gas evolution reaction of AWE, OER performance is also subject to the generated O2 bubble. Figure 5A shows the LSV curves of OER under the condition of electrolytes flowing electrode pores. Similar trends can be found that the potential of OER can be decreased as the electrolyte flux increases, but the decreased values (Figure 5B-C) are less than that of the HER process. Similarly, the current density can be increased as the electrolyte flux increases (Figure S23, Table S2), but the current density increments (Figure 5D) in the OER process are less than that of the HER process. This can be attributed to two factors: For one thing, OER is a 4-electron transfer process and HER is a 2-electron transfer process. At the same current density, the O2 product is half of H2. Hence the masking of the catalytic active sites caused by generated bubbles is not as severe as in the HER process. In other words, the effect of O2 evolution process on electrode performance is weaker. For another thing, compared with lower H2solubility (1.8 vol% at 293 K), higher O2 solubility (3.1 vol% at 293 K) leads to a lesser gas evolution amount. As a result, the decrease in potential for OER by flowing electrolyte through electrode pores is less, compared with that of HER.
The durability of Co(OH)2/Ni cathode and CoOOH/Ni anode was investigated (Figure 5E) by performing a long term catalytic test at 300 mA cm-2. It can be found that both cathode and anode show excellent catalytic stability without any reduction in current density and no catalyst shedding was observed during a continued test for 18 hours. This can be mainly attributed to the advantages of the electrode preparation method, where unstable catalysts within the electrode pores can be removed with the continuous flushing of the precursor fluid during flowing synthesis. In addition, it was also found that the current density during the HER/OER process could be changed in time as the pump was switched on and off, demonstrating good instantaneous responsiveness.