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.