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Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30 Page 23 of 30

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100 mA cm with the iR compensation in 0.1 M KOH + 0.5 M NaCl and showed similar LSV curves with
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that in 0.1 M KOH [Figure 14E]. The overpotential was less than 480 mV at 100 mA cm , which
demonstrated the high selectivity for OER electrocatalysis. No reactive chlorine species were detected after
the OER test in the chlorine-containing electrolyte. Moreover, the excellent long-term stability of NCFPO/C
NPs was tested by CV for 10,000 cycles and chronopotentiometry for 100 h at 10 mA cm and 50 mA cm .
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Up to now, most seawater electrolytes contain alkaline additives for seawater catalysis. The direct natural
seawater catalysis without purification and alkali addition is an important technology in the future. A novel
and efficient strategy is to create a favorable local reaction microenvironment on the surface of
electrocatalysts for the direct natural seawater electrocatalysis. Guo et al. introduced a hard Lewis acid layer
on the TM oxides to manipulate the local pH values for the direct seawater electrocatalysis [Figure 14F] .
[130]
Due to the strong ability to water dissociation, a large amount of OH could be in situ generated on the
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surface of catalysts with the increased content of hard Lewis acid Cr O . In particular, the pH value on the
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6 at% Cr O -CoO could approach 14.0 under the applied potential of 1.60 V (vs. RHE), which
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x
demonstrated that it was nearly in alkaline seawater [Figure 14G]. The normalized OER activity to the
electrochemical surface area of 6 at% Cr O -CoO was much higher than those of CoO and benchmark Fe-
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x
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doped NiOOH and RuO [Figure 14H]. Furthermore, the Cr O -CoO could stably catalyze the natural
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seawater catalysis for more than 200 h at 1.8 V vs. RHE, higher than the potential required to trigger
chloride oxidation. Most OH could participate in OER processes, and some residual OH could interact
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with the positive charge on the catalyst surface to constitute a stable electrical double layer (EDL)
[Figure 14I]. The enrichment of OH prevents the diffusion and adsorption of Cl to the catalyst in seawater
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electrolytes, thus inhibiting the chlorine redox chemistry and the corrosion of the electrode. When the
Cr O -CoO content is 6 at%, the current density of the as-developed flow type seawater electrolyzer at
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1.87 V at 60 °C meets an industrial requirement of 1.0 A cm , and it runs stably for 100 h at 500 mA cm .
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STUDY ON METAL ELECTRODES OF SMABS
Metal anodes, such as Na, Mg, Al, and Zn, play a critical role in determining the properties of cycling life,
capacity, and energy density of the SMABs. Especially in R-SMABs, mainly the odium-air batteries, where
the anode is made of pure sodium or sodium alloys. During the discharge process, sodium ions are released
from the negative electrode, generating electrical energy. During the charging process, sodium ions in the
seawater at the negative electrode are reduced to metallic sodium. Sodium is abundantly available and is
often used as anode material due to its high theoretical capacity of up to 1,166 mA g . However, the
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uncontrolled growth of sodium dendrites hinders the safe operation of cells, damages the separator
membrane, and shortens the device lifetime. Additionally, seawater-based sodium-air batteries still have a
low coulombic efficiency and cell performance. Currently, the problem of sodium dendrites in seawater-
based sodium-air batteries can be addressed through the following approaches: (i) Electrolyte optimization.
By modifying the composition and concentration of the electrolyte, the formation of sodium dendrites can
be reduced. Adding additives that inhibit the growth of sodium dendrites, such as polymers or ionic liquids,
can effectively suppress their growth; (ii) Electrode coating. Applying a protective coating on the surface of
the negative electrode can prevent the formation and growth of sodium dendrites. This coating can be a
polymer or other materials that provide sufficient mechanical strength and chemical stability to prevent
penetration and damage caused by dendrites; and (iii) Design optimization. By optimizing the structure and
design of the battery, the formation of sodium dendrites can be minimized. For example, using porous
electrode materials or nanostructures can provide more surface area and a more uniform distribution of
sodium ions, reducing dendrite formation.