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Page 2 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

[1,2]
especially on the sea . Harvesting energy directly from the sea provides great potential for developing the
marine economy and promoting marine research. Thus, developing renewable energy utilization
techniques, such as wind power generation, photovoltaics, and hydroelectric generation, is highly
[3-5]
demanded for providing green electricity to the marine equipment . However, the intermittent power
supply property makes the clean energy sources difficult to be utilized efficiently. Moreover, traditional
secondary batteries, such as Lithium (Li)-ion batteries and Lead-acid batteries, cannot meet the long-term
and high-power density requirements of the equipment working in the deep and open sea . Therefore,
[6,7]
harvesting energy directly from the ocean is critically demanded.

Seawater metal-air batteries (SMABs) are considered as extremely promising power sources for providing
electricity for the equipment working on the sea or in the deep sea due to their high theoretical energy
[8,9]
density, low cost, eco-friendly nature, etc. . During the discharge of SMABs, seawater is directly used as
the electrolyte, the dissolved oxygen (O ) in seawater is reduced on the cathode, and the metallic anode
2
[Magnesium (Mg), Aluminum (Al), Sodium (Na), etc.] is oxidized [10,11] . The working processes are
summarized as follows:

The dissolved O was harvested from seawater and subsequently reduced through the oxygen reduction
2
reaction (ORR), while the metals (alloys) were oxidized, forming metallic hydroxides [12,13] . Due to the open
structure of SMABs, oxygen, as an active species, can infinitely diffuse to the cathode for ORR, which
ensures the SMABs display high theoretical energy density [14,15] . One critical issue is that the un-optimized
electronic structure of metallic sites and the four reaction steps for the ORR process would result in sluggish
catalytic kinetics . Although the OER is thermodynamically favored over the hypochlorite formation
[16]
reaction in seawater (pH ≈ 8), both reactions were a balancing relationship during seawater catalysis owing
to the slow catalytic kinetics in seawater [17,18] . Currently, we consider OER to be the primary cathode
reaction upon the charging of seawater batteries. Thus, designing and exploring efficient and low-cost
electrocatalysts is an urgent task to enhance the battery performance. In the past few years, various types of
electrocatalysts were designed to accelerate the ORR/OER catalytic kinetics in seawater, including noble
metals of Pt, Ru, Ir, and their alloys [19-21] . However, the noble metals are more easily being poisoned in
seawater electrolytes. The non-noble metal electrocatalysts, such as Fe groups (Fe, Co, Ni) and carbon-based
materials with abundant and inexpensive advantages, have been explored, displaying satisfying ORR
performance and deserving further exploration as promising ORR/OER electrocatalysts in seawater
conditions [22-24] . At the same time, the circulating seawater electrolyte can promote the heat and charge
transfer rate, thereby improving the intrinsic safety and cycle life of the SMABs [25,26] . Therefore, the SMABs
are particularly suitable as long-term power supply systems for the equipment working in the sea.

However, in comparison to the alkaline metal-air battery systems, the high content of chloride ions (Cl ) in
-
seawater (19.345 g/kg) is easily absorbed on the surface of electrocatalysts. This absorption changes the
electronic structure and poisons the metallic sites, thereby greatly reducing the power density of
SMABs [27,28] . Although some electrocatalysts display good ORR catalytic performance in conventional
electrolytes, they cannot maintain active and stable in seawater electrolytes due to the poison of high
-
concentration of Cl . The recent results suggest that, on the one hand, the adsorption of Cl on the surface
-[29]
catalytic sites can hinder the breakage of O-O bonds and thus inducing the reaction pathway change from a
four-electron to a sluggish two-electron pathway for the ORR process [30,31] . Moreover, the adsorption of O
2
molecules would also be suppressed. On the other hand, due to competitive chlorine evolution reaction