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Page 6 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

The theoretical voltage of the R-SMABs can reach 3.48 V. However, due to the sluggish catalytic kinetics of
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electrocatalysts in cathodes, the Cl corrosion on both cathodes and anodes, and the competing relationship
between the two electrodes, the battery system is still far from reaching its theoretical properties [56-60] . To put
forward the practical application of SMABs, such as the typical sodium-based seawater batteries, the power
density, capacity, and stability of the SMABs should be further optimized. The SMABs can be divided into
small (< 1 kWh), medium (1-10 kWh), and large-scale (> 1 MWh) power sources according to their
discharging capacity. Some low-power seawater-based metal-air battery prototypes have been utilized for
underwater observation and ocean buoys. Medium-power seawater batteries can be used for exploratory
unmanned aerial vehicles and maritime search and rescue operations. The stability of large-scale seawater
batteries should be further optimized for their large-scale application in the future.

THE COMPONENTS AND EVALUATION OF SEAWATER METAL-AIR BATTERY
The large-scale commercialization of efficient and stable SMABs remains a significant challenge, which
requires the optimization of all individual components in the battery. As discussed above, SMABs feature an
open structure. The essential components in the battery contain electrode materials (cathodes and anodes),
electrolytes (anolytes, catholytes), current collectors, ceramic solid electrolytes, electrocatalysts, and the
general cell type, depending on different categories of SMABs.

Electrocatalysts and electrode materials
SMABs are basically composed of anodes and cathodes with current collectors. The electrocatalysts were
mixed with binders and were pasted onto the current collectors for preparing electrodes [61,62] . Highly
efficient SMABs require electrodes (both anodes and cathodes) with optimized electronic conductivity,
porous density, and wettability [63,64] . The electrodes should meet several prerequisites for developing efficient
and stable SMAB devices [Figure 3A]. Developing anode materials should meet the following requirements:
(i) avoid the side reactions that may lead to cell swelling and failure; (ii) possess good electronic
conductivity and excellent stability in seawater conditions; and (iii) narrow voltage window, low-cost and
low toxicity. At present, SMABs with anodes consisting of Li, Na, Mg, Al, Zn, and their alloys have been
systematically studied [65,66] . Among various types of SMABs, seawater lithium-air batteries theoretically
possess the highest energy density, reaching up to 11,140 Wh·kg . On the other hand, metals such as Zn,
-1
Mg, and Al display the advantages of environmental friendliness, abundance in the Earth's crust, low cost,
and intrinsic safety. In addition, Al is readily available for recycling in massive amounts and has a high
[67]
energy density of 8,100 Wh·kg and a significant theoretical voltage of 2.7 V . As a result, seawater
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aluminum-air batteries are considered as the most promising systems for developing SMABs.
The air cathode is composed of a gas diffusion layer (GDL), electrocatalyst, and current collector. As
mentioned above, the OER and ORR occur in the cathodes when seawater batteries are charged and
discharged, respectively . Therefore, an efficient air electrode should possess strong oxygen adsorption
[68]
capacity, fast oxygen diffusion, and high electrochemical activity for oxygen redox reactions in SMABs. The
electrocatalyst layer in the cathode has a significant influence on the performance of SMABs. The
electrocatalysts on the cathodes may be noble metals, transition metals (TM), and non-metallic materials.
Moreover, the electrocatalysts should not only display efficient ORR and OER activity and stability but also
-
-
possess strong Cl corrosion resistance with stable physical structures in seawater conditions. The Cl in
seawater can easily destroy the oxide thin film on the surface of the electrocatalysts and form complexes
with metal ions, resulting in corrosion of metallic sites in seawater [53,69] . The electrocatalysts can be divided
into the following three categories: (1) noble metals and their alloys, such as Pt as ORR electrocatalysts while
IrO and RuO as OER electrocatalysts; (2) non-noble metal-based catalysts, such as metal-supported carbon
x
x
materials and metal oxides and sulfides; and (3) metal-free electrocatalysts, such as nitrogen (N) doping