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Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30 Page 9 of 30

discharge voltage, temperature, geometry dimension of the anode and cathode, and the mass loading of
[77]
active materials . Generally, it is difficult to reach the theoretical capacity in practical applications due to
the polarization effects and the side reactions during charging and discharging. Therefore, increasing the
capacity of seawater batteries requires not only performance improvement of cathode/anode materials but
also the optimization of the structure of the whole device.

Coulombic and energy efficiencies are typical parameters in electrochemical systems of R-SMABs, which
represent the ratio of the amount of charge (Q) flowing through the battery and the voltage ratio between
charging and discharging, respectively [Figure 3D]. A coulomb efficiency is usually used as a comparative
value to determine the capacity loss for each cycle in the rechargeable battery systems, which is an
important parameter to predict the remaining life of the battery [78,79] . However, the performance of
coulombic and energy efficiencies can be influenced by many factors, such as the environmental
temperature, the humidity, and the uniformity of each package and electrode. Therefore, to precisely
evaluate the key factors and the mechanisms that influence enhanced battery performance, the standard
experimental procedure should be established.

Stability and safety are key parameters in assessing the performance of SMABs. The stability of positive
electrodes in the seawater medium is one of the biggest factors for determining the long-term performance
-
[61]
of the whole battery . Due to the existence of Cl in seawater electrolytes, more surface metallic sites would
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be attacked, thus decreasing the exposure of metallic sites. Moreover, the Cl adsorption would change the
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reaction pathway. For instance, the ORR pathway would be transferred from 4e in alkaline electrolytes to
-
[80]
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2e in seawater electrolytes . As a result, it is a critical challenge to develop Cl corrosion-resistant
electrocatalysts to enhance the stability of cathodes and the whole SMAB devices. Moreover, because
seawater was employed as the electrolyte without the use of any organic additives, the SMABs possess highly
intrinsic safety and are eco-friendly [81-83] . In addition, the seawater battery is a semi-open or fully open
electrochemical system, which is beneficial for gas release and temperature diffusion, thus keeping the
system working in relatively low temperature conditions (generally lower than 60 °C).
ORR ELECTROCATALYSTS IN SEAWATER ELECTROLYTE
The oxygen electrocatalytic process in seawater is considered as a complicated pathway, which may involve
the simultaneous occurrence of oxygen reduction and chlorine corrosion, depending on the condition of
pH values, oxygen/chlorine concentration, temperature, etc. [83-85] . The absorption and corrosion of Cl play a
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critical role in enhancing the catalytic efficiency and stability of the catalysts, thus affecting the battery
performance. In this section, we discussed the development of ORR electrocatalysts and the influence
behavior of Cl toward the ORR process .
[86]
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ORR mechanism and Cl -resistance mechanism in seawater batteries
The ORR mechanism in seawater involves the reduction of dissolved O by a four-electron process to
2
hydroxide ions (OH ) or by a two-electron process to form hydrogen peroxide (H O ) on the cathode
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2
2
electrocatalysts. In the four-electron process, each oxygen molecule (O ) accepts four electrons and
2
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undergoes a complete reduction to form OH . This is the most favorable pathway for ORR as it does not
produce any intermediate reactive species. The reaction process can be represented as follows: