Page 191 - Read Online

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

(ClER) and chloride corrosion, the oxygen evolution reaction (OER) performance of the catalysts in
[32]
seawater electrolytes decays rapidly during charging . Therefore, it remains a great challenge for the
development of efficient chlorine-resistant ORR/OER electrocatalysts for their application in seawater
electrolytes for SMABs.

In order to put forward the wide application of SMABs in the field of marine techniques, three key issues
-
need to be solved urgently: (i) The poisoning mechanism of Cl on ORR/OER processes in seawater
electrolytes should be clarified, thus providing guidance for designing high-efficient chlorine-resistant
catalysts; (ii) The electronic structure of noble metal catalysts should be optimized to enhance the
electrocatalytic activity and stability in seawater electrolytes. Specifically, this can be achieved through
several strategies. Firstly, by modifying the ligands or surface modifiers, the electronic structure of the
catalyst can be adjusted. This influences the distribution of electron density and the reactivity of the active
sites, leading to improved electrocatalytic performance. Secondly, introducing suitable elements into the
noble metal catalyst can modify its electronic structure. This alters the binding strength of reactants and
intermediates, facilitating the desired electrochemical reactions and enhancing catalytic activity and
stability. Thirdly, modifying the surface of the catalyst through techniques such as surface deposition or
functionalization can regulate its electronic structure. This can enhance the interaction with reactants and
ions in the electrolyte, improving catalytic activity and stability. At the same time, chlorine-resistant non-
noble metal-based electrocatalysts with excellent ORR/OER performance should be developed to broaden
the types of catalysts and reduce the cost of catalysts; and (iii) The integrated structure of SMABs should be
optimized to improve the power density and stability of batteries. Based on the above consideration, this
review presents the progress in the development of chlorine-resistant cathode electrocatalysts for SMABs. In
this review, we first summarized the development of various types of SMABs to understand their working
principle and battery performance [Figure 1]. Subsequently, the poisoning mechanism of Cl on cathode
-
electrocatalysts during charging and discharging was studied and summarized. Then, we classified the
reported chlorine-resistant electrocatalysts and comprehensively summarized the composition and
structural designing strategies of high-efficient chlorine-resistant ORR/OER electrocatalysts in seawater
electrolytes. Finally, the main challenges to be addressed in the commercialization of SMABs were
discussed.

THE DEVELOPING HISTORY OF SEAWATER METAL-AIR BATTERIES
The SMABs were developed through three stages, starting from seawater-activated batteries (SABs) to
[33]
primary SMABs (P-SMABs). and further evolving into rechargeable SMABs (R-SMABs) . The SAB was
originally developed in the 1940s by Bell Telephone Laboratories to meet the requirement for high energy
density, prolonged shelf-life, and outstanding low temperature performance. It was mainly used as a power
source for military torpedoes. The SAB, which used magnesium as the anode, silver chloride (AgCl) as the
cathode, and flowing seawater as the electrolyte, was first commercialized in 1943, as shown in Figure 2A.
The battery can be stored for up to five years in a dry condition and can be activated by the addition of
seawater when being used, thus being called SAB. During the discharge, the following reactions occur :
[34]

The above reaction possesses fast catalytic kinetics. For discharging, the AgCl on the cathode of the battery
is reduced to Ag, leading to an increase in conductivity as the process proceeds. However, the discharge
voltage would drop dramatically when AgCl is reduced to Ag completely. Moreover, another advantage of
the Mg-AgCl battery system is that the device can keep working efficiently in wide temperature ranges. The