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Page 12 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

structure, TMs can increase the degree of graphitization of carbon materials during carbonization. At the
same time, encapsulating carbon materials on the surface of metal sites can effectively prevent metal
agglomeration and promote electron transfer. Suh et al. designed a graphene-nanotube-cobalt hybrid
[97]
electrocatalyst (S-rGO-CNT-Co) as a highly active seawater cathode catalyst [Figure 5A] . The reported
S-rGO-CNT-Co is composed of tubular CNTs and partially anchored 10-30 nm Co NPs [Figure 5B]. The
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composite nanostructure of the Co-C and graphene protective layer prevents the adsorption of Cl on the
cobalt and enhances the catalytic activity and stability of ORR. The S-rGO-CNT-Co sample was used as an
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electrocatalyst for the air cathode for SMABs. At a current density of 0.01 mA cm , the charging voltage was
3.42 V, and the discharge voltage was 3.0 V. [Figure 5C]. Although the as-prepared electrocatalysts exhibit
excellent performance, the discharge performance is still lower than that of 20 wt% Pt/C catalysts in SMABs
[Figure 5D]. S-rGO-CNT-Co degraded rapidly in SMABs, even at a low current, and the cathode catalyst
still suffered rapid Cl corrosion. Therefore, it is necessary to further modify the graphene-coated cobalt
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catalysts to improve their catalytic activity and stability in seawater electrolytes.

Wu et al. have developed a three-step method to construct defect-rich Fe-doped Co NPs coated by N-doped
hierarchical carbon (D-FeCo@NHC) [Figure 6A] . The as-prepared D-FeCo@NHC has a typical
[98]
core-shell structure with metal NPs encapsulated in carbon, which can promote the electrical conductivity
and corrosion resistance of catalysts [Figure 6B]. Besides, Fe doping can not only promote the formation of
metal defects but also adjust the electronic structure of D-FeCo@NHC. Meanwhile, DFT theoretical
calculations show that the combination of metal and carbon defect synergistically optimized the d-band
center of the sample and thus boosted the ORR activity [Figure 6C]. As shown in Figure 6D and E, the
D-FeCo@NHC exhibits a high E of 0.874 V in alkaline seawater electrolytes, and the as-assembled battery
1/2
shows a high peak power density and long cycling stability.

In addition, TM-based single-atom electrocatalysts have been widely employed, which show promising
potential for being utilized as ORR electrocatalysts. Typically, Fe-N moieties on carbon matrix (Fe-N-C)
catalysts show excellent ORR activity in alkaline and acidic electrolytes. Furthermore, compared with Pt/C
and other noble metal-based catalysts, the Fe-N-C catalysts generally show excellent resistance to chlorine
poisoning. Based on the mentioned above, Fe-N-C catalysts have the potential as cathode materials in the
SMABs. However, up to now, little attention has been paid to its practical application in real seawater
environments. Gao et al. reported a microwave heating method and synthesized the atomically dispersed
Fe-N-G/CNT catalyst in a short time, which possesses high activity and a strong oxygen-philic interface
[99]
between graphene and CNTs [Figure 7A] . In addition, DFT calculations and experimental results indicate
that the high oxygen affinity of the catalyst is caused by the double adsorption sites on the G/CNT interface,
and the high activity of Fe-N active sites is due to charge separation [Figure 7B-E]. The Fe-N-G/CNT
4
shows an excellent ORR performance in both O -saturated alkaline solution and seawater with E of 0.929
1/2
2
and 0.704 V, respectively, which are much better than commercial Pt/C [Figure 7F]. In addition, the SMAB
with Fe-N-G/CNT as the cathode exhibits good battery performance in oxygen-poor seawater
(≈ 0.4 mg L ), where the discharge voltage at 10 mA cm is 1.18 V [Figure 7G].
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Although big progress has been made in the development and the application of Fe-N-C as the ORR
electrocatalysts in seawater batteries, the catalytic influencing mechanism of the Cl resistance of Fe-N-C
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catalysts is still unclear and remains a challenge. Zhan et al. prepared an atomic electrocatalyst by anchoring
Fe-N sites on N-doped activated carbon substrates (Fe-N /NAC) to explore the effect of Cl on Fe-N /NAC
-
x
x
x
ORR performance . The isolated single Fe atom is well dispersed and embedded on porous NAC, and the
[100]
Fe, C, N, and O elements are uniformly distributed throughout the active carbon matrix [Figure 8A and B].
Benefiting from the abundance of Fe-N active sites, high surface area of activated carbon, and good
x