Page 198 - Read Online

Page 10 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

In the two-electron process, each dissolved O molecule accepts only two electrons, resulting in the
2
formation of H O . This pathway is less desirable as it can lead to the generation of reactive oxygen species
2
2
and undesirable byproducts. The overall reaction can be represented as follows:

-
Cl is among the most abundant ions in seawater electrolytes. However, due to the specific Cl blocking
-
effect, the Cl tends to adsorb on the surface catalytic sites of the cathodes. The adsorption of Cl would
-
-
cause several serious results: (1) the active site exposure would be reduced, thus causing the decrease of
ORR current density; (2) the electronic structure of surface metallic sites would be changed, thus causing
the poison of catalytic sites [52,86] ; and (3) the local bonding environment of active sites would be tuned in the
catalysts and cause the dissolution of metal atoms, thus worsening the stability of cathodes. Currently, the
reversibility of Cl adsorption is still debatable. Along with the blocking effect, Cl adsorption behavior
-
-
would affect the breakage of O-O bonds during ORR processes, thus changing the ORR pathway from a
four-electron to a two-electron mechanism, leading to the generation of H O . More importantly, the
2
2
existence of H O at the three-phase interface (gas-solution-solid) will produce free radicals that can attack
2
2
the metal anodes or/and electrocatalysts, thus resulting in the reduction of durability [87,88] . In order to
overcome the bad influence of Cl on the catalytic process in seawater, the ORR electrocatalyst generally
-
should possess the prerequisites of high electronic conductivity and electrochemical stability in saltwater
electrolytes. In addition, other criteria such as large specific surface area, high intrinsic catalytic activity, and
[88]
low mass loading are also should be considered . Based on the related reports, ORR electrocatalysts can be
classified into three categories: noble metal-based materials, non-noble metal-based electrocatalysts, and
metal-free electrocatalysts. This section will summarize the research efforts that have focused on enhancing
the catalytic performance of different electrocatalysts in SMABs.

Noble metal-based ORR electrocatalysts
Noble metal-based ORR electrocatalysts, especially Pt-based catalysts, usually have high ORR efficiency, but
they are expensive and can be easily destroyed by a large amount of Cl in seawater. Besides, the formation
-
of soluble chlorine complexes (such as PtCl ) can also lead to the dissolution of platinum and the surface
2-
4
passivation of metallic sites, which adversely affects the stability of ORR [89,90] . Recently, Ryu et al. designed a
Pt-Co alloy electrocatalyst using carbothermal shock (CTS) for use in high-performance seawater
batteries . The authors prepared single and blended aqueous solutions of H PtCl ·6H O and CoCl
[91]
2
6
2
2
precursors with different ratios to synthesize the Pt-Co alloy on heated carbon felt (HCF) within 10 s
[Figure 4A and B]. Figure 4C enabled us to observe that the peak at q = 2.77 A shifted to a higher value,
-1
indicating that the formation of Pt-Co alloy leads to lattice contraction. Moreover, the Pt 4f XPS spectra of
the samples mentioned above show that the peaks of Pt 4f and Pt 4f shift to high binding energy, mainly
5/2
7/2
due to the loss of 5d electrons in Pt-based alloys, indicating the formation of Pt-Co alloys. As shown in
Figure 4D, the catalyst with Pt:Co = 2:1 achieved high current density and low overpotential in the ORR
process for the synergistic effect between the Pt and Co metallic sites. Moreover, the Pt-Co alloy
electrocatalyst can remain stable in the seawater catholyte, and thus, the seawater batteries using Pt-Co
alloys as cathodes exhibit excellent cycling stability with no obvious degradation [Figure 4E]. Compared to
the SMAB using pristine HCF as a current collector, the battery using a composition of Pt-Co nanoparticle
(NP)-decorated CF can be cycled at 0.3 mA cm for up to 500 h without substantial performance
-2
deterioration. In addition, Jin et al. reported well-dispersed Rh S NPs supported on carbon nanotube
17 15
(CNT) substrates . The diameter of the embedded particles was evenly about 8 nm, and the mass loading
[92]
o
of Rh reaches 25 wt% when pyrolyzed at 650 C. The ORR catalytic activity of the nanocomposites is highly
active under Cl electrolysis conditions, which could exhibit considerable stability in the chloride
-
environment.