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Page 20 of 30 Guo et al. Microstructures 2023;3:2023038 https://dx.doi.org/10.20517/microstructures.2023.30

Therefore, moderate current density and high pH values are the key conditions for catalysts to maintain
excellent electrocatalytic activity, selectivity, and long-term stability for OER processes.

Ni-based LDH electrocatalysts are known as the best OER candidates in seawater electrolytes, and NiFe
LDH, in particular, is considered to be the benchmark among noble metal-free catalysts. Furthermore, in
view of the considerable stability of Ir metals for OER, You et al. introduced Ir to develop a monolayer
[115]
NiIr-LDH as an OER catalyst for seawater electrocatalysis . The NiIr-LDH catalyst showed 313 mV and
361 mV overpotentials at 500 mA cm in artificial seawater (1 M potassium hydroxide + 0.5 M NaCl) and
-2
alkaline seawater (1 M potassium hydroxide + seawater), respectively, along with nearly 100% O Faradaic
2
efficiency in alkaline seawater [Figure 12C]. Impressively, the NiIr-LDH catalyst delivered excellent long-
term stability, maintaining its performance for up to 650 h under an industrial 500 mA cm current density
-2
in alkaline seawater [Figure 12D]. Compared with the OER benchmark NiFe-LDH, both Ir and Ni are
considered as the active sites for the OER in NiIr-LDH, and with the introduction of Ir, the Ni atom
becomes more electrochemically active. The incorporation of Ir into the Ni(OH) layer optimized the
2
electron density of Ir and Ni sites and accelerated the rate-limiting step of the intermediate *O and *OOH
generation on Ni and Ir sites. The synergistic effects of multi-component metallic sites are also one of the
most effective strategies for constructing efficient and stable electrocatalysts in seawater conditions.
Consequently, Enkhtuvshin et al. reported multi-metallic Ni-Fe-Al-Co-layered double hydroxides
(NFAC-MELDHs) as OER catalysts (1.0 M KOH + 0.5 M NaCl), which showed an overpotential of 280 mV
[116]
-2
at a current density of 100 mA cm [Figure 12E] . The Fe sites are considered to be the real active sites for
redox flexibility, which cooperate with the adjacent metals to stabilize the adsorption of oxygen
intermediates while promoting charge transfer.

Transition metal-based electrocatalysts
In the past decades, TM compound-based electrocatalysts have been considered as the most promising
alternatives for replacing noble metal-based electrocatalysts due to their earth-abundance, popular price,
potential multi-catalytic performance, and adjustable crystal and electronic structures [117-119] . Based on these
advantages, strategies have been developed in recent reports to promote the OER performance of TM
compound-based catalysts in seawater-based electrolytes. Accordingly, Yu et al. synthesized a 3D core-shell
NiMoN@NiFeN OER catalyst for seawater catalysis . The seawater diffusion and gas releasing processes
[120]
were facilitated, benefiting from the 3D core-shell structure with multiple levels of porosity. Therefore, on
the one hand, the high conductivity and large surface area of interior NiMoN nanorods led to an efficient
charge transfer rate and more active sites. On the other hand, thin amorphous NiFe oxy(hydroxide) layers
in situ evolved from the outer NiFeN NPs under the OER-applied voltage, acting as the active species and
-
effectively resisting the invasion of Cl [Figure 13A]. The NiMoN@NiFeN catalyst showed 368 mV and 398
mV OER overpotentials at industrial current densities of 500 mA cm and 1,000 mA cm , respectively, in
-2
-2
alkaline natural seawater, which are both below the 480 mV overpotential required to trigger the HCFR
process [Figure 13B]. Kuang et al. designed a multilayer electrode NiFe/NiS -Ni as OER electrocatalysts in
x
alkaline seawater [Figure 13C] . NiS was formed from the Ni foam by a solvothermal method, and NiFe
[121]
x
hydroxide was prepared by electrodeposition using NiS -Ni foam as precursors. After anodic activation in
x
an alkaline condition, the polyanion sulfate/carbonate-passivated NiFe/NiS -Ni foam anode was generated.
x
After 1,000 h seawater catalysis in alkaline simulated seawater electrolyte (1 M KOH + 0.5 M NaCl),
NiFe/NiS -Ni delivered an OER overpotential of 510 mV at the current density of 400 mA cm , which could
-2
x
reach 300 mV at 400 mA cm after iR compensation and was below the 480 mV to limited the chloride
-2
oxidation to hypochlorite [Figure 13D]. Even under the condition of high temperature (80 °C), alkalinity
(6 M KOH), and salinity (1.5 M NaCl), the catalyst still shows similar excellent activity. Generally, NiFe is a
highly active and selective OER catalyst for seawater electrolysis, and the interior NiS layer is a conductive
x