A Hierarchical CuO Nanowire@CoFe-Layered Double Hydroxide Nanosheet Array as a High-Efficiency Seawater Oxidation Electrocatalyst

Seawater electrolysis has great potential to generate clean hydrogen energy, but it is a formidable challenge. In this study, we report CoFe-LDH nanosheet uniformly decorated on a CuO nanowire array on Cu foam (CuO@CoFe-LDH/CF) for seawater oxidation. Such CuO@CoFe-LDH/CF exhibits high oxygen evolution reaction electrocatalytic activity, demanding only an overpotential of 336 mV to generate a current density of 100 mA cm−2 in alkaline seawater. Moreover, it can operate continuously for at least 50 h without obvious activity attenuation.


Introduction
Hydrogen (H 2 ) is extensively considered an ideal future carbon-neutral energy carrier with high energy density, and water electrolysis is a facile, cost-effective, and environmentally friendly approach to producing high-purity H 2 [1][2][3][4][5][6]. However, considering the limited reserves and uneven distribution on earth of freshwater, direct freshwater electrolysis will be a non-negligible problem for large-scale H 2 production in the future [7,8]. Seawater, as an abundant water resource on earth, is regarded as a candidate to substitute for freshwater feedstock. However, the practical applications of seawater electrolysis still face formidable challenges, including the slow reaction kinetics of oxygen evolution reaction (OER) and the competitive chlorine evolution reaction occurring on the anode [9][10][11][12][13][14][15]. Thus, it is highly necessary to explore high-active OER electrocatalysts in seawater.
Currently, IrO 2 and RuO 2 exhibit excellent electrocatalytic properties for OER, but their exorbitant price and scarcity severely limit their widespread commercial applications [16][17][18][19]. Among transition metal-based electrocatalysts, layered double hydroxides (LDHs), especially CoFe-LDH, have recently aroused interest on account of their cost-effectiveness, good intrinsic sites, and relative ease of preparation [20][21][22]. It is widely known that three-dimensional core-shell nanostructures have the advantages of durability and more active sites to contact with electrolytes, promoting catalytic activity [23][24][25]. Although several studies have reported seawater oxidation enabled by CoFe-LDH [26][27][28], CoFe LDH-based hierarchical core-shell structures for boosting seawater oxidation has not been explored so far.
Herein, we report the development of a CoFe-LDH nanosheet decorated CuO nanowire array on Cu foam (CuO@CoFe-LDH/CF) for seawater oxidation. Such CuO@CoFe-LDH/CF exhibits high OER electrocatalytic activity, demanding only overpotentials of 336 and 375 mV to generate large current densities (j) of 100 and 300 mA·cm −2 in alkaline seawater, respectively. Furthermore, CuO@CoFe-LDH/CF achieves stable continuous electrolysis for 50 h at a j of 200 mA·cm −2 in alkaline seawater without obvious activity attenuation.
(120.32 mV dec −1 ), reflecting CuO@CoFe-LDH/CF has the fastest OER reaction kinetics. Notably, the double-layer capacitance (Cdl) value is evaluated by cyclic voltammetry tests in the non-Faraday region ( Figure S8) of CuO@CoFe-LDH/CF is 5.08 times as large as the CoFe-LDH/CF (23.4 vs. 4.6 mF·cm −2 ) (Figure 3c), signifying CuO@CoFe-LDH/CF can expose abundant active sites. The multi-step chronopotentiometry curve (Figure 3d) shows that the potentials are rapidly stabilized at each step, indicating CuO@CoFe-LDH/CF has a remarkable mass transfer capability. Motivated by the superior OER catalytic performance of CuO@CoFe-LDH/CF in alkaline freshwater, it was further evaluated in alkaline simulated seawater and alkaline seawater. When measured in alkaline seawater (black curve), the catalytic activity of CuO@CoFe-LDH/CF is less desirable than that in 1 M KOH (red curve) and alkaline simulated seawater (blue curve) in Figure 4a, which may result from the complex composition of seawater. Noticeably, the needed overpotentials of CuO@CoFe-LDH/CF to achieve j of 100, 300, and 500 mA cm −2 are only 336, 375, and 399 mV, respectively, revealing CuO@CoFe-LDH/CF has excellent seawater oxidation activity ( Figure 4b). As displayed in Figure S9, CuO@CoFe-LDH/CF shows a Tafel slope of 55.14 mV dec −1 , 56.62 mV dec −1 , and 70.2 mV dec −1 in alkaline freshwater, alkaline simulated seawater, and alkaline seawater, respectively. Notably, the electrocatalytic performance of CuO@CoFe-LDH/CF to Motivated by the superior OER catalytic performance of CuO@CoFe-LDH/CF in alkaline freshwater, it was further evaluated in alkaline simulated seawater and alkaline seawater. When measured in alkaline seawater (black curve), the catalytic activity of CuO@CoFe-LDH/CF is less desirable than that in 1 M KOH (red curve) and alkaline simulated seawater (blue curve) in Figure 4a, which may result from the complex composition of seawater. Noticeably, the needed overpotentials of CuO@CoFe-LDH/CF to achieve j of 100, 300, and 500 mA cm −2 are only 336, 375, and 399 mV, respectively, revealing CuO@CoFe-LDH/CF has excellent seawater oxidation activity ( Figure 4b). As displayed in Figure  S9, CuO@CoFe-LDH/CF shows a Tafel slope of 55.14 mV dec −1 , 56.62 mV dec −1 , and 70.2 mV dec −1 in alkaline freshwater, alkaline simulated seawater, and alkaline seawater, respectively. Notably, the electrocatalytic performance of CuO@CoFe-LDH/CF to generate the j of 100 mA cm −2 also stands out from most of the reported OER self-supported seawater electrocatalysts (Figure 4c and Table S1). Figure 4d exhibits the LSV curves of CuO@CoFe-LDH/CF before (red curve) and after 3000 CV scans (black curve), and it shows no noticeable decay in comparison to the initial one before scanning. Furthermore, the chronopotentiometry tests conducted at j of 100 and 200 mA cm −2 are also applied to show remarkable OER stability of the CuO@CoFe-LDH/CF in alkaline seawater, which shows no significant decay after 50 h operation (Figure 4e). In contrast, CoFe-LDH/CF exhibits obvious performance degradation after only 24 h of continuous electrolysis ( Figure  S10). Colorimetric test papers are used to confirm the existence of hypochlorite production during seawater oxidation. Figure S11 shows no apparent color change in the test papers, indicating that hypochlorite is not produced in the stability tests. The SEM images ( Figure S12) and XRD pattern ( Figure S13) of the post-OER CuO@CoFe-LDH/CF confirm that the morphology and crystal structure of CuO@CoFe-LDH/CF are almost unchanged, suggesting the excellent stability of CuO@CoFe-LDH/CF in alkaline seawater. Notably, no Molecules 2023, 28, 5718 5 of 9 peak associated with Cl is observed in the XPS survey spectrum of post-OER CuO@CoFe-LDH/CF ( Figure S14a). Moreover, the cobalt in CuO@CoFe-LDH/CF is oxidized to higher valance Co 3+ (CoOOH) after a long-term durability test ( Figure S14b), which may be the active site for OER. The production of CoOOH is useful for the resistance to chloride ion corrosion in seawater [26].
LDH/CF exhibits obvious performance degradation after only 24 h of continuous electrolysis ( Figure S10). Colorimetric test papers are used to confirm the existence of hypochlorite production during seawater oxidation. Figure S11 shows no apparent color change in the test papers, indicating that hypochlorite is not produced in the stability tests. The SEM images ( Figure S12) and XRD pattern ( Figure S13) of the post-OER CuO@CoFe-LDH/CF confirm that the morphology and crystal structure of CuO@CoFe-LDH/CF are almost unchanged, suggesting the excellent stability of CuO@CoFe-LDH/CF in alkaline seawater. Notably, no peak associated with Cl is observed in the XPS survey spectrum of post-OER CuO@CoFe-LDH/CF ( Figure S14a). Moreover, the cobalt in CuO@CoFe-LDH/CF is oxidized to higher valance Co 3+ (CoOOH) after a long-term durability test (Figure S14b), which may be the active site for OER. The production of CoOOH is useful for the resistance to chloride ion corrosion in seawater [26].

Preparation of CuO/CF, CuO@CoFe-LDH/CF, and CoFe-LDH/CF
Firstly, 5 g NaOH and 1.428 g (NH 4 ) 2 S 2 O 8 were dissolved in 50 mL of ultrapure water and then put a Cu foam (2 cm × 3 cm) into the aqueous solution for 20 min. The obtained sample was dried in the air, followed by air annealing at 180 • C for 1 h (2 • C min −1 ) to obtain CuO/CF. After that, potentiostatic electrodeposition was performed in a threeelectrode setup. The working electrode, reference electrode, and counter electrode were the CuO/CF, Ag/AgCl, and a graphite rod, separately. Typically, Co(NO 3 ) 2 ·6H 2 O (2.2 g) and FeSO 4 ·7H 2 O (2.08 g) were dissolved in 50 mL of water and mixed to form the electrolyte. Then, the CoFe-LDH was electrodeposited on the CuO (1 cm × 2 cm) at −1.0 V vs. Ag/AgCl for 100 s. The synthesized catalyst was washed with water several times and dried in air. CoFe-LDH/CF was similarly prepared.

Conclusions
In summary, we report a hierarchical CuO@CoFe-LDH nanoarray on Cu foam as a high-active and robust seawater oxidation electrocatalyst. Such CuO@CoFe-LDH/CF offers excellent electrocatalytic activity for seawater oxidation with low overpotentials of only 336 and 375 mV to attain j of 100 and 300 mA cm −2 , respectively. It also shows long-term electrochemical durability to retain its activity for at least 50 h at a j of 200 mA cm −2 . This work not only offers an efficient and stable catalyst for seawater oxidation but also paves the strategy for the construction of core-shell hierarchical nanoarray as attractive catalyst materials for seawater oxidation.