Construction of an efficient CuCo-TA@FeOOH heterojunction for high-performance electrocatalytic seawater oxidation

Abstract

To mitigate the adverse effects of high concentrations of Cl⁻ ions in seawater on electrolysis efficiency, it is essential to develop efficient and stable electrocatalysts. Based on this need, CuCo-ZIF NCs were used as a precursor to synthesize a CuCo-TA@FeOOH heterojunction composites, specifically designed for the oxygen evolution reaction (OER) in alkaline seawater, through a combination of acid etching and a self-growth method. The resulting material exhibits an OER overpotential of 234 mV at 10 mA/cm² in alkaline freshwater and 256 mV at 10 mA/cm² in seawater electrolyte. This performance is attributed to synergistic interactions at the heterojunction interfaces, which enhances the specific surface area, offers abundant active sites, and improves mass transfer efficiency, thereby increasing catalytic activity. Moreover, at a current density of 100 mA/cm², it maintains stable performance for up to 300 h without deactivation. This remarkable stability and corrosion resistance stems from the synergistic effect at the CoOOH and FeOOH interface formed during reconstruction, which facilitates electron transfer, optimizes the electronic structure during the reaction process, and effectively suppresses the chlorine evolution reaction (CER). This study offers a valuable reference for the rational design of high-performance electrocatalysts for alkaline seawater oxidation.

Graphical abstract

Keywords

CuCo-TA@FeOOH / heterojunction / hollow structure / electrocatalytic seawater oxidation / oxygen evolution reaction (OER)

1 Introduction

Currently, alkaline electrolyzed water systems have shown great potential for application in the field of energy conversion and storage [1], and are regarded as one of the key technologies to promote the green energy revolution [2]. However, it is a reality that cannot be ignored that these efficient systems almost invariably rely on freshwater resources as electrolytes, while the scarcity of global freshwater resources has become a major bottleneck restricting sustainable development [3]. Meanwhile, the vast availability of seawater resources worldwide presents an opportunity for electrocatalytic seawater splitting technology to enable sustainable and eco-friendly hydrogen production [4].

Nevertheless, the high concentration of chloride ions Cl⁻ in seawater favors the kinetically faster two-electron chlorine evolution reaction (CER) over the desired oxygen evolution reaction (OER) on the anode [5]. This preference not only hinders the efficient progression of OER but also triggers anode catalyst corrosion and hypochlorite formation, which are detrimental to system stability and efficiency [6]. Therefore, the design of low-cost, high-catalyst-activity OER catalysts is particularly critical for advancing seawater electrolysis technology [7]. Although Ir, Ru, and their oxides are used as efficient OER electrocatalysts [8,9], their high cost and limited availability significantly restrict widespread application. Thus, there is an urgent need to develop non-precious metal catalysts with high electrocatalytic activity [10].

In recent years, cobalt-based bimetallic organic frameworks (MOFs) have demonstrated exceptional electrochemical performance due to their metallic synergistic effects and adjustable metal nodes, leading to widespread application in water electrolysis [11]. However, their OER activity remains suboptimal due to the limited exposure of active sites [12]. Recent studies have demonstrated that enhancing the water electrolysis performance of MOF materials can be achieved by structural modifications—such as the creation of hollow structures—and construction of heterojunctions [13–15]. To improve the interaction between the electrolytes and active sites within the catalyst, as well as to enhance ion transport, acid etching (e.g., using tannic acid (TA)) has emerged as an effective strategy for preparing functionalized MOFs materials [16]. The hollow structure derived from acid etching can effectively promote the exposure of more active sites and enhance mass transfer efficiency [17–19].

Moreover, heterojunction structures incorporating transition metal hydroxides exhibit significant potential in water splitting due to their ability to facilitate water decomposition in alkaline environments [22]. On the other hand, hydroxy oxides possess self-healing capabilities during the electrolysis process, providing excellent OER stability. Among these, FeOOH emerges as a highly promising OER candidate material due to its simple synthesis, cost-effectiveness, and superior intrinsic activity compared to NiOOH and CoOOH [23,24].

Based on these insights, the present study has developed a seawater electrocatalyst with a hollow heterostructure using an etching and self-growth strategy (CuCo-TA@FeOOH). Experimental results indicate that the CuCo-TA@FeOOH electrocatalyst demonstrates favorable OER performance and maintains stable operation at a current density of 100 mA/cm² for 300 h. This performance is attributed to the electronic structure modification induced by the heterostructure, as well as the synergistic interactions across different interfaces, which accelerates electron transfer and suppresses the CER. These effects collectively contribute to enhanced corrosion resistance.

In summary, this strategy provides a valuable approach for the development of cost-effective, high-performance catalysts for alkaline seawater electrolysis.

2 Experimental section

2.1 Material preparation

2.1.1 Synthesis of CuCo-ZIF nanocubes (NCs)

CuCo-ZIF NCs were synthesized using a room-temperature precipitation method [25]. Specifically, 0.873 g of Co (NO₃)₂·6H₂O and 0.483 g of Cu (NO₃)₂·3H₂O were dissolved in 30 mL of deionized water and stirred vigorously until the solution became clear. Next, 15 mg of cetyltrimethylammonium bromide (CTAB) was added and dissolved in the solution, followed by rapidly stirring at room temperature for 10 min, to form Solution A. Separately, 13.62 g of 2-methylimidazole (2-MI) was dissolved in 210 mL of deionized water with continuous stirring to firm Solution B. Solution A was rapidly poured into Solution B and stirred vigorously for 30 min at room temperature. The mixture was then left undisturbed for 12 h. The resulting purple precipitate was collected by centrifugation, washed three times each with deionized water and absolute ethanol, and dried in a vacuum oven at 60 °C overnight.

2.1.2 Synthesis of CuCo-TA nanoboxes (NBs)

A total of 150 mg of the synthesized CuCo-ZIF NCs was dispersed in 100 mL of anhydrous ethanol, using ultrasound treatment for 10 min. This dispersion was then poured into 800 mL of a mixed solution (V_{{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}}:V_{{\mathrm{H}}_2\mathrm{O}} = 1:1) containing tannic acid (TA) at a concentration of 1 mg/mL. The mixture was stirred at room temperature for 15 min. The resulting brownish-yellow powder was collected by centrifugation at 5000 r/min, washed several times with anhydrous ethanol, and dried in an oven at 60 °C overnight. To evaluate the influence of TA on electrocatalytic efficiency, the amount of CuCo-ZIF NCs added was varied (150, 300, and 450 mg) [26].

2.1.3 Synthesis of CuCo-TA@FeOOH

CuCo-TA@FeOOH was also synthesized via a room-temperature precipitation method. In total, 500 mg of CuCo-TA NBs was dissolved in 15 mL of anhydrous ethanol under ultrasonic agitation at room temperature until complete dispersed. Then, 10 mL of an FeCl₂·4H₂O solution (1 g in ethanol), was rapidly added, and the mixture was stirred vigorously at room temperature for 4 h. The resulting precipitate was collected by centrifugation, washed multiple times with anhydrous ethanol, and dried in a vacuum oven at 60 °C overnight.

2.1.4 Synthesis of FeOOH

FeOOH samples as prepared using a similar method to that of CuCo-TA@FeOOH, except the addition of CuCo-TA NBs.

2.2 Electrochemical measurements

Electrochemical measurements were performed using a VMP-300 multichannel electrochemical workstation (BioLogic, France) in a standard three-electrode setup. The working electrodes consisted of the prepared samples (CuCo-TA NBs, FeOOH, and CuCo-TA@FeOOH), with an Hg/HgO electrode as the reference electrode and a graphite rod as the counter electrode [27].

Cyclic voltammetry (CV) was conducted at a scanning rate of 50 mV/s. For polarization curve measurements, a reverse scan at a slower rate of 5 mV/s was employed in three electrolytes: 1 mol/L KOH, 1 mol/L KOH + 0.5 mol/L NaCl, and 1 mol/L KOH + seawater. This approach was taken to avoid overestimation in activity evaluation. Tafel slopes were extracted from the linear sweep voltammetry (LSV) curves using the equation η = a + blogj. Electrochemical impedance spectroscopy (EIS) was performed across a frequency range from 0.01 Hz to 0.1 MHz with an amplitude of 5 mV.

The electrochemical double-layer capacitance without Faradaic processes, was determined from CV in the potential range of 0–0.1 V versus saturated calomel electrode (SCE), providing an estimate of the electrochemical active surface area (ECSA) of the catalysts. Corrosion resistance was evaluated using chronopotentiometry. The potentials measured at room temperature were converted to the reversible hydrogen electrode (RHE) scale using

{\mathit{E}}_{\mathrm{R}\mathrm{H}\mathrm{E}}={\mathit{E}}_{\mathrm{H}\mathrm{g}/\mathrm{H}\mathrm{g}\mathrm{O}}+0.098+0.0592\times \mathrm{p}\mathrm{H}.

All electrochemical measurements were corrected with 85% iR compensation [28].

3 Results and discussion

3.1 Structural characterization

The synthesis process of CuCo-TA@FeOOH is illustrated in Fig. 1(a). Initially, cubic-shaped CuCo-ZIF NCs were prepared through precipitation at room temperature. Compared with ZIF-67 (Fig. S1), the obtained CuCo-ZIF NCs exhibited a similar morphology (Fig. 1(b)). Subsequently, TA etching was employed to optimize the coordination environment of the precursors, resulting in transformation of CuCo-ZIF NCs into CuCo-TA NBs with a hollow structure, exhibiting uniform distribution and smooth surfaces, as shown in Fig. 1(c)). Following the introduction of Fe²⁺ ions, a surface cation-exchange reaction occurred on the CuCo-TA NBs. Due to the abundance of phenolic hydroxyl groups, Fe²⁺ ions formed coordination polymers on the surface, rendering material surface rough. Under appropriate experimental conditions, such as mechanical stirring, the FeOOH nanosheet subsequently formed on the surface (Fig. 1(d)).

To further investigate the microstructure of CuCo-TA@FeOOH, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were conducted, as depicted in the Figs. 1(e)–1(g). Compared with the previously synthesized ZIF-67 and CuCo-ZIF NCs (Fig. S2), the CuCo-TA@FeOOH structure exhibited a hollow nanocage morphology. This morphology is favorable for facilitating electron and mass transport and for increasing the exposure of unsaturated metal sites along the edges and surfaces of the nanosheets, which collectively enhance catalytic performance for OER. HRTEM images and selected electron diffraction (SAED) patterns (inset in Fig. 1(g)) revealed faint diffraction rings and indistinct lattice fringes, suggesting that the FeOOH component was present in a microcrystalline form with weak crystallinity. Furthermore, elemental mapping results shown in Figs. 1(h)–1(m), confirmed the uniform distribution of C, O, Cu, Co, and Fe elements across the entire structure.

To study the crystal phases of CuCo-TA NBs, FeOOH, and CuCo-TA@FeOOH, X-ray diffraction (XRD) analysis was conducted. As shown in Fig. S3, the synthesized CuCo-ZIF NCs retain the characteristic structure of ZIF-67. As shown in Fig. 2(a), the CuCo-TA NBs exhibit only a broad diffraction peak at 20°, indicating an amorphous structure. The XRD pattern of FeOOH corresponds closely with that of standard β-FeOOH, with diffraction peaks appearing at 2θ values of 11.8°, 16.7°, 26.7°, 35.1°, 39.2°, 46.4°, 52.0°, 55.9°, and 64.3°, which are assigned to the (110), (200), (310), (211), (301), (411), (600), (521), and (541) crystal planes, respectively, in accordance with JCPDS No. 34-1266 [29]. These results confirm the successful synthesis of FeOOH.

In contrast, the XRD pattern of CuCo-TA@FeOOH reveals only a broad peak indicative of an amorphous phase, and no distinct diffraction peaks corresponding to FeOOH are observed. This absence of characteristic peaks may be due to the low crystallinity of the synthesized FeOOH component within the heterojunction structure, consistent with the observations from TEM.

In addition, FT-IR was used to characterize the molecular structures of the synthesized materials, as illustrated in Fig. 2(b). For the CuCo-TA NBs, characteristic absorption bands are observed at 3452, 1720, and 1622 cm⁻¹, corresponding to the O−H, C=O, and C=C stretching vibrations present in TA [30]. Additional peaks observed at 1381 and 1201 cm⁻¹ are attributed to aromatic C−O vibrations, while the vibrational band at 1064 cm⁻¹ is associated with C−O−C stretching vibrations [31].

In the spectrum of FeOOH, the broad peak at 3361 cm⁻¹ corresponds to the stretching vibration of the −OH groups, while the bands at 854 and 1630 cm⁻¹ are assigned to the bending vibrations of −OH and adsorbed water, respectively. In the spectrum of FeOOH, a broad peak at 3361 cm⁻¹ is observed, which is attributed to the stretching vibration of the O−H bond within FeOOH. Furthermore, the bending vibration of Fe-OH likely correlates with the peaks situated near 1630 and 1197 cm⁻¹, whereas the peaks spanning from 854 to 434 cm⁻¹ correspond to metal-oxygen (Fe-O) vibrations. A distinct feature at 684 cm⁻¹ is associated with the deformation vibration of Fe−O−Cl hydrogen bonding [32].

Regarding the CuCo-TA@FeOOH composite, the FT-IR spectrum largely mirrors that of CuCo-TA NBs, indicating the preservation of the original TA structure. But the vibration peak corresponding to the Fe-O-Cl hydrogen bond disappears, the vibration peak of Mmuro (metal-oxygen) remains and the peak strength increases. This suggests that the interaction between the metal centers (Cu and Co) in CuCo-TA NBs and the oxygen atoms in FeOOH modifies the original chemical bonding and vibrational characteristics. These spectral changes support the successful integration of FeOOH onto the CuCo-TA NBs.

Figure S4 shows the Raman spectra of CuCo-TA NBs, FeOOH, and CuCo-TA@FeOOH. In the Raman spectrum of CuCo-TA NBs, a broad G-band centered around 1650 cm⁻¹ is evident, indicating the presence of carbonaceous species. FeOOH and CuCo-TA@FeOOH show five main characteristic peaks at 215, 276, 387, and 581 cm⁻¹, indicative of FeOOH [33]. However, the intensity of these peaks in the CuCo-TA@FeOOH spectrum is relatively weak, which may be caused by the amorphous nature of CuCo-TA NBs in the material.

XPS was employed to examine the surface chemical state of the composite. As shown in Figs. 2(c), 2(d) and S5(a), the presence of Cu, Co, and Fe elements in CuCo-TA@FeOOH is confirmed. In the Cu 2p spectrum of CuCo-TA@FeOOH (Fig. S5(a)), two spin orbit peaks 932.94 and 952.74 eV correspond to Cu⁺ 2p_3/2 and 2p_1/2, respectively, while the satellite peak at 940.68 eV indicates the partial oxidation of Cu⁺ to Cu²⁺ [34]. Compared to the CuCo-TA NBs, a shift of the Cu 2p_1/2 and Cu 2p_3/2 peaks toward higher binding energies is observed in CuCo-TA@FeOOH, indicating altered electronic environments.

The Co 2p spectrum (Fig. 2(c)) displays spin-orbital peaks at 780.6 and 796.4 eV corresponding to Co³⁺, and a peak at 782.8 eV attributed to Co²⁺ [35]. In the Fe 2p spectrum (Fig. 2(d)), binding energies of 711.04 and 724.42 eV are assigned to Fe²⁺ 2p_1/2 and Fe 2p_3/2, respectively, which is the characteristic of Fe²⁺ in CuCo-TA@FeOOH. while additional peaks at 714.1 and 727.36 eV correspond to Fe³⁺ [36].

In contrast, the O1s spectrum in Fig. S5(b) shows a marked decrease in the peak areas related to C=O and C−C, while peaks assigned to Fe-O bonds become prominent. This observation suggests the preferential coordination of Fe³⁺ with the phenolic −OH hydroxyl group in TA. Additionally, the Cu 2p and Co 2p peaks exhibit shifts toward lower binding energies, likely due to intermolecular interactions that lead electron transfer to the Cu and Co sites [37].

3.2 Electrocatalytic performance

To investigate the impact of TA concentration on the performance of CuCo-ZIF NCs, an LSV with IR (85%) compensation was conducted using a three-electrode system in 1.0 mol/L alkaline KOH solution. This enabled identification of the CuCo-TA NBs with optimal performance for subsequent modification (as shown in Fig. S6). The results reveal that CuCo-TA NBs prepared with 150 mg of precursor exhibit the best electrocatalytic performance. However, increasing the amount of precursor lead to decreased LSV performance, which can be attributed to the aggregation of some of unreacted precursor particles, leading to a decline in electrocatalytic performance.

The LSV curves in Fig. 3(a) indicate that at a current density of 10 mA/cm², the overpotential of CuCo-TA@FeOOH is merely 234 mV, significantly lower than that of CuCo-TA NBs (299 mV) and FeOOH (257 mV), demonstrating superior electrocatalytic performance of OER. This enhancement may arise from the coupling of FeOOH at the crystal interface, which optimizes synergistic interactions between metal ions, thus improving the OER performance of CuCo-TA@FeOOH.

Tafel slope analysis was used to explore the kinetic characteristics of the catalyst in the OER process (Fig. 3(b)). The CuCo-TA@FeOOH catalyst exhibits a Tafel slope of 31 mV/dec, outperforming CuCo-TA NBs (60 mV/dec) and FeOOH (39 mV/dec), indicating that CuCo-TA@FeOOH has the fastest reaction kinetics. These results demonstrate that the CuCo-TA@FeOOH heterojunction effectively enhances electron transfer between the electrode and electrolyte, facilitating a more efficient electrochemical process.

The double-layer capacitance (C_dl), derived from the CV curves, is utilized to estimate the electrochemical surface area (ECSA) of the catalyst. Figure 3(d), displays the linear plot of the capacitive current density versus scan rate, which is obtained by analyzing the respective CV curves presented in Fig. S7. The figure shows that CuCo-TA@FeOOH (3.13 mF/cm²) has a higher C_dl value compared to CuCo-TA NBs (2.74 mF/cm²) and FeOOH (1.29 mF/cm²). Subsequently, based on the formula ECSA = C_dl/C_s (where C_s represents the capacitance of the electrode surface, typically 0.04 mF/cm²) (Fig. 3(f)), the ECSA values were calculated as 78.3 cm² for CuCo-TA@FeOOH, 68.5 cm² for CuCo-TA NBs and 30.8 cm² for FeOOH. These results demonstrate that the CuCo-TA@FeOOH heterojunction exhibits a larger electrochemical surface area and more active catalytic sites, leading to superior catalytic activity.

The inherent catalytic activity was evaluated by normalizing the LSV curve to the ECSA. The results shows that CuCo-TA@FeOOH has the highest intrinsic catalytic activity compared with CuCo-TA and FeOOH.

In addition, the charge transfer behavior was further examined via Nyquist plots. As shown in Figs. 3(g) and S8, and Table S1, CuCo-TA NBs (11.67 Ω) and FeOOH (1.07 Ω), CuCo-TA@FeOOH (0.61 Ω) has the lowest R_ct value. This indicates that CuCo-TA@FeOOH exhibits excellent charge transfer kinetics.

The durability was assessed by chronopotential (CP) stability testing (Fig. 3(h)). It can be seen that the CuCo-TA@FeOOH sample maintains high activity for 100 h at a current density of 10 mA/cm². As can be observed from Fig. 3(i), the LSV curves recorded before and after stability test are nearly identical, further confirming its excellent stability. Overall, these findings demonstrate that CuCo-TA@FeOOH has outstanding OER catalytic performance and excellent electrochemical stability.

Given the remarkable OER catalytic activity of CuCo-TA@FeOOH in 1 mol/L KOH alkaline freshwater solution, its performance was further evaluated in simulated seawater electrolysis. Figure S9 presents a comparative analysis of the catalytic activities of all the prepared samples. As shown in Fig. S9(a), CuCo-TA@FeOOH exhibits robust OER catalytic activity with an overpotential of 249 mV at 10 mA/cm² in simulated seawater solution (1 mol/L KOH with 0.5 mol/L NaCl). As depicted in Fig. S9 (b), CuCo-TA@FeOOH possesses the smallest Tafel slope (32 mV/dec) compared to CuCo-TA NBs and FeOOH, indicating faster reaction kinetics in simulated seawater.

It is noteworthy that Fig. S9(c) demonstrates that, compared to alkaline freshwater solutions, the CuCo-TA@FeOOH exhibits a decreased C_dl in its double-layer capacitance value (C_dl = 2.49 mF/cm²). However, despite this reduction, overall catalytic activity remains enhanced, likely due to the hollow structure, which boosts the catalytic capacity of CuCo-TA@FeOOH in marine environments. Nyquist plots (Fig. S9(d) and Table S2 demonstrate a marginal increase in R_ct in seawater but still support the exceptional performance of CuCo-TA@FeOOH, concurrently validating the outcomes of the Tafel analysis.

Figures 4(h) and 4(i) exhibit the long-term stability of CuCo-TA@FeOOH at a current density of 100 mA/cm² for 100 h and the corresponding LSV changes after stability test. These results highlight the rapid mass transport, superior conductivity, and remarkable stability of CuCo-TA@FeOOH for OER catalysis in simulated seawater.

To validate the environmental adaptability of the materials, the catalytic activities of all prepared samples were evaluated in an alkaline seawater solution (1 mol/L KOH with seawater) (Fig. 4). Among the materials tested, CuCo-TA@FeOOH exhibited the most superior OER catalytic activity, with an overpotential of 256 mV at 10 mA/cm². Furthermore, analysis of the Tafel plot (Fig. 4(b)), C_dl fitting curves (Fig. 4(c)), and EIS data (Fig. 4(d), Table S3) revealed that CuCo-TA@FeOOH possessed the lowest Tafel slope (37 mV/dec), the smallest Rct (0.93 Ω), and the maximum value of C_dl (1.89 mF/cm²). In terms stability, CuCo-TA@FeOOH demonstrated the best durability for seawater catalysis (Fig. 4(e)), maintaining performance over an extended period of 300 h at a current density of 100 mA/cm². Additionally, a stable performance was maintained throughout the multi-step chronoamperometric test (Fig. 4(f)), where the current density increased from 50 to 250 mA/cm² at a rate of 50 mA/cm² every 2 h. These results indicate that the heterojunction structure effectively promotes the electrocatalytic OER performance in seawater solutions.

Beyond activity and stability, selectivity is crucial for the effectiveness of a catalyst. Figures S10(a) and S10(b) present a comparison of the electrocatalytic oxidation performance of CuCo-TA@FeOOH in both freshwater and seawater environments. The results reveal that both simulated seawater and actual seawater solutions exhibit reduced OER activity compared to freshwater, likely due to the presence of trace elements, dissolved organics, microorganisms, and undesirable precipitates in seawater, which may poison the electrode or block active sites, thereby hindering catalytic activity [38]. Nevertheless, CuCo-TA@FeOOH exhibits an overpotential of 420 mV at 100 mA/cm² in alkaline seawater, significantly below the theoretical voltage for chloride precipitation reactions (480 mV) [39]. This indicates that the electrode remains unaffected by Cl⁻ corrosion, allowing for a longer operational lifespan.

In addition, the Faraday efficiency of CuCo-TA@FeOOH in alkaline seawater at 100 mA/cm² was determined using the water displacement method, as shown in Fig. S11. The results in Fig. S11(b) show a Faraday efficiency of 95%, confirming that CuCo-TA@FeOOH has excellent electrocatalytic OER activity.

3.3 Surface species analysis

To further investigate the origin of the catalyst activity, characterization of the samples after long-term stability testing was conducted using Raman spectroscopy, XPS, SEM, and TEM. This analysis aimed to determine the stability of the material’s structure and surface by assessing whether the CuCo-TA@FeOOH catalyst underwent any structural transformation following the OER reaction.

Figure S12 depicts changes in the Raman peak positions of CuCo-TA@FeOOH before and after stability testing. Notably, the Raman peaks associated with Fe-O vibrations in FeOOH (at 215, 276, and 387 cm⁻¹) disappear, while a new peak emerges at 547 cm⁻¹, attributed to the formation of a Co-OOH intermediate [40]. This suggests that the transfer of electrons to Co during the stability process promoted the further formation of CoOOH. These observations indicate a structural transformation in the material after the stability test.

Figure 5(a) displays the SEM image of CuCo-TA@FeOOH following long-term stability assessment. The material clearly consists of nanosheets, indicating that the original cubic structure collapsed after the stability test, undergoing a transformation from a 3D to a 2D nanosheet morphology. To confirm this observation, TEM analysis was conducted on the samples (Figs. 5(b) and 5(d)). The lamellar structures observed are consistent with the SEM results. Moreover, the HRTEM image in Fig. 5(d) reveals a lattice fringe spacing of 0.210 nm, corresponding to the (321) lattice plane of FeOOH (JCPDS No. 34-1266), and a lattice spacing of 0.206 nm, matching the (021) plane of CoOOH (JCPDS No.26-1107), thereby corroborating the findings from Raman spectroscopy.

To substantiate these results, XPS measurements were conducted to further explore the electronic structure of CuCo-TA@FeOOH before and after stability testing. As shown in Figs. 5(e), 5(f) and S13, XPS analysis confirms the presence of metallic Co and Fe, but Cu is absent, likely due to the dissolution of Cu⁺/Cu²⁺ ions into the electrolyte during the stability process. A leftward shift of the Co²⁺ peak in the Co 2p spectrum and an increase in peak area of Co³⁺ ions are observed. Additionally, the high-resolution Fe 2p XPS spectra exhibit a shift of Fe³⁺ to a lower binding energy, suggesting the electron transfer between Fe and Co, consistent with the formation of CoOOH species.

These results demonstrate that the CuCo-TA@FeOOH catalyst possesses a heterojunction interface that undergoes phase reconstruction in solution, resulting in the formation of metal hydroxides on its surface. This structural transformation contributes significantly to enhancing the durability of the catalyst.

4 Conclusions

In summary, a MOFs-derived CuCo-TA@FeOOH heterostructure was successfully synthesized as an efficient alkaline OER electrocatalyst using ZIF-67 as the precursor via a straightforward three-step process. Studies reveal that the transformed (Co,Fe)OOH from the CuCo-TA@FeOOH heterojunction serves as the genuine active catalyst, with Cu acting as a sacrificial metal to facilitate the conversion to CoOOH. This catalyst exhibits remarkable OER performance in 1 mol/L KOH freshwater electrolyte, achieving an overpotential of 234 mV at 100 mA/cm². In simulated seawater and alkaline seawater electrolytes, it demonstrates overpotentials of 249 and 256 mV, respectively, at a current density of 10 mA/cm². Notably, the catalyst can operate continuously for 300 h at 100 mA/cm² without deactivation, demonstrating excellent stability.

The enhancement in OER activity can be attributed to the altered electronic structure in the heterojunction, which reduces the reaction energy barrier, and the outer FeOOH layer which effectively protects the electrode and reduces the corrosion caused by Cl⁻, thus enhancing the stability of the material during seawater electrolytic oxidation. Consequently, the CuCo-TA@FeOOH electrocatalyst displays superior electrocatalytic OER performance. This work provides significant insights into the design of heterogeneous catalysts for seawater electrolysis.

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