Two-dimensional bimetallic selenium-containing metal-organic frameworks and their calcinated derivatives as electrocatalysts for overall water splitting

Abstract

The use of two-dimensional (2D) layered metal-organic frameworks (MOFs) as self-sacrificial templates has been proven to be a successful method to create high-efficiency Selenium (Se)-containing electrocatalysts for overall water splitting. Herein, two strategies are then utilized to introduce Se element into the Co–Fe MOF, one being the etching of as-prepared MOF by SeO₂ solution, and the other, the replacing of SCN⁻ with SeCN⁻ as the construction unit. The electrochemical activity of the pristine 2D MOF and their calcinated derivatives for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is evaluated and further discussed. It is found that the effect of introducing Se on improving electrochemical catalytic activity is significant for the HER process. Specifically, the calcinated derivative in the replacing method exhibits an overpotential of 235 mV for HER and 270 mV for OER at a current density of 10 mA/cm²_. For comparing the two methods of introducing Se element into MOF, similar electrocatalytic activity can be achieved on the their calcinated derivatives. The high electrochemical performance of 2D CoFe-MOF derivatives may be resulted from the unique 2D hierarchical porous structure and strong synergistic effect between different components in the material.

Graphical abstract

Keywords

2D MOF / selenium (Se)-containing materials / hydrogen evolution reaction (HER) / oxygen evolution reaction (OER)

1 Introduction

Hydrogen is thought to be a suitable energy vehicle for energy conservation and delivery for its extreme energy density and zero-carbon nature. In the industrial field, hydrogen is also a critical chemical raw ingredient. The conventional hydrogen production process (e.g., natural gas reforming) will not only increase fossil fuel use, but also significantly increase global carbon dioxide emissions [1–4]. Electrolysis of water is a highly economical and sustainable method of synthesizing hydrogen [5–8]. Two crucial reactions in the entire process of water splitting are the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) [9–16]. It is significant to investigate catalytically active materials for the application of electrochemical hydrogen production. Different materials, such as metal chalcogenides, metal carbides, and metal oxides, have been reported to have good catalytic effects on producing oxygen and hydrogen [17–20]. Metal sulfides have been reported to exhibit outstanding catalytic performance for water splitting [21]. Compared to sulfur atoms, selenium atoms have lower ionization energies, more metallicity (greater conductivity), and a larger atomic radius. Correspondingly, transition metal selenides have a higher metallicity, a narrower band gap, a great stability, and a low cost, making them a good choice for electrocatalytic water splitting [22–24].

Metal-organic frameworks (MOFs) are a sort of high porosity crystal material created by combining metal ions and organic ligands [25–30]. Because MOFs have a unique coordination connection mechanism, they can be paired with a variety of metal centers and organic ligands. The structural advantages of MOFs, such as porous structure, structural tailor-ability, and easy component control, dictate their practical implementation in a broad diversity of fields, and they also show tremendous promise as a potential precursor for total water splitting [31–36]. In addition, the modification of bimetallic MOFs and their derived materials has been used to make catalysts with good electrocatalytic capabilities [37–41]. Traditionally, the MOF-derived selenides are produced via the self-sacrificing MOF template method, i.e., the pyrolysis of MOF followed by selenidation with selenium powder at high temperature. However, this strategy is high-energy consuming. In addition, it is difficult to precisely control the structure and component homogeneity of the product during pyrolysis. Moreover, the advantages of the component homogeneity and tailor-ability of MOFs can be fully employed to resolve the above problems and govern the number of specific components. For example, selenium (Se)-containing MOFs can be prepared in a single step using a Se-containing ligand (SeCN⁻) as initial component, which can be used as the catalysts directly or as the templates to prepare selenides by pyrolysis, avoiding the difficulties of component in-homogeneity and excessive defects caused by post-doping of selenium powders.

2D layered MOF, in particular, usually has a lot of interlayer electrons and intermediate adsorption/desorption pathways, making it a good template for water splitting catalysis [42–48]. To promote the application of MOF-based electrocatalysts, it is critical to create an MOF with high efficiency and large-scale production. The two-dimensional (2D) MOF can be created in a variety of ways, such as solvothermal reaction, mechanical grinding, and diffusion technique. Therein, the reaction conditions of the diffusion method are mild, and the solvent can be changed to tailor the structure of MOF [49–55]. As a result, the cost of MOF-based electrocatalysts can be lowered from the initial step, and large-scale preparation would be achieved by using the diffusion method.

In this work, a 2D CoFe-MOF constructed via the diffusion method was selected as self-sacrificing template to explore the viability of making Se-containing electrocatalysts via two methods. In the first method, CoFe-MOF is used as raw material, the selenium source is introduced by SeO₂ solution etching, and then the CoFe-MOF is calcined. In the second method, SeCN⁻ is used as the construction unit to prepare Se-containing MOF, and then the Se-containing MOF is calcined. It is found that the calcination obviously enhances the activity of MOF for HER. Co/Fe/Se-400 displays an overpotential of 270 mV (10 mA/cm²) for OER and 235 mV (10 mA/cm²) for HER, and a remarkable stability over the course of a 24-h consecutive test. The current findings may provide a simple and feasible way to manufacture metal selenides as catalysts for HER and OER.

2 Experimental

2.1 Materials

Potassium hydroxide (KOH, 99.9%), ferrous sulfate heptahydrate (FeSO₄·7H₂O,99.9%), potassium selenocyanate (KSeCN, 98.5%), selenium dioxide (SeO₂, 99.9%), N,N-dimethylformamide (DMF, 99.8%), and 1-methyl-2-pyrrolidinone (NMP, 99.9%) were obtained from the Aladdin Co. (Shanghai, China), while cobalt sulfate heptahydrate (CoSO₄·7H₂O, 99.5%), methanol solution (CH₃OH, 99.5%), potassium thiocyanate (KSCN, 98.5%), and poly (vinylidene fluoride) (PVDF) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received without any purification.

2.1.1 Synthesis of Co_0.6Fe_0.4-MOF

The synthetic procedure for CoFe-MOF was slightly modified from Ref.[56]. CoSO₄·7H₂O (0.06 mmol), KSCN (0.2 mmol), and FeSO₄·7H₂O (0.04 mmol) were combined in 5 mL of methanol solution and vigorously stirred for several minutes. The mixture appeared to precipitate and was then filtered. The filtrate was transferred to a rigid test tube, defined as layer A (Fig.1(a)), followed by the slow addition of methanol solution (5 mL), as layer B (Fig.1(a)), finally the addition of methanol solution containing L (0.005 mol/L) placed in layer C (Fig.1(a)). After leaving the reaction for a few days, shiny pink crystals (35% in L) appeared at the bottom of the test tube. L represents tetrakis (3-pyridyloxymethylene) methane which was produced in accordance with Wang et al. [57]. The X-ray diffraction (XRD) pattern of the Co_0.6Fe_0.4-MOF precursor is consistent with the original Co-MOF, which determined successfully to synthesize by powder X-ray diffraction (PXRD), as shown in Fig. S1 in Electronic Supplementary Material (ESM). In addition, Co and Fe contents were measured by inductively coupled plasma−mass spectrometry (ICP/MS) to further confirm the above results, which are shown in Table S1.

2.1.2 Synthesis of Co/Fe/S/Se-400

The pristine Co_0.6Fe_0.4-MOF was submerged in the aqua solution of SeO₂ for 5 h. The solid samples were dried for later use after being filtered, then washed three times with water. Then the solid samples (named as CoFe/Se-MOF-5h) were placed in the tubular furnace at N₂ atmosphere, and then heated at 400 °C for 2 h, and the calcinated derivative is named as Co/Fe/S/Se-400. For comparison, CoFe/Se-MOF-1h, CoFe/Se-MOF-3h, and Co/Fe/S/Se-500 were fabricated and their structures and electrocatalytic properties were also characterized to determine the optimal temperature and SeO₂ etching times. The detailed results are shown in ESM.

2.1.3 Synthesis of CoFe-MOF-Se

The precise synthesis method and reagent amount were the same for Co_0.6Fe_0.4-MOF, with the exception that KSeCN (0.2 mmol) was employed in place of KSCN. A sparkling pink crystal was obtained after several days (30% depending on L). For comparison, a series of CoFe-MOF-Se based on different amount of SeCN⁻ were synthesized and the electrocatalytic results are shown in ESM.

2.1.4 Synthesis of Co/Fe/Se-400

The pristine pink crystal named as CoFe-MOF-Se was placed in the tubular furnace at N₂ atmosphere, and then heated at 400 °C for 2 h and the last calcinated product is named as Co/Fe/Se-400.

2.2 Characterizations

PXRD was detected by using a Panalytical X’pert Powder Diffractometer at 40 kV and 40 mA, in which the Cu Kα radiation source (λ = 1.5406 Å). A Sigma HD thermal field emission scanning electron microscope (SEM) was performed to observe the morphologies of 2D MOFs and their derivatives. Meanwhile, the Thermo ESCALAB 250XI spectrometer was used to collect the singles of X-ray photoelectron spectroscopy (XPS) spectra. N₂ adsorption–desorption isotherms were determined by Micromeritics ASAP2020 PLUS HD. Electrochemical tests were conducted with CHI 760E (Shanghai Chenhua Science Technology Corp., Ltd., China) electrochemistry workstation operating at room temperature. The apparatus employed has three electrodes, including a saturated calomel electrode, a graphite rod counter electrode, and a working electrode. The reversible hydrogen electrode (RHE) was widely used for potential conversion in this work, and the specific procedure for electrode preparation is described in ESM.

3 Results and discussion

3.1 Structural analysis of materials

A 2D MOF layer (Co_0.6Fe_0.4-MOF) was successfully constructed via the diffusion method by utilizing the teterpyridyl ligand, Co²⁺, Fe²⁺, and SCN⁻, according to Fig.1 [56]. Subsequently, there are two methods to introduce the Se element into the material, i.e., the etching of Co_0.6Fe_0.4-MOF (the pristine 2D MOF in the etching method) by SeO₂ solution (process as shown in Fig.1(a)) and synthesizing Se-containing MOFs (named as CoFe-MOF-Se, the pristine 2D MOF in the replacing method) by replacing SCN⁻ with SeCN⁻ (process as shown in Fig.1(b)), both are calcined to obtain the final materials, Co/Fe/S/Se-400 (the calcinated derivative in the etching method) and Co/Fe/Se-400 (the calcinated derivative in the replacing method), respectively.

For the first method, Se-containing MOF was prepared by etching 2D Co_0.6Fe_0.4-MOF in a SeO₂ solution. The intermediate products are named CoFe/Se-MOF-1h, CoFe/Se-MOF-3h, and CoFe/Se-MOF-5h after etching for 1, 3, and 5 h. XRD images also demonstrate that the etched materials can still maintain the original crystalline structure with the pristine Co_0.6Fe_0.4-MOF even after etching for 5 h, which can be seen from Fig. S2(a). The peaks at 9.81° and 12.09° show a rightward shift with increasing etching time, accompanied by an increase in the lattice spacing as determined by Bragg calculation (Fig. S2(b)). Furthermore, CoFe/Se-MOF-5h was utilized as a self-sacrificial template to prepare MOF-derived materials by calcination. The thermogravimetric measurement of MOF demonstrates that its structure is stable until 380 °C [56]. As a result, the calcination temperature is restricted to 400 and 500 °C, and the Co/Fe/S/Se-400 derived from CoFe/Se-MOF-5h by calcination at 400 °C shows diffraction peaks (30.7°, 35.2°, and 47.2°) ascribed to CoSe₂ (PDF#10-0408) in addition to Co_1−xS (PDF#42-0826) (Fig.2). As the calcination temperature increased to 500 °C, the main diffraction peaks (30.8° and 34.7°) of Co/Fe/S/Se-500 were matched with CoSe₂ (PDF#10-0408) (Fig. S3), and the other diffraction peaks were matched with Co_1−xS. Comparably, the characteristic peaks of CoSe₂ are more obvious and sharper in Co/Fe/S/Se-400, indicating that the calcination temperature of 400 °C is more favorable for the formation of CoSe₂. However, no diffraction peaks of FeSe₂ could be detected by XRD (Fig.2), indicating that the calcination does not promote the formation of FeSe₂.

The second method to introduce Se into 2D MOF is the replacement of SCN⁻ by SeCN⁻, and the corresponding product obtained is named CoFe-MOF-Se. The characteristic diffraction peaks of CoFe-MOF-Se based on different amount of SeCN⁻ (Fig.2 and S4) are comparable to those of Co_0.6Fe_0.4-MOF except that the diffraction intensity is significantly increased, demonstrating that the replacement of SCN⁻ by SeCN⁻ for the synthesis of Se-containing MOFs is successful. After calcination of CoFe-MOF-Se at 400 °C, Co/Fe/Se-400 was fabricated. The diffraction peaks belonging to pristine MOFs vanished completely, and new peaks appeared (Fig.2). The diffraction peaks at 30.8°, 34.7°, 36.3°, 47.8°, and 57.2° can be ascribed to CoSe₂ (PDF#10-0408), while peaks at 44.3°, 57.4°, and 77.7° coming from FeSe₂ (PDF#48-1881). The presence of FeSe₂ may be attributed to the higher doping amount of selenium in CoFe-MOF-Se than that of CoFe/Se-MOF-5h, which allows the formation of FeSe₂.

The morphology of materials is characterized by SEM. As for the samples prepared by the etching method, the 2D MOF, i.e., the Co_0.6Fe_0.4-MOF shows as a lamellar stacking structure as shown in Fig.3(a) and 3(b). As for the corresponding calcinated derivative, Co/Fe/S/Se-400 presents a fragmented flake-like structure with many small particles on the surface (Fig.3(c), 3(d), and S5(a)). The relatively smaller sheet size of Co/Fe/S/Se-400 is conducive to exposing more active sites. The transmission electron microscope (TEM) images of Co/Fe/S/Se-400 (Figs. S5(c) and S5(e)) clearly show the fragmented structure, which is in consistent with the SEM image.

When it comes to the replacing method, the SEM image of CoFe-MOF-Se shows a flower-like lamellar structure which is slightly different from that of Co_0.6Fe_0.4-MOF as presented in Fig.3(e) and Fig.3(f). After calcination, the surface of Co/Fe/Se-400 becomes rough, as can be observed from Fig.3(g), Fig.3(h), and S5(b). The irregular layered structure consists of randomly assembled nanosheets stacked close to each other. The thinner lamellar structure, with significantly smaller particle size and gaps between layers, can provide more marginal active sites. The TEM images of Co/Fe/Se-400 (Figs. S5(d) and S5(f)) clearly show a 2D lamellar structure, which is in agreement with the SEM image.

The energy dispersive spectrometer (EDS) mapping images (Fig. S6) show that the Co, Fe, and Se compositions in Co/Fe/S/Se-400 and Co/Fe/Se-400 are uniformly distributed, including the presence of S within Co/Fe/S/Se-400. The specific surface area of the as-synthesized calcinated derivatives was characterized through N₂ adsorption–desorption measurements. The N₂ adsorption–desorption isotherms of Co/Fe/S/Se-400 and Co/Fe/Se-400 display typical type I isotherms (Fig. S7), indicating the presence of microporous structures in both materials (Table S2). The Brunauer–Emmett–Teller (BET) surface area of Co/Fe/Se-400 was 178.72 m²/g compared to 102.44 m²/g of Co/Fe/S/Se-400.

Fig.4 illustrates the XPS analysis findings applied to investigate the elements content and surface valence states of Co/Fe/S/Se-400, CoFe-MOF-Se, and Co/Fe/Se-400. Regarding Co/Fe/S/Se-400, the overall spectrum exhibits distinct peaks corresponding to O 1s, C 1s, Co 2p, S 2p, Fe 2p, and Se 3d (Figs. S2 and S3). In the Co 2p spectrum observed from Fig.4(a), Co 2p splits into two spin orbits of Co 2p_3/2 (781.8 eV) and Co 2p_1/2 (794.5 eV), and two satellite peaks (at 785.5 and 802.4 eV), which indicates the existence of Co²⁺ [58,59]. In the Fe 2p spectrum, the peaks around 711.6 and 713.9 eV might be originated from Fe²⁺ 2p_3/2, and the peak at 724.8 eV from Fe²⁺ 2p_1/2 [60,61]. In the S 2p spectrum (Fig. S8), the peaks which centered on 168.7 and 164.1 eV belong to SO₄²⁻ and metal-S bond, respectively, indicating the existence of metal-S species. In the XPS spectrum of Se 3d, Se 3d_5/2 exhibits the peaks at 56.0 eV, along with the peak of Se 3d_3/2 at 55.2 eV, indicating the existing metal-Se species [62,63], and the peak with binding energy at 61.1 eV allocated for Se oxide [63]. The formation of S and Se oxide is attributed to the thin surface oxide layer formed by exposure to air.

The XPS spectra of CoFe-MOF-Se are exhibited in Fig.4(d)–Fig.4(f). The peaks at 796.6 and 780.8 eV are linked to Co 2p_1/2 and Co 2p_3/2. The high intensity of satellite peaks at 785.5 and 802.4 eV indicates the presence of Co²⁺ [58,59]. The two peaks of Fe 2p which appeared around 711.3 and 721.4 eV correspond to Fe 2p_3/2 and Fe 2p_1/2. The peak at 54.8 and 55.1 eV of Se 3d is assigned to Se 3d_5/2 and Se 3d_3/2. Like CoFe-MOF-Se, the Co 2p and Fe 2p spectrum of Co/Fe/Se-400 in Fig.4(g)–Fig.4(i) also demonstrates that the Co and Fe element exist in the form of Co²⁺ and Fe²⁺ state. The peaks which centered at 54.4 and 55.4 eV should be ascribed to Se 3d_5/2 and Se 3d_3/2, indicating that there exists a metal–Se bond in the sample [64].

The related XPS spectra of carbon and oxygen in different samples are shown in Figs. S8 and S9. All the peaks of carbon which occur around 285 eV are assigned to the binding energy of C 1s [65]. Meanwhile, the related peaks of oxygen are presented around 531 eV [66].

3.2 Electrocatalytic performances

The electrocatalytic behaviors of MOF materials etched by SeO₂ reveal that HER performances have been improved to some extent, as shown in Figs. S10(a) and S10(b). When the etching time is 5 h, the overpotential of HER regulated by the etching time is the lowest. Thus, the corresponding CoFe/Se-MOF-5h was utilized as a self-sacrificial template and calcined at a high temperature, and the derived materials were also characterized by electrocatalysis. The results reveal that there exists calcination at 400 and 500 °C, that the HER performance of the derived materials has been improved, and that the overpotential of Co/Fe/S/Se-400 is the smallest, which may be due to the exposure of more active sites of co-doped Co, S, and Se at 400 °C as illustrated in Figs. S11(a) and S11(b). Both the OER and HER polarization curves of CoFe-MOF-Se (based on different amounts of SeCN⁻) are essentially the same (Fig. S12), suggesting that Se-containing MOFs formed with different amounts of SeCN⁻ has no effect on improving the OER and HER performance.

Comparing the electrochemical characteristics of the materials obtained by the two selenisation methods, the OER and HER performance of the original 2D MOFs including Co_0.6Fe_0.4-MOF and CoFe-MOF-Se, as well as the corresponding calcinated derivatives including Co/Fe/S/Se-400 and Co/Fe/Se-400, were evaluated systematically. The values of the electrocatalytic results are shown in Table S3.

First, the OER polarization curves and Tafel slopes of Co/Fe/S/Se-400 are close to that of the original Co_0.6Fe_0.4-MOF, as shown in Fig.5(a) and Fig.5(b), indicating that the calcination has no effect on the activity of the OER. The overpotential of Co/Fe/S/Se-400 for OER is comparable to that of Co_0.6Fe_0.4-MOF, such as their overpotential comparison at a specific current density: Co/Fe/S/Se-400 (273 mV) and Co_0.6Fe_0.4-MOF (260 mV) at 10 mA/cm².

In contrast, as presented in Fig.5(c) and 5(d), HER is obviously improved after calcination. Co/Fe/S/Se-400 displays a considerably lower overpotential than Co_0.6Fe_0.4-MOF (381 mV@10 mA/cm²), 225 mV at 10 mA/cm². After SeO₂ etching and calcination, the overpotential of the final Co/Fe/S/Se-400 in HER is reduced by 156 mV, which is a great improvement compared to the original sample. The improved HER activity is probably accounted for the formation of CoSe₂ and Co_1−xS (according to XRD and XPS results) in Co/Fe/S/Se-400, which are active species for HER catalysis. The comparison reveals that following SeO₂ etching, HER performances to some extent has been improved. When the etching time is 5 h, the overpotential of HER regulated by the etching time is the lowest, which could be due to the fact that as time passes, Se becomes more complete, exposing more active sites, hence improving the HER performance.

Secondly, the linear sweep voltammetry (LSV) and Tafel curves were obtained to further evaluate the electrochemical performance, and the corresponding results for CoFe-MOF-Se and Co/Fe/Se-400 are shown in Fig.5. The OER performance of CoFe-MOF-Se is inferior to that of Co_0.6Fe_0.4-MOF, while the HER performance of CoFe-MOF-Se is similar to that of Co_0.6Fe_0.4-MOF, suggesting that the replacement of SCN⁻ with SeCN⁻ cannot increase the electrochemical performance of CoFe-MOF. In contrast, the calcination at 400 °C (Co/Fe/Se-400) leads a significant enhancement in the catalytic activity. When the current density is 10 mA/cm² for OER, the overpotential of CoFe-MOF-Se and Co/Fe/Se-400 are 300 and 270 mV, respectively. The Tafel slopes are 55.2 and 49.7 mV/dec, respectively. The overpotential of Co/Fe/Se-400 is 30 mV less than that of CoFe-MOF-Se, which may be due to the synergistic impact of CoSe₂ and FeSe₂ (according to XRD and XPS results) to enhance the performance of OER.

As presented in Fig.5(c) and Fig.5(d), the overpotential of HER is 235 mV for Co/Fe/Se-400 and 360 mV for CoFe-MOF-Se at 10 mA/cm², and the Tafel slopes are 134.5 and 174.4 mV/dec, respectively. The final calcination Co/Fe/Se-400 indicates an overpotential reduction of 125 mV in HER, which is an improvement in comparison with the original MOF. The associated findings indicate that the calcination increases the OER and HER activities of Co/Fe/Se-400, and the effect on HER is more significant, which may be due to fact that the CoSe₂ formed during calcination is the active species for HER.

The overpotential values of Co/Fe/S/Se-400 and Co/Fe/Se-400, which has been synthesized after calcination by the two selenide methods, are lower than those of the previously reported metal selenides derivative (Table S4) from MOF at 10 mA/cm², such as FeSe₂ (240 mV) [67], CoSe₂/C (253 mV) [68], 30%FeSe₂/GO (250 mV) [69] for HER, and CoSe₂-450 (330 mV) [70], CoSe₂-30 (287 mV) [71], 6%Se–Co₃O₄ (281 mV) [72], Co_0.8Fe_0.2Se₂ (345 mV) [73], CoSe₂/CNT (300 mV) [74], commercial IrO₂ (331 mV) [75] for OER, proving that the synthetic method is feasible and effective for the synthesis of metal selenides as water splitting catalysts.

Aside from the above evaluation index for electrochemical performance, the electrochemical double layer capacitance (C_DL) is regarded as another properties characterization to indirectly reflect the activity of the catalyst [76]. The cyclic voltammetry (CV) curves of four materials at different scanning rates are shown in Fig.6. The C_DL is 7.9, 4.2, 3.9, and 12.0 mF/cm² for Co_0.6Fe_0.4-MOF, CoFe-MOF-Se, Co/Fe/S/Se-400, and Co/Fe/Se-400, respectively, of which, the C_DL of Co/Fe/Se-400 is much larger than that of Co/Fe/S/Se-400, indicating that the number of active sites is the largest. However, for Co/Fe/S/Se-400, the smallest C_DL value might be caused by the random etching in the solution of SeO₂, which sacrifices the effective area. Comparably, the introduction of SeCN⁻ might be appropriate to preserve the effective electrochemical active area for the electrocatalytic performance. Electrochemical impedance spectroscopy (EIS) has been used to further demonstrate the electrocatalytic performance for these materials [77]. It can be found that CoFe-MOF-Se has the highest charge transfer resistance for OER (Fig.6(f)), while the other three materials have a lower and similar charge transfer resistance, which is consistent with the OER electrocatalytic performance results. However, the Nyquist plots of various materials demonstrate that calcined derivatives such as Co/Fe/S/Se-400 and Co/Fe/Se-400 exhibit a lower charge transfer resistance for HER (Fig. S13 and Table S5), which is compatible with the results of HER electrocatalytic performance.

The turnover frequency (TOF) has been used for the evaluation of the intrinsic activity of the catalyst [78]. In general, the efficiency of a catalyst increases with a higher TOF value. According to calculation (see ESM), the TOF values of Co/Fe/Se-400 and Co/Fe/S/Se-400 at a potential of 270 mV are 5.6 s⁻¹ and 5.3 s⁻¹ higher than those of pristine (CoFe-MOF-Se) (3.8 s⁻¹) and Co_0.6Fe_0.4-MOF (2.2 s⁻¹) for HER, respectively, which represents the superior HER reactivity of the calcined selenides.

The morphology and crystalline phase composition of Co/Fe/Se-400 and Co/Fe/S/Se-400 after cycling reaction have been characterized. Irregular agglomeration of the nanosheets of Co/Fe/S/Se-400 after cycling for HER can be observed from Figs. S14(a), S14(c), and S14(e), forming agglomerated particles, indicating that some of the active sites will be blocked, which is detrimental to the inter-transfer of electrolyte ions. The nanosheets of Co/Fe/Se-400 (Figs. S14(b)–S14(f)) are not completely destroyed after cycling for HER, but the edges of the nanosheets are corroded and passivated. The alteration of the nanosheet layer facilitates the full contact of the electrolyte with the reactants and the exposure of more active sites, which is responsible for the better stability. The nanosheets are severely agglomerated and aggregated and changed significantly after OER cycles, as shown in the morphologies of Co/Fe/S/Se-400, and Co/Fe/Se-400 (Fig. S15). The results of XRD (Fig. S16) also show that the crystalline phase of CoSe₂/FeSe₂ are also retained in Co/Fe/Se-400.

The stability of the electrocatalyst is a crucial factor in the evaluation of high-performance catalysts. As shown in Fig. S17, during the 24-h chronopotentiometry testing, Co/Fe/Se-400 only has a little step at the 4th hour from 1.49 to 1.50 V, and there is no obvious attenuation after that. The LSV curve after the 24-h testing is close to the pristine one, indicating that Co/Fe/Se-400 displays a good durability for OER.

However, it should be noted that the electrochemical properties of Co/Fe/S/Se-400 and Co/Fe/Se-400 are similar, which suggests that the two methods of introducing Se into MOF have similar effects on the electrochemical properties. According to the relaxed experimental results, the second method, i.e., the replacement of SCN⁻ with SeCN⁻, is a simpler process, easier to control the amount of doped Se, which contributes to the formation of bimetallic selenides (CoSe₂/FeSe₂) while improving the OER and HER activity. It provides a new idea for designing 2D MOF materials for doping.

The results of the related experiments show that both the pristine MOF (Co_0.6Fe_0.4-MOF) and (CoFe-MOF-Se) exhibit an unsatisfactory OER and HER catalytic activity, but the calcined samples both show a great improvement in HER. The high electrochemical performance of CoFe-MOF derivatives may be resulted from the following factors: First, the two similar electron intrinsic configurations of Co²⁺ and Fe²⁺ and their synergistic effects are the foundation for achieving efficient catalytic activity and long-term stability. Next, the unique 2D hierarchical structure provides a high porosity feature (i.e., adequate gaps) that allows for an effective contact of electrolyte and active sites, as well as a quick release of H₂/O₂ gas bubbles. Finally, direct integration of CoSe₂/FeSe₂ or CoSe₂/Co_1−xS nanosheets ensures the effective charge transport between the catalyst and the substrate, considerably lowering the charge transfer resistance.

4 Conclusions

In this work, two strategies are utilized to introduce Se element into the 2D CoFe MOF to form the final calcinated derivatives for catalyzing HER and OER in overall water splitting. One is the etching of as-prepared MOF by SeO₂ solution, and the other is the replacing SCN⁻ with SeCN⁻ as the construction unit. As a consequence, it is found that both two Se introducing approaches can obviously improve the HER performance, and the HER electrocatalytic activity of the final calcinated derivatives from those two methods are comparable during overall water splitting. The high electrochemical performance of 2D CoFe MOF derivatives may be resulted from the unique 2D hierarchical porous structure and strong synergistic effect between different components in the material. Based on the results, the rational design of layered MOFs with S- or Se-containing linkers as water splitting catalysts is a feasible option for the development of economical and low-energy-consuming electrocatalysts, which provides an innovative approach for the synthesis of MOF-based metallic selenides.

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