Investigation of Oxygen Evolution Performance of Highly Efﬁcient Water Electrolysis Catalyst: NiFe LDH/BPene

: The oxygen evolution reaction (OER) plays a crucial role in hydrogen production through water electrolysis. However, the high overpotential and sluggish kinetics of the OER pose signiﬁcant challenges. Layered double hydroxides (LDHs) have been widely used as highly active electrocat-alysts to tackle these issues. To further enhance the catalytic activity of LDHs and optimize their composition and morphology, the rational design of highly efﬁcient electrocatalysts is desirable. Considering the ﬂexibility of heterogeneous structures in terms of their electronic structure and surface chemistry, this study employs a simple and effective hydrothermal synthesis method. By leveraging van der Waals (vdW) interactions, a heterostructure is constructed between nickel-iron bimetallic hydroxide (NiFe LDH) nanosheets and black phosphorene (BPene). The OER electrochemical test results demonstrate the superior electrocatalytic properties of the NiFe LDH/BPene heterostructure. The heterostructure exhibits remarkably low overpotential (180 mV) and Tafel slope (72.36 mV dec − 1 ) at a current density of 10 mA cm − 2 . Furthermore, the stability test conducted for 30,000 s showed a current retention rate exceeding 93.00%. This work provides new perspectives into the electronic structure regulation of 2D heterostructures and highlights new avenues for tuning the electrocatalytic adsorption of emerging phosphorus-based materials.


Introduction
As a global energy crisis approaches and environmental pollution becomes increasingly severe, the exploration of low-cost and efficient new clean energy has become a common goal for humanity [1].Hydrogen, one of the cleanest energy sources in the 21st century [2], is considered one of the most convenient and effective ways to obtain hydrogen through electrolysis [3].The hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are the two half-reactions used in the electrolysis of water [4].OER is a slow kinetic process that significantly lowers the overall efficiency of water splitting due to the fact that it is comprised of several proton-coupled electron transfer stages [5].Therefore, overcoming the high reaction energy barrier has become a pressing issue that needs to be solved [6].Fortunately, electrocatalysts can be employed to solve this issue effectively.Currently, precious metal oxides such as RuO 2 /IrO 2 have high catalytic activity in OER [7], but their expensive cost and scarcity limit their industrial applications.For this purpose, some researchers are devoted to the study of electrocatalysts that can replace precious metals, such as metal-organic frameworks [8], transition metal dichalcogenides [9], and transition metal phosphides [10].The OER mechanisms are generally considered through two pathways: adsorbate evolving mechanism (AEM) and lattice oxygen mechanism (LOM), both considering the different origins of generated oxygen.AEM shows that O 2 derived from H 2 O in electrolytes is considered to be the conventional OER mechanism, proceeding as H 2 O → *OH → *O → *OOH → O 2 [11].In the LOM, the first two steps are the same as AEM, proceeding as H 2 O → *OH → *O.Next, the *O couples with the lattice oxygen to produce O 2 , and oxygen vacancy (VO) is formed in the lattice of the catalyst at the same time [12].These electrocatalysts have many advantages, such as high activity, high stability, and low cost [13], which can effectively promote the development of the field of electrocatalysis [14].It should be noted that although these alternative electrocatalysts have great potential, their performance and applications still require further research and improvement.
In the field of electrocatalysis, LDHs are important transition metal oxides.Alternating layers of negatively charged oxide ions and positively charged metal ions form LDHs, as electrocatalysts possess high catalytic activity, high stability, tunable composition and morphology, and excellent biocompatibility.They have been widely used in the field of electrocatalysis.However, LDHs also have some drawbacks, such as poor conductivity and difficulty controlling the number of layers and crystal facets, leading to fluctuations in catalytic activity.To address these challenges and enhance the electrocatalytic activity of LDHs, researchers have recently proposed a variety of enhancement options.These strategies include controlling the interlayer distance, surface modification, and loading techniques, among others [14].These efforts have provided new insights and methods for the application of LDHs in electrocatalysis.
In recent years, BPene has gained significant attention as a highly promising non-metal electrocatalytic material [15].Research has shown that BPene can be prepared through liquid-phase exfoliation [16] of black phosphorus [17].BPene exhibits unique electronic and optical properties, including high charge carrier mobility [18], large specific surface area [19], excellent on/off ratio [20], in-plane anisotropy [21], and adjustable bandgap width [22].These properties are advantageous for promoting electrocatalytic reactions.However, BPene faces challenges due to the presence of defects on its edges and surfaces [23], which make it prone to oxidation reactions and result in poor adsorption capacity for oxygen intermediates.These issues contribute to the inherent low activity and stability of BPene in oxygen evolution reactions [24].To address these problems, researchers have adopted various approaches.
For instance, Sun et al. [25] achieved the in-situ synthesis of a cobalt oxide/black phosphorus nanosheet heterostructure using a three-electrode electrochemical method.They also employed edge or in-plane coupling of other transition metal catalysts to enhance OER performance.Gan et al. [26], on the other hand, improved BPene through chemical modifications.They doped C/S/N/O elements into BPene and introduced OH/NH 2 functional groups to enhance its hydrogen evolution reaction (HER) catalytic performance.While these efforts have made significant contributions to suppressing the hydrolysis of BPene, the experimental procedures remain complex, and further improvements in electrocatalytic performance are needed.
Two-dimensional nanomaterials loaded with catalysts frequently have a large surface area and superior mechanical [27], electrical, and thermal characteristics [28].These properties are attractive for the preparation of durable and stable metal nanoparticle catalysts with easily accessible active sites.Due to the unique physicochemical properties of LDHs in transition metals, they have shown great potential in the fields of hydrogen evolution and oxygen evolution [29].With their typical two-dimensional layered structure and numerous tunable properties [30] due to their rich composition and structure, LDHs are considered one of the most promising materials to replace precious metals.Numerous papers over the last few years have emphasized the use of LDHs in electrocatalytic water splitting.The distinctive layered structure of LDHs can not only serve as precursors but also be incorporated into composite structures [31].For example, Sharma [32] studied the potential dual functionality of a layered material, the surface-active agent-intercalated α-Co(OH) 2 -type layered metal hydroxide, using an ion exchange method for electrocatalytic applications.Additionally, Que et al. [33] developed core-shell structures of ZnCo 2 O 4 @NiFe LDH that had a Tafel slope of 96.7 mV dec −1 and an overpotential of 249 mV at a current density of 10 mA cm −2 .As a bifunctional electrocatalyst, Wang et al. [34].created a hybrid nanostructure of CoNiN@NiFe LDH and tested its OER performance with overpotentials of 227 mV at current densities of 10 mA cm −2 .
Therefore, in this study, we grew NiFe LDH on the surface of BPene using a straightforward and practical one-pot hydrothermal method.During the OER process, BPene can act as an electron transport channel to promote electrons from the catalyst surface to oxygen molecules and participate in the release reaction of oxygen atoms.The generated NiFe LDH layers have a low thickness, preventing layer aggregation and facilitating the exposure of more active sites.Additionally, the synergistic effect of the heterostructure formed between BPene and NiFe LDH enhances OER activity and stability while reducing the required adsorption energy for intermediate reaction steps, thereby improving the catalyst's stability.Finally, this method provides a new approach and solution for the preparation of efficient electrocatalytic materials.

Materials
Block black phosphorus, N-methylpyrrolidone (NMP), Ethanol, Ferric nitrate nonahydrate, Nickel nitrate hexahydrate, Sodium hydroxide, Sodium carbonate, and Isopropanol.The purity of the reagents used was greater than 99%, and no additional purification was required.

Fabrication of BPene
BPene is formed via simple black phosphorus liquid exfoliation using the dispersant.Ground black phosphorus is taken out from an inert glove box and transferred to a dehydrated and inert gas-saturated dispersant for sonication, resulting in a brown dispersion.The dispersion is centrifuged to collect the supernatant for future use.A portion of the centrifuged supernatant is further centrifuged, washed, dried, and weighed, and the concentration of the BPene dispersion is calculated.

Preparation of NiFe LDH
Compared to preparation methods such as electrodeposition, co-precipitation, and sol-gel, the hydrothermal method for synthesizing NiFe LDH offers advantages such as high purity, controllable morphology and size, avoidance of contamination, and an improved reaction rate.Therefore, based on the relevant techniques from previous work, the hydrothermal method was adopted in this study for synthesizing NiFe LDH samples.In order to create blended solutions, Fe(NO 3 ) 3 •9H 2 O and Ni(NO 3 ) 2 •6H 2 O were first separately dissolved in 50 mL deionized water at molar ratios of 1:1, 2:1, 3:1, 4:1, and 5:1.The blended solutions were then gradually adjusted to a pH level of 9-10 by adding a combined solution of Na 2 CO 3 and NaOH, and stirred continuously for 2 h at 50 • C. The resultant mixture was then placed into a reaction vessel and heated for 12 h at 120 • C. The mixture spontaneously cooled to room temperature following the reaction.Subsequently, the mixture was centrifuged at 4000 rpm and washed three times in ethanol and deionized water.Finally, samples of NiFe LDH with molar ratios of 1:1, 2:1, 3:1, 4:1, and 5:1 were prepared by drying and gathering them at 80 • C in a vacuum oven.For convenience of identification, these samples were labeled as NiFe LDH-1, NiFe LDH-2, NiFe LDH-3, NiFe LDH-4, and NiFe LDH-5.

Fabrication of NiFe LDH/BPene
To explore the effect of BPene on the synthesis of NiFe LDH, we prepared NiFe LDH/BPene using the previously mentioned BPene and NiFe LDH preparation processes.Fe(NO 3 ) 3 •9H 2 O and Ni(NO 3 ) 2 •6H 2 O were mixed separately with varying amounts of BPene supernatant (10 mL, 15 mL, 21 mL, 30 mL, and 39 mL), followed by centrifugation at 10,000 rpm for 30 min and adjustment of the pH to 9-10 using a mixed solution of Na 2 CO 3 and NaOH.The mixture was then transferred to a reaction kettle, stirred uniformly, and reacted at 120 • C for 12 h.After completion of the reaction, the composite material was collected by centrifugation at 10,000 rpm, washed thrice with ethanol, and dried at 80 • C in a vacuum oven.The resulting products were designated as NiFe LDH/BP-0.3,NiFe LDH/BP-0.5,NiFe LDH/BP-0.7,NiFe LDH/BP-1.0,and NiFe LDH/BP-1.3for distinction.
This study utilizes the novel material BPene to synthesize NiFe LDH/BPene, investigating the influence of heterostructure on the oxygen evolution performance of electrocatalysts.By mixing the dispersion of phosphorene with Fe(NO 3 ) 3 •9H 2 O and Ni(NO 3 ) 2 •6H 2 O, fol- lowed by a hydrothermal reaction, we successfully prepared a series of NiFe LDH/BPene composite materials with different ratios.These materials will be further characterized and subjected to electrochemical testing to investigate the potential application value of NiFe LDH/BPene.

Characterization
To gain a comprehensive understanding of the morphology, structure, and composition of the synthesized NiFe LDH/BPene sample for better analysis and research, a series of characterization techniques were employed.First, a copper K (=0.15406 nm) X-ray diffraction (XRD) examination was performed utilizing the D8 ADVANCE X-ray diffractometer from Bruker (Leipzig, Germany), scanning the angle range of 10~80 • .Additionally, the JEOL JEM-2100F transmission electron microscope (TEM, JEM-2100 PLUS, JEOL Ltd., Tokyo, Japan) was used to further analyze the catalyst's microstructure, and TEM/EDS analysis was used to assess the element distribution of NiFe LDH/BPene.Finally, other characterization techniques such as atomic force microscopy (AFM) were utilized for a more meticulous analysis.

Electrochemical Measurements
To investigate the catalytic reaction mechanism and active sites of the synthesized NiFe LDH/BPene, electrochemical measurement was employed to evaluate the electrocatalytic performance of the catalysts.In this study, a standard three-electrode cell system was used, and the electrochemical properties of NiFe LDH/BPene heterostructure catalysts were tested using the CHI660E electrochemical workstation.The specific operating steps were as follows: first, a 2 mL sample bottle containing 5 mg of the catalyst, 750 µL deionized water, 230 µL isopropanol, and 20 µL Nafion (5wt%) solution were treated with ultrasonic energy for 30 min.The working electrode was then created by drop-casting 10 µL combined solution onto a glassy carbon electrode and allowing it to dry naturally.Additionally, a graphite electrode served as the counter electrode and an electrode made of silver or silver chloride served as the reference electrode.Finally, oxygen was passed into the electrolyte for 30 min to achieve oxygen saturation prior to the electrochemical measurement.

Characterization of NiFe LDH/BPene
Based on the preparation method mentioned in the previous sections, Scheme 1 illustrates the process of synthesizing NiFe LDH/BPene using a one-step hydrothermal method.Furthermore, to further investigate the structural characterization of the prepared samples, we employ various techniques for characterization and testing.
We initially performed a characterization analysis using the XRD technique to examine the samples' structural properties.Figure 1a shows XRD plots of prepared catalysts.The XRD pattern of BPene can be seen to have three distinctive diffraction peaks at 16.5 • , 34.2 • , and 52.4 • , which correspond to the (020), (040), and (060) crystal planes, respectively, and are consistent with the JCPDS card (No. 73-1358) for BPene.The sharp and intense diffraction peaks, along with the absence of other impurity peaks, confirm the high crystallinity and high purity of BPene.The XRD plot of NiFe LDH displays evident characteristic peaks We initially performed a characterization analysis using the XRD technique to examine the samples' structural properties.Figure 1a shows XRD plots of prepared catalysts.The XRD pattern of BPene can be seen to have three distinctive diffraction peaks at 16.5°, 34.2°, and 52.4°, which correspond to the (020), ( 040  We initially performed a characterization analysis using the XRD technique to examine the samples' structural properties.Figure 1a shows XRD plots of prepared catalysts.The XRD pattern of BPene can be seen to have three distinctive diffraction peaks at 16.5°, 34.2°, and 52.4°, which correspond to the (020), (040), and (060) crystal planes, respectively, and are consistent with the JCPDS card (No. 73-1358) for BPene.The sharp and intense diffraction peaks, along with the absence of other impurity peaks, confirm the high crystallinity and high purity of BPene.The XRD plot of NiFe LDH displays evident characteristic peaks at 11.2°, 22.8°, 59.69°, and 61.1°, corresponding to the (003), ( 006  To further investigate the molecular structure, vibrational information, and interactions of the samples, we present the Raman scattering spectra of the samples in Figure 1b.As can be seen, BPene shows three distinct peaks at 323.5, 400.5, and 427.6 cm −1 .These peaks correspond to BPene's three vibrational modes, which are Ag 1 , B2g, and Ag 2 .Ag 1 is associated with the out-of-plane vibration of phosphorus atoms, whereas B2g and Ag 2 correspond to the in-plane vibrations of phosphorus atoms; The peaks of NiFe LDH/BPene are located at 417.8, 388.2, and 316.1 cm −1 , which are related to the Ag 2 , B2g, Ag 1 and vibrational modes of BPene.The position of the peaks is shifted towards higher wavenumbers due to the van der Waals interactions between NiFe LDH and BPene layers, leading to a Raman redshift, which indicates a definite interaction between NiFe LDH and BPene.The To further investigate the molecular structure, vibrational information, and interactions of the samples, we present the Raman scattering spectra of the samples in Figure 1b.As can be seen, BPene shows three distinct peaks at 323.5, 400.5, and 427.6 cm −1 .These peaks correspond to BPene's three vibrational modes, which are A g 1 , B 2g , and A g 2 .Ag 1 is associated with the out-of-plane vibration of phosphorus atoms, whereas B 2g and A g 2 correspond to the in-plane vibrations of phosphorus atoms; The peaks of NiFe LDH/BPene are located at 417.8, 388.2, and 316.1 cm −1 , which are related to the A g 2 , B 2g , A g 1 and vibrational modes of BPene.The position of the peaks is shifted towards higher wavenumbers due to the van der Waals interactions between NiFe LDH and BPene layers, leading to a Raman redshift, which indicates a definite interaction between NiFe LDH and BPene.The peak around 480 cm −1 of NiFe LDH can correspond to the Ni-O bond.Compared to NiFe LDH, the position of the NiFe LDH/BPene peak has an obvious blue shift, which is due to the generation of structural defects around the Ni-O bond [35].Studies have shown that the structural defects present in the samples can promote the catalytic activity of the material [36]. In this study, we employed several advanced characterization techniques to further explore the microstructure of NiFe LDH/BPene.Figure 2a,b are the SEM images of BPene and NiFe LDH/BPene, respectively.It can be observed that the BPene prepared through ultrasonic liquid exfoliation exhibits a distinct layered structure with clear layering of black phosphorus.Furthermore, the layers are relatively thin.The SEM image of NiFe LDH/BPene reveals that the synthesized product possesses unevenly sized layered structures that are stacking together, and the crystal size varies from 30 nm to 100 nm.Furthermore, the TEM image confirms that the prepared BPene indeed possesses a few-layer structure.These results fully validate the effectiveness and rationality of our BPene preparation method.Figure 2c-e display the TEM images of the prepared BPene, NiFe LDH, and NiFe LDH/BPene samples, respectively.The images show that the NiFe LDH synthesized by the hydrothermal method exhibits uniform and regular hexagonal sheet-like structures with a crystal size of about 100 nm and a relatively higher number of layers.Additionally, the exposed edges of the prepared NiFe LDH are well-presented, which is advantageous for the exposure of active sites.Moreover, TEM images show overlapping hexagonal and irregularly layered materials, with non-uniform thickness.This indicates that in certain regions, the addition of BPene prevents the aggregation of NiFe LDH during the synthesis process, resulting in a material with fewer layers.The microstructure and morphology of NiFe LDH/BPene are expected to maximize the exposure of active sites and edges of nanosheets while maintaining a significant number of active sites.According to the microstructural characteristics of the composite material mentioned above, it is anticipated that the composite material with abundant surface active sites will significantly enhance its mechanical stability and durability.Additionally, the prepared nanosheet array-type composite material can increase the effective contact area between the electrode and electrolyte, thereby facilitating the rapid diffusion of active electrochemical species and efficient electron transfer.This feature holds potential advantages for water splitting reactions.Furthermore, the HRTEM image in Figure 2f reveals partial crystal structure information of the NiFe LDH/BPene nanosheets at the microscale.With interplanar spacings of 0.197 and 0.179 nm, respectively, the densely packed lattice fringes are plainly visible.The NiFe LDH (112) plane is represented by the lattice fringe at 0.197 nm, which has a width comparable to that of the NiFe LDH crystal.The BPene (112) plane is represented by the lattice fringe at 0.179 nm, which has a width corresponding to the BP crystal.This result demonstrates the coexistence of the NiFe LDH (018) plane and BPene (112) plane in the same region of composite material, providing preliminary evidence for the formation Furthermore, the HRTEM image in Figure 2f reveals partial crystal structure information of the NiFe LDH/BPene nanosheets at the microscale.With interplanar spacings of 0.197 and 0.179 nm, respectively, the densely packed lattice fringes are plainly visible.The NiFe LDH (112) plane is represented by the lattice fringe at 0.197 nm, which has a width comparable to that of the NiFe LDH crystal.The BPene (112) plane is represented by the lattice fringe at 0.179 nm, which has a width corresponding to the BP crystal.This result demonstrates the coexistence of the NiFe LDH (018) plane and BPene (112) plane in the same region of composite material, providing preliminary evidence for the formation of the heterostructure in NiFe LDH/BPene and further confirming the high crystallinity of the NiFe LDH/BPene material.We utilized the STEM-EDS technique (Figure 2g) and elemental mapping images to demonstrate the uniform distribution of oxygen (O), phosphorus (P), iron (Fe), and nickel (Ni) elements in NiFe LDH/BPene.Compared to the original shape of NiFe LDH/BPene, there was almost no change in the morphology of the elemental distribution, further confirming the uniform growth of NiFe LDH on the surface of BPene and determining the crystal structure of the composite material.These findings clearly indicate the effective combination of highly crystalline NiFe LDH and BPene.
In order to investigate the surface information of the catalytic material, we characterized the samples using AFM.The measurement results in Figure 3a indicate that the thickness of the prepared BPene is approximately 1.3 nm.By studying the thickness of phosphorus atoms, we determined that the thickness of a single layer of phosphorus atoms is 0.33 nm.Therefore, it can be estimated that the number of layers corresponding to the thickness of BPene is about three to four layers, further confirming that the exfoliation met our expectations.In Figure 3b, we present the AFM image of NiFe LDH/BPene, and the measurement results indicate that the average thickness of the sample is approximately 3 nm.We hypothesize that the thinner layers of NiFe LDH and BPene undergo crossstacking, because the thickness of a single layer of NiFe LDH is around 0.28 nm, which is comparable to the thickness of a single layer of phosphorus atoms.Thus, it can be inferred that the thickness of 3 nm is about five layers of NiFe LDH/BPene.This ultrathin nanosheet structure allows for increased exposure of active sites and reduced mass transfer resistance, further supporting our structural inference and providing a theoretical basis.Finally, we conducted a thorough XPS study on NiFe LDH/BPene, confirming the components of nickel, iron, phosphorus, and oxygen.In order to observe the distribution of each element more intuitively, we performed an XPS surface scan analysis.Figure S1 shows the XPS surface scan spectrum of the NiFe LDH/BPene sample, and the analysis results indicate that the mass ratios of O, Ni, Fe, and P are 50.4%,34.2%, 10.9%, and 4.5%, respectively.After calculation, the atomic ratios were determined to be 22:4:1:1.The high proportion of oxygen atoms may be due to the oxidation phenomenon caused by the offsite test of NiFe LDH/BPene.Finally, it can be concluded that the NiFe LDH/BPene material was successfully prepared.To analyze the chemical environment of NiFe LDH/BPene, we characterized the surface valence states of the catalyst using XPS.The XPS spectra of P 2p is shown in Figure 4a, with double peaks at 130.10 eV and 130.99 eV that correspond to P 2p3/2 and P 2p1/2 of BPene, respectively.The twin peaks in NiFe LDH/BPene (Figure 4e) at 129.3 eV and 130.2 eV are P 2p3/2 and P 2p1/2, respectively.In addition, a second peak Finally, we conducted a thorough XPS study on NiFe LDH/BPene, confirming the components of nickel, iron, phosphorus, and oxygen.In order to observe the distribution of each element more intuitively, we performed an XPS surface scan analysis.Figure S1 shows the XPS surface scan spectrum of the NiFe LDH/BPene sample, and the analysis results indicate that the mass ratios of O, Ni, Fe, and P are 50.4%,34.2%, 10.9%, and 4.5%, respectively.After calculation, the atomic ratios were determined to be 22:4:1:1.The high proportion of oxygen atoms may be due to the oxidation phenomenon caused by the off-site test of NiFe LDH/BPene.Finally, it can be concluded that the NiFe LDH/BPene material was successfully prepared.To analyze the chemical environment of NiFe LDH/BPene, we characterized the surface valence states of the catalyst using XPS.The XPS spectra of P 2p is shown in Figure 4a, with double peaks at 130.10 eV and 130.99 eV that correspond to P 2p 3/2 and P 2p 1/2 of BPene, respectively.The twin peaks in NiFe LDH/BPene (Figure 4e) at 129.3 eV and 130.2 eV are P 2p 3/2 and P 2p 1/2 , respectively.In addition, a second peak is seen at 133.20 eV, which could be the result of P-O bond formation.The formation of P-O bonds in the catalyst after loading promotes the coupled electron transfer between NiFe LDH and BPene, while further enhancing the stability of the catalyst [37].The peak of O 1s XPS spectrum in NiFe LDH (Figure 4b) is split into two peaks: one peak at 531.25 eV corresponds to OH, and the other peak at 530.57eV corresponds to M-O bonds, representing two types of oxygen species, H 2 O and M-OH groups.Apart from these OH and M-O groups, the ratio of OH heights decreases, and the chemical state of the elements remains almost unchanged in comparison with NiFe LDH/BPene.Therefore, it can be concluded that P does not bond with the O element in NiFe LDH, which may be due to oxidation reaction caused by contact with air.The XPS spectrum of Ni 2p corresponds to Ni 2p 1/2 , Ni 2p 3/2 , and two satellite peaks.Compared to NiFe LDH, the binding energy of NiFe LDH/BPene at 853.01 eV may be related to Ni 2+ , indicating electron transfer from NiFe LDH [38].Peak fitting analysis of Fe 2p XPS spectra shows that iron in NiFe LDH is Fe 2+ and Fe 3+ , with two fitting peaks at 708.68 eV and 711.68 eV, respectively (Figure 4d).Iron in NiFe LDH/BPene is also Fe 2+ and Fe 3+ , with fitting peaks at 712.1 eV and 715.4 eV, respectively, corresponding to Fe 2p 3/2 and Fe 2p 1/2 , indicating electron transfer from Fe species.Therefore, electrostatic interaction plays a major role in the NiFe LDH/BPene composite, forming a heterojunction structure, which facilitates carrier transport and enhances the electron transfer rate of the catalyst.The electrostatic interaction between surface phosphorus atoms and metal cations allows for uniform growth of NiFe LDH on BPene nanosheets, which modulates the electronic environment of Ni and Fe in LDH nanosheets, creating optimal reaction conditions for the catalytic process.Meanwhile, the interaction between positive and negative charges adjusts the interlayer spacing of BPene, preventing layer stacking.On the other hand, it has been reported that during the OER process, the presence of nickel ions will form NiOOH, which are considered to be active species for OER that can enhance the transfer of proton-coupled electrons, thereby improving the electrocatalytic performance [39].
Processes 2023, 11, x FOR PEER REVIEW 9 of 14 the electron transfer rate of the catalyst.The electrostatic interaction between surface phosphorus atoms and metal cations allows for uniform growth of NiFe LDH on BPene nanosheets, which modulates the electronic environment of Ni and Fe in LDH nanosheets, creating optimal reaction conditions for the catalytic process.Meanwhile, the interaction between positive and negative charges adjusts the interlayer spacing of BPene, preventing layer stacking.On the other hand, it has been reported that during the OER process, the presence of nickel ions will form NiOOH, which are considered to be active species for OER that can enhance the transfer of proton-coupled electrons, thereby improving the electrocatalytic performance [39].

OER Performance Testing of NiFe LDH/BPene
Based on the above experiments, electrochemical measurements were conducted on the catalytic materials using an electrochemical workstation to investigate their electrochemical performance.To improve the synthetic conditions, we conducted preliminary studies on the electrochemical characteristics of NiFe LDH with various metal ion ratios.As seen in Figure 5a

OER Performance Testing of NiFe LDH/BPene
Based on the above experiments, electrochemical measurements were conducted on the catalytic materials using an electrochemical workstation to investigate their electrochemical performance.To improve the synthetic conditions, we conducted preliminary studies on the electrochemical characteristics of NiFe LDH with various metal ion ratios.As seen in Figure 5a Therefore, we determined that the optimal electrochemical performance of NiFe LDH was achieved at a molar ratio of Ni to Fe of 3:1.Thus, we used a Fe 3+ :Ni 2+ = 3:1 ratio to prepare NiFe LDH/BPene.By adjusting the amount of different NiFe LDH added to the BPene solution, a series of NiFe LDH/BPene with different mass ratios were obtained, namely NiFe LDH/BP-0.3,NiFe LDH/BP-0.5,NiFe LDH/BP-0.7,NiFe LDH/BP-1.0,and NiFe LDH/BP-1.3.(Figure 5b).We found that the sample with a ratio of 0.7 exhibited the lowest OER overpotential, reaching only 180 mV, which is significantly lower than other samples and commercial RuO2.As the current density increased, the overpotential difference between NiFe LDH/BP-0.7 and NiFe LDH and BPene became increasingly apparent, indicating that the composite of NiFe LDH and BPene can significantly enhance OER activity.Additionally, the Tafel slope (Figure 5c) of NiFe LDH/BP-0.7 was 72.36 mV dec −1 , lower than other samples and commercial RuO2, indicating excellent reaction kinetics.The smaller Tafel slope of the composite material can accelerate the rate of OH − conversion to O2 and reduce the negative charge on electrodes.Furthermore, the heterostructure of NiFe LDH and BPene forms an electron transfer channel, promoting electron transport and enhancing the electron conduction performance of the catalyst, improving the efficiency of the catalytic reaction.We also fitted the Tafel slope curve to explain the OER kinetics.These results indicate that NiFe LDH/BPene-0.7 is a composite material with excellent catalytic performance.
To evaluate the electrochemical active surface area (ECSA) of the catalysts, we measured the electrochemical double-layer capacitance (Cdl), which is directly related to the electrochemical active surface area.Under the same conditions, the Cdl value of NiFe LDH/BPene composite is 0.278 mF cm −2 , which is about 1.9 times that of NiFe LDH (0.15 mF cm −2 ) and 2.5 times that of BPene nanosheets (0.113 mF cm −2 ).Since the ECSA value is proportional to the Cdl value, it suggests that the NiFe LDH/BPene composite material has more exposed catalyst active sites and a greater electrochemically active surface area [40].
To demonstrate the electronic transport characteristics of NiFe LDH/BPene, we present the Nyquist plot and corresponding equivalent circuit diagram in Figure 5e.The equivalent circuit consists of resistance Rs in series with a high-frequency arc constant Therefore, we determined that the optimal electrochemical performance of NiFe LDH was achieved at a molar ratio of Ni to Fe of 3:1.Thus, we used a Fe 3+ :Ni 2+ = 3:1 ratio to prepare NiFe LDH/BPene.By adjusting the amount of different NiFe LDH added to the BPene solution, a series of NiFe LDH/BPene with different mass ratios were obtained, namely NiFe LDH/BP-0.3,NiFe LDH/BP-0.5,NiFe LDH/BP-0.7,NiFe LDH/BP-1.0,and NiFe LDH/BP-1.3.(Figure 5b).We found that the sample with a ratio of 0.7 exhibited the lowest OER overpotential, reaching only 180 mV, which is significantly lower than other samples and commercial RuO 2 .As the current density increased, the overpotential difference between NiFe LDH/BP-0.7 and NiFe LDH and BPene became increasingly apparent, indicating that the composite of NiFe LDH and BPene can significantly enhance OER activity.Additionally, the Tafel slope (Figure 5c) of NiFe LDH/BP-0.7 was 72.36 mV dec −1 , lower than other samples and commercial RuO 2 , indicating excellent reaction kinetics.The smaller Tafel slope of the composite material can accelerate the rate of OH − conversion to O 2 and reduce the negative charge on electrodes.Furthermore, the heterostructure of NiFe LDH and BPene forms an electron transfer channel, promoting electron transport and enhancing the electron conduction performance of the catalyst, improving the efficiency of the catalytic reaction.We also fitted the Tafel slope curve to explain the OER kinetics.These results indicate that NiFe LDH/BPene-0.7 is a composite material with excellent catalytic performance.
To evaluate the electrochemical active surface area (ECSA) of the catalysts, we measured the electrochemical double-layer capacitance (C dl ), which is directly related to the electrochemical active surface area.Under the same conditions, the C dl value of NiFe LDH/BPene composite is 0.278 mF cm −2 , which is about 1.9 times that of NiFe LDH (0.15 mF cm −2 ) and 2.5 times that of BPene nanosheets (0.113 mF cm −2 ).Since the ECSA value is proportional to the C dl value, it suggests that the NiFe LDH/BPene composite ma-terial has more exposed catalyst active sites and a greater electrochemically active surface area [40].
To demonstrate the electronic transport characteristics of NiFe LDH/BPene, we present the Nyquist plot and corresponding equivalent circuit diagram in Figure 5e.The equivalent circuit consists of resistance Rs in series with a high-frequency arc constant phase element (CPE1) and Rct, where Rs represents the ionic transport resistance solution and CPE1 relates to the interface between the electrolyte and electrode [40].The results reveal that the semi-circular trend of NiFe LDH/BPene is significantly smaller than the control samples NiFe LDH, BPene, and RuO 2 .This indicates that charge transfer resistance generated by it is minimized, leading to superior charge transfer kinetics during electrochemical processes.The combination of NiFe LDH and BPene effectively accelerates the electron transfer rate and reduces the electrochemical impedance, resulting in optimal electrocatalytic OER activity.According to the exceptional OER properties of NiFe LDH/BP-0.7 and the lowest Tafel slope value, we can infer that the synergistic effect between BPene and NiFe LDH is beneficial for electrochemical reaction kinetics.The OER properties of NiFe LDH/BP-0.7 surpasses that of commercial RuO 2 , NiFe LDH, and BPene.
To investigate the potential commercial application of NiFe LDH/BPene, an essential parameter to consider is its long-term stability.Therefore, we used chronoamperometry to assess the catalyst's stability.The chronoamperometry curve of NiFe LDH/BPene is shown in Figure 5f.At a potential of 0.45 V and after 30,000 s, the initial slow increase in current density of NiFe LDH/BPene indicates the activation process of the electrocatalyst.Eventually, the outstanding electrocatalytic activity was demonstrated by current density, which was approximately 10.62 mA cm −2 .After conducting a stability test for 30,000 s, the current density retention of NiFe LDH/BPene was 93.00%, indicating its outstanding electrochemical stability.Combining all the characterization and electrochemical tests, it is evident that NiFe LDH/BPene has advantages such as low overpotential, fast charge transfer rate, low electrochemical impedance, and high stability in the OER in alkaline electrolyte solution.Therefore, NiFe LDH/BPene can be considered an ideal OER catalyst with remarkable features, and it can substantially decrease the energy barrier of the OER four-electron transfer reaction, which is responsible for the excellent overpotential of 180 mV at a current density of 10 mA cm −2 compared to the precursor materials NiFe LDH and BPene.
In general, the OER in the metal hydroxide electrode in an alkaline solution is caused by the adsorbate evolution mechanism (AEM) [41].In this work, the OH − of the electrolyte attached to the surface of NiFe LDH/BPene combines and releases water.Then, owing to the combination of two other OH − ions in the electrolyte, it is gradually reduced to O and OOH, forming and releasing O 2 .Under the action of the AEM, NiFe LDH/BPene has excellent electrocatalytic oxygen evolution activity (180 mV@ 10 mA cm −2 and a Tafel slope of 72.36 mV dec −1 ).Finally, as shown in Table S1, we list the latest experimental studies on BPene-based OER, and the results also demonstrate that the NiFe LDH/BPene heterostructure has outstanding electrocatalytic performance.

Conclusions
This study offers a novel method for modifying phosphorene by simply combining nickel-iron hydroxide layered double hydroxide (NiFe LDH) to generate more electrocatalytic active sites, promote charge transfer, and enhance chemical stability.Firstly, the combination of an NiFe LDH layered structure and metal ions provides numerous active sites for OER, which can adsorb and catalyze the oxidation reaction of water molecules.Second, the heterostructure formed by black phosphorene (BPene) and NiFe LDH creates an electron transfer channel that facilitates electron transport and boosts the catalyst's rate of electron transfer, increasing the effectiveness of the catalytic process.NiFe LDH evenly develops on the BPene carrier due to the electrostatic interaction between surface phosphorus atoms and metal cations.Additionally, the interaction between positive and negative charges adjusts the interlayer spacing of BPene, preventing layer stacking.At a current density of 10 mA cm −2 , the ultra-low overpotential and fast reaction kinetics of NiFe LDH/BPene demonstrate its inherent potential.This study presents a convenient method for constructing a heterostructure (NiFe LDH/BPene) of NiFe LDH nanosheets and BPene using van der Waals (vdW) interactions.It offers new insights for the advancement of non-precious metal OER catalysts.
), and (060) crystal planes, respectively, and are consistent with the JCPDS card (No. 73-1358) for BPene.The sharp and intense diffraction peaks, along with the absence of other impurity peaks, confirm the high crystallinity and high purity of BPene.The XRD plot of NiFe LDH displays evident characteristic peaks at 11.2°, 22.8°, 59.69°, and 61.1°, corresponding to the (003), (006), (110), and (113) lattice planes, respectively.This result is consistent with the standard diffraction data in JCPDS card (No.40-0215), indicating successful preparation of NiFe LDH.The characteristic peaks of both NiFe LDH and BPene are clearly visible, sharp, and symmetrical in the XRD plot of the NiFe LDH/BPene composite material at 11.2° (003), 16.5° (020), 22.8° (006), 34.2° (040), 52.4° (060), 59.69° (110), and 61.1° (113).Further evidence that the synthesized NiFe LDH/BPene sample is exclusively composed of the structures of NiFe LDH and BPene is present in the form of a lack of any impurity diffraction peaks.The strength and symmetry of the diffraction peaks further demonstrate the great crystallinity and regular form of NiFe LDH/BPene.

Figure 1 .
Figure 1.(a) XRD patterns of NiFe LDH, BPene, and NiFe LDH/BPene; (b) Raman spectra of NiFe LDH, BPene, and NiFe LDH/BPene.To further investigate the molecular structure, vibrational information, and interactions of the samples, we present the Raman scattering spectra of the samples in Figure1b.As can be seen, BPene shows three distinct peaks at 323.5, 400.5, and 427.6 cm −1 .These peaks correspond to BPene's three vibrational modes, which are Ag 1 , B2g, and Ag 2 .Ag 1 is ), (110), and (113) lattice planes, respectively.This result is consistent with the standard diffraction data in JCPDS card (No.40-0215), indicating successful preparation of NiFe LDH.The characteristic peaks of both NiFe LDH and BPene are clearly visible, sharp, and symmetrical in the XRD plot of the NiFe LDH/BPene composite material at 11.2° (003), 16.5° (020), 22.8° (006), 34.2° (040), 52.4° (060), 59.69° (110), and 61.1° (113).Further evidence that the synthesized NiFe LDH/BPene sample is exclusively composed of the structures of NiFe LDH and BPene is present in the form of a lack of any impurity diffraction peaks.The strength and symmetry of the diffraction peaks further demonstrate the great crystallinity and regular form of NiFe LDH/BPene.

Processes 2023 ,
11, x FOR PEER REVIEW 8 of 14structure allows for increased exposure of active sites and reduced mass transfer resistance, further supporting our structural inference and providing a theoretical basis.
are cited in the supplementary materials.Author Contributions: Y.W.: Data curation, Writing-original draft, Conceptualization, Methodology, Validation, Writing-review.X.W.: Conceptualization, Methodology, Validation.Y.M.: Visualization, Investigation.Q.L.: Funding acquisition, Methodology, Writing-review & editing, Conceptualization.Q.X.: Funding acquisition, Project administration, Supervision, Writing-review & editing, Investigation.All authors have read and agreed to the published version of the manuscript.Funding: This work was supported by the National Natural Science Foundation of China (NSFC) (No. 22172098) and the Science and Technology Commission of Shanghai Municipality (No. 23ZR1424900, No. 19DZ2271100 and No. 20520740900).