Fe‐Induced Electronic Transfer and Structural Evolution of Lotus Pod‐Like CoNiFePx@P,N‐C Heterostructure for Sustainable Oxygen Evolution

Transition metal phosphides with metallic properties are a promising candidate for electrocatalytic water oxidation, and developing highly active and stable metal phosphide‐based oxygen evolution reaction catalysts is still challenging. Herein, we present a facile ion exchange and phosphating processes to transform intestine‐like CoNiPx@P,N‐C into lotus pod‐like CoNiFePx@P,N‐C heterostructure in which numerous P,N‐codoped carbon‐coated CoNiFePx nanoparticles tightly anchors on the 2D carbon matrix. Meanwhile, the as‐prepared CoNiFePx@P,N‐C enables a core‐shell structure, high specific surface area, and hierarchical pore structure, which present abundant heterointerfaces and fully exposed active sites. Notably, the incorporation of Fe can also induce electron transfer in CoNiPx@P,N‐C, thereby promoting the oxygen evolution reaction. Consequently, CoNiFePx@P,N‐C delivers a low overpotential of 278 mV (vs RHE) at a current density of 10 mA cm−1 and inherits excellent long‐term stability with no observable current density decay after 30 h of chronoamperometry test. This work not only highlights heteroatom induction to tune the electronic structure but also provides a facile approach for developing advanced and stable oxygen evolution reaction electrocatalysts with abundant heterointerfaces.


Introduction
Inspired by the worldwide "carbon neutral" strategy, hydrogen energy is flourishing as an efficient, clean, and sustainable energy source. [1]Currently, most of the hydrogen is produced from fossil fuels through the steam reforming process, where carbon dioxide is generated as a by-product. [2]8][9] Effective electrocatalysts are highly demanded to reduce the potential and accelerate the OER reaction.[15] Despite some achievements, these electrocatalysts still have some issues to be improved, such as poor electrical conductivity, insufficient active sites, and possible structural collapse during the catalytic process to enhance the OER activity and stability.
[18][19] In particular, the adsorption-desorption free energy of oxygen and hydroxyl groups of the TMPs can be optimized to kinetically promote the electrocatalytic OER process. [20]However, TMPs catalysts suffer from poor stability and severe aggregation, resulting in poor electrocatalytic performance.In this regard, active TMPs particles are usually supported on conductive substrates with high specific surface area to avoid the aggregation and improve stability. [21]eanwhile, constructing a core-shell structure is a reliable approach to prevent TMPs particles from corroding, exfoliating, and agglomerating during the electrocatalytic reaction.For example, Li and co-workers [22] embedded a layer of close-packed CoFeP nanocages into an interconnected carbon-cage framework to form a CoFeP@C core-shell superlattice, which showed excellent activity and stability for OER reaction.Recently, metal-organic framework (MOF)-derived multi-metal phosphides have attracted much attention due to their unique electronic structures and abundant active sites.Interestingly, the functional nanomaterials derived from the MOF with unique structures, dispersed metal ions, and high specific surface areas can well meet the above needs. [23]For instance, Duan et al. [24] doped Fe into CoP@Ni 2 P nanowires by combining the solvothermal route, chemical bath deposition, and phosphating process.The unique electronic structure in Fe-doped CoP@Ni 2 P heterostructure promotes the Gibbs free energy of hydrogen adsorption, leading to enhanced electrocatalytic activity.Especially, multi-metal phosphides with numerous heterointerfaces have been proposed as one of the most effective strategies to tune intrinsic activity since the electrocatalytic reactions often occur at the interfaces of electrodes, electrolytes, and target products. [25]28] Encouraged by the aforementioned motivations, we demonstrate a feasible ion exchange and phosphating process to transform intestinelike CoNiP x @P,N-C into lotus pod-like CoNiFeP x @P,N-C heterostructure.In the engineered CoNiFeP x @P,N-C heterostructure, a larger number of P,N-codoped carbon-coated CoNiFeP x nanoparticles are tightly anchored on the 2D carbon matrix, thus presenting abundant heterointerfaces and fully exposed active sites.Furthermore, the hierarchical pore structure and large specific surface area in CoNiFeP x @P,N-C heterostructure further increase the exposed active site.Importantly, the incorporation of Fe in the CoNi-MOF also modulates the electronic structure of CoNiP x @P,N-C, thereby promoting the OER reaction.As expected, CoNiFeP x @P,N-C achieves superior OER activity and stability in alkaline media.

Results and Discussion
As detailed in the experimental above, the lotus pod-like CoNiFeP x @P, N-C (Figure 1a) were synthesized in three steps.Firstly, spherical CoNi-MOF was synthesized using Co 2+ /Ni 2+ and H 3 BTC by solvothermal reaction.The CoNi-MOF spheres with a diameter of ~43.5 lm are assembled from a large number of thin cuboids with smooth surfaces (22 lm in length and 3 lm in width) (Figure 1b).Afterward, the etching process is controlled by hydrolytic ion exchange, in which iron nitrate undergoes a hydrolysis reaction in hot ethanol solution leading to the formation of H + ions.Fe 3+ and H + are simultaneously etched and gradually passed through the surface of the CoNi-MOF, followed by the release of Co 2+ and Ni 2+ , which immediately co-precipitate with hydroxide ions and Fe 3+ to produce nanosheets targeting nanoparticle assemblies. [29]The nanoparticles become larger, driving the ionic reaction and transforming into CoNi-MOF-Fe (Figure 1c).After the ion exchange reaction, the reaction kinetics of the internal and external coordination bonds in the MOF framework is different, [23] so the lamellar morphology of CoNi-MOF is well preserved.Finally, CoNi-MOF-Fe is phosphating under an Ar atmosphere.During this phosphating process, the smooth thin cuboid structures are transformed into porous lotus pod-like microstructures (Figure 1d), and the metal sources (Co, Ni, and Fe) are transformed into metal phosphides (CoNiFeP x ).
The X-ray diffraction (XRD) patterns (Figure S1, Supporting Information) prove the similar crystal structure of CoNi-MOF reported in the literature, [30,31] and the pristine MOF phase can be maintained in as-prepared CoNi-MOF-Fe after ion exchange. [32]After phosphating CoNi-MOF, the main XRD peaks of the CoNiP x @P,N-C can be assigned to Co 2 P (PDF#54-0413) and Ni 2 P (PDF#03-0953) (Figure 2a). [18,33]or the phosphating product CoNiFeP x @P,N-C of CoNi-MOF-Fe, in addition to the Co 2 P and Ni 2 P, the diffraction peaks of Fe 2 P (PDF#88-1803) are also detected. [33,34]It can be seen that the addition of iron atoms can induce their nucleation and derivation of new Fe 2 P phases from the separated binary metal phosphide phases (Ni 2 P and Co 2 P), which may provide more abundant types and numbers of active sites.
The X-ray photoelectron spectroscopy (XPS) survey spectra of CoNiP x @P,N-C and CoNiFeP x @P,N-C in Figures S2 and S3, Supporting Information demonstrate the presence of Co, Ni, C, N, O, and P elements.The CoNiFeP x @P,N-C contains Fe with an atomic ratio of 4%.In the high-resolution Fe 2p XPS spectra (Figure 2b), the peaks at 710.5 and 719.5 eV are ascribed to the Fe-P bonds in the Fe 2 P species, [35] while the other two peaks at 712.9 and 723.7 eV arise from Fe-O bonds.The remaining two broad peaks (716.1 and 727.8 eV) correspond to the satellite peaks.Figure 2c shows the XPS spectra of Co 2p for CoNiP x @P,N-C and CoNiFeP x @P,N-C.In addition to the two pairs of satellite peaks and Co 2+ peaks, a Co-P peak also appears at 778.2 eV, which is attributed to the Co 2 P species. [36]Notably, the incorporation of Fe leads to a negative shift in the binding energy of Co 2p 3/2 and Co 2p 1/2 , which is attributed to the implantation of lowelectronegativity Fe atoms into CoP.The ability of Fe to extract electrons from bonded Co atoms is weaker than that of P, which results in fewer electrons for Co atoms participating in Co-Fe bonding and makes the Co oxidation state in CoNiFeP x @P,N-C lower than that in CoNiP x @P,N-C. [37]In Figure 2d, Ni 2p spectra have three pairs of peaks corresponding to the Ni-P bond, Ni 2+ , and the satellite peak. [38]Similarly, compared with the Ni 2+ peaks corresponding to the binary metal CoNiP x @P,N-C, the Ni 2+ peaks (Ni 2p 3/2 and Ni 2p 1/2 ) of the ternary metal CoNiFeP x @P,N-C has a negative shift in the binding energy.This result indicates that doped Fe modulates the electronic configuration of Ni, and strong electronic interactions occur between metal ions. [29,39,40]herefore, more active sites are introduced into CoNiFeP x @P,N-C, and its OER performance will be improved.Naturally, the positive metal center and negative center can function as proton acceptor and hydride acceptor sites during electrocatalysis. [41]In the P 2p spectra (Figure 2e), the fitted peak centered at 129.9 eV corresponds to the metal-P bond, [42] while the P-O bond between 133 and 134 eV is assigned to the oxidized P species (P-O).Meanwhile, the dopant Fe can also indirectly adjust the electron density of adjacent P. The high-resolution P 2p XPS spectra of CoNiFeP x @P,N-C reveal a negative shift of the P 2p binding energy, indicating that more electrons occupy the P 2p of the P atom due to the enhanced electron donating ability of the Co atom. [37]Four main peaks at 398.3, 399.9, 401.3, and 402.5 eV are observed in the N 1s high-resolution XPS spectra (Figure 2f), corresponding to pyridine nitrogen, pyrrolic nitrogen, graphitic nitrogen, and nitrogen oxide, respectively.Besides, in the high-resolution XPS spectra of C 1s (Figure S4, Supporting Information), three main peaks were assigned to the C=C, C-P, and C-O peaks. [23]Additionally, the O 1s spectra in Figure S5, Supporting Information display two peaks of C-O and P-O. [23]he Raman spectra of CoNiP x @P,N-C, and CoNiFeP x @P,N-C in Figure S6, Supporting Information show two peaks at 1359.7 and 1592.6 cm À1 , corresponding to the typical D and G bands of carbon, respectively.The D band indicates relative graphitic disorder and the G band indicates the presence of crystalline graphitic carbon. [43]Among them, the I D /I G ratio of the ternary metal phosphide CoNiFeP x @P,N-C (1.87) is higher than that of the binary metal phosphide CoNiP x @P,N-C (1.78), which indicates that more defects are introduced into the carbon shell of CoNiFeP x @P,N-C.Generally, more defects are beneficial to provide additional active sites. [44,45]he as-synthesized CoNi-MOF is assembled with one-dimensional (1D) thin and smooth cuboid nanostructures with a thickness of ~150 nm, as shown in the field emission scanning electron microscope Energy Environ.Mater.2024, 7, e12628   Energy Environ.Mater.2024, 7, e12628 dimensional (2D) nanosheet structure with a thickness of ~30 nm, but its surface is loaded with a large number of ultrasmall nanoparticles with a diameter of ~20 nm (Figure 3g,h and Figure S9, Supporting Information), which is P,N-C-coated CoNiFeP x , as demonstrated by the TEM discussed below.Apparently, a large number of CoNiFeP x active sites are tightly loaded on the 2D carbon matrix, forming lotus pod-like CoNiFeP x @P,N-C, which fully exposes the active sites and further improves the physical stability of the catalysts.The corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mapping images of CoNiFeP x @P,N-C are displayed in Figure 3i and Figure S10, Supporting Information.It can be seen that Co, Ni, Fe, and P are uniformly distributed throughout the lotus pod-like heterostructure, demonstrating that the target Fe and P heteroatoms are successfully doped into the catalyst.
The transmission electron microscope (TEM) images of the CoNi-MOF (Figure 4a) show a cuboid-shaped assembly unit.The introduction of Fe 3+ ions has little effect on the morphology of CoNi-MOF (Figure 4b and Figure S11, Supporting Information), and the surface becomes rough.After phosphating, the cuboid-shaped CoNi-MOF converts into intestine-like CoNiP x @P,N-C with massively porous structures (Figure 4c,d), which can facilitate the contact of the active site with the electrolyte.Furthermore, the ultrathin and porous nanosheets (thickness of ~15 nm) with certain pore channels (diameter of ~3 nm) are loaded with a large number of uniformly dispersed ultrasmall nanoparticles (diameter of ~7 nm) (Figure S12, Supporting Information).The lotus pod-like CoNiFeP x @P,N-C shows core-shell nanoparticles uniformly embedded on the MOF-derived carbon matrix (Figure 4e and Figure S13, Supporting Information).The inner core nanoparticle is a metal phosphide with a size increased to 30 nm, and the outer shell is an ultrathin graphitized carbon layer (about ~4 nm), which can act as a protective shell for the active nanoparticles and protect them from oxidation during the electrocatalytic process.The high-resolution TEM (HRTEM) image in Figure 4f and Figure S14, Supporting Information depicts the core-shell structure.The lattice fringes of 0.181 and 0.331 nm are ascribed to the (130) and (020) facets of Co 2 P, [46,47] respectively.The lattice fringe of 0.19 nm is assigned to the (210) plane of Ni 2 P. [48] Moreover, the lattice fringe of 0.22 nm can be attributed to the (111) plane of Fe 2 P, [49,50] (111) plane of Ni 2 P, [50,51] or (121) plane of Co 2 P. [49,52] These results illustrate the abundant heterointerfaces in the CoNiFeP x catalyst, which is beneficial to optimize the electronic structure and facilitate the adsorption and desorption of oxygen intermediates.The HRTEM images in Figure S15, Supporting Information further confirm the lattice fringes of Co 2 P, Ni 2 P, and Fe 2 P, as well as numerous heterointerfaces between them.High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding EDS elemental mapping images of CoNiFeP x @P,N-C further demonstrates the homogeneous distribution of Co, Ni, Fe, P, N, and C elements throughout the electrocatalyst (Figure 4g,h and Figure S16, Supporting Information).
To gain insight into the specific surface area and porous structure of the catalysts, N 2 adsorption-desorption isotherms were performed.The Brunauer-Emmett-Teller (BET) curves show that the specific surface areas of intestine-like CoNiP x @P,N-C and lotus pod-like CoNiFeP x @P, N-C reaches 87.4 and 39.0 m 2 g À1 , respectively (Figure 4i,k).The slightly reduced specific surface area of CoNiFeP x @P,N-C is mainly due to the growth of metal phosphide nanoparticles caused by the incorporation of Fe, and the collapse of part of the ultrathin nanosheet structure, resulting in a partial reduction in specific surface area.From the pore size distribution curves (Figure 4j,l), it can be seen that both samples have hierarchical pore structures.Additionally, CoNiFeP x @P,N-C also presents macropores (74.7 nm), and the hierarchical pores are more abundant, which is conducive to the entry of electrolytes and the transport of reactants on the active sites.These results are in good agreement with the above SEM and TEM results.As expected, the carbonization and phosphating processes are very important for the formation of a porous carbon matrix, and the intestine/lotus pod-like structure promotes the formation of multiple heterointerfaces and maintains a highly developed porous structure.
Given the Fe-induced electronic transfer and structural evolution of heterostructures, lotus pod-like CoNiFeP x @P,N-C with abundant heterointerfaces and fully exposed active sites was employed as a catalyst for the electrolysis of water to O 2 .OER activity was investigated in 1 M KOH, and the linear sweep voltammetric (LSV) polarization curves are shown in Figure 5a.As expected, CoNiFeP x @P,N-C delivers robust activity with an overpotential of 278 mV versus RHE at a current density of 10 mA cm À2 , which is better than that of commercial RuO 2 (316 mV vs RHE), CoNiFe@N-C (341 mV vs RHE), and CoNiP x @P,N-C (350 mV vs RHE).The iR-compensated LSV curves were also recorded based on the equation of E iR-corrected = E À iR, [53,54] where E is the original potential, R is the internal resistance measured by EIS, and i is the corresponding current.As shown in Figure S17, Supporting Information, the OER activity trend of the catalyst is consistent with that of the catalyst without iR correction, and CoNiFeP x @P,N-C exhibits the best OER activity (268 mV vs RHE @ 10 mA cm À2 ).The low activity of CoNiFe@N-C and CoNiP x @P,N-C indicates the positive effect of Fe incorporation and phosphating in promoting the OER activity.The outstanding OER activity of CoNiFeP x @P,N-C outperforms numerous lately-reported metal phosphide-based electrocatalysts (Table S1, Supporting Information).Furthermore, a series of optimizations were performed by varying the molar ratios of Co 2+ and Ni 2+ in the Co x Ni y -MOF (x:y = 1:0, 2:1, 3:1, and 0:1) precursors, revealing the optimal molar ratios of Co 2+ and Ni 2+ is 2:1 (Figure S18, Supporting Information).Additionally, the influence of CoNi-MOF precursor content and phosphating temperature on the OER activity was also explored in Figures S19 and S20, Supporting Information, and the corresponding optimal values are 0.1 g and 500 °C.The amount of CoNi-MOF precursor added and the phosphating temperature has an effect on the active site exposure and morphology of the target catalysts, so there are certain differences in their OER activity.Apparently, the large specific surface area of and a larger number of dispersed  MOF, c, d) CoNiP x @P,N-C, and e) CoNiFeP x @P,N-C.f) HRTEM images, g) HAADF-STEM images, and h) EDS elemental mapping images of CoNiFeP x @P,N-C.i, k) N 2 adsorption-desorption isotherms and j, l) pore size distribution curves of i, j) CoNiP x @P,N-C and k, l) CoNiFeP x @P,N-C.and well-exposed active sites in the lotus pod-like CoNiFeP x @P,N-C heterostructure can promote the OER activity.
Besides, to investigate the OER kinetics of the catalysts, the Tafel slope was calculated from the LSV polarization curves, as shown in Figure 5b.CoNiFeP x @P,N-C enables a Tafel slope of 49.5 mV dec À1 , lower than that of RuO 2 (63.6 mV dec À1 ), CoNiFe@N-C (50.7 mV dec À1 ), and CoNiP x @P,N-C (78.3 mV dec À1 ), indicating faster electrocatalytic kinetics for CoNiFeP x @P,N-C.Obviously, the abundant heterointerfaces in CoNiFeP x @P,N-C can facilitate the adsorption and desorption of oxygen intermediates, thereby promoting the OER process.[57] C dl was calculated from cyclic voltammetry (CV) curves at different scan rates (20-100 mV s À1 ) in the non-faradaic region (Figure S21, Supporting Information).As illustrated in Figure 5c, CoNiFeP x @P,N-C exhibits the lowest C dl value (7.03 mF cm À2 ) compared to RuO 2 (5.77 mF cm À2 ), CoNiFe@N-C (3.35 mF cm À2 ), and CoNiP x @P,N-C (1.85 mF cm À2 ).Therefore, CoNiFeP x @P,N-C shows the highest ECSA value (Figure S22, Supporting Information).Additionally, electrochemical impedance spectroscopy (EIS) measurements were performed to further explore the conductivity of CoNiFeP x @P,N-C in alkaline media.As displayed in Figure 5d, CoNiFeP x @P,N-C achieves the lowest charge transfer resistance (R ct ) of 17.4 Ω, indicating its favorable electron transfer.It is clear that the incorporation of Fe and phosphating treatment promotes the formation of P, N co-doped graphitized carbon and metalloid multi-metal phosphides in CoNiFeP x @P,N-C, which greatly improves the conductivity of the catalyst.and d) Nyquist plots of commercial RuO 2 , CoNiFe@N-C, CoNiP x @P,N-C, and CoNiFeP x @P, N-C catalysts.e) LSV polarization curves CoNiFeP x @P,N-C before and after 3000 CV cycles.f) Chronoamperometry of CoNiP x @P,N-C and CoNiFeP x @P,N-C at a constant potential for 30 h. g, h) TEM and i) HRTEM images of CoNiFeP x @P,N-C after chronoamperometry test.
Energy Environ.Mater.2024, 7, e12628 To disclose the durability of the catalyst, a CV cycling test was performed to evaluate the feasibility of the actual OER system process.As depicted in Figure 5e, after a continuous 3000 CV cycles, CoNiFeP x @P, N-C shows only a slight potential decay (0.008 V vs RHE) at a current density of 10 mA cm À2 .However, CoNiP x @P,N-C exhibits worse durability with a potential decay of 0.014 V (vs RHE) at the same conditions.It is believed that the ultrathin graphitized carbon layer in the lotus pod-like CoNiFeP x @P,N-C can well act as a protective shell to protect the multi-metal phosphides from oxidation during electrocatalysis.Meanwhile, a chronoamperometry test was also carried out to probe the stability of the catalyst.Figure 5f elucidates that CoNiFeP x @P, N-C maintains a stable current density after 30 h of the OER process, while CoNiP x @P,N-C reveals a ~3.5% drop in current density after the chronoamperometry test.Undoubtedly, the tightly loaded CoNiFeP x active sites on the 2D carbon matrix contribute to the excellent stability of CoNiFeP x @P,N-C during the OER process.The structural stability of CoNiFeP x @P,N-C was also evaluated by comparing the morphology before and after the long-term stability test.As illustrated by the TEM images (Figure 5g,h and Figure S23, Supporting Information), the core-shell nanoparticles and uniformly dispersed metal phosphides in the lotus pod-like heterostructure are well maintained after the longterm OER process.At the same time, the HRTEM images in Figure 5i and Figure S24, Supporting Information clearly show the lattice fringes of 0.22 nm, which can be assigned to the metal phosphides (Co 2 P, Ni 2 P, and Fe 2 P), which again confirms the remarkable stability of CoNiFeP x @P,N-C.Therefore, these merited stability features make CoNiFeP x @P,N-C a promising candidate for efficient OER catalysts.

Conclusions
In summary, a lotus pod-like CoNiFeP x @P,N-C with core-shell structure, high specific surface area, and hierarchical pore structure is constructed as a highly active and stable OER catalyst via ion exchange and phosphating processes.In-depth ex situ characterizations reveal that after exchange heteroatomic Fe into the CoNi-MOF precursor, the phosphate products will transform from the intestine-like CoNiP x @P, N-C to the lotus pod-like CoNiFeP x @P,N-C, which delivers numerous P,N-codoped carbon-coated CoNiFeP x nanoparticles tightly anchors on the 2D carbon matrix, thus presenting abundant heterointerfaces and fully exposed active sites.In addition to the structural evolution, the incorporation of Fe also induces electron transfer in CoNiP x @P,N-C, thereby promoting the OER reaction.Leveraging the hierarchical pore structure, abundant heterointerfacial structure, and favorable electronic structure, CoNiFePx@P,N-C inherits excellent OER performance with a low overpotential of 278 mV (vs RHE) at a current density of 10 mA cm À2 and outstanding long-term stability in alkaline media.This study not only highlights heteroatom induction to tune the electronic structure but also provides a facile strategy for developing advanced and stable OER electrocatalysts with abundant heterointerfaces.Synthesis of CoNi-MOF: 0.1163 g Ni(NO 3 ) 2 Á6H 2 O was dissolved in the mixed solvents of 48 mL DMF and 12 mL methanol, and then 0.1164 g Co (NO 3 ) 2 Á6H 2 O was dissolved in the solution.Finally, 0.75 g H 3 BTC was added into the mixed Ni-Co solution.After stirring for 30 min, the precursor was poured into a 100 mL Teflon-lined autoclave.The autoclave was soaked at 120 °C for 24 h in oven and then cooled down naturally.The products were collected by centrifugation and washed with methanol repeatedly for three times.After drying in vacuum oven at 80 °C for 12 h, CoNi-MOF was obtained.Co x Ni y -MOF with various molar ratio of Co/Ni (x:y = 1:0, 2:1, 3:1, and 0:1) were synthesized to study the effect of Co/Ni molar ratio on the OER activities of MOF.

Experimental Section
Synthesis of CoNi-MOF-Fe: Typically, 0.1 g as-synthesized CoNi-MOF and 0.025 g Fe(NO 3 ) 3 Á9H 2 O were added to 22.5 mL ethanol.The mixture was magnetically stirred for 30 min and then transferred into a 50 mL Teflon-lined autoclave.After soaking at 120 °C for 4 h, the CoNi-MOF-Fe powders were washed with ethanol, and then dried in a vacuum oven at 40 °C for 12 h.For comparison, the CoNi-MOF precursor content (0.05 and 0.15 g) was also optimized.
Synthesis of CoNiP x @P,N-C and CoNiFeP x @P,N-C: The CoNiFeP x @P,N-C was prepared by phosphating the CoNi-MOF-Fe at 500 °C for 2 h with a heating rate of 3 °C min À1 under argon atmosphere using NaH 2 PO 2 ÁH 2 O as phosphorus source located at the upstream side of the tube furnace.Similarly, CoNi-MOF was also phosphated by the same procedure to obtain the intestine-like CoNiP x @P,N-C electrocatalysts.The effects of phosphating temperature on the OER performance of CoNiFeP x @P,N-C were studied by varying the phosphating temperature of 350 °C and 650 °C, respectively.For comparison, CoNi@N-C and CoNiFe@N-C were synthesized by directly pyrolyzing CoNi-MOF and CoNi-MOF-Fe under argon atmosphere without phosphating process.
The material characterizations and electrochemical measurements for the catalysts are detailed in the Supporting Information.

(
FESEM) of Figure 3a,b and Figure S7, Supporting Information.After Fe exchange, the surface of CoNi-MOF is etched, but the 1D cuboidlike morphology remained intact, but the surface become rough (Figure 3c,d).Interestingly, when the CoNi-MOF undergoes the simultaneous carbonization and phosphorylation in argon, the cuboid-shaped CoNi-MOF is derived into CoNiP x @P,N-C with ultrathin nanosheet structures (~15 nm in thickness) and wrinkled surfaces (Figure 3e,f and Figure S8, Supporting Information), which provides large specific surface area for exposing more active sites.Notably, after phosphating, CoNi-MOF-Fe also forms a similar ultrathin two-

Figure 1 .
Figure 1.a) Schematic illustration of the in-situ conversion of CoNi-MOF to lotus pod-like CoNiFeP x @P,N-C heterostructure.SEM images of b) CoNi-MOF, c) CoNi-MOF-Fe, and d) CoNiFeP x @P,N-C.

Figure 5 .
Figure 5. a) LSV polarization curves, b) Tafel slopes, c) C dl values,and d) Nyquist plots of commercial RuO 2 , CoNiFe@N-C, CoNiP x @P,N-C, and CoNiFeP x @P, N-C catalysts.e) LSV polarization curves CoNiFeP x @P,N-C before and after 3000 CV cycles.f) Chronoamperometry of CoNiP x @P,N-C and CoNiFeP x @P,N-C at a constant potential for 30 h. g, h) TEM and i) HRTEM images of CoNiFeP x @P,N-C after chronoamperometry test.