Anchoring Active Sites by Pt2FeNi Alloy Nanoparticles on NiFe Layered Double Hydroxides for Efficient Electrocatalytic Oxygen Evolution Reaction

Zhicheng Zheng,Yanru Guo*,Hao Wan ,Gen Chen ,Ning Zhang,Wei Ma,Xiaohe Liu*,Shuquan Liang,and Renzhi Ma*

Strategy of anchoring alloy nanoparticles made up of the efficient catalytic element(e.g.,Ni,Fe)on dodecyl sulfate(DS-)-intercalated NiFe layered double hydroxides(DS--NiFe LDH)obtained by a convenient one-step hydrothermal coprecipitation method for essentially enhancing oxygen evolution reaction(OER)performance was proposed.The results of structural characterization indicate Pt2FeNi alloy nanoparticles evenly distribute on the surface of DS--NiFe LDH.The sizes of the Pt2FeNi nanoparticles,closely related to their OER performance,could be wellcontrolled by adjusting the amount of H2PtCl6addition.The composite structure of as-prepared product was stable during processes of synthesis,exfoliation,self-assembly,and subsequent electrocatalytic OER.Rigorous electrochemical test proving the contributing catalytic active sites was located at the interface between Pt2FeNi and DS--NiFe LDH,and the Ni and Fe were the major active elements while O atoms are adsorption sites.The formation of Pt2FeNi nanoparticles could greatly prompt the reduction of Tafel slope.The best-performing Pt2FeNi/DS--NiFe LDH with a Pt content of 0.98 wt% achieved low overpotential of 204 mV at 10 mA cm-2and 262 mV at 50 mA cm-2.This work provides a convenient and effective strategy to create additional active sites for enhancing OER performance of NiFe LDH and make contribution to its wide application.

Keywords

electrocatalysis,NiFe Layered double hydroxides,oxygen evolution reaction,Pt2FeNi nanoparticles

1.Introduction

With the aggravation of energy crisis and environmental pollution,it is significant to seek sustainable and eco-friendly energy.[1-3]Oxygen evolution reaction(OER)has attracted extensive research interests as a kinetically sluggish process in water splitting[4-6]and metal air batteries.[7-9]In the electrocatalytic water splitting,compared with hydrogen evolution reaction(HER),OER demands a higher overpotential to drive the complex four proton coupled electron transfers and the formation of oxygen-oxygen bond.[10-12]Normally,noble metal-based catalysts,such as iridium oxide(IrO2)and ruthenium oxide (RuO2), possess efficient electrocatalytic OER performance under alkaline conditions.However,scarcity and high cost vastly limited their wide application.[13]Accordingly,earth-abundant non-noble metals,such as tradition metals,were extensively researched for achieving high OER performance.

NiFe-based LDH are the most promising materials to substitute the noble metal-based catalyst owing to its low cost,layered structure,and efficient OER performance.[14,15]The OER performance of NiFe LDH could be optimized by adjusting Ni2+/Fe3+ratio,interlayer spacing,and degree of crystallinity.[16]However,the single NiFe LDH is of poor electronic conductivity and intrinsic catalytic activity,restricting its practical application.[17,18]To further reduce the overpotential of electrocatalytic OER,abundant approaches were developed such as exfoliation[19-21]for increasing specific surface area,superlattice structure[16,22,23]for improving electron conductivity and activity,in-situ growth on metal surface[24,25]for optimizing spatial structure and third-metal element doping[13,17,26]for adjusting local electronic structure.Recently,a new efficient approach for modifying the surface of LDH was proposed by introducing heterogeneous nanoparticles over its surface.[27-29]With the help of heterogeneous nanoparticles,additional active sites were formed at the interface,resulting in the lower overpotential.Up to now,various heterogeneous nanoparticles were proposed, such as FeOOH,[27]CeOx,[29]Co0.85Se.[30]Gao et al.[28]found that Ni cation locate at NiO/NiFe LDH intersection and neighbor upon the lattice O of LDH atop Fe-Ni-Ni triangle,displayed a unique catalytic behavior and delivered a very low overpotential.That work well demonstrated that catalytic active sites could locate at outside the NiFe LDH host via suitable design and thus get rid of intrinsic activity of NiFe LDH.

Normally,noble metal Pt is chemical inert,while H2PtCl6and other Pt-based salts are not stable under heating.Conventional Pt-based bimetallic or polymetallic alloy nanoparticles were obtained via coreduction of Pt and other metallic elements to form Pt3Mn,[31]Pt3Ni-Fe,[32]Pt2CuNi[33]and Pt3(NiCo)2.[34]The synergistic effect between Pt and other metallic elements helps to improve the catalytic performance largely.Nonetheless,most reports of Pt-based alloys focused on their ORR or HER performance,and performance for OER was rarely studied.[35]Furthermore,the addition of other metals could decrease the total cost of Pt-based alloys.

In this work,a strategy of anchoring Pt2FeNi alloy nanoparticles on surface of DS--NiFe LDH to intrinsically enhance its OER performance was proposed via a simple one-step hydrothermal coprecipitation method.Similar to classical coprecipitation method,PtCl62-,Ni2+,and Fe2+coexisted in precursor solution which contains intercalated anion(sodium dodecyl sulfate,DS-)and alkali source(hexamethylenetetramine,HMT).As the temperature increases,the pyrolyzation of hexamethylenetetramine resulted in the rise of pH and generation of formaldehyde.[36]PtCl62-united the efficient catalytic element(Ni2+and Fe2+)and formed hybrid hydroxides with the concurrent reaction of crystallization of DS--NiFe LDH.and then the hybrid hydroxides were reduced by formaldehyde and formed Pt2FeNi alloy nanoparticles supported by DS--NiFe LDH.Characterization analysis indicated Pt2FeNi alloy nanoparticles evenly distribute on the surface of DS--NiFe LDH with a great affinity.The as-prepared electrocatalyst requires overpotentials of only 204 mV and 262 mV to achieve benchmarks of 10 mA cm-2and 50 mA cm-2,respectively.The key active sites were verified to be the Ni and Fe on surface of Pt2FeNi/DS--NiFe LDH.This work provides a convenient and effective strategy of creating additional active sites for enhancing the OER performance of NiFe LDH.

2.Results and Discussion

A high efficiency composite catalyst of Pt2FeNi alloy nanoparticles supported by DS--NiFe LDH was prepared via a simple one-step hydrothermal coprecipitation method.The XRD patterns of catalysts are shown in Figure 1a.Typical characteristic peaks of DS--intercalated NiFe LDH are observed and consistent with the literature.[16,37]The position of(003)diffraction peak is 3.39°and corresponding d(003)is about 2.6 nm which is larger than that of the LDH synthesized by ion-exchange reaction(about 2.4 nm[16,37]).As the addition of trace amount of H2PtCl6(0.1 at% and 1 at% ),diffraction peaks have no obvious changes due to the low content of Pt2FeNi nanoparticles.FT-IR spectra show(as shown in Figure S2)that a stretching vibration at 2362 cm-1stand out and suggests the formation of new phase(Pt2FeNi)which could bonds with air molecules.[34]And stretching vibration at 653 cm-1corresponding to M-O[38]attributed to the oxidation of Pt2FeNi nanoparticles or the oxygen atom bonded to NiFe LDH and Pt2FeNi nanoparticles.As observed in the XRD pattern of DS--NiFe LDH-0.05Pt,three strong diffraction peaks at 41.1°,46.9°,and 71.2°appear which are fully in accord with crystallography structure of Pt2FeNi(PDF#350702).Results of XRD pattern prove that the obtained products are consist of DS--NiFe LDH and Pt2FeNi alloy.The EDS data(as shown in Table S1)indicate NiFe element ratios of as-prepared samples are similar(close to 3:1).As exhibited in the HAADF-STEM image of DS--NiFe LDH-0.01Pt(Figure 1b),a mass of tiny bright particles corresponding to Pt2FeNi nanoparticles distribute in the darker LDH carrier homogenously.Figure S3 displays similar distribution while the other two sizes of Pt2FeNi nanoparticles(DS--NiFe LDH-0.001Pt and DS--NiFe LDH-0.05Pt).The size of Pt2FeNi nanoparticles is about dozens of nanometers.The HRTEM image(see Figure 1c)shows distinct lattice fringes with spacing values of 0.219 and 0.193 nm,corresponding to the interplanar crystal spacing of Pt2FeNi(111)and(020)planes,respectively.The EDS mapping images exhibit that the compositional distributions of the three elements(Pt,Fe,and Ni)are uniform,verifying the presence of Pt2FeNi nanoparticles.The binding strength between Pt2FeNi nanoparticles and DS--NiFe LDH was evaluated qualitatively via processes of exfoliation and assembly.As shown in Figure S4,the thickness of NiFe LDH nanosheets is about 1 nm and that of Pt2FeNi nanoparticles is about 6 nm(representative thickness).Meanwhile,most of Pt2FeNi nanoparticles were located on the surface of the DS--NiFe LDH nanosheets.After self-assembled by KOH,HAADF-STEM image seems similar to the previous one,showing Pt2FeNi nanoparticles are still homogenously distributed in DS--NiFe LDH,confirming the tight adhesion between Pt2FeNi nanoparticles and DS--NiFe LDH.Elemental analysis given by inductively coupled plasma(ICP)spectroscopy indicates that the Pt content changes slightly from 0.98 wt% to 1.40 wt% after exfoliation and self-assembly,confirming the great affinity between Pt2FeNi and DS--NiFe LDH.

Figure 1.a)XRD patterns of(i)DS--NiFe LDH,(ii)DS--NiFe LDH-0.001Pt,(iii)DS--NiFe LDH-0.01Pt and(iv)DS--NiFe LDH-0.05Pt;b)HAADF-STEM;and c)HRTEM image of DS--NiFe LDH-0.05Pt;d)EDS elemental mapping images of Pt2FeNi/DS--NiFe LDH composite structure.

The XPS measurements were further carried out on DS--NiFe LDH and DS--NiFe LDH-xPt to investigate the surface chemical information.The high-resolution Ni 2p spectrum(as shown in Figure 2a)can be fitted with two spin-orbit doublets of Ni 2p1/2at 873.8 eV and Ni 2p3/2at 856.2 eV.Two shakeup satellite with a lower intensity(identified as“Sat.”)locate at 862.2 eV and 880.2 eV,respectively.[29,39]The fine fitting peaks of Ni2+and Ni3+certify the coexistence of Ni2+and Ni3+in NiFe LDH,and the peak at 867.5 eV might be shakeup satellite corresponding to Ni3+.[40-42]In the case of Fe 2p spectrum(Figure 2b),two main peaks at 726.8 eV and 713.4 eV corresponded to Fe 2p1/2and Fe 2p3/2,respectively.And two shakeup satellite at 715.8 eV and 734.3 eV,respectively.[43,44]Figure S5a and Figure S4b show Ni 2p and Fe 2p curves of DS--NiFe LDH and DS--NiFe LDH-xPt.The great similarity suggests the great stability of the DS--NiFe LDH in all of asprepared samples.Figure 2c shows O 1s XPS spectra of DS--NiFe LDH and DS--NiFe LDH-0.05Pt.In the DS--NiFe LDH,two peaks at 531.7 eV and 532.8 eV assigned to M-O-H(M stands for metal atoms,H stands for hydrogen atoms)and H-O-H molecule.[10,27]Importantly,the O 1s of DS--NiFe LDH-0.05Pt displays an additional characteristic peak at 530.8 eV corresponding to M-O-M,[10,29,45]which attributed to the oxidation of Pt2FeNi nanoparticles and the oxygen atom binding DS--NiFe LDH and Pt2FeNi nanoparticles.The appearance of M-O-M characteristic peak is consistent with the enhancement of M-O in FT-IR spectra(Figure S2).Pt 4f fine XPS spectra can be fitted with two spin-orbit doubles of Pt 4f5/2and Pt 4f7/2at 74.4 eV and 70.2 eV,respectively.Compared with Pt nanoparticles(Pt 4f7/2at about 70.8 eV)in other reports,[45,46]the Pt 4f7/2of Pt2FeNi nanoparticles in this work have a negative shift of about 0.6 eV,which suggest the difference between Pt nanoparticles and Pt2FeNi nanoparticles.

Figure 2.XPS fine spectra.a)Fe 2p of DS--NiFe LDH-0.01Pt,b)Ni 2p of DS--NiFe LDH-0.01Pt,c)O 1s of(i)DS--NiFe LDH and(ii)DS--NiFe LDH-0.05Pt,d)Pt 4f of(i)DS--NiFe LDH-0.001Pt,(ii)DS--NiFe LDH-0.01Pt and(iii)DS--NiFe LDH-0.05Pt.

The OER electrochemical activity of as-prepared samples was tested in 1.0 M KOH aqueous solution using a standard three-electrode system.Figure 3a shows the linear sweep voltammetry(LSV)curves of asprepared samples and blank carbon paper electrode,and Figure 3b exhibits the overpotential at geometrical current density of 10 mA cm-2and 50 mA cm-2.For the carbon paper electrode had the slow-rising current density with the rise of applied voltage indicate blank carbon paper electrode,showing feeble contribution for electrochemical OER activity.DS--NiFe LDH exhibited overpotentials of 318 mV and 412 mV at 10 mA cm-2and 50 mA cm-2,respectively,which is similar with other works.[10,13,16]In contrast with DS--NiFe LDH,all of DS--NiFe LDH-xPt show extremely low overpotential and the DS--NiFe LDH-0.01Pt have the lowest overpotential of 204 mV at 10 mA cm-2and 262 mV at 50 mA cm-2,indicating the Pt2FeNi nanoparticles have a great promotion to the OER activity of DS--NiFe LDH.Meanwhile,the OER activity is not in direct proportion to the addition of Pt.As shown in Figure S3,Pt2FeNi nanoparticles in DS--NiFe LDH-0.001Pt are rare and Pt2FeNi nanoparticles in DS--NiFe LDH-0.05Pt agglomerate into large particles,resulting in the lower overpotential compared with DS--NiFe LDH-0.01Pt.As the Pt2FeNi alloys nanoparticles form,the oxidation peaks emerge in the LSV curves,and the intensity increased with the rising amounts of Pt2FeNi nanoparticles.This result suggested the increased oxidation was caused by the oxidation of Pt2FeNi nanoparticles.Obviously,excessive Pt2FeNi nanoparticles inhibited the OER performance.

Figure 3.The electrochemical activity tests of OER.a)Polarization curves at a scan rate of 10 mV s-1;b)overpotential at geometrical current density of 10 mA cm-2and 50 mA cm-2,(i)DS--NiFe LDH,(ii)DS--NiFe LDH-0.001Pt,(iii)DS--NiFe LDH-0.01Pt,iv)DS--NiFe LDH-0.05Pt;c)the corresponding Tafel plots;d)Nyquist plots of the catalysts recorded at 1.45 V vs RHE;e)difference between anodic and cathodic current densities as a function of the scan rates;f)durability test of DS--NiFe LDH-0.01Pt at a current density of 10 mA cm-2and 50 mA cm-2,insert shows the polarization curves of DS--NiFe LDH-0.01Pt before and after 4000 cycles of CV.

The OER kinetics of as-prepared samples was evaluated via Tafel slopes obtained from the polarization curves.As shown in Figure 3c,the Tafel slopes of DS--NiFe LDH,DS--NiFe LDH-0.001Pt,DS--NiFe LDH-0.01Pt,and DS--NiFe LDH-0.05Pt are 111.1,103.1,90.3,and 88.4 mV dec-1,respectively.Obviously,with the increase of additive amount of Pt,the value of Tafel slope gradually decrease.DS--NiFe LDH-0.05Pt possesses the lowest Tafel slope,which exhibited the fastest reaction kinetics.This result indicates Pt2FeNi alloy nanoparticles have a promotion on kinetics of OER.To further investigate the kinetics of the electrocatalytic process,electrochemical impedance spectroscopy(EIS)is employed.Figure 3d shows Nyquist plots of the catalysts recorded at 1.45 V vs.RHE.Normally,The semicircles in the EIS curves correspond to the charge transfer resistance(Rct),which is related to the kinetic of OER occurring at the electrode/electrolyte interface.[47,48]All of the DS--NiFe LDH-xPt exhibited much smaller diameter of semicircle than that of the DS--NiFe LDH,suggesting the quicker reaction kinetics.The DS--NiFe LDH-0.01Pt which has the smallest diameter of semicircle shows the optimal kinetic performance.However,the DS--NiFe LDH-0.05Pt with a lower Tafel slope has a wider semicircle indicate process of charge transfer occurs at not surface of Pt2FeNi but at other sites,and further proof will be discussed in detail later.The double-layer capacitance(Cdl)determined on the basis of the CV curves is used to roughly represent the corresponding electrochemical surface area(ECSA)of the samples.As shown in Figure 3e,the linear slope of DS--NiFe LDH-0.01Pt is 0.35 mF cm-2,which is much higher than the linear slope of DS--NiFe LDH(0.20 mF cm-2),exhibited the highest ECSA.

The stability and durability of the DS--NiFe LDH-0.01Pt catalyst during OER was tested at constant current densities of 10 mA cm-2and 50 mA cm-2.As shown in Figure 3f,DS--NiFe LDH-0.01Pt exhibits a very high stability in all of the cases.The overpotential changed tinily at the constant current densities of 10 mA cm-2and 50 mA cm-2after 15 h.For the CV test,regardless of the part of the oxidation peak,LSV curves of DS--NiFe LDH-0.01Pt before and after 4000 cycles coincide very well,illustrating the great durability of prepared catalyst in alkaline media.The XRD and HRTEM images of DS--NiFe LDH-0.01Pt after 4000 cycles of CV curves are shown in Figure S6.The results indicate that the composite structure between NiFe LDH and Pt2FeNi nanoparticles remain stable even after the continuous 4000 CV cycles.By the way,(003)diffraction peak(as shown in Figure S6a)suggest that the interlayer spacing of NiFe LDH after 4000 cycles was greatly reduced.This is due to ion-exchange reaction occur in the interlayer during OER process and the DS-was replaced by OH-.[49]

For investigating the location of active sites of Pt2FeNi/DS--NiFe LDH on OER,a comparison test among DS--NiFe LDH,Pt2FeNi/DS--NiFe LDH,Pt/DS--NiFe LDH,and Pt2FeNi nanoparticles was carried out.XRD(as shown in Figure 4a)and TEM(as shown in Figure S7)results indicate that the Pt/DS--NiFe LDH and Pt2FeNi nanoparticles were successfully obtained.According to the LSV curves(as shown in Figure 4b),overpotential of Pt2FeNi/DS--NiFe LDH(204 mV)at 10 mA cm-2is much lower than both Pt2FeNi nanoparticles(294 mV)and DS--NiFe LDH(318 mV),which indicate that Pt2FeNi nanoparticles and DS--NiFe LDH have a synergistic effect on electrocatalytic OER.Importantly,Figure 4c exhibits the Pt2FeNi nanoparticles have an extreme low Tafel slope(60.5 mV dec-1),which is much lower than the DS--NiFe LDH(111.1 mV dec-1)and Pt2FeNi/DS--NiFe LDH(90.3 mV dec-1).While the higher overpotential and larger diameter of Nyquist plot semicircle(as shown in Figure 4d)of Pt2FeNi nanoparticles suggest the high-activity of OER is not occur on the surface of Pt2FeNi nanoparticles.This result is consistent with the previous results of Figure 3 that excessive Pt2FeNi nanoparticles inhibited the OER performance.Obviously,the interface between Pt2FeNi nanoparticles and DS--NiFe LDH is the most significant contributor during OER,as both single Pt2FeNi nanoparticles and DS--NiFe LDH cannot achieve low overpotential.

Figure 4.Comparison of Pt2FeNi/DS--NiFe LDH,Pt/DS--NiFe LDH and Pt2FeNi nanoparticles.a)XRD,(i)Pt2FeNi/DS--NiFe LDH,(ii)Pt/DS--NiFe LDH,(iii)Pt2FeNi nanoparticles;b)LSV curves;c)Tafel slopes and d)Nyquist plots.

Figure 5 reveals the schematic diagram of the proposed process of OER on the Pt2FeNi/DS--NiFe LDH.As a result of hydrothermal treatment,chemical bonds would be formed between Pt2FeNi and NiFe LDH,resulting in the considerable change of the chemical environment of metal atoms on the surface of Pt2FeNi nanoparticles and thus a great enhancement of OER performance.The Nyquist plots(as shown in Figure 4d)indicate the Rctof Pt2FeNi nanoparticles is higher than the composite structure of Pt2FeNi/DS--NiFe LDH,which suggests a promotion for DS--NiFe LDH on the reaction rates occurring at the electrode/electrolyte interface.Compared with Pt/DS--NiFe LDH,the lower overpotential and Tafel slope of Pt2FeNi/DS--NiFe LDH manifest that element of nickel and iron have a pivotal effect on improving the intrinsic activity of active sites.This result is consistent with the observation that the amounts of loaded Pt2FeNi lead to different OER performance.The quantity and area of interface between Pt2FeNi and DS--NiFe LDH should be optimal.In the one case,with the increase of the amount of Pt,the quantity of nucleation of Pt2FeNi and corresponding quantity of interface increased,lead to an increasing OER activity.In the other case,when the amount of Pt was excess(DS--NiFe LDH-0.05Pt),agglomeration occur after the initial nucleation and the OER performance becomes worse.To further explore the active sites,we used DS--NiFe LDH-0.01Pt-loaded carbon paper after 4000 cycles of CV to get XPS spectra.As shown in Figure S10,there are no shifts on Ni 2p,Fe 2p,and Pt 4f before and after 4000 cycles of CV.However,the satellite peaks of Ni 2p3/2and Fe 2p3/2have a significant increase in relative intensity after 4000 cycles CV test.This result indicates the electronic structure of Ni and Fe had changed greatly.Hence,we regard as the Ni and Fe are the active element in OER process.On the other hand,there is slight shift of O 1s between before and after stability test.And a wide peak emerged at near 535.5 eV.It suggests some intermediates during OER adsorbed on the O atoms and we regard O atoms as important adsorption sites.The possible reason of synergistic effect between Pt2FeNi and NiFe LDH is that Ni and Fe in Pt2FeNi could change density of charge distribution of O atom binding Pt2FeNi and NiFe LDH,and then promoting the process of mass transfer and heightening the activity of OER.

Figure 5.Schematic diagram of the proposed process of OER on the Pt2FeNi/NiFe LDH.

3.Conclusion

In summary,Pt2FeNi alloy nanoparticles stably supported by DS--NiFe LDH was successfully obtained by a convenient one-step hydrothermal coprecipitation method.The sizes of Pt2FeNi nanoparticles,which have a great influence on OER performance in alkaline electrolyte,could be controlled by adjusting the amount of H2PtCl6without affecting the structure of DS--NiFe LDH.The DS--NiFe LDH-0.01Pt nanocomposites shows the best OER performance with an overpotential of 204 mV at 10 mA cm-2and 262 mV at 50 mA cm-2in 1 M KOH.The excellent OER activity could be attributed to the synergistic effect and highly active interfacial surface between Pt2FeNi and DS--NiFe LDH.This work provides a convenient and effective strategy to create abundant active sites for enhancing OER performance of NiFe LDH.

4.Experimental Section

Materials:Nickel(II)chloride hexahydrate(NiCl2·6H2O,Sinopharm Group Co.Ltd,China),ferrous chloride tetrahydrate(FeCl2·4H2O,Wako Pure Chemical Industrial Ltd,Japan),sodium dodecyl sulfate(SDS,Wako Pure Chemical Industrial Ltd),hexamethylenetetramine(HMT,Alfa Aesar,China),chloroplatinic acid(H2PtCl6,Sinopharm Group Co.Ltd,China),isopropanol(Sinopharm Group Co.Ltd,China),nafion solution(10 wt% ,Sigma-Aldrich,Inc.),carbon paper(TORAY,Japan),potassium hydroxide(KOH,Sinopharm Group Co.Ltd).

Synthesis of DS--NiFe LDH:DS--intercalated Nickel Iron layered double hydroxides(DS--NiFe LDH)was synthesized via a simple one-step hydrothermal coprecipitation with oil bath and re fluxing.Deionized water was purged with nitrogen gas for 1 h to remove carbon dioxide under continuous magnetic stirring.Then,2.801 g HMT and 2.160 g SDS was dissolved into the 300 mL DI water.Subsequently,0.446 g NiCl2·6H2O and 0.124 g FeCl2·4H2O(the molar ratio of Ni and Fe is 3:1)was added to the solution.The final mixed solution was heated at 120°C for 8 h with continuous nitrogen protection.After cooling down naturally,the precipitate was collected by centrifugation,washed 3 times with deionized water and ethanol,and eventually dried at 60°C for 12 h.

Synthesis of DS--NiFe LDH-xPt:The synthesis method of DS--NiFe LDH-xPt is similar with the synthesis of DS--NiFe LDH.Before adding SDS and HMT,a certain volume of H2PtCl6aqueous solution(0.01 g mL-1)was mixed into the water,and the rest of steps same as above.In this work,the addition of volume of H2PtCl6was 0.130 mL,1.295 mL,and 6.475 mL,the corresponding atomic ratio of Pt is 0.1% ,1% ,and 5% of total of the additive Ni and Fe.The collected precipitate noted as DS--NiFe LDH-0.001Pt,DS--NiFe LDH-0.01Pt,and DS--NiFe LDH-0.05Pt,respectively.

Exfoliation and Self-assembly of DS--NiFe LDH-0.01Pt:30 mg of as-prepared DS--NiFe LDH-0.01Pt was dispersed in 50 mL formamide.The suspension was shaken for 8 h with a speed of 100 rpm,and then, the NiFe LDH nanosheets(NiFe LDH NSs)colloidal solution were collected by centrifugation to remove the bulk and polylaminate LDH. For self-assembly,1 M KOH solution was dropwise added to colloidal solution,and then floccule(noted as NiFe LDH/Pt2FeNi)was formed.The floccule was collected by centrifugation and washed for one time with ethanol and 2 times with deionized water, and eventually dried by vacuum freeze-drying.

Synthesis of Pt/DS--NiFe LDH:20 mg as-prepared DS--NiFe LDH was dispersed in the mixed solution of 30 mL deionized water and 10 mL ethanol.Subsequently,2 mL K2PtCl6aqueous solution with a concentration of 2 mg mL-1was added in above suspension liquid.The mixtures were stirred intensely and illuminated by a 300 W mercury amp for 120 min with a distance of 10 cm at room temperature.The black precipitates were collected by centrifugation and washed with deionized water for five times.

Synthesis of Pt2FeNi Nanoparticles:29.7 mg glucose,3.838 mg FeCl2·4H2O,4.588 mg NiCl2·6H2O,and 1 mL 0.01 g mL-1H2PtCl6aqueous solution were dissolved into 10 mL deionized water.Stirring well for 10 min.Adjusting pH to 11.0 and then stirring into a homogeneous solution for 10 min.10 mL 0.01 M sodium borohydride was slowly added into above solution for reduction reaction.The final Pt2FeNi powder was collected by centrifugation and vacuum drying.

Materials Characterization: The crystallographic composition was investigated by X-ray diffraction (XRD, Rigaku D/max 2500) with Cu Kα radiation(λ = 1.54178 ˚A) and 40 kV/15 mA. Nicolet 6700 Fourier Transform Infrared (FT-IR)spectrometer in KBr matrix was employed to record IR spectrums. A Tecnai G2 F20transmission electron microscope (TEM) is used to record more detailed information of microstructural and element. Inductively coupled plasma (ICP) spectroscopywas carried out on Optima 5300 DV. X-ray photoelectron spectroscopy (XPS) wasrecorded on a Thermo Fisher ESCALAB 250Xi spectrophotometer.

Electrochemical Measurements:Electrochemical measurements were obtained on a CHI 760E electrochemical analyzer(CH Instruments,Inc.,Shanghai)in a standard three-electrode system.3 mg as-prepared catalyst was dissolved into 1 mL of the mixed solution of isopropanol and deionized water(volume ratio is 1:1),and then 20 μL 10 wt% nafion aqueous solution was added.The suspension was dispersed by ultrasound for 30 minutes to obtain a homogeneous ink.The work electrode was prepared by dripping the catalyst ink onto a carbon paper with a size of 1×2 cm2(a surface of the carbon paper was covered completely by Teflon tape and the other one surface was covered partly,and the area of catalytic reaction region is 1×1 cm2,as shown in Supporting Information Figure S1).The catalyst loading in this work was unified 0.6 mg cm-2.Hg/HgO electrode and platinum plate with a size of 1×1 cm2as reference and counter electrodes,respectively.All potentials in this study were calibrated with respect to the reversible hydrogen electrode(RHE)scale according to the following equation:

40 cycles of cyclic voltammetry(CV)scan were used to activate the catalysts.The polarization curves were measured in 1 M KOH aqueous solution at room temperature with a scan rate of 10 mV s-1and 95% iR correction.Electrochemical surface area(ECSA)was measured by cyclic voltammetry(CV)using the same working electrodes at a potential window of 0.35-0.4 V vs Hg/HgO(1 M KOH).CV curves were obtained at different scan rates of 20,30,40,50 and 60 mV s-1.After plotting charging current density differences(Δj=ja-jcat 0.375 V vs Hg/HgO,where jais the anode current density and jcis the cathode current density)versus the scan rates,the slope,twice of the double-layer capacitance Cdl,is used to represent ECSA.Electrochemical impedance spectra were investigated at 1.45 V vs.RHE in the frequency range of 0.1-100000 Hz with an AC voltage amplitude of 5 mV.

Acknowledgment

The authors acknowledge the financial support by the National Natural Science Foundation of China(51874357,51872333,U20A20123)and Innovative Research Group of Hunan Provincial Natural Science Foundation of China(2019JJ10006).X.L.acknowledges support from Shenghua Scholar Program of Central South University.R.M.acknowledges support from JSPS KAKENNHI(18H03869).

Conflict of Interests

The authors declare no conflict of interest.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.