Engineering of Bifunctional Nickel Phosphide@Ni-N-C Catalysts for Selective Electroreduction of CO2-H2O to Syngas

Chengyu Ye,Xiaofei Yu,Wencui Li,Lei He,Guangping Hao,Anhui Lu

State Key Laboratory of Fine Chemicals,Liaoning Key Laboratory for Catalytic Conversion Carbon Resources,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,Liaoning Province,China.

Abstract:Electroreduction of CO2 is one of the most promising CO2 conversion pathways because of its moderate reaction conditions,controllable product composition,and environment-friendliness.However,most of the current CO2 electroreduction technologies have not reached the techno-economic threshold for a competitively profitable electrochemical process.Based on a simple two-electron transfer process,the electroreduction of CO2 to CO,which is further processed into syngas with the reduction of H2O to H2,is postulated to be the most promising pathway for a profitable electrochemical process.Such a process urgently requires nonprecious electrocatalysts that can precisely control the CO/H2 ratio.Herein,we present a tailored synthesis of bifunctional electrocatalysts with high activity,which can realize the preparation of syngas with controlled compositions via molecular engineering of a ternary nanocomposite.Specifically,a mixture of melamine,triphenylphosphine,and nickel acetate was milled and dissolved in ethanol;the ternary nanocomposite was obtained after rotary evaporation of the mixture.We prepared the catalysts by pyrolyzing the obtained composites at 850 °C for 2 h.The synthesis strategy was facile and easy to scale.The specific surface area and pore volume of the bifunctional electrocatalyst were both significantly enhanced upon increasing the concentration of the phosphorus source,triphenylphosphine,during the precursor preparation.The obtained bifunctional electrocatalysts had hierarchically porous structures,which had well-dispersed active sites and could promote mass transport.Raman spectra revealed higher degrees of disorder with higher P/Ni ratios in the precursor.X-ray photoelectron spectroscopy verified the presence of Ni-Px and Ni-Nx functionalities,which were the active sites for hydrogen evolution and CO2 reduction,respectively.Hence,the electrocatalytic performance of this series of bifunctional electrocatalysts can be tuned from CO-dominant to H2-dominant.The electrochemical performance was evaluated using a CO2-saturated 0.5 mol·L-1 KHCO3 aqueous solution at ambient temperature by linear sweep voltammetry and potentiostatic electrolysis.Through these experiments,we determined that the activity of the catalysts was influenced by the surface phosphorus/Ni-Nx site ratio.The highest CO faradaic efficiency(91%)was achieved at -0.8 V(vs a reversible hydrogen electrode,RHE)with Ni-N-C in the absence of Ni-P.The CO/H2 molar ratio in the syngas stream was tunable from 2:5 to 10:1 in the potential range from -0.7 to -1.1 V(vs RHE)with a total faradic efficiency of 100%.The syngas composition directly links to the molar ratio of the two integrated components,nickel phosphide and Ni-N-C.Additionally,the stability of the optimized bifunctional electrocatalyst at -0.7 V for 8 h was tested,in which the CO/H2 ratio was maintained between 1.2 and 1.3,indicating excellent stability.This study provides a new perspective for the engineering of bifunctional electrocatalysts for the conversion of abundant CO2 and water into syngas with tailorable CO/H2 ratios.

Key Words:Nonprecious catalyst;CO2 electroreduction;Syngas;Porous carbon;Nickel phosphide

1 Introduction

With rapid expansion of industry and high consumption of fossil fuels,global atmospheric carbon dioxide is increasing dramatically to a new record high level(407.4 ± 0.1 ppm(1 ppm=1 × 10-6,volume fraction)in 20181),which results in climate problems such as global warming,ocean acidification,etc.Therefore,extensive efforts have been made to CO2capture and conversion,aiming to carbon footprint neutralization.So far,fundamental studies2-5and commercially available industrial technologies on CO2capture and storage6are quite mature.

Among various CO2conversion technologies,CO2electrochemical reduction(CO2RR)has become the hot spot in fundamental researches,under the consideration of future electricity supply from clean renewable energy such as solar,wind and tide,etc.The currently developed CO2RR technologies are still far from the techno-economic threshold for a competitively profitable electrochemical process7,8.

An alternative is through less impeditive two-electron conversion,reducing CO2to CO,meanwhile reducing H2O to H2for syngas production.The syngas product could be employed as feedstock for Fischer-Tropsch process to produce value-added chemicals9,10.In principle,generation of syngas with different proportions of CO/H2can be achieved by increasing the cell potentials that cause unbearably large energy consumption.To this end,it is crucial to engineer bifunctional electrocatalysts that can simultaneously and selectively reduce CO2and H2O to CO and H2in a narrow potential range,with the aim of retaining high energy efficiency11-18.

To date,studies on electrocatalysts with high selectivity for CO2RR and high activity for hydrogen evolution reaction(HER)individually have been well-documented.For example,noble metal catalysts including Au19,Ag20,and functionalized carbons21-24catalysts exhibited decent performance for CO2-to-CO reduction.Among them,transition metal and nitrogen codoped carbons(M-N-C)are generally considered to be the most promising catalysts,because of their excellent conductivity,low cost as well as high activity and selectivity25-30.Particularly,the nickel and nitrogen-doped carbon(Ni-N-C)31-39,was proved to be one of the most successful type M-N-C electrocatalysts for CO2-to-CO due to its high current density at medium potential range.For example,Ju and co-workers compared a series of MN-C(M=Mn,Fe,Co,Ni,Cu)electrocatalysts for CO2-to-CO,in which Ni-N-C material was identified as the most active materials40.Alternatively,Jiang et al.41reported that the atomically dispersed Ni on N-doped CNTs(NiSA-N-CNTs)by one-step pyrolysis of nickel acetate and dicyandiamide precursors exhibited high activity and selectivity toward CO2RR,with the Faradic efficiency(FE)of CO being up to 91%.Similarly,researches on HER catalysts are also trending from the benchmark Pt/C and related materials to nonprecious electrocatalysts including metal carbides42,nitrides43,44,sulfide45-48,and phosphide nanomaterials49-52,etc.Notably,nanostructured transition metal phosphide such as nickel phosphide shows not only excellent activity and stability but also nearly unit FE for HER in all electrolyte including acid,alkaline and neutral media,and thus becomes one of the most studied non-precious metalbased HER catalysts in recent years53.

Motivated by these advancements,herein,we propose a surface coupling strategy to in situ grow the nickel phosphide on the Ni-N-C,based on the molecular engineering of an easy-made ternary nanocomposite.Thus,the obtained electrocatalysts feature a precisely controlled two integrated functions.The bifunctional catalysts show the capability of tuning the CO/H2molar ratio in a wide range from 10:1 and 2:5 with a total unity FE.

2 Experimental and computational section

2.1 Materials preparation

Agents with analytic grade were employed without further purification in our work.The electrocatalysts were prepared by a facilely paragenetic growth method54.Typically,Ni(Ac)2·4H2O(2 mmol)was dissolved in ethanol(40 mL).Then a certain amount of triphenylphosphine(TPP)was added into the slurry and kept stirring until completely dissolved.Next,melamine(0.03 mol)was added to the solutions.Then,ethanol was removed by rotary evaporation.Finally,a light green product was obtained and transferred into a porcelain boat,heated to 850 °C with a heating rate of 5 °C·min-1and held at this temperature for 2 h in a tube furnace under argon atmosphere.The obtained products were treated with 4 mol·L-1HCl,stirring for 24 h at room temperature to wash off nickel particles exposed on the surface.We denote the electrocatalyst by P/Ni-x@Ni-N-C,where x means the P/Ni molar ratio that was relevant to the amount of TPP used in the synthesis.

2.2 Materials characterization

The microstructures of the catalysts were characterized by scanning electron microscope(SEM,FEI Nova NanoSEM 450)and transmission electron microscope(TEM,FEI Tecnai F30)equipped with energy dispersive X-ray(EDX)spectrometer for element mapping.Nitrogen adsorption isotherms were measured using a Tristar 3000 analyzer(Micromeritics Instruments,USA)at 77 K,the specific surface areas(SBET)and pore size distributions(PSDs)were calculated based on the multi-point Brunauer-Emmett-Teller(BET)model and Barrett-Joyner-Halenda(BJH)methods,respectively.Prior to measurement,all samples were degassed at 200 °C for at least 4 h.The crystalline structures of materials were characterized by X-ray diffraction(XRD)using a PANalytical X’Pert X-ray diffractometer equipped with Cu Kαradiation(λ=0.15418 nm).Raman spectra were collected on a Raman spectrometer equipped with a 532 nm laser excitation(DXR Smart Raman).The X-ray photoelectron spectroscopy(XPS)measurements were further carried out using ESCA LAB250 electron spectrometer.

2.3 Electrochemical measurements

All electrochemical tests were performed on an electrochemical workstation IviumStat(Ivium Technologies,The Netherlands).The electrocatalytic performance were tested in an airtight two-compartment H-type cell(100 mL volume for each cell)separated by a Nafion 117 membrane using a standard three-electrode system with 75 mL 0.5 mol·L-1KHCO3solution in each chamber.An Ag/AgCl(saturated KCl)and Pt plate were used as reference and counter electrode,respectively.The working electrode was either a glassy carbon electrode(3 mm diameter)or a carbon fiber paper(12 mm diameter)loaded with catalyst.The applied potential(E)in this paper was converted vs reversible hydrogen electrode(RHE)by the Eq.(1):

The pH of CO2saturated 0.5 mol·L-1KHCO3solution is about 7.2 and all curves were reported with iR compensation.

The linear scan voltammetry(LSV)experiments were recorded using glassy carbon electrode loaded with catalyst as working electrode,and the preparation procedure was as followed.6 mg electrocatalyst was dispersed into 1 mL 0.25% nafion solution.After sonication,10 μL catalyst ink was dropcasted onto the glassy carbon electrode which was first polished with 1.5 μm and 50 nm Al2O3powder to a mirror surface and sonicated in water and ethanol for 5 min,respectively.For comparison,the LSV was first tested in Ar atmosphere before electrolyte was purged with high-purity CO2(99.999%)for at least 30 min.The scan rate for LSV is 5 mV·s-1.

The potentiostatic electrolysis experiments were conducted using carbon fiber paper loaded with catalyst as working electrode,and the preparation procedure was as followed.Catalyst slurry was drop-casted onto Toray carbon paper achieving a catalyst loading of 1 mg·cm-2(two sides were modified with catalyst),then dried under 80 °C overnight in a vacuum oven.Potentiostatic electrolysis was controlled at the range of -0.5 - -1.1 V vs RHE.Prior to every test,KHCO3electrolyte solution was first bubbled with high-purity CO2(99.999%)for at least 30 min until it was saturated reaching the pH of 7.2.During electrolysis,CO2continues to flow into the cathode chamber at a flow rate of 20 mL·min-1.Gaseous products were detected by online gas chromatography(Aglient 7890B,USA)equipped with a TCD detector.The Faradaic efficiency(FECOorH2)and partial current density(jCOorH2)of products and were calculated following the Eq.(2-3):

The QCOorH2can be calculated by the Eq.(4):

where n refers to the moles of product CO or H2,and z is the number of electron transfer,which is equal to 2 for both CO and H2,and F corresponds to faradaic constant,96485 C·mol-1,while the QTotalis the integration of current and electrolysis time.ITotalin Eq.(3)is the total current during electrolysis.

3 Results and discussion

3.1 Material synthesis and characterization

The typical synthesis procedure is sketched in Fig.1a.Firstly,a mixture of melamine,triphenylphosphine(TPP)and nickel acetate was milled and dissolved in ethanol,ending with a uniform,light green ternary nanocomposite.Then,the ternary nanocomposite was converted into Ni,N,P-codecorated carbon structure after pyrolysis at 850 °C in Ar.During carbonization,melamine was employed as nitrogen and carbon source,TPP as P-dopants,while Ni species were evolved into metallic nickel,N-coordinated Ni and NixPy.Notably,the metallic nickel served as catalysts for the in situ growth of carbon nanotubes31,55,56,benefiting the enhancement of the conductivity.Finally,P/Nix@Ni-N-C catalysts were harvested after leaching the exposed metallic Ni particles out in 4 mol·L-1HCl solution.Such a facilely paragenetic growth method enables it readily to scale up(Fig.1b).The obtained powder was easily processed into 40-50 cm2flexible membranes in a laboratory condition by using PTFE(5%(w,mass fraction))as binders(Fig.1c).This flexible membrane is self-supportive with relatively strong mechanical strength,which retained its flexibility after hundreds of bending tests(Fig.1d).

Fig.1 (a)Synthesis procedure of P/Ni-x@Ni-N-C,(b)digital images of electrocatalyst powder(c)electrocatalyst film and(d)flexible electrocatalyst membrane.

X-ray powder diffraction measurements were used to characterize the phase of this series of catalysts with different P/Ni ratios(Fig.2a).The diffraction peak at 2θ value of ～26° can be attributed to the(002)plane of graphitic carbon.Together with the SEM and TEM images(Fig.S1(Supporting Information(SI))),we confirmed that carbon nanotubes were formed.The other peaks located at 44°,51.8°,and 76.4°correspond to(111),(200),(220)planes of metallic nickel(ICDD No.00-004-0850),respectively,indicating the existence of nickel particles in P/Ni-0@Ni-N-C.Notably,all the products were extensively leached in 4 mol·L-1HCl,while the Ni nanoparticles encapsulated in graphitic carbon shells were remained,revealing the barely accessible way to the Ni species.Further increasing the P/Ni ratio to 4 and above,the peaks ascribed to metallic Ni disappeared,revealing the complete phosphorization of all the detectable Ni species.Fig.2b illustrates the morphology changes of this series of the bifunctional catalysts as increasing P/Ni ratio(P/Ni molar ratio=0,4,10).

The Ni species were gradually phosphorized into higher density of NixPyas higher concentration of phosphor sources used,which was confirmed by HAADF-STEM images and corresponding EDX element mappings of N(Fig.S2(SI)),Ni,and P(Fig.2c).Notably,the growth of CNT support was severely restricted when the ratio of P/Ni reaching 10,as shown in TEM images(Fig.2d-l).CNTs matrix in P/Ni-0@Ni-N-C was longer than others.After the addition of TPP,CNTs was shortened and stacked together.When the ratio of P/Ni was up to 10,the CNTs structures almost disappeared,while twodimensional Ni-N-C matrix was formed.This can be interpreted by strong activation effect of TPP and the fact that Ni species were first reacted with phosphorus to form nickel phosphide(Ni12P5,Ni2P)during thermal treatment,leaving insufficient nickel to catalyze the growth of CNTs.During the phase transformation,the ratio of Ni-coordinated N-sites to the NixPysites was altered.Considering their activity nature toward CO2RR and HER,a bifunctional catalyst is expected.The surface composition of Ni,P and N elements(Table S1(SI))again agree well with the above observations,confirming the successful preparation of the bifunctional catalysts.

The N2adsorption isotherms(Fig.3a)were obtained to further characterize the pore structure of the P/Ni-x@Ni-N-C.Obviously,all the catalysts exhibit typical type-IV isotherms with a H3-type hysteresis loop which indicate the existence of mesopores.The peak of pore size distributions(PSDs,Fig.3b)shifted from ca.5 to 15-20 nm as increasing the P/Ni ratio.N2adsorption uptakes increase sharply as pressure approaching saturation point of N2(P/P0=1),indicating the existence of the interparticle voids.

This reveals their hierarchically porous structure,which is consistent with that of SEM and TEM observations(Fig.S1).The samples prepared with higher P/Ni ratio showed larger specific surface area(SBET)and pore volume reaching 1077 m2·g-1and 1.99 cm3·g-1for P/Ni-10@Ni-N-C(Fig.3c).We speculate that the enhancement of SBETand pore volume was relevant to the activation process caused by the decomposition of TPP,releasing active gases and creating wide mesopores.Raman spectra of typical samples are given in Fig.3d.The D band locating at 1344 cm-1originates from distortion of sp2carbon(mainly graphene edges),indicating the existence of some defects in this kind of catalysts;while the G band at 1582 cm-1is the signature feature of the in-plane vibration of sp2carbon.Hence,the ratio of ID/IGbecomes a reliable index to evaluate graphitization degree of carbon materials.In our case,the values of ID/IGare 0.96,1.1,2.0,2.6 for P/Ni-x(x=0,2,4,10),respectively.The higher P/Ni ratio in the precursor leads to higher degree of disorder(ID/IGratio),which agrees with the above observation that additional porosities were created in the presence of higher concentration of TPP.

Fig.2 (a)XRD patterns of this series of P/Ni-x@Ni-N-C,(b)illustration of the evolution trend of nickel phosphide as increasing the P/Ni ratio in the precursor,(c)EDX element mappings of N and P in P/Ni-0@Ni-N-C,P/Ni-4@Ni-N-C and P/Ni-10@Ni-N-C,(d-l)high resolution TEM images of P/Ni-0@Ni-N-C(d,e,f),P/Ni-4@Ni-N-C(g,h,i)and P/Ni-10@Ni-N-C(j,k,l).

The X-ray photoelectron spectroscopy(XPS)was conducted to further analyse the composition and the chemical state of surface constituents of the samples.The XPS survey spectra(Fig.S3a-c(SI))confirms the existence of the Ni,N,C,O elements on P/Ni-0@Ni-N-C.On P/Ni-x@Ni-N-C(x=2,10),besides the above-mentioned elements,P element was also detected.Their composition was listed in Table S1.The surface content of P was gradually increased from 0 to 1.56%(atomic fraction),while the Ni content was kept almost constant.The deconvolution of O 1s peaks(Fig.S3d(SI))can be assigned to the components of Ni―O,P＝O and H2O.Furthermore,peaks in N 1s spectra(Fig.4a)can be deconvoluted into pyridine N,Ni―N,pyrrole N,graphite N and oxide N,corresponding to the peaks centred at about 398.5,399.1,400.1,401.1 and 402.7 eV,respectively37.

Particularly,Ni-coordinated N-sites are believed to be the origins of the CO2RR activity.Particularly,Ni-coordinated N-sites are believed to be the origins of the CO2RR activity.Different coordination structures of Ni have been studied,and their specific role has been reported in previous findings.Based on density functional theory,researchers found that metallic Ni cluster would be more prone to H2formation while NiN2and NiN3barely contribute to CO formation and NiN4could be the active centre for CO2RR57.Similarly,NiN4was reported to be the active centres for CO2-to-CO due to a strong binding energy for *COOH to reduce the required overpotential as well as a weak binding energy for *CO to realize the fast desorption of*CO from the active site31,37.According to XPS results,the content of Ni-coordinated N-sites can reach the value of 1.91%,0.66% and 1.79%(atomic fraction)on the P/Ni-x@Ni-N-C(x=0,2,10),respectively.A lower Ni-N value in P/Ni-2@Ni-N-C is owing to the formation of Ni12P5,reducing the content of nickel to coordinate with N.Besides,more exposed surface of P/Ni-10@Ni-N-C,as shown in TEM images,makes more Ni―N sites be detected in spite of the formation of Ni2P.The C 1s spectra of three samples(Fig.4b)are identical,which can be assigned to the C―C(284.7 eV),C―N(285.6 eV),C―O(286.3eV),and O―C＝O(288.3 eV),further proving the formation of Ni-N-C active sites for CO2RR.

Fig.3 (a)N2 sorption isotherms,(b)pore size distributions of P/Ni-x@Ni-N-C,(c)structure parameters of P/Ni-x@Ni-N-C and(d)Raman spectra of P/Ni-x@Ni-N-C.

Fig.4 High-resolution XPS spectra of(a)N 1s of P/Ni-x@Ni-N-C,(x=0,2,10),(b)C 1s of P/Ni-0@Ni-N-C,(c)Ni 2p of P/Ni-x@Ni-N-C(x=0,2,10),and(d)P 2p of P/Ni-x@Ni-N-C,(x=2,10).

High-resolution XPS spectra of Ni and P are shown in Fig.4c and d,respectively.In Ni 2p spectra of P/Ni-0@Ni-N-C,we assign the binding energies(BEs)of 854.4,871.8 and 879.8 eV to the Ni 2p3/2,Ni 2p1/2,and corresponding satellite peak for Ni0respectively,which is closely similar to that of Ni@NiO/N-C reported previously by Luo et al.32.It is worth noting that the BEs’ positive shift compared to standard 852.8 eV for zero valence Ni is resulted from the introduction of N-doped CNTs58.The electronegativities of N and Ni are 3.04 and 1.91,respectively,thereby promoting electron transfer from Ni to N.The electron binding energies of 852.9,870.1 eV for Ni 2p3/2and Ni 2p1/2in P/Ni-2@Ni-N-C can be ascribed to Niδ+in Ni12P5species,while 854.3 and 871.6 eV for Ni 2p3/2and Ni 2p1/2in P/Ni-10@Ni-N-C assigned to Ni2P species59.For P/Ni-10@Ni-N-C,the positive shift of binding energy compared to P/Ni-2@Ni-N-C may arise from the increase of valence state of Ni caused by the addition of phosphorus60.This is because Ni can supply electrons to P to form a positive charge state.Furthermore,the peaks at binding energies of 854.8 and 872.5 eV in P/Ni-2@Ni-N-C,and 855.3 and 872.7 eV in P/Ni-10@Ni-N-C are the characteristic peaks of Ni 2p3/2and Ni 2p1/2in oxidized Ni52.In P 2p spectrum of P/Ni-2@Ni-N-C,the peaks centred at ～129.9,130.8 and 133.8 eV can be ascribed to 2p3/2,2p1/2of Ni―P and oxidized phosphorus,respectively.Meanwhile,there are no peaks for Ni2P in P/Ni-10@Ni-N-C but only peaks at 132.1 and 133 eV,which can be attributed to the P 2p3/2and P 2p1/2of nickel phosphate,probably caused by the exposure of the sample to the air61.The above results confirm the existence of Ni―N active sites for CO2RR,Ni12P5and Ni2P active ingredients for HER.

3.2 Electrocatalytic performance

We further evaluated the electrocatalytic performance of this series of P/Ni-x@Ni-N-C(x=0,2,4,10)bifunctional catalysts toward the CO2-H2O reduction with a three-electrode system in an airtight H-type cell.The gaseous products were qualified by gas chromatography(GC),which showed the generation of H2and CO with the sum of their FE being close to 100% within the potential range.The electrochemical activity for CO2RR measured by linear scan voltammetry(LSV)method is shown in Fig.S4a(SI).The electrocatalysts were uniformly loaded on the glassy carbon electrode acting as work electrode,and LSV curves were obtained in Ar and CO2saturated KHCO3electrolytes,respectively.It is worth noting that four samples show different degrees of activity for CO2RR.Among them,the current density of P/Ni-2@Ni-N-C and P/Ni-4@Ni-N-C in CO2atmosphere was up to 50-55 mA·cm-2,which is much larger than that in Ar,indicating the high activity for CO2RR as well as good conductivity of the electrocatalysts,while P/Ni-0@Ni-NC and P/Ni-10@Ni-N-C in CO2atmosphere are relatively low,less than 20 mA·cm-2.Theoretically,CNTs in situ formed during preparation provide P/Ni-0@Ni-N-C with higher conductivity,that is,have a higher current density.However,a relatively lower specific surface area as well as part of Ni-N-C are coated with CNTs,made it hard to full contact with electrolyte,resulting in lower current density.Though P/Ni-10@Ni-N-C owns a unique 2D morphology and high surface area,made Ni-N-C almost distribute on surface,but it has a lower graphitization degree according to XRD patterns in Fig.2a,indicating poor conductivity as well as the oxidized of Ni2P to nickel phosphate,these facts may result in a lower current density of the sample.For comparison,the activity of N-doped carbon without Ni-Nxsites or NixPycomponent was also examined,showing that the sample is inactive toward CO2RR to CO which further proves the function of Ni-Nxin CO2RR.Overall,the activity trend reflects their structural features in terms of HER function and CO2RR function quite well.We first analysed the two samples obtained under boundary conditions.On the P/Ni-0@Ni-N-C in the absence of nickel phosphide,the detected current was mainly relevant to the proton and electron coupling process for CO2RR,which will be further examined in the electrolysis experiments.In contrast,on P/Ni-10@Ni-N-C with the highest P/Ni ratio,the HER current contributes the most of the overall current.

FECOas well as partial current density(j)of CO(Fig.5a,b)and H2(Fig.S4b,c(SI))reveal that the selectivity toward CO decreases with the increase of P content after the potential of-0.7 V vs RHE.Specifically,P/Ni-0@Ni-N-C shows the highest FE of 91% among this series of samples at -0.8 V vs RHE,whereas P/Ni-10@Ni-N-C exhibited the lowest FECOof 52%.Notably,on P/Ni-2@Ni-N-C,the highest partial current density of CO of 27.4 mA·cm-2and H2of 16 mA·cm-2at -1.1 V vs RHE was reached,respectively.To investigate the kinetic mechanism,Tafel plots of CO2reduction on the four materials are exhibited in Fig.5c.The P/Ni-4@Ni-N-C exhibits the lowest Tafel slope of 233 mV·dec-1,which is smaller than that of P/Ni-0@Ni-N-C(261 mV·dec-1),P/Ni-2@Ni-N-C(236 mV·dec-1)and P/Ni-10@Ni-N-C(283 mV·dec-1),indicating that electron transfer to CO2is faster on P/Ni-4@Ni-N-C and the rate-limiting step is the initial CO2activation process involving single-electron transfer to CO262.These results regarding the selectivity of CO/H2,together with Tafel plots are all consistent well with the activities derived from the LSV curves,exhibiting P/Ni-4@Ni-N-C having the best activity.

Commonly H2was the concomitant product when conducting CO2RR.And the formation of H2dominates when large overpotential applied.In this regard,the CO/H2selectivity could be tuned by applying different electrolysis potential.However,the large energy consumption remains the biggest barrier for actual application7,63,64.In this work,we can produce syngas with a wide range of CO/H2ratio in much lower potential range,as shown in Fig.6a.P/Ni-0@Ni-N-C produces the max CO/H2ratio of 9.7 at -0.8 V vs RHE,indicating that the direct formation of 90% purity CO is possible,which is consistent to other reports on Ni-N-C materials65,the high ratio of CO/H2can used as CO production by separation process.Whereas the minimum ratio of 0.4 was generated by P/Ni-10@Ni-N-C at -1.1 V vs RHE.Moreover,CO/H2ratio in the range of 0.8-1.3,a useful window of the typical thermocatalytic process such as ethanol synthesis reaction and Fischer-Tropsch reaction65,can be reached by P/Ni-4@Ni-N-C and P/Ni-10@Ni-N-C in the potential of -0.7 - -1.1 V vs RHE,corresponding to a relatively low overpotential and thus reducing energy consumption.We conducted a long-term electrolysis of CO2with P/Ni-4@Ni-N-C at -0.7 V for 8 h(Fig.6b).Clearly,the electrocatalysis is relatively stable,keeping the CO/H2ratio in the range of 1.2-1.3 in 8 h.When P/Ni-2@Ni-NC catalyst was applied,the syngas with the CO/H2ratio in the range of 6.5-7 was obtained for 18 h period(Fig.S5(SI)).

Fig.5 (a)CO Faradaic efficiency and(b)CO partial current density of P/Ni-x@Ni-N-C at different potentials,(c)Tafel slope of this family of bifunctional electrocatalyst.

Fig.6 (a)Ratio of CO/H2 produced by P/Ni-x@Ni-N-C at different potentials,(b)stability of P/Ni-4@Ni-N-C at -0.7 V,(c)linear correlation diagram of surface P/Ni-Nx and ratio of CO/H2 at different potentials.

In order to understand the relationship between the structural features and the performances,we plotted the relation between the syngas CO/H2ratio with the surface ratio of P sites to Ni coordinated N sites.As expected,a linear relationship was obtained,indicating the controllable structural feature of this family of bifunctional electrocatalysts(Fig.6c).Overall,this family of bifunctional electrocatalysts demonstrated the controllable structural feature,and delivered the expected catalytic performance for electroreduction of CO2-H2O to syngas with tailorable CO/H2ratios.

4 Conclusions

A family of bifunctional nickel phosphide@Ni-N-C catalysts were synthesized by a one-step thermal treatment of designed ternary nanocomposites and tested for electrocatalytic reduction of CO2-H2O to syngas.In accordance with other reports,in the absence of nickel phosphide,the as-prepared Ni-N-C catalysts displayed higher than 90% selectivity to CO in the voltage range from ca.-0.7 to -1.1 V vs RHE in an H-cell electrolyser.Imparting the HER functions resulted in a clear enhancement of catalytic HER process.We studied the activity trend through electrochemical and structural characterization of a series of nickel phosphide@Ni-N-C with varied P loadings.Overall,increasing P loading lead to an increase in the partial current density to H2.This work provides a new perspective to engineer bifunctional electrocatalyst for simultaneous CO2and H2O to syngas with controlled CO/H2proportions and may inspire other combination of efficient electrocatalysts for syngas production in an economic and facile manner.

Supporting Information:available free of charge via the internet at http://www.whxb.pku.edu.cn.