A dual metal-organic framework strategy for synthesis of FeCo@NC bifunctional oxygen catalysts for clean energy application

Jun Chen ,Liandong Li ,Liu Yang,Chang Chen,Shitao Wang,Yan Huang,Dapeng Cao

State Key Laboratory of Organic-Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Metal Organic Frameworks Zeolite imidazole framework Bifunctional electrocatalysts FeCo alloy Zn-air battery

ABSTRACT Developing high efficient bifunctional oxygen electrocatalysts for clean energy applications like Zin-air battery (ZAB) is highly desired,because it would reduce the cost and speed up the practical application of ZAB.Here we use a dual metal–organic framework (MOF) synthesis strategy to prepare the N-doped carbon supported bimetallic FeCo nanoparticle catalysts (marked as FeCo@NC) by pyrolysis of ZnCo-ZIF/MIL-101(Fe) composite.The FeCo@NC exhibits remarkable electrocatalytic activity for ORR with half-wave potential of 0.89 V vs.the reversible hydrogen electrode (RHE) and robust durability for both ORR and OER(oxygen reduction reaction and oxygen evolution reaction),which is attributed to the generation of Fe0.26Co0.74 crystalline phase and mesopores due to the dual-MOF synthesis strategy.The rechargeable ZAB based on FeCo@NC air electrode shows a maximum energy density of 139.6mW.cm-2 and excellent cyclic stability over 130 h,significantly surpassing the Pt and Ir-based ZAB.The present work provides a useful dual-MOF synthesis strategy for preparing high-performance multifunctional catalysts for ORR,OER and hydrogen evolution reaction (HER).

1.Introduction

Global warming is the consequence of the climate change caused by excessive use of fossil energy [1,2].Currently,most of countries around the world have adopted global agreements to reduce greenhouse gas emissions,and China has subsequently proposed the plans for peak emissions by 2030 and carbon neutrality targets by 2060[3].Therefore,it calls for speeding up the application of new energy and the development of clean energy technologies.

Rechargeable zinc-air batteries (ZABs) are high-efficiency and sustainable devices in current energy storage/conversion systems and have attracted widespread attention [4–8],due to the superiorities of zinc abundance,high theoretical energy density,low price and reliable safety.One of the main difficulties hindering the further application of ZABs lies in the development of high efficient oxygen reduction reaction (ORR) and oxygen evolution reaction(OER)catalysts,because ORR and OER determine the discharge and charging voltages of the battery,respectively [9–14].How to reduce the overpotential of the two reactions is significantly important [15–19].Currently,the state-of-the-art ORR and OER catalysts are still precious metal Pt-and Ru-/Ir-based materials,and the high expense and rarity of these materials hinder their widespread application [20–28].Hence,it is urgently needed to design bifunctional non-precious metal-based alternatives with high activity and reliable durability for ORR and OER.

The bifunctional oxygen catalysts based on transition metal(mainly Fe,Co,Ni and Mn etc.)single atoms,metallic and bimetallic particles,hydroxides,carbides,nitrides,oxides,phosphides,sulfides and metal-free heteroatom (N,P,S) doped carbon structure have been studied extensively[29–32].Among them,the bimetallic alloy embedded porous carbon electrocatalysts have attracted more attention due to the synergistic effects of bimetal and carbon supports.Most of the bimetallic alloy-based catalysts (including FeCo [33,34],FeNi [35],CoNi [36,37] etc.) have shown excellent bifunctional ORR and OER properties,which is beneficial for application in ZABs because the oxygen bifunctional catalysts can simultaneously preform the discharge and charge of ZABs [33].Typically,metal alloy loaded porous carbon catalysts were prepared by simultaneously introducing two metal salts into the carbon substrate or precursor [36,38].Specifically,the cobaltcontaining zeolite imidazole framework (Co-ZIF) or Fe-containing MOF were often selected as precursors,and alternatively,the bimetallic MOFs containing two metal atoms were also considered as precursors.Then,another metal salt (including Fe,Co,Ni,Cu salts) was introduced into the precursors to synthesize metal nanoparticle–based porous carbon materials by pyrolysis [39].The resulting samples may be bimetallic alloy-embedded porous carbon [39,40],atomically-dispersed M-N-C catalyst [41–43],and their composites [44].Generally,pyrolysis of MOFs (such as ZIF-67,MIL-101) leads to metal salt aggregation,which significantly reduce the exposure of metal active sites.In ZnCo-ZIF(Co partially replaces Zn on the metal nodes of ZIF-8),the steric barrier effect of Zn and its volatilization during subsequent pyrolysis contribute to metal distribution[45],while in the step of introducing the second metal Fe,Fe-MOF has its uniqueness compared with iron salts[46].Actually,to avoid the fact that metal salts agglomerate,combining two MOFs together as precursors may be more efficient for the exposure of metal active sites of the synthesized materials.

In this work,we used dual MOF strategy to synthesize the FeCo alloy embedded porous carbon catalysts by simple mechanical grinding and pyrolysis of ZnCo-ZIF/MIL-101(Fe) composite,where ZnCo-ZIF acts as structural support and provides cobalt/nitrogen sources,while Fe-based MIL-101 provides Fe sources.Importantly,in the pyrolysis process,the Zn metal evaporation in ZnCo-ZIF is beneficial for the formation of porous channel and the exposure of FeCo alloy in the synthesized carbon.As a result,the synthesized FeCo@NC catalysts exhibit excellent ORR and OER bifunctional properties.ZAB with FeCo@NC as air electrode shows maximum power density of 139.6 mW.cm-2and superior cycle life.

Fig.1.(a)The dual-MOF synthetic route of FeCo@NC.(b)XRD patterns of catalysts.(c)TEM images of FeCo@NC,(d)HRTEM images of FeCo@NC and the lattice fringes of the carbon shells and the embedded bimetallic nanoparticles,(e)TEM-EDX elemental mapping of FeCo@NC.(f)TG curves of three precursors in Ar atmosphere,(g)TG curves of ZnCo-ZIF,FeCl3.H2O and ZIF/FeCl3 in Ar atmosphere.

2.Results and Discussion

2.1.Synthesis and characterizations of FeCo@NC electrocatalyst

The‘‘Dual-MOF”synthesis strategy of the FeCo@NC catalyst was illustrated in Fig.1(a).First,ZnCo-ZIF and MIL-101 (Fe) were prepared by previous methods in literature [47,48] and SEM images(Fig.S1,in Supplementary Material) were used to confirm the well-defined dodecahedral structure of ZnCo-ZIF and the regular octahedral structure of MIL-101 (Fe).The sizes of both ZnCo-ZIF and MIL-101 (Fe) are about 400 nm.Subsequently,the asprepared ZnCo-ZIF and MIL-101(Fe)were mixed directly at different mass ratios of 50:1,30:1,20:1,10:1 and 5:1,and were pyrolyzed to obtain FeCo@NC-X,where X represents the mass ratio.In particular,FeCo@NC-20 is also abbreviated as FeCo@NC for convenience.In order to demonstrate advantage of the dual-MOF method,a comparative catalyst FeCo@NC_FeCl3was also synthesized by pyrolyzing the composite of ZnCo-ZIF with FeCl3.H2O and TPA,where Fe salt was then added into the ZnCo-ZIF precursor as the common method mentioned in Introduction.Another comparative sample Co@NC was also prepared by carbonizing ZnCo-ZIF at 900 °C without adding the iron source.

After the pyrolysis at 900 °C,ZnCo-ZIF-X basically maintained their original structure and a part of carbon nanotube appears in all three samples (Fig.S2 and Fig.S3),while MIL-101 (Fe) usually losses its original structure after pyrolysis due to relatively inferior thermal stability.As shown in Fig.S3,the comparison samples show a different morphology with FeCo@NC without regular structure.

The XRD patterns of the three samples were given in Fig.1(b).All the samples exhibit a diffraction peak near 26°,which can be corresponding to the (0 0 2) lattice plane for graphite carbon(JCPDS No.26-1076).The Co@NC shows three diffraction peaks at 44.1°,51.4°and 75.8°,which are attributed to the (1 1 1),(2 0 0)and (2 2 0) crystal planes of Co.The characteristic peaks of FeCo@NC show a slight left shift compared with Co@NC due to the larger atomic radius of Fe,which is assigned to Fe0.26Co0.74(JCPDS No.04-002-3691),indicating that Fe was doped into Co metal lattice,while the characteristic peaks of FeCo@NC_FeCl3located around 45.0°,65.4°and 83.1°corresponding to the classical FeCo alloy(JCPDS No.04-004-9065)The ICP test indicates that the Fe and Co contents in FeCo@NC are 1.67%(mass)and 4.63%(mass),respectively (Table S1),which is consistent with the composition ratio of Fe0.26Co0.74.The Raman spectra in Fig.S4 indicate that FeCo@NC has the same ratio of D band against G band (ID/IG) of 0.98 as the FeCo@NC_FeCl3,which is less than the 1.09 of Co@NC,suggesting that Fe doping causes a relatively higher degree of graphitization [49].

A representative TEM image of FeCo@NC was displayed in Fig.1(c),where the carbon substrate is decorated with evenly distributed metal nanoparticles (NPs) with diameter of 10–15 nm.The lattice fringes of 0.179 nm in Fig.1(d)are related to the Fe0.26-Co0.74(2 0 0)facet,and the lattice distance of 0.34 nm matches the graphitic carbon(0 0 2)plane,which confirms that Fe0.26Co0.74NPs in FeCo@NC are tightly wrapped by several thin layers of carbon,and it is beneficial to the durability of the electrocatalyst [44],because it can prevent Fe0.26Co0.74NPs from leaching out in catalytic reactions.Moreover,the EDS mappings in Fig.1(e) demonstrated the Fe signals basically match the Co signals,suggesting that the NPs of FeCo@NC is an alloy rather than monometallic Fe or Co NPs,which was further proved by similar EDS mappings in Fig.S7.In contrast,Fig.S5 shows that the average size of NPs in FeCo@NC_FeCl3is obviously larger compared to that of FeCo@NC.The lattice fringe of 0.2 nm in Fig.S6 matches the characteristic lattice plane of the FeCo (110),which implies the formation of FeCo NPs in FeCo@NC_FeCl3.Similarly,the Fe and the Co signals in the EDS mapping also match well in the FeCo@NC_FeCl3samples,indicating the formation of the FeCo alloys (Fig.S8).

Interestingly,although both samples of FeCo@NC and FeCo@NC_FeCl3contain the bimetallic alloy NPs,the two NPs exhibits different crystal structures,which may be attributed to the difference of synthesis method.In the dual-MOF strategy,with the increase of pyrolysis temperature,MIL-101 (Fe) was gradually decomposed into fragments (Fig.1(f)) and loaded on the surface of ZnCo-ZIF to obtain the final sample,while at～200°C,FeCl3.H2O was basically converted to Fe2O3(Fig.1(g)) to obtain the FeCo@NC_FeCl3.The gradual decomposition of MIL-101(Fe)(Fig.1(f)) is beneficial for the formation of Fe0.26Co0.74crystalline,which is attributed to the dual-MOF strategy.Otherwise,the fast decomposition of FeCl3.H2O would lead to the formation of FeCo alloys.

Nitrogen adsorption–desorption isotherms (Fig.2(a) and Table S2) indicate that Co@NC holds the maximum BET specific surface area (SSA) of 918 m2.g-1.After adding Fe species,the BET SSA of FeCo@NC_FeCl3and FeCo@NC are reduced to 613 m2.g-1and 583 m2.g-1,respectively.The pore size distributions (PSDs)in Fig.3(b) prove that all samples have sufficient micropores.It should be pointed out that the isotherms of the FeCo@NC sample exhibit obvious hysteresis loops,suggesting the existence of mesoporous structures [50],which was also confirmed by the PSDs in Fig.2(b).The hierarchical pore structure in FeCo@NC would promote the exposure of reactive sites and facilitate the transfer of O2molecule during ORR and OER process.XPS spectra in Fig.2(c)show that both FeCo@NC and FeCo@NC_FeCl3contains C,N,O,Fe and Co elements (Table S3),indicating that Fe element were successfully doped in both samples.The high-resolution N1s spectrum of FeCo@NC could be reasonably decomposed into four peaks belonging to different N-species,as shown in Fig.2(d).The FeCo@NC contains a high content of nitrogen,which is beneficial for electrocatalytic reaction,because a high N content is favorable for tailoring the local electronic structure of the carbon matrix,and thereby improving its catalytic activity [51].The high-resolution spectra of Co 2p and Fe 2p in CoFe@NC sample are displayed in Fig.2(e)-(f),where the peaks of zero-valent metallic Co(779.4 eV)and metallic Fe(707.1 eV)are attributed FeCo alloy,further demonstrating the existence of FeCo alloy [33].It should be mentioned that XPS can detect the elemental composition of the catalyst surface,and most FeCo NPs in FeCo@NC samples were encapsulated by several carbon layers during the pyrolysis process[52].Near-surface FeCo mainly presents in the form of oxidation state,because the surface of FeCo alloy NPs is likely oxidized into their oxides or hydroxides,causing the content of zero-valent Fe and Co is relatively low.Overall,besides the element content,the valence states of FeCo@NC_FeCl3are also similar with the FeCo@NC sample (Figs.S9 and S10).

Fig.2.(a) N2 adsorption–desorption isotherms and (b) pore size distributions of Co@NC,FeCo@NC and FeCo@NC_FeCl3 (c) XPS survey spectrum of FeCo@NC_FeCl3 and FeCo@NC.(d) XPS spectra of (d) N 1s,(e) Co 2p and (f) Fe 2p of FeCo@NC.

2.2.Electrochemical activity of FeCo@NC electrocatalyst

Fig.3(a) and Fig.S11(a)-(c) show that all the tested samples display the clear cathodic peak in the range of 0.8–0.85 V in O2-saturated 0.1 mol.L-1KOH by cyclic voltammetry (CV),indicating that they hold good ORR activity.The LSV curves in Fig.S12 show that the onset potential and half-wave potential (E1/2) of all FeCo@NC-X samples exceed these of Co@NC and Pt/C,indicating that the addition of Fe has improved ORR performance significantly.In particular,E1/2of FeCo@NC reaches 0.89 V and its limited current density is highest among four samples (Fig.3(b)),while E1/2of FeCo@NC_FeCl3is 0.87 V,and they are 0.83 V for Co@NC and 0.85 V for 20% Pt/C.Apparently,the ORR performance of FeCo@NC is better than most recently reported transition-metal catalysts (Table S4).The Tafel slope (74.2 mV.dec-1) of FeCo@NC is also almost the same as 20%Pt/C(73.5 mV.dec-1)and lower than Co@NC (88.5 mV.dec-1) and FeCo@NC_FeCl3(122.1 mV.dec-1)(Fig.3(c)),suggesting its fast ORR kinetics.Compared to FeCo@NC_FeCl3,FeCo@NC exhibits better ORR performance,which is attributed to the generation of Fe0.26Co0.74crystalline phase and production of mesopores due to the dual-MOF synthesis strategy.The number of electron transfers (n) and the H2O2yield (H2O2%)of four samples were obtained through the RRDE test curves,and shown in Fig.3(d).Apparently,the n values of FeCo@NC are closest to Pt/C,which is approximately equal to 4,indicating its fourelectron reaction pathway.Besides,based on the RRDE curve,the H2O2% of FeCo@NC was less than 8.5% between 0.2 and 0.6 V (vs.RHE),which belows the two samples of Co@NC and FeCo@NC_FeCl3.

Fig.3.ORR performance in 0.1 M KOH.(a)CV curves of FeCo@NC,(b)ORR LSV curves and(c)Tafel slope of four samples.(d)H2O2 yield and electron transfer number of four samples derived from RRDE results.(e–f) Chronoamperometric curves of FeCo@NC and Pt/C (e) at 0.65 V and (f) after adding methanol.

The durability test of FeCo@NC was explored by chronoamperometric measurement (Fig.3(e)).The current retention rate of FeCo@NC catalyst is 97.04%after 40000 s operation at 0.65 V(versus RHE),which is more stable than the current retention rate of 66.25% for Pt/C.We investigated the morphology of the samples after 40000 s accelerated degradation test (ADT),FeCo@NC still maintained the original morphology,in which the metal particles are embedded in the carbon matrix(Fig.S13).The excellent stability of FeCo@NC benefits from the stable ZIF-derived carbon matrix and the structure of Fe0.26Co0.74NPs covered by the carbon layers,which prevents the NPs from directly contacting the electrolyte solution.As depicted in Fig.3(f),there was no noticeable change in cathodic current of FeCo@NC after 3 ml of methanol was added,indicating the strong methanol tolerance,while it exhibited a sudden drop for Pt/C.

The OER catalytic properties of FeCo@NC in 0.1 mol.L-1KOH were also tested,and the LSV curves were provided in Fig.4(a).FeCo@NC showed an overpotential of 410 mV to deliver 10 mA.cm-2(E10),which is lower than Co@NC (540 mV) and,FeCo@NC_FeCl3(477 mV) and commercial IrO2(438 mV).In addition,electrochemical impedance spectroscopy (EIS) measurement indicated that the charge-transfer resistance (Rct) value of FeCo@NC is smallest among all the prepared electrocatalysts,which is a favorable feature for enhancing OER activity (Fig.S14).

Fig.4.Electrocatalytic OER performance.(a)LSV curves of the three synthesized materials and IrO2.(b)Chronopotentiometry response of FeCo@NC and IrO2 at 10 mA.cm-2.(c) OER stability of FeCo@NC after 2000 CV scans.(d) Combined ORR/OER overpotential curves of two synthesized materials and Pt/C+IrO2.

The durability of FeCo@NC was studied at 10 mA.cm-2.As displayed in Fig.4(b),the chronopotentiometry curve shows that the overpotential of FeCo@NC has only a slight increase of 8 mV after 40000 s test,suggesting a good stability,while the overpotential of IrO2increases by 50.5 mV after the 40,000 s test.The long-term catalytic durability was further performed by CV cycling in range of 1.3 to 1.8 V.As shown in Fig.4(c),FeCo@NC only showed a loss of 4 mV in overpotential after 2000 CV scans,suggesting its outstanding catalytic stability.We also observed the TEM images of the FeCo@NC after 2000 CV cycles,and found no obvious change in the morphology of FeCo@NC before and after ADT (Fig.S15).

The electrochemical active surface area (ECSA) was calculated by electrochemical double-layer capacitance (Cdl) based on (cyclic voltammetry) CV method (Fig.S16(a)-(c)).The Cdlvalues of 3.77,6.14,and 6.48 mF.cm-2for Co@NC,FeCo@NC_FeCl3and FeCo@NC(Fig.S16(d)) suggest that the introduction of Fe can improve ECSA of Co@NC,and FeCo@NC shows highest ECSA.

The bifunctional performance of the catalysts can be evaluated by the difference of OER and ORR potentials(ΔE=E10-E1/2).[19]Impressively,FeCo@NC exhibits a signifcantly decreased ΔE value(0.75 V) in comparison to FeCo@NC_FeCl3and Pt/C+IrO2(Fig.4(d)),indicating that FeCo@NC is a good bifunctional electrocatalyst.The superior performance of FeCo@NC may be attributed to the following several points.(1) the generation of Fe0.26Co0.74crystalline phase due to dual-MOF synthesis strategy,where Fe0.26Co0.74crystalline phase can signifcantly improve conductivity and produce M-Nxsites with moderate adsorption strength to ORR intermediate products.(2)Production of mesopores,which is beneficial for mass transfer in elelctrocataltic reaction.(3) Carbon shell-encapsulated Fe0.26Co0.74NPs,where synergism of the carbon shell and Fe0.26-Co0.74can redistribute the surface charge and tailor the electronic structure to improve catalytic properties.

2.3.Zn–air battery

To explore the practical application of FeCo@NC as a bifunctional electrocatalyst in ZAB,a primary homemade ZAB was assembled with zinc foil anode,where FeCo@NC catalyst loaded on carbon paper (1 mg.cm-2) served as air cathode,and 6 mol.K-1KOH+0.2 mol.K-1ZnO as the electrolyte.For comparison,the ZAB was also assembled by using commercial catalysts (20% Pt/C and IrO2with a mass ratio of 1:1,the total mass loading is 2 mg.cm-2).Fig.5(a) shows that FeCo@NC-based ZAB has an open-circuit potential of 1.405 V.Fig.5(b)displays the charge–discharge polarization curves of two assembled ZABs.The ZAB based on FeCo@NC gave a significantly narrower voltage gap between discharge–charge than the other one,and its maximum power density of reaches 139.6 mW.cm-2at around 234 mA.cm-2(Fig.5(c)),which even surpasses that of ZAB based on commercial Pt/C+IrO2catalysts(109.6 mW.cm-2).The long-term stability and rechargeability were also measured by a 130 h charge/discharge cycle at 10 mA.cm-2(Fig.5(d)).The initial charging/discharging potentials of FeCo@NC–based ZAB are 2.02 V and 1.20 V,and after 130 h charge/discharge cycles,the voltage gap only increases to 0.87 V from initial 0.82 V (the final charging potential and discharging potential are 2.07 V and 1.20 V).By comparison,the voltage gap of Pt/C+IrO2-based ZAB showed a larger initial voltage of 0.87 V,and then the voltage gap increases to 1.13 V after the 130 h test.As an interesting example,two FeCo@NC-based ZABs connected in series can power a red LED (2.5 V) (Fig.5(e)).All the above results demonstrate that the FeCo@NC is an excellent bifunctional electrocatalyst for rechargeable ZABs.

Fig.5.(a)A prototype ZAB using FeCo@NC serving as air cathode,and it shows an open-circuit voltage of 1.405 V.(b)Charging-discharging curves.(c)Discharging curves and corresponding power densities.(d)Galvanostatic cycling tests at 10 mA.cm-2 for 130 h of FeCo@NC and Pt/C+IrO2 electrocatalysts assembled ZABs.(e)Photograph of a red LED (2.5 V) powered by two FeCo@NC-based ZABs.

3.Conclusions

In summary,we have used the dual MOF synthesis strategy to successfully prepare the FeCo@NC electrocatalyst with FeCo alloy embedded in N-doped carbon by pyrolyzing ZnCo-ZIF/ MIL-101(Fe)composite.The FeCo@NC exhibits excellent bifunctional properties and robust stability for ORR and OER in alkaline media,which is attributed to the generation of Fe0.26Co0.74crystalline phase and production of mesopores due to the dual-MOF synthesis strategy,in which Fe dopant was added by the form of MIL-101(Fe)rather than by the form of Fe salts.The rechargeable Zn-air battery using FeCo@NC air electrode shows a maximum energy density of 139.6 mW.cm-2and excellent cyclic stability over 130 h,significantly surpassing the Pt and Ir-based ZAB.This work provides a useful dual-MOF synthesis strategy for preparing advanced electrocatalysts for ORR,OER and HER.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Key Research and Development Program of China (2019YFA0210300) and the National Natural Science Foundation of China (21905016).

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.02.017.