FeCo-based hybrid MOF derived active species for effective oxygen evolution

Yunjun Liu,Cheng Wng,Suxio Ju,Mo Li,Aihu Yun,b,Guoxing Zhu

a School of Environmental and Chemical Engineering,Jiangsu University of Science and Technology,Zhenjiang,202018,China

b Marine Equipment and Technology Institute,Jiangsu University of Science and Technology,Zhenjiang,212003,China

c School of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang,212013,China

ABSTRACT Metal-organic frameworks (MOFs) have been regarded as promising catalyst materials due to the richness of coordinately unsaturated metal sites on the surface,which can act as catalytic active centres.In this study,a hybrid MOF material composed of Fe-based MOF and Co-based MOF was prepared with the involvement of graphene oxide nanosheets as additive.It was demonstrated that the hybrid MOF materials showed much higher electro-catalytic activity towards oxygen evolution than the single-phase counterparts.To drive current density of 10 mA cm-2,the hybrid CoFe-based MOF only needed an over potential of 290 mV in 1 M KOH.The catalytic activity could sustain for a longer time with only slight current density decrease.During oxygen evolution operation,the MOF catalyst evolved into catalytic active species but kept well the microscale sheet-like structure.It is thus believed that this study will provide an avenue for the development of advanced electrocatalysts.

Keywords:Metal-organic frameworks Oxygen evolution Cobalt Iron Electrocatalysts Water oxidation

1.Introduction

Hydrogen is believed to be an alternative energy carrier due to its high energy density and the generation of water when used [1-3].Hydrogen can be produced by renewable energy (such as solar energy,wind energy) driven electrochemical water splitting,which provides a green route for the large-scale hydrogen production.Electrochemical water splitting involves two half reactions,oxygen evolution reaction(OER) at the anode end and hydrogen evolution reaction (HER) at the cathode[4,5].Both of the reactions need higher over potentials.Active electrocatalysts are thus urgently required to improve the sluggish kinetics of these two processes.

Metal-organic frameworks (MOFs) is an important kind of inorganic-organic hybrid materials with strong chemical bonds linking of metal centres and various organic ligands[6].MOF materials have been applied in fields like gas storage,catalysis,and biomedicine because of their tunable microstructures,high specific surface areas,and richness of coordinated unsaturated metal sites on the surface [7-9].The MOF derived material including porous carbon and various composites show potential applications in energy storage and conversion devices[10-15].Recently,MOF materials that can directly be used as electrocatalysts have got much attention.Their high porosity induces the exposing of active metal sites as catalytic centres and facilitates the mass transfer.Especially,tuning the ligands,the electronic structure of the involved metal sites in MOF can be finely tailored to get catalytic sites with high activity [16,17].During OER operation,the MOF-based catalyst will change and evolve into catalytic active species on the surface.The thus obtained catalytic active phase possibly has different microstructures and surface features as compared to the directly prepared active materials.A significant drawback of MOF materials is the lower conductivity.

Up to now,the noble metal-based materials(such as IrO2and RuO2)have been confirmed to be the excellent catalysts for OER [18].Nevertheless,their scarcity,high-cost,and poor stability in alkaline environment restrict their wide use.Designing and developing of nonprecious metal based electrocatalysts for OER is thus urgently needed[19,20].Earth-abundant and cheap transition metal-based materials including Ni-based and Co-based oxides,hydroxides,sulfides are then tested as alternatives for OER catalysts[21].It is generally believed that the high activity of IrO2and RuO2catalysts is due to their moderate bonding interactions for the intermediates,e.g.OH and O-O,in OER[22,23].In contrast,the adsorption energy of these intermediates on nonprecious transition metals(e.g.Fe,Co,Ni)is either too strong or too weak,causing a relatively lower catalytic activity[24,25].It was found that the coupling of iron and cobalt sites can tune the bond strength of these species on metal sites,inducing enhanced OER activity [26-28].For FeCo-based bimetal catalysts,it is proposed that the involved Fe sites are the main catalytic active centres,while the Co species provides conductive networks and synergistic effects for Fe sites [24].Herein,nanosheet-like MOF-based hybrid materials were synthesized with a stepwise synthesis strategy,which were then investigated as electrocatalysts for oxygen evolution reaction.The results show that loading of electrochemically inert Fe-MOF species on Co-MOF surface can substantially enhance the catalytic performance.

2.Experimental

2.1.Materials

FeCl3∙6H2O and trimethylamine were purchased from Sinopharm Chemical Reagent Co.Ltd.CoCl2∙6H2O,N-N-dimethylformomide(DMF)and terephthalic acid(H2BDC)were purchased from J&K Scientific Ltd.Graphene oxide nanosheets (GO) were synthesized by a modified Hummer's method.All of the chemicals were used as received.

2.2.Synthesis of Fe-MOF or Co-MOF

Typically,0.02 g of GO was firstly dispersed in 20 mL of DMF with ultrasonic irradiation.Then 1 mmol of FeCl3was dissolved into the above solution with vigorous stirring and allowed to stand for 1 h.2 mL of ethanol,2 mL of deionized water,and 100 mg of terephthalic acid dispersed in 10 mL of DMF were sequently added into the above solution.After that,1 mL of triethylamine was added to induce the reaction.After reacting at room temperature for 8 h under stirring,the thus obtained product was collected,washed with deionized water,ethanol for several times,and dried.The obtained product is noted as Fe-MOF.Another sample of Co-MOF was also synthesized with Co2+under the same procedure.

2.3.Synthesis of Co-MOF@Fe-MOF and Fe-MOF@Co-MOF

70 mg of Co-MOF and 20 mg of FeCl3∙6H2O were firstly dispersed in 30 mL of DMF under ultrasonication irradiation.Then,2 mL of ethanol,2 mL of deionized water and 60 mg of terephthalic acid were added.Stirring for at least 10 min,1 mL of triethylamine was then added in.After reacting for 8 h,the formed solid product,hybrid MOF material,was collected,washed by ethanol and deionized water,and dried.The synthesized hybrid MOF product is named as Co-MOF@Fe-MOF.Another contrast sample was prepared with the same method but tuning Co-MOF and FeCl3∙6H2O,to Fe-MOF and CoCl2∙6H2O.The corresponding product is noted as Fe-MOF@Co-MOF.

2.4.Materials characterization

Powder X-ray diffraction(XRD)was examined on a Shimadzu XRD-6000 diffractometer (Cu-Kα radiation).Scanning electron microscopy and transmission electron microscopy images were recorded on a Hitachi S4800 microscope and Tecnai G2 F30 S-Twin microscope,respectively.Element mapping analysis of the samples were performed with an Oxford X-ray detector set on the TEM.X-ray photo-electron spectroscopy(XPS)was performed on an UIVAC-PHI spectrometer.The metal contents in the samples were checked by a Vista-MPX inductively coupled plasma optical emission spectrometer.Fourier transform infrared spectra (FT-IR) were performed on an ATR spectrometer.

2.5.Electrochemical measurements

The electrochemical tests were performed in 1 M KOH on V54810 Ivium electrochemical work station.Hg/HgO electrode was used as a reference electrode,a carbon rod was used as counter electrode.The working electrode is a glassy carbon electrode (GCE,sectional area of 0.0707 cm-2) loaded with the catalyst ink,which was prepared according to previous work[29].Typically,12 mg of the catalyst powder was dispersed in the mixture of 60 μL of Nafion solution (5wt%) and 6 ml of ethanol by sonication.Then,10 μL of the catalyst ink was dropped onto GCE and was dried at room temperature in air.The catalyst loading density on GCE was determined to be 0.28 mg cm-2.

Polarization curves (LSVs) were collected with a sweep rate of 5 mV s-1.Tafel slopes were derived from the LSV curves by plotting overpotential (η) against log (J).Electrochemical impedance spectroscopy (EIS) test was performed over a frequency range of 105-0.1 Hz and under an added potential of 1.55 V (vs RHE).All potential values relative to the Hg/HgO reference electrode are converted to potentials relative to the reversible hydrogen electrode (RHE) as follows:ERHE=EHg/HgO++0.059 pH (in volts).

3.Results and discussion

Two-dimensional Co-MOF@Fe-MOF hybrid nanosheets were prepared by a facile solution synthesis method in an amine alkaline system(Fig.1).CoCl2and FeCl3were used as the metal precursors.A usual molecular,terephthalic acid was selected as the organic ligand.As a relatively smaller ligand,the thus formed MOF with terephthalic acid ligand will expose high density of metal centres that act as catalytic active sites.Secondly,the formed MOF materials with terephthalic acid and metal ions often have conjugated structure,which would show higher conductivity and is favorable for the electrocatalytic behavior.In our study,Co-MOF materials was firstly prepared,on which Fe-MOF materials was then generated.GO nanosheets were used as the additive for the catalyst.

As shown in Fig.2,monometallic MOF products,Fe-MOF and Co-MOF material,possess flexible thin nanosheet morphology with lateral size in sub-microscale (Fig.2a-c).It is difficult to distinguish the involved graphene nanosheets,which were added in the reaction system to improve the conductivity of the final product.High-resolution TEM images indicated that the obtained MOF materials were amorphous or only with very weak crystalline(Fig.2b).Because of the thin sheet-like structure and good dispersity in the reaction system,the 2D MOF nanosheets could be loaded with another MOF species via the interactions between surface anchored metal sites and H2BDC ligands.In this case,the loading of Fe-MOF species on Co-MOF species are shown(Fig.2d-f).The hybrid Co-MOF@Fe-MOF product also shows obvious sheet-like microstructure with amorphous feature or very weak crystalline.Only on the partial smaller region very weak lattice fringes could be observed.The elemental mapping analysis of the samples are shown in Fig.2g,which reveals the presence of Fe,Co,O,and C elements.The relatively stronger mapping signals of Fe suggest that iron elements were on the surface,confirming the successful synthesis of MOF composite.

Fig.3 shows the FT-IR spectra of Fe-MOF,Co-MOF,and Co-MOF@Fe-MOF products.For all of the product,the peak at 1572 cm−1can be indexed into the asymmetric stretching vibrations of carboxylate groups[30,31].The bands at 1354 cm−1originates from the symmetric vibrations of carboxylic groups,proving the presence of benzenedi-carboxylate anion (BDC2−) [32-34].The obvious peak at 3429 cm-1is attributed to the stretching vibration of OH-[30].The IR bands for benzene ring were shown in the range of 740-810 cm-1.Raman spectroscopy was also used to further examine the structures of the products(Fig.3b).The samples present the typical and separate D (1350 cm−1)and G(1588 cm−1)bands due to the involved graphene oxide[35].The values of ID/IGratios calculated from the peak intensity are in a narrow range of 1.04-1.06 for the three samples,suggesting the similar state of the graphene oxide in the products.The new present bands at 509,671,1420 cm-1for all of the three samples are due to the M-O species [30].

The chemical composition,oxidation states,and the involved electronic interactions of the as-obtained products were then checked by Xray photoelectron spectroscopy(XPS)analysis(Fig.4).For comparison,the XPS spectra for Fe-MOF and Co-MOF materials were also tested.The full XPS survey spectra of the products are shown in the Supporting Information (Figs.S1 and SI),demonstrating the presence of Fe,Co,O,and C elements in the corresponding products.In the XPS spectrum of Fe 2p for Fe-MOF and Co-MOF@Fe-MOF products,the bands at binding energies of 712.0 eV and 725.3 eV can be corresponded into Fe 2p2/3and Fe 2p1/2,respectively[36].These two bands indicates that Fe in the products are in Fe(III) oxidation state [37].The fitted bands in the range of 714-723 eV is the shake-up satellite bands.After the formation of hybrid MOF,the shake-up band shows an obvious enhance,suggesting more paramagnetic state of iron ions [38].The high-resolution Co 2p XPS spectra of the as-obtained products are shown in Fig.4b.The Co 2p spectrum of Co-MOF exhibits two prominent bands at～781.6 and 797.3 eV with a separation of 15.3 eV assigning to Co 2p3/2and Co 2p1/2respectively,indicating that cobalt species is mainly in its +2 oxidation state [39,40].After the generation of Fe-MOF on the surface of Co-MOF,this two bands have obvious blue-shifts to 782.0 and 797.8 eV,respectively,suggesting the strong electronic interactions between the two MOFs.The presence of satellite peaks was observed at about 786.1 and 803.3 eV.As shown in Fig.4c,the O 1s regions of all products can be fitted into three peaks:M-O bonds (O2−,～530.7 eV),hydroxyl species (-OH,～531.7 eV),and C＝O or surface adsorbed oxygen(H2O,～532.6 eV).Besides,XPS spectra of C 1s for the products showed almost the same bands that can be assigned to C＝C-C units at 284.6 eV,C-O units at 285.4 eV,and C＝O units at 288.3 eV[40].The carbon composition not only comes from the ligands but also from the graphene oxide additive.

The 2D hybrid MOF materials were then directly used as electrocatalysts to catalyse OER without any further treatment process.The test was conducted in 1 M KOH on a glassy carbon electrode in a typical three-electrode cell.The electrocatalytic performances of all Fe-MOF,Co-MOF,and Fe-MOF@Co-MOF hybrid products towards OER were also examined with the same mass loading.The polarization curves are presented in Fig.5a.When scanning to potentials more positive than 1.5 V (vs RHE),the collected current density sharply increased,corresponding to water oxidation.As shown in Fig.5a,the hybrid Co-MOF@Fe-MOF catalyst exhibits the lowest overpotential of 290 mV at 10 mA cm−2.In contrast,the monometallic MOF counterparts,Co-MOF and Fe-MOF have higher overpotentials of 394 and 454 mV to drive the same current density,respectively.It is noted that,if in reverse,loading of Co-MOF on Fe-MOF surface,the formed Fe-MOF@Co-MOF hybrid presents overpotential of 348 mV for 10 mA cm-2.This value is also obviously lower than those of Co-MOF and Fe-MOF catalysts,although it is higher than that of Co-MOF@Fe-MOF hybrid.This indicates the crucial interactions between Co and Fe species that induce the improved electrocatalytic activity.The optimized MOF hybrid catalyst is also superior to the state-of-the-art OER catalyst IrO2reported in the references[41].A comparison table for the catalytic performance of the hybrid MOF catalyst is shown in Table S1 (see SI).It can be seen that this MOF hybrid catalyst has catalytic activity much better than or comparable to most of reported FeCo-based catalysts.

Tafel plots derived from the LSV curves are shown in Fig.5b.The Tafel slope not only can suggest the possible rate-limit step,but also indicate the reaction kinetics.A lower Tafel slope is favorable for practical applications,since a lower Tafel slope means that an improved OER rate can be achieved with a slight increase of the overpotential.The Co-MOF@Fe-MOF electrode exhibits a smallest Tafel slope of 40.9 mV dec−1,which is lower than those of Fe-MOF(68.3 mVdec−1),Co-MOF(81.3 mVdec−1),and comparable to Fe-MOF@Co-MOF hybrid(44 mVdec−1)electrodes.The two hybrid MOF,Co-MOF@Fe-MOF and Fe-MOF@Co-MOF,show similar Tafel slopes,indicating that on these hybrid bimetal catalysts,the OER possibly has the same rate-limit step.Fig.5c shows the comparison results of the typical four samples,which clearly exhibits the improved electrocatalytic performance of hybrid MOF materials.

The electrochemically active surface area (ECSA) of the MOF-based catalyst was checked by double layer capacitance values(Cdl),since the ECSA of an electrode is proportional to its Cdlvalue.The double-layer capacitance values of different catalysts were estimated by CV tests in a non-Faradaic region with different scanning rates.As shown in Fig.5d,the Cdlvalue of Co-MOF@Fe-MOF is 4.2 mF cm−2,which is significantly greater than those of Fe-MOF (0.36 mF cm−2) and Co-MOF(1.29 mF cm−2).Relatively,Fe-MOF or Fe-MOF loaded with Co-MOF on the surface showed smaller Cdlvalues (0.36 and 0.2 mF cm−2),which would be related to the weaker conductivity of iron oxide or oxyhydroxide formed during oxygen evolution process.The above results indicate that the Co-MOF@Fe-MOF hybrid possesses the highest ECSA value among of the investigated samples.

The electrochemically active surface area of a catalyst is influenced by the conductivity.For a better understanding of the OER kinetics on these catalysts,the electrochemical impedance spectroscopy (EIS)measurements were performed under a potential of 1.55 V (Fig.6).It can be observed that the Nyquist plots of Co-MOF@Fe-MOF hybrid and Co-MOF consist of two semicircles,attributing to the possible ionic resistance (Ri) in the higher frequency region and the charge transfer resistance (Rct) in the low frequency region,while the Fe-MOF@Co-MOF hybrid and Fe-MOF products only consist of one semicircle.These Nyquist plots were then fitted by the resistance-capacitance equivalent circuit (Fig.S2,see SI),which show that the Co-MOF@Fe-MOF hybrid had a Rctvalue of 33.9 Ω,much smaller than that of Co-MOF(371.3 Ω)and Fe-MOF (1319 Ω).In addition,in the Nyquist plots of Fe-MOF and Fe-MOF@Co-MOF products,there was a return tendency in the end of the plots.This return tendency suggests the presence of inductance in the circuit.

A promising electrode for practical application should show not only high electrocatalytic performance,but also good long-term stability.As shown in Fig.7,no obvious potential shift was observed in the LSV curves for the Co-MOF@Fe-MOF catalyst after 3000 cyclic voltammetry (CV) cycles in the range of 1.0-1.55 V (vs RHE).The longterm stability of the Co-MOF@Fe-MOF catalyst at current density of 10 mA cm-2was also tested with chronoamperometric (i-t) measurements for 36 h.After 36 h of operation,the current density only showed a slight decrease of 10%,further suggesting the good stability.The above result indicates that no significant catalytic performance degradation of the MOF-based catalyst was observed.

However,it should be noted that no catalytic performance degradation doesn't mean no catalyst material phase transformation.To further check the catalyst phase transformation proceeding progressively during OER operation,the Co-MOF@Fe-MOF catalyst after OER operation was then characterized.

As shown in Fig.8a,the Co-MOF@Fe-MOF catalyst after 36 h of OER operation at current density of 10 mA cm-2 demonstrated typical sheet-like shape.On these nanosheets,there were some small sized particles (Fig.8b).These particles may be the newly-generated oxyhydroxides formed due to the strong alkaline environment and high overpotential for OER.Especially,it seems that there are amounts of defects or voids on the nanosheets.ICP results indicate that after OER test,the catalyst contained cobalt and iron of 31.8 wt% with Fe:Co molar ratio of 0.45:1,which is basically consistent with the results before OER test (metal content of 34.9 wt% with Fe:Co molar ratio of 0.48:1),suggesting no obvious cobalt or iron dissolving in the electrolyte during OER process (Table S2,see SI).The XPS spectra of the catalyst after OER operation was shown in Fig.4,which were compared with those before OER.Clearly,the XPS bands of Fe 2p and Co 2p changed;the bands shift to lower binding energy,indicating the change of chemical environment of metal sites.The Co 2p bands of the catalyst after OER located at 779.7,783.2,794.8,and 796.6 eV,suggesting the formation of CoOOH.As for the O 1s band,the catalyst after OER showed three fitted bands at 530.3,531.0,and 531.7 eV; the first two bands are corresponded to M-O species and M-OH species.The last one is attributed to absorbed oxygen species.The O1s XPS spectrum showed almost the same percentage of M-O species and M-OH species,which is consistent with the formation of CoOOH and/or FeOOH,as they possess equal amounts of Co-O and Co-OH groups.During OER testing,some Nafion was added,which caused the introduction of carbon element.Thus,XPS band of C 1s after OER testing was not analyzed here.The formation of oxyhydroxides on the surface after OER operation is further confirmed by the FT-IR spectrum of the Co-MOF@Fe-MOF catalyst(Fig.3a),which shows stronger bands at 1000 cm-1corresponding to bending vibration of M-OH and at 587 cm-1corresponding to M-O units.These further confirm the formation of oxyhydroxides during OER operation.

As shown above analysis of the catalyst after OER operation,it can be concluded that the catalyst layer surface was oxidized,forming oxyhydroxides during OER operation.This surface oxidation phenomenon is similar to the other kinds of OER catalysts such as oxides,sulfides,and so on.The in situ formed oxyhydroxides would act as stable catalytic active centres for the water oxidation process.Thus,only a slight catalytic current density decrease was observed for 36 h of OER operation.

Obviously,the Co-MOF@Fe-MOF hybrid obtained from the loading of Fe-MOF species on Co-MOF nanosheets exhibit improved OER performance than the counterparts of monometallic Fe-MOF and Co-MOF products due to the involved synergistic effect between Co and Fe species.In the hybrid,the closely packed Fe-and Co-MOF nanosheets could expose more metal centres for effective intermolecular synergistic interactions.In the hybrid,Fe-MOF was on the surface; Fe species that act as main catalytic active centres were thus easily exposed to the electrolyte.This is favorable for the catalytic process.The demonstrated superior OER activity of Co-MOF@Fe-MOF can also be contributed by the formation of the richness of defects or voids during electrochemical activation to facilitate active sites expose.At last,the involved graphene oxide nanosheets with higher electrical conductivities will facilitate electron transport necessary for the redox reactions [42].The presence of graphene oxide nanosheets and carbon-based frameworks in MOF would also possibly inhibit the agglomeration of active Co-Fe species.All of the factors contribute the excellent electrocatalytic activity.

4.Conclusions

This work demonstrates an efficient route for the preparation of 2D MOF hybrid materials composed of Co-MOF and Fe-MOF by solution phase chemical method for advanced oxygen evolution catalyst.The advanced 2D Co-MOF@Fe-MOF hybrid is considered to be an excellent electrocatalyst,exporting 10 mA cm-2at an overpotential of only 290 mV and a Tafel slope of 40.9 mV dec-1in 1 M KOH for OER.Moreover,in the 36 h of OER study,only slight current density deviation can be observed in the hybrid MOF catalyst.It is believed that the strong synergistic interaction between Fe species and Co species boosts the reaction.After 36 h of OER operation,the microscale sheetlike structure of the catalyst keeps well.The demonstration of hybrid MOF materials as OER catalysts would stimulate extensive exploration of MOF materials for catalyst application,since the structural parameters and composition of MOF materials can be easily tuned by different ligands.Further study should be paid on the structural and composition evolution of MOF materials during OER operation.It is thus anticipated that such strategies for hybrid MOF materials should be further expanded to other various electrocatalysts.

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

The authors are grateful for National Natural Science Foundation of China (No.21776115) and Jiangsu Natural Science Foundation (No.BK20161343).Six talent peaks project in Jiangsu Province (XCL-2018-017).Foundation from Marine Equipment and Technology Institute for Jiangsu University of Science and Technology,China (HZ20190004).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pnsc.2020.02.006.