Metal-organic framework-derived Ni doped Co3S4 hierarchical nanosheets as a monolithic electrocatalyst for highly efficient hydrogen evolution reaction in alkaline solution

Jiankang Wang,Yajing Wang ,Zhongping Yao,Zhaohua Jiang

1 School of Chemistry and Chemical Engineering,State Key Laboratory of Urban Water Resource and Environment,Harbin Institute of Technology,Harbin 150001,China

2 College of Materials Science and Engineering,Yangtze Normal University,Chongqing 408100,China

Keywords:Metal-organic framework Electronic structure Ni doped Co3S4 Electrochemistry Catalysis Hydrogen production

ABSTRACT Hierarchical nanostructure construction and electronic structure engineering are commonly employed to increase the electrocatalytic activity of HER electrocatalysts.Herein,Ni doped Co3S4 hierarchical nanosheets on Ti mesh (Ni doped Co3S4 HNS/TM) were successfully prepared by using metal organic framework(MOF)as precursor which was synthesized under ambient condition.Characterization results confirmed this structure and Ni incorporation into Co3S4 lattice as well as the modified electronic structure of Co3S4 by Ni doping.Alkaline HER performance showed that Ni doped Co3S4 HNS/TM presented outstanding HER activity with 173 mV overpotential at -10 mA·cm-2,surpassing most of metal sulfide-based electrocatalysts.The hierarchical structure,superior electrical conductivity and electronic structure modulation contributed to the accelerated water dissociation and enhanced intrinsic activity.This work provides a new avenue for synthesizing hierarchical nanostructure and simultaneously tuning the electronic structure to promote HER performance,which has potential application in designing highly efficient and cost-effective HER nanostructured electrocatalyst.

1.Introduction

The ongoing concerns of fossil fuel depletion and severe environmental problem require us to develop green and sustainable energy.Hydrogen energy with the merits of zero-carbon emission,renewable nature and high gravimetric energy density is considered as a promising alternative to fossil fuels [1-3].Electrochemical water splitting is an effective method for hydrogen production with high purity and selectivity.However,the great energy barrier for hydrogen evolution reaction (HER) on cathode contributes to the sluggish reaction kinetics and then reduces the energy conversion efficiency due to high HER overpotential.Pt/C as the ‘‘Holy Grail” HER electrocatalyst suffers from the limited reserves,high cost and unsatisfied stability,thus constraining its widely commercial application.

Recently,the earth abundant transitional metal-based electrocatalysts have been exploited for hydrogen production [4-12].Among them,cobalt sulfide stands out due to its metallic nature,low cost and high HER activity.However,the HER activity of single cobalt sulfide is still far from that of the state-of-the-art Pt/C,mainly due to the inferior intrinsic activity.To boost the intrinsic activity,heteroatom incorporation is usually adopted,through which the electron density of pristine active sites is modulated and then the adsorption free energy of hydrogen intermediates on active sites is optimized,meantime charge transfer resistance is reduced,leading to significant improvement in HER performance[13,14].For instance,CoS2HER catalytic activity was significantly enhanced after Ni incorporation due to suitable hydrogen adsorption energy influenced by Ni doping[15].In addition,surface morphology engineering is also widely applied in enhancing HER performance.It has been demonstrated that three-dimensional(3D) hierarchical structure can provide large surface area for affording more accessible active sites for water reduction and also shortening ion or reactant diffusion pathway for enhancing mass transport,eventually improving HER kinetics [16,17].Motivated by these,it is meaningful to design Ni doped Co3S4hierarchical nanostructure for HER application.

Currently,metal-organic framework(MOFs)as a relatively new class of material have gained tremendous attention in the fields of electrocatalysis,supercapacitor and fuel cell due to its high surface area,fascinating structures and multifunctional properties [18-24].However,synthesizing Ni doped Co3S4with hierarchical nanostructure by using Co-MOF as precursor has not been reported.Furthermore,some MOF-derived HER electrocatalysts[25-29]are usually subjected to high pyrolysis temperature during preparation process,which is energy-consuming,complicated and high cost.Meantime,the as-prepared electrocatalysts are prone to aggregation under high temperature.Moreover,such electrocatalysts are in powder form and polymeric binder is needed to coat current collector,which will result in the blockage of some active sites and easier detachment of electrocatalysts during continuous gas releasing.Therefore,direct growth of MOF and MOF-derived Ni doped Co3S4hierarchical nanosheets on Ti mesh (TM) under mild condition will be desirable and meaningful.

Herein,we adopted an innovative method to synthesize a novel Ni doped Co3S4hierarchical nanosheets on Ti mesh(denoted as Ni doped Co3S4HNS/TM) and the schematic diagram of Ni doped Co3S4HNS/TM preparation was shown in Fig.1.This synthesis approach involved three steps:firstly,Co-MOF was synthesized under ambient condition by using 2-methylimidazole as ligand;secondly,CoNi hydroxide was synthesized by ion exchange and etching with Ni2+;finally,anion exchange of CoNi hydroxide with S2-contributed to the formation of Ni doped Co3S4hierarchical nanosheets.The as-prepared Ni doped Co3S4HNS/TM exhibited superior HER activity in 1.0 M KOH with an overpotential of 173 mV at -10 mA·cm-2,and long-term durability.

2.Experimental

2.1.Chemicals and materials

Co(NO3)2·6H2O,Ni(NO3)2·6H2O and 2-methylimidazole(C4H6N2) were supplied by Sinopharm chemical reagent Co.,Ltd.,China.Na2S·9H2O was purchased from Tianjin Kemiou Chemical Reagent Co.,Ltd.,China.Deionized water was used throughout the whole experiments.

2.2.Preparation of Ni doped Co3S4 hierarchical nanosheets on Ti mesh

Firstly Co-MOF and CoNi hydroxide on TM were prepared according to the published literature [30].In detail,25 ml 0.4 mol·L-12-methylimidazole aqueous solution was poured into 25 ml 50 mmol·L-1Co(NO3)2solution under vigorous stirring.After homogeneous mixing,2 cm2Ti mesh (TM)was immersed into the above mixture.After the reaction for 4 h under room temperature,Co-MOF was successfully deposited on TM surface with deep blue color.The obtained Co-MOF/TM was submersed into 50 ml ethanol-water solution (volume ratio 1:4) containing 0.15 g Ni(NO3)2-·6H2O and then the ion exchange and etching reaction with Ni2+were triggered under 85 °C water bath.The final CoNi hydroxide/TM was achieved after the reaction for 15 min.

Ni doped Co3S4HNS/TM was prepared by hydrothermal vulcanization with Na2S as sulfur source.In detail,40 ml 10 mmol·L-1Na2S aqueous solution was poured into 50 mL Teflon-lined stainless autoclave with 2 cm2CoNi hydroxide/TM.The sealed autoclave was placed into an oven and then heated to 120 °C for 8 h.Finally,Ni doped Co3S4HNS/TM sample was taken out,washed with ethanol and water for several times,dried at 60 °C for 6 h in a vacuum oven.Ni doped Co3S4HNS/TM prepared with 0.15 g Ni(NO3)2·6H2O was utilized to characterization and HER performance test.The loading content of Ni doped Co3S4HNS on TM was determined to be 1.40 mg·cm-2by weighing the mass difference before and after loading catalyst on TM surface.

2.3.Characterization

X-ray diffraction (XRD) patterns were obtained from a Rigaku D/max-γB diffractometer with Cu Kα (λ=0.15406 nm) radiation at a scanning range from 10°to 90°.Scanning electron microscope(SEM) measurements were performed on NanoLab 600i scanning electron microscope coupled with energy dispersive spectroscopy(EDS) to investigate the surface morphology and microstructure.Transmission electron microscopy (TEM) images were achieved on FEI Tecnai G2 F30,America.X-ray photoelectron spectroscopy(XPS) of the samples before and after long-term HER tests were accomplished on Perkin-Elmer PHI 5400 ESCA System and Thermo Scientific ESCALAB 250Xi system,respectively,with Al Kα radiation at 1486.6 eV.

2.4.Electrochemical measurement

Fig.1.Schematic diagram of Ni doped Co3S4 HNS/TM formation.

Electrochemical measurement was performed on CHI706E electrochemical workstation by using three-electrode system in 1.0 mol·L-1KOH solution,where Ag/AgCl (sat.KCl) and Pt foil were labelled as reference and counter electrode,respectively.2 cm2as-prepared sample was employed as working electrode.Before HER measurement,consecutive cyclic voltammetry (CV)was carried out at a scan rate of 50 mV/s until a stable CV curve was observed.After that,the HER linear sweep voltammetry(LSV) was executed at 10 mV·s-1.Tafel plots were derived from LSV curves.Electrochemical impedance spectra were conducted at the HER overpotential of 0.2 V in a frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 5 mV,from which charge transfer resistance (Rct) and serial resistance (Rs) could be obtained.The electrochemically active surface area (ECSA)was calculated by acquiring electrical double-layer capacitor(Cdl) according to Eq.(1) [31].Cdl(mF·cm-2) was determined by measuring non-Faradaic capacitive current at the scan rates of 10,20,40,60,80,100 and 120 mV·s-1.Cdlis given by Eq.(2):

Δjand v represent the current density difference between anodic and cathodic current density,and scan rate,respectively.Note that all the potentials unless otherwise mentioned are relative to reversible hydrogen electrode (RHE) and meantime subjected toiR compensation.Pt/C electrode was prepared by dropping the ink onto glassy carbon electrode (Φ 5 mm) with the loading of≈0.2 mg·cm-2.The homogeneous ink was formed by dispersing 1 mg 20% (mass) Pt/C in 1 ml isopropanol,water and Nafion mixture (V/V 1:3:0.16).

3.Results and Discussion

The surface morphologies of as-synthesized Co-MOF,Co hydroxide,CoNi hydroxide and Co3S4NS and Ni doped Co3S4HNS on TM were analyzed in detail by SEM and the relevant results were shown in Figs.S1(Supplementary Material)and Fig.2.Fig.S1(a and d) showed that Co-MOF presented micro-sized sheet-like structure with an average lateral dimension of 4.2 μm in the middle region and thickness of 1.25 μm.After Co-MOF was converted to Co hydroxide,micro-sized sheet-like structure turned into nanosheet structure with only 20-40 nm thickness and these nanosheets were interconnected to each other,shown in Fig.S1(b and e),contributing to numerous pores formation.However,after the reaction with Ni(NO3)2,the hierarchical nanosheet structure was produced,displayed in Fig.S1(c and f),i.e.the much thinner nanosheets with random arrangement were further grown on pristine nanosheet surface.Fig.2 revealed that Co3S4and Ni doped Co3S4on TM inherited the surface morphology of Co hydroxide and CoNi hydroxide,respectively,but the nanosheets surface became rough and curly.Note that Co3S4nanosheet and Ni doped Co3S4hierarchical nanosheet on TM were labelled as Co3S4NS/TM and Ni doped Co3S4HNS/TM,respectively,unless otherwise specified.The elemental mapping of Ni doped Co3S4HNS/TM in Fig.S2 showed uniform distribution of Co,Ni and S at the sample surface.This fascinating hierarchical nanosheet structure and abundant pores on Ni doped Co3S4HNS/TM are in favor of accelerating mass transfer and providing more active sites for water reduction,thus improving HER performance.

Fig.2.SEM images of Co3S4 NS/TM (a,b) and Ni doped Co3S4 HNS/TM (c,d).

Fig.3.TEM image (a),HRTEM image (b) and corresponding SAED pattern (c),EDS spectrum (d) for Ni doped Co3S4 HNS/TM.

The microstructure of as-prepared Ni doped Co3S4HNS/TM was further investigated by TEM,shown in Fig.3a.It could be observed that the curly nanosheets were intertwined with each other,which was consistent with the SEM results.High resolution TEM image in Fig.3b showed the clear lattice fringes with interplanar spacing of 3.33 ? (1 ?=0.1 nm),corresponding to Co3S4(2 2 0) plane.The selected area electron diffraction (SAED) pattern in Fig.3c presented the obvious diffraction rings,indexed to (3 1 1) and(4 4 0) planes of Co3S4phase,which suggested the polycrystalline nature of Co3S4.TEM-EDS in Fig.3d showed the presence of Co,Ni,S and O in Ni doped Co3S4HNS sample.

The crystal structures of as-prepared Co3S4NS/TM and Ni doped Co3S4HNS/TM were analyzed by XRD.Fig.4 showed that except for the diffraction peaks of substrate Ti mesh,the well-defined peaks at 31.46° and 54.9° were assigned to the cubic Co3S4(JCPDS No.47-1738).After Ni doping,no other diffraction peaks appeared,indicating that no new substance was produced.In addition,a slight positive shift was observed at around 31.4°in the magnified XRD patterns (inset),which might be caused by lattice shrinking induced by Ni incorporation into Co3S4lattice,since the ionic radius of Ni2+(0.072 nm) is a little smaller than that of Co2+(0.074 nm) [32].The above XRD results confirmed the successful incorporation of Ni into Co3S4lattice.The XRD patterns of Co-MOF/TM,CoNi hydroxide/TM and Co hydroxide/TM samples were shown in Figs.S3 and S4.It was seen from Fig.S3 that only diffraction peaks of substrate Ti existed and this phenomenon might be the reason that Co-MOF and CoNi hydroxide samples had a poor crystallinity.However,the diffraction peaks corresponding to hexagonal Co(OH)2(JCPDS No.30-0443) could be well distinguished from Fig.S4.All in all,MOF-derived Ni doped Co3S4was successfully synthesized.

Fig.4.XRD patterns of Co3S4 NS/TM and Ni doped Co3S4 HNS/TM (Inset is magnified XRD patterns at the range from 27° to 34°).

The surface elemental composition and valence state of Ni doped Co3S4HNS/TM sample were investigated by XPS pattern shown in Figs.5 and S5.The XPS survey spectrum in Fig.5a revealed the presence of Ni,Co,S and O in the Ni doped Co3S4HNS/TM sample,among which the oxygen element was due to the spontaneous oxidation by air.The high-resolution Co 2p XPS spectrum of Co3S4NS/TM was showed in Fig.S5.After deconvolution,two spin-orbit doublets with two satellite peaks were observed.The peaks at 778.5 eV and 795.0 eV were derived from Co3+2p3/2 and 2p1/2,respectively,and the peaks at 780.4 and 796.3 eV were due to 2p3/2 and 2p1/2 of Co2+,which agreed with the previously reported Co 2p XPS peaks of Co3S4[33,34].Compared with Co 2p XPS spectrum of Co3S4NS/TM,a noticeably positive shift of Co 2p3/2 peak was identified after Ni doping,shown in Fig.5b.This meant that electron transfer from Co to Ni occurred,making the decrease of electronic density of Co,which was ascribed to the larger electronegativity of Ni2+(1.91) than Co2+(1.88) [35,36].The electronic structure change of Co in Ni doped Co3S4HNS/TM sample was in favor of optimizing sorption behavior of H2O and hydrogen intermediate and then affected the alkaline HER performance.The deconvoluted Ni 2p XPS spectrum in Fig.5c displayed that the binding energies of Ni 2p3/2 and 2p1/2 at 853.9 and 872.9 eV,respectively,were observed due to spinorbital splitting,accompanied with two satellite peaks at 861.6 and 879.2 eV.These results were in accordance with the characteristic spectrum of Ni2+[37,38].The high resolution S 2p XPS spectrum in Fig.5d was divided into three peaks,among which the peaks located at 162.0 and 163.4 eV were the corresponding S 2p3/2 and 2p1/2 in M-S bond,while the peak at 168.8 eV was ascribed to the sulfate species owing to the surface oxidation[39,40].Thus,XPS results further confirmed the successful synthesis of Ni doped Co3S4.

Fig.5.(a) Full XPS spectra of Ni doped Co3S4 HNS/TM.(b) High resolution Co 2p XPS spectra of Co3S4 HNS/TM and Ni doped Co3S4 HNS/TM.(c) High resolution Ni 2p XPS spectra and (d) S 2p XPS spectra for Ni doped Co3S4 HNS/TM.

The electrocatalytic performances of various samples toward HER in 1.0 mol·L-1KOH aqueous solution were tested by using linear sweep voltammetry.To assess Ni source content on HER activity of as-prepared Ni doped Co3S4HNS,the cathodic polarization curves of Ni doped Co3S4HNS prepared with 0.10,0.15 and 0.20 g Ni(NO3)2·6H2O were achieved and presented in Fig.S6.It was clearly seen that Ni doped Co3S4HNS prepared with 0.15 g Ni source exhibited the best HER performance.Therefore,Ni doped Co3S4HNS prepared with 0.15 g Ni source was used for the following HER performance evaluation.As illustrated in Fig.6a and b,Pt/C exhibited the highest HER activity with 72 mV at-10 mA·cm-2.Ni doped Co3S4HNS/TM sample,which required 173 mV to reach the current density of-10 mA·cm-2,was distinctly lower than that of Co3S4NS/TM(266 mV),CoNi hydroxide/TM(484 mV),Co-MOF/TM(369 mV) and black TM (655 mV).Compared with the Table S1 in Supplementary Material,the HER activity also outperforms most of the transitional metal sulfide-based electrocatalysts such as CoS2HNSs(193 mV@-10 mA·cm-2)[41],Co3S4(270 mV@-10 mA·cm-2)[42],CoSx/Ni3S2@NF (204 mV@-10 mA·cm-2) [43],Co3S4@MoS2(210 mV@-10 mA·cm-2) [34],NiCo2S4NA/NF (210 mV@-10 mA·c m-2) [38],etc.In order to study the HER kinetics of as-prepared electrocatalysts,Tafel plots derived from LSV curves were presented in Fig.6c.Tafel slope of Ni doped Co3S4HNS/TM was as low as 97 mV/dec,much lower than that of Co3S4NS/TM(114 mV/dec),Co-MOF/TM (121 mV/dec),CoNi hydroxide/TM(130 mV/dec) and the black TM (160 mV/dec).The lowest Tafel slope of Ni doped Co3S4HNS/TM indicated that the electrocatalytic hydrogen reaction followed the Volmer-Heyrovsky process.Moreover,Ni doping could significantly enhance the HER kinetics of Co3S4and accelerate the water dissociation step (Volmer step),thus promoting HER activity.

To get deep insight into the improved HER performance of Ni doped Co3S4HNS/TM,the electrochemical impedance spectra(EIS) were performed under HER overpotential of 200 mV in 1.0 mol·L-1KOH solution.The EIS spectra and its enlarged parts in high frequency showed that the semicircle diameter for various samples followed the order of CoNi hydroxide/TM ＞Co-MoF/TM ＞Co3S4NS/TM ＞Ni doped Co3S4NS/TM.It is well acknowledged that the semicircle diameter represents the charge transfer resistance(Rct) and the bigger the arc diameter is,the largerRctis [44,45].Hence,the lowestRctfor Ni doped Co3S4NS/TM implied the fastest charge transfer rate.

To investigate the electrochemically active surface area (ECSA)of Co-MOF/TM,CoNi hydroxide/TM,Co3S4NS/TM and Ni doped Co3S4HNS/TM samples,the cyclic voltammograms were recorded in electric double layer charging-discharging region from 0.59 to 0.69 V with scan rate ranging from 10 to 120 mV·s-1in 1.0 mol·L-1KOH,shown in Figs.S7 and 7.After calculation according to Eq.(2),the double layer capacitances for Co-MOF/TM,CoNi hydroxide/TM,Co3S4NS/TM and Ni doped Co3S4NS/TM were 0.38,0.49,9.64 and 13.53 mF·cm-2,presented in Fig.7b and d.Thus,the ECSA for various samples was in the sequence of Ni doped Co3S4NS/TM(338.25 cm2) ＞Co3S4NS/TM (241 cm2) ＞CoNi hydroxide/TM(12.25 cm2) ＞Co-MOF/TM (9.5 cm2) based on Eq.(1).The hierarchical nanostructure of Ni doped Co3S4NS/TM assured its highest ECSA among them,which was conducive to providing much more accessible active sites for electrocatalyzing hydrogen production and shortening diffusion pathway for reactant and products.

Fig.6.(a)Polarization curves for TM,Co-MOF/TM,CoNi hydroxide/TM,Co3S4 NS/TM,Ni doped Co3S4 HNS/TM and Pt/C in 1.0 mol·L-1 KOH,(b) corresponding overpotential histogram at -10 mA·cm-2,(c) Tafel plots for various samples except for TM,(d) Nyquist plots at HER overpotential of 200 mV (inset is the enlarged EIS spectra in high frequency region).

Fig.7.Cyclic voltammograms of Co3S4 NS/TM(a) and Ni doped Co3S4 HNS/TM(c) measured in non-faradic region from 0.59 V to 0.69 V with scan rates ranging from 10 to 120 mV·s-1 in 1.0 mol·L-1 KOH,and corresponding linear relationship between current density difference(Δj)and scan rate(v)for Co3S4 NS/TM(b)and Ni doped Co3S4 HNS/TM (d).

Fig.8.(a)Polarization curves for Ni doped Co3S4 HNS/TM in 1.0 mol·L-1 KOH before and after 2000 CV cycles,(b)HER overpotential at the current density of-10 mA·cm-2 vs.time curve.

The long-term durability of Ni doped Co3S4HNS/TM was also evaluated by polarization curves before and after 2000 CV cycles and chronopotentiometric curve at the constant current density of-10 mA·cm-2.Compared with the initial LSV curve,Ni doped Co3S4HNS/TM showed a negligible decay of current density after 2000 consecutive CV cycles,observed in Fig.8a.The chronopotentiometry measurement at the constant current density of -10 mA·cm-2in Fig.8b exhibited a slight increase in HER overpotential by 45 mV after running 24 h,signifying a slight decrease in HER activity of Ni doped Co3S4HNS/TM occurred.

To reveal the reason why a minor attenuation of HER performance occurred on Ni doped Co3S4HNS/TM,the surface morphology and elemental composition of Ni doped Co3S4HNS/TM after long-term durability test were collected by SEM,XRD and XPS analysis.Fig.S7(a) and (b) demonstrated that the hierarchical nanosheet structure maintained well even experiencing chronopotentiometry test at-10 mA·cm-2for 24 h,thus,this structure was highly stable.The elemental compositions were also analyzed by SEM-EDS measurement.EDS patterns and corresponding elemental compositions were presented in Fig.S8(c,d) and Table S2.The results showed that after long-term durability test,the relative content of Ni,Co,S were decreased from 21.4%,14.1% and 34.5%to 17.6%,7.3% and 3.7%,respectively,while O had an obvious increase,suggesting that the sample suffered from superficial oxidation during the stability test.The XRD patterns of the initial and used Ni doped Co3S4HNS/TM were presented in Fig.S9,the diffraction peaks corresponding to Co3S4still existed after stability test and no other peaks except for the peaks of TM were observed,indicating the phase composition had no change during stability test.The XPS results in Fig.S10 showed that the peak intensity of the survey spectra and the high-resolution Co 2p,Ni 2p,S 2p spectra for the used Ni doped Co3S4HNS/TM sample were much higher than the counterpart,which might be caused by the test instrument difference.That is to say,XPS spectra of the samples before and after long-term HER tests were accomplished on Perkin-Elmer PHI 5400 ESCA System and Thermo Scientific ESCALAB 250Xi system,respectively.However,from the view of the peak position,Co 2p and S 2p for the used sample had no significant change while Ni 2p had an obvious positive shift.The relatively higher binding energy of Ni 2p after HER test meant much more high-valent Ni species due to surface oxidation,which was consistent with the SEM-EDS results.In conclusion,the HER activity loss of Ni doped Co3S4HNS/TM during long-term stability test might be related to the surface oxidation,which contributed to the decrease of the relative amount of sulfur in the as-prepared sample.

The improved HER activity of Ni doped Co3S4HNS/TM in alkaline solution could be ascribed to the following reasons.Firstly,the hierarchical nanosheet structure guaranteed much more active sites available for water reduction,confirmed by the largest ECSA,and shortened the reactant diffusion length and thus facilitated mass transfer.Secondly,Ni doping could increase the electric conductivity of the pristine Co3S4and was then in favor of improving HER performance.Lastly but most importantly,incorporation of Ni2+into Co3S4lattice could modulate the electronic structure of Co2+,leading to the decrease of d band center and thus weakened the adsorption energy of hydrogen intermediates,finally promoted the HER kinetics and then lowered HER overpotential,just like the reported literatures [46,47].

4.Conclusions

In summary,MOF-derived Ni doped Co3S4with hierarchical nanosheet structure was synthesized.The HER activity of Ni doped Co3S4HNS/TM in basic solution was evaluated in detail.The Ni doped Co3S4HNS/TM exhibited extraordinary HER performance with only 173 mV overpotential to drive the current density of-10 mA·cm-2,exceeding the most of metal sulfide-based electrocatalysts.The superior HER activity of Ni doped Co3S4HNS/TM could be assigned to the intriguing hierarchical structure,enhanced electrical conductivity and electronic structure modulation of Co sites,which gave rise to the improved intrinsic activity.

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 was funded by National Natural Science Foundation of China (Nos.21906008 and 51571076),Open Project of State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology(No.HCK201716),Chongqing Basic and Frontier Research Program (cstc2018jcyjAX0774) and Science and Technology Research Program of Chongqing Municipal Education Commission (Nos.KJQN201901420 and KJQN202001413).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.02.010.