Jingsha Li,Tao Hu,Changhong Wang,Chunxian Guo
Institute of Materials Science and Devices,Suzhou University of Science and Technology,Suzhou,215011,China
Abstract Surface engineering of active materials to generate desired energy state is critical to fabricate high-performance heterogeneous catalysts.However,its realization in a controllable level remains challenging.Using oxygen evolution reaction (OER) as a model reaction,we report a surface-mediated Fe deposition strategy to electronically tailor surface energy states of porous Co3O4(Fe-pCo3O4)for enhanced activity towards OER.The Fe-pCo3O4 exhibits a low overpotential of 280 mV to reach an OER current density of 100 mA cm-2,and a fast-kinetic behavior with a low Tafel slop of 58.2 mV dec-1,outperforming Co3O4-based OER catalysts recently reported and also the noble IrO2.The engineered material retains 100%of its original activity after operating at an overpotential of 350 mV for 100 h.A combination of theoretical calculations and experimental results finds out that the surface doped Fe promotes a high energy state and desired coordination environment in the near surface region,which enables optimized OER intermediates binding and favorably changes the rate-determining step.
Keywords: Surficial Fe doping;Cobalt oxides;High energy state;Water oxidation;Oxygen evolution reactions
Heterogeneous catalysis plays an important role in developing efficient energy storage and conversion systems[1,2].Since heterogeneous catalysis reaction occurs on the surface of a specific catalyst and is always stepwise,surface engineering of the catalyst to achieve desired energy state is of great significance [3-6].For example,a heterogeneous catalyst of PdMo bimetallene exhibits a highly curved surface for strain effect and sub-nanometer thickness for quantum size effect,achieving a desired energy state and coordination environment for optimized oxygen binding [7].The PdMo bimetallene enables a mass activity of 16.37 Aat 0.9 V vs.RHE towards oxygen reduction reaction(ORR) and also exhibits excellent performance for oxygen evolution reaction (OER) in alkaline electrolyte.Another hetero-structured catalyst comprising 2D nickel metalorganic framework and Pt nanocrystals(Ni-MOF-Pt)induces the formation of Ni-O-Pt surficial bond for a high energy surface state [8].Such a high energy surface leads to the increased reaction intermediate OH*adsorption energy and reduced H*adsorption energy,promoting enhanced hydrogen evolution reaction with an activity of 7.92 Aamong the best reported for alkaline electrocatalysts.If properly designed with desired surface properties,a heterogeneous catalyst could offer favorable energy state and coordination environment,which promote their interactions with target molecules and reaction intermediates (e.g.,OH*for OER) for high-performance catalysis [7-12].Thereby,surface engineering of active materials to generate desired surficial energy state is critical in fabricating high-performance heterogeneous catalysts.
Electrocatalytic water splitting [9,13-15] and metal-air batteries [16-19] have attracted increasing attention recently,and both of them involve the heterogeneous reaction of OER.Being a four-electron transfer process,OER always suffers from sluggish kinetic,which has impeded their industrialization.Hence,it is indispensable to develop efficient electrocatalysts for accelerating the reaction rate and reducing the overpotential [20,21].Various non-noble metal materials have been explored as OER electrocatalysts to replace expensive IrO2and RuO2catalysts,including heteroatom-doped carbon materials[22,23],transition-metal oxides,sulfides,phosphides or selenides[6,14,15,24-29],transition-metal and nitrogen codoped carbon materials [13,30].Among them,cobalt-based OER electrocatalysts,especially spinel Co3O4materials,have received widespread attention owing to their abundant reserve,promising catalytic activity and good stability [14,24,25,31-33].Recent efforts about cobalt-based OER catalysts have been devoted to improving their catalytic activities through cationic incorporation in bulk-like strategies using transitional metals such as nickel [34],manganese [35-37] to fabricate transitional metal compounds of MCo2O4materials (M=Ni,Fe,Co).For instance,NiCo2O4core-shell nanowires with a three-dimensional (3D) structure have shown a promising OER performance with an overpotential of～320 mV [34].MnCo2O4with mesoporous structure,dominant Mn(IV)in the surface demonstrates improved OER catalytic activity [35].Considering that the catalytic reaction of a heterogeneous catalyst always occurs on the surface,incorporation of cationic metal species in bulk forms could induce limited effect on surficial energy state of Co3O4-based materials,which will restrict their OER catalytic activities.On the other hand,surface doping could efficiently tune electronic structures to generate desired surficial energy state,which may facilicate the the adsorption of reactants and charge transport,thus promoting enhanced catalytic performance.
Metal-free element doping/defecting strategies that include heteroatom doping [24,25] and oxygen vacancies [25,38,39]have also been exploited to improve OER catalytic activities of Co3O4-based materials.For example,P-doped Co3O4demonstrated a promising OER catalytic activity with a low overpotential of 260 mV to reach OER current density of 20 mA cm-2[24].Plasma-engraved Co3O4with oxygen vacancies exhibits a high specific activity of 0.055 mAat 1.6 V vs.RHE,about 10 times higher than that of pristine Co3O4[39].Through treating Co3O4with Ar plasma in the presence of NaH2PO4,a Co3O4material with oxygen vacancies for improved electrical conductivity and P doping for desired binding demonstrates a much better catalytic activity than pristine Co3O4[14].These metal-free doping/defecting strategies have demonstrated the possibilities to tailor surface properties of Co3O4materials in atomic level.On the other hand,considering the unique property of unsaturated coordination of some transitional metals (e.g.,Ni and Fe) as well as their similar atom size as that of Co,it is expected that the metal-free doping/defecting strategy could be extended to atomically incorporate transitional metal on the surface of Co3O4materials for enhanced OER catalytic activity.Nevertheless,such an exploration has been rarely reported.
In this study,we report a surface-mediated Fe incorporation strategy using an electrochemical cycling technique to tailor surface energy state of Co3O4materials for significantly enhanced OER catalytic activity.The as-fabricated model electrocatalyst comprises surficial Fe on the surface of porous Co3O4(Fe-pCo3O4).Morphology and structure,in particular surface energy state in the near surface region of the FepCo3O4are characterized.Electrochemical behaviors and OER catalytic activity of the Fe-pCo3O4are investigated,and compared with control samples that include pristine pCo3O4,porous FeCo2O4(pFeCo2O4) and noble metal OER catalyst.Kinetic behaviors and redox transitions of the samples as well as relationship between the amount of incorporated Fe and their OER catalytic performance are explored and analyzed.Synergistically combining experimental results and theoretical calculations is further designed and carried out,aiming to gain deep fundamental understanding of catalytic mechanism for the enhanced performance.
2.1.Materials synthesis
2.1.1.Synthesis of porous Co3O4on Ni foam (pCo3O4)
3.492 g of Co(NO3)2·7H2O and 3.6 g of CO(NH2)2were dissolved into 120 mL of deionized water under stirring for 15 min,and then transferred into a glass container with glass cover.Subsequently,the cleaned Ni foams were vertically added to the above glass container and placed into the furnace at 90°C for 14 h.After cooling down to room temperature,the Ni foam was taken out,followed by washing with deionized water several times and dried.The obtained sample was placed into a muffle furnace,heated to 250°C at a ramp rate of 1°C min-1and kept for 4 h,producing porous Co3O4(pCo3O4) on Ni foam .
2.1.2.Surface incorporation of Fe on pCo3O4(Fe-pCo3O4)
The surface Fe incorporation was realized by electrochemical cycling in Fe-containing electrolyte based on a threeelectrode system,of which the pCo3O4was used as working electrode,Pt foil and saturated calomel electrode (SCE) as counter electrode and reference electrode,respectively.The Fe-containing electrolytes were prepared as followings:3.89 g of sodium acetate (NaAc) was dissolved into 500 mL water,and then 149.4 μL of acetic acid was added to the above NaAc solution under stirring to obtain the transparent acetic acid buffer solution.Then 200 mL of acetic buffer solution was purged with N2for 20 min,followed by adding 100 mg of FeSO4·7H2O.The solution was then purged with N2for another 20 min,and kept at N2during the electrochemical incorporation.The Fe incorporation amount was controlled by different electrochemical cyclic voltammetry cycles,which scan from-0.15 V to 1.25 V with a scan rate of 50 mV s-1in the Fe-containing electrolyte.After electrochemical cycling,the working electrode was taken out and washed,producing Fe-incorporated pCo3O4(Fe-pCo3O4).
2.1.3.Synthesis of control sample of porous FeCo2O4on Ni foam (pFeCo2O4)
Synthesis procedure of pFeCo2O4was the same as that of the pCo3O4except the use of 1.08 g of FeCl3·7H2O and 2.328 g of Co(NO3)2·7H2O instead of 3.492 g of Co(NO3)2·7H2O.
2.2.Material characterizations and electrochemical measurement
Scanning electron microscopy (SEM) was performed on a JEOL JSM-6700F,and transmission electron microscopic(TEM) images were recorded with a JEOL JEM-2100F.For TEM and HRTEM measurements,the sample preparation details are as followings.Firstly,the Fe-pCo3O4decorated Ni foam were dispersed in 2 mL ethanol by ultrasound to obtain the catalyst ink.And then,100 μL catalyst ink were dropped on the microgrid and dried under the infrared lamp for TEM/HRTEM measurement.Elemental mappings were recorded by energy dispersive X-ray spectroscopy (EDS) on JSM-6700F equipped with an Oxford EDS.X-ray diffraction (XRD) patterns were measured using X-ray diffractometer(Bruker AXS)with a Cu Kα source (λ=1.54 Å).X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantera X-ray photoelectron spectrometer.
Electrochemical measurements were performed using CHI760e electrochemical workstation.Cyclic voltammogram(CV) and linear sweep voltammetry (LSV) curves were recorded to evaluate electrochemical behaviors and catalytic activity of the working electrode.The testing was based on a three-electrode system,in which the obtained samples was served as the working electrode,SCE and Pt foil as the reference and counter electrodes,respectively.All the potentials were converted to reversible hydrogen electrode (RHE)according to the equation: E(RHE)=E(SCE)+0.059*pH +0.242 V.The current density of the electrodes was calculated by dividing the current by the real area of the specific electrode.
2.3.Density functional theory (DFT) calculations
Spin-polarization structural optimizations were performed based on DFT using the Vienna Ab initio Simulation Package(VASP) [40].The exchange correlation energy was modelled by using the Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) [41].Projector augmented wave(PAW)pseudo-potentials[42]were used to describe ionic cores,while electron-ion interactions were described by ultrasoft pseudopotentials with O 2s,2p and Co,Fe 3d,4s valence electrons.The cutoff energy was 500 eV.The Co 3d and Fe 3d electron correlation was considered as an on-site Coulomb term (U=5.0 eV and 4.3 eV respectively).Since the previous study indicate that the Gibbs free energy is sensitive to the choice of U for Co,we used the well-tested U value 5.0 eV[43,44].Hubbard U of Fe is referred to the work of Ceder [45].Based on the optimized conventional cell of Co3O4,Co3O4(110)-B plane was built [46].A 15 Å vacuum was inserted in the z direction to prevent image interactions.The top two layers were fully relaxed,while the other layers were fixed at their bulk positions during structural optimizations.
3.1.Morphology and structure characterizations
Morphology and structure of the pCo3O4and Fe-pCo3O4were characterized by SEM and TEM.SEM images inform that the pCo3O4with nanowire shape and an average length of around 3.5 μm was uniformly grown on the Ni foam(Figs.S1 and S2).TEM and high-magnification TEM images tell that the pCo3O4has a porous structure(Figs.S3 and S4).After Fe incorporation through the electrochemical cycling deposition,the as-fabricated Fe-pCo3O4well retains the same shape as the pCo3O4(Figs.S5 and S6).TEM image in Fig.1a shows that the average diameter of the Fe-pCo3O4nanowires was around 30 nm.High-magnification TEM image in Fig.1b informs that the Fe-pCo3O4has a porous structure,similar as the pCo3O4.To prove the successful incorporation of Fe for the Fe-pCo3O4,EDS elemental mappings of the Fe-pCo3O4were recorded.As shown in Fig.1c,except the Co and O,there is also Fe,which is uniformly distributed on the whole Fe-pCo3O4.More evidence for the successful incorporation of Fe was also provided(Fig.S7).These results indicate that Fe with a uniform distribution was successfully incorporated through the electrochemical cycling deposition for the Fe-pCo3O4.
The structure of the Fe-pCo3O4was studied by HRTEM with images given in Fig.2.As shown in Fig.2a,an edge part was observed in HRTEM image of the Fe-pCo3O4.Different parts of the Fe-pCo3O4were given in Fig.2b and 2c.As shown in Fig.2b,central part of the Fe-pCo3O4exhibits distinct lattices with the interplanar spacing of 0.28 nm,which corresponds to the (220) plane of cubic spinel Co3O4.The edge part of the Fe-pCo3O4in Fig.2c has the same lattice structure as that of the inner part,indicating that the Fe incorporation does not affect the lattice structure of the pCo3O4for the FepCo3O4,and avoids the formation of aggregation for a surficial deposition behavior.Effect of the Fe deposition on crystal structure of the Fe-pCo3O4with different Fe deposition cycle was studied by XRD (Fig.S8).The XRD patterns could be indexed to the cubic Co3O4spinel phase (JCPDS #42-1467).Even for the Fe-pCo3O4sample with the Fe deposition cycle up to eight,except the patterns for cubic Co3O4spinel phase,no other peaks were observed,suggesting that the surface Fe deposition does not affect crystal structure of the Fe-pCo3O4.Nevertheless,it cannot be excluded that some Fe-based aggregated nanoparticles with amorphous structure may form on surface of the Fe-pCo3O4.
3.2.Surface properties and energy state analysis
Fig.1.TEM (a) and high magnification TEM (b) images of the Fe-pCo3O4.Elemental mapping images of the Fe-pCo3O4.(c) TEM image and EDX elemental mapping images of Co Ka1,O Ka1 and Fe Ka1 of the Fe-pCo3O4.
Fig.2.(a) High-magnification TEM image of the Fe-pCo3O4.(b,c) HRTEM images of the Fe-pCo3O4,showing atomic structure in the different areas.
Fig.3.XPS Co 2p spectra for the(a)pCo3O4 and(b)Fe-pCo3O4 samples.(c)XPS Fe 2p spectrum of the Fe-pCo3O4.(d)Relationship between Fe incorporation amount of the Fe-pCo3O4 and electrochemical deposition cycle.
To investigate the surface properties and valence states of the samples,XPS measurements of the Fe-pCo3O4and pCo3O4were carried out.The XPS Co 2p spectra of the pCo3O4and Fe-pCo3O4were recorded and divided using the widely used Gaussian fitting method with results given in Fig.3a and 3b.As shown in Fig.3a,for the pCo3O4sample,two peaks of the XPS Co 2p3/2at 779.8 and 780.9 eV that are related to respective Co(III)and Co(II),and another two peaks of the XPS Co 2p1/2at 794.9 and 796.0 eV corrspond to Co(III) and Co(II),respectively [14,25,47].While for the FepCo3O4,both binding energies of the Co(III)and Co(II)peaks shifted positively in comparison with those of the pCo3O4(Fig.3b).A positively shifted binding energy means a high oxidation state for the surface species in metal oxide materials.The results indicate a higher oxidation state for cobalt species in the Fe-pCo3O4over the pCo3O4.Owing different configurations in the d-subshell,cobalt species of Co(II) and Co(III)exhibit different spectral fingerprints in the XPS Co 2p spectrum [50].Co(III) always shows relatively narrow 2p3/2and 2p1/2lines with minor satellites,While Co(II) generally exhibits broad 2p3/2and 2p1/2lines accompanied by intense satellite structures.Thereby,intensity ratio of satellite/main peak can provide information about relative amount of Co(II)/Co(III) in the surface region.Ratios of the satellite/main peak area as well as cor111responding satellite energy of Co 2p3/2for the Fe-pCo3O4and pCo3O4are shown in Fig.S9.In comparison with the pCo3O4,the Fe-pCo3O4has a positively shifted satellite energy and also a higher ratio of Co(II)/Co(III).The high binding energy further proves that surface Fe incorporation induces a high energy state for the cobalt species in the near surface region of the Fe-pCo3O4.
The high-resolution XPS Fe 2p spectra of Fe-pCo3O4are shown in Fig.3c,which provides information about energy states in the near surface region.As shown in Fig.3c,highresolution XPS Fe 2p spectrum of the Fe-pCo3O4could be divided into three peaks: Fe 2p3/2and Fe 2p1/2characteristic peaks of Fe(III) at respective 711.2 eV and 717.5 eV,and a satellite peak at 725.0 eV [16-18].In other words,the incorporated Fe in the Fe-pCo3O4should be mainly in the form of Fe(III).Thus,surficial Fe incorporation promotes a high energy state for the cobalt species in the near surface region of the Fe-pCo3O4[48,49].The amount of incorporated Fe in the Fe-pCo3O4is closely associated with the electrochemical deposition cycle number (Fig.3d),achieving an atomic ratio ranging from 0 (for pCo3O4) to～18%.It is also noted that there is no obvious aggerate on the surface of the Fe-pCo3O4even with the Fe deposition cycles up to 8 (Figs.S10-S13).These results indicate that the surface Fe incorporation is a controllable manner.
3.3.Electrocatalytic activity towards OER
Electrochemical activities of Fe-pCo3O4and pCo3O4towards OER were evaluated by LSV in 1 mol L-1KOH electrolytes.For comparison,except the substrate that was used to grow the Fe-pCo3O4,another control sample of porous FeCo2O4(pFeCo2O4) that was fabricated using the same procedure as the pCo3O4was also fabricated.The difference between the Fe-pCo3O4and pFeCo2O4is the Fe incorporation,e.g.,Fe incorporation on surface vs bulk for the Fe-pCo3O4and pFeCo2O4,respectively.
OER catalytic activities of the samples were investigated by recording their LSV curves that are given in Fig.4a.The substrate alone displays quite poor OER catalytic activity.The pCo3O4exhibits remarkably enhanced OER catalytic performance with an overpotential of 270 mV (the thermodynamic OER potential of 1.23 V as the reference).While for the FepCo3O4,it exhibits an overpotential as low as 215 mV,much smaller than that of pCo3O4.While for the pFeCo2O4,it shows an overpotential of 245 mV,which is higher than that of the Fe-pCo3O4.The overpotential results indicate that Fe incorporation on the surface greatly promotes the OER catalytic activity of the Fe-pCo3O4.To deliver an OER current density of 100 mA cm-2,the Fe-pCo3O4requires an overpotential of 280 mV,which is much smaller than that of pCo3O4(410 mV),pFeCo3O4(385 mV),and noble metal catalyst of IrO2(320 mV) (Fig.4b).Moreover,the Fe-pCo3O4demonstrates higher OER catalytic performance than other cobalt-based electrocatalysts reported in the literatures,such as P8.6-Co3O4/NF(η=260 mVat 20 mA cm-2)[24],functionalized oxygen-deficient Co3O4nanorods (η=275 mV at 10 mA cm-2) [32],cobalt-defected Co3-xO4(η=268 mV at 10 mA cm-2) [51],porous CoxOynanosheets with N-doping and oxygen vacancies (η=280 mV at 10 mA cm-2) [38],CoOxspecies in ZIF-67 (η=318 mV at 10 mA cm-2) [13],and 3D NiCo2O4core-shell nanowires (η=320 mV at 10 mA cm-2) [34].More OER performance comparisons of the Fe-pCo3O4with recently reported cobalt-based electrocatalysts were summarized in Table 1.Meanwhile,the current density of Fe-pCo3O4at a fixed overpotential of η=0.29 V is also compared with that of other samples with results given in Fig.4b.The OER current density for the Fe-pCo3O4at η=0.29 V reaches 136 mA cm-2,which is much higher than that of pCo3O4(15 mA cm-2),pFeCo2O4(29 mA cm-2) and IrO2(58 mA cm-2),further demonstrating the excellent OER catalytic activity of the Fe-pCo3O4.
Fig.4.(a) LSV curves of the samples including substrate,pCo3O4,Fe-pCo3O4,pFeCo2O4 and IrO2.(b) Overpotential (η) at the fixed current density of 100 mA cm-2 and the current density at η=0.29 V for the samples.(c)Tafel plots of the samples derived from their cor111responding LSV curves.(d)Relationship between the current density and the Fe deposition cycle number at the overpotential(η)of 0.27 V.(e)Capacitive current density against scan rate for the samples.(f) Stability test for the Fe-pCo3O4 at an overpotential of 0.35 V.
Table 1 Summary for OER performance of the Fe-pCo3O4 and other cobalt-based electrocatalysts recently reported.
Kinetic behaviors of the samples were studied by Tafel plots,which are calculated from their LSV curves.As shown in Fig.4c,Tafel slop of Fe-pCo3O4is 58.2 mV dec-1,which is much smaller than that of pCo3O4(115.8 mV dec-1),pFe-Co2O4(117.6 mV dec-1) and IrO2(88.5 mV dec-1),representing the Fe-pCo3O4with a fast kinetic OER process.We also investigated the relationship between the Fe incorporated amount and the OER catalytic activity for the Fe-pCo3O4.As shown in Fig.4d,the OER current density increases gradually with the increasement of the deposition cycle,reaching a maximum value at the 4th Fe deposition cycle.While the OER current density decreased when further increasing the Fe deposition cycle.Active site density of an OER electrocatalyst is highly related to its electrochemically active surface area(ECSA),which can be evaluated by slope of difference between anodic and cathodic currents against scan rate,as shown in Fig.4e.The slope of pCo3O4,Fe-pCo3O4and pFeCo2O4was calculated to be around 76.3,75.2 and 75.5,respectively,demonstrating that ECSA is not the main factor to determinate catalytic activity of the Fe-pCo3O4.Long term stability is also critical in evaluating an electrocatalyst.As shown in Fig.4f,the Fe-pCo3O4can retain 100% of its original activity after running for 100 h at a fixed overpotential of 0.35 V,indicating an excellent stability.
3.4.Redox transition and catalytic behavior investigations
To study mechanism for the enhanced catalytic activity of the Fe-pCo3O4,redox transitions of cobalt species were investigated.Cobalt oxides always undergo two redox transitions during the oxidation process,namely,Co2+/Co3+at 1.32 V and Co3+/Co4+at 1.40 V vs.RHE.Additionally,a higher valent state of the metal species for transition metal oxides always have higher oxidation capability,which is favorable in achieving high-performance activity towards oxidation reactions such as OER.Narrow-range LSV curves of the Fe-pCo3O4and the two control samples of pCo3O4and pFeCo2O4were recorded with results given in Fig.5.
Fig.5.(a) Narrow-range LSV curves of pCo3O4,Fe-pCo3O4 and pFeCo2O4,showing the redox behaviours.A1 stands for redox transition peak of Co(II)/Co(III) and A2 for Co(III)/Co(IV).(b) Corresponding Co(III)/Co(IV) redox transition peak potentials as well as A1/A2 ratio of the pCo3O4,Fe-pCo3O4 and pFeCo2O4.
As shown in Fig.5a,the pCo3O4exhibits two characteristic peaks around 1.32 V and 1.40 V that correspond to Co(II)/Co(III)(A1,low oxidation state)and Co(III)/Co(IV)(A2,high oxidation state)redox transition,respectively.The ratio of A2/A1(high oxidation state/low oxidation state)for the pCo3O4is around 1.81,as given in Fig.5b.In comparison with the pCo3O4,the Fe-pCo3O4well retains the two characteristic peaks of Co(II)/Co(III) (A1) and Co(III)/Co(IV) (A2) but displays a much higher ratio of A2/A1with a value of 2.85.This observation demonstrates that surficial Fe incorporation does not affect the redox transitions of Co(II)/Co(III) and Co(III)/Co(IV) but leads to significantly increased amount of the higher oxidation state cobalt species.This observation is in good agreement with the XPS measurement,indicating that Fe incorporation induces a higher energy state in surface of the Fe-pCo3O4over the pCo3O4.While for the pFeCo2O4,the bulk Fe incorporation largely affects the redox transitions of Co(II)/Co(III) and Co(III)/Co(IV),and more seriously,disturbs the higher oxidation state cobalt species.Therefore,it is the surficial Fe doping that greatly promotes the high oxidation state of active cobalt oxides in the near surface region of the Fe-pCo3O4,eventually facilitating OER catalytic activity for high performance.
3.5.Combing theoretical calculations and experimental results
To further verify the important role of Fe incorporation on surface of the Fe-pCo3O4,density functional theory (DFT)calculations utilizing the Vienna Ab initio Simulation Package(VASP) have been performed.Two models with different lateral sizes for the Co3O4-based materials with active(110)-B were built according to previous work [24,46].The slab models of pristine pCo3O4(110)-B and Fe-pCo3O4(110)-B used in this study are presented in Fig.6.Free energy changes were calculated according to the pathways of OER in Eqs.(1)-(4):
where the superscript*represents an active site of the pCo3O4and Fe-pCo3O4configurations and*OH,*O and*OOH for the absorbed reaction intermediates during OER.
Free energy diagrams of the pCo3O4and Fe-pCo3O4are presented in Fig.7.Energy diagram of the pCo3O4in Fig.7a indicates that the rate-determining-step(rds) is the OH*→O*step for the pCo3O4,which has an overpotential of 0.82 V.This observation is in consistent with the previous study [24].As for the Fe-pCo3O4(Fig.7b),the binding energies with the OER reaction intermediates of OH*,O*and OOH*change.The rds for the Fe-pCo3O4is the O*→OOH*step with an overpotential of only 0.59 V,which is much smaller than that of the pCo3O4.Combining the DFT calculations and the experimental XPS characterization results that surficial Fe deposition promotes a high energy state in the near surface region of the Fe-pCo3O4(Fig.3b and c),it can be concluded that the surficial Fe deposition greatly promotes high energy state and desired coordination environment.These desired electronic structures lead to optimized intermediator binding,which further favorably changes the rate-determining-step for a high OER catalytic activity.The DFT calculation results are also consistent with the electrochemical measurement results(Fig.4a and 4b),which demonstrate that the Fe-pCo3O4delivers a much higher OER catalytic activity with a lower overpotential and faster kinetics than the pCo3O4.
Fig.6.Atomic configurations of OER mechanism on(a-d)pCo3O4(110)-B and(e-h)Fe-pCo3O4(110)-B.50%active sites,surface Co(III),are covered with OH*,O*,or OOH*.Blue,cyan,gold,red,and pink spheres represent Co(III),Co(II),Fe,O,and H atom respectively.
Fig.7.Free energy diagrams for OER on (a) pCo3O4 (110)-B and (b) FepCo3O4(110)-B.50%active sites,surface Co(III),are covered with OH*,O*,or OOH*.
In summary,a surface-mediated Fe deposition strategy is presented to tailor electronic structures of spinel Co3O4for enhanced activity towards OER.Electrochemical and surface characterizations indicate that the incorporation of surficial Fe does not affect the redox transitions of Co(II)/Co(III) and Co(III)/Co(IV) but leads to significantly increased amount of the high oxidation state cobalt species in the near surface region.To deliver an OER current density of 100 mA cm-2,the Fe-pCo3O4requires an overpotential of 280 mV,which is much smaller than that of pCo3O4(410 mV),pFeCo2O4(385 mV) and other spinel Co3O4-based materials.The FepCo3O4also exhibits a fast kinetic with a Tafel slop of 58.2 mV dec-1,and retains 100% of its original activity after operating at a fixed overpotential of 0.35 V for 100 h.Synergistic combination of the DFT calculations and experimental results proves that the surficial Fe promotes a high energy state and desired coordination environment,which optimize OER intermediator binding and favourably change the rate-determining step.This surface engineering strategy with controllable manner could be used to fabricate other highperformance heterogeneous catalysts.
Conflict of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (21972102),Natural Science Foundation of Jiangsu province (BK20200991),Suzhou Science and Technology Planning Project (SS202016),Jiangsu Laboratory for Biochemical Sensing and Biochip,Jiangsu Key Laboratory for Micro and Nano Heat Fluid Flow Technology and Energy Application,Collaborative Innovation Center of Water Treatment Technology ＆Material.
Appendix A.Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2020.11.009.