Approaching the activity limit of CoSe2 for oxygen evolution via Fe doping and Co vacancy

Electronic structure engineering lies at the heart of efficient catalyst design. Most previous studies, however, utilize only one technique to modulate the electronic structure, and therefore optimal electronic states are hard to be achieved. In this work, we incorporate both Fe dopants and Co vacancies into atomically thin CoSe2 nanobelts for /coxygen evolution catalysis, and the resulted CoSe2-DFe–VCo exhibits much higher catalytic activity than other defect-activated CoSe2 and previously reported FeCo compounds. Deep characterizations and theoretical calculations identify the most active center of Co2 site that is adjacent to the VCo-nearest surface Fe site. Fe doping and Co vacancy synergistically tune the electronic states of Co2 to a near-optimal value, resulting in greatly decreased binding energy of OH* (ΔEOH) without changing ΔEO, and consequently lowering the catalytic overpotential. The proper combination of multiple defect structures is promising to unlock the catalytic power of different catalysts for various electrochemical reactions.

D esign of highly efficient water splitting electrocatalysts for clean hydrogen production is essential for the sustainable development of modern society 1,2 . Electronic structure engineering via incorporating dopants, vacancies, strains, heterostructures, etc. lies at the heart of efficient catalyst design, as it effectively tunes the binding energies of reaction intermediates 3,4 . Most previous studies, however, utilize only one technique to manipulate the electronic structure, and thus the catalytic performance is barely satisfactory 5,6 . For instance, CoSe 2 has moderate catalytic activity toward oxygen evolution reaction (OER), and the doping of Fe has been demonstrated to be an effective approach to improving the performance 7,8 . However, whether the activity could be further enhanced in a synergetic manner by incorporating other defects, such as anion and cation vacancies, has not been reported. We believe that the proper combination of two or more defect structures is essential to achieve near optimal electronic states and ideal intermediate binding energies, which holds the key for the construction of highly efficient water splitting electrocatalysts.
In this work, we seek to fully excavate the catalytic potential of atomically thin CoSe 2 nanobelts for OER by incorporating Fe dopants and Co/Se vacancies. Through both experiments and theoretical calculations, we find that the best catalyst is CoSe 2 -D Fe -V Co and the most active center is the Co 2 site adjacent to the V Co -nearest surface Fe site. Fe doping and Co vacancy work synergistically to optimize the electronic states of Co 2 , and therefore the binding energy of OH* is dramatically decreased and high catalytic activity is achieved. By contrast, Se-derived O vacancy has an obvious impact on the binding energy of O* at Co 2 site, which results in relatively high overpotential and low catalytic activity.

Results
Synthesis and structural characterization. Figure 1a  lamellar intermediates 9 . Fe ions were then incorporated into CoSe 2 /DETA via a wet-impregnation method involving chemical adsorption and cation exchange 10 . After that, atomically thin CoSe 2 -D Fe nanobelts were obtained by liquid-phase exfoliation using low-power ultrasonication. Se vacancies were created on the surface of exfoliated CoSe 2 -D Fe nanobelts through Ar plasma treatment 11 , and Co vacancies were extracted by DETA molecules during the exfoliation of CoSe 2 /DETA under high ultrasonic power 12 . Figure 1b shows the scanning transmission electron microscopy (STEM) image of the CoSe 2 -D Fe -V Co nanobelts. The selected area electron diffraction (SAED) pattern (left inset) reveals the coexistence of cubic (yellow) and orthorhombic (green) phases of CoSe 2 matrix, which is also identified by the Xray diffraction (XRD) patterns ( Supplementary Fig. 1) 13 . The cross-sectional view of the nanobelts (right inset) suggests an atomic thickness of~1.20 nm, which is further confirmed by the atomic force microscopy analysis (Fig. 1c). Energy dispersive Xray spectroscopy (EDS) mapping shows that Co, Se, and Fe dopants are homogeneous distributed throughout the nanobelts (Fig. 1d) (3), E g , and A 1g modes of CoO x , respectively 15 . The created Se and Co vacancies are readily visible via high-angle annular darkfield STEM (HAADF-STEM) imaging ( Fig.1e-g). In addition, the intensity profiles along the selected rectangular regions also suggest the missed surface Se and Co atoms in CoSe 2 -D Fe -V Se and CoSe 2 -D Fe -V Co , respectively.
OER catalytic activity evaluation. To understand how Fe dopants and Co/Se vacancies affect the OER catalytic performance, the prepared catalysts were subjected to systematic electrochemical evaluation via the rotating disk electrode (RDE) method in purified 1 M NaOH (Fig. 2). Cyclic voltammetry (CV) was first conducted to activate the electrodes (Supplementary Fig. 4), and large current densities were recorded during the first few cycles, indicating the instability of CoSe 2 in alkaline solution at anodic potentials 14 . EDS analysis shows that Se signal can hardly be detected after CV test, and SAED pattern indicates the formation of CoOOH phase, which serves as the real host under OER conditions ( Supplementary Fig. 5). The thickness of the nanobelts increases from 1.24 to 1.35 nm after CV activation ( Supplementary Fig. 6), which is resulted from the significantly increased surface roughness due to CV-induced structural reconstruction. The catalytic activities were then assessed by linear sweep voltammetry (LSV, Fig. 2a). The polarization curves were corrected with 95% iR-compensation and then normalized by the electrochemical double-layer capacitance. As shown in Fig. 2b, c, to reach a current density (j) of 2.5 A F −1 , the required overpotentials (η) for CoSe 2 , CoSe 2 -D Fe , CoSe 2 -D Fe -V Se , and CoSe 2 -D Fe -V Co are 385.7, 300.4, 307.3, and 294.2 mV, respectively, and at η = 350 mV, the recorded j are 0.92, 8.73, 7.06, and 12.95 A F −1 , respectively. Therefore, Fe doping, in accordance with previous reports, enhances the OER catalytic performance of CoSe 2 , and further incorporation of Se and Co vacancies leads to slightly decreased and dramatically increased catalytic activity, respectively. It is important to note that the current density achieved by CoSe 2 -D Fe -V Co at η = 350 mV is 30.2% higher than the sum of the current densities of CoSe 2 -D Fe and CoSe 2 -V Co ( Supplementary Fig. 7), indicating a synergistic effect between Fe dopants and Co vacancies. This synergistic effect was further confirmed by changing the sequence of Fe doping and Co vacancy. As shown in Supplementary Fig. 8, the CoSe 2 -V Co -D Fe exhibits higher overpotential and lower current density than CoSe 2 -D Fe -V Co , probably due to that the afterward doping fills the vacancy sites and weakens the role of Co vacancies 16 .
The high catalytic activity of CoSe 2 -D Fe -V Co was further evidenced by analyzing the turnover frequency (TOF, assuming all cations to be catalytically active) and Tafel slope (derived from the polarization curve in low overpotential region). As shown in Fig. 2d, the apparent TOF of CoSe 2 -D Fe -V Co at η = 280 mV reaches 0.045 s −1 , exceeding 0.003, 0.035, and 0.024 s −1 for CoSe 2 , CoSe 2 -D Fe , and CoSe 2 -D Fe -V Se , respectively, indicative of the high intrinsic catalytic activity. In addition, CoSe 2 -D Fe -V Co exhibits a small Tafel slope of 53.5 mV dec −1 , much lower than 72.6, 66.0, and 70.5 mV dec −1 for CoSe 2 , CoSe 2 -D Fe and CoSe 2 -D Fe -V Se , respectively (Fig. 2e), suggesting the remarkably enhanced reaction kinetics 6 . The Nyquist plots at η = 370 mV display typical semicircles for different catalysts (Fig. 2f), corresponding to the charge transfer resistances (R ct ). Using a relevant equivalent circuit (inset), the fitted results indicate the indicating the efficient electron transfer and the fast ion diffusion 8 . As a result, the incorporation of Fe dopants and Co vacancies creates highly active catalytic sites with excellent mass transport properties for OER catalysis.
The catalytic stability was further assessed via chronoamperometry (CA) for 6 h and chronopotentiometry (CP) for 8 h. As shown in Fig. 2h, CA test shows that the current density increases with increasing overpotential. More specifically, . CP test at j = 10 mA cm −2 shows that the overpotential of CoSe 2 suffers an obvious increase of 13.6 mV, while those of CoSe 2 -D Fe , CoSe 2 -D Fe -V Se , and CoSe 2 -D Fe -V Co see slight rises of 8.6, 7.0, and 3.7 mV, respectively, which should be attributed to the enhanced electrical conductivity caused by Fe doping 29 . Generally, the trend in the activity of different catalysts does not change after long-term durability test.
Understanding defect structures before and after catalysis. Prior to the mechanism investigation, it is essential to study the stability of dopants and vacancies and probe the evolution of electronic structures during catalysis as they are directly relevant to the catalytic activity 30 . X-ray photoelectron spectroscopy (XPS) was first performed to understand the primary defects before catalysis. As shown in Fig. 3a, b and Supplementary Fig. 12, Co, Fe, and Se signals are clearly detected, indicating the successful doping of Fe into CoSe 2 matrix, and the Co/Fe 2p spectra show the existence of Co/Fe-O bonds due to oxide impurities. The Co/ Fe 2p 3/2 peaks of Co/Fe-Se and Co/Fe-O in CoSe 2 -D Fe -V Se exhibit negative shifts to lower binding energies, and those in CoSe 2 -D Fe -V Co show positive shifts to higher binding energies, indicating the reduction and oxidation states of metal sites caused by Se/O and Co vacancies, respectively (Fig. 3c) 31 . The defect structures after CA test at η = 370 mV for 1.5 h were then investigated via electron energy loss spectroscopy (EELS). As shown in Fig. 3d, e, both Co and Fe signals can be detected after catalysis, and no significant changes in their atom ratios were found due to fully purified electrolyte (Supplementary Table 1). For better analysis, the L 2,3 -edge spectra of Co and Fe were fitted by multiple Gaussian functions, which represent different oxidation states and coordination environments 32 . Compared with In comparison, extra peaks located at 783.0 and 797.9 eV (orange) in Co L 2,3 -edge spectra, and 712.6 and 724.8 eV (magenta) in Fe L 2,3 -edge spectra of CoSe 2 -D Fe -V Co , which should be ascribed to the local chemical environment of Co vacancies. Therefore, the surface reconstruction induced by OER catalysis has little impact on the defect structures. To provide further evidence, X-ray absorption near-edge structure (XANES) spectra at the L-edges of Co and Fe before and after catalysis were conducted ( Fig. 3f and Supplementary Fig. 13). Obviously, the L 3 -edge centroids of Co before catalysis shift by 0.10 and 0.12 eV to lower and higher energy positions after the incorporation of Se/O and Co vacancies, respectively 33,34 . After catalysis, slight broadenings of the white line peaks were observed due to OER-induced complex coordination environment 35 . Despite the electronic-structure changes,    Fig. 13). As a result, both dopants and vacancies are well preserved during OER catalysis, and they consequently determine the catalytic performance of different catalysts.

Discussion
To provide clear insight into the effects of dopants and vacancies on the catalytic activities, we performed Hubbard-corrected density functional theory (DFT + U) calculations. The crystal structure of CoOOH was utilized to build up the periodical surface models as it serves as the real host under OER conditions ( Supplementary Figs. 4 and 5). The high-index (01-12) facet was selected as the surface termination due to the proved good coincidence between theoretical and experimental results 36 , and all potential active sites on (01-12) facets of models with different atomistic arrangements of dopants and vacancies were considered to gain insight into the catalytic mechanism ( Supplementary  Fig. 14). Figure 4a shows the FeCoOOH-V Co structure and the OER pathway, which involves four proton coupled electron transfer steps and three intermediates of OH*, O*, and OOH* 37 . The corresponding Gibbs free energies (ΔG i , i = OH*, O*, and OOH*) were calculated and provided in Fig. 4b. As can be seen, the catalytic activities of Co sites are restricted by the transformation of OH* to O*, due to the relatively large difference between ΔG O* and ΔG OH* (namely ΔG 2 ). In comparison, the potential-limiting step of Fe sites is assigned to the transformation of O* to OOH*, owing to the relatively large difference between ΔG OOH* and ΔG O* (i.e., ΔG 3 ). Therefore, Co and Fe sites in CoOOH matrix exhibit different catalytic behaviors for OER. The ΔG i of FeCoOOH-V O -Co 1 and FeCoOOH-V Co -Co 1 deviate from the general principle probably due to that they are adjacent to the vacancies and their electronic states are greatly altered. Scaling relations exist between energetics of OH* and OOH* over the above considered metal sites [36][37][38] , which are expressed as ΔG OOH* = ΔG OH* + 2.90 eV (Gibbs free energies, Fig. 4c), ΔE OOH* = ΔE OH* + 2.87 eV (adsorption energies, Supplementary  Fig. 15a), and ΔE OOH = ΔE OH + 1.03 eV (binding energies, Supplementary Fig. 15b) (Supplementary Table 2). Based on the scaling relation, a universal volcano relationship can be constructed by plotting η OER as a function of ΔG O* − ΔG OH* 38 . As shown in Fig. 4d, Fe sites are distributed around the left leg of the volcano which is restricted by the transformation of O* to OOH*, and Co sites are located on the right leg that is determined by the transformation of OH* to O*. Both Fe and Co could serve as OER active centers by analyzing their η OER , which     Fig. 4e, an intense negative peak near the Fermi level was observed in CoOOH. Fe doping causes a small shift of the peak away from the Fermi level and Co vacancy induces the splitting of the peak, both of which lead to slightly decreased binding energy of OH* 42 . Remarkably, the combination of Fe doping and Co vacancy greatly decreases the peak intensity near the Fermi level, and therefore results in dramatically weakened binding with OH*. It is important to note that, the low overpotential is achieved due to that Fe doping and Co vacancy jointly have little impact on the binding energy of O* (dashed ellipse in Fig. 4e). In comparison, even though the combination of Fe doping and O vacancy decreases the binding energy of OH* as well, they have a more obvious influence on the binding energy of O*, which leads to increased overpotential and declined catalytic activity.

FeCoOOH-V O-Co1
FeCoOOH-VO-Co2 FeCoOOH-VCo-Co2 in the green region, and the incorporation of Fe doping and Co vacancy individually moves the point to the green-yellow boundary. FeCoOOH-V Co -Co 2 , sitting close to the center of the red region, possesses near-optimal intermediate binding energies, and near-minimum catalytic overpotential that different systems can research in this study. Therefore, Fe doping and Co vacancy work synergistically to approach the activity limit of Co-based catalysts. This contour map provides valuable guidance for the design of efficient OER catalysts via modulating the electronic structures and intermediate binding energies.
In conclusion, different defect structures were incorporated into atomically thin CoSe 2 nanobelts for OER catalysis. We found that the combination of Fe doping and Co vacancy synergistically optimize the electronic states and intermediate binding energies at Co 2 site, which is responsible for the dramatically enhanced catalytic activity. This electronic-structure modulation strategy with proper combination of two or more defect structures could efficiently unlock the catalytic power, showing great promise for the rational design of advanced catalysts for various electrochemical reactions.

Methods
Material synthesis. Atomically thin CoSe 2 nanobelts were synthesized via a lamellar intermediate-assisted exfoliation approach. In detail, 14.0 ml DETA was slowly poured into 7.0 ml H 2 O under stirring as severe exothermic reaction occurs. Then, 0.125 g Co(Ac) 2 ·4H 2 O and 0.130 g Na 2 SeO 3 were added and stirred until completely dissolved. After that, hydrothermal reaction was performed at 160°C for 10 h, and CoSe 2 /DETA lamellar intermediates were finally obtained after reaction. Characterizations. The morphology of nanobelts was observed via STEM (JEOL JEM-ARM200F), and the composition elements were identified by atomicresolution elemental maps via EDS (Oxford XMax100TLE EDS spectrometer). The element contents before catalysis and after CA test at η = 370 mV for 1.5 h were determined by an Optima 2000 DV ICP-emission spectrometer. SAED, XRD (MMA, GBC Scientific Equipment LLC, Hampshire, IL, USA) and Raman scattering (Renishaw 100, 632.8 nm He-Ne laser) were employed to determine the phase structure. The missing lattice atoms were observed by high-resolution HAADF-STEM and analyzed via intensity profile. The electronic states of Co and Fe before catalysis were studied by XPS (PHOIBOS 100 Analyzer from SPECS, Berlin, Germany; Al K α X-rays), and those after CA test at η = 370 mV for 1.5 h were investigated by EELS (Gatan GIF Quantum ER EELS spectrometer). The EELS characterization was performed at liquid nitrogen temperature to avoid sample damaging and electronic-state changes, and the spectrometer was calibrated against the zero-loss peak at the start of each session. The deconvolution and curve-fitting of the spectra were performed by employing nonlinear least squares fitting tools written in DigitalMicrograph software. The coordination environments of Co and Fe before and after catalysis (CA test at η = 320 mV for 0.5 h) were also studied by XANES (National Synchrotron Radiation Laboratory (NSRL), Hefei, China). The Co and Fe L-edges XANES spectra of catalysts were measured at the photoemission end-station at beamline BL10B in the NSRL, Hefei, China. A bending magnet is connected to the beamline, which is equipped with three gratings covering photon energies from 100 to 1000 eV. The samples were kept in the total-electron-yield mode under an ultrahigh vacuum at 5 × 10 −10 mbar. The resolving power of the grating was typically E/ΔE = 1000, and the photon flux was 1 × 10 10 photons per s. Spectra were collected at energies from 770 to 814 eV for Co and 695 to 740 eV for Fe in 0.2 eV energy steps. The XANES raw data were normalized by a procedure consisting of several steps.
Electrochemical measurements. The catalytic performances were evaluated via the RDE method with continuous rotation at 1200 rpm (CHI 760, Shanghai Chenhua Instruments Co. Ltd.), where O 2 -saturated 1 M NaOH, graphite rod and Hg/HgO electrode were utilized as the electrolyte, counter electrode and reference electrode, respectively. The working electrode was prepared by placing 12 μl of catalyst suspension onto glassy carbon electrode which was dried in a fume cupboard. The catalyst ink was prepared by suspending 2 mg catalyst in 1 ml water/ isopropanol/Nafion® mixed solution (3:1:0.2, v/v) under ultrasonication. Before catalytic evaluation, the electrolyte was fully purified with the working electrode via CV cycles between 0.30 and 0.75 V vs. Hg/HgO at 10 mV s −1 until a stable response was achieved. New working electrodes were then activated by six CV cycles, during which CoSe 2 was fully converted into CoOOH. LSV was then recorded within the voltage range of 0.30-1.00 V vs. Hg/HgO at 10 mV s −1 , and the polarization curves were corrected with 95% iR-compensation and then normalized by electrochemical double-layer capacitance that was estimated from the slope of the difference between anodic and cathodic current densities at 0.35 V vs. Hg/HgO against the scan rate (10,20,30,40,50, and 60 mV s −1 ). After that, the electrodes were stabilized at 0.70 V vs. Hg/HgO for 3 min, and then electrochemical impedance spectroscopy was performed at the same potential with an amplitude of 10 mV and a frequency range of 200 kHz-100 mHz. The catalytic durability was finally assessed by CA test at stepwise potentials of 0.70, 0.75, 0.80, and 0.85 V vs. Hg/HgO for 6 h, and CP test at a constant current density of 10 mA cm −2 for 8 h.
Overpotentials were determined by η OER = E Hg/HgO + (0.098 + 0.059 pH − 1.23) V, where E Hg/HgO is the recorded potential vs. Hg/HgO. TOF values were calculated based on TOF = jS/4Fn, where j (A cm −2 ) is the current density at η = 280 mV, S (cm 2 ) is the surface area of glassy carbon electrode, F is the Faraday constant (96,485 C mol −1 ), and n is the number of moles of the cations assuming all of them are catalytically active. Tafel plots were derived from the polarization curves in low overpotential regions by plotting overpotential against log(current density). The error bars of reported overpotentials, current densities and TOFs were obtained from five replicates after discarding the maximum and minimum values.
Computational methods. In this work, the crystal structure of CoOOH was chosen to build up the periodical surfaces including Fe doping and Co/O vacancy models. The high-index (01-12) facet was adopted as the surface termination 36,37 , as it exhibits theoretical activities in good agreement with the experimental results. The periodic surface was 14.79 Ǻ × 15.22 Ǻ with a vacuum slab of 18 Å in thickness to separate the layer from its periodic images. All the calculations were performed by spin-polarized plane-wave DFT, as implemented in the CASTEP program 43 . The geometry optimizations were performed using the BFGS algorithm. The Perdew-Burke-Ernzerhof exchange-correlation functional within the generalized gradient approximation as well as ultrasoft pseudopotentials was selected 44,45 . The cut-off energy for the plane-wave basis and the Brilliouin zone k-points were chosen as 340.0 eV and 0.04 Ǻ −1 spacing in the Monkhorst-Pack scheme 46 , respectively, which causes little difference in the calculation results when further increase. To sufficiently consider the on-site Columbic repulsion between the d electrons, the Hubbard U corrections were applied to transition metal d-electrons and the values of U-J parameters for Co (3.42) and Fe (3.29) atoms were taken from the reference 47 . The convergence criteria for the total energy, forces, stress, atomic displacement, and self-consistent field iterations were set to 1 × 10 −5 eV atom −1 , 3 × 10 −2 eV Å −1 , 5 × 10 −2 GPa, 1 × 10 −3 Å, and 1 × 10 −6 eV atom −1 , respectively.
The OER reaction steps and Gibbs free energy changes can be expressed by 37 H 2 Oþ* ! OH*þe À þH þ ΔG 1 ¼ ΔG OH* ÀeUþk B Tln10 pH ; ð1Þ OOH* ! * þ O 2 þ e À þ H þ ΔG 4 ¼ 4:92 eV À ΔG OOH* À eU þ k B Tln10 pH ; ð4Þ where * represents an active site on (01-12) facet, and ΔG i (i = OH*, O*, and OOH*) are Gibbs free energies of OER intermediates. The theoretical overpotential under standard conditions (T = 298.15 K, p = 1 bar, pH = 0) is then given by 37 The Gibbs free energies, ΔG i , were determined by the adsorption energies combined with corrections for zero-point energy and entropy, according to ΔG i = ΔE i + ΔZPE i − TΔS i . The ZPE and TS were calculated using DFT calculations of vibrational frequencies and using standard tables for gas-phase molecules 48   The binding energies, ΔE j (j = OH, O, and OOH), were calculated directly by ΔE j = E j* − E j − E * , where * represents an active site on (01-12) facet, and E is the total energy calculated by using the spin polarization DFT method.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.