Electrocatalysts for High-Efficiency Oxygen Evolution Reaction

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Layered Fe-Substituted LiNiO Electrocatalysts for High-Efficiency Oxygen Evolution Reaction Kaiyue Zhu, Tao Wu, Yue Zhu, Xuning Li, Mingrun Li, RuiFeng Lu, Junhu Wang, Xuefeng Zhu, and Weishen Yang ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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Layered Fe-Substituted LiNiO2 Electrocatalysts for High-Efficiency Oxygen Evolution Reaction Kaiyue Zhu,†,‡ Tao Wu,# Yue Zhu,†,‡ Xuning Li,‡,§ Mingrun Li,† Ruifeng Lu,# Junhu Wang,§ Xuefeng Zhu,*,† and Weishen Yang † †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P R China.

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University of Chinese Academy of Sciences, Beijing, 100049, P R China

#

Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, 210094, P R China.

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Mössbauer Effect Data Center & Laboratory of Catalysts and New Materials for Aerospace, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P R China

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ABSTRACT: LiNi1-xFexO2 (0 ≤ x ≤ 0.3) and LiyNi0.8Fe0.2O2 (0.8 ≤ y ≤ 1.2) catalysts for OER were systematically investigated to discovery the influence of the composition and layered structure on electrochemical activity. LiNi0.8Fe0.2O2 exhibits better OER activity than LiNiO2 and other Fe-substituted LiNiO2 catalysts, while Li1.2Ni0.8Fe0.2O2 shows much higher OER activity than LiNi0.8Fe0.2O2 and Li0.8Ni0.8Fe0.2O2. The best OER activity is achieved on Li1.2Ni0.8Fe0.2O2 with a Tafel slope of 59 mV dec-1 and a current density of 10 mA cm-2 at an overpotential of 302 mV, better than that for the benchmark IrO2 catalyst. Combined with the density of functional theory (DFT) calculations, the enhanced OER activity is mainly attributed to the unique electronic structure derived from the interaction of Li, Ni and Fe in the materials, and the layered structure which played an important role in stabilizing the high valence states of Ni and Fe during OER.

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Oxygen evolution reaction (OER) is a key for many energy storage and conversion devices, including electrolysis cells for hydrogen production and rechargeable metal-air batteries for electric energy storage.1-9 However, OER proceeds via a multistep proton-coupled electron transfer process, thus is kinetically sluggish.9-12 Precious metal oxides of IrO2 and RuO2, which are thought to be the most active OER electrocatalysts, have been used to catalyze OER and improve the efficiency of the energy conversion devices.12-14 However, the two precious metal oxides are not appropriate for large-scale applications considering their high cost and low reserves. Therefore, it is imperative to develop alternative OER electrocatalysts based on low cost and earth-abundant elements. Layered transitional metal (Ni, Fe, Co) hydroxides, consisting of metal-hydroxyl host slabs and charge-balancing ions in the interlayer galleries, have attracted much attention of researchers because of their special redox character and good accessibility for the reaction species.15-18 Layered cobalt-iron oxyhydroxides exhibit promising OER activities. It was reported that layered CoOOH can provide a conductive, chemically stable host for Fe, and there are strong electronic interactions between Co and Fe.19 Chen et al. found Fe4+ species in the NiFe hydroxide catalyst, but not in Fe hydroxide during the steady-state water oxidation using an operando Mössbauer spectroscopy, and suggested the important stabilizing effect on Fe4+ species by the layered NiOOH lattice and the synergistic roles of Ni and Fe during OER.20 Trotochaud et al. also found a partial-charge-transfer activation between Ni and Fe in the nickel-iron oxyhydroxide with a layered structure.21 Besides, Ni4+ and/or Fe4+ are demonstrated to be present in (NiFe)OOH at OER potentials by using X-ray absorption spectroscopy and Mössbauer spectroscopy.22,23 The previous investigations indicate that Ni4+ and/or Fe4+ can be stabilized in the layered hydroxides,


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and synergistic effects between Ni and Fe are favorable for OER. Thus, materials with structures that can stabilize the ions (Ni4+, Fe4+) at high valence state may be promising catalysts for OER. LiNiO2 is a layered structure consisting of NiO2 slabs made of edge-sharing NiO6 octahedrons and lithium ions inserting the slabs at the octahedral sites, similar to the structure of hydroxides and oxyhydroxides.24-26 In this layered LiNiO2 material, Ni3+ ions are in the low-spin configuration (d7 t2g6eg1), and they are correlated with high OER activity in perovskite oxides from theoretical predication that a near-unity occupancy of the eg orbital of surface transition metal ions can enhance the intrinsic OER activity of perovskite transition metal oxides.27,28 Although the layered structure of LiNiO2 can stabilize significant amounts of Ni3+ species, the OER activity of LiNiO2 is still low. Doping is an efficient strategy to tailor the electronic structure of transitional metal ions in oxides. In the realm of basic electrochemistry, studies of the redox processes in Fe-substituted LiNiO2 have shown that iron ions can be partially oxidized to tetravalent state forming an apparent valence state of (3+δ), which results from rapid hole hopping between Fe3+ and Fe4+ ions or Fe3+ and Ni4+ ions.28-31 Considering the excellent OER activity of (Ni,Fe)OOH, we hypothesize that Fe-substituted LiNiO2 would show a promising OER activity due to partial-charge-transfer activation between Ni and Fe ions and the structural similarity between LiNi1-xFexO2 and (Ni,Fe)OOH.32,33 Herein, a series of LiNi1-xFexO2 with different Fe doping amounts and LiyNi0.8Fe0.2O2 with different Li contents were prepared to investigate the influence of the composition and layered structure on electrochemical activity toward OER. Combined with XRD and Mӧssbauer data, we found that the contents of Ni and Fe ions in the Li layer increase with the increase of Fe doping amount in LiNi1-xFexO2 (0 ≤ x ≤ 0.3), and the best OER activity is achieved on LiNi0.8Fe0.2O2 with a Tafel slope of 59 mV dec-1 and an overpotential of 330 mV at a current density of 10 mA


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cm-2, better than that of the IrO2 benchmark catalyst. Besides, the ordering of the layered structure can be tuned by changing Li content, and Li1.2Ni0.8Fe0.2O2 with the most ordered layered structure shows much higher OER activity than LiNi0.8Fe0.2O2 and Li0.8Ni0.8Fe0.2O2. Combined with the DFT calculations, the high OER activity is related to the layered structure and the interaction of Li, Ni and Fe in the materials.

Figure 1. Rietveld refined powder XRD patterns of (a) LiNiO2, (b) LiNi0.9Fe0.1O2, (c) LiNi0.8Fe0.2O2 and (d) LiNi0.7Fe0.3O2 with experimental data in red dots, the refined patterns in black line, tick marks indicating allowed Bragg reflections in olive, and the difference curve in blue. (e) Theoretical XRD patterns of LiNiO2 with different z (z is the percent of Ni mixed in the Li layer) calculated by Materials Studio 5.5 program. (f) SEM image and (g) HRTEM image of LiNi0.8Fe0.2O2 sample. The inserted figure in (g) is the SAED image of HRTEM along the [42-1] zone axis and the indexed points refer to the lattice planes of (012) and (104).

Figure 1 shows the Rietveld refined XRD patterns of the LiNi1-xFexO2 (0 ≤ x ≤ 0.3) samples prepared by a combined ethylene diamine tetraacetic acid (EDTA)-citrate acid method; the observed lattice parameters are summarized in Table S2. All the diffraction peaks of LiNi1xFexO2

(0 ≤ x ≤ 0.3) samples are consistent with the standard pattern of layered structure

consisting of [MO6] (M=Ni,Fe) octahedral layers with Li atoms lying between the octahedral


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layers.17,30 Compared to the parent LiNiO2 (I003/I104=1.4), an increase in the intensity ratio of the (003) peak to (104) peak for LiNi0.9Fe0.1O2 (I003/I104=1.5) indicates the better ordering of Li+ and Ni3+ in the Li-rich layer and Ni-rich layer, respectively. However, the intensity ratio of (003) peak to (104) peak decreases from 1.5 to 0.6 with the increase of the Fe doping amount from 0.1 to 0.3, indicating the layered structure becomes more and more disordered.26,42 The change also is confirmed by the theoretical XRD patterns of LiNiO2 with different amounts of Ni in the Li layers. As shown in Figure 1e, for LiNiO2, the intensity ratio of (003) peak to (104) peak decreases significantly with the increase of the content of Ni in Li layers. Therefore, it is inferred that the disordered layered structure is resulting from the site-exchange between lithium atoms and Ni/Fe atoms. The granular morphology of LiNi1-xFexO2 (0 ≤ x ≤ 0.3) samples (Figure 1f and S1) was characterized by scanning electron microscopy (SEM). And, there is no significant difference in the Brunauer-Emmett-Teller (BET) specific areas between these materials (Table S3). To investigate the phase structure of LiNi1-xFexO2 (0 ≤ x ≤ 0.3), the HRTEM and the selected-area electron-diffraction (SAED) patterns along [42-1] zone axes of LiNi0.8Fe0.2O2 (as a representative) were collected on a sample thinned by a focused ion beam (FIB) sampling. The HRTEM and the SAED patterns (Figure 1g) reveal that LiNi0.8Fe0.2O2 has a hexagonal structure with a space group of R3ത m, which is consistent with the results from Rietveld refined XRD. From the room temperature 57Fe Mӧssbauer spectra of the as-prepared samples (Figure 2a and Table S4), the symmetric single quadrupole doublet shows that Fe3+ ions only present in the high spin state.31,43 Besides, the spectra indicate that the full width at half maximum (FWHM) of the Mӧssbauer line increases with the increase of iron amount. In layered structures, this phenomenon can be explained by the existence of a distribution of local environments around the


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iron ions leading to a distribution of quadrupole splittings. Figure 2b gives the relative populations of iron

Figure 2. Structure and composition characterization. (a) Room temperature

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Fe Mӧssbauer spectra, (b)

corresponding quadrupole splitting distributions, (c) Fe 2p XPS spectra and (d) Ni 2p XPS spectra of LiNi1xFexO2

with different Fe contents.

ions for each value of quadrupole splitting. In all cases, the distributions of quadrupole splitting reflect the characteristic of unusual iron ion environments in the structure.42,43 For LiNi1-xFexO2 (0.1 ≤ x ≤ 0.3), the distribution of quadrupole splitting tends to widen as Fe content increases, i.e. the loss of the lamellar characteristic. Combined with the XRD results, that the intensity ratio of (003) peak to (104) peak decreases with the increase of Fe doping amount from x=0.1 to 0.3, and the results of

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Fe Mӧssbauer spectra, the disordering of the layered structure results from the

increase in the amount of Ni and Fe ions in the lithium layers. As shown in Figure 2c and Table S5, the core-level binding energy spectra of Fe 2p are similar and the valence state of Fe is only +3 for all LiNi1-xFexO2 (0.1 ≤ x ≤ 0.3) samples.44,45 However, the electronic structure of the


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surface Ni ions changes greatly due to the Fe doping, as indicated by the surface-sensitive XPS spectra shown in Figure 2d and Table S6. Compared to that of LiNiO2, the higher binding energy of the main peak in Ni 2p spectra of LiNi1-xFexO2 (0.1 ≤ x ≤ 0.3) indicates a partial oxidation of Ni2+ to Ni3+ in catalyst surfaces.45-47 The Ni2+ should be situated in the lithium layer owing to steric considerations (the radii of Li+, Ni2+ and Ni3+ are 0.76, 0.68 and 0.50 Å, respectively).25 Ni 2p XPS spectra of LiNiO2 and LiNi0.9Fe0.1O2 catalysts show a minor presence of Ni2+ along with the major Ni3+, while Ni 2p XPS spectra of the LiNi0.8Fe0.2O2 and LiNi0.7Fe0.3O2 could be fitted with only one chemical state of Ni, i.e. +3, indicating that the doping of Fe is prone to keeping Ni in its higher oxidation state.45

Figure 3. OER performance of LiNi1-xFexO2 catalysts with various Fe contents. (a) Linear sweeping voltammograms for LiNi1-xFexO2 (0 ≤ x ≤ 0.3) catalysts and the commercial IrO2 as the benchmark, (b) overpotentials at a current density of 10 mA cm-2, (c) OER activity normalized by BET surface area (SA) and electric double layer capacitance (EA) of LiNi0.8Fe0.2O2 and IrO2 at an overpotential of 350 mV, (d) Plots of charging current density differences ∆j vs the scan rate for LiNi0.8Fe0.2O2 and IrO2, (e) Tafel plots of LiNi1-


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xFexO2

(0 ≤ x ≤ 0.3) catalysts and the commercial IrO2, (f) EIS at 1.687 V (versus RHE) of the LiNi1-xFexO2

with different Fe contents.

The OER activity of the LiNi1-xFexO2 (0 ≤ x ≤ 0.3) catalysts with a loading amount of 0.25 mg cm-2 was evaluated on a rotating disk electrode in an O2-saturated 0.1 M KOH solution in a typical three electrode system. For comparison purpose, the commercial IrO2, which was identified as the most active OER catalyst, was tested under the same condition. The BET area and SEM image of IrO2 were shown in Figure S2. The potential of reversible hydrogen electrode (RHE) was calibrated using a platinum working electrode in an H2-saturated solution and the electrolyte resistance in the O2-saturated 0.1 M KOH solution was measured by AC impedance spectroscopy to eliminate iR drops before each test. From the linear sweeping voltammograms (LSVs) shown in Figure 3a, we observed that the overpotential at the same OER current density decreased with increasing Fe content to 20% and then increased sharply upon further increase in the Fe content beyond 20%. The LiNi0.8Fe0.2O2 shows the highest activity among LiNi1-xFexO2 (0 ≤ x ≤ 0.3) and is better than the benchmark IrO2 in OER process. To reach a current density of 10 mA cm-2 (on a basis of 10% solar-to-fuel conversion efficiency), the required overpotential for LiNi0.8Fe0.2O2 is 330 mV, which is 97 mV, 24 mV, 113 mV and 11 mV smaller than that for LiNiO2, LiNi0.9Fe0.1O2, LiNi0.7Fe0.3O2, and IrO2, respectively (Figure 3b). Moreover, under an overpotential of 350 mV, the OER current density of LiNi0.8Fe0.2O2 reaches 18.1 mA cm-2, about 1.5 times that of IrO2 (11.9 mA cm-2). SA (normalized to real oxide surface area as estimated from BET measurements) of the LiNi0.8Fe0.2O2 and the benchmark IrO2 catalyst were also measured under an overpotential of 350 mV, the SA of LiNi0.8Fe0.2O2 is 2.5 mA cm-2, 25 times that of IrO2 (0.1 mA cm-2). Besides, the effective surface areas of the LiNi0.8Fe0.2O2 and IrO2 were estimated by measuring the capacitance of the electric double layer with cyclic


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voltammetry. As shown in Figure 3d, the capacitance of IrO2 was measured to be 11 mF cm-2, while that of LiNi0.8Fe0.2O2 was only 2.4 mF cm-2. The current density normalized by electrochemical active surface area (EA) for LiNi0.8Fe0.2O2 is 7.5 mA mF-1, about 7 times that of IrO2 (1.1 mA mF-1). All the results reveal that LiNi0.8Fe0.2O2 has the superior OER activity than IrO2. The Tafel slope, which describes the influence of potential by current density, is an important factor to evaluate the OER kinetics. As shown in Figure 3e, the Tafel slope for LiNi0.8Fe0.2O2 (59 mV dec-1) is much smaller than that for LiNiO2 (82 mV dec-1), LiNi0.9Fe0.1O2 (63 mV dec-1), LiNi0.7Fe0.3O2 (96 mV dec-1) and the benchmark IrO2 (86 mV dec-1), implying LiNi0.8Fe0.2O2 shows much faster reaction kinetics than others. Moreover, LiNi0.8Fe0.2O2 possesses the best charge transfer ability among all the catalyst. The electrochemical impedance spectroscopy (EIS) analysis at a potential of 1.687 V vs RHE was tested to study the OER kinetics of catalysts, as shown in Figure 3f. The semicircle is related to the charge-transfer resistance (Rct). Doping Fe in the LiNiO2 catalyst can significantly reduce the Rct. A minimum Rct was achieved on LiNi0.8Fe0.2O2, while the order of Rct for LiNi1-xFexO2 (0 ≤ x ≤ 0.3) catalysts coincides with the order of OER activity. Besides, the stability of LiNi0.8Fe0.2O2 was evaluated by chronopotentiometry shown in Figure S3. Under a galvanostatic current density of 10 mA cm2

, LiNi0.8Fe0.2O2 displays a nearly constant overpotential of 0.34 V for 3h, whereas the potential

of IrO2 increased distinctly, implying better durability of LiNi0.8Fe0.2O2. But, there is a gradual decay of catalytic activity after 3h, which may be attributed to the damage of LiNi0.8Fe0.2O2 under the long-term oxygen evolution potential. As noted above, LiNi0.8Fe0.2O2 with moderate content of Fe in LiNiO2 possesses the best OER activity among the LiNi1-xFexO2 (0 ≤ x ≤ 0.3) catalysts. The remarkable enhancement of OER performance may be attributed to the following factors associated with the synergistic interplay


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of Ni and Fe. Firstly, the Fe doping modified the electronic structure of the surface Ni ions (Figure 2d), and the octahedral surroundings of the iron ions become more asymmetric with increasing Fe content as indicated by a large increase in the distribution of quadrupole splitting (Figure 2a and b). The phenomenon that interaction between Ni and Fe in (Ni,Fe)OOH seems to occur in our LiNi1-xFexO2 (0 ≤ x ≤ 0.3) system because of their similar structures.21 That is, the synergistic electronic effects between the Ni and Fe can modify the OER activity. Besides, according to previous studies, the Fe4+ and Ni4+ in LiNi1-xFexO2 can be stabilized by the layered structure during

Figure 4. (a) XRD patterns, (b) an expanded XRD patterns over 21-32.5º of LiyNi0.8Fe0.2O2 with different Li content. (c) Ni 2p XPS spectra and (d) OER performance of LiNi0.8Fe0.2O2 prepared at low oxygen pressure and LixNi0.8Fe0.2O2 with different Li content. The standard XRD pattern of Li2CO3 (JCPDS No. 22-1141) is shown in violet lines at the bottom of (b) as a reference. The inserted figure in (d) is Tafel plot of Li1.2Ni0.8Fe0.2O2 sample.

OER.29,42 It has been widely accepted that high-valence transitional cations (Ni4+, Fe4+, Co4+) are crucial to enable OER because they facilitate the adsorption and reaction of OH- with the catalysts to form adsorbed –OOH species.27,48-50 Secondly, for LiNi1-xFexO2 (0 ≤ x ≤ 0.3) and


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LiyNi0.8Fe0.2O2 (0.8 ≤ y ≤ 1.2), the ordering of the layered structure is effected by Fe doping and lithium content. For LiNi1-xFexO2 (0 ≤ x ≤ 0.3), pure phases crystallize in the rhombohedral system with the space group of R3ത m. For LiNiO2, LiNi0.9Fe0.1O2 and LiNi0.8Fe0.2O2, the layered structure is ordered because the (003) peak is the strongest peak. However, the strongest peak turns to (104) peak for LiNi0.7Fe0.3O2, indicating more transitional metal ions entering the Li layers. From XRD shown in Figure 4a and Rietveld refined powder XRD patterns shown in Figure S4 of LiyNi0.8Fe0.2O2, the intensity ratio of (003) peak to (104) peak decreases with the decrease of Li content, indicating more transitional metal ions entering in the Li layers results in a more disordering of the layered structure. The results can be further confirmed by Mӧssbauer results (Figure S5). The higher intensity ratio of (003) peak to (104) peak of Li1.2Ni0.8Fe0.2O2 (I003/I104=1.6) compared to that of LiNi0.8Fe0.2O2 (I003/I104=1.3) indicates the content of Li in the LiNi0.8Fe0.2O2 is insufficient resulting from the evaporation of Li at high temperatures, while the content of Li in the Li1.2Ni0.8Fe0.2O2 is excess because there are Li2CO3 impurites in Li1.2Ni0.8Fe0.2O2 but no Li2CO3 impurites in Li0.8Ni0.8Fe0.2O2 and LiNi0.8Fe0.2O2 from Figure 4b and S4. Compared to Li0.8Ni0.8Fe0.2O2, the binding energy of the main peak in Ni 2p for LiNi0.8Fe0.2O2 becomes higher (shown in Figure 5c), indicating a partial oxidation of surface Ni2+ to Ni3+ with the increase of Li content. The fitting result of the Ni 2p XPS spectra of Li1.2Ni0.8Fe0.2O2 demonstrates the only existence of Ni3+ as that of LiNi0.8Fe0.2O2. Thus, the valence state of Ni is affected by the lithium content, i.e. the ordering of the layered structure influences the electronic structures of transitional metal ions. From SEM (Figure S6 and S7), Li0.8Ni0.8Fe0.2O2 and Li1.2Ni0.8Fe0.2O2 have the granular morphology similar to the LiNi0.8Fe0.2O2 sample, and there is no significant difference in BET specific areas between these samples (Table S3). Therefore, the OER performance is related to the ordering of the layered structure.


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Although the inactive Li2CO3 impurity appears in Li1.2Ni0.8Fe0.2O2 (Figure S8) sample, the best OER activity was achieved by Li1.2Ni0.8Fe0.2O2 with a Tafel slope of 59 mV dec-1 and a current density of 10 mA cm-2 at an overpotential of 302 mV (Figure 4d). Besides, we prepared a nonlayered LiNi0.8Fe0.2O2 sample crystallizing in a cubic structure with Li, Ni and Fe alternating in the basal planes (Figure 4a and S9). The importance of the layered structure is further confirmed by the fact that the layered LiNi0.8Fe0.2O2 exhibits much higher OER activity than the nonlayered sample shown in Figure 4d. Previous studies have shown that the Ni3+ and Fe3+ can be oxidized to Ni4+ and Fe4+ by de-intercalating Li ions from the Li layer, while the transitional metal ions in the Li layer inhibit Li ions migration.30,42,51 Thus, the layered structure is not only prerequisite of Li de-intercalation, but is also important to stabilize the unusual valence of Ni4+ and Fe4+. In general, it has been proposed that the oxygen binding energy on the surface is closely related to the OER activity.13,52,53 We conducted a series of DFT calculations to investigate the oxygen binding energies on the surface of layered LiNi0.8Fe0.2O2 and non-layered LiNi0.8Fe0.2O2 (cubic structure), shown in Figure 5. Calculations of oxygen binding energies for layered LiNi0.8Fe0.2O2 were performed on the natural (0001) surface which is the most frequently exposed surface of the layer structure,39 while dominated (100) surface was selected to calculated for cubic LiNi0.8Fe0.2O2 because the (100) surface energy( 0.22 eV Å-2) is much lower than that of (111) surface (0.47 eV Å-2). In Figure 5, we can see that the exposed O atoms on the (100) surface of cubic LiNi0.8Fe0.2O2 are 5-fold coordinate while the O atoms on the (0001) surface of layered LiNi0.8Fe0.2O2 are 3-fold coordinated leading to the lower oxygen binding energy. From DFT calculation, the average oxygen binding energies in the (0001) surface of layered LiNi0.8Fe0.2O2


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Figure 5. The chosen (2×2) slab models for layered and cubic LiNi0.8Fe0.2O2. (a) Side view and (b) top view of the layered (0001) surface with different O sites shown on the top of surface. (c) Side view and (d) top view of the cubic (100) surface with different O sites shown on the top of surface. The Li, Ni, Fe, O are depicted with violet, royal blue, green and red solid spheres, respectively. Calculated density of states (DOS) of (e) layered LiNi0.8Fe0.2O2 and (f) cubic LiNi0.8Fe0.2O2. The black dotted line represents the Fermi level, meanwhile the orange shaded areas highlight the DOS contribution around the Fermi level.

(5.45 eV) is much lower than that in (100) surface of cubic LiNi0.8Fe0.2O2 (5.92 eV), indicating layered LiNi0.8Fe0.2O2 is favorable for the generation of oxygen vacancies and oxygen desorption. The results further confirm the better OER activity of layered LiNi0.8Fe0.2O2, since the OER activity is closely related to the oxygen binding energy. The density of states (DOS) of layered and cubic LiNi0.8Fe0.2O2 were calculated using density functional theory (DFT). The total density of states (DOS) reveal that the electrons near Fermi level between layered and cubic LiNi0.8Fe0.2O2 are obviously different (shown in Figure 5e and f). Besides, the partial DOS of Li, Ni, Fe, and O in layer LiNi0.8Fe0.2O2 are quite different from those in cubic LiNi0.8Fe0.2O2, in consistent with the XPS spectra shown in Figure 2c, 2d and 4c. These results are strong evidences of that the ordering of the layered structure influences the electronic structures of transitional metal ions, further affecting the OER activity. Apart from that, DFT calculation found the average oxygen binding energy of layered LiNi0.8Fe0.2O2 (5.45 eV) is higher than that


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of layered Li1.2Ni0.8Fe0.2O2 (4.27 eV). Thus, doping Li in the surface of layered LiNi0.8Fe0.2O2 can lower the average oxygen binding energy in the surface, and make it much easier to generate oxygen vacancies and desorb oxygen, thus improving the OER activity. Although it is hard to determine whether Fe or Ni is the active site of OER in LiNi1-xFexO2 oxides, the reasons why LiNi0.8Fe0.2O2 possesses the best OER activity among the LiNi1-xFexO2 (0 ≤ x ≤ 0.3) catalysts can be mainly attributed to the electronic interaction between Li, Ni and Fe in the materials and the ordering of the layered structure based on above analysis. In summary, we prepared LiNi1-xFexO2 (0 ≤ x ≤ 0.3) and LiyNi0.8Fe0.2O2 (0.8 ≤ y ≤ 1.2) catalysts for OER and systematically investigated the influence of the composition and layered structure on electrochemical activity. LiNi0.8Fe0.2O2 exhibits the best OER activity among all the LiNi1-xFexO2 (0 ≤ x ≤ 0.3) samples, while Li1.2Ni0.8Fe0.2O2 shows higher OER activity than LiNi0.8Fe0.2O2 and Li0.8Ni0.8Fe0.2O2. The best OER activity was achieved on Li1.2Ni0.8Fe0.2O2 with a Tafel slope of 59 mV dec-1 and a current density of 10 mA cm-2 at an overpotential of 302 mV, superior to that for the benchmark IrO2 catalyst. The high OER activity appears to be mainly attributed to the interaction between Li, Ni and Fe in the materials, and the layered structure used to stabilize the high valence states of Ni and Fe. The importance of the layered structure is further demenstrated by the DFT calculations and the fact that that layered LiNi0.8Fe0.2O2 exhibits much higher OER activity than cubic LiNi0.8Fe0.2O2. This work not only provides a high active noble-metal-free catalyst for OER, but also gives guidance for designing earth-abundant and high-performance electrocatalysts for OER. ASSOCIATED CONTENT Supporting Information Electrochemical data, SEM, HRTEM, BET, XPS and Mössbauer spectra.


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AUTHOR INFORMATION Corresponding Author [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT All the authors thank the financial support from National Natural Science Foundation of China (21476225, U1508203 and 91545202), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020400), Youth Innovation Promotion Association of the Chinese Academy of Sciences and Dalian Outstanding Young Talents in Science & Technology (2016RJ07). REFERENCES (1) Suen, N. T.; Hung, S. F.; Quan, Q.; Zhang, N.; Xu, Y. J.; Chen, H. M. Electrocatalysis for the Oxygen Evolution Reaction: Recent Development and Future Perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (2) Zhu, J.; Xiao, M.; Zhang, Y.; Jin, Z.; Peng, Z.; Liu, C.; Chen, S.; Ge, J.; Xing, W. MetalOrganic Framework-Induced Synthesis of Ultrasmall Encased NiFe Nanoparticles Coupling with Graphene as an Efficient Oxygen Electrode for a Rechargeable Zn-Air Battery. ACS Catal. 2016, 6, 6335-6342. (3) Wang, H.; Lee, H. W.; Deng, Y.; Lu, Z.; Hsu, P. C.; Liu, Y.; Lin, D.; Cui, Y. Bifunctional Non-Noble Metal Oxide Nanoparticle Electrocatalysts through Lithium-Induced Conversion for Overall Water Splitting. Nat. Commun. 2015, 6, 7261-7268.


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