Surface Termination and Composition Control of Activity of the CoxNi1

Research Article Cite This: ACS Catal. 2018, 8, 11773−11782

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Surface Termination and Composition Control of Activity of the CoxNi1−xFe2O4(001) Surface for Water Oxidation: Insights from DFT+U Calculations Hamidreza Hajiyani and Rossitza Pentcheva*

ACS Catal. Downloaded from pubs.acs.org by CALIFORNIA STATE UNIV FRESNO on 11/30/18. For personal use only.

Department of Physics, Theoretical Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany S Supporting Information *

ABSTRACT: Using density functional theory calculations with an on-site Hubbard term (DFT+U), we explore the effect of surface termination and cation substitution on the performance of the CoxNi1−xFe2O4(001) surface (x = 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Different reaction sites (Fe, Co, Ni, and an oxygen vacancy) were investigated at three terminations: the B-layer with octahedrally coordinated Co/Ni and with an additional half and full monolayer of Fe (0.5A and A-layer, respectively). Ni substitution with an equal concentration of Co and Ni (x = 0.5) reduces the overpotential over the end members for the majority of reaction sites. Surface Co cations are identified as the active sites and the ones at the A-layer termination for x = 0.5 exhibit one of the lowest theoretically reported overpotentials of 0.26 V. The effect of the additional iron layer on the active site modification is 2fold: analysis of the electronic properties and spin densities indicates that the additional Fe layer stabilizes a bulk-like oxidation state of +2 for Co and Ni at the A-layer termination, whereas at the B-layer termination, they are oxidized to 3+. Moreover, the unusual relaxation pattern enables the formation of a hydrogen bond of the OOH intermediate to a neighboring surface oxygen that lowers the reaction free energy of this formerly rate-limiting step, leading to a deviation from the scaling relationship and almost equidistant reaction free-energy steps of intermediates. This renders an example of how a selective surface modification can result in a significant improvement of OER performance. KEYWORDS: oxygen evolution reaction, water splitting, transition metal oxides, spinels, density functional theory

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the Td and Oh sites are occupied by M2+ and Fe3+, respectively. In contrast, in the inverse spinel Fe3+ occupies the tetrahedral (Td) sites and half of the octahedral (Oh) sites, the rest being filled by M2+. Since the 1970s different ferrite electrodes have been investigated for PECs including CdFe2O4,14 MgFe2O4,15 CaFe2O4,16−18 ZnFe2O4,17,19 CoTixFe2−xO4,20 ZnTixFe2−xO4,21 CoFe2O4,22 and NiFe2O4.23 The wide range of cation miscibility allows to tune the electrical conductivity, catalytic activity, and redox chemistry. Most of the early work focused on determining the positions of the valence and conduction bands and flat band potential. Previous theoretical investigations based on density functional theory (DFT) have provided insight into the material properties on the atomistic level, including the electronic structure, cation ordering, and magnetic properties of different bulk ferrites MFe2O4, (M = Co,24−26 Ca,27,28 Ni,24,29 Mg,28 Zn,28,30 Cu31). Moreover, the surface termination and the adsorption of water on spinel

INTRODUCTION Hydrogen production efficiency in photoelectrochemical cells (PECs) is limited due to the sluggish kinetics of the oxygen evolution reaction (OER) taking place at the photoanode. To improve the performance, cost-effective and stable compounds with a low overpotential for water oxidation are required. For this purpose, different types of transition metal oxides (TMO) have been studied in the last decades. Some of the common materials comprise TiO2,1,2 WO3,3,4 and Fe2O3.5,6 Despite of significant efforts, the solar-to-hydrogen conversion efficiency of the majority of these materials is still limited, necessitating further research and improvement.7,8 Among the TMO spinel ferrites, MFe2O4 (M being a transition metal ion) have been in the focus of research for different catalytic applications because of their high surface reactivity, suitable gap size, and availability of different cation coordinations and multiple oxidation states.9,10 The spinel structure comprises a slightly distorted face-centered cubic lattice of oxygen ions where cations occupy one-eighth of the tetrahedral (Td) and half of the octahedral (Oh) sites. Depending on the cation distribution spinels can range from normal to inverse.11−13 In the normal spinel ferrites © XXXX American Chemical Society

Received: February 9, 2018 Revised: October 2, 2018 Published: October 29, 2018 11773

DOI: 10.1021/acscatal.8b00574 ACS Catal. 2018, 8, 11773−11782

Research Article

ACS Catalysis surfaces32−39 has been considered, whereas the OER process has so far been addressed in only a few cases.40−43 Recently, we have explored the role of Ni and V substitution on the catalytic activity of CoFe2O4 for OER applications.44,45 In particular, measurements on CoxNi1−xFe2O4 nanoparticles indicate that substitution of Co by Ni in equal stoichiometry leads to an overpotential of 0.34 V which is one of the lowest experimentally reported values.44 DFT simulations on the CoxNi1−xFe2O4(111) surface44 confirmed the beneficial effect of Ni subsitution at x = 0.5 over the end members, which was correlated to a modification of binding energy differences of the intermediate species to the surface. However, the theoretical overpotential for the (111)-surface is significantly higher (0.55 V46 ) than the experimental value. The discrepancy between the theoretical and experimental overpotential may be related to the fact that the nanoparticles used in experiment expose facets with different crystallographic orientation. This has motivated us to consider here the catalytic activity of the (001)-surface besides the previously studied (111)-orientation. In particular, we investigate the role of the surface termination by considering three distinct cases, the so-called B-layer termination with octahedrally coordinated cations and oxygen and with an additional 0.5 ML Fe (0.5 A) and 1 ML Fe (A-layer) using density functional theory (DFT +U) calculations. The stability of these terminations is assessed as a function of the oxygen chemical potential also in comparison to the (111) oriented surfaces studied previously44 and under reaction conditions as a function of applied potential and pH. Besides the surface termination and composition also the reaction sites for OER were varied to identify the active species. The trends in OER activity are related to binding energy differences as well as the underlying structural and electronic properties.

studies52 have shown that the zero-point energy (ZPE) and entropic contribution (TS) of intermediates do not change considerably for different metal oxide surfaces. Indeed, similar values were reported for TiO250 and Co3O4.41 We have used here the values from ref 52. The external potential, that turns all individual free energies downhill (ΔGi ≤ 0), ϕOER, equals the highest value among ΔGi. The theoretical overpotential is then calculated as η = (ϕOER − ϕeq), where ϕeq = 4.92/4e = 1.23 V. In order to establish trends in reactivity, the overpotential is often related to binding energy differences.49,53 The binding energy of the intermediate species to the surface are defined as52 ΔG BO = E O − E − E H2O + E H2 + ΔZPE − T ΔS * * * ΔG BOH = E OH − E − E H2O + (1/2)E H2 + ΔZPE − T ΔS * * * (7) ΔG BOOH *

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THEORETICAL MODELING OF OER The oxygen evolution reaction is a four step process involving a coupled proton and electron transfer that leads to the formation of the oxygen molecule from water (2H2O → O2 + 4H+ + 4e−). A widely used approach to model OER, developed by Rossmeisl et al.,47 considers the formation of four reaction intermediates on the surface. (1)

*OH → *O + H+ + e−

(2)

H 2O + *O → *OOH + H+ + e−

(3)

*OOH → * + O2 + H+ + e−

(4)

where * denotes the bare surface and *OiHj the surface with different chemisorbed species. Alternative mechanisms, such as, for example, the formation of two adjacent *O, lead to a high activation energy48 and have not been considered here. The reaction free energy of the individual steps can be calculated as ΔGi = ΔE + ΔZPE − T ΔS − eϕ + kBT ln a H +

= E OOH − E − 2E H2O + (3/2)E H2 + ΔZPE − T ΔS * * (8)

COMPUTATIONAL DETAILS The density functional theory (DFT) calculations were carried out with the VASP code54−56 which uses the projected augmented wave (PAW) method.57 For the exchangecorrelation functional we used the generalized-gradient approximation (GGA-PBE).58 As spinel ferrites are strongly correlated systems, static electronic correlations were taken into account within the GGA+U method, where the rotationally invariant formulation of Dudarev et al.59 was employed with an effective U value of 3.0 eV for Fe, Ni and Co. Similar values were used in a number of previous OER studies for Fe, Co and Ni containing oxides40,41,52 as well as of the bulk and surface properties of spinels.24,37,38 From the calculation of oxidation energies between bulk oxides, Wang, Maxisch, and Ceder60 determined U values of 3.3, 4.0, and 6.4 eV for Co, Ni, and Fe, respectively. Using those values, we find only a small enhancement of the overpotential by 0.04 eV for the A-layer. This is consistent with a recent study of perovskite oxides showing only a small change in overpotential for U = 3 and 5 eV at LaXO3 (X = Fe, Co, Ni) catalysts.61 With U = 3 eV, we obtained band gaps of 1.66 eV, 1.73, and 1.69 eV for CoFe2O4, NiFe2O4, and Co0.5Ni0.5Fe2O4, respectively (see Figure S1 in SI). Experimental values ranging between 1.3 and 1.58 eV62,63 have been reported for CoFe2O4 and 1.52 eV for NiFe2O4.63 The electronic but also catalytic properties are strongly sensitive to cation order. Previous studies have shown that in NiFe2O4 alternating layers of octahedral Ni and Fe along the [001]-direction are most favorable.24 In our previous work, we have identified that the most stable cation ordering for Co0.5Ni0.5Fe2O4 comprises ordered Co−Ni octahedral layers alternating with Fe octahedral layers along the [001] direction44 and have used this configuration here. For the bulk calculations, we used a plane-wave cutoff of 500 eV and a k-point mesh of 8 × 8 × 8 and performed full relaxation of the unit cell and the internal parameters until the residual forces were less than 0.01 eV/Å. The (001)-oriented spinel surfaces were simulated using slabs containing 9 and 11 layers for the B- and the A-layer terminations, respectively. Consistent with the above-mentioned bulk cation ordering, the surface layer of the former contains M = Ni, Co, or ordered Co and Ni sites for x = 0.0, 1.0, and 0.5, respectively, and oxygen.

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H 2O + * → *OH + H+ + e−

(6)

(5)

where ΔE is the energy difference of intermediates presented in reactions (eqs 1−4), ZPE is the zero-point energy, TΔS is the entropic contribution, and ϕ is the external potential. The expressions for ΔGi are presented in SI, for a detailed derivation see e.g.49−51 The free energies are given at standard conditions (ϕ = 0, pH = 0, p = 1 bar, T = 298 K). Previous 11774


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ACS Catalysis The lateral size of the slabs corresponds to the calculated bulk lattice constants 8.351, 8.355, and 8.377 Å for x = 0.0, 0.5, and 1.0, respectively. The slabs are separated by vacuum of ∼10 Å to avoid interaction between the slab and its periodic images along the z-direction. For the surface calculations, we used a plane-wave cutoff of 500 eV and a Monkhorst−Pack k-point mesh64 of 8 × 8 × 1 for converged results. The internal positions for the slabs were relaxed until the residual forces were less than 0.01 eV/Å.

While in the A layer termination of the latter, the additional Fe builds dimers,35 at the Co0.5Ni0.5Fe2O4(001) surface the cation environment leads to a distinct relaxation pattern of Fe along the [110] direction toward the Ni (A-layer) or Co neighbors (0.5 A layer). These configurations are favored by 85 and 51 meV/Fe adatom compared to relaxation in the opposite direction. As we will see below, this relaxation pattern influences the OER activity of these surfaces. The stability of different terminations was assessed in the framework of ab initio thermodynamics,33−35,66 adapting the scheme of Krishnaswamy et al.67 The lower limit of the oxygen chemical potential marks the region where the ferrite decomposes into the elemental bulk metals and O2 gas and the upper limit (zero reference) is determined by the stability of the O2 molecule. The surface phase diagram of the (001) and previously studied (111)44 surfaces of CoxNi1−xFe2O4, displayed in Figure 2, shows overall little variation with Ni concentration. The lowest surface energy terminations are the A-layer at oxygen poor and the B-layer termination of the (001) surface for oxygen rich conditions, while the 0.5 A-layer gets close in stability only near the crossing point between the A and B layer and the B-layer with oxygen vacancies is higher in energy. The 0.25 ML Fetet1 termination of the (111) surface is slightly higher in energy than the B-layer, whereas the most favorable (111) oriented termination toward the oxygen-poor conditions is 0.5 ML Feoct2. Under reaction conditions, the relevant surfaces can be covered by different adsorbates such as *O and *OH groups.41,42 Therefore, we have explored the stability of the A- and B-layer terminations of Co0.5Ni0.5Fe2O4(001) with different coverages of functional groups as a function of the external potential and pH. The Pourbaix diagrams are constructed from the free energies of different surface structures and w.r.t. reversible hydrogen electrode. Detailed description of this approach can be found in ref 68. While we show the whole range of pH, we note that bulk dissolution that takes place in acidic environments (pH < 7)68,69 is not considered here; however, most of the experiments on TMO anodes are performed under alkaline conditions. Our results in Figure 3 indicate that the clean B-layer is stable up to URHE = 1.86 V, at higher values a mixed 1/3 ML *O and 2/3 ML *OH forms at the surface by dissociation of water molecules. Further deprotonation leads to an oxygenated surface at URHE > 2.17 V. A similar trend is also observed for the A-layer where the clean surface is stable up to 1.10 V. Beyond that, a fully hydroxylated surface is stable, followed by a partial deprotonation (2/3 ML *O and 1/3 ML *OH) at 1.20 V < URHE < 1.45 V and finally, for URHE > 1.45 V by an oxygenated surface. Thus, besides the clean surfaces, we have considered the OER process at the latter for the A-layer and at the mixed 1/3 ML *O and 2/3 ML *OH of the B-layer. Overpotential as a Function of Termination and Reaction Site. In the following we investigate the catalytic activity of different reaction sites at the three terminations of the CoxNi1−xFe2O4(001) surface and compare those with our previous results for the (111) surface.44 At the B-layer termination both Ni and Co are exposed on the surface and consequently both have been considered as possible reaction sites. Furthermore, the water oxidation at an O vacancy is explored. For the 0.5A-layer and A-layer terminations, OER at an iron site is also evaluated together with Co and Ni. Additionally, OER is considered at a Co reaction site for the Blayer covered with 1/3 ML *O and 2/3 ML *OH and the

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RESULTS AND DISCUSSION Stability of Surface Terminations and Pourbaix Diagrams. We have investigated the OER process on three distinct (001) surface terminations with a B-layer and additional half and full monolayer of Fe, denoted 0.5A-layer and A-layer, respectively. The former was previously found to be stabilized for other spinel surfaces such as Fe3O4(001).33−35 Recently, a more complex model with subsurface vacancies was proposed for the Fe3O4(001) surface,65 but upon adsorption of water defects tend to heal, thus they will not be considered for the OER study here. In Figure 1 we show the side and top

Figure 1. Side and top views of the (a) B-layer, (b) 0.5 A-layer, and (c) A-layer at the Co0.5Ni0.5Fe2O4(001) surface. Relative displacements of cations w.r.t. to the bulk positions (pale colors) are denoted by black arrows. Note the strong downward and lateral relaxation of the Fe adatom at the 0.5 A and A terminations from an initially tetrahedral surface site toward an empty octahedral site in the B-layer.

views of the relaxed structure of the three terminations for Co0.5Ni0.5Fe2O4(001). In both the 0.5A- and A-layer, the additional Fe which is initially at a surface tetrahedral site prefers to relax toward an available octahedral site in the surface B-layer with a total displacement of more than 1.1 Å. This trend is observed in all Ni−Co stoichiometries and similar to previous findings for the Fe3O4(001) surface.35 11775


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Figure 2. Surface phase diagram of CoxNi1−xFe2O4 with (x = 1.0, 0.5 and 0.0) of the (001) and (111) surface orientation showing the stability of different terminations as a function of the oxygen chemical potential, which varies between O-poor and O-rich limits, with 1/2 EO2 as zero reference.

overpotential of NiFe2O4(001) with a Ni active site is very high, 1.07 V, and is not reduced by Co mixing. Recently Li and Selloni 43 reported an overpotential of 0.42 V for NiFe2O4(001), however for a Fe reaction site on a mixed Ni and Fe surface layer and the OER mechanism involves the initial adsorption and partial dissociation of water molecules on top of each surface cation, followed by repeated deprotonation and formation of O2 involving lattice oxygen. On the other hand, for a Co reaction site at the end member CoFe2O4(001) an external potential ϕOER = 2.01 V leads to ∑ΔGi ≤ 0. Therefore, the lower limit of the overpotential at a Co site is η = 2.01 − 1.23 = 0.78 V. Interestingly, in the mixed oxide Co0.5Ni0.5Fe2O4(001) at the Co site the overpotential is reduced to 0.63 V which is lower than for both end members. We note that a slightly higher value (0.76 V) was reported for the Co3O4(001) surface.41 To explore the influence of surface defects on the catalytic performance, we investigated the OER process at an O vacancy site at the B-layer termination. The corresponding reaction free energies and overpotentials are also listed in Table 1. While the overpotential of the mixed Ni−Co compound is lower than for the end members, the overall values for OER at an oxygen vacancy site (B-layer+VO) are much higher than for a Co reaction site. In conclusion, mixing of Ni and Co in Co0.5Ni0.5Fe2O4(001) reduces the overpotential compared to the end members. Moreover, Co is identified as the active site, but the overall values of η are higher than the ones for the (111) surface.44 In the Pourbaix diagram Figure 3 the overpotential of the most active Co site at the B-layer of Co0.5Ni0.5Fe2O4(001) (η = 0.63) lies at the border between the clean and the mixed 1/3 ML *O and 2/3 ML *OH covered B-layer. Therefore, we have calculated the overpotential for a vacancy site on top of the Co reaction site at the latter termination. As can be seen from the reaction free energies of intermediates in Table 1, because of the stabilization of *OOH (see discussion below), the most demanding step is now the formation of OH groups upon absorption of H2O, but the overpotential is only slightly higher than for the clean surface (0.03 V).

Figure 3. Surface Pourbaix diagram of Co0.5Ni0.5Fe2O4(001) for the B-layer (left) and A-layer (right) terminations. The dashed lines denote the reversible hydrogen electrode (RHE) and the H2O/O2 equilibrium. For comparison, the overpotentials of the clean A and B terminations with Co active site as a function of pH are marked by black lines.

oxygenated A-layer of the mixed oxide Co0.5Ni0.5Fe2O4(001). The reaction free energies and overpotentials of the abovementioned reaction sites on the B-layer, 0.5A-layer and A-layer terminations are displayed in Table 1. The corresponding cumulative free energies of intermediates and overpotentials of reaction sites at the Co0.5Ni0.5Fe2O4(001) are shown in Figure 4 together with side views of the relaxed geometries of intermediates at the Co reaction site of the mixed spinel surface. B-Layer. For the B-layer termination available Co and Ni reaction sites were investigated for the end members and the mixed oxide. Comparison of the reaction free energies shows that for Co as a reaction site in both CoFe2O4(001) and Co0.5Ni0.5Fe2O4(001) the most endoergic step is the formation of *OOH upon dissociative adsorption of a second H2O molecule at the *O intermediate, while for a Ni reaction site the formation of *O is the most energetically costly step. The relaxed structures of intermediates in Figure 4 indicate that all groups are singly bonded to the Co-reaction site. The 11776


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Table 1. Reaction Free Energies ΔGi of Intermediate Steps (U = 0) and Overpotentials for the B-Layer, 0.5A, and A-Layer Terminationsa ΔG1[eV] B-Layer CoFe2O4 (Co) Co0.5Ni0.5Fe2O4 Co0.5Ni0.5Fe2O4 Co0.5Ni0.5Fe2O4 NiFe2O4 (Ni) B-Layer+VO CoFe2O4 (O) Co0.5Ni0.5Fe2O4 NiFe2O4 (O) 0.5A-Layer CoFe2O4 (Fe) Co0.5Ni0.5Fe2O4 NiFe2O4 (Fe) Co0.5Ni0.5Fe2O4 A-Layer CoFe2O4 (Fe) Co0.5Ni0.5Fe2O4 NiFe2O4 (Fe) CoFe2O4 (Co) Co0.5Ni0.5Fe2O4 Co0.5Ni0.5Fe2O4

(Co) (Co) (1/3*O+2/3*OH) (Ni)

(O)

(Fe) (Ni)

(Fe)

(Co) (Co) (1 ML *O)

ΔG2[eV]

ΔG3[eV]

ΔG4[eV]

η [V]

1.63 1.39 1.89 1.68 1.93

1.59 1.60 1.64 2.42 2.30

2.01 1.86 0.89 0.88 0.80

−0.32 0.02 0.49 0.02 −0.11

0.78 0.63 0.66 1.18 1.07

0.27 −0.28 −0.35

0.63 1.02 0.90

2.37 2.11 2.24

1.64 2.06 2.12

1.14 0.88 1.01

0.32 0.33 0.36 1.87

2.10 1.95 1.97 1.88

1.32 1.47 1.42 1.15

1.16 1.15 1.16 0.01

0.88 0.72 0.75 0.65

0.12 0.17 0.17 1.32 1.40 1.54

1.77 1.85 1.87 1.40 1.44 1.50

1.69 1.62 1.60 1.55 1.49 1.53

1.32 1.26 1.27 0.63 0.57 0.33

0.54 0.62 0.63 0.32 0.26 0.30

a

The reaction site for each case is shown in parentheses. The most endoergic step for each reaction site (ϕOER) is highlighted in bold font.

Figure 4. Cumulative free energies ∑ΔGi of the intermediates for the B-layer (left) and the A-layer (right) terminations. Side view of the corresponding intermediate structures for the mixed oxide are also shown. The intermediate steps 1−4 are denoted by their products *OH, *O, *OOH, and *().

0.5A-Layer. Besides the B-layer termination, we explore here the influence of an additional 0.5 ML of Fe on the OER performance. The corresponding reaction free energies and overpotentials are shown in Table 1. For a Fe reaction site, we obtain overpotentials of 0.75, 0.72, and 0.88 eV, for x = 0.0, 0.5, and 1.0, respectively. Because of the relaxation of Fe toward the Co neighbors, discussed above and shown in Figure 1b, intermediates at a Co active site tend to shift to the Fe site, whereas at the Ni active site with longer Ni−O bonds, the overpotential is lowered to 0.65 V. A-Layer. As shown in Figure 1c, the additional Fe cations relax toward the Ni neighbors, and the proximity of Fe and the reduced bond lengths hinder the OER process at the Ni

reaction site. The energetics of intermediates for a Fe reaction site show that the most demanding step is the formation of *O groups by deprotonating *OH, while for a Co reaction site the highest energy cost is for the formation of *OOH. Overall, the reaction free energies are noticeably lower than for the B-layer and Co is identified as the active site with a strongly reduced overpotential of 0.32 V at the end member CoFe2O4(001). The overpotential is further decreased for the mixed oxide Co0.5Ni0.5Fe2O4(001) to 0.26 V. This value is significantly lower than for the Co0.5Ni0.5Fe2O4(111) surface44 and to our knowledge one of the lowest theoretically reported overpotentials for a TMO anode. The value is also close to the experimentally measured 0.34 V,44 although we note that a 11777


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spinel ferrite surfaces studied here. We find that the scaling relationship is generally fulfilled, however the slope is reduced to 0.92, still very close to the unity line with a reasonable goodness of fit (R2 = 0.94) and maximum deviation from the linear fit of ±0.40 eV. An interesting feature is that the deviation from the fitted line is particularly strong for the Co active site: at the B-layer the binding energy difference is 3.55 eV, in contrast, for the A-layer it is 2.94 eV, which correlates with the lower overpotential at the latter termination. The binding energies of *OH and *OOH vs *O are shown in Figure S6 and S7, respectively. These exhibit a stronger deviation from the 0.5 slope expected from the formal single and double bond of the adsorbates to the surface. This can be attributed to further stabilization of the *OH and *OOH cases due to a rather flat bonding geometry of the former and the formation of a hydrogen bond to the surface of the latter. As a consequence of the scaling relationship, the binding energy difference of *O and *OH has been used as a descriptor of OER activity and the overpotential as a function of ΔGB*O − ΔGB*OH gives rise to a volcano plot.49 In Figure 6,

direct comparison to experimental values has to be taken with caution as no kinetic effects are considered here and the surface structure/faceting of the nanoparticles is not known. Because the calculated overpotential of the Co active site coincides with the fully oxygenated region of the Pourbaix diagram (Figure 3) of the A-layer, we have calculated also the overpotential of the latter. OER is performed at an oxygen vacancy on top of Co which has the lowest formation energy compared to sites on top of Fe or Ni. Similar to the *O and *OH precovered B-layer, the most endoergic step is the formation of *OH upon dissociative adsorption of H2O, but overall the reaction free energies for the formation of *OH, *O, and *OOH for the Co active site at the clean and the oxygenated surface are nearly equal, meaning equidistant steps of the cumulative reaction free energya coveted property for an OER catalyst. We note that surface oxygenation has only a small effect on the overpotential (increase by 0.04 V). In the following, we explore the origin of the superior performance of the A-layer and compare also to the results for the (111) surface orientation. Understanding the Energetic Trends: Overpotential vs Binding Energy Differences of Intermediate Species. Previous studies49,53 have shown that the binding energies of intermediates are correlated, which on one hand simplifies the theoretical modeling of OER and allows to define descriptors but on the other hand limits the independent optimization of binding energies to improve the OER activity.70−74 In particular, the binding energies of *OOH and *OH were found to differ by a constant ∼3.20 ± 0.4 eV for a variety of oxides, thus leading to a lower limit for the overpotential of 0.4−0.2 eV.49,53 Recently, several studies have reported deviation from the unity slope between the binding energies of *OOH and *OH, which offers a chance to overcome the limitations posed by the scaling relationship.70−75 Figure 5 illustrates ΔGB*OOH as a function of ΔGB*OH for the different reaction sites, terminations and orientation of the

Figure 6. Overpotential versus the binding energy difference of ΔGB*O B − ΔG*OH for different reaction sites and terminations of the CoxNi1−xFe2O4(001) and (111) surfaces (open and filled symbols, respectively). Labels of compounds are abbreviated as Ni and Co for the end members NiFe 2 O 4 and CoFe 2 O 4 and Ni−Co for Co0.5Ni0.5Fe2O4, additionally the reaction site is given in the legend. The red dashed line follows from eq 9.

we compile the results for different reaction sites and terminations of CoxNi1−xFe2O4(001), together with previous results for the (111)-surface.44 Under the assumption that the scaling relationship applies, one can express the overpotential alternatively as (displayed with a dashed red line in Figure 6, e.g., see also ref 49): ηsc = [max((ΔG BO − ΔG BOH), 3.35eV * * − (ΔG BO − ΔG BOH))/e] − 1.23V (9) * * The positive slope of the volcano plot, indicating low binding energy differences, is composed mainly of Co reaction sites in its upper region and the unfavorable O reaction sites of the Blayer of the (001) surface and the 0.25 ML O1 termination of

Figure 5. Scaling relationship between binding energies of *OOH and *OH for different reaction sites and terminations of the (001) and (111) surfaces. The yellow and purple regions mark deviations of ±0.2 and ±0.4 to the linear fit. For comparison the classical scaling relationship with unity slope49,53 is denoted by a black short-dashed line. 11778


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Figure 7. Side view of the spin density of the Co0.5Ni0.5Fe2O4(001) surface for a Co reaction site at the clean and 1/3*O and 2/3*OH precovered B-layer (left two rows) and the clean and oxygenated A-layer (right two rows) terminations. The spin density is shown for an iso-surface value of 0.02 e/Å3. The blue (lighter) and red (darker) colors represent the majority and minority spin density, respectively.

for Fe (MFe = 4.0 μB). In Figure 7, the spin densities of intermediates for a Co reaction site at B-layer and A-layer (clean and *O covered) terminations of Co0.5Ni0.5Fe2O4(001) are compared. For completeness, we have shown besides the clean surfaces also the 1/3*O and 2/3*OH precovered B-layer and the oxygenated A-layer. We recall that these terminations have similar overpotentials to the A and B layer but the potential limiting step is changed to the formation of *OH. Additionally, in Figure S4, the projected density of states of surface TM 3d and O 2p states is given. Together with further results including other active sites and Ni concentrations shown Figure S3, they indicate that the oxidation state of surface cations varies strongly, depending on the composition, termination, and type of chemisorbed species on the surface. At the bare surface of the B-layer termination in Figure 7, the magnetic moments of surface Co and Ni cations are lower (1.8 and 0.8 μB, respectively) than the corresponding bulk values indicating a higher oxidation state of +3. At the Co active site, the magnetic moment is further reduced to 0.9 μB for adsorbed *OH and 1.1 μB for *O, and finally quenched to 0.0 μB for *OOH, indicating Co 3+ in the low spin state, whereas the magnetic moments of other surface ions remain unchanged. A comparison to the electronic behavior at the 1/3*O and 2/ 3*OH precovered B-layer reveals that the oxidation state at the reaction site is largely not influenced by adsorbates at the remaining cation sites, except for the *OH step where the magnetic moment at the Co site is 2.3 μB. In contrast, at the clean A-layer termination, the surface Fe adopts the oxidation state of +2 with a magnetic moment of ∼3.5 μB (note that the oxidation state for Fe at the oxygenated surface is +3, ∼ 3.9− 4.0 μB). Most importantly, surface Co and Ni obtain an almost bulk-like oxidation state of +2 with magnetic moments of 2.6 μB and 1.6 μB, respectively. For the Co active site, the magnetic moment changes only weakly during the intermediate steps: slightly higher, 3.0 μB for *OH, slightly lower 2.1 μB for *O and bulk-like 2.6 μB for *OOH. The values are similar also for the oxygen precovered case. We can conclude that the excess Fe stabilizes a bulk-like oxidation state of surface Co and Ni cations which remain largely unaltered during the OER

the (111) surface in the lower part, whereas the negative slope contains predominantly Fe and Ni reaction sites. While the Co active sites exhibit a binding energy difference between 1.4 and 1.6 eV, the Fe iron sites show higher values (1.6−2.2 eV), followed by Ni (1.9−2.2 eV). Overall, Ni−Co mixing reduces the overpotential by modification of the binding energy difference ΔGB*O − ΔGB*OH toward the optimum value of ∼1.6 eV. Interestingly, the top of the volcano plot is dominated by Fe reaction sites at the (111)-surface46 and the A-layer of the (001) surface. While the Fe and Ni reaction sites lie almost on top of the volcano plot, the Co sites exhibit stronger deviations. This is related to the deviations from the linear fit mentioned above and demonstrates that the scaling relationship applies only approximately for the system we discuss here. Most prominently, the most active case, that is, Co at the A layer of the Co0.5Ni0.5Fe2O4(001) surface, lies above the top of the volcano plot with η = 0.26 eV, which correlates with the reduced binding energy difference of *OOH and *OH to 2.94 eV. Oxidation States of Surface Cations and Their Variation during OER. Although the binding energy difference between *O and *OH for the Co active sites for the A- and B-layer of the Co0.5Ni0.5Fe2O4(001) surface fall in a narrow region (1.5 ± 0.1 eV), the corresponding overpotentials differ significantly. This can be attributed to the large variance in the other binding energy difference of *OOH and *OH, which is 2.94 eV for the A layer and 3.55 eV for the B-layer. To shed more light on the origin of this difference, we analyze here the structural, electronic, and magnetic properties of CoxNi1−xFe2O4(001) throughout OER. Since the total occupation of d states has been found to vary typically by only a few percent for different oxidation states,76,77 the spin density and local magnetic moments can be used as a much clearer indicator for changes in valence state. In particular the variation of spin density gives access to the dynamical changes of the electronic properties of the catalyst throughout the reaction cycle. In the bulk compounds with x = 0.0, 0.5, 1.0 (see the spin-density in Figure S2) the oxidation state of cations is +2 for Co (MCo = 2.6 μB), Ni (MNi = 1.6 μB), and +3 11779


Research Article

ACS Catalysis

performance of spinel catalysts. The formation of a hydrogen bond to a surface oxygen at the *OOH intermediate bears parallels to recent proposals to circumvent the limitations of the universal scaling relationship (e.g., in Ni and Co doped rutile surfaces or Me−N−C catalysts)70,71 by involving a secondary active site. Beyond the presented results, further possible mechanisms, activation barriers, cation order and the effect of solvation need to be addressed in future studies.

process. This likely has a favorable influence on the reduction of overpotential. An interesting structural feature can also be observed from the side view of the *OOH intermediate in Figure 7: while the H of the OOH-group points away from the surface at the clean B-layer, it is oriented toward the surface and forms a hydrogen bond of 1.58 Å to a surface oxygen at the A layer. This is enabled by the structural distortion around the active site due to the unusual relaxation pattern of the additional Fe and explains the significant reduction of the reaction free energy of *OOH from the B-layer where it is the reaction limiting step to the A layer where it has a much lower energy (see Table 1). We note that such a hydrogen bond formation of *OOH to the surface occurs also for the B and A layers covered by *OH and/or *O groups shown in Figure 7. In both cases, it stabilizes this intermediate and shifts the potential limiting step to the formation of *OH.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b00574. Definition of the reaction free energies of the individual steps, data on ZPE and TS, the bulk electronic and magnetic properties (density of states, spin density) as well as the spin density of intermediates for different reaction sites, terminations and compositions of the (001) surface. For the A and B layer with Co active site additionally the projected density of states of the adsorbates and surface cations is given, as well as the coordinates of the A and B layer of CoFe2O4(001) (PDF)

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CONCLUSIONS Using of DFT+U calculations, we provide a detailed insight into the oxygen evolution reaction (OER) at the CoxNi1−xFe2O4(001) surface. In particular, three distinct surface terminations, a B-layer, 0.5A-, and A-layer, and different reaction sites: Co, Ni, Fe, and an oxygen vacancy were investigated for x = 0.0, 0.5, and 1.0. Co is identified as the active site with a substantially reduced overpotential for the Alayer termination of the CoFe2O4 (001) surface of 0.32 V which can be further lowered to 0.26 V by substitution of half of the Co cations by Ni in Co0.5Ni0.5Fe2O4(001). These values are significantly lower than the ones predicted for the B-layer termination (0.63 V) and for the (111) surface and close to the measured overpotential of 0.34 eV;44 however, for a direct comparison future experiments on nanoparticles78,79 or films with well-defined (001)-surface orientation will be useful. The overpotentials for the other studied reaction sites Fe, Ni, and O vacancy are significantly higher (0.54−1.18 V). Overall, the scaling relationship largely applies for this system, but strong deviations are observed in particular for the Co active sites. While the binding energy difference of *OH and *O lies somewhat lower than the optimum value of 1.6 eV, the one between *OOH and *OH (2.94 eV) is much lower than the 3.2(3.35) eV found from the scaling relationship. As a consequence, the overpotential of the A-layer lies above the top of the volcano plot formed by η of the remaining reaction sites. The reduction of overpotential at the A-layer with Co active site is associated with a significant lowering of the reaction free energy for the *O and *OOH intermediates. The stabilization of *OOH is traced back to a hydrogen bonding to a surface oxygen. Most importantly, the reaction free energies for the three intermediates *OH, *O, and *OOH are close to equidistant, pointing toward the much sought-after nearly ideal catalyst.49 Analysis of the dynamic variation of magnetic moments and the spin-density of intermediates during OER indicates that at the A-layer termination the additional Fe supports a bulk-like oxidation state of +2 at the Co active sites, while at other terminations, Co is oxidized to +3 at the bare surface and varies its valence strongly throughout OER. Our results illuminate the intricate interplay of surface orientation, termination, and active site and provide guidelines for optimizing anode materials for OER. In particular, the additional monolayer of Fe leads to a controlled modification of oxidation state and bonding mechanism at the active site and represents thus a promising strategy to improve the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 (0)203 379 2238. Fax: +49 (0)203 379 1679. ORCID

Rossitza Pentcheva: 0000-0002-4423-8980 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) within Priority Program SPP1613 (PE883/9-2), Collaborative Research Center TRR247 (B4) and computational time at the Leibniz Rechenzentrum (grant pr87ro) and at magnitUDE of the Center of Computer Science and Simulation (DFG grant INST 20876/209-1 FUGG,INST 20876/243-1 FUGG).

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Surface Termination and Composition Control of Activity of the CoxNi1

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