Surface termination and composition control of activity of the CoxNi1

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Surface termination and composition control of activity of the CoNi FeO(001) surface for water oxidation: insights from DFT+U calculations

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Hamidreza Hajiyani, and Rossitza Pentcheva ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00574 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Catalysis

Surface termination and composition control of activity of the CoxNi1xFe2O4(001) surface for water oxidation: insights from DFT+U calculations

Hamidreza Hajiyani and Rossitza Pentcheva∗ Department of Physics, Theoretical Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, Lotharstraÿe 1, 47057 Duisburg, Germany

E-mail: [email protected],[email protected]

Phone: +49 (0)203 379 2238. Fax: +49 (0)203 379 1679

Abstract Using density functional theory calculations with an on-site Hubbard term (DFT+U ) we explore the eect of surface termination and cation substitution on the performance of the CoxNi1xFe2O4(001) surface (x= 0.0, 0.5, 1.0) as an anode material in the oxygen evolution reaction (OER). Dierent 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 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 identied 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

eect of the additional iron layer on the active site modication is twofold: analysis of 1

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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, while 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 how a selective surface modication can result in a signicant improvement of OER performance.

Keywords oxygen evolution reaction, water splitting, transition metal oxides, spinels, density functional theory

Introduction Hydrogen production eciency 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-eective and stable compounds with a low overpotential for water oxidation are required. For this purpose, dierent 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 signicant eorts, the solar-to-hydrogen conversion eciency of the majority of these materials is still limited, necessitating further research and improvement. 7,8 Among the TMO spinel ferrites M Fe2O4 (M being a transition metal ion) have been in the focus of research for dierent catalytic applications due to their high surface reactivity, suitable gap size and availability of dierent 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 2


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ACS Catalysis

of the octahedral (Oh ) sites. Depending on the cation distribution spinels can range from normal to inverse. 1113 In the normal spinel ferrites the Td and Oh sites are occupied by M 2+ 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 lled by M 2+ . Since the 1970s dierent ferrite electrodes have been investigated for PECs including CdFe2O4, 14 MgFe2O4, 15 CaFe2O4, 1618 ZnFe2O4, 17,19 CoTixFe2xO4, 20 ZnTixFe2xO4, 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 at 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 dierent bulk ferrites M Fe2O4, (M =Co, 2426 Ca, 27,28 Ni, 24,29 Mg, 28 Zn, 28,30 Cu 31 ). Moreover, the surface termination and the adsorption of water on spinel surfaces 3239 has been considered, while the OER process has so far been addressed in only a few cases. 4043 Recently, we have explored the role of Ni and V substitution on the catalytic activity of CoFe2 O4 for OER applications. 44,45 In particular, measurements on CoxNi1xFe2O4 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 CoxNi1xFe2O4(111) surface 44 conrmed the benecial eect of Ni subsitution at x= 0.5 over the end members which was correlated to a modication of binding energy dierences of the intermediate species to the surface. However, the theoretical overpotential for the (111)-surface is signicantly higher (0.55 V 46 ) 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 dierent 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 oc3


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tahedrally 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 previously 44 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 dierences as well as the underlying structural and electronic properties.

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 (2H2 O → 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.

where

∗

H2 O+∗ → ∗ OH + H+ + e−

(1)

∗

(2)

OH → ∗ O + H+ + e−

H2 O + ∗ O → ∗ OOH + H+ + e−

(3)

∗

(4)

OOH →

∗

+ O2 + H+ + e−

denotes the bare surface and ∗ Oi Hj the surface with dierent chemisorbed species.

Alternative mechanisms, such as e.g. the formation of two adjacent ∗ O lead to a high activation energy 48 and have not been considered here. The reaction free energy of the individual steps can be calculated as:

4


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ACS Catalysis

(5)

∆Gi = ∆E + ∆ZPE − T ∆S − eφ + kB T ln aH +

where ∆E is the energy dierence of intermediates presented in reactions (1-4), ZPE is the zero-point energy, T ∆S is the entropic contribution and φ is the external potential. The expressions for ∆Gi are given in SI, for a detailed derivation see e.g.

4951

The free

energies are given at standard conditions (φ = 0, pH = 0, p = 1 bar, T = 298 K). Previous studies 52 have shown that the zero point energy (ZPE) and entropic contribution (TS) of intermediates do not change considerably for dierent metal oxide surfaces. Indeed, similar values were reported for TiO2 50 and Co3O4. 41 We have used here the values from Ref. 52 The external potential, that turns all individual free energies downhill (∆Gi ≤ 0), φOER , equals the highest value among ∆Gi . 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 dierences. 49,53 The binding energy of the intermediate species to the surface are dened as: 52

∆GB ∗ O = E∗ O − E∗ − EH2 O + EH2 + ∆ZPE − T ∆S

(6)

∆GB ∗ OH = E∗ OH − E∗ − EH2 O + (1/2)EH2 + ∆ZPE − T ∆S

(7)

∆GB ∗ OOH = E∗ OOH − E∗ − 2EH2 O + (3/2)EH2 + ∆ZPE − T ∆S

(8)

Computational Details The density functional theory (DFT) calculations were carried out with the VASP code 5456 which uses the projected augmented wave (PAW) method. 57 For the exchange-correlation 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 5


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the GGA+U method, where the rotationally invariant formulation of Dudarev et al. 59 was employed with an eective 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 oxides 40,41,52 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 Ceder 60 determined U values of 3.3, 4.0 and 6.4 eV for Co, Ni and Fe, respectively. Using those values we nd 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 LaX O3 (X =Fe, Co, Ni) catalysts. 61 With U = 3 eV we obtained band gaps of 1.66 eV, 1.73 eV and 1.69 eV for CoFe2O4, NiFe2O4 and Co0.5Ni0.5Fe2O4, respectively (see Fig. S1 in SI). Experimental values ranging between 1.3 and 1.58 eV 62,63 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 identied that the most stable cation ordering for Co0.5Ni0.5Fe2O4 comprises ordered Co-Ni octahedral layers alternating with Fe octahedral layers along the [001] direction 44 and have used this conguration here. For the bulk calculations, we used a plane-wave cut-o 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. 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 cut-o of 500 eV and a 6


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ACS Catalysis

Monkhorst-Pack k -point mesh 64 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.

Results and discussion Stability of surface terminations and Pourbaix diagrams

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 towards an empty octahedral site in the B-layer.

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 7


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as Fe3O4(001). 3335 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 Fig. 1 we show the side and top views of the relaxed structure of the three terminations for Co0.5Ni0.5Fe2O4(001). In both the 0.5Aand A-layer, the additional Fe which is initially at a surface tetrahedral site prefers to relax towards 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 ndings for the Fe3O4(001) surface. 35 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 towards the Ni (A-layer) or Co neighbors (0.5 A layer). These congurations are favored by 85 and 51 meV/Fe adatom compared to relaxation in the opposite direction. As we will see below, this relaxation pattern inuences the OER activity of these surfaces. The stability of dierent terminations was assessed in the framework of ab initio thermodynamics 3335,66 , 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 CoxNi1xFe2O4, displayed in Fig. 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 Blayer with oxygen vacancies is higher in energy. The 0.25ML Fetet1 termination of the (111) surface is slightly higher in energy than the B-layer, while the most favorable (111) oriented termination towards the oxygen-poor conditions is 0.5ML Feoct2 . Under reaction conditions, the relevant surfaces can be covered by dierent adsorbates such as ∗ O and ∗ OH groups. 41,42 Therefore, we have explored the stability of the A- and 8


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ACS Catalysis

Figure 2: Surface phase diagram of CoxNi1xFe2O4 with (x =1.0, 0.5 and 0.0) of the (001) and (111) surface orientation showing the stability of dierent terminations as a function of the oxygen chemical potential. The oxygen chemical potential varies between O-poor and O-rich limits, with 1/2 EO2 as zero reference.

B-layer terminations of Co0.5Ni0.5Fe2O4(001) with dierent coverages of functional groups as a function of the external potential and pH. The Pourbaix diagrams are constructed from the free energies of dierent 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 Fig. 3 indicate that the clean B-layer is stable up to

U RHE= 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 U RHE>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 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 dierent reaction sites at the three terminations of the CoxNi1xFe2O4(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 B-layer covered with 1/3 ML ∗ O and 2/3 ML ∗ OH and the oxygenated A-layer of the mixed oxide Co0.5Ni0.5Fe2O4(001). The reaction free energies and overpotentials of the above mentioned reaction sites on the B-layer, 0.5A-layer and Alayer 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 10


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ACS Catalysis

Fig. 4 together with side views of the relaxed geometries of intermediates at the Co reaction site of the mixed spinel surface. Table 1: Reaction free energies ∆Gi of intermediate steps (U = 0) and overpotentials for the B-layer, 0.5A and A-layer terminations. The reaction site for each case is shown in parentheses. The most endoergic step for each reaction site (φOER ) is highlighted in bold font.

B-layer

∆G1 [eV]

CoFe2O4 (Co) Co0.5Ni0.5Fe2O4 (Co)

Co0.5Ni0.5Fe2O4 (Ni)

1.68

Co0.5Ni0.5Fe2O4 (Co) (1/3∗ O+2/3∗ OH) NiFe2O4 (Ni)

∆G2 [eV]

∆G3 [eV]

∆G4 [eV]

1.63

1.59

0.78

1.60

2.01 1.86

-0.32

1.39

0.02

0.63

1.89

η

[V]

1.64

0.89

0.49

0.66

2.42 2.30

0.88

0.02

1.18

0.80

-0.11

1.07

2.37 2.11 2.24

1.64

1.14

2.06

0.88

2.12

1.01

B-layer+V

1.93

CoFe2O4 (O)

0.27

0.63

Co0.5Ni0.5Fe2O4 (O)

-0.28

1.02

NiFe2O4 (O)

0.5A-layer

-0.35

0.90

CoFe2O4 (Fe)

0.32

1.16

0.88

0.33

1.47

1.15

0.72

NiFe2O4 (Fe)

0.36

1.42

1.16

0.75

Co0.5Ni0.5Fe2O4 (Ni)

1.87

2.10 1.95 1.97 1.88

1.32

Co0.5Ni0.5Fe2O4 (Fe)

1.15

0.01

0.65

CoFe2O4 (Fe)

0.12

1.32

0.54

0.17

1.62

1.26

0.62

NiFe2O4 (Fe)

0.17

1.77 1.85 1.87

1.69

Co0.5Ni0.5Fe2O4 (Fe)

1.60

1.27

0.63

CoFe2O4 (Co)

1.32

1.40

0.63

0.32

Co0.5Ni0.5Fe2O4 (Co) Co0.5Ni0.5Fe2O4 (Co) (1 ML ∗ O)

1.40

1.44

1.55 1.49

0.57

0.26

1.54

1.50

1.53

0.33

0.30

O

A-layer

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 H2 O 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 Fig. 4 indicate that all groups are singly bonded to the Co-reaction site. The 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 11


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P Figure 4: The cumulative free energies ∆Gi 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 ∗ ().

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 P to ∆Gi ≤ 0. Therefore, the lower limit of the overpotential at a Co site is η = 2.011.23 = 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 inuence 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 identied as the active site, but the overall values of η are higher

12


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ACS Catalysis

than the ones for the (111) surface. 44 In the Pourbaix diagram Fig. 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, due to 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). 0.5A-layer:

Besides the B-layer termination, we explore here the inuence of an ad-

ditional 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. Due to the relaxation of Fe towards the Co neighbors, discussed above and shown in Fig.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 Fig. 1c, the additional Fe cations relax towards the Ni neighbors,

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 identied 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) surface 44 and to our knowledge one of the lowest theoretically reported overpotentials for a TMO anode. The value is also close to the experimentally measured value 0.34 V, 44 although we note that a direct comparison to experimental values has to be taken with caution as no kinetic eects are considered here 13


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and the surface structure/faceting of the nanoparticles is not known. Since the calculated overpotential of the Co active site coincides with the fully oxygenated region of the Pourbaix diagram (Fig.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 eect 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 dierences of intermediate species Previous studies 49,53 have shown that the binding energies of intermediates are correlated, which on one hand simplies the theoretical modeling of OER and allows to dene descriptors but on the other hand limits the independent optimization of binding energies to improve the OER activity. 7074 In particular the binding energies of ∗ OOH and ∗ OH were found to dier by a constant ∼ 3.20 ± 0.4eV 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 oers a chance to overcome the limitations posed by the scaling relationship. 7075 B Figure 5 illustrates ∆GB ∗ OOH as a function of ∆G∗ OH for the dierent reaction sites,

terminations and orientation of the spinel ferrite surfaces studied here. We nd that the scaling relationship is generally fullled, however the slope is reduced to 0.92, still very close to the unity line with a reasonable goodness of t (R2 = 0.94) and maximum deviation 14


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Figure 5: Scaling relationship between binding energies of ∗ OOH and ∗ OH for dierent 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 t. For comparison the classical scaling relationship with unity slope 49,53 is denoted by a black short-dashed line.

from the linear t of ±0.40 eV. An interesting feature is that the deviation from the tted line is particularly strong for the Co active site: at the B-layer where the binding energy dierence 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 the Fig. 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 at bonding geometry of the former and the formation of a hydrogen bond to the surface of the latter.

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B Figure 6: The overpotential versus the binding energy dierence of ∆GB ∗ O -∆G∗ OH for dierent reaction sites and terminations of the CoxNi1xFe2O4(001) and (111) surfaces (open and lled symbols, respectively). Labels of compounds are abbreviated as: Ni and Co for the end members NiFe2O4 and CoFe2O4 and Ni-Co for Co0.5Ni0.5Fe2O4, additionally the reaction site is given in the legend. The red dashed line follows from Eq. (9).

As a consequence of the scaling relationship, the binding energy dierence of ∗ O and ∗

OH has been used as a descriptor of OER activity and the overpotential as a function of

B 49 ∆GB In Fig. 6 we compile the results for dierent ∗ O -∆G∗ OH gives rise to a volcano plot.

reaction sites and terminations of CoxNi1xFe2O4(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 Fig. 6, see also e.g.

16


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49

): B B B η sc = [M ax((∆GB ∗ O − ∆G∗ OH ), 3.35 eV − (∆G∗ O − ∆G∗ OH ))/e] − 1.23 V

(9)

The positive slope of the volcano, indicating low binding energy dierences, is composed mainly of Co reaction sites in its upper region and the unfavorable O reaction sites of the B-layer of the (001) surface and the 0.25 ML O1 termination of 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 dierence 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, NiCo mixing reduces B the overpotential by modication of the binding energy dierence ∆GB ∗ O -∆G∗ OH towards

the optimum value of ∼ 1.6 eV. Interestingly, the top of the volcano is dominated by Fe reaction sites at the (111)-surface 46 and the A-layer of the (001) surface. While the Fe and Ni reaction sites lie almost on top of the volcano, the Co sites exhibit stronger deviations. This is related to the deviations from the linear t mentioned above and demonstrates that the scaling relationship applies only approximately for the system we discuss here. Most prominently, the most active case - Co at the A layer of the Co0.5Ni0.5Fe2O4(001) surface lies above the top of the volcano with η = 0.26 eV, which correlates with the reduced binding energy dierence of ∗ OH and ∗ OOH to 2.94 eV.

Oxidation states of surface cations and their variation during OER Although the binding energy dierence between ∗ O and ∗ OH for the Co active sites for the Aand B-layer of the Co0.5Ni0.5Fe2O4(001) orientation fall in a narrow region (1.5 ± 0.1 eV), the corresponding overpotentials dier signicantly. This can be attributed to the large variance in the other binding energy dierence 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 dierence, we analyze here the structural, electronic and magnetic properties of CoxNi1xFe2O4(001) throughout OER. Since

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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 isosurface value of 0.02 e/Å3. The blue (lighter) and red (darker) colors represent the majority and minority spin density, respectively.

the total occupation of d states has been found to vary typically by only a few percent for dierent 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 Fig. S2) the oxidation state of cations is +2 for Co (M Co = 2.6 µB ), Ni (M Ni = 1.6 µB ) and Fe (M Fe = 4.0 µB ). In Fig. 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 18


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have similar overpotentials to the A and B layer but the potential limiting step is changed to the formation of ∗ OH. Additionally, in Fig. 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 Fig. 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 Fig. 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 at ∗ OH and 1.1 µB at ∗ O, and nally quenched to 0.0

µB at ∗ 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 inuenced 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, 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 +2, ∼ 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 at ∗ OH, slightly lower 2.1 µB at ∗ O and bulk-like 2.6 µB at ∗ 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 process. This likely has a favourable inuence on the reduction of overpotential. An interesting structural feature can also be observed from the side view of the ∗ OOH intermediate in Fig. 7: while the H of the OOH-group points away from the surface at the clean B-layer, it is oriented towards 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 19


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the active site due to the unusual relaxation pattern of the additional Fe and explains the signicant 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 Fig. 7. In both cases it stabilizes this intermediate and shifts the potential limiting step to the formation of ∗ OH.

Conclusion Using of DFT+U calculations we provide a detailed insight into the oxygen evolution reaction (OER) at the CoxNi1xFe2O4(001) surface. In particular, three distinct surface terminations, a B-layer, 0.5A- and A-layer, and dierent reaction sites: Co, Ni, Fe and an oxygen vacancy were investigated for x =0.0, 0.5 and 1.0. Co is identied as the active site with a substantially reduced overpotential for the A-layer termination of the CoFe2 O4 (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 signicantly 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 nanoparticles 78,79 or lms with well-dened (001)-surface orientation will be useful. The overpotentials for the other studied reaction sites Fe, Ni and O vacancy are signicantly 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 dierence 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 formed by

η of the remaining reaction sites. The reduction of overpotential at the A-layer with Co active site is associated with a signicant lowering of the reaction free energy for the ∗ O and

20


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∗

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 towards the much sought-after nearly ideal

catalyst. 49 Analysis of the dynamic variation of magnetic moments and the spin-density of intermediates 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 modication of oxidation state and bonding mechanism at the active site and represents thus a promising strategy to improve the 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 eect of solvation need to be addressed in future studies.

Acknowledgement 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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Supporting Information Available The Supporting Information contains the denition of the reaction free energies of the individual steps, data on ZPE and T S , the bulk electronic and magnetic properties (density of states, spin density) as well as the spin density of intermediates for dierent 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).

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Surface termination and composition control of activity of the CoxNi1

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