Importance of Correlation in Determining Electrocatalytic Oxygen

Sep 10, 2012 - Yu Pei , Yang Yang , Fangfang Zhang , Pei Dong , Robert Baines , Yuancai Ge , Hang Chu , Pulickel M. Ajayan , Jianfeng Shen , and Mingx...
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Importance of Correlation in Determining Electrocatalytic Oxygen Evolution Activity on Cobalt Oxides Mónica García-Mota,† Michal Bajdich,‡ Venkatasubramanian Viswanathan,† Aleksandra Vojvodic,† Alexis T. Bell,‡,§ and Jens K. Nørskov*,† †

SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States ‡ JCAP-North, The Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Co-based oxides are suitable electrode materials for the electrocatalytic oxygen evolution reaction (OER) with promising activity and stability, in addition to being widely available and relatively cheap. We investigate OER on Co3O4(001) and β-CoOOH (0112̅ ) surfaces using density functional theory calculations (DFT). We construct surface Pourbaix diagrams and investigate the theoretical overpotential for the elementary steps involved in OER on these surfaces. We show that inclusion of the Hubbard-U correction to DFT (DFT+U) is necessary to recover experimentally observed trends in the activity for the strongly correlated cobalt oxides. We find that the inclusion of the Hubbard-U correction lowers the activity of both Co3O4(001) and β-CoOOH(011̅2) when compared to results from pure DFT. In addition, the Hubbard-U correction shifts the location of Co3O4 and βCoOOH from the strong binding leg to the weak binding leg of the OER volcano plot. The calculations also suggest that the theoretical overpotentials for Co3O4 and β-CoOOH are very nearly the same. We ascribe this to a similar local coordination environment of the active Co site in Co3O4 and CoOOH under OER conditions.

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in the spinel structure of Co3O4 are converted to CoIIIcontaining species, such as CoOOH and Co2O3.8−10 Recent experimental studies suggest CoOOH as one of the stable structures under OER working conditions by in situ Raman spectroscopy,7 electron paramagnetic resonance,11 and X-ray absorption spectroscopy.12 The bulk and surface properties of spinel Co3O4 have been studied extensively using density functional theory calculations (DFT);13−15 however, there is a lack of literature addressing the reactivity of stable cobalt oxides under OER working conditions. Furthermore, it is well-known that semilocal forms of DFT fail to describe correctly the electronic structure for such strongly correlated systems due to large self-interaction errors. In a recent work, Selloni et al. showed the importance of including a Hubbard correction to properly describe the electronic states and the magnetic structure at the Co3O4(110) surface.16 The purpose of the present study is to understand the electrocatalytic activity of Co oxides. The approach we take

INTRODUCTION Electrochemical water splitting to form molecular oxygen, protons, and electrons is a key step in routes for electrochemical fuel production.1 The protons and electrons can be used at the cathode to form hydrogen or reduce CO2 to hydrocarbons or alcohols. If the electrical energy is produced from sources like wind, hydro, or solar power, the fuel production becomes sustainable.2 However, the substantial energy loss associated with electrochemical water splitting severely limits the overall efficiency of such systems. The energy losses are mainly due to the high overpotentials associated with the oxygen-evolving anode.3−5 Therefore, to minimize these energy losses, it is important to find active oxygen-evolving electrocatalysts. Co-based oxides have received a significant amount of interest as electrode materials with promising activity and stability in alkaline media, in addition to being based on widely available and relatively cheap elements.4,6 For these reasons, considerable effort has been devoted to the understanding of the changes in composition and structure of cobalt anodes as a function of applied potential and the influence of these changes on the activity for oxygen evolution reaction (OER).4,7 It has been proposed that, under oxidative conditions relevant to OER, the CoII species present © 2012 American Chemical Society

Received: June 26, 2012 Revised: August 24, 2012 Published: September 10, 2012 21077

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energy for the oxidation of CoO to Co3O4 according to the equation

here is to improve the accuracy of DFT by using the DFT+U method, in which a Hubbard-type term is added to account for the on-site Coulomb interactions of the localized d orbitals.17−19 We begin by addressing the discrepancy between the previously calculated and measured activity trend between RuO2 and Co3O4 surfaces for electrocatalytic OER. In an earlier theoretical study,15 it was predicted that Co3O4 is slightly more active than RuO2, whereas experimental studies suggest that Co3O4 has a higher overpotential than RuO2 by 0.2−0.25 V.3 We resolve this issue by including a Hubbard-U correction to improve the description of the localized Co d electrons in Co3O4. We find that the Hubbard-U correction on Co3O4 results in a lowering of its activity toward OER. As a consequence, we find a higher theoretical overpotential on Co3O4 than on RuO2 in agreement with the experimental trend. In addition, there is also a change in the potentialdetermining step found for the OER, which results in a shift from the strong binding energy leg to the weak binding energy leg of the OER volcano plot. As noted earlier, under oxidative conditions, CoII species in the spinel structure can be oxidized.7−12 We consider that possibility by examining the electrocatalytic activity for one of the stable structures under OER working conditions, namely, βCoOOH. We find that the inclusion of the Hubbard-U correction on β-CoOOH results in a lowering of the activity similar to that obtained when the correction is applied on Co3O4. We find theoretical overpotentials on β-CoOOH similar to that found on Co3O4.

6CoO + O2 → 2Co3O4

(1)

We fully optimize the cobalt oxide structures for each of the tested U values. We avoid the well-known errors associated with the DFT-calculated energy of O2 by using the H2O reference as carried out earlier.28 The experimental reaction enthalpy for reaction 1 at room temperature is −3.68 eV. We find that the calculated reaction enthalpy is equal to the experimental one at a value of U = 3.52 eV (see Figure 1 in the Supporting Information). The calculated U value is in good agreement with the one found in the study by Ceder et al.17 We consider the most energetically preferred termination of the Co oxides, namely, Co3O4(001) and β-CoOOH (011̅2), under oxygen-rich conditions, which are cobalt-terminated slabs with Co in an octahedral position13,29 (see Figure 1). We model the

Figure 1. Side views of the top layers of the relaxed CoxOy surface slabs used in the DFT calculations. Oxygen atoms and cobalt atoms in tetrahedral and octahedral positions are colored red, green, and purple, respectively.

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THEORETICAL METHODS We investigate the theoretical overpotential (ηOER) for OER on CoxOy surfaces (Co3O4 and CoOOH) by first-principles density functional theory (DFT) calculations using a computational hydrogen electrode model.20 We perform all calculations with the GPAW code using the ASE simulation package.21 We replace the inner cores by projector augmented wave pseudopotentials (PAW),22 and we use a uniform real-spaced grid with a spacing smaller than 0.2 Å for the representation of the electronic wave functions.23 We use the revised Perdew− Burke−Ernzerhof functional (RPBE) to describe the exchange and correlation.24 To address the discrepancy between the previously calculated and measured activity trend between RuO2 and Co3O4 surfaces for electrocatalytic OER, we apply a Hubbard-U correction (DFT+U method) as implemented in GPAW25 to improve the description of localized Co d-electrons in the Co oxides. It has been shown that the bulk formation energies of rutile oxides are well described within standard DFT, and thus we would expect the thermodynamics of oxygen intermediates to be well described on RuO2.17,26−28 We use the theoretical overpotential on the RuO2(110) surface previously estimated by DFT calculations 15 as a benchmark for comparison purposes. There is no universally accepted method for the choice of the value of U for the Hubbard correction. Ceder and co-workers carried out a systematic study of the oxidation energies of transition metals and suggested that the choice of U should be made to describe accurately the formation energies of different oxides.17 A similar approach has been done in recent studies, where it was suggested that the U value should be taken to fit the reaction energy of the different oxide forms relevant for the catalytic reaction under study.18,19Accordingly, as we are interested in the electrooxidation of water, we chose the value of U to fit the reaction

Co3O4(001) surface with slabs consisting of eight Co layers in a (1 × 1) supercell, while the CoOOH (011̅2) surface is modeled with five Co layers in a (2 × 2) supercell. The k-point sampling consists of 3 × 3 × 1 Monkhorst−Pack points.30 During structure relaxation, we allow relaxation of both the adsorbates involved in OER (O*, OH*, and OOH*) and the top four Co layers for Co3O4(001) and the top two Co layers for CoOOH(0112̅ ). The rest of the Co layers of the Co oxides were kept frozen at their bulk positions. We separate the slabs by more than 16 Å of vacuum. We carry out spin-polarized calculations including the dipole correction.31 The Bader charges reported in this article were calculated by using the method developed by Henkelman et al.32

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RESULTS AND DISCUSSION Surface Pourbaix Diagrams. The theoretical overpotential depends strongly on the coverage of the different reactant species at the surface.33 We construct surface Pourbaix diagrams where we identify the most stable structures for Co3O4(001) and CoOOH (011̅2) for a range of potentials and pH values. It is to be noted that the bulk dissolution of cobalt ions occurs spontaneously in acidic environment.34 Thus, we compare the results of our model to experiments carried out in alkaline environment where these dissolution effects are not present. We determine the free energies of different surface structures based on RPBE+U calculations and the computational standard hydrogen electrode model.20 This model has previously been successful in predicting theoretical cyclic voltammograms for hydrogen and oxygen species on Pt surfaces,35,36 trends in oxygen reduction reaction (ORR) activity for Pt and Pt alloys,37,38 and trends in electrochemical activity on oxide surfaces.15,33,39 The surface Pourbaix diagram 21078

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Figure 2. Surface Pourbaix diagram obtained from DFT+U calculations. (a) Co3O4(001) and (b) CoOOH (011̅2). The dashed black lines mark the reversible hydrogen electrode (RHE) and the H2O/O2 equilibrium. The dotted red lines indicate the self-consistent theoretical overpotential (ηOER).

ΔG4 OER = 4.92[eV] − ΔGOOH * − eU + kBT ln aH +

is constructed by assuming an equilibrium of the surface with water, protons, and electrons according to the equation

where * represents an active site on the oxide. A detailed derivation of these expressions can be found elsewhere.40 It is to be noted that an analogous derivation can be carried out for alkaline environment since the intermediates are the same in both acidic and alkaline environment. We calculate the free energy differences ΔG1−4OER using the computational standard hydrogen electrode model.20 The free energies of the intermediates along the reaction path, ΔGO*, ΔGOH*, and ΔGOOH*, are calculated relative to H2O, at a vapor pressure of 0.035 bar where it is in equilibrium with liquid water, and H2 in the gas phase at U = 0 V and standard conditions. We neglect any additional kinetic barriers that may be present between the intermediates. This analysis can be viewed as a first step toward a complete picture of the reaction pathway. We note that calculations for the oxygen reduction reaction (which is the reverse of the OER) on Pt(111) indicate that these additional barriers are typically small.41 The potential determining step, a concept that has been discussed and reviewed in previous works on OER and ORR,20,42−44 can be deduced from the free energy diagram. The potential determining step is the highest free energy step in the process (GOER) and, therefore, the last step to become downhill in free energy as the potential increases. With this approach, the theoretical overpotential (ηOER) at standard condition is defined as

Cox Oy + zH 2O ↔ Cox Oy + z H w + (2z − w)(H+ + e‐) (2)

We consider the adsorption of O, OH, and OOH on the CoxOy surfaces. The adsorption of O and OH is considered in all possible combinations (see Table 2 in Supporting Information). We identify the most stable coverage of these intermediate species as a function of potential and pH. From the Surface Pourbaix diagram shown in Figure 2a, the bare Co3O4 surface is stable up to 1.8VRHE, where the voltage, VRHE, is reported versus the reversible hydrogen electrode. Beyond 1.8VRHE, the formation of OOH occurs on an oxygen-covered Co3O4 surface. This finding is in agreement with previous DFT studies.15 A similar trend is also observed on the CoOOH surface (see Figure 2b). For the sake of comparison with the GGA description, we show the surface Pourbaix diagram obtained with both RPBE and RPBE+U in Figure 2 in the Supporting Information. A similar stabilization trend is observed for both of these descriptions with a higher potential for the stability transitions when the GGA+U approach is used. Oxygen Evolution Reaction on Co3O4(100) and βCoOOH (011̅2). In the water splitting reaction, 2H 2O(l) → O2 (g) + 2H 2(g)

(3)

hydrogen is produced at the cathode, and oxygen is evolved at the anode. As outlined in previous works, we will consider the following mechanism for the oxygen evolution reaction (OER) in an acidic environment H 2O(l) + * → OH* + H+ + e−

ηOER = (GOER /e) − 1.23 V

In a recent study, Man et al. showed the existence of a universal activity volcano to describe OER on oxide surfaces.15 This is based on the existence of a scaling relation between the binding energies of the intermediates OOH* and OH* on oxide surfaces. They showed that the binding energies of OOH* and OH* scale according to the relation ΔEOOH* = ΔEOH* + 3.2, with 95% of the points within ±0.4 eV. This approximate scaling means that the trends in activity between oxide materials are to a first approximation determined by the O* binding energy. The implication of this scaling relation is that either reaction 3b or 3c is the potential determining step on materials of interest for OER. Therefore, eq 4 can be rewritten as

(3a)

ΔG1OER = ΔGOH * − eU + kBT ln aH +

OH* → O* + H + + e−

(3b)

ΔG2OER = ΔGO * − ΔGOH * − eU + kBT ln aH +

O* + H 2O(l) → OOH* + H+ + e−

(3c)

ΔG3OER = ΔGOOH * − ΔGO * − eU + kBT ln aH +

OOH* → * + O2 (g) + H+ + e−

(4)

(3d) 21079

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nearly 1.1 eV weaker than the binding energy obtained with the RPBE functional. However, the destabilization of OH* and OOH* binding energies due to the Hubbard-U correction is only about 0.6 eV (see Table 1). Due to the more pronounced destabilization of O*, it is not the oxidation of O* to OOH* which is the potential determining step for OER on Co3O4 but it is the oxidation of water to form OH*. The oxidation of OH* to O* is found to be the second elementary step with highest free energy change. As a consequence, the inclusion of the Hubbard-U correction shifts the location of Co3O4 from the strong binding leg to the weak binding leg of the OER volcano plot, as shown in Figure 4b. It is to be noted that the universal activity volcano assumes that the potential determining step for the weak binding leg of the volcano is the oxidation of OH* to O*. However, for the systems studied here, the potential determining step is the oxidation of H2O to OH*, and as a result, these points are not expected to lie on the universal line as the activity descriptor is not ΔGOOH* − ΔGOH*. We find that the inclusion of the Hubbard-U correction increases the theoretical overpotential on Co3O4 from 0.41 to 0.76 V. Hence, the theoretical overpotential on Co3O4 calculated with the DFT+U approximation is around 0.3 V higher than the theoretical overpotential on RuO2 (∼0.4 V).15 This trend is consistent with experiments and resolves the discrepancy found in the earlier study that used DFT to estimate the activity toward OER on Co3O4.3,15 We also investigate the electrocatalytic OER on one of the stable structures under OER working conditions, CoOOH. We find that the Hubbard-U correction results in modifications of the interaction between the adsorbates and the CoOOH (011̅2) surface. The modification is similar to that found on the Co3O4 surface (see Table 1). The consequence of applying the Hubbard-U correction is a similar destabilization of OH* and OOH* species and a nearly double destabilization for O*. This trend is consistent with the principle of bond order conservation, which predicts a slope of 1/2 between the binding energies of OH/OOH and O.45,46 We find the selfconsistent theoretical overpotential on CoOOH to increase from 0.27 eV with DFT-RPBE to 0.78 eV, when the HubbardU correction is applied. Hence, we find the activity on CoOOH and Co3O4 to be comparable. This finding gives confidence in the theoretically calculated OER activity based on the spinel Co3O4 structure since we would expect similar trends on many

ηOER = Max{[(ΔGO * − ΔGOH *)/e], 3.2[eV] − (ΔGO * − ΔGOH *)/e} − 1.23 V

(5)

The binding energies of O* (ΔEO*), OH* (ΔEOH*), and OOH* (ΔEOOH*), calculated relative to H2O and H2 in the gas phase, on the oxygen-covered Co3O4(001) and CoOOH (011̅2) surfaces are compiled in Table 1. We find that the Table 1. Binding Energies of O*, OH*, and OOH* (ΔEO*, ΔEOH*, and ΔEOOH* in eV) on the Oxygen-Covered Co Oxide Surfacesa Co3O4 β-CoOOH Co3O4 β-CoOOH

RPBE RPBE RPBE+U RPBE+U

ΔEO*

ΔEOH*

ΔEOOH*

pds

ηOER

2.56 2.62 3.71 3.72

0.90 0.81 1.55 1.57

3.76 3.68 4.32 4.34

OOH* OOH* OH* OH*

0.41 0.27 0.76 0.78

a

Potential determining steps (pds), either O*, OH*, or OOH* formation, and self-consistent theoretical overpotentials (ηOER in V) associated with OER on the oxygen-covered Co oxide surfaces.

binding energies of OH* and OOH* on the cobalt oxide surfaces, calculated with RPBE or RPBE+U, follow the constant scaling relationship ΔEOOH* = ΔEOH* + 3.2 within ±0.43 eV (see Figure 4a). The existence of this scaling relation with the inclusion of U has not been demonstrated yet and is out of the scope of this paper. For the sake of simplicity, we assume that the universal OER activity volcano calculated using DFT also holds for the calculations carried out using DFT+U for the cobalt oxide surfaces considered here. We find that the self-consistent theoretical overpotential calculated with DFT-RPBE on the oxygen-covered Co3O4 surface is 0.41 V. We obtain self-consistency in the sense that the surface is stable under the potential at which that particular surface can perform OER. We find the oxidation of O* to OOH* to be the potential determining step for OER, as shown in Figure 3a. These results agree with the previously reported theoretical overpotential value and potential determining step on the Co3O4(001) surface.15 In a second step, we apply a Hubbard-U correction (DFT+U method) to improve the description of localized Co d-electrons. We find this correction leads to a destabilization of all oxygen-containing intermediates (see Table 1). The O* binding energy calculated by DFT+U is

Figure 3. Standard free energy diagram for the oxygen evolution reaction (OER) on oxygen-covered cobalt oxide surfaces calculated with RPBE and with RPBE+U, black and red line, respectively. The dashed lines indicate the free energy diagram for an ideal electrocatalyst. The potential determining steps for the OER on cobalt oxide surfaces calculated with both RPBE and with RPBE+U are marked as pds. 21080

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Figure 4. (a) Adsorption energies of OOH* (ΔEOOH*) as a function of the adsorption energies of OH* (ΔEOH*) on the oxygen-covered Co3O4(001) and CoOOH (011̅2) surfaces, shown in triangles and circles, respectively. The black and red colors correspond to calculations performed with RPBE and with RPBE+U, respectively. The small black circles correspond to ΔEOOH* as a function of ΔEOH* on oxides surfaces (data adapted from ref 15). The black star indicates the position of RuO2(110) (data adapted from ref 15). The binding energies of OOH* and OH* scale according to the relation, ΔEOOH* = ΔEOH* + 3.2, with 95% of the points within ±0.4 eV. (b) Calculated theoretical overpotentials (ηthe) plotted as a function of (ΔGO* − ΔGOH*) for oxygen-covered CoxOy surfaces. The small black circles correspond to ηthe as a function of (ΔGO* − ΔGOH*) on rutiles and perovskites (data adapted from ref 15). The volcano curve is established by using the scaling relation between (ΔGOOH* − ΔGO*) and (ΔGO* − ΔGOH*) as described in ref 15 . Same color code as in figure (a).

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of the other possible stable structures reported under these oxidative conditions.7,11,12 The similar theoretical overpotential found on Co3O4 and βCoOOH can be understood by comparing the local coordination environment of the Co active site in Co3O4 and CoOOH. The Co active sites of both Co3O4 and CoOOH share the same octahedral coordination with only minor differences in the O−O distances (see Figure 3 and Table 4 in the Supporting Information). Furthermore, an analysis of the magnetic moment and the effective charge via the Bader method for the two investigated surfaces yields almost identical values. We find a slightly higher Bader charge of ∼+1.6 on the Co active site in CoOOH and Co3O4 surfaces compared to +1.44 for the bulk Co3+. We also find a non-null magnetic moment on the Co active site (∼1 μB). The calculated values of Bader charge and magnetic moment of Co in the cobalt oxide surfaces suggest that the Co active site has a formal charge that is slightly higher than Co3+ at OER conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Choice of U for DFT+U calculations for Co oxides, surface Pourbaix diagrams for Co3O4(001) and β-CoOOH (0112̅ ) obtained with RPBE and RPBE+U approach, ZPE and TS at T = 298 K for all the relevant species involved in OER, and a comparison between the local coordination and charge of the Co active site on Co3O4(001) and β-CoOOH (0112̅ ) are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computational work on Hubbard-U correction and OER on Co3O4 by MGM, AV, and JKN was supported by Center of Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001060. The computational work on OER on β-CoOOH by MB and ATB was supported by the Joint Center for Artificial Photosynthesis (JCAP), a DOE Energy Innovation Hub, through the Office of Science of the U.S. Department of Energy under Award No. DE-SC0004993. The computational work on Hubbard-U correction by VV was supported by an UTRC fellowship.

CONCLUSIONS

In conclusion, we have investigated the electrocatalytic oxygen evolution reaction (OER) on the Co3O4(001) and β-CoOOH (011̅2) surfaces using density functional theory calculations. We find that the inclusion of the Hubbard correction (DFT+U) resolves the discrepancy between the calculated and measured Co3O4 activity. We find that the theoretical overpotential on Co3O4(001) is about 0.3 V higher than that on RuO2 which is in agreement with the experimental trend. We also calculate the theoretical overpotential on the β-CoOOH (011̅2) surface and find it to be almost identical to that of the spinel Co3O4(001) surface. We argue that this similarity is due to an identical local coordination environment of the Co active site. On the basis of these findings, we would expect similar trends on other possible stable Co oxide structures reported in experiments under OER conditions.

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dx.doi.org/10.1021/jp306303y | J. Phys. Chem. C 2012, 116, 21077−21082