First Principles Study of Cobalt (Hydr)oxides under ElectrochemicalConditionsJia Chen* and Annabella Selloni

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States

*S Supporting Information

ABSTRACT: Density functional theory (DFT) calculations withon-site Coulomb repulsion are carried out to study the relativestabilities of crystalline cobalt oxides and hydroxidesCoO,Co(OH)2, Co3O4, CoO(OH), and CoO2in electrochemicalenvironment. Co(OH)2 is the thermodynamic ground state underreducing conditions, i.e., at voltages V < 0 relative to the standardhydrogen electrode (SHE) potential in acidic solution, whereasCoO(OH) and CoO2 are stable under oxidizing conditions, i.e., atexternal voltage larger than 1.23 eV vs SHE in basic solution.These results, combined with surface structure studies of the(0001) natural cleavage surface of CoO(OH), show that a CoO2

x−

(x = 0−0.5) layer is present when the surface is exposed tosolution under oxidizing conditions, in agreement with recentexperimental findings. Study of the energetics of water oxidation at regular surface sites of CoO(OH)(0001) indicates howeverthat water deprotonation to form a surface OH species is energetically very costly. Different active sites, e.g. steps, are thusresponsible for the observed high activity of crystalline cobalt oxide for electrochemical oxygen evolution.

1. INTRODUCTION

As the rate-limiting step of electrochemical and photochemicalwater splitting,1 the oxygen evolution reaction (OER) is aprocess of both fundamental and technological interest. Amongthe possible anode materials for the OER, much attention hasbeen recently focused on the spinel cobalt oxide Co3O4, whichhas an OER activity only slightly lower than that of the noblemetal oxides RuO2, IrO2, and PtO2, with the advantage thatcobalt is an earth-abundant element.2 Recent efforts atdeveloping catalysts based on Co3O4 include the synthesis ofnanomaterials with various morphologies,3,4 the use of activesupporting materials,5,6 and the growth of thin films containingCo3O4 and other metal oxides.7 Another advance has been thedevelopment of an efficient amorphous cobalt phosphatecatalyst.8 Crystalline Co3O4 thin film catalysts have also beenprepared,9 which could be used for detailed model catalyststudies of the OER.In addition to these synthetic efforts, studies aimed at

understanding the atomic scale mechanisms of the oxidationreaction have been carried out. By in-situ Raman spectroscopy,Yeo and Bell showed that Co3O4 undergoes progressiveoxidation to CoO(OH) under reaction conditions, suggestingthat the Co electrode is largely covered by CoO(OH) duringthe OER.6 However, ex-situ XPS analysis on thin film catalystsrevealed that the transformation of the spinel Co3O4 to alayered hydroxide/oxyhydroxide is incomplete, suggesting thatin-situ transformation to the layered structure is allowed fromthe rock salt structure whereas it is inhibited from the spinelstructure.7

The above observations give rise to several questions.Particularly important questions concern the thermodynamicground state structure of cobalt oxide under OER conditions,the role of the kinetics of the structural transformation, andespecially which component is mainly responsible for thecatalyst’s OER activity. Focusing on crystalline materials, in thiswork we address these questions by first principles DFTcalculations of the bulk and surface stabilities of CoO,Co(OH)2, Co3O4, CoO(OH), and CoO2 as a function of pHand applied voltage in an electrochemical environment. Ourresults show that Co3O4, which has a low overpotential, isunstable in electrochemical environment, whereas the CoO2

x−

layered structures, which are thermodynamically stable, have alarge overpotential, and thus are poorly active for the OER.

2. COMPUTATIONAL DETAILS

Our calculations were performed within the spin-polarizeddensity functional theory and plane-wave-pseudopotentialscheme as implemented in the QUANTUM ESPRESSOpackage.10 Following our recent studies of bulk Co3O4 andthe Co3O4(110) surface,11−13 the Perdew−Burke−Ernzerhof(PBE)14 exchange-correlation functional with on-site Coulombrepulsion U term15 was used. This approach was foundnecessary to reduce the delocalization error of pure DFT-PBE16

and improve the description of the electronic structure of

Received: June 26, 2013Revised: September 9, 2013Published: September 11, 2013

Article

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© 2013 American Chemical Society 20002 dx.doi.org/10.1021/jp406331h | J. Phys. Chem. C 2013, 117, 20002−20006

Co3O4. However, whereas in our previous studies the value U =5.9 eV given by linear response theory17 was employed, herethe value of U was carefully determined so as to provide asatisfactory overall description of the bulk electronic (e.g., bandgap) and magnetic properties of the different cobalt oxide andhydroxide phases of interest (see section S1 of the SupportingInformation). The value obtained after extensive testing, U = 3eV, is also close to that (U = 3.3 eV) recommended by Cederand co-workers on the basis of oxide formation energies.18

Studies of the dependence of the bulk stability diagram andsome OER free energy profiles on the value of U are reportedin section S2 of the Supporting Information. While thedependence on U is sometimes significant, calculations with U= 3 eV are again found to provide the best agreement with theavailable experimental information.Electron−ion interactions were described by ultrasoft

pseudopotentials,19 with O (2s, 2p) and Co (3d, 4s) electronstreated explicitly. Plane wave kinetic energy cutoffs were set at35 and 350 Ry for the wave functions and augmented densities,respectively. K-point samplings varied from system to system(see Supporting Information); their convergences with respectto computed quantities were carefully tested.We modeled the (0001) surface of CoO(OH) using seven-

layer symmetric slabs with different terminations and vacuumwidths of ∼15 Å. For the CoO2 (0001) surface, we simply usedone CoO2 layer as surface model, since interactions betweenlayers are weak. A rectangular (2 × 1) surface supercell ofdimensions 2.85 × 4.94 Å2 exposing 4 oxygens and 2 cobaltcations was used. Adsorption calculations were performed withadsorbed species present on one side only of the slab.Experimental values of the lattice parameters were used forall systems.

3. RESULTS AND DISCUSSION3.1. Bulk Phase Diagram in Electrochemical Environ-

ment. A detailed discussion of the bulk electronic andmagnetic properties of CoO, Co(OH)2, Co3O4, CoO(OH),and CoO2 under normal conditions is reported in theSupporting Information. In particular, for Co(OH)2 we focusedon β-Co(OH)2, which is the best known form of cobalthydroxide.20 β-Co(OH)2 is isostructural with the mineralbrucite, Mg(OH)2, and consists of a layered structure, withcobalt ions coordinated to O2− anions and layers held togetherby hydrogen bonds (Figure 1). For cobalt oxyhydroxide,CoO(OH), we focused on heterogenite-3R, which the mostcommon form of CoO(OH). This belongs to the hexagonalcrystal family21 and has a simple structure with Co(III) ions inoctahedral sites and oxygen ions in tetrahedral sites,

coordinated to three cobalt ions and one hydrogen (Figure1). Finally, Co(IV)O2 is also a layer compound and the x = 0end member of the LixCoO2 and NaxCoO2 families

22 (Figure1).To study the relative stabilities of CoO, Co(OH)2, Co3O4,

CoO(OH), and CoO2 under electrochemical conditions, wecalculated the Gibbs free energy changes ΔG for the followingreactions:

+ →CoO(s) H O(l) Co(OH) (s)2 2 (1)

→ + ++ −Co(OH) (s) CoO(OH)(s) H e2 (2)

+ → + ++ −Co O (s) 2H O(l) 3CoO(OH)(s) H e3 4 2(3)

→ + ++ −CoO(OH)(s) CoO (s) H e2 (4)

where the symbols (s) and (l) indicate a solid and liquidsystem, respectively. The chemical potentials of the proton andelectron depend on the pH and the external voltage Vaccording to

μ μ μ μ+ = + + −+ − + − +k T a eVlnH e H0

e0

B H

where aH+ denotes the activity of the proton in water solution,−log aH+ = pH, and μH+

0 and μe−0 represent the chemical

potentials of electron and proton under standard conditions (T= 298 K, pH2

= 1 bar). To evaluate the vibrational contributionsto the free energies, we determined the vibrational frequenciesof the different materials at the Γ point using density functionalperturbation theory.23 The only exception was the vibration ofthe proton along the O−H−O direction in CoO(OH), forwhich the structure with the proton in the middle between thetwo oxygens was found to have an imaginary frequency. Thealmost symmetric O−H−O bond structure present in CoO-(OH) (Figure 1) is similar to that found in high-pressure ice,24

where significant quantum effects in the motion of the protonare observed. To investigate the possible role of nuclearquantum effects in CoO(OH), we determined the potentialenergy surface for a proton moving along the O−H−Odirection and then solved the Schrodinger equation for theproton numerically. The computed potential energy surface is adouble-well potential (Figure 2), and solution of the

Schrodinger equation shows that the proton is delocalizedacross the two wells, an indication that at low T quantumeffects should be indeed relevant. We thus determined the zeropoint energy for the proton vibration along O−H−O from theproton ground state energy in the double well of Figure 2.Finally, to complete our free energy calculations, we also

Figure 1. Layer structure of (a) Co(OH)2, (b) CoO(OH), and (c)CoO2 . The main difference among these structures is the number ofprotons between CoO2 layers.

Figure 2. Potential energy (blue) and probability density (black) for aproton moving along the O−H−O direction between two adjacentCoO2 layers in bulk CoO(OH).

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evaluated the free energy change associated with the magneticphase transition of CoO at 289 K using experimentalthermodynamic data at 298 K25 and standard condition. Inthis way, the free energy change for reaction 1 is

Δ = Δ − Δ − Δ

= −

G G G G

1.24 eV/mol

r f Co(OH)0

f CoO0

f H O0

2 2

indicating that CoO in contact with liquid water actuallytransforms to Co(OH)2.The resulting computed Pourbaix diagram (Figure 3a) is in

qualitative agreement with the Pourbaix diagram of cobalt-based electrocatalysts determined with electrochemical techni-ques and EPR spectroscopy.26 Similarly to CoO, also Co3O4 isnot present in the diagram of Figure 3a. Co(OH)2 is thethermodynamic ground state under reduction conditions, i.e., atnegative voltages in acidic solution, whereas CoO(OH) areCoO2 are stable under oxidation conditions, i.e., at externalvoltage larger than 1.23 eV in basic solution. Interestingly, allthree compounds in the phase diagram of Figure 3a, i.e.,Co(OH)2, CoO(OH), and CoO2, share a similar CoO2

x− (x =0−2) layered structure (Figure 1).3.2. Surface Phase Diagram under Electrochemical

Conditions and OER Activity. The surface structures ofCoO2 and CoO(OH), the compounds which are stable underelectrochemical oxidation conditions, were determined by thesame approach used for the bulk phases (section 3.1). For bothmaterials under normal conditions, the natural cleavage surfaceis the (0001) one, but for CoO(OH) different terminations arepossible corresponding to different proton concentrations onthe surface. Three surface models were thus consideredonewith no protons on top (O-terminated), one fully covered byprotons (H-terminated), and one-half-covered by protonsand their surface energies were calculated as a function of pHand applied voltage. The resulting surface phase diagram(Figure 3b) shows that the O- and H-terminated surfaces arestable under oxidizing and reducing conditions, respectively.The stability of the surface with half H-coverage in a large partof the phase diagram suggests that the surface H coverage mayvary during electrochemical experiments.To test the surface reactivity under oxidizing conditions, we

focused on the O-terminated CoO(OH)(0001) and theCoO2(0001) surfaces and studied the energetics of the OERaccording the simplified scheme developed by Nørskov etal.27−29 In this scheme the OER is assumed to consist of four

elementary reaction steps, each involving the coupled transferof an electron to the electrode and a proton to water:

+ ∗ → * + ++ −H O(l) HO H e2 (5)

* → * + ++ −HO O H e (6)

* + → * + ++ −O H O(l) HOO H e2 (7)

* → + ++ −HOO O (g) H e2 (8)

where ∗ denotes a surface site and X* indicates an adsorbed Xspecies; H2O(l) is a water molecule in liquid phase. The freeenergy changes for these steps are calculated including thedependencies on pH and external bias potential as additionalterms; the reference potential is that of the standard hydrogenelectrode at which the chemical potential of added (H+ + e−)equals that of 1/2H2 (at pH = 0). As shown by kinetic studies,the OER mechanism and activity can be influenced by thecoverage of the intermediates and by possible reaction channelsnot included in the scheme mentioned above.30−33 The waterenvironment, where the OER actually takes place, is also notincluded in our calculations. We used these approximationsbecause our goal is to compare the reactivities of differentphases rather than to identify the detailed OER mechanisms.Our computed Gibbs free energy changes for reactions 5−8

on the O-terminated CoO(OH)(0001) and CoO2(0001)surfaces with a half monolayer (ML) of adsorbed species aresummarized in Figure 4. For comparison, the free energy

Figure 3. Phase diagram for (a) bulk cobalt (hydr)oxide and (b) the CoO(OH) (0001) surface (right) in electrochemical environment, from PBE+U calculations with U = 3.0 eV . The value of the external voltage is referred to the standard hydrogen electrode (SHE).

Figure 4. Free energy changes (in eV) at T = 298 K, pH = 0 for thefour steps of the OER at V = 0 vs SHE (full line) and at theequilibrium potential for the OER, V = 1.23 V vs SHE (dashed line),on (a) O-terminated CoO(OH) (0001) (1/2 ML coverage); (b)CoO2 (0001) (1/2 ML coverage); and (c) Co-4f site of the A-terminated Co3O4 (110) surface (1/4 ML coverage). Overpotentialvalues are in parentheses. 1 ML coverage is defined as one adsorbateper cobalt site. All calculations are performed at the PBE +U level, withU = 3 eV.

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diagram for the OER on the Co3O4(110) surface (at 0.25 MLcoverage) is also shown. For the layered structures, the firstdeprotonation is rate limiting and gives rise to a substantialoverpotential (given by the difference between the largest freeenergy change voltage at V = 0 and the minimum voltagerequired for the OER, 1.23 V), indicating that layered CoO2

x−

is not active for the OER. This can be attributed to the fact thatCoO2

x− layers have a very stable and closed structure, andtherefore their interactions with the OER intermediates arerather weak, as illustrated in Figure 5. The overpotential ismuch smaller for the Co3O4(110) surface and also correspondsto a different step of the considered OER mechanism.

Our finding of poor reactivity of CoO(OH) and CoO2 isconsistent with various experimental observations. For instance,LiCoO2, which consists of CoO2

1− layers, was reported of beingnot catalytically active.34 Other studies also pointed out thatCoO2 layers are resistant to water oxidation.35 Thus, othereffects should be considered to explain the OER activity ofcobalt (hydr)oxide based electrodes. For instance, Co3O4 has arather small OER overpotential (see Figure 4 and refs 36 and37), and recent XPS studies have found that the transformationof Co3O4 to layered CoO(OH) or CoO2 is kinetically inhibitedand incomplete.7 However, it seems unlikely that thermody-namically unstable Co3O4 regions remain at the surface of theelectrode. A more plausible explanation could be that defects inthe layered structure, particularly steps, are responsible for thereactivity. In particular, recent computational work37 has shownthat CoOOH(011 2), a minority surface whose structure maybe relatively close to that of a step on the majorityCoOOH(0001) surface, has an OER overpotential close tothat of Co3O4. Moreover, it is well established that step edgesare responsible for the hydrogen evolution activity of MoS2,

38 alayered compound with a structure similar to CoO(OH).

4. SUMMARY AND CONCLUSIONSIn summary, in this work we carried out DFT+U calculations tostudy the stability diagram of cobalt oxides and hydroxides inelectrochemical environment. The value of U (U = 3 eV) wascarefully determined so as to well describe the bulk propertiesof all the different cobalt oxides and hydroxides underinvestigation under normal conditions. Our results show thatthe thermodynamically stable phases of cobalt (hydr)oxides inoxidizing environment are CoO(OH) and CoO2, in agreementwith in situ Raman spectroscopy studies.6 To probe the surfacereactivity under oxidation conditions, we also carried outcalculations of the energetics of the OER, eqs 5−8, on thenatural (0001) surfaces of these layered CoO2

x− structures. Ourresults show that the regular sites of these surfaces are poorlyactive for water oxidation, due to the weak binding energies ofthe various intermediate species. These findings, combined withother available experimental and theoretical information,7,37

suggest that the observed high activity of cobalt oxideelectrodes for oxygen evolution may originate from the kinetic

stability (incomplete transformation) of the catalytically activeCo3O4 phase under electrochemical conditions or, more likely,from surface defects on converted CoO2

x− layers.

■ ASSOCIATED CONTENT*S Supporting InformationBulk properties of cobalt (hydr)oxides and validation of thecomputational approach; influence of U value on calculatedproperties (bulk Pourbaix diagrams and OER free energyprofiles). This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail jiachen@princeton.edu (J.C.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by DoE-BES, Division of MaterialsSciences and Engineering, under Award DE-FG02-06ER-46344and Division of Chemical Sciences, Geosciences and Bio-sciences, under Award DE-FG02-12ER16286. We usedresources of the National Energy Research Scientific Comput-ing Center (DoE Contract DE-AC02-05CH11231). We alsoacknowledge use of the TIGRESS high performance computercenter at Princeton University which is jointly supported by thePrinceton Institute for Computational Science and Engineeringand the Princeton University Office of Information Technol-ogy.

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