Surface Proton Hopping and Fast-Kinetics Pathway of Water Oxidation

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Surface Proton Hopping and Fast-Kinetics Pathway of Water Oxidation on Co3O4 (001) Surface Hieu H. Pham, Mu-Jeng Cheng, Heinz Frei, and Lin-Wang Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00713 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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Surface Proton Hopping and Fast-Kinetics Pathway of Water Oxidation on Co3O4 (001) Surface Hieu H. Pham a, Mu-Jeng Cheng a, b, Heinz Frei c, Lin-Wang Wang a, d* a

Joint Center for Artificial Photosynthesis and Chemical Sciences Division Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA b Department of Chemistry, University of California, Berkeley, California 94720, USA c Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory Berkeley, California 94720, USA d Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley, California 94720, USA Abstract We propose a mechanism of water splitting on cobalt oxide surface with atomistic thermodynamic and kinetic details. The density-functional theory studies suggest that the oxidation process could proceed with several non-electrochemical (spontaneous) intermediate steps, following the initial electrochemical hydroxyl-to-oxo conversion. More specifically, the single oxo sites CoIV=O can hop (via surface proton/electron hopping) to form oxo pair CoIV(=O)-O-CoIV=O, which will undergo nucleophilic attack by a water molecule and form the hydroperoxide CoIII-OOH. Encounter with another oxo would generate a superoxo CoIII-OO-, followed by the O2 release. Finally the addition and deprotonation of a fresh water molecule will restart the catalytic cycle by forming the hydroxyl CoIII-OH at this active site. Our theoretical investigations indicate that all non-electrochemical reactions are kinetically fast and thermodynamically downhill. This hypothesis is supported by recent in situ spectroscopic observations of surface superoxo that is stabilized by hydrogen bonding to adjacent hydroxyl group as an intermediate on fast-kinetics Co catalytic site. Keywords: cobalt oxide, water splitting, electrochemical catalysis, proton dissociation, oxygen evolution, earth-abundant oxides *Corresponding Author: Lin-Wang Wang ([email protected])

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1. Introduction One of the biggest scientific and technical challenges of our time would be the transition from fossil fuels into carbon-neutral energy sources 1. Sustainable resources, such as wind and sunlight, are abundant but intermittent. The commercial and large-scale utilization of these power sources thus necessitates the development of efficient methods for energy conversion and storage. In this context, the splitting of water to produce molecular hydrogen and oxygen, or the direct reduction of carbon dioxide to liquid fuel under oxidation of water (artificial photosynthesis) could be key processes in future green technology to address the climate change problem and the growing energy demand. The development of cost-effective and efficient materials operating at low overpotential has remained a grand challenge. This is in part due to the lack of detailed understanding of the electrochemical catalysis, especially at the oxygen evolution (anode) side

2,3

. Among different

catalysts, great attention has been given to transition metals oxides, due to their Earth abundance, eco-friendliness and stability. Cobalt oxides especially have been shown to be efficient for water splitting 4-7. The thermodynamic model of water oxidation, which constitutes a four-hole cycle, has been widely accepted. Norskov et al. 8 have established a stepwise process that consists of HO*, O*, HOO* and O2 (gas) states on catalyst’s surface (where “*” denotes the adsorbed species). The conversion from one intermediate state to another involves the absorption of one hole (h+), and the overpotential for the whole cycle will be determined by the highest-energy step. Despite this thermodynamic description, the kinetics of such oxygen evolution reaction (OER) remains a subject of rigorous debate 9-12. In particular, the exact nature of hole transfer and how the charge is delivered to a catalytic site is not clear. One simple hypothesis assumes that, at each deprotonation step, one hole must be available to instantly enable the electrochemical conversion. However, if the evolution reactions are relatively rare, it will require the storage and accumulation of free charge carriers nearby, which might not always be feasible or represent the actual situation. In this work, we propose an alternative way by which a hole can be stored on the model Co3O4 (001) surface. More specifically, we assume that when holes are injected, they might first react with the surface CoIII-OH (hydroxyl) to generate CoIV=O (oxo) plus the proton H+ in the electrolyte (CoIII-OH + h+ → CoIV=O + H+). This is plausible since, as will be shown in

the surface Pourbaix diagram, the oxide surface would preferably be passivated with either CoIII-


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OH or CoIII-O termination. We will show that mobile CoIV=O moieties then could serve as the hole sources to drive subsequent OER steps, i.e. the remaining deprotonation steps proceed by redox reaction of the hopping proton (coupled H+/e- surface hopping CoIII-OH + CoIV=O ↔ CoIV=O + CoIII-OH). In fact, the role of surface proton diffusion in oxygen reduction reaction (ORR) has also been studied previously by Rossmeisl et al. using density-functional theory, which suggested that some electrochemical ORR steps on Pt(111) could occur via simple proton transfer

13

. Our proposed hypothesis is supported by a recent study by Zhang and Frei et al.,

where the time-resolved spectroscopy revealed that two adjacent active Co sites on Co3O4 nanoparticle are simultaneously involved in the catalytic activity for each fast-kinetics cycle 4. In contrast, the catalytic oxidation based on a single oxo CoIV=O is shown to be much less active 4. The participation of dual sites is consistent with our model, in which the drift-in of additional CoIV=O would provide the required hole for the next conversion. Our mechanism also agrees with another key finding in this study 4, which indicates that the oxidation reaction continues after termination of initial injection of the hole charge. This suggests that some reaction steps might not require the participation of freshly added charge carriers. In this work, we will study the oxidation reaction on Co3O4 spinel, in particular the energetically preferred (001) surface 14. Several studies have shown that in OER conditions, the sub-shell of Co3O4 could transform to CoOx(OH)y15,16. Still, Co3O4 spinel would serve as a good test model, where its catalytic property has been extensively studied

14,17

. In addition, if major

structural phase change to Co oxy hydroxide were to precede catalytic water oxidation, an induction period would be observed, which however is not the case in Zhang and Frei et al. experiment 4. For the spectroscopic observations reported 4, the active phase is Co3O4, not oxy hydroxide. Here, our model surface is truncated by octahedral Co, which was experimentally indicated as the active OER site 4. From density-functional theory (DFT) calculations, we present a detailed oxidation pathway with clear kinetic and mechanistic descriptions. Key features of our proposed mechanism include: i) The hydroxyl-to-oxo is the only electrochemical step that later provide energy to drive the rest of OER cycle as chemical conversions. ii) The oxo sites are highly mobile due to fast surface proton/electron hopping and are able to form pairs or clusters in the same area with negligible energy penalty. iii) O-O bond formation could be initiated by the attack of H2O on two adjacent surface oxo CoIV=O moieties to form the hydroperoxide CoIIIOOH, subsequently followed by surface H+/e- hopping to another nearby oxo to form the


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superoxo CoIII-OO, which is the precursor of O2 release. All these non-electrochemical reactions have favorable (downhill) thermodynamics and low kinetic barriers. 2. Computational Method Density functional theory (DFT)

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, as implemented in the Vienna ab initio Simulation

Package (VASP), 19 was employed to perform first-principles calculations. Our calculations used the electron projector-augmented wave methods approximation (GGA) exchange-correlation 22

21

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with the PBE generalized gradient

, plus on-site Co d state U corrections (DFT+U)

. A plane-wave cut-off of 400 eV was used and the magnetic moment was accounted for by

performing spin-polarized energy calculations, in which tetrahedral (CoII) and octahedral Co (CoIII) have high and low spin densities, respectively. The value of U = 3.52 eV was used for the Coulomb corrections to the Co 3d states that has been shown to reproduce well the phase stability and electrocatalytic activity of cobalt oxides

15

. The (001) surface truncated with

octahedral Co (CoIII) was modeled using a slab of nine Co layers in a (1 x 1) supercell (Figure 1a) and a vacuum of 25-Å thickness is added to simulate the open space. The calculations of oxo-covered and hydroxyl-covered surfaces were performed with O and OH groups added on either slab surfaces to minimize the dipole moment (atomic coordinates are given in the Supporting Information). All the geometries were optimized under the influence of water (the implicit Poisson-Boltzmann solvent model with dielectric constant ε = 80).23 During the structure optimization, the middle three Co layers were fixed and top three Co layers on both sides are relaxed. For the k-space sampling, we used a 2x2x1 Monkhorst-Pack grid

24

. In this work, the

transition states and kinetic barriers were studied using the dimer method, which was developed by Henkelman et al.

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and the saddle points were then verified by the vibrational frequency

calculations, obtained through the finite difference approach . The zero point energy, thermal enthalpy and entropy corrections (ZPE, ∆Hther and TS) for water, hydrogen molecule and adsorbed species were included to Gibbs free energy calculations of different surface compositions as listed in Supporting Information Tables S1 and S2. These corrections were neglected for the bulk structure (Co3O4 spinel) and the initial CoIII-terminated slab. This approach proposed by Rossmeisl and Norskov et al.

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has been widely used in the

studies of the oxygen reduction reaction and oxygen evolution reaction.


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Figure 1. a) Illustration of top layers of the HO-covered Co3O4 (001) surface (the green phase in b). Red sphere – oxygen, blue – tetrahedral cobalt (CoII), yellow – octahedral cobalt (CoIII). The stacking sequence of this surface in [0 0 1] is CoIII – O – CoII – CoIII – O – CoII. The distances between two CoIII in [1 1 0] and [1 -1 0] directions are approximately 2.8 and 5.7 Å, respectively. b) Surface Pourbaix diagram for Co3O4 (001) obtained from DFT+U calculations. The potential is the potential for positive charge relative to NHE. The electron potential is the negative of the potential V due to the electron negative charge.


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3. Results and Discussions 3.1 Surface Pourbaix Diagram We construct the surface Pourbaix diagram to determine the relative stable coverage for Co3O4 (001) as a function of applied potentials U and pH values. The free energies of different surface terminations (assuming the CoxOyHz composition) are calculated in equilibrium with water, proton and electron, using the following reaction (in which the energy of spinel bulk structure, G(Co3O4), is used as the reference):

+ −

↔ + −

+ 2 − +

(1)

The composition (x,y,z) that minimizes the free energy of formation, Δ , , will be the most stable one to display on the diagram (Figure 1b) for a given pH value and applied potential U, with Δ , being calculated as:

# 4# Δ , = ! " − − % − ' 3 3 + −

(

+ 2 − − − )* +,-10 × )

(2)

where U is the applied electron potential referenced to the normal hydrogen electrode (NHE) at standard conditions. At U = 0 vs. NHE, the Gibbs energy of a hydrogen molecule equals those of two proton

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. For the surface Pourbaix diagram construction, we consider the coverage of oxo

and hydroxyl as one monolayer (on both top and bottom slab sides). It was seen that at pH = 0, this bare surface is stable up to the potential of 0.9 VNHE. However, at high pH and voltages the surface favors the hydroxyl and oxo coverage (Figure 1b). Note, in the above calculation, we can assume that U is determined by the Fermi energy of the n-type carrier electron in Co3O4. For large positive potential (U), there could be a surface depletion layer for the electron. Hence in the oxidation reaction, when hole is generated (e.g., by light absorption), there are two Fermi energies, one is for electron, another for the hole. Within the short duration of the reaction, we can ignore the electron from the interior of the Co3O4 due to the energy barrier created by the depletion layer. The reaction (starting with the Pourbaix diagram stable phase) is then induced by the arrival of the holes. On the other hand, in the absence of hole, the surface Pourbaix diagram will be determined by the Fermi energy of the electron, which shows what surface structure is most stable in the inactive stage. This structure can then be used as the starting point of the reaction (the default structure) when the hole has been used up. In the following study, we will


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calculate the oxidation based on the hydroxyl-covered surface, which is stable within a large region of U and pH values relevant to experimental conditions.

3.2. Oxygen Evolution by a Single Active Site We first examined the OER Gibbs free energy surface that involves single active center. We used the procedure proposed by Norskov and co-workers 8, in which the thermodynamic energies of intermediate states are calculated. The reaction is composed of four electrochemical steps, each of which constitutes one H+/e- transfer with the formation of the Co-exposed surface (*), HO* (hydroxyl), O* (oxo) and HOO* (hydroperoxide) intermediates, respectively. H2O + * → HO* + H+ + e-

(3)

HO* → O* + H+ + e-

(4)

H2O + O* → HOO* + H+ + e-

(5)

HOO* → * + O2 + H+ + e-

(6)

Note, in the above equations, the “+e-“ is equivalent to taking one hole in the reaction. Although denoted as the product of the first step, the HO* is actually the default surface configuration as discussed above. The Gibbs free energy changes for electrochemical steps 3-6 are calculated as: (

∆G1(U, pH) = G(HO*) – G(*) – G(H2O) + G(H2) – eU – kBTln(10)×pH (

∆G2(U, pH) = G(O*) – G(HO*) + G(H2) – eU – kBTln(10)×pH (

(7) (8)

∆G3(U, pH) = G(HOO*) – G(O*) - G(H2O) + G(H2) – eU – kBTln(10)×pH

(9)

∆G4(U, pH) = 4.92 - ∆G1(0, 0) – ∆G2(0, 0) – ∆G3(0, 0) – eU – kBTln(10)×pH

(10)

where G(*), G(HO*), G(O*) and G(HOO*) denote the Gibbs free energies of surfaces with bare Co site, HO*, O* and HOO* ligands, respectively. In order to avoid the calculation of oxygen molecule O2 energy which might have errors within some DFT framework

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, ∆G4 is obtained

from the experimental Gibbs free energy formation of water oxidation equation at standard conditions 2H2O → O2 + 2H2, ∆G = - 4.92 eV. The equilibrium potential to produce oxygen by water oxidation is therefore (eU + kBTln(10)×pH)/e = 1.23 V for the ideal case when ∆G1 = ∆G2 = ∆G3 = ∆G4 = 0. In practical cases, the overpotential is determined by requiring all of ∆G1, ∆G2, ∆G3, ∆G4 to be negative (free energy goes downhill in all intermediate steps) by choosing the


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lowest eU + kBTln(10)×pH. In this section, for simplicity we only discuss U at pH = 0 condition. More specifically, the OER overpotential (at pH=0) can be calculated as: 1234 = 56# 7∆( 0,0, ∆ 0,0, ∆ 0,0, ∆ 0,09⁄ − 1.23

Surface Proton Hopping and Fast-Kinetics Pathway of Water Oxidation

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