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Hydrogen Oxidation and Water Dissociation over an Oxygen-Enriched Ni/YSZ Electrode in the Presence of an Electric Field: A First Principles-Based Microkinetic Model Fanglin Che, Su Ha, and Jean-Sabin McEwen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04028 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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Industrial & Engineering Chemistry Research

Hydrogen Oxidation and Water Dissociation over an Oxygen-Enriched Ni/YSZ Electrode in the Presence of an Electric Field: A First Principles-Based Microkinetic Model Fanglin Che,a Su Ha,a Jean-Sabin McEwen*abc a

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, WSU, WA, 99164 b

Department of Physics and Astronomy, WSU, WA, 99164 c

Department of Chemistry, WSU, WA, 99164

Abstract Elucidating the sulfur poisoning or coking for electrochemical cells (e.g., a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC)) is highly dependent on studying such mechanisms by which said catalysts deactivate under experimentally relevant conditions. For a SOFC (or a SOEC) system, this requires the inclusion of the effect of a negative (or a positive) electric field when modeling the elementary catalytic reactions. In this contribution, the field effects on hydrogen oxidation and water decomposition over the triple phase boundary (TPB) region of the Ni/YSZ electrode are investigated using a field-dependent microkinetic model. Our results firstly show that the field effects on the Ni surface of the Ni/(YSZ+O) model are different as compared to a pure Ni(111) surface due to a difference in the charge distribution on the said surfaces. Between 400 K to 1200 K, the negative fields assist in hydrogen oxidation over the TPB region of the Ni/(YSZ+O) cermet, which can potentially result in a larger probability for the said model to have oxygen vacancies at the TPB. Consequently, deactivation from sulfur poisoning or coking can increase since such vacancies are active for sulfur adsorption or coke formation. On the other hand, a high positive electric field can decrease the water decomposition rate to form hydrogen as compared to when the field is

*

Corresponding author: Email: [email protected] (J.-S. McEwen); Phone: (+1)509-335-8580

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absent. Overall, this study provides insights for considering the electric field effects on the hydrogen oxidation and water dissociation over Ni/(YSZ+O) electrodes.

1. Introduction Electrochemical cells containing Ni with a yttria-stabilized zirconia support (Ni/YSZ) have become an attractive and high efficient energy conversion technology, which produces less harmful greenhouse emissions as compared to the traditional fuel combustion process.1, 2 For example, direct-methane solid oxide fuel cells (SOFCs) at high temperatures can convert chemical energy (CH4) directly to electricity.3-9 Methane can also be activated in a solid electrolyte (i.e. Ni/YSZ) single-chamber cell, which applies electricity to dramatically alter the production yield or selectivity.10, 11 In such cases, the nonfaradaic electrochemical modification of catalytic activity (NEMCA) caused by the applied electric potential is observed and contributes to changes in the catalyst work function, as well as the interaction between the catalytic surface and the fuel.12 However, one of the major issues facing the use of a Ni/YSZ cermet for hydrocarbon fuel activation is its catalytic degradation by sulfur poisoning and coking.1319

Since such a degradation usually occurs at the oxygen vacancies in the triple phase

boundary (TPB) region of the Ni/YSZ cermet,20 it is essential to understand the kinetics involved in the formation of oxygen vacancies over the TPB area of Ni/YSZ via H2 oxidation in a direct-methane electrochemical cell environment. Additionally, as the reverse mode of a SOFC, solid oxide electrolysis cells (SOEC) provide hydrogen via water decomposition over the Ni/YSZ surface at high temperatures.21-25 For a SOEC system, besides the sulfur poisoning at the TPB region of the Ni/YSZ electrode, the Ni cluster in the Ni/YSZ cermet also can get easily oxidized 2 ACS Paragon Plus Environment

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under high temperature and high steam pressure conditions. Therefore, having a lowtemperature SOEC is of importance since it can potentially lower the oxidation of the Ni cluster of the Ni/YSZ cermet and enhance its stability. As such, it is necessary to understand the working mechanisms of the Ni/YSZ electrode for the decomposition of water in a SOEC as well. In recent years, many theoretical and experimental studies have reported mechanisms for forming oxygen vacancies or water decomposition at the Ni-YSZreactant TPB area of the Ni/YSZ cermet.26-31 Ammal and Heyden studied the oxidation of hydrogen on Ni/YSZ (Ni18Zr40Y8O92) via a microkinetic model based on DFT calculations. They concluded that the O migration from the YSZ surface to the Ni cluster was much faster than the H spillover from the Ni cluster to the YSZ surface and OH migration from the YSZ surface to the Ni cluster.32 Theoretical calculations from Y. Zhang et al. 16 and Ziegler et al.33 found that S or C diffused easily to an oxygen vacancy at the TPB area of the Ni-based electrode and that S or C gets trapped at the oxygen vacancy, which is very difficult to remove by hydrogen fuel. In their manuscript, they suggested that the trapped sulfur or carbon at the Ni-based electrode interface is key to the sulfur poisoning or coking mechanism in a solid oxide fuel cell system. Li et al. experimentally investigated the H2S poisoning of the Ni/YSZ anode in a SOFC environment.34 Based on impedance spectroscopy analysis, they showed that H2S poisoning was associated with the blocking of oxygen ion migration to the TPB region as well as forming Ni3S2 that could not be removed by hydrogen, which results in permanent damage to the cell.34, 35 To help provide insights into how one can mitigate the degradation due to sulfur poisoning, experimental work from L. Yang et al. 36 found that 3 ACS Paragon Plus Environment


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sulfur poisoning was caused by the strong adsorption of elemental sulfur on the Ni surface and the TPB between Ni, the electrolyte, and fuel. In their experiments, they enhanced the sulfur or coke resistance of the Ni/YSZ electrode via changing the YSZ support to a BZCYYb (BaZr0.1Ce0.7Y0.2–xYbxO3–δ) cermet. Zhang et al. performed DFT calculations to examine the effects of oxygen vacancy concentration on sulfur poisoning within a Ni/YSZ model.37 Compared to the Ni/YSZ model with higher interface oxygen vacancy concentrations (> 18.8 mol%), the corresponding models with lower oxygen vacancy concentrations can weaken the adsorption of sulfur at the TPB vacancy site as well as increase the hopping rate of a sulfur atom from the TPB region to the Ni cluster. In a SOEC, Gu and Nikolla38 performed DFT calculations and reported that the Ni/Fe alloy had a higher water dissociation rate to provide hydrogen as compared to the pure Ni slab or other Ni/M alloy systems (M=W, Co, Cu, Zn, Ga, and Ge). This indicates that in a SOEC, the Ni/Fe alloy can potentially lower the temperature for water dissociation as compared to the other examined alloys. Although current DFT studies carefully examine the kinetics and the energetics involved in the formation of oxygen vacancies via hydrogen oxidation or the formation of hydrogen via water decomposition over the TPB area of a Ni/YSZ electrode, as well as the degradation mechanisms over its TPB region, the influence of the electrochemical cell environment on the underlying electro-catalytic mechanisms has remained largely uninvestigated. Theoretically, the Janik, Heyden and Nikolla groups38-40 studied electrochemical reactions (borohydride oxidation, hydrogen oxidation, or water dehydrogenation) on metal surfaces (or perovskite surfaces, or metal/metal-oxide slabs). These studies treated the electrochemical environment in a similar way. In their reported 4 ACS Paragon Plus Environment

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results, for an elementary reaction in a fuel cell or an electrolysis cell environment, the change in Gibbs free energy (∆) and the activation energy barrier ( ) in the presence of an electric field (F) were defined as Eq. (1) and Eq. (2): ∆ = ∆ = 0 − ∆

(1)

= = 0 − ∆

(2)

where ∆ = 0 and = 0 represent the change of the Gibbs free energy and the activation energy barrier for an elementary step under experimentally relevant temperatures and pressures in the absent of the electric field; n stands for the number of electrons involved in the elementary step; α is the transfer coefficient, which ranges from 0.3 to 0.7 for most electrochemical systems; and ∆ is the electrical potential between the anode and the cathode. Such calculations where performed by combining DFT calculations with statistical mechanical techniques in the absence of an electric field. The calculated ∆ = 0 and = 0 values were then used to determine the rate and equilibrium constants for each elementary reaction to establish a field-dependent microkinetic model. However, these studies neglected the effects of the electric fields generated by the potential drop across the electrode/electrolyte interface on the reaction energies and activation energies of the elementary steps. In an electrochemical cell (i.e., SOFC and SOEC), the Helmholtz layer (a thinlayer interface between the electrode and the electrolyte) has a large potential drop caused by the electronic structure of the electrode, species adsorption at the surface, and the potential from the electrolyte bulk solutions.41 In general, the values of the electrode potential are ~ ±1.5 VNHE. If an extra electrode potential of ~ ± 1.5 VNHE is applied in an electrolysis cell, the total electrode potential can be up to ~ ± 3 VNHE.41, 42 In addition, the 5 ACS Paragon Plus Environment


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thickness of a typical Helmholz layer can be 3 Å to 8 Å.5, 43, 44 As such, a large interfacial uniform electric field in an electrochemical cell, on the order of ±1.0 V/Å, can be generated and it is strong enough to influence the electronic structure and bonding nature of molecules. Our previous work has already shown that a large negative electric field and a high positive electric field due to the presence of the Helmholz layer in a SOFC and a SOEC, respectively.45 Such a large electric field can largely alter the electronic interactions of the metal and adsorbates,46-48 the catalytic activities and selectivity of heterogeneous reactions,49-51 and the electronic properties of the TPB region of the metal/metal-oxide cermet.45 Therefore, to bridge the gap between the field-dependent reaction kinetics and the experimental observations on the degradation and working mechanisms of the TPB region of the Ni/YSZ electrode in a SOFC and a SOEC system, we present a field-dependent microkinetic model of the TPB region of Ni/YSZ for hydrogen oxidation and water decomposition with varying operating temperatures and electric fields.

Scheme 1. Proposed H-spillover, O-spillover, and OH-spillover pathways for forming an oxygen vacancy at the triple phase boundary (TPB) area of the Ni/YSZ model. The blue 6 ACS Paragon Plus Environment

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and green circles represent two different O atoms at the TPB region (filled) and the generated oxygen vacancy via H2 oxidation (empty), respectively. Based on our previous work45 and the phase diagram of the Ni/(YSZ+O) model as a function of the hydrogen or oxygen chemical potentials at a range of temperatures (Section 1 in the SI), we concluded that for the electrode of a SOFC and a SOEC, the most favorable configuration is the Ni/(YSZ+O) model at low hydrogen pressures (< 1.2 × 104 Pa) or with a small amount of oxygen gas (> 0.1 Pa). In this present paper, we will present the electric field effects on: (i) the energetic profiles of different reaction pathways of forming a TPB oxygen vacancy inside a Ni/(YSZ+O) model via hydrogen oxidation, including the H spillover and O spillover pathways (Scheme 1); (ii) a microkinetic model for the formation and decomposition of water via all possible elementary steps in Scheme 1 under different operating temperatures and various electric fields; (iii) the rate-limiting steps as a function of temperature during the electrocatalytic hydrogen oxidation and water decomposition over the TPB area of a Ni/YSZ model. This study can deliver both valuable insights into the influence of electric fields on hydrogen oxidation and water decomposition over the Ni/YSZ cermet, and provide some insights for future studies on the deactivation mechanisms of the said electrode during methane activation (e.g., direct methane SOFC and internal reforming SOFC).

2. Methods 2.1 DFT Computational Setup All of our calculations were performed by spin-polarized plane wave density functional theory with the Vienna Ab Initio Simulation Package (VASP) code.52, 53 The projector-augmented wave (PAW) method54 was used to calculate the ion-electron 7 ACS Paragon Plus Environment


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interaction and the generalized gradient approximation55,

56

with the Perdew-Burke-

Ernzerhof (PBE)56 functional was used to solve the exchange-correlation interactions in the Kohn-Sham equations in a periodic system.53 We set up an energy cutoff of 400 eV for the plane wave basis sets with the Gaussian smearing method (σ = 0.1 eV). When increasing the energy cutoff to 500 eV, the oxygen vacancy formation energies for a YSZ system changed by less than 0.03 eV regardless of the applied field strength.45 To model the triple phase boundary between a Ni cluster and a YSZ support, a Ni/YSZ slab with a unit cell dimension of 7.25 Å × 12.56 Å (Fig. 1) was used, which is similar to the Ni/YSZ model proposed by Shishkin and Ziegler.4, 37, 57-62 In the calculations, a Monkhorst-Pack mesh63 with a (4 × 2 × 1) k-points grid was chosen for the Brillouin zone integration. To avoid field emission as well as the interaction between each periodic supercell in the perpendicular direction, a 12 Å vacuum was used in our model. The structure optimization of all systems was converged to 10-5 eV and the forces were smaller than 0.03 eV/Å. More details regarding the accuracy and validity of the Ni/YSZ model, including layer effects, size effects, oxygen vacancy concentration effects, the Y atom location effects and the reason of performing pure DFT calculations rather than applying Hubbard-type U parameter,32, 64, 65 were all addressed in our previous work.45


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Fig. 1. Side and top views of an oxygen enriched Ni/YSZ model (Ni/(YSZ+O)). The black dotted line shows that the bottom O-Zr-O layer of the YSZ support is fixed during the calculations. We used dark green, light green, white, and red atoms to present Y, Zr, Ni, and O species. The oxygen atom highlighted in yellow is the interface O that is most favorably removed via hydrogen oxidation.45 We also note that after removing one TPB O atom, the Ni/YSZ+½O model is formed and the concentration of oxygen vacancies at the TPB is 25%.

Additionally, we explain the nomenclature here. Ni/(YSZ+O) is used to represent the oxygen-enriched Ni/YSZ model and Ni/(YSZ+½O) is used to represent the Ni/(YSZ+O) model with one interface oxygen removed − corresponding to a TPB vacancy concentration of 25%. For the H spillover path, 2H/Ni/(YSZ+O) corresponds to the Ni/(YSZ+O) model with two H atoms adsorbed on the Ni cluster. Similarly, OH_H/Ni/(YSZ+½O) and H2O/Ni/(YSZ+½O) represent Ni/(YSZ+½O) with either one OH group at the TPB region and one H still adsorbed on the Ni cluster or one H2O molecule formed at the TPB region, both of which have H incorporated into the TPB O atom. To apply a simulated electric field to our periodic system, we used the Neugebauer and Scheffler method,66 wherein an artificial dipole layer is introduced to the middle of the vacuum layer to generate a uniform electric field. To assist the reader, we assigned negative electric field values to the simulated fields that point from the vacuum to the surface (making the surface more negatively charged), while positive values denote electric fields that point in the opposite direction (making the surface more positively charged). To make this explicit, the field in all locations is perpendicular to the a-b plane of the model (i.e. the YSZ surface). As a result, it is not perpendicular to the Ni cluster but on top of the YSZ surface. More details regarding the local charge distribution and 9 ACS Paragon Plus Environment


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electronic structure analysis of the Ni/YSZ system with oxygen vacancies at different concentrations in the presence and absence of an electric field can be found in our previous work.45 2.2 Microkinetic Model 2.2.1 Rate constants for reactions on the surface Between the initial and final states (IS and FS), all the images along the elementary reaction pathways of the proposed mechanisms (Scheme 1) were, as a first step, optimized using the nudged elastic band (NEB) method and further refined using the climbing image nudged elastic band (CINEB) method to find the true transition states (TS).67,

68

The field-dependent reaction energy (∆ ) and the activation energy

( ) are defined as: ∆ = −

(3)

= −

(4)

where , , and are the field-dependent total energies of the IS, TS, and FS along the minimum energy path. The field-dependent forward and backward rate constants ( and ), and equilibrium constant (K(F)), at a certain temperature can be calculated using transition state theory:69-71 = =

∗

)

∗

"

'

# $%& ⁄ '

'

# $%& $∆*+,- ⁄

. = / 0

where kB and h are the Boltzmann's

and Planck's

(5) (6) (7)

constants; 1 and

1 represent ∆ the zero-point energy (ZPE) corrected activation energy and reaction


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energy; T is the absolute temperature; 3 , 3∗ , and 3 are the fielddependent harmonic vibrational partition functions of the IS, TS, and FS: 3 = ∏
?@A

(10) (11) ∗

M where K

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