Fundamental Insights into High-Temperature Water Electrolysis Using

Oct 26, 2015 - Our studies show that H2O dissociation is the rate-limiting step on Ni(111) and Ni(211), with Ni(211) exhibiting the lowest barrier for...
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Fundamental Insights into High-Temperature Water Electrolysis Using Ni-Based Electrocatalysts Xiang-Kui Gu and Eranda Nikolla* Department of Chemical Engineering and Materials Science, Wayne State University, Detroit 48202, United States ABSTRACT: Hydrogen production from water electrolysis using solid oxide electrolysis cells (SOECs) has attracted considerable attention because of favorable kinetics and thermodynamics associated with operation at elevated temperatures. In the present work, we employ density functional theory calculations combined with microkinetic modeling to investigate the factors that govern this process on Ni and Ni-based alloy electrocatalysts. Our studies show that H2O dissociation is the rate-limiting step on Ni(111) and Ni(211), with Ni(211) exhibiting the lowest barrier for this step. The effect of alloying Ni with another metal on the energetics associated with this process is also investigated. Our studies show that the binding energies of the most abundant intermediates, OH and O, become gradually weaker, and the barriers for water dissociation become gradually higher as Ni is alloyed with metals from left to right in the periodic table. A volcano-type relationship between the calculated electrochemical rates and the binding energies of O is found, with the Ni/Fe alloy exhibiting the highest rate among the Ni alloys considered. These predictions are consistent with the reported experimental results, suggesting that these structure/performance relationships can be used to guide the design of heterogeneous electrocatalysts for high-temperature water electrolysis.

1. INTRODUCTION Hydrogen is an important chemical and energy source, widely used in several key industrial processes,1−3 such as desulfurization of petroleum,4 ammonia,5,6 and Fischer−Tropsch synthesis,7 and as fuel for generation of electricity in fuel cells.8 Electrolysis of water using the energy generated from renewable sources, such as the sun and the wind, has attracted significant attention to generate clean hydrogen.9,10 This process also provides an avenue for storing the electrical energy generated from renewable sources in a chemical form. Water electrolysis using high-temperature solid oxide electrolysis cells (SOECs) has been considered as an efficient route for this process because of favorable kinetics and thermodynamics at elevated temperatures.11−22 In addition to high efficiency, this route also leads to the evolution of pure oxygen, significantly enhancing the overall economics of the process.23,24 SOECs are solid-state electrochemical devices that under an applied potential facilitate the electrochemical splitting of water at the cathode generating hydrogen and oxygen ions. Oxygen ions are transported through an ion-conducting electrolyte to the anode, where they are evolved as gas-phase oxygen. The half-cell electrochemical water splitting reaction at the SOEC cathodes is shown via eq 1. This process involves water (in the form of steam) drawing two electrons from the SOEC cathode electrocatalyst to form O2− anions at the electrolyte and gasphase hydrogen.

Currently, the most commonly used cathode electrocatalyst for SOECs is monometallic Ni because of its low cost, high electron conductivity, and thermal compatibility with the other cell components.20,22 To enhance the triple-phase boundary (the interface between the electrocatalyst, electrolyte, and gasphase reactive species) and to facilitate ion transport at the cathode, monometallic Ni is typically mixed with the electrolyte oxide, yttria-stabilized zirconia (YSZ).14,17 Although Ni has shown promise as an SOEC cathode electrocatalyst,20 it exhibits high overpotential losses for water electrolysis leading to high input potentials for obtaining reasonable electrolysis rates.25 Optimization of the catalytic activity of Ni has been quite challenging due to limited understanding of the factors that govern water electrolysis on Ni-based cathode electrocatalysts. Experimental studies from Ishihara et al. have shown that combining Ni with Fe and Pt can enhance the H2 generation rate at the SOEC cathode.12 They reported that Ni/Fe bimetallic electrocatalyst exhibited the highest H2 generation rate as compared to monometallic Ni and Ni/Pt, suggesting that mixing Ni with another metal can enhance the catalytic activity toward water electrolysis.12 In this contribution, we utilize density functional theory (DFT) calculations, statistical thermodynamics, and microkinetic modeling to understand the underlying mechanism that governs electrochemical water splitting on Ni electrocatalysts under SOEC operating conditions. Our calculations show that

H 2O(g) + 2e− → Oelectrolyte 2 − + H 2(g) (overall half‐cell reaction) © XXXX American Chemical Society

Received: August 11, 2015 Revised: October 20, 2015

(1) A

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The binding energies of related species are calculated as

on Ni(111) and Ni(211) surfaces water dissociation is the ratelimiting step with Ni(211) exhibiting the lowest barrier for this step. We also discuss the effect of alloying Ni with another metal on the electrochemical activity toward water electrolysis. It is well-known that the surface chemistry of metals can be tuned via alloying with another metal because of the alloyinduced electronic and geometric effects.26−31 We find that alloying Ni with another metal alters the electrolysis activity, and this activity can be predicted by the binding energies of O on the Ni alloy surfaces. A volcano-type relationship between the calculated electrochemical rates and the binding energy of O on various Ni-based alloys is obtained. Our results suggest that alloying Ni with Fe would lead to a higher rate for water electrolysis as compared to monometallic Ni and the other Ni alloys considered here, consistent with experimental report.12

BE = Ead/sub − Ead − Esub

(2)

Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the gas phase, and the clean substrate, respectively. The transition state of the elementary steps is determined using the climbing-image nudged elastic band (CI-NEB) method,38,39 and it is confirmed using a single imaginary frequency. Quantum tunneling effect on the energetics of this process is neglected since the focus is on water electrolysis at high temperatures (∼1073 K) and defect surfaces, conditions under which this phenomenon is not significant.40,41 In our calculations, we have also neglected the zero-point energies. Our test calculations (not included here) showed that, while the zero-point energy correction affected the absolute value of the rates, the relative trends of activity for water electrolysis on Ni and Ni alloys were not affected.

2. COMPUTATIONAL METHODOLOGY Spin polarized periodic DFT calculations are performed with the Vienna ab initio simulation package (VASP).32,33 The exchange-correlation interaction is described by the generalized gradient approximation (GGA) and Perdew−Burke−Ernzerhof (PBE) functional.34 Other functionals, such as the Perdew− Wang exchange and correlation functional (PW91), were also tested, and a very small difference in the calculated binding energies of species involved in water dissociation on Ni(111) was found, consistent with literature reports for water dissociation on Cu(111).35 The Kohn−Sham equations are solved in a plane wave basis set with a kinetic energy cutoff of 400 eV. The Ni(111) and Ni(211) surfaces are modeled using a four-layer slab model with (2 × 2) and (1 × 4) unit cells, respectively. The Ni3M(211) alloy surfaces (where M represents the alloy element) with homogeneous structures are modeled using (1 × 2) unit cells with similar unit cell structures as Ni(211) (Figure 1). The YSZ electrolyte is

3. RESULTS AND DISCUSSION 3.1. Mechanisms for Water Electrolysis. Three potential mechanisms for water electrolysis at high temperatures are considered, as shown in Scheme 1. The mechanisms considered Scheme 1. Proposed Half-Cell Reaction Mechanisms for Water Electrolysis at the SOEC Cathode

Figure 1. Side view of the model systems used for Ni(111), Ni(211), and Ni3M(211) surfaces. Green atoms represent Ni, while blue atoms represent the alloy element.

here are based on the reverse mechanisms proposed for H2 electro-oxidation on Ni anode electrocatalysts in solid oxide fuel cells, since the mechanism for water electrolysis is not known.42 The elementary steps involved in these mechanisms are listed in Table 1. In mechanism 1, H2O dehydrogenates to OH* and H* adsorbed on the metal surface followed by dehydrogenation of OH* to H* and O*. The adsorbed oxygen atom (O*) on the metal surface, in the presence of two electrons, is then reduced to oxygen ions (O2−) in the electrolyte. On the other hand, H* desorbs from the metal surface via the formation of gas-phase hydrogen. In mechanism 2, the first step is similar to mechanism 1, but the second step involves the dehydrogenation of the OH* species on the metal surface in the presence of electrons leading to the formation of oxygen ions (O2−) in the electrolyte and adsorbed H* on the metal. In mechanism 3, we assume that the electron transfer is completed in two steps. The first step involves electrochemical water reduction via one electron to form OH− ions in the electrolyte and adsorbed hydrogen (H*) on metal, followed by H spillover from the OH− ions in the electrolyte to the metal,

modeled using the most stable cubic-ZrO2(111) (composed of three O−Zr−O layers) with one of the Zr atoms replaced by an Y atom. In this model, the concentration of Y is ∼8.33%, close to that of 8% used in the experimental studies.22 Our calculations show that surface and subsurface substitutions of Y in ZrO 2 are comparable, consistent with literature reports.36,37 In our calculations, the surface substitution of Y in ZrO2 is used. We discuss the effect of the YSZ model system on the electrolysis rates below. A (4 × 4 × 1) k-point mesh is used to sample the surface Brillouin zone, and a 12 Å vacuum is introduced between the repeated slabs along the z-direction. Convergence of the binding energies with respect to all electronic parameters is confirmed. During optimization, the bottom two layers of the slab are fixed, while the remaining atoms and adsorbates are relaxed until the residual forces are less than 0.02 eV/Å. B

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The Journal of Physical Chemistry C Table 1. Elementary Steps Involved in the Three Potential Mechanisms for Water Electrolysis at High Temperatures mechanism 1

mechanism 2

mechanism 3

H 2O + 2* → OH* + H*

H 2O + 2* → OH* + H*

H 2O + * + VO(YSZ) + e− → OH− + H*

OH* + * → O* + H*

OH* + 2e− → O2 − + H*

OH− + * + e− → O2 − + H* + VO(YSZ)

O* + 2e− → O2 − + *

2H* → H 2 + 2*

2H* → H 2 + 2*

2H* → H 2 + 2*

be 0.98 V at 1073 K on the basis of literature reports.43,44 In the free-energy calculations (eqs 7−10), the entropic contributions for both H2O and O2 are considered. The entropic contribution for H2O adsorption is calculated to be 1.91 eV at 1073 K and standard pressure. 45 The entropic contribution for O 2 desorption from the surface to gas phase is calculated to be 1.99 eV under the same conditions. In these studies, we have neglected the effect of the electric field induced by the potential drop across the cathode/electrolyte interface on the free energies of the chemical intermediates, since it has been shown that this effect is fairly small at realistic values of the electric field.42,43 The activation barriers for water dissocation to OH or OH− are calculated with respect to H2O in the gas phase, taking into account the entropic contribution of 1.91 eV for H2O adsorption. The barrier for OH dissociation is calculated with respect to adsorbed OH*. Equation 11 is used to account for the effect of the potential on the activation barriers associated with the elementary reaction steps involving electron transfer. This methodology was adopted from literature.46−48

leaving behind O2− ions in the electrolyte. Given the complexity of an interfacial metal-alloy/electrolyte model, here we assume that the water dissociation step occurs on the metal or metal alloy surface. We discuss the effect of this approximation on the activity trends below. In all three mechanisms, it is assumed that the O2− ions formed in the electrolyte are transported through the electrolyte to the anode, where they are evolved as gas-phase O2 without significant losses. Since the losses associated with these processes are neglected in all three mechanisms and all the electrocatalytic systems, we anticipate that the activity trends for the half-cell reactions at the cathode will not be affected because of cancellation of errors. The Gibbs free energies for the chemical elementary steps that do not involve electrons in the three mechanisms discussed above are calculated using DFT combined with statistical thermodynamics. In the case of the elementary steps involving charged species (i.e., OH− and O2− ions), which are directly affected by the cathode potential bias, the free energies are calculated with respect to a standard oxygen electrode (SOE, 1/ 2O 2 + 2e − →O YSZ 2− ), as previously discussed in the literature.42,43 On the basis of this approach, the free energy of steps 3−6 (the electrochemical steps in the three mechanisms) can be expressed using eqs 7−10, respectively. O* + 2e− → O2 − + *

(3)

OH* + 2e− → O2 − + H*

(4)

H 2O + * + VO(YSZ) + e− → OH− + H*

(5)

OH− + * + e− → O2 − + H* + VO(YSZ)

(6)

Ea = Ea0 − nαΔV

E0a is the activation barrier of the chemical step,46 n is the number of electrons involved in the elementary step, and α is the transfer coefficient, which ranges from 0.3 to 0.7 for most electrochemical systems.48 Here, the reported barriers for the electrochemical steps are based on an α value of 0.3. The dependence of the electrochemical rate on α is discussed later. From eq 11, it is clear that the barrier becomes lower with an increase in the applied potential, ΔV. In the case of hydrogen spillover and hydrogen association steps, Ea is calculated from the thermodynamic barrier. 3.2. Water Electrolysis on Ni. 3.2.1. Energetics of Water Electrolysis on Ni(111) and Ni(211). The calculated binding energies of the intermediates involved in water splitting on Ni(111) and Ni(211) surfaces are shown in Table 2. The calculated H binding energy of −2.81 eV on Ni(111) is comparable to that on Ni(211) (−2.81 eV) and is close to the

Gibbs free energies: ΔG3 = 1/2GO2 + G − GO * − 2eΔV *

(7)

ΔG4 = 1/2GO2 + G H * − GOH * − 2eΔV

(8)

(11)

ΔG5 = GOH(YSZ) + G H * − G H2O − GVO(YSZ) + eVcathode (9)

Table 2. Calculated Binding Energies (eV) of Chemical Species Involved in Water Splitting on Ni and Ni Alloys

ΔG6 = 1/2GO2 + G H * + GVO(YSZ) − G − GOH(YSZ) * + eVcathode (10)

where GOH(YSZ) and GVO(YSZ) are the total energy of OH adsorbed on the surface oxygen vacancy of YSZ(111) and the total energy of YSZ(111) with one surface oxygen vacancy, respectively. ΔV is the applied potential defined as the difference in the potential between the SOE and the cathode (ΔV = VSOE − Vcathode). Since the anode of the SOEC involves the same chemical transformation as the SOE, if in equilibrium and under standard pressure conditions, it can play the role of the standard electrode. Under these conditions, the anode potential can be described as Vanode = VSOE and is calculated to

Ni(111) Ni(211) Ni3Fe(211) Ni3Co(211) Ni3Cu(211) Ni3Zn(211) Ni3Ga(211) Ni3Ge(211) Ni3Os(211) Ni3W(211) C

H2O

OH

O

H

−0.21 −0.54 −0.63 −0.64 −0.50 −0.42 −0.48 −0.58 −0.79 −1.18

−3.14 −3.73 −4.03 −3.87 −3.53 −3.45 −3.36 −3.21 −3.94 −4.87

−5.62 −5.76 −6.02 −5.91 −5.68 −5.65 −5.41 −5.20 −6.46 −7.56

−2.81 −2.81 −2.82 −2.78 −2.83 −2.96 −3.06 −3.12 −2.92 −2.86

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Table 3. Calculated Energy Barriers (eV) and Gibbs Free Energies (eV) for the Elementary Steps Involved in Water Electrolysis at 1073 K, Standard Pressure, and ΔV = 1.30 V H2O + 2* → OH* + H* Ni(111) Ni(211) Ni3Fe(211) Ni3Co(211) Ni3Cu(211) Ni3Zn(211) Ni3Ga(211) Ni3Ge(211) Ni3Os(211) Ni3W(211)

OH* + * → O* + H*

OH* + 2e− → O2− + H*

H2O + * + VO(YSZ) + e− → OH− + H*

OH− + * + e− → O2− + H* + VO(YSZ)

2H* → H2 + 2*

Ea

ΔG

Ea

ΔG

Ea

ΔG

Ea

ΔG

ΔG

ΔG

2.55 2.30 1.99 2.15 2.38 2.52 2.64 2.81 1.72 1.01

1.34 0.75 0.45 0.65 0.94 0.89 0.87 0.96 0.44 −0.43

0.90 1.48 1.44 1.49 1.31 1.31 1.50 1.42 1.13 0.58

−0.33 0.12 0.15 0.13 −0.02 −0.20 −0.15 −0.16 −0.48 −0.60

0.12 0.70 0.66 0.71 0.53 0.53 0.72 0.64 0.35 0

−1.58 −1.00 −0.71 −0.83 −1.22 −1.43 −1.62 −1.83 −0.89 0.09

2.16 1.91 1.60 1.76 1.99 2.13 2.25 2.42 1.33 0.62

−1.56 −1.56 −1.57 −1.53 −1.58 −1.71 −1.81 −1.87 −1.67 −1.61

1.32 1.32 1.31 1.35 1.30 1.17 1.06 1.00 1.21 1.27

−0.51 −0.51 −0.49 −0.57 −0.47 −0.21 0 0.12 −0.29 −0.41

Figure 2. Gibbs free energy diagrams for (a) thermochemical water splitting and water electrolysis using (b−d) three potential mechanisms on Ni(111) and Ni(211) at 1073 K, standard pressure, and applied potential (ΔV) of 1.30 V.

previously reported value of −2.84 eV.49 As expected, the binding energies for H2O, OH, and O on a step Ni(211) surface (−0.54, −3.73, and −5.76 eV, respectively) are stronger than those on a terrace Ni(111) surface (−0.21, −3.14, and −5.62 eV, respectively). This is especially the case for OH, for which the binding energy on Ni(211) is ∼0.59 eV stronger than on Ni(111). The binding enegies for H2O and OH on Ni(111) are consistent with the reported values of −0.29 and −3.14 eV.50 The OH binding energy has a significant impact on the energetics associated with H2O and OH dissociation. A higher binding energy of OH results in a lower barrier for H2O dissociation but a higher barrier for OH dissociation to O. As

shown in Table 3, the barrier for thermochemical (nonelectrochemical) water dissociation (2.30 eV) on Ni(211) is 0.25 eV lower than that on Ni(111) (2.55 eV). However, the barrier for thermochemical OH dissociation (1.48 eV) on Ni(211) is 0.58 eV higher than that on Ni(111) (0.90 eV). The Gibbs free energy diagrams for thermochemical water splitting and water electrolysis using the three mechanisms dicussed earlier on Ni(111) and Ni(211) at 1073 K, standard pressure, and ΔV = 1.30 V are shown in Figure 2. In the case of thermochemical water splitting and water electrolysis via mechanisms 1 and 2, the first step (water dissociation) is highly endothermic and has the highest barrier as compared to D

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Figure 3. Simulated plots of current density versus voltage for water electrolysis on Ni(211) at 1073 K and standard pressure via (a) mechanisms 1 and 2 and (b) mechanism 3 using different values of transfer coefficient.

steps, respectively, are used to determine the coverage of the intermediates and the vacant active sites. As discussed above, H2 desorption is treated as a fast step and thus is considered to be in quasi-equilibrium, similar to literature.50 The electrolysis current density is calculated as i = neσr, where n, σ, and r are the number of electrons involved in the process, the number of active sites per surface area of the unit cell, and the calculated rate, respectively.47 In these calculations, we have assumed that the active sites are equal to the number of metal edge sites per unit cell, since these are the most active for water dissociation (the rate-limiting step). The number of active sites is assumed to be the same for all mechanisms. While this might be an overapproximation especially in the case of mechanism 3 where the edge sites near the interface might be more relevant, we find that the impact of the number of active sites only affects the absolute value of the current density and not the shape of the current−voltage curve. As such, conclusions drawn below are not significantly affected by this approximation. The simulated electrolysis current density versus potential plots at 1073 K and standard pressure for water electrolysis via mechanisms 1−3 are shown in Figure 3. Figure 3a shows that the current density (electrochemical rate) for water electrolysis using mechanisms 1 and 2 has a weak dependence on the applied potential. This stems from the fact that the rate-limiting step in these two mechanisms is thermochemical water activation, which is independent of the applied potential. In these mechanisms, the applied potential indirectly affects the rate by providing a driving force to facilitate OH dissociation and O removal from the metal surface, freeing the active sites. However, in mechanism 3, the simulated electrolysis current density has a strong dependence on the applied potential. Current density increases gradually with an increase in the applied potential (Figure 3b), which is consistent with the experimental findings for high-temperature water electrolysis on Ni-based SOECs.14,20 Figure 3b shows that varying the transfer coefficient, α, mainly affects the magnitude of the current density and has limited impact on the dependence of the current density on the applied voltage (the shape of the current−voltage curve). In Figure 3b, the current densities calculated using α = 0.4 and 0.5 have been decreased to fit in the same scale as the current densities for α = 0.3. The effect of the YSZ model on the activity of water electrolysis via mechanism 3 is also tested (not shown here). We find that the calculated current densities using other YSZ model systems,

the other steps. In the case of thermochemical water splitting, the oxygen desorption step on both Ni(111) and Ni(211) is also highly endothermic by at least 1.35 eV per oxygen atom. This suggests that under thermochemical conditions, Ni would oxidize and deactivate quickly over time, leading to low rates for thermochemical water splitting. However, in the case of water electrolysis via mechanisms 1 and 2, the removal of O and dissociation of OH are facilitated by the electric potential of the electrode. For example, under an applied potential (ΔV) of 1.3 V, the removal of O becomes exothermic, and the barriers for OH dissociation via mechanism 2 decrease to 0.70 for Ni(211) and 0.12 eV for Ni(111) from the corresponding values of 1.48 and 0.90 eV, respectively, under no applied potential (ΔV = 0V). In contrast to thermochemical water splitting and water electrolysis via mechanisms 1 and 2, the first step in water electrolysis via mechanism 3 involves water dissociation to OH− ions in the electrolyte, which is a highly exothermic process. This is due to strong OH binding on an oxygen vacancy on the YSZ(111) surface. On the other hand, the hydrogen spillover from YSZ to Ni is endothermic by 1.32 eV. Figure 2 shows that in all cases, H2 desorption from Ni(111) and Ni(211) surfaces is exothermic by 0.51 eV. When taking into account the entropic contribution of ∼1.60 eV for H2 desorption at 1073 K and standard pressure, this step is faster than the water dissociation step. The free-energy diagrams show that water dissociation is the rate-limiting step for water electrolysis via all three mechanisms on Ni(211) and Ni(111), with Ni(211) exhibiting the lowest barrier for this step. This suggests that nanostructured electrocatalytic particles highly populated by surface defect sites would result in higher electrocatalytic activity than large particles, highly dominated by terrace surface sites. 3.2.2. Microkinetic Modeling Analysis for Water Electrolysis on Ni(211). To evaluate the electrochemical rates as a function of the applied potential for water electrolysis using the different mechanisms on Ni(211), a microkinetic modeling analysis is used. Equation 12 is applied to calculate the rate constants for the elementary steps. k=

k bT exp( −Ea /RT ) h

(12)

where kb, h, and Ea are the Boltzmann constant, Planck’s constant, and activation barrier, respectively. Steady-state and quasi-equilibrium assumptions for the rate-limiting and fast E

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Figure 4. Optimized most stable geometries of intermediates involved in water splitting on Ni-based alloys. The green, blue, red, and white spheres represent the Ni, alloy metal, O, and H atoms, respectively.

composed of three Ni atoms near the step. In the case of Ni3W, O adsorption occurs on top of a step W atom. Our calculations show that the binding energies with respect to atomic gas phase O on Ni3Fe/Co/Os/W alloys are stronger than on Ni(211). In the case of Ni3Cu/Zn/Ga/Ge alloys, O binding energies are weaker than on Ni(211). The most favorable binding site for atomic H is the 3-fold hollow site composed of three terrace Ni atoms on Ni3Cu/Zn/ Ga/Ge alloys, and one terrace Ni atom, one step Ni atom, and one step alloy metal atom on Ni3Fe/Co alloys. Our calculations show that on Ni3Os/W alloys, atomic H binds to the step bridge site composed of one Ni and one alloy atom. The calculated binding energies of H with respect to atomic gas phase H on these alloys are comparable to monometallic Ni(211), indicating that alloying Ni with another metal does not significantly affect H binding. The above results show that alloying Ni with another metal mainly impacts the binding energies of OH and O intermediates. These binding energies gradually decrease on Ni alloys as the alloy element is varied from a metal on the left to the right in the periodic table (Table 2). This is due to the more electrophilic character of the metals on the left of the periodic table that facilitates a stronger interaction with Ocontaining compounds. 3.3.2. Energetics of Elementary Steps for Water Electrolysis on Ni Alloys. The barriers and Gibbs free energies of the elementary steps involved in water electrolysis on Ni alloys at 1073 K, standard pressure, and ΔV = 1.30 V are shown in Table 3. Compared to the barrier of 2.30 eV for H2O dissociation to OH and H on Ni(211), the barriers for this step on Ni3Fe/Co/Os/W alloys are lower, especially in the case of Ni3W (1.01 eV). This is due to the strong binding energies for OH on these alloy surfaces. In the case of alloy surfaces that bind OH weaker than Ni(211), i.e., Ni3Cu/Zn/Ga/Ge alloys, higher energy barriers for H2O dissociation are obtained. The barriers for H2O dissociation to OH− ions are lower than those for H2O dissociation to OH on all surfaces considered because of the effect of the electric potential on the barrier. This suggests that compared to mechanisms 1 and 2, water dissociation via mechanism 3 is more favorable, similar to the case of monometallic Ni. We also find that the barriers for OH dissociation on most of the Ni alloys considered here are lower

such as YSZ subsurface doping with Y, are comparable to that of the model used here. The plots of current density versus applied potential for the three mechanisms imply that mechanism 3 is the most consistent with the experimental results on the basis of the dependence of the current density on the applied potential.14,20 The simulated electrolysis current density is overestimated when compared to the reported experimental values,20 as a consequence of neglecting ohmic losses and mass diffusion limitation, assuming that the SOEC anode acts as a standard oxygen electrode and the approximation of the active sites per surface area. 3.3. Water Electrolysis on Ni Alloys. 3.3.1. Binding Energies of Intermediates on Ni Alloys. In this section, we discuss the effect of alloying Ni(211) with another metal on the electrochemical activity for water electrolysis. We mainly focus on Ni alloys with 3d metals, such as Fe, Co, Cu, Zn, Ga, and Ge. Ni alloys with Os and W are also considered, since Os and W alloys have shown promissing activity for water dissociation.51,52 The optimized most stable geometries of the intermediates on these surfaces involved in the electrolysis process are shown in Figure 4, and the corresponding binding energies can be found in Table 2. Our calculations show that H2O binds to the top of a step Ni and metal alloy atom on Ni3Cu/Zn/Ga/Ge and Ni3Fe/Co/Os/W alloys, respectively. The calculated binding energies of H2O on these surfaces are comparable to that on Ni(211), except for Ni3W, where the binding energy is 0.64 eV stronger than that on Ni(211). Our studies show that OH prefers to bind to the step bridge site composed of one Ni atom and an adjacent alloy atom on all Ni alloys considered here, except for Ni3W, on which the OH group prefers to bind to the top of a step W atom. The binding energies of OH on Ni3Cu/Zn/Ga/Ge alloy surfaces are weaker than that on Ni(211), whereas those on Ni3Fe/Co/Os/W alloys are stronger than on Ni(211). The strongest OH binding energy is obtained on Ni3W (1.14 eV stronger than on Ni(211)). Atomic oxygen (O) prefers to adsorb on a 3-fold hollow site on most of the Ni alloys considered here. In the case of Ni3Fe/ Co/Ga/Ge/Os alloys, the hollow site is composed of one Ni atom at the terrace and an alloy atom with a Ni atom at the step. On the other hand, on Ni3Cu/Zn, the hollow site is F

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The Journal of Physical Chemistry C than those for H2O dissociation, except in the case of Ni3Os/W alloys. The Gibbs free energy diagram for water electrolysis via mechanism 3 on Ni alloys at 1073 K, standard pressure, and ΔV = 1.30 V are shown in Figure 5. We find that water

Figure 6. Volcano-type relationship between the calculated rates for water electrolysis and the binding energies of O on Ni and Ni alloys.

Figure 5. Gibbs free energy diagram of water electrolysis via mechanism 3 on Ni alloys at 1073 K, standard pressure, and ΔV = 1.30 V.

dissociation is the rate-limiting step for water electrolysis on most of these alloys, as in the case of monometallic Ni. The only exceptions are Ni3Os and Ni3W alloys. On Ni3Os/W alloys, the diffusion barriers for OH (1.82 and 1.92 eV, respectively) from the metal to the electrolyte are higher than the barriers for water dissociation, making this the rate-limiting step. We find that the barrier for water dissociation to OH− increases gradually on alloys composed of an alloy metal from left to right in the periodic table. 3.3.3. Activity Trends for Water Electrolysis on Ni Alloys. Using the energetics associated with the elementary steps discussed above and microkinetic modeling analysis, the rates for water electrolysis via mechanism 3 on different Ni alloys are calculated at 1073 K, standard pressure, ΔV = 1.30 V, and α = 0.3. We find that the rates on Ni3Cu/Zn/Ga/Ge/W alloys are lower than on monometallic Ni, whereas the rates on Ni3Fe/ Co/Os are higher than on monometallic Ni. Our results show that Ni3Fe alloy exhibits the highest activity among the alloys considered here. This is consistent with the previously reported experimental findings, which showed that the addition of Fe to Ni enhanced the rate of H2 formation from water electrolysis at 873 K.12 We find a volcano-type relationship between the calculated rates of water electrolysis and the binding energies of O on Ni and Ni alloys, as shown in Figure 6. This suggests that the binding energy of O can be used as an activity descriptor for high-temperature water electrolysis. This is also supported by the fact that a linear relationship is obtained between the energy barriers associated with H2O dissociation to OH− (the rate-limiting step) and the binding energies of O on Ni and Ni alloys, as shown in Figure 7. Although the stronger binding of O results in lower energy barriers for H2O dissociation on Ni3Os and Ni3W, the rates for H2O electrolysis on these surfaces are lower because these alloys bind O too strongly and are most likely oxidized under reaction conditions. Similar volcano-type relationships for the rate of water electrolysis using mechanisms 1 and 2 and binding energy of oxygen on the different alloys are also obtained (not shown here). We find that the trends of the activity are very similar to mechanism 3

Figure 7. Plot of the barriers for water dissociation to OH− as a function of the binding energies of O on Ni and Ni alloys at 1073 K, standard pressure, and ΔV = 1.30 V.

again reinforcing the fact that irrespective of the mechanism, if the metal alloy surfaces are involved in water dissociation, the binding energies of O can be used as an activity descriptor for water electrolysis. We would like to note that, in this contribution, we have mainly focused on the water dissociation step occurring on the Ni or Ni alloy surfaces. The interface between the metal and the electrolyte can also act as the active site for this process. Ammal et al., has shown that the energy barrier for H2O dissociation (reverse reaction for the association of OH and H in the literature37) with respect to H2O in gas phase on a Ni cluster/YSZ(111) interface is similar to that on Ni(211). This implies that the interfacial sites might become interesting on the Ni alloys that exhibit higher activation barrier for water dissociation as compared to monometallic Ni. On the other hand, the Ni alloys that exhibit lower water dissociation barriers than Ni would be the dominant active sites for this process. As such, the Ni alloy electrocatalysts at the top of the volcano plot in Figure 6 would still result in higher rates for water electrolysis as compared to the traditional Ni/YSZ SOEC cathodes, consistent with the experimental observations.12 In addition, we would like to note that due to high operating temperatures, the coverage of the intermediates during water electrolysis in SOECs should be low, unless the binding energy of an intermediate is very strong, as in the case of O/OH for Ni alloys to the left of the volcano plot, where the intermediate G

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acts as a poison for the active sites. This is the case for Ni3Os and Ni3W alloys where the binding of O/OH is so strong that they poison the active sites, resulting in lower activity for water electrolysis on these surfaces.

4. CONCLUSIONS Spin-polarized DFT calculations were performed to study hightemperature water electrolysis on Ni and Ni alloy electrocatalysts under solid oxide electrolysis cell conditions. Three different potential mechanisms for this process were considered. We found that the defect Ni(211) surface is more active than the terrace Ni(111) surface for high-temperature water electrolysis. Our calculations showed that alloying Ni with another metal impacts the electrochemical activity for water electrolysis. The binding energies of the most abundant intermediates, OH and O, become gradually weaker, and the barriers for water dissociation become gradually higher as Ni is alloyed with metals from left to right in the periodic table. A volcano-type relationship between the calculated rates and the binding energies of O was found, implying that the binding energy of O can be used as an activity descriptor to screen for SOEC cathode electrocatalysts with optimal activity for hightemperature water electrolysis.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CBET-BRIGE 1226569, CBET-CAREER 1350623) and Wayne State University Office of Vice President for Research (OVPR) for the financial support. We would like to acknowledge the support from the Extreme Science and Engineering Discovery Environment (XSEDE) for computational resources.

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