In Situ Surface Monitoring of Charge Transfer during Oxidation of

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

In Situ Surface Monitoring of Charge Transfer during Oxidation of Zirconia at Elevated Temperatures Tadeusz Bak,† Eric D. Wachsman,‡ Kathryn E. Prince,§ Kazi A. Rahman,† and Janusz Nowotny*,† †

Solar Energy Technologies, Western Sydney University, Penrith, New South Wales 2751, Australia Maryland Energy Innovation Institute, University of Maryland, College Park, Maryland 20742, United States § Australian Nuclear Science and Technology Organisation, Kirrawee DC, New South Wales 2232, Australia

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 95.85.80.101 on 03/28/19. For personal use only.

‡

ABSTRACT: This work considers the reactivity at the gas/solid interface for energy conversion systems based on ZrO2, such as solid oxide fuel cells, SOFCs, and the related charge transfer. We consider the effect of a quasi-isolated surface structure, QISS, on the reactivity of yttria-stabilized zirconia, YSZ, with oxygen at elevated temperatures. The charge transfer associated with oxidation of both YSZ and Nbdoped YSZ in the range 973−1173 K was determined by in situ surface monitoring using work function, WF, measurements. We show that the reactivity at the O2/YSZ interface can be enhanced by incorporation of pentavalent cations, such as Nb5+ ions, into the QISS that exhibits the functions of both fast ionic oxygen conductor and metallic electrode. It has been documented that surface doping of YSZ with niobium results in removal of oxygen chemisorption-related surface potential barrier that prevents oxygen incorporation into the lattice of YSZ. This finding paves the way for the development of novel materials for energy conversion devices, such as SOFCs, with enhanced performance through surface processing. KEYWORDS: yttria-stabilized zirconia, surface structure, surface engineering, reactivity, energy conversion, defect engineering

1. INTRODUCTION The development of novel materials for energy conversion requires better understanding of the effect of surface properties on reactivity and performance.1,2 At the same time awareness is growing that the performance of energy materials based on crystalline solids is determined by atomic size structural defects, such as point defects, rather than the crystalline structure itself.3−5 The present work considers the effect of surface defect disorder on the reactivity of niobium-doped ZrO2-based materials with oxygen and the associated performance of solid oxide fuel cells, SOFCs. It has been documented that the reactivity of oxide materials, such as CoO, is critically affected by defect disorder of a quasi-isolated surface structure, QISS, which is formed during processing at elevated temperatures.5 It has been shown that the surface layer, which exhibits extraordinary properties, is quasi-isolated from the bulk phase in terms of its crystalline structure, defect disorder, and the related semiconducting properties. The formation of the QISS has also been observed during oxidation and reduction of yttria-stabilized zirconia, YSZ, at elevated temperatures.6 These findings indicate that the development of novel energy materials with enhanced performance imposes the need of better understanding the local properties of the QISS. The latter could pave the way for the development of processing procedures that enable controlled modification of these properties. While YSZ is known as a fast oxygen ionic conductor,7,8 there are still intensive studies aiming at increasing the rate of oxygen incorporation into the oxide lattice. The latter process © XXXX American Chemical Society

requires supply of electrons for oxygen reduction. In this work we show that the reactivity of YSZ with oxygen may be markedly enhanced when the QISS of YSZ is doped with pentavalent ions, such as Nb5+. The experimental approach applied in this work for characterization of the local surface properties is based on the measurements of work function, WF, which is uniquely sensitive to the outermost surface layer.5 The obtained WF data are used in derivation of theoretical models that explain the effect of niobium on the reactivity mechanism at the YSZ-O2 interface.

2. OXIDATION OF YSZ Oxidation of YSZ, which is the most commonly applied oxygen ion conductor, involves several reactions, including oxygen adsorption, the formation of several chemisorbed oxygen species, oxygen incorporation into the YSZ lattice, and oxygen lattice diffusion. The reactivity between YSZ and oxygen may be represented by a chain of reactions at the gas/solid interface resulting in oxygen reduction, leading ultimately to oxygen incorporation into the oxide lattice:9 I

II

III

IV

O2 + e′ → O−2 + e′ → 2O− + 2e′ → (2O2 −)surf → (2O2 −)bulk (1) Received: January 23, 2019 Accepted: March 5, 2019 Published: March 5, 2019 A

DOI: 10.1021/acsaem.9b00161 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials where e′ denotes quasi-free electron in the oxide lattice, O−2 and O− are singly charged molecular and atomic chemisorbed oxygen species, respectively, O2− is doubly ionized oxygen species in the oxygen sublattice of YSZ, and I−IV correspond to different oxidation stages. The last stage IV represents propagation of ionized oxygen species within the lattice via oxygen vacancies that are the predominant defects in YSZ.7 Assuming that oxygen lattice transport is the slowest process, the studies on YSZ oxidation have been concentrated on the determination the effect of processing on oxygen diffusion.8 So far, however, little is known about the steps related to oxygen chemisorption at elevated temperatures and its impact on performance of SOFCs. While high-performance SOFCs require fast oxygen diffusion in ionic form, it becomes increasingly clear that the critical issue is the need to enhance incorporation of chemisorbed oxygen species. SOFCs operate at elevated temperatures when the adsorbed molecular oxygen species dissociate. Consequently, the molecular form of chemisorbed oxygen may be ignored. Therefore, reaction 1 may be represented in the general form: O2 + z e′ → 2O(z /2) −

to oxygen chemisorption and the formation of surface charge preventing oxygen incorporation.15 The present work is an attempt to reduce the negative effect of the chemisorption-related surface charge through defect engineering of the QISS that is expected to play a critical role in the charge transfer at the gas/solid interface. In this work we show that incorporation of traces of pentavalent ions, such as Nb5+, into the surface layer of YSZ leads to the formation of a bifunctional QISS that exhibits the functions of both fast oxygen ion conductor and high electronic conduction within the surface layer. Such QISS enables fast oxygen incorporation into the bulk phase without the formation of the surface potential barrier. The experimental approach in the determination of the effect of niobium is based on in situ surface monitoring of YSZ during oxidation at elevated temperatures by using WF measurements.16 The related Experimental Section is preceded by considering the basic relationships on the charge transfer at the gas/solid interface formed of oxide semiconductors, such as YSZ, and oxygen.

3. WORK FUNCTION COMPONENTS The effect of oxidation on WF of oxide semiconductors is represented by the band model in Figure 2, showing the WF

(2)

where z is the number of electrons involved in the formation of oxygen species from ionization of a single oxygen molecule, which assumes 2 or 4. The values of z reported in the DFT studies, which can be considered as related to an average quantity, may be lower than unity. It is important to note that the charge transfer associated with oxygen chemisorption leads to the formation of a surface potential barrier that prevents oxygen incorporation. The common approach to enhance oxygen reduction in SOFCs consists of deposition of a thin layer of lanthanum− strontium−manganese oxides, La1−xSrxMnO3 (LSM), and their solid solutions acting as a cathode.10−13 The effect of the LSM electrode on the performance of SOFC is represented schematically in Figure 1.

Figure 2. Schematic representation of the WF components for an ntype semiconductor with negative surface charge.

components associated with the Fermi level, EF, the band bending related to the surface charge, ΦS, and external WF, χ, which is determined by the structure of the outermost surface layer. The absolute WF value (Φ = EF + ΦS + χ) as well as its components represent materials-related data when the specimen is in gas/solid equilibrium. However, the determination of the individual WF components is very awkward. Yet when the surface structure during oxidation remains unchanged, then Δχ = 0 and the measured values of ΔΦ may be considered in terms of the changes of either EF or ΦS or both. Then the WF changes of a monophase specimen mainly consists of the component related to (i) the formation of singly ionized atomic species, (ΔΦS)z=2, and (ii) oxygen incorporation (ΔEF) that is associated with the transfer of 4 electrons (z = 4) per oxygen molecule. Consequently ΔΦ = (ΔE F)z = 4 + (ΔΦS)z = 2

Figure 1. Schematic representation of the YSZ-based SOFC structure comprising solid electrolyte, the electrodes (LSM cathode and NiYSZ anode), the gas flow system, and the external circuit.

(3)

Figure 3 represents the WF changes associated with a change in the Fermi level (Figure 3a,b) and oxygen chemisorption (Figure 3c,d). The performance of YSZ as an ionic conductor requires that oxidation results in fast oxygen incorporation and fast transport within the lattice. Here we show that the component ΔΦS for YSZ, which is responsible for the formation of the surface barrier preventing oxygen incorporation, can be either diminished or removed by incorporation of traces of pentavalent ions, such as niobium, into the surface layer.

The main feature of the LSM-based cathode, in addition to fast oxygen ion conduction, is high porosity leading to an enhanced three-phase boundary area. However, the main shortcoming of this approach is undesired reactivity of the electrode material and YSZ compromising functionality of SOFCs.14 Moreover, oxidation of the LSM materials also leads B


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phase. The effect of oxidation on properties of the outermost surface layer of YSZ, which may be determined by the WF measurements, may be considered in terms of the formation of chemisorbed oxygen species (steps II and III) and the changes in defect disorder within the surface layer as a result of step IV in eq 1 leading to the incorporation of O2− ions. The free enthalpy change associated with YSZ oxidation, involving oxygen chemisorption is ΔG = [2μ°(O(z /2) −) − μ°(O2)] − zμ(e′) − kT ln p(O2 ) + 2kT ln xz

(7)

where μ°(O ) is the standard chemical potential of the chemisorbed oxygen species involved, μ°(O2) is the standard chemical potential of oxygen, μ(e′) is the chemical potential of electrons, k is the Boltzmann constant, T is absolute temperature, and x is the activity of the adsorbed oxygen species. Considering that the chemical potential of electrons is equivalent to the Fermi level, oxygen chemisorption on YSZ results in the WF changes that are equal to the changes of EF with opposite sign (ΔΦ = −ΔEF). In gas/solid equilibrium ΔG = 0 and the effect of p(O2) on WF may be expressed as ÉÑ ÄÅ ÅÄ ÑÉÑ ÑÑ ∂xz ÑÑ 1 ÅÅÅ 1 1 ÅÅÅ 2 ∂Φ ÑÑ ÅÅ ÑÑ = ÅÅ1 − = Ñ Å Å Ñ mΦ kT ÅÅÇ ∂ ln p(O2 ) ÑÑÖ z ÅÅÇ xz ∂ ln p(O2 ) ÑÑÑÖ (8) (z/2)−

Figure 3. Schematic representation of the WF changes associated with oxygen incorporation, leading to a decrease of the Fermi level, ΔEF (a, b), and oxygen chemisorption resulting in WF increase, ΔΦS, that is represented by the band bending (c, d).

4. EFFECT OF OXYGEN ACTIVITY ON SURFACE VERSUS BULK PROPERTIES OF YSZ 4.1. Bulk-Related Properties. Zirconia, ZrO2, is an oxygen deficient oxide. Its predominant defects are oxygen vacancies that are formed due to the reaction of ZrO2 with oxygen that can be represented by the Krö ger−Vink notation:17 1 OO ⇆ O2 + V •• O + 2e′ (4) 2 The incorporation of acceptor-type ions, such as yttrium, into the ZrO2 lattice results in an increase of the concentration of oxygen vacancies leading to stabilization of tetragonal or cubic structure and forming pathways for oxygen transport: Y2O3 ⇆ 2Y′Zr + V •• O + 3OO

The activity of chemisorbed oxygen species is mainly determined by the concentration of surface active sites rather than oxygen activity in the gas phase. Therefore: ∂xz =0 ∂ ln p(O2 )

Then 1 1 = mΦ z

(10)

4.3. Effect of Oxygen Incorporation on Work Function. Assuming that oxygen activity in the oxide lattice is equal to unity, the free enthalpy change related to oxygen incorporation into YSZ, leading to a change in the concentration of the related defects (oxygen vacancies) in gas/solid equilibrium, can be expressed as

(5)

The Y2O3−ZrO2 solid solution, termed yttria-stabilized zirconia, YSZ, is a commonly applied oxygen ion conductor for SOFCs. Oxidation and reduction of YSZ result in a change in the concentration of both ionic and electronic defects and the related electrical properties. The effect of oxygen activity on defect disorder of the bulk phase may be considered in terms of the changes in the concentrations of electronic charge carriers that can be examined by electrical conductivity measurements.4 The effect of reduction and oxidation on bulk properties in gas/solid equilibrium can be expressed as inverse of the partial derivative of the logarithm of electrical conductivity with respect to the logarithm of oxygen activity, p(O2), yielding the parameter 1/mσ: ∂ log μn ∂ log σ ∂ log n 1 = = + mσ ∂ log p(O2 ) ∂ log p(O2 ) ∂ log p(O2 )

(9)

− ΔG = 2μ°(O2bulk ) − μ°(O2 ) − kT ln p(O2 ) − 4μ(e′) = 0

(11)

Consequently EF =

1 kT − [2μ°(O2bulk ) − μ°(O2 )] − ln p(O2 ) 4 4

(12)

where μ(e′) is considered as the Fermi level. Therefore, the parameter 1/mΦ, describing the effect of p(O2) on WF of YSZ, assumes the value18,19 ÅÄ ÑÉÑ ÑÑ 1 1 1 ÅÅÅ ∂Φ ÅÅ ÑÑ = = mΦ kT ÅÅÅÇ ∂ ln p(O2 ) ÑÑÑÖ 4 (13)

(6)

Relationships 10 and 13 enable derivation of the effect of YSZ oxidation on the WF changes components related to the specific oxygen species in eq 1 as represented in Figure 4.

where μn is the mobility of electrons. Assuming that the μn term is independent of p(O2), the parameter 1/mσ is equal to the term (∂ log n)/(∂ log p(O2)) that is reflective of bulk defect disorder.4 4.2. Effect of Oxygen on Surface Properties. The picture at the surface is entirely different from that of the bulk

5. EXPERIMENTAL SECTION 5.1. Work Function Measurements. The WF changes, which are reflective of the electrical effects associated with chemical C


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flowing through the reactor was imposed by the argon−oxygen mixtures of controlled composition. The p(O2) was determined by the zirconia-based electrochemical probe. 5.2. Specimen. The powder of YSZ (10 mol % Y2O3) obtained from Tosoh Corp. (average grain size was 2−3 μm) was isostatically pressed at 200 MPa into discs (1 mm thick and 7 mm in diameter) and sintered at 1673 K for 2 h in air, then polished, and reannealed in the same conditions to the density of 99.9−100%. The surface layer of YSZ was subsequently doped with niobium by depositing 0.02 mL of 0.1 mol/L solution of NbCl 5 (corresponding to a surface concentration of 10−7 mol/cm2), and the specimen was annealed at 1473 K. The prolonged annealing of Nb-doped YSZ at 1474 K in air, formed according to a similar procedure, results in a stable composition within the surface layer of 4 nm thickness assuming the ratio of Nb/Zr = 0.1 within 300 h and this ratio remains constant within 700 h.23 These results, showing that the QISS of Nb-doped YSZ specimen exhibits stable local concentration of niobium, indicate that both niobium evaporation to the gas phase and its diffusion toward the bulk phase can be ignored despite prolonged annealing. The reported segregation-induced surface concentration of yttrium in YSZ (10 mol %) is approximately 40 at. %.24 However, after incorporation of niobium and subsequent annealing at 1473 K during 700 h, the concentration of yttrium within 6 nm was 35 at. % Y.23

Figure 4. Effect of oxidation, associated with the p(O2) change in the range 13.7 Pa to 21 kPa, on the WF components corresponding to (i) oxygen incorporation (z = 4) and (ii) oxygen chemisorption leading to the formation of singly ionized atomic oxygen species (z = 2), for YSZ at elevated temperatures (973−1173 K).

6. RESULTS AND DISCUSSION The WF changes, determined in situ during isothermal oxidation of both YSZ and Nb-doped YSZ within the same interval of p(O2), are shown in Figure 6 as a function of time at 1173 K. As seen, the character of WF changes indicates rapid chemisorption of oxygen and its subsequent incorporation into the lattice. The WF changes, which assume stable values after 25 and 0.1 h at 973 and 1173 K, respectively, are reflective of the gas/solid equilibrium (the observed small fluctuations are related to the external noise of the heating element). The related WF changes as a function of temperature are shown in Figure 7a,b for YSZ and Nb-doped YSZ, respectively. The WF changes related to oxygen incorporation, ΔEF, represented by curve 1, and oxygen chemisorption, ΔΦS, represented by curve 2 (Figure 7), indicate the following: (1) The chemisorption-related potential barrier for YSZ in the range 973−1173 K assumes a substantial value ranging between 0.15 eV at 973 K and 0.2 eV at 1073 K (Figure 7a). (2) Surface doping of YSZ with niobium results in reduction of the chemisorption-related value to negligibly low levels (Figure 7b).

reactions within the QISS, may be determined by the measurements of the contact potential difference, CPD, between the studied specimen and the reference electrode according to the concept proposed by Kelvin16 (Figure 5):

Figure 5. Principle of WF measurement using the Kelvin method16 (CPD, contact potential difference; EC, conduction band; EV, valence band; (EF)x and (EF)ref, Fermi levels for the studied specimen and reference surface, respectively). This approach allows the determination of the WF changes from the following formula:

ΔΦx = eΔCPD + ΔΦref

(14)

where ΔΦx and ΔΦref denote the WF changes of the studied specimen and the reference electrode, respectively, and e is elementary charge (the parameter 1/mΦ associated with the ΔΦref component for platinum oxide that covers the reference electrode is equal to 1/ 418,19). Consequently, the ΔΦx term may be determined when both ΔCPD and ΔΦref are known. The effect of oxygen activity, p(O2), on the WF changes of the studied specimen may be represented by the sum of the following components: ΔΦx = ΔE F + ΔΦS

7. REACTIVITY OF YSZ WITH OXYGEN The reactivity of oxygen ion conductors, such as YSZ, with oxygen is the key issue in performance of SOFCs. Essentially, the reactivity of solids, which is associated with charge transfer, is profoundly influenced by the outermost surface layers. The ability to charge transfer may be considered in terms of the following defect-related properties:25 (i) Surface active sites that represent local properties responsible for the affinity to oxygen, and (ii) Fermi level that represents a collective property. The present section considers the effect of niobium on the reactivity of YSZ with oxygen in terms of several theoretical models that are based on different mechanisms of niobium incorporation into the QISS and the resulting reactivity factors. The effect of niobium on surface properties of YSZ requires recognition that annealing of undoped YSZ results in yttrium

(15)

where EF and ΦS denote the Fermi level and the surface chargerelated WF component, respectively. The pioneering approach in using the Kelvin probe at elevated temperatures by Vayenas et al.20−22 for in situ monitoring of the catalytic activity of platinum films deposited on YSZ allowed quantification of the electronic factor in catalysis during oxidation and reduction at elevated temperatures ranging between 573 and 673 K. In the present work we utilize the WF measurements for in situ surface monitoring at elevated temperatures up to 1200 K. The high-temperature Kelvin probe, HTKP, used in this work is described elsewhere.5,18 Desired oxygen activity in the gas phase D


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Figure 6. WF changes as a function of time during isothermal oxidation of YSZ (a−c) and Nb-doped YSZ (d−f) at elevated temperatures (973− 1173 K).

segregation. While the bulk phase of YSZ involving 10 mol % Y2O3 may be represented by the general formula Y0.2Zr0.8O2−y, the composition and the related formula for the QISS are much more complex. Segregation results in an enrichment of the surface layer in yttrium to the average level of 40 at. %;24 however, the enrichment factor of the outermost surface layer is expected to be much larger. Consequently, the resulting defect interactions already lead to the formation of a QISS before the incorporation of niobium.6 The incorporation of niobium and subsequent prolonged annealing results in a decrease of yttrium content to 35 at. %.23 The resulting local defect disorder, which has a controlling effect on the reactivity factors, will be considered in terms of four models. 7.1. Niobium Incorporated into Cation Sublattice: Ionic Charge Compensation. Using the Kröger−Vink notation,17 the mechanism of niobium incorporation into YSZ in oxidizing conditions may be represented by the following reaction: 2Nb2 O5 ⇆

4Nb•Zr

⁗ + 10OO + VZr

where square brackets represent concentrations. The positively charged niobium ions may act as active sites for oxygen chemisorption; however, the defects formed according to reaction 16 are not expected to easily provide electrons that are required for oxygen incorporation. Moreover, since reaction 16 does not lead to the formation or removal of electrons, its impact on the Fermi level can be ignored. Therefore, this model does not explain the effect of niobium on the WF data. 7.2. Niobium Incorporated into Cation Sublattice: Electronic Charge Compensation. In reducing conditions niobium enters the cation sites and provides electrons to the conduction band: Nb2 O5 ⇆ 2Nb•Zr + 2e′ + 4OO +

(18)

Then the charge neutrality requires that

[Nb•Zr ] = n

(16)

(19)

where n is the concentration of electrons. This mechanism leads to formation of donor-type local active sites for oxygen chemisorption and increase of the Fermi level. While both functions explain the obtained WF data, reaction 18 considers

This reaction requires the following charge neutrality: ⁗] [Nb•Zr ] = 4[VZr

1 O2 2

(17) E


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NiO,28 and Fe2O3,29 results in long range lateral reconstructions of the surface layer yielding QISS that differs from the bulk in defect disorder and the related semiconducting properties. While the bulk-related parameter 1/mσ (where subscript σ is related to electrical conductivity) during the annealing of all these oxides remains constant, the p(O2)related exponent of WF (1/mΦ) exhibits a substantial change providing evidence of long range lateral interactions and the formation of a QISS. The formation of such a QISS has also been revealed for YSZ.6 It appears that doping of the related QISS with niobium ions results in the formation of Nb-doped YSZ that exhibits exceptional stability of its chemical composition that remains unchanged despite long term annealing at high temperatures (1474 K). It is essential to note, however, that defect disorder of the QISS may be considered in terms of both the substitutial mechanisms 16 and 18 as well as the interstitial mechanisms: Nb2 O5 ⇆ 2Nb••••• + 2e′ + 4OO + i Nb2 O5 ⇆ 2Nb••••• + 10e′ + i

1 ⁗ O2 + 2VZr 2

5 O2 2

(21)

(22)

While the interstitial mechanism is not feasible in the bulk phase, this mechanism seems possible for the outermost surface layers as this has been observed for cobalt ions in CoO,5 lithium ions in NiO,30 and chromium ions in TiO2.27 Consequently, the surface of Nb-doped YSZ may be represented by the following general formula: × (Nb•Zr )α (Nb••••• )β (Y′Zr)γ (Zr ×Zr)δ (OO )ε (V O••)κ i

Figure 7. Effect of temperature on WF changes during oxidation of (a) pure YSZ and (b) niobium-doped YSZ at elevated temperatures. Line 1 represents the theoretical WF changes associated with oxygen incorporation, ΔEF, and line 2 shows the experimental WF changes (the difference between the WF data corresponding to lines 2 and 1 is equal to the component ΔΦS). The inserts represent the band models for (a) pure YSZ and (b) niobium-doped YSZ.

Nb•Zr,

(23)

V•• O,

All donor-type defects, such as and may act as local cathodic surface active sites for oxygen chemisorption and incorporation. The alternative approach recognizes that the Fermi level of the QISS is markedly larger than that of YSZ owing to the predominant impact of all donor-type species. This model may be summarized in terms of the following points:

the effect of niobium in terms of an ideal defect disorder. However, taking into account high concentrations of both intrinsic and extrinsic defects, the defect disorder within the QISS should be considered in terms of larger defect aggregates. 7.3. Short Range Interactions. The defect disorder of Nb-doped YSZ involves niobium ions incorporated according to mechanisms 16 and 18, yttrium ions incorporated according to mechanism 5, and oxygen vacancies. Moreover, the concentration of yttrium ions at the surface is larger than that in the bulk phase owing to the phenomenon of segregation.23 Since the resulting interactions between the defects can be substantial,26,27 one may expect that these lattice species lead to the formation of larger defect aggregates that can be represented, for example, by the following reaction: •• Nb•Zr + Y′Zr + V •• O ⇆ {NbZr YZrVO}

Nb••••• , i

(1) While defect reactions have been derived for the YSZ structure, the local structure of the QISS may be entirely different. (2) The high chemical stability of the surface layer of Nbdoped YSZ indicates that the predominant driving force of niobium surface segregation is lowering free energy during the formation of the QISS. (3) The local properties of the QISS in gas/solid equilibrium are defined by the equilibrium conditions, such as temperature and oxygen activity. Considering the thickness of the QISS, the following points should be taken into account: (1) WF of oxide semiconductors is predominantly influenced by the electronic structure of the outermost surface layer and is less influenced by the inner layers.31 Consequently, the WF changes are reflective of the properties related to 2−3 outermost atomic layers. (2) Prolonged annealing of YSZ at elevated temperatures results in substantial changes of defect disorder of the surface layer (determined by WF measurements). However, the thickness of the QISS is below the detectability limit of the electrical conductivity data that are reflective of the bulk defect disorder.6

(20)

Assuming short range interactions, the resulting defect aggregates can be expected to form surface islets of increased concentration of donor-type active sites and increased Fermi level. However, while certain inhomogeneity within the surface layer can be expected, especially due to the presence of grain boundary-related linear defects, the model based on long range interactions seems more feasible. 7.4. Long Range Lateral Interactions. Former studies show that prolonged annealing of binary oxides, such as CoO,5 F


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(2) Tao, L.; Luttrell, T.; Batzill, M. A two-dimensional phase of TiO2 with reduced bandgap. Nat. Chem. 2011, 3, 296−300. (3) Kröger, F. A. The Chemistry of Imperfect Crystals; North-Holland: Amsterdam, 1974. (4) Kofstad, P. Nonstoichiometry, Diffusion, and Electrical Conductivity in Binary Metal Oxides; Wiley-Interscience: New York, 1972. (5) Bak, T.; Gür, T. M.; Sharma, V. K.; Dodson, J.; Rahman, K. A.; Nowotny, J. Evidence of low-dimensional surface structure for oxide materials, Impact on energy conversion. ACS Appl. Mater. Interfaces 2018, 1, 6469−6476. (6) Nowotny, J.; Sloma, M.; Weppner, W. Surface relaxation of Y2O3-stabilized ZrO2. Solid State Ionics 1988, 28−30, 1445−1450. (7) Weppner, W. Electrochemical transient investigations of the diffusion and concentration of electrons in yttria stabilized zirconiasolid electrolytes. Z. Naturforsch., A: Phys. Sci. 1976, 31, 1336−1343. (8) Kilo, M.; Borchardt, G.; Argirusis, C.; Jackson, R. A. Oxygen diffusion in yttria-stabilized zirconia. Experimental results and molecular dynamics calculations. Phys. Chem. Chem. Phys. 2003, 5, 2219−24. (9) Bielanski, A.; Haber, J. Oxygen in Catalysis; CRC Press: Boca Raton, FL, USA, 1990. (10) Dai, H.; He, S.; Chen, H.; Yu, S.; Guo, L. Performance enhancement for solid oxide fuel cells using electrolyte surface modification. J. Power Sources 2015, 280, 406−409. (11) Park, J. S.; An, J.; Lee, M. H.; Prinz, F. B.; Lee, W. Effects of Surface chemistry and microstructure of electrolyte on oxygen reduction kinetics of solid oxide fuel cells. J. Power Sources 2015, 295, 74−78. (12) Decorse, P.; Caboche, G.; Dufour, L. C. A comparative study of the surface and bulk properties of lanthanum-strontium-manganese oxides La1−xSrxMnO3±δ as a function of Sr-content, oxygen potential and temperature. Solid State Ionics 1999, 117, 161−169. (13) Lee, Y. N.; Lago, R. M.; Fierro, J. L. G.; Cortés, V.; Sapiña, F.; Martınez, E. Surface properties and catalytic performance for ethane combustion of La1−xKxMnO3+δ perovskites. Appl. Catal., A 2001, 207, 17−24. (14) Yokokawa, H.; Sakai, N.; Kawada, T.; Dokiya, M. Thermodynamic Analysis on Interface Between Perovskite Electrode and YSZ Electrolyte. Solid State Ionics 1990, 40-41, 398−401. (15) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Nonstoichiometry and work function of (La,Sr)MnO3. II. Verification of theoretical model. J. Phys. Chem. Solids 2001, 62, 737−742. (16) Kelvin, L. V. Contact electricity of metals. Philos. Mag. 1898, 46, 82−120. (17) Kröger, F.; Vink, H. Relations between the concentrations of imperfections in crystalline solids. Solid State Phys. 1956, 3, 307−435. (18) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Application of high temperature Fermi probe in studies of charge transfer at the gas/ solid interface. Example of the oxygen/zirconia system. Key Eng. Mater. 2003, 253, 151−178. (19) Odier, P.; Rifflet, J. C.; Loup, J. P. Defect and surface phenomena at high temperature for oxides by thermal emission of electrons. In Reactivity of Solids. Proceedings of the 9th International Synposium on the Reactivity of Solids; Dyrek, K., Haber, J., Nowotny, J., Eds.; Elsevier: Amsterdam, 1982; pp 458−470. (20) Vayenas, C. G.; Bebelis, S.; Ladas, S. Dependence of catalytic rates on catalyst work function. Nature 1990, 343, 625−627. (21) Tsiplakides, D.; Nicole, J.; Vayenas, C. G.; Comninellis, C. Work function and catalytic activity measurements of an IrO2 film deposited on YSZ subjected to in situ electrochemical promotion. J. Electrochem. Soc. 1998, 145, 905−908. (22) Vayenas, C. G.; Tsiplakides, D. On the work function of the gas-exposed electrode surfaces in solid state electrolyte cells. Surf. Sci. 2000, 467, 23−34. (23) Zhang, Z.; Bak, T.; Gong, B.; Lamb, R. N.; Nowotny, J.; Prince, K. E. Effect of Long Term Annealing on the Surface Chemical composition of Surface Dopants. J. Australian Ceram. Soc. 1998, 34, 219−224.

(3) The QISS formed as a result of long range lateral interactions is quasi-isolated. The quasi-isolation means that the local properties of the QISS and the local functions are different from those of the bulk phase. The concentration gradients of both niobium and yttrium within the QISS and the bulk phase of YSZ are represented schematically in Figure 8.

Figure 8. Preliminary model of the QISS for Nb-doped YSZ in gas/ solid equilibrium.

8. CONCLUSIONS This work considers local surface properties of Nb-doped YSZ. It is shown that surface doping of YSZ with niobium results in enhanced affinity to oxygen at elevated temperatures (973− 1173 K) leading to reduction of the chemisorption-related surface charge. The obtained results indicate that, in an analogy to the LSM electrode, the QISS of Nb-doped YSZ results in enhanced charge transfer at the O2/YSZ interface. The observed effect indicates that surface doping of YSZ with niobium may either lead to enhanced performance of SOFCs involving the LSM electrode or the formation of SOFCs without the LSM electrode.

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Corresponding Author

*E-mail: [email protected]. ORCID

Tadeusz Bak: 0000-0003-1461-951X Eric D. Wachsman: 0000-0002-0667-1927 Janusz Nowotny: 0000-0002-1822-7508 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS It is acknowledged that this work was performed within the UN program of Future Earth. This project was commenced in partnership with Dr. Eric (Lou) Vance during 1997−98, while J.N. was with the Australian Nuclear Science and Technology Organisation.

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REFERENCES

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