Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials

SOSI-14069; No of Pages 6 Solid State Ionics xxx (2016) xxx–xxx

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Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials R. Zohourian, R. Merkle ⁎, J. Maier Max Planck Institute for Solid State Research, Stuttgart, Germany

a r t i c l e

i n f o

Article history: Received 17 June 2016 Received in revised form 1 September 2016 Accepted 13 September 2016 Available online xxxx

a b s t r a c t Proton uptake in mixed-conducting cathode materials is of particular interest as it allows the oxygen reduction reaction to proceed via the bulk path in proton conducting ceramic fuel cells (PCFC). This work investigates the proton concentration of BaCo0.4Fe0.4Zr0.2O3−δ (BCFZr) and the predominant proton uptake reactions using thermogravimetry. Based on the obtained proton concentrations increasing from 0.5 mol% at 400 °C to 1.5 mol% at 200 °C, the bulk path is expected to be active for BCFZr. The variation of proton concentrations with the concentration of electronic defects indicates nonideal behavior with detrimental interactions between protons and (trapped) holes. The comparison of BCFZr with other materials emphasizes that several factors determine the proton concentration such as oxide ion basicity, B-site cations and their oxidation state and B-O covalency. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Fuel cells based on proton conducting electrolytes (proton conducting ceramic fuel cells, PCFCs) such as Ba(Zr,Ce,Y)O3−δ have advantages such as a higher electrolyte conductivity at lower temperatures (see e.g. ref. [1] and references therein) compared to cells based on oxide ion conducting electrolytes. Furthermore, the water formation at the cathode side without diluting the fuel allows for higher fuel utilization and increased efficiency [2]. While PCFCs are still in an early stage of development regarding electrolyte layer manufacturing and optimization of cathode materials when compared to conventional solid oxide fuel cells, performances of about 0.7 W/cm2 at 600 °C have recently been reported using Sm0.5Sr0.5CoO3‐δ, [3] BaCo0.4Fe0.4Zr0.1Y0.1O3‐δ, [4] and NdBa0.5Sr0.5Co1.5Fe0.5O5+δ [5] as porous cathode materials. Performances in the range of 0.3–0.4 W/cm2 were obtained using BaPr0.8In0.2O3‐δ, Ba0.5Sr0.5Co0.8Fe0.2O3‐δ and Sr3Fe2O7‐δ cathodes [6,7,8]. In order to obtain better cathode and overall cell performances, the active zone for the reduction of oxygen to water should extend beyond the gas/electrode/electrolyte triple phase boundary. Thus, the electrode materials should exhibit some proton conductivity (see e.g. refs. [9,10, 11]). An estimate from a simple numerical model indicates that proton conductivities of about 10−5 S/cm in the electrode material suffice to make large parts of the electrode active [12]. However, the measurement of proton conductivity in a predominantly electron conducting cathode material (which often exhibits also oxide ion conductivity) is ⁎ Corresponding author. E-mail addresses: [email protected] (R. Zohourian), [email protected] (R. Merkle).

challenging. There is a lack of suitable materials to be used as selectively proton-permeable electrodes since the typical proton conducting Ba(Zr,Ce,Y)O3‐δ perovskites exhibit a non-negligible electronic conductivity unless exposed to reducing conditions (which on the other hand is detrimental for most cathode materials). An estimate of proton mobility from the diffusion-limited transient in thermogravimetry after a pH2O change for dense Ba0.5Sr0.5Fe0.8Zn0.2O3 ceramic samples indicated that the mobility is in the same range as for Ba(Zr1‐ xYx)O3 electrolytes [13,14]. In the present work, we focus on the investigation of proton concentration in the perovskite BaCo0.4Fe0.4Zr0.2O3‐δ, which was used as cathode material in ref. [15] and is closely related to BaCo0.4Fe04Zr0.1Y0.1O3‐δ used in ref. [4]. Deviations from ideally dilute behavior will be discussed. Furthermore, we compare proton uptake of electrolyte and electrode materials and discuss parameters decisive for a high proton concentration. 2. Experimental 2.1. Sample preparation BaCo0.4Fe0.4Zr0.2O3‐δ (BCFZr) powder was synthesized using a citric acid and EDTA complexing route and calcined in air for 10 h at 1100 °C [15,16,17,18,19]. By inductively coupled plasma-optical emission spectroscopy (Spectro Ciros CCD, Spectro Analytical Instruments, Germany), the cation concentrations were found to match the nominal values within 98%. The XRD pattern (PANalytical Empyrean X-ray diffractometer, Cu Kα) of the powder showed solely a cubic perovskite phase (a = 406 pm), correspondingly all oxygen sites are equivalent for forming oxygen vacancies or protonic defects. The ballmilled

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Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012

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powder was isostatically pressed and sintered in air at 1200 °C for 8 h (300 Kh−1), yielding a density N95%. After grinding off the outermost layer, the pellet was crushed and sieved to particles of 100–250 μm size. Such a particle size helps to avoid errors by surface water adsorption while still allowing for moderate equilibration times after pO2 and pH2O changes. As grain boundaries are not blocking for such materials (see e.g. [13]), the particle size is the main determining factor for equilibration times (the grain size of sintered BCFZr was not determined, but is expected to be of the order of several micrometers for such cathode materials). Thermogravimetry (STA449C Jupiter, Netzsch, Germany) was carried out with typically 3–4 g of sample in an alumina crucible and a gas flow of 60 ml/min. Oxygen partial pressure pO2 was controlled by mixing O2, 1% O2 in N2 and N2 using mass flow controllers. Under humid conditions, water partial pressure pH2O was adjusted by bubbling the gas stream through a thermostated water evaporator (the flow of 10 ml/min “protective gas” through the balance compartment of the STA449 was always kept dry). For nonisothermal measurements buoyancy changes were measured with a crucible filled with an appropriate amount of inert alumina ceramics and then subtracted from the sample weight changes. For isothermal pH2O changes, the measured buoyancy effects were negligible. According to the literature [15,17], BCFZr is stable against carbonate formation due to presence of Zr. XRD after long TG runs proved phase stability, in agreement with literature [15,17,19].

on T. For a BCFZr weight of 4 g (density: 5.72 g cm− 3) and 100 μm size, the total surface area would be 0.042 m2 resulting in b 1 μg weight gain due to water adsorption which is negligible compared to total weight gain of the sample in similar conditions (e.g. approx. 400 μg in Fig. 2a). 3. Results and discussion 3.1. Thermogravimetry under dry conditions The TG results in dry 100 ppm, 1% and 100% O2 are shown in Fig. 1a. BCFZr has a high concentration of oxygen vacancies (V⦁⦁ O ) which approaches 0.4 at high T (Fig. 1a). This V⦁⦁ O concentration (= δ) corresponds to the point where the average oxidation state of Co and Fe is 3+. Only for the lowest pO2 of 100 ppm, δ increases further within the studied T range. In analogy to the findings for Ba1‐ xSrxCo0.8Fe0.2O3‐δ, one can assign this to a further reduction of Co below Co3+ [21]. For the investigation of proton uptake, the relevant temperatures are below 600 °C. In this range with δ b 0.4, the average oxidation state of the B site cation is ≥3. The equilibrium with pO2 could be formulated describing the electronic defects as delocalized holes. However, the van 't Hoff plot (Fig. 1b) exhibits less deviations from linearity and between the data for different pO2 when the electronic carriers are expressed as localized defects (Fe4+ = Fe⦁Fe [22]) according to ˟ ˟ ⦁ 1=2O2 þ V⦁⦁ O þ 2FeFe ⇌OO þ 2FeFe

ð1Þ

2.2. Oxygen nonstoichiometry (δ) The sample weight was monitored in dry 100 ppm, 1% and 100% O2 for stepwise temperature (T) changes between 250 and 800 °C. The necessary equilibration times in 100 ppm O2 decreased from 60 h at 250 °C to 8 h (or less) at 500 °C and above, in pure O2 the equilibration (largely surface reaction controlled) was faster. The absolute oxygen nonstoichiometry was determined from reducing a small amount of sample (≈ 100 mg) in 7% H2 (in N2) at 1100 °C in the TG such that BCFZr fully decomposed to oxides with known oxidation state and metals (Co and Fe) as confirmed by XRD. 2.3. Proton concentration

([OH⦁O])

For fully equilibrated measurements, the sample was equilibrated with given T, pO2 and an initial low pH2O (typically 6.6 mbar), then pH2O was changed stepwise (steps from and to nominally dry conditions are impractical since the change from humid to dry conditions is very slow). The weight changes can be related to proton concentrations as discussed later. Some measurements of proton uptake were performed under “quenched” conditions: the sample was equilibrated with 100 ppm O2 at 600 °C and then quenched (20 K/min) to the desired temperature. This preserves the oxygen stoichiometry owing to the sluggish oxygen exchange surface reaction at low T and low pO2 while the faster water uptake (see reaction (4) later in the text) still equilibrates. Then, proton uptake was measured by pH2O changes. Karl-Fischer titration (KFT, 831 KF-Coulometer, Metrohm, Germany) was performed as a chemical ex-situ method to check the proton concentration [20]. Samples were hydrated at selected conditions and quenched to room temperature. They were inserted to the cold zone of the KFT setup and flushed with dry argon until a stable baseline was achieved. Then, the sample was moved to the hot zone (600 °C) and the released water in the Ar carrier gas flow measured. To ensure that potential water adsorption on the surface of the BCFZr particles is negligible, pH2O changes were performed with fine-grained powders of related materials that however do not show bulk hydration. For this, 0.375 g of the BaCO3 powder (BET surface area: 1.9 m2/g) as well as 0.545 g Ba(OH)2 (BET surface area: 1.3 m2/g) were equilibrated at 200–400 °C in humid N2 and pH2O was stepwise changed as in other humid experiments. The samples gained about 7–14 μg/m2 depending

assuming that iron acts as preferred hole trapping center (if the electronic defects are expressed as delocalized holes, the KOX values for 100 ppm and 100% O2 at the lowest T in Fig. 1b differ by more than 1.5 orders of magnitude). The oxidation mass action constant (KOX) can then be written as ð3−δÞð0:8−2δÞ2 K OX ¼ pffiffiffiffiffiffiffiffiffi pO2 δð2δ−0:4Þ2

ð2Þ

with the acceptor concentration being [A'] = 0.8 = 1 − [Zr⦁Fe] and [Fe˟Fe] = 2δ− 0.4, [Fe⦁Fe] = 0.8 − 2δ. The alternative approach of considering Fe and Co equally participating in the redox reaction (Tr stands for the transition metal, Fe or Co in this case) ˟ ˟ ⦁ 1=2O2 þ V⦁⦁ O þ 2TrTr ⇌OO þ 2TrTr

ð3Þ

with [Tr˟Tr] = 2δ, [Tr⦁Tr] = 0.8 − 2δ yields slightly larger deviations between data for different pO2 in the van 't Hoff plot. This supports using (1),(2) for the oxygenation reaction in the relevant T range. However, Fig. 1b shows deviations from linearity, i.e. from ideally dilute behavior. This becomes most evident when calculating the enthalpy of oxidation from the slope of the van 't Hoff plot for each T, and drawing it as function of the respective [Fe⦁Fe] in Fig. 1c [23]. With increasing [Fe⦁Fe], ΔH°OX becomes less negative, pointing towards a repulsive interaction of the electronic defects. Such a nonideal behavior with less negative ΔH°OX for a higher average oxidation state has been previously observed for other perovskites such as SrCoyFe1‐ yO3‐δ, [24] LaxSr1‐ xCoyFe1‐ yO3‐δ, [25] BaxSr1‐ xCoyFe1‐ yO3‐δ, [26] and LaSryCo1‐ yO3‐δ [27]. For BaCo0.70Fe0.22Nb0.08O3‐δ, a defect chemical model comprising free and trapped holes was suggested in ref. [28]. However, while deviations from ideal behavior in BCFZr are obvious, the data do not yet suffice to set up a specific model of defect interactions. ΔH°OX for BCFZr (− 45 ±10 kJ mol−1, determined in the range of 250–400 °C, 0.01–1 bar O2) is comparable with other cathode materials such as Ba0.5Sr0.5Fe0.8Zn0.2O3‐δ (−40±30 kJ mol−1, 350 °C, 10 mbar O2), [14] Ba0.5Sr0.5FeO3‐δ (−58 ± 10 kJ mol−1, 750 °C, 0.001–1 bar O2), [26] Ba0.5Sr0.5Co0.8Fe0.2O3‐δ (− 45 ±5 kJ mol−1, 350 °C, 40 mbar O2) [26] and La0.5Sr0.5CoO3‐δ (−75 kJ mol−1) [27].

Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012

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Fig. 1. TG results in dry oxidizing atmosphere for BCFZr: (a) absolute oxygen nonstoichiometry. (b) Oxidation van 't Hoff plot according to Eq. (2). (c) Oxidation enthalpy calculated from ΔHOX0 = −R⋅∂lnKOX/∂(1/T) at each T, plotted versus the Fe⦁Fe (hole) concentration at the respective T.

3.2. Thermogravimetry under humid conditions In the perovskites containing oxygen vacancies and (trapped) holes, the proton uptake can occur through two different reactions. The first is the hydration reaction (dissociative water incorporation by acid-base reaction), • H2 O þ V••O þ O O ⇌2OHO

K hydrat ¼

 • 2 OHO pH2 Oδð3−δÞ

ð4Þ

which dominates proton uptake when the vacancy concentration ⦁ ⦁ exceeds that of the electronic defects 2[V⦁⦁ O ] N [h ],[FeFe]. The second is the hydrogenation reaction (combination of oxygenation and hydration reaction), •  H2 O þ 2Fe•Fe þ 2O O ⇌2OHO þ 1=2O2 þ 2FeFe  • 2 pffiffiffiffiffiffiffiffiffi OHO pO2 ð2δ−0:4Þ2 K hydrog ¼ pH2 Oð3−δÞ2 ð0:8−2δÞ2

ð5Þ

which becomes important for a large electronic defect concentration ⦁ ⦁ (2[V⦁⦁ O ] b [h ],[FeFe]). This boundary condition between the different predominant reactions was derived in refs. [14,29,30] from the general defect chemical model comprising the hydration and hydrogenation reaction (please note that it is not identical to the boundary between two-fold and single-fold relaxation kinetics after pH2O increase [30, 31]). An example of the proton uptake measurements is shown in Fig. 2a. The sample was equilibrated at certain T and pO2, and then pH2O was changed stepwise. The weight changes Δm in Fig. 2a are quite pronounced and nicely reversible. When the pH2O changes are performed with the sample equilibrated with 100% O2, the weight changes are considerably smaller (e.g. to 70 μg at 250 °C), which indicates that the predominant contribution of proton uptake is shifting from hydration (4)

towards hydrogenation (5). The proton concentration can be obtained using one of the following approaches: (i) from fitting with the full defect chemical model (i.e. using Eqs. (2) and (4)), then the contributions from hydration and hydrogenation are obtained from the fit; or (ii) assuming which is the predominant reaction, and then directly fitting with either Eqs. (4) or (5). Both approaches implicitly use the condition [OH⦁O] = 0 for pH2O = 0. In the present investigation, point (i) indicated in the previous paragraph is problematic because the TG results under dry conditions (Fig. 1) show that the defect concentrations deviate from the ideally dilute model, so that Eq. (2) will not strictly be valid, and deviations might also affect the proton concentrations. Thus, we use approach (ii) instead. For the measurements in 100 ppm O2, it is obvious that 2[V⦁⦁ O ][Fe⦁Fe] holds (cf. Fig. 1a) and therefore proton uptake is expected to proceed via hydration (4). The resulting values of Khydrat are shown in Fig. 2b by the blue symbols. They yield a straight van 't Hoff plot with ΔH°hydrat = − 33±5 kJ mol−1, ΔS°hydrat = − 103 ±5 J mol−1 K−1. The linearity suggests that there is no significant proton-proton interaction. The corresponding proton concentrations are shown in Fig. 3b, they increase up to 1.5 mol% at 200 °C, i.e. the majority of V⦁⦁ O remains unhydrated. The situation is more complex for the sample equilibrated in humid 100% O2. At 250 °C in pure O2 the amount of oxygen vacancies and Fe⦁Fe is ⦁ ⦁⦁ ⦁ almost equal ([V⦁⦁ O ] = 0.27, [FeFe] = 0.26), thus still 2[VO ] N [FeFe] and hydration should be the predominant process (with only a slight ⦁⦁ [OH⦁O] decrease owing to [V⦁⦁ O ] = 0.27 instead of [VO ] = 0.33 for 100 ppm O2). However, Δm is about four times lower than in 100 ppm O2. This can be rationalized by two scenarios, which both suggest the presence of pronounced interactions between protons and electronic de⦁ fects (trapped holes): (i) Since 2[V⦁⦁ O ] N [FeFe] holds, the proton uptake occurs still predominantly by hydration, but the value of Khydrat decreases with increasing [Fe⦁Fe]. (ii) The activity of the trapped holes exceeds their nominal concentration and thus in terms of activity 2aV⦁⦁O b aFe⦁Fe, and correspondingly the system switches to predominant 100

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Fig. 2. TG results in humid atmosphere for equilibrated BCFZr: (a) Example of changing pH2O from 6.6 mbar (1) to 10.7 mbar (2) and 15.7 mbar (3) and back at 250 °C in 100 ppm O2. (b) ⦁ van 't Hoff plot in 100 ppm O2 (predominantly hydration reaction) and 100% O2 (mixed hydration and hydrogenation reaction, V⦁⦁ O and FeFe concentrations indicated).

Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012

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3 2 1 0

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Fig. 3. TG results for BCFZr in 15.7 mbar H2O with equilibrated or quenched oxygen stoichiometry: (a) Blue diamonds: equilibrium values of 3-δ. Green line: in the quenching experiment, δ is frozen at its 600 °C equilibrium value of 0.4. (b) Proton concentration for the samples with equilibrated (blue circles) and quenched (green diamonds) oxygen stoichiometry. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

hydrogenation reaction (5). If the proton concentration in 100% O2 is calculated assuming proton uptake solely by hydrogenation reaction (5), the corresponding Khydrat values (red circles in top part of Fig. 2b) come close to those in 100 ppm O2 (blue circles) at the lowest T (highest [Fe⦁Fe]). At higher T (lower) the deviation to the blue circles increases, pointing towards an increasing amount of proton uptake by hydrogenation (which comes with 9 times lower Δ[OH⦁O] for the same Δm than hydrogenation) when [Fe⦁Fe] decreases. The lower red circles correspond to the lower limit of proton concentration calculated from the hydration reaction. At present, it is not possible to finally discriminate between these two cases. The fact that (trapped) holes give rise to defect interactions is in line with the results from oxygen uptake under dry conditions that also showed increasingly nonideal behavior with increasing [Fe⦁Fe]. Regarding the “effective radius” within which the presence of a Fe⦁Fe influences the materials properties, it seems a reasonable assumption that - owing to the partially covalent Fe-O bonds - the 6 oxygens in the first coordination sphere of a Fe⦁Fe are affected. Correspondingly, a Fe⦁Fe molar fraction of 0.1–0.15 already suffices to modify the basicity of a large part of the oxide ions. This is in line with the decreased water uptake in pure O2 in Fig. 2b, and the results of quenching experiments (Fig. 3b) discussed in the next paragraph. Such an effective radius also matches with the strong increase of hole conductivity in BaZr1‐ xFexO3 with x already below the geometrical percolation limit [32]. Further evidence for defect interactions between protons and trapped holes is obtained from ΔpH2O experiments on samples with frozen-in oxygen stoichiometry (Fig. 3). The sample was equilibrated ⦁ at 600 °C ([V⦁⦁ O ] = 0.4, [FeFe] = 0) and then quenched to a certain T in qffiffiffiffiffiffiffiffiffiffi 100 ppm O2. As known from Eq. (4), [OH⦁O] ∝ ½V⦁⦁ O  (the change in the term 3‐δ is negligible), and therefore [OH⦁O] is expected to be pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:4=0:33 = 1.1 times larger for the quenched sample than the equilibrated one at 250 °C. However, the proton concentration differs by a factor of 1.7 revealing the fact that already a small concentration of (trapped) holes can suppress the proton uptake (or partially change it to hydrogenation with correspondingly smaller Δm). In order to check the proton concentrations in BCFZr by an independent method, Karl-Fischer measurements were performed for samples hydrated at 100 ppm and 100% O2 both at 250 and 500 °C. The results (not shown) yield qualitatively the same trends as the TG analysis (smaller proton concentration at higher T and more oxidizing conditions) and semiquantitatively agree regarding the absolute [OH⦁O] (percent range at 250 °C). However, we speculate that issues in the quenching procedure and crack formation in the particles (probably leading to water surface adsorption during quenching resulting in too high [OH⦁O]) cause the Karl-Fischer technique to overestimate the proton content.

The comparison of the proton uptake to that of Ba0.5Sr0.5Fe0.8Zn0.2O3‐δ (BSFZn) shows that BSFZn takes up more protons (1.5 mol% at 350 °C, corresponding to its more negative hydration enthalpy of −70 kJ mol−1) than BCFZr [14]. The proton concentration of BCFZr is comparable to that determined for the closely related perovskite BaCo0.4Fe0.4Zr0.1Y0.1O3‐δ (0.21 mol% at 500 °C in 0.043 bar O2, calculated assuming predominant hydration reaction, cf. supplementary information in ref. [4]). If we assume a similar proton mobility in BCFZr as determined for BSFZn, the proton concentration of BCFZr is sufficient to activate the bulk path for oxygen reduction to water in this material. This is in line with the good cathode performances of BCFZr [15] and BCFZrY [4]. 3.3. Trends in proton uptake thermodynamics In order to place the proton concentrations obtained in the present work into larger context, Fig. 4 shows a collection of data ranging from typical electrolyte materials such as acceptor doped BaZrO3 and BaCeO3 to perovskites used as cathode materials with (predominantly) redox-active transition metals Fe and Co in the B site. As a measure for the proton uptake, we choose ΔG°hydrat at a typical temperature of 700 K. As abscissa, in a first attempt we choose the Pauling electronegativity averaged over the elements on the A- and B-site of the perovskite [43]. The motivation for this choice is that for the proton conducting electrolytes a higher basicity of the oxide ions was found to lead to a more negative ΔG0hydrat, cf. the trend of decreasing proton uptake from BaCeO3 via BaZrO3 to SrZrO3 and SrTiO3 [44] (two alternative correlations previously suggested in literature [45,46] are displayed in the Appendix A). The basicity of the oxide ions is expected to be the larger, the lower the electronegativity of the A-and B-site elements in the perovskite. At a first glance, Fig. 4a is in line with this interpretation. It shows a separation of the data into electrolyte materials (black circles) with negative ΔG°hydrat, large proton uptake and low average electronegativity, and electrode materials (red diamonds) with positive Δ G°hydrat, low proton concentration and larger electronegativity. This plot reveals also another interesting feature: as indicated by the dashed lines, the perovskites with 4+ cations on the B site seem to follow a correlation line which is separate from that for perovskites with (mainly) 3+ B cations. This indicates that the basicity of the oxide ions might not be the sole decisive parameter for proton uptake. Hypothetically, one can first split the water molecule to be incorporated into a proton and a hydroxide ion. For the proton incorporation (attachment to a regular oxide ion) H⦁i þ O˟O ⇌OH⦁⦁ O

ð6Þ

Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012

R. Zohourian et al. / Solid State Ionics xxx (2016) xxx–xxx

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average "ion electronegativity" Fig. 4. Standard Gibbs energy of the hydration reaction (ΔG°hydrat) at 700 K plotted (a) versus average Pauling electronegativity of A- and B-site elements, (b) versus average “ion electronegativity” (see text; assuming predominant Fe3+ and Co3+ for the electrode-type perovskites) of A- and B-site cations. Circles = electrolyte-type perovskites, diamonds = electrode-type perovskites. 1 = BaPr0.9Gd0.1O3−δ, [33] 2 = BaCe0.9Y0.1O3‐δ, [44] 3 = BaZr0.9Gd0.1O3‐δ, [34] 4 = BaZr0.9Y0.1O3−δ, [44] 5 = BaZr0.9Sc0.1O3‐δ, [44] 6 = La0.9Ca0.1ErO3‐δ, [35] 7 = La0.9Ba0.1YbO3‐δ, [36] 8 = SrZr0.95Yb0.05O3‐δ, [37] 9 = CaZr0.9Sc0.1O3‐δ, [38] 10 = BaTi0.5Sc0.5O3‐δ, [39] 11 = SrTi0.98Sc0.02O3‐δ, [34] 12 = La0.9Sr0.1ScO3‐δ, [40] 13 = Ba2SnYO5.5‐δ, [44] 14 = Ba0.5Sr0.5Fe.8Zn0.2O3‐δ, [13] 15 = BaGd0.8La0.2Co2O6‐δ, [41] 16 = Ba0.95La0.05FeO3−δ, [42] 17 = Ba0.5Sr0.5FeO3‐δ, [42] 18 = BaCo0.4Fe0.4Zr0.2O3‐δ (present work). Some typical materials compositions are indicated in the plots.

a high basicity of O˟O should obviously be favorable. On the other hand, for the incorporation of the hydroxide ion into an oxygen vacancy

⦁ OHi' þ V⦁⦁ O ⇌OHO

ð7Þ

a high charge of the B cation (corresponding to a larger Madelung potential at the O site) is expected to be beneficial. Since this high charge is expected to decrease the O˟O basicity by withdrawing some electron density from the oxide ion, the B cation charge has opposing trends. While the plot ofΔG°hydrat versus average Pauling electronegativity obviously leads to some instructive correlations, one has to keep in mind that the Pauling values refer to the electronegativity of the elements (neutral atoms). Correspondingly, the less noble Zr has a much lower Pauling electronegativity (1.33) than Fe (1.83). On the other hand, here we deal with largely ionic solids, and we would like to use a term such as electronegativity as a measure of how much the basicity of the oxide ions is decreased by the electron-withdrawing (polarizing) power of the B cations in the perovskite. Here, qualitatively one might expect a stronger effect of Zr4+ compared to Fe3+ (despite the slightly larger radius of Zr4+). This brought us to explore another quantity as the abscissa in Fig. 4b. Motivated by the atom electronegativity definition of Allred and Rochow [47] from the effective charge of the incompletely shielded atom nucleus and the atom radius χAllrRoch ∝zeff/r2atom which is a

5

measure for the electrostatic force of the core on the outer electrons, one could tentatively define an “ion electronegativity” χion ∝zion/r2ion from the ion charge and radius to describe the polarizing force of a cation on the outer electrons of the neighboring oxide ions. The average of χion for the A and B cations (calculated from Shannon ion radii [48] for respective oxidation state and coordination number) is used as abscissa in Fig. 4b. This definition yields similar values of χion for Zr4+ and Fe3+, [49] it also leads to larger difference among the alkali earth cations and between alkali earth and rare earth cations than the Pauling definition. In Fig. 4b one can still recognize the overall trend of more favorable proton uptake with lower cation electronegativity. Interestingly, the “electrolyte-type” perovskites without redox-active B cations are located in one zone (marked grey) which comprises 2+/4+ as well as 3+/ 3+ perovskites. The cathode materials with Fe and Co on the B site lie in a different zone (red diamonds), with less negative ΔG°hydrat for the same χion. While Fig. 4a and b may not be the only possible correlation plots, the fact that “electrolyte-type” and “electrode-type” (redox-active) perovskites are clearly demarcated suggests that parameters in addition to χion also influence the proton uptake thermodynamics. One could be the stronger hybridization of transition metal and O orbitals for later transition metals in a row (increased covalency of the B-O bond), making protonation of the oxide ions less favorable. This may also be the origin of the observed deviations from ideally dilute defect models. Apparently, holes lead to decreased proton uptake beyond simple dilute defect models (cf. Fig. 3b), which can tentatively be rationalized by holes as well as protons competing for oxygen as the binding site and the fact that one hole may affect more than a single O as discussed above. More experimental as well as quantum-chemical investigations are necessary to really understand the complex defect chemistry of these perovskites with simultaneous presence of oxygen vacancies, protons and holes. These insights may then serve as guidelines for the optimization of PCFC cathode materials, where a good compromise between proton conductivity and catalytic activity for oxygen reduction has to be found. 4. Conclusions For the mixed conducting BCFZr perovskite, proton concentrations up to 1.5 mol% (equilibrated conditions) and 4 mol% (quenched oxygen nonstoichiometry) were obtained from TG measurements at 200 °C. Such proton concentrations are sufficient to allow for oxygen reduction to water via the “bulk path” assuming a comparable proton mobility in BCFZr as in BSFZn. The TG measurements for samples equilibrated in different pO2 and with quenched oxygen nonstoichiometry clearly indicate that the system shows nonideal behavior owing to unfavorable proton-hole interactions. The comparison of the hydration thermodynamics of a large range of perovskites shows that potential electrode materials generally exhibit much lower proton uptake compared to proton conducting electrolyte materials. It further emphasizes that several parameters are important to describe the material's protonation ability, including the basicity of the oxide ions, charge of the B cations and the covalency of the B-O bonds. Acknowledgements We thank Dr. H. Hoier (MPI for Solid State Research Stuttgart) for measuring X-ray diffractograms. Appendix A. Some correlation plots as suggested in literature Fig. A1a shows Δ G°hydrat at 700 K plotted versus the difference in Allred-Rochow electronegativity (of the metal atoms), a correlation that was suggested in ref. [45]. Since the variation in electronegativity is much larger for the B-site elements than for the A-elements, this plot largely reflects the correlation of ΔG°hydrat with the B site electronegativity (and is - apart from the position of A3+B3+O3 electrolyte

Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012

6

R. Zohourian et al. / Solid State Ionics xxx (2016) xxx–xxx

materials - quite similar to Fig. 4a). It separates perovskites with early transition metals on the B site (left part of the plot, comparably low electronegativity) from those with late transition metals and main group elements on the B site (high electronegativity) in the right part. Within each group there is a quite good correlation. Its physical origin is probably related to the fact that the basicity of the oxide ions closely depends on the B site cation. The splitting into the two groups appears a bit arbitrary because it reflects the strong changes in element electronegativity for early and late transition metals, which is however less pronounced if instead the electronegativity of the cations was considered (which should be the relevant quantity for the oxides ions' basicity). Fig. A1b shows ΔG°hydrat at 700 K plotted versus the Goldschmidt tolerance factor t as suggested in ref. [46]. This plot exhibits a quite pronounced scatter of the data. On one hand there is the trend of a more positive Δ G°hydrat for larger t when comparing perovskites with the same A-site cation (cf. BaCeO3-BaZrO3, SrZrO3-SrTiO3, LaErO3-LaScO3). This can be rationalized from the fact that a larger B cation increases t and in parallel decreases the oxide ions' basicity. The cathode materials investigated so far have mainly Ba on the A site and roughly follow the correlation line drawn for Ba-based perovskites. On the other hand, comparing materials with the same B cation (CaZrO3-SrZrO3-BaZrO3) rather leads to an opposite trend (and thus increases the scatter in the overall plot) because larger A cations increase t but tend to lead to a

0

ΔGhydrat(700K) / kJ/mol

50

(Ba,Sr)FeO3 17 15 16 14 18

SrTiO3 11

(a) 10 CaZrO3 9

25 0

LaScO3 12

-25

BaCeO3 -50

2

7

3 SrZrO 4,8 BaZrO3 3 5

Ba(Sn,Y)O3 13

1

-75

6

LaErO3

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

difference of Allred-Rochow electronegativity (metal atoms)

0 ΔG hydrat (700K) / kJ/mol

50

(Ba,Sr)FeO3 SrTiO3 11 17

(b)

25

CaZrO3

14

9

0

LaScO3

-25

12

13

-50

2

18

3 4 BaZrO3

8 SrZrO3

BaCeO3

10

15 16

5

1

7 -75

6

LaErO3

0.85

0.90

0.95

1.00

1.05

Goldschmidt tolerance factor Fig. A1. Standard Gibbs energy of the hydration reaction (ΔG°hydrat) at 700 K plotted (a) versus the difference of Allred-Rochow electronegativity of A- and B-site elements, (b) versus Goldschmidt tolerance factor (assuming predominant Fe3+ and Co3+ for the electrode-type perovskites). Circles = electrolyte-type perovskites, diamonds = electrode-type perovskites. Numbering and references are the same as in Fig. 4.

larger basicity and thus more negative ΔG°hydrat. References

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Please cite this article as: R. Zohourian, et al., Proton uptake into the protonic cathode material BaCo0.4Fe0.4Zr0.2O3-δ and comparison to protonic electrolyte materials, Solid State Ionics (2016), http://dx.doi.org/10.1016/j.ssi.2016.09.012