Effects of NiO addition on sintering and proton uptake of Ba(Zr,Ce,Y)O3−δ

Proton conducting Ba(Zr,Ce,Y)O3−δ: solid state reactive sintering with NiO, transient liquid phase, complex phase evolution, increased grain size, density and decreased proton uptake.


Introduction
Fuel cells based on ceramic proton conducting electrolytes (protonic ceramic fuel cells, PCFCs) have several advantages over oxide conductor based solid oxide fuel cells (SOFCs) such as a higher electrolyte conductivity at intermediate temperatures (300-600 ℃), and water formation at the cathode side which facilitates operation at high fuel utilization (see e.g. [1][2][3][4][5][6][7]. In addition, operation in electrolysis mode (directly producing dry compressed hydrogen) or catalytic membrane reactors become increasingly attractive. [8][9][10][11] The basis for these applications is the availability of proton-conducting electrolytes in the form of planar or tubular membranes. Ydoped BaZrO 3 (BZY) combines a high bulk proton conductivity with good chemical stability. 2,12 However, its poor sinterability and blocking nature of the grain boundaries (GBs) have severely impeded its application (see e.g. [12][13][14][15] ). Partial substitution of Zr by Ce improves both aspects, but too high ceria contents are detrimental for chemical stability against CO 2 and H 2 O. 12,16,17 The use of NiO as sintering aid in a solid state reactive sintering approach (SSRS [18][19][20][21][22] ) allows for improved densification and grain growth at decreased temperature (typically 1400-1550 °C) via formation of a transient (Ba,Ni,Y)O x liquid phase. SSRS has been successfully applied for PCFCs (see e.g. 5 ) and tubular reactors. 10,11 While other additives such as ZnO, 19,23 CoO, 19,24 and CuO 19,25 also improve the sintering, NiO is preferred because it is present in PCFCs on the anode side anyway and -if not deliberately added to the electrolyte layer -diffuses in from the anode BZY-Ni mixture. 7,26,27 Although SSRS with NiO tends to yield better bulk and grain boundary conductivities than the use of other sintering aids and procedures, NiO addition has also detrimental effects (see e.g. 20,21,26,[28][29][30][31]. They are related to Ni-rich residues of the transient liquid phase which, in particular for high NiO addition, may lead to cracking and potentially even electronic shortcircuits when the material is exposed to reducing conditions. In addition, the liquid phase formation may affect the cation composition of the grain interior, and thus proton uptake as well as bulk and GB conductivity. Specific temperature treatments have been suggested (see e.g. 21,28,29 ) to decrease these problems. It is obvious that a balance between improved sintering and decreased bulk conductivity must be found.
For this purpose, in the present work the effect of different amounts of NiO addition to Ba 1.015 Zr 0.664 Ce 0.20 Y 0.136 O 3-δ and Ba 1.015 Zr 0.63 Ce 0.20 Y 0.17 O 3-δ are investigated with respect to the sintering properties (densification and grain size as function of time and temperature) and bulk proton uptake (from which the effective acceptor concentration can be derived). Limiting the Ce substitution to 20 mol% keeps the material safely in the stability range versus CO 2 and H 2 O, and a slight nominal Ba excess is used to compensate for potential BaO loss in the sintering.
1.0 wt% NiO is frequently used in literature, [18][19][20][21] but occasionally also 0.5 wt% 32,33 and 2.0 wt% 18,20,34 have been tested. The NiO addition is varied from 0.125 wt% to 1.0 wt% to identify the optimum NiO content, and 2.0 wt% NiO addition is used for investigating the liquid phase sintering mechanisms. A detailed investigation of bulk and GB conductivity will be reported separately.  Figure S3. Therefore, the discussion will focus on Ba 1.015 Zr 0.664 Ce 0.20 Y 0.136 O 3-δ unless stated differently.

Sample Preparation
For comparison, some samples were sintered by spark plasma sintering (SPS). For these samples, powders having the perovskite structure were prepared by conventional solid state reaction from BaCO 3 , CeO 2 , ZrO 2 -Y 2 O 3 (Tosoh TZ-10) without/with NiO by repeated dry ball milling and calcination (4 h at 1100 ℃, followed by three times 8 h at 1300 ℃).

Characterization
The density of samples was calculated from weight and geometry, the theoretical density was calculated from X-ray diffraction (XRD) lattice constants and nominal composition. XRD was performed using Cu Kα radiation in Bragg-Brentano geometry (Panalytical Empyrean) on the sintered samples after manually grinding off the surface layer.
To check the chemical composition, about 50 mg of ground powder from the sintered sample was dissolved into 3.0 vol% HCl solution in a microwave autoclave, and measured by inductively coupled plasma-optical emission spectrometry (ICP-OES, Spectro Ciros). The ICP-OES results are shown and discussed in Table S1, Table S2 and Figure S1. Some samples were thermally etched for 1 h at 50 K below their sintering temperature (or the annealing temperature for SPS samples) for better image quality.
The proton uptake was measured by thermogravimetry (TG, Netzsch STA 449) on sintered pellets crushed and sieved to a particle size ≤ 300 µm (typical sample weight ca. 500 mg). The samples were first heated in dry N 2 (60 ml/min) to 900 ℃, then the atmosphere was switched to 17 mbar H 2 O in N 2 , the respective weight change yields the absolute water content at 900 ℃.
The samples were cooled with rates of 1 K/min down to 600 ℃, 0.6 K/min down to 400 ℃, 0.3 K/min down to 300 ℃. For selected samples the reversibility was checked by heating again with the respective rate; the absence of hysteresis indicated that the hydration reaction had Inc., USA), after which a final ion-beam milling with low energy Ar ions (1 keV) was applied in a PIPS1 (Gatan, Inc., USA) at room temperature. Bright-field (BF) images were obtained with a resolution of 0.2 nm and high-angle annular dark-field (HAADF) with a resolution < 0.1 nm (electron scattered to angles 0-111 mrad). Acceptor-doped BaZrO 3 materials are known to require drastic sintering conditions. Conventional sintering at 1500 ℃ without NiO addition yields low densities, 20 a temperature of 1600 ℃ yields dense samples but the grain size remains  1 m 35 despite a long soaking time of 24 h. In the SPS process for Ni-free samples, uniaxial pressure is applied at a temperature where plastic deformation of the grains becomes possible. This results in a high density of  95 %, but the grain size remains close to that in the starting powder at about 0.5 µm (Figure 1a). Even extended annealing of SPS samples at 1700 ℃ for 20 h does not lead to perceptible grain growth. 36 Depending on the added NiO amount, the situation differs strongly for the SSRS process. It is important to recognize that an addition of 1.0 wt% NiO corresponds to ≈ 4.0 at% Ni (relative to the B-site cations), which yields about 3 -6 vol% of a transient liquid phase (assuming the melt consists of BaNiO 2 or BaY 2 NiO 5 , see discussion below and in 37 ). The beneficial effect of SSRS with NiO addition is illustrated in Figure 1. For the SSRS sample with 0.25 wt% NiO, the grain size remains small ≈ 1 µm (this agrees with the results in 38 for 0.2 wt% NiO). When a higher amount of 0.5 or 1.0 wt% NiO is used, the grains grow up to 2 -6 µm. For the latter sample the grains actually develop the equilibrium hexagonal shape predicted by the Wulff construction. These results clearly demonstrate the effect of NiO addition in promoting BZCY grain growth and densification. While the average grain size of the sample with 1.0 wt% NiO addition is much larger than the one with 0.25 wt.% NiO, the shape and width of the grain size distribution is similar (the average grain sizes from Figure 1c,f are larger than those from intercept linear method because the size distribution was calculated from individual grain whose size was measured by the longest diagonal). The relative density and grain size of samples with different NiO content are plotted in Figure   2. For 0.25 wt% NiO or less, high sintering temperatures and long soaking times are required for full densification, but the grains remain small (≈ 1 µm). Figure S2a demonstrates similar behavior for 0.125 wt%. However, for ≥ 0.375 wt% NiO (Figure 2b-f, Figure S2b,d), already lower sintering temperature and shorter soaking time results in densities ≥ 95% (similar to the trend for Ce-rich BaZr 0.1 Ce 0.7 Y 0.2 O 3-  ). In parallel to the densification the grains grow significantly. To access shorter soaking time, the heating and cooling rate was increased to 600 K/h for 0.5-1.0 wt% NiO (blue dots in Figure 2). These data demonstrate that a high NiO content of 1.0 wt% yields a high density already at 1400 °C. These results show that dense samples can be obtained above 1400-1450 ℃ for 1.0-0.5 wt% NiO addition. However, in order to achieve pronounced grain growth (decreasing the number of low-conductive GBs), 1500-1550 ℃ and extended soaking times are necessary. The lattice parameters of the SSRS samples show no systematic dependence on the sintering conditions but a characteristic variation with NiO content, as discussed in section 3.3.  gives further insight into the dependence of the SSRS sintering process on the NiO content. When ≤ 0.25 wt% NiO is used, the densification process largely occurs without pronounced grain growth, the grain size remains below 1 µm (Figure 3a). On the other hand, higher NiO contents ≥ 0.375 wt% NiO lead to grain growth in parallel to the densification. The maximum grain size that is achieved for 16 h at 1550 ℃ then increases with NiO addition, and exceeds 5 µm for 1.0 wt% NiO (Figure 3b). The grain growth behavior indicates a qualitative difference in the SSRS sintering process between samples with low and high NiO addition, which is most probably related to the amount of transient liquid phase formed. Characteristic differences are also observed in the microstrain that is extracted from the XRD peak widths according to the Williamson-Hall method 39 (Figure 3c). This micro-strain is high for the Nifree SPS sample as well as for the SSRS sample with 0.125 wt% NiO (independent of soaking time). The strain decreases strongly for 0.25 wt% NiO, and then remains negligible for higher  It is interesting to analyze the grain growth kinetics in more detail. Figure 4 shows that the grain size follows the frequently observed grain growth relation 40 an addition of 2 mol% CoO strongly increased the grain growth, 43 however the liquid phase formation was not unambiguously proven. For sintering of BaTiO 3 , 1 wt% LiF liquid phase increased densification without grain growth, while 2 -5 wt.% LiF were required to increase also the grain size. 44 The liquid phase fraction in the present investigation is in a comparable range and expected to increase with NiO content. Correspondingly, with a larger NiO addition the increased amount of liquid phase can wet a larger area of forming perovskite grains, and thus accelerate the overall grain growth and densification.  The behavior with a low NiO content of 0.125 wt% is shown in Figure 7. The perovskite phases also start to form above 1000 ℃, with some Y 2 O 3 residue. In this sample even three perovskite phases (BaZrO 3 , (Ce,Y)-rich BZCY, and homogeneous BZCY) are present in an extended temperature range, and more than 1500 °C are required to reach a single homogeneous BZCY phase. Clearly, a low NiO content (low transient liquid phase volume) makes it more difficult to obtain a homogeneously distributed B site cation occupancy.   Interestingly, for a given NiO content the (Ce,Y)-rich BZCY phase vanishes at lower T than the BZ phase , i.e. it seems to dissolve more easily in the transient liquid phase. The comparison

Sintering behavior
for the different NiO contents shows that a higher liquid phase volume facilitates the cation transport, and the perovskite phase becomes uniform more easily.
For selected samples, the element distribution and possible remainders of the transient liquid phase were investigated by TEM-EELS. Figure 9 shows the TEM-EELS mapping for the sample with 2.0 wt% NiO quenched from 1250 ℃. There is a clear element inhomogeneity on a 100 nm length scale. While some grains show high Ce content, others are high in Ba and Zr (owing to the low Y concentration, the Y map is too noisy for any conclusions). This pronounced inhomogeneity agrees with the XRD results in Figure 6 (presence of an undoped BZ and a Ce-rich BZCY perovskite phase at low temperature). Further TEM results are given in Figure S7,8 and ref. 56 .  So far it might seem that the higher the NiO addition is, the better are BZCY processing (lower sintering temperature) and properties (larger grains and improved cation homogeneity).

Proton uptake
However, a clear detrimental effect of sintering additives such as NiO, CuO, ZnO on the proton conductivity of Y-doped BaZrO 3 has been shown e.g. in 23,26,29 . For BaZr 1-x Y x O 3 , a systematic decrease of proton uptake with increasing NiO addition was found. 37,57,58 Figure 10b shows that for the Ni-free sample the maximum proton uptake almost equals the nominal dopant concentration = 13.6 at% at 300 ℃. This holds also for Ni-containing samples when Ni is incorporated as substitutional dopant on the B-site (i.e., BaZr 1-x-y Y x Ni y O 3 ), and is independent of the sintering method (SPS or conventional sintering). However, Figure 10a demonstrates that when NiO is added in excess to the perovskite's cation composition (i.e., BaZr 1-x Y x O 3 +yNiO) as typical for the SSRS process, the maximum proton uptake decreases.
This decrease is the more severe the higher the NiO addition is, e.g. for 1 wt% NiO the proton uptake decreases by about one half (1 wt% NiO corresponds to ≈ 4.0 at% Ni on B site if fully incorporated into the perovskite lattice).
The mass action constant for the hydration reaction K hydrat is calculated according to:     The possible sintering mechanism of SSRS as concluded from the former discussion is schematically shown in Figure 13. Direct contact points of BaO with ZrO 2 and CeO 2 lead to the formation of Zr-rich and Ce-rich perovskite grains (Figure 6-8). The key of SSRS sintering of highly refractory Ba(Zr,Ce,Y)O 3 perovskites is the formation of a transient liquid phase, which allows for comparably fast cation transport which facilitates densification, grain growth, and homogenization of B site cation composition. It is rich in Ba, Ni and Y, and forms more easily when NiO is added to the mixture of BaO (from BaCO 3 ), ZrO 2 , CeO 2 and Y 2 O 3 starting materials, than when NiO is added to an already formed BZCY perovskite phase. The melt has a solubility for Zr 4+ , Ce 4+ (probably increasing with temperature) and thus allows for grain growth (Ostwald ripening) by a solution-reprecipitation mechanism. The final product are large BZCY grains with close to equilibrium shape.

Concluding discussion
The transient liquid phase is formed at low temperature and is continuously consumed during the sintering with increasing temperature and time. BaNiO 2 is thermally stable even above 1800 ℃ (it melts at about 1200 °C 60 but does not decompose into phases with different cation composition), while BaY 2 NiO 5 was reported to decompose to Y 2 O 3 , BaO and NiO at 1500 ℃. 61 With increasing progress of the BZCY phase formation, the Ba is expected to be at least partially re-incorporated into the perovskite lattice. 20 However, the decreased effective acceptor concentration as well as the decreased lattice constant suggest that the BZCY phase retains some Ba deficiency.
The thermodynamic data and the lattice constants of the SSRS samples (Figure 10-12 The liquid phase induced by NiO promotes cation transport and therefore results in much better densification, grain growth, and homogeneous perovskite phase at moderate temperature, which is beneficial for membrane fabrications. In full cells, the anode layer is usually composed of BZCY and 50 -60 wt% NiO, providing an infinite source of Ni for the electrolyte layer. Ni can diffuse a few micrometers from the anode layer into the electrolyte layer 63,64 even without NiO addition in the electrolyte, which can trigger SSRS. To have controlled conditions, it is advisable to deliberately add some NiO into the electrolyte. However, too high NiO addition decreases the proton uptake (Figure 10), and may lead to crack formation or electronic short-circuiting in reducing conditions. An increased Y dopant concentration may partially compensate the decreased effective acceptor dopant concentration and result in a higher proton uptake. 56 Based on its effects on sintering behavior and proton uptake of the present Zr-rich composition, a NiO content of 0.4 -0.5 wt% in the BZCY layer is recommended as a good compromise for BZCY membrane fabrication. For Ce-rich BaZr 0.1 Ce 0.7 Y 0.2 O 3- , a similar NiO content has been recommended in ref. 33 .

Summary
The solid state reactive sintering process using NiO for Ba(Zr,Ce,Y)O 3 proton conductors was investigated in detail. The results strongly indicate that solid state reactive sintering proceeds via the formation of a (Ba,Ni,Y)O x transient liquid phase, which facilitates homogeneous perovskite phase formation, ceramics densification, and grain growth (up to 6 µm with 1.0 wt% NiO addition) at temperatures of 1400-1500 °C, which are also feasible for the sintering of large-area membranes. Proton uptake measurements evidence a decrease of the effective acceptor concentration with increasing NiO addition, which is related to an incomplete reincorporation of Ba into the perovskite. From the comprehensive analysis of the NiO effects on sintering behavior and proton uptake, an addition of 0.4 -0.5 wt% is recommended as a good balance between advantageous and detrimental effects.