Impact of sin-
tering procedures on the densification and conductivity of

BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3-δelectrolyte forprotonicceramici

Lozane Hamze, Annie Le Gal La Salle, Olivier Joubert, Eric Quarez

► To cite this version:

Lozane Hamze, Annie Le Gal La Salle, Olivier Joubert, Eric Quarez. Impact of sintering procedures on the densification and conductivity of BaZr_0.4Ce_0.4Y_0.1Yb_0.1O_3-δelectrolyte forprotonicceramicfuelcells.International Journal of Hyd 324.10.1016/j.ijhydene.2025.05.241.hal - 05165881

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Distributed under a Creative Commons Attribution 4.0 International License
Impact of Sintering Procedures on the Densification and
Conductivity of BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>3-δ</sub> Electrolyte for Protonic
Ceramic Fuel Cells

Lozane Hamze¹, Annie Le Gal La Salle¹, Olivier Joubert¹, Eric Quarez¹*

¹ Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, F-44000 Nantes,
France

*Corresponding author: eric.quarez@cnrs-imn.fr

**Abstract**

BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>2.9</sub> (BZCYYb4411), a promising proton-conducting electrolyte, requires high sintering temperatures, typically 1600 °C, to be densified to acceptable relative densities, greater than 90 %. This study explores three sintering strategies to reduce the sintering temperature to 1400 °C. The approaches include (1) external ZnO addition, (2) internal Zn doping, and (3) fluorite-phase Zr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>1.9</sub> (ZCYYb4411) addition as a sintering aid. All methods yielded pure ceramics adopting the I4/mcm tetragonal perovskite phase, with Zn-based strategies achieving relative densities above 90 %. In contrast, the addition of the fluorite phase proved less effective, particularly at higher concentrations. In most cases, the conventional calcination step was avoided, and a relative density above 90 % was achieved through single-step sintering at 1400 °C, thereby improving energy efficiency and simplifying the process. The selected samples were subjected to SEM analysis and conductivity testing, revealing a high conductivity level: BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> reached 0.9 x 10<sup>-2</sup> S cm<sup>-1</sup>, while BZCYYb4411 with 0.5 wt% ZnO addition achieved 0.6 x10<sup>-2</sup> S cm<sup>-1</sup> at 700 °C in humidified N<sub>2</sub> underscoring the effectiveness of these approaches in enhancing the sintering of BZCYYb4411 while maintaining a high level of conductivity.

**Keywords:** — BZCYYb, densification, sintering aids, proton-conducting electrolytes, and conductivity.
Introduction

The growing environmental concerns associated with the reliance on fossil fuels and the emission of greenhouse gases, particularly CO2, have intensified the global push for new-generation renewable energy power systems, which are cleaner and more sustainable energy solutions. Recent research shows the importance of sustainable energy technology in attaining environmental sustainability goals. For instance, Hung Van Tran highlighted the disparate impacts of economic expansion, globalization, and renewable energy on environmental quality, emphasizing the crucial need for clean technology in reducing ecological degradation [1]. At the same time, Aziz et al. developed new Ni/ZnO nanocomposites for the efficient photocatalytic degradation of malachite green dye, highlighting the growing emphasis on advanced material design for environmental remediation and green energy applications [2]. These findings emphasize the necessity for generating highly efficient, durable proton-conducting ceramic electrolytes, which provide a sustainable energy solution with low impact on the environment. In such a context, solid oxide fuel cells (SOFCs) associated with electrolyzers are one of the key areas of research that have the potential to cope with the present energy demand, offering high efficiency, low emissions, fast reaction kinetics, and the ability to use renewable fuels [3]. These factors are essential for transitioning to a low-carbon economy, which is critical for future energy conversion systems [4-6]. Unfortunately, one key issue of first-generation oxygen-ion solid oxide fuel cells (O-SOFCs) based on stabilized zirconia (e.g., YSZ/ScSZ) or rare-earth doped ceria (Ce1-xMxO2; M=GD, Y) is the low durability due to the high operating temperatures (typically 800-1000 °C), facilitating the migration of certain elements and the formation of isolating interfaces [7,8]. One achievable solution is to operate at reduced temperatures compared to traditional oxide-ion conductors [9-12]. Over the years, the discovery of BaCeO3- and BaZrO3-based proton-conducting electrolytes by Iwahara et al. in 1981, operating at an intermediate temperature (500-700 °C), has proven to be a valuable contender in developing solid oxide fuel/electrolyzer cell technologies [12-14]. This has attracted considerable attention as a promising electrolyte material for intermediate-temperature proton-conducting fuel cells (PCFCs) [15,16], due to their advantages over oxide-ion-conducting fuel cells (O-SOFC), including the generation of water at the cathode side, preventing fuel dilution in the anode compartment, and enabling the working cell to maintain a high voltage. Furthermore, the fuel at the anode remains pure, requiring no treatment during recirculation, which enhances the performance [17,18].
Proton-conducting solid solutions based on barium cerates (BaCeO₃) crystallize in a perovskite structure and attract considerable attention due to their high protonic conductivity, reaching around 10⁻² S cm⁻¹ in the temperature ranges between 400 °C and 600 °C [19–21]. However, this material has demonstrated chemical instability owing to its low resistance to humid (H₂O) and carbon dioxide (CO₂) rich atmospheres, as demonstrated in equations 1 and 2.

$$BaCeO_3 + H_2O \rightarrow Ba(OH)_2 + CeO_2 \quad \text{Equation 1}$$

$$BaCeO_3 + CO_2 \rightarrow BaCO_3 + CeO_2 \quad \text{Equation 2}$$

The formation of hydroxide or carbonate from the reaction with water and carbon dioxide further contributes to the poor durability performance of the cell [22]. In contrast, many researchers have shed light on the chemical stability and introduced other promising electrolytes for PCFC, including barium zirconates (BaZrO₃). Although these materials possess superior mechanical durability, greater bulk proton conductivity, and higher chemical stability, their proton conductivity values are relatively lower compared to BaCeO₃-based materials due to a large blocking grain boundary effect [17,22]. Moreover, compositions with high zirconium content tend to be extremely refractory and exhibit resistive grain boundaries, necessitating elevated sintering temperatures (> 1600 °C) and extended annealing times (> 24 hours) to achieve dense pellets and promote grain growth. These demanding conditions can disrupt the material's stoichiometry, as partial evaporation of barium from the perovskite lattice and potential segregation of yttria (Y₂O₃) from the bulk phase may occur, further reducing proton conductivity and hindering the overall performance of the dense electrolytes [23–26]. Nevertheless, these limitations can be overcome when compromising the chemical stability of BaZrO₃ with the high conductivity of BaCeO₃ to maintain a high level of protonic conductivity at elevated temperatures [27,28]. Hence, solid solutions of doped BaCeO₃ and BaZrO₃, such as BaZrₓCe₁-xYᵧO₃-δ, have gained significant attention due to their potential to enhance both chemical stability and proton conductivity [29]. It is worth mentioning that the introduction of trivalent cations, ytterbium (Yb³⁺) in addition to the yttrium (Y³⁺) in the co-doped barium zirconate-cerate, has been shown to promote proton transport [30] and exhibit higher conductivity than BZCY as well as the conventional oxide ion conductors such as YSZ and GDC [31].

In this study, BaZr₀.₄Ce₀.₄Y₀.₁Yb₀.₁O₂.₉ (BZCYYb4411), which crystallizes in a perovskite structure (ABO₃), is a promising proton-conducting material due to its high conductivity and improved stability compared to (BZCY)-based oxides with lower Zr content [32]. However,
achieving a dense BZCYYb4411 ceramic generally requires a high sintering temperature of 1600 °C and prolonged sintering times [33,34]. These conditions can lead to the partial evaporation of barium at sintering temperatures exceeding 1250 °C and the segregation of doped trivalent cations, thus disrupting the material's stoichiometry [35]. Barium loss, which results in the consumption of hydroxyl vacancies [(OH)O] affects both the knowledge of the material composition as well as a decrease in the total ionic conductivity [36,37].

Several strategies can be employed to address this issue. One approach is to minimize the barium evaporation by covering the pellet with a bed of powder of the same composition and partially enclosing the crucible [35]. While this method reduces barium loss, it does not entirely prevent it. Alternatively, lowering the sintering temperature while maintaining densification efficiency is a promising solution. This can be achieved by either a cold sintering process [38,39] or by incorporating transition metal oxides as sintering aids, such as MOₓ (where M= Ni, Cu, Ti, Fe, Co, or Zn), at the sintering stage, which enhances pellet densification and avoids high-temperature evaporations [40–42]. Using this method, Xu et al. enhanced the densification at a relatively low sintering temperature of 1200 °C by coupling the addition of NiO sintering aid with microwave heating [43]. Liu et al. reported that the addition of 1.0 wt% NiO to BaZr₀.₁Ce₀.₇Y₀.₁Yb₀.₁O₃-δ significantly enhanced the material's densification, achieving approximately 96 % of the theoretical density after sintering at 1350 °C in air for 3 hours [44]. Similarly, Lyagaeva et al. obtained at 1450 °C a relative density exceeding 94 % for BaCe₀.₅Zr₀.₃Y₀.₂O₃-δ by incorporating CuO as a sintering aid [45]. Compared to the MOₓ sintering aids, adding ZnO has demonstrated the ability to facilitate sintering in both BaCeO₃- and BaZrO₃-based electrolytes. Although NiO and CuO facilitate densification, they may also trigger the formation of undesirable secondary phases (BaY₂NiO₅ and Ba₂YCu₃Oₓ for NiO and CuO, respectively) or introduce electronic conductivity, which may compromise the ionic selectivity of the electrolyte [43,45]. By contrast, ZnO demonstrates remarkable chemical compatibility with BaZr₀.₄Ce₀.₄Y₀.₁Yb₀.₁O₂.₉ (BZCYYb4411). The interaction between ZnO and BaO results in the formation of a eutectic phase at a low melting temperature, approximately 1100 °C, facilitating densification while maintaining the material's protonic conductivity [30,35,46].

Zinc cation in ZnO, with a single valence of +2 and ionic radius of 0.74 Å, is closer to the ionic radius of Zr (0.72 Å), which has an oxidation state of +4. This closeness in the ionic radii allows Zn to be doped into the B-site of the perovskite structure, introducing B-site defects and additional oxygen vacancies. These defects enhance mass transport and improve protonic conductivity. Moreover, it also helps in reducing the electronic conductivity of the
material caused by the change in Ce valence [35,47]. Moreover, ZnO is widely available, affordable, and exhibits a low tendency to produce harmful effects in comparison to Ni- and Co-based additives [35,46]. These characteristics justify the focus on ZnO as a sintering aid in this study, which enables high densification at a lower temperature while preserving good proton transport properties.

Numerous studies have explored the use of ZnO as a sintering aid. Baral et al. successfully densified BaCe<sub>0.35</sub>Zr<sub>0.5</sub>Y<sub>0.15</sub>O<sub>3-δ</sub> composition at an exceptionally low temperature of 1100 °C using a ZnO sintering additive [48]. Matsuda et al. reduced the sintering temperature to 1300 °C while achieving a relative density ≥ 90 % by adding ZnO sintering aid to BaZr<sub>0.1</sub>Ce<sub>0.8</sub>Y<sub>0.1</sub>O<sub>3-δ</sub> (BZCY181) [49]. Castellani et al. also reduced the sintering temperature of Zr-rich BaZr<sub>0.7</sub>Ce<sub>0.2</sub>Y<sub>0.1</sub>O<sub>3-δ</sub> (BZCY721) from 1600 °C to 1300 °C with the addition of ZnO-Li<sub>2</sub>O sintering aid, achieving a relative density of 93 % [38]. Additionally, Wang et al. reported that ZnO significantly enhances the densification of BaCe<sub>0.5</sub>Zr<sub>0.3</sub>Y<sub>0.2</sub>O<sub>3-δ</sub> (BZCY532), reaching a relative density of 97 % at 1300 °C for 10 hours [46]. Besides, Naeem Khan et al. reported that ZnO-added BaZr<sub>0.3</sub>Ce<sub>0.5</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>3-δ</sub> (BZCYYb3511) achieved a high density of approximately 98 % at 1400 °C [50]. Furthermore, Tao et al. demonstrated that the internal doping of Zn into BaCe<sub>0.5</sub>Zr<sub>0.3</sub>Y<sub>0.16</sub>Zn<sub>0.04</sub>O<sub>3-δ</sub> enabled the material to reach a relative density of 96 % after sintering at 1325°C for 10 hours [51].

The goal of this study is to explore three different sintering strategies to reduce the sintering temperature of BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>2.9</sub> (BZCYYb4411) to 1400 °C, minimize the number of post-synthesis steps needed to produce highly dense pellets, and achieve high conductivity levels. Two of the strategies involve adding a small weight percentage of a sintering aid. In one case, ZnO was used due to its effectiveness in lowering the sintering temperature of BZCY electrolyte materials [36,46,48–52]. In the second case, we incorporated a few percent of a fluorite-type AO<sub>2</sub> phase with the composition Zr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>1.9</sub> (ZCYYb4411) in which Zr, Ce, Y, and Yb occupy the A site of the fluorite structure. This phase was chosen because it preserves the same Zr:Ce:Y:Yb cation ratio as that found in the B site of the perovskite BZCYYb4411. Unlike the perovskite phase, this fluorite phase can be sintered at lower temperatures. In this approach, the goal is to combine the two phases without introducing any foreign element, and to examine whether the better sinterability of the fluorite phase can facilitate the sintering of the mixture. The third strategy is to partially replace Y and/or Yb elements with Zn in BZCYYb4411, which was also experimented with the BZCY electrolyte [36,51,53,54]. The glycine nitrate process was selected as the synthesis method for preparing BZCYYb4411 powder. After combustion, the powder is usually calcined at 900–
1100 °C to remove organic residues and complete the reaction [39,55,56]. To reduce the number of steps, we directly sintered the pellets resulting from the combustion using the proposed sintering strategies, skipping the calcination step. The pure powder obtained after sintering treatment and the high relative densities demonstrate that our sintering strategies make it possible to avoid the calcination step. Finally, conductivity measurements were conducted on selected pellets sintered at 1400 °C using the sintering strategies.

# Experimental section

## Powder synthesis

All samples, including the reference BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>2.9</sub> (BZCYYb4411) material and the fluorite Zr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>1.9</sub> (ZCYYb4411), were prepared following the glycine-nitrate process as detailed in our previous article [57]. The nanoparticles were manually ground in an agate mortar after synthesis. One strategy is adding zinc nitrate hexahydrate (Zn(NO<sub>3</sub>)<sub>2</sub>.6H<sub>2</sub>O, 98 %, Acros) to the dissolved metal nitrate solution used to prepare BZCYYb4411 to synthesize Zn-containing materials. The synthesized compositions included the substitution of yttrium by zinc, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, the substitution of ytterbium by zinc, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.05</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, and the double substitution of yttrium and ytterbium by zinc, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>.

On the other hand, external ZnO (0.5 and 1.0 wt%, 99.99 % Alfa Aesar) and fluorite Zr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.1</sub>O<sub>1.9</sub> (ZCYYb4411) (0.5, 1.0, and 3.0 wt%) sintering additives were added to BZCYYb4411. The additives were manually mixed with BZCYYb4411 in an agate mortar to ensure a homogenous powder mixture, then pressed into pellets with a 10 mm diameter at a pressure of 375 MPa for 10 minutes and placed in a yttrium-stabilized zirconia crucible on a bed of BZCYYb4411 powder.

The prepared pellets were then processed in two routes to evaluate the effect of calcination on the sintering behavior. In the first route, a conventional method was used [56]; the pellets were initially calcined at 900 °C for 10 hours in air to burn the organic compounds and complete phase formation. They were then sintered in air at two different temperatures: 1600 °C for 1 hour and 1400 °C for 10 hours. In the second method, the pellets were directly sintered at both temperatures in air, without the calcination step.
# Characterization techniques

## X-ray and neutron powder diffraction

X-ray powder diffraction (XRPD) analysis was performed at room temperature in air to determine the crystal structure of the sintered pellets. A Bruker D8 Advance diffractometer, operating in Bragg-Brentano geometry with a LynxEye detector, was used. A germanium monochromator and copper anode source with a wavelength $\lambda_{Cu K\alpha}=1.5406$ Å were employed. Data were recorded in the range of 10–130° 2θ with a step size of 0.0018° and a time per step of 3.2 s.

Neutron powder diffraction (NPD) data were collected using the D2B powder diffractometer at the Institute Laue-Langevin (ILL) in Grenoble, France. The BZCYYb4411 sample was measured at room temperature inside a vanadium can under vacuum conditions (10⁻⁴–10⁻⁵ mbar). Data acquisition covered an angular range of 0–160° with a step size of 0.05° and a wavelength of 1.59440 Å. The diffraction pattern was recorded over a duration of 3 hours at room temperature.

Rietveld analysis was carried out using Jana2006 software [58] in a tetragonal symmetry (I4/mcm space group), allowing the refinement of cell parameters and atomic coordinates. The relative density was obtained from the ratio of the experimental density of the sintered pellet to its theoretical density obtained from diffraction data. The experimental density is calculated geometrically by measuring the pellet's mass and volume. The theoretical density is calculated according to equation 3.

$$ \rho_{theo} = \frac{Z \times M}{Na \times V} \qquad \text{Equation 3} $$

where Z is the number of formula units, M is the molar mass, $N_a$ is the Avogadro number ($N_a = 6.022 \times 10^{23}$) and V is the lattice volume taken after refinement from Jana2006.

## Scanning electron microscopy (SEM)

The microstructure of the ceramics was characterized by scanning electron microscopy (JEOL JSM 7600LV, SEM) on the fractured cross-sections. To better visualize the grains and grain boundaries, the samples were mirror-polished and cleaned with ethanol in an ultrasonic bath and then subjected to a new heat treatment at 1300 °C for 3 hours (thermal etching). Since the electrolyte BZCYYb4411 is an electron insulator, a metallization process with Pt is necessary to avoid electron charging.
## Conductivity measurements

Electrochemical impedance spectroscopy (EIS) was performed to measure the ionic conductivity of the symmetrical cell. Symmetrical cells were prepared by screen-printing gold (Au) paste on both sides of the pellets as the current collector. The studied compositions included BZCYYb4411 + 0.5 wt% ZnO, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, and BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> pellets.

Impedance spectra were recorded using a Bio-Logic SAS model VSP-300 frequency response analyzer. Measurements were carried out every 50 °C while decreasing the temperature from 700 °C to 300 °C in 3 % H<sub>2</sub>O – 97 % N<sub>2</sub> (Alphagaz™ 2 grade, Air Liquide) with a stabilization time of 1 hour. The validity of the data was verified using the built-in Kramer-König test in EC-LaB software (Biologic SAS). Frequency sweeps were performed from 1 MHz to 100 mHz with an applied amplitude of 50 mV, chosen to optimize the signal-to-noise ratio while maintaining the linearity range. In order to determine the conductivity, the impedance diagrams of the symmetrical cells were fitted with appropriate equivalent circuits using the Zview software (Scribner Associates).

## Results and discussion

The X-ray powder diffraction (XRPD) patterns of the non-calcined, directly sintered BZCYYb4411 at 1400 °C for 10 hours are represented in Figure 1. Notably, all observed peaks on the XRPD patterns correspond to a perovskite phase, with no additional diffraction peaks detected for the externally sintering aids, such as ZnO or fluorite-phase ZCYYb4411, nor for the internally doped Zn. However, minor peaks corresponding to the cubic fluorite ZCYYb4411 phase were identified in the BZCYYb4411 + 3.0 wt% ZCYYb4411 sample. Overall, the diffraction patterns show that the samples were pure and well crystallized without secondary phase formation.
<figure><img src="image_2.png" /></figure>

Figure 1: XRPD patterns of the reference BZCYYb4411, BZCYYb4411 with 0.5–1.0 wt% ZnO, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.05</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> and BZCYYb4411 with 0.5–3.0 wt% ZCYYb4411 fluorite. All powders were prepared by the nitrate glycine process [57] and sintered at 1400 °C for 10 hours in air at room temperature.

The tetragonal *I*4/*mcm* space group was chosen for the BZCYYb4411 structure instead of the commonly reported cubic *P*m*3*m space group found in the literature [32,59]. Although this composition is often refined as cubic in *P*m*3*m, the selection of *I*4/*mcm* for BZCYYb4411 was based on the observation of the superlattice reflections (211) and (213) in the neutron diffraction diagram, as shown in Figure 2, which suggests a symmetry lower than cubic [57]. These reflections are difficult to detect using X-ray diffraction but are more clearly visible on a neutron diffraction pattern. The refinement produced a favorable agreement factor, confirming that the structure is tetragonal in *I*4/*mcm* at room temperature.

To study the effect of sintering strategies on the BZCYYb4411 lattice parameters, Rietveld refinements in *I*4/*mcm* space group were performed, as shown in Figure 3a-d. The unit cell parameters and the unit cell volume of all compounds show close values compared to the reference material, as summarized in Table 1.
<figure><img src="image_3.png" /><figcaption>Figure 2: Rietveld refinement based on neutron powder diffraction data of the reference BZCYYb4411 in tetragonal I4/mcm space group where a = b = 6.09989(14) Å, c = 8.6423(4) Å, χ² = 2.05, Robs = 3.24 %, Rp = 2.86 %, and Rwp = 3.75 %.</figcaption></figure>

Figure 4 illustrates the relative density of pellets processed using the three different sintering strategies, both with (C) and without (NC) calcination, with sintering at 1600 °C for 1 hour. BZCYYb4411 shows a relative density of only 69 % (NC) and 76 % (C), indicating its poor sinterability and the need for improvement strategies. The relative density increases to 96 % (C) and 93 % (NC) using other sintering strategies. These promising results prompted us to lower the sintering temperature from 1600 °C to 1400 °C (Figure 5), extending the duration to 10 hours. While the relative density of the reference material drops to 58 % (NC) and 70 % (C), the relative density of some pellets using sintering strategies, with (C) or without (NC) calcination, remains above 90 % (Figure 5). Below, we discuss the data obtained at 1600 °C and 1400 °C for each strategy.

For ZnO sintering aid, pellets that underwent calcination followed by sintering at 1600 °C (Figure 4) displayed superior relative density compared to those processed without the
calcination step, with relative density values of 96 % and 93 % (C) compared to 91 % and 79 % (NC) for 0.5 and 1.0 wt%, respectively. This increase is expected since calcination provides an additional thermal treatment step that promotes the formation of materials with enhanced crystallinity, contributing to higher densification. However, at 1400 °C (Figure 5), the relative density trend with ZnO was reversed. Samples processed without calcination exhibited higher relative densities than those that were calcined prior to sintering.

<figure><img src="image_4.png" /><figcaption>Figure 3: Rietveld refinements based on XRD data of (a) reference BZCYYb4411, (b) BZCYYb4411 + 0.5 wt% ZnO, (c) BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, (d) BZCYYb4411 + 0.5 wt% ZCYYb4411 (inset) ZCYYb4411 fluorite without calcination step, sintered at 1400 °C for 10 hours.</figcaption></figure>

<figure><img src="image_5.png" /></figure>

and R-factors determined by Rietveld analysis.

<table><thead><tr><th>Composition</th><th>a (Å)</th><th>b (Å)</th><th>c (Å)</th><th>V (Å<sup>3</sup>)</th><th>Space Group</th><th>χ<sup>2</sup></th><th>R<sub>obs</sub> (%)</th><th>R<sub>p</sub> (%)</th><th>R<sub>wp</sub> (%)</th></tr></thead><tbody><tr><td>BZCYYb4411</td><td>6.09658(16)</td><td>6.09658(16)</td><td>8.6647(4)</td><td>322.051(18)</td><td>I4/mcm</td><td>1.35</td><td>3.20</td><td>8.32</td><td>11.04</td></tr><tr><td>BZCYYb4411+0.5 wt% ZnO</td><td>6.1058(2)</td><td>6.1058(2)</td><td>8.6516(7)</td><td>322.54(3)</td><td>I4/mcm</td><td>2.08</td><td>8.04</td><td>11.28</td><td>16.69</td></tr><tr><td>BZCYYb4411+1.0 wt% ZnO</td><td>6.09453(7)</td><td>6.09453(7)</td><td>8.6265(2)</td><td>320.417(9)</td><td>I4/mcm</td><td>1.38</td><td>2.77</td><td>10.14</td><td>13.95</td></tr><tr><td>BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub></td><td>6.09783(17)</td><td>6.09783(17)</td><td>8.6628(4)</td><td>322.112(19)</td><td>I4/mcm</td><td>1.31</td><td>4.32</td><td>7.82</td><td>10.49</td></tr><tr><td>BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.05</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub></td><td>6.10292(17)</td><td>6.10292(17)</td><td>8.6686(4)</td><td>322.87(2)</td><td>I4/mcm</td><td>1.46</td><td>4.51</td><td>8.50</td><td>11.86</td></tr><tr><td>BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub></td><td>6.10017(19)</td><td>6.10017(19)</td><td>8.6792(4)</td><td>322.97(2)</td><td>I4/mcm</td><td>1.39</td><td>3.38</td><td>8.81</td><td>11.88</td></tr><tr><td>ZCYYb4411</td><td>5.27091(10)</td><td>5.27091(10)</td><td>5.27091(10)</td><td>146.439(5)</td><td>Fm&#773;m</td><td>1.67</td><td>1.67</td><td>12.24</td><td>17.63</td></tr><tr><td>BZCYYb4411+0.5 wt% ZCYYb4411</td><td>6.09820(14)</td><td>6.09820(14)</td><td>8.6528(4)</td><td>321.780(17)</td><td>I4/mcm</td><td>1.55</td><td>4.20</td><td>9.56</td><td>13.13</td></tr><tr><td>BZCYYb4411+1.0 wt% ZCYYb4411</td><td>6.09967(13)</td><td>6.09967(13)</td><td>8.6453(3)</td><td>321.657(15)</td><td>I4/mcm</td><td>1.36</td><td>3.36</td><td>8.28</td><td>11.19</td></tr></tbody></table>
Specifically, samples sintered without a prior calcination step showed relative density values of 96 % and 95 % compared to 91 % and 92 % for calcined specimens for 0.5 and 1.0 wt%, respectively. This result suggests that the necessity of calcination may be more temperature-dependent and that at lower sintering temperatures, the additional thermal treatment may not be as advantageous for ZnO-assisted densification. Interestingly, increasing ZnO content from 0.5 wt% to 1.0 wt% does not necessarily correlate with higher relative density values. Pellets with 0.5 wt% ZnO achieved relative densities comparable to those with 1.0 wt%, indicating that an optimal ZnO content may exist for maximizing densification efficiency. Furthermore, using a lower ZnO concentration, such as 0.5 wt%, improves densification while preserving the critical defect chemistry needed for high protonic conductivity [60]. However, higher ZnO concentrations may disturb the balance of protonic defects by promoting the formation of undesirable defect complexes, which reduce the number of mobile protons [60]. Consequently, it is essential to control ZnO content to ensure effective proton transport while preserving the materials' microstructure and ionic conductivity.

For the second strategy, involving the substitution of Zn by Y and/or Yb, pellets achieved higher relative density without calcination compared to calcined samples in most cases, both at 1600 °C (Figure 4) and 1400 °C (Figure 5). This behavior contrasts with that observed for the first strategy, where calcination improved relative densities at 1600 °C but not at 1400 °C. Additionally, substituting Zn with Y (BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>) resulted in slightly higher relative density values compared to substitution with Yb (BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.1</sub>Yb<sub>0.05</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>), regardless of the sintering temperature. However, despite achieving relative densities as high as 93 % at 1600 °C and 98 % at 1400 °C, these samples exhibited significant cracking and brittleness upon cooling due to hydration effects. This issue is attributed to the rapid absorption of moisture when cooling from high temperatures to ambient conditions. To prevent this, we implemented a controlled cooling protocol: samples were cooled to 500 °C before being removed from the furnace and then placed in a desiccator to reach room temperature gradually. This approach mitigates moisture absorption and prevents hydration-related cracking [61].

In the third strategy, the fluorite phase ZCYYb4411 was used as a sintering additive with its good sinterability (92 % at 1600 °C and 87 % at 1400 °C). The relative densities of ZCYYb4411-added samples were higher at 1600 °C (Figure 4) than those at 1400 °C (Figure
5). However, neither sintering condition achieved a relative density of at least 90 %, regardless of the sintering approach or additive concentration. Notably, higher fluorite concentration led to a decrease in relative density from 86 % to 75 % at 1600 °C (Figure 4) and from 79 % to 61 % at 1400 °C (Figure 5). This trend suggests that higher concentrations of ZCYYb4411 may inhibit densification, possibly due to two main factors. First, an excess of fluorite-phase could react with the primary BZCYYb4411 perovskite phase, leading to the formation of a secondary phase, not detected by XRD, which impedes grain boundary movement and thus limits overall densification [60]. Secondly, a high concentration of sintering aid might disrupt the optimal packing of BZCYYb4411 particles, resulting in larger pores that are challenging to eliminate during sintering, a phenomenon known as a reduction in effective particle packing [60].

<figure><img src="image_6.png" /><figcaption>Figure 4: Relative density (%) of all samples using sintering strategies with and without a calcination step, sintered at 1600 °C for 1 hour.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5: Relative density of all samples using sintering strategies with and without a calcination step, sintered at 1400 °C for 10 hours.</figcaption></figure>
The three sintering strategies improve densification via different mechanisms. The external ZnO addition facilitates densification mainly through a liquid-phase sintering mechanism. ZnO reacts with BaO in the system to generate a eutectic phase with a low melting point of around 1100 °C, which promotes grain rearrangement, accelerates pore elimination, and enhances grain boundary diffusion during sintering at 1400 °C [30,46]. In contrast, internal Zn doping functions via defect engineering. The substitution of Y³⁺ or Yb³⁺ by Zn²⁺ in the B-site generates defects and additional oxygen vacancies, hence enhancing the diffusion pathways and promoting the grain boundary migration. This method enhances densification without creating a liquid phase. Additionally, it helps stabilize Ce⁴⁺ ions and reduces electronic conduction pathways [30,43]. The incorporation of the fluorite-phase ZCYYb4411 is intended to facilitate sintering by solid-state rearrangement. The highly sinterable fluorite phase ZCYYb4411 facilitates particle packing and rearrangement, but at elevated concentrations, it hinders optimum grain boundary movement, resulting in larger pores that are challenging to eliminate during sintering [62] and induces secondary phase formation from the reaction with BZCYYb4411, which might not be detected by X-ray, thus limiting the overall densification [63].

<figure><img src="image_8.png" /><figcaption>Figure 6: SEM images are obtained on non-calcined samples directly sintered at 1400 °C for 10 hours, cross-sectioned, polished, cleaned with ethanol, and with a thermally etched at 1350 °C for 3 hours: a) BZCYYb4411 reference pellet b) BZCYYb4411 + 0.5 wt% ZnO c) BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> d) BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>.</figcaption></figure>

The SEM micrographs in Figure 6 show the microstructure of the fracture surface of sintered pellets under various conditions, highlighting the impact of doping and sintering strategies on porosity and grain morphology.

The reference sample (Figure 6a) exhibits relatively large grains (≈ 0.5 µm) with moderate porosity, reflecting the inherent limitations of sintering without any additives. In contrast, the addition of 0.5 wt% ZnO (Figure 6b) exhibits a more compact microstructure, promoting grain connectivity and boundary cohesion, indicative of enhanced sinterability. This improvement is attributed to ZnO's role as a sintering aid, likely facilitating grain boundary
diffusion during the densification process through liquid phase sintering. The effect is even more pronounced in the doped sample with the composition BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> (Figure 6c), where a finer and more homogenous grain distribution with significantly reduced porosity is observed, reflecting the beneficial effect of defect engineering on sintering behavior. Finally, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> (Figure 6d) reveals elongated grain growth and aligned microstructural features, which may be associated with anisotropic grain boundary migration influenced by the modified B-site occupancy. Overall, the observed improvements in microstructure, particularly in the doped samples, highlight the significance of doping strategies and sintering conditions in tailoring the performance of BZCYYb-based electrolytes.

<figure><img src="image_9.png" /><figcaption>Figure 7: Comparison of the total conductivity for samples with external ZnO addition (BZCYYb4411+ 0.5 wt % ZnO) and internal Zn doping (BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>, and BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub>) from this work against literature data from Choi et al. [32], Wang et al. [64], Guan et al. [59].</figcaption></figure>
An ionic conductivity in the range of 10⁻² to 10⁻³ S cm⁻¹ at 700 °C is generally required for the practical application of proton-conducting oxides as electrolytes. Above this temperature, due to dehydration of the material, the proton concentration is close to zero, and the material exhibits mainly oxide-ion conductivity [32].

In order to get an estimate of the samples' conductivity level, the total electrolyte resistance was determined from electrochemical impedance spectroscopy in 3 % H₂O – 97 % N₂ of three selected pellets. These pellets were sintered at 1400 °C for 10 hours without prior calcination and achieved a relative density exceeding 94 %. At temperatures lower than 500 °C, impedance diagrams show three depressed semicircles that can be fitted by a series of three (R//CPE) elements. The impedance of the CPE element is given by equation 4:

$$Z_{CPE} = \frac{1}{Q_{CPE}(j\omega)^p} \qquad \text{Equation 4}$$

where $Q_{CPE}$ is the pseudo capacitance and $p$ is the depression angle parameter. The equivalent capacitance, in F cm⁻², is calculated using the equation 5 [65].

$$C_{eq} = \frac{(R_{meas} \cdot Q_{CPE})^p}{R_{meas}} \qquad \text{Equation 5}$$

where $C_{eq}$ is the equivalent capacitance and $R_{meas}$ the measured resistance of the R-CPE parallel circuit. The capacities of the two first phenomena are respectively in the 10⁻¹² – 10⁻¹⁰ F cm⁻² and 10⁻¹⁰ – 10⁻⁸ F cm⁻² ranges, respectively, corresponding to bulk (Rb) and grain boundary (Rgb) resistances. The total conductivities of the samples were finally calculated using the sum of Rb and Rgb and taking into account the geometry of the samples. As temperature increases, all contributions are shifted to higher frequencies, and the high-frequency part of the diagram is reduced to a series resistance corresponding directly to the sum of bulk and grain boundary contributions.

The results are plotted in Figure 7 and compared with the literature data on BZCYYb4411 synthesized by different methods: Choi et al. [32] prepared it by solid-state reaction, calcined at 1100 °C for 10 hours, and densified at 1600 °C for 24 hours. Wang et al. [64] prepared it by solid-state reaction, calcined at 1450 °C for 10 hours, and sintered at 1500 °C for 5 hours, and Guan et al. [59] prepared it by EDTA-citric sol-gel method, calcined at 1100°C for 10 hours, and densified at 1500 °C.
Among the tested compositions, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> and BZCYYb4411 + 0.5 wt% ZnO exhibit conductivity values comparable to those reported by Wang et al. [64] and Guan et al. [59], despite being sintered at a lower temperature (1400 °C). At 700°C, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> reaches a conductivity of 0.9 x 10<sup>-2</sup> S cm<sup>-1</sup>, while BZCYYb4411 + 0.5 wt% ZnO achieves 0.6 x 10<sup>-2</sup> S cm<sup>-1</sup>. Conversely, BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.075</sub>Yb<sub>0.075</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> demonstrates a lower conductivity of 0.2 x 10<sup>-2</sup> S cm<sup>-1</sup>.

## Conclusion

This study explored various sintering strategies to enhance the sinterability of the BZCYYb4411 protonic conducting electrolyte. All samples were synthesized by the glycine nitrate process and sintered at 1600 °C for 1 hour and 1400 °C for 10 hours. Remarkably, a one-step sintering process without a prior calcination step achieved a relative density above 90 % even at the lower temperature of 1400 °C, representing a significant advancement in processing efficiency and energy reduction. Neutron diffraction reveals that BZCYYb4411 crystallizes in a tetragonal space group *I*4/mcm. Among the three sintering strategies studied, the Zn-based methods, both external ZnO addition and internal Zn doping, showed better potential for practical application in protonic ceramic electrolytes. These approaches enabled achieving a high relative density above 90 % even at 1400 °C, and resulted in good conductivity values, reaching 0.9 x 10<sup>-2</sup> S cm<sup>-1</sup> for BaZr<sub>0.4</sub>Ce<sub>0.4</sub>Y<sub>0.05</sub>Yb<sub>0.1</sub>Zn<sub>0.05</sub>O<sub>3-δ</sub> and 0.6 x 10<sup>-2</sup> S cm<sup>-1</sup> for BZCYYb4411 + 0.5 wt% ZnO at 700°C in humidified N<sub>2</sub>. In contrast, the fluorite phase (ZCYYb4411) addition was less effective in enhancing densification, particularly at higher concentrations. Thus, Zn-based sintering strategies are more promising for developing high-performance BZCYYb-based electrolytes for proton ceramic devices.

## Acknowledgments

The authors acknowledge support from the French National Research Agency (ANR) under France 2030 program and reference ANR-PEHY-0006 (project PEPRH2-PROTEC). The author would like to thank ILL for providing beam time and Dr. Emmanuelle Suard for her help with the measurements on the D2B powder diffractometer, Dr Pierre Emmanuel Petit responsible for the diffractometers in the IMN's characterization platform PLASSMAT, Nantes, France.
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