Role of Sintering Aids in Electrical and Material Properties of Yttrium- and Cerium-Doped Barium Zirconate Electrolytes

Abstract

Ceramic proton conductors have the potential to lower the operating temperature of solid oxide cells (SOCs) to the intermediate temperature range of 400–600 °C. This is attributed to their superior ionic conductivity compared to oxide ion conductors under these conditions. However, prominent proton-conducting materials, such as yttrium-doped barium cerates and zirconates with specified compositions like BaCe_1−xY_xO_3−δ (BCY), BaZr_1−xY_xO_3−δ (BZY), and Ba(Ce,Zr)_1−yY_yO_3−δ (BCZY), face significant challenges in achieving dense electrolyte membranes. It is suggested that the incorporation of transition and alkali metal oxides as sintering additives can induce liquid phase sintering (LPS), offering an efficient method to facilitate the densification of these proton-conducting ceramics. However, current research underscores that incorporating these sintering additives may lead to adverse secondary effects on the ionic transport properties of these materials since the concentration and mobility of protonic defects in a perovskite are highly sensitive to symmetry change. Such a drop in ionic conductivity, specifically proton transference, can adversely affect the overall performance of cells. The extent of variation in the proton conductivity of the perovskite BCZY depends on the type and concentration of the sintering aid, the nature of the sintering aid precursors used, the incorporation technique, and the sintering profile. This review provides a synopsis of various potential sintering techniques, explores the influence of diverse sintering additives, and evaluates their effects on the densification, ionic transport, and electrochemical properties of BCZY. We also report the performance of most of these combinations in an actual test environment (fuel cell or electrolysis mode) and comparison with BCZY.

1. Introduction

Solid oxide cells (SOCs) have been a remarkable source of renewable power generation (in fuel cell mode) as well as green chemical energy production (in electrolysis mode) with higher efficiency, fuel tolerance, and use of non-precious metal catalysts with minimal GHG (Green House Gas) emissions [1,2,3,4,5]. Although this technology is quite mature, and has already been deployed at the MW scale, it has some inherent challenges related to material stability. Most importantly, the oxide ion conducting electrolytes used in SOCs require operating temperature as high as 800 °C to ensure high ionic conductivity and better electrocatalytic activity of the electrodes [6,7,8,9,10]. However, such high temperature operation (1) limits the choice of materials used for electrodes and sealing and current collectors; (2) results in the faster degradation of materials; and (3) leads to lengthy start-up and shut-down periods [11]. Compared to SOCs, proton-conducting solid oxide cells (H-SOCs) offer some unique features like intermediate temperature operation (500 to 700 °C) and higher theoretical electromotive force (EMF) [12] and lower activation energy. Such lower-temperature operation broadens the choice of materials that can be used as electrodes [13,14]. Also, such lower-temperature operation is thermodynamically more suitable for certain reactions like methane generation (Sabatier process) and ammonia synthesis (Haber–Bosch process) [15,16,17,18,19]. Thus, green hydrogen generated in a proton-conducting solid oxide electrolytic cell can undergo a further reaction in the cell itself to produce methane (in the presence of CO₂) or ammonia (in the presence of N₂), provided that appropriate electrocatalysts are used in the fuel electrode (cathode), and broadens the choice of materials that can be used as electrodes [20,21,22,23].

H-SOCs have attracted a lot of attention since Iwahara et al. [24] discovered in 1981 that doped strontium cerate sintered between 1300 and 1450 °C was an almost pure protonic conductor in a hydrogen-containing atmosphere. Figure 1a shows the simple operation of a proton-conducting solid oxide fuel cell with the proton-conducting oxide layer sandwiched between the anode and the cathode, where the reaction happening on the anode side is the hydrogen splitting reaction (Equation (1)) [25,26,27]:

H₂ ⟶ 2H⁺ + 2e⁻

(1)

The other reaction happening on the cathode side, where the protons get transported through the solid oxide electrolyte and react with the oxygen on the cathode side to form water (Equation (2)), is

0.5O₂ + 2H⁺ + 2e⁻ ⟶ H₂O

(2)

Figure 1b shows the operation of a proton-conducting solid oxide electrolysis cell. At the cathode, steam splits into hydrogen ions and oxygen (Equation (3)) [28,29,30]. The protons are conducted across the electrolyte to the anode, where pure hydrogen evolves (Equation (4)).

H₂O ⟶ 0.5O₂ + 2H⁺ + 2e⁻

(3)

2H⁺ + 2e⁻ ⟶ H₂

(4)

The proton-conducting ceramics like doped barium cerate- and barium zirconate-based materials have been considered as promising electrolytes for H-SOCs due to their high ionic conductivity that can be attributed to a fast proton transport via the classic Grotthus mechanism [31].

Barium-based perovskites have been studied in depth and shown promise as proton-conducting electrolytes [32,33,34,35]. While barium cerate (BaCeO₃) oxides exhibit high protonic conductivity, they are not stable at low temperatures and easily react with steam and CO₂ due to their larger lattice structure. In contrast, barium zirconates (BaZrO₃) are chemically stable, but they are rather refractory and demand very high sintering temperatures (above 1600 °C) for sufficient densification [36,37]. Insufficient densification reduces protonic conductivity, and often leads to gas cross-over, as well. In fact, according to the literature, the total conductivities of zirconates are lower than those of cerates. Thus, to take advantage of both cerates and zirconates, solid solutions of BaZr_1−xCe_xO_3−δ-based ceramics have been proposed and investigated in depth [11,38]. They do exhibit enhanced stability and conductivity; however, the sintering temperature required to densify the materials for electrolyte use is still higher than 1500 °C. Further research led to the evolution of single-doped BaZr_1−xCe_xO_3−δ-based ceramics, where the dopant can be any aliovalent rare earth element like Y, Sm, Gd, and Dy. Y-doped BaZr_1−xCe_xO_3−δ has shown the highest conductivity in dry air (2.5 × 10⁻² S cm⁻¹) and wet air (2.1 × 10⁻³ S cm⁻¹) atmospheres at 600 °C, and attains >95% relative density at ~1400  °C [39,40]. The amount of the yttrium dopant was doped in the range of 10–20 mol%, and cerium was doped from 20 to 30 mol%. Thus, BaZr_1−x−yCe_xY_yO_3−δ is the most widely used electrolyte for most of the studies on proton-conducting solid oxide ceramics.

Although higher sintering temperatures enhance the densification of BCZY electrolytes, studies have shown that it can cause the evaporation of barium and the interdiffusion of Ni from the green cermet electrode (composite of NiO and BCZY) to the BCZY electrolyte for co-sintering [41]. Such Ni diffusion often causes a decrease in the protonic conductivity of BCZY as reported by Donglin et al. [42]. In fact, Babilo et al. [43] reported an increase in the electronic conductivity of electrolytes due to Ni cation diffusion from the anode support to the electrolyte. Additionally, at higher co-sintering temperatures, NiO-based cermet electrodes become more densified, which reduces the number of open pores and builds up higher diffusion and mass transfer losses across the electrode. Loss of porosity also reduces the number of active sites for an oxygen reduction reaction and increases the charge-transfer losses [44]. Moreover, higher sintering temperatures lead to barium evaporation from the lattice that lowers the occupancy of A-sites and attenuates oxygen vacancies [45,46].

Ideally, a lower sintering temperature is desirable without affecting the density and protonic conductivity of the electrolyte or imparting any electronic conductivity. This is where sintering aids come into play. Sintering aids are oxides of transition metals like Ni, Cu, Zn, Co, Fe, etc., that are added in very low concentration to enhance the sinterability of proton-conducting electrolytes [47,48,49]. However, sintering aids in a higher concentration cause structural distortions in the grain boundary region that may lead to a decrease of proton mobility or a depletion in protonic defects, resulting in increased resistance for proton conduction.

This review focuses on the different sintering aids that have been used so far for BCZY electrolytes and discusses the pros and cons of each one [42,50,51,52]. It provides a synopsis of various potential sintering techniques, explores the influence of diverse sintering additives, and evaluates their effects on densification, ionic transport, and electrochemical properties of BCZY. Similar reviews are available in the literature [44,53,54,55,56]; however, they focus on (1) only one sintering aid like NiO or ZnO and its effect on barium cerates and zirconates or (2) a specific synthesis technique and how sintering aids incorporated by that technique affect the material and ionic properties of barium cerates and zirconates. Here, we capture and present, for the first time, the effect of various regulatory parameters like the synthesis technique, sintering profile, etc., for different sintering aids on the material and ionic properties of state-of-the-art intermediate-temperature proton-conducting ceramic BCZY. We also report based on the literature the performance of most of these combinations in an actual test environment (fuel cell or electrolyser mode).

2. Current Challenges with BCZY Electrolyte

To understand the current challenges with state-of-the-art material BCZY, it is important to explain how proton conduction takes place in BCZY and how density affects proton conductivity.

Perovskite material BCZY has oxygen vacancies arising from the valency difference of the B-site cations. Zr has a valency of +4, Ce has variable valency of +3/+4 depending on oxygen partial pressure, and Y has a valency of +3. Thus, in the presence of air/oxygen, BCZY can transport oxide ions via ion hopping from one oxygen vacancy to another [57,58]. However, in the presence of hydrogen (highly reducing environment), the hydrogen reacts with interstitial lattice oxygen through hydroxyl bond formation (also known as protonic defect), undergoes re-orientation, breaks the bond, and hops onto the next available oxygen site. This is the well-established Grotthuss mechanism [59,60,61] of proton transport in ceramics in the form of protonic defects (OH^.). It is accepted that in the presence of humidity, the water interacts [62,63] with oxygen vacancies through the formation of more protonic defects, resulting in a higher proton conduction. However, the exact reaction mechanism of proton transport, especially in humid conditions, is very complex and still debated. Further details can be found in a recent review by Wang et al. [64]. In general, the total conductivity (σ_total) of BCZY is the sum of proton (σ_H+), oxygen vacancy (σ_Vo), and hole (σ_h) conductivities (Equation (5)) that depend on the concentration of charge and mobility (µ) of charge carriers (Equation (6)).

$${\sigma }_{total}={\sigma }_{H+}+{\sigma }_{Vo}+{\sigma }_h$$

(5)

$${\sigma }_{total}=({\mu }_h\left[h\right]+\left|-2\right|{\mu }_{Vo}\left[V_O\right]+{\mu }_{H+}\left[H^{+}\right]){\displaystyle \frac{e{N}_A}{V_m}}$$

(6)

In the above equations, μ_i is the mobility of the charge carrier i, e is the electron charge, N_A is Avogadro’s number, and V_m is the molar volume of BCZY. At temperatures below 600 °C, σ_H+ dominates in reducing atmospheres, whereas σ_Vo and σ_h dominate at higher oxygen partial pressures [65,66]. It is to be noted here that any ion hopping in ceramic materials takes place along and/or between the grain boundaries, and across the bulk of the grain. The resistance arising from the grain boundaries is known as the grain boundary resistance (R_GB) and that arising from the grain bulk is called bulk resistance or grain interior resistance (R_GI). The arrows in Figure 2a indicate the arcs in a typical Nyquist plot corresponding to R_GB and R_GI. The ohmic resistance (R_ohm) is the sum of R_GB and R_GI. Two things happen when grains are not fully developed with well-defined boundaries overlapping or touching each other:

• Ions cannot migrate across the grains, thus increasing R_ohm and lowering ionic conductivity.
• The gap between the individual grains manifests as pores or pinholes (depending on the size and number of such gaps) that can lead to gas cross-over, affecting the open-circuit voltage.

In general, an increase in grain size is accompanied by a decrease in the number of grains and thus the number of grain boundaries. As a result, the protons have to travel a longer distance across the grains that increases grain bulk resistance (R_GI). In contrast, smaller grain sizes are accompanied by a higher number of grain boundaries that increases the grain boundary resistance (R_GB). According to the DFT calculations of the Grotthuss mechanism by Jing and Aluru, a proton is not able to migrate until it has passed two energy barriers, one being the energy needed for the proton to hop from one oxygen vacancy to another and the other being the energy needed for the hydroxide ion to spin. The first energy barrier is highly affected by the BCZY microstructure. If grains are not well developed with pores or pinholes present, the energy barrier of proton hopping spikes since protons can hop only to nearby lattice oxygen vacancies in the intermediate temperature range [67]. Hence, grain growth and densification are necessary to achieve proper ionic conduction pathways and ensure high ionic conductivity and no gas cross-over. The major challenge with BCZY is the high sintering temperature (~1500 °C) [68,69,70] needed to achieve a high densification, and in turn, a high proton conductivity. Muhammad et al. [71] reported that crystalline size increased from 10.48 nm to 18.51 nm with increased sintering temperature of BCZY (BaCe_0.2Zr_0.6Y_0.2O_3−δ perovskite) from 900 °C to 1100 °C. The increase in sintering temperature leads to a better grain boundary movement as grain boundaries start migrating too quickly with temperature, leading to increased crystalline size [71]. However, studies have shown that high sintering temperatures can cause the evaporation of barium and interdiffusion of Ni from the green cermet electrode (composite of NiO and BCZY) to the BCZY electrolyte for co-sintering [41]. The most common techniques used to minimize Ba evaporation are (1) placing the samples on a sacrificial BCZY layer while sintering [48,54,72], and (2) using the excess Ba strategy [73,74] whereby the A-site Barium ratio is initially increased from 1 to 1.3 by adding barium carbonate (BaCO₃) to the sample to compensate for any Ba loss. In spite of adopting these preventive measures, Ba loss is still envisaged at higher sintering temperatures (~1500 °C).

Ni interdiffusion from electrodes to electrolytes is something that is difficult to control or fully mitigate in the case of co-sintering. Toshiaki et al. [75] reported Ni diffusing into the BCZY electrolyte from the NiO-based anode substrate during high-temperature co-sintering (>1300 °C). Due to higher co-sintering temperature, the diffused Ni effectively works like a sintering aid that promotes the densification of BCZY electrolytes [76]. However, such Ni diffusion is uncontrolled, unlike in the case of sintering aids. While sintering aids are added in only 1–5 wt%, NiO used in cermet electrodes varies from 45 to 65 wt%. Thus, Ni diffusion from electrodes to electrolytes during co-sintering can be enough to form electron conduction pathways that then decrease the protonic conductivity of the electrolyte [42]. In fact, Babilo et al. [43] reported an increase in the electronic conductivity of electrolytes due to Ni cation diffusion from the anode support to the electrolyte, and doping with 4 mol% ZnO reduces sintering temperature from 1700 °C to 1300 °C along with the bulk conductivity due to strong proton trapping at Zn_Zr sites. One possible strategy to minimize Ni interdiffusion is to first sinter the electrode to allow the grain growth and densification of the electrode, and then coat it with the electrolyte [77,78,79,80,81]. However, in this case, the electrolyte has to be coated either via tape casting or as a slurry via techniques like spraying, spinning, dipping, or brushing. Research involving these techniques has so far shown negligible electronic conduction across the electrolyte [77,78,79,80,81]. However, these techniques pose additional challenges related to slurry composition optimization and coating procedure optimization and are also time-consuming compared to co-pressing/co-sintering.

Additionally, at higher co-sintering temperatures, cermet (metal + BCZY) electrodes become more densified, which reduces the number of open pores and builds up higher diffusion and mass transfer losses across the electrode. Nur Hashina et al. [44] studied the impact of sintering temperature on the properties of NiO-BCZY composite anodes, which revealed that the conductivities of NiO-BCZY anodes sintered at 1200 °C, 1300 °C, and 1400 °C were 443 Scm⁻¹, 633 Scm⁻¹, and 1124 Scm⁻¹, respectively, and the porosity results are shown in Figure 2b. A possible and widely adapted solution is the addition of pore formers in the cermet electrode. However, assuring pore connectivity is a challenge and hard to achieve using facile synthesis techniques like dry/wet mechanical mixing. Secondly, for two-step fabrication processes where the electrode is pre-sintered followed by coating (spray coating, dip coating, drop casting, tape casting, spin coating) [18,77,80,82] and sintering the electrolyte, the presence of pores at the electrode–electrolyte interface leads to electrolyte percolation and delamination. This calls for incorporating a dense cermet interlayer that is a few microns thick between the electrode and the electrolyte, which adds to the number of steps and overall fabrication complexity.

In view of the above challenges posed by high sintering temperature of BCZY, the addition of sintering aids has been proposed as a solution and widely studied in the literature. The different sintering aids reported so far and their pros and cons are discussed in depth in the next section.

3. State-of-the-Art Sintering Aids

3.1. Nickel Oxide

NiO is one of the most widely tested sintering aids. Duan et al. [79] tested BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ, doped with 1 wt % NiO, as the electrolyte (20−30 μm in thickness), sintered at 1400 °C; a composite of 40 wt% BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ and 60 wt% NiO as the anode; and a mixture of BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY0.1) and BaCe_0.6Zr_0.3Y_0.1O_3−δ (BCZY63) as the cathode to prepare the SOFC. They operated the SOFC under H₂/air and obtained a peak power density of 455 mW cm⁻² at 500 °C. The open-circuit voltage was higher than 1.05 V, suggesting that both electronic and mechanical leakages were minimal.

Recently, Liu et al. [83] studied anode-supported SOFCs with a BaCe_0.7Zr_0.1Y_0.2O_3−δ electrolyte having 2 mol % of Ni_1−xFe_x (x range: from 0 to 1.0) oxides and 4 mol % of FeO_1.5 sintering aids. All of the electrolytes with additives were >95% densified after pressing at 150 MPa followed by sintering at 1400 °C for 5 h, while BCZY without an additive was still porous. X-ray diffraction spectra show that Ni and Fe were doped into the lattice of BCZY, and it is backed by the hypothetical deduction stated in most studies that transition metal ions from the metal oxide additives tend to occupy the B-site of the ABO₃ perovskite structure [84,85]. With 3% humidified H₂, the cell with the 2 mol % Ni_0.5Fe_0.5-doped BCZY electrolyte, with an unoptimized cathode, gave a power density of 973 mW cm⁻² at 700 °C and 1 V as opposed to only 370 mW cm⁻² with pristine BCZY. Others have also reported the presence of sintering aid metal in the BCZY lattice that gets manifested as a peak shift (shown using a solid yellow vertical line along the major BCZY peak in Figure 2c) and is often accompanied by an occasional increase or decrease in proton conductivity [54,86,87,88]. However, any correlation between the lattice incorporation of sintering aid metal and proton conductivity is not yet well established. As has been discussed later, the enhancement or suppression of proton conductivity depends on various factors like the concentration of the sintering aid, synthesis technique, ratio of Ce/Zr in BCZY, sintering profile, etc. Nonetheless, reports indicate that during the sintering, the NiO decomposition and bulk diffusion of Ni into BCZY perovskites cause a commensurate increase in the bulk oxygen vacancy concentration [89]. The higher oxygen vacancy concentration could further promote bulk diffusion and enhance sintering.

Wang et al. [72] studied NiO addition in different concentrations for BaZr_0.1Ce_0.7Y_0.2O_3−δ electrolytes. The X-ray diffractogram reflected that with an increase in the sintering temperature, the BCZY peak gradually shifted towards a higher angle, indicating a substantial substitution of Zr by Ni in BCZY since Ni²⁺ (0.069 nm) has a slightly smaller ionic radius than that of Zr²⁺ (0.072 nm) at the B-site of perovskite. However, 10% NiO addition showed a reduction of NiO aggregation to Ni, thus forming the electronic pathway between electrodes. The author reported an ohmic resistance of 3.51Ω cm², which should theoretically be 0.07 Ω cm² considering BCZY conductivity of 0.01 Scm⁻¹ at 800 °C. This can either be from poor electrode–electrolyte interfacial contacts or even due to suppressed protonic conductivity of BCZY as a result of the sintering aid. Along similar lines, Lee et al. [90] studied the effect of different NiO wt% on BaCe_0.6Zr_0.2Y_0.2O_3−d (BCZY) electrolytes. The fuel cell with a BCZY-3%NiO electrolyte (7 µm thickness) sintered at 1500 for 10 h exhibited the highest maximum power density of ~106.6 mW cm⁻² with a corresponding OCV of 1.09 V (at 800 °C) as opposed to only 6 mW cm⁻² for pristine BCZY. In another work, Lee et al. [90] fabricated a BaCe_0.6Zr_0.2Y_0.2O_3−d (BCZY) electrolyte by spin coating using a NiO sintering aid in 1, 3, and 5 wt%, co-sintered with an anode at 1500 °C for 10 h. Increasing NiO sintering aid addition from 1 wt% to 3 wt% increased the OCV of the fuel cell from 0.95 V to 1.09 V since the number and size of pinholes decreased as was also seen from scanning electron microscopic images. Pristine BZCY gave a power density of 6 mW/cm² at 800 °C in humidified hydrogen (3% H₂O), whereas BZCY with 1 wt%, 3 wt%, and 5 wt% achieved power densities of ~ 75 mW/cm², ~110 mW/cm², and 65 mW/cm² of the single cells measured at 800 °C. Here, we also see that increasing the NiO% from 1 to 3 improved cell performance, whereas a further increase to 5% significantly dropped the power density. Based on the research published so far, it can be concluded that an appropriate NiO addition has a significant positive contribution to the densification and grain growth of BCZY electrolytes. However, too much addition gives rise to NiO aggregation within the electrolyte and leads to electronic conduction pathways, thus deteriorating the cell performance.

The optimum concentration depends on various factors like the sintering profile, sintering time, electrolyte composition, powder particle size, etc. Thus, a certain % NiO addition in one scenario may give very high conductivity, whereas that same % in another scenario may show a massive drop in conductivity. For example, Zongping et al. [91] fabricated pellets of composites with varying weight percentages of x-NiO and BCY (where x is 0, 1.0, 5.0, and 10.0 wt %) through dry pressing and sintering at 1450 °C for a duration of 5 h. The electrochemical performance was determined using humidified hydrogen (3 vol% H₂O). The relative densities of all sintered pellets with varying NiO contents were around 96–98%. The total ionic conductivity of pristine BCZY pellets was 23.5 mScm⁻¹ at 600 °C and the addition of 1.0, 5.0, and 10.0 wt% NiO to the electrolyte led to a decrease in ionic conductivity by 27.0%, 42.3%, and 49.4% at 600 °C. The peak power densities that were reported at 700 °C, 600 °C, and 500 °C were 566 mW cm⁻², 377 mW cm⁻², and 181 mW cm⁻², respectively. Similarly, Nasani et al. [54] reported a drop in the conductivity of BaCe_0.3Zr_0.55Y_0.15O_3−δ (BCZY35) upon doping 1 wt% NiO, CuO, and ZnO through the citrate–nitrate combustion method and sintering at 1400 °C for a duration of 5 h. A detailed analysis of Arrhenius plots revealed a sharp decline in bulk conductivity upon introducing the sintering aids, whereas there was no change in grain boundary conductivity (Figure 3a–c). The reason could be the dissolution of the sintering aid into the host perovskite phase and subsequent proton trapping effects as pointed out by some authors [50,92,93,94]. It is to be noted that all these references have used wet chemistry techniques like the sol–gel method, citrate–nitrate combustion, exsolution, etc., as the method of sintering aid doping. We surmise that the synthesis technique plays a pivotal role in determining how much the sintering aid percolates or dissolves into the BCZY lattice. While wet chemistry doping techniques may lead to complete dissolution into the bulk phase, mechanical mixing accomplishes uniform particle distribution followed by the lattice penetration of the sintering aid to some extent only during the sintering period. However, detailed streamlined studies are needed for conclusive remarks and a better understanding of the phenomenon.

Chen et al. [95] fabricated BaZr_0.9Y_0.1O_3−d (BZY10) with 0.2 wt% NiO electrolyte disks sintered at 1500 °C and 1600 °C for 6 h. Higher sintering temperature had a beneficial effect on the grain size for BZY10+0.2 wt% NiO-1600 compared to the BZY10+0.2 wt% NiO-1500 sample, but this was also traded off by the barium evaporation. Thus, a total conductivity of 2.4 × 10⁻³ Scm⁻¹ was obtained at 600 °C for the sample sintered at 1600 °C whereas the same conductivity was achieved at just 400 °C for the sample sintered at 1500 °C. The authors also characterized the temperature dependance of grain interior (GI), grain boundary (GB), and total electrical conductivity in an Arrhenius plot for both BZY10+0.2 wt%NiO-1500 and BZY10+0.2 wt%NiO-1600 in a cell testing condition of 2.7% humidified H₂ and Ar (1:19 v/v). In the lower temperature range of 150 to 300 °C, conductivity at the grain interior (σ_GI) as well as activation energy (E_a) values of GI and GB for both samples were very similar but independent in their microstructural behavior and their fabrication process. This phenomenon was highly related to the charge carrier transport properties, which is also an intrinsic parameter. The Arrhenius plots are shown in Figure 3d,e for both the samples. Yamazaki et al. [96] proved that the proton trapping behavior will occur in the low temperature range (150 to 350 °C), which induces a downward curvature in the Arrhenius plot but due to a limited number of experimental data points, the downward curvature was not identified in Figure 3d,e.

Cheng et al. [97] studied the performance of BaCe_0.6Zr_0.2Y_0.2O₃ (BCZY) electrolytes with a 3 wt% NiO sintering additive under fuel cell conditions. The electrolyte powders obtained via nanomilling (average particle diameter: 131 nm) were co-sintered at 1350 °C, 1400 °C, 1450 °C, and 1480 °C for 8 h. The respective conductivities were 0.008 Scm⁻¹, 0.01 Scm⁻¹, 0.013 Scm⁻¹, and 0.014 Scm⁻¹ (in the temperature range of 800 °C to 500 °C in an H₂ environment). Corresponding peak power density at 800 °C was 409.6 mW/cm², 489.8 mW/cm², 467.1 mW/cm², and 398.9 mW/cm². Interestingly, same composition electrolyte powder prepared through ballmilling (average particle diameter: 297 nm) and co-sintered at 1480 °C for 8 h gave a peak power density of only 354.9 mW/cm². Nanomilling reduced the electrolyte particle size that helped attain > 95% densification at only 1400 °C as compared to 1480 °C for ballmilling. The lower co-sintering temperature caused lesser coarsening of the NiO anode, thus protecting the electrocatalytic active sites for the H₂ spillover [98,99] reaction by Ni during cell operation. This was reflected as a 50% drop in charge-transfer resistance from electrochemical impedance spectra. However, what has not been explained is the correlation between particle size and densification. It is possible that nanomilling the NiO-BCZY electrolyte not only reduced the BCZY particle size but also the NiO particle size, thus creating a more uniform distribution of fine NiO and BCZY particles. Such a uniform distribution and better coverage of NiO may have assisted the bridging of the BCZY particles via liquid phase sintering [100,101,102] at a lower temperature. More streamlined studies are needed to clearly understand this phenomenon.

A summary of cell performances and electrolyte conductivities of different compositions of BZCY and BZY electrolytes is summarized in

Table 1. BCZY and BZY with nickel oxide sintering aids reported in the literature.

Electrolyte Composition	Sintering Aids	mol% or weight% Added	Sintering Temperature (°C), Duration	Relative Density (%)	Atmosphere	T (°C)	Conductivity (×10−3 Scm−1)	T (°C)	Peak Power Density (mWcm−2)	Open-Circuit Voltage (V)	Synthesis Method of Powder
BaZr0.1Ce0.66Ni0.04Y0.2O3−δ [72]	NiO	4% (mol)	1400 °C, 5 h	N/A	Wet H2	600	6.30	600	477	1.053	Wet chemistry
BaZr0.1Ce0.66Ni0.04Y0.2O3− δ [72]	NiO	4% (mol)	1400 °C, 5 h	N/A	Wet H2	600	6.30	650	674	1.038	Wet chemistry
BaZr0.85Y0.15O3−δ [72]	NiO	4% (mol)	1450 °C, 8 h	>95%	Humid N2 atmosphere (pH2O = 0.026 atm)	800	1.03	N/A	N/A	N/A	Wet chemistry
BaZr0.91Ni0.01Y0.08O3− δ [94]	NiO	1% (mol)	1600 °C, 5 h	>99%	Wet 1% H2 + Ar	900	>1.00	N/A	N/A	1.000	Solid-state reaction
BaZr0.8Y0.2O3− δ [103]	NiO	2% (wt)	1450 °C, 5 h	>95%	3% H2O/Ar	600	3.10	700	5.5	N/A	Conventional solid-state reaction
BaZr0.9Y0.1O3− δ [104]	NiO	0.6% (mol)	1600 °C, 4 h	97.3	3% humidified H2	600	1.20	900	2.35	1.124	Conventional solvent mixing
BaZr0.9Y0.1O3− δ [104]	NiO	0.9% (mol)	1500 °C, 4 h	96.3	3% humidified H2	600	5.90	900	2.35	0.794	Conventional solvent mixing
BaZr0.9Y0.1O3− δ [104]	NiO	1% (mol)	1450 °C, 8 h	N/A	Dry air	600	~7.50	N/A	N/A	N/A	Conventional solvent mixing
BaZr0.9Y0.1O3− δ [104]	NiO	1% (mol)	1450 °C, 8 h	N/A	Humid air	600	3.70	N/A	N/A	N/A	Conventional solvent mixing
BaZr0.9Y0.1O3− δ [104]	NiO	1% (mol)	1450 °C, 8 h	N/A	5% H2/Ar	600	2.00	N/A	N/A	N/A	Conventional solvent mixing
BaZr0.9Y0.1O3− δ [104]	NiO	1.5% (mol)	1500 °C, 4 h	96.5	3% humidified H2	600	0.44	900	2.35	0.793	Conventional solvent mixing
BaZr0.9Y0.1O3−d [105]	NiO	1% (wt)	1600 °C, 12 h	N/A	5% H2 in N2	600	1.03	N/A	N/A	N/A	Solid-state reactive sintering
BaZr0.9Y0.1O3−d [105]	NiO	1% (wt)	1600 °C, 12 h	N/A	5% H2 in N3	500	1.10	N/A	N/A	N/A	Solid-state reactive sintering
BaZr0.8Y0.2O3− δ [79]	NiO	1% (wt)	1450 °C	N/A	H2/air	N/A	N/A	500	335	1.050	Synthesis from raw precursor
BaCe0.7Zr0.1Y0.1Yb0.1O3− δ [79]	NiO	1% (wt)	1400 °C	N/A	H2/air	N/A	N/A	500	455	>1.050	Solution infiltration
BaZr0.1Ce0.7Y0.2O3− δ [106]	NiO	2% (mol)	1400 °C, 6 h	N/A	Wet air Wet H2	700	25 19	700	855	1.000	Solid-state reaction
BaZr0.8Y0.2O3−δ [106]	NiO	2% (mol)	1400 °C, 6 h	N/A	Wet air Wet H2	700	14 4	700	360	0.950	Solid-state reaction
BaZr0.8Y0.2O3−δ [14]	NiO	1% (wt)	1450 °C, 18 h	N/A	Humidified H2 and air	N/A	N/A	600	660	N/A	Solid-state reactive sintering
BaCe0.7Zr0.1Y0.1Yb0.1O3−δ [107]	NiO	1% (wt)	rapid laser reactive sintering defocus distance: 20 mm laser energy: 95 W scan speed: 0.1 mm/s	N/A	Humidified H2 and air	600	3.70	600	121	0.970	Solid-state reactive sintering + rapid laser reactive sintering
BaCe0.7Zr0.1Y0.2O3−δ [108]	NiO	0.5% (wt)	1400 °C, 6 h	~98	Humidified H2 and air	N/A	N/A	600	60	1.110	Solid-state reaction
BaZr0.8Y0.2O3−δ [14]	NiO	1% (wt)	1450 °C, 18 h	N/A	Humidified H2 and air	N/A	N/A	600	660	N/A	Solid-state reactive sintering

below.

3.2. Oxides of Copper and Zinc

Various other transition metal oxides like CuO and In₂O₃ have also been investigated in depth. Nikodemski et al. [109] studied the effect of different metal oxide additives (CuO, ZnO, Fe₂O₃, MnO₂, PdO, and Cr₂O₃, all in 5 mol %) on sintering and transport properties of BaCe_0.6Zr_0.3Y_0.1O_3−δ (BCZY63). Only CuO led to an enhanced densification and grain growth of ~6.2 µm (following 12 h sintering at 1450 °C), whereas PdO addition resulted in a similar grain size of ~1.4 µm to the pristine BCZY63 (following 12 h sintering at 1600 °C) as was seen from Field Emission Scanning Electron Microscopic images (Figure 4A–F). However, most additives (e.g., ZnO, Fe₂O₃, MnO₂, Cr₂O₃) led to a decrease in conductivity (Figure 4G–I), whereas CuO and PdO dopants yielded an increase in total conductivity compared to pristine BCZY63. They explained the factors for enhancement in the total conductivity as follows: (1) the additive can act as a sintering aid to produce larger grain size, and (2) substitutional doping of the BCZY63 structure by metal ions from additives can lead to an increased bulk proton concentration. However, why certain metals succeeded in enhancing proton conductivity while the others did not is necessary to understand, and requires further studies. The authors tested a fuel cell with the BCZY63+5 mol%CuO electrolyte, 40 wt% BCZY63+60 wt% NiO anode, and BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCFZY0.1) cathode in 5%H₂/2.5%H₂O/92.5%Ar. OCV between 1.0 and 1.2 V was obtained in the temperature range of 450 to 750 °C, indicating no significant electronic transport upon adding CuO. There was a peak power density of 500 mW cm⁻² at 600 °C, and the cell was stable over 500 h of operation at 500 °C, 260 mA cm⁻², and 0.8 V.

It has been reported that a low melting temperature of Bi₂O₃ significantly improves the material density due to the introduction of the liquid phase sintering effect [100,101,102]. On the other hand, CuO contributes significantly towards the increased conductivity of materials as compared to other sintering aids. Thus, the synergy of CuO and Bi₂O₃ has been reported as an effective sintering aid in terms of both lowering the sintering temperature and improving proton conductivity. Recently, Babar et al. [110] achieved an appreciable densification of BCZY sintered at 1150 °C for 5 h with a high ionic conductivity of 0.85 × 10⁻² (at 600 °C) by adding a 2 mol% CuO-Bi₂O₃ sintering aid. The authors also reported that a relatively higher conductivity was obtained for the sample with additive content as a 2 mol% sintering aid in comparison to that of 3 mol %. The difference in conductivities was indicated prominently at a lower temperature. The low content of additives promoted the densification and sintering process for BCZY ceramics whereas higher content had negative effects upon the conductivity. The increased amount of sintering additives initiated the accumulation of secondary phases at the ends of grain boundaries, attributing to the increased resistance (Figure 4J). It is generally accepted that excess of a sintering aid accumulates as secondary phases and attenuates protonic conductivity. However, a literature review by Franscisco et al. [111] discusses how the addition of transition metal oxides promotes the exsolution of excess metal cations to the grain boundaries, forming secondary phases in a liquid state, from which metal cations can diffuse inside the lattice grain boundary, creating secondary proton-conducting phases and densifying the structure. This result was also confirmed by Tong et al. [51], who obtained only a 36% densification of unmodified BZY at 1500 °C, but the addition of BaY₂NiO₅ as a sintering aid increased densification to 97% at 1400 °C. Above ~1000 °C, BaY₂NiO₅ starts breaking down and the bulk diffusion of Ba, Y, and Ni into the perovskite causes an elevated level of bulk oxygen vacancy concentration and Ba deficiency in the A-site of the ABO₃ framework and as its carried out, this allows for the transport of oxygen vacancies, which allows for protonic conductivity in the reverse direction, which improves the overall ionic conductivity. Tong et al. [52] reported in their previous work that employing 2 wt% of a NiO sintering aid helped achieve >95% relative density and large grains (3–5 µm) and obtained total conductivity as high as 3.3 × 10⁻² Scm⁻¹, measured under a 600 °C wet Argon atmosphere.

Babilo and Haile added 4 mol % of transition metals, from Sc to Zn, into BaZr_0.85Y_0.15O_3−δ (BZY15), respectively. They found that Ni, Cu, and Zn were effective sintering aids. Ni and Cu improved relative density from 60% for pristine BZY15 to 86-88%. However, with the use of 4 mol% zinc oxide as a sintering aid, a relative density of 93% for BZY15 was obtained by sintering at 1300 °C for 4 h [43]. They also compared the effect of ZnO homogeneity by testing three different synthesis methods. In the first method, ZnO and BZY15 were mechanically mixed. In the second method (glycine–nitrate synthesis), zinc nitrate was added to a BZY15 nitrate solution followed by evaporation and calcination to ensure the incorporation of Zn into the BZY15 lattice. In the third method, ZnO powder was added to a BZY15 nitrate solution followed by evaporation and calcination. Densification was greatest for ZnO introduced as a dispersed oxide into the BYZ nitrate solution. The improvement over mechanical mixing of the two oxides is attributed to the greater homogeneity of the ZnO distribution within the ceramic sample. The poorer densification of the presumably most homogenous sample, in which zinc nitrate was added to the BYZ nitrate solution, is attributed to the complete dissolution of Zn into the bulk crystalline structure of BYZ without significant preferential segregation to the grain boundaries. In comparison with the unmodified sample, however, even this sample shows enhanced shrinkage over simple BYZ. However, the measured protonic conductivity was lower than that of a Zn-free barium zirconate by a factor of two. Possibly, the Zn²⁺ ions residing on the tetravalent Zr site, with an effective −2 charge, acted as deep proton traps, both decreasing the effective concentration of protons able to participate in the charge transport process and raising the activation energy. Similar findings have been reported by Park et al. [112], who tested the effect of a 4 mol% ZnO sintering aid on BZY conductivity under different reducing atmospheres with varying water vapor pressures at 600 and 700 °C. Samples synthesized by solid-state reactions were pressed into rectangular pellets at 200 MPa and sintered at 1300 °C for 10 h. They inferred that proton mobility varied as a function of water vapor pressure, and was almost an order of magnitude lower with ZnO addition.

In another work [92], BaCe_0.35Zr_0.5Y_0.15O_3−δ (BCZY) prepared by the sol–gel method exhibited the lowest sintering temperature of 1100 °C, after adding 2 wt.% of ZnO. The bulk conductivity of the Zn-doped sample decreased by one order of magnitude due to sever trapping of protons, although its grain boundary conductivity doubled. As a result, the overall conductivity was halved (8 × 10⁻³ Scm⁻¹ for BCZY and 4.02 × 10⁻³ Scm⁻¹ for BCZY-ZnO at 700 °C). Several studies have reported the reduction in sintering temperature from 1300 to 1200 °C by the addition of ZnO but at the cost of decreased conductivity [110]. Baral et al. [92] studied the sinterability of BaCe_0.35Zr_0.5Y_0.15O_3−δ with a 2 wt% ZnO sintering additive sintered at 1100 °C for 5 h and the total conductivity was measured at 700 °C with a wet H₂ atmosphere to be 4.02 mScm⁻¹ whereas pristine BCZY had a total conductivity of 8.00 mScm⁻¹; looking closely, this was due to the decreased bulk conductivity from 9.97 mScm⁻¹ (pristine) to 4.24 mScm⁻¹ (ZnO-aided), and this was inferred as the influence of the space charge effect on the grain boundary activity as sol–gel-prepared perovskite appears to allow more Zn into the lattice compared to the solid-state route. It is again to be noted here that all such studies have doped ZnO by a wet synthesis technique (sol–gel method, citrate–nitrate combustion, etc.) and envisaged a suppression of proton conductivity. This observation complies with the general trend noticed for NiO added using wet synthesis techniques as discussed in the previous section.

In comparison, mechanical mixing like ball milling has shown enhancement in proton conductivity. Tao and Irvine [93] studied the effect of 1 wt% ZnO addition (via ball milling) to BCZY electrical and thermal properties and reported promising results. ZnO showed 97% densification on firing BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3–d pellets (pressed at 3.7 × 10⁵ MPa) at 1325 °C, whereas samples without ZnO substitution only achieved 70% density under these conditions. Its total conductivity in wet 5% H₂ was 3.14 mScm^–1 at 400 °C and over 10 mScm^–1 above 600 °C, which is the highest of the known stable single-phase ceramic proton-conducting oxides, although comparable conductivity has been reported for BaCe_0.17Zr_0.68Y_0.15O_3–d that was sintered at 1700 °C for 20 h but with the presence of some Y₂O₃/YSZ impurities [34]. For BaCe_0.5Zr0.3Y_0.16Zn_0.04O_3–d, open-circuit voltage over 0.8 V was recorded at 650 °C between 97% H₂/3% H₂O and 97% O2/3% H₂O, confirming protonic conductivity domination over most of the partial-pressure range at this temperature. The potential is slightly lower than those reported by Coors [113] but comparable to that reported by Kreuer [34] for similar electrolyte materials, which could be related to leakage and/or the presence of some electronic conduction. Similarly, Wang et al. [114] added 4 mol% ZnO to BaCe_0.5Zr_0.3Y_0.2O_2.9 via mechanical mixing as well as a solid-state reaction (SSR) and reported higher conductivity (1.35 × 10⁻² S/cm at 600 °C) and high relative density (98.5% sintered at 1300 °C) for both cases. They scanned from 0 to 20 mol% of the ZnO range and observed that 4 mol% gave the best results for both SSR and mechanical mixing. They concluded that a BaO·ZnO eutectic mixture improves densification since BaO and ZnO undergo significant volatilization at 1300 °C. Residual ZnO mainly accumulated at grain boundaries rather than percolating inside the perovskite lattice and substituting the B-site cations. Zhang et al. [115] observed remarkable improvement in stability, relative density (>95%), and conductivity upon adding ZnO to Ba_1.03Ce_0.5Zr_0.4Y_0.1O_3−δ via mechanical mixing. They also scanned a wide range from 0 to 3 wt% ZnO and observed no further enhancement in densification above a threshold wt%. This threshold value varied according to the sintering temperature (Figure 5a). A 1 wt% ZnO addition followed by sintering at 1300 for 10 h gave > 97% densification and fully developed grains (Figure 5b,c). Increasing the concentration from 0.5 to 2 wt% had minimal effect on grain boundary conductivity, but increased bulk conductivity. The increase in bulk conductivity was attributed to the generation of more oxygen vacancies through Ce⁴⁺/Zr⁴⁺ substitution by Zn²⁺. Matsuda et al. [116] also reported similar observations that (1) ZnO improves densification via ZnO.BaO eutectic mixture formation (Figure 5d) and (2) there is a threshold wt% of ZnO beyond which any excess of the ZnO liquid phase along with derived products of BaO, ZnO, and Y₂O₃ deposit as impurity phases at the grain boundary of BCZY, thus impeding protonic conduction. They sintered BaCe_0.8Zr_0.1Y_0.1O_3−δ with 0.1 wt% to 3 wt% additions of ZnO (mechanical mixing using ethanol solvent) at 1400 °C for 6 h and achieved relative densities of 90%, 92%, and 94%, respectively. The conductivity measurements were taken under a 3.0% H₂O, 19.4% H₂, and 77.6% N₂ mixed gas environment from 500 °C to 700 °C, with 0.1 wt% ZnO showing the highest value at all test temperatures. For example, at 700 °C, 0.1 wt% ZnO-added BCZY achieved a total conductivity of ~19 mScm⁻¹ whereas 1 wt% and 3 wt% reached a total conductivity of ~14 mScm⁻¹ and ~13 mScm⁻¹, respectively. The value recorded with 0.1 wt% ZnO was significantly higher than what had been previously reported for BaCe_0.8Zr_0.1Y_0.1O_3−δ but without any sintering aid. For instance, Katahira et al. [32] reported that pristine BaCe_0.8Zr_0.1Y_0.1O_3−δ sintered at 1700 °C for 10 h gave a total conductivity of 10 mScm⁻¹ under 1.7% H₂O and 99.3% H₂; Taillades et al. [117] reported a total conductivity of 5 mScm⁻¹ under 3% H₂O and 97% Argon for pristine BaCe_0.8Zr_0.1Y_0.1O_3−δ sintered at 1350 °C for 10 h; and Thabet et al. [118] reported 16 mScm⁻¹ under dry gas conditions for BaCe_0.8Zr_0.1Y_0.1O_3−δ sintered at 1200 °C for 10 h.

Thus, it is clear that the sintering aid addition technique and its concentration significantly affect (1) the BCZY lattice structure including A-site or B-site cation substitution and (2) the incorporation of a sintering aid into the bulk or its accumulation at the grain boundaries. In general, most wet chemistry addition techniques have shown a reduction in proton conductivity, possibly due to excess metal incorporation into the bulk. Recently, Baral et al. [119] came to the same conclusion by studying space charge variation in the bulk and grain boundaries of BaCe_0.35Z_0.5Y_0.15O_3−δ with and without ZnO as a sintering aid. In total, 2 wt% of ZnO was added to BCZY (via ball milling in presence of ethanol solvent) followed by pelletization at 100 MPa and sintering at 1325 °C for 5 h whereas at 1600 °C for 30 h was chosen for just BCZY. The relative densities of the pellets were 98% and 95%, respectively. The impedance measurements were carried out in a wet hydrogen atmosphere (3% H₂O + 4% H₂ balanced with Ar) using two probe methods up to 420 °C. As a general trend, bulk conductivities were similar for both samples, suggesting that the sintering aid (ZnO) did not affect bulk properties of BCZY. However, grain boundary conductivity was an order of magnitude lower with ZnO addition due to its accumulation at the grain boundaries as confirmed through SEM images. The activation energy of the interfacial space charge-transfer process below 350 °C was about 0.35 eV for both BCZY and BCZY-ZnO, whereas it was about 0.45 eV above 350 °C for BCZY, and these values are close to the activation energy in various other ceramic proton conductors reported in the literature [12,32,92,120]. Further conclusive understandings require in-depth studies where (1) BCZY + a sintering aid are fabricated using wet synthesis techniques as well as mechanical mixing, (2) covering a wide range of sintering aid wt% and (3) backed by a Nyquist plot analysis using a distribution function of relaxation times, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and scanning electron microscopy.

Figure 5. Relative density of Ba_1.03Ce_0.5Zr_0.4Y_0.1O_3−δ with various ZnO concentrations sintered at different temperatures (a) and SEM images of Ba_1.03Ce_0.5Zr_0.4Y_0.1O_3−δ without (b) and with 1 wt% (c) ZnO sintered at 1300 °C for 10 h, reproduced with permission from Elsevier [115]; BaCe_0.8Zr_0.1Y_0.1O_3−δ densification mechanism through ZnO.BaO eutectic formation reproduced with permission from Elsevier [116] (d); thermogravimetric analysis (TGA) curve of pristine BaCe_0.3Zr_0.55Y_0.15O_3−δ [54] in pure CO₂ environment (e), and TGA curve of BaCe_0.55Zr_0.3Y_0.15O_3−d and BaCe_0.35Zr_0.5Y_0.15O_3−d in CO₂ and air (f) [121], replotted from original papers.

Figure 5. Relative density of Ba_1.03Ce_0.5Zr_0.4Y_0.1O_3−δ with various ZnO concentrations sintered at different temperatures (a) and SEM images of Ba_1.03Ce_0.5Zr_0.4Y_0.1O_3−δ without (b) and with 1 wt% (c) ZnO sintered at 1300 °C for 10 h, reproduced with permission from Elsevier [115]; BaCe_0.8Zr_0.1Y_0.1O_3−δ densification mechanism through ZnO.BaO eutectic formation reproduced with permission from Elsevier [116] (d); thermogravimetric analysis (TGA) curve of pristine BaCe_0.3Zr_0.55Y_0.15O_3−δ [54] in pure CO₂ environment (e), and TGA curve of BaCe_0.55Zr_0.3Y_0.15O_3−d and BaCe_0.35Zr_0.5Y_0.15O_3−d in CO₂ and air (f) [121], replotted from original papers.

Narendar et al. [54] reported how densification and total conductivity of BaCe_0.3Zr_0.55Y_0.15O_3−δ (BCZY35) electrolyte membranes were affected by the addition of CuO, ZnO, and NiO. In this work, 1 wt% of each sintering aid was used to fabricate a series of electrolyte membranes labeled as BCZY35, BCZY35-NiO, BCZY35-ZnO, and BCZY35-CuO, all of which were sintered at 1400 °C. Sintering aid-added BCZY35 samples had a relative density > 95%, with BCZY35-ZnO having an exceptionally high relative density at 97.61%. A thermogravimetric analysis (TGA) was used to study the chemical stability of pristine material as a function of temperature under flowing pure CO₂. Pristine BCZY35 was highly stable in CO₂ with no mass gains, and merely a slight weight was lost at high temperatures as shown in Figure 4E. Similar observations were reported by Ashok et al. [92] for BaCe_0.35Zr_0.5Y_0.15O_3−δ (Figure 4F). Nasani et al. [54] also reported the shrinkage behavior of green compact pellets with a diameter of 10 mm under air in the temperature range of 50 to 1500 °C at a heating rate of 10 °C/min. The pristine sample shrunk only ~4.5%, whereas BCZY35-ZnO exhibited ~14% shrinkage, typically well below 1400 °C. Overall, the shrinkage followed the pattern BCZY35-ZnO > BCZY35-NiO > BCZY35-CuO.

In some instances, the addition of a promoter to the sintering aid has proved beneficial, although the exact mechanism is still not clearly understood. For example, Yong et al. [121] reported that the total conductivity of BaCe_0.5Zr_0.3Y_0.2O_{3− δ} sintered at 1320 °C for 2 h with 4 mol% of ZnO and 0.5 mol% Na₂CO₃ was 7.66 mScm⁻¹ at 700 °C in wet hydrogen and it was also reported that the fuel cell with this electrolyte could achieve a peak power density of 302 mWcm⁻² at 700 °C. Li et al. [121] used 4 mol% ZnO with 0.5 mol% Na₂CO₃ and observed 17.5% shrinkage (95% relative density) at 1320 °C. Similarly, a combination of 2 mol% Li₂O and 2 mol% ZnO reduced the sintering temperature of BZY to 1400 °C (corresponding relative density of 95.6%) with the conductivity of 6.90 X 10⁻³ Scm⁻¹ at 700 °C [122]. Along similar lines, Castellani et al. [123] made a mixture of Li₂O and ZnO (1:4 molar ratio), added 1 wt% of the same to BaZr_0.7Ce_0.2Y_0.1O_3−δ, and reported 95% relative density and a protonic conductivity of 1.3 × 10⁻³ Scm⁻¹ (at 450 °C, 3% humidified air) after sintering at 1300 °C for 25 h. Although density was higher than pure BaZr_0.7Ce_0.2Y_0.1O_3−δ (90% densification post sintering at 1600 °C for 15 h), overall conductivity was only slightly higher (1.13 × 10⁻³ Scm⁻¹ at 450 °C, 3% humidified air). Transmission electron microscopic images revealed a bigger grain size (0.33 µm with Li-ZnO versus 0.035 µm without sintering aid) and a thinner grain boundary thickness (3 $\times$ 10⁻³ µm with Li-ZnO versus 8 $\times$ 10⁻³ µm without sintering aid). However, electrochemical impedance spectroscopy of the samples revealed similar bulk resistance for both with the grain boundary resistance being slightly lower for the Li-ZnO-added sample, resulting in the slightly improved conductivity value. While the authors suggest that bulk conductivity is an intrinsic property of a material and does not vary with microstructure, some research has shown that grain size affects bulk conductivity [54,124,125]. It is generally believed that increasing the grain size increases the distance to be traveled by protons across the grain, thus increasing the grain bulk resistance. However, in the case of sintering aids, these metals partially replace the B-site cations, thus generating more oxygen vacancies in the grain bulk, which enhances the proton hopping. Thus, there are two opposing factors likely involved and the maximum conductivity will be achieved only at the right threshold. This threshold will be governed by the BCZY composition (especially Ce/Zr ratio), sintering aid concentration, and addition technique as well as the sintering profile.

The Ce/Zr ratio is an important parameter that affects the overall as well as protonic conductivity of BCZY with or without a sintering aid. According to the DFT calculations of the Grotthuss mechanism by Jing and Aluru [126], an excess trivalent B-site dopant generates negatively charged defects that attract protons, leading to proton trapping that in turn prevents its additional migration. Since cerium has a variable valence state (3+/4+), in partially reducing atmospheres, Ce⁴⁺ to Ce³⁺ in situ reduction takes place along with the generation of oxygen vacancies. This affects overall proton transport in two ways. Firstly, excess Ce³⁺ creates negatively charged defects that trap protons, preventing their further movement. Secondly, oxide ion conduction can start taking place through the excess oxygen vacancies depending on partial pressure in both anode and cathode chambers. The overall effect is an attenuation of proton transference across the electrolyte, which is manifested as a drop in the open-circuit voltage of the fuel cell or electrolyser being tested. For example, Kim et al. [127] fabricated Ba(Zr_1−xCe_x)_0.85Y_0.15O_3−δ (x = 0.0, 0.05, 0.1, and 0.2, which are denoted as BZCY0, BZCY5, BZCY10, and BZCY20, respectively) via the solid-state reaction method along with the addition of a 1 wt% ZnO sintering aid to each of the series of BZCY powders and then sintered at 1350 °C for 10 h. EIS measurements were also taken to study the electrical conductivity of the sintering under humid Ar (water partial pressure, P_H2O ~ 3.1%). The total conductivities were ~5.00 mScm⁻¹, ~1.60 mScm⁻¹, ~1.25 mScm⁻¹, and ~1.00 mScm⁻¹ for BZCY20, BZCY10, BZCY5, and BZCY0, respectively, under humid Ar at 600 °C.

Table 2. Total conductivities of BZCY20 and BZCY40 reported in the literature by other authors.

Electrolyte Composition	Sintering Temperature (°C), Duration	Atmosphere	T (°C)	Total Conductivity, σTotal (S/cm)	Synthesis Procedure
BZCY20 [128]	1400 °C, 12 h	humidified 10%-H2/Ar	600 °C	~0.00520	solid-state reactive sintering
BZCY20 [129]	1500 °C, 8 h	humidified N2	600 °C	~0.00170	new nitrate-free acetate–H2O2 combustion method
BZCY40 [129]	1500 °C, 8 h	humidified N2	600 °C	~0.00550	new nitrate-free acetate–H2O2 combustion method
BZCY20 [130]	1350 °C + sintering pressure of 50 MPa	humidified N2 + H2	600 °C	~0.00310	solid-state reaction with spark plasma sintering

below shows the total conductivity of BCZY20 and BCZY40 reported in the literature at 600 °C.

In general, total conductivity increases with an increase in the Ce/Zr ratio due to (1) Ce⁴⁺ to Ce³⁺ in situ reduction in reducing environments that generate oxygen vacancies and (2) the proton activation energy increasing with a decrease in Ce content due to the difference in electronegativity of Zr⁴⁺ and Ce⁴⁺ ions occupying the perovskite B-site and higher repulsive interactions between Zr and H than Ce and H [131]. Sawant et al. [131] reported a total ionic conductivity of 0.2 mScm⁻¹ (at 400 °C) for BaCe_0.2Zr_0.6Y_0.2O_3−δ sintered at 1400 °C for 10 h that increased to 0.5 mScm⁻¹ for BaCe_0.6Zr_0.2Y_0.2O_3−δ. This was attributed to a monotonic drop in both bulk and grain boundary activation energies with increasing Ce content. Similar findings were reported by Ricote et al. [132] in that increasing Ce content increases BCZY conductivity. However, an increase in total conductivity does not mean an increase in protonic conductivity since Ce⁴⁺ getting reduced to Ce³⁺ creates oxygen vacancies that can contribute some oxide ion conductivity, especially under wet conditions. In fact, Zhao et al. [133] showed that the proton transport number decreases with an increase in Ce content, although overall conductivity increases. While most of the research reports total conductivity values, only very few [125,133] have shown the variation in proton conductivity (proton transference $\times$ total conductivity [134,135]) as a function of the sintering aid wt% or Ce/Zr ratio or other critical parameters. Thus, more work focused on proton transference is needed for a better understanding of this material, BCZY, with or without a sintering aid.

It is to be mentioned here that the sintering time is also a crucial parameter that affects the grain growth, grain boundary thickness, and thus overall conductivity. The same sintering aid in equal amounts will render completely different conductivities depending on the sintering time and temperature [136]. Reddy et al. [124] studied the effect of sintering time (5, 10, 15 h) on the densification (at 1450 °C) and conductivity of BaCe_0.4Zr_0.4Y_0.2O_3−δ. The grain size, densification, and conductivity monotonically increased with sintering time as shown in

Table 3. Variation in grain size, densification, and bulk conductivity as a function of sintering time [124].

Sample	Time (h)	Temperature (°C)	Grain Size (µm)	Densification (%)	Bulk Conductivity in Wet Air (Scm−1), 500 °C
BCZY	5	1450	0.48	85	0.50 $\times$ 10−3
	10		0.54	89	0.65 $\times$ 10−3
	15		1.05	92	0.78 $\times$ 10−3
BCZY-ZnO	10	1300	0.91	97	0.65 $\times$ 10−3

. Interestingly, the addition of 4 mol% ZnO rendered 97% densification upon sintering at just 1300 °C for 10 h, and the conductivity was similar to just BCZY sintered for 10 h at 1450 °C.

The relative densities, conductivity, and peak power densities reported in the literature by other authors’ work are tabulated in

Table 4. BCZY and BZY with zinc oxide sintering aids reported in the literature.

Electrolyte Composition	Sintering Aids	mol% or weight% of Sintering Aid	Sintering Temperature (°C), Duration	Relative Density (%)	Atmosphere	T (°C)	Conductivity (×10−3 Scm−1)	T (°C)	Peak Power Density (mWcm−2)	Open-Circuit Voltage (V)	Synthesis Method of Powder
BaCe0.7Zr0.1Y0.1Zn0.1O3− δ [137]	ZnO	10% (wt)	1200 °C, 2 h	97.47	wet 5% H2	600	8.59	N/A	N/A	N/A	conventional solid-state reaction
BaCe0.7Zr0.1Y0.1Zn0.1O3− δ [137]	ZnO	10% (wt)	1200 °C, 2 h	97.47	dry H2	600	6.67	N/A	N/A	N/A	conventional solid-state reaction
BaCe0.6Zr0.2Y0.15Sm0.05O3− δ [138]	ZnO	4% (wt)	1400 °C, 10 h	98.96	wet 5% H2	700	3.89	700	420	1.01	conventional solid-state reaction
BaCe0.5Zr0.3Y0.16Zn0.04O3− δ [139]	ZnO	4% (mol)	1200 °C, 5 h	97.4	humidified hydrogen (∼3% H2O)	700	2.73	700	240	1.00	modified Pechini method—using citrate and ethylenediamine tetraacetic acid (EDTA), which are used as parallel complexing agents
BaZr0.80Y0.2O3− δ [94]	N/A	N/A	1400 °C, 10 h	68	dry 5% H2	900	9.00	N/A	N/A	N/A	solid-state reaction
BaZr0.80Y0.2O3− δ [94]	ZnO	4% (mol)	1325 °C, 10 h	96	dry 5% H2	600	1.00	N/A	N/A	N/A	solid-state reaction
BaCe0.5Zr0.3Y0.2O2.9 [114]	ZnO	4% (mol)	1300 °C, 10 h	98.5	humidified hydrogen (3%H2O + 97%H2)	600	13.5	N/A	N/A	N/A	solid-state reaction
BaCe0.5Zr0.3Y0.2O2.9 [114]	ZnO	7% (mol)	1300 °C, 10 h	96	humidified hydrogen (3%H2O + 97%H2)	600	10.0	N/A	N/A	N/A	solid-state reaction
BaCe0.5Zr0.3Y0.2O2.9 [114]	ZnO	20% (mol)	1300 °C, 10 h	94	humidified hydrogen (3%H2O + 97%H2)	600	71.1	N/A	N/A	N/A	solid-state reaction
BaZr0.80Y0.16Zn0.04O3− δ [140]	ZnO	4% (mol)	1450 °C, 5 h	N/A	H2 (~3% H2O)	600		600	~75	0.99	citric acid–nitrate auto-combustion method
BaZr0.85Y0.15O3−δ [141]	ZnO	1% (wt)	1500 °C, N/A	>95	(3% H2O) 5% H2 atmosphere	650	10.0	800	24.5	1.00	solid-state reaction
BaZr0.80Y0.16Zn0.04O3−δ [141]	ZnO	1% (wt)	1400 °C, 5 h	N/A	humidified H2 and air	N/A	N/A	800	27	1.00	solid-state reaction
BaZr0.80Y0.16Zn0.04O3−δ [141]	ZnO	1% (wt)	1450 °C, 5 h	N/A	humidified H2 and air	N/A	N/A	600	75	N/A	solid-state reaction
BaCe0.6Zr0.3Y0.1O3−δ [34]	ZnO	5% (mol)	1400 °C, 12 h	98	N/A	N/A	N/A	500	N/A	1.094	solid-state reaction
BaZr0.4Ce0.4Y0.2O3−δ [128]	Zn(NO3)2	3.56% (wt)	1400 °C, 8 h	>97	humidified H2/Ar and air	600	4.20	600	279	1.07	solid-state reactive sintering
BaZr0.6Ce0.2Y0.2O3−δ [128]	Zn(NO3)2	3.56% (wt)	1400 °C, 8 h	>97	humidified H2/Ar and air	600	5.2	600	336	1.08	solid-state reactive sintering
BaZr0.7Ce0.1Y0.2O3−δ [128]	Zn(NO3)2	3.56% (wt)	1400 °C, 8 h	>97	humidified H2/Ar and air	600	2.7	600	111	1.05	solid-state reactive sintering
BaZr0.6Ce0.2Y0.2O3−δ [142]	Zn(NO3)2	3.56% (wt)	1400 °C, 12 h	>97	humidified H2/Ar and air	600	3.00	600	336	0.92	solid-state reaction + single-step cofiring (electrolyte)

below.

Fan et al. [143] fabricated a BaCe_0.7Zr_0.1Y_0.2O_3−δ (BCZY) thin-film electrolyte of a thickness of 20 µm by simple spray coating along with reduced sintering temperature, and the electrolyte conductivity of the spray-coated thin-film BCZY was studied under H₂/H₂O (3 vol.% H₂O). Spray-coated electrolyte film with 1 wt% ZnO was sintered at 1400 °C for 4 h and the film without ZnO was sintered at 1500 °C for 3 h to achieve densification, although the achieved densification was not reported. The conductivity of BCZY with ZnO was 0.0102 Scm⁻¹ at 650 °C and 0.0046 Scm⁻¹ at 450 °C; the activation energy of the thin electrolyte was as low as 0.31 eV, which was comparatively lower than the activation energies recorded by other authors’ work of similar BCZY composition with no ZnO additives. Zuo’s work [144] reported that Ba(Ce_0.7Zr_0.1Y_0.2)O_3−δ, BCZY7 prepared by sintering at 1550 °C for 10 h, was tested for ionic conductivity under a humid 4% H₂/Ar atmosphere and the total ionic conductivity of BCZY7 reported was 0.009 Scm⁻¹ at 500 °C along with the activation energy of 0.36 eV; compared to Zuo’s work, the activation energy was relatively low, indicating a high protonic conductivity, and the total conductivity was significantly high, almost five folds, indicating that ZnO addition facilitated liquid-aided sintering for enhanced protonic conduction. Fan et al. [143] further investigated the fuel cell performance and the EIS characterization of BCZY single cells; the short circuit current density and peak power density at 650 °C were 1.1 A cm⁻² and 268 mW cm⁻², respectively, for the single cell sintered at 1500 °C with no ZnO addition whereas the short circuit current density and the peak power density exhibited by the single cell sintered at 1400 °C were 1.4 A cm⁻² and 362 mW cm⁻² at 650 °C, indicating that ZnO addition also enhanced the microstructure of the electrolyte film and the anode. Zhuo et al. [145] evaluated the effects of 1 wt% ZnO addition on BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_{3− δ}, which was sintered at 1450 °C for 5 h. The maximum conductivity achieved at 20 vol%H₂O-Air was ~0.01357 S·cm⁻¹; however, the relative density achieved was approximately 85%. Loureiro et al. [146] investigated the effects of 4 mol% of a ZnO sintering additive on BaZr_0.852Y_0.148O_3−d where the samples were sintered at 1300 °C for 5 h, and the electrical impedance spectroscopy was carried out in a cooling in flowing dry (p_H2O = 1.66 × 10⁻⁵ atm) and wet (p_H2O = 0.022 atm) atmospheres of N₂ and O₂. The maximum total conductivity measured was around 20.0 mS·cm⁻¹ and this conductivity was mostly dominated by the bulk conductivity of the perovskite structure [147,148].

3.3. Oxides of Cobalt, Iron, and Other Metals

The Cobalt oxide (CoO) sintering aid has not attracted much attention for BCZY, although it has been studied widely for ceria and zirconia electrolytes. Ricote et al. [149] used Cobalt doping into BZCY, wherein the doped BaZr_(0.9−x)Ce_xY_(0.1−y)Co_yO_(3−δ) for x = 0 and 0.2 and y = 0.01, 0.02, and 0.05 was synthesized using a solid-state reaction at 1400 °C. They took into account the fact that replacing cerate for zirconate facilitates sintering but increases oxygen conductivity and, however, diminishes chemical stability and mechanical strength. Additionally, 1 wt% Cobalt- and 1 wt% nickel-doped samples achieved >80% relative densities, while 2% Cobalt-doped BCZY27 reached a maximum of 97.1%. Notably, just 1wt% of Ni or Co doping at 1450 °C significantly increased density, resulting in a bigger grain size and subsequently lesser number of grain boundaries. In general, barium zirconates are well known to exhibit very resistive grain boundaries [150], and thus a decrease in sintering temperature eventually reduced the ionic conductivity by an order of magnitude for Ni- and Co-doped BZY10 samples. However, reduction in sintering temperature did not have any significant effect on the conductivity of Co- and Ni-doped BCZY27. Both gave ionic conductivity similar to just BCZY27 sintered in 1700 °C (1 $\times$ 10⁻³ Scm⁻¹ to 2 $\times$ 10⁻³ Scm⁻¹). The ionic (σ_i) and p-type (σ_p) conductivities recorded before and after the correction of porosity at varying partial pressures of oxygen (P(O₂)) using Equation (7) are tabulated in

Table 5. The ionic (σ_i) and p-type (σ_p) conductivities of different materials recorded before and after the correction of porosity.

Parameter (mS·cm−1)	BZY10	BZY10 + 1% Co	BZY10 + 2% Co	BZY10 + 1% Ni	BZY10 + 2% Ni	BCZY27	BCZY27 + 1% Co	BCZY27 + 2% Co	BCZY27 + 1% Ni	BCZY27 + 2% Ni
σi measured	1.66	0.17	0.49	0.36	1.02	1.89	0.98	1.23	1.77	1.4
σi corrected	1.88	0.18	0.53	0.39	1.16	2.04	1.04	1.28	1.93	1.53
σp measured	2.48	0.39	1.71	1.85	9.41	2.32	1.65	1.73	4.09	3.78
σp corrected	2.81	0.42	1.84	1.99	10.68	2.5	1.74	1.81	4.46	4.12

. The p-type conductivity was higher for Ni doping than Co since Ni has a lower oxidation state (2⁺) than Co (3⁺), leading to higher oxygen vacancies. Interestingly, 2% Co showed higher ionic conductivity than 1% Co (Figure 6a), whereas Ni showed completely orthogonal trends (Figure 6b). This indicates that the optimum concentration of the sintering aid depends on the metal oxide being used.

$${\sigma }_{Total}={\sigma }_i+{\sigma }_p.P{\left(O_2\right)}^{1/4}$$

(7)

The temperature dependance of the conductivity of different BCZY10 and BCZY27 samples under a wet reducing atmosphere (9% H₂N₂, P_H2O = 0.015 atm) is shown in Figure 5. At high temperature, the conductivity dropped due to a loss of protonic defects from the perovskite structure via the Grotthuss diffusion mechanism [151].

Figure 6. The temperature dependence of the conductivity of different BCZY10 and BCZY27 samples under a wet reducing atmosphere (9% H₂N₂, P_H2O = 0.015 atm) with (a) Co doping and (b) Ni doping [132]; Arrhenius plots of the conductivities in 3% H₂O of the BZCY, BZCY-2, BZCY-5, and BZCY-10 samples sintered at 1400 °C for 5 h, reproduced with permission from Elsevier [152] (c).

Figure 6. The temperature dependence of the conductivity of different BCZY10 and BCZY27 samples under a wet reducing atmosphere (9% H₂N₂, P_H2O = 0.015 atm) with (a) Co doping and (b) Ni doping [132]; Arrhenius plots of the conductivities in 3% H₂O of the BZCY, BZCY-2, BZCY-5, and BZCY-10 samples sintered at 1400 °C for 5 h, reproduced with permission from Elsevier [152] (c).

Wan et al. [153] investigated effects of doping Cobalt in BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ for sinterability and protonic conductivity. The authors reported the use of the EDTA–citric acid method [154,155] to prepare BaZr_0.1Ce_0.7Y_0.1Yb_0.1Co_xO_3−δ where x = 0.00 (stoichiometrically), x = 0.02, x = 0.05, and x = 0.1 denote BZCYYb, BZCYYbC2, BZCYYbC5, and BZCYYbC10. An electrochemical impedance spectroscopy study was also performed on the fabricated single cells under humidified hydrogen as fuel and the static air as an oxidant from 600 °C to 700 °C. The pellets were sintered at 1300, 1350, and 1400 °C for 5 h. The authors reported the shrinkage of the samples as 12.4%, 15.5%, 17.8%, and 18.4% for samples BZCYYb, BZCYYbC2, BZCYYbC5, and BZCYYbC10, respectively, at 1400 °C, which implied that the Cobalt doping promoted grain growth and densification. They also reported the trend in the relative densities of the samples with varying sintering temperatures, where BZCYYbC10 sintered at 1300 °C gave a relative density of 98.75% whereas BZCYYbC10 sintered at 1350 °C and 1400 °C achieved a relative density almost as high as 99.5%. The authors reported the electrolyte film conductivities for BZCYYbC2 at different testing temperatures from 600 °C to 700 °C as 3.8 mScm⁻¹, 3.2 mScm⁻¹, and 2.7 mScm⁻¹ along with the peak power densities of 0.67 W cm⁻², 0.52 W cm⁻², and 0.37 W cm⁻² at 700 °C, 650 °C, and 600 °C, respectively.

Viechineski et al. [156] studied the sinterability and electrical conductivity of BaCe_0.2Zr_0.7Y_0.1O_3−δ sintered at 1500 °C for 4 h using 4 wt% ZnO, Fe₂O₃, and Mn₂O₃. This research has gained interest regarding Iron (III) oxides as Zhang et al. [157] used an Iron (III) oxide sintering aid for ceria-based ceramic Ce_0.8Gd_0.2O_2−d, and the addition of 0.5 wt% Fe₂O₃ increased the total conductivity of Ce_0.8Gd_0.2O_2−d from 6.14 S/m in the pristine condition to 6.17 S/m in the sintering aid-added form. Viechineski et al. [156] reported that the relative density of the sample sintered with Fe₂O₃ had a relative density of 96.4% but the relative density achieved by other samples sintered with Mn₂O₃ and ZnO sintering aids had a higher value. The total conductivity measured under a humid N₂ atmosphere followed the order ZnO > Fe₂O₃ > Mn₂O₃. Both Mn₂O₃ and Fe₂O₃ reflected the formation of secondary phases like BaFe_0.2Zr_0.7Y_0.1O₃, Y_0.4Ce_0.6O₂, BaFe₂O₄, Ba₄CeMn₃O₁₂, and Y_0.4Ce_0.6O₂, unlike ZnO that was incorporated into the perovskite, thus stabilizing the phase. The formation of such unwanted impurity phases caused the drop in conductivity for Fe₂O₃ and Mn₂O₃.

Bi₂O₃ is another sintering aid that can stabilize the zirconia sintering process and reduce the sintering temperature [101,102]. Gui’s [152] work used the citric acid–EDTA process to fabricate BaZr_0.3Ce_0.5Y_0.2O_3−δ [154,155], which was then mixed with 0 mol%, 2 mol%, 5 mol%, and 10 mol% Bi₂O₃ before being co-sintered at 1400 °C for 5 h. The electrolyte disk shrinkage was also measured in which the shrinkage was non-uniformly proportional to the addition of Bi₂O₃, where the disk shrinkage was ~13.5%, ~17%, ~18%, and ~20.5% for 0%, 2%, 5%, and 10% additions of Bi₂O₃, respectively. The total conductivities (Figure 6c) of all the series of samples were ~1.0 mScm⁻¹, ~3.5 mScm⁻¹, ~6.3 mScm⁻¹, and 5.0 mScm⁻¹ for BZCY-10, BZCY-5, BZCY-2, and pristine BZCY, respectively, at 600 °C under humidified hydrogen (~3% H₂O). The authors also characterized the open-circuit voltage and the peak power density of BZCY-2 at 600 °C, 650 °C, and 700 °C using humidified (~3%) hydrogen as fuel and ambient air as an oxidant; the recorded peak power density values were 0.67, 0.44, and 0.27 W cm⁻² and were recorded at 700, 650, and 600 °C. This was also higher than the peak power density achieved with a ZnO sintering aid on BaZr_0.3Ce_0.5Y_0.16O_3−δ, wherein the maximum power density recorded by Lin et al. [139] was ~0.236 W cm⁻² at 700 °C.

Li et al. [158] used copper nitrate, barium nitrate, and boric acid to fabricate a BaO-CuO-B₂O₃ mixture, which was used as a sintering aid for BZCY (BaZr_0.1Ce_0.7Y_0.2O_3−δ), as CuO-B₂O₃ undergoes a eutectic reaction at temperatures below 1000 °C [159], meaning that BaO forms a low-temperature eutectic liquid phase and any excess BaO reduces the tendency of the reaction of CuO and B₂O₃ with BaO in the crystalline structure. This, in turn, reduces the sintering temperature and increases the densification of the perovskite structure. The BaO-CuO-B₂O₃ sintering aid mixture was made in molar ratios of 1%, 2%, and 3% to the BZCY powder and sintered at 1150 °C and 1200 °C for 8 h. The authors reported that the pristine sample had a relative density of about 59.26% whereas the sintering aid-added samples achieved relative densities of 67.27%, 83.76%, and 97.29% for 1%, 2%, and 3% mol, respectively. The 3% mol sample (BCZY-3) sintered at 1150 °C showed a conductivity of 0.030 Scm⁻¹ at 700 °C (3% humidified hydrogen), which dramatically decreased to 0.010 Scm⁻¹ for the sintering temperature of 1200 °C since the thickness of the grain boundaries increases at higher sintering temperatures. This indicates that an optimal temperature is needed to attain sufficient grain growth and densification, but beyond that temperature, the grain boundaries start thickening and adding to grain boundary resistance that decreases the overall conductivity of the material. Similar findings were reported by Badwal et al. [136], who tested the resistivity of 3 mol% yttria-stabilized zirconia as a function of sintering temperature and obtained inflection points. According to them, the sintering temperature not only affects the grain size and density but also affects the location and wetting properties of the grain boundary phase.

Tsai et al. [160] used lithium fluoride (LiF) as an additive for the low-temperature sintering of Ba(Zr_0.8−xCe_xY_0.2)O_3−δ. This research was inspired by previous studies wherein LiF additions were used for the sintering of BaTiO₃ and CaZrO₃ at temperatures of 250–300 °C lower than that necessary for pure material densification [161,162,163,164,165]. Tsai et al. reported that BCZY pellets with and without a 7 wt% LiF sintering additive were sintered at 1400 °C for 5 h. BZCYs, BZY82, BZCY712, BZCY622, BZCY532, BZCY442, and BZCYs/LiF were denoted to represent Ba(Zr_0.8−xCe_xY_0.2)O_3−δ (0 ≤ x ≤ 0.4), Ba(Zr_0.8Y_0.2)O_2.9, Ba(Zr_0.7Ce_0.1Y_0.2)O_2.9, Ba(Zr_0.6Ce_0.2Y_0.2)O_2.9, Ba(Zr_0.5Ce_0.3Y_0.2)O_2.9, Ba(Zr_0.4Ce_0.4Y_0.2)O_2.9, and Ba(Zr_0.8−xCe_xY_0.2)O_3−δ (0 ≤ x ≤ 0.4) mixed with a LiF sintering additive, respectively. BZCY442 exhibited the highest relative density of about 96.00% along with the maximum power density of ~50 mW/cm². The authors also reported the total conductivities of BCZY and BCZY/LiF at 700 °C as 1.6 × 10⁻² S/cm and 1.8 × 10⁻² S/cm, respectively.

Although other sintering aids as mentioned above have been investigated to a certain extent, more work is needed for a thorough assessment of the impact of the sintering profile, synthesis techniques, and BCZY composition on protonic conductivity for each such sintering aid.

4. Future Recommendations

The addition of a sintering aid helps to achieve bigger grains and thus higher densification at lower temperatures compared to pristine BCZY. Significant work has been carried out so far on different sintering aids, and how overall conductivity gets affected by the sintering aid concentration and sintering profile. However, as is evident from comparing the different literature available, the sintering aid addition technique also plays an important role in modifying the conductivity. Wet synthesis techniques like the sol–gel method, citrate–nitrate combustion, co-precipitation, Pechini method, etc., cause the complete incorporation of the sintering aid into the grain bulk, thus leading to an excess oxygen vacancy generation and proton trapping effect that finally reduces the material’s conductivity. Such incorporation is also accompanied by the removal of Ba and/or Y from the lattice that then accumulate as secondary impurity phases at the grain boundaries, further increasing the grain boundary resistance. On the other hand, dry or wet mechanical mixing promotes bulk incorporation to only a certain extent, which improves the conductivity. However, in this case, as well, excess of a sintering aid can lead to accumulation at the grain boundaries and a drop in conductivity. Further conclusive understanding requires in-depth studies comparing BCZY sintered with different sintering aids using wet synthesis techniques as well as mechanical mixing, backed by a Nyquist plot analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and scanning electron microscopy.

While the authors suggest that bulk conductivity is an intrinsic property of a material and does not vary with microstructure, some research has shown that grain size affects bulk conductivity. It is generally believed that increasing the grain size increases the distance to be traveled by protons across the grain, thus increasing the grain bulk resistance. However, in the case of sintering aids, these metals partially replace the B-site cations, thus generating more oxygen vacancies in the grain bulk, which enhances the proton hopping. Thus, there are two opposing factors likely involved and the maximum conductivity will be achieved only at the right threshold. This threshold will be governed by the BCZY composition (especially Ce/Zr ratio), sintering aid concentration, and addition technique as well as the sintering profile.

The Ce/Zr ratio is also an important parameter that affects the overall as well as protonic conductivity of BCZY with or without a sintering aid. In general, total conductivity increases with an increase in the Ce/Zr ratio. However, an increase in total conductivity does not mean an increase in protonic conductivity since Ce⁴⁺ getting reduced to Ce³⁺ creates oxygen vacancies that can contribute some oxide ion conductivity, especially under wet conditions. While most of the research reports total conductivity values, only very few have shown the variation in proton conductivity as a function of the sintering aid wt% or Ce/Zr ratio. Thus, more work focused on proton transference is needed for a better understanding of this material BCZY, with or without a sintering aid.

To further enhance the scope and efficacy of H-SOCs, a comprehensive investigation into the impact of achieving a uniform phase distribution within the electrolyte structure could be pursued. This could involve a meticulous exploration of the relationship between the homogeneity of the phase distribution and the overall performance and stability of the perovskite-based materials. In terms of optimizing the electrolyte fabrication process, refining the techniques used in powder milling stands out as a crucial area for improvement. Adopting a nano-miller instead of a planetary ball miller can offer advantages such as enhanced precision in particle size reduction, ensuring a more consistent and fine-grained powder. This finer powder can facilitate improved mixing, reducing the likelihood of agglomeration and promoting a more uniform distribution of components within the electrolyte. Such modifications aim to address challenges related to uneven mixing, which can adversely impact the overall structural integrity of the perovskite material. Furthermore, the incorporation of binders into the electrolyte fabrication process represents another avenue for enhancement. Introducing binders can augment the mechanical strength of the electrolyte, providing structural support to the perovskite lattice. This reinforcement is crucial in fortifying the overall stability of the perovskite structure, thereby minimizing the occurrence of structural defects and fractures. A thorough investigation into the selection and incorporation of binders would be essential to strike a balance between mechanical strength and other electrochemical properties.

Considering the thermal aspects of the manufacturing process, optimizing the heating and cooling ramp rates as well as the dwell times emerges as a potential solution. Employing slower ramp rates can mitigate thermal stress on the perovskite structure, preventing the imposition of excessive mechanical strain. This, in turn, can mitigate the development of hairline cracks or major fractures in the electrolyte. A systematic exploration of the impact of different heating and cooling profiles on the structural and electrochemical characteristics of the perovskite material would contribute valuable insights into the optimal thermal processing conditions.

By addressing these aspects (Figure 7), the aim should be to advance the fabrication process, ultimately leading to perovskite structures with enhanced mechanical strength, improved stability, and superior electrochemical performance.