Designing Composite BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3-δ-Sm_0.2Ce_0.8O_2-δ Heterostructure Electrolyte for Low-Temperature Ceramic Fuel Cell (LT-CFCs)

Abstract

In recent years, tuning perovskite and fluorite-based materials and modifying them to ionic conductors has been an interesting but challenging topic for advanced low-temperature ceramic fuel cells (LT-CFCs). In this regard, we prepared a new composite heterostructure, BaCe_0.4Zr_0.4Y_0.1Yb_0.1O3-Sm_0.2Ce_0.8O₂ (BCZYYb-SDC), and evaluated it as an electrolyte to realize the fuel cell reaction. The developed electrolyte could be a hybrid ionic conductor, possess a very small ohmic area-specific resistance, and exhibit excellent fuel cell performance of over 1.0 W/cm² along with higher OCV of more than 1.1 V at a low operating temperature of 550 °C. The attained performance and ionic conductivity are specially accredited to constructing the heterostructure of BCZYYb-SDC. Moreover, various spectroscopy and microscopic analysis methods have been used to investigate the ions’ transportation, while on the other hand suppressing the electronic conduction. The developed composite heterostructure proposes and suggests new insight to design new electrolytes for LT-CFCs.

1. Introduction

Recently, ionic conduction has become more crucial and highly desired for ceramic fuel cells (CFCs). It is speculated that the fast ion transportation of electrolytes and electrodes is directly connected to the benefits of ceramic fuel cells, such as quick start-up, long durability, better efficiency, and higher modulation [1,2,3]. The electrolyte is the main component of SOFCs. It needs sufficient ionic conductivity of 0.1 S/cm to guarantee excellent power output; likewise, YSZ and doped CeO₂ (rare earth-doped CeO₂) can deliver high ionic conductivity compared to other common ionic conductors [4]. However, these electrolytes can provide high ionic conductivity due to a thermally activated process that demands a high operating temperature of 800–1000 °C. The high temperature of 800–1000 can deliver high ionic conduction. Still, it leads to certain challenges, including delays in start-up and shut-down cycles, component price issues, and quick degradation rate of OCV and performance, hindering the marketing of SOFCs [1,2,3,4,5]. Therefore, from the perspective of performance, cost, and durability, designing new electrolytes with high ionic conduction is the leading prerequisite for the optimization and marketing of SOFCs. In addition, following Goodenough’s statement, chemists should put much more effort into designing novel electrolytes to attain high ionic conduction at <550 °C to be commercially available and useful.

Therefore, in the past decade, to reach the low operating temperature and high ionic conductivity target, much effort has been put forward to enhance the oxide ion conduction in doped CeO₂ and proton conduction in proton conductors and mix conduction (H⁺/O²⁻) in composite or heterostructure systems. Different techniques and phenomena have been considered to activate ionic conduction and defect structural modulation design and thin films [6,7,8,9,10]. The abovementioned phenomena are feasible, but the heterostructure phenomena like semiconductors’ ionic heterostructure and composite heterostructure are of great interest, with many benefits described in recent years. Garcia–Barriocanal et al. proposed a planar heterostructure (composite heterostructure SrTiO₃/YSZ) where SrTiO₃ is a perovskite YSZ fluorite that has reported eighth-order higher ionic conductivity than the YSZ, suggesting that the heterostructure gained oxide ion conduction compared to the bulk structure [10,11]. Similar behaviour has been detected in the newly proposed heterostructure of SDC nanocolumns embedded in the matrix of STO and used as an electrolyte delivering one-order higher ionic conductivity than the bulk SDC [12]. The above reports suggest that a high ion carrier owes to the atomic reconstruction, especially at the interface providing multiple paths for charge transportation, which eventually enhances the ionic conduction [13,14,15,16,17,18,19]. Later, Zhu et al. developed the bulk heterostructure by using perovskite LSCF and ionic conductors of fluorite structure to shape the semiconductor ionic heterostructure LSCF-SDC. The prepared electrolyte is pressed between two Ni-NCAL (symmetrical electrodes) and exhibits a higher power output of 1080 and reasonably high ionic conductivity of >0.1 S/cm at 550 °C [14,15,16]. For instance, Rauf, Mushtaq, Xia, and Shah et al. have developed new heterostructure materials, including BCZY-SCDC, SFT-SDC, BCFZY-ZnO, and SFT-ZnO and have used an electrolyte delivering impressive fuel cell performance and high ionic conductivity at a low operational temperature [14,15,16,17]. The main reason for attaining high ionic conductivity is due to the formation of a composite heterostructure due to charge redistribution, which further establishes a space charge region constituting the built-in electric field (BIEF), which additionally excels the ions’ transportation and suppresses the electronic conduction through the electrolyte layer.

Hence, these studies have proven that composite heterostructure enables the operation of SOFC at low temperatures and plays an essential role in promoting ionic conduction due to the interfacial effects between the semiconductor–semiconductor and ionic–ionic or semiconductor–ionic conductors. In addition to heterostructure materials, single-phase perovskite has recently become more popular and used as an alternative electrolyte (SmNiO₃, SFT, LBZY, CF-LBZY, and SrCoSnO₃) in LT-CFCs [20,21,22,23,24,25,26,27]. Moreover, some new perovskite electrolytes have not been explored and composited with the ionic conductor to function as a composite electrolyte for the LT-CFCs. The above finding reveals that semiconductors and heterostructures hold great potential for LT-CFCs in terms of higher fuel cell performance.

Inspired by the abovementioned literature, and taking the plus point of composite heterostructure electrolytes, we aim to develop a novel electrolyte via compositing perovskite BCZYY with ionic conductor SDC fluorite structure (composite heterostructure BCZYY-SDC) for LT-CFCs. The detailed study includes (i) developing a composite electrolyte by using the solid-blender technique and a fabrication of cross-ponding pellets of BCZYY-SDC for LT-CFCs (low-temperature ceramic fuel cells) application; (ii) testing and classifying the practicability of proposed composite BCZYY-SDC heterostructure as an electrolyte for LT-CFCs; and (iii) studying the different characterizations of X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV-visible spectroscopy, ultra-photoelectron spectroscopy (UPS), TGA/DSC, etc., and electrochemical analysis for prepared electrolytes and devices. Moreover, the experimental results have been compared with the previously reported literature on composite electrolytes. In addition, the influence of the proposed composite heterostructure has been further investigated. Based on attained outcomes, the prepared bulk heterostructure has been developed and utilized as a novel strategy for advanced LT-CFCs.

2. Materials & Methods

2.1. Material Preparation

At first, the BaCe_0.4Zr_0.4Y_0.1Yb_0.1O₃ and Sm_0.2CeO₂ powders were synthesized by following the co-precepting method combined with a hydrothermal reaction. For preparing BCZYYbO₃, suitable amounts of Ba (NO₃)₂.6H₂O, Ce (NO₃)₂.9H₂O, Zr (NO₃)₂.9H₂O, and Y (NO₃)₂.9H₂O and Yb NO₃)₂.9H₂O bought from Alfa Aesar with 99.98% purity were detached into the deionized water to make the transparent solution. After this, a double molar solution of Na₂CO₃ compared to the moles used in BCZYYbO₃ solution, was added dropwise into the BCZYYbO₃ solution. The resulting products were milky white precipitates, which were further transferred into a Teflon autoclave. Then, the Teflon autoclave was placed at 160 °C for 12 h for hydrothermal treatment by using a vacuum oven. Consequently, the obtained BCZYYbO₃ fine suspension was cleaned and purified with absolute ethanol and deionized water many times to avoid surface-adsorbed water. The obtained precursors were then dried via the drying oven to evaporate the absorbed water. Lastly, the collected powders were calcined at 1000 °C in the air for 4 h to obtain BCZYYbO₃. Moreover, SDC powders were synthesized by using Ce (NO₃)₂.9H₂O and Sm (NO₃)₂.9H₂O and the same protocols mentioned above. At the same time, SDC precursors were calcinated at 700 °C for 4 h rather than 1000 °C. The BCZYYb-SDC heterostructure composite was prepared by mixing the above prepared BCZYYb and SDC powders in different mass ratios, such as 9BCZYYb-1SDC, 7BCZYYb-3SDC, 6BCZYYb-4SDC, 5BCZYYb-5SDC, and 3BCZYYb-7SDC.

2.2. Characterizations Tools and Electrochemical Measurements

The structural crystallinity of prepared individual BCZYYb, SDC, and different BCZYYb-SDC heterostructures was examined by using X-ray diffraction (XRD) performed via Bruker D8 Advanced X-ray diffractometer (Germany, Bruker Corporation) with Cu Kα monochromator radiation, λ = 1.5418 Å to in 2θ ranges from 10–90°. The HR-TEM (FEI Tecnai GI F30 and JEOL JSM7100F, HR-TEM) was used to investigate the morphology of prepared materials in depth. The field emission scanning electron microscope (FE-SEM) was deployed to evaluate the microstructure, and chemical composition of synthesized BCZYYb, SDC, and different BCZYYb-SDC heterostructures. Furthermore, the chemical oxidation states were studied. The UV-visible absorption was measured by using a UV-Vis 3600 spectrophotometer. Moreover, thermogravimetric, and differential scanning (TG/DSC) analysis was employed (thermal analyzer 449F3) to investigate the weight and mass loss under the air environment and over a wide temperature range of 30–800 °C. Moreover, the NT-MDT (Russia) spectrometer was used to analyze the Raman spectra by using a 532-nm solid laser excitation source with 20 mW laser power.

2.3. Fabrication of Fuel Cells

Herein, we have employed a simple dry-pressing technique to construct the solid-fuel cell devices incorporating dense BCZYYb, SDC and different BCZYYb-SDC heterostructure electrolyte and porous Ni_0.8Coi_0.15Al_0.05LiO_2-δ (NCAL) electrodes. In detail, the layered structured NCAL powders were purchased (Bamo-Sci. & Tech. Joint-Stock Company Ltd., Tianjin, China), and added into C₁₀H₁₈O to make a homogeneous slurry painted with a brush on porous Ni-foam. Afterwards, Ni-painted NCAL foam was fired in an oven at 120 °C for 2 h and used for symmetrical electrodes. Finally, synthesized BCZYYb, SDC, and different BCZYYb-SDC heterostructure powders were pressed between two NCAL/Ni-foam electrodes in a steel mold with a diameter of 13 mm (pressure, 240 Mpa) to fabricate solid fuel cell devices with a thickness of 1.0 mm and active-area 0.64 cm², respectively. The performance of the fabricated fuel cell was determined via ITECH8511dc electronic load instrument (ITECH Electrical Co., Ltd., new Taipei, Taiwan). Under the H₂, it acts as a fuel, and the atmospheric air acts as an oxidant with the following flow rates: 100–110 mL/min, and 100 mL/min, respectively. The obtained I-V and I-P curves of the measured data were presented to show the electrochemical properties of the fabricated FC devices. A Gamry Reference 3000 (USA) workstation was used to measure electrochemical impedance spectroscopy (EIS) under the open-circuit voltage (OCV) with 10 mV of DC signal over the frequency range of 0.1 to 10⁶ Hz. The recorded data was analyzed by using ZSIMPWIN software to be obtained EIS data.

3. Results

3.1. Structure and Composition Analysis

Figure 1a shows the XRD pattern of BaCe_0.4Zr_0.4Y_0.1Yb_0.1O₃ (BCZYYb), Sm-doped CeO₂ (SDC), and BCZYYb-SDC heterostructure composite in 2θ range of 10–90°. The dominant diffraction peaks of BCZYYb are located at 24, 31.2, 38.8, 45.4, 56.8, 67.6, and 75.6 and correspond to (100), (110), (111), (200), (211), (220), and (310) planes, respectively, which correspond to the Pm-3m (221) space group and JCPDF # 33-0677, as shown in Figure 1a. The XRD diffraction pattern of SDC located at 28.5°, 33.08°, 47.47°, 56.3°, 69.4°, 76.7°, and 79.07°, correspond to (111), (200), (220), (311), (222), (400), (331), and (420) planes, respectively [22,23]. Although the diffraction pattern of the BCZYYb-SDC heterostructure composite shows the mixed phase of perovskite oxide (BCZYYb) and fluorite structure (SDC) and indicates the coexistence of both diffraction patterns. No peaks other than BCZYYb and SDC diffraction patterns were observed in the BCZYYb-SDC heterostructure composite sample, eliminating the possibility of any chemical reaction forming new phases, as shown in Figure 1c. Moreover, a clear difference in the peak intensity of the diffraction pattern of the BCZYYb and SDC phase can be observed in the different mass ratios of samples.

Moreover, Figure 2 shows the HR-TEM images (crystallographic structure) of SDC, BCZYYb, and bulk composite BCZYYb-SDC heterostructures. Figure 2a, b reveals the HR-TEM images of SDC, where particles seem coherently distributed and establish a uniform network path, whereas Figure 2c shows the d-spacing (0.267 nm) value of SDC. The observed d-spacing of 0.267 nm is most precisely concerned with the (111) plane of SDC, as depicted in Figure 2c. Figure 2d,e reveals the HR-TEM images of BCZYYb where particles are uniformly distributed at the nanoscale and seem to be spherical. At the same time, Figure 2f shows the d-spacing of BCZYYb, which is most probably linked to the (110) planes of BCZYYb. In addition, bulk composite heterostructure HR-TEM images have been displayed in Figure 2g,h, where different particles with different shapes can be noticed and seem to be well glued with each other, which overall favors the device performance. Figure 2h shows the particles’ contacts between SDC and BCZYYb, which have been further magnified in Figure 2i. Moreover, the particles seem uniformly distributed with plenty of connections with different amplification observed among the particles or grains at the nanoscale. Moreover, the various planes can be seen in Figure 2i, confirming the formation of composite heterostructure at particle levels as shown by yellow circles. The composite heterostructure BCZYYb-SDC with different planes (111) and (110) are assigned to the observed d-spacing value for the crystal structure of 0.23 nm and 0.27 nm, respectively.

Furthermore, scanning tunnelling electron microscopy (HAADF-STEM) was performed to reveal the dark field imaging with a high-angle annular image of the hybrid heterostructure, as illustrated in Figure 3a, where particles at the nanoscale are coherently connected with uniform distribution. Moreover, EDS elemental mapping supported with HAADF-TEM was deployed to confirm the uniform distribution of each element at the nanoscale, as depicted in Figure 3b. Figure 3b–h shows the elemental mapping of SDC-BSFSb bulk heterostructure, where homogenous chemical spreading of each component such as Ba, Ce, Zr, Sm, Yb, and O can be seen. This confirms the formation of heterostructure composite at particle/grain levels. Moreover, Figure 3i shows the EDS spectrum of BCZYYb-SDC, where the distribution of all elements can be seen.

3.2. Electrochemical Performance Measurements

To study the success of our developed BCZYYb and SDC heterostructure in fuel cells, different heterostructure composites of BCZYYb and SDC were used as ionic-conducting electrolytes to measure the electrochemical performance. Figure 4 shows the typical current voltage (I-V) and associated power density (I-P) characteristics of fuel cells utilizing pure SDC, BCZYYb, and different heterostructure composites composed of SDC and BCZYYB as an electrolyte operating at 475–550 °C. Our synthesized 4BCZYYb-6SDC electrolyte demonstrated an OCV of 1.14 V and a maximum power density (Pmax) of 1040 mW cm², in contrast to the (Pmax) of only 765 mW cm⁻² displayed by SDC (Pmax) of only 610 mW cm² shown by BCZYYb and 920 mWcm² by 5SDC-5BCZYYb. This improvement in power density is substantial at our operating temperature of 550 °C [23,24,25]. Additionally, as shown in Figure 4d, our prepared 6SDC-4BCZYYb electrolyte fuel cell showed excellent electrochemical performance at low temperatures to 475 °C. At 525 °C and 475 °C, it showed peak power densities of 610 and 450 mW cm². The improved electrochemical performance of synthesized 6SDC-4BCZYYb electrolyte-based fuel cells over pure SDC, BCZYYb, and other BCZYYb-SDC heterostructure composites points to the critical importance of the mass ratio of SDC and BCZYYb that creates a highly oxygen-deficient structure for establishing the path for oxygen ions or proton transport [20]. As a result, it can be concluded from the findings that BCZYYb-SDC stoichiometry produced in three steps could be useful in controlling the ionic properties.

Figure 5 illustrates the cross-sectional SEM image of a 6SDC-4BCZYYB electrolyte-based fuel cell after fuel cell operation, e.g., sintering and testing. It is evident from SEM images that the fabricated SDC-BCZYYb electrolyte layer appears dense and tightly held with the NCAL adjacent anode and cathode without any cracks after the operation of fuel cell performance. Such an adhesive and dense structure of SDC-BCZYYb electrolyte could guarantee no gas leakage for better fuel cell performance [23,26]. Moreover, Figure 5b–i shows the EDS mapping of a cross-sectional SEM image of the SDC-BCZYYb-based fuel cell, where the distribution of Ni, Co and Al (Figure 5b–d) from the NCAL electrode at both edges of the cell, where Ba, Ce, Zr, Ce and O at the intermediate layer can be seen clearly.

3.3. Electrochemical Impedance and Electrical Conductivity

Additionally, EIS characterizations of the cell with BCZYYb, SDC, 5SDC-5BCZYYb-5SDC, and 6SDC-4BCZYYb electrolyte were carried out in an atmosphere of H₂ and air at 550 °C under OCV conditions. Figure 6a–d illustrate the measured EIS spectra’s Nyquist curve. The equivalent circuit modelling of R_o − (R₁ − CPE₁) − (R₂ − CPE₂) was carried out by using the ZSIMPWIN software (as shown in Figure 6), where R_o is the ohmic resistance from the electrolyte and R₁ and R₂ are the charge transfer and mass transfer losses from the electrode of the fuel cell with BCZYYb, SDC, 5SDC-5BCZYYb-5SDC, and 6SDC-4BCZYYb electrolyte, respectively. The EIS spectra show that the R_o of the fuel cell with 6SDC-4BCZYYb significantly decreased in H₂/air (fuel cell operating conditions) at a high temperature compared to SDC and BCZYYb cell a. For instance, a 6SDC-4BCZYYb electrolyte fuel cell showed R_o of only 0.108 Ωcm² in H₂/air at 550 °C, whereas pure SDC showed a R_o of more than 0.18 Ωcm². Then, as observed for EIS results in Figure 6a–d, it dissolves the 6SDC-6BCZYYb electrolyte, reduces both ohmic and polarization losses, and helps to reduce the accumulation layer at the electrode–electrolyte interface, lowering the fuel cell’s charge transfer resistance (R₁) and mass transport resistance (R₂) compared to SDC and BCFZYYb. The capacitance value may follow the mass and charge transfer value decrease at each cell’s electrode–electrolyte interface. (i.e., R₁∼C₁, R₂∼C₂), where capacitance can be determined by ${\mathrm{C}}_{\mathrm{i}}=\frac{{({\mathrm{R}}_{\mathrm{i}}{\mathrm{Q}}_{\mathrm{i}})}^{1/n}}{{\mathrm{R}}_{\mathrm{i}}}$, where R is the corresponding resistance and n is the frequency power [0 ≥ n ≤ 1] of the Q’s values [11,24,25,27].

3.4. Spectroscopic Analysis

Additionally, a wide range of spectroscopic methods, including UV–visible, TGA, Raman, and X-ray photoelectron spectroscopy, were used to investigate additional structural characteristics of SDC and BCZYYb and BCZYYb-SDC powders. Moreover, the TGA of the pure SDC, BCZYYb and BCZYYb-SDC was conducted in air at 25 to 800 °C to investigate the heterostructure formation’s thermal effects further, as demonstrated in Figure 7a. The evaporation of the absorbed water is designated by the rapid weight loss in all three samples around 100 °C. The other rapid weight changes begin at around 300 °C for the BCZYYb and SDC powders, whereas the other rapid weight changes start at 500–600 °C to release lattice oxygen in the form of oxygen vacancies. The BCZYYb-SDC sample exhibits the highest mass change, which is fully evident and supports our hypotheses. The large mass change in BCZYYb could be released easily because of the O bands’ softening when forming a heterostructure. Figure 7b shows the absorbance spectra of pure SDC, BCZYYb and BCZYYb-SDC measured by UV-visible spectroscopy. It is possible to see a substantial difference in the absorbance spectra of the synthesized BCZYYb, SDC, and BCZYYb-SDC powders. The variations in absorbance spectra indicate that the BCZYYb-SDC energy band gap is lowered with the formation of the heterostructure composite; this is only possible because of the creation of oxygen vacancies. This is widely reported when many oxygen vacancies are produced, reducing the metal oxides’ energy band gaps [21,24,25,28].

Figure 7c shows the Raman spectroscopy of BCZYYb, SDC, and BCZYYb-SDC in comparison. The specific Raman bands of BZCYYb and SDC are centred at 220, 463, and 1310 cm⁻¹, respectively. However, a downshift in bands was observed for the BZCYYb-SDC heterostructure sample. Typically, the downshift in the BCZYYb-SDC peak could be due to the decrease in energy for exciting the vibrations along gradually looser bonds in both phases. This confirms the high BCZYYb-SDC activity could have resulted after flexible oxygen bonding at the interface of BCZYYb and SDC and hence high densities of active surface-oxygen species to enhance ORR electrocatalytic activity.

Additionally, a material’s O1s spectra impact its ionic conductivity [29,30]. The O1s XPS spectrum of BCZYYb-SDC shows a lopsided peak, indicating the presence of different oxygen species levels. The curve was fitted to three distinct peaks (1, 2, and 3), as shown in Figure 7d. Peak 1 centered at 528.2 eV, which could be attributed to O²⁻ ions bonded to BCZYYb and SDC, whereas peak 2 situated at 530.1 eV, which could be assigned oxygen defects/vacancies, supporting the supposition of forming more nonstoichiometric properties. Finally, peak 3 appeared at 531.8 eV and could be correlated to weak bonds of oxygen on the surface, such as OH groups. Increased oxygen vacancies, crucial for high fuel cell performance, are indicated by an enhanced area percentage ratio of Olat/Ovac of BCZYYb-SDC [21,28]. As a result, our developed BCZYYb-SDC may offer a new approach for producing high-performance LT-SOFC electrolyte materials [31]. It indicates that the BCZYYb-SDC has more oxygen vacancies than pristine SDC and BCZYYb. Figure 8 illustrates the schematic illustration of the process for configuring a deficient BCZYYb-SDC layer necessary for the mobility of oxygen and proton ions [32].

4. Conclusions

In summary, we successfully prepared and characterized perovskite-structured Ba-cerates based BaCeZrYYb and its heterostructure composite with fluorite-structured Sm-doped CeO₂ material. Moreover, the synthesized BCZYYB-SDC were appointed as electrolytes in fuel cells. It exhibited brilliant fuel cell performance of over 1000 mWcm⁻² when operating at 550 °C with a very small area-ohmic specific resistance. The ohmic ASR of only 0.08 ohm-cm⁻² shows that BCZYYb-SDC-based electrolytes could exhibit very high ionic conductivity and high fuel cell performance. At 550 °C, the fuel cell with different heterostructure composites as the electrolyte also demonstrated good electrochemical performance. In addition, in this study, we have investigated the mechanism for the upsurge in ionic conductivity of BCZYYb-SDC in comparison to pure BCZYYb, SDC, and different heterostructure composites by using microscopic and spectroscopic analyses. We noticed that the ionic conductivity depends on the various mass ratios of BCZYYb and SDC to get low ohmic area-specific resistance and to form the highly oxygen-deficient structure that helps O^2- transport. In conclusion, this technique might inspire the formation of novel oxygen ion-conducting electrolytes based on the heterostructure of proton conducting and oxygen ion, which may benefit all energy devices and material systems.