Moritz
Kindelmann
*abc,
Sonia
Escolastico
d,
Laura
Almar
d,
Ashok
Vayyala
b,
Dylan
Jennings
bc,
Wendelin
Deibert
c,
Wilhelm A.
Meulenberg
c,
Wolfgang
Rheinheimer
e,
Martin
Bram
c,
Jose M.
Serra
*d,
Joachim
Mayer
ab and
Olivier
Guillon
*cf
aRWTH Aachen University, Central Facility for Electron Microscopy (GFE), Aachen 52074, Germany. E-mail: m.kindelmann@fz-juelich.de
bForschungszentrum Jülich GmbH Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons (ER-C), Jülich 52425, Germany
cForschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1), Jülich, 52425, Germany. E-mail: o.guillon@fz-juelich.de
dUniversitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Instituto de Tecnología Química, Valencia, 46022, Spain. E-mail: jmserra@itq.upv.es
eUniversity of Stuttgart, Institute for Manufacturing Technologies of Ceramic Components and Composites, Stuttgart 70569, Germany
fJülich Aachen Research Alliance, JARA-Energy, Jülich, 52425, Germany
First published on 27th December 2023
Proton-conducting barium zirconate ceramics have shown large potential for efficient electrochemical conversion and separation processes at intermediate operation temperatures. The high energy efficiency, robustness, and intermediate-temperature operation (500–650 °C) make proton-conducting cells promising candidates for future energy conversion systems. However, the major disadvantages of these materials are the inevitable high-sintering temperatures (>1500 °C), leading to Ba-evaporation and formation of high-resistance grain boundaries, which dominate the electrochemical performance. Here, we introduce a novel processing route for proton-conducting barium zirconates, which, on the one hand, significantly lowers the maximum processing temperature and, on the other hand, overcomes the dominating influence of grain boundaries on total conductivity. The key step of this novel processing route is the cold sintering of the powder using pure water as a sintering aid to consolidate BaZr0.7Ce0.2Y0.1O3−δ (BZCY) at 350 °C. We show that clean grain boundaries with a high acceptor-dopant concentration are preserved thanks to the recovery of the perovskite phase during thermal treatment at 1300 °C. This compensates the interfacial core charge, resulting in highly conductive grain boundaries, which do not limit the total conductivity. Consequently, dense BZCY electrolytes produced by our novel approach outperform the conductivity of conventionally sintered BZCY irrespective of the significantly lower maximum processing temperature and its nanocrystalline microstructure. Our presented approach opens up new possibilities for grain boundary engineering and might facilitate novel co-sintering pathways for barium zirconate-based components.
The refractory nature of BaZrO3-based perovskites requires processing temperatures of around 1600 °C to achieve sufficient densification. This is alleviated by using solid-state sintering with additives that enable a low-cost, single-step process called solid-state reactive sintering (SSRS).18–20 SSRS combines in situ phase formation and sintering promoted by transition metal oxides (NiO, CoO, or ZnO), leading to lower sintering temperatures (around 1500 °C) and enhanced grain growth for some BZCY compositions.21–23 However, applying sintering aids leads to losing key conduction features, i.e., reduced proton uptake due to lower acceptor concentrations24 and cationic segregation at grain boundaries.25
Further reduction in processing temperatures is desirable but comes at the expense of fine-grained microstructures reducing the total proton conductivities,26 resulting in low conductivity due to the high resistance of their grain boundaries.27 As with other perovskite ceramics, the grain boundaries of BaZrO3 exhibit a positive core charge, leading to the formation of a negatively charged space charge layer (SCL) adjacent to the interface.28,29 This redistribution of point defects leads to a blocking character of the grain boundary, hindering the transport of protons.30,31 Additionally, segregation of acceptor dopants to the interface can be observed, leading to an enrichment in the grain boundary core and/or the SCZ.29,32,33 This can partially compensate for the core charge, leading to an increase in grain boundary conductivity.34 Proposed strategies to exploit this effect include high-temperature annealing, which allows redistribution of dopants, or incorporation of large A-side acceptor dopants.35 Neither of these approaches has led to a sustainable and successful solution to simultaneously lower the sintering temperature and maintain sufficient grain boundary conductivity.
This study presents a novel processing route combining cold sintering and thermal post-treatments (TPT) to produce dense, Zr-rich BZCY electrolytes with highly conductive grain boundaries. Cold sintering is an emerging pressure- and liquid-assisted sintering process that enables significantly lower densification temperatures (below 400 °C) for a variety of ceramic materials (e.g., ZnO,36–38 BaTiO3,39,40 CeO2 (ref. 41 and 42) and LLZO43,44) and composites.45–47 Adapting hydrothermal processes found in geology, cold sintering applies these mechanisms to synthetic systems to enable low-temperature densification.48,49 Here, we apply cold sintering to densify BZCY at 350 °C and 400 MPa using only deionized water as an additive, backed by the initial BZCY densification studies pinpointing the importance of precursors' phase composition.54
Here, we now characterize the microstructure and phase formation during cold sintering at the macro- and nanoscale to elucidate the densification mechanisms. Under these sintering conditions, a high degree of densification can be achieved, but for electrochemical applications, a TPT (1300 °C for 10 h) is required to recover the target phase fully. To demonstrate the advantage of our approach compared to conventional processing, the grain boundary structure and chemistry of BZCY processed at low temperatures is characterized by high-resolution scanning transmission electron microscopy (STEM) and 3-dimensional atom probe tomography (3D-APT). In addition, electrochemical characterization by electrochemical impedance spectroscopy (EIS) and 4-point DC measurements confirms the high grain boundary conductivity. Our novel approach can overcome the incompatibility of fine microstructures and high protonic conductivity in barium zirconate ceramics by reforming the perovskite phase after densification. This allows acceptor dopants to segregate to the grain boundary and balance the core charge, significantly increasing the interfacial conductivity.
The cold sintering experiments were carried out in a field-assisted sintering technology/spark plasma sintering machine (FAST/SPS, FCT Systeme HP-D5, Germany) with a TZM die set-up (TZM = Mo-based alloy from Plansee SE, Austria). The powders were pre-compacted in the die, and 5 wt% deionized water was homogeneously added to the green body using a micropipette (Eppendorf, Germany). The water/powder mixture was densified at a sintering temperature of 350 °C, with a uniaxial pressure of 400 MPa, a heating rate of 20 K min−1, and a dwell time of 5 min. To derive the actual shrinkage from cold sintering experiments, the displacement curve was corrected for the thermal expansion of the mold assembly. After cold sintering, all samples were subjected to thermal post-treatment (TPT) at different temperatures (1100, 1300, 1500, 1600 °C) for 10 h in air to investigate the reformation of the perovskite phase.
Conventionally processed samples used for comparison were uniaxially pressed at 50 MPa and subsequently sintered in a powder bed of the same powder at 1600 °C for 5 h in air.
Total conductivity was measured on sintered bars by the standard four-point DC technique. Rectangular bars used in conductivity measurements were obtained by polishing the disk samples from cold sintering and conventional manufacturing techniques. Silver paste and wires were used for contacting. The constant current was supplied by a programmable current source (Keithley 2601), while the voltage drop was detected by a multimeter (Keithley 3706, both Keithley Instruments, USA). Total conductivity measurements were performed in different atmospheres, and the isotopic effect was evaluated. The employed atmospheres were: wet 5% H2 in Ar and 5% D2 in Ar (where wet means 2.5% H2O and 2.5% D2O, respectively).
EIS analysis was conducted with a Solartron Analytical 1470E CellTest System frequency response analyzer (Ametek Inc., USA) in the 0.03–106 Hz frequency range. The disk samples were placed in a quartz reactor with Ag paste painted contacts and between Pt meshes, tested in a temperature range between 800 °C and 500 °C and humidified (3% H2O) atmospheres: (a) synthetic air, (b) Ar and (c) 5% H2-95% Ar with a total flow rate of 50 mL min−1 at each sample side. The collected impedance data was analyzed and fit using the ZView software.
The two-step processing applied in this study is displayed in a schematic process scheme in Fig. 1a. First, the powders are filled into a metal pressing die, and deionized water is added. Then, cold sintering is performed at 350 °C using a uniaxial pressure of 400 MPa for 5 min in an instrumented field-assisted sintering/spark plasma sintering device (FAST/SPS). The total processing time for densification did not exceed 30 min. After cold sintering, the samples are removed from the pressing die and subjected to a TPT at 1300 °C for 10 h to reform the perovskite phase fully.
The shrinkage curve (corrected for instrument and tool elasticity, 1b) shows that a clear acceleration of the densification under high uniaxial pressures starts at around 100 °C when water starts to evaporate (increase in SPS chamber pressure, orange curve). The densification during cold sintering may involve particle rearrangement, particle surface cleaning, local dissolution, subsequent reprecipitation processes, and dislocation-based plastic deformation.48,50,51 In BZCY ceramics, this process starts below the evaporation temperature of the water and continues up to 350 °C.
A scanning electron microscopy (SEM) image of a fracture surface of cs-BZCY is displayed in 1c, revealing the fine-grained microstructure and high relative density. However, X-ray diffraction (XRD) results (1e) suggest that parts of the powder decompose during cold sintering, forming several secondary phases (Ba(OH)2, BaCO3, and CeO2). This behavior is caused by the instability of Ce-rich BZCY compositions, which are prone to decomposition under water/steam environments.52,53 Here, we intentionally used a starting powder that consists of two perovskite phases (1e, blue diffractogram), one being a Ce-rich and the other a Zr-rich BZCY. This approach allows for controlling the dissolution behavior, enabling densification at low temperatures. Details on the impact of the composition of the starting powder were reported elsewhere.54
To understand the densification behavior during cold sintering, we examined the sintering process in detail. In the first step, the Ce-rich BZCY reacts under cold sintering conditions with water and forms Ba(OH)2, Y doped-CeO2, and the residual, stable perovskite BaZr1−xYxO3. In the second step, the formed Ba(OH)2 either solves in residual H2O or reacts with carbon-containing molecules (CO, CO2, or carbon tape used during sintering) to form BaCO3 and H2O. All of these phases could be detected using XRD.
The secondary phases forming during cold sintering have a low strength (especially BaCO3 (ref. 55)), easing particle rotation and sliding under high mechanical pressures enabling densification. However, residual secondary phases prevent electrochemical testing of the cold-sintered BZCY under application conditions (wet atmospheres at temperatures from 400 to 800 °C). Therefore, a systematic variation of TPT (from 1100 to 1600 °C for 10 h in air) was conducted to investigate the reformation of the target phase and the associated microstructural development. Fig. 1f shows the phase changes of samples annealed at different temperatures after cold sintering.
A TPT at 1100 °C is insufficient to reform the target phase, leaving two BaZrO3 compositions and residual BaCO3 and Y2O3 in the sample. In contrast, annealing treatments at 1300 °C and above result in a single BZCY phase with only minor residuals of BaZrO3. Through a careful thermal decomposition of BaCO3 above 1200 °C56 and the dissolution of Y2O3 and CeO2 into the perovskite structure, it is possible to produce dense (rel. density 96%) and fine-grained BZCY polycrystals (average grain size of 365 nm). The microstructure of such a BZCY ceramic annealed at 1300 °C is displayed in Fig. 1d. Details on the microstructure evolution of thermally post-treated samples are furthermore displayed in Fig. 2. Increased annealing temperatures at 1500 and 1600 °C led to coarser microstructures with grain sizes of 1.9 and 2.7 μm, respectively. Compared to the reference sample it is clearly possible to decrease the overall sintering temperature to values around 1300 °C, while maintaining high densities. Additional investigations of the microstructure and phase evolution by SEM and HT-XRD are shown in supplementary Fig. S1–S3.†
To further understand the mechanisms that enable low-temperature consolidation through cold sintering, the microstructure and chemical composition were investigated by STEM-EDS. The multi-phase microstructure observed by XRD is resolved at a high spatial resolution. Fig. 3a shows an overview and its associated EDS mappings. Three different phases can be differentiated as highlighted: (I) a Zr-depleted and C-rich phase, which can be assigned to BaCO3, (II) Ce and Y-rich nanoparticles, and (III) a Zr-rich matrix with a homogeneous composition. Additionally, high-resolution STEM imaging confirms that Y and Ce-rich nanoparticles (Fig. 3b and c) are cubic CeO2 particles with a high amount of Y. The lattice spacing was measured (3.24 Å) in 3c, which makes it possible to derive the visible lattice plane (111) in the cubic crystal structure of CeO2. Furthermore, a high-resolution EDS mapping (3d) confirms the chemical composition and reveals that precipitated CeO2 particles are embedded into a BaCO3 matrix. However, the high electron-beam sensitivity of the carbonate matrix complicates conventional spectral imaging leading to a comparably low EDS signal for Ba.
Combining the findings of the macroscopic phase and microstructure analysis (Fig. 1) with the high-resolution STEM investigation of cs-BZCY (Fig. 3) allows us to elaborate on the mechanisms enabling cold sintering of BZCY.
In BZCY, the densification mechanism during cold sintering can be divided into two stages, as schematically displayed in Fig. 3e. First, the dissolution of Ce-rich components, which can be affected by the applied pressure, and the associated formation of reaction products Ba(OH)2 and BaCO3. After the evaporation of water from the powder compact, these residuals remain in the microstructure. Second, the pressure-assisted densification mechanism takes place, which is characterized by the stress exponent,51 generally determined by systematically varying the applied pressure. We assume that under the high uniaxial pressure applied during cold sintering, the low shear strength of the precipitated phases allows grain sliding and rotation mechanisms. Third, removing the residual phases and reincorporating Ce and Y back in the perovskite using TPT at elevated temperatures yields stable low-temperature processed BZCY ceramics.
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Fig. 4 Electrochemical properties of low-temperature processed BZCY. Electrochemical impedance spectroscopy (EIS) of CONV (1600 °C, 5 h) and TPT1300 (1300 °C, 10 h) BCZY in wet air (a), wet Ar (b), and wet H2 (c) (3% H2O). Full circles represent bulk conductivity, and empty circles represent grain boundary conductivity. (d) Total conductivity determined by DC conductivity measurements in a reducing atmosphere (wet 5% H2). Filled circles represent the corresponding EIS measurements. (e) DC measurements using D2 in D2O revealing the isotopic effect and highlighting the high protonic conductivity of low-temperature processed BZCY. (f) Literature overview of the total conductivity data at 600 °C as a function of processing temperature summarizing different sintering approaches and material systems.14,22,23,61–66 The material compositions and experimental details are displayed in Table S1 and highlighted in Fig. S6.† (g–j) H2/D2 isotopic effect for cold sintered samples after different TPT from 1300 to 1600 °C and CONV, respectively. |
In Fig. 4a–c, the bulk and the grain-boundary conductivities obtained after impedance analysis for the low-temperature processed BZCY (1300 °C, 10 h) are directly compared with a conventional SSRS sample (1600 °C, 5 h, details can be found in the ESI†). The TPT1300 electrolytes revealed superior proton-conduction properties compared to samples produced by SSRS in all tested atmospheres (additionally corroborated by EIS measurements on the TPT1500 sample, Fig. S4†). Bulk conductivities are in the same range in wet Ar and H2. Remarkably, the grain-boundary conductivity in low-temperature processed BZCY is not limiting the overall conductivity despite its nanocrystalline microstructure. This behavior directly contrasts the conventionally sintered sample and observations made for BaZrO3-based proton conductors in general,30,57,58 which exhibit a high grain boundary resistivity. Additionally, the differences in activation energy for grain boundary conductivity point towards a change in the proton-transport mechanism across the grain boundaries.
From the H2/D2 isotope effect (Fig. 4g–j) observed in reducing atmospheres, it can be concluded that the improvement in transport properties is clearly due to the enhanced proton transport through the nanocrystalline microstructure in TPT1300 (ref. 15, 59 and 60) (additional measurements in Ar/H2 and ArD2 which also show high protonic mobility in TPT1300 are shown in Fig. S4†). The total conductivity reported here is the highest for Zr-rich BZCY compositions at these low overall sintering temperatures, highlighted using an overview graph summarizing total conductivities at 600 °C from different literature sources (Fig. 4f).14,22,23,61–66
The chemical composition of grain boundaries of both sample types was investigated using STEM-EDS and is depicted in Fig. 5a and b. Both grain boundaries are imaged in an edge-on state (i.e. the grain boundary plane is parallel to the electron beam) to avoid overlap and unwanted broadening of the spectral image at the interface. Clear differences between the grain boundaries can already be seen in the bright field (BF) images of the two samples. The TPT1300 sample shows a thin and clean interface (thickness of about 0.6 nm), while the conventionally processed samples show a structurally different interlayer (thickness of about 2.1 nm). This observation is confirmed by elemental mapping of the two-grain boundaries. In TPT1300, Y and Ni segregate on the interface (segregation thickness of 3 nm), and a slight depletion of Ba, Zr, and Ce is visible. These elemental mappings indicate the integration of Y and Ni cations into the perovskite structure near the grain boundary, although the exact atomic positions cannot be determined. In the CONV sample, significant differences in grain boundary composition are evident. Ni forms a 5 nm thick grain boundary layer, while all other components – Ba, Zr, Ce, and Y – are significantly depleted at the interface. These two observations indicate a Ni-rich grain boundary film, which could be a remnant of the transient BaY2NiO5 phase formed during SSRS.19,20,24,68
To further investigate the chemical composition of the grain boundaries in TPT1300, atom probe tomography (APT) was utilized in addition to electron microscopy. Fig. 5c–e presents a three-dimensional reconstruction from an APT analysis of a nanocrystalline TPT1300 sample, which includes an exemplary grain boundary proxigram. The data set displays two-grain boundaries near each other, with clear segregation of Y and Ni at the grain boundaries. Furthermore, slight variations in the composition are observed within grains, particularly for Ce. The composition of the interfaces shown in Fig. 5d and e corroborates the findings previously revealed by STEM. The grain boundaries show significant segregation of Y and Ni and a decrease in Zr concentration from about 17.0 at% in bulk to about 14.2 at% at the interface due to the higher amount of Y and Ni accumulating at the interface. Although the strong curvature of grain boundaries in nanocrystalline microstructures causes slight broadening during data analysis, these results offer quantitative insight into the segregation behavior of Y and Ni at grain boundaries in TPT1300. The complete APT data set is provided in supplementary Fig. S7.†
The detailed examination of the grain boundaries illustrates the different characteristics of the interfaces created by our novel process. A comparison with conventionally processed samples revealed significant differences in chemical composition and structure at the grain boundaries. The pre-densification step by cold sintering at 350 °C allows to achieve high relative densities at very low temperatures, removing the need to apply sintering temperatures over 1500 °C, which are required to reach sufficient densities by conventional processing. Subsequent thermal treatments at intermediate temperatures bypass the formation of the transient BaY2NiO5 phase in conventional SSRS processing and prevent Ni-rich secondary phases from remaining at the grain boundary.18 Instead, we process the cold-sintered samples at temperatures suitable to decompose residual BaCO3 (ref. 56) and reform the perovskite phase by dissolving the precipitated Y-rich CeO2 particles. The interdiffusion coefficients of Ce and Zr are sufficiently high at 1300 °C to form a single perovskite phase and allow equilibrium of the charged dopants at interface.69 Bypassing the remaining transition phases at the grain boundaries allows Y and Ni to dissolve into the perovskite crystal structure and form a sharp segregation profile at the grain boundary. It is generally accepted that dopant segregation phenomena in oxide ceramics are driven by electrostatic and elastic driving forces that reduce the Gibbs free energy.70 Dopants whose charge and ion size do not match the host lattice exhibit negative segregation energy toward surfaces71,72 and grain boundaries.73 In our case, both cases apply, i.e., the increased concentration of oxygen vacancies at the grain boundary core in BaZrO3-based proton conductors28,58,74 leads to a driving force for negatively charged point defects towards the grain boundary. Assuming that both Y and Ni are located at the B site of the perovskite (instead of Zr or Ce), both can be interpreted as negative point defects and
, which are subjected to an electrostatic driving force toward the positively charged grain boundary core.75 There, they can compensate for the core charge, reducing the magnitude of the space charge potential formed.35 In addition to electrostatic effects, the cationic size mismatch between the host lattice (Zr4+: 79 pm, Ce4+: 103 pm) and the dopants (Y3+: 93 pm and Ni2+: 72 pm) leads to an additional elastic strain that drives segregation toward the interface.
Since the conduction path of protons in BaZrO3-based ceramics is across the grain boundary rather than along it,28,57,76–80 the observed segregation behavior in cs1300-BZCY is considered to be the main reason for the high proton conductivities, despite low processing temperatures and very fine microstructures. This allows us to overcome the blocking behavior of grain boundaries without applying SSRS or high-temperature annealing treatments, thus removing the main obstacle in lowering the sintering temperatures in BZCY ceramics. Another advantage of small grain size is the expected higher fracture strength, as usually observed in brittle ceramic materials,81 and previously confirmed for cold sintered ZnO.82 Therefore, not only the ionic conductivity but also the mechanical reliability is enhanced by our novel approach.
Electrochemical characterization by impedance spectroscopy and DC measurements in all application-relevant atmospheres proves that low-temperature processed BZCY outperforms conventionally processed reference electrolytes sintered at 1600 °C. Furthermore, high protonic conductivity down to around 400 °C could be observed by measuring the isotopic effect in wet H2 and D2 atmospheres. The protonic conductivity in low-temperature processed BZCY is not limited by the grain boundary resistivity despite its nanograined microstructure. The high conductivity of grain boundaries can be associated with their pristine nature, a lack of residual Ni-rich secondary phases, and a high amount of segregated acceptor dopants. This allows to compensate parts of the intrinsic core charge, reducing the blocking behavior of grain boundaries against the conduction of protons through the electrolyte.
The cold sintering pretreatment applied here deliberately changes the order between powder densification and final phase formation at the grain boundary, allowing us to bypass thermal regimes, which typically would lead to detrimental secondary phase formation and high interfacial resistances. Hereby, our novel sintering route solves one of the major obstacles in developing future ceramic proton conductors – the incompatibility between low sintering temperatures and high protonic conductivities83 and provides optimal mechanical properties related to the fine microstructure. Therefore, a further implementation of cold sintering on thin layered architectures is needed to unlock the potential of this novel processing strategy for grain boundary engineering and novel material combinations in proton-conducting ceramics, which were not possible beforehand due to the significant mismatch in required sintering temperatures.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta07076j |
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