Overall, screen printing can be considered a promising method for the large-scale production of SOFCs. However, it can only be applied on planer SOFC configurations and possesses similar limitations to the tape casting method, requiring high sintering temperatures (especially when YSZ and GDC are used) and showing large shrinkage levels associated with the removal of organic additives. Thus, it is commonly used for electrode fabrication, where an improved porosity is required.
2.2. Other Fabrication Methods
2.2.1. Solid-State Reaction
It is well known that different synthesis processes can greatly alter the microstructure of the products, resulting in changes in (a) the grain size and grain boundaries in the electrolyte, and (b) the durability and electrochemically active surface area of the electrodes
[140][141]. In addition, the number of impurities observed in different SOFC components is also closely dependent on the synthesis and fabrication procedures used. Thus, the activation energy and electrical conductivity can be greatly affected by different procedures, as impurity levels significantly alter the characteristics of the grain boundary, grain, and density levels. The solid-state reaction is a well-known preparation method for its high yield, high selectivity, simplicity, and the absence of numerous solvents and side reactions. This method requires intimate mixing of the ceramic compounds in the form of oxides or other forms such as carbonates, where repetitive cycles of grinding and heating are commonly applied in order to reach a complete reaction between all reagents
[142][143]. However, it requires a long process time and can lead to high contamination levels. In addition, due to the large number of uncertain factors involved in multiphase reactions, the final structure and composition of the samples are often inhomogeneous and nonstoichiometric
[144].
When compared with data obtained with other methods, lower electrolyte conductivities, and much higher activation energies were reported for samples prepared using a solid-state reaction
[145][146]. Zhan et al.
[147] studied the contribution of grain boundary resistance to the total resistance measured for electrolytes produced through this method and reported an overwhelming contribution of more than 90%, resulting in low total conductivities. It was further reported that it is the presence of high levels of impurities in the grain boundaries of samples prepared through solid-state processing that blocked oxygen ion migration in the electrolyte structure. In addition, high temperatures required for the annealing processes during solid-state reactions commonly result in the formation of strongly agglomerated particles with large particle sizes
[148]. Thus, very high sintering temperatures are often required for the full densification of the resulting electrolyte powder. Such high sintering temperatures can be extremely problematic when the electrolyte is co-fired with the electrode material
[149]. As was mentioned in the previous section, it is often required for a thin electrolyte layer to be supported on an electrode substrate in order to improve cell performance by decreasing the ohmic resistance of the electrolyte layer
[42]. In such configurations, the prepared electrode-electrolyte bilayers are co-fired to reach a dense electrolyte film while preserving the electrode porosity
[150]. High sintering temperatures required for electrolyte powders prepared by the solid-state reaction can either lead to extreme interfacial reactions between the electrode and electrolyte materials or a loss in the electrode porosity
[42]. Both of which are fatal for the fuel cell performance. Thus, alternative preparation methods, such as wet chemical methods, have been proposed in order to overcome such drawbacks.
2.2.2. Sol–Gel Process
The sol–gel chemical synthesis method has also been used as a useful wet chemical route for laboratory synthesis of ceramic oxides
[151]. Since, during a wet chemical reaction, the precursors are commonly dissolved and mixed in a solution, it is possible to reach a high level of homogeneity in the final product. Thus, by using the sol–gel method, it is possible to obtain stoichiometric ceramic powders at relatively low temperatures, avoiding problems aroused from solid-state reactions. However, numerous properties need to be controlled to reach superior properties for produced ceramics when compared to solid-state processing
[152][153]. Sol–gel synthesis is an increasingly popular wet chemical synthesis method, first adopted for the synthesis of glass materials and novel compositions of ceramic oxides in the 1960s
[154]. However, numerous properties need to be controlled to reach superior properties for produced ceramics when compared to solid-state processing
[152][155][156]. The synthesis process includes the transition of monomers from a solution or colloidal liquid system (sol) into an integrated solid network (gel)
[157]. The formed gel is basically a solid-state network in a liquid. In general, the synthesis process can be characterised by the following steps: (1) the formation of stable solutions of the solvated metal precursors (the sol); (2) gel formation resulting from the formation of an alcohol- or oxide-bridged network (the gel), commonly through polycondensation reactions. The initiation of such reactions can be observed in the form of a dramatic increase in the viscosity of the solution; (3) Gel ageing, where a solid mass is gradually formed. During this step, the continuous polycondensation reaction contracts the gel network, also expulsing the solvent from the gel pores; (4) drying the gel by removing water and other volatile liquids from the gel network; (5) decomposition and densification of the dried gel at high temperatures. Such processes would result in the formation of smaller particles with improved sinterability
[158].
An important advantage of the sol–gel process or, in general, wet chemical processes is the possibility of incorporating different types of dopants at different stages of the process, where, due to the mixing of the precursors at the molecular level, an enhanced distribution of the dopant in the final solid solution can be achieved
[159]. Thus, a greater grain interior conductivity can be reached for the electrolyte layer. Although the properties of materials synthesised through sol–gel processes are commonly superior to solid-state reactions, the relatively low powder yield observed for the sol–gel process limits its use in small-scale productions
[21]. In addition, there have been cases where even lower conductivities and higher activation energies were reported for samples prepared using the sol–gel process in comparison to solid-state reactions
[147][160][161]. Haung et al.
[160] reported the synthesis of Ce
0.2Sm
0.8O
1.9 using the sol–gel process and investigated the structure, thermal properties, and ionic conductivity of the solid electrolyte. The electrolyte powder revealed a lower sintering temperature (1400 °C) than powders prepared by solid-state processes (1650 °C). In the study, a considerably low conductivity (5.0 × 10
−3 S·cm
−1) and high activation energy (0.97 eV) were reported for the Ce
0.2Sm
0.8O
1.9 sample prepared using the sol–gel process (at 600 °C). Similar observations were also reported for GDC powders prepared using the sol–gel process, where a conductivity and activation energy of 3.6 × 10
−3 S·cm
−1 and 1.08 eV, respectively, were reported for Gd
0.1Ce
0.9O
1.95 [162]. Since such high activation energies and low conductivities are not expected for samples prepared using the sol–gel process, most studies have addressed this issue to the possible presence of impurities in the precursors and solvents used in the process. However, due to the considerable number of similar observations reported by different groups, especially for the synthesis of doped ceria, a more detailed investigation seems to be required.
2.2.3. Coprecipitation Process
The coprecipitation process is a well-known wet chemical method used in the earliest syntheses of nanoparticles, and it is based on the simultaneous precipitation of multiple species
[163][164][165][166]. In this method, metal cations can be coprecipitated in different forms, e.g., carbonates, bicarbonates, and oxalates, followed by calcination and decomposition steps. The required calcination step will, however, result in particle agglomeration, which, at relatively high temperatures, could lead to aggregation and sintering. Fortunately, nanoparticulates of carbonate and oxalate have been shown to decompose at relatively low temperatures, minimising the level of agglomeration of the final powder
[167][168][169][170]. Both carbonate and oxalate coprecipitation methods have been proven to produce highly sinteractive and homogeneous products and show improved performance over samples prepared using the solid-state reaction and sol–gel techniques
[171][172], especially when used for the synthesis of doped ceria
[168][170]. However, different problems, such as relatively low green density (for oxalate) and composition deviation from feed ratios (for carbonate), limit their application as an ideal synthesis method for SOFC applications
[171].
In the oxalate coprecipitation synthesis process, oxalate acid (C
2H
2O
4) is commonly used to react with the present metal cations in the solution, forming fine precipitates. The precipitates are then calcined to obtain the desired metal oxide composition. Generally, oxalate precipitates show good stability towards different treatment conditions, such as washing and drying, and possess a relatively high production yield
[173]. In addition, the high homogeneity of the electrolyte powders produced by this method commonly results in the report of high bulk conductivities
[170]. Such observations can be traced back to the early applications of this method for the production of doped ceria-based electrolyte powders
[174]. Nevertheless, samples prepared using the oxalate coprecipitation method do require higher sintering temperatures when compared to other wet chemical methods, i.e., carbonate coprecipitation
[175]. It has been seen that the oxalate coprecipitation method usually results in the formation of large rod-like particles, being the agglomerates of smaller irregular-shaped particles. Thus, electrolytes prepared using this method illustrate relatively low green densities and commonly require high sintering temperatures in order to reach full densification
[176][177][178].
The carbonate synthesis route is similar to a coprecipitation method where ammonium carbonate is commonly used as the precipitant. Carbonates are known as precursors for highly sinterable oxides
[168]. The non-gelatinous nature of carbonate precursors allows for low agglomeration of the resultant powders, resulting in well-distributed nano-sized particles with relatively low sintering temperatures
[179]. Thus, most of the literature surrounding the carbonate coprecipitation method is mainly focused on the aspects of microstructure and sinterability, whereas a smaller number of studies address its effect on the electrical properties of the synthesised samples. Although the reported conductivity data for electrolytes prepared by carbonate precipitation show a slight inconsistency, especially for doped ceria, high conductivities and considerably low activation energies are often reported
[180][181][182]. Tok et al.
[183] reported a conductivity of 18.3 × 10
−3 S·cm
−1 (600 °C) for Gd
0.1Ce
0.9O
1.95, while even higher conductivities of 22 × 10
−3 S·cm
−1 (600 °C) have been reported for Sm
0.2Ce
0.8O
1.9 prepared using carbonate coprecipitation
[184]. Both studies revealed activation energies lower than 0.7 eV. Nanosized spherical-shaped particles with high surface areas are usually reported for powders prepared by the carbonate method
[182][185]. Such microstructural properties have been proven to facilitate the sinterability of the synthesised electrolyte powders, making it possible to reach high densification levels at relatively low temperatures
[186]. In addition, low sintering temperatures offer the possibility of fabricating thin electrolyte layers with submicron grain sizes which can greatly enhance the grain boundary conductivity of the electrolyte layer, especially for doped ceria solid electrolytes.
2.2.4. Glycine Nitrate Process (Combustion)
Recently, the development of novel combustion preparation processes has made it possible to produce ultrafine ceramic oxide powders at a surprisingly low reaction time and calcination temperatures with improved powder characteristics. The method includes an exothermic redox reaction between suitable oxidisers (e.g., metal nitrates) and an organic fuel (e.g., glycine, urea, and citric acid)
[42][187][188][189]. In this synthesis process, important characteristics such as surface area, crystallite size, and the nature of the agglomerates can be controlled by the flame temperature and level of generated gaseous products, which in turn show great dependence on both the nature of the fuel and the oxidant-to-fuel ratio
[190][191][192]. In general, all fuels used in this synthesis process serve two purposes: (1) acting as the H and C source, whereupon combustion forms H
2O, CO
2, and liberated heat; (2) forming complexes with the metal ions, improving the homogeneous mixing of metal cations in the final product
[193]. When compared to conventional synthesis techniques, the combustion method provides attractive advantages. First, the relatively high heat generated from the redox reaction can greatly decrease the need for a rather energy-intensive high-temperature furnace, commonly used as an external energy source for conventional synthesis methods. Furthermore, this simple method is capable of producing ultrafine ceramic powders with very high purity
[188][194]. The generated high temperature purges the powder of any possible volatile impurities present in the reactants. The combination of high-temperature gradients with rapid cooling rates in the generated combustion wave can result in the formation of unique microstructures, making it possible to produce powders with small average particle sizes and high porosity
[188][190][194]. In addition, the rapid formation of large volumes of gaseous products generated in this method greatly dissipates the combustion heat, limiting the temperature rise and, therefore, preventing premature particle sintering between primary particles
[195]. Furthermore, the gas evolution limits the formation of hard agglomerates by reducing the interparticle contacts, thus improving the sinterability of the final product
[196]. Compared to solid-state and sol–gel processes, combustion reactions occur at high temperatures, typically in the range of several hundred to several thousand degrees Celsius. These high temperatures are necessary to initiate and sustain the exothermic reactions involved in combustion. While combustion processes typically involve the rapid oxidation of a fuel source, such as hydrocarbons, in the presence of oxygen, and the temperature required for combustion depends on the specific fuel and the nature of the combustion reaction, controlled reactions and phase transformation are the main characteristics of solid-state reactions
[189][197]. On the other hand, the sol–gel process, in which hydrolysis and condensation take place at relatively lower temperatures ranging from room temperature to a few hundred degrees Celsius, enables controlling the composition, structure, and morphology of the product
[158]. Regarding fuel cell application, combustion processes are not typically utilised directly in fuel cell applications or materials synthesis due to their high temperatures and rapid, uncontrolled nature. However, the heat generated from combustion reactions can be harnessed to generate high-temperature steam or thermal energy for fuel cell systems
[198]. Solid-state processes are commonly employed in fuel cell materials syntheses, such as the fabrication of ceramic electrolytes or electrodes. The high temperatures used in solid-state processes enable the formation of dense, crystalline structures with optimised properties for fuel cell operation
[146][199][200]. Sol–gel processes are particularly relevant in fuel cell applications for the production of thin films and nanoscale materials. They provide precise control over composition, morphology, and surface properties, allowing for the development of advanced fuel cell components with improved performance and stability
[154][201][202].
Glycine (NH
2CH
2COOH), with an amino group at one end and a carboxylic acid group at the other end, has been commonly used as a complexing agent when reacted with different metal ions
[192]. It is this zwitterionic characteristic of the molecule that allows for an efficient mixture of a variety of metal ions with different ionic sizes. Such characteristics can greatly inhibit selective precipitation among the reactive elements, greatly improving the homogeneity of the metal composites. In addition, glycine can act as a reliable fuel in the combustion reaction when oxidised by nitrate ions
[203][204]. A great deal of research has reported the rapid and simple production of single- or multicomponent oxide ceramic powders using the glycine nitrate process
[205]. The rapid and self-sustaining nature of glycine nitrate combustion reaction has made it possible to immediately produce homogeneous composite powders with relatively high surface area and very low carbon residue
[206][207].
It is often claimed that a well-adjusted stoichiometry can result in an almost complete reaction, where the gaseous combustion products would mostly consist of CO
2, N
2, and H
2O
[208]. However, due to the fast reaction rates and, thus, the possible limitations imposed on the reaction kinetics, it is more likely that the combustion processes end up with incomplete reactions
[209]. In fact, Pine et al.
[210] reported a relatively high emission level of NOx and CO for a wide range of fuel-to-oxidant ratios used in this method. Despite such disadvantages, this synthesis technique has proven to be very successful for the laboratory-scale production of ceria-based SOFCs.
Figure 8a,b illustrate a highly porous foam-like GDC particle using the glycine nitrate process before and after calcination. The powder was reported to possess an extremally porous structure with loosely agglomerated particles. Such properties have made it possible to prepare thin doped ceria films (<20 μm) using dry pressing, where the resulting powders were pelletised using hardened metal dies (
Figure 7c,d)
[197]. When compared with the carbonate precipitation method, lower conductivities have been reported for electrolyte material, especially doped ceria, prepared using the glycine nitrate process
[197][211]. Despite the differences between the conductivity values reported by different groups, it can be generally said that electrolyte powders, especially doped ceria, prepared using the coprecipitation and glycine methods show higher conductivity and lower activation energy than samples prepared using the solid-state and sol–gel techniques.
Figure 158. (
a) SEM image of as-prepared GDC powder; (
b) NiO-GDC powders calcined at 1000 °C; cross-section SEM image of the cell structure, along with N–-GDC anode layers fabricated using glycine nitrate process (
c,
d)
[197].