Recent Novel Fabrication Techniques for Proton-Conducting Solid Oxide Fuel Cells

: Research has been conducted on solid oxide fuel cells (SOFCs) for their fuel ﬂexibility, modularity, high eﬃciency, and power density. However, the high working temperature leads to the deterioration of materials and increased operating costs. Considering the high protonic conductivity and low activation energy, the proton conducting SOFC, i


Introduction
Nowadays, the escalating levels of global environmental pollution and energy consumption are significant issues in human society [1,2].The growing concern about climate change has led to the development of environmental protection technologies primarily based on renewable resources [3].Electricity, as the most widely used resource, is closely related to human life and has become an indispensable energy source at present.Improving the efficiency of existing power sources or developing new green energy technologies has become a hot topic in current research [4].Thus, the storage and conversion of energy through energy devices are becoming the way forward.Among the various energy sources, fuel cells (FCs) play a significant role in improving efficiency and reducing greenhouse gas emissions, as they are clean, efficient, and sustainable [5,6].Fuel cells can directly convert the chemical energy of fuel into electricity with an efficiency exceeding 50% and ultra-low emissions.Therefore, FCs are regarded as promising green power-generation devices [7,8].
Crystals 2024, 14, 225 2 of 23 Among the various types of fuel cells that have been developed, the solid oxide fuel cell (SOFC) is more efficient than other types [9].SOFCs have garnered significant attention due to their fuel flexibility, modularity, high efficiency, and power density [7,10].SOFCs can withstand very high temperatures [9,11], while such a high working temperature also presents several limitations.The high operating temperatures cause material deterioration and increase operational costs.Considering the high ionic conductivity and low activation energy of a protonic conductor, the protonic ceramic fuel cell (PCFC), which operates at a lower temperature, is proposed to address the detrimental issues associated with high working temperatures.This approach has garnered tremendous interest [12].Protons serve as the charge carriers in protonic ceramics, possessing much lower transport activation energy than oxide ions.This characteristic has made protonic ceramics suitable for extensive use in intermediate temperature (400-650 • C) electrochemical devices [13][14][15][16][17].Over the past decade, the PCFC has emerged as a leading energy conversion device due to its low operating temperature, in which various renewable fuels can be directly converted into electricity at low temperatures [14,18].The protonic ceramic fuel cell is considered an effective upgrade to the traditional solid oxide fuel cell [19].
The construction of the PCFC is directly related to its performance; therefore, the fabrication techniques are crucial.To attain the necessary phase and microstructure, along with other desired qualities of PCFC components (such as porous electrodes, dense electrolytes), some processes (such as several high-temperature and energy-intensive procedures) are crucial [19,20].For example, the electrolytes involved in PCFCs need to be sintered at temperatures as high as 1700 • C for more than 10 h to achieve a thin layer with a high relative density [21,22].This high temperature and long dwell time has been criticized for its energy and time consumption, as well as poor performance resulting from material volatilization [23][24][25][26].In a study, it was estimated that material manufacturing costs account for approximately 30% of the total manufacturing costs of proton-conducting solid oxide fuel cells [27].Therefore, it is necessary to develop next-generation flexible advanced manufacturing technologies, such as in situ 3D printing laser processing technology, to achieve high-performance PCFCs required for low-cost, clean, and fast manufacturing.

Introduction to Solid Oxide Fuel Cells
The solid oxide fuel cell is an energy conversion system based on ceramics.It is a type of high-temperature fuel cell [28].Its characteristics include the direct conversion of chemical energy into electrical energy, high efficiency, low emissions, and flexibility in fuel options [19,[29][30][31].SOFCs do not suffer from issues such as electrolyte evaporation and precipitation.Additionally, they do not experience corrosion caused by the electrolyte or electro-segregation problems, and they have a long cell life.It operates at temperatures above 800 • C and has an efficiency of over 60% [32].Gases such as CO and CH 4 , as well as other fossil fuels, can be utilized as fuel after they are reformed inside the cell.The SOFC is an ideal choice for utilizing fossil fuels for power generation [33].However, SOFCs have a lengthy start-up time and are not suitable for emergency power supply.

Solid Oxide Fuel Cell Working Principle
The main components of the SOFC structure include a porous anode, dense electrolyte, porous cathode, and connectors [34].The cathode and anode are the primary components for gas-catalyzed reactions and electron transfer.The dense electrolyte layer serves as an electronic insulator, preventing cathode-anode contact, internal short circuits, and isolating the fuel gas from the oxidizing gas, while also conducting oxygen ions or protons.Depending on the type of ions conducted by the electrolyte, SOFCs can be categorized into oxygen ionic and protonic SOFCs [35].For an oxygen ionic SOFC, Figure 1 illustrates its simple working principle.Fuel gas is introduced from the anode side of a SOFC system.The fuel gas undergoes an oxidation reaction at high temperatures to release electrons.These electrons then pass through an outer circuit and reach the cathode, providing power to an external circuit load.The electrons flowing to the cathode undergo a reduction reaction with the incoming oxygen, producing oxygen ions.Subsequently, the oxygen ions are transported to the anode through the electrolyte of the oxygen ion conductor, where they react with the fuel to release electrons.
protons.Depending on the type of ions conducted by the electrolyte, SOFCs can be categorized into oxygen ionic and protonic SOFCs [35].For an oxygen ionic SOFC, Figure 1 illustrates its simple working principle.Fuel gas is introduced from the anode side of a SOFC system.The fuel gas undergoes an oxidation reaction at high temperatures to release electrons.These electrons then pass through an outer circuit and reach the cathode, providing power to an external circuit load.The electrons flowing to the cathode undergo a reduction reaction with the incoming oxygen, producing oxygen ions.Subsequently, the oxygen ions are transported to the anode through the electrolyte of the oxygen ion conductor, where they react with the fuel to release electrons.cathode, electrolyte, anode.Reprinted with permission from Ref. [36].Copyright 2018 WIREs Energy and Environment.

Introduction to Proton Ceramic Fuel Cell
The Protonic ceramic fuel cell is currently the focus of intense development and research.It is a promising electrochemical device for the efficient and clean conversion of hydrogen and hydrocarbon fuels into electrical energy [37].It combines the thermal and kinetic benefits of solid oxide fuel cells with the inherent advantages of proton exchange membrane fuel cells (PEMFCs) and proton conductivity of phosphoric acid fuel cells (PAFCs).PCFCs are among the most promising energy conversion devices due to their low cost and good durability [38].They can be operated at much lower temperatures (400-650 °C) compared to their oxide-ion-conducting counterparts, which require temperatures above 700 °C.Lower operating temperatures offer several cost-effective benefits, including shorter start-up times, reduced energy input required to heat the cell to operating temperature, and extended material lifetimes [39,40].In recent years, research on PCFCs has garnered increasing attention, and the research fervor continues to grow.

Working Principles of Protonic Ceramic Fuel Cells
There are differences between PCFCs and conventional SOFCs, which are based on oxygen ion conduction.The fuel (H2, CH4, NH3, etc.) supplied at the anode side is catalytically dissociated into protons and electrons.Afterward, heat-activated protons diffuse through the electrolyte to the cathode side, while electrons are transmitted through an

Introduction to Proton Ceramic Fuel Cell
The Protonic ceramic fuel cell is currently the focus of intense development and research.It is a promising electrochemical device for the efficient and clean conversion of hydrogen and hydrocarbon fuels into electrical energy [37].It combines the thermal and kinetic benefits of solid oxide fuel cells with the inherent advantages of proton exchange membrane fuel cells (PEMFCs) and proton conductivity of phosphoric acid fuel cells (PAFCs).PCFCs are among the most promising energy conversion devices due to their low cost and good durability [38].They can be operated at much lower temperatures (400-650 • C) compared to their oxide-ion-conducting counterparts, which require temperatures above 700 • C. Lower operating temperatures offer several cost-effective benefits, including shorter start-up times, reduced energy input required to heat the cell to operating temperature, and extended material lifetimes [39,40].In recent years, research on PCFCs has garnered increasing attention, and the research fervor continues to grow.

Working Principles of Protonic Ceramic Fuel Cells
There are differences between PCFCs and conventional SOFCs, which are based on oxygen ion conduction.The fuel (H 2 , CH 4 , NH 3 , etc.) supplied at the anode side is catalytically dissociated into protons and electrons.Afterward, heat-activated protons diffuse through the electrolyte to the cathode side, while electrons are transmitted through an external circuit to power external devices.On the cathode side, the adsorbed oxygen gains electrons and is reduced to oxygen ions, which then react with protons delivered from the anode to form water [41].
In the case of H 2 fuel, the electrochemical reaction that takes place inside the anode is simply represented by the following steps: Anodic reaction: 2H  The PCFC produces water at the cathode rather than the anode.This approach avoids fuel dilution, enhances fuel utilization, and improves overall system efficiency.Schematic representation of a PCFC structure is shown in the Figure 2. Protonic ceramic fuel cells can conduct protons through their lattice with low activation energies, resulting in higher ionic conductivities than SOFCs; thus, they can be operated at lower temperatures [42].
external circuit to power external devices.On the cathode side, the adsorbed oxygen gains electrons and is reduced to oxygen ions, which then react with protons delivered from the anode to form water [41].
The PCFC produces water at the cathode rather than the anode.This approach avoids fuel dilution, enhances fuel utilization, and improves overall system efficiency.Schematic representation of a PCFC structure is shown in the Figure 2. Protonic ceramic fuel cells can conduct protons through their lattice with low activation energies, resulting in higher ionic conductivities than SOFCs; thus, they can be operated at lower temperatures [42].The stack schematic is shown in Figure 3 and was originally intended for use in a 50 W portable PCFC military application.The design is centered on three repetitive components: a protonic ceramic membrane electrode assembly (MEA), a composite ceramic frame combined with a MEA, and a metal interconnect/bipolar plate that conducts electricity between adjacent cells [44].The stack schematic is shown in Figure 3 and was originally intended for use in a 50 W portable PCFC military application.The design is centered on three repetitive components: a protonic ceramic membrane electrode assembly (MEA), a composite ceramic frame combined with a MEA, and a metal interconnect/bipolar plate that conducts electricity between adjacent cells [44].

Manufacturing Method of PCFCs
There are various methods used for manufacturing PCFCs, including solid-state reactive sintering (SSRS), spark plasma sintering (SPS), microwave sintering, tape casting, and 3D printing.One of the primary challenges in the fabrication of protonic ceramicbased energy conversion devices is the necessity of high temperatures (1600-1700 °C) and

Manufacturing Method of PCFCs
There are various methods used for manufacturing PCFCs, including solid-state reactive sintering (SSRS), spark plasma sintering (SPS), microwave sintering, tape casting, and 3D printing.One of the primary challenges in the fabrication of protonic ceramicbased energy conversion devices is the necessity of high temperatures (1600-1700 • C) and long firing time (>10 h) Additionally, the refractory nature of the ceramics makes them well-suited for use as structural materials.However, it is sometimes seen as a hindrance when they are used as a functional material, which makes the sustainable and clean manufacturing of proton ceramic devices impractical [45].Here, we offer a comprehensive introduction to sustainable and clean manufacturing techniques for proton ceramic energy devices, along with conventional traditional methods, and briefly outline some alternative approaches.
3.1.Conventional Processing Technology for PCFC 3.1.1.Solid-State Reactive Sintering SSRS is a commonly used method for preparing powder materials.Specifically, solidphase reaction refers to a chemical reaction involving two or more solid materials and generating a new compound.This method combines solid-phase reaction and sintering in a single step [46].This method greatly simplifies the production of protonic-conducting ceramics by integrating phase formation, densification, and grain growth into a single high-temperature sintering step [47].This reaction occurs through solid surface contact, so the reaction efficiency is generally improved by grinding the powder into small particles, thereby increasing the contact area between the particles.Solid-state reactive sintering is the process of sintering a pressed mixture at a temperature below the melting point, based on solid-phase reaction.During sintering, the air between the powder particles is expelled, and particle aggregation, crystallization, and densification occur among the reactants [48].Therefore, solid-state reaction sintering is one cost-effective method for synthesizing and manufacturing [19,49].The experiment by Zhao et al. [50] also indicates that the one-pot SSRS method can be used to prepare ideal components for proton ceramic electrochemical devices.Tong et al. [51] report that large particle size BaZr 0.8 Y 0.2 O 3−δ (BZY20) can be prepared at a low cost using simple SSRS methods.In the SSRS process, the sintering time and the sintering temperature can be significantly reduced by adding proper sintering additives.

Spark Plasma Sintering
Spark plasma sintering (SPS) is an innovative sintering technology that utilizes highenergy, low-voltage pulsed current to instantly generate a discharge plasma in the local area between the particles.SPS is a promising method for obtaining solid electrolytes for PCFCs at lower temperatures (by 400-500 • C), compared to traditionally used temperature conditions [52].For the sintering mechanism of SPS, it is generally believed that in addition to the Joule heat caused by hot-press sintering and the plastic deformation caused by pressure promoting the sintering process, the SPS process also generates a DC pulse voltage between powder particles.At the same time, it effectively utilizes the surface activation and self-heating effects generated by the discharge between powder particles.Therefore, a phenomenon unique to the SPS process that is beneficial for sintering has emerged [53].SPS rapidly heats the materials under uniaxial pressure to produce high-density ceramics.The ability to apply very high heating and cooling rates, short residence times, and sintering at relatively low temperatures limits grain growth during densification [54].SPS can be categorized as a low-temperature sintering technique, and the principle of discharge plasma sintering is shown in Figure 4 [55].
emerged [53].SPS rapidly heats the materials under uniaxial pressure to produce high density ceramics.The ability to apply very high heating and cooling rates, short residenc times, and sintering at relatively low temperatures limits grain growth during densifica tion [54].SPS can be categorized as a low-temperature sintering technique, and the prin ciple of discharge plasma sintering is shown in Figure 4 [55].

Microwave Sintering
Microwave sintering is the utilization of microwaves, which have a special band wit the basic fine structure of the material coupled to generate heat.This method exploits th material's dielectric loss, causing an overall heating of the material to the sintering tem perature and realizing densification.It transfers heat directly to the material through th coupling of electromagnetic waves, thus realizing volumetric heating in the microwav field, which improves the heating efficiency, and also has the feasibility of being used i the fabrication of lumped components compared with discharge plasma sintering [56]. is characterized by fast heating speed, high energy utilization, high heating efficiency safety, hygiene without pollution, and the ability to improve the uniformity and yield o the product, as well as improve the microstructure and property of the sintered materia Microwave sintering is widely used in the ceramic industry due to its self-heating nature which enables fast and efficient sintering [57].Due to the use of lower temperatures i microwave sintering, less grain growth or coarsening can be seen without the use of an

Microwave Sintering
Microwave sintering is the utilization of microwaves, which have a special band with the basic fine structure of the material coupled to generate heat.This method exploits the material's dielectric loss, causing an overall heating of the material to the sintering temperature and realizing densification.It transfers heat directly to the material through the coupling of electromagnetic waves, thus realizing volumetric heating in the microwave field, which improves the heating efficiency, and also has the feasibility of being used in the fabrication of lumped components compared with discharge plasma sintering [56].It is characterized by fast heating speed, high energy utilization, high heating efficiency, safety, hygiene without pollution, and the ability to improve the uniformity and yield of the product, as well as improve the microstructure and property of the sintered material.Microwave sintering is widely used in the ceramic industry due to its self-heating nature, which enables fast and efficient sintering [57].Due to the use of lower temperatures in microwave sintering, less grain growth or coarsening can be seen without the use of any commercial grain growth inhibitors, and due to the advantages of microwave sintering, it has become possible to fabricate hard materials at lower than usual temperatures [58].

Hot-Press Sintering
Hot-press sintering (HPS) is a useful method to synthesize low-temperature densification ceramics [59][60][61].It is a sintering technique in which the green body is heated and pressurized at the same time to promote sintering, thus eliminating the pores in the ceramic and realizing the high densification of the ceramic; an example of preparing SiC ceramics using hot-press sintering is shown in Figure 5. Hot-press sintering can only be used to prepare ceramic parts with simple shapes, but the ceramics have a high density and excellent high-temperature mechanical properties, which have great application prospects [55].
OR PEER REVIEW 7 of 24 commercial grain growth inhibitors, and due to the advantages of microwave sintering, it has become possible to fabricate hard materials at lower than usual temperatures [58].

Hot-Press Sintering
Hot-press sintering (HPS) is a useful method to synthesize low-temperature densification ceramics [59][60][61].It is a sintering technique in which the green body is heated and pressurized at the same time to promote sintering, thus eliminating the pores in the ceramic and realizing the high densification of the ceramic; an example of preparing SiC ceramics using hot-press sintering is shown in Figure 5. Hot-press sintering can only be used to prepare ceramic parts with simple shapes, but the ceramics have a high density and excellent high-temperature mechanical properties, which have great application prospects [55].

Printing Technology for PCFC
In recent years, with the rapid development of 3D printing technology, more and more manufacturing industries are beginning to apply it to the production process.3D

Printing Technology for PCFC
In recent years, with the rapid development of 3D printing technology, more and more manufacturing industries are beginning to apply it to the production process.3D printing technology is a kind of processing technology to make three-dimensional objects by stacking materials layer by layer.Its working principle is mainly applied through computer-aided design software to transform the three-dimensional model into a digital model, and then through the 3D printer control system to divide the digital model into a series of two-dimensional slices, and finally through the layer-by-layer stacking of materials using layer-by-layer printing; as a result, two-dimensional slices are gradually stacked into a three-dimensional object [62].3D printing technology can greatly shorten the product manufacturing cycle, and improve production efficiency and flexibility, to meet the needs of personalized and small batch production, and at the same time can achieve a very complex and detailed product design, thus bringing significant improvements in the performance and quality of products.Compared with traditional manufacturing methods, 3D printing technology has the advantages of low manufacturing costs, high production efficiency, and large degree of freedom of design.Its unique advantages are especially reflected in the manufacturing of complex parts; thus, some people even call it "the most iconic production tool of the third industrial revolution" [63].Tape casting, punching, screenprinting, laminating, and stacking are common procedures to fabricate planar configuration SOFC stacks, as shown in Figure 6a.The tubular structure SOFC stacks first prepare each single cell individually and then assemble all the cells into a stack, as shown in Figure 6b.3D printing technology can produce SOFC battery stacks in one step, as shown in Figure 6c   Reprinted with permission from Ref. [69].Copyright 2019 Elsevier B.V.  The main 3D printing technologies include laser-based printing technology and nonlaser-based printing technology.Laser-based printing technologies include light-curing printing, selective laser sintering (SLS), and laser 3D printing.In addition, light-curing printing includes stereolithography apparatus (SLA) and digital light processing (DLP).Non-laser-based printing technology includes inkjet printing and extrusion 3D printing.Extrusion 3D printing is also known as the extrusion free forming (EFF), layered extrusion molding, or direct ink writing (DIW) of paste materials.Comparison of 3D printing technology is shown in the Table 1.Light-curing printing belongs to a type of rapid prototyping technology, which can prepare structures with complex, small, and hollow structural items.At present, the lightcuring printing can be divided into stereolithography apparatus (SLA) and digital light processing (DLP) according to the different molding methods.
SLA is one of the more popular 3D printing technologies, which generally uses a ceramic paste of photosensitive resin and micro-and nano-ceramic powders mixed in a certain ratio.Then, a specific wavelength of ultraviolet light is used to solidify the paste from point to line, and from line to surface scanning.The paste in the light irradiation occurs after the curing bond.The light-curing printing process involves different methods, either from bottom to top or from top to bottom.Whenever the curing of the paste is completed, the printing platform is raised or lowered to a certain height according to the thickness of the layer, and the above operation is repeated until the complex structures of the ceramic parts are fully formed.Due to the particularly fine beam size used in SLA, the technology can achieve micron-level high-precision ceramic parts manufacturing.The formed blank has a very high density, effectively improving the mechanical properties of ceramic parts.This method is suitable for most ceramic powders, and has high versatility and uncomplicated processes, leading to a wide range of applications [63].Stereolithography apparatus creates 3D objects by selectively curing liquid resins via a photopolymerization reaction.Lightcuring rapid prototyping technology has attracted much attention due to its ability to produce high-precision objects and a wide variety of materials [70].Griffith et al. [71] prepared SiO 2 , Al 2 O 3 , and Si 3 N 4 powders with UV-curable solutions to form suspensions with a solid content ranging from 40% to 55%, and used them for ceramic body molding.This is the first report that stereolithography has been combined with the preparation of complex ceramic components.
The DLP process is the evolution of the SLA printing process.They have similar printing processes and mechanisms, with the main difference being that the light sources used in DLP are digital and directly used for surface molding, which greatly improves printing efficiency, as shown on Figure 7 [72].In recent years, DLP rapid prototyping has been widely used in the preparation of structural ceramics such as SiO 2 , Al 2 O 3 , and ZrO 2 [73][74][75][76][77][78], in which the high hardness, high strength and mechanical properties are focused [69,73,79,80].Wei et al. [69] looked at the fabrication of a dense 8YSZ electrolyte in a batch for SOFCs using the digital light stereolithography-based 3D printing technique.The SOFCs with the structure of Ag-GDC|YSZ|Ag-GDC showed good performance.They achieved an OCV of 1.04 V, and a maximum power density of 176 mW cm −2 with a hydrogen flow rate of 40 mL min −1 at 850 • C. Zhang et al. [81] successfully fabricated fully dense cube-shaped 8YSZ monoliths of tube bundles using digital light stereolithography 3D printing technology.The successful fabrication of complex-shaped 8YSZ monoliths via digital light stereolithography 3D printing provides a good example of how the 3D printing method can be applied to the energy field.
Mu et al.
[82] developed a new laser 3D printing (L3DP) method by integrating 3D printing and laser processing.The characteristics of laser 3D printing are the use of commercial raw materials, a small amount of binder, and the use of a CO 2 laser for rapid in situ drying [83].The in situ 3D printing laser sintering technique combines digital micro-extrusion-based 3D printing with precise and rapid laser processing, which includes sintering, drying, cutting, and polishing.The L3DP method enables the fabrication of protonic ceramic (PC) components into different shapes, including cylinders, cones, straight or leaf-like tubes with sealed ends, microchannel membranes, and half-cells for assembling PC energy devices.This L3DP technology not only demonstrates the potential to put PCs into practical use, but also enables the rapid direct digital fabrication of ceramic-based devices [84][85][86].By integrating 3D printing and laser processing (e.g., rapid drying, fast sintering, precise polishing, and accurate cutting), it enables the cost-effective material preparation and efficient attainment of well-defined shape and dimension-controlled uniform microstructures in porous anode supports, dense electrolytes, and porous cathodes [83].
Zou et al. [83] used the L3DP to prepare a scalable tubular PCFC.The preparation process is simple and fast.This study suggested that the L3DP technique can manufacture PCFCs with high-power output and long life spans, ushering in new possibilities for commercializing scalable tubular PCFCs.Figure 8 shows the process of manufacturing a complete tubular PCFC using L3DP.Figure 8b shows a large uniform green anode tube with an outer diameter of approximately 13.5 mm and a height of approximately 73 mm, with a tapered end.
process is simple and fast.This study suggested that the L3DP technique can manufacture PCFCs with high-power output and long life spans, ushering in new possibilities for commercializing scalable tubular PCFCs.Figure 8 shows the process of manufacturing a complete tubular PCFC using L3DP.Figure 8b shows a large uniform green anode tube with an outer diameter of approximately 13.5 mm and a height of approximately 73 mm, with a tapered end.Selective laser sintering (SLS) technology primarily utilizes a laser as the heat source.The ceramic powder is melted, bonded, and sintered under the laser irradiation to realize the ceramic powder melting, bonding, and sintering, layer by layer, to achieve three-dimensional solid molding manufacturing.SLS equipment mainly comprises a laser source, scanner, preheating device, control system, and other components.The process of this forming technology is as follows: first, a layer of heat-sensitive ceramic powder is spread on the forming platform.Then, the preheating device is heated to a temperature below the melting point of the powder.Next, the computer transmits the data of the CAD solid model to the control system.The control system then directs the laser to perform selective sintering based on the cross-section information of the model.Finally, the cured ceramic powder is stacked layer by layer to form the desired solid part by stacking one layer after another [87].There are two types of SLS processes: indirect selective laser sintering (iSLS) [88] and selective laser melting (SLM) [89], depending on whether a binder is used during manufacturing.The main advantage of selective laser sintering technology lies in the ability to print a variety of single materials and composite materials with complex structures, Selective laser sintering (SLS) technology primarily utilizes a laser as the heat source.The ceramic powder is melted, bonded, and sintered under the laser irradiation to realize the ceramic powder melting, bonding, and sintering, layer by layer, to achieve threedimensional solid molding manufacturing.SLS equipment mainly comprises a laser source, scanner, preheating device, control system, and other components.The process of this forming technology is as follows: first, a layer of heat-sensitive ceramic powder is spread on the forming platform.Then, the preheating device is heated to a temperature below the melting point of the powder.Next, the computer transmits the data of the CAD solid model to the control system.The control system then directs the laser to perform selective sintering based on the cross-section information of the model.Finally, the cured ceramic powder is stacked layer by layer to form the desired solid part by stacking one layer after another [87].There are two types of SLS processes: indirect selective laser sintering (iSLS) [88] and selective laser melting (SLM) [89], depending on whether a binder is used during manufacturing.The main advantage of selective laser sintering technology lies in the ability to print a variety of single materials and composite materials with complex structures, high precision, high strength, and good mechanical properties.However, there are some limitations, such as a relatively narrow range of material selection, particularly complex process, and high cost [63].

Non-Laser-Based Printing Technology
In the field of ceramics, inkjet printing is a common printing technology used in industrial production to date [72].As a non-contact and maskless manufacturing method, inkjet printing technology is mainly used for manufacturing by precisely positioning the print head and controllably ejecting and depositing the ink containing the desired material in the form of fine ink droplets onto a pre-determined substrate surface.3D printing manufacturing can be achieved by repeated deposition to achieve layer-by-layer cumulation.This technology has been used in the manufacturing of some precision ceramic parts [90], especially ceramics with thin layers.Inkjet printing has the advantages of low-cost, easy, fast, and precise manufacturing.Firstly, the printing can be carried out at atmospheric pressure, without the need for vacuum conditions, thus significantly saving production costs.Secondly, inkjet printing is environmentally friendly and avoids material waste.Finally, the technology exhibits a high degree of flexibility [91].Some inkjet printing techniques also offer the precise regulation of droplet volume and droplet number delivered per unit area of substrate, thus precisely controlling the thickness of the layer.To date, inkjet printing technology is widely used in solar cells [92,93], sensors [94,95], electronic circuits [96,97], and other fields.It is expected to be one of the attractive alternative methods to fabricate thin layer components such as SOFC electrodes.
Inkjet printing technology prepares ceramic components, such as porous or dense electrodes and electrolyte thin layers, by employing ceramic nanopowder suspensions as inks to be sprayed drop by drop onto the surface of the substrate for spreading and fusing and stacking into a thin layer.Post-processing is also required, using drying and sintering methods.In recent years, there are also some scholars combining inkjet printing and other processes to prepare composite ceramic electrode components with good performance.One of the most important factors in realizing high-quality inkjet printing is the preparation of ceramic inks with matching printing requirements, which generally requires good stability, dispersion, and homogeneity.The composite also needs to be able to have a flat surface, uniform structure, and material distribution after post-treatment, using drying and sintering processes, in order to meet the requirements for SOFC usage [98].Han et al. [99] proposed a method to manufacture an entire SOFC using a low-cost commercial inkjet printer, as shown in Figure 9.
[90], especially ceramics with thin layers.Inkjet printing has the advantages of easy, fast, and precise manufacturing.Firstly, the printing can be carried out pheric pressure, without the need for vacuum conditions, thus significantly sa duction costs.Secondly, inkjet printing is environmentally friendly and avoids waste.Finally, the technology exhibits a high degree of flexibility [91].Some inkje techniques also offer the precise regulation of droplet volume and droplet numb ered per unit area of substrate, thus precisely controlling the thickness of the date, inkjet printing technology is widely used in solar cells [92,93], sensors [94 tronic circuits [96,97], and other fields.It is expected to be one of the attractive al methods to fabricate thin layer components such as SOFC electrodes.
Inkjet printing technology prepares ceramic components, such as porous electrodes and electrolyte thin layers, by employing ceramic nanopowder suspe inks to be sprayed drop by drop onto the surface of the substrate for spreading a and stacking into a thin layer.Post-processing is also required, using drying and methods.In recent years, there are also some scholars combining inkjet printing a processes to prepare composite ceramic electrode components with good perf One of the most important factors in realizing high-quality inkjet printing is the tion of ceramic inks with matching printing requirements, which generally requ stability, dispersion, and homogeneity.The composite also needs to be able to h surface, uniform structure, and material distribution after post-treatment, usin and sintering processes, in order to meet the requirements for SOFC usage [98].H [99] proposed a method to manufacture an entire SOFC using a low-cost commer printer, as shown in Figure 9.  Extrusion 3D printing is centered on the idea of paste extrusion molding, in which solids are formed from an extruded paste stacked on a printing platform, also known as the extrusion free forming (EFF), layered extrusion molding, or direct ink writing (DIW) of paste materials.Extrusion 3D printing methods can be considered as an extension of traditional inkjet printing techniques and can be used as a substrate-friendly method for printing unique microstructures for electrochemical energy storage devices [91].Its advantages are that the process and equipment are simple and easy to implement; it does not require the high energy output consumption that laser printing requires, thus consuming less energy and lower costs.However, the problems, such as lower molding accuracy and a slurry that is not easy to be preserved, still need to be improved and solved [100].

Other Advanced Manufacturing Methods
Artem Tarutin et al. [39] performed the one-step fabrication of protonic ceramic fuel cells using a convenient tape calendering method.The success of this fabrication approach is due to two main factors: the rational choice of chemically and mechanically compatible components, as well as the selection of a convenient preparation (tape calendering) method.Figure 10 shows the preparation process of fabricating PCFCs using this method.Firstly, four separate powders for the corresponding functional layers were prepared, and then a mixture of the corresponding powders and an organic binder was prepared.Then, the mixtures were dried overnight to evaporate and remove the solvent.The dried residues were rolled to fabricate the corresponding film layers with the required thicknesses, and finally, sintering was performed.
suming less energy and lower costs.However, the problems, such as lower molding accuracy and a slurry that is not easy to be preserved, still need to be improved and solved [100].

Other Advanced Manufacturing Methods
Artem Tarutin et al. [39] performed the one-step fabrication of protonic ceramic fuel cells using a convenient tape calendering method.The success of this fabrication approach is due to two main factors: the rational choice of chemically and mechanically compatible components, as well as the selection of a convenient preparation (tape calendering) method.Figure 10 shows the preparation process of fabricating PCFCs using this method.Firstly, four separate powders for the corresponding functional layers were prepared, and then a mixture of the corresponding powders and an organic binder was prepared.Then, the mixtures were dried overnight to evaporate and remove the solvent.The dried residues were rolled to fabricate the corresponding film layers with the required thicknesses, and finally, sintering was performed.

Microstructure of PCFCs
Xu et al. [101] prepared proton-conducting electrolyte membranes for solid oxide fuel cells using the microwave sintering strategy.The preparation of a dense proton-conducting BaCe0.7Zr0.1Y0.2O3-δ(BCZY) electrolyte membrane can be prepared at 1200 °C using the

Microstructure of PCFCs
Xu et al. [101] prepared proton-conducting electrolyte membranes for solid oxide fuel cells using the microwave sintering strategy.The preparation of a dense protonconducting BaCe 0.7 Zr 0.1 Y 0.2 O 3−δ (BCZY) electrolyte membrane can be prepared at 1200 • C using the microwave sintering method.Figure 11a-d compares the BCZY electrolyte membranes sintered in an traditional electric furnace and microwave furnace.The presence of apparent pores were observed in the BCZY electrolyte membrane that was sintered using a traditional electric furnace.In contrast, the BCZY membrane sintered in the microwave furnace appears dense after sintering at the same temperature of 1200 • C.This suggests that microwave heating is advantageous for the formation of dense protonconducting membranes and effectively reduces the densification temperature of BCZY electrolyte materials.
Tong et al. membranes.This simple and cost-effective solid-state reactive sintering (SSRS) method involved only a single high-temperature sintering step.Figure 11e provides the effect of the sintering aid type on the BZY20 pellet morphology.The most dramatic changes were observed for the 1 wt.%NiO-modified BZY20 pellet, which exhibited a non-porous crosssection with equiaxed grains as large as 5 µm.Importantly, the large grain size and high density of this pellet should bode well for its protonic conductivity and use in applications related to functional protonic ceramics.

als.
Tong et al. [51] used cost-effective precursors of BaCO3, ZrO2, and Y2O3 to prepare proton-conducting ceramic pellets of BaZr0.8Y0.2-O3−δ(BZY20).A range of sintering aids, including LiF, NiO, Al2O3, and SnO2, were used to help the densification of BZY20 membranes.This simple and cost-effective solid-state reactive sintering (SSRS) method involved only a single high-temperature sintering step.Figure 11e provides the effect of the sintering aid type on the BZY20 pellet morphology.The most dramatic changes were observed for the 1 wt.%NiO-modified BZY20 pellet, which exhibited a non-porous crosssection with equiaxed grains as large as 5 µm.Importantly, the large grain size and high density of this pellet should bode well for its protonic conductivity and use in applications related to functional protonic ceramics.Han et al. [99] proposed a method for manufacturing an entire SOFC using a lowcost commercial inkjet printer.In Figure 12a-c, a fully dense YSZ electrolyte with a very small thickness of approximately 0.8 µm was successfully formed on the Ni-YSZ anode functional layer (AFL).The thin YSZ electrolyte formed after two printing scan cycles maintained its gastight and uniform microstructure even after the anode reduction process at 600 °C, following two printing scan cycles.As depicted in Figure 12d, the surface morphology of the YSZ electrolyte exhibited a tightly bound grain structure, clearly indicating that inkjet printing can be successfully used to form fully dense and extremely thin electrolytes at submicron levels.Han et al. [99] proposed a method for manufacturing an entire SOFC using a low-cost commercial inkjet printer.In Figure 12a-c, a fully dense YSZ electrolyte with a very small thickness of approximately 0.8 µm was successfully formed on the Ni-YSZ anode functional layer (AFL).The thin YSZ electrolyte formed after two printing scan cycles maintained its gastight and uniform microstructure even after the anode reduction process at 600 • C, following two printing scan cycles.As depicted in Figure 12d, the surface morphology of the YSZ electrolyte exhibited a tightly bound grain structure, clearly indicating that inkjet printing can be successfully used to form fully dense and extremely thin electrolytes at submicron levels.
Zhang et al. [102] prepared 8 mol% yttria-stabilized zirconia (8YSZ) using DLP equipment.Figure 13a-c depict the green body prepared from the 30 vol% suspension via digital light 3D printing after degreasing under air, N 2 , and vacuum conditions.There are visible cracks in the green body degreased in air or N 2 , whereas there are no cracks in the green body degreased under vacuum conditions.The sintered compact forms are shown in Figure 13d.Figure 13(e11,e3) show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths prepared from the 30 vol% suspension using digital light 3D printing.There are obvious cracks in the green body degreased in air or N 2 , while there are no cracks in the green body degreased under vacuum conditions.Figure 13(e11,e3) show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths.Fully dense cube-shaped 8YSZ monoliths were successfully fabricated using digital light stereolithography 3D printing technology [102].Zhang et al. [102] prepared 8 mol% yttria-stabilized zirconia (8YSZ) using DLP equipment.Figure 13a-c depict the green body prepared from the 30 vol% suspension via digital light 3D printing after degreasing under air, N2, and vacuum conditions.There are visible cracks in the green body degreased in air or N2, whereas there are no cracks in the green body degreased under vacuum conditions.The sintered compact forms are shown in Figure 13d.Figure 13(e11,e3) show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths prepared from the 30 vol% suspension using digital light 3D printing.There are obvious cracks in the green body degreased in air or N2, while there are no cracks in the green body degreased under vacuum conditions.Figure 13(e11) and 13e3 show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths.Fully dense cube-shaped 8YSZ monoliths were successfully fabricated using digital light stereolithography 3D printing technology [102].Zhang et al. [102] prepared 8 mol% yttria-stabilized zirconia (8YSZ) using DLP equipment.Figure 13a-c depict the green body prepared from the 30 vol% suspension via digital light 3D printing after degreasing under air, N2, and vacuum conditions.There are visible cracks in the green body degreased in air or N2, whereas there are no cracks in the green body degreased under vacuum conditions.The sintered compact forms are shown in Figure 13d.Figure 13(e11,e3) show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths prepared from the 30 vol% suspension using digital light 3D printing.There are obvious cracks in the green body degreased in air or N2, while there are no cracks in the green body degreased under vacuum conditions.Figure 13(e11) and 13e3 show SEM images of the surface and cross-section, respectively, of the sintered 8YSZ monoliths.Fully dense cube-shaped 8YSZ monoliths were successfully fabricated using digital light stereolithography 3D printing technology [102].14a-d.The porous anode support exhibits a uniform and defect-free microstructure with a pore size of several micrometers.The electrolyte is uniform and dense, without visible pinholes or cracks, and adheres well to the porous electrode without exfoliation.The cathode has a fine porosity and nanoscale grains, which facilitate rapid gas transport and a large number of three-phase boundary positions, resulting in excellent electrochemical performance [83].
size of several micrometers.The electrolyte is uniform and dense, without visible pinholes or cracks, and adheres well to the porous electrode without exfoliation.The cathode has a fine porosity and nanoscale grains, which facilitate rapid gas transport and a large number of three-phase boundary positions, resulting in excellent electrochemical performance [83].

Crystal Structure and Performance
Ullah et al. [103] synthesized nanocomposite electrolytes using a microwave sintering technique instead of the conventional sintering method.Through an X-ray diffraction (XRD) analysis, it was indicated that the material was crystalline, exhibiting only one CeO2 phase.The crystallite sizes of MW-SDC (the microwave sintering technique was applied to synthesis of an SDC electrolyte called microwave-SDC) and CON-SDC (a conventional sintered electrolyte labelled CON-SDC) were calculated using Scherrer's formula, and the relative densities of MW-SDC and CON-SDC were also determined to be 96% and 90%, respectively; the relative densities were also calculated from XRD data.Therefore, the major advantage of the microwave sintering is the ability to achieve highly dense materials [104].
Nikodemski et al. [47] elucidated the solid-state reactive sintering (SSRS) mechanism by systematically studying the effects of a series of metal oxide sintering additives on the

Crystal Structure and Performance
Ullah et al. [103] synthesized nanocomposite electrolytes using a microwave sintering technique instead of the conventional sintering method.Through an X-ray diffraction (XRD) analysis, it was indicated that the material was crystalline, exhibiting only one CeO 2 phase.The crystallite sizes of MW-SDC (the microwave sintering technique was applied to synthesis of an SDC electrolyte called microwave-SDC) and CON-SDC (a conventional sintered electrolyte labelled CON-SDC) were calculated using Scherrer's formula, and the relative densities of MW-SDC and CON-SDC were also determined to be 96% and 90%, respectively; the relative densities were also calculated from XRD data.Therefore, the major advantage of the microwave sintering is the ability to achieve highly dense materials [104].
Nikodemski et al. [47] elucidated the solid-state reactive sintering (SSRS) mechanism by systematically studying the effects of a series of metal oxide sintering additives on the phase formation and densification of proton-conducting ceramics.An XRD analysis indicated that the BCZY63 pellets sintered with the assistance of certain sintering additives showing, exhibiting an almost pure cubic perovskite phase of BCZY63 without any harmful second phase.
The Summary of XRD and SEM testing analysis is shown in the Table 2.

SSRS
At a certain sintering temperature and sintering aids, the porosity of the electrolyte is small and the degree of densification is high [105].
Under a certain sintering temperature and with sintering additives, the phase was pure In Ramos [106] et al.'s study, powders were prepared by solidstate reaction through three procedures, and it was possible to obtain dense pellets of BaCe 0.9 Y 0.1 O 3−δ at sintering temperature as low as 1200 • C without sintering aid.

Microwave Sintering
Liu et al. [57] prepared BSCF cathodes for H-SOFC applications using microwave sintering.It allowed the BSCF cathode to adhere well to the electrolyte without destroying its microstructure, and the low sintering temperature also mitigated the Ba interdiffusion.

Verification of phase structure
Li et al. [107] found that the microwave sintering process significantly improved the densification behavior of ZnO varistors.

Stereolithography Apparatus
Under certain printing mechanisms, the samples have a good microstructure with a high degree of densification and uniform distribution.

Verification of phase structure
Suitable for parts with complex shapes, good surface quality, relatively smooth, suitable for fine parts, can show the best details, ideal for small parts, the equipment is integrated and relatively easy to operate.

Inkjet Printing
The surface microstructure quality is better when dry sintering under certain printing parameters, and the number and size of cracks are less, which has less of an effect on the electrode performance.The pore size and distribution are also more uniform.

Verification of phase structure
The advantages of low-cost, easy, fast, and precise fabrication, which can be achieved using repeated deposition for layer-by-layer cumulative 3D printing fabrication.This is currently used for the preparation of some precision ceramic artifacts, especially thin layers of ceramics [98].

Laser 3D Printing
Under certain printing parameters and sintering conditions, the sintered layer is well-bonded and a fully densified electrolyte film with a large grain size can be obtained.
Certain conditions give the desired crystal structure.
Fabrication of sintered plasmonic ceramic components for use in mesothermal plasmonic ceramic devices with a variety of complex geometries and controlled microstructures [82].

A single Sintering Step
The samples prepared by Tarutin et al. [39] using this method exhibited basic properties including the dense state of the electrolyte and the porous structure of the functional cathode and anode.

Verification of the phase structure of electrolytes and electrodes
It achieves the required morphological properties-full densification of the electrolyte and sufficient electrode porosity [39].

Electrochemical Properties
Ng et al. [108] investigated the effect of microwave sintering on the properties of 1 mol% ceria-doped scandia stabilized zirconia.This study has demonstrated the beneficial effect of microwave sintering in promoting the densification of 10Sc1CeSZ at lower temperatures and with a short sintering time.The sintered bodies show high ionic conductivity without sacrificing the cubic phase and mechanical properties for SOFC application.
Artem Tarutiet al. [39] reported a one-step fabrication of PCFCs using a convenient tape calendering method.The electrochemical impedance spectroscopy (EIS) results presented in Figure 15a,b indicate that the spectra have different shapes depending on the measurement temperatures and/or cell bias.In detail, at least three parts (described by RQ elements, where R is the resistance, Q is the constant phase element) can be clearly distinguished at low temperatures; these merge to enable the detection of at least two (low-and highfrequency) processes at higher temperatures.Kang et al. [109] successfully synthesized high performance components for solid oxide fuel cells and proton ceramic fuel cells by inkjet printing.Figure 16a-c shows that SOFCs with inkjet-printed LSCF and LSCF/GDC composite cathodes exhibit the maximum power of 95-274 mW cm −2 and 98-328 mW cm −2 at 550-650 °C, respectively.PCFCs with inkjet-printed PBSCF cathodes produced up to 430-720 mW cm −2 at 550-650 °C, respectively.This enhancement is due to the high ionic conductivity of proton-conducting materials compared to oxide-ion conductors.The results indicate that inkjet printing is effective for the manufacturing of PCFC.
Tong et al. [51] evaluated the NiO-modified BZY 20 material using the DC four-point probe technique in both dry and wet argon gases, showing that the material has a fairly high total conductivity, as shown in Figure 16d.Although the tubular PCFCs prepared by Zou et al. using L3DP have a larger effective area, they exhibit an area-specific resistance comparable to that of tubular PCFCs manufactured using state-of-the-art traditional methods.The prepared single tubular cell exhibits excellent electrochemical performance, with an effective area of 12.5 cm 2 and a 2.45 W at 650 • C, as shown in Figure 15c.To evaluate the long-term stability of the manufactured tubular PCFCs, we tested a single battery for 200 h at a current of 200 mA cm −2 at 650 • C, as shown in Figure 15d.During the first 15 h of operation, the power density and cell terminal voltage slightly dropped (~4.3%) and in the remaining time thereafter, the degradation rate was 0.00039 V h −1 , which can be negligible, confirming its long-term stability [83].
Kang et al. [109] successfully synthesized high performance components for solid oxide fuel cells and proton ceramic fuel cells by inkjet printing.Figure 16a-c shows that SOFCs with inkjet-printed LSCF and LSCF/GDC composite cathodes exhibit the maximum power of 95-274 mW cm −2 and 98-328 mW cm −2 at 550-650 • C, respectively.PCFCs with inkjet-printed PBSCF cathodes produced up to 430-720 mW cm −2 at 550-650 • C, respectively.This enhancement is due to the high ionic conductivity of proton-conducting materials compared to oxide-ion conductors.The results indicate that inkjet printing is effective for the manufacturing of PCFC.
Tong et al. [51] evaluated the NiO-modified BZY 20 material using the DC four-point probe technique in both dry and wet argon gases, showing that the material has a fairly high total conductivity, as shown in Figure 16d.
The summary of electrochemical properties is shown in the Table 3, and comparison of electrical properties of devices with different preparation methods is shown in the Table 4.
spectively.This enhancement is due to the high ionic conductivity of proton-conducting materials compared to oxide-ion conductors.The results indicate that inkjet printing is effective for the manufacturing of PCFC.
Tong et al. [51] evaluated the NiO-modified BZY 20 material using the DC four-point probe technique in both dry and wet argon gases, showing that the material has a fairly high total conductivity, as shown in Figure 16d.

SSRS
The simplicity of fabrication provided by SSRS greatly enhances the potential for The deployment of deploying proton-conducting ceramics in various electrochemical devices [47].

Microwave Sintering
Li et al. [107] found that the microwave sintering enhanced the electrical properties of ZnO varistors.Liu et al. [57] proposed that batteries with microwave-sintered cathodes exhibit significantly better battery performance than those using conventionally sintered cathodes.Wang et al. [110] found that microwave sintering improved fuel cell performance.
Stereolithography Apparatus E. M. et al. [3] showed that the fabrication process does not have any adverse effect on the electrical properties of structured materials.

Inkjet Printing
Many micropores and small cracks were formed in the cathode layer prepared under certain printing parameters, forming a porous, homogeneous structure with good interfacial bonding, which promotes oxygen penetration and increases the specific surface area to participate in the electrochemical ORR, which is favorable to the electrochemical performance [111].And when the effective connection of the cathode layer is maintained, the oxygen ion conductivity is improved to enhance the efficiency of the cathode reduction reaction, which in turn improves the electrochemical performance output of the battery [98].

Laser 3D Printing
Mu et al. [23] conducted the electrochemical impedance measurement of the fully dense strips and showed promising protonic conductivities.
A Single Sintering Step Artem Tarutin et al. [39] performed the one-step fabrication of protonic ceramic fuel cells using a convenient tape calendering method.The PCFC fabricated in this way demonstrated both gas-tightness (1.03 V at 600 • C in the current-free mode) and high performance (~400 mW cm −2 at 600 • C).

Future Directions
The current challenges faced by traditional solid oxide fuel cells have prompted researchers to consider proton-conducting solid oxide fuel cells as a feasible solution for achieving cleaner, more efficient, and more economical energy alternatives.The structure of the PCFC directly affects its performance, so the manufacturing technology is of vital significance to PCFC preparation methods.The future development direction of the PCFC preparation process will mainly focus on the following aspects: (1) At present, the problems existing in the preparation of proton conductor ceramic energy-integrated devices mainly stem from the characteristics of proton ceramics themselves, resulting in a large amount of energy consumption, high waste rate, low energy density, poor performance, and other problems in the actual preparation process.Therefore, it is necessary to continuously develop new preparation processes for ceramic energy equipment to achieve high-performance proton ceramic electrochemical devices.(2) Nowadays, although many new technologies have been used for the processing of PCFCs, most of them do not have the processing ability for complex and fine structures, which limits the development limit of equipment.Therefore, it is necessary to develop new technologies for ceramic energy equipment, achieve cost-effective requirements, and quickly prepare proton ceramic electrochemical devices with controllable structures, high energy density, and excellent performance, which become the key to the widespread application of such materials and equipment.(3) At present, most of the characterized superior properties of medium temperature proton ceramic energy devices come from small structures, which cannot meet the actual needs of large energy density devices.This is because the operating temperature range of the device imposes demanding requirements on the sealing, operation, and long-term stability of the cell stack.How to design and manufacture proton conductor ceramic devices with high integration, flexible and controllable structures, and meet different test requirements has become the direction of researchers' efforts.

Figure 1 .
Figure 1.Components and working principles of an oxygen ionic SOFC.Layers from up to down:

Figure 1 .
Figure 1.Components and working principles of an oxygen ionic SOFC.Layers from up to down: cathode, electrolyte, anode.Reprinted with permission from Ref. [36].Copyright 2018 WIREs Energy and Environment.

Figure 2 .
Figure 2. Schematic representation of a PCFC structure.Reprinted with permission from Ref. [43].Copyright 2022 Royal Society of Chemistry.

Figure 2 .
Figure 2. Schematic representation of a PCFC structure.Reprinted with permission from Ref. [43].Copyright 2022 Royal Society of Chemistry.

Crystals 2024 , 24 Figure 3 .
Figure 3. Proton-conducting ceramic fuel cell stack: (a) schematic; (b) unit-cell stack with thin metallic interconnects and internal voltage taps at key interfaces.Note that V5 is connected to the surface of the cermet anode support, while V6 is connected to the outside of the anode endplate.(c) Photo of three-cell stack.Reprinted with permission from Ref. [44].Copyright 2021 Elsevier B.V.

Figure 3 .
Figure 3. Proton-conducting ceramic fuel cell stack: (a) schematic; (b) unit-cell stack with thin metallic interconnects and internal voltage taps at key interfaces.Note that V5 is connected to the surface of the cermet anode support, while V6 is connected to the outside of the anode endplate.(c) Photo of three-cell stack.Reprinted with permission from Ref. [44].Copyright 2021 Elsevier B.V.

Figure 5 .
Figure 5. Process flow of SiC ceramics prepared by hot-press sintering .

Figure 5 .
Figure 5. Process flow of SiC ceramics prepared by hot-press sintering.

Figure 8 .
Figure 8. Schematic illustration of the manufacturing of a single tubular PCFC by 3DP.(a) Schematic of the manufacturing process.(b) Photographs of the tubular PCFC at different manufacturing steps.Reprinted with permission from Ref. [83].Copyright 2023 American Chemical Society.

Figure 8 .
Figure 8. Schematic illustration of the manufacturing of a single tubular PCFC by 3DP.(a) Schematic of the manufacturing process.(b) Photographs of the tubular PCFC at different manufacturing steps.Reprinted with permission from Ref. [83].Copyright 2023 American Chemical Society.

Figure 12 .
Figure 12.Microstructures of the inkjet-printed SOFC composed of NiO−YSZ|YSZ|GDC|PBS CF.(a) Cross-sectional SEM image of the anode-supported SOFC and schematic of the inkjet printing (scale bar: 10 µm).(b) Cross-sectional SEM image of the SOFC with the approximat ely 0.5 µm thick GDC buffer layer (scale bar: 5 µm).(c) Cross-sectional and (d) surface SEM images of the SOFC with the approximately 0.8 µm thick YSZ electrolyte layer (scale bar: 2.5 µm).Reprinted with permission from Ref. [99].Copyright 2020 American Chemical Society.

Figure 12 .
Figure 12.Microstructures of the inkjet-printed SOFC composed of NiO−YSZ|YSZ|GDC|PBS CF.(a) Cross-sectional SEM image of the anode-supported SOFC and schematic of the inkjet printing (scale bar: 10 µm).(b) Cross-sectional SEM image of the SOFC with the approximat ely 0.5 µm thick GDC buffer layer (scale bar: 5 µm).(c) Cross-sectional and (d) surface SEM images of the SOFC with the approximately 0.8 µm thick YSZ electrolyte layer (scale bar: 2.5 µm).Reprinted with permission from Ref. [99].Copyright 2020 American Chemical Society.

Figure 13 .
Figure 13.Body after debinding in (a1,a2) air, (b1,b2) N 2 , and (c1,c2) under vacuum conditions; (d1-d3) body after sintering; (e1,e11,e2,e21,e22,e3) microstructure of 8YSZ monoliths after sintering.Reprinted with permission from Ref. [102].Copyright 2020 Elsevier B.V. The single tube PCFC prepared using in situ 3D printing laser rapid reaction sintering technology displays a tubular BaCe 0.2 Zr 0.7 Y 0.1 O 3−δ (BCZY27)-Ni comprising an anode support, a dense BCZY27 electrolyte film, and a porous cathode, BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3−δ (BCFZY0.1).The well-controlled microstructure of the PCFC is shown in Figure14a-d.The porous anode support exhibits a uniform and defect-free microstructure with a pore size of several micrometers.The electrolyte is uniform and dense, without visible pinholes or cracks, and adheres well to the porous electrode without exfoliation.The cathode has a fine porosity and nanoscale grains, which facilitate rapid gas transport and a large number of three-phase boundary positions, resulting in excellent electrochemical performance[83].

s 24 Figure 15 .
Figure 15.Typical impedance spectra of the fabricated PCFC at different temperatures under conditions corresponding to open circuit voltage (OCV) (a) and Pmax (b).Reprinted from Ref.[39].(c) Power output of single tube PCFCs prepared by different methods at 650 °C; (d) long-term stability of tube PCFCs at a constant current density of 200 mA cm −2 at 650 °C.Reprinted with permission from Ref. [83].Copyright 2023 American Chemical Society.

Figure 15 .
Figure 15.Typical impedance spectra of the fabricated PCFC at different temperatures under conditions corresponding to open circuit voltage (OCV) (a) and Pmax (b).Reprinted from Ref. [39].(c) Power output of single tube PCFCs prepared by different methods at 650 • C; (d) long-term stability of tube PCFCs at a constant current density of 200 mA cm −2 at 650 • C. Reprinted with permission from Ref. [83].Copyright 2023 American Chemical Society.

Table 1 .
Comparison of 3D printing technology.
Precise and fast processing (sintering, cutting, polishing, melting, welding)Thermal stress residual and pre-processing required

Table 1 .
Comparison of 3D printing technology.

Table 2 .
Summary of XRD and SEM testing analysis.

Table 3 .
Summary of electrochemical properties.

Table 4 .
Comparison of electrical properties of devices with different preparation methods.