Recent advancement of solid oxide fuel cells towards semiconductor membrane fuel cells

SOFCs: conventional electrolyte to semiconductor membranes

SOFCs are classified into three kinds based on cell operating temperatures. Firstly, high-temperature SOFCs (HT-SOFCs) were developed in the initial development that worked at high temperatures in the range of 900-1,000 °C. A high operating temperature is required to ensure adequate ionic conductivity. On the one hand, a high working temperature improves the reaction rates and charge conduction; on the other hand, it possesses critical hurdles for practical applications such as high material cost, material degradation, and other challenges for material compatibility. Therefore, after so much effort, intermediate temperature SOFCs (IT-SOFCs) were developed to reduce the operating temperature, which can work at a temperature range from 650 to 800 °C. Moreover, nanostructured materials are currently well-preferred, particularly at low temperatures, with an operating temperature range of up to 650 °C. These are easy to operate, reduce the cell fabrication cost by using the lower-cost materials, and have long-term durability. Among growing research fields, low-temperature SOFCs (LT-SOFCs) with high conversion efficiency are the need of the present era.

To work effectively at low temperatures, the development of critical components of the cell (cathodes, anodes, and electrolytes) and an in-depth understanding of materials science to obtain desired materials having exceptional properties, such as desired electrical conductivity, electrocatalytic activity, thermal stability, and chemical compatibility, are needs. In order to achieve the excellent performance of SOFC devices, there is a necessity for more investigations into versatile materials and their combinations to accomplish the materials requirements of SOFCs. Moreover, the development of SOFC devices for practical deployment is associated with tremendous technological challenges in energy material research, for example, material selection challenges in terms of cost, operating temperatures, and fabrication procedures.

As the core component of the SOFCs, the electrolyte plays an essential part in the performance of single cells since it decides the running temperature of the fuel cell and the related other components of the cell. Materials used as electrolytes in SOFCs could be an oxide ion conductor, a proton ion conductor, or a dual ion conductor. The characteristics of applied electrolyte materials decide the electrode reaction and performance of each cell configuration. In a SOFC, the primary role of a solid electrolyte is to conduct ions, either O^2- or H⁺ (nature of the specific material), between the electrodes in order to avoid blocking electrons for internal conduction, driving the electrons to move through the outside external circuit to complete the overall electrochemical reaction. Materials used as electrolytes in SOFCs must have high ionic conductivity, chemically, thermally, and mechanically companionable with the adjoining cell components, highly dense microstructure, and low electronic conductivity to avoid internal short-circuits during the electrochemical reaction^[4-6].

In the last few decades, SOFC electrolyte materials have received significant attention from materials researchers and scientists in order to achieve good SOFC performance. Despite the rigorous necessity for electrolyte materials, the practical application of limited ionic conducting electrolyte materials has been a challenge. Various ionic conductors have been explored as electrolyte materials^[7-10], such as commonly used doped CeO₂/Bi₂O₃/ZrO₂ and LaGaO₃-based perovskite oxide and newly developed Ca₁₂Al₁₄O₃₃^[8],Ln₁₀(SiO₄)₆O₃ [Ln = La, Nd, Gadolinium (Gd), Samarium (Sm), and Dy]^[9], Na_0.5Bi_0.5TiO₃ (NBT) and La₂Mo₂O₉-based fast oxide ion conductors^[10]. Among these materials, oxygen (O₂) ionic conductors of the acceptor-doped ceria, yttrium stabilized zirconia, doped LaGaO₃-based oxides, and proton conducting materials of barium zirconate and barium cerate have been the selected for SOFCs industrial and academic utilization. Yttrium-stabilized zirconia (YSZ) is the most common choice due to its high ionic conduction property at elevated operating temperatures (≥ 700 °C) and its capability to work effectively across a wide range of oxygen partial pressure. Still, for low-temperature operations, its ionic conductivity is not sufficient. The currently extensively implemented electrolyte materials, such as acceptor doped ceria and doped LaGaO₃-based fast ionic conductor oxide, always undergo fundamental problems, such as the short circuit inside the system due to reduction in Ce⁴⁺ to Ce³⁺ and extreme chemical reactivity with the other common electrode materials for LaGaO₃-based electrolyte materials. Other recently developed electrolytes, such as rare-earth-doped ortho-niobates and tantalates^[11], NBT^[12] and Sr_1-xNa_xSiO_3-0.5x^[13], still require more in-depth studies before being widely used akin to the above-stated electrolyte materials.

Worldwide, researchers and scientists put their efforts into reducing the operating temperature of SOFCs. The reduced operating temperature permits prolonged material and device lifespan and facilitates less investment cost. These are the two foremost concerns for product consumption in the market. The operating temperature reduction of electrolytes from a high-temperature range (800-1,000 °C) to a low to intermediate temperature range (300-700 °C) requires significantly less ohmic resistance. Two major approaches are implemented. In order to develop new electrolyte materials with high ionic conductivity, reducing the electrolyte thickness from its current micrometer scale to the nanometer scale is crucial. This reduction is necessary because, according to the ohmic laws, the ohmic resistance of electrolytes is directly related to the electrolyte thickness. Few interesting phenomena are observed when the thin film is reduced to in the range of nanometers, such as distinct improvement/diminution of the charge conduction transition such as electronic-to-ionic conduction or ionic-to-electronic conduction or proton charge conduction in the common oxide ion conductor in the range of low operating temperatures. For that reason, an alternative technique is dedicated to designing nanograin electrolytes with superionic charge conduction in a composite interface along the grain boundary (GB) and interface. Over the past few years, nanocomposite materials for advanced fuel cells have initiated an emergent Research & Development and global academia surge^[14]. Invented from this approach, fast ion conducting nanocomposite electrolyte materials, such as core-shell and nanowire structural ceria-based nanocomposites, have exceptional ionic conductivity in temperatures over 300 °C in the order of 0.1 S cm^-1 and a distinctive property of simultaneous O^2-/H⁺ charge transfer has been developed for low temperatures in the range of 300-600 °C. Many investigations have proven that ceria-carbonate-based nanocomposite materials have the benefit of multi-ion charge conduction, and outstanding performances in LT-SOFCs enable them to be superior electrolytes for SOFC applications. The favorable characteristics of these proposed composite materials also provide them with possible applications in the safety of the environment, for example, the separation of CO₂^[15,16].

When protons pass through the electrolyte instead of oxide ions, the SOFC is also called protonic ceramic fuel cells (PCFCs). Compared to SOFCs, PCFCs may operate at reduced temperatures as diffusion of protons requires lower conducting activation energy (0.3-0.5 eV) than that of oxygen ions. Additionally, most proton-conductive ceramics have very high ion conductivity at a lower temperature than oxygen-conductive ceramics. Furthermore, PCFCs show relatively high efficiency and working performance in a low operating temperature range (600 °C or less). Another advantage of PCFCs is that the water, which is generated at cathodes as a by-product of the reaction, prevents fuel dilution in the anode.

Semiconductor membrane fuel cell: a novel concept of SOFCs

Recently, a major revolution in the development of SOFCs devices has been informed, i.e., single-layer/component or electrolyte-free fuel cell (SLFC or EFFC) and double layer fuel cell (DLFC)^[17,18]. Figure 1A illustrates the conventional three-layer configuration of a SOFC with oxygen ion conducting electrolyte without electron passage. Here, the H₂ fuel is oxidized, and electrons are released at the anode side. While oxygen (O₂) is reduced to oxygen ions (O^2-) at the cathode side, which combine with electrons from external circuit to produce electricity. Interestingly, the double layer fuel cell (DLFC) device (as shown in Figure 1B) can perform even better than the electrolyte based SOFCs. As a result, DLFC demonstrates great potential for further advancement. The SCFCs on the other hand, as shown in Figure 1C, are consist of simply only one component having neither electrolyte membrane or electrodes (cathode and anode); however, they offer comparable, even superior performance than the traditional three-component fuel cells. The simple structure having a “three in one” system allows promoting their cost-effectiveness, which further enables them to exhibit a great potential to accelerate fuel cell commercialization. The vital part of SCFC or EFFC is the co-existence of semiconductors and ion conductors. These structures help in avoiding the internal short-circuiting problem and realize fuel cell nano-redox and processes at the nanoscale or at the particle level, as shown in Figure 1D.

Figure 1. Schematic representation of different kinds of fuel cells: (A) conventional solid oxide fuel cells (SOFCs). (B) double-layer fuel cells (DLFCs), (C) single component fuel cells (SCFCs), (D) SCFC–nano redox fuel cells, (E) band alignment for SCFCs. Adapted and modified from Ref.^[32].

Another significant area of research interest is the interfaces developed among semiconductor-ionic, semiconductor-insulator, semiconductor-semiconductor, or oxides, known as Semiconductor membrane/Semiconductor-ionic membrane fuel cells, which influences the variation in bandgap and charge conduction properties. The primary interest in these fuel cells is to lead to changes of ionic transport properties in the form of interfacial or surface electronic band and state changes. It has been discovered that these changes can be reached in different semiconductors and heterostructure composites of semiconductors can possess similar ionic conduction and fuel cell power. This points out that the ionic charge transport process may have a similar origin across various interfaces modified by energy bands. The role of electron band states and structures in composite interfaces of these materials is important as ion transportation is determined by the local electron charge density and the electric field along the interfaces of heterostructure composite oxide materials, as shown in Figure 1E. Recent literature articles reveal that semiconductor-ionic composite heterostructure materials drastically improve ionic conductivity, as earlier reported that a YSZ/SrTiO₃ composite mixture has exhibited an improved ionic conductivity value compared to YSZ^[19].

A new characteristic of fuel cell technology from semiconductor physics was first presented by Singh et al. in 2013^[20], where amphoteric oxide semiconductor cathodes and anodes separated by an ionic conducting electrolyte are renowned as p and n-type regimes, respectively. The ion conducting electrolyte materials possess the same properties as intrinsic semiconductors, having a bandgap of a larger value; for example, the bandgap of YSZ is 5.79 eV^[21,22]. Meanwhile, the cathode and anode, placed in an oxidizing and reducing environment, respectively, are responsible for ORR and HOR and possess high hole/electronic conductivity^[23]. Therefore, extrinsic semiconductors having low bandgaps are required to enable sufficient p-type and n-type charge conduction, as shown in Figure 1D. A common example is the typical family of perovskite oxide electrode materials having mixed ionic and electronic conductivity^[24].

In order to summarize by using the semiconductor physics aspect, the SOFCs can correspond to a three-layer p-i-n semiconductor device by applying a corresponding p-type cathode, intrinsic semiconductor-type electrolyte, and n-type anode^[25-27]. Furthermore, a p-n two-component/layer device, i.e., DLFC, as represented in Figure 1B, similar to a p-n heterojunction device, is designed when the middle layer of electrolyte (i-type) is removed. The p-n junctions consisting of bulk heterojunction p-n type materials, i.e., Single-Layer Fuel Cells (SLFC) having ionic conductors mixed with n- and p-type semiconductors, which have been widely reported for the formation of electric field barriers to avert the further movement of the hole from p to n layer and electron transportation from n-type to p-type layers until the bending of the band is accomplished^[17,28,29]. New designing approaches of semiconductor heterojunction-based fuel cells using important aspects of energy band/alignment and built-in electric field (BIEF) create barriers for the electron transport while supporting superionic charge transport along the interfaces and afterward improve the performances of the device in working conditions^[30-32].

Based on rigorous investigations and developments of semiconductor membrane fuel cell (SMFC) materials and device technologies, this review will cover the principal understandings, historical development, and achievements in the last few decades. Its further objectives are to afford a novel method and approach by presenting the semiconductor energy band and BIEF theories as a common technical foundation to design Semiconductor Ionics to develop next-new-generation superionic materials with self-field driving ionic transport at low temperatures of 300-500 °C.

Fluorite

The fluorite oxides with a general formula AO₂ (A = mainly large tetravalent cations such as Ce, U, etc.) are known to be good oxygen ion (O^2-) conductors. This structural configuration results in an open structure consisting of interstitial octahedral voids, which helps tolerate doping on a large scale and endorses ionic conduction. One example is ZrO₂, where Zr⁴⁺ is partially substituted by a larger cation Y³⁺, resulting in yttria-stabilized zirconia (YSZ) composition with high ionic conductivity due to the creation of oxygen vacancy for the sake of charge neutrality of the composition, as shown in Figure 2A^[35]. Another popular example is ceria, which has a fluorite structure and Ce⁴⁺ mainly partially replaced by lower valence Gd³⁺ or Sm³⁺ to create gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC), as illustrated in Figure 2B^[36]. The compositions have been used extensively as electrolytes in SOFCs. In recent years, many novel research developments have reported that fluorite can be used as a proton conductor, which is an interesting topic and will be further discussed in the following chapter.

Figure 2. (A) Yttria-stabilized zirconia (YSZ) is a cubic crystal structure of zirconium dioxide with the addition of yttrium oxide. (B) Ceria has a fluorite structure and is mainly partially replaced by lower-valence Mⁿ⁺ (Gd³⁺ or Sm³⁺) to create gadolinium/samarium-doped ceria (GDC)^[36].

From the semiconductor aspect, fluorite-structured oxides are wide bandgap materials with a bandgap larger than 3.0 eV. Therefore, these materials naturally show very low or neglectable electronic conduction compared to their ionic (O^2- or H⁺) one, thus, good for electrolyte applications. Moreover, recent developments have shown that the ceria and zirconia fluorite oxides also possess proton conductivity, which is getting more interesting in the new development of LT-SOFCs. We will start a new session in the next chapter “Proton conducting electrolyte”.

Perovskite oxide

The perovskite oxides are technologically important materials having applications in different fields due to their unique properties^[37-39]. The general formula of perovskite oxide is ABO_3, where A-site cations are alkaline, alkaline earth, and lanthanide ions, whereas B-site cations are mainly transition metal ions^[40]. Although the ideal ABO₃ structure has a cubic phase with a Pm3m space group, other phases, such as orthorhombic and tetragonal, are also found^[41]. In the cubic unit cell, the B-site cation sits at the center of the cube, forming BO₆ octahedra with oxygen atoms residing at the center of the faces, while the A-site cation sits on the corner of the cube. The structure can also be viewed in an alternative way, as shown in Figure 3A-C, emphasizing the formation of BO₆ octahedra in the unit cell. In real situations, most of the perovskite oxides show non-cubic phases, i.e., orthorhombic, tetragonal, and rhombohedral phases, due to distortion in BO₆ octahedra, which is mainly caused by the large size of A-site cations. The Goldschmidt tolerance factor (t), introduced by Goldschmidt by the following equation (Equation 1), actually gives an idea of the distortion^[42].

Figure 3. ABO₃ perovskite structure: (A) B-site cation at the center of the unit cell, (B) A-site cation at the center of the unit cell, (C) emphasizes the BO₆ octahedra formation and (D) unit cell of fluorite CeO₂. Reproduced under the terms of the CC BY 3.0 License Copyright 2020, Book Chapter Published by Intechopen^[51], (E) MgAl₂O₄ normal spinel structure, (F) MgGa₂O₄ inverse spinel structure. Unit cells, along with different coordination environments, are Reproduced under the terms of the CC BY 4.0 License Copyright 2020, Published by Springer Nature^[53]. (G) Proton insertion in layered Li compound where Li⁺ is the red-colored sphere, and H⁺ is the blue-colored sphere. Reproduced with permission^[54] copyright 2014 John Wiley and Sons.

where r_A, r_B, and r₀ are the ionic radii for A, B, and O, respectively.

For cubic structures, the t value resides between 0.9-1, whereas if it is greater than 1, then a hexagonal or tetragonal structure can be formed. An orthorhombic or rhombohedral structure can be formed when the value is between 0.71 and 0.9. The properties of perovskite, such as oxygen ion vacancy, structural defects, ionic or mixed ionic and electronic conductivity, oxygen exchange kinetics, creation of redox couples, and stability, can be tailored by introducing more than one cation both at A- and B-sites of the structure^[43]. The structural flexibility allows perovskite oxide for various applications in SOFC/PCFC, e.g., LaGaO₃-based electrolytes as O^2- conducting electrolytes^[44] and BaZrO₃ and BaCeO₃-based perovskites for H⁺ conducting electrolytes used for PCFCs^[45]; more commonly, mixed electronic and ionic or triple charge (O^2-/H⁺/e^-) conducting perovskites have been widely used as electrodes, mostly for cathodes^[46-48], but also some for anodes^[49-50].

Spinel

Basic ABO₃ perovskite-type structures typically feature B-site cations at the center of the unit cell and A-site cations at the center of the unit cell, as shown in Figure 3A and B. Further, Figure 3C emphasizes the BO₆ octahedra formation. The unit cell consists of cations sitting at the corner and center of faces in the cubic lattice while the oxygen atoms occupy the tetrahedral sites, as demonstrated in Figure 3D^[51]. Spinels are known for their application in the energy storage, but recently, they are being explored for other applications because of their encouraging catalytic properties. The normal spinel is generally represented by the formula [A]^t[B₂]^oO₄ (t and o represent the tetrahedral and octahedral sites, respectively). The structure is a face-centered cubic lattice where A-site atoms occupied 18 tetrahedral voids and B-site atoms occupied 12 octahedral voids, making them tetrahedrally and octahedrally coordinated, respectively. On the other hand, the structure of inverse spinels is the opposite, where B-site atoms sit on the tetrahedral sites and both A and B-site atoms are in the octahedral sites^[52,53]. The structures of normal spinels (MgAl₂O₄) and inverse spinels (MgGa₂O₄) are shown in Figure 3E and F.

Other structures

Apart from the abovementioned oxide materials, there are other structures that have been recently used as electrolytes in SOFCs. Lithium-based layered oxides are now investigated as potential electrolyte materials^[54]. At first, Sebastian et al. showed possible modification of material from lithium-ion to proton conductors by replacing Li⁺ with H⁺ in lithium-ion conductors using reversible ion exchange methods^[55]. This work opens up new possibilities for layered structured materials^[54,55]. Lan et al. showed that hydrogen (H₂)-inserted structure, Li_xH_yAl_0.5Co_0.5O₂, can be made from a layered lithium-based oxide material, Li_xAl_0.5Co_0.5O₂, at high temperature in a hydrogen atmosphere^[54]. The proton insertion process is shown in Figure 3G. The resultant material showed an enhanced value of proton ion conductivity.

Moreover, many researchers have applied the heterostructure concept to further improve the performance of the electrolytes. When a composite containing p- and n-type semiconductors is formed, electrons will move from n-type to p-type during experimental conditions, giving an opportunity to create superionic conductors. Composites containing semiconductors and ionic conductors have also been explored extensively, resulting in improved ionic conductivity several times^[19]. The concept will be discussed in detail in the upcoming sections.

Oxygen ion conducting electrolyte

A significant amount of work has been done for decades on oxygen ion conducting electrolytes for SOFCs. In this subsection, we will discuss the compositions that have been studied extensively.

YSZ is the most studied electrolyte composition to date, mainly due to its high ionic conductivity. Pure ZrO₂ shows different phases depending upon the temperature, such as monoclinic at room temperature, tetragonal above 1,100 °C, and cubic fluorite above 2,370 °C, along with low oxide ion conductivity because of limited oxygen ion vacancies. It is observed that doping of Y³⁺ in the ZrO₂ structure results in stabilization of the fluorite phase at room temperature along with enhancement of oxygen on conductivity by creating more vacancies. YSZ [8 mol% YSZ (8YSZ)] shows high ionic conductivity of 0.14 S cm^-1 at 1,000 °C. This high temperature is not suitable for ultimate application purposes due to cell degradation^[56]. Therefore, the main challenge for YSZ electrolytes is to reduce their operational temperatures without compromising their ionic conductivity. So, the researchers have tried to lower the temperature by applying various innovative modification techniques such as densification, sintering modification, and thin layer formation. Thin-film formation is found to be very effective in this regard.

Esposito et al. made thin-film YSZ electrolytes by using an inkjet printer that contains YSZ nanoparticles, which yielded a high-power density of 1.5 W cm^-2 at 800 °C^[57]. Park et al. used the atomic layer deposition (ALD) technique to fabricate thin-film YSZ electrolytes, resulting in higher conductivity and astonishing fuel-cell performance with an open-circuit voltage (OCV) near 1 V at 1,000 °C^[58]. Apart from thin films, different unique types of modifications have also been applied by various groups. For example, Lu et al. reported lower area-specific resistance (ASR) of YSZ electrolytes by the introduction of Ca₃Co₂O₆ and Sm_0.2Ce_0.8O_1.9 as interlayers^[59]. Lee et al. achieved several benefits, such as sintering temperature drop, grain growth, and eutectic liquid creation, by introducing Li as a dopant in YSZ^[60]. Xu et al. prepared a composite containing YSZ and mineral CuFe-oxide minerals, which shows high ionic conductivity and power density^[61]. Liu et al. prepared the electrolyte by a combination of conventional YSZ and YSZ nanowires synthesized by electrospinning techniques, resulting in ionic conductivity of 0.01 S cm^-1 at 375 °C, which is 290 times better than the bulk YSZ^[62]. Han et al. observed that the effect of grain size reduction is more prominent in the low and intermediate temperature range (up to 800 °C)^[63]. This is due to the enhancement of thickness of the intergranular region with grain size reduction, which mainly contains a maximum of three atom layers. Because of this, oxygen ion conduction is not interrupted due to segregation or impurity. So, the mixed grain and GB give an easy road for ionic movement, as shown in Figure 4A.

Figure 4. (A) Schematic representation of large grain and grain boundary area, along with routes for ionic conduction. Reproduced with permission^[63] Copyright 2007 Elsevier, (B) ionic conductivity comparison of various compositions [Ln₂O₃ (Ln = lanthanide)-doped ZrO₂] at 1,000 °C. Reprinted with permission^[64] Copyright 2000 Elsevier, (C) HAADF image of grain boundary of CeO_2-δ@CeO₂ core-shell, (D) mapping of Ce at the interface (E) ration of Ce⁴⁺/Ce³⁺ at the interface. Reprinted under the terms of the CC BY 4.0 License Copyright 2019, Published by Springer Nature^[67]. (F) Voltage-current density-power density plot SrTiO₃ at different temperatures (450, 500, 550 °C). Reprinted from Ref.^[68] with permission from the Royal Society of Chemistry, 2019.

Yamamoto et al. show the trend of ionic conductivity with lanthanide doping in ZrO₂ shown in Figure 4B^[64]. It has been observed that Sc-doped ZrO₂ establishes the greatest conductivity of 0.3 S cm^-1 at 1,000 ^oC. Therefore, much focus has been given to the scandia-stabilized zirconia (ScSZ) composition. The introduction of Sc₂O₃ in zirconium increases the oxygen ion vacancy, resulting in fast ionic movement and less ohmic resistance. The optimized Sc₂O₃ concentration is an important factor for structure stabilization and high performance. It has been observed that doping of 5-9 mol% Sc₂O₃ generates the cubic phase while 10-15 mol% results in rhombohedral phase structures.

Another fluorite-structured oxide, CeO₂, has enticed the concentration of the research community because of its potential application as an electrolyte for low-temperature fuel cells. Anirban et al. described that ceria is basically an insulator; the partial substitution of acceptor cations into the ceria lattice introduces oxygen vacancy defects that influence the ionic conductivity at relatively low operating temperatures^[65]. The ionic conductivity of ceria can be significantly enhanced by doping rare earth cations, such as Ce_0.8RE_0.2O_1.9(RE = Er, Ho, Gd, Eu, Sm, Nd, and La), which create defects associated with the structure. The variation of ionic radii of the dopant cations plays an important role in the value of ionic conductivity and activation energy, which can be explained with the help of existence of defect associates. Among other rare earth-doped ceria electrolytes, the Gd³⁺-doped ceria shows the highest conductivity and the lowest activation energies. Although significant progress has been achieved with this material, some critical challenges include reducing the electrolyte by hydrogen, which reduces OCV and power density, and enhancing electronic conduction on the transition from micro to nanometer scale remains as bottlenecks for its practical application. Doped ceria compositions are suitable for low-temperature applications (below 600 °C) because it is observed that OCV and the efficiency of SDC lower with an increase in temperature. High-temperature sintering (above 1,600 °C) for densification is another issue for ceria-based electrolyte fabrication. Therefore, novel synthesis techniques, such as solvothermal, co-precipitation sol-gel, etc., have been adopted to create ceria nanomaterials that reduce the required sintering temperature. Apart from synthesis techniques, sintering aid has also been applied. Singh et al. revealed that co-doping approaches are useful for getting better conductivity of electrolytes simply at an intermediate temperature range and at an optimum concentration of dopants^[66]. Wang et al. conducted a study where they deliberately introduced various surface defects and altered the electrical properties of CeO₂ in order to examine the relationship between conductivity and surface states^[67]. The CeO₂-δ@CeO₂ core-shell heterostructure was clearly revealed by using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS), as demonstrated in Figure 4C-E. These techniques conclusively established that the surface buried layer, spanning a few nanometers, consisted of Ce³⁺ on ceria particles.

Apart from fluorite structures, perovskite oxides have also been studied as electrolytes. Doped lanthanum gallates show promising results. Ishihara et al. found that La_1-xSr_xGa_1-yMg_yO_3-δ (LSGM) shows very good oxygen ion conductivity (~0.14 S cm^-1 at 800 °C), which is better than stabilized zirconia along with stability^[44]. Although LSGM shows potential, it faces several challenges such as phase purity, internal diffusion, and mechanical strength up to the present. Chen et al. prepared SrTiO₃ perovskites as an electrolyte, which showed superior ionic conductivity of 0.24 S cm^-1 and a high-power density of 620 mW cm^-2 at 550 °C, as shown in Figure 4F^[68]. Although many potential candidates are found, much more research and introduction of new concepts are required to get the ultimate composition for practical use.

Proton conducting electrolyte

Proton conducting fluorite electrolyte

GDC has a fluoride structure, which has been extensively studied as an O^2- conducting electrolyte in SOFC applications. However, in recent years, new developments in fluorite proton conducting properties have gained a lot of importance^[72-76]. Zhao et al. provided evidence of introduction of surface reactive oxygen species to the CeO₂ substrate through DFT calculations^[72]. They found that desorption of adsorbed hydrogen (H⁺) intermediates can be promoted. Scherrer et al. reported YSZ thin films that have proton conduction via chemisorbed water at the inner surface in addition to oxygen ion conductivity, while above 400 °C, it turns to oxygen ion conduction^[73]. Shirpour et al. found doped CeO₂ exhibited higher conductivity than pure CeO₂^[74]. When the wet atmosphere was applied, a slight improvement in GB conductivity could be observed. Another research mentioned a transference of adsorbed water layers from an “ice-like” to a “water-like” structure for ceria fluoride materials, along with the shift in proton conduction mode^[75]. However, there was no clear understanding of why the proton mobility at the GB of a nanocrystalline material might be so high compared to the bulk. In this research, protonic surface conduction has been proposed with two series of connected processes that together compose a parallel rail to the bulk and GB conduction of oxide ions. These are intra-grain transport along the single grain’s surface and inter-grain transport conduction across the intersection between two adjacent grains, where the latter is highly resistive at low relative humidity^[75,76].

However, it should be noted that all these reports concerning water or humidity as a basic need when intra-grain transport occurs based on water contents, but the water cannot sustain at high temperatures, say 500 °C, then water molecular-associated proton conduction rapidly disappears. In this circumstance, some research could be carried out with dry H₂ to identify proton conduction directly under fuel cell operations. In this case, protons are produced through the following defect formation:

(2) $$\begin{array}{r}{\mathrm{H}}_2+2{\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}\to 2{\mathrm{OH}}_{\mathrm{O}}^{\cdot }+2{\mathrm{e}}^{\prime }\end{array}$$

Recent research indicates protons can conduct along a pathway at the interface of the CeO₂ core and CeO_2-δ shell^[77]. The maximum power density of 697 mW cm^-2 was achieved for CeO₂/CeO_2-δ fuel cell configurations^[77]. It has demonstrated that proton conductivity can sustain at high temperatures, e.g., above 500 °C. In fact, a number of ceria-based materials have been reported at 400-600 °C region with high proton conductivity and successful fuel cell applications^[78,79]. For example, Li et al. reported some proton conducting CeO₂ nanocubes^[78] (as shown in Figure 5), and Paydar et al. reported surficial proton conducting CeO₂ nanosheets, as shown in Figure 6A^[79]. Figure 6 summarizes proton conducting ceria with various morphologies such as CeO₂ nanosheets and CeO₂ nanoparticles [Figure 6B]. Between 300-600 °C and below 200 °C, there has been numerous ceria, and zirconia-based fluorites have been reported for proton conductivity.

Figure 5. Different CeO₂ morphologies (A) Nanocube. Copyright 2018, International Journal of Hydrogen Energy^[78]; (B) core-shell and (C) proton conduction in the shell layer^[78].

Figure 6. SEM images of (A) CeO₂ nanosheets, (B) CeO₂ nanoparticles, HRTEM images of (C) CeO₂ nanosheets, (D) CeO₂ nanoparticles. Copyright 2023, Ceramics International^[79].

Mixed ion conducting electrolyte

Concern about the low-temperature operation of SOFCs has been raising emergent interest in recent years. Recent progress in perovskite oxide phases has led to the development of H⁺/O^2-/e^- triple-conducting electrode BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) for the low-temperature operation of fuel cells. Xia et al. have developed a high ionic conductor, BCFZY electrolytes, and fabricated a BCFZY -ZnO composite p-n heterojunction-structured material in order to suppress its electronic conduction^[30]. Through this methodology, it has been established that BCFZY could be used in the operation of fuel cells with good performance as electrolytes, attaining extraordinary ionic conductivity, durability, and cell working performance. The energy band alignment mechanism can explain the electronic conductivity suppression and improvement of oxide ion conductivity in the p-n heterostructure. The research outcomes reveal that BCFZY is a promising electrode and an excellent electrolyte.

Heterostructure composite materials

Structural doping and bulk conduction

In a SOFC, the primary role of solid electrolytes is to conduct ions, either O^2- or H⁺ (nature of the specific material), between the electrodes in order to avoid blocking electrons’ internal conduction, driving the electrons to move through the outside external circuit to complete the overall electrochemical reaction. Materials used as electrolytes in SOFCs must have high oxygen ion conductivity, chemically, thermally, and mechanically companionable with the adjoining cell components, highly dense microstructure, and low electronic conductivity to avoid internal short-circuits during electrochemical reactions^[4,6,102]. In the last few decades, SOFC electrolyte materials have received great attention from researchers and scientists in order to achieve good SOFC performance^[85,103]. Therefore, a wide variety of compounds have been investigated as suitable candidates for electrolyte materials for SOFCs. The major research work has been focused mainly on four categories of electrolyte materials, viz. YSZ, GDC/SDC, doped bismuth, and doped lanthanum gallate^[103-105]. In order to improve the ionic conductivity of these systems, single or multiple dopants in each of these four systems of materials have been further sub-categorized.

The ionic conductivity of electrolytes, such as doped ceria, originates through ion movement in lattice under the effect of an electric field. The electrical conductivity σ can be expressed as:

Where σ, n, e, and represent the ionic conductivity, concentration of charge carrier in a given volume of lattice, electronic charge, and mobility of charge carriers, respectively. In the case of pure oxygen ion conductors, ion conduction takes place via oxygen ion vacancies. Therefore, the ionic conductivity of such conductors can be also expressed as:

The N_o is the number of oxygen sites per unit volume, $$V_O^{֟}$$ represents the fraction of oxygen vacancies, and μ_i is the oxygen-ions mobility having charge denoted as q_i^[81].

Stabilized zirconia is the most extensively investigated and most commonly used SOFC electrolyte material working at high temperatures. In order to stabilize cubic fluorite structures of zirconia, several divalent and trivalent oxide dopants are incorporated into the cation sublattice such as CaO, Y₂O₃, MgO, Sm₂O₃, Yb₂O₃. Among these compositions, 8YSZ provides the best properties for electrolyte materials, having the required characteristics for SOFC application, such as improved ionic conductivity, durability, and chemical and thermal stability in reducing and oxidizing environments^[106-108].

In further investigation of alternative electrolyte materials, Scandium-doped ceria (ScSZ) exhibits extensively higher ionic conductivity as compared to YSZ. Further, 1 mol% substitution of Bi₂O₃ in ScSZ exhibits an improved ionic conductivity of 0.33 S/cm at a temperature of 1,000 °C. At room temperature, the undoped system has a rhombohedral crystal structure, which results in lower ion conductivity compared to cubic phase structures. A cubic phase transformation occurs at higher temperatures. The 2 mol% addition of Bi₂O₃ in 10ScSZ leads to lower temperature range stability of cubic phases and exhibits the highest conductivity value (0.18 S/cm) at a temperature of 600 °C^[109]. However, the high cost of the scandium limits its practical use in SOFC applications as an electrolyte on a larger scale^[110].

Lanthanum gallate LaGaO₃-based oxides with cubic perovskite structures have been widely investigated as oxygen conductor electrolytes for IT-SOFC applications. In the crystal lattice, La³⁺ cations are situated in twelve coordination sites of O^2- anions, and Ga³⁺ cations occupy six-coordinated octahedral sites with the corner-sharing arrangement of GaO₆ octahedra. The GaO₆ octahedral tilting causes a deviation from ideal cubic unit cells^[111]. In order to obtain higher ionic conductivity, La can be partly substituted by Ca, Sr, Ba, Nd, and Sm, whereas Ga may also be partly substituted by Mg, Al, In, or Zn, thereby increasing the concentration of oxygen vacancies, as in LSGM. In this regard, Sr and Mg-substituted lanthanum gallate with the general formula LSGM showed a higher value of ionic conductivity than that of YSZ in the temperature range of 770-1,100 K. The TEC is also similar to the other common components of cells. The composition La_0.8Sr_0.2Ga_0.83Mg_0.17O_3-δ was found to exhibit a higher value of ionic conductivity (σ_ion = 0.17 S.cm^-1 at 800 °C)^[112].

Among various oxide ion conducting electrolyte materials, the series of oxides derived from Bi₂O₃ is particularly attention-grabbing because of their high oxide ion conductivity as compared to many solid oxide electrolytes. Bi₂O₃ exhibits a noteworthy polymorphism, having α and δ as two stable phases^[113]. The δ-phase having fluorite crystal structures shows high ionic conductivity in high-temperature regions. The phase stability of the δ-phase appears above 730 °C^[111].

At temperatures below 973-1,073 K, the phase stability of the δ-Bi₂O₃ phase can be attained by doping rare-earth cations, such as Y, Er, or Dy, in bismuth and their groupings with upper oxidation state cations, for example, V, Nb, or W^[85]. The maximum ionic conductivity has been obtained in Y (Bi_1-xY_xO_1.5, x = 0.23-0.25) and Er (Bi_1-xEr_xO_1.5, x = 0.20)-doped compounds. The Phase stability of these systems appears only in the temperature region 1,043-1,143 °C. Bi₂O₃-based materials show a number of shortcomings, such as volatilization of Bi₂O₃ at moderate temperatures, thermal instability in reducing atmospheres, high corrosion activity, and minimal mechanical strength, which affect the ion conductivity. The 20 mol% addition of Er₂O₃ in Bismuth oxide (ESB) also exhibits the maximum conductivity among all reported electrolytes of stabilized Bi₂O₃. However, ESB cannot be employed as oxide ion conductor solid electrolytes for IT-SOFCs because of its low phase stability in reducing environment p(O₂) < 10^-6-10^-7 Pa at 973 K and p(O²) > l0^-8-10^-9 Pa at 873 K^[114-116].

The partial substitution of acceptor cations into the ceria lattice introduces oxygen vacancy defects that influence the ionic conductivity at relatively low operating temperatures. These defects, mainly oxygen vacancies, are responsible for influencing electrical and many other properties of solid oxides^[117,118]. In order to generate oxygen vacancies, generally, trivalent rare earth and divalent alkaline earth metals are incorporated as dopants into the ceria lattice. Among several rare earth dopants, GDC and SDC were observed as the best ionic conductors for IT-SOFCs. The value of ionic conductivity of Ce_0.90Gd_0.10O_1.95 (CGO10) was found to be 0.01 S/cm at 500 °C^[119,120]. In the past few decades, both theoretically and experimentally, various co-doped ceria-based materials have been extensively observed as solid electrolytes for SOFC applications, such as Ce_1-xHo_0.1Sr_xO_2-δ^[121], Gd_0.2-xDy_xCe_0.8O_1.9^[122], Ce_1-aGd_a-ySm_yO_2-0.5^[123], Sm_xNd_0.15-xCe_0.85O_2-δ^[124],Ce_1-x(Y_0.5Dy_0.5)_xO_2-δ^[125], Ce_0.8Sm_0.2-xPr_xO_2-δ^[126], Ce_1-x(Gd_0.5Pr_0.5)_xO_2-δ^[127], etc. Nonetheless, expensive elements in precursors, such as Sm and Gd, continue to inhibit the development of these materials as the foremost electrolytes in the SOFC market. It has been reported that the incorporation of numerous transition metal ions (Cu, Fe, Mn, Ni, Cr, Co) in minor amounts (< 1-2 mol %) into ceria serves as a sintering aid, reducing the sintering temperature and alleviating the blocking effect along GB^[128-130].

Surface and interfacial ion transport

Emerging research & development growth in the LT-SOFC field has been resulted by recent activities on the composite electrolytes, especially the doped ceria-alkali carbonate composites. One of the most important benefits of the doped ceria-carbonate composites is to resolve the problem of ceria reduction, as reported in much earlier research^[131-133]. In addition, the core-shell structure of composite electrolytes addresses the localized electrons within the ceria lattice, which are unable to travel from one particle of ceria to the other since the amorphous carbonate shell does not permit electrons to move or conduct^[90]. The description of the mechanism of charge conduction in ceria-carbonate composite materials is still not well understood. Numerous hypotheses have been proposed in recent years, and this section will provide a concise review of them. Zhu et al. proposed the oxide ion conduction in the oxide phase in addition to proton conduction in the carbonate salt phase in their first work on carbonate-ceria materials^[134].

Zhu et al. highlighted an in-depth study of oxygen ion and proton conduction mechanisms, along with the strong enhancement in conductivity^[135,136]. The most common description for the appealed interfacial superionic conductivity persists in the Maier’s “space charge” layer theory^[137]. The space charge interfacial region formed along the phase boundaries as a result of the interfacial interaction between carbonate-oxide phases is usually believed to be responsible for the superior electrochemical properties; for instance, cations of the carbonate in addition to oxygen ions could accumulate along the interface. The enrichment of the defects/ions along the interfaces rather than that of the bulk phase of ceramic essentially provides “superionic conduction highways” at the interfaces between the two constituents. Zhu et al. proposed that various types of negatively charged oxygen species (O₂^-, O^-, and O^2-) covering the oxide surface can also occur, and distinct considerations should be paid to the interfacial interactions among the oxygen ions and the alkali cations M⁺ (Li⁺, Na⁺, or K⁺) of the molten carbonate phase that creates an induced electrical field along the interfaces between the two phases, as shown in Figure 7^[134]. They considered that the formation of this electric field distribution plays a vital role in recognizing interfacial ionic conduction, particularly oxide ion charge conduction, permitting ions to transport along the interfaces as high conductivity highways^[134,136]. Also, Schober, using the space charge layer theory, considered that the space charge layer nearby to the interfacial region containing a higher concentration of defects than that of the bulk phases of ceramic is the main source of the superior ionic conductivity^[138]. Huang et al. suggested a more systematic explanation of enhanced conduction mechanisms using defect chemistry^[139]. For the alkali carbonate phase (M₂CO₃, M=Li, Na, or K), there could be a surface reaction, owing to the interfacial surface interaction, resulting in enrichment in vacancies of cation in the bulk phase of carbonate. Thus, a space charge layer is created, increasing cation disorder that offers rise to improved conductivity in ceria-carbonate composites below the melting point temperatures of the carbonate phase in the air atmosphere^[139].

Figure 7. Interfacial superionic charge conduction pathway based on space charge layer theory. Adopted and modified from Ref.^[134].

Schottky junction

In subsequent advancements of fuel cell technology, the SJ became a pivotal component with a profound impact on fuel cell performance, as SJ-based fuel cells are a cost-effective and straightforward fuel cell technology. Notably, the SJ fuel cell device, as depicted in Figure 9, employs a single semiconductor, commonly n-type or p-type, or heterostructure^[28]. A potential difference can be easily established at the interface between a metal and an n- or p-type semiconductor, commonly known as a Schottky barrier, in an SJ fuel cell device. Various types of SJ devices, including solar cells and sensors, have been developed using this concept and taking advantage of the depletion layer that forms between the metal and the semiconductor. A well-known function of SJ is to separate electron/hole pairs by establishing an internal device voltage. The formation of a Schottky barrier occurs between the surface of the anodically reduced metal and the semiconductor in a fuel cell.

Figure 9. Energy level diagram of (A) perovskite solar cells, (B) the fuel-to-electricity conversion device inspired by Perovskite solar cell structures (Type I), and (C) symmetrical fuel-to-electricity conversion device (Type II) with an in-situ formed Schottky junction. Adapted and modified from Ref.^[28].

Figure 10 shows the operating mechanism of the Ce_0.8Sm_0.2O_2-δ and SrTiO₃ (SDC-STO) fuel cell based on the SJ effect and defines the energy band that builds up across the junction^[82]. The SJ is formed in-situ by the reduced metallic Ni/Co from NCAL on the H₂ side at the NiCo/STO and NiCo/SDC interfaces. The barrier of SJ plays a vital role in electronic suppression to avoid the short circuit issue^[82]. This is similar to the junction described for the NCAL and LZO-SDC systems^[152].

Figure 10. Schematic diagram of the Ce_0.8Sm_0.2O_2-δ and SrTiO₃ (SDC-STO) fuel cell that reveals the electronic blocking mechanism by Schottky junction effect at anodic NiCo/electrolyte interface. Adapted and modified from Ref.^[82].

p-n heterojunction

As previously mentioned, the elimination of the intermediate electrolyte layer from a conventional SOFC allows the fuel cell to transform into a p-n junction-based device. In the study conducted by He et al. in 2000^[91], SLFC was fabricated using La_0.9Sr_0.1InO_3-δ as the functional single layer, exhibiting p and n-type conduction at high and low oxygen partial pressures. This SLFC behavior closely resembles the traditional SOFC, wherein the p- and n-type conduction properties are crucial for electrode functionality.

This SLFC-based fuel cell is also highlighted by Singh et al.^[20]. Generally, a fuel cell device can be considered as an assembly consisting of n (anode), p (cathode), and an ionic conducting electrolyte/component, as shown in Figure 11A, with material aspects. If the middle electrolyte layer is removed, Figure 11A transforms into a p-n junction-based fuel cell device [Figure 11B]. Asghar et al. have also confirmed that wide bandgap semiconductors can be used as an ionic electrolyte material^[22]. It means that a SOFC could be conceptually treated as a semiconductor fuel cell system, while the fuel cell effectively functions as a p-n junction device by removing the middle electrolyte layer of the SOFC [Figure 11B]. As expected, SLFC showed the same functions or performances as the traditional SOFC but can be designed with different configurations.

Figure 11. Transition of fuel cell from (A) ionic conductor-based materials to (B) p-n junction assembly. Adapted and modified from Ref.^[89].

Band alignments

The electrochemical reaction and charge transfer processes in a fuel cell device involve two half-cell reactions (oxidation and reduction) from both sides of the electrolyte interfaces. The anode and cathode sides of the electrochemical cell separate electron-hole pairs (e^-/h⁺) through the band structure, encompassing the conduction band (CB), valence band, and BIEF. This energy band serves as a crucial pathway for the electrochemical reaction. Within the anode and cathode's CB (EC) and valence band (EV), free electrons and holes engage in charge exchange with the electrolyte via diffusion-based processes, facilitating charge neutrality at the interface. Consequently, these processes induce a reconfiguration of the energy band structures, resulting in band bending, a phenomenon known as energy band alignment, which manifests within fuel cells. In semiconductors, the position of the Fermi level (EF) predominantly relies on the type of doping (n-type or p-type). Additionally, there are no alternative reference energy states to access energy levels; thus, the position of EF can only be determined if the distances between EF, EC, or EV are known.

Generally, two types of band bending can occur, namely upward band bending and downward band bending, which depend on the material's nature and the modification of the energy band structure near the surface or interface of a material. Regarding an n-type semiconductor (anode), the EF typically resides at a higher energy compared to that of the electrolyte. Consequently, electrons tend to migrate from the semiconductor (anode) toward the electrolyte in order to equalize the EFs. This process causes the EF of semiconductors to gradually decrease in energy until both levels align, resulting in a shift of EF to a lower energy (down-band bending). The corresponding illustration in Figure 12 visually depicts this phenomenon, showcasing the creation of a potential barrier for electrons. As a result, their movement is impeded, leading to the formation of a depletion layer near the surface of the semiconductor. Conversely, for p-type semiconductors, the EF is usually positioned at a lower energy (near the valence band) relative to the electrolyte. As a result, the EF shifts to higher energy (up-band bending) at the interface. To establish electronic equilibrium, an electron deficiency (h⁺) occurs near the anode surface within the positive space charge region as electrons are transferred to the electrolyte. This positive space charge signifies a scarcity of electrons within this specific region.

Figure 12. Schematic of the working principle and energy diagram for a solid oxide fuel cell electrode (semiconductors) interface with electrolytes in Fermi level equilibrium. Adapted and modified from Ref.^[89].

Accordingly, the energy gap between the EF and the CB is increased at the interface, resulting in a corresponding band bending; therefore, a charge accumulation depletion region is formed on the semiconductor side (anode). Conversely, when the electrolyte side reaches full charge, unabsorbed ions of opposite charges persist at the electrolyte interface, giving rise to the formation of a Helmholtz bilayer, as depicted in Figure 12. Due to the disparity in electrostatic potential during the process, a space charge zone is anticipated on the semiconductor side, resulting in a shift in the position of the EF. As a consequence, charge transfer occurs from the space charge zone toward the electrolyte, leading to energy loss and subsequent reorganization of the band structure (refer to Figure 12).

The space charge zone (created by the change of EF positions) can be anticipated on the semiconductor side because of the difference in electrostatic potential created during the process, causing the transfer of charges to the electrolyte; as a result, the semiconductor loses its energy, leading to a rearrangement in the band structure (see Figure 12). The potential difference between the semiconductor and the electrolyte has an influence on the concentration of charges in the space charge region. The potential drop within the space charge region is notably greater than that of the Helmholtz layer. This discrepancy arises due to the extensive ionization of the acceptor solid, which covers a wide area, in contrast to the ions situated far from the electrolyte surface that form the Helmholtz layer. The non-uniform distribution of charges in the interfacial region results in the formation of a BIEF that extends across the electrode/electrolyte interface. This BIEF can facilitate the movement of ions or charge carriers across the interface, albeit necessitating some energy. In simpler terms, the produced electric field possesses an inherent potential difference. The significance of this inherent potential at the interface is its role in impeding the movement of electrons and holes across the junction, establishing a potential barrier that determines the voltage of the device. In real-world applications, it may be a single interface, and the built-in field exists because of the advanced technology used to construct the device with an electrode in more or less ohmic contact to reduce polarization loss.

The utilization of band alignments and junctions in fuel cells, which are constructed using energy bands of semiconductors and heterostructure materials, plays a crucial role in optimizing the efficiency of fuel conversion^[153]. Although ion and electron conductions coexist in the heterostructure semiconductor membranes, there is no electronic short-circuit issue due to the design energy band, and the alignment forming the junction plays a key role in preventing the electrons from passing through the internal devices while generating high power outputs > 1,000 mW cm^-2 at 550 °C. By lowering the necessary activation energy, band alignment can significantly enhance the efficiency of the electrode HOR and ORR processes. This phenomenon is frequently observed in the realm of photoelectrochemical water splitting. This approach holds promise as a universal method for advancing the performance of semiconductor-based conversion devices.

Built-in-field effects

A unique feature of the compound properties is the formation of a built-in field that can promote the transport of ions (H⁺ or O^2-). Assuming no voltage is applied to the p-n junction, the junction attains thermal equilibrium, where the Fermi energy level remains constant throughout the system. Figure 13 illustrates the energy band diagram for the p-n junction under thermal equilibrium. As the conduction and valence band energies traverse the space charge region, they undergo bending due to the changing relative position between the conduction and valence bands with respect to the Fermi energy in the p- and n-domains. Electrons in the n-region CB encounter a potential barrier while entering the p-region CB. This potential barrier is called the built-in potential barrier. This barrier preserves the equilibrium between the majority carrier electrons in the n-region and the minority carrier electrons in the p-region and between the majority carrier holes in the p-region and the minority carrier holes in the n-region. Since the potential preserves equilibrium, no current is generated at this voltage. Due to the transition, the intrinsic EF is equally distant from the CB edge. Therefore, the difference between the intrinsic EFs in the p and n regions can be used to determine the built-in potential barrier^[154].

Figure 13. Energy-band diagram of a p-n junction formed in thermal equilibrium. Adapted and modified from Ref.^[89].

Solid oxide fuel cell (SOFC) route towards semiconductor membrane fuel cell (SMFC) from material aspects

Since the last few decades, the three layers, anodes, electrolytes, and cathodes, of conventional SOFCs have been continuously modified with new materials, especially electrolyte and cathode materials. In SOFC technology, basic requirements for suitable electrolyte materials, which are sandwiched between anodes and cathodes, include several factors: sufficient high oxygen ionic conductivity (0.1 S/cm) at operating temperatures, compatibility with electrodes, prevention of electronic conduction, adequate mechanical properties, and thermodynamic and chemical stability.

Various materials developed for electrolytes are fluorite-structure-type-based, such as ZrO₂, CeO₂, and Bi₂O₃, and perovskite-structure-based, which mainly includes LaGaO₃, and pyrochlore^[50,155]. Despite the fact that all these fluorite-type and perovskite-structure-based electrolytes are being used as SOFC electrolytes, YSZ electrolyte-based SOFC is the only available commercial product in the market. The operating temperature of YSZ-based SOFC should be 700-1,000 °C to get sufficiently high fuel cell performance (if no thin film technology is involved). High temperature operation and complex technology results in high-cost manufacturing.

Interestingly, all these advanced SOFC materials developed can be applied in SMFCs using both electrode and electrolyte membranes. Here, both electrode and electrolyte membranes are based on semiconductors for SMFCs and have shown no compatibility issue. These types of SMFCs can easily avoid compatibility issues between the electrolyte and electrode as an electrolyte itself contains the electrode with good compatibility. Also, developed compatible SMFC electrode materials showed better catalytic activity and redox ability, which helps improve the performance and durability of SMFC devices.

Further, conventional SOFCs are high temperature manufacturing technology, and their electrolytes have limited flexibility towards electrode materials. Contrary, SMFC technology is considered low-cost manufacturing and a wide range of materials, which are based on semiconductors/metal oxide semiconductors, such as ZnO, TiO₂, NiO, CuO, and perovskite-type oxide, such as SrTiO₃, SrCoSnO₃, etc. In particular, SOFC perovskite cathode materials, which are usually p-type semiconductors, can function as electrolytes following the SJ principle. Semiconductor-based electrolytes exhibit some electronic conduction, which makes SMFCs work with better performance based on the junction principle due to high ionic conductivity of 0.1 S/cm in a low-temperature range (< 500 °C)^[18,52,87]. More research and development efforts are needed to work on the current limitations and issues related to SMFC-based devices, e.g., long-term stability, scalability, etc. By designing novel materials, new fabrication technologies, advanced synthesis routes, and new technologies, these limitations can be addressed, and we may realize better functioning and enhanced overall efficiency of SMFC devices.

Solid oxide fuel cell (SOFC) towards SMFC from technologies aspects

Thin film fabrication of SOFCs using physical and chemical methods, such as pulsed laser deposition, ALD, chemical vapor deposition, physical vapor deposition, atmospheric plasma spraying, and sol-gel, results in better fuel cell performance in the low-temperature range of 600 °C when compared to bulk electrolyte-supported SOFCs^[156,157]. The internal resistance of the device is decreased due to these thin-film technologies. In contrast to bulk electrolyte-supported SOFCs, thin film-fabricated SOFCs have durability issues for long-life operation. The expensive and challenging nature of the thin film fabrication process is another drawback. For long-life SOFC products (over 40,000 h), reliable thick-electrolyte-supported technologies serve as the foundation. Contrary to thin film fabrication techniques, the low-cost bulk shaping technique is well suited for use in advanced SMFCs and has a number of benefits, including minimal power loss due to interfacial polarization and simple cell fabrication. For example, perovskite materials, which are used as cathode components in conventional SOFCs, can be used as the semiconductor electrolyte membrane in SMFC without having any interfacial gap problems. This interfacial issue is predominant in SOFC technology and results in lots of disadvantages, e.g., major power loss.

Here, in the fabrication of SMFCs, the semiconductor electrolyte membrane, LSCF-Sm/Ca co-doped CeO₂ (SCDC) nanocomposites, is sandwiched between the two NCAL thin electrode layers; i.e., Symmetrical electrodes are applied to the SMFC device, as shown in Figure 14^[158,159]. Also, in SMFC, the cell is easy to fabricate, compared to SOFC, by shaping drying powder into pellets under applied pressure without using the high-temperature sintering densification method as in SOFC. Here, the intrinsic semiconductor and heterostructure material layer is used as a key layer in SMFCs rather than the ionic-conductive electrolyte layer as in SOFCs. The key issue difference for SMFCs is the low temperature (= 600 °C)-treated semiconductor-based functional layer, whereas the SOFC requires a high temperature (≥ 1,300 °C) treatment with a multi-step co-sintering process. Moreover, the SMFC can also be designed by a material of single layers of conventional SOFC cathode components to realize SOFC anode, electrolyte, and cathode three-component functions, which leads to simple fuel cell advanced technology. Additionally, the advanced engineering of the SMFC device will be crucial not only for its scalability but also for unlocking its full potential.

Figure 14. Schematic representation of Solid oxide fuel cell (SOFC) shifting towards Semiconductor membrane fuel cell (SMFC) from materials, technologies, and science aspects.

SOFCs towards SMFCs from cost and commercialization aspects

From a commercial point of view, the cost of SOFCs mainly depends on fuel cell stack cost, which depends on factors such as material cost, machining cost, energy cost, bipolar plates, and labor cost^[160,161]. Among these different expanses, the materials cost of SOFCs and bipolar plates constitutes the majority portion of the total cost for the fuel cell stack. Also, conventional SOFCs operate at temperatures above 700 °C, so the cost of bipolar plates is much higher than that of SMFCs due to the use of special and expensive alloy materials.

Future research and development may be focused on Semiconductor-ionic-based materials to develop next-generation functional materials by combining semiconductor energy band and BIEF theories to design and develop new-generation superionic materials with self-field driven ionic transport at low temperatures (300-500 °C). Further, advanced research should focus on fuel cell devices based on semiconductor band alignment and the BIEF effect, which can improve HOR and ORR and charge (electrons and ions) transfer and ion transport. These advancements are aimed at enhancing fuel cell performance, lowering device costs, and creating a vital alternative path for commercialization.