1. Introduction
A solid oxide cell (SOC) is an electrochemical device with three operating modes: fuel cell (solid oxide fuel cell (SOFC)) [
1], electrolysis cell (solid oxide electrolysis cell (SOEC)) [
2], and reversible cell (reversible solid oxide cell (RSOC)), which is capable of operating in both modes [
3]. An SOC is a high-temperature (600-1000 °C), all-solid-state cell consisting of a fully-dense ion-conducting oxide electrolyte sandwiched between two porous electrodes (a fuel/hydrogen electrode and an oxygen electrode) [
4], [
5]. SOC technology has several advantages over other power generation and chemical production technologies such as higher efficiency and fuel flexibility. This is particularly true when compared to proton exchange membrane (PEM) fuel cells for power generation or low-temperature electrolyzers (PEM/alkaline) for hydrogen production. Additionally, SOCs have the unique capability to electrolyze carbon dioxide and co-electrolyze carbon dioxide-water mixtures [
6]. Solid-state construction using ceramics and metals together with a high operating temperature (≥ 600 °C) are key features of SOCs. The unique combination of these attributes offers several distinct advantages: flexible cell/stack designs, diverse manufacturing processes, multi-fuel capability, and a wide operating temperature range [
4], [
7], [
8] (
Fig. 1). SOC technology serves two primary applications [
9]: ① power generation, producing electricity from diverse fuels—including hydrogen, hydrocarbons, alcohols, and ammonia—in systems ranging from watt-sized devices to multi-megawatt power plants; and ② chemical production, generating chemicals such as hydrogen, oxygen, and syngas in both distributed and central systems.
The SOC has the potential to become a base technology for future energy systems due to its desired characteristics including
compatibility (i.e., environmentally compatible),
flexibility (i.e., fuel flexible),
capability (i.e., multifunctional),
adaptability (i.e., integrable into different energy systems and applications), and
affordability (i.e., cost effective) [
10] (
Fig. 2). Over the past two decades, SOC technology has garnered significant attention, leading to considerable advancements in the development and commercialization of SOC systems. Growing interest in the technology stems from the ongoing demand for more efficient and cleaner energy conversion methods.
Recent advances in materials science, engineering, and manufacturing processes have driven increased interest in SOCs. Specifically, progress has been made in developing new material compositions and microstructures, tailoring relationships between composition, structure, and properties, forming thin layers with required characteristics, and fabricating and processing intricate structures. These technological advancements have led to improved designs, better performance, increased durability and reliability, and more cost-effective manufacturing methods. Consequently, SOC systems up to the megawatt-scale have been successfully demonstrated. In this paper, we review the progress and status of SOC technology and provide our perspectives on ongoing transformational approaches to move the technology toward practical applications. These technical approaches focus on integrating recent advances in design, engineering, materials, and manufacturing to enhance performance, improve durability, and reduce costs—key elements for widespread commercialization.
2. Applications and technology status
2.1. Applications
SOCs have been developed for a broad spectrum of power generation and chemical production applications. Each application utilizes a specific system configuration, which incorporates SOC modules/stacks and other required components—known as balance-of-plant (BoP) components—to create a fully functional, stand-alone unit [
11]. Consequently, each system configuration must be designed and defined to specify the components and operating parameters that meet the requirements of the target application. In SOFC mode, SOC applications range from small (e.g., portable watt-sized devices) to large-scale systems (e.g., central multi-megawatt power plants). In SOEC mode, applications include distributed systems (e.g., producing 1500 kg hydrogen per day) and central systems (e.g., producing 150 000 kg hydrogen per day).
Table 1 summarizes these applications, with readiness levels ranging from research and development (R&D) and prototype demonstration to commercial stages.
There are several applications specific to the SOC, each with its own system architecture:
(1) In the fuel-cell operating mode, for example, the SOC operating as an SOFC can be hybridized with other power-generating equipment, such as a gas turbine (GT), to enhance performance. In these SOFC/GT hybrid systems, the SOFC exhaust is mixed with air and directed to the GT’s turbine section to generate additional electricity. In a generic design, the SOFC produces about 65%-80% of the power with the remaining 20%-35% coming from the GT [
8]. Hybridization of the SOFC with a GT significantly boosts net efficiency (e.g., up to 65% for 500 kW SOFC/GT hybrid systems) and makes it suitable for power plants (e.g., hundreds of kilowatts or higher) in stationary and transportation (aircraft power) applications.
(2) In the electrolysis operating mode, the SOC functions as an SOEC for oxygen production—a capacity crucial for space applications. A primary example is the Mars Oxygen
In-Situ Resource Utilization Experiment (MOXIE) aboard National Aeronautics and Space Administration (NASA)’s Perseverance rover, which used an SOEC to convert the carbon dioxide-rich Martian atmosphere into oxygen through applied electrical energy. The successful demonstration of this
In-Situ Resource Utilization (ISRU) technology for oxygen production on Mars proves that such systems can reliably provide breathable air and rocket propellant for future human missions [
12].
2.2. Technology status
SOFCs. The development of SOC systems (SOFC) for power generation has progressed to prototype and pre-commercial/commercial stages for a number of applications ranging from W-scale devices to MW-power plants (
Fig. 3). For instance, a 250 kW power system has been demonstrated to run on natural gas, biogas, or hydrogen [
13]. The natural gas operating specifications are detailed in
Table 2. To date, an 80 MW single-site installation is the largest SOFC power system to be constructed and operated for stationary applications [
14].
SOECs. The development of SOC-based systems for chemical production typically leverages more technologically advanced SOFC designs. Consequently, many SOEC systems—those for hydrogen production via steam electrolysis—utilize SOFC stack technology. An example is a 150 (kg H
2)·d
−1 SOEC system [
15]. The specifications for this system are detailed in
Table 3. Currently, a 2.6 MW system represents the largest operational SOEC plant demonstrated to date [
16].
RSOCs. The development of RSOC systems is technologically derived from SOFCs. An example is a demonstration RSOC system designed to generate 10 kW alternating current (kW
AC) in fuel cell mode and produce hydrogen with a power input of 40 kW
AC [
17]. The operating characteristics of this system are given in
Table 4. The largest RSOC system demonstrated to date is an integrated, containerized unit with a power input of 120 kW in electrolysis mode and an output of 50 kW in fuel cell mode [
18].
3. Recent advancements
Substantial progress has been made over the past decade in several technology areas, such as scale-up, durability, and system operation, critical to the development and commercialization of SOCs. However, significant challenges, especially regarding cost, must be resolved for widespread adoption in the future (current SOC stack cost is roughly 1500-3000 USD·kW
−1 with US Department of Energy 2025/2030 targets of 225 USD·kW
−1 for SOFCs [
19] and < 125 USD·kW
−1 for SOECs [
20]). At present, SOCs offer efficiency and performance levels suitable for many applications. While the reliability and durability of SOCs demonstrated to date are adequate, further improvements are desirable because performance, durability, and cost are inherently interdependent.
Further improvements are needed in several areas for SOC cells and stacks, relating to the key drivers (efficiency, reliability, and cost) to enable commercialization (
Fig. 4).
Power density (increase), especially under practical operating conditions such as high fuel utilization (fuel cell mode), to increase power output per unit cell area (efficiency), thus lowering the number of cells per stack (cost) and similarly, in electrolysis mode, increased current density under specified voltage.
Material quantity (decrease) to minimize stack weight and volume (cost), thus reducing the stack mass that it requires (e.g., thermal management such as cooling (efficiency)).
Cell size (increase), especially for scale-up to large systems, to decrease the parts count (cost), thus reducing cell-to-cell connections and potential mal-distributions (e.g., cell stacking and gas flow distribution (reliability)).
Reproducibility (increase), especially stacking reproducibility, to improve stack yields based on acceptance criteria for the intended application (reliability), thus minimizing rejects (cost).
Life (increase), especially cell life, to prevent untimely failures (reliability) and performance losses during long-term operation (efficiency).
Degradation (decrease), especially performance poisoning caused by chemical interactions and impurities, to meet cell/stack specifications (reliability) and minimize excessive performance losses (efficiency).
In addition to the above areas, improvements specific to the SOC in reversible mode include:
Electrode reversibility. Electrode performance for the RSOC must be reversible to ensure efficient operation in fuel cell, electrolysis, and dual (cyclic) modes. This challenge primarily relates to the oxygen electrode, whereas the hydrogen electrode is inherently reversible.
Electrode stability. Oxygen electrodes for the RSOC must possess appropriate interfacial and microstructural characteristics to maintain performance while avoiding electrode pitting/delamination caused by oxygen evolution when switching from fuel cell to electrolysis mode. Furthermore, the material and microstructure of the hydrogen electrode must be stable within high-steam environments,
High-level roadmaps have been developed for the SOC, focusing on transitioning the technology to large-scale, cost-competitive, and sustainable energy systems [
21], [
22]. The SOFC roadmap prioritizes reducing operating temperatures and increasing longevity along with cost reduction and mass production to secure a share of future power generation market. The focus for SOECs is on advancing efficiency and lowering costs of green hydrogen production and decarbonizing industrial sectors like steel and ammonia. Key roadmap elements and strategic directions include material innovation, system integration and efficiency, durability and degradation improvements, and cost reductions.
Cost reduction is the primary priority for the SOC. Reducing the cost of SOCs depends on transformative breakthroughs in materials, design, engineering, and manufacturing beyond simple economies of scale. Innovations in several key areas such as novel materials, nanostructures, microstructure engineering, thin-film technology, and digital manufacturing offer distinct pathways to minimize both capital expenditure and long-term operation and maintenance (O&M) costs. The following sections explore these advancements, drawing upon our ongoing work in SOC design, engineering, manufacturing, and concept development.
3.1. Novel materials and nanomaterials
In recent years, several novel electrode materials, including nanomaterials, have been developed to enhance the performance and durability of SOCs. Approaches in this area involve tailoring and engineering materials and microstructures via a combination of modified electrode compositions, alternative designs, and advanced manufacturing techniques. These approaches are discussed in this and the following sections.
One example of such novel materials is a nickel-substituted perovskite for the fuel or hydrogen electrode. The development of these materials addresses two key limitations of conventional Ni-YSZ hydrogen electrodes: nickel particle agglomeration/depletion (coarsening or depletion near electrode/electrolyte interfaces during long-term operation) and redox instability (chemo-mechanical instability of the electrode under oxygen partial pressure changes during redox cycles at high temperatures). Further discussion on this issue is given in the microstructure engineering section. When a Ni-substituted perovskite hydrogen electrode is exposed to a reducing environment, Ni will exsolve to form fine particles embedded in the oxide skeleton [
23]. Nickel nanoparticles formed through this process have been shown to resist agglomeration and migration, significantly reducing electrode polarization resistance and enhancing long-term stability. Furthermore, in a redox cycle environment where oxidation and reduction are repeated, these nanoparticles can be regenerated via re-dissolution (oxidation) into the lattice and subsequent re-exsolution (reduction), rendering the electrode inherently redox-stable.
An example is our effort to develop a stable and redox-resistant hydrogen electrode for the SOC. The electrode is composed of nickel-substituted lanthanum strontium cobalt iron perovskite (LSCFN) and gadolinium doped ceria (GDC). LSCFN is a perovskite with the composition La
0.97−xSr
xFe
0.8−yNi
yCo
0.2O
3−δ (a solid solution of lanthanum strontium cobalt perovskite (LSCF) and LaNiO
3). While LSCF has been shown to be stable and is used as an anode material for SOFCs [
24], our work has demonstrated exceptionally high performance—achieving a peak power density of about 3 W·cm
−2 at 600 °C—using a hydrogen electrode consisting of a Ni-LSCF mixture. Because LSCFN has shown nickel exsolution/dissolution (
Fig. 5) and excellent redox resistance when exposed to redox cycles, this Ni-substituted perovskite can serve as a stable alternative to Ni-LSCF for the SOC hydrogen electrode. In addition, when combined with LSCF oxygen electrodes, cells with LSCFN hydrogen electrodes become symmetrical, offering improved thermochemical compatibility, enhanced reversibility, and simplified fabrication.
3.2. Nanostructures
A novel development in SOC R&D work is the use of nanostructures to enhance electrode and, consequently, cell performance. As operating temperatures are reduced, incorporating these nanostructures becomes more feasible due to their improved stability at lower temperatures. Nanostructures can be introduced into the cell by adding nanomaterials to existing porous electrode structures (e.g., by infiltration of a precursor solution followed by decomposition [
25]), by forming from elements present in existing structures (e.g., by exsolution [
23] as discussed earlier) or by depositing using different deposition techniques (e.g., by sputtering [
26], [
27]). When incorporated into the electrode, these nanostructures—owing to their significantly enlarged triple phase boundaries (TPBs)—impart high electroactivity to the electrode, resulting in minimized polarization resistance and superior electrochemical performance.
An example is our effort to develop high-performance nanostructured electrodes—especially oxygen electrodes—for the SOC. The work leverages manufacturing techniques to create an oxygen electrode with enhanced TPB density and improved gas diffusion in and out of the electrode. This structure aims to increase performance and prevent oxygen gas pressure buildup, which can cause electrode delamination during electrolysis [
2]. Using sputtering, we have produced oxygen electrodes (composed of a perovskite and an electrolyte material such as YSZ or GDC) and hydrogen electrodes (composed of Ni and YSZ or GDC) with a unique feature: a columnar YSZ/GDC nanostructure with diameters of tens of nanometers and nanofiber-like branches of Ni or perovskite growing from the columns (
Fig. 6). This type of nanostructure significantly enlarges the electrode’s active areas, resulting in exceptionally high electrochemical performance for the cell. For instance, co-sputtering has been used to fabricate a RSOC with a nanostructured oxygen electrode of lanthanum nickel cobalt perovskite (LNC)-GDC and a nanostructured hydrogen electrode of Ni-GDC. This cell demonstrates a low area-specific polarization resistance of 0.031 Ω·cm
2 at 600 °C, and exhibits a peak power density of 2.49 W·cm
−2 with 97% H
2-3% H
2O in fuel cell mode and a current density of 1.68 A·cm
−2 with 50% H
2-50% H
2O in electrolysis mode [
28]. In addition, this columnar electrode nanostructure with a tortuosity of nearly one facilitates gas transport to reactive sites and prevents any gas entrapment and the associated pressure buildup (such as oxygen in electrolysis mode) which can otherwise cause delamination and cracking of the oxygen electrode.
The scalability of this sputtering process has been demonstrated with the fabrication of thin-film cells up to a commercial size of 15 cm × 15 cm [
29]. The capital cost of stacks (based on a specific design) incorporating all-sputtered thin-film cells is estimated at approximately 188 USD·kW
−1 [
29].
3.3. Microstructure engineering
Current SOC cells based on YSZ electrolytes almost exclusively use Ni-YSZ (or Ni-GDC) compositions for the hydrogen electrode. These compositions also serve as cell substrates for mechanical support in planar designs. However, redox instability remains the primary disadvantage of Ni-YSZ hydrogen electrodes and supports. In conventional planar cells, the Ni-YSZ component can become unstable due to nickel oxidation if fuel supply is interrupted, the component is exposed to an oxidizing gas, or seal leakage occurs. Furthermore, instability may arise under extremely high fuel utilization and high current conditions. The oxidation of Ni to NiO increases the component’s volume, generating compressive stress in the electrode and tensile stress in the electrolyte. This often results in cracking, particularly in thin electrolytes, because weak links within the YSZ network lack the structural integrity to withstand the stresses generated by Ni volume expansion upon re-oxidation [
30].
We have developed a microstructure engineering approach to address the redox instability, particularly in hydrogen electrode supports. This concept utilizes an engineered 3D network that consists of a specifically designed, dense YSZ phase and a porous Ni-YSZ phase. The former, characterized by uniform dimensions, is free of weak links, while the latter facilitates fuel gas transport and accommodates NiO within its pores. This arrangement accommodates volume expansion during Ni oxidation, as shown in
Figs. 7(a) and
(b). As illustrated in
Fig. 7(a), each phase is topologically interconnected throughout the support structure. A fabricated 3D hydrogen electrode support is shown in
Fig. 7(c); notably, this structure withstood multiple redox cycles without exhibiting structural damage. The 3D support structure has been scaled up to 5 cm × 5 cm. Since this microextrusion process involves printing and firing, the cost is expected to be comparable to or lower than conventional processes such as tape casting.
3.4. Thin-film technology
Thin-film SOCs have been developed to facilitate efficient operation at lower temperatures (for improved durability and reliability) and to minimize cell size, weight, and volume (for higher specific power and energy) [
31]. Operating the SOC at lower temperatures (≤ 600 °C) without sacrificing performance offers significant advantages such as the integration of lower-cost materials, the mitigation of deleterious chemical interdiffusion, and the reduction of thermal stresses. This reduction in scale increases specific power and energy, providing a viable pathway for miniaturization in applications ranging from portable devices such as handheld electronics to high-power-density, high-energy-density systems like electric aircraft.
Various deposition methods have been evaluated for fabricating thin-film SOCs on ceramic or metal substrates. These include chemical vapor deposition (CVD) techniques such as metal-organic CVD (MOCVD) and atomic layer deposition (ALD), as well as physical vapor deposition (PVD) methods such as pulsed laser deposition (PLD) and sputtering. Our research specifically focuses on developing a sputtering-based process for thin-film SOC fabrication. The process involves the sputtering or co-sputtering of cell components using ceramic or metal/alloy targets with radio frequency (RF) or direct current (DC) power, respectively. The key process parameters, which can be tailored or optimized to deposit thin films with desired characteristics, include power, pressure, atmosphere, substrate temperature, and target-to-substrate distance and angle [
26], [
28], [
32]. For example, our LNC-GDC oxygen electrodes are produced by co-sputtering LNC and Gd/Ce alloy targets; the Gd/Ce alloy deposit is subsequently oxidized into GDC during or after the deposition. By tailoring process specifications and parameters, we have successfully fabricated thin-film SOC units with the required properties and characteristics.
Fig. 8(a) displays a typical cross-section of an all-sputtered thin-film SOC deposited on a nanoporous anodized aluminum oxide (AAO) substrate. We have found that under appropriate conditions, sputtering is capable of producing both the dense electrolytes and porous electrodes required for cell operation, including electrodes with a unique nanofibrous columnar structure (
Fig. 6). The combination of a thin electrolyte and nanostructured porous electrodes has resulted in exceptional SOC performance in both fuel cell and electrolysis modes at reduced temperatures, as demonstrated in
Fig. 8(b). The durability of thin-film cells has been demonstrated with tests up to 600 h, exhibiting a projected degradation rate (based on curve fitting of the experimental data) of 2.71% per thousand hours (kh) during 1000 h of operation at 600 °C in fuel cell mode [
33].
3.5. Digital manufacturing
In recent years, a new class of manufacturing techniques (referred to as digital manufacturing) has become available, enabling the fabrication of complex 3D components that were previously impossible to create using conventional processes [
34]. Collectively known as additive manufacturing (AM), these processes build intricate shapes directly from 3D model data, typically layer by layer. Unlike subtractive manufacturing (e.g. machining) and formative manufacturing (e.g. casting, molding), the AM process does not require additional tooling or labor costs to accommodate complex designs.
The 3D hydrogen electrode support design, as described in the section on microstructural engineering, requires an AM process. Among various techniques, such as microextrusion, inkjet printing, and aerosol jet printing, we selected microextrusion. Microextrusion AM is a high-resolution, layer-by-layer process that dispenses materials through a micronozzle (often < 50 µm) to create complex miniature 3D structures [
35], [
36]. This is made possible by extruding inks or pastes of metal or ceramic powder mixed with a binder, which is removed from the part after 3D printing. Key parameters include nozzle diameter, extrusion pressure, print speed, and ink/paste rheology (viscosity). This choice of microextrusion over inkjet and aerosol jet printing is primarily based on its less stringent ink/paste requirements, as well as its high solid-loading capacity. The process is illustrated in
Fig. 9(a), where the YSZ and NiO/YSZ networks are printed by extruding pastes of the materials suspended in organic binders. These deposited layers dry to form a green body, which is configured layer-by-layer using computer-aided design (CAD) software. The green body is then fired at high temperatures to remove the organics and yield a final sintered sample.
Fig. 9(b) displays a representative NiO-YSZ hydrogen electrode support in both its green and sintered states. The fabrication process developed for this 3D support has been scaled up to produce samples of up to 5 cm × 5 cm in size after firing. With 70 wt% NiO content in the NiO/YSZ network, this support demonstrated sufficient conductivity and a stable microstructure after redox cycling. The electrochemical performance of cells using this support remains to be evaluated.
4. Concluding remarks
SOC technology offers a highly efficient solution for power generation and chemical production. It is defined by five core attributes: fuel flexibility, environment compatibility, multifunction capability, system adaptability, and cost affordability. Over the past two decades, the technology has transitioned from R&D to successful demonstration and early commercialization. Units have scaled from watt-level components to kilowatt-level systems, with megawatt-level plants currently under development. Recent breakthroughs in materials science, design, engineering, and advanced manufacturing have significantly enhanced durability and performance, while reducing capital costs. However, achieving market-competitive price points and resolving remaining technical hurdles are essential for widespread adoption.
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