Preamble

Process Analysis of Nuclear Hydrogen Production via Intermediate Temperature SOEC Electrolysis

Qing Shao,¹,²,† Yue Lu,²,† Dun Jin,² Ling-Hong Luo,¹,‡ Xiu-Lin Wang,³ Hui-Chao Yao,³ Ruo-Yun Dai,³ Cheng-Zhi Guan,²,⁴,⁵,§ Guo-Ping Xiao,²,⁴,¶ and Jian-Qiang Wang,⁴,⁵,∗∗

¹School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, China
²Department of Hydrogen Technique, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
³CNOOC Gas and Power Group/R&D Center, Beijing 100020, China
⁴Key Laboratory of Interfacial Physics and Technology, Chinese Academy of Sciences, Shanghai 201800, China
⁵Shanghai Hyenergy Technology Co., Ltd., Shanghai 201800, China

When the operating temperature of a Solid Oxide Electrolysis Cell (SOEC) is lower than the outlet temperature of a nuclear reactor, the reactor can be directly coupled with the SOEC as a high-temperature heat source. However, the efficiency and return on investment of this hybrid energy system critically depend on the expected lifetime of the SOEC. This study assessed the long-term stability of Ni-YSZ|YSZ|GDC|LSC fuel electrode support cells during electrolysis at 650 °C with a current density of −0.5 A·cm⁻² over 1818 h. The average voltage degradation rate of 2.63%·kh⁻¹ occurred in two distinct phases: an initial rapid decay (90–1120 h at 3.58%·kh⁻¹) followed by a stable decay period (1120–1818 h at 2.14%·kh⁻¹), demonstrating the feasibility of coupling SOECs with nuclear reactors at 650 °C. Post-test analysis after 1818 h revealed nickel particle formation associated with Ni(OH)ₓ diffusion and re-deposition, alongside a strontium-containing layer that caused interface cracking. Despite minimal strontium segregation in EDS data, XPS indicated surface segregation of Sr. These findings provide crucial insights into prolonged SOEC operation, highlighting both its potential and challenges.

Keywords: Nuclear hydrogen production, SOEC, Stability, Intermediate temperature

Introduction

Rising global energy demand, coupled with ambitious decarbonization targets, has catalyzed a significant shift toward sustainable energy systems. As traditional energy sources fall out of favor due to environmental impacts, attention has turned to alternatives that promise reduced carbon footprints alongside robust reliability and scalability [1,2]. Nuclear power is particularly promising in this context. Unlike intermittent renewable sources such as solar and wind, nuclear power offers a steady, reliable energy flow without direct greenhouse gas emissions [3,4,5]. Moreover, nuclear energy's role extends beyond electricity generation; it is increasingly recognized as a versatile asset capable of supporting industrial processes through combined heat and power production [6]. This multipurpose utility is crucial for creating integrated, efficient energy systems. One of the most significant aspects of nuclear energy's potential is its ability to produce hydrogen [7].

Hydrogen is increasingly viewed as a critical element for transitioning to a sustainable energy future, serving as a clean energy carrier, an effective energy storage solution, and a valuable feedstock for numerous industrial processes [8–10]. Producing hydrogen via nuclear power satisfies the energy system's need for low-carbon solutions while aligning with global efforts to establish a more sustainable, reliable, and diversified energy supply chain [11].

To harness this potential effectively, advanced nuclear reactor technologies such as Very High-Temperature Reactors (VHTR), Supercritical Water Reactors (SCWR), and Molten Salt Reactors (MSR) have been developed to provide the high-temperature heat necessary for efficient large-scale hydrogen production [12]. VHTRs, which use helium coolant to achieve temperatures exceeding 1000 °C, are ideal for thermochemical hydrogen production at high efficiencies [13,14]. Intermediate-temperature reactors operating at lower temperatures can also play a pivotal role in hydrogen production through technologies like Solid Oxide Electrolysis Cells (SOEC) [7,15].

SOECs represent a compelling technology for hydrogen production, capable of leveraging both electrical and thermal energy from nuclear reactors. These cells operate more efficiently at higher temperatures, which enhances reaction kinetics, making them promising for sustainable hydrogen production [16]. Recognizing this potential, the European Union has funded numerous projects under the Fuel Cells and Hydrogen Joint Undertaking (FCH-JU) to advance SOEC technology and integrate it with existing energy systems [17,18].

Research on SOECs and their integration with nuclear power has been ongoing for years. In 2003, Idaho National Laboratory initiated a major project to explore hybrid nuclear energy systems incorporating SOECs [19,20]. Subsequent studies have examined system efficiencies under various conditions. Peters et al. [21] investigated SOEC system efficiencies with external heat sources, finding they could vary from 90% to 104% depending on configuration and operation. Milewski et al. [22] explored integration of protonic and ionic SOECs with nuclear reactors, calculating hydrogen production energy consumption at 38.83 and 37.55 kWh·kg⁻¹ respectively, which they found significantly efficient. Fütterer et al. [23] examined coupling high-temperature gas-cooled reactors (HTGR) with SOECs within the European GEMINI+ project, suggesting that extending SOEC service life could substantially enhance hybrid system feasibility and efficiency. Yalamati et al. [24] posited that SOEC efficiency depends on the nuclear heat source, suggesting that utilizing both electricity and waste heat from nuclear plants could elevate hydrogen production efficiency to as much as 60%. Zhang et al. [25] calculated thermo-hydrogen conversion efficiencies for different coupling methods, finding that when all SOEC heat is provided by a nuclear reactor, the highest efficiency is achieved with a reactor outlet temperature of 900 °C.

Most research has focused on anode-supported and electrolyte-supported SOECs operating above 750 °C, whereas MSR outlet temperatures are below 700 °C [26]. Since direct coupling offers significant advantages for large-scale hydrogen production, the nuclear reactor's outlet steam temperature should exceed the SOEC operating temperature [25]. In this configuration, nuclear waste heat can be used for heating the SOEC module, gas preheating, and water vaporization [4]. To align with MSR outlet temperatures, the SOEC working temperature should be lowered to an intermediate range of approximately 650 °C.

Although high-temperature SOEC operation (>800 °C) is advantageous, it introduces challenges [27]. At these temperatures, mutual diffusion of cell components can occur at the molecular level, potentially degrading structural integrity and reducing efficiency and operational lifespan. Particle aggregation is another significant concern, as high temperatures can cause particles to clump together, altering material properties crucial for efficient operation. Additionally, selecting appropriate sealants is problematic; many common sealant materials degrade or lose their sealing properties at high temperatures, compromising system effectiveness and safety.

In response to these challenges, research has shifted toward intermediate-temperature SOECs (IT-SOECs) operating at approximately 650 °C. IT-SOECs significantly mitigate high-temperature issues by imposing reduced demands on connectors and seals, aligning with the trend toward mid- to low-temperature operation of SOECs [28].

Given that hybrid energy system efficiency and return on investment are constrained by SOEC service life, investigating long-term stability at intermediate temperatures is crucial. This study aims to reveal alterations in electrochemical behavior through stability tests on commercial Ni-YSZ|YSZ|GDC|LSC planar cells targeting electrolysis at 650 °C. Through voltage-over-time curves and post-test microscopic analysis, we explored degradation mechanisms within various cell components under intermediate temperatures. Extensive R&D in this field highlights the synergy between nuclear power and hydrogen production as a beacon of hope for a sustainable energy future. By leveraging advanced nuclear technologies and innovative hydrogen production methods like SOECs, we can move closer to achieving a low-carbon, high-efficiency energy system that meets global demands while adhering to stringent environmental standards.

Experimental

A. Description of the Hybrid Energy System

The schematic in Fig. 1 [FIGURE:1] illustrates a nuclear-powered hydrogen production system with MSR as the primary energy source. In this system, coolant salt heated in the MSR core flows into a heat exchanger, which plays a pivotal role in transferring thermal energy throughout the system. Following heat exchange, thermal energy drives electricity generation via a turbine-generator setup. The produced electricity is strategically distributed: a portion feeds into the electrical grid, while the remainder powers SOECs for water electrolysis and drives system components including pumps, separators, and compressors essential for operational continuity.

The exchanged heat is utilized for heating the SOEC module, gas preheating, and water vaporization. Notably, the gas produced by the SOEC still possesses high heat value, which is subsequently recovered for processes like water vaporization through another heat exchanger (simplified in the diagram). This waste heat repurposing significantly enhances thermal efficiency.

In the system shown in Fig. 1, heating of the SOEC module, water vaporization, and gas superheating are powered by MSR waste heat. Within this framework, only the electricity required for SOEC water electrolysis (Pₑₗₑ) and for running other components like pumps and compressors (Pₒₜₕₑᵣ) is supplied by MSR-generated electricity. This design effectively utilizes MSR thermal energy for process heating, optimizing energy efficiency by repurposing waste heat for critical thermal processes while relying on MSR-generated electricity for power-driven operations. This integrated approach enhances overall energy efficiency and sustainability, with corresponding efficiency calculated by Eq. (1) [25]:

η = (m·HHV)/(Pₑₗₑ + Pₒₜₕₑᵣ)

where m is the mass flow rate of produced hydrogen, and HHV is hydrogen's high heating value (285.8 kJ·mol⁻¹). The HHV includes energy released from hydrogen combustion and subsequent water vapor condensation.

B. Cell and Test System

This study examined commercial fuel electrode-supported cells, with detailed preparation documented previously [16]. As shown in Fig. 2 [FIGURE:2], cross-sectional microstructure was analyzed via scanning electron microscopy (SEM) to understand material interfaces and layer continuity essential for optimizing performance. The cell structure comprises:

• A fuel electrode support layer of approximately 400 µm Ni/3YSZ (yttria-stabilized zirconia), providing mechanical robustness to withstand operational stresses at elevated temperatures
• An active fuel electrode layer of Ni/8YSZ (≈12 µm) tailored for enhanced catalytic activity and ionic conductivity
• A 3 µm electrolyte layer critical for maintaining high ionic conductivity with minimal electronic leakage, substantially reducing ohmic losses
• A 3 µm gadolinium-doped ceria (GDC) barrier layer mitigating chemical reactivity and inter-diffusion between YSZ electrolyte and La₀.₆Sr₀.₄CoO₃₋δ (LSC) oxygen electrode, stabilizing the triple-phase boundary (TPB)
• A 12 µm LSC oxygen electrode compatible with intermediate-temperature operations

Cell dimensions were 5 cm × 5 cm, with an air electrode measuring 4 cm × 4 cm and an active area of 16 cm², providing meaningful data for industrial applications.

To align with MSR outlet temperatures below 700 °C [26], the SOEC operating temperature was reduced to approximately 650 °C. The cell was placed in a furnace at 650 °C to simulate MSR-SOEC coupling. Electrochemical performance was evaluated using a custom-designed integrated testing station (Fig. 3 [FIGURE:3]) engineered to provide a highly controlled environment for reproducible measurements. The gas supply system managed feed gases for both electrodes using high-quality mass flow controllers. The fuel electrode received 80% steam (230 ml·min⁻¹) and 20% hydrogen (58 ml·min⁻¹), with hydrogen serving as a protective gas to prevent Ni oxidation. The oxygen electrode received pure air at 1152 ml·min⁻¹. The testing station performed various electrochemical tests including Electrochemical Impedance Spectroscopy (EIS), current-voltage (I-V) measurements, and durability testing, with accurate temperature control via a thermal management system ensuring reliability and reproducibility [16].

C. Experimental Procedures

Experimental assessments began with current-voltage polarization curves to explore initial cell performance. The fuel electrode received 80% steam while operating temperature varied between 600, 650, 700, and 750 °C to investigate temperature effects. Stability testing was then conducted at 650 °C with a current density of −0.5 A·cm⁻² and 80% steam partial pressure, which increases cell voltage and gas diffusion impedance [29,30].

To understand degradation mechanisms, post-test characterization was performed using SEM and energy dispersive spectrometry (EDS) on the "aging cell" after 1800 h of stability testing. SEM provided insights into morphological changes, while EDS revealed compositional shifts such as active element depletion and secondary phase formation. A reference cell from the same production batch, subjected only to reduction, served as a comparison baseline.

Results and Discussion

Initial Performance and Cell Evolution

Initial I-V polarization tests (Fig. 4a [FIGURE:4]) provided crucial data on electrochemical behavior under operational conditions, establishing a baseline for durability assessment. Tests were conducted at 230 ml·min⁻¹ steam (fuel electrode) and 1152 ml·min⁻¹ air (air electrode). Performance varied across 600–750 °C, showing an inverse relationship between temperature and open-circuit voltage, consistent with the Nernst equation [31]:

E = E⁰ − (RT/2F) ln(PH₂O/PH₂ · PO₂¹ᐟ²)

where E⁰ is the theoretical standard-state voltage (Nernst electromotive force), R is the gas constant, T is absolute temperature, F is Faraday's constant, PH₂O and PH₂ are partial pressures of steam and hydrogen at the fuel electrode, and PO₂ is oxygen partial pressure at the air electrode.

At higher temperatures (750 °C and 700 °C), I-V curves showed current limitations with disproportionate voltage increases relative to current density, indicating non-linear characteristics. This behavior, with turning points at 48.48% steam conversion, likely results from concentration polarization due to gas diffusion limitations [31]. While enhanced kinetic processes occur at higher temperatures, physical transport of reactants to the TPB becomes limiting.

Although higher temperatures enable greater current density and hydrogen production efficiency, this study selected 650 °C to align with nuclear reactor outlet temperatures. At 650 °C, the I-V curve showed linearity, suggesting adequate gas supply for efficient electrolysis.

EIS results at different temperatures (Fig. 4b) reveal how temperature affects cell efficiency. Lower temperatures increase both ohmic and polarization impedances due to pronounced effects on oxygen ion transport kinetics, Ni catalytic effectiveness, and charge transfer dynamics at the TPB [32,33]. Temperature significantly influences oxygen ion mobility; lower temperatures slow kinetics and decrease ionic conductivity. Nickel catalytic activity also decreases at lower temperatures, impairing electrochemical reaction efficiency, particularly for hydrogen evolution. At the TPB, lower temperatures hinder charge transfer dynamics, increasing polarization impedance.

Specific values highlight more favorable conditions at 650 °C: ohmic impedance of 0.22 Ω and polarization impedance of 0.14 Ω, compared to 600 °C where a significantly increased impedance arc was observed for charge transfer at the fuel electrode. This suggests 650 °C is more suitable for long-term stability research, providing better conductivity and catalytic activity within the temperature window for MSR-SOEC coupling while minimizing impedance losses.

Long-Term Stability

Long-term stability is crucial for practical SOEC application, particularly in nuclear-hydrogen systems. Figure 5 [FIGURE:5] presents the voltage-time (V-T) curve in galvanostatic mode, showing performance and degradation over 1818 h at −8 A (−0.5 A·cm⁻²) and 650 °C with the same atmosphere as initial performance tests. The cell demonstrated stable SOEC-mode operation for over 1800 h.

The V-T curve divides into three distinct phases with different degradation rates:

1. Initial phase (0–90 h): Curve fitting yielded a slope of −3.604 × 10⁻⁵, indicating a degradation rate of −3.42%·kh⁻¹. The negative slope suggests an activation process where cell components stabilize and improve performance [33].

2. Rapid decay phase (90–1120 h): Voltage increased from 1.101 V to 1.136 V, representing a linear degradation rate of 3.58%·kh⁻¹, reflecting significant degradation as reported by Wang et al. [16,34].

3. Steady decay phase (1120–1818 h): Voltage continued increasing but at a slower rate, with average degradation of 2.14%·kh⁻¹. The reduced slope indicates transition to a more stable phase, consistent with patterns reported in Ref. [35].

The overall average degradation rate from 90–1818 h was 2.63%·kh⁻¹. This long-term stability suggests substantial potential for coupling SOECs with nuclear energy systems for water electrolysis. The relatively low degradation rates, particularly during the steady decay phase, indicate that SOECs can maintain efficiency over extended periods, making them suitable for continuous hydrogen production.

Post-Test Cell Analysis

Understanding degradation factors in intermediate-temperature SOECs is critical. SEM and EDS analyzed microstructure and elemental distribution to examine physical and chemical changes in the fuel electrode over time.

Cross-sectional SEM images (Fig. 6 [FIGURE:6]) reveal significant microstructural changes. Unlike the blocky Ni-YSZ structure in the reference cell (Fig. 6a), numerous ≈100 nm particles appeared near the electrolyte interface after long-term testing (Fig. 6b), indicating alterations that could impact performance and stability [36,37]. Microcracks and voids between the electrolyte and GDC barrier layer were also observed. These defects impede ionic conductivity and exacerbate degradation, further reducing cell efficiency [38]. Additionally, a dense layer between the electrolyte and GDC barrier suggests secondary phase formation from electrode-electrolyte interactions, introducing additional resistive interfaces that impact electrochemical performance [39].

Further analysis of the Ni-YSZ fuel electrode (Fig. 7 [FIGURE:7]) showed no obvious Ni migration. Ni migration in SOECs is a complex phenomenon influenced by electrolysis current density, electric field distribution, hydrogen/steam concentration gradients, and temperature gradients [40]. The absence of pronounced Ni migration after 1800 h suggests that conditions didn't reach thresholds for significant migration, or that migration rates were too slow for detection. This critical finding indicates Ni-YSZ stability can be maintained under certain operational conditions, potentially enabling longer cell lifetimes.

However, EDS mapping (Fig. 8 [FIGURE:8]) revealed minute nickel-containing particles. Mogensen et al. [41,42] attributed Ni migration dynamics to Ni(OH)ₓ volatile species diffusion driven by vapor pressure gradients. Under elevated water vapor pressure (>80%), nickel reacts with water, forming Ni(OH)ₓ (x = 1 or 2). In SOEC mode, current flows from the oxygen electrode through the electrolyte to the fuel electrode, which receives steam-rich gas. Steam electrolyzes at the electrode into hydrogen and oxygen ions, with x = 1 predominant. Ni(OH)ₓ diffuses from high to low concentration along the PH₂O gradient. In SOEC mode, diffusion occurs from the support layer to the fuel electrode interior, driven by reactions where nickel generates and deposits, contributing to migration.

EDS point scanning (Fig. 9 [FIGURE:9]) of particle aggregation and block-like regions revealed stark Ni content contrast: 6.67% Ni in blocky regions versus 73.56% in particle agglomeration areas. This supports previous observations and underscores the complexity of Ni migration, highlighting the need for further research to understand and mitigate these processes through optimized operational parameters, microstructural analysis, and material innovations.

Microstructural analysis of the oxygen electrode post-electrolysis using SEM and EDS revealed changes affecting performance and durability. SEM images (Fig. 10 [FIGURE:10]) show microcracks at the oxygen electrode-electrolyte interface, undermining mechanical stability and accelerating degradation under operational stresses [38]. A dense interfacial layer likely resulted from Sr segregation. EDS mapping indicates this layer comprises strontium-containing phases such as SrZrO₃, an electrical insulator with poor ionic conductivity (1.87 × 10⁻⁶ S·cm⁻¹ at 800 °C) [43]. SrZrO₃ formation obstructs ion migration [44], while Sr diffusion to YSZ surfaces depletes Sr from the LSC electrode, promoting formation of Sr-free LaCoO₃ with reduced electrocatalytic activity for the oxygen reduction reaction (ORR) [45]. These high-resistance Sr-containing phases, combined with potential electrolyte cracking, increase overall cell resistance [46,47], leading to higher energy losses and decreased efficiency that pose significant challenges for long-term operation.

While EDS mapping indicated no significant Sr segregation toward the surface, XPS provided deeper insight into surface chemistry. Figure 11 [FIGURE:11] shows Sr 3d photoelectron spectra for reference and aged SOECs. The Sr 3d energy level exhibited dual states: low binding energy (BE) 3d₅/₂ (≈131.9 eV) indicating lattice Sr, and high BE 3d₃/₂ (≈133.3 eV) corresponding to surface Sr [48], speculated to comprise SrO, Sr(OH)₂, and SrCO₃ [49,50]. Quantitative analysis revealed surface Sr proportion increased from 39.42% in the reference sample to 49.5% in the aged sample, indicating enhanced Sr surface segregation after 1818 h despite minimal EDS intensity differences.

Sr segregation in LSC electrodes is well-documented, influenced by material stoichiometry [51], strain [52], temperature [51], oxygen partial pressure [53], LSC microstructure [54], and electrochemical polarization [55]. Under this study's relatively mild electrolysis conditions, Sr segregation likely resulted from Sr reaction with atmospheric CO₂ or H₂O, forming stable SrCO₃ and Sr(OH)₂ compounds that promote Sr migration from bulk to surface [47].

Conclusion

Achieving direct MSR-SOEC coupling requires robust SOEC stability below nuclear reactor outlet temperatures. Stability tests were conducted on Ni-YSZ|YSZ|GDC|LSC fuel electrode support cells at 650 °C with −0.5 A·cm⁻² current density. From 90–1818 h, an average voltage degradation rate of 2.63%·kh⁻¹ was observed, occurring in distinct stages: initial rapid decay (90–1120 h at 3.58%·kh⁻¹) followed by stable decay (1120–1818 h at 2.14%·kh⁻¹). These results demonstrate SOEC viability for direct MSR coupling at intermediate temperatures, showing moderate voltage degradation and commendable long-term stability at 650 °C.

After 1818 h of electrolysis, minute nickel particles were observed in the Ni-YSZ fuel electrode, possibly associated with Ni(OH)ₓ diffusion and re-deposition. Concurrently, a compact strontium-containing layer formed at the oxygen electrode-electrolyte interface, causing interface microcracking. Although pronounced Sr surface segregation was not evident in EDS analyses, XPS data revealed a Sr segregation tendency.

To further improve cell stability and enhance hybrid energy system efficiency and economic return, efforts should focus on mitigating Ni migration in the fuel electrode and preventing Sr segregation in the oxygen electrode. Strategies may include optimizing material compositions, refining microstructural designs, and controlling operational parameters to reduce degradation impacts.

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