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Page 14 of 27 Chen et al. Energy Mater. 2025, 5, 500045 https://dx.doi.org/10.20517/energymater.2024.144

Electrolytes of H-SOECs
The H-SOEC can operate at low temperatures (< 500 °C) due to the use of a proton conductor as the
[115]
electrolyte . BaCeO exhibits high proton conductivity and facile firing, but its diminished chemical
3
stability in steam makes it unfeasible for direct utilization as a SOEC electrolyte . BaCe Y O (BCY),
[116]
3-δ
0.8 0.2
prepared by Y doping, exhibits higher proton conductivity; however, it is not stable in the high
concentration of H O and CO atmospheres. Yang et al. prepared BaCe Zr Y Yb O (BZCYYb) solid
0.1
3
2
2
0.7
0.1 0.1
electrolyte using Yb doping . Results showed that high ionic conductivity and chemical stability were
[117]
archived at low temperatures (500~700 °C), along with excellent resistance to sulfur and coking. As shown
in Figure 7D and E, Gd doping or adding a barrier layer at the electrolyte interface can further
[118]
[119]
improve the performance of SOECs.
In SOEC systems, simultaneous electrolytic oxidation of the reactants at both the cathode and anode can be
achieved, resulting in an enhanced yield. Kim et al. prepared a hybrid SOEC (Hybrid-SOEC, Figure 7F)
[120]
with BaZr Ce Y Yb O as an electrolyte . The electrolysis of 10% H O/H at 700 °C and 1.3 V resulted
0.1
2
2
0.7 0.1
0.1
3-δ
-2
in a current density of 3.16 A·cm , which far exceeded that of the electrolyte material with a single
conductor. Furthermore, the cell shows no significant performance degradation during more than 60 h of
continuous operation. This means that it is a stable and efficient hydrogen production system. In
conclusion, the conventional oxygen ion conductor electrolytes are not suitable for the current
requirements for low-temperature electrolysis. The higher ionic conductivity of proton-conducting
electrolytes at low temperatures aligns with the development trend of low- and medium-temperature SOCs,
making further research in the this area worthwhile. Hybrid-SOEC exhibits a high hydrogen production
rate, but it remains in the laboratory stage of development. In order to accurately represent the performance
differences between different electrolyte materials, the conductivities of various materials have been collated
and are presented in Table 5.
SOEC CONFIGURATIONS
It is essential that the SOECs have a stable structure to ensure the mechanical strength of the cell. As
illustrated in Figure 8, the common SOECs can be classified into three distinct categories based on the type
of support layer: fuel electrode-supported cells (FESCs), oxygen electrode-supported cells (OESCs), and
electrolyte-supported cells (ESCs).

Electrode-supported cells
The resistance of SOECs is attributable to the ohmic resistance of the electrolytes and the polarization
resistance of the electrodes. The utilization of fuel electrodes as the support layer can diminish the
electrolyte thickness, thereby attenuating the impact of ohmic resistance on the electrolytic cell. However,
the anode in FESCs is insufficiently thick, and the oxygen generated during the reaction increases the
oxygen partial pressure at the anode/electrolyte interface, which causes delamination or fracture at the
interface and significantly reduces the stability of SOECs . The selection of an appropriate thickness
[131]
(5-20 µm) for the active electrode layer of the fuel electrode and the reduction of the electrolyte thickness
can result in a reduction of ohmic resistance and operating temperature for the cells. Thus, it can lead to an
enhancement in the electrochemical performance and redox stability of the FESCs .
[132]

OESCs facilitate the reduction of electrolyte and cathode thickness, thereby diminishing the ohmic
resistance of the electrolytic cell. Additionally, they prevent delamination caused by volume changes in the
cathode material. However, conventional perovskites result in a reduction in anode pore structures during
high-temperature sintering, which subsequently causes barriers for gas diffusion. The construction of
asymmetric thick OESCs with a robust anode-electrolyte interface and dendritic anode gas diffusion