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Page 2 of 14 Zhuang et al. Energy Mater. 2025, 5, 500015 https://dx.doi.org/10.20517/energymater.2024.90

INTRODUCTION
Energy and environmental issues have become increasingly prominent in recent years . Developing highly
[1]
efficient, sustainable, and clean technologies is a top priority to achieve the goals of carbon neutrality and
sustainable development. Compared to conventional coal-based thermal power generation technology, fuel
cells are power generators that can convert the chemical energy in the fuels directly into electricity through
electrochemical reactions. Unlike conventional systems limited by the Carnot cycle, fuel cells can achieve
[2]
high energy conversion efficiency . Among all fuel cell systems, solid oxide fuel cells (SOFCs) are the most
promising electrochemical conversion devices due to their high efficiency, reliance on non-precious metal
[3,4]
catalyst, and all-solid-state structure . SOFCs generally can be divided into two categories according to the
mobile carriers. The first one is oxygen ion-conducting SOFCs (called O-SOFCs), and the second one is
proton-conducting ceramic fuel cells [named H-SOFCs or protonic ceramic fuel cells (PCFCs)] . Yttrium-
[5]
stabilized zirconia (YSZ) and scandium-stabilized zirconia (SSZ) are the commonly used oxygen ion-
conducting electrolytes for O-SOFCs. They have a higher activation energy (E , ~0.9 eV) for oxygen ion
a
conduction and typically operate at a higher temperature (700-1,000 °C), owning to the larger ionic radius
of the oxygen ions . In contrast, the carrier of PCFCs, proton (H ), has a much smaller radius and needs
[6,7]
+
[8]
lower E (0.3~0.5 eV), making it possible to operate at low temperatures (< 700 °C) . A lower operating
a
temperature can effectively reduce the cost of accessory materials and improve the thermo-mechanical
stability of the cell. In addition, water is generated on the cathode side of PCFCs, which helps to avoid the
fuel dilution and improves the fuel utility [9-11] .

To date, both planar and tubular PCFCs have been widely investigated and significant improvements
[12]
reported . Compared to planar PCFCs, tubular PCFCs have been receiving increasing attention because of
their advantages of high mechanical strength, fast start-up, easy sealing, and good thermal cycle stability .
[13]
However, tubular PCFCs typically exhibit low power density due to the elongated current path, which
[14]
increases the internal resistance . The microtubular structure combines the advantages of both tubular and
planar, featuring a smaller tube radius and shorter current channels, effectively reducing the loss for current
collection and transmission, ultimately enhancing cell performance [15,16] . At the same time, the smaller size
minimizes the impact of stresses from thermal gradients, allowing for rapid start-up, better mechanical
strength, and low thermal shock damage . However, most existing research has been focused on planar
[17]
PCFCs, while there is little information available on developing microtubular PCFCs.

For the fabrication of anode functional layer (AFL) and electrolyte layer of microtubular PCFCs, the dip-
coating method is one of the most commonly used techniques, which only requires one to three dip-coating
cycles to achieve the desired thickness (10-30 μm). However, for the anode supports, which play a critical
role in supporting the whole cell structure, it need to reach a certain thickness of around hundred microns.
Currently, several techniques have been used for the thick anode support fabrication of microtubular
PCFCs, such as phase inversion, dip-coating, and 3D printing. Hou et al. fabricated a high-performance
microtubular PCFC with an outer cell diameter of 0.38 mm via phase inversion method (support layer)
combined with a dip-coating process (electrolyte layer), achieving a maximum power density of 2.62 W cm
-2
at 700 °C . Tong et al. also developed a phase inversion method for the batch preparation of microtubular
[18]
PCFCs with an extremely small outer diameter of 0.15 mm, showing a maximum power density of
601.2 mW cm at 700 °C . The phase inversion method can prepare electrode supports with an open-
-2
[19]
straight pore structure, which facilitates gas transfer and improves electrochemical performance. However,
the large open-straight pore structure PCFCs may have insufficient strength, even in the microtubular
structure, limiting its large-scale practical application . The dip-coating was also widely used for the
[20]
preparation of microtubular PCFCs. Chen et al. prepared a microtubular PCFC with an outer diameter of