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La/Pr co-doped ceria (LCP) is processed to fabricate low-temperature ceramic fuel cell based on industrial-grade rare-earth carbonate electrolyte that is reached above a maximum power density of 750 mW/cm 2 at 520 °C. The charge carriers are investigated through LCP fuel cell having symmetric NCAL (Ni 0.8 Co 0.15 Al 0.05 LiO 2-δ ) electrodes using proton conductor BCZY (BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ ) as a blocking layer and are found protons that dominate during the cell operation. The results of associated characterizations for HCC (hydrogen concentration cell) and the OCC (oxygen concentration cell) reveal that LCP material is mixed conductor of both protons and oxygen ions simultaneously. Transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analysis before and after the electrochemical testing of the cell are performed which show an amorphous layer of LiOH/Li 2 CO 3 mixture that is formed after the tests on the surface of LCP structure. Conceptually, it looks that LiOH/Li 2 CO 3 mixture in molten state in the interface region of two-phase material promotes the proton conduction through LCP electrolyte, with negligible oxygen ion conduction.

Among various types of alternative energy devices, solid oxide fuel cells (SOFCs) operating at low temperatures (300‐600°C) show the advantages for both stationary and mobile electricity production. Proton‐conducting oxides as electrolyte materials play a critical role in the low‐temperature SOFCs (LT‐SOFCs). This review summarizes progress in proton‐conducting solid oxide electrolytes for LT‐SOFCs from materials to devices, with emphases on (1) strategies that have been proposed to tune the structures and properties of proton‐conducting oxides and ceramics, (2) techniques that have been employed for improving the performance of the protonic ceramic‐based SOFCs (known as PCFCs), and (3) challenges and opportunities in the development of proton‐conducting electrolyte‐based PCFCs. Protonic ceramic fuel cells have attracted the increased attention in the last 20 years. This review summarizes progress in proton‐conducting solid‐oxide electrolytes for low temperature protonic ceramic fuel cells.

Protonic ceramic electrochemical cells hold promise for operation below 600 °C (refs. 1 , 2 ). Although the high proton conductivity of the bulk electrolyte has been demonstrated, it cannot be fully used in electrochemical full cells because of unknown causes 3 . Here we show that these problems arise from poor contacts between the low-temperature processed oxygen electrode–electrolyte interface. We demonstrate that a simple acid treatment can effectively rejuvenate the high-temperature annealed electrolyte surface, resulting in reactive bonding between the oxygen electrode and the electrolyte and improved electrochemical performance and stability. This enables exceptional protonic ceramic fuel-cell performance down to 350 °C, with peak power densities of 1.6 W cm −2 at 600 °C, 650 mW cm −2 at 450 °C and 300 mW cm −2 at 350 °C, as well as stable electrolysis operations with current densities above 3.9 A cm −2 at 1.4 V and 600 °C. Our work highlights the critical role of interfacial engineering in ceramic electrochemical devices and offers new understanding and practices for sustainable energy infrastructures. A simple acid treatment can improve high-temperature annealed electrolyte surfaces, resulting in improved performance and stability at lower temperatures for protonic ceramic fuel/electrolysis cells, offering new understanding for sustainable energy infrastructures.

AbstractsProton-conducting ceramics (PCCs) are of considerable interest for use in energy conversion and storage applications, electrochemical sensors, and separation membranes. PCCs that combine performance, efficiency, stability, and an ability to operate at low temperatures are particularly attractive. This review summarizes the recent progress made in the development of low-temperature proton-conducting ceramics (LT-PCCs), which are defined as operating in the temperature range of 25–400 °C. The structure of these ceramic materials, the characteristics of proton transport mechanisms, and the potential applications for LT-PCCs will be summarized with an emphasis on protonic conduction occurring at interfaces. Three temperature zones are defined in the LT-PCC operating regime based on the predominant proton transfer mechanism occurring in each zone. The variation in material properties, such as crystal structure, conductivity, microstructure, fabrication methods required to achieve the requisite grain size distribution, along with typical strategies pursued to enhance the proton conduction, is addressed. Finally, a perspective regarding applications of these materials to low-temperature solid oxide fuel cells, hydrogen separation membranes, and emerging areas in the nuclear industry including off-gas capture and isotopic separations is presented.

Protonic ceramic fuel cells, like their higher-temperature solid-oxide fuel cell counterparts, can directly use both hydrogen and hydrocarbon fuels to produce electricity at potentially more than 50 per cent efficiency 1 , 2 . Most previous direct-hydrocarbon fuel cell research has focused on solid-oxide fuel cells based on oxygen-ion-conducting electrolytes, but carbon deposition (coking) and sulfur poisoning typically occur when such fuel cells are directly operated on hydrocarbon- and/or sulfur-containing fuels, resulting in severe performance degradation over time 3 – 6 . Despite studies suggesting good performance and anti-coking resistance in hydrocarbon-fuelled protonic ceramic fuel cells 2 , 7 , 8 , there have been no systematic studies of long-term durability. Here we present results from long-term testing of protonic ceramic fuel cells using a total of 11 different fuels (hydrogen, methane, domestic natural gas (with and without hydrogen sulfide), propane, n -butane, i -butane, iso-octane, methanol, ethanol and ammonia) at temperatures between 500 and 600 degrees Celsius. Several cells have been tested for over 6,000 hours, and we demonstrate excellent performance and exceptional durability (less than 1.5 per cent degradation per 1,000 hours in most cases) across all fuels without any modifications in the cell composition or architecture. Large fluctuations in temperature are tolerated, and coking is not observed even after thousands of hours of continuous operation. Finally, sulfur, a notorious poison for both low-temperature and high-temperature fuel cells, does not seem to affect the performance of protonic ceramic fuel cells when supplied at levels consistent with commercial fuels. The fuel flexibility and long-term durability demonstrated by the protonic ceramic fuel cell devices highlight the promise of this technology and its potential for commercial application. Tests on a versatile protonic ceramic fuel cell resistant to carbon deposition and sulfur poisoning show that its durability and the wide range of fuels it can accept make it suitable for use in industry in the near future.

Because of the generally lower activation energy associated with proton conduction in oxides compared to oxygen ion conduction, protonic ceramic fuel cells (PCFCs) should be able to operate at lower temperatures than solid oxide fuel cells (250° to 550°C versus ≥600 °C) on hydrogen and hydrocarbon fuels if fabrication challenges and suitable cathodes can be developed. We fabricated the complete sandwich structure of PCFCs directly from raw precursor oxides with only one moderate-temperature processing step through the use of sintering agents such as copper oxide. We also developed a proton-, oxygen-ion–, and electron-hole–conducting PCFC-compatible cathode material, BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1), that greatly improved oxygen reduction reaction kinetics at intermediate to low temperatures. We demonstrated high performance from five different types of PCFC button cells without degradation after 1400 hours. Power densities as high as 455 milliwatts per square centimeter at 500°C on H2 and 142 milliwatts per square centimeter on CH4 were achieved, and operation was possible even at 350°C.

Protonic ceramic fuel cells (PCFCs) are more suitable for operation at low temperatures due to their smaller activation energy ( E a ). Unfortunately, the utilization of PCFC technology at reduced temperatures is limited by the lack of durable and high-activity air electrodes. A lot number of cobalt-based oxides have been developed as air electrodes for PCFCs, due to their high oxygen reduction reaction (ORR) activity. However, cobalt-based oxides usually have more significant thermal expansion coefficients (TECs) and poor thermomechanical compatibility with electrolytes. These characteristics can lead to cell delamination and degradation. Herein, we rationally design a novel cobalt-containing composite cathode material with the nominal composition of Sr 4 Fe 4 Co 2 O 13+ δ (SFC). SFC is composed of tetragonal perovskite phase (Sr 8 Fe 8 O 23+ δ , I 4/ mmm , 81 wt.%) and spinel phase (Co 3 O 4 , Fd 3̄ m , 19 wt.%). The SFC composite cathode displays an ultra-high oxygen ionic conductivity (0.053 S·cm −1 at 550 °C), superior CO 2 tolerance, and suitable TEC value (17.01 × 10 −6 K −1 ). SFC has both the O 2− /e − conduction function, and the triple conducting (H + /O 2− /e − ) capability was achieved by introducing the protonic conduction phase (BaZr 0.2 Ce 0.7 Y 0.1 O 3− δ , BZCY) to form SFC+BZCY (70 wt.%:30 wt.%). The SFC+BZCY composite electrode exhibits superior ORR activity at a reduced temperature with extremely low area-specific resistance (ASR, 0.677 Ω·cm 2 at 550 °C), profound peak power density (PPD, 535 mW·cm −2 and 1.065 V at 550 °C), extraordinarily long-term durability (> 500 h for symmetrical cell and 350 h for single cell). Moreover, the composite has an ultra-low TEC value (15.96 × 10 −6 K −1 ). This study proves that SFC+BZCY with triple conducting capacity is an excellent cathode for low-temperature PCFCs.

Protonic ceramic electrochemical cells (PCECs) can be employed for power generation and sustainable hydrogen production. Lowering the PCEC operating temperature can facilitate its scale-up and commercialization. However, achieving high energy efficiency and long-term durability at low operating temperatures is a long-standing challenge. Here, we report a simple and scalable approach for fabricating ultrathin, chemically homogeneous, and robust proton-conducting electrolytes and demonstrate an in situ formed composite positive electrode, Ba 0.62 Sr 0.38 CoO 3 − δ –Pr 1.44 Ba 0.11 Sr 0.45 Co 1.32 Fe 0.68 O 6 − δ , which significantly reduces ohmic resistance, positive electrode–electrolyte contact resistance and electrode polarization resistance. The PCECs attain high power densities in fuel-cell mode (~0.75 W cm −2 at 450 °C and ~0.10 W cm −2 at 275 °C) and exceptional current densities in steam electrolysis mode (−1.28 A cm −2 at 1.4 V and 450 °C). At 600 °C, the PCECs achieve a power density of ~2 W cm −2 . Additionally, we demonstrate the direct utilization of methane and ammonia for power generation at <450 °C. Our PCECs are also stable for power generation and hydrogen production at 400 °C. The typically high temperatures (≥500 °C) at which ceramic electrochemical cells operate place constraints on device materials and construction. Here Liu and colleagues design reversible proton-conducting electrochemical cells that can operate with high performance at temperatures of 450 °C and below.

With the material system operating at lower temperatures, protonic ceramic electrochemical cells (PCECs) can offer high energy efficiency and reliable performance for both power generation and hydrogen production, making them a promising technology for reversible energy cycling. However, PCEC faces technical challenges, particularly regarding electrode activity and durability under high current density operations. To address these challenges, we introduce a nano-architecture oxygen electrode characterized by high porosity and triple conductivity, designed to enhance catalytic activity and interfacial stability through a self-assembly approach, while maintaining scalability. Electrochemical cells incorporating this advanced electrode demonstrate robust performance, achieving a peak power density of 1.50 W cm⁻ 2 at 600 °C in fuel cell mode and a current density of 5.04 A cm −2 at 1.60 V in electrolysis mode, with enhanced stability on transient operations and thermal cycles. The underlying mechanisms are closely related to the improved surface activity and mass transfer due to the dual features of the electrode structure. Additionally, the enhanced interfacial bonding between the oxygen electrode and electrolyte contributes to increased durability and thermomechanical integrity. This study underscores the critical importance of optimizing electrode microstructure to achieve a balance between surface activity and durability. Protonic ceramic electrochemical cells, operating at lower temperatures, offer efficient power generation and hydrogen production, but they face challenges related to electrode activity and durability. Here, a scalable nano-porous electrode design enhances performance, stability, and long-term reliability.

Low temperature ionic conducting materials such as OH − and H + ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH − /H + conduction in ceramic materials. SrZr 0.8 Y 0.2 O 3- δ exhibits a high ionic conductivity of approximately 0.01 S cm −1 at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr 0.8 Y 0.2 O 3- δ . The OH − ionic conduction of CaZr 0.8 Y 0.2 O 3- δ in water was demonstrated by electrolysis of both H 2 18 O and D 2 O. The ionic conductivity of CaZr 0.8 Y 0.2 O 3- δ in 6 M KOH solution is around 0.1 S cm −1 at 90 °C, 100 times higher than that in pure water, indicating increased OH − ionic conductivity with a higher concentration of feed OH − ions. Density functional theory calculations suggest the diffusion of OH − ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices. Low temperature ionic conducting materials such as OH - and H + ionic conductors are important electrolyte materials. Here the authors report the discovery of fast mixed OH - /H + conductors in ceramic materials, SrZr0.8Y0.2O3-δ and CaZr0.8Y0.2O3-δ, for potential use as electrolytes in fuel cells.