Moving beyond a single d-band descriptor: Coordinated d-p orbital regulation enhances K2NiF4-type cathodes for protonic ceramic fuel cells

Generating electricity efficiently with a low environmental footprint remains a major challenge in the transition toward a sustainable energy future. Protonic ceramic fuel cells (PCFCs) are attracting growing interest as efficient electrochemical energy-conversion devices for their high conversion efficiency, fuel flexibility, and environmentally friendly operation. The intermediate operating temperatures, typically 300-700 °C, can reduce material degradation and enable less costly system components. However, reduced temperatures also lead to lower cathodic activity and sluggish oxygen reduction reaction (ORR) kinetics, making the cathode a major bottleneck in PCFC performance. Developing cathode materials that support efficient electron, oxygen-ion, and proton-related transport is crucial for advancing PCFC technology.

K₂NiF₄-type Ruddlesden-Popper (R-P) oxides have emerged as promising alternatives due to their layered structures, favorable oxygen transport, oxygen non-stoichiometry, and thermal compatibility with proton-conducting electrolytes. Nevertheless, their cathodic performance has generally lagged behind that of reported high-performing perovskite electrodes, particularly under the humid conditions relevant to PCFC operation. Meanwhile, although the d-band center is widely used as an electrocatalytic descriptor, its absolute position alone is insufficient to describe the complex proton-coupled ORR (p-ORR) processes. The relationship among transition-metal 3d states, O 2p states, defect chemistry, and proton transport therefore provides an important opportunity for rational cathode design.

Recently, a team of material scientists led by Jie Hou from the University of South China investigated how electronic-structure regulation could accelerate p-ORR in layered K₂NiF₄-type cathodes. Rather than considering the transition-metal d-band center as an isolated descriptor, the researchers combined d-band center regulation with d-p orbital alignment, oxygen-redox behavior, and defect thermodynamics. They compared Mn-based La_0.5Sr_1.5MnO_4+δ (LSMO) with Fe-based La_0.5Sr_1.5FeO_4+δ (LSFO), integrating density functional theory (DFT) calculations with structural and surface chemical characterization, electrical conductivity relaxation (ECR), atmosphere-dependent electrochemical impedance spectroscopy (EIS) and single-cell testing to establish an electronic structure-defect-transport-performance relationship. This work not only clarifies how coordinated regulation of the d-band center and Fe 3d-O 2p orbital alignment influences oxygen-defect chemistry, proton transport, and p-ORR in K₂NiF₄-type cathodes, but also provides an electronic structure-guided strategy for designing high-performance PCFC cathode materials.

The team published their work in Journal of Advanced Ceramics on September 10, 2026.

“In this article, we investigated how coordinated regulation of the d-band center and metal-oxygen orbital alignment can improve layered K₂NiF₄-type cathodes for PCFCs. Rather than treating the d-band center as a single descriptor, we linked electronic structure with oxygen-defect formation, hydration-related proton incorporation, proton transport, and oxygen reduction,” said Jie Hou, professor at the School of Resource Environment and Safety Engineering, University of South China.

Theoretical calculations show that LSFO possesses a more favorable Fe 3d-O 2p orbital alignment than LSMO. The smaller d-p energy separation and stronger Fe-O interaction provide a favorable electronic environment for oxygen-redox activity, oxygen-defect formation, hydration-related proton incorporation, and interfacial charge redistribution.

“Notably, the Fe 3d-O 2p orbital alignment is closely coupled with the defect and proton-transfer properties of LSFO. The optimized electronic structure lowers the energetic cost of oxygen-vacancy formation and hydration-related proton incorporation. Meanwhile, the proton-hopping barrier decreases from 0.74 eV in LSMO to only 0.24 eV in LSFO, facilitating long-range proton transport through successive hopping processes,” said Hou.

Experimental results further support the theoretical predictions. LSFO exhibits a higher relative abundance of oxygen-vacancy-related surface species, enhanced electronic conductivity, and substantially faster oxygen/proton surface exchange and diffusion. In proton-related ECR measurements, LSFO reaches conductivity equilibrium in approximately 150 s, compared with about 1500 s for LSMO. These improvements indicate that electronic structure regulation facilitates multiple coupled oxygen/proton-related processes involved in the cathodic ORR rather than improving only a single reaction step. As a result, the LSFO cathode delivers peak power densities of 2192, 1477, 853, and 465 mW cm^-2 at 700, 650, 600, and 550 °C, respectively. At 700 °C, its peak power density is approximately 109% higher than that of the LSMO reference cell, while its polarization resistance is only 0.040 Ω cm²,

Crucially, the enhanced electrochemical performance does not compromise operational stability. The LSFO cell maintains stable operation for more than 400 h, with a normalized degradation rate of approximately 3.2% per 1000 h. After durability test, the polarization resistance increases only slightly from 0.209 to 0.215 Ω cm², while post-test characterization reveals no obvious continuous interfacial reaction or severe elemental interdiffusion, supporting the electrochemical and interfacial stability.

“Our findings show that high cathodic activity cannot be attributed solely to the position of the d-band center. Instead, the coordinated relationship among d-p orbital alignment, defect chemistry, electronic conduction, and oxygen/proton transport is important for accelerating p-ORR. This provides a useful strategy for designing high-performance R-P cathodes for PCFCs and related electrochemical energy-conversion devices” said Hou.

Other contributors include Zhe Song, Shaozhi Wang, Dezhi Geng, Zhangjin Xie, and Shenchi Qu from School of Resource Environment and Safety Engineering at University of South China. Zhe Song and Shaozhi Wang contributed equally to this work.

About Author

Jie Hou is a distinguished professor and doctoral supervisor at School of Resource Environment and Safety Engineering, University of South China. He received his PhD degree in Materials Science from University of Science and Technology of China in 2017. From 2016 to 2017, he worked as a visiting scholar in Professor John Irvine’s group at the University of St Andrews, UK, and from 2018 to 2020, as an associate research fellow in Professor Jing-Li Luo’s group at Shenzhen University. He serves as a young editorial board member of Energy Reviews and a senior member of the Chinese Materials Research Society. His research mainly focuses on energy conversion and storage materials, ceramic fuel/electrolysis cells, and functional membrane reactors. He has led several research projects, including those supported by the National Natural Science Foundation of China, and has published more than 50 SCI-indexed papers in journals including Journal of Advanced Ceramics, Fuel, Acta Materialia, Journal of Rare Earths, Rare Metals, Chemical Engineering Journal, Journal of Materials Chemistry A, Journal of Materials Science & Technology, Journal of Power Sources, International Journal of Hydrogen Energy, Electrochimica Acta, Ceramics International, and Journal of Alloys and Compounds. He also holds one authorized national invention patent.

Funding

This work was supported by National Natural Science Foundation of China (No. 51802200) and the Startup Funding for Talents at University of South China.

DOI LINK: 10.26599/JAC.2026.9221375

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC’s 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in “Materials Science, Ceramics” category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508