A critical challenge hindering the practical application of lithium-oxygen batteries (LOBs) is the inevitable problems associated with liquid electrolytes, such as evaporation and safety problems. Our study addresses these problems by proposing a modified polyrotaxane (mPR)-based solid polymer electrolyte (SPE) design that simultaneously mitigates solvent-related problems and improves conductivity. mPR-SPE exhibits high ion conductivity (2.8 × 10^-3 S cm^-1 at 25 °C) through aligned ion conduction pathways and provides electrode protection ability through hydrophobic chain dispersion. Integrating this mPR-SPE into solid-state LOBs resulted in stable potentials over 300 cycles. In situ Raman spectroscopy reveals the presence of an LiO₂ intermediate alongside Li₂O₂ during oxygen reactions. Ex situ X-ray diffraction confirm the ability of the SPE to hinder the permeation of oxygen and moisture, as demonstrated by the air permeability tests. The present study suggests that maintaining a low residual solvent while achieving high ionic conductivity is crucial for restricting the sub-reactions of solid-state LOBs.

Hydrogen (H 2 ) is a clean and efficient energy carrier with high gravimetric energy density, making it ideal for storing renewable energy. Proton-exchange membrane water electrolyzers (PEMWEs) are a promising technology for producing green hydrogen due to their high current density, compact design, and low operating temperature. However, the sluggish oxygen evolution reaction (OER) at the anode, which requires four-electron transfer in acidic media, remains a major bottleneck. While iridium oxide (IrO 2 ) is the current benchmark OER catalyst, its high cost and scarcity limit widespread adoption. Ruthenium oxide (RuO 2 ) has emerged as a promising alternative due to its superior OER activity and lower cost, but its poor stability under acidic conditions, caused by Ru leaching and over-oxidation, hinders practical applications. To address this, we developed RuO 2 nanolayers epitaxially grown on rutile titanium dioxide (TiO 2 ) nanofibers (NFs) as a highly efficient and stable acidic OER catalyst (NL-RuO 2 -250). The rutile TiO 2 support was chosen for its excellent stability in acidic environments, moderate conductivity, and isostructural compatibility with RuO 2 , which minimizes interfacial energy and facilitates controlled catalyst growth. The one-dimensional TiO 2 NF structure provides a high surface area and enhances electron transfer, while the RuO 2 (101) crystal facet, predominantly exposed in NL-RuO 2 -250, offers optimized catalytic activity. The catalysts were synthesized through a hydrothermal process at varying pH conditions followed by heat treatment. At neutral pH (7), amorphous RuO x nanolayers formed on the TiO 2 surface and were converted into crystalline nanolayers after heating at 250°C. In contrast, at higher pH (11.5), crystalline RuO 2 nanosheets (NS-RuO 2 ) formed with dominant exposure of the less active (110) facets. At acidic pH (2.5), weak interactions between Ru species and TiO 2 resulted in sparsely distributed RuO x nanoparticles (NP-RuO 2 ). XRD, TEM, and Raman analyses confirmed the epitaxial growth and strong interfacial interactions in NL-RuO 2 -250, which enhanced the stability and electronic properties of the catalyst. Electrochemical testing in a three-electrode system demonstrated that NL-RuO 2 -250 outperformed other catalysts. It required a low overpotential of 230 mV to achieve 10 mA cm -2 and a Tafel slope of 43 mV dec -1 , indicating fast OER kinetics. NL-RuO 2 -250 also showed the highest electrochemical surface area (ECSA) and low charge transfer resistance, attributed to its nanolayer structure and optimized facet exposure. Stability tests revealed minimal performance degradation over 50 hours, with NL-RuO 2 -250 achieving a significantly higher stability number (S-number) compared to other catalysts, indicating reduced Ru leaching. Density functional theory (DFT) calculations revealed that the Ru (101) facets of NL-RuO2-250 facilitate the adsorbate evolution mechanism (AEM), which improves activity and stability by suppressing lattice oxygen participation. In situ Raman spectroscopy further confirmed that NL-RuO 2 -250 maintained stable Ru oxidation states during OER, avoiding over-oxidation and dissolution. When tested in a PEMWE single cell, NL-RuO 2 -250 achieved superior performance, requiring only 1.75 V to deliver 2 A cm -2 . It also demonstrated excellent stability with negligible voltage drop over 24 hours at 0.2 A cm -2 , outperforming commercial RuO 2 . This study highlights the importance of interfacial engineering and facet control in enhancing the performance and stability of Ru-based OER catalysts, offering a viable strategy for advancing PEMWE technology. Acknowledgement: This work is supported by the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Education [NRF-00463589].