Structurally engineered CNT-confined Mn _<i>x</i> Ru _{1− <i>x</i>} O ₂ catalysts for efficient acidic oxygen evolution at low Ru loading

Developing acidic oxygen evolution reaction (OER) catalysts with low noble metal loading and high activity remains a critical challenge for advancing proton exchange membrane water electrolyzers. Herein, we report structurally engineered Mn _<i>x</i> Ru_1-<i>x</i> O₂ catalysts confined on carbon nanotubes (CNTs), enabling highly dispersed active sites and remarkable catalytic activity at low Ru content. The uniform nanoscale coating of Mn _<i>x</i> Ru_1-<i>x</i> O₂ along CNT sidewalls promotes Mn-O-Ru interfacial bonding and establishes an electron-bridge for enhanced charge transfer. The optimized CNT-(Mn_0.75Ru_0.25)O₂ catalyst delivers a low overpotential of 120 mV at 10 mA cm^-2 and an exceptional mass activity of 5549 A g_Ru ^-1 at 270 mV-252 times that of commercial RuO₂ (22 A g_Ru ^-1). Combined X-ray spectroscopy, <i>in situ</i> Raman spectroscopy, and differential electrochemical mass spectrometry reveal that the electron-rich Ru centers stabilized by Mn-O bridges accelerate charge transfer and suppress Ru dissolution during the OER. Moreover, the CNT substrate and Ru incorporation synergistically generate abundant oxygen vacancies, significantly enhancing the catalytic activity through an improved lattice oxygen-mediated mechanism. This work highlights the critical role of CNT confinement and interfacial electronic modulation in decoupling noble metal usage from performance, offering a versatile design strategy for next-generation acidic OER catalysts.