PNNL

Abstract

Nitrogen doped carbon (NC) materials exhibit magnetic behavior arising from the interplay of conduction electrons and defect localized spins, and this balance is highly sensitive to gas adsorption. Using temperature dependent electron paramagnetic resonance (EPR) spectroscopy under vacuum and low pressure O2, N2, and H2, we show that defect-rich and defect-poor NCs, despite their differing nitrogen speciation and defect densities, display markedly stronger paramagnetic responses than nitrogen poor Vulcan carbon. Above ~200 K, exposure to all gases enhances the Curie-like contributions in NCs, primarily by promoting electron localization and modifying exchange interactions. Below this threshold, all gases induce pronounced linewidth broadening and reduced EPR intensity, signaling a transition toward antiferromagnetic exchange, with defect-rich NC showing extreme broadening consistent with gas mediated interlayer magnetism. Density functional theory calculations corroborate these findings, revealing that even weakly interacting gases introduce states near the Fermi level, strengthening interlayer exchange in vacancy free regions, while chemisorption at defect sites suppresses spin localization and restores intralayer coupling. Collectively, these results demonstrate that both physisorbed and chemisorbed gases reversibly tune magnetic coupling in N doped carbons by modulating electron localization, defect-driven exchange, and interlayer interactions.