Multiple Dynamic Nuclear Polarization Mechanisms in Carbonaceous Materials: From Exogenous to Endogenous 13C Dynamic Nuclear Polarization-Nuclear Magnetic Resonance up to Room Temperature

Abstract

Carbonaceous materials are ubiquitous in energy storage and conversion systems due to their versatile chemical and physical properties. The surface chemistry of carbonaceous materials strongly affects their properties, and there is therefore great interest in determining the chemical composition of different carbon allotropes. Solid-state nuclear magnetic resonance spectroscopy is well suited for providing atomic-level structural information, especially when equipped with sensitivity from dynamic nuclear polarization (DNP). Exogenous DNP from nitroxide biradicals is the most efficient approach, typically providing 102–104 fold enhancement in surface sensitivity. Herein, we consider the application of DNP in the study of the surface chemistry of carbonaceous materials through natural abundance 13C detection. We found that with TEKPol biradicals, the polarization transfer via 1H-13C cross-polarization is limited to the solvent and does not propagate to the sample surface. We thus investigated in detail polarization transfer directly to 13C nuclei and found multiple interfering mechanisms when employing the exogenous DNP approach: (a) direct 13C polarization from the biradicals, (b) opposite enhancements due to heteronuclear cross-relaxation leading to the solvent Overhauser effect, and (c) solid effect from defects and delocalized electrons within the carbons. While the endogenous electron polarization interferes with the utilization of exogenous DNP, it provides significant surface sensitivity with signal enhancements of up to 50 and 20 for 13C and 1H, respectively. Moreover, we show that endogenous DNP can be used at a wide range of temperatures, providing close to a 10-fold increase in 13C and 1H signals at room temperature through differing DNP mechanisms. This approach opens the way for efficient detection of carbon surface chemistry under ambient conditions.