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

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.