Nitrogen-Doped Carbon Materials with Unusual Structural and Electronic Properties Enhancing Electrocatalytic CO₂ Conversion and N₂ Fixation

Nitrogen (N) doping in carbon materials yields carbon nanospikes (CNS) with tip sizes down to 1 nm, which enable electrochemical CO₂ reduction reaction (CO₂RR) selectively to ethanol when coupled with Cu nanoparticles (CuNP),¹ and electrochemical N₂ reduction reaction (NRR) to ammonia if used alone.² Through joint experimental and theoretical studies, we uncovered the unusual effects of N doping that induce metallic properties in graphitic materials via unconventional interlayer covalent pi-pi bonding (so-called pancake bonding) and create an ultrastrong electric field at the sharp tips of CNS. In the first case, we identified that N-doped graphene (NGP) with various doping levels can form 2D covalent pancake bonds, significantly reducing interlayer separations.³ The binding energies can be enhanced by 50% compared to pristine graphene, which relies mainly on van der Waals (vdW) interactions. This unusual chemical bonding results from the covalent pi-pi overlap across the vdW gap, while the individual layers maintain the in-plane pi-conjugation. In NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked, and the 3D bulk exhibits metallic characteristics in both in-plane and stacking directions, beneficial for enhancing electrode conductivity. In the second case, we found that N doping can afford the unique morphology of CNS, featuring intense folds and sharp spikes.⁴ When coupled with electro-nucleated CuNP, CNS can electrochemically reduce CO₂ to ethanol at a rate of 286 μg cm^-2 h^-1 with a high selectivity of 84% and an overall Faradaic efficiency of 90%.¹ If used alone without cocatalysts, CNS can electrochemically reduce N₂ to ammonia at a rate of about 100 μg cm^-2 h^-1 under ambient conditions.² Using multiscale modeling efforts, we found that the strongly positive surface curvature at the sharp tips of CNS can produce an ultrastrong electric field above 10 V/nm, which is 10⁴x stronger than that in cloud-to-ground lightning. This ultrastrong electric field activates the otherwise inert and stable CO₂ and N₂ molecules, both characterized by the triple and double bonds, large ionization potentials, negative electron affinities, and relatively small proton affinities compared to solvent water molecules. The field effects also facilitate electron and proton transfers from CNS and solvent molecules, respectively. The insights gained offer valuable guidance for scientists aiming to enhance carbon-based nanomaterials through N doping toward practical technological applications.<br/><br/><b>Acknowledgments: </b>Part of this work was performed at the Center for Nanophase Materials Sciences, a US Department of Energy Office of Science User Facility.<br/><br/><b>References:</b><br/>1. Song, Y.; Peng, R.; Hensley, D. K.; Bonnesen, P. V.; Liang, L.; Wu, Z.; Meyer III, H. M.; Chi, M.; Ma, C.; Sumpter, B. G.; Rondinone, A. J. <i>ChemistrySelect</i> <b>2016</b>, <i>1</i>, 6055-6061.<br/>2. Song, Y.; Johnson, D.; Peng, R.; Hensley, D. K.; Bonnesen, P. V.; Liang, L.; Huang, J.; Yang, F.; Zhang, F.; Qiao, R.; Baddorf, A. P.; Tschaplinski, T. J.; Engle, N. L.; Hatzell, M. C.; Wu, Z.; Cullen, D. A.; Meyer III, H. M.; Sumpter, B. G.; Rondinone, A. J. <i>Sci. Adv.</i> <b>2018</b>, <i>4</i>, e1700336.<br/>3. Tian, Y.-H.; Huang, J.; Sheng, X.; Sumpter, B. G.; Yoon, M.; Kertesz, M. <i>Nano Lett.</i> <b>2015</b>, <i>15</i>, 5482-5491.<br/>4. Sheridan, L. B.; Hensley, D. K.; Lavrik, N. V.; Smith, S. C.; Schwartz, V.; Liang, C.; Wu, Z.; Meyer III, H. M.; Rondinone, A. J. <i>J. Electrochem. Soc.</i> <b>2014</b>, <i>161</i>, H558–H563.