PNNL

The Science 

Researchers developed a synthesis method to precisely control dopant speciation and clustering in N-containing layered carbon materials. They systematically varied the synthesis conditions of N-doped carbon materials to facilitate or inhibit N-doping of a defect within the graphitic lattice. Their studies show that the choice of precursor material—either graphene oxide or carbon nitride—and synthesis temperature can control the formation of N-motifs within the carbon-based materials. The formation of specific N-motifs was confirmed using coupled spectroscopic, microscopic, and computational techniques.

The Impact

N-doping of carbon-based materials led to changes in their electronic properties and behavior under a hydrogen atmosphere. Properly tuned N-doped carbons hold immense promise as low-cost materials for hydrogen activation and storage. Healing the thermal-generated defects with nitrogen provides the ability to control the thermodynamics of hydrogen binding, and the kinetics of hydrogen mobility strongly depend on how their dopants are arranged. This research unveils how to precisely control the structure and location of dopants in carbon-based materials. Precise control of dopant distribution is critical for achieving structures with reversible activity toward activation of molecular hydrogen.

Summary

Researchers synthesized the nitrogen-doped carbon materials from either graphene oxide or carbon nitride precursors. Materials from graphene oxide precursors form extended graphitic domains. N substitution predominantly occurs at defect sites and leads to a large fraction of pyridinic N. The most stable motifs are N3 and N4 at single vacancy and 5–8–5 defects, respectively. The most common motif in carbon nitride–derived materials (NCs) is CN3, which is analogous to the N3 configuration at a single vacancy site. The increase in annealing temperature results in a loss of isolated graphitic N, thereby increasing the relative content of CN3 motifs and enhancing NC activity. N3 and CN3 motifs have different local electron-rich environments, which control hydrogen activation kinetics. These complementary synthesis approaches provide a method for controlling N speciation at constant N content for fine-tuning the activity of N-doped carbon materials for hydrogen storage.

Computational studies predicted the most favorable N-motifs, which were compared against experimental data from the synthesized materials. The formation of the most stable N-motifs was confirmed through extensive characterization via X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy, in situ high-pressure nuclear magnetic resonance, and core-level XPS simulations. The thermocatalytic activity for molecular hydrogen activation of the synthesized materials was tested through an evaluation of hydrogen–deuterium exchange.

Contact

Maria Sushko 
Pacific Northwest National Laboratory 
maria.sushko@pnnl.gov

Funding

This research was performed at Pacific Northwest National Laboratory (PNNL) with support from the Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) program, Division of Materials Sciences and Engineering, FWP 80110. XPS simulations were supported by the DOE BES, Division of Chemical Sciences, Geosciences, and Biosciences under the Center for Scalable Predictive methods for Excitations and Correlated phenomena (SPEC), which is funded as part of Computational Chemical Sciences, FWP 70942. X-ray diffraction and transmission electron microscopy experiments were performed under user proposal 61041 at the Environmental Molecular Sciences Laboratory, a DOE, Office of Science, Biological and Environmental Research program user facility located at PNNL. Simulations were performed using the resources of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility supported under contract no. DE-AC02-05CH11231 using NERSC award BES-ERCAP0027895. PNNL is operated by Battelle for DOE under contract no. DE-AC05-76RLO1830.