N-Doped Carbon Nanocatalyst with Zn Single Atom Catalytic Sites from N-Doped Graphene and Metal-Organic Frameworks

Over the past decade, nitrogen-doped carbon materials, particularly nitrogen-doped graphene (N-G), have emerged as highly promising catalysts for various electrochemical reactions. N-G has demonstrated noteworthy catalytic activity for oxygen reduction reactions (ORR), which are crucial for the performance of electrochemical energy systems such as PEM fuel cells, metal-air batteries, supercapacitors, etc. To further enhance N-G's catalytic performance, researchers have integrated it with metal-organic frameworks (MOFs), resulting in N-G/MOF composites. These composites benefit from the combination of N-G's catalytic properties, as well as the micro- and mesoporous structures, and diverse chemically functional sites offered by MOFs, which together promote ORR catalysis. In specific electrochemical conditions, these N-G/MOF composites have sometimes outperformed benchmark ORR catalysts like Pt/C. Typically, the preparation of these catalysts involves high-temperature treatments that remove most of the metal atoms, creating metal-free porous carbon structures along with N-G-like structures. However, the potential of retaining and utilizing these metal sites within N-G/MOF catalysts for ORR catalysis remains an area that has yet to be thoroughly explored.<br/>In our study, we synthesized a nitrogen-doped carbon nanocatalyst by integrating an N-G catalyst with a metal-organic framework (Zeolitic Imidazolate Frameworks-8, ZIF-8) using a Nanoscale High Energy Wet (NHEW) ball milling process. Initially, we produced the N-G catalyst by wet ball milling graphene oxide (GO) with melamine, serving as the carbon and nitrogen precursors, respectively. This N-G catalyst showed performance close to a 10 wt% Pt/C catalyst. Subsequently, we integrated the N-G catalyst with ZIF-8 through a similar NHEW ball milling process, optimizing the structural and functional properties by adjusting the grinding speed and time. The optimal N-G/MOF nanocatalyst was synthesized at a milling speed of 350 rpm for 16 hours, resulting in the best performance as an ORR catalyst. This N-G/MOF catalyst exhibited a higher ORR current density compared to a 10 wt% Pt/C catalyst in both alkaline and acidic environments, with almost the same ORR onset potential. The calculated electron transfer number indicated a 4e- transfer ORR process on the N-G/MOF catalyst. Moreover, the N-G/MOF catalyst retained over 90% of its ORR current density after 2000 cycles of operation. The N-G/MOF nanocatalyst was thoroughly characterized to examine changes in morphology, elemental composition, and chemical bonds using various analytical techniques: scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). The results showed that the N-G/MOF nanocatalyst retained the morphological features of both N-G and ZIF-8. Elemental composition analysis revealed the presence of zinc (Zn) in the material. Detailed XPS analysis of the C 1s and N 1s spectra confirmed the presence of Zn-NC-C type bonds, indicating the formation of Zn single-atom catalytic sites advantageous for ORR catalysis. This finding was further supported by comparing FTIR peaks among N-G, pure ZIF-8, and N-G/MOF. Energy Dispersive Spectroscopy (EDS) analysis showed the atomic-scale dispersion of Zn sites within the nanocatalyst. The N-G/MOF nanocatalyst effectively leveraged the synergistic advantages of combining N-G and ZIF-8, along with the formation of beneficial Zn sites. Hence, this N-G/MOF nanocatalyst performed robustly across different electrochemical environments.