Defects on Graphene for Hydrogen and Oxygen Electrocatalysis

Abstract

Electrocatalytic energy conversion reactions as a branch of catalysis, are widely utilized in industry and continuously hot in academic research. Hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are among the most common reactions in electrocatalysis, which are applied in water splitting, fuel cells and zinc-air batteries etc. Normally, the ideal benchmark catalysts for HER, OER and ORR are noble metal based catalysts, such as platinum and iridium. However, the low storage and high cost of noble metal catalysts become the distinct barrier impeding the large-scale deployment. To develop a commercially affordable electrocatalyst with comparable activity to noble metal catalysts, a series of catalysts originated from the defective carbon emerged. These defective carbon catalysts (DCCs) not only exhibited high performances for electrocatalysis, but presented robust stabilities in the long term test under rigorous conditions from acidic media to alkaline media. With the aim to further promote the performance of DCC, I functionalized the carbon defects using a couple of methods, including trapping single metal atoms on the defective sites and hybridizing the defective graphene (DG) with exfoliated nickel/iron-layers doubled hydroxide (NiFe-LDH) sheets. The defects derived catalysts showed remarkable activities in the electrocatalysis, much superior to the corresponding noble metal catalysts. Moreover, the intrinsic nature of the high activity was investigated and the interactions between the defective sites and the activity was subsequently proposed. First of all, in previous studies, DCC was reported as a potential catalysts for ORR experimentally. However, the mechanism was still inexplicit, since the carbon defects had never been directly observed in those amorphous carbon catalysts. More evidence and mechanism study were highly desired to bring defect catalysis to light. With this respect, I chose graphene as the precursor considering that defects could be witnessed on this two-dimensional morphology. Unexpectedly, DG prepared from pristine graphene was found to be tri-functional in HER, OER and ORR. The microscopic images confirmed the existence of the abundant defects on DG. The later Density Functional Theory (DFT) calculations further revealed that different types of defects showed different activities for HER, OER and ORR. Although the defective carbon catalysts exhibited superior activities among metal free catalysts, it is worth to note that, compared to noble metal catalysts, defective carbon catalysts are not competitive and hard to challenge the market place of noble metal catalysts (especially for HER and ORR). Functionalization of defective carbon with trace metal elements was a plausible strategy, not only improving the activities, but controlling the consumption of the unsustainable metal resources thereby the cost. Transition metal catalysts were found to possess high activities in the electrocatalysis. To this end, I dispersed the single nickel (Ni) atoms on the defective sites of DG, enhancing the HER and OER activities, as well as reducing the usage of Ni. The X-ray adsorption characterization and the DFT calculation revealed that the diverse defects on graphene can induce different local electronic densities of state (DOS) of atomic Ni, which suggests that atomic Ni@Defect serving as active sites appealing to unique electrocatalytic reactions. As examples, atomic Ni@G585 is responsible for OER, while atomic Ni@G5775 activates HER. The derived catalyst exhibits exceptionally excellent activities for both HER and OER, e.g. an overpotential of 70 mV at 10 mA/cm2 for HER (similar to the commercial Pt/C), and 270 mV at 10 mA/cm2 for OER (much superior to Ir oxide). To facilitate the ORR activity of defective carbon catalysts, I fabricated a Pt/Co single atoms co-trapped carbon catalyst (A-CoPt-NC), which showed profoundly high activities in ORR and HER, both outperforming commercial Pt/C catalyst. In common sense, atomically deployed Pt catalyst displayed a high selectivity of 2e- transfer pathway in ORR. On the contrary, the catalyst we developed showed a high selectivity of 4e- transfer pathway, which was resulted from the unique locally distributing strategy of the metal atoms. DFT calculations revealed that the electronic structure, especially the d orbital fillings and charge distribution around the metal atoms, was significantly impacted by the coordinated nitrogen/carbon atoms and other adjacent metal atoms. This alteration contributed to the high activity and selectivity of the catalyst. Besides manipulating single metal atoms on the defective carbon materials, coupling DG with NiFe-LDH is another method to enhance the activity. The coupled DG/NiFe-LDH was found to be extremely active in HER and OER. DFT calculation demonstrated that the heterogeneous coupling leaded to the charge redistribution on the surface of DG and NiFe-LDH, resulting to the holes accumulation on the NiFe-LDH and electrons accumulation on the DG, thereby enhancing the HER activity of DG and OER activity of NiFe-LDH simultaneously. In summary, this thesis mainly focus on two topics, one is the characterizations and reaction mechanism of carbon defects, and the other one is modification of carbon defects to achieve higher activities but with low cost.