Design And Catalytic Mechanism Of Carbon-Based Metal-free Electrocatalysts For Oxygen Reduction Reaction

Fuel cells can directly convert the chemical energy of fuels into electricity with high efficiency and low emission. The main challenge for the large-scale application of fuel cells is exploring cheap and stable electrocatalysts for cathodic oxygen reduction reaction (ORR). In recent years, the metal-free sp2 carbon nanomaterials demonstrate excellent ORR activity with high stability and CO and methanol tolerance and, thus, become the frontier subject in the field of ORR electrocatalysis. This dissertation focuses on the design and catalytic mechanism of carbon-based metal-free electrocatalysts for oxygen reduction reaction. Based on the understanding towards the origin of ORR activity and the design principle at molecular level, taking the intrinsic carbon defect and doping configurations as the research objects, both experimental and theoretical methods are employed to study the contribution of carbon defect and doping configurations to oxygen reduction activity of carbon-based metal-free electrocatalysts. The main progresses are summarized as followed.1. Revealing the significant contribution of intrinsic carbon defects to oxygen reduction activity. While the field of carbon-based metal-free electrocatalysts for ORR has experienced great progress in recent years, the fundamental issue of the origin of ORR activity is far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been explored little. Herein, chemical vapor deposition has been developed to prepare the pure and defective carbon nanocages by the in situ MgO template method with benzene as the precursor. The defective carbon nanocages (CNCs) possess the high ORR activity, which is the highest for pure carbon materials and even better than some doped carbon nanostructures. Density functional theory calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon defects to ORR activity. This work demonstrates that besides the dopants, the ORR activities of doped carbon materials should originate partially from the intrinsic defects.2. Evaluating the ORR activity of carbon-based nanomaterials in alkaline medium by their work functions. We noticed that the carbon nanomaterials with different dopants demonstrate different ORR activity in alkaline medium, but a simple and general descriptor to assess their performance has not been proposed yet. Based on the understanding that the ORR process is strongly correlated with the formation of the O2- intermediate, we propose that the ORR activity of the carbon-based nanomaterials depends on their work functions. We calculated the work function of the different N doping structures and intrinsic carbon defects. Theoretical results demonstrate that the graphtic N possess the lowest work function and thus, the best ORR activity, while the ORR activity of carbon nanostructures with predominant pyridinic N comes from the intrinsic carbon defects at a much lower synthesis temperature. The theoretically predicted ORR onset potentials of graphtic N and pyridinic N structures confirmed this result, and verified the feasibility of evaluating the ORR activity of carbon-based nanomaterials by their work functions. Further calculations on the work functions of different doping configurations indicate that the P and S doping could reduce the work function of carbon nanomaterials effectively, and facilitate the ORR process. On the basis of this study, we point out the subsequent research direction for carbon-based metal-free electrocatalysts in alkaline medium.3. Design and explore of efficient carbon-based metal-free ORR electrocatalysts in acidic medium. Metal-free carbon-based nanomaterials have been quite successful as the electrocatalysts for ORR in alkaline medium. But they usually show much inferior activity in acidic medium, and the strategy to modify sp2 carbons to facilitate the ORR in acidic medium is critically needed. By analyzing the different reaction step of ORR in alkaline and acidic medium, we demonstrate that an effective ORR electrocatalyst in acidic medium should possess strong O2-catalyst interaction. The O2 chemisorption ability and ORR free energy diagram of different doping carbon configurations, i.e., N, B, P, S, and O mono/mutil doping, are tested by theoretical calculation and the result indicate that the BCN2 structure favours all steps involved in the ORR process in acidic medium from O2 chemisorption to recovery of active sites. Based on the synthesis method that the structural character of the precursor molecules could pass down to the products under a proper condition, we obtained a new kind of carbon-based metal-free ORR electrocatalysts working effectively in acidic medium, i.e. B and N codoped carbon nanocages with BCN2 structures. The optimized electrocatalyst presents a state-of-the-art ORR performance with an onset potential of 0.76 V vs. RHE and a predominant 4e process, as well as the high durability and immunity to methanol crossover.These three studies improve the understandings on the catalytic mechanism of carbon-based metal-free ORR electrocatalysts in alkaline or acidic medium, which are crucial for understanding the ORR origin and further exploring the advanced carbon-based metal-free electrocatalysts.