Non-Precious-Metal Carbon-Based Nanomaterials for Oxygen Electrocatalysis


Student thesis: Doctoral Thesis

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Award date29 Dec 2020


To reduce the dependence of traditional fossil fuels and build renewable energy governing systems, tremendous studies about the power systems predominated by electrochemical reactions, such as fuel cells, water-splitting devices, and metal-air batteries, have been carried out in the last few decades. For these advanced technologies, the oxygen electrocatalysis process including the oxygen reaction (ORR) and evolution reactions (OER) have been intensively investigated. To enhance the sluggish reaction rate due to multiple charge-transfer processes and a high activation barrier during oxygen redox reactions, the design and utilization of effective electrocatalysts are of vital significance. In this thesis, the non-precious-metal carbon-based nanomaterials have been explored as the promising alternatives of precious-metal-catalysts in oxygen electrocatalysis.

In the first study, the metal-free nanomaterials are fabricated via the chemical vapor deposition method using the thiophene and basic magnesium carbonate nanoflakes as carbon and sulfur resources and hard templates, respectively. The edge-rich carbon nanocages with high sulfur doping amount (6.04 at.%) are successfully obtained after this facile synthesis process. These metal-free carbon-based catalysts perform remarkable ORR catalytic activity and outstanding stability, exceeding the commercial Pt/C benchmark. When applied in the primary Zn-air batteries, the as-prepared nanomaterials demonstrate superior electrochemical performances under different discharging current densities. The unique hollow structure, large specific surface area, excellent conductivity of carbon-based materials, and the edge decoration derived from the heteroatom doping are attributed to the superior ORR catalytic of this unique metal-free catalysts.

The active non-precious-metal species are introduced to the carbon-based materials in the second research. The fine Fe/Fe5C2 nanocrystals (~10 nm) are well encapsulated into the bubble-like porous carbon nanofibers by a facile one-pot pyrolysis strategy. The novel catalysts are equipped with high Fe content (37 wt.%), and synergetic N and S doping, which are favorable to the efficiently catalyzing the oxygen electrocatalysis reactions. Moreover, as catalysts on the air electrodes of rechargeable Zn-air batteries, the optimal non-precious-metal catalysts show a high peak power density of 59.6 mW cm-2 and extraordinary discharge-charge cycling performance for 200 h with negligible voltage gap change of only 8% at the current density of 20 mA cm-2, surpassing the commercial noble-metal counterpart. The strikingly bifunctional catalytic performance can benefit from the synthetic effect between graphitic carbon nanofibers and active Fe/Fe5C2, and impressive battery stability can be attributed to the carbon wrapping to prevent oxidation, agglomeration, and dissolution of Fe nanoparticles during battery cycling. Undoubtedly, the successful design of these non-precious-metal catalysts will promote the development of inexpensive metal-carbon hybrid materials for oxygen electrocatalysis and rechargeable metal-air batteries.

To maximize the utilization efficiency of metal active sites in oxygen electrocatalysis, the third study focus on the design of non-precious-metal single-atom-catalysts. Supported on the surface of nitrogen-doped carbon spheres, the Fe atomic sites are successfully anchored into the carbon skeletons. These cost-effective nanomaterials exhibit more positive onset potential (0.98V vs. RHE), higher diffusion-limited current density (7.6 mA cm-2), and better stability than the commercial Pt/C benchmark. Moreover, the as-prepared catalysts also deliver comparable OER performance, resulting in excellent bifunctional catalytic activities with the lowest ORR/OER potential gap (∆E, 0.722 V) compared with previously reported single-atom-catalysts. This work opens a new avenue to the scale-up production of non-precious-metal single-atoms confined on the carbon supports for various energy storage and conversion devices.

In summary, this thesis provides several facile preparation methods of non-precious-metal carbon-based nanomaterials for catalyzing oxygen redox reactions. The role of carbon supports, metal nanoparticles, and metal atomic sites has been specifically discussed. In particular, their applications in the metal-air batteries are additionally investigated and our final target is to accelerate the development of the renewable energy storage and conversion systems.