Design, Preparation and Electrocatalytic Properties of Biomass-based Functional Carbon Materials

Abstract

In the face of increasingly severe energy and environmental problems, sustainable and green energy conversion systems are considered the most promising energy efficiency way to alleviate these problems. Among them, zinc-air batteries have attracted more and more attention due to their high energy density and energy conversion efficiency. The bottleneck of the commercial development of zinc-air batteries lies in the slow power rate of the cathode oxygen reduction reaction (ORR), and the high price, scarce reserves, and poor stability of the platinum-based catalyst used to catalyze the ORR reaction. Therefore, the development of cost-effective and high-performance alternatives to ORR platinum-based electrocatalysts is a promising strategy. Cheap and readily available microalgae and lignocellulosic biomass waste (wood) are considered effective carbon precursors, but how to convert them into highly efficient non-platinum-based electrocatalysts remains a huge challenge.

The main research projects and results of my works are summarized as follows:
1. The microalgae biomass with a high N content was used as a precursor to prepare N-doped biochar material via an ionic liquid-assisted carbonization process. Experimental results show that the sp²/sp³ carbon interface and pore structure of the as- synthesized biochar material could be tuned by the polymerization of the ionic liquids and the catalysis of Fe species in the carbonization process. The as-synthesized N- doped biochar material exhibited robust electrocatalytic ORR performance with a highly catalytic activity, excellent long-term stability and great tolerance to methanol and CO. The pyridinic N species was identified as the active catalytic site for the ORR. A Zn-air battery was assembled using the N-doped biochar as the air cathode catalyst and showed favourable performance.

2. A facile pyrolysis method to construct N, P-dually doped biochar materials from the lignocellulosic biomass wastes were demonstrated. The as-synthesized N, P-dually doped biochar samples could act as electrocatalysts for oxygen reduction and evolution reactions (ORR/OER), showing excellent catalytic performance and long-term durability, as well as robust tolerance to CO and methanol. The unique hierarchical porous structure, favorable electronic structure modified by the N and P doping, as well as a variety of defect sites induced by the N and P doping into the carbon framework were identified as the main contributions to the prominent catalytic activity of the as- synthesized N, P-dually doped biochar materials.

3. A simple method to upgrade biomass via hydrothermal treatment and pyrolysis of a mixture of microalgae and g-C₃N₄. The use of g-C₃N₄ as the nitrogen source and self-sacrificing template not only improves the catalytic activity by increasing the graphitic nitrogen content, but also benefits the mass transport due to higher porosity. Near edge X-ray absorption fine structure unravels the distinct carbon and nitrogen structure. With optimal pyrolysis temperature, the prepared NBC-900 catalyst exhibits a remarkable oxygen reduction ability (E_1/2 = 0.875 V vs. RHE), outstanding long-term stability, and strong resistance to CO and CH3OH poisoning. When assembled in a primary zinc-air battery, the NBC-900 catalyst reaches a maximum power density of 220.7 mW cm^-2 with negligible voltage decay over the 6 h test, which was higher than that of commercial Pt/C at 112.1 mW cm^-2.

4. A series of cobalt-based multicomponent embedded in biomass-derived porous carbon materials for ORR were fabricated through thermal pyrolysis of 1,10- phenanthroline-cobalt (II) metal-complex and 1,10-phenanthroline onto biomass- derived carbon surfaces from 750 to 950 °C. The biomass derivative carbon was prepared by carbonizing the mixture of wood biomass and Zn₂(OH)₂CO₃. The optimized WPC-950 electrocatalyst presented splendid ORR electrocatalytic performances, preferable survivability to methanol, and outstanding long-term stability in alkaline electrolyte. The excellent electrochemical properties can be attributed to unique structure with CoO_x embedded into the structure of N-doped porous carbon materials, rich Co-N_x and graphitic-N active species, as well as lower I_D/I_G value of carbon with more graphite carbon.

My works provide a series of straightforward strategies to prepare efficient and low-cost functional carbon materials for clean energy conversion from naturally abundant biomass. In addition, my works would spur more efforts into developing advanced materials with tailored electronic structure and porosity from biomass wastes

Date of Award	15 Nov 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jianxiong ZENG (External Supervisor), Kwan Sing Paul LAM (Supervisor) & Ruquan YE (Supervisor)