Design And Synthesis Of Carbon-based Nanomaterials And The Study Of Their Electrocatalytic Properties

With the rapid developping of the economy,the global energy consumption is increasing. However, the traditional energy resources are unrenewable, and have negtive impact in environmental. So, it is urgent for human to search the substitution of economic and sustainable clean energy for traditional fossil fuels. Electrochemistry can offer solutions to energy challenges in several ways, which studies the conversion between electricity and energy stored in chemical bonds. Carbon-based nanomaterials with excellent physical and chemical properties have been found broad applying prospect in many fields, such as biosensor, electrochemistry and so on. In this paper, we prepared three carbon-based nanomaterials by mild methods, including heteroatom（N, P and B） co-doped nanocarbons（NPBC）, carbon quantum dots（CQDs）/SnO₂-Co₃O₄ composite and the carbon quantum dots（CQDs）/MoS₂ composite, and further studied their good electrochemical catalytic properties and the reasons. The main works are as follows:（1） By altering the amount of phosphoric acid and boracic acid in raw materials, we have fabricated several heteroatom（N, P and B） co-doped nanocarbons（NPBC） with nanoporous morphology via a facile one-step pyrolysis method and researched the catalytic properties for oxygen reduction reaction（ORR）. It was found that NPBC-2（the initial molar ratio of P/B was 3:7） outperformed the catalysts with other ratio of P and B. By SEM, TEM and BET characterizations, it was certified the film-shaped catalysts with porous structure and large special surface area(89.5 m²Â·g^-1). Electrochemical test indicated that NPBC-2 possesse of much higher electrocatalytic activity toward ORR than single-（nitrogen-doped carbon（NC）） or dual-doped（nitrogen and phosphorus co-doped carbon（NPC） or nitrogen and boron co-doped carbon（NBC）） catalysts in 0.1M KOH solution. The N, P and B co-doped nanocarbons exhibited a positive onset potentials（-0.11 V vs SCE）, long time stability, and high selectivity toward O RR with excellent performance in avoiding crossover effects.（2） We synthesized a series of nanocomposite based on carbon quantum dots（CQDs）, SnO₂ and Co₃O₄ as the electrocatalyst for oxygen evolution reaction（O ER） by simple hydrothermal method. It is found that the CQDs/SnO₂-Co₃O₄ with a molar ratio of Sn:Co at 1:3（as CQDs/SnO₂-Co₃O₄） revealed the highest catalytic activity toward OER among other composites. In alkaline media, the complex exhibited high electrocatalytic activity（a negative onset potentials of ⁰.56 V） and good stability, which was better than either pristine Co₃O₄ or SnO₂-Co₃O₄ composite. Experimental results indicated that the CQDs on the surface effectively protected the catalyst from dissolution in alkaline solution, resulting in a stable structure and long-term stability.（3） We developed a facile hydrothermal method to fabricate the carbon quantum dots（CQDs） and MoS₂ composites with different weight ratios of CQDs and Mo as the hydrogen evolution reduction（HER） catalyst. Among them, the electrocatalyst with the CQDs/Mo of 1.13%（as CQDs/MoS₂） performed the best HER activity. After irradiation with visible light for 30 minutes, the CQDs/MoS₂ composite（as CQDs/MoS₂-L30） performed improved HER activity, featured by a small overpotential of ～0.125 V at 10 mA cm², a high current density, a small Tafel slope of 45 mV/decade and a long term stability in 0.5 M H₂SO₄. By FT-IR and XPS spectra, we suggested that this excellent HER properties may be due to the high charge transfer efficiency, the decrease of S⁴⁺ and the increase of S₂^2- and S^2- after the light treatment.