Synthesis of CNT from atmospheric CO$_2$-based chemical vapor deposition and its application to electrochemical devices

Abstract

Synthesis of carbon materials from CO$_2$ is an attractive strategy to reduce CO$_2$ emission, but involves extreme reaction conditions and has low scalability. To overcome this, sodium borohydride is used as a reducing agent to synthesize CO$_2$ into porous carbon under mild conditions at 500 °C and 1 atm., which is discussed in Chapter 1.Chapter 2 introduces continuous chemical vapor deposition for CO$_2$-derived CNTs (CCNTs) using NaBH$_4$ reductant and NiCl2 catalyst at 500 ~ 700 ℃ and 1 atm. Based on in situ analyses, the proposed mechanism behind the formation of CCNTs is CO$_2$ activation and subsequent hydroboration for the generation of methane, which can induce the growth of CCNTs on the catalyst. In Chapter 3, the intrinsic properties of CCNTs give enhanced supercapacitive performance. The boron and oxygen of CCNTs provide pseudo-capacitance, showing a value of 302 F g$^{-1}$ at a low charging rate of 0.1 A g$^{-1}$ in 1 M TEABF4/acetonitrile. The mesoporous networks between CCNT fibers enhance ion transport at a high current density of 204.8 A g$^{-1}, leading to an outstanding energy density of 13 W h kg$^{-1} at a high power density of 115 kW kg$^{-1}. A well-developed graphitized structure of CCNTs also contributes to reducing the electrochemical resistance and leads to superior stability at 65℃ during 10,000 cycles. In Chapter 4, CCNT is coated by an amorphous boron layer, which acts as a protection layer of carbon networks by oxidizing first to become boron oxide, so it has a remarkable stability on thermal oxidation around 1000 ℃. CCNT shows the stable electrical conductivity even up to 1000 ℃ under air conditions while conventional carbon-based materials including commercial CNTs cannot maintain electrical properties due to the oxidative expire below 400 ℃.