Development of novel permeable electrode system for bio-electrochemical $CO_2$ conversion = 투과형 전도성 전극을 이용한 생물 전기화학적 이산화탄소 전환

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Carbon dioxide ($CO_2$) from the injudicious use of fossil fuels has been considered as the main source of greenhouse gas. Hence, $CO_2$ reduction is the most important due to widespread concerns regarding their contribution to climate change. Many processes are being implemented to sequester $CO_2$ including physical, chemical fixation, geological sequestration, membrane technology. On the other hands, biological processes could potentially make a significant contribute to carbon capture and utilization. Although $CO_2$ was stable that it was difficult to conversion of $CO_2$ to useful resources due to strong covalent bonds, microbial catalyzed electro-chemical reduction in microbial electro-synthesis systems (MES) could reduce the $CO_2$ to useful resources with additional energy. A key advantage of MES is that renewable energy and renewable products (e.g., methane) could be generated at low costs. However, the slow conversion rate should be overcome for the practical application of MES. The primary objective of this study was to fabricate the hollow-fiber shape of electrode with carbon nanotubes (CNTs) apply on the MES system and improve $CO_2$ conversion rate using the mass transfer by convectional flow into CHF electrode used as a cathode. This is a new approach in comparison to previous work in which novel electrode could overcome the mass transfer system on MES To achieve this, carbon hollow fiber (CHF) was fabricated to a used electrode on the MES system. The properties of CHF showed high specific surface area (77.81 $m^2$/g), fair conductivity (57.5 S/cm) and mesoporous structure (13.8 nm). Hydrogenotrophic methanogens which are known to metabolize $H_2$ and $CO_2$ to methane were inoculated from anaerobic sludge, and successfully prepared as the biotic electrode on CHF by 1.5 V from the power supply for 60 days. As a result, it was confirmed that the current density increased with incubation time from the linear sweep voltammograms (LSV) test at the onset potential about 0.9 V. Methane production rate and coulombic efficiency gradually increased to 3.07 L$m^{-2}d^{-1}$ and 82% after 60 days. The microbial community at the biocathode was dominated by a genus of Archaea and one bacteria. The Archaeal genotypes were most closely related to Methanobrevibacter and Methanobacterium. Besides methanogenic Archaea, bacteria with hydrogenophaga seemed to be associated with methane production, producing hydrogen as an intermediate. Next, several of operation parameter (pH, potential, the concentration of $NaHCO_3$) conditions on the MES system was operated to find optimal condition. Respectively, methane production rate was the highest (4.79L$m^{-2}d^{-1}$) at 1.1 V, while the highest coulombic efficiency achieved at 0.9 V (vs. Ag/AgCl) due to electrons were consumed during the side reaction such as the production of hydrogen by water decomposition. Also, microorganisms were more active at neutral pHs than acidic conditions. In case of concentration of $NaHCO_3$, 6g/L of $NaHCO_3$ showed the highest coulombic efficiency among tested range from 0.5 g/L (0.006 mol-$Na^+$/L) to 12 g/L (0.14 mol-$Na^+$/L). However, it can be confirmed that methane production rate and coulombic efficiency are low at 12 g/L (0.14 mol-$Na^+$/L) despite the high concentration of $HCO3^-$ due to the inhibition of sodium ions. In the third session, we develop the novel flow-through system for reduction of $CO_2$. The results of LSV showed the onset potential of the flow-through system was less negative voltage than a static system. In addition, higher electron recovery in the form of methane (~96.14%) was also observed. Overall, the flow-through system explained the higher methane production rate of 6.62 L/$m^2$/day from direct $CO_2$ flow through the attached microorganisms of CHF electrode, compared to 3.07 L/$m^2$/day for the static electrode. In conclusion, the novel CHF electrode design and modified the flow-through operation can overcome the methane conversion rate of conventional bio-reactor.
Kang, Seoktaeresearcher강석태researcher
한국과학기술원 :건설및환경공학과,
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학위논문(석사) - 한국과학기술원 : 건설및환경공학과, 2019.2,[vii, 37 p. :]


Microbial elecotro-synthesis (MES)▼aCarbon dioxide ($CO_2$)▼aCarbon nanotubes (CNTs)▼ahollow-fiber membrane; 미생물 전기화학▼a이산화탄소▼a탄소나노튜브▼a중공사막

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