膜電極組件應用於3D列印空氣陰極微生物燃料電池放大之研究

Abstract

微生物燃料電池是一種新興之生質能技術，透過消耗水中有機物質，將之轉為可用電能，因其能同時達到去除水中有機物及回收電能，故受到科學家們重視。但目前微生物燃料電池有其應用上的瓶頸，受限於其產電能力，通常單一微生物燃料電池的開路電壓在1伏特以下，雖然許多研究已開始透過電池之串並聯，嘗試將微生物燃料電池的產電能力放大，但微生物燃料電池自身過大的內阻，可能導致單一微生物燃料電池輸出效能的損失並進一步限制放大的效果，故優化微生物燃料電池構型以降低內阻為其往後放大應用的關鍵。而膜電極(membrane electrode assembly，MEA)技術為透過冷/熱壓的方式，將電極以及交換膜壓合在一起，使交換膜以及電極之距離，達到小於1 mm，以致幾乎沒有空隙，能大幅降低溶液所造成的阻抗，並獲得更高的性能。 本研究結合膜電極與3D列印技術，運用冷壓法將陰極與離子交換膜於10000 psi的壓力下緊密壓合，並客製化印製能與膜電極組件良好耦合的槽體，從整體構型設計上降低空氣陰極微生物燃料電池之內阻，優化其產電能力。 研究結果顯示，有做膜電極之組別，各項性能皆明顯優於其它未做膜電極之組別。產電效能部份，其輸出電壓穩定度高、個體差異小，開路電壓皆能突破600毫伏特，且在閉路運行時，有做膜電極之組別陰極單極電位，皆能維持於正電位，顯見其極化的程度較其他組別低，最大功率密度結果則發現有做膜電極之組別效能為未做的組別的1.3倍。水質分析部份，在水力停留時間為三天的條件下，COD去除率可達到75%的去除率。電化學結果顯示有膜電極之組別在功率密度曲線法所得到的電池內阻值為680 Ω，交流阻抗分析的總內阻阻抗結果為359 Ω，兩者相比其他組別的結果都明顯較低。此外亦針對不同條件之陰極進行阻抗分析，結果顯示有做膜電極的組別，其陰極阻抗僅為未做膜電極組別的一半。16S rRNA微生物群落結果則發現，已知產電菌Geobacter 在大部分的組別皆為最優勢之屬，其餘的產電菌屬像是Pseudomonas、Thiopseudomonas，也都占了一定的比例，間接證實了組別間的產電差異，關鍵是在使用膜電極技術。串並聯放大試驗結果顯示，開路情況下串聯五個電池，電壓可以達到3.4伏特，另外在並聯的情況下，電壓也可保持在0.68伏特，並可成倍增加電流。功率密度結果則發現串聯放大的過程中，由於電壓反轉的發生，其功率密度逐漸下降。並聯放大時，其功率輸出下降幅度較小，能較穩定地放大電能。串並聯前後之16S rRNA微生物群落結果顯示，無論是在串聯還是並聯，都可以得到在產電菌屬Geobacter以及Pseudomonas皆有下降之趨勢。電化學交流阻抗分析結果則顯示，串並聯後電池內阻皆增加。進一步分析內阻組成，發現歐姆阻抗之上升同時發生於串聯與並聯。極化阻抗部分，電荷轉移阻抗增加主要發生於串聯放大，擴散阻抗上升則主要發現於並聯放大。在電容充電試驗之結果，串聯五個電池之情況下，經過48小時之充電，能夠分別將1.5 F以及2.5 F之電容充至3.7 V、3 V；在並聯五個電池之情況下，能將1.5 F和2.5 F之電容充至3.6 V和3.1 V。經過48小時充電之電容所收集的電能，也成功地為桌上型電風扇進行充電。因此透過此方式優化之微生物燃料電池，對於微生物燃料電池的產電放大，具有良好的發展潛力。

Microbial fuel cell (MFC) is an emerging bioenergy technology that converts organic matter in wastewater into usable electrical energy. This technology has been proved that it can remove organic matter from the wastewater and recover electrical power simultaneously, attracting increasing attention. However, at present, MFCs still have bottlenecks in their development due to the low voltage and current output. Although several studies have been conducted to scale up the power generation of MFC by connecting them in series or parallel, the considerable internal resistance of a single microbial fuel cell may still cause significant power loss and limit the power-output efficiency of MFC. Optimizing the construction of the MFC is the key to large-scale application. The technology of membrane electrode assembly (MEA) can shorten the distance between the electrode and exchange membrane to less than 1 mm by cold/hot-pressing the electrode and exchange membrane together. Hence, the internal resistance can be reduced. In this study, technologies of the MEA and 3D printing were combined. The cathode and membrane were pressed together under the pressure of 10000 psi by cold-pressing. Then, a 3D-printed reactor was designed, and it can be well coupled with the MEA to collectively reduce the internal resistance of the air cathode microbial fuel cell and high power generation can be harvested. The results showed that the performance of the group with MEA is better than that of other groups without it. With regard to power generation, the output voltage has high stability and slight differences with open-circuit voltage exceeding 600 mV. During a closed-circuit operation, only the cathodic potential of the group with MEA can be maintained at a positive potential, which indicated no apparent polarization was observed. The maximum power density results found that the group's performances with MEA was 1.3 times that of those without MEA. Water quality analysis showed that under the hydraulic retention time of three days, the removal rate of COD can reach 75%. The electrochemical results showed that the internal resistance value of the air-cathode MFC obtained by the power density curve method for the group with MEA is 680 Ω, and the total internal resistance impedance result of the EIS analysis is 359 Ω, both of which are lower compared with the results of other groups. In addition, impedance analysis was carried out for the cathodes under different conditions. The results showed that the cathode impedance of the group with MEA was only half that of the group without MEA. The 16S rRNA microbial community results showed that the known electrogenic bacteria Geobacter was the most dominant genus in most of the groups, and the rest of the electrogenic bacteria genera, such as Pseudomonas and Thiopseudomonas, also accounted for a certain proportion. The results confirmed that the use of MEA is responsible for the difference in electricity production. The results of the scale-up test showed that the voltage could reach 3.4 V when five batteries are connected in series in the open circuit condition, and the voltage can also be maintained at 0.68 V in the case of a parallel connection. The power density results showed that in the process of series connection, the power density gradually decreases due to the occurrence of voltage reversal. When connected in parallel, its power output drops less and can scale up electric energy more stably. The results of the 16S rRNA microbial community before and after the series/parallel connection showed that the abudance of electrogenic bacteria Geobacter and Pseudomonas had a decreasing trend whether in the series or parallel connection. The electrochemical EIS analysis showed that the internal resistance increases after the series and parallel connection. Further investigating the internal resistance composition, it showed that the rise of ohmic resistance co-occurs in series and parallel. Concerning the polarization resistance, the increase in charge transfer resistance mainly occurs in series connections, and the increase in diffusion resistance is primarily found in parallel connections. With regard to the capacitor charging test, when five air-cathode MFCs are connected in series for 48-hour charging, the 1.5 F and 2.5 F capacitors can be charged to 3.7 V and 3 V, respectively; as for those in parallel, the 1.5 F and 2.5 F capacitors can be charged to 3.6 V and 3.1 V. In conclusion, air-cathode MFC with MEA performed outstanding potential for scale-up in the future.