Performance of microbial electrolysis cells with separator-electrode assembly
Open Access
- Author:
- Sengupta, Arupananda
- Graduate Program:
- Environmental Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 18, 2013
- Committee Members:
- John Michael Regan, Thesis Advisor/Co-Advisor
- Keywords:
- MEC
separator electrode assembly
membrane electrode assembly
EIS
buffer strength
internal resistance - Abstract:
- Microbial electrolysis cells (MECs) use microorganisms as biocatalysts to recover energy from organic matter in the form of hydrogen, thus providing the cleanest fuel without using a fossil-fuel precursor. MECs, together with microbial fuel cells (MFCs) that directly produce electricity, are speculated to offer an energy-neutral or -positive wastewater treatment technology. In this context, MECs have advantages over MFCs since theoretically there are fewer losses in MEC systems and the hydrogen recovered has wider uses than electricity. However, large-scale implementation of this technology has been delayed due in part to low hydrogen production rates compared to existing hydrogen-production technologies. Reducing the distance between the electrodes in bioelectrochemical systems offers a strategy to increase the current in these systems by reducing the internal resistance. In this study, MECs with separator electrode assembly (SEA) and membrane electrode assembly (MEA) configurations were used to test the performance of MECs with minimal electrode separation. All MECs were constructed using carbon cloth as the anode and stainless steel (SS) mesh of pore size 60 with Pt coating (0.5 mg/cm2) as the cathode. The electrodes were electrically insulated from each other in the system using 46% cellulose-54% polyester textile separators (SEA MECs) or CMI7000 cation exchange membrane (MEA MECs), with a standard two-chamber MEC having a 2-cm electrode spacing used as a benchmark. The adjacent electrode configurations resulted in higher current densities (187 ± 44 A m-3 for the SEA MECs with 50 mM buffer and 121 ± 16 A m-3 for the MEA MECs with 100 mM buffer) as compared to the benchmark design (103 ± 7 A m-3 with 100 mM buffer). A problem of hydrogen recycling was anticipated due to the electrodes being placed close together. The reduced electrode spacing facilitates hydrogen crossover from cathode to anode, which can support current from recycled hydrogen as well as the growth of hydrogen-consuming methanogens, thereby reducing the hydrogen gas recovery. This was observed in the SEA MECs, which had negligible production of hydrogen in later cycles and coulombic efficiencies (CEs) of over 100%, as well as high CH4 production. This problem was operably arrested by using the membrane instead of the textile-type separator in the MECs. However, the use of a membrane resulted in high pH imbalances, which necessitated a higher buffer capacity. Using 100 mM phosphate buffer, cathodic efficiencies of 121 ± 26% and an overall energy efficiency of 92 ± 10% were achieved. The pH imbalance of the MEA MECs was still not fully addressed with this buffer strength, resulting in reduced batch times and residual acetate in the effluent. The MECs were further studied using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to obtain the internal resistances of the MECs. The CV results suggested that the anodic biofilm in both the benchmark and MEA MECs were similar. EIS results showed that at the operational applied potential of 0.7 V the total internal resistance of the MEA MECs was 25.2 ± 4.0 Ω as compared to 41.4 ± 6.8 Ω for the benchmark MECs. The ohmic resistances were of the order MEA MEC (8.8 ± 4.1 Ω, 100 mM buffer) < SEA MECs (9.8 ± 0.0 Ω, 50 mM buffer) < benchmark system MECs (25.5 ± 0.3 Ω, 100 mM buffer). Collectively these results show that the SEA and MEA configurations decrease the system ohmic resistance and can yield higher currents than an MEC design with an electrode spacing of 2 cm, but the proximity of cathodic hydrogen production in these alternate designs increases the potential for hydrogen recycling to the anode and losses.