Hydrogen production in microbial electrolysis cells: Choice of catholyte

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9620international journal of hydrogen energy 38 (2013) 9619e9624reducing the MEC’s internal resistance [5]. However, a greatconcern with the single-chamber MECs is methane production,which is caused by methanogens inhabiting the adjacentcathode and has been observed in several MECs including apilot test [6]. Therefore, to produce high-purity hydrogen gasin an MEC, IEM may still be a preferable separator to preventmicrobial contamination in the cathode. With an IEM and aseparated cathode chamber, a simple but important questionarises: what should be the catholyte?A commonly used catholyte is a phosphate buffered solution(PBS), which can minimize the pH elevation and provide acertain electrolyte conductivity; however, the use of PBS maynot be economical and environmentally friendly. The anodeeffluent (treated wastewater) could be a low-cost catholyte,which has been studied in microbial fuel cells (MFCs) [7], but itcan introduce microorganisms including methanogens intothe cathode chamber, resulting in methane production. Thus,we should avoid any catholytes containing microbial sources.In addition to PBS, several other catholytes have been studied,including salt solutions and bicarbonate buffers. In general,there is very limited information available about the comparisonbetween these and other potential catholytes in termsof performance and cost. In this study, we aim to examinefour different catholytes and provide suggestions on optimalchoices of a catholyte in an abiotic cathode for sustainablehydrogen production in an MEC.2. Materials and methods2.1. MEC set up and operationAn MEC was made from two glass bottles, which were joinedby a cation exchange membrane (Membranes International,Inc., GlenRock, NJ, USA). Each bottle had a liquid volume of120 mL. The anode electrode was a carbon brush (GordonBrush Mfg. Co., Inc., Commerce, CA, USA), and the cathodeelectrode was a piece of carbon cloth (Zoltek Corporation, St.Louis, MO, USA) with a length of 4 cm and a width of 3 cm. Thecathode electrode was coated with Pt as a catalyst (0.5 mg Pt/cm 2 ) [8]. Both the anolyte and the catholyte were continuouslymixed by magnetic stirrers. An external voltage (0.8 V) wasapplied to the circuit by connecting the negative pole of apower supply (3644 A, Circuit Specialists, Inc., Mesa, AZ, USA)Fig. 1 e Current generation in the MEC with different catholytes: (A) 100, 50, 25 and 10 mM PBS; (B) 134, 100 and 200 mMNaCl; (C) DI and tap water; and (D) acidified water at pH 4 and pH 2 adjusted by H 2 SO 4 or HCl.
international journal of hydrogen energy 38 (2013) 9619e9624 9621Table 1 e Coulombic efficiencies and hydrogen production in the MEC with different catholytes.Catholytes Time period (day) Coulombicrecovery: CR (%)Cathodic H 2recovery: R cat (%)Overall H 2recovery: R H2 (%)H 2 productionrate: Q H2 (m 3 H 2 /m 3 /d)100 mM PBS 1.44 0.10 34.6 1.0 89.2 9.2 30.8 2.3 0.237 0.03150 mM PBS 1.06 0.06 18.7 2.0 89.2 6.6 16.7 1.4 0.165 0.00525 mM PBS 1.11 0.07 13.2 0.4 80.1 3.7 10.6 0.8 0.100 0.00710 mM PBS 1.29 0.07 8.0 0.5 69.9 8.1 5.6 0.3 0.048 0.007134 mM NaCl 2.01 0.12 26.5 1.5 120.4 4.4 31.8 1.2 0.171 0.012100 mM NaCl 1.55 0.06 19.8 1.4 91.2 3.4 18.0 0.8 0.116 0.004200 mM NaCl 1.30 0.07 19.7 3.5 93.7 5.0 18.4 2.8 0.143 0.009DI water 5.10 1.47 42.8 6.8 67.9 12.6 28.5 1.4 0.065 0.021Tap water 3.14 0.61 10.9 2.6 93.8 22.1 9.8 0.1 0.034 0.007Acidified water (pH 4) a 2.81 0.18 11.0 2.1 156.3 17.3 17.0 1.3 0.065 0.002Acidified water (pH 2) a 1.19 0.05 21.5 0.8 84.7 1.8 18.2 0.5 0.171 0.004Acidified water (pH 2) b 1.37 0.07 18.5 2.0 65.5 8.4 12.0 0.8 0.089 0.002a Adjusted by sulfuric acid.b Adjusted by hydrochloric acid.to an external resistor (10 U), then to the cathode electrode,and the positive pole to the anode electrode.The MEC was operated in a fed-batch mode at 20 C. Theanode chamber was inoculated using anaerobic sludge fromSouth Shore Water Reclamation Facility (Milwaukee, WI,USA). The anode feeding solution was prepared in tap waterconsisting of 1.0 g/L sodium acetate, 0.3 g/L NH 4 Cl, 1.0 g/LNaCl, 0.03 g/L MgSO 4 , 0.04 g/L CaCl 2 , 0.2 g/L NaHCO 3 , 5.3 g/LKH 2 PO 4 , 10.7 g/L K 2 HPO 4 and 1 mL/L trace elements [8]. Fourgroups of catholytes were examined in the MEC: PBS, NaClsolution, water, and acidified water. The PBS was tested infour concentrations, 10, 25, 50, and 100 mM. The NaCl solutionwas prepared by dissolving NaCl in tap water to 100, 134, and200 mM. Two waters, tap water and deionized (DI) water, werestudied. The acidified water was prepared in two pHs, 2 and 4,using hydrochloric acid (HCl) or sulfuric acid (H 2 SO 4 ). Thecatholyte was completely replaced when the voltage droppedbelow 2 mV. The new catholyte was flushed with nitrogen gasfor 15 min to remove any oxygen before starting MEC operation.The anolyte was replaced by 80% with a fresh anodefeeding solution at the same time when the catholyte wasreplaced. During the extended-catholyte study, the anolytewas replaced when the voltage decreased below 2 mV, but thecatholyte was not replenished.2.2. Measurement and analysisThe voltage was monitored and recorded every 5 min by adigital multimeter (Keithley Instruments Co., Ltd., USA). Theconcentration of chemical oxygen demand (COD) wasanalyzed by a DR/890 colorimeter (HACH Co., Ltd., USA). ThepH was measured using a bench-top pH meter (Oakton InstrumentsCo., Ltd., USA) and the electrical conductivity wasmeasured by a bench-top conductivity meter (Mettler-ToledoCo., Ltd., USA). The production of hydrogen gas was measuredby water replacement and analyzed by using a gasTable 2 e COD removal in anolyte and characteristics of effluent catholyte with different catholytes.Catholytes COD removal (%) COD removal rate Effluent pHConductivity (ms/cm)(g COD/m 3 /d)InitialEffluent100 mM PBS 94.6 0.3 0.54 0.04 9.2 0.2 13.97 0.01 12.15 0.2350 mM PBS 97.0 0.4 0.69 0.07 8.9 0.2 8.52 0.80 9.67 0.3525 mM PBS 97.5 0.4 0.66 0.06 10.8 0.2 4.06 0.04 5.40 0.3010 mM PBS 97.2 0.4 0.59 0.05 11.3 0.2 1.44 0.48 2.63 0.57134 mM NaCl 96.7 0.7 0.37 0.03 12.2 0.1 13.88 0.24 21.07 0.65100 mM NaCl 95.9 1.8 0.44 0.02 12.0 0.1 7.68 0.66 15.28 0.92200 mM NaCl 96.7 0.2 0.54 0.05 11.9 0.1 20.00 0.20 25.05 0.30DI water 94.6 2.3 0.15 0.04 12.6 0.1 0.01 0.00 1.56 0.08Tap water 86.6 0.8 0.24 0.05 11.6 0.4 0.30 0.02 2.53 0.50Acidified water (pH 4) a 97.4 0.1 0.27 0.01 11.9 0.4 0.05 0.01 2.94 0.38Acidified water (pH 2) a 98.2 0.3 0.66 0.02 9.0 0.7 7.37 0.05 3.61 0.25Acidified water (pH 2) b 96.8 0.2 0.51 0.03 11.0 0.4 5.46 0.02 3.27 0.49a Adjusted by sulfuric acid.b Adjusted by hydrochloric acid.
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