Hydrogen production in microbial electrolysis cells: Choice of catholyte

international journal of hydrogen energy 38 (2013) 9619e9624Available online at www.sciencedirect.comjournal homepage: www.elsevier.com/locate/heShort CommunicationHydrogen production in microbial electrolysis cells:Choice of catholyteSiriporn Yossan a , Li Xiao b , Poonsuk Prasertsan c,d , Zhen He b, *a Program of Science, Faculty of Liberal Arts and Science, Sisaket Rajabhat University, Sisaket 33000, Thailandb Department of Civil Engineering and Mechanics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USAc Department of Industrial Biotechnology, Faculty of Agro-Industry, Prince of Songkla University, Songkhla 90112,Thailandd Palm Oil Products and Technology Research Center (POPTEC), Faculty of Agro-Industry, Prince of SongklaUniversity, Songkhla 90112, Thailandarticle infoArticle history:Received 18 January 2013Received in revised form16 May 2013Accepted 19 May 2013Available online 21 June 2013Keywords:HydrogenMicrobial electrolysis cellsCatholyteBufferabstractA catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs).Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffersolution (PBS) resulted in the highest hydrogen production rate of 0.237 0.031 m 3 H 2 /m 3 /d,followed by 0.171 0.012 m 3 H 2 /m 3 /d with a 134 mM NaCl solution and 0.171 0.004 m 3 H 2 /m 3 /d with the acidified water adjusted with sulfuric acid. The MEC with all catholytesachieved good organic removal efficiency, but the removal rate varied following the trendof the hydrogen production rate. The reuse of the catholyte for an extended period led to adecreasing hydrogen production rate, affected by the elevated pH. The cost of both theacidified water and the NaCl solution was much lower than the PBS, and therefore, theycould be a better choice as an MEC catholyte with further consideration of cost reductionand chemical reuse/disposal.Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rightsreserved.1. IntroductionHydrogen is a promising alternative to fossil fuel and isconsidered as a clean fuel because its combustion generatesno pollutants but water [1]. Hydrogen gas can be produced byboth thermochemical (pyrolysis/gasification) and biological(biophotolysis, dark and photo fermentation) processes [2]. Inrecent years, a new method of hydrogen production has beendeveloped based on bioelectrochemical principles in microbialelectrolysis cells (MECs). The advantage of MEC technologyis to produce hydrogen gas from a non-fermentingsubstrate (e.g., in wastewater) with a very low input ofexternal energy. Therefore, MECs can function as an energyefficientprocess for simultaneous waste treatment and energy(hydrogen) production.Our understanding of MECs has been greatly advanced byintensive research on microbiology, substrates, and reactorconfiguration and operation [3,4]. MECs usually contain twochambers, an anode and a cathode separated by an ion exchangemembrane (IEM). A single-chamber MEC was developedby omitting IEM to considerably simplify the reactorstructure and potentially improve hydrogen production by* Corresponding author. Tel.: þ1 414 229 5846; fax: þ1 414 229 6958.E-mail address: zhenhe@uwm.edu (Z. He).0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijhydene.2013.05.094

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.

9622international journal of hydrogen energy 38 (2013) 9619e9624chromatograph (Focus GC, Thermo Scientific, USA). The keyparameters were calculated as previously described [8]: thecoulombic recovery (CR) is defined as the total output coulombsover the total input coulombs in acetate; the cathodichydrogen recovery (R cat ) is the ratio between the electronscontained in the produced hydrogen gas and the electronsproduced as current; the overall hydrogen recovery ðR H2 Þ is thesubstrate used for the hydrogen production; and the hydrogenproduction rate ðQ H2 Þ is the hydrogen production per anodeworking volume per time.3. Results and discussionThe MEC performance with different catholytes was describedby using three main parameters, electricity generation,hydrogen production, and COD removal.Fig. 1 shows the profile of batch current generation withfour groups of catholytes at different concentrations. In general,the PBS catholytes with higher concentrations had ahigher peak current, followed by the acidified water (pH 2),and the NaCl solution. Both tap water and DI water resulted invery low peak currents. Decreasing the PBS concentrationfrom 100 to 10 mM also decreased the peak current from31.1 1.5 to 8.2 0.4 A/m 3 (Fig. 1(A)), while increasing the NaClconcentration from 100 to 200 mM slightly improved the peakcurrent from 15.0 0.5 to 18.7 1.1 A/m 3 (Fig. 1(B)). Theexceptional performance of the 100-mM PBS is due to itsbuffer capacity and conductivity (Table 2). The high currentwith the NaCl catholyte is because of its high conductivity(Table 2), while the high current with the acidified water islikely due to the additional proton input. The higher conductivityof the tap water over that of the DI water did not result inmore current production: the current generation with the DIFig. 2 e Current generation (white dot) and hydrogen production rate (white square) in the MEC in the extended period ofcatholyte reuse: (A) 100 mM PBS; (B) 134 mM NaCl; and (C) acidified water at pH 2 adjusted by H 2 SO 4 .

international journal of hydrogen energy 38 (2013) 9619e9624 9623water (11.7 0.1 A/m 3 ) was twice that with the tap water(5.5 0.4 A/m 3 ), possibly because of the pH influence: theinitial pH of the DI water (4.7 0.0) was much lower than thatof the tap water (7.3 0.3), indicating that the proton supplymay have a more important role in current generation thanthe electrolyte conductivity. When the initial pH was similar,for example, with the pH-2 water prepared by H 2 SO 4 or HCl, ahigher influent conductivity of the H 2 SO 4 catholyte led to ahigher peak current of (31.1 0.8 A/m 3 ) than that of25.7 0.3 A/m 3 with the HCl catholyte.Hydrogen production efficiencies and rates with differentcatholytes are summarized in Table 1. The efficiency of thesubstrate-to-hydrogen (overall hydrogen recovery) varied between5% and 32%; interestingly, the DI water and the tapwater had comparable overall hydrogen recovery, as well ascoulombic recovery, with the other catholytes. However, ittook a much longer time (3e5 days) for the MEC with the DI ortap water to complete a cycle, resulting in a much lowerhydrogen production rate. For the catholytes of the PBS, theNaCl solution or the acidified water, the hydrogen productionrates matched their current generation, and a higher currentled to a higher hydrogen production rate.The primary function of an MEC using wastewater as itsanode substrate is considered to be contaminant removal;thus, COD reduction is a key parameter to evaluate the MECperformance. In general, the MEC achieved more than 94%of COD removal with all catholytes except the tap water(Table 2), demonstrating a good performance in organicremoval. However, the COD removal rate was clearlyaffected by current generation and the period of a batchcycle, and the catholytes such as the DI water and the tapwater with low hydrogen rates also yielded low CODremoval rates.We chose three catholytes, 100-mM PBS, 134-mM NaCl, andacidified water pH 2 (H 2 SO 4 ), and conducted an extendedstudy without replacing the catholyte. The peak currentdensities with the PBS and the acidified water clearlydecreased over time, but the NaCl maintained a relativelystable current output. The hydrogen production rates, on theother hand, all decreased in the extended operating period(Fig. 2). Interestingly, the hydrogen production rate with theacidified water showed a dramatic decrease in the second daywith a significant increase in pH from 2.0 to 11.2, and thenslowly recovered in the following six days with a further increaseof pH to 12.6 (Fig. 2(C)). The pH of the PBS increasedfrom 7.3 to 12.3 in ten days and the NaCl catholyte had anelevated pH from 7.5 to 12.9 in six days. The final conductivityof the acidified water was 12.57 mS/cm, half of that of the PBSor NaCl catholytes. All three catholytes had a similar hydrogenproduction rate of 0.10 m 3 H 2 /m 3 /d at the end of their testingperiod, indicating that pH had more influence than catholyteconductivity on hydrogen production.The cost of a catholyte was estimated from the amountof the chemicals used and their bulk prices obtained fromSigmaeAldrich (St. Louis, USA). For an operation with frequentreplacement of the catholyte (as shown in Fig. 1), it would cost$3.39/m 3 H 2 with the 100-mM PBS, much higher than thehydrogen price ($0.42/m 3 estimated from $4.75/kg H 2 [9]). Theuse of the 134-mM NaCl or the acidified water (pH ¼ 2 adjustedby sulfuric acid) would cost $0.29/m 3 and $0.21/m 3 , respectively.In an extended use of the catholyte (as shown in Fig. 2), thecost of those three catholytes (calculated from the first fourcycles) decreased because of the reuse: $1.50/m 3 with the100-mM PBS, $0.19/m 3 with the 134-mM NaCl, and $0.11/m 3with the acidified water (pH ¼ 2). The cost can be even less withadditional operating cycles using the same catholyte (Fig. 2(A)and (C)) and lower prices of raw chemicals in a larger quantity.Both the NaCl solution and the acidified water may be a betterchoice as an MEC catholyte than the PBS, because of thesignificantly lower cost. The use of PBS can also potentiallycause environmental problems by releasing phosphorus intonatural water bodies. Of course, challenges also exist with theuse of the NaCl or the acidified water and should be furtheraddressed; for example, the anode needs to have a sufficientalkalinity when using a high-concentration NaCl catholyte[10], or the waste sulfate from the acidified water should beproperly disposed.4. ConclusionsHydrogen production in an MEC is affected by the pH bufferingcapacity and the electrolyte conductivity of a catholyte. The100-mM PBS catholyte exhibited the best buffering capacityand also led to the highest hydrogen production rate, followedby the NaCl solution with a high electrolyte conductivity andthe acidified water that supplied additional protons to thecathode reaction. The tap/DI water, the low-concentration PBSand the acidified water at pH 4 had the lowest hydrogen productionrates. The reuse of the cathode for an extended periodled to decreased hydrogen production rates, but could bejustified by cost saving. PBS may not be a good choice as an MECcatholyte because of its high cost and potential environmentaleffects. Both NaCl and acidified water could be considered asbetter choices. Future applications of those catholytes in MECswill need to consider issues such as the catholyte cost (withfurther reductions), the catholyte reuse, and the disposal.AcknowledgmentsThis work was financially supported by the Research Groupfor the Development of Microbial Hydrogen Production Processesfrom Biomass, Office of the Higher Education Commission,Thailand, and a research grant from the NationalScience Foundation (Award 1033505). We also thank Dr. MarjoriePiechowski (UW-Milwaukee) for proofreading themanuscript.references[1] Suwansaard M, Choorit W, Zeilstra-Ryalls JH, Prasertsan P.Isolation of anoxygenic photosynthetic bacteria fromSongkhla Lake for use in a two-staged biohydrogenproduction process from palm oil mill effluent. Int JHydrogen Energy 2009;34:7523e9.[2] Manish S, Banerjee R. 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