Hydrogen production in microbial electrolysis cells: Choice of catholyte

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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