- Email: [email protected]
Comparison of different electrodes in hydrogen gas production from electrohydrolysis of wastewater organics using photovoltaic cells (PVC)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparison of different electrodes in hydrogen gas production from electrohydrolysis of wastewater organics using photovoltaic cells (PVC)5 Fikret Kargi* Department of Environmental Engineering, Dokuz Eylul University, Buca 35160, Izmir, Turkey
article info
abstract
Article history:
Electrical power generated by a photovoltaic cell (PVC) was supplied to diluted industrial
Received 13 October 2010
wastewater in a mechanically mixed and sealed stainless-steel reactor for hydrogen gas
Received in revised form
production. Three different electrodes, graphite, stainless steel and aluminum rods were
30 November 2010
used for comparison. Protons released from decomposition of organic compounds and
Accepted 2 December 2010
electrons provided by the DC current reacted to form hydrogen gas. The highest cumulative
Available online 31 December 2010
hydrogen gas formation (CHF) was obtained with the aluminum electrode (120 L in 8 days) and the lowest was with the graphite electrode (4 L). Hydrogen gas production from
Keywords:
wastewater was 2.4 times higher than that produced from water when aluminum elec-
Electrodes
trodes were used. TOC content of wastewater was reduced from 2400 to 1700 mg L1 with
Electrohydrolysis
nearly 29% TOC removal within 6 days. CHF from wastewater was 76 L within 18 days with
Hydrogen gas
the stainless-steel electrodes while CHF from water was only 9.5 L. Fermentative hydrogen
Photovoltaic cells (PVC)
gas production from wastewater was negligible in the absence PVC. Energy conversion
Wastewater
efficiency for hydrogen gas production (hydrogen energy/electric energy) was found to be 74% with the aluminum electrodes. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Search for clean and high energy fuels has increased rapidly over the last fifty years. Fossil fuels used as major source of energy emit undesirable gases to atmosphere (SOx , NOx, COx) leading serious air pollution problems. Hydrogen gas is a clean and high energy (122 kJ g1) fuel as compared to fossil fuels. Hydrogen can also be used in fuel cells to generate electricity and is considered to be the major energy carrier of the future. The need for hydrogen gas is increasing rapidly with a growth rate of nearly 10% per year [1]. Unlike fossil fuels, hydrogen gas is not readily available in nature and is produced by steam reforming of natural gas or by electrolysis of water which are energy intensive processes 5
[2]. Fermentative production of hydrogen gas using carbohydrate rich renewable raw materials is more advantageous due to operation under mild conditions. Major obstacles in biohydrogen production by fermentation are low hydrogen yields and formation rates due to slow bacterial metabolisms [2e4]. Sequential dark and photo fermentations were used for fermentative hydrogen gas production from starch containing wastes [5e10]. Photo-fermentation of carbohydrates or volatile fatty acids (VFAs) produced by dark fermentation effluent requires strict control of environmental conditions and constant light intensity [8e10]. Hydrogen gas production by electrohydrolysis of volatile fatty acids (VFA) produced by dark fermentation has been studied as an alternative to photofermentation [11]. A few recent patents on hydrogen gas
All rights of the findings of this study are reserved by a patent application by Dr. F. Kargi. * Tel.: þ90 232 4127109. E-mail address: [email protected]. 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
production from organic wastes and fermentation effluents by electrical power application have been reported [12e14]. Hydrogen gas production using microbial electrolysis cells (MEC) has also been investigated [15e18]. Electrolysis of water for hydrogen gas production has been studied extensively [19e21,30]. The energy content of hydrogen gas produced by electrolysis of water is 3.55 kWh/m3 H2 (39.70 kWh/kg H2) while water electrolysis requires nearly 4 kWh/m3 H2 (44.75 kWh/kg H2) electrical energy. The rate and extent of hydrogen gas production by water electrolysis were improved by using catalytic and high temperature steam electrolysis. Water electrolysis using PEM containing cells requires net energy input for hydrogen gas production [22]. Photovoltaic cells (PVC) have been recently used for electrolysis of water [23e28]. No studies were reported on H2 gas production from electrohydrolysis of organic compounds present in wastes. The major objective of this study is to investigate hydrogen gas production from electrohydrolysis of organic compounds present in industrial wastewater using photovoltaic cells (PVC) for electrical power generation. Different electrodes (graphite, stainless steel and aluminum rods) were used in a well sealed and mechanically mixed reactor. Control experiments with DC power application to water and no DC power application to the wastewater were also performed to determine hydrogen gas evolution by water electrolysis and by natural anaerobic digestion of wastewater, respectively.
2.
Materials and methods
2.1.
Experimental set up
The experimental system (Fig. 1) consisted of a PVC panel with 80 cm 120 cm dimensions, a voltage regulator, a battery and a stainless-steel sealed reactor with dimensions of Do ¼ 21 cm, H ¼ 48 cm and volume of 16.8 L [29]. Wastewater volume in the reactor was 13.2 L with a head space of 3.6 L. The PVC panel contained 32 cells with a total power supply of 115 W (ie, 3.6 W for each cell) and provided 18 V voltage with 6 A current. A voltage regulator was used to adjust voltage to the desired level of 13.5 V. A battery was used to store electrical energy generated by the PVC panel and to provide constant current to the reactor when the sunlight was not available. Mechanically mixed and sealed stainless-steel reactor contained aqueous organic waste, electrodes and a pressure gauge. Electrodes were used to transmit the electrical current (electrons) generated by the PVC to the aqueous medium in the reactor. Three different electrodes, stainless steel, aluminum and graphite rods were used for comparison. Dimensions of the electrodes were L ¼ 49.5 cm and Do ¼ 0.9 cm which were mounted on the head plate of the reactor and immersed in wastewater inside the reactor [29].
2.2.
Experimental procedure
Experiments were started by filling the reactor with diluted wastewater, closing the head plate tightly and connecting the PVC to the electrodes through a voltage regulator and battery. Three sets of experiments were performed with different electrodes (stainless steel, aluminum and graphite) using
3451
diluted wastewater. Control experiments were performed with water to determine hydrogen gas production by water electrolysis. Hydrogen formation by anaerobic fermentation was also tested by inoculating the wastewater by heat-treated anaerobic sludge with no supply of electrical current. Wastewater was obtained from PAK MAYA Bakers Yeast Company, Izmir, Turkey and diluted properly to reduce TOC content to desired level. No external chemicals were added to diluted wastewater. Raw wastewater had COD and total solids contents of nearly 10 g L1 and 0.5 g L1 with a pH of 6.9 and conductivity of 1500 mS cm1.
2.3.
Analytical methods
Voltage and electrical current supplied to the reactor was measured using a voltage regulator. Voltage from the regulator was nearly 13.5 V and the current varied between 100 mA and 2 A depending on the availability of sunlight. Ampere hour (I t) were recorded by the voltage regulator everyday and used for electrical energy (P ¼ V I t) calculations. Hydrogen gas was sampled from the head space of the reactor by using gas-tight glass syringes and hydrogen concentration was determined by using a gas chromatograph (HP Agilent 6890). The GC column was Alltech, Hayesep D 80/100 600 1/800 0.8500 . Nitrogen gas was used as carrier with a flow rate of 30 ml min1 and the head pressure was 22 psi. The amount of total gas produced was determined everyday by water displacement method using sulfuric acid (2%) and NaCl (10%) containing solution. The cumulative hydrogen gas production was determined as described in our previous publications [5e10, 29]. pH and oxidationereduction potential (ORP) of the fermentation medium were monitored by using pH and ORP meters with relevant probes (WTW Scientific, Germany). Samples removed from fermentation media everyday were centrifuged at 7000 g to remove solids. Total organic carbon (TOC) content of wastewater was determined using a TOC analyzer (Teledyne Tekmar Apollo 9000 Combustion TOC Analyzer, USA) after centrifugation. Metal ions (Fe(II) and Al(III)) released to the medium were measured by using an ICP analyzer (Optical Emission Spectrometer Optima 2100 OV ICP Analyzer, Perkin Elmer, USA).
3.
Results and discussion
3.1.
Experiments with the stainless-steel electrodes
Hydrogen gas production from wastewater by application of PVC power strongly depends on the availability of free electrons from the electrodes. The experiments with stainless-steel electrodes were done for comparison of hydrogen evolution from water and wastewater. Fig. 2 depicts hydrogen evolution from water (Nov 10e28, 2008) and wastewater (Dec 18, 08eJan 9, 09) with the application of PVC power [29]. Approximately, 9.4 L hydrogen gas was produced within 432 h (18 days) from water electrolysis with hydrogen gas evolution rate of 0.522 L d1 [29]. Water loss from the reactor was nearly 700 mL. Medium pH increased from 7.7 to 8.3 due to removal of hydrogen ions (Hþ) from water to form
3452
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
Fig. 1 e A schematic diagram of the experimental set up used for hydrogen gas production from wastewater using photovoltaic cells. 1. Photovoltaic cell (PVC), 2. voltage regulator, 3. battery, 4. stainless-steel reactor containing aqueous organic waste, 5. electrodes, 6. mixing motor and blades, 7. hydrogen storage tank, 8. liquid feed/effluent port, 9. gas release port, 10. aqueous organic waste [29].
hydrogen gas. Nearly 95 L hydrogen gas was released within 22 days with hydrogen evolution rate of 4.32 L d1 when the same electrical power was applied to wastewater with initial TOC content of 4500 mg L1 [29]. Hydrogen gas production from wastewater was nearly 8 times greater than that obtained from water. Percentage of hydrogen gas in the head space varied between 90 and 98%. TOC content of wastewater decreased from nearly 4500 to 4200 mg L1. Total wastewater volume decreased from 13.2 to 11.9 L with a 1.32 L water loss within 22 days. The amount of TOC removed from wastewater was 9.5 g resulting in a hydrogen yield of nearly 10 L H2 g1 TOC. pH of wastewater decreased slightly from 6.83 to 6.7 indicating negligible hydrogen ion removal from water for hydrogen gas formation. Apparently, organic matter in wastewater was dissociated and/or hydrolysed with application of electrical current releasing hydrogen ions (Hþ) into
media which reacted with electrons released from the anode to form hydrogen gas. Anode reaction :
Fe/FeðIIÞ þ 2e
Organic compound dissociation : 2 HOC/2 OC þ 2 Hþ H2 gas formation :
2 Hþ þ 2 e / H2 ðgÞ
(1) (2) (3)
1
Approximately, 2.03 mg L Fe(II) was detected in the medium at the end of operation. Part of Fe(II) released to the media was deposited on cathode surfaces in form of pure Feo as described below. Cathode reaction :
FeðIIÞ þ 2e /FeO
(4)
Energy conversion efficiency (h) with the stainless-steel electrodes was calculated to be 42% as described below:
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
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Fig. 2 e Comparison of hydrogen production from water and wastewater using photovoltaic cell and stainless-steel electrodes. C wastewater B water [29].
Epvc ¼ V I t ¼ 13:6Vð0:1 AÞð269 hÞ ¼ 365:8 W h VH2 ¼ 56:2 L; mH2 ¼ 4:53 gð30 C; 1 atmÞ EH2 ¼ mH2 122 kJ=g ¼ 4:53ð122Þ ¼ 552:7 kJ ¼ 153:5 W h h ¼ EH2 =Epvc ¼ 0:42
3.2.
Experiments with the graphite electrodes
Graphite has been the most widely used electrode in water electrolysis. Fig. 3 depicts CHF from water and wastewater by solar power application using graphite electrodes. When solar power was applied to water, only 3.22 L H2 was produced within 11 days (June 8e19, 2009) yielding hydrogen production rate of 0.29 L d1. Despite high sunlight intensity in June 2009,
hydrogen production was low due to low electrical conductivity of graphite electrodes. Head space of the reactor contained 85e90% hydrogen with no total gas release from the reactor. Hydrogen gas production from wastewater was higher than that of water yielding 3.81 L H2 production within 8 days (June 24eJuly 2, 2009). In this case, head space contained nearly 95% H2 with no total gas release from the reactor. Hydrogen gas formation rate from wastewater was considerably higher (0.476 L d1) than that of the water (0.29 L d1). The results indicated that electrode selection is very important to obtain high rates of hydrogen gas formation. TOC concentration of wastewater decreased from 2726 to 2404 mg L1 with a 4.25 g total TOC removal within 8 days. The hydrogen yield (0.90 L H2 g1 TOC) was considerably lower than that obtained with the stainless-steel electrodes. Energy conversion efficiency with the graphite electrodes was calculated to be 0.92% as presented below.
Fig. 3 e Cumulative hydrogen formation o water and wastewater using photovoltaic cells and graphite electrodes. C wastewater B water.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
Fig. 4 e Cumulative hydrogen formation from water and wastewater using photovoltaic cells and aluminum electrodes. C wastewater B water.
Epvc ¼ V I t ¼ 13:6Vð8:3 A hÞ ¼ 112:9 W h VH2 ¼ 3:81 L; mH2 ¼ 0:307 g ð30 C; 1 atmÞ EH2 ¼ mH2 122 kJ=g ¼ 0:307ð122Þ ¼ 37:5 kJ ¼ 1:04 W h h ¼ EH2 =Epvc ¼ 0:0092 This efficiency is significantly lower than that obtained with the stainless-steel electrodes indicating that graphite is not a suitable electrode for hydrogen production due to low free electron density and therefore, low electrical conductivity.
3.3.
Experiments with aluminum electrodes
Aluminum electrodes were used in the third set of experiments in July 2009. Fig. 4 depicts variation of CHF from water and wastewater using Al electrodes. Total hydrogen formation from water was 41.4 L within 6 days (July 4e10, 2009) yielding
HFR of 6.85 L d1. Gas phase contained nearly 98% hydrogen gas. pH of water decreased from 7.3 to 6.7 at the end of 6 days. When wastewater with an initial TOC content of 2350 mg L1 was used in the reactor, 98 L H2 was produced within 6 days (July 14e20, 2009) with nearly 99% H2 in gas phase. HFR (16.33 L d1) from wastewater was nearly 2.4 times higher than that obtained from water. Hydrogen production rate obtained with Al electrodes were much higher than those obtained with the stainless steel and graphite electrodes due to high free electron density and high electrical conductivity of aluminum. Hydrogen peroxide (1% H2O2) was added to wastewater for the last two days in order to determine if chemical decomposition of organic compounds by peroxide would improve hydrogen gas production. In fact hydrogen production increased to 118 L within the next 2 days with an HFR of 10 L d1. Fig. 5 depicts variation of TOC concentration with time using Al electrodes. TOC decreased from 2350 to 1640 mg L1 at the end of 6 days yielding a total TOC removal of 9.37 g.
Fig. 5 e Variation of TOC concentration with time for H2 gas production from wastewater using aluminum electrodes.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
Table 1 e Comparison of hydrogen production rates and yields by electrohydrolysis of wastewater using PVC power. H2 gas formation rate (L d1)
Electrode
Graphite Stainless steel [29] Aluminum
Hydrogen yield (L H2 g1 TOC)
Water
Wastewater
Water
Wastewater
Water
Wastewater
0.292 0.522 6.85
0.476 4.32(Feb) 16.33
e e e
0.90 4.5 10.45
e 34% 55%
0.92% 42% 74%
Hydrogen yield (10.45 L H2 g1 TOC) was much higher than that obtained from stainless-steel electrode. Peroxide addition decreased COD content further to 1035 mg L1 due to decomposition of organic matter in wastewater with no extra hydrogen gas production. Al(III) concentration in wastewater was 634 mg L1 at the end of 8 days of operation. Part of Al(II) was deposited on cathode surfaces in form of pure Alo. Energy efficiency (h) for H2 production from water using Al electrodes was calculated to be 55% as described below. Epvc ¼ V I t ¼ 13:6Vð15:2 A hÞ ¼ 206:72 W h VH2 ¼ 41:42 L;
mH2 ¼ 3:34 g
EH2 ¼ mH2 122 kJ=g ¼ 3:34ð122Þ ¼ 407:5 kJ ¼ 113:2 W h h ¼ EH2 =Epvc ¼ 0:55 Energy efficiency (h) for H2 production from wastewater using Al electrodes was 74% as calculated below. Epvc ¼ V I t ¼ 13:6Vð26:6 A hÞ ¼ 361:8 W h VH2 ¼ 98 L;
mH2 ¼ 7:9 g
EH2 ¼ mH2 122 kJ=g ¼ 7:9ð122Þ ¼ 964 kJ ¼ 267:7 W h h ¼ EH2 =Epvc ¼ 0:74 Energy conversion efficiency with the wastewater was much higher than that of water due to extra H2 gas production from organic compounds. Due to high sunlight intensity in July 2009, large volumes of H2 was produced with much higher rates and yields as compared to those of winter months. The yields and formation rates of hydrogen gas were compared in Table 1 for different electrodes along with energy efficiencies. Aluminum electrodes yielded the highest hydrogen yield (10.45 L H2 g1 TOC) as compared to the stainless steel and graphite electrodes. This is due to high electrical conductivity of aluminum electrodes releasing 3 e for every Al(III) ion formation. Metal ions (Fe(II) and Al(III)) released to the wastewater can be recovered by precipitation in form of metal hyroxides with lime (Ca(OH)2) addition.
4.
Energy efficiency ðEH2 =Eelec Þ
Conclusions
This study is one of the first reports on production of hydrogen gas from electrohydrolysis of organics present in wastewater using photovoltaic cells. Electrical current generated by PVC panel was applied to wastewater for H2 gas production using three different types of electrodes: graphite, stainless steel and
aluminum rods. The highest cumulative hydrogen gas evolution and the highest rate were obtained with the aluminum electrodes. Hydrogen gas formation from electrolysis of pure water varied depending on the sunlight intensity and the type of electrode. Hydrogen gas production from wastewater was 2.4 times higher than that obtained from water indicating that the major fraction of hydrogen was generated by electrohydrolysis of organic compounds. Energy efficiencies of hydrogen production from electrohydrolysis of wastewater varied between 0.92%(graphite) and 74% (aluminum). Hydrogen gas fraction in the gas phase varied between 90 and 99%. Hydrogen gas production from electrohydrolysis of wastewater using PVC power was proven to be a fast and energy efficient method as compared to electrolysis of water.
references
[1] Winter CC. Into the hydrogen energy economy-milestones. Int J Hydrogen Energy 2005;30:681e5. [2] Kapdan IK, Kargi F. Biohydrogen production from waste materials. Enzym Microb Technol 2006;38:569e82. [3] Levin DB, Pitt L, Love M. Biohydrogen production: prospects and limitations to practical application. Int J Hydrogen Energy 2004;29:173e85. [4] Das D, Veziroglu TN. Hydrogen production by biological processes: survey of literature. Int J Hydrogen Energy 2001;26: 13e28. [5] Argun H, Kargi F, Kapdan IK, Oztekin R. Biohydrogen production by dark fermentation of wheat powder solution: effects of C/N and C/P ratio on hydrogen yield and formation rate. Int J Hydrogen Energy 2008;33:1813e9. [6] Oztekin R, Kapdan IK, Kargi F, Argun H. Optimization of media composition for hydrogen gas production from hydrolyzed wheat starch by dark fermentation. Int J Hydrogen Energy 2008;33:4083e90. [7] Argun H, Kargi F, Kapdan IK, Oztekin R. Batch dark fermentation of powdered wheat starch to hydrogen gas: effects of the initial substrate and biomass concentrations. Int J Hydrogen Energy 2008;33:6109e15. [8] Argun H, Kargi F, Kapdan IK, Oztekin R. Light fermentation of dark fermentation effluent for bio-hydrogen production by different Rhodobacter species at different initial volatile fatty acid (VFA) concentrations. Int J Hydrogen Energy 2008;33:7405e12. [9] Kapdan IK, Kargi F, Oztekin R, Argun H. Biohydrogen production from acid hydrolyzed wheat starch by photofermentation using different Rhodobacter sp. Int J Hydrogen Energy 2009;34:2201e7. [10] Argun H, Kargi F. Photo-fermentative hydrogen gas production from dark fermentation effluent of ground wheat solution: effects of light source and light intensity. Int J Hydrogen Energy 2010;35:1595e603. [11] Tuna E, Kargi F, Argun H. Hydrogen gas production by electrohydrolysis of volatile fatty acid (VFA) containing dark fermentation effluent. Int J Hydrogen Energy 2009;34:262e9.
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[12] Day DL, Steinberg MP. Electrochemical conversion of organic material. U.S. patent 4,200,505, April 1980. [13] Roychowdhury S. Process for production of hydrogen from anaerobically decomposed organic materials. U.S. patent 6,090,266, July 2000. [14] Paterek JR. Method and apparatus for hydrogen production from organic wastes and manure. U.S. patent 6,887,692, May 2005. [15] Liu H, Grot S, Logan BE. Electrochemically assisted microbial production of hydrogen from acetate. Environ Sci Technol 2005;39:4317e20. [16] Cheng S, Logan BE. Sustainable and efficient biohydrogen production via electro hydrogenesis. Proc Nat Acad Sci USA 2007;104:18871e3. [17] Ditzig J, Liu H, Logan BE. Production of hydrogen from domestic wastewater using a bioelectrochemically assisted microbial reactor (BEAMR). Int J Hydrogen Energy 2007;32: 2296e304. [18] Call D, Logan BE. Hydrogen production in a single chamber microbial electrolysis cell lacking a membrane. Environ Sci Technol 2008;42:3401e6. [19] Nagai N, Takeuchi M, Kimura T, Oka T. Existence of optimum space between electrodes on hydrogen production by water electrolysis. Int J Hydrogen Energy 2003;28:35e41. [20] Varkaraki E, Lymberopoulos N, Zoulias E, Guichardot D, Poli G. Hydrogen based uninterruptible power supply. Int J Hydrogen Energy 2007;32:1589e96. [21] Dubey PK, Sinha ASK, Talapatra S, Koratkar N, Ajayan PM, Srivastava ON. Hydrogen generation by water electrolysis using carbon nano-tubes. Int J Hydrogen Energy 2010;35: 3945e50.
[22] Grigoriev SA, Porembsky VI, Fateev VN. Pure hydrogen production by PEM electrolysis for hydrogen energy. Int J Hydrogen Energy 2006;31:171e5. [23] Obara S. Study of water electrolysis system using a compact solar cell module with a plant shoot configuration. Int J Hydrogen Energy 2010;35:26e36. [24] Gibson TL, Kelly NA. Predicting efficiency of solar powered hydrogen generation using photovoltaic electrolysis devices. Int J Hydrogen Energy 2010;35:900e11. [25] Gibson TL, Kelly NA. Optimization of solar powered hydrogen production using photovoltaic electrolysis devices. Int J Hydrogen Energy 2008;33:5931e40. [26] Khaselev O, Bansal A, Turner JA. High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production. Int J Hydrogen Energy 2001;26:127e32. [27] Liu Z, Qiu Z, Luo Y, Mao Z, Wang C. Operation of first solarhydrogen system in China. Int J Hydrogen Energy 2010;35: 2762e6. [28] Sayigh AAM. The use of solar energy-photovoltaic- in hydrogen production and arid zones like Saudi Arabia. In ‘Hydrogen Energy Progress’ Proceedings of the third World Hydrogen Energy Conference Tokyo, Japan. June 1980. Pergamon Press; 1981. pp. 1431e39. [29] Eker S, Kargi F. Hydrogen gas production from electrohydrolysis of industrial wastewater organics by using photovoltaic cells (PVC). Int J Hydrogen Energy 2010;35: 12761e6. [30] Stojic D, Marcta M, Sovilj S, Miljanic S. Hydrogen generation from water electrolysis-possibilities of energy saving. J Power Sources 2003;118:315e9.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Comparison of different electrodes in hydrogen gas production from electrohydrolysis of wastewater organics using photovoltaic cells (PVC)5 Fikret Kargi* Department of Environmental Engineering, Dokuz Eylul University, Buca 35160, Izmir, Turkey
article info
abstract
Article history:
Electrical power generated by a photovoltaic cell (PVC) was supplied to diluted industrial
Received 13 October 2010
wastewater in a mechanically mixed and sealed stainless-steel reactor for hydrogen gas
Received in revised form
production. Three different electrodes, graphite, stainless steel and aluminum rods were
30 November 2010
used for comparison. Protons released from decomposition of organic compounds and
Accepted 2 December 2010
electrons provided by the DC current reacted to form hydrogen gas. The highest cumulative
Available online 31 December 2010
hydrogen gas formation (CHF) was obtained with the aluminum electrode (120 L in 8 days) and the lowest was with the graphite electrode (4 L). Hydrogen gas production from
Keywords:
wastewater was 2.4 times higher than that produced from water when aluminum elec-
Electrodes
trodes were used. TOC content of wastewater was reduced from 2400 to 1700 mg L1 with
Electrohydrolysis
nearly 29% TOC removal within 6 days. CHF from wastewater was 76 L within 18 days with
Hydrogen gas
the stainless-steel electrodes while CHF from water was only 9.5 L. Fermentative hydrogen
Photovoltaic cells (PVC)
gas production from wastewater was negligible in the absence PVC. Energy conversion
Wastewater
efficiency for hydrogen gas production (hydrogen energy/electric energy) was found to be 74% with the aluminum electrodes. Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Search for clean and high energy fuels has increased rapidly over the last fifty years. Fossil fuels used as major source of energy emit undesirable gases to atmosphere (SOx , NOx, COx) leading serious air pollution problems. Hydrogen gas is a clean and high energy (122 kJ g1) fuel as compared to fossil fuels. Hydrogen can also be used in fuel cells to generate electricity and is considered to be the major energy carrier of the future. The need for hydrogen gas is increasing rapidly with a growth rate of nearly 10% per year [1]. Unlike fossil fuels, hydrogen gas is not readily available in nature and is produced by steam reforming of natural gas or by electrolysis of water which are energy intensive processes 5
[2]. Fermentative production of hydrogen gas using carbohydrate rich renewable raw materials is more advantageous due to operation under mild conditions. Major obstacles in biohydrogen production by fermentation are low hydrogen yields and formation rates due to slow bacterial metabolisms [2e4]. Sequential dark and photo fermentations were used for fermentative hydrogen gas production from starch containing wastes [5e10]. Photo-fermentation of carbohydrates or volatile fatty acids (VFAs) produced by dark fermentation effluent requires strict control of environmental conditions and constant light intensity [8e10]. Hydrogen gas production by electrohydrolysis of volatile fatty acids (VFA) produced by dark fermentation has been studied as an alternative to photofermentation [11]. A few recent patents on hydrogen gas
All rights of the findings of this study are reserved by a patent application by Dr. F. Kargi. * Tel.: þ90 232 4127109. E-mail address: [email protected]. 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.12.010
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
production from organic wastes and fermentation effluents by electrical power application have been reported [12e14]. Hydrogen gas production using microbial electrolysis cells (MEC) has also been investigated [15e18]. Electrolysis of water for hydrogen gas production has been studied extensively [19e21,30]. The energy content of hydrogen gas produced by electrolysis of water is 3.55 kWh/m3 H2 (39.70 kWh/kg H2) while water electrolysis requires nearly 4 kWh/m3 H2 (44.75 kWh/kg H2) electrical energy. The rate and extent of hydrogen gas production by water electrolysis were improved by using catalytic and high temperature steam electrolysis. Water electrolysis using PEM containing cells requires net energy input for hydrogen gas production [22]. Photovoltaic cells (PVC) have been recently used for electrolysis of water [23e28]. No studies were reported on H2 gas production from electrohydrolysis of organic compounds present in wastes. The major objective of this study is to investigate hydrogen gas production from electrohydrolysis of organic compounds present in industrial wastewater using photovoltaic cells (PVC) for electrical power generation. Different electrodes (graphite, stainless steel and aluminum rods) were used in a well sealed and mechanically mixed reactor. Control experiments with DC power application to water and no DC power application to the wastewater were also performed to determine hydrogen gas evolution by water electrolysis and by natural anaerobic digestion of wastewater, respectively.
2.
Materials and methods
2.1.
Experimental set up
The experimental system (Fig. 1) consisted of a PVC panel with 80 cm 120 cm dimensions, a voltage regulator, a battery and a stainless-steel sealed reactor with dimensions of Do ¼ 21 cm, H ¼ 48 cm and volume of 16.8 L [29]. Wastewater volume in the reactor was 13.2 L with a head space of 3.6 L. The PVC panel contained 32 cells with a total power supply of 115 W (ie, 3.6 W for each cell) and provided 18 V voltage with 6 A current. A voltage regulator was used to adjust voltage to the desired level of 13.5 V. A battery was used to store electrical energy generated by the PVC panel and to provide constant current to the reactor when the sunlight was not available. Mechanically mixed and sealed stainless-steel reactor contained aqueous organic waste, electrodes and a pressure gauge. Electrodes were used to transmit the electrical current (electrons) generated by the PVC to the aqueous medium in the reactor. Three different electrodes, stainless steel, aluminum and graphite rods were used for comparison. Dimensions of the electrodes were L ¼ 49.5 cm and Do ¼ 0.9 cm which were mounted on the head plate of the reactor and immersed in wastewater inside the reactor [29].
2.2.
Experimental procedure
Experiments were started by filling the reactor with diluted wastewater, closing the head plate tightly and connecting the PVC to the electrodes through a voltage regulator and battery. Three sets of experiments were performed with different electrodes (stainless steel, aluminum and graphite) using
3451
diluted wastewater. Control experiments were performed with water to determine hydrogen gas production by water electrolysis. Hydrogen formation by anaerobic fermentation was also tested by inoculating the wastewater by heat-treated anaerobic sludge with no supply of electrical current. Wastewater was obtained from PAK MAYA Bakers Yeast Company, Izmir, Turkey and diluted properly to reduce TOC content to desired level. No external chemicals were added to diluted wastewater. Raw wastewater had COD and total solids contents of nearly 10 g L1 and 0.5 g L1 with a pH of 6.9 and conductivity of 1500 mS cm1.
2.3.
Analytical methods
Voltage and electrical current supplied to the reactor was measured using a voltage regulator. Voltage from the regulator was nearly 13.5 V and the current varied between 100 mA and 2 A depending on the availability of sunlight. Ampere hour (I t) were recorded by the voltage regulator everyday and used for electrical energy (P ¼ V I t) calculations. Hydrogen gas was sampled from the head space of the reactor by using gas-tight glass syringes and hydrogen concentration was determined by using a gas chromatograph (HP Agilent 6890). The GC column was Alltech, Hayesep D 80/100 600 1/800 0.8500 . Nitrogen gas was used as carrier with a flow rate of 30 ml min1 and the head pressure was 22 psi. The amount of total gas produced was determined everyday by water displacement method using sulfuric acid (2%) and NaCl (10%) containing solution. The cumulative hydrogen gas production was determined as described in our previous publications [5e10, 29]. pH and oxidationereduction potential (ORP) of the fermentation medium were monitored by using pH and ORP meters with relevant probes (WTW Scientific, Germany). Samples removed from fermentation media everyday were centrifuged at 7000 g to remove solids. Total organic carbon (TOC) content of wastewater was determined using a TOC analyzer (Teledyne Tekmar Apollo 9000 Combustion TOC Analyzer, USA) after centrifugation. Metal ions (Fe(II) and Al(III)) released to the medium were measured by using an ICP analyzer (Optical Emission Spectrometer Optima 2100 OV ICP Analyzer, Perkin Elmer, USA).
3.
Results and discussion
3.1.
Experiments with the stainless-steel electrodes
Hydrogen gas production from wastewater by application of PVC power strongly depends on the availability of free electrons from the electrodes. The experiments with stainless-steel electrodes were done for comparison of hydrogen evolution from water and wastewater. Fig. 2 depicts hydrogen evolution from water (Nov 10e28, 2008) and wastewater (Dec 18, 08eJan 9, 09) with the application of PVC power [29]. Approximately, 9.4 L hydrogen gas was produced within 432 h (18 days) from water electrolysis with hydrogen gas evolution rate of 0.522 L d1 [29]. Water loss from the reactor was nearly 700 mL. Medium pH increased from 7.7 to 8.3 due to removal of hydrogen ions (Hþ) from water to form
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Fig. 1 e A schematic diagram of the experimental set up used for hydrogen gas production from wastewater using photovoltaic cells. 1. Photovoltaic cell (PVC), 2. voltage regulator, 3. battery, 4. stainless-steel reactor containing aqueous organic waste, 5. electrodes, 6. mixing motor and blades, 7. hydrogen storage tank, 8. liquid feed/effluent port, 9. gas release port, 10. aqueous organic waste [29].
hydrogen gas. Nearly 95 L hydrogen gas was released within 22 days with hydrogen evolution rate of 4.32 L d1 when the same electrical power was applied to wastewater with initial TOC content of 4500 mg L1 [29]. Hydrogen gas production from wastewater was nearly 8 times greater than that obtained from water. Percentage of hydrogen gas in the head space varied between 90 and 98%. TOC content of wastewater decreased from nearly 4500 to 4200 mg L1. Total wastewater volume decreased from 13.2 to 11.9 L with a 1.32 L water loss within 22 days. The amount of TOC removed from wastewater was 9.5 g resulting in a hydrogen yield of nearly 10 L H2 g1 TOC. pH of wastewater decreased slightly from 6.83 to 6.7 indicating negligible hydrogen ion removal from water for hydrogen gas formation. Apparently, organic matter in wastewater was dissociated and/or hydrolysed with application of electrical current releasing hydrogen ions (Hþ) into
media which reacted with electrons released from the anode to form hydrogen gas. Anode reaction :
Fe/FeðIIÞ þ 2e
Organic compound dissociation : 2 HOC/2 OC þ 2 Hþ H2 gas formation :
2 Hþ þ 2 e / H2 ðgÞ
(1) (2) (3)
1
Approximately, 2.03 mg L Fe(II) was detected in the medium at the end of operation. Part of Fe(II) released to the media was deposited on cathode surfaces in form of pure Feo as described below. Cathode reaction :
FeðIIÞ þ 2e /FeO
(4)
Energy conversion efficiency (h) with the stainless-steel electrodes was calculated to be 42% as described below:
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 3 4 5 0 e3 4 5 6
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Fig. 2 e Comparison of hydrogen production from water and wastewater using photovoltaic cell and stainless-steel electrodes. C wastewater B water [29].
Epvc ¼ V I t ¼ 13:6Vð0:1 AÞð269 hÞ ¼ 365:8 W h VH2 ¼ 56:2 L; mH2 ¼ 4:53 gð30 C; 1 atmÞ EH2 ¼ mH2 122 kJ=g ¼ 4:53ð122Þ ¼ 552:7 kJ ¼ 153:5 W h h ¼ EH2 =Epvc ¼ 0:42
3.2.
Experiments with the graphite electrodes
Graphite has been the most widely used electrode in water electrolysis. Fig. 3 depicts CHF from water and wastewater by solar power application using graphite electrodes. When solar power was applied to water, only 3.22 L H2 was produced within 11 days (June 8e19, 2009) yielding hydrogen production rate of 0.29 L d1. Despite high sunlight intensity in June 2009,
hydrogen production was low due to low electrical conductivity of graphite electrodes. Head space of the reactor contained 85e90% hydrogen with no total gas release from the reactor. Hydrogen gas production from wastewater was higher than that of water yielding 3.81 L H2 production within 8 days (June 24eJuly 2, 2009). In this case, head space contained nearly 95% H2 with no total gas release from the reactor. Hydrogen gas formation rate from wastewater was considerably higher (0.476 L d1) than that of the water (0.29 L d1). The results indicated that electrode selection is very important to obtain high rates of hydrogen gas formation. TOC concentration of wastewater decreased from 2726 to 2404 mg L1 with a 4.25 g total TOC removal within 8 days. The hydrogen yield (0.90 L H2 g1 TOC) was considerably lower than that obtained with the stainless-steel electrodes. Energy conversion efficiency with the graphite electrodes was calculated to be 0.92% as presented below.
Fig. 3 e Cumulative hydrogen formation o water and wastewater using photovoltaic cells and graphite electrodes. C wastewater B water.
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Fig. 4 e Cumulative hydrogen formation from water and wastewater using photovoltaic cells and aluminum electrodes. C wastewater B water.
Epvc ¼ V I t ¼ 13:6Vð8:3 A hÞ ¼ 112:9 W h VH2 ¼ 3:81 L; mH2 ¼ 0:307 g ð30 C; 1 atmÞ EH2 ¼ mH2 122 kJ=g ¼ 0:307ð122Þ ¼ 37:5 kJ ¼ 1:04 W h h ¼ EH2 =Epvc ¼ 0:0092 This efficiency is significantly lower than that obtained with the stainless-steel electrodes indicating that graphite is not a suitable electrode for hydrogen production due to low free electron density and therefore, low electrical conductivity.
3.3.
Experiments with aluminum electrodes
Aluminum electrodes were used in the third set of experiments in July 2009. Fig. 4 depicts variation of CHF from water and wastewater using Al electrodes. Total hydrogen formation from water was 41.4 L within 6 days (July 4e10, 2009) yielding
HFR of 6.85 L d1. Gas phase contained nearly 98% hydrogen gas. pH of water decreased from 7.3 to 6.7 at the end of 6 days. When wastewater with an initial TOC content of 2350 mg L1 was used in the reactor, 98 L H2 was produced within 6 days (July 14e20, 2009) with nearly 99% H2 in gas phase. HFR (16.33 L d1) from wastewater was nearly 2.4 times higher than that obtained from water. Hydrogen production rate obtained with Al electrodes were much higher than those obtained with the stainless steel and graphite electrodes due to high free electron density and high electrical conductivity of aluminum. Hydrogen peroxide (1% H2O2) was added to wastewater for the last two days in order to determine if chemical decomposition of organic compounds by peroxide would improve hydrogen gas production. In fact hydrogen production increased to 118 L within the next 2 days with an HFR of 10 L d1. Fig. 5 depicts variation of TOC concentration with time using Al electrodes. TOC decreased from 2350 to 1640 mg L1 at the end of 6 days yielding a total TOC removal of 9.37 g.
Fig. 5 e Variation of TOC concentration with time for H2 gas production from wastewater using aluminum electrodes.
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Table 1 e Comparison of hydrogen production rates and yields by electrohydrolysis of wastewater using PVC power. H2 gas formation rate (L d1)
Electrode
Graphite Stainless steel [29] Aluminum
Hydrogen yield (L H2 g1 TOC)
Water
Wastewater
Water
Wastewater
Water
Wastewater
0.292 0.522 6.85
0.476 4.32(Feb) 16.33
e e e
0.90 4.5 10.45
e 34% 55%
0.92% 42% 74%
Hydrogen yield (10.45 L H2 g1 TOC) was much higher than that obtained from stainless-steel electrode. Peroxide addition decreased COD content further to 1035 mg L1 due to decomposition of organic matter in wastewater with no extra hydrogen gas production. Al(III) concentration in wastewater was 634 mg L1 at the end of 8 days of operation. Part of Al(II) was deposited on cathode surfaces in form of pure Alo. Energy efficiency (h) for H2 production from water using Al electrodes was calculated to be 55% as described below. Epvc ¼ V I t ¼ 13:6Vð15:2 A hÞ ¼ 206:72 W h VH2 ¼ 41:42 L;
mH2 ¼ 3:34 g
EH2 ¼ mH2 122 kJ=g ¼ 3:34ð122Þ ¼ 407:5 kJ ¼ 113:2 W h h ¼ EH2 =Epvc ¼ 0:55 Energy efficiency (h) for H2 production from wastewater using Al electrodes was 74% as calculated below. Epvc ¼ V I t ¼ 13:6Vð26:6 A hÞ ¼ 361:8 W h VH2 ¼ 98 L;
mH2 ¼ 7:9 g
EH2 ¼ mH2 122 kJ=g ¼ 7:9ð122Þ ¼ 964 kJ ¼ 267:7 W h h ¼ EH2 =Epvc ¼ 0:74 Energy conversion efficiency with the wastewater was much higher than that of water due to extra H2 gas production from organic compounds. Due to high sunlight intensity in July 2009, large volumes of H2 was produced with much higher rates and yields as compared to those of winter months. The yields and formation rates of hydrogen gas were compared in Table 1 for different electrodes along with energy efficiencies. Aluminum electrodes yielded the highest hydrogen yield (10.45 L H2 g1 TOC) as compared to the stainless steel and graphite electrodes. This is due to high electrical conductivity of aluminum electrodes releasing 3 e for every Al(III) ion formation. Metal ions (Fe(II) and Al(III)) released to the wastewater can be recovered by precipitation in form of metal hyroxides with lime (Ca(OH)2) addition.
4.
Energy efficiency ðEH2 =Eelec Þ
Conclusions
This study is one of the first reports on production of hydrogen gas from electrohydrolysis of organics present in wastewater using photovoltaic cells. Electrical current generated by PVC panel was applied to wastewater for H2 gas production using three different types of electrodes: graphite, stainless steel and
aluminum rods. The highest cumulative hydrogen gas evolution and the highest rate were obtained with the aluminum electrodes. Hydrogen gas formation from electrolysis of pure water varied depending on the sunlight intensity and the type of electrode. Hydrogen gas production from wastewater was 2.4 times higher than that obtained from water indicating that the major fraction of hydrogen was generated by electrohydrolysis of organic compounds. Energy efficiencies of hydrogen production from electrohydrolysis of wastewater varied between 0.92%(graphite) and 74% (aluminum). Hydrogen gas fraction in the gas phase varied between 90 and 99%. Hydrogen gas production from electrohydrolysis of wastewater using PVC power was proven to be a fast and energy efficient method as compared to electrolysis of water.
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