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Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology
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 x x x ( 2 0 1 8 ) 1 e5
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Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology Xiao-Wen Fang a, Lili Wang b, Wen-Fang Cai a, Deng-Wei Jing c,**, Qing-Yun Chen c, Yun-Hai Wang a,* a
Department of Environmental Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049, China Oil and Gas Technology Research Institute of Changqing Oilfield Company, Xi'an, 710018, China c State Key Lab of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China b
article info
abstract
Article history:
It is known that the reaction from methanol to hydrogen has a positive Gibbs free energy
Received 1 June 2018
and therefore cannot occur spontaneously. In the present work, by utilizing the chemical
Received in revised form
energy of neutralization, a new electrochemical technology was developed to produce
19 June 2018
hydrogen and electricity from methanol solution simultaneously, without needing external
Accepted 25 June 2018
energy input. In our designed electrochemical cell, hydrogen can be produced on cathode
Available online xxx
while methanol can be oxidized on anode with additional electricity production. The effect of anode surface area on hydrogen production rate and power output was also investi-
Keywords:
gated. With anode apparent surface area of 6.15 cm2, initial hydrogen production rate can
Bipolar membrane
reach up to 1.07 m3 H2 m3 d1 and the maximum power density output of 1.26 W m2 can
Electrochemical
be achieved, at the same time. Although it is only a preliminary work, our work is supposed
Fuel cell
to provide a new approach for the on-board hydrogen production for the application of
Hydrogen
various fuel cell technologies, which is urgently needed nowadays.
Methanol
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, as an important clean energy carrier, has been paid increasing attention due to its various advantages [1]. The benefit of using hydrogen is that the product of its combustion is only water and no greenhouse gases, or acid rain would be generated [2]. Adopting hydrogen as fuels will therefore significantly decrease the energy-linked environmental impacts [3]. In fact, hydrogen has been widely attempted for use in zero emission vehicles [4e6] and mobile technology [7,8].
However, several problems have to be addressed, such as safety, distribution, storage, and refueling [5,9] if it is intended to be used commercially. In order to overcome the above problems, a strategy of onsite hydrogen production from liquid fuels has been proposed [7,8,10]. On the other hand, due to the relatively high energy density, good availability, and high hydrogen-to-carbon ratio, methanol is considered to be such potential liquid fuel for the purpose. Traditionally, methanol is converted to hydrogen by a steam reforming technology, in which methanol is partially oxidized [11e13]. The main product of methanol reforming is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.-W. Jing), [email protected] (Y.-H. Wang). https://doi.org/10.1016/j.ijhydene.2018.06.151 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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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 x x x ( 2 0 1 8 ) 1 e5
a mixture of hydrogen and carbon dioxide and it has to be purified to obtain pure hydrogen. Methanol electrolysis is an alternative technology to obtain pure hydrogen directly with much lower electricity consumption than water electrolysis. It was reported that the electricity consumption by methanol electrolysis can be decreased by 65% compared to that of water electrolysis [14]. However in methanol electrolysis, a power supply is still needed and certain amount of electricity will be consumed in the process [14e16]. The need of a power supply is obviously inconvenient for its practical applications and it is also not cost effective. Generally, methanol to hydrogen conversion needs additional energy input because of the thermal dynamic barrier. In the present work, a novel electrochemical cell is designed to overcome the thermal dynamic barrier by integrating additional chemical energy into the process. In the present electrochemical cell, methanol can be oxidized in alkaline electrolyte while hydrogen is produced on cathode in acid electrolyte. The anolyte and catholyte can be separated by a bipolar membrane as illustrated in Fig. 1. Thus protons can be reduced to H2 on the cathode, while methanol can be oxidized on the anode. The cathodic, anodic and overall reaction can be shown in Eqs. (1)e(3), respectively. Cathodic: 2Hþ þ 2e /H2
Eqcat ¼ 0 V
Anodic: CH3 OH þ 8OH /CO2 3 þ 6H2 O þ 6e
(1) Eqan ¼ 0:81 V (2)
Overall: CH3 OH þ 6Hþ þ 8OH /CO2 3 þ 6H2 O þ 3H2
(3)
The overall reaction (Eq. (3)) has a negative Gibbs free energy change [17] of 469.73 kJ mol1 by integrating the acidalkali neutralization energy, thus the reaction can occur spontaneously. The above equations illustrate that the cathode potential is higher than the anode potential, the cell may produce hydrogen and electricity simultaneously. The present design is therefore expected to produce hydrogen and electricity simultaneously.
Materials and methods Reactor construction The electrochemical cell employed in our work is illustrated in Fig. 1. The anode is carbon cloth (HCN-030, Shanghai Hesen Co., China) coated with 60% PteRu/C (Shanghai Hesen Co., China. Pt:Ru ¼ 1:1) and Nafion® solution (5%, DuPont Company) and the apparent area is 3.5 cm2 except otherwise mentioned. The catalyst loaded on carbon cloth is 1 mg cm2 and the anode preparation procedure is referred to previous reports [18,19]. The cathode is a platinum foil with apparent surface area of 1.7 cm2. Both the anode and cathode are connected with Ti wires as current collectors. The anode and cathode are connected by a resistor and the voltage across the resistor is recorded with a data collector (DAM-3059R, Beijing Art Technology Development Co. Ltd., China) and a computer. The anode potential is monitored with a Ag/AgCl reference electrode on an IviumStat electrochemical workstation and the cathode potential is calculated by adding the anode potential and cell voltage. The anode and cathode chambers are separated by a bipolar membrane (BPM-I, Beijing Ting Run Co., China). The anolyte consists of 0.5 mol L1 sodium hydroxide, 2.5 mol L1 methanol and 0.26 mol L1 sodium sulfate while the catholyte is 0.1 mol L1 sulfuric acid which is previously bubbled with nitrogen for 15 min to remove the dissolved oxygen. The anolyte and catholyte pH during the experiments is measured with a pH meter (pHS-3C, Shanghai Leici Instruments, China).
Analytical methods Methanol concentration is calculated from the solution COD (1 mg L1 COD equal to 0.67 mg L1 methanol), while the COD concentration is monitored with a COD analyzer (CODMax, HACH) according to a standard method [20]. The concentration of H2 collected in the glass tube is analyzed by a gas chromatography equipped with a thermal conductivity detector (TCD) with nitrogen as carrier gas and the amount of generated hydrogen is calculated according to its concentration. The power density output is calculated according to the method reported before [18].
Results and discussion Hydrogen production
Fig. 1 e Illustration of the electrochemical cell.
To determine the hydrogen production performance of the electrochemical cell, the amount of generated hydrogen and charge transferred vs. reaction time are recorded as shown in Fig. 2. The experiment was carried out with 60 U external resistance. It can be found that both the hydrogen generation and charge transferred increases steadily with reaction time. From the experimental results, it is noted that at the initial 3 h the hydrogen production rate (normalized to catholyte volume of 20 mL) can reach as high as ca. 1.01 m3 H2 m3 d1, which then undergoes a steady decrease. The charge transfer
Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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 x x x ( 2 0 1 8 ) 1 e5
Fig. 2 e Hydrogen volume and charge transferred vs. the reaction time.
Fig. 3 e The cell voltage and electrodes potential vs. the reaction time.
rate shows similar trend. By comparing the hydrogen production rate and charge transfer, the cathodic efficiency is estimated to ca. 81% according to the method reported earlier [18]. It is also worth noting that during the reaction, methanol concentration also decreases steadily from 2.5 mol L1 to 1.47 mol L1 with reaction time indicating its consumption during the process. It is also worth noting that the ratio between methanol consumption and hydrogen production deviates from the stoichiometric ratio of 1 mol methanol to 3 mol hydrogen which may be a result of methanol evaporation to air and methanol crossover through the bipolar membrane [21]. In the present technology hydrogen and electricity coproduction was realized from methanol oxidation. Normally it is known that the reaction from methanol to hydrogen has a positive Gibbs free energy change as shown in Eq. (4). CH3 OH þ H2 O ¼ CO2 þ 3H2
DG ¼ 9:20 kJ mol
1
3
(4)
The positive Gibbs free energy change indicates that the above reaction cannot occur spontaneously due to the thermal dynamic barrier. However, our present technology can overcome this and realize hydrogen production together with electricity because our design can also utilize the chemical energy of neutralization. It can be more easily seen from the overall reaction (Eq. (3)) in the present technology. In present technology, hydrogen is produced on cathode by consuming protons in cathode chamber, while methanol is oxidized on anode to release electrons, protons and CO2. The released electrons are transported to cathode via external circuit, while released protons and CO2 are neutralized by hydroxide ions in anode chamber. So hydrogen production and methanol oxidation occurs together with acid-base neutraliztion. Compared with traditional methanol electrolysis method, hydrogen production in the present technology can be realized without additional electricity consumption.
Electricity production In a typical run of the experiment, the recorded cell voltage on a 60 U resistor vs. the reaction time is shown in Fig. 3.
As shown in Fig. 3, the voltage decreases steadily with the experiment running on and after about 960 min the cell voltage decreases to be below 10 mV. It was possibly caused by methanol consumption and alkaline consumption in anode chamber and protons consumption in cathode chamber. The consumption of protons on cathode increased the pH value in cathode chamber and caused the cathodic potential decreasing (Eq. (1) and Fig. 3). While the released protons and CO2 on anode decreased the pH in anode chamber and caused the anodic potential increasing (Eq. (2) and Fig. 3). After the experiment, pH value in cathode chamber was determined to be ca. 2.5 (initial pH ¼ 0.8) while it is ca. 12.7 in anode chamber (initial pH ¼ 13.1). This pH change is relatively larger compared with that from protons consumption for hydrogen production or the hydroxide ions consumption for neutralizing protons from methanol. So it is proposed that pH change may be also resulted from the protons and hydroxide ions transportation via the bipolar membrane. As mentioned above, the bipolar membrane was used to separate the anolyte and catholyte, but the ion-exchange layers were not strictly permselective [22]. Protons and hydroxide ions can diffuse through the membrane to accelerate the acid-base neutralization process. All the above factors can significantly decrease the electricity production performance of the electrochemical cell. Based on the analysis, using improved ion exchange membranes should be one option to solve this problem. Indeed, researchers have found many kinds of membranes to separate different electrolytes and improve cell performance [23e28]. Using membrane-electrolyte assembly (MEA) is another effective way to decrease the crossover [29]. In order to further investigate the electricity production performance, different external resistance (10 Ue1000 U) was applied to the electrochemical cell and the corresponding voltages were recorded to get the polarization curve. The anode apparent area was 3.5 cm2. The voltages were recorded after 10 min reaction to reach its steady state. The electricity production performance is shown in Fig. 4. As shown in Fig. 4, the cell voltage decreased from 368 mV to 6 mV with the external resistance changing from 1000 U to 10 U. While the maximum power output of 0.42 mW was
Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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Fig. 4 e The cell voltage and power output vs. the external resistance.
achieved with external resistance of 60 U, that was 1.2 W m2 by normalizing to the anode apparent area of 3.5 cm2. It indicates the internal resistance of the cell being about 60 U. Though at this external resistance, the electricity output reached the maximum, the hydrogen production rate was not the highest because hydrogen production rate is directly related to the current flow through the external circuit. The current usually increased with the external resistance decrease. This internal resistance is much higher than that in traditional direct methanol fuel cells [30,31]. Thus it suggests that further optimization of the cell configuration to minimize the internal resistance and to enhance the cell performance, should be the future work. One is expected to improve the performance of the cell by optimizing the cell design and operation [32,33] and lowering the distance between anode and cathode [34]. Integration of several cells in series or in parallel are also supposed to be promising approaches to increase the power output [35,36].
Effect of anode apparent area on hydrogen production rate and power output In order to reduce the anodic catalyst loading without decreasing the cell performance, the influence of different anode apparent area on hydrogen production rate as well as power output has also been tested and the results are shown in Fig. 5. In our work, the anode apparent area was changed from 0.25 to 6.15 cm2. The external resistance was kept as 60 U. The hydrogen production rate was evaluated within initial 3 h. Fig. 5 illustrates that the anode apparent area has significant effect on hydrogen production rate and power output. The initial hydrogen production rate increases from 0.46 to 1.01 m3 H2 m3 d1 with the anode apparent area increasing from 2.5 to 3.5 cm2, while the power output increases from 0.08 to 0.42 mW. This indicates that the anodic methanol oxidation may be the reaction rate limiting step. With the increase of the anode apparent area, more active sites on anode are available for methanol oxidation and the reaction rate accelerates. As a result, the hydrogen production rate and power output increase, too. When the anode apparent area increases to 6.15 cm2, the hydrogen production rate reaches 1.07 m3
Fig. 5 e Hydrogen production rate and power output vs. the anode apparent area.
H2 m3 d1 while power output reaches 0.44 mW (1.26 W m2). Compared with 3.5 cm2, the cell performance doesn't show considerable change with anode apparent area further increasing.
Conclusions The present work reported a new method of recovering simultaneously electricity and hydrogen from methanol solution. By using 0.5 mol L1 sodium hydroxide, 2.5 mol L1 methanol and 0.26 mol L1 sodium sulfate as anolyte and 0.1 mol L1 sulfuric acid as catholyte, PteRu/C as anode catalyst and bipolar membrane as separator, the constructed cell can be used to produce hydrogen and electricity simultaneously without additional energy input. The hydrogen production rate can reach as high as 1.07 m3 H2 m3 d1. In addition, the power output can reach up to 1.26 W m2. Although it is only a preliminary work, the present technology shows a promising method to recover hydrogen and electricity from methanol solution.
Acknowledgments This work was financially supported by Natural Science Basic Research Plan of Shaanxi Province, China [No. 2017JM5004].
references
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Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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 x x x ( 2 0 1 8 ) 1 e5
[4] Petkov T, Veziroglu TN, Sheffield JW. An outlook of hydrogen as an automotive fuel. Int J Hydrogen Energy 1989;14:449e74. [5] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Progr Energy Combust Sci 2009;35:490e527. [6] Song CS. Fuel processing for low-temperature and hightemperature fuel cells: challenges, and opportunities for sustainable development in the 21st century. Catal Today 2002;77:17e49. [7] Nielsen M, Alberico E, Baumann W, Drexler HJ, Junge H, Gladiali S. Low temperature aqueous phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013;495:85e9. [8] Krumpelt M, Krause TR, Carter JD, Kopasz JP, Ahmed S. Fuel processing for fuel cell systems in transportation and portable power applications. Catal Today 2002;77:3e16. [9] Durbin DJ, Malardier-Jugroot C. Review of hydrogen storage techniques for on board vehicle applications. Int J Hydrogen Energy 2013;38:14595e617. [10] Trimm DL, Onsan ZI. On-board fuel conversion for hydrogenfuel-cell-driven vehicles. Catal Rev 2001;43:31e84. [11] Shen CC, Jian TY, Wang YT. Steam reforming of methanol in a compact copper microchannel foam reactor. Fuel Cell 2013;13(6):965e70. [12] Iruretagoyena D, Hellgardt K, Chadwick D. Towards autothermal hydrogen production by sorption enhanced water gas shift and methanol reforming: a thermodynamic analysis. Int J Hydrogen Energy 2018;43:4211e22. [13] Zhou W, Ke YZ, Pei PC, Yu W, Chu XY, Li SL, et al. Hydrogen production from cylindrical methanol steam reforming microreactor with porous Cu-Al fiber sintered felt. Int J Hydrogen Energy 2018;43:3643e54. [14] Uhm S, Jeon H, Kim TJ, Lee J. Clean hydrogen production from methanolewater solutions via power-saved electrolytic reforming process. J Power Sources 2012;198:218e22. [15] Tuomi S, Santasalo-Aarnio A, Kanninen P, Kallio T. Hydrogen production by methanol-water solution electrolysis with an alkaline membrane cell. J Power Sources 2013;229:32e5. [16] Menia S, Tebibel H, Lassouane F, Khellaf A, Nouicer I. Hydrogen production by methanol aqueous electrolysis using photovoltaic energy: Algerian potential. Int J Hydrogen Energy 2017;42:8661e9. [17] Haynes WM, Lide DR. CRC handbook of chemistry and physics: a ready reference book of chemical and physical data. 92nd edition. Boca Raton: CRC Press; 2011. p. 5. 13-67. [18] Wang YH, Wang BS, Liu YP, Chen QY. Electricity and hydrogen co-production from a bio-electrochemical cell with acetate substrate. Int J Hydrogen Energy 2013;38:6600e6. [19] Wu Z, Sun G, Jin W, Wang Q, Hou H, Chan KY, et al. Use of in situ polymerized phenol-formaldehyde resin to modify a Nafion® membrane for the direct methanol fuel cell. J Power Sources 2007;167:309e14. [20] APHA. Standard methods for the examination of water and wastewater. Eighteenth ed. Washington, DC: American Public Health Association; 1992.
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[21] Liu J, Wang HT, Cheng SA, Chan KY. Nafion-polyfurfuryl alcohol nanocomposite membranes with low methanol permeation. Chem Comm 2004;6:728e9. [22] Strathmann H, Krol JJ, Rapp HJ, Eigenberger G. Limiting current density and water dissociation in bipolar membranes. J Membr Sci 1997;125:123e42. [23] Seo SH, Lee CS. A study on the overall efficiency of direct methanol fuel cell by methanol crossover current. Appl Energy 2010;87:2597e604. [24] Casalegno A, Santoro C, Rinaldi F, Marchesi R. Low methanol crossover and high efficiency direct methanol fuel cell: the influence of diffusion layers. J Power Sources 2011;196:2669e75. [25] Heinzel A, Barragan VM. A review of the state-of-the-art of the methanol crossover in direct methanol fuel cells. J Power Sources 1999;84:70e4. [26] Yamaguchi T, Zhou H, Nakazawa S, Hara N. An extremely low methanol crossover and highly durable aromatic porefilling electrolyte membrane for direct methanol fuel cells. Adv Mater 2007;19:592e6. [27] Krishnan L, Yeager G, Clark K, Kerr J, Soloveichik G. Enhanced fuel cell performance of decalin treated Nafion® membranes. Fuel Cell 2015;15(1):239e45. [28] Manjula N, Balaji R, Ramya K, Dhathathreyan KS, Ramachandraiah A. Studies on development of titania nanotube (TNT) based ePTFE-Nafion(R)-composite membrane for electrochemical methanol reformation. Int J Hydrogen Energy 2016;41:8777e84. [29] Liu F, Lu G, Wang CY. Low crossover of methanol and water through thin membranes in direct methanol fuel cells. J Electrochem Soc 2006;153:A543e53. [30] Martin JJ, Qian WM, Wang HJ, Neburchilov V, Zhang JJ, Wilkinson DP, et al. Design and testing of a passive planar three-cell DMFC. J Power Sources 2007;164:287e92. [31] Wang YT, Zheng L, Han GM, Lu LX, Wang M, Li JL, et al. A novel multi-porous and hydrophilic anode diffusion layer for DMFC. Int J Hydrogen Energy 2014;39:19132e9. [32] Li X, Faghri A. Review and advances of direct methanol fuel cells (DMFCs) part I: design, fabrication, and testing with high concentration methanol solutions. J Power Sources 2013;226:223e40. [33] Ayyaru S, Dharmalingam S. Development of MFC using sulphonated polyether ether ketone (SPEEK) membrane for electricity generation from waste water. Bioresource Technol 2011;102:11167e71. [34] Raman K, Lan JCW. Performance and kinetic study of photo microbial fuel cells (PMFCs) with different electrode distances. Appl Energy 2012;100:100e5. [35] Kim Y, Hatzell MC, Hutchinson AJ, Logan BE. Capturing power at higher voltages from arrays of microbial fuel cells without voltage reversal. Energy Environ Sci 2011;4:4662e7. [36] Kim D, Lee J, Lim TH, Oh IH, Ha HY. Operational characteristics of a 50 W DMFC stack. J Power Sources 2006;155:203e12.
Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology Xiao-Wen Fang a, Lili Wang b, Wen-Fang Cai a, Deng-Wei Jing c,**, Qing-Yun Chen c, Yun-Hai Wang a,* a
Department of Environmental Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049, China Oil and Gas Technology Research Institute of Changqing Oilfield Company, Xi'an, 710018, China c State Key Lab of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China b
article info
abstract
Article history:
It is known that the reaction from methanol to hydrogen has a positive Gibbs free energy
Received 1 June 2018
and therefore cannot occur spontaneously. In the present work, by utilizing the chemical
Received in revised form
energy of neutralization, a new electrochemical technology was developed to produce
19 June 2018
hydrogen and electricity from methanol solution simultaneously, without needing external
Accepted 25 June 2018
energy input. In our designed electrochemical cell, hydrogen can be produced on cathode
Available online xxx
while methanol can be oxidized on anode with additional electricity production. The effect of anode surface area on hydrogen production rate and power output was also investi-
Keywords:
gated. With anode apparent surface area of 6.15 cm2, initial hydrogen production rate can
Bipolar membrane
reach up to 1.07 m3 H2 m3 d1 and the maximum power density output of 1.26 W m2 can
Electrochemical
be achieved, at the same time. Although it is only a preliminary work, our work is supposed
Fuel cell
to provide a new approach for the on-board hydrogen production for the application of
Hydrogen
various fuel cell technologies, which is urgently needed nowadays.
Methanol
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, as an important clean energy carrier, has been paid increasing attention due to its various advantages [1]. The benefit of using hydrogen is that the product of its combustion is only water and no greenhouse gases, or acid rain would be generated [2]. Adopting hydrogen as fuels will therefore significantly decrease the energy-linked environmental impacts [3]. In fact, hydrogen has been widely attempted for use in zero emission vehicles [4e6] and mobile technology [7,8].
However, several problems have to be addressed, such as safety, distribution, storage, and refueling [5,9] if it is intended to be used commercially. In order to overcome the above problems, a strategy of onsite hydrogen production from liquid fuels has been proposed [7,8,10]. On the other hand, due to the relatively high energy density, good availability, and high hydrogen-to-carbon ratio, methanol is considered to be such potential liquid fuel for the purpose. Traditionally, methanol is converted to hydrogen by a steam reforming technology, in which methanol is partially oxidized [11e13]. The main product of methanol reforming is
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.-W. Jing), [email protected] (Y.-H. Wang). https://doi.org/10.1016/j.ijhydene.2018.06.151 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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a mixture of hydrogen and carbon dioxide and it has to be purified to obtain pure hydrogen. Methanol electrolysis is an alternative technology to obtain pure hydrogen directly with much lower electricity consumption than water electrolysis. It was reported that the electricity consumption by methanol electrolysis can be decreased by 65% compared to that of water electrolysis [14]. However in methanol electrolysis, a power supply is still needed and certain amount of electricity will be consumed in the process [14e16]. The need of a power supply is obviously inconvenient for its practical applications and it is also not cost effective. Generally, methanol to hydrogen conversion needs additional energy input because of the thermal dynamic barrier. In the present work, a novel electrochemical cell is designed to overcome the thermal dynamic barrier by integrating additional chemical energy into the process. In the present electrochemical cell, methanol can be oxidized in alkaline electrolyte while hydrogen is produced on cathode in acid electrolyte. The anolyte and catholyte can be separated by a bipolar membrane as illustrated in Fig. 1. Thus protons can be reduced to H2 on the cathode, while methanol can be oxidized on the anode. The cathodic, anodic and overall reaction can be shown in Eqs. (1)e(3), respectively. Cathodic: 2Hþ þ 2e /H2
Eqcat ¼ 0 V
Anodic: CH3 OH þ 8OH /CO2 3 þ 6H2 O þ 6e
(1) Eqan ¼ 0:81 V (2)
Overall: CH3 OH þ 6Hþ þ 8OH /CO2 3 þ 6H2 O þ 3H2
(3)
The overall reaction (Eq. (3)) has a negative Gibbs free energy change [17] of 469.73 kJ mol1 by integrating the acidalkali neutralization energy, thus the reaction can occur spontaneously. The above equations illustrate that the cathode potential is higher than the anode potential, the cell may produce hydrogen and electricity simultaneously. The present design is therefore expected to produce hydrogen and electricity simultaneously.
Materials and methods Reactor construction The electrochemical cell employed in our work is illustrated in Fig. 1. The anode is carbon cloth (HCN-030, Shanghai Hesen Co., China) coated with 60% PteRu/C (Shanghai Hesen Co., China. Pt:Ru ¼ 1:1) and Nafion® solution (5%, DuPont Company) and the apparent area is 3.5 cm2 except otherwise mentioned. The catalyst loaded on carbon cloth is 1 mg cm2 and the anode preparation procedure is referred to previous reports [18,19]. The cathode is a platinum foil with apparent surface area of 1.7 cm2. Both the anode and cathode are connected with Ti wires as current collectors. The anode and cathode are connected by a resistor and the voltage across the resistor is recorded with a data collector (DAM-3059R, Beijing Art Technology Development Co. Ltd., China) and a computer. The anode potential is monitored with a Ag/AgCl reference electrode on an IviumStat electrochemical workstation and the cathode potential is calculated by adding the anode potential and cell voltage. The anode and cathode chambers are separated by a bipolar membrane (BPM-I, Beijing Ting Run Co., China). The anolyte consists of 0.5 mol L1 sodium hydroxide, 2.5 mol L1 methanol and 0.26 mol L1 sodium sulfate while the catholyte is 0.1 mol L1 sulfuric acid which is previously bubbled with nitrogen for 15 min to remove the dissolved oxygen. The anolyte and catholyte pH during the experiments is measured with a pH meter (pHS-3C, Shanghai Leici Instruments, China).
Analytical methods Methanol concentration is calculated from the solution COD (1 mg L1 COD equal to 0.67 mg L1 methanol), while the COD concentration is monitored with a COD analyzer (CODMax, HACH) according to a standard method [20]. The concentration of H2 collected in the glass tube is analyzed by a gas chromatography equipped with a thermal conductivity detector (TCD) with nitrogen as carrier gas and the amount of generated hydrogen is calculated according to its concentration. The power density output is calculated according to the method reported before [18].
Results and discussion Hydrogen production
Fig. 1 e Illustration of the electrochemical cell.
To determine the hydrogen production performance of the electrochemical cell, the amount of generated hydrogen and charge transferred vs. reaction time are recorded as shown in Fig. 2. The experiment was carried out with 60 U external resistance. It can be found that both the hydrogen generation and charge transferred increases steadily with reaction time. From the experimental results, it is noted that at the initial 3 h the hydrogen production rate (normalized to catholyte volume of 20 mL) can reach as high as ca. 1.01 m3 H2 m3 d1, which then undergoes a steady decrease. The charge transfer
Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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 x x x ( 2 0 1 8 ) 1 e5
Fig. 2 e Hydrogen volume and charge transferred vs. the reaction time.
Fig. 3 e The cell voltage and electrodes potential vs. the reaction time.
rate shows similar trend. By comparing the hydrogen production rate and charge transfer, the cathodic efficiency is estimated to ca. 81% according to the method reported earlier [18]. It is also worth noting that during the reaction, methanol concentration also decreases steadily from 2.5 mol L1 to 1.47 mol L1 with reaction time indicating its consumption during the process. It is also worth noting that the ratio between methanol consumption and hydrogen production deviates from the stoichiometric ratio of 1 mol methanol to 3 mol hydrogen which may be a result of methanol evaporation to air and methanol crossover through the bipolar membrane [21]. In the present technology hydrogen and electricity coproduction was realized from methanol oxidation. Normally it is known that the reaction from methanol to hydrogen has a positive Gibbs free energy change as shown in Eq. (4). CH3 OH þ H2 O ¼ CO2 þ 3H2
DG ¼ 9:20 kJ mol
1
3
(4)
The positive Gibbs free energy change indicates that the above reaction cannot occur spontaneously due to the thermal dynamic barrier. However, our present technology can overcome this and realize hydrogen production together with electricity because our design can also utilize the chemical energy of neutralization. It can be more easily seen from the overall reaction (Eq. (3)) in the present technology. In present technology, hydrogen is produced on cathode by consuming protons in cathode chamber, while methanol is oxidized on anode to release electrons, protons and CO2. The released electrons are transported to cathode via external circuit, while released protons and CO2 are neutralized by hydroxide ions in anode chamber. So hydrogen production and methanol oxidation occurs together with acid-base neutraliztion. Compared with traditional methanol electrolysis method, hydrogen production in the present technology can be realized without additional electricity consumption.
Electricity production In a typical run of the experiment, the recorded cell voltage on a 60 U resistor vs. the reaction time is shown in Fig. 3.
As shown in Fig. 3, the voltage decreases steadily with the experiment running on and after about 960 min the cell voltage decreases to be below 10 mV. It was possibly caused by methanol consumption and alkaline consumption in anode chamber and protons consumption in cathode chamber. The consumption of protons on cathode increased the pH value in cathode chamber and caused the cathodic potential decreasing (Eq. (1) and Fig. 3). While the released protons and CO2 on anode decreased the pH in anode chamber and caused the anodic potential increasing (Eq. (2) and Fig. 3). After the experiment, pH value in cathode chamber was determined to be ca. 2.5 (initial pH ¼ 0.8) while it is ca. 12.7 in anode chamber (initial pH ¼ 13.1). This pH change is relatively larger compared with that from protons consumption for hydrogen production or the hydroxide ions consumption for neutralizing protons from methanol. So it is proposed that pH change may be also resulted from the protons and hydroxide ions transportation via the bipolar membrane. As mentioned above, the bipolar membrane was used to separate the anolyte and catholyte, but the ion-exchange layers were not strictly permselective [22]. Protons and hydroxide ions can diffuse through the membrane to accelerate the acid-base neutralization process. All the above factors can significantly decrease the electricity production performance of the electrochemical cell. Based on the analysis, using improved ion exchange membranes should be one option to solve this problem. Indeed, researchers have found many kinds of membranes to separate different electrolytes and improve cell performance [23e28]. Using membrane-electrolyte assembly (MEA) is another effective way to decrease the crossover [29]. In order to further investigate the electricity production performance, different external resistance (10 Ue1000 U) was applied to the electrochemical cell and the corresponding voltages were recorded to get the polarization curve. The anode apparent area was 3.5 cm2. The voltages were recorded after 10 min reaction to reach its steady state. The electricity production performance is shown in Fig. 4. As shown in Fig. 4, the cell voltage decreased from 368 mV to 6 mV with the external resistance changing from 1000 U to 10 U. While the maximum power output of 0.42 mW was
Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151
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Fig. 4 e The cell voltage and power output vs. the external resistance.
achieved with external resistance of 60 U, that was 1.2 W m2 by normalizing to the anode apparent area of 3.5 cm2. It indicates the internal resistance of the cell being about 60 U. Though at this external resistance, the electricity output reached the maximum, the hydrogen production rate was not the highest because hydrogen production rate is directly related to the current flow through the external circuit. The current usually increased with the external resistance decrease. This internal resistance is much higher than that in traditional direct methanol fuel cells [30,31]. Thus it suggests that further optimization of the cell configuration to minimize the internal resistance and to enhance the cell performance, should be the future work. One is expected to improve the performance of the cell by optimizing the cell design and operation [32,33] and lowering the distance between anode and cathode [34]. Integration of several cells in series or in parallel are also supposed to be promising approaches to increase the power output [35,36].
Effect of anode apparent area on hydrogen production rate and power output In order to reduce the anodic catalyst loading without decreasing the cell performance, the influence of different anode apparent area on hydrogen production rate as well as power output has also been tested and the results are shown in Fig. 5. In our work, the anode apparent area was changed from 0.25 to 6.15 cm2. The external resistance was kept as 60 U. The hydrogen production rate was evaluated within initial 3 h. Fig. 5 illustrates that the anode apparent area has significant effect on hydrogen production rate and power output. The initial hydrogen production rate increases from 0.46 to 1.01 m3 H2 m3 d1 with the anode apparent area increasing from 2.5 to 3.5 cm2, while the power output increases from 0.08 to 0.42 mW. This indicates that the anodic methanol oxidation may be the reaction rate limiting step. With the increase of the anode apparent area, more active sites on anode are available for methanol oxidation and the reaction rate accelerates. As a result, the hydrogen production rate and power output increase, too. When the anode apparent area increases to 6.15 cm2, the hydrogen production rate reaches 1.07 m3
Fig. 5 e Hydrogen production rate and power output vs. the anode apparent area.
H2 m3 d1 while power output reaches 0.44 mW (1.26 W m2). Compared with 3.5 cm2, the cell performance doesn't show considerable change with anode apparent area further increasing.
Conclusions The present work reported a new method of recovering simultaneously electricity and hydrogen from methanol solution. By using 0.5 mol L1 sodium hydroxide, 2.5 mol L1 methanol and 0.26 mol L1 sodium sulfate as anolyte and 0.1 mol L1 sulfuric acid as catholyte, PteRu/C as anode catalyst and bipolar membrane as separator, the constructed cell can be used to produce hydrogen and electricity simultaneously without additional energy input. The hydrogen production rate can reach as high as 1.07 m3 H2 m3 d1. In addition, the power output can reach up to 1.26 W m2. Although it is only a preliminary work, the present technology shows a promising method to recover hydrogen and electricity from methanol solution.
Acknowledgments This work was financially supported by Natural Science Basic Research Plan of Shaanxi Province, China [No. 2017JM5004].
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Please cite this article in press as: Fang X-W, et al., Simultaneous production of electricity and hydrogen from methanol solution with a new electrochemical technology, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.06.151