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A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell
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 ) 2 7 7 3 e2 7 7 6
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell Yu-Xi Huang a, Xian-Wei Liu a, Xue-Fei Sun b, Guo-Ping Sheng a, Yuan-Yuan Zhang a, Guo-Ming Yan a, Shu-Guang Wang b, An-Wu Xu c, Han-Qing Yu a,* a
Department of Chemistry, University of Science & Technology of China, Hefei 230026, China School of Environmental Science and Engineering, Shandong University, Jinan, 250100, China c Hefei National Laboratory for Physical Sciences at the Microscale, University of Science & Technology of China, Hefei 230026, China b
article info
abstract
Article history:
Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from
Received 25 August 2010
organic matters, but it still suffers from the lack of efficient and cost-effective cathode
Received in revised form
catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using
20 November 2010
electrochemical deposition method and used as the cathodic catalyst in an MEC to facili-
Accepted 27 November 2010
tate hydrogen production. The electrode coated with Pd nanoparticles showed a lower
Available online 30 December 2010
overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst
Keywords:
were slightly higher than those with a Pt cathode, while the Pd loading was one order of
Cathodic catalysts
magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd
Electrochemical deposition
nanoparticles was about fifty times higher than that of the Pt black catalyst. These results
Hydrogen
demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the
Microbial electrolysis cell (MEC)
costs of the cathodic catalysts when maintaining the MEC system performance.
Pd nanoparticle
1.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen can be produced from renewable energy resources, water and biomass, by a variety of processes, e.g., photolysis, electrolysis, thermochemical, and biochemical [1,2]. One of them is microbial electrolysis cell (MEC) technology, which provides an efficient and sustainable approach for hydrogen production from organic matters [3e5]. This system uses microbes to oxidize organic materials at anode, and hydrogen gas is evolved at cathode by adding a supplemental voltage to that produced by the bacteria to overcome the endothermic barrier of hydrogen formation [5].
Despite of its recent advances in reactor design and operation, the MEC still faces many challenges for practical applications, among which the high costs of cathode catalyst is one of the most critical ones. Platinum is the most effective catalyst to facilitate hydrogen evolution reaction (HER) and has been widely employed as the cathodic catalyst for HER in MECs [4,6]. Such a dependence on Pt and an expensive and scarce resource definitely limit the wide deployment and application of MECs. To resolve this problem, attempts have been made to maximize Pt utilization and search for Pt-alternative catalysts for HER in MECs. Recently, tungsten carbide [7] and Ni-based alloy catalysts [8,9] have been evaluated. There catalysts display
* Corresponding author. Fax: þ86 551 3601592. E-mail address: [email protected] (H.-Q. Yu). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.114
2774
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 ) 2 7 7 3 e2 7 7 6
good performance in terms of HER, but their hydrogen evolution rate is lower than that with Pt. And the use of the high cost Nafion binder also leads to the increase of the MEC construction cost. On the other hand, extensive studies have been carried out on non-Pt catalysts for HER in water electrolysis [10,11]. Palladium, the most Pt-like metal, is being a focus as a catalyst due to its excellent catalytic capabilities and a relatively abundant resource. Although hydrogen production from water electrolysis and MEC shares the same cathodic reaction, the operational conditions in water electrolyzers are significantly different from those in MECs [8]. Also, the catalytic performance of Pd at neutral pH has not been well evaluated. In addition, it is well known that the physicochemical properties of catalysts are dependent heavily on their size, shape and oxidation state. Thus, it is of significance to develop cathodic catalysts with controllable morphology and stable performance by using facile methods to enhance the HER and lower the MEC costs. Thus, the aim of the present study is to investigate the feasibility of using the Pd nanoparticles prepared by the electrochemical deposition method for HER in an MEC. The Pd nanoparticles were firstly deposited on carbon paper. Then, the electrochemical properties towards hydrogen evolution of MECs with different catalysts were evaluated and compared.
platinum wire counter electrode in 50 mM phosphate buffer solution (pH 7.0). LSV scans from 0.4 to 1.8 V (vs. Ag/AgCl reference electrode) were conducted at a scan rate of 2 mV/s. The MEC system was constructed according to previous report [14]. The anode and cathode each in a 480 mL chamber were separated by a PEM (GEFC-10N, GEFC Co., China). The anode chamber was filled with 400-mL medium containing (in 1 L of 50 mM phosphate buffer, pH 7.0): NaAc, 100 mg; NH4Cl, 310 mg; KCl, 130 mg; CaCl2, 10 mg; MgCl2$6H2O, 20 mg; NaCl, 2 mg; FeCl2, 5 mg; CoCl2$2H2O, 1 mg; MnCl2$4H2O, 1 mg; AlCl3, 0.5 mg; (NH4)6Mo7O24, 3 mg; H3BO3, 1 mg; NiCl2$6H2O, 0.1 mg; CuSO4$5H2O, 1 mg; ZnCl2, 1 mg. The cathode chamber of the MEC was filled with 400-mL of 50 mM phosphate buffer at pH 7.0. Activated carbon fibers [15] with enriched bacteria from an acetate-fed microbial fuel cell for over 6 months were used as the anode. The cathode electrodes were 4 4 cm carbon papers coated by Pd nanoparticles as prepared. The commercial Pt-coated carbon paper (0.5 mg cm2) and raw carbon paper were also evaluated as the cathode for HER. The voltage adding to the MEC was 0.6 V using potentiostat. After each fed-batch cycle (when H2 production stopped), the MEC was drained, and refilled with substrate solution, and sparged with nitrogen gas for 30 min. Each cathode material was tested three times for one month.
2.4.
2.
Materials and methods
2.1.
Materials
K2PdCl6 was purchased from Aladdin Reagent Co. and the remaining chemicals were from Sinopharm Chemical Reagent Co. Cathode electrodes were made of carbon papers (TGP-H-090, Toray Co., Japan) with or without Pt/C (20 wt% 3.2-nm Pt nanoparticles on Vulcan XC-72 carbon support) on it (GEFC Co., China), which were sonicated for 30 min in deionized water before further use.
2.2.
Electrochemical deposition of Pd nanoparticles
Pd nanoparticles were prepared via the electrochemical deposition method onto a 4 4 cm plain carbon paper using a potentiostat (660C, CH Instruments, Inc., USA). Before the electrochemical deposition, electrochemical oxidation of the carbon paper was proceeded to form the adequate catalyst support. Briefly, a constant potential of 2.0 V was applied in HClO4 (0.5 M) for 2 min using a saturated Ag/AgCl reference electrode (þ0.198 V vs. Standard Hydrogen Electrode) and a Pt counter electrode [12]. Then, the electrochemical deposition of Pd was conducted in 0.1 M NaCl solution containing 1.26 mM K2PdCl6 as the precursor [13]. The deposition potential was fixed at 0.4 V, and the deposition time was 200 s.
2.3.
Evaluation of catalysts
The hydrogen evolution overpotential was evaluated using linear sweep voltammetry (LSV). The cathode electrodes before and after the MEC tests were placed in cubic electrochemical cells (5 cm length) with an Ag/AgCl reference electrode and
Analysis and calculation
The produced gases were measured two times everyday using a gas chromatograph (model SP-6800A, Lunan Co., China). The morphology of the Pd-coated carbon paper was observed using a Sirion 200 scanning electron microscopy (SEM) (FEI Co., the Netherlands). The MEC performance was evaluated in terms of: coulombic efficiency (CE) (%) based on total added substrate; cathodic hydrogen recovery (RH2 ;CAT ) (%) based on the recovered electrons as hydrogen compared to the current transferred; total hydrogen recovery (RH2 ;COD ) (%) obtained as RH2 ;COD ¼ CE*RH2 ;CAT ; hydrogen production rate (Q) (L m2 d1) based on hydrogen produced normalized to the cathode electrode area [6]; catalyst loading (CL) (mg) based on the electrons transferred in the electrochemical deposition process; catalytic efficiency (CaE) is defined as CaE ¼ Q/CL with a unit of L m2 d1 mg1.
3.
Results and discussion
3.1.
Preparation and morphology of the Pd nanoparticles
The electrochemical oxidation enriched the oxygen-containing species on the carbon paper surface with carboxylic and hydroxy groups. This led to the formation of more-dispersed Pd particles, attributed to the increase in surface accessibility. In this case, the oxygen-containing species on the carbon surface acted as acidic sites for the adsorption of the catalyst precursors [12]. Typical SEM image of the Pd nanoparticles clearly shows that a low surface density of nanoparticles with uniform shape was formed under the tested conditions (Fig. 1). Assuming that the electrodeposition coulombic efficiency was 100%, the maximum loadings of the Pd nanoparticles were calculated as
2775
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 ) 2 7 7 3 e2 7 7 6
Fig. 1 e SEM images of carbon papers without (a) and with (b) Pd nanoparticles deposited. 0.17 mg, or 0.0106 mg cm2, which was nearly 50 times less than Pt (0.5 mg cm2).
3.2.
Hydrogen evolution activity of the Pd nanoparticles
The Pd-coated carbon paper was characterized by LSV and compared with the Pt black and control cathodes before and after MEC tests. Fig. 2 shows the hydrogen evolution over potentials of the three electrodes before and after 1-month MEC experiments, which could provide further evidence of the activity and durability of the electrodeposited Pd nanoparticles. Both the Pd and Pt catalysts had much lower over potentials than the control electrode without any catalyst, and the Pt catalyst exhibited the highest current density because of its larger loading. The over potential of the Pd catalyst had a slightly change, while the Pt catalyst showed a significant negative shift, suggesting that the Pd catalyst exhibited better stability than the Pt catalyst. The LSV results indicate that the electrodeposited Pd nanoparticles had excellent activity in HER and reliable durability, which is of great importance for long term operation of MEC.
3.3. Hydrogen production of MEC with the Pd nanoparticles The Pd-coated carbon paper was then used as the cathode in an MEC, and its performance was compared to other MECs
respectively with Pt black and control carbon paper cathodes. The anode was inoculated with pre-enriched consortium of exoelectrogens and large volume buffer (400 mL each chamber) was used, while the cathode area was relatively small to make sure the cathode to be the main limitation in the MECs. The MEC with the Pd cathode produced slightly more hydrogen than that with the Pt cathode at the end of the experiments (Fig. 3), and consequently resulted in a higher coulombic efficiency, hydrogen recovery and hydrogen production rate (Table 1). This result demonstrates that the Pd nanoparticles are more effective for the HER catalysis in MECs, though the Pt catalyst showed higher current density in LSV test. Since the Pd loading amount (0.0106 mg cm2) was significant less than that of Pt (0.5 mg cm2), the MEC performance with the Pd catalyst was still better than that with the Pt catalyst. Utilization of the Pd catalyst prepared in this work would significantly reduce the cathodic catalyst costs for MECs. However, it should be noticed that the coulombic efficiency and hydrogen production rate in this study are low because the MEC was not optimized. To further compare the different catalysts for HER, the catalytic properties of the catalysts toward to their loadings were benchmarked. The catalytic efficiency of the Pd nanoparticles was 15.42 2.83 (L m2 d1 mg1), over fifty times larger than the Pt black (0.27 0.04 L m2 d1 mg1) (Table 1), but the difference between their total hydrogen production rates
0
16
Hydrogen volume / mL
-2
j (mA cm )
-20 -40 -60 -80
Control Pt Pd
18
Control
-100 -2.0
Pd
Pt
14 12 10 8 6 4 2
-1.5 -1.0 -0.5 E (V vs. Ag/AgCl)
0.0
Fig. 2 e LSV plots of the three cathodes before (dot line) and after (solid line) the MEC experiments in 50 mM phosphate buffer solution (pH 7.0) and scan rate of 2 mV sL1.
0 0
20
40
60
80
100
120
Operating time / hour Fig. 3 e Hydrogen production for the MEC with the three cathodes for three cycles.
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Table 1 e MEC results for Pd, Pt and control catalyst cathodes at an applied voltage of 0.6 V for three cycles. Catalyst None Pt Pd
CE (%)
RH2 ;CAT (%)
RH2 ;COD (%)
Q (L m2 d1)
Catalytic efficiencies normalized by loadings (L m2 d1 mg1)
23.3 0.6 52.0 7.4 56.0 10.2
25.4 12.1 41.6 6.1 46.4 8.5
5.9 2.8 21.6 3.2 26.0 4.8
0.4 0.1 2.1 0.3 2.6 0.5
/ 0.27 0.04 15.42 2.8
was not significant, which clearly demonstrates the excellent catalytic efficiency of the Pd nanoparticles for HER. Since both the Pd nanoparticles and the Pt black have a large specific surface area, the excellent catalytic capacity to facilitate HER of the Pd catalyst might be mainly attributed to its surface nanosized morphology. As for the exact roles of size, facet, and morphology of Pd nanoparticles in the catalytic HER, a further investigation is needed. From an application point of view, the costs associated with the MEC construction should be lowered, while the overall MEC performance is required to be kept at a reasonable level. We proposed such a simple approach to facilitate HER with the Pd nanoparticles using the electrochemical deposition method. A common concern regarding Pd-based catalysis is its cost. While Pd is more expensive than common non noble metals, plenty of studies using Pd catalysis in environment and energy science demonstrated that the efficiency of Pd catalysis, in contrast, is equally efficient. Due to the absence of the high cost binder and the trace loading amount, Pd catalysis is a competitive option for HER in MEC system.
4.
Conclusions
We demonstrated that the carbon paper electrode coated with Pd nanoparticles could be prepared using the electrochemical deposition method and that the electrode was an efficient and cost-effective catalyst for hydrogen production in an MEC. The LSV result indicates that the Pd nanoparticles were active, and had a very low over potential of HER at neutral pH and good stability after 1-month experiment. The MEC tests also show its greater activity than the commercially Pt black, which might be mainly attributed to its surface morphology formed under the electrodeposition conditions. Utilization of this Pd nanoparticle-dominated cathode catalyst with good catalytic capacity and stability would reduce the MEC construction costs.
Acknowledgements The authors wish to thank the CAS (KJCX2-YW-H21-01) for the partial support of this study.
references
[1] Turner JA, Sverdrup G, Mann MK, Maness PC, Kroposki B, Ghirardi M, et al. Renewable hydrogen production. Int J Energ Res 2008;32:379e407. [2] Westermann P, Jorgensen B, Lange L, Ahring B, Christensen C. Maximizing renewable hydrogen production from biomass in a bio/catalytic refinery. Int J Hydrogen Energ 2007;32:4135e41. [3] Rozendal RA, Hamelers HVM, Euverink GJW, Metz SJ, Buisman CJN. Principle and perspectives of hydrogen production through biocatalyzed electrolysis. Int J Hydrogen Energ 2006;31:1632e40. [4] Cheng S, Logan BE. Sustainable and efficient biohydrogen production via electrohydrogenesis. P Natl Acad Sci USA 2007;104:18871e3. [5] Selembo PA, Merrill MD, Logan BE. Hydrogen production with nickel powder cathode catalysts in microbial electrolysis cells. Int J Hydrogen Energ 2010;35:428e37. [6] Liu H, Grot S, Logan BE. Electrochemically assisted microbial production of hydrogen from acetate. Environ Sci Technol 2005;39:4317e20. [7] Harnisch F, Sievers G, Schro¨der U. Tungsten carbide as electrocatalyst for the hydrogen evolution reaction in pH neutral electrolyte solutions. Appl Catal B Environ 2009;89:455e8. [8] Hu HQ, Fan YZ, Liu H. Hydrogen production in single-chamber tubular microbial electrolysis cells using non-precious-metal catalysts. Int J Hydrogen Energ 2009;34:8535e42. [9] Jeremiasse AW, Hamelers HVM, Saakes M, Buisman CJN. Ni foam cathode enables high volumetric H2 production in a microbial electrolysis cell. Int J Hydrogen Energ 2010;35: 12716e23. [10] Perez J, Gonzalez ER, Villullas HM. Hydrogen evolution reaction on gold single-crystal electrodes in acid solutions. J Phys Chem B 1998;102:10931e5. [11] Uchida T, Mogami H, Yamakata A, Sasaki Y, Osawa M. Hydrogen evolution reaction catalyzed by proton-coupled redox cycle of 4,4’-bipyridine monolayer adsorbed on silver electrodes. J Am Chem Soc 2008;130:10862. [12] Uhm S, Lee Hye J, Kwon Y, Lee J. A stable and cost-effective anode catalyst structure for formic acid fuel cells. Angew Chem Int Edition 2008;47:10163e6. [13] Xiao L, Zhuang L, Liu Y, Lu JT, Abruna HD. Activating Pd by morphology tailoring for oxygen reduction. J Am Chem Soc 2009;131:602e8. [14] Sun M, Sheng GP, Zhang L, Xia CR, Mu ZX, Liu XW, et al. An MEC-MFC-coupled system for biohydrogen production from acetate. Environ Sci Technol 2008;42:8095e100. [15] Zhang SJ, Yu HQ, Feng HM. PVA-based activated carbon fibers with lotus root-like axially porous structure. Carbon 2006;44:2059e68.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell Yu-Xi Huang a, Xian-Wei Liu a, Xue-Fei Sun b, Guo-Ping Sheng a, Yuan-Yuan Zhang a, Guo-Ming Yan a, Shu-Guang Wang b, An-Wu Xu c, Han-Qing Yu a,* a
Department of Chemistry, University of Science & Technology of China, Hefei 230026, China School of Environmental Science and Engineering, Shandong University, Jinan, 250100, China c Hefei National Laboratory for Physical Sciences at the Microscale, University of Science & Technology of China, Hefei 230026, China b
article info
abstract
Article history:
Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from
Received 25 August 2010
organic matters, but it still suffers from the lack of efficient and cost-effective cathode
Received in revised form
catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using
20 November 2010
electrochemical deposition method and used as the cathodic catalyst in an MEC to facili-
Accepted 27 November 2010
tate hydrogen production. The electrode coated with Pd nanoparticles showed a lower
Available online 30 December 2010
overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst
Keywords:
were slightly higher than those with a Pt cathode, while the Pd loading was one order of
Cathodic catalysts
magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd
Electrochemical deposition
nanoparticles was about fifty times higher than that of the Pt black catalyst. These results
Hydrogen
demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the
Microbial electrolysis cell (MEC)
costs of the cathodic catalysts when maintaining the MEC system performance.
Pd nanoparticle
1.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen can be produced from renewable energy resources, water and biomass, by a variety of processes, e.g., photolysis, electrolysis, thermochemical, and biochemical [1,2]. One of them is microbial electrolysis cell (MEC) technology, which provides an efficient and sustainable approach for hydrogen production from organic matters [3e5]. This system uses microbes to oxidize organic materials at anode, and hydrogen gas is evolved at cathode by adding a supplemental voltage to that produced by the bacteria to overcome the endothermic barrier of hydrogen formation [5].
Despite of its recent advances in reactor design and operation, the MEC still faces many challenges for practical applications, among which the high costs of cathode catalyst is one of the most critical ones. Platinum is the most effective catalyst to facilitate hydrogen evolution reaction (HER) and has been widely employed as the cathodic catalyst for HER in MECs [4,6]. Such a dependence on Pt and an expensive and scarce resource definitely limit the wide deployment and application of MECs. To resolve this problem, attempts have been made to maximize Pt utilization and search for Pt-alternative catalysts for HER in MECs. Recently, tungsten carbide [7] and Ni-based alloy catalysts [8,9] have been evaluated. There catalysts display
* Corresponding author. Fax: þ86 551 3601592. E-mail address: [email protected] (H.-Q. Yu). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.114
2774
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 ) 2 7 7 3 e2 7 7 6
good performance in terms of HER, but their hydrogen evolution rate is lower than that with Pt. And the use of the high cost Nafion binder also leads to the increase of the MEC construction cost. On the other hand, extensive studies have been carried out on non-Pt catalysts for HER in water electrolysis [10,11]. Palladium, the most Pt-like metal, is being a focus as a catalyst due to its excellent catalytic capabilities and a relatively abundant resource. Although hydrogen production from water electrolysis and MEC shares the same cathodic reaction, the operational conditions in water electrolyzers are significantly different from those in MECs [8]. Also, the catalytic performance of Pd at neutral pH has not been well evaluated. In addition, it is well known that the physicochemical properties of catalysts are dependent heavily on their size, shape and oxidation state. Thus, it is of significance to develop cathodic catalysts with controllable morphology and stable performance by using facile methods to enhance the HER and lower the MEC costs. Thus, the aim of the present study is to investigate the feasibility of using the Pd nanoparticles prepared by the electrochemical deposition method for HER in an MEC. The Pd nanoparticles were firstly deposited on carbon paper. Then, the electrochemical properties towards hydrogen evolution of MECs with different catalysts were evaluated and compared.
platinum wire counter electrode in 50 mM phosphate buffer solution (pH 7.0). LSV scans from 0.4 to 1.8 V (vs. Ag/AgCl reference electrode) were conducted at a scan rate of 2 mV/s. The MEC system was constructed according to previous report [14]. The anode and cathode each in a 480 mL chamber were separated by a PEM (GEFC-10N, GEFC Co., China). The anode chamber was filled with 400-mL medium containing (in 1 L of 50 mM phosphate buffer, pH 7.0): NaAc, 100 mg; NH4Cl, 310 mg; KCl, 130 mg; CaCl2, 10 mg; MgCl2$6H2O, 20 mg; NaCl, 2 mg; FeCl2, 5 mg; CoCl2$2H2O, 1 mg; MnCl2$4H2O, 1 mg; AlCl3, 0.5 mg; (NH4)6Mo7O24, 3 mg; H3BO3, 1 mg; NiCl2$6H2O, 0.1 mg; CuSO4$5H2O, 1 mg; ZnCl2, 1 mg. The cathode chamber of the MEC was filled with 400-mL of 50 mM phosphate buffer at pH 7.0. Activated carbon fibers [15] with enriched bacteria from an acetate-fed microbial fuel cell for over 6 months were used as the anode. The cathode electrodes were 4 4 cm carbon papers coated by Pd nanoparticles as prepared. The commercial Pt-coated carbon paper (0.5 mg cm2) and raw carbon paper were also evaluated as the cathode for HER. The voltage adding to the MEC was 0.6 V using potentiostat. After each fed-batch cycle (when H2 production stopped), the MEC was drained, and refilled with substrate solution, and sparged with nitrogen gas for 30 min. Each cathode material was tested three times for one month.
2.4.
2.
Materials and methods
2.1.
Materials
K2PdCl6 was purchased from Aladdin Reagent Co. and the remaining chemicals were from Sinopharm Chemical Reagent Co. Cathode electrodes were made of carbon papers (TGP-H-090, Toray Co., Japan) with or without Pt/C (20 wt% 3.2-nm Pt nanoparticles on Vulcan XC-72 carbon support) on it (GEFC Co., China), which were sonicated for 30 min in deionized water before further use.
2.2.
Electrochemical deposition of Pd nanoparticles
Pd nanoparticles were prepared via the electrochemical deposition method onto a 4 4 cm plain carbon paper using a potentiostat (660C, CH Instruments, Inc., USA). Before the electrochemical deposition, electrochemical oxidation of the carbon paper was proceeded to form the adequate catalyst support. Briefly, a constant potential of 2.0 V was applied in HClO4 (0.5 M) for 2 min using a saturated Ag/AgCl reference electrode (þ0.198 V vs. Standard Hydrogen Electrode) and a Pt counter electrode [12]. Then, the electrochemical deposition of Pd was conducted in 0.1 M NaCl solution containing 1.26 mM K2PdCl6 as the precursor [13]. The deposition potential was fixed at 0.4 V, and the deposition time was 200 s.
2.3.
Evaluation of catalysts
The hydrogen evolution overpotential was evaluated using linear sweep voltammetry (LSV). The cathode electrodes before and after the MEC tests were placed in cubic electrochemical cells (5 cm length) with an Ag/AgCl reference electrode and
Analysis and calculation
The produced gases were measured two times everyday using a gas chromatograph (model SP-6800A, Lunan Co., China). The morphology of the Pd-coated carbon paper was observed using a Sirion 200 scanning electron microscopy (SEM) (FEI Co., the Netherlands). The MEC performance was evaluated in terms of: coulombic efficiency (CE) (%) based on total added substrate; cathodic hydrogen recovery (RH2 ;CAT ) (%) based on the recovered electrons as hydrogen compared to the current transferred; total hydrogen recovery (RH2 ;COD ) (%) obtained as RH2 ;COD ¼ CE*RH2 ;CAT ; hydrogen production rate (Q) (L m2 d1) based on hydrogen produced normalized to the cathode electrode area [6]; catalyst loading (CL) (mg) based on the electrons transferred in the electrochemical deposition process; catalytic efficiency (CaE) is defined as CaE ¼ Q/CL with a unit of L m2 d1 mg1.
3.
Results and discussion
3.1.
Preparation and morphology of the Pd nanoparticles
The electrochemical oxidation enriched the oxygen-containing species on the carbon paper surface with carboxylic and hydroxy groups. This led to the formation of more-dispersed Pd particles, attributed to the increase in surface accessibility. In this case, the oxygen-containing species on the carbon surface acted as acidic sites for the adsorption of the catalyst precursors [12]. Typical SEM image of the Pd nanoparticles clearly shows that a low surface density of nanoparticles with uniform shape was formed under the tested conditions (Fig. 1). Assuming that the electrodeposition coulombic efficiency was 100%, the maximum loadings of the Pd nanoparticles were calculated as
2775
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 ) 2 7 7 3 e2 7 7 6
Fig. 1 e SEM images of carbon papers without (a) and with (b) Pd nanoparticles deposited. 0.17 mg, or 0.0106 mg cm2, which was nearly 50 times less than Pt (0.5 mg cm2).
3.2.
Hydrogen evolution activity of the Pd nanoparticles
The Pd-coated carbon paper was characterized by LSV and compared with the Pt black and control cathodes before and after MEC tests. Fig. 2 shows the hydrogen evolution over potentials of the three electrodes before and after 1-month MEC experiments, which could provide further evidence of the activity and durability of the electrodeposited Pd nanoparticles. Both the Pd and Pt catalysts had much lower over potentials than the control electrode without any catalyst, and the Pt catalyst exhibited the highest current density because of its larger loading. The over potential of the Pd catalyst had a slightly change, while the Pt catalyst showed a significant negative shift, suggesting that the Pd catalyst exhibited better stability than the Pt catalyst. The LSV results indicate that the electrodeposited Pd nanoparticles had excellent activity in HER and reliable durability, which is of great importance for long term operation of MEC.
3.3. Hydrogen production of MEC with the Pd nanoparticles The Pd-coated carbon paper was then used as the cathode in an MEC, and its performance was compared to other MECs
respectively with Pt black and control carbon paper cathodes. The anode was inoculated with pre-enriched consortium of exoelectrogens and large volume buffer (400 mL each chamber) was used, while the cathode area was relatively small to make sure the cathode to be the main limitation in the MECs. The MEC with the Pd cathode produced slightly more hydrogen than that with the Pt cathode at the end of the experiments (Fig. 3), and consequently resulted in a higher coulombic efficiency, hydrogen recovery and hydrogen production rate (Table 1). This result demonstrates that the Pd nanoparticles are more effective for the HER catalysis in MECs, though the Pt catalyst showed higher current density in LSV test. Since the Pd loading amount (0.0106 mg cm2) was significant less than that of Pt (0.5 mg cm2), the MEC performance with the Pd catalyst was still better than that with the Pt catalyst. Utilization of the Pd catalyst prepared in this work would significantly reduce the cathodic catalyst costs for MECs. However, it should be noticed that the coulombic efficiency and hydrogen production rate in this study are low because the MEC was not optimized. To further compare the different catalysts for HER, the catalytic properties of the catalysts toward to their loadings were benchmarked. The catalytic efficiency of the Pd nanoparticles was 15.42 2.83 (L m2 d1 mg1), over fifty times larger than the Pt black (0.27 0.04 L m2 d1 mg1) (Table 1), but the difference between their total hydrogen production rates
0
16
Hydrogen volume / mL
-2
j (mA cm )
-20 -40 -60 -80
Control Pt Pd
18
Control
-100 -2.0
Pd
Pt
14 12 10 8 6 4 2
-1.5 -1.0 -0.5 E (V vs. Ag/AgCl)
0.0
Fig. 2 e LSV plots of the three cathodes before (dot line) and after (solid line) the MEC experiments in 50 mM phosphate buffer solution (pH 7.0) and scan rate of 2 mV sL1.
0 0
20
40
60
80
100
120
Operating time / hour Fig. 3 e Hydrogen production for the MEC with the three cathodes for three cycles.
2776
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 ) 2 7 7 3 e2 7 7 6
Table 1 e MEC results for Pd, Pt and control catalyst cathodes at an applied voltage of 0.6 V for three cycles. Catalyst None Pt Pd
CE (%)
RH2 ;CAT (%)
RH2 ;COD (%)
Q (L m2 d1)
Catalytic efficiencies normalized by loadings (L m2 d1 mg1)
23.3 0.6 52.0 7.4 56.0 10.2
25.4 12.1 41.6 6.1 46.4 8.5
5.9 2.8 21.6 3.2 26.0 4.8
0.4 0.1 2.1 0.3 2.6 0.5
/ 0.27 0.04 15.42 2.8
was not significant, which clearly demonstrates the excellent catalytic efficiency of the Pd nanoparticles for HER. Since both the Pd nanoparticles and the Pt black have a large specific surface area, the excellent catalytic capacity to facilitate HER of the Pd catalyst might be mainly attributed to its surface nanosized morphology. As for the exact roles of size, facet, and morphology of Pd nanoparticles in the catalytic HER, a further investigation is needed. From an application point of view, the costs associated with the MEC construction should be lowered, while the overall MEC performance is required to be kept at a reasonable level. We proposed such a simple approach to facilitate HER with the Pd nanoparticles using the electrochemical deposition method. A common concern regarding Pd-based catalysis is its cost. While Pd is more expensive than common non noble metals, plenty of studies using Pd catalysis in environment and energy science demonstrated that the efficiency of Pd catalysis, in contrast, is equally efficient. Due to the absence of the high cost binder and the trace loading amount, Pd catalysis is a competitive option for HER in MEC system.
4.
Conclusions
We demonstrated that the carbon paper electrode coated with Pd nanoparticles could be prepared using the electrochemical deposition method and that the electrode was an efficient and cost-effective catalyst for hydrogen production in an MEC. The LSV result indicates that the Pd nanoparticles were active, and had a very low over potential of HER at neutral pH and good stability after 1-month experiment. The MEC tests also show its greater activity than the commercially Pt black, which might be mainly attributed to its surface morphology formed under the electrodeposition conditions. Utilization of this Pd nanoparticle-dominated cathode catalyst with good catalytic capacity and stability would reduce the MEC construction costs.
Acknowledgements The authors wish to thank the CAS (KJCX2-YW-H21-01) for the partial support of this study.
references
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