- Email: [email protected]
Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode Qingping Hu, Guangwei Li, Jing Pan*, Lisheng Tan, Juntao Lu, Lin Zhuang College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China
article info
abstract
Article history:
Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of fuel cell that has been
Received 29 April 2013
expected to combine the advantages of alkaline fuel cells (AFCs) and polymer electrolyte
Received in revised form
fuel cells (PEFCs). In recent decade, APEFCs have drawn much attention in the fuel cell
16 September 2013
world. While great efforts have been devoted to the development of high-performance
Accepted 20 September 2013
alkaline polymer electrolytes (APEs), prototypes of APEFC using nonprecious metal cata-
Available online 21 October 2013
lysts in both the anode and the cathode have not been well implemented, except for our previous report where NieCr was used as the anode catalyst and Ag was employed as the
Keywords:
cathode catalyst. In the present work, we report our recent progress in this regard. The self-
Alkaline polymer electrolyte
crosslinked quaternary ammonia polysulfone (xQAPS), a high-performance APE that pos-
H2eO2/air fuel cell
sesses both good ionic conductivity and extremely high dimensional stability, is applied as
Self-crosslinked quaternary
both the electrolyte membrane and the ionomer impregnated in the electrodes. Carbon-
ammonia polysulfone
supported Co-polypyrrole (CoPPY/C) is employed as the cathode catalyst and a new
Co-polypyrrole composite
Ni-based catalyst, W-doped Ni, is used as the anode catalyst, which features in high
W-doped Ni
oxidation tolerance. H2eO2 and H2-air APEFCs are thus fabricated and show a decent performance with peak power density being 40 and 27.5 mW/cm2 at 60 C, respectively. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) have been considered as an ideal power source for electric vehicles and mobile devices. Nafion is a perfluorosulfonic PEM that has outstanding performance in both ionic conductivity (over 0.1 S/cm at 80 C) and stability, and thus been widely employed in PEMFC [1e3]. Albeit superior, Nafion is, in nature, a strong acid, which highly restricts the choice of electrocatalyst in fuel cells, and almost only noble metals can remain relatively stable in such an acidic environment. This has been one of the major barriers hindering the widespread application of PEMFC.
To fundamentally get rid of the dependence of noble metal catalysts, alkaline media should, in principle, be used. However, alkaline fuel cells (AFCs) have been suffering from the carbonation issue when operated in air. To combine the advantages of AFC and PEMFC, alkaline polymer electrolyte fuel cells (APEFCs) is now considered as a very promising candidate [4e9]. On one hand, under alkaline environment, some nonprecious metals or compounds (such as oxides and sulfides) could be used as efficient and stable catalyst for fuel cell reactions [10e14]. On the other hand, the carbonation issue that causes water-proof breaking and electrolyte leakage will no longer be a trouble because of the use of solid polymer electrolyte.
* Corresponding author. Tel.: þ86 15927695399. E-mail addresses: [email protected], [email protected] (J. Pan). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.125
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
In last decade, APEFC has been a focus research in the fuel cell world. While great efforts have been devoted to the development of alkaline polymer electrolyte (APE) [15e17], prototypes of APEFC completely using nonprecious metal catalysts in both the anode and the cathode have not been well implemented, the only preliminary report was our previous work [5], where Cr-doped Ni catalyst was used as the anode catalyst for hydrogen oxidation reaction (HOR) and Ag was the cathode catalyst for oxygen reduction reaction (ORR). In the present work, we demonstrate a prototype of H2eO2/ air APEFC using self-crosslinking quaternary ammonia polysulfone (xQAPS) as the APE, NieW as the anode catalyst, and Copolypyrrole/C (CoPPY/C) as the cathode catalyst. xQAPS has been reported in our previous work [18,19]. By deliberately introducing a ternary amine group onto the polysulfone backbone, the QAPS will form a short-range crosslinking structure upon solidification, which significantly restrict the swelling of the APE membrane. In 90 C water, the xQAPS membrane will only swell by 3% in size and its ionic conductivity can reach 60 mS/cm. Therefore, xQAPS is very practical for fabricating APEFC. As for nonprecious metal catalysts, CoPPY/C was reported to be a promising Pt-alternative catalyst that can be stable even in acidic media [20], we now test it in alkaline media and use it as the cathode catalyst for H2eO2/air APEFC. The study of nonprecious metal catalyst for HOR in alkaline media has not been quite successful, although Ni is able to catalyze the HOR, it is too active to remain stable [21]. We have been working on tailoring the electronic property of Ni surface by suppressing its d band reactivity; modifying the Ni surface with appropriate transition metals seems to be effective to make the Ni surface more tolerant to oxidation. In this work, we report a new type of Ni-based catalyst, tungsten doped nickel (NieW), which can act as a stable catalyst in alkaline media toward the HOR.
2.
Experimental
2.1.
Materials
The materials used for preparing xQAPS had been previously described [18,19]. BP-2000 carbon powder (Cabot Co., US),
16265
Cobalt(II) nitrate hexahydrated (Co(NO3)2$6H2O, Sinopharm Chemical Reagent Co. Ltd., 99%), pyrrole (PY, Sinopharm Chemical Reagent Co. Ltd., 99%), hydrogen peroxide (H2O2, Sinopharm Chemical Reagent Co. Ltd., 30% aqueous solution), potassium borohydride (KBH4, Sinopharm Chemical Reagent Co. Ltd., 99%), Nickel(II) nitrate hexahydrated (Ni(NO3)2$6H2O, Sinopharm Chemical Reagent Co. Ltd., 99%), ammonium tungstate (H40N10O41W12$xH2O, Sinopharm Chemical Reagent Co. Ltd., 99%), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co. Ltd., 99%), and acetic acid (Sinopharm Chemical Reagent Co. Ltd., 99%) were used as received.
2.2.
Methods
xQAPS membranes were synthesized following the procedures reported in our previous works [18]. The synthesis of ORR catalysts, CoPPY/C, was similar to those reported by Bashyam and Zelenay [20]. Briefly, 1.5 g of carbon powders (BP-2000, Cabot) and 1 mL of glacial acetic acid were added into 100 mL of de-ionized water and the solution was stirred for 30 min at room temperature. Then, 0.38 mL of pyrrole was added to the resulted carbon dispersion and stirred for 10 min, followed by the addition of 9 mL of 10% H2O2. The mixture was stirred at room temperature for 3 h. The carbon-supported polypyrrole (PPY/C) was then filtered and washed with de-ionized water, and dried at 60 C under vacuum for 8 h. 0.3 g of such PPY/C was dispersed in 100 mL of de-ionized water and heated to 75 C with constant stirring. 3 mL of aqueous solution containing 0.16 g of Co(NO3)2$6H2O was then added into the PPY/C dispersion. The obtained mixture was vigorously stirred for 30 min at 75 C, followed by addition of KBH4 (0.5 g) and KOH (0.035 g) solution, which were dissolved in 30 mL of de-ionized water, and the reaction system was stirred for 30 min at 75 C to complete the reduction process. The resulting CoPPY/C was filtered, washed repeatedly with de-ionized water. Finally, the catalyst was dried overnight at 60 C under vaccum and the Co content in CoPPY/C was measured to be 10 wt%. The HOR catalyst, NieW, was prepared by dissolving 1 g H40N10O41W12$xH2O and 1.14 g Ni(NO3)2$6H2O in 200 mL deionized water. After stirring at 100 C for 5 h, the solution was dried in an oven at 110 C. The resulting powder was
Fig. 1 e RDE tests of the CoPPY/C catalyst for ORR. (a) The KouteckyeLevich plot; (b) 10 circles of potential sweeping within the potential range of ORR test. Data were recorded at scan rate of 5 mV/s in O2-saturated 1 mol/L KOH solution at rotation rate of 900 rpm.
16266
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
placed in a glazed ceramic boat and calcined under Ar atmosphere at 500 C for 2 h, followed by a reduction process under H2 atmosphere at 550 C for 2 h. After cooling down to room temperature, the NieW catalyst is obtained. CoPPY/C and NieW catalysts were mixed with the xQAPS ionomer solution and sprayed on a Teflon-treated carbon paper (Toray-250) to make the cathode and anode electrodes (the area of each electrode was 9 cm2), respectively. To replace the Cl anion in the ionomer with OH, the resulting electrodes were immersed in a KOH solution (1 mol/L) for 12 h. Then the electrodes were rinsed with de-ionized water until the pH of residual water was neutral. The CoPPY/C and NieW loading in the cathode and anode were 2 mg/cm2 and 17.5 mg/cm2, respectively. The content of xQAPS in the catalyst layer was controlled to be 30 wt% in both electrodes. Finally, an xQAPS membrane was sandwiched between the two electrodes and pressed at room temperature for 5 min under a pressure of 500 N/cm2. H2eO2/air fuel cell tests were conducted with H2 and O2/air gases flowing at 50 mL/min flow rate and controlled at 100% relative humidity (RH) under 70 C. The IeV curves were measured point by point galvanostatically and the voltage was taken after 10 min of discharging when steady state was established.
the number of electrons (n) of ORR, the RDE data for CoPPY/C electrode with different rotation rates were measured, and the KouteckyeLevich plots shows a linear dependence on potentials (Fig. 1(a)). The number of electrons of ORR can be calculated using the KouteckyeLevich equation: 1 1 1 1 1 ¼ þ ¼ þ I Ik Id Ik Bu1=2
(1)
3.
Results and discussion
where I is the measured current, Ik is the kinetic current, and u is the rotation rate. B is a constant, B ¼ 0.62 nFg1/6COD2/3pr2o , where F is the Faraday constant, D is the diffusion coefficient of O2 (1.43 105 cm2/s), g is the kinetic viscosity of the solution (1.128 102 cm2/s), CO is the concentration of dissolved O2 in solution (0.843 106 mol/cm3), and pr2o is the electrodes geometric area (0.1964 cm2). The n can then be determined from the slope of KouteckyeLevich plot, which was found to be 3.7, indicating that the ORR on CoPPY/C proceeds mainly via a four-electron pathway in the studied potential range. To evaluate the stability of the CoPPY/C catalyst, the electrode potential was cycled within the potential range of the ORR test. Fig. 1(b) shows a result of 10 circles of such potential sweeping, the IeV curves remains essentially unchanged during this test, except that the diffusion limiting current decreases slightly, most probably due to the consumption of dissolved O2 during the test. The stability of CoPPY/C for ORR catalysis is thus rather acceptable under this test condition.
3.1.
Electrochemistry of CoPPY/C cathode catalyst
3.2.
The electrocatalytic activity of CoPPY/C toward the ORR was evaluated by rotating disk electrode (RDE) tests. To estimate
Characterizations of NieW anode catalyst
The XRD pattern of NieW is shown in Fig. 2(a). The complicated diffraction peaks indicates that NieW is a mixture,
Fig. 2 e XRD (a) and XPS (bed) characterizations of the NieW catalyst.
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
consisting of W (JCPDS 47-1319), WO2 (JCPDS 32-1339), and WO3 (JCPDS 46-1096). However, there are no XRD peaks corresponding to Ni or Ni oxides, which suggest that Ni should be either in disordered structure or very small particles. The composition and electronic properties of NieW were also investigated using XPS analysis. The wide-range XPS spectrum shows that the sample contains Ni, W, and O (Fig. 2(b)), and the XPS spectrum of W 4f (Fig. 2(c)) proves the existence of W (BE ¼ 31.4 eV), WO2 (BE ¼ 34.2 eV), and WO3 (BE ¼ 35.8 eV), which is quite consistent with the XRD analysis. Meanwhile, through fitting the spectra of Ni 2p (Fig. 2(d)), the Ni signal can be decomposed into Ni0 (BE ¼ 853.06 eV) and NiO (BE ¼ 856.21 eV and 857.86 eV) [21]. To test the oxidation tolerance of the prepared NieW catalyst, the open circuit potential of the NieW anode under H2 atmosphere was monitored. The elapsed time for establishing the equilibrium potential of HOR is a sensitive index for how quickly the catalyst surface can be refreshed from an oxidized state [22]. As displayed in Fig. 3(a), the equilibrium potential of HOR on NieW catalyst can be established in 200 s, in comparison to the elapsed time of more than 1000 s for pristine Ni catalyst, indicating that, upon doping with W, the oxidation tolerance of Ni surface has been significantly enhanced. The NieW catalyst also exhibits a relatively high activity toward the HOR at room temperature, which increases significantly when the temperature is elevated to 60 C (Fig. 3(b)).
3.3.
Fuel cell performances
An APEFC single cell using xQAPS as the membrane and the ionomer in electrodes, CoPPY/C and NieW as the cathode and anode catalysts, respectively, was tested in the present work. Such a single cell was operated at 60 C under pure H2 and O2 gases of 100% related humidity. As demonstrated in Fig. 4, as the first APEFC using nonprecious metal catalysts for both the anode and the cathode, the cell exhibits a decent performance, the peak power density is 40 mW/cm2 under a current density of 100 mA/cm2. This performance is comparable to that of our previous report where Ag was employed as the cathode catalyst [5]. When changing the cathode gas from O2
16267
Fig. 4 e Cell performance of H2eO2 and H2-air APEFCs using xQAPS membrane (60 mm thick), NieW anode, and CoPPY/C cathode. The weight percentage of xQAPS in the catalyst layers is 30 wt%, and the loading of NieW and Coppy/C in anode and cathode are 17.5 mg/cm2 and 2 mg/cm2, respectively. Testing conditions: 60 C, RH [ 100% H2 and O2/air gas were flowing at a rate of 50 mL/min, no back pressures added.
to air, the peak power density of the cell reduces to 27.5 mW/ cm2 under the same conditions, which should be because of the lower partial pressure of O2 and the CO2 contamination effect on the ionic conductivity and catalyst activity [23,24].
4.
Conclusion
An alkaline polymer electrolyte fuel cell (APEFC) is reported in the present work using the self-crosslinked quaternary ammonia polysulfone (xQAPS) as the electrolyte. NieW catalyst and CoPPY/C catalyst are employed for the anode and cathode reactions, respectively. The xQAPS membrane exhibits a good OH conductivity and excellent anti-swelling property and stability under fuel cell operating conditions. The CoPPY/C turns out to be an efficient catalyst for 4-electron
Fig. 3 e Electrochemical tests of the NieW anode catalyst. (a) The change in open circuit potential of NieW catalyst under H2 atmosphere; (b) RDE test of NieW catalyst for HOR. Data were recorded in a H2-saturated 1.0 mol/L KOH solution at rotation rate of 1600 rpm and scan rate of 2 mV/s.
16268
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
reduction of O2 in alkaline media, and the NieW catalyst has a much enhanced tolerance to oxidation and can be activated at room temperature and used directly for the HOR. The H2eO2 and H2-air APEFCs thus fabricated gain a decent performance with a peak power density of 40 and 27.5 mW/cm2 at 60 C, respectively. Although the cell performance is still not high, this report represents an important step toward the goal of realizing APEFC based completely on nonprecious metal catalysts.
Acknowledgments This work was financially supported by the National Basic Research Program (2012CB215503, 2012CB932800), the National Science Foundation of China (20933004, 21125312, 21303124, 21303123), the National Hi-Tech R&D Program (2011AA050705), the Doctoral Fund of Ministry of Education of China (20110141130002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1030).
references
[1] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [2] Schmidt-Rohr K, Chen Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nat Mater 2008;7:75e83. [3] Mauritz KA, Moore RB. State of understanding of Nafion. Chem Rev 2004;104:4535e85. [4] Pan J, Chen C, Zhuang L, Lu J. Designing advanced alkaline polymer electrolytes for fuel cell applications. Acc Chem Res 2012;45:473e81. [5] Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc Natl Acad Sci U S A 2008;105:20611e4. [6] Robertson NJ, Kostalik IV HA, Clark TJ, Mutolo PF, Abrun˜a HD, Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;132:3400e4. [7] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. [8] Asazawa K, Yamada K, Tanaka H, Oka A, Taniguchi M, Kobayashi T. A platinum-free zero-carbon-emission easy fuelling direct hydrazine fuel cell for vehicles. Angew Chem Int Ed 2007;46:8024e7.
[9] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005;5:187e200. [10] Piana M, Boccia M, Filpi A, Flammia E, Miller HA, Orsini M, et al. H2/air alkaline membrane fuel cell performance and durability, using novel ionomer and non-platinum group metal cathode catalyst. J Power Sources 2010;195:5875e81. [11] Mamlouk M, Kumar SMS, Gouerec P, Scott K. Electrochemical and fuel cell evaluation of Co based catalyst for oxygen reduction in anion exchange polymer membrane fuel cells. J Power Sources 2011;196:7594e600. [12] Olson TS, Switzer EE, Atanassov P. Alkaline fuel cell employing novel anion exchange membrane. In: 211th ECS meeting abstract, 701; 2007. p. 282. [13] Li XG, Popov BN, Kawahara T, Yanagi H. Non-precious metal catalysts synthesized from precursors of carbon, nitrogen, and transition metal for oxygen reduction in alkaline fuel cells. J Power Sources 2011;196:1717e22. [14] Hu WK, Kiros Y, Nore´us D. AB5-type hydrogen storage alloys as catalysts in hydrogen-diffusion electrodes for novel H2/ hydride//perovskite/O2 alkaline fuel cells. J Phys Chem B 2004;108:18530e4. [15] Tanaka M, Fukasawa K, Nishino E, Yamaguchi S, Yamada K, Tanaka H, et al. Anion conductive block poly(arylene ether)s: synthesis, properties, and application in alkaline fuel cells. J Am Chem Soc 2011;133:10646e54. [16] Robertson NJ, Kostalik IV HA, Clark TJ, Mutolo PF, Abrun˜a HD, Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;133:3400e4. [17] Pan J, Lu S, Li Y, Huang A, Zhuang L, Lu J. High-performance alkaline polymer electrolyte for fuel cell applications. Adv Funct Mater 2010;20:312e9. [18] Pan J, Li Y, Zhuang L, Lu J. Self-crosslinked alkaline polymer electrolyte exceptionally stable at 90 C. Chem Commun 2010;46:8597e9. [19] Pan J, Tan L, Zhuang L, Lu J. Synthesis and characterization of self-crosslinked alkaline polymer electrolytes operable at 90 C. Sci China Chem 2011;41:1848e56. [20] Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63e6. [21] Schulze M, Gu¨lzow E, Steinhilber G. Activation of nickelanodes for alkaline fuel cells. Appl Surf Sci 2001;179:251e6. [22] Tang D, Lu J, Zhuang L, Liu P. Calculations of the exchange current density for hydrogen electrode reactions: a short review and a new equation. J Electroanal Chem 2010;644:144e9. [23] Kiss AM, Myles TD, Grew KN, Peracchio AA, Nelson GJ, Chiu WKS. Carbonate and bicarbonate ion transport in alkaline anion exchange membranes. J Electrochem Soc 2013;160:F994e9. [24] Matsui Y, Saito M, Tasaka A, Inaba M. Influence of carbon dioxide on the performance of anion-exchange membrane fuel cells. ECS Trans 2010;25:105e10.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Alkaline polymer electrolyte fuel cell with Ni-based anode and Co-based cathode Qingping Hu, Guangwei Li, Jing Pan*, Lisheng Tan, Juntao Lu, Lin Zhuang College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China
article info
abstract
Article history:
Alkaline polymer electrolyte fuel cells (APEFCs) are a new class of fuel cell that has been
Received 29 April 2013
expected to combine the advantages of alkaline fuel cells (AFCs) and polymer electrolyte
Received in revised form
fuel cells (PEFCs). In recent decade, APEFCs have drawn much attention in the fuel cell
16 September 2013
world. While great efforts have been devoted to the development of high-performance
Accepted 20 September 2013
alkaline polymer electrolytes (APEs), prototypes of APEFC using nonprecious metal cata-
Available online 21 October 2013
lysts in both the anode and the cathode have not been well implemented, except for our previous report where NieCr was used as the anode catalyst and Ag was employed as the
Keywords:
cathode catalyst. In the present work, we report our recent progress in this regard. The self-
Alkaline polymer electrolyte
crosslinked quaternary ammonia polysulfone (xQAPS), a high-performance APE that pos-
H2eO2/air fuel cell
sesses both good ionic conductivity and extremely high dimensional stability, is applied as
Self-crosslinked quaternary
both the electrolyte membrane and the ionomer impregnated in the electrodes. Carbon-
ammonia polysulfone
supported Co-polypyrrole (CoPPY/C) is employed as the cathode catalyst and a new
Co-polypyrrole composite
Ni-based catalyst, W-doped Ni, is used as the anode catalyst, which features in high
W-doped Ni
oxidation tolerance. H2eO2 and H2-air APEFCs are thus fabricated and show a decent performance with peak power density being 40 and 27.5 mW/cm2 at 60 C, respectively. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cells (PEMFCs) have been considered as an ideal power source for electric vehicles and mobile devices. Nafion is a perfluorosulfonic PEM that has outstanding performance in both ionic conductivity (over 0.1 S/cm at 80 C) and stability, and thus been widely employed in PEMFC [1e3]. Albeit superior, Nafion is, in nature, a strong acid, which highly restricts the choice of electrocatalyst in fuel cells, and almost only noble metals can remain relatively stable in such an acidic environment. This has been one of the major barriers hindering the widespread application of PEMFC.
To fundamentally get rid of the dependence of noble metal catalysts, alkaline media should, in principle, be used. However, alkaline fuel cells (AFCs) have been suffering from the carbonation issue when operated in air. To combine the advantages of AFC and PEMFC, alkaline polymer electrolyte fuel cells (APEFCs) is now considered as a very promising candidate [4e9]. On one hand, under alkaline environment, some nonprecious metals or compounds (such as oxides and sulfides) could be used as efficient and stable catalyst for fuel cell reactions [10e14]. On the other hand, the carbonation issue that causes water-proof breaking and electrolyte leakage will no longer be a trouble because of the use of solid polymer electrolyte.
* Corresponding author. Tel.: þ86 15927695399. E-mail addresses: [email protected], [email protected] (J. Pan). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.125
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
In last decade, APEFC has been a focus research in the fuel cell world. While great efforts have been devoted to the development of alkaline polymer electrolyte (APE) [15e17], prototypes of APEFC completely using nonprecious metal catalysts in both the anode and the cathode have not been well implemented, the only preliminary report was our previous work [5], where Cr-doped Ni catalyst was used as the anode catalyst for hydrogen oxidation reaction (HOR) and Ag was the cathode catalyst for oxygen reduction reaction (ORR). In the present work, we demonstrate a prototype of H2eO2/ air APEFC using self-crosslinking quaternary ammonia polysulfone (xQAPS) as the APE, NieW as the anode catalyst, and Copolypyrrole/C (CoPPY/C) as the cathode catalyst. xQAPS has been reported in our previous work [18,19]. By deliberately introducing a ternary amine group onto the polysulfone backbone, the QAPS will form a short-range crosslinking structure upon solidification, which significantly restrict the swelling of the APE membrane. In 90 C water, the xQAPS membrane will only swell by 3% in size and its ionic conductivity can reach 60 mS/cm. Therefore, xQAPS is very practical for fabricating APEFC. As for nonprecious metal catalysts, CoPPY/C was reported to be a promising Pt-alternative catalyst that can be stable even in acidic media [20], we now test it in alkaline media and use it as the cathode catalyst for H2eO2/air APEFC. The study of nonprecious metal catalyst for HOR in alkaline media has not been quite successful, although Ni is able to catalyze the HOR, it is too active to remain stable [21]. We have been working on tailoring the electronic property of Ni surface by suppressing its d band reactivity; modifying the Ni surface with appropriate transition metals seems to be effective to make the Ni surface more tolerant to oxidation. In this work, we report a new type of Ni-based catalyst, tungsten doped nickel (NieW), which can act as a stable catalyst in alkaline media toward the HOR.
2.
Experimental
2.1.
Materials
The materials used for preparing xQAPS had been previously described [18,19]. BP-2000 carbon powder (Cabot Co., US),
16265
Cobalt(II) nitrate hexahydrated (Co(NO3)2$6H2O, Sinopharm Chemical Reagent Co. Ltd., 99%), pyrrole (PY, Sinopharm Chemical Reagent Co. Ltd., 99%), hydrogen peroxide (H2O2, Sinopharm Chemical Reagent Co. Ltd., 30% aqueous solution), potassium borohydride (KBH4, Sinopharm Chemical Reagent Co. Ltd., 99%), Nickel(II) nitrate hexahydrated (Ni(NO3)2$6H2O, Sinopharm Chemical Reagent Co. Ltd., 99%), ammonium tungstate (H40N10O41W12$xH2O, Sinopharm Chemical Reagent Co. Ltd., 99%), potassium hydroxide (KOH, Sinopharm Chemical Reagent Co. Ltd., 99%), and acetic acid (Sinopharm Chemical Reagent Co. Ltd., 99%) were used as received.
2.2.
Methods
xQAPS membranes were synthesized following the procedures reported in our previous works [18]. The synthesis of ORR catalysts, CoPPY/C, was similar to those reported by Bashyam and Zelenay [20]. Briefly, 1.5 g of carbon powders (BP-2000, Cabot) and 1 mL of glacial acetic acid were added into 100 mL of de-ionized water and the solution was stirred for 30 min at room temperature. Then, 0.38 mL of pyrrole was added to the resulted carbon dispersion and stirred for 10 min, followed by the addition of 9 mL of 10% H2O2. The mixture was stirred at room temperature for 3 h. The carbon-supported polypyrrole (PPY/C) was then filtered and washed with de-ionized water, and dried at 60 C under vacuum for 8 h. 0.3 g of such PPY/C was dispersed in 100 mL of de-ionized water and heated to 75 C with constant stirring. 3 mL of aqueous solution containing 0.16 g of Co(NO3)2$6H2O was then added into the PPY/C dispersion. The obtained mixture was vigorously stirred for 30 min at 75 C, followed by addition of KBH4 (0.5 g) and KOH (0.035 g) solution, which were dissolved in 30 mL of de-ionized water, and the reaction system was stirred for 30 min at 75 C to complete the reduction process. The resulting CoPPY/C was filtered, washed repeatedly with de-ionized water. Finally, the catalyst was dried overnight at 60 C under vaccum and the Co content in CoPPY/C was measured to be 10 wt%. The HOR catalyst, NieW, was prepared by dissolving 1 g H40N10O41W12$xH2O and 1.14 g Ni(NO3)2$6H2O in 200 mL deionized water. After stirring at 100 C for 5 h, the solution was dried in an oven at 110 C. The resulting powder was
Fig. 1 e RDE tests of the CoPPY/C catalyst for ORR. (a) The KouteckyeLevich plot; (b) 10 circles of potential sweeping within the potential range of ORR test. Data were recorded at scan rate of 5 mV/s in O2-saturated 1 mol/L KOH solution at rotation rate of 900 rpm.
16266
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
placed in a glazed ceramic boat and calcined under Ar atmosphere at 500 C for 2 h, followed by a reduction process under H2 atmosphere at 550 C for 2 h. After cooling down to room temperature, the NieW catalyst is obtained. CoPPY/C and NieW catalysts were mixed with the xQAPS ionomer solution and sprayed on a Teflon-treated carbon paper (Toray-250) to make the cathode and anode electrodes (the area of each electrode was 9 cm2), respectively. To replace the Cl anion in the ionomer with OH, the resulting electrodes were immersed in a KOH solution (1 mol/L) for 12 h. Then the electrodes were rinsed with de-ionized water until the pH of residual water was neutral. The CoPPY/C and NieW loading in the cathode and anode were 2 mg/cm2 and 17.5 mg/cm2, respectively. The content of xQAPS in the catalyst layer was controlled to be 30 wt% in both electrodes. Finally, an xQAPS membrane was sandwiched between the two electrodes and pressed at room temperature for 5 min under a pressure of 500 N/cm2. H2eO2/air fuel cell tests were conducted with H2 and O2/air gases flowing at 50 mL/min flow rate and controlled at 100% relative humidity (RH) under 70 C. The IeV curves were measured point by point galvanostatically and the voltage was taken after 10 min of discharging when steady state was established.
the number of electrons (n) of ORR, the RDE data for CoPPY/C electrode with different rotation rates were measured, and the KouteckyeLevich plots shows a linear dependence on potentials (Fig. 1(a)). The number of electrons of ORR can be calculated using the KouteckyeLevich equation: 1 1 1 1 1 ¼ þ ¼ þ I Ik Id Ik Bu1=2
(1)
3.
Results and discussion
where I is the measured current, Ik is the kinetic current, and u is the rotation rate. B is a constant, B ¼ 0.62 nFg1/6COD2/3pr2o , where F is the Faraday constant, D is the diffusion coefficient of O2 (1.43 105 cm2/s), g is the kinetic viscosity of the solution (1.128 102 cm2/s), CO is the concentration of dissolved O2 in solution (0.843 106 mol/cm3), and pr2o is the electrodes geometric area (0.1964 cm2). The n can then be determined from the slope of KouteckyeLevich plot, which was found to be 3.7, indicating that the ORR on CoPPY/C proceeds mainly via a four-electron pathway in the studied potential range. To evaluate the stability of the CoPPY/C catalyst, the electrode potential was cycled within the potential range of the ORR test. Fig. 1(b) shows a result of 10 circles of such potential sweeping, the IeV curves remains essentially unchanged during this test, except that the diffusion limiting current decreases slightly, most probably due to the consumption of dissolved O2 during the test. The stability of CoPPY/C for ORR catalysis is thus rather acceptable under this test condition.
3.1.
Electrochemistry of CoPPY/C cathode catalyst
3.2.
The electrocatalytic activity of CoPPY/C toward the ORR was evaluated by rotating disk electrode (RDE) tests. To estimate
Characterizations of NieW anode catalyst
The XRD pattern of NieW is shown in Fig. 2(a). The complicated diffraction peaks indicates that NieW is a mixture,
Fig. 2 e XRD (a) and XPS (bed) characterizations of the NieW catalyst.
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
consisting of W (JCPDS 47-1319), WO2 (JCPDS 32-1339), and WO3 (JCPDS 46-1096). However, there are no XRD peaks corresponding to Ni or Ni oxides, which suggest that Ni should be either in disordered structure or very small particles. The composition and electronic properties of NieW were also investigated using XPS analysis. The wide-range XPS spectrum shows that the sample contains Ni, W, and O (Fig. 2(b)), and the XPS spectrum of W 4f (Fig. 2(c)) proves the existence of W (BE ¼ 31.4 eV), WO2 (BE ¼ 34.2 eV), and WO3 (BE ¼ 35.8 eV), which is quite consistent with the XRD analysis. Meanwhile, through fitting the spectra of Ni 2p (Fig. 2(d)), the Ni signal can be decomposed into Ni0 (BE ¼ 853.06 eV) and NiO (BE ¼ 856.21 eV and 857.86 eV) [21]. To test the oxidation tolerance of the prepared NieW catalyst, the open circuit potential of the NieW anode under H2 atmosphere was monitored. The elapsed time for establishing the equilibrium potential of HOR is a sensitive index for how quickly the catalyst surface can be refreshed from an oxidized state [22]. As displayed in Fig. 3(a), the equilibrium potential of HOR on NieW catalyst can be established in 200 s, in comparison to the elapsed time of more than 1000 s for pristine Ni catalyst, indicating that, upon doping with W, the oxidation tolerance of Ni surface has been significantly enhanced. The NieW catalyst also exhibits a relatively high activity toward the HOR at room temperature, which increases significantly when the temperature is elevated to 60 C (Fig. 3(b)).
3.3.
Fuel cell performances
An APEFC single cell using xQAPS as the membrane and the ionomer in electrodes, CoPPY/C and NieW as the cathode and anode catalysts, respectively, was tested in the present work. Such a single cell was operated at 60 C under pure H2 and O2 gases of 100% related humidity. As demonstrated in Fig. 4, as the first APEFC using nonprecious metal catalysts for both the anode and the cathode, the cell exhibits a decent performance, the peak power density is 40 mW/cm2 under a current density of 100 mA/cm2. This performance is comparable to that of our previous report where Ag was employed as the cathode catalyst [5]. When changing the cathode gas from O2
16267
Fig. 4 e Cell performance of H2eO2 and H2-air APEFCs using xQAPS membrane (60 mm thick), NieW anode, and CoPPY/C cathode. The weight percentage of xQAPS in the catalyst layers is 30 wt%, and the loading of NieW and Coppy/C in anode and cathode are 17.5 mg/cm2 and 2 mg/cm2, respectively. Testing conditions: 60 C, RH [ 100% H2 and O2/air gas were flowing at a rate of 50 mL/min, no back pressures added.
to air, the peak power density of the cell reduces to 27.5 mW/ cm2 under the same conditions, which should be because of the lower partial pressure of O2 and the CO2 contamination effect on the ionic conductivity and catalyst activity [23,24].
4.
Conclusion
An alkaline polymer electrolyte fuel cell (APEFC) is reported in the present work using the self-crosslinked quaternary ammonia polysulfone (xQAPS) as the electrolyte. NieW catalyst and CoPPY/C catalyst are employed for the anode and cathode reactions, respectively. The xQAPS membrane exhibits a good OH conductivity and excellent anti-swelling property and stability under fuel cell operating conditions. The CoPPY/C turns out to be an efficient catalyst for 4-electron
Fig. 3 e Electrochemical tests of the NieW anode catalyst. (a) The change in open circuit potential of NieW catalyst under H2 atmosphere; (b) RDE test of NieW catalyst for HOR. Data were recorded in a H2-saturated 1.0 mol/L KOH solution at rotation rate of 1600 rpm and scan rate of 2 mV/s.
16268
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 8 ( 2 0 1 3 ) 1 6 2 6 4 e1 6 2 6 8
reduction of O2 in alkaline media, and the NieW catalyst has a much enhanced tolerance to oxidation and can be activated at room temperature and used directly for the HOR. The H2eO2 and H2-air APEFCs thus fabricated gain a decent performance with a peak power density of 40 and 27.5 mW/cm2 at 60 C, respectively. Although the cell performance is still not high, this report represents an important step toward the goal of realizing APEFC based completely on nonprecious metal catalysts.
Acknowledgments This work was financially supported by the National Basic Research Program (2012CB215503, 2012CB932800), the National Science Foundation of China (20933004, 21125312, 21303124, 21303123), the National Hi-Tech R&D Program (2011AA050705), the Doctoral Fund of Ministry of Education of China (20110141130002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1030).
references
[1] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [2] Schmidt-Rohr K, Chen Q. Parallel cylindrical water nanochannels in Nafion fuel-cell membranes. Nat Mater 2008;7:75e83. [3] Mauritz KA, Moore RB. State of understanding of Nafion. Chem Rev 2004;104:4535e85. [4] Pan J, Chen C, Zhuang L, Lu J. Designing advanced alkaline polymer electrolytes for fuel cell applications. Acc Chem Res 2012;45:473e81. [5] Lu S, Pan J, Huang A, Zhuang L, Lu J. Alkaline polymer electrolyte fuel cells completely free from noble metal catalysts. Proc Natl Acad Sci U S A 2008;105:20611e4. [6] Robertson NJ, Kostalik IV HA, Clark TJ, Mutolo PF, Abrun˜a HD, Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;132:3400e4. [7] Gu S, Cai R, Luo T, Chen Z, Sun M, Liu Y, et al. A soluble and highly conductive ionomer for high-performance hydroxide exchange membrane fuel cells. Angew Chem Int Ed 2009;48:6499e502. [8] Asazawa K, Yamada K, Tanaka H, Oka A, Taniguchi M, Kobayashi T. A platinum-free zero-carbon-emission easy fuelling direct hydrazine fuel cell for vehicles. Angew Chem Int Ed 2007;46:8024e7.
[9] Varcoe JR, Slade RCT. Prospects for alkaline anion-exchange membranes in low temperature fuel cells. Fuel Cells 2005;5:187e200. [10] Piana M, Boccia M, Filpi A, Flammia E, Miller HA, Orsini M, et al. H2/air alkaline membrane fuel cell performance and durability, using novel ionomer and non-platinum group metal cathode catalyst. J Power Sources 2010;195:5875e81. [11] Mamlouk M, Kumar SMS, Gouerec P, Scott K. Electrochemical and fuel cell evaluation of Co based catalyst for oxygen reduction in anion exchange polymer membrane fuel cells. J Power Sources 2011;196:7594e600. [12] Olson TS, Switzer EE, Atanassov P. Alkaline fuel cell employing novel anion exchange membrane. In: 211th ECS meeting abstract, 701; 2007. p. 282. [13] Li XG, Popov BN, Kawahara T, Yanagi H. Non-precious metal catalysts synthesized from precursors of carbon, nitrogen, and transition metal for oxygen reduction in alkaline fuel cells. J Power Sources 2011;196:1717e22. [14] Hu WK, Kiros Y, Nore´us D. AB5-type hydrogen storage alloys as catalysts in hydrogen-diffusion electrodes for novel H2/ hydride//perovskite/O2 alkaline fuel cells. J Phys Chem B 2004;108:18530e4. [15] Tanaka M, Fukasawa K, Nishino E, Yamaguchi S, Yamada K, Tanaka H, et al. Anion conductive block poly(arylene ether)s: synthesis, properties, and application in alkaline fuel cells. J Am Chem Soc 2011;133:10646e54. [16] Robertson NJ, Kostalik IV HA, Clark TJ, Mutolo PF, Abrun˜a HD, Coates GW. Tunable high performance cross-linked alkaline anion exchange membranes for fuel cell applications. J Am Chem Soc 2010;133:3400e4. [17] Pan J, Lu S, Li Y, Huang A, Zhuang L, Lu J. High-performance alkaline polymer electrolyte for fuel cell applications. Adv Funct Mater 2010;20:312e9. [18] Pan J, Li Y, Zhuang L, Lu J. Self-crosslinked alkaline polymer electrolyte exceptionally stable at 90 C. Chem Commun 2010;46:8597e9. [19] Pan J, Tan L, Zhuang L, Lu J. Synthesis and characterization of self-crosslinked alkaline polymer electrolytes operable at 90 C. Sci China Chem 2011;41:1848e56. [20] Bashyam R, Zelenay P. A class of non-precious metal composite catalysts for fuel cells. Nature 2006;443:63e6. [21] Schulze M, Gu¨lzow E, Steinhilber G. Activation of nickelanodes for alkaline fuel cells. Appl Surf Sci 2001;179:251e6. [22] Tang D, Lu J, Zhuang L, Liu P. Calculations of the exchange current density for hydrogen electrode reactions: a short review and a new equation. J Electroanal Chem 2010;644:144e9. [23] Kiss AM, Myles TD, Grew KN, Peracchio AA, Nelson GJ, Chiu WKS. Carbonate and bicarbonate ion transport in alkaline anion exchange membranes. J Electrochem Soc 2013;160:F994e9. [24] Matsui Y, Saito M, Tasaka A, Inaba M. Influence of carbon dioxide on the performance of anion-exchange membrane fuel cells. ECS Trans 2010;25:105e10.