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A single-component fuel cell reactor
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 ) 8 5 3 6 e8 5 4 1
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
A single-component fuel cell reactor Bin Zhu a,*, Haiying Qin a,b, Rizwan Raza a,c, Qinghua Liu a, Liangdong Fan a, Janne Patakangas d, Peter Lund d a
Department of Energy Technology, Royal Institute of Technology, KTH, SE-10044, Stockholm, Sweden Department of Chemical and Biological, Zhejiang University, 310027, Hangzhou, China c Department of Physics, COMSATS Institute of Information Technology, 54000, Lahore, Pakistan d Department of Applied Physics, Aalto University, FI-00076 AALTO, Espoo, Finland b
article info
abstract
Article history:
We report here a single-component reactor consisting of a mixed ionic and semi-
Received 17 January 2011
conducting material exhibiting hydrogen-air (oxygen) fuel cell reactions. The new single-
Received in revised form
component device was compared to a conventional three-component (anode/electrolyte/
2 April 2011
cathode) fuel cell showing at least as good performance. A maximum power density of
Accepted 10 April 2011
300e600 mW cm2 was obtained with a LiNiZn-oxide and ceria-carbonate nanocomposite
Available online 5 May 2011
material mixture at 450e550 C. Adding a redox catalyst element (Fe) resulted in an
Keywords:
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
improvement reaching 700 mW cm2 at 550 C. Single-component
reserved.
Fuel cell Nanocomposites Ionic conductor Low-temperature
1.
Introduction
Electrolyte plays a key role in electrochemical devices, such as batteries, fuel cells, electrolysers, electro-ceramic sensors and catalytic membrane reactors. In a fuel cell, the electrolyte provides several functions: electronic insulator, ion conductor and gas separator. An electrolyte-based fuel cell (FC) [1] consists of a three-component device structure: electrolyte, anode and cathode in a membrane electrode assembly (MEA) with the electrolyte as the core [2]. Each of these components needs to be stable and compatible with each other. The interfaces between the electrolyte and electrodes (anode and cathode) contribute to major polarization loss [3,4]. Reducing the thickness of the electrolyte and improving the ion conductivity of electrolyte can decrease the polarization loss and thereby improve the fuel cell performance [5e7].
Fuel cells are often classified based on the type of electrolyte used, e.g. polymer electrolyte membrane, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cell (SOFC). SOFC with yttria stabilized zirconia electrolyte require high temperatures of around 800 C to obtain a sufficiently high ionic conductivity [8]. A high operational temperature leads to major material and durability problems increasing costs and slowing down the commercialization of this technology [2,3]. Lowering the operational temperature is hence highly motivated and can be achieved through the so-called lowtemperature SOFCs (LTSOFCs) concepts which operate at 300e600 C [9e18]. Within the EU-NANOCOFC (Nanocomposites for advanced fuel cell technology, www.nanocofc.com collaboration), nanocomposites have been developed for LTSOFC resulting in a simple device structure employing only one homogenous component exhibiting simultaneously electrode
* Corresponding author. Tel.: þ46 8 7907403. E-mail address: [email protected] (B. Zhu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.082
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 ) 8 5 3 6 e8 5 4 1
and electrolyte functionalities [19]. This new concept developed differs from a traditional LTSOFCs which is based on a threecomponent MEA. The single-component consists of a homogenous mixture from ionic and semi-conducting materials.
2.
Experimental
Two types of materials were used for the single-component: a semiconductor and an ionic conductor. Different semiconducting (n and p types) metal oxides were considered, mainly based on transition metals oxides such as NiO, CuO, FeOx, ZnO and CoOx. Semiconducting LiNiZn-mixed oxides were prepared through a solid state reaction as follows: stoichiometric amounts of Li2CO3, NiCO3$2Ni(OH) 2$6H2O and Zn (NO3)2$6H2O (all from sigma-Aldrich) were grounded and mixed with a molar ratio of Li:Ni:Zn ¼ 1:4:5 or 2:4:4, then this mixture was sintered at 800 C for 2e4 h. For the ionic conducting material, Na2CO3eCe0.8Sm0.2O2d (SDC) nanocomposites (NSDC) were used and they were prepared by a one-step co-precipitation [9]. The single-component was prepared from above synthesized LiNiZn-mixed oxides (semi-conducting materials) and NSDC (ionic conducting materials) in various weight ratios. A weight ratio of 40 (semiconductor)/60 (ionic conductor) yielded good fuel cell performance. The mixture was heated at 700 C for 1 h used for studies. Further improvement was made by adding 5e10 wt.% Fe (NO3)3$6H2O solution into the above mixture. The resulting new mixture was sintered at 700 C for 1 h. The crystal structure of the prepared LiNiZn-oxides was analyzed by X-ray diffraction (XRD) with Rigaku-D/Max-3A ˚ ). The diffractometer using Cu Ka radiation (l ¼ 1.5406 A morphology of the LiNiZn-oxides was studied with a Zeiss Ultra 55 field emission scanning electron microscopy (FESEM). The single-component devices were fabricated by pressing the prepared mixture powders uniaxially with a 100e300 MPa load to form tablets. A nickel-foam was amounted on one surface and on the other surface silver paste was pasted to collect current. The diameter of a tablet was 13 mm and its thickness was 0.6e1 mm.
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For comparison, we also constructed a conventional threecomponent fuel cells using a symmetrical configuration; the above material (LiNiZn-oxides and NSDC) was used both as anode and cathode, separated with a NSDC layer as an electrolyte. The three-component tablet was prepared as above and with the same dimensions. The fuel cells were measured using a computerized instrument (L43 Inc, Tianjin, China) over the temperature range of 400e550 C. Hydrogen and air were supplied at the flow rate range of 80e110 ml min1 under 1 atm on each side of the cells. AC impedance spectra and conductivity measurements were performed using a Versa STAT-4 (Princeton Applied Research, USA) analyzer from 0.01 Hz to 1 MHz with an amplitude of 10 mV at 550 C both in air and H2.
3.
Results
Fig. 1 shows a schematic of a three-component and a singlecomponent fuel cell device. The single-component is made of a homogenous mixture of both ionic and electronic (n and p type) conductors instead of the layered anode, electrolyte and cathode structure in the three-component fuel cell. Fig. 2 displays the XRD pattern of synthesized materials, LiNiZn-oxides and NSDC materials. It reveals a mixture of individual metal oxides of NiOx and ZnO. Li has doped into NiOx and ZnO. CeO2 phase diffraction pattern can also be clearly identified for NSDC in the component. These results suggest a composite structure for the material. A SEM analysis of the composite material (Fig. 3) shows a homogenous distribution of all material particles. The particle size in the composite ranges from tens up to a few hundred nanometers. Fig. 4 shows the IeV and IeP characteristics of the singlecomponent and three-component devices. Fig. 5 displays the performance of the single-component device operated at different temperatures for 400, 450, 500, and 550 C, respectively. It can be seen from Figs. 4 and 5 that the open circuit voltages (OCVs) of both devices reach 1.0 V. The performance is also comparable, 400e600 mW cm2, the single-component device showing slightly better performance than the three-
Fig. 1 e Schematic illustration of (a) a conventional three-component fuel cell, A: anode, C: cathode, and (b) a singlecomponent fuel cell.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 5 3 6 e8 5 4 1
Fig. 2 e XRD patterns of the as-prepared LiNiZn-oxidesNSDC composite and NSDC.
component device. Fig. 6 shows the device performance when adding a Fe redox catalyst. A maximum power density of 700 mW cm2 has been achieved at 550 C which may be due to increased catalytic activity [20e22]. The ratio of the ionic and semi-conducting material components was also varied showing that a 30e60 wt.% share of the ion conducting material can avoid electronic short circuiting and reach the same OCV level as a conventional threecomponent cell. The effects of the metal current collector were also investigated. A single-component device was constructed using only Ni-foam and silver as the electrodes (anode and cathode) and NSDC instead of LiNiZn-oxide/SDC, but this arrangement yielded only a few mA though a good OCV around 1 V With pure semi-conducting LiNiZn-oxide for the single component surrounded by Ni-foam and silver on both sides resulted in shortcircuiting (w100 mV). Only when both a semi-conductor (LiNiZn-oxides) and an ionic conductor (NSDC) are mixed with a proper match ratio (e.g. 40:60), a proper operation could be demonstrated (see Figs. 4e6). This implies that pure metallic current collectors (Ni-foam and silver) as the electrodes alone insignificantly to the device performance. Current levels of several hundreds up to a thousand mA shown in Figs. 4e6
Fig. 3 e SEM image of the as-prepared LiNiZn-oxides-NSDC composite.
Fig. 4 e Comparison of IeV and IeP characteristics of a single-component and a three-component fuel cell at 550 C.
would not be possible from this effect. It is clear that a well balanced electronic and ionic conductivity is necessary to achieve a good performance which actually does not differ from a conventional SOFC electrode in which a composite electrode is used by mixing a catalyst electrode and an electrolyte. Based on the experimental evidence, a single-component fuel cell device produces electricity through a “normal” fuel cell reaction route by the following reactions: anode : H2 /2Hþ þ 2e
(1)
cathode : 1=2O2 þ 2e/O2
(2)
overall reaction : H2 þ 1=2O2 /2Hþ þ O2 2Hþ þ O2 /H2 O
(3a) (3b)
Combining Eq. (3)a and b yields: H2 þ 1=2O2 /H2 O
(4)
For comparison, a conventional three-component SOFC has following reactions:
Fig. 5 e IeV and IeP characteristics of the singlecomponent device operated at various temperatures.
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 ) 8 5 3 6 e8 5 4 1
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Fig. 6 e IeV and IeP characteristics of the singlecomponent device using the as-prepared LiNiZn-oxidesNSDC composite modified by Fe.
anode : H2 þ O2 /H2 O þ 2e
(5)
cathode : 1=2O2 þ 2e/O2
(6)
overall reaction : H2 þ 1=2O2 /H2 O
(7)
Eq. (3)a and b indicates a two step reaction to complete the overall reaction in the single-component fuel cells whereas in the SOFC, O2 transport through the electrolyte to react with H2 (anode) is the critical reaction. In the single-component case, electricity generation is completed directly between the Hþ and O2 ions as there is no electrolyte separator indicating a co-ion process. This is possible because LiNiZn-oxideNSDC possesses a bi-catalyst function for ionizing H2 and O2 and the NSDC phase conducts both Hþ and O2 [23]. The single-component device exhibits fast reversible response when exchanging H2 and air supplies which leads to same OCV but with opposite polarity as shown in Fig. 7. This indicates fast bi-catalytic reaction processes for both H2 and O2, i.e. both sides of the single-component fuel cell can function as an anode for hydrogen oxidation and a cathode for oxygen reduction. For current collection, both a silver paste and a Ni-foam were used here, but a Ni-foam would perform slightly better for better contacting and would also provide a better mechanical support than an Ag paste. These experiments were also repeated with the three-component fuel cell made of same anode and cathode materials as above obtaining similar results but with much longer response time explained by the electrolyte and electrode interfaces that delay the kinetic processes.
4.
Discussion
A traditional fuel cell consists of a separate anode, electrolyte and cathode. Ion transportation through the electrolyte is critical for the performance, e.g. in a SOFC O2 transport through an O2 conducting electrolyte. In a single-component fuel cell device these functionalities seem to be more sophisticated; for example the electrolyte would in our case loose its traditional
Fig. 7 e OCV polarity change in the single-component fuel cell device when exchanging gas supplies (H2 and air) on electrodes.
functions meaning of providing both ionic transport and electronic separation properties, whereas here only the ionic transport property would be relevant. So we elaborate further the possible working principles of the device to explain the differences to the traditional “electrolyte” fuel cell. When the two sides of the single-component device are in a H2 and an air atmosphere respectively, both H2 and O2 can be catalytically dissociated into Hþ and O2 for the bi-catalystic functions of the component material. Hþ and O2 combine together on the particle surfaces inside the material and produce H2O and electricity at the same time. During this process, the H2 contacting side acts as an anode releasing electrons by forming Hþ. The air (O2) contacting side acts as a cathode receiving electrons. The device reactions are completed as long as Hþ and O2 appear in close proximity, preferentially at particle surfaces. This redox may be realized at particle level composed of both electronic (n and p) and ionic conducting particles, as illustrated in Fig. 8. We think that a p-n junction forms a barrier to block the internal electronic conduction in the single-component fuel cell when exposed to H2/O2 atmosphere. So the electrons are conducted to corresponding current collector without passing internally through the device. All ion and electron transfer and electricity generation may take place within a composite particle that shows both ionic and semiconducting properties. In principle, the cathodic process of the single-component device may be similar to the SOFC. Several step-reactions are needed to realize the overall reaction in Eq. (2): Step 1 : O2 ðgÞ/2Oad
(8)
Step 2 : Oad þ e/O ad
(9)
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Fig. 8 e A micro-view of the redox process in the single-component material.
Step 3 : O ad /OTPB
(10)
x Step 4 : O TPB þ VO,, þ e/Oo
(11)
We use here same notations as for SOFC, e.g. the threephase boundary (TPB) is also of importance to the singlecomponent device. On the anode side we have the following steps: Step 1 : H2 ðgÞ/2Had
(12)
Step 2 : Had /Hþ ad þ e
(13)
þ Step 3 : either Hþ ad /HTPB
(14)
þ or Hþ ad /H
(15)
As a matter of fact, the protons most preferentially move through the particle surfaces and interfaces between the electronic conductor (metal oxides) and ionic conductor (NSDC) particles. When protons approach the component from the H2 contacting side, it can form meta-stable hydrogen bonds with oxygen ions from both NSDC and metal oxide, e.g. LiNiZn-oxide’s surfaces. The ceria-based composite electrolyte possess both Hþ and O2- conduction [17,18,23] leading to effective proton and oxygen ion mobility driven by hydrogen and oxygen concentration gradients from both sides of the
single-component device. On the other hand, it has been reported that Hþ can be transported through lithiated nickel oxide [24] and that the Ni element shows good catalyst properties for converting H2 to Hþ. Protons may be directly generated at the anode according to Eq. (12) and (13). This means that the overall performance of the single-component device may be determined by the cathode kinetics/processes determined since the anode process can be simpler and faster. The impedance spectra of the composite material in both hydrogen and air atmosphere are shown in Fig. 9.The electrochemical impedance spectrum (EIS) consists of a “semicircle” followed by a “tail”. The small intercepts of the semicircle portion at the real axis in H2 and air indicates that both the functional anode and cathode reactions are fast kinetic processes [25]. This implies that LiNiZn-oxides-NSDC materials have high catalytic activity toward the hydrogen oxidation reaction and oxygen reduction reaction. From the EIS spectra, the value of total conductivity (including both ionic and electronic contributions) reaches about 0.1 S cm1(0.12 S cm1 for air side and 0.23 S cm1 for H2 side) when calculated from the formula s ¼ L/(RS), where L is the thickness of the pellet, R is the intercept of the EIS high frequency semicircle on real axis, and S is the effective area of the pellets. Finally, we comment shortly on the possibility of electrochemical and mechanical leakage of H2 and O2 in the single-
Fig. 9 e Typical electrochemical impedance spectra of the as-prepared LiNiZn-oxides-NSDC composite electrodes both in H2 (a) and air (b).
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 ) 8 5 3 6 e8 5 4 1
component fuel cell. Firstly, it should be noticed that in the mechanical leakage case H2 and O2 molecules and reactions are involved, not Hþ and O2. The half-device reactions (Eq. (1) and (2)) produce Hþ and O2 and have already generated electricity to the external circuit, which is different from the mechanical leakage to cause H2eO2 combustion. Wherever the Hþ and O2 reaction takes place, it will contribute to electricity generation without producing any “electrochemical” leakage. Furthermore, H2 and O2 in mesoporous media do not result in combustion due to quenching. The quenching distance for H2eO2 combustion is 1 mm [26]. In our case fuel cell reactions occurred at a nanometer or micrometer level far below the critical distance thus eliminating the possibility of combustion as a leakage path. It is obvious that there is still a need for research to fully understand the chemistry and physics of the new device as well as to construct optimal device structures. For example, the charge (electrons and ions) and phase separation (electronic and ionic) phenomena and no short circuiting problem observed in this fuel cell device deserve further attention.
5.
Conclusions
We reported here about a single-component fuel cell reactor which was compared to a conventional three-component electrolyte based fuel cell. The performance of the singlecomponent fuel cell is comparable to and in some cases slightly better than the three-component fuel cells. A power density of 300e600 mW cm2 was obtained with a LiNiZnoxide and NSDC mixture at 450e550 C. When adding a redox catalyst element (Fe), cell performance improved to 700 mW cm2 at 550 C. It is suggested that bi-catalyst function and dual Hþ and O2- and surface conduction explain single-component device performance. Future work will focus on charge and phase separation between electronic and ion materials as well as on electrochemical impedance studies and simulations of the new device.
Acknowledgments This work was supported by the Swedish agency for Innovation Systems (VINNOVA), the Swedish Research Council and the Swedish Agency for International Development Cooperation (SIDA), and the Swedish Agency for Energy (STEM), KIC InnoEnergy and Finnish Funding Agency for Technology and Innovation (TEKES).
references
[1] Grove WR. On voltaic series and the combination of gases by platinum. Phil Mag Ser 1839;314:127e30. [2] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52.
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[3] Steele BCH. Material science and engineering: the enabling technology for the commercialization of fuel cell systems. J Mater Sci 2001;36:1053e68. [4] Wilson JR, Kobsiriphat W, Mendoza R, Chen HY, Hiller JM, Thornton K, et al. Three-dimensional reconstruction of a solid-oxide fuel-cell anode. Nat Mater 2006;5:541e4. [5] Shao ZP, Haile SM. High-performance cathode for the next generation solid-oxide fuel cells. Nature 2004;431:170e3. [6] Steele BCH. Appraisal of Ce1yGdyO2y/2 electrolytes for ITSOFC operation at 500 C. Solid State Ionics 2000;129:95e110. [7] Will J, Mitterdorfer A, Kleinlogel C, Perednis D, Gauckler LJ. Fabrication of thin electrolytes for second-generation solid oxide fuel cells. Solid State Ionics 2000;131:79e96. [8] Nernst W. Material for electric-lamp glowers. U.S. Patent US685730, 1902. [9] Raza R, Wang XD, Ma Y, Zhu B. Improved ceria carbonates composite electrolytes. Int J Hydrogen Energy 2010;5:2684e8. [10] Zhu B, Liu XR, Zhou P, Yang XT, Zhu ZG, Zhu W. Innovative solid carbonateeceria composite electrolyte fuel cells. Electrochem Commun 2001;3:566e71. [11] Zhu B. Functional ceriaesalt-composite materials for advanced ITSOFC applications. J Power Sources 2003;114:1e9. [12] Zhu B. Solid oxide fuel cell (SOFC) technical challenges and solutions. Int J Energ Res 2009;33:1126e37. [13] Zhu B. Next generation fuel cell R&D. Int J Energ Res 2006;30: 895e903. [14] Huang JB, Yang LZ, Gao RF, Mao ZQ, Wang C. A highperformance ceramic fuel cell with samarium doped ceriaecarbonate composite electrolyte at low temperatures. Electrochem Commun 2006;8:785e9. [15] Huang JB, Mao ZQ, Liu ZX, Wang C. Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes. J Power Sources 2008;175:238e43. [16] Ma Y, Wang XD, Li SH, Toprak MS, Zhu B, Muhammed M. Samarium-Doped ceria nanowires: novel Synthesis and Application in low-temperature solid oxide fuel cells. Adv Mater 2010;22:1640e4. [17] Zhu B, Albinsson I, Andersson C, Borsand K, Nilsson M, Mellander BE. Electrolysis studies based on ceria-based composites. Electrochem Commun 2006;8:495e8. [18] Huang JB, Mao ZQ, Liu ZX, Wang C. Development of novel low-temperature SOFCs with co-ionic conducting SDCcarbonate composite electrolytes. Electrochem Commun 2007;9:2601e5. [19] Zhu B, Wang XD, Ma Y, Raza R, Qin HY, Fan LD. A fuel cell with a single component functioning simultaneously as the electrodes and electrolyte. Electrochem Commun 2011;13:225e7. [20] Lefe`vre M, Proietti E, Jaouen F, Dodelet JP. Iron-Based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science 2009;324:71e4. NM. Just a Dreamdor Future Reality? [21] Gasteiger HA, Markovic Science 2009;324:48e9. [22] Jacoby M. Low-cost iron for fuel cells. Chem Eng News 2009; 87:8. [23] Wang XD, Ma Y, Li S, Kashyout AH, Zhu B, Muhammed M. Ceria-based nanocomposite with simultaneous proton and oxygen ion conductivity for low-temperature solid oxide fuel cells. J Power Sources 2011;196:2754e8. [24] Giron JC. Proton conduction in the LiNiOx-GDC composite is possible. U.S. Patent US6277523, 2001. [25] Bard J, Faulkner LR. Electrochemical methods: fundamentals and applications. 2nd ed. New York: John Wiley & Sons, Inc.; 2001. p.380e387. [26] Veser G. Experimental and Theoretical Investigation of H2 oxidation in a high-temperature catalytic micro-reactor. Chem Eng Sci 2001;56:1265e73.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
A single-component fuel cell reactor Bin Zhu a,*, Haiying Qin a,b, Rizwan Raza a,c, Qinghua Liu a, Liangdong Fan a, Janne Patakangas d, Peter Lund d a
Department of Energy Technology, Royal Institute of Technology, KTH, SE-10044, Stockholm, Sweden Department of Chemical and Biological, Zhejiang University, 310027, Hangzhou, China c Department of Physics, COMSATS Institute of Information Technology, 54000, Lahore, Pakistan d Department of Applied Physics, Aalto University, FI-00076 AALTO, Espoo, Finland b
article info
abstract
Article history:
We report here a single-component reactor consisting of a mixed ionic and semi-
Received 17 January 2011
conducting material exhibiting hydrogen-air (oxygen) fuel cell reactions. The new single-
Received in revised form
component device was compared to a conventional three-component (anode/electrolyte/
2 April 2011
cathode) fuel cell showing at least as good performance. A maximum power density of
Accepted 10 April 2011
300e600 mW cm2 was obtained with a LiNiZn-oxide and ceria-carbonate nanocomposite
Available online 5 May 2011
material mixture at 450e550 C. Adding a redox catalyst element (Fe) resulted in an
Keywords:
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
improvement reaching 700 mW cm2 at 550 C. Single-component
reserved.
Fuel cell Nanocomposites Ionic conductor Low-temperature
1.
Introduction
Electrolyte plays a key role in electrochemical devices, such as batteries, fuel cells, electrolysers, electro-ceramic sensors and catalytic membrane reactors. In a fuel cell, the electrolyte provides several functions: electronic insulator, ion conductor and gas separator. An electrolyte-based fuel cell (FC) [1] consists of a three-component device structure: electrolyte, anode and cathode in a membrane electrode assembly (MEA) with the electrolyte as the core [2]. Each of these components needs to be stable and compatible with each other. The interfaces between the electrolyte and electrodes (anode and cathode) contribute to major polarization loss [3,4]. Reducing the thickness of the electrolyte and improving the ion conductivity of electrolyte can decrease the polarization loss and thereby improve the fuel cell performance [5e7].
Fuel cells are often classified based on the type of electrolyte used, e.g. polymer electrolyte membrane, alkaline, phosphoric acid, molten carbonate, and solid oxide fuel cell (SOFC). SOFC with yttria stabilized zirconia electrolyte require high temperatures of around 800 C to obtain a sufficiently high ionic conductivity [8]. A high operational temperature leads to major material and durability problems increasing costs and slowing down the commercialization of this technology [2,3]. Lowering the operational temperature is hence highly motivated and can be achieved through the so-called lowtemperature SOFCs (LTSOFCs) concepts which operate at 300e600 C [9e18]. Within the EU-NANOCOFC (Nanocomposites for advanced fuel cell technology, www.nanocofc.com collaboration), nanocomposites have been developed for LTSOFC resulting in a simple device structure employing only one homogenous component exhibiting simultaneously electrode
* Corresponding author. Tel.: þ46 8 7907403. E-mail address: [email protected] (B. Zhu). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.082
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 ) 8 5 3 6 e8 5 4 1
and electrolyte functionalities [19]. This new concept developed differs from a traditional LTSOFCs which is based on a threecomponent MEA. The single-component consists of a homogenous mixture from ionic and semi-conducting materials.
2.
Experimental
Two types of materials were used for the single-component: a semiconductor and an ionic conductor. Different semiconducting (n and p types) metal oxides were considered, mainly based on transition metals oxides such as NiO, CuO, FeOx, ZnO and CoOx. Semiconducting LiNiZn-mixed oxides were prepared through a solid state reaction as follows: stoichiometric amounts of Li2CO3, NiCO3$2Ni(OH) 2$6H2O and Zn (NO3)2$6H2O (all from sigma-Aldrich) were grounded and mixed with a molar ratio of Li:Ni:Zn ¼ 1:4:5 or 2:4:4, then this mixture was sintered at 800 C for 2e4 h. For the ionic conducting material, Na2CO3eCe0.8Sm0.2O2d (SDC) nanocomposites (NSDC) were used and they were prepared by a one-step co-precipitation [9]. The single-component was prepared from above synthesized LiNiZn-mixed oxides (semi-conducting materials) and NSDC (ionic conducting materials) in various weight ratios. A weight ratio of 40 (semiconductor)/60 (ionic conductor) yielded good fuel cell performance. The mixture was heated at 700 C for 1 h used for studies. Further improvement was made by adding 5e10 wt.% Fe (NO3)3$6H2O solution into the above mixture. The resulting new mixture was sintered at 700 C for 1 h. The crystal structure of the prepared LiNiZn-oxides was analyzed by X-ray diffraction (XRD) with Rigaku-D/Max-3A ˚ ). The diffractometer using Cu Ka radiation (l ¼ 1.5406 A morphology of the LiNiZn-oxides was studied with a Zeiss Ultra 55 field emission scanning electron microscopy (FESEM). The single-component devices were fabricated by pressing the prepared mixture powders uniaxially with a 100e300 MPa load to form tablets. A nickel-foam was amounted on one surface and on the other surface silver paste was pasted to collect current. The diameter of a tablet was 13 mm and its thickness was 0.6e1 mm.
8537
For comparison, we also constructed a conventional threecomponent fuel cells using a symmetrical configuration; the above material (LiNiZn-oxides and NSDC) was used both as anode and cathode, separated with a NSDC layer as an electrolyte. The three-component tablet was prepared as above and with the same dimensions. The fuel cells were measured using a computerized instrument (L43 Inc, Tianjin, China) over the temperature range of 400e550 C. Hydrogen and air were supplied at the flow rate range of 80e110 ml min1 under 1 atm on each side of the cells. AC impedance spectra and conductivity measurements were performed using a Versa STAT-4 (Princeton Applied Research, USA) analyzer from 0.01 Hz to 1 MHz with an amplitude of 10 mV at 550 C both in air and H2.
3.
Results
Fig. 1 shows a schematic of a three-component and a singlecomponent fuel cell device. The single-component is made of a homogenous mixture of both ionic and electronic (n and p type) conductors instead of the layered anode, electrolyte and cathode structure in the three-component fuel cell. Fig. 2 displays the XRD pattern of synthesized materials, LiNiZn-oxides and NSDC materials. It reveals a mixture of individual metal oxides of NiOx and ZnO. Li has doped into NiOx and ZnO. CeO2 phase diffraction pattern can also be clearly identified for NSDC in the component. These results suggest a composite structure for the material. A SEM analysis of the composite material (Fig. 3) shows a homogenous distribution of all material particles. The particle size in the composite ranges from tens up to a few hundred nanometers. Fig. 4 shows the IeV and IeP characteristics of the singlecomponent and three-component devices. Fig. 5 displays the performance of the single-component device operated at different temperatures for 400, 450, 500, and 550 C, respectively. It can be seen from Figs. 4 and 5 that the open circuit voltages (OCVs) of both devices reach 1.0 V. The performance is also comparable, 400e600 mW cm2, the single-component device showing slightly better performance than the three-
Fig. 1 e Schematic illustration of (a) a conventional three-component fuel cell, A: anode, C: cathode, and (b) a singlecomponent fuel cell.
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Fig. 2 e XRD patterns of the as-prepared LiNiZn-oxidesNSDC composite and NSDC.
component device. Fig. 6 shows the device performance when adding a Fe redox catalyst. A maximum power density of 700 mW cm2 has been achieved at 550 C which may be due to increased catalytic activity [20e22]. The ratio of the ionic and semi-conducting material components was also varied showing that a 30e60 wt.% share of the ion conducting material can avoid electronic short circuiting and reach the same OCV level as a conventional threecomponent cell. The effects of the metal current collector were also investigated. A single-component device was constructed using only Ni-foam and silver as the electrodes (anode and cathode) and NSDC instead of LiNiZn-oxide/SDC, but this arrangement yielded only a few mA though a good OCV around 1 V With pure semi-conducting LiNiZn-oxide for the single component surrounded by Ni-foam and silver on both sides resulted in shortcircuiting (w100 mV). Only when both a semi-conductor (LiNiZn-oxides) and an ionic conductor (NSDC) are mixed with a proper match ratio (e.g. 40:60), a proper operation could be demonstrated (see Figs. 4e6). This implies that pure metallic current collectors (Ni-foam and silver) as the electrodes alone insignificantly to the device performance. Current levels of several hundreds up to a thousand mA shown in Figs. 4e6
Fig. 3 e SEM image of the as-prepared LiNiZn-oxides-NSDC composite.
Fig. 4 e Comparison of IeV and IeP characteristics of a single-component and a three-component fuel cell at 550 C.
would not be possible from this effect. It is clear that a well balanced electronic and ionic conductivity is necessary to achieve a good performance which actually does not differ from a conventional SOFC electrode in which a composite electrode is used by mixing a catalyst electrode and an electrolyte. Based on the experimental evidence, a single-component fuel cell device produces electricity through a “normal” fuel cell reaction route by the following reactions: anode : H2 /2Hþ þ 2e
(1)
cathode : 1=2O2 þ 2e/O2
(2)
overall reaction : H2 þ 1=2O2 /2Hþ þ O2 2Hþ þ O2 /H2 O
(3a) (3b)
Combining Eq. (3)a and b yields: H2 þ 1=2O2 /H2 O
(4)
For comparison, a conventional three-component SOFC has following reactions:
Fig. 5 e IeV and IeP characteristics of the singlecomponent device operated at various temperatures.
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 ) 8 5 3 6 e8 5 4 1
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Fig. 6 e IeV and IeP characteristics of the singlecomponent device using the as-prepared LiNiZn-oxidesNSDC composite modified by Fe.
anode : H2 þ O2 /H2 O þ 2e
(5)
cathode : 1=2O2 þ 2e/O2
(6)
overall reaction : H2 þ 1=2O2 /H2 O
(7)
Eq. (3)a and b indicates a two step reaction to complete the overall reaction in the single-component fuel cells whereas in the SOFC, O2 transport through the electrolyte to react with H2 (anode) is the critical reaction. In the single-component case, electricity generation is completed directly between the Hþ and O2 ions as there is no electrolyte separator indicating a co-ion process. This is possible because LiNiZn-oxideNSDC possesses a bi-catalyst function for ionizing H2 and O2 and the NSDC phase conducts both Hþ and O2 [23]. The single-component device exhibits fast reversible response when exchanging H2 and air supplies which leads to same OCV but with opposite polarity as shown in Fig. 7. This indicates fast bi-catalytic reaction processes for both H2 and O2, i.e. both sides of the single-component fuel cell can function as an anode for hydrogen oxidation and a cathode for oxygen reduction. For current collection, both a silver paste and a Ni-foam were used here, but a Ni-foam would perform slightly better for better contacting and would also provide a better mechanical support than an Ag paste. These experiments were also repeated with the three-component fuel cell made of same anode and cathode materials as above obtaining similar results but with much longer response time explained by the electrolyte and electrode interfaces that delay the kinetic processes.
4.
Discussion
A traditional fuel cell consists of a separate anode, electrolyte and cathode. Ion transportation through the electrolyte is critical for the performance, e.g. in a SOFC O2 transport through an O2 conducting electrolyte. In a single-component fuel cell device these functionalities seem to be more sophisticated; for example the electrolyte would in our case loose its traditional
Fig. 7 e OCV polarity change in the single-component fuel cell device when exchanging gas supplies (H2 and air) on electrodes.
functions meaning of providing both ionic transport and electronic separation properties, whereas here only the ionic transport property would be relevant. So we elaborate further the possible working principles of the device to explain the differences to the traditional “electrolyte” fuel cell. When the two sides of the single-component device are in a H2 and an air atmosphere respectively, both H2 and O2 can be catalytically dissociated into Hþ and O2 for the bi-catalystic functions of the component material. Hþ and O2 combine together on the particle surfaces inside the material and produce H2O and electricity at the same time. During this process, the H2 contacting side acts as an anode releasing electrons by forming Hþ. The air (O2) contacting side acts as a cathode receiving electrons. The device reactions are completed as long as Hþ and O2 appear in close proximity, preferentially at particle surfaces. This redox may be realized at particle level composed of both electronic (n and p) and ionic conducting particles, as illustrated in Fig. 8. We think that a p-n junction forms a barrier to block the internal electronic conduction in the single-component fuel cell when exposed to H2/O2 atmosphere. So the electrons are conducted to corresponding current collector without passing internally through the device. All ion and electron transfer and electricity generation may take place within a composite particle that shows both ionic and semiconducting properties. In principle, the cathodic process of the single-component device may be similar to the SOFC. Several step-reactions are needed to realize the overall reaction in Eq. (2): Step 1 : O2 ðgÞ/2Oad
(8)
Step 2 : Oad þ e/O ad
(9)
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Fig. 8 e A micro-view of the redox process in the single-component material.
Step 3 : O ad /OTPB
(10)
x Step 4 : O TPB þ VO,, þ e/Oo
(11)
We use here same notations as for SOFC, e.g. the threephase boundary (TPB) is also of importance to the singlecomponent device. On the anode side we have the following steps: Step 1 : H2 ðgÞ/2Had
(12)
Step 2 : Had /Hþ ad þ e
(13)
þ Step 3 : either Hþ ad /HTPB
(14)
þ or Hþ ad /H
(15)
As a matter of fact, the protons most preferentially move through the particle surfaces and interfaces between the electronic conductor (metal oxides) and ionic conductor (NSDC) particles. When protons approach the component from the H2 contacting side, it can form meta-stable hydrogen bonds with oxygen ions from both NSDC and metal oxide, e.g. LiNiZn-oxide’s surfaces. The ceria-based composite electrolyte possess both Hþ and O2- conduction [17,18,23] leading to effective proton and oxygen ion mobility driven by hydrogen and oxygen concentration gradients from both sides of the
single-component device. On the other hand, it has been reported that Hþ can be transported through lithiated nickel oxide [24] and that the Ni element shows good catalyst properties for converting H2 to Hþ. Protons may be directly generated at the anode according to Eq. (12) and (13). This means that the overall performance of the single-component device may be determined by the cathode kinetics/processes determined since the anode process can be simpler and faster. The impedance spectra of the composite material in both hydrogen and air atmosphere are shown in Fig. 9.The electrochemical impedance spectrum (EIS) consists of a “semicircle” followed by a “tail”. The small intercepts of the semicircle portion at the real axis in H2 and air indicates that both the functional anode and cathode reactions are fast kinetic processes [25]. This implies that LiNiZn-oxides-NSDC materials have high catalytic activity toward the hydrogen oxidation reaction and oxygen reduction reaction. From the EIS spectra, the value of total conductivity (including both ionic and electronic contributions) reaches about 0.1 S cm1(0.12 S cm1 for air side and 0.23 S cm1 for H2 side) when calculated from the formula s ¼ L/(RS), where L is the thickness of the pellet, R is the intercept of the EIS high frequency semicircle on real axis, and S is the effective area of the pellets. Finally, we comment shortly on the possibility of electrochemical and mechanical leakage of H2 and O2 in the single-
Fig. 9 e Typical electrochemical impedance spectra of the as-prepared LiNiZn-oxides-NSDC composite electrodes both in H2 (a) and air (b).
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component fuel cell. Firstly, it should be noticed that in the mechanical leakage case H2 and O2 molecules and reactions are involved, not Hþ and O2. The half-device reactions (Eq. (1) and (2)) produce Hþ and O2 and have already generated electricity to the external circuit, which is different from the mechanical leakage to cause H2eO2 combustion. Wherever the Hþ and O2 reaction takes place, it will contribute to electricity generation without producing any “electrochemical” leakage. Furthermore, H2 and O2 in mesoporous media do not result in combustion due to quenching. The quenching distance for H2eO2 combustion is 1 mm [26]. In our case fuel cell reactions occurred at a nanometer or micrometer level far below the critical distance thus eliminating the possibility of combustion as a leakage path. It is obvious that there is still a need for research to fully understand the chemistry and physics of the new device as well as to construct optimal device structures. For example, the charge (electrons and ions) and phase separation (electronic and ionic) phenomena and no short circuiting problem observed in this fuel cell device deserve further attention.
5.
Conclusions
We reported here about a single-component fuel cell reactor which was compared to a conventional three-component electrolyte based fuel cell. The performance of the singlecomponent fuel cell is comparable to and in some cases slightly better than the three-component fuel cells. A power density of 300e600 mW cm2 was obtained with a LiNiZnoxide and NSDC mixture at 450e550 C. When adding a redox catalyst element (Fe), cell performance improved to 700 mW cm2 at 550 C. It is suggested that bi-catalyst function and dual Hþ and O2- and surface conduction explain single-component device performance. Future work will focus on charge and phase separation between electronic and ion materials as well as on electrochemical impedance studies and simulations of the new device.
Acknowledgments This work was supported by the Swedish agency for Innovation Systems (VINNOVA), the Swedish Research Council and the Swedish Agency for International Development Cooperation (SIDA), and the Swedish Agency for Energy (STEM), KIC InnoEnergy and Finnish Funding Agency for Technology and Innovation (TEKES).
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