Communication pubs.acs.org/cm
Thermoelectric Solid-Oxide Fuel Cells with Extra Power Conversion from Waste Heat Tao Wei,† Yun-Hui Huang,†,* Qin Zhang,† Li-Xia Yuan,† Jun-You Yang,† Yong-Ming Sun,† Xian-Luo Hu,† Wu-Xing Zhang,† and John B. Goodenough‡,* †
State Key Laboratory of Material Processing and Die and Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China ‡ Texas Materials Institute, ETC 9.102, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
KEYWORDS: solid-oxide fuel cell, thermoelectric cathode, NaCo2O4, waste heat utilization
S
conductor, the two criteria necessary for the cathode of a SOFC. The cathode need not be a good oxide-ion conductor, as has been demonstrated by the La1−xSrxMnO3 perovskite cathodes where the oxide ions generated by the ORR are transported on the cathode-particle surfaces to a triple phase boundary (TPB) of air/electrode/electrolyte. 14 If the NaCo2O4-based compounds can also act as a cathode material for a SOFC, it may create an extra thermoelectric voltage (ΔV) by utilizing the created temperature gradient. The basic concept illustrated in Figure 1A uses the SOFC cathode as a
olid oxide fuel cells (SOFCs) are important electrochemical devices to convert chemical energy directly into electricity with fuel flexibility.1−3 The fuel conversion efficiency of conventional SOFCs is usually about 50%;4 much of the chemical energy converts into waste heat energy. Thermoelectric materials can generate electricity from the waste heat.5−8 Here, we demonstrate a new type of SOFC based on a thermoelectric cathode material with extra power conversion from the waste thermal energy. Thermoelectric ptype Na1−xCuxCo2O4 (0 ≤ x ≤ 1) was employed as the cathode material of a SOFC to generate an additional potential. SOFCs with Na0.7Cu0.3Co2O4 as cathode delivered a maximum power density of 641 mW cm−2 at 800 °C. By converting waste heat to electric power with the thermoelectric cathode, the open circuit voltage of the cell obviously increased, which is ascribed to the additional thermoelectric voltage. In SOFCs, the electrochemical H2 or CH4 oxidation reaction is exothermic (for example, H2(g) + 1/2O2(g) = H2O(g), ΔH727 °C = −236.46 kJ mol−1). In addition, the internal resistance of a cell will inevitably give rise to heat.9 Therefore, operational SOFCs produce a lot of additional heat energy, which provides an appreciable temperature gradient to ambient temperature. In a single-chamber fuel cell, the temperature gradient can be sustained under the operating temperature.10 As we know, a thermoelectric generator can convert waste heat into electricity through thermoelectric power. If a thermoelectric material can serve at the same time as the cathode or anode material for a SOFC, it is possible to enhance the cell voltage and, hence, to generate an additional thermoelectric energy conversion in the SOFC. Fabrication of such a thermoelectric SOFC is exactly the same as that of a conventional SOFC. When it is used in a SOFC stack, it does not influence the design. Furthermore, since it makes use of the released waste heat, it is helpful to sustain a uniform temperature distribution within the cell and thus to reduce the rate of cell degradation. NaCo2O4 is a p-type thermoelectric compound that exhibits an excellent thermoelectric property at high temperature.11−13 Moreover, a mixed Co4+/Co3+ valence on CoO6/3 octahedra sharing edges offers the potential to be a good catalyst for the oxygen−reduction reaction (ORR) as well as an electronic © 2012 American Chemical Society
Figure 1. (A) Schematic diagram and constructional drawing for the porous elongated cathode fuel cell. Conventional fuel cell consists of anode, electrolyte, and cathode; elongated thermoelectric cathode produce thermoelectric voltage caused by temperature gradient. (B) The photo of NaCo2O4-based elongated cathode fuel cell supported on an LSGM electrolyte (2 cm diameter, 220 μm thickness).
thermoelectric generator. The area close to the electrolyte is the hot end at a temperature (T1), which is higher than that (T2) of the cold end far from the electrolyte. For a p-type thermoelectric material, the temperature gradient drives charge carriers from the hot end to the cold end to generate a positive thermoelectric voltage at the cold end. The thermoelectric voltage can be expressed as ΔV = S * (T1 − T2), where S is the Seebeck coefficient.15 The whole cell consists of two parts: a conventional solid-oxide fuel cell and a thermoelectric Received: January 15, 2012 Revised: March 31, 2012 Published: April 2, 2012 1401
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Communication
excellent electrochemical performance of NCCO is related to the catalytic activity for oxygen reduction and the low polarization resistance. For insight into the additional contribution from the thermoelectric voltage produced by the thermoelectric cathode owing to the temperature gradient in the SOFC, a porous elongated cathode was fabricated for testing. 85 wt % NCCO was mixed carefully with 15 wt % tragantine and a quantity of polymer binder (PVB, polyvinyl butyral); it was then pelletized into a column with 1 cm in diameter and 1.5 cm in height. The column was sintered at 800 °C for 2 h to achieve a porous elongated cathode. The porous configuration is to enhance air movement inside the cathode to provide more oxygen flow for the fuel cell. The thermal conductivity (k) of NaCo2O4 is very low.17 The introduction of CuCo2O4 can improve the thermal conductivity, but it is still low. For example, the k value of Na0.7Cu0.3Co2O4 only reaches 3.1 W m−1 K−1 at 900 °C (Figure S7, Supporting Information). Since the k value of air is only 0.076 W m−1 K−1 at 1000 °C,18 the air in the cathode will not increase the thermal conductivity of the cathode. Therefore, the cathode can effectively sustain the created temperature gradient. The porous elongated cathode fuel cell was fabricated via an unusual route. First, a layer of Pt slurry was coated onto the cathode side of the LSGM electrolyte (20 mm diameter, 220 μm thickness) to enhance the catalytic activity for oxygen reduction. The elongated porous cathode was attached to the Pt catalyst surface with Ag paste as a binder. The configuration of the cell is shown in Figure 1B. Interestingly, a stable power output with maximum power density of around 300 mW cm−2 is obtained at 800 °C in such a fuel cell (Figure S8, Supporting Information). It is notable that both the conventional fuel cell and the elongated cathode fuel cell show very stable performance (Figure S9, Supporting Information). Even after running in air at 800 °C for 200 h, no impurity phase has been detected in NCCO. In the cell, air steadily diffuses through the cathode, which gives a power density independent of the operating time. To measure accurately the generated thermoelectric voltage by the thermoelectric cathodes, we directly tested the voltage difference between #1 and #2 points of Figure 1B. Since p-type thermoelectric materials produce a positive voltage at the cold junction, the voltage at #2 point is more positive than that at #1 point. The difference in voltage between #1 and #2 points (ΔV) should be the thermoelectric voltage caused by the temperature gradient between the two points. As shown in Figure 3A, the value of ΔV for a Na1−xCuxCo2O4 elongated
generator. The total voltage (Vtotal) of the whole cell is expressed as Vtotal = Vcell + ΔV, where Vcell is cell voltage of the SOFC. Thermoelectric SOFCs were fabricated on La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) supporting electrolyte with NCCO as cathode and a (65:35 by weight) mixture of NiO and Sm-doped ceria (SDC) as anode; SDC was also used as a buffer layer between the anode and the electrolyte to prevent chemical reaction between NiO and LSGM. Figure 2 shows cell voltage
Figure 2. Electrochemical performance for conventional SOFC with configuration of NCCO∥LSGM∥SDC∥NiO+SDC. The thicknesses of NCCO, LSGM, SDC, and NiO+SDC are 30, 200, 20, and 50 μm, respectively. H2 was supplied to the anode (30 mL min−1) and the cathode was exposed to air. (A) Cell voltage and power density as functions of current density for SOFC with Na0.7Cu0.3Co2O4 as cathode. (B) Power density as a function of Cu content x for NCCObased SOFCs.
and power output for the above cells operating on H2 fuel. For the cell with Na0.7Cu0.3Co2O4 (x = 0.3) as cathode, the maximum power density (Pmax) is 641 mW cm−2 at 800 °C and 489 mW cm−2 at 750 °C (Figure 2A). The dependence of Pmax on Cu content x is presented in Figure 2B. For NaCo2O4 (x = 0), Pmax only reaches 370 mW cm−2 at 800 °C. With increasing x, Pmax first increases and then decreases; the x = 0.3 cell shows the highest Pmax. The CuCo2O4 (x = 1) cell exhibits 496 mW cm−2 at 800 °C, indicating that CuCo2O4 itself is a fairly good candidate cathode material for a SOFC. From scanning electron microscopy (SEM) images (Figure S3, Supporting Information), we can see that CuCo2O4 connects well with LSGM. In other words, the incorporation of CuCo2O4 improves the connection between cathode and electrolyte, which facilitates the transportation of oxide ions from the cathode surface to the TPB with the electrolyte. Analyzed by Xray photoelectron spectroscopy (XPS, Figure S4, Supporting Information), the valence state of Na in NCCO is +1 and Cu is mostly bivalent. Thus in the CoO2 octahedra of both phases, the Co ions are of mixed Co4+ and Co3+ valences, which gives rise to catalytic efficiency for oxygen reduction and good electronic conduction through Co3+−O−Co4+ electron transfer. 16 The NCCO samples exhibit excellent electronic conductivity in air (Figure S5, Supporting Information). The conductivity decreases gradually with increasing CuCo2O4 content. At 850 °C, the conductivity is 138 S cm−1 for NaCo2O4 and drops to 50 S cm−1 for CuCo2O4. NaCo2O4 shows metallic conduction. Polarization resistances of the NCCO cathodes were measured at 800 °C under ambient air pressure on NCCO∥LSGM∥NCCO symmetric cells (Figure S6, Supporting Information). NaCo2O4 shows the highest resistance of 0.47 Ω cm2. With incorporation of an appropriate amount of CuCo2O4, the resistance decreases dramatically. For example, the measured resistance values are 0.23 and 0.17 Ω cm2 for Na0.9Cu0.1Co2O4 and Na0.7Cu0.3Co2O4, respectively. The resistance for the Na-free CuCo2O4 is 0.28 Ω cm2. The
Figure 3. (A) Thermoelectric voltage ΔV generated by Na1−xCuxCo2O4 (x = 0 − 0.4) porous elongated cathode from 650 to 800 °C. The inset shows the OCV data tested within 1 h for conventional SOFC and thermoelectric SOFC. (B) Temperature dependence of Seebeck coefficient for Na1−xCuxCo2O4 (x = 0 − 0.4) tested in vacuum from 300 to 600 °C. 1402
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cathode decreases somewhat with increasing CuCo2O4 content from x = 0 to 0.4. The ΔV at 800 °C is 13.9 mV for NaCo2O4 and 11.1 mV for Na0.7Cu0.3Co2O4. The direction of the thermoelectric voltage is the same as that of the fuel cell voltage,19 which means that the thermoelectric voltage enhances the total voltage of the fuel cell. We took a NaCo2O4-based fuel cell as an example to check the cell voltage. The bottom position (#1) and top position (#2) were used as the cathode testing points (see Figure 1B). Under a conventional testing condition for a fuel cell, we continuously monitored the cell open circuit voltage (OCV) from the two testing points for 1 h at 800 °C. The average voltage is 1.1639 V at #2 and 1.1497 V at #1 (the inset of Figure 3A), which shows that an electric voltage difference was created between the two ends of the cathode. The average voltage difference between #1 and #2 points is 14.2 mV, which agrees well with the ΔV value. For both the conventional fuel cell and the elongated cathode fuel cell, the power density increases gradually with increasing operating temperature from 700 to 800 °C (Figure S8B, Supporting Information). The Seebeck coefficient S of the NCCO family is shown in Figure 3B. It can be seen that the magnitude of S strongly depends on x and temperature. At temperatures higher than 500 °C, S decreases as x increases; NaCo2O4 exhibits the highest S. Since the generated thermoelectric potential ΔV varies with the Seebeck coefficient and the temperature gradient, ΔV = S * (T1 − T2), the dependence of ΔV on x and temperature shows the same trend as S, as is displayed in Figure 3A. For example, ΔV of NaCo2O4 monotonically increases from 650 to 800 °C. At 800 °C, ΔV is 13.9 mV for NaCo2O4 and 5.6 mV for Na0.6Cu0.4Co2O4. Increasing the CuCo2O4 content lowers the value of ΔV. For comparison, we further employed a Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) porous elongated cathode for testing of the thermoelectric potential. It shows a ΔV value of only about 1 mV from 650 to 800 °C. BSCF provides no thermoelectric voltage at these temperatures. The comparison indicates that only a thermoelectric cathode can generate additional thermoelectric potential. In summary, with (1−x)(NaCo2O4)·x(CuCo2O4) solids as cathode materials, SOFCs show an additional thermoelectric potential generated by the temperature gradient in the waste heat. The thus-designed thermoelectric SOFCs exhibit a high power density and a stable performance during operation. Our data suggest that insertion of a cathode material of high catalytic activity between the electrolyte and electrode material providing a thermoelectric voltage at the operating temperature gradients may prove to be an interesting strategy. The preliminary results indicate that developing thermoelectric SOFCs can enhance the electrical conversion from waste heat energy in a fuel cell. More efficient thermoelectric materials deserve being explored as cathode or anode components for SOFCs to achieve a higher power output.
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Y. H. H); jgoodenough@ mail.utexas.edu (J. B. G). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Natural Science Foundation of Distinguished Young Scientists (Grant No. 50825203) and the PCSIRT (Program for Changjiang Scholars and Innovative Research Team in University, Grant No. IRT1014) for support of this work. J.B.G. thanks the Robert A. Welch Foundation of Houston, TX, for financial support.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, fuel cell fabrication and test, crystal structure, X-ray diffraction, SEM, XPS, conductivity, electrochemical impedance spectroscopy, thermal conductivity, elongated cathode SOFC power density, chemical stability and cell durability, and Seebeck coefficients. This material is available free of charge via the Internet at http://pubs.acs.org. 1403
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