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Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system
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Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system Yoshihiro Hirata a,n, Shinji Daio a, Ayaka Kai a, Taro Shimonosono a, Reiji Yano a, Soichiro Sameshima a, Katsuhiko Yamaji b a
Department of Chemistry, Biotechnology, and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Fuel Cell Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-11 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 August 2016 Accepted 27 August 2016
This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H2 fuel and circulate CO2 gas in the production process of electric power. A large electric power was measured in the H2–O2 gas system and the CO–O2 gas system at 1073 K. The formation process of O2 ions in the endothermic cathodic reaction (1/2O2 þ 2e -O2 ) controlled the cell performance in both the gas systems. The electric power of the H2–CO2 gas system, which allowed to change CO2 gas into a CO fuel (H2 þCO2-H2O þCO) in the cathode, was 1/31–1/11 of the maximum electric power for the H2–O2 gas system. This result is related to the larger endothermic energy for the formation of O2 ions from CO2 molecules at the cathode (CO2 þ2e -COþO2 ) than from O2 molecules. The CO–H2O gas system and the H2–H2O gas system was expected to produce a H2 fuel in the cathode (COþH2O-H2 þCO2, H2 þH2O-H2 þH2O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Electrical properties D. ZrO2 E. Fuel cells
1. Introduction The decrease in the CO2 concentration in atmosphere is an important target to stop the global warming [1]. To overcome this situation, many renewable energies such as wind power, solar energy, water power, geothermal energy, tidal power, and biomass energy have been developed instead of fossil fuels. The above energies are converted to electric power or in the form of clean H2 molecules. The produced H2 molecules are supplied as a fuel to several types of fuel cells [2]. The conversion efficiency from a chemical energy of H2 molecules to an electric power is closely related to the structures and materials of electrolyte, anode and cathode of a fuel cell [2]. On the other hand, the research on the circulation of CO2 gas or energy development using CO2 gas has been scarcely reported as compared with the papers and patents on a H2 fuel [3–7]. In our previous papers, (1) the electrochemical reforming of biogas (60% CH4–40% CO2) to produce a H2–CO mixed fuel n
Corresponding author. E-mail address: [email protected] (Y. Hirata).
(CH4 þCO2-2H2 þ2CO) [8–11], (2) the electrochemical shift reaction of CO gas to produce a H2 fuel (COþH2O-H2 þCO2) [12,13] and (3) electrochemical decomposition of CO gas into solid carbon and O2 gas (2CO-2C þO2) [14,15] were investigated to achieve a complete closed system (Eq. (1)) of H2 production and CO2 decomposition from a biogas. Eq. (1) shows the ideal combined chemical process and proceeds by the supply of an external electric power.
Biogas (3CH4 + 2CO2 ) + CO2 (circulated) + H2 O →7H2 + CO2 (circulated) + 5C + 5/2O2
(1)
Our next challenge is to establish the combined fuel cell systems which produce both the electric power and a H2 fuel during the decomposition process of CO2. As a first stage of this strategy, we analyze 6 reactions for H2 and CO fuels in Table 1. In a usual fuel cell, Eq. (2) for the H2–O2 gas system is applied to produce an electric power. Eq. (5) for the CO–O2 gas system is also possible in a solid oxide fuel cell to produce an electric power. CO gas in Eq. (5) is formed from Eq. (3) for the H2–CO2 gas system, which can also produce an electric power during the formation process of H2O and CO gas. Eq. (7) for the CO–H2O system is an attractive reaction
http://dx.doi.org/10.1016/j.ceramint.2016.08.170 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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2
Table 1 Possible fuel–oxidant gas systems for fuel cell. Fuel
Oxidant
Chemical reaction
H2
O2
H2 + 1/2O2 → H2O
(2)
H2
CO2
H2 + CO2 → H2O + CO
(3)
H2
H2O
H2 + H2O → H2O + H2
(4)
CO
O2
CO + 1/2O2 → CO2
(5)
CO
CO2
CO + CO2 → CO2 + CO
(6)
CO
H2O
CO + H2O → CO2 + H2
(7)
H2
CO2
CO2
electric power H2O
CO
H2
electric power CO
Eq. (3)
Eq. (7)
H2O
Combined system A CO2
H2
electric power
electric power H2O
CO
O2
CO2
CO
Eq. (3)
Eq. (5)
O2
Combined system B Fig. 1. Combined reaction systems for the production of a H2 fuel and the circulation of CO2 gas in the production process of electric power.
2. Standard Gibbs free energy change Fig. 2(a) shows the temperature dependence of the standard Gibbs free energy change of Eqs. (2), (3), (5) and (7) in Table 1. Eqs. (2) and (5) have the large negative ΔG° values in a wide temperature range of 273–1273 K. The corresponding emf (electromotive force, E°) is higher than 1 V (E° ¼ ΔG°/2F, F: Faraday constant). These chemical reactions are used to produce a great
Standard Gibbs free energy change, ΔGº (kJ/mol)
circulated
of possible chemical reactions produces electric power in addition to the useful chemical product which can be supplied as an active starting gas in the neighbor electric power generation process. The circulation of CO2 gas in the above combined system provides an ideal solution for both the global warming and clean energy development. In this paper, the evaluation of electric power was experimentally studied using an yttria-stabilized zirconia cell (YSZ) for the basic reactions of Eqs. (2)–(5) and (7) in Table 1 at 1073 K.
Electromotive force, Eº (V)
[12,13] which creates a H2 fuel during the production of an electric power. The produced CO2 gas is supplied to Eq. (3) to change again into CO gas during the production process of an electric power. Eqs. (4) and (6) do not seem to proceed in a fuel cell. Although the standard Gibbs free energy change (ΔG°) of Eq. (4) or (6) is 0 kJ/ mol, the ΔG value of Eq. (4) or (6) becomes a negative value by controlling the partial pressures of H2, H2O, CO and CO2 gases in the anode and cathode of a fuel cell. When the activation energy required for the decomposition of H2O and CO2 molecules is sufficiently low, Eqs. (4) and (6) are the possible reaction systems producing an electric power. Fig. 1 shows two types of the combined reaction systems for the production of a H2 fuel and the circulation of CO2 gas in the production process of electric power. The combined system A uses Eqs. (3) and (7) in Table 1. The two reactions can produce two types of electric power and the formed CO and H2 gases are circulated between Eqs. (3) and (7) as a closed system. Another combined system B includes Eqs. (3) and (5), and can produce two types of electric power. The produced CO2 gas is treated in a closed system. In the combined system B, a H2 fuel produced in another process is to be supplied. As shown in Fig. 1, the combined system
100 H2 + CO2 = H2O + CO (3) 0 CO + H2O = H2 + CO2 (7) -100 H2 + 1/2O2 = H2O (2) -200 CO + 1/2O2 = CO2 (5) 1.6 1.2 0.8
CO + 1/2O2 = CO2 (5)
(a) (b)
H2 + 1/2O2 = H2O (2)
0.4 CO + H2O = H2 + CO2 (7) 0.0 H2 + CO2 = H2O + CO (3) -0.4 273
673
1073
1473
Temperature (K) Fig. 2. (a) Standard Gibbs free energy change of Eqs. (2), (3), (5) and (7), and (b) the corresponding electromotive force (emf, E°).
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
3. Experimental procedure An end-closed tube electrolyte of 8 mol% Y2O3-stabilized zirconia (YSZ-8, Nikkato Co. Ltd., Japan) was used in this experiment. Its outer diameter and thickness are 15 and 2 mm, respectively. In the fabrication of anode electrode, a YSZ powder (92 mol% ZrO2– 8 mol% Y2O3, TZ-8Y, Tosoh Co. Ltd., Japan) was dispersed in a 0.4 M Ni(NO3)2 aqueous solution at a volume ratio of YSZ:Ni¼70:30. The prepared suspension was stirred for 24 h and then freeze-dried. The freeze-dried powder was calcined at 873 K for 1 h to form a NiO–YSZ mixed powder. In the fabrication of a cathode powder, the above-mentioned YSZ powder was dispersed in a RuCl3 aqueous solution at a volume ratio of YSZ:Ru ¼ 70:30. The pH value of the suspension was adjusted to 10 to precipitate Ru(OH)3. After stirring for 24 h, the suspension was freeze-dried. The freeze-dried powder was calcined at 1073 K for 1 h to form a RuO2–YSZ mixed powder. Each electrode powder was dispersed in a solvent of 75 vol% isopropyl alcohol–25 vol% toluene, and was mixed with polyethylene glycol (lubricant) of 40 mass% and polyvinyl butyral (binder) of 20 mass% against the mass of YSZ powder. The prepared non-aqueous pastes of anode and cathode powders were spread on the outer and inner surfaces of the YSZ tube electrolyte, respectively. The anode and cathode pastes on the YSZ electrolyte were heated at 1473 and 1173 K for 1 h, respectively. Then, a Pt mesh welded to a Pt lead wire was attached to each electrode with a Pt paste and then heated at 1273 K for 1 h. The fabricated cell was fixed in an Al2O3 tube using a brass fixater and then placed in an electric furnace. Before a cell test, NiO and RuO2 in the anode and cathode were reduced to metal Ni and Ru, respectively, by supplying a H2 gas at 1073 K for 1 h. The performance of the YSZ cell was examined for the H2–O2 gas system, H2–CO2 gas system, H2–H2O gas system, CO–O2 gas system and CO–H2O gas system at 1073 K. The relationship between terminal voltage and electric current was measured by a potentiostat (HAL-3001, Hokuto Denko Corp., Japan). Since it was difficult to measure the exact surface areas of both the electrodes, the measured current was not converted to the current density in this paper. The oxygen partial pressure of the outlet gas of anode side was measured by a YSZ oxygen sensor at 973 K. The gas compositions of the outlet gases of anode and cathode sides were measured by a gas chromatograph (GT7100T, J-ScienceLab Co. Ltd., Japan). The flow rates of outlet gases were measured by a flow meter (Soapfilm Flow Meter, 300111002, GL Science Inc., Japan).
4. Results and discussion 4.1. YSZ cell performance using the H2–O2 gas system Fig. 3 shows (a) the terminal voltage and (b) electric power of an yttria-stabilized zirconia cell (YSZ) at 1073 K using a 97% H2–3%
3
H2/3%H2O 50 ml/min, air 100 ml/min H2/3%H2O 50 ml/min, air 200 ml/min 1073K
1.0
0.5
0.0
Electric power (mW)
electric power. On the other hand, Eq. (7) provides a negative ΔG° value below 1097 K and is suitable to produce an electric power at a low temperature. Eq. (3) shows an opposite tendency of ΔG° value against heating temperature as compared with Eq. (7) and gives a negative ΔG° value above 1097 K. The E° values for Eqs. (3) and (7) are small as compared with the E° values for Eqs. (2) and (5). In addition to the ΔG° values of possible chemical reactions, the temperature dependence of the self diffusion coefficient of oxide ions in a solid electrolyte affects the electric power produced. The self diffusion coefficient of O2 ions decreases at a low temperature, causing the increase in the internal resistance of a fuel cell [16–19]. The combination of ΔG° and the self diffusion coefficient of O2 ions controls the electric power of a fuel cell.
Terminal voltage (V)
Y. Hirata et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
300 200 100 0 0
200 400 Electric current (mA)
600
Fig. 3. (a) Terminal voltage and (b) electric power of an yttria-stabilized zirconia cell at 1073 K using a H2/3% H2O fuel and air.
H2O fuel and an O2 (air) oxidant. The results of Fig. 3 were used as reference data for comparison with the H2–CO2 gas system. The OCV (open circuit voltage) and maximum electric power were independent of the flow rate (100–200 ml/min) of the supplied air, and 1.075–1.100 V and 280–286 mW (at 595–597 mA of electric current), respectively. The measured result is basically explained by the ΔG° and E° values for the H2–O2 gas system in Fig. 2. The terminal voltage (E) of the cell for the H2–O2 gas system is expressed by Eq. (8),
ΔG 2F ⎧ ⎫ PH2O(anode) ⎪ 1⎪ ⎨ ΔG° + RT ln ⎬ =− 1/2 ⎪ 2F ⎪ P P ( ) ( ) anode cathode H2 ⎩ ⎭ O2
E=−
=E ° −
PH2O(a) RT ln 2F PH2(a)PO1/2(c) 2
(8)
Fig. 3(a) indicates a small influence of the flow rate of the supplied air (cathode) on E value, suggesting that the partial pressures of H2 (anode), H2O (anode) and O2 (cathode) are not sensitive to the flow rates of a H2 fuel and air. The decrease in E is caused by the increase of electric current as seen in Fig. 3(a). The electric current is transported by O2 ions through the YSZ electrolyte [16,17,19,20]. The flux (J) of O2 ions diffusing the YSZ electrolyte is given by Eq. (9),
J=−D
∂C ΔC ≈−D ∂x Δx
(9)
where D is the self diffusion coefficient of O2 ions, ΔC is the difference of the concentration of O2 ions between cathode side and anode side of the YSZ electrolyte, and Δx is the thickness of the electrolyte. The increased current in Fig. 3 is related to the increase in J in Eq. (9) and leads to the increase in ΔC at given D and Δx values. The increased ΔC accelerates the chemical reaction of H2 gas and O2 ions (H2 þO2 -H2Oþ2e ) at the anode and causes the decrease of PH2 (anode) and the increase of PH2O
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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Terminal voltage (V)
1.0
Key Experiment Measured Calculated No.1-3 No.1-4 No.1-5
0.9 0.8
0.5 30
Electric power (mW)
10−3I (8.314 J/mol K)(298.15 K)(106 ml)(60 s) 2(9.6485 × 104 C/mol)(1.01325 × 105 Pa)(1 m3)(1 min)
PCO(c) = PCO2(c)
20 10
0
10
20 30 Electric current (mA)
40
50
Fig. 4. (a) Terminal voltage and (b) electric power of a YSZ cell using the H2/3% H2O (50 ml/min)–CO2 (100 ml/min) gas system at 1073 K.
(anode), resulting in the decrease of E (terminal voltage) in Eq. (8). 4.2. YSZ cell performance using the H2–CO2 gas system Fig. 4 shows (a) the terminal voltage and (b) electric power of the YSZ cell using the H2–CO2 gas system at 1073 K. The same YSZ cell used in Section 4.1 (the H2–O2 gas system) was examined for the H2–CO2 gas system. The OCV value was 0.790–0.916 V in the repeated experiments of three times. The maximum power was 9.1–26.6 mW at 14.4–41.0 mA of electric current. These electric powers were 1/31–1/11 of the maximum power (280–286 mW) for the H2–O2 gas system at 1073 K. The terminal voltage and electric power were almost independent of the flow rate (100–200 ml/ min) of supplied CO2 gas. When the terminal voltage decreased below about 0.6 V, it was impossible to get an electric current. The emf for Eq. (3) in Table 1 is expressed by Eq. (10).
PH O(a)PCO(c) ΔG° RT − ln 2 2F 2F PH2(a)PCO2(c)
(11)
The equilibrium constant (K) is given by Eq. (12).
PH2(a)PO1/2(a) 2
PH2O(a)
V (CO) V (CO) + V (CO2) V (CO2) V (CO) + V (CO2)
) = V (CO) ) V (CO ) 2
(15)
(10)
Resistance (Ω)
1 K H2 O⟷H2 + O2 2
( (
The terminal voltage E in Eq. (13) was calculated using the following ΔG°, K and PO2(a) measured by YSZ oxygen gas sensor for the anode outlet gas: ΔG° (1100 K) ¼0.107 kJ/mol, K (1100 K) ¼ 1.3129 10 9 atm1/2, PO2(a)¼ 1.1322 10 21 Pa. Fig. 4(a) shows the emf (E) as a function of measured current. The measured tendency of E is explained by the calculated E (Eq. (13)) which decreases with an increase in electric current. The ratio of PCO(c)/PCO2(c) was calculated to be 2.662 10 4– 3.241 10 3 for the electric current of 3.5–42.6 mA in the 1st experimental (No. 1–3). The composition of cathode outlet gas was analyzed by a gas chromatography but the formation of CO gas was not recognized. This result is due to the low CO concentration below the detection limit of the gas chromatography used. Therefore, the increase in the electric current reduces the terminal voltage which is related to the partial pressures of gases in the anode and cathode. As seen in Fig. 4(b), the measured electric power was also well explained by the calculation based on Eq. (13). The electric current is greatly influenced by the total resistance of transportation of O2 ions through the electrolyte and the overpotentials for the chemical reactions at the anode (H2 þO2 -H2Oþ2e ) and cathode (CO2 þ2e -COþO2 ) [21]. Fig. 5 compares the resistance of the YSZ cell for (a) the H2–O2 gas system and (b) the H2–CO2 gas system at 1073 K as a function of electric current. The resistance (R) was determined from the difference of electric current (ΔI) per 0.02 V of the drop of terminal voltage (ΔE) in the measured E–I relation (Figs. 3 and 4). The measured R values showed strong and weak dependence on electric current in the H2–CO2 gas system and the H2–O2 system, respectively. The magnitude of R was larger for the H2–CO2 gas
In the anode, the following equilibrium is assumed.
K=
(14)
That is, the PCO/PCO2 ratio at the cathode in Eq. (13) is expressed by the ratio of the flow rates of CO and CO2 gases (Eq. (15)) and the flow rate of CO gas is determined by the measured electric current (Eq. (14)).
0.6
E=−
=
10−3IRT 2FP
=7.607 × 10−3I ( mA)
0.7
0
V (CO, ml/ min) =
10 (b) H2/3% H2O –CO2 gas system
1
(a) H2/3% H2O –O2 gas system
(12)
The substitution of Eq. (12) for Eq. (10) results in Eq. (13).
E=−
PO1/2(a)PCO(c) ΔG° RT − ln 2 2F 2F KPCO2(c)
0.1 (13)
The flow rate (V(CO) ml/min, 298 K) of CO gas formed at the cathode by the reaction of CO2(c) þ2e -CO(c)þ O2 , is related to the measured current (I mA) by Eq. (14).
1
10
100
1000
Electric current (mA) Fig. 5. Resistance of a YSZ cell for (a) the H2/3% H2O–O2 gas system and (b) H2/3% H2O–CO2 gas system at 1073 K.
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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5
Terminal voltage (V)
1073K 1.0
Key
Flow rate Flow rate (ml/min) Oxidant (ml/min) 50 Air 100 H2 50 Air 100 H2/3% H2O 10%CO-90%Ar 50 Air 200 10%CO-90%Ar 100 200 Air 10%CO-90%Ar 200 Air 200 Fuel
0.5
Fig. 6. Enthalpy for the reactions at the cathode and anode in (a) the H2–O2 gas system and (b) the H2–CO2 gas system.
system rather than for the H2–O2 gas system. The same YSZ cell was used in both the experiments in Figs. 3 and 4, and the same kind of a H2/3% H2O fuel was supplied to the anode. The R values in Fig. 5 reflect mainly the resistance associated with the different chemical reactions at the cathode (1/2O2 þ2e -O2 , CO2 þ2e -COþO2 ). A large energy (a large drop of electric power) is required for the formation of O2 ions from CO2 oxidant rather than from O2 oxidant. Fig. 6 shows the enthalpy for the reactions at the cathode and anode in (a) the H2–O2 gas system. The anodic reaction with a H2 fuel (H2 þO2 -H2Oþ2e ) is accompanied by a large exothermic energy ( 1085 kJ/mol-H2, 298 K). The cathodic reaction in the H2– O2 gas system (1/2O2 þ 2e - O2 ) needs a large endothermic energy (948.8 kJ/mol-O2 ions). The total reaction forms H2O gas with the exothermic energy of 136.3 kJ/mol-H2O (298 K). The formation process of O2 ions from O2 gas and electrons controls the cell performance and the endothermic energy for the formation of O2 ions is treated as an activation energy for the electrochemical reaction of H2 and O2 gases. The cathodic reaction in the H2–CO2 gas system (CO2 þ2e -COþO2 ) needs a large endothermic energy of 1231.7 kJ/mol-O2 ions because CO2 is more chemically stable than O2 as seen in Fig. 6(b). The summation of the anodic and cathodic reactions gives an endothermic reaction of H2 þCO2-H2O þCO. That is, the H2–O2 gas system has a higher reactivity in a fuel cell than the H2–CO2 gas system because of (1) the lower endothermic energy for the formation of O2 ions at the cathode and (2) the exothermic reaction for the formation of H2O gas (H2 þ 1/2O2-H2O). As discussed above, the small electric power in the H2–CO2 gas system is mainly explained by the increased activation energy of the formation of O2 ions from CO2 at the cathode.
Electric power (mW)
0.0 60
40
20
0
0
50
100 150 Electric current (mA)
200
Fig. 7. (a) Terminal voltage and (b) electric power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K.
4.3. YSZ cell performance using the CO–O2 gas system Fig. 7 compares (a) the terminal voltage and (b) electric power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K. Since the YSZ cell examined in the experiments of Figs. 3 and 4 was broken by mixing an excess amount of liquid H2O with a N2 carrier gas in the cathode, another different YSZ cell was used in the experiment of Fig. 7. The OCV (1.050 V) of the H2/3% H2O fuel (reference gas system) was close to the OCV (1.075– 1.100 V) of the YSZ cell used in Fig. 3 but the electric current measured in Fig. 7 was small as compared with Fig. 3. The ΔG° for the CO–O2 gas system in Fig. 2 was comparable to the ΔG° for the H2–O2 gas system. The emf for the CO–O2 gas system is expressed by Eq. (16).
E = E° −
PCO2(a) RT ln 2F PCO(a)PO1/2(c)
(16)
2
Since PO2(c) in air is treated as a constant value (2.1225 104 Pa), the emf depends on the ratio of the partial pressures of CO2 and CO in the anode. The measured OCV (0.755– 0.778 V, Table 2) was lower than that for the H2–O2 gas system. However, the electric current for both the gas systems was measured in a similar magnitude range. The maximum power for the CO–O2 gas system was independent of the flow rate of a CO fuel and 59–66% of the maximum power for the H2–O2 gas system. This
Table 2 Summary of OCV and maximum power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K. Experiment
No. No. No. No. No.
2-1 2-2 2-3 2-4 2-5
Anode
Cathode
Gas
Flow rate (ml/min)
Gas
Flow rate (ml/min)
H2 H2-3%H2O 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar
50 50 50 100 200
Air Air Air Air Air
100 100 200 200 200
Volume ratio
OCV (V)
Maximum power (mW)
H2/O2 ¼ 1/0.42 H2/O2 ¼ 1/0.43 CO/O2 ¼1/8.38 CO/O2 ¼1/4.19 CO/O2 ¼1/2.09
1.267 1.050 0.778 0.758 0.755
63.19 58.10 38.56 37.37 37.18
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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1 2 5 10
1 2 5
H2O H2O H2O – – – – H2O H2O H2O H2O 99 98 95 20 20 20 20 99 98 95 90 0 2 4 10
Flow rate (ml/min) Liquid Flow rate (ml/min) Flow rate (ml/min)
N2 N2 N2 Air Air Air Air N2 N2 N2 N2 – – – H2O H2O H2O H2O – – – – 50 50 50 20 18 16 10 20 20 20 20 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar H2 H2 H2 H2 3-1 3-2 3-3 4-1 4-2 4-3 4-4 5-1 5-2 5-3 5-4 No. No. No. No. No. No. No. No. No. No. No.
This cell is attractive because the cathode outlet gas includes a H2 fuel as seen in Fig. 2 [12,13]. This cell may be operated at a low temperature. However, the E° value is relatively small as compared with the E° for the H2–O2 gas system or the CO–O2 gas system. The increase in PCO(a) and PH2O(c) contributes to the enhancement of E and the resultant electric power. The experimental results using the CO–H2O gas system (experiment No. 3-1 to No. 3-3) are presented in Table 3. The same YSZ cell used in Table 2 was also examined in the experiments in Table 3. Liquid H2O was mixed with a N2 carrier gas and supplied to the YSZ cell heated at 1073 K. The OCV of 0.577–0.585 V was measured for the flow rates of 1–5 ml liq.H2O/min. However, it was impossible to get an electric current, indicating no decomposition of H2O vapor at the cathode. This result may be explained by a high endothermic energy for the formation of O2 ions at the cathode as seen in Fig. 6(a). As
Gas
(17)
Liquid
PCO2(a)PH2(c) RT ln 2F PCO(a)PH2O(c)
Flow rate (ml/min)
E = E° −
Gas
The terminal voltage of a YSZ cell using the CO–H2O gas system is expressed by Eq. (17).
Cathode
4.4. YSZ cell performance using the CO–H2O gas system and the H2– H2O gas system
Anode
CO–O2 gas system as well as the H2–O2 gas system, can produce a large power in a YSZ cell. Fig. 8 shows the enthalpy for the reactions at the cathode and anode in the CO–O2 gas system. The large exothermic energy ( 1231.74 kJ/mol-CO) at the anode (CO þO2 -CO2 þ2e ) is supplied to the cathodic reaction to form O2 ions (1/2O2 þ2e O2 ). The formation process (endothermic reaction) of O2 ions dominates the cell performance. This situation is the same as the H2–O2 gas system in Fig. 6(a). According to Eq. (16), the electric power ( ¼E(terminal voltage)I(electric current)) is enhanced by increasing PCO(a) and PO2(c). In this experiment, the mixed gas of 10% CO–90% Ar was supplied to keep a safe experiment. A fuel of a high PCO(a) may be used in the industrial application of the fuel cell. Similarly, the emf for the H2–O2 gas system is expressed by Eq. (8) and increases with an increase in PH2(a). The high E and electric power for the H2–O2 gas system shown in Fig. 7 and Table 2 are due to the high PH2(a) (97–100% H2, 0.9828 105–1.0132 105 Pa).
Experiment
Fig. 8. Enthalpy for the reactions at the cathode and anode in the CO–O2 gas system.
Table 3 Summary of OCV and maximum power of a YSZ cell using the CO–H2O gas system, the CO/H2O–O2 gas system and the H2–H2O gas system at 1073 K.
Volume ratio
CO/H2O(g) ¼ 1/2.71 102 CO/H2O(g) ¼ 1/5.41 102 CO/H2O(g) ¼ 1/1.35 103 CO/H2O(g)/O2 ¼1/0/2.09 CO/H2O(g)/O2 ¼1/1.50 103/2.33 CO/H2O(g)/O2 ¼1/3.38 103/2.62 CO/H2O(g)/O2 ¼1/1.35 104/4.19 H2/H2O(g) ¼ 1/6.77 10 H2/H2O(g) ¼ 1/1.35 102 H2/H2O(g) ¼ 1/3.38 102 H2/H2O(g) ¼ 1/6.77 102
0.580 0.585 0.577 0.765 0.767 0.768 0.772 0.855 0.846 0.831 0.867
OCV (V)
00.00 00.00 00.00 18.06 17.69 17.55 17.54 00.00 00.00 00.00 00.00
Maximum power (mW)
6
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compared with O2 oxidant, more endothermic energy of 136.3 kJ/ mol-H2O (298 K) is required to form O2 ions. However, the decomposition energy (136.3 kJ/mol H2O) of H2O vapor is about the half of the decomposition energy of CO2 gas (282.94 kJ/mol CO2, Fig. 6(b)). As discussed in Section 4.2, the H2–CO2 gas system produced a small electric power. Therefore, another factor except for the decomposition energy of H2O vapor is to be investigated to explain the results in Table 3. In the experiments of No. 4-1 to 4-4, liquid H2O was mixed with a CO fuel in the anode to accelerate the decomposition of H2O vapor and the formation of a H2 fuel. Air of 20 ml/min was supplied to the cathode. This cell system showed the OCV of 0.765– 0.772 V and it was possible to get a small electric power of about 18 mW at 1073 K. This reaction system was based on the CO–O2 gas system in Section 4.3. Basically, Eq. (5) in Fig. 2 (COþ 1/2O2CO2) is applied to produce an electric power. In addition, the excess CO was expected to produce a H2 fuel in the anode through the shift reaction with H2O vapor (Eq. (7), COþ H2O-H2 þCO2). The measured OCV for the CO/H2O–O2 gas system was comparable to that for the CO–O2 gas system. The addition of H2O vapor in the anode gave little influence on the maximum power. The H2 gas fraction of the anode outlet gas, which was analyzed by a gas chromatography, was about 0.2 vol% in the experiments No. 4-2 to 4-4. The low H2 fraction may be due to the small ΔG° value ( 0.7375 kJ/mol) at 1073 K in Fig. 2. In the experiments of No. 5-1 to 5-4, H2 and H2O vapor were supplied to the anode and cathode of the YSZ cell at 1073 K, respectively. The OCV of 0.831–0.867 V was measured but no electric power was measured. This situation was the same as the experiments of the CO–H2O gas system (No. 3-1 to 3-3). At this moment, it is difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.
5. Conclusions In this paper, fuel cell performance of five kinds of gas systems was examined in an yttria-stabilized zirconia cell (YSZ) to establish the combined fuel cell systems which can produce a H2 fuel and circulate CO2 gas in the production process of an electric power. As a reference gas system, the performance of the YSZ cell using the H2–O2 (air) gas system was measured at 1073 K. The maximum electric power for the H2–CO2 gas system, which allowed to change CO2 gas into a CO fuel (H2 þCO2-H2O þCO), was 1/31–1/ 11 of the maximum power for the H2–O2 gas system. The increase in the electric current reduced the terminal voltage which was closely related to the partial pressures of gases in the anode and cathode. In both the gas systems, the formation process of O2 ions from O2 or CO2 and electrons controlled the cell performance. The endothermic energy for the formation of O2 ions was treated as an activation energy for the electrochemical reaction of H2 gas and O2 or CO2 gas. The cathodic reaction in the H2–CO2 gas system (CO2 þ2e -COþO2 ) needs a large endothermic energy than the H2–O2 gas system because CO2 molecules are more chemically stable than O2 molecules. In addition, the summation of the anodic (H2 þ O2 -H2O þ2e ) and cathodic (CO2 þ 2e -COþ O2 ) reactions gives an endothermic reaction of H2 þCO2-H2O þCO. That is, the H2–O2 gas system has a higher reactivity in a fuel cell than the H2–CO2 gas system. On the other hand, the CO–O2 gas system as well as the H2–O2 gas system, produced a large electric power in
7
a YSZ cell. The maximum power for 10% CO/90% Ar–O2 gas system was independent of the flow rate of the CO fuel and 59–66% of the maximum power for the 97% H2/3% H2O–O2 gas system at 1073 K. The emf and electric power of the CO–O2 gas system is to be enhanced by increasing the partial pressure of CO in the anode. In both the H2–H2O gas system and the CO–H2O gas system, the OCV values in the range of 0.83–0.87 V and 0.58–0.59 V were measured at 1073 K, respectively. However, no electric current was measured. At this moment, it is difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.
References [1] Fifth IPCC Assessment Report: Climate Change, 2014, 〈http://ar5-syr.ipcc.ch/ ipcc/ipcc/resources/pdf/IPCC_SynthesisReport.pdf〉 (accessed 02.06.16). [2] A. Kirubakaran, S. Jain, R.K. Nema, A review on fuel cell technologies and power electronic interface, Renew. Sustain. Energy Rev. 13 (2009) 2430–2440. [3] K.S. Udupa, G.S. Subramanian, H.V.K. Udupa, The electrolytic reduction of carbon dioxide to formic acid, Electrochim. Acta 16 (1971) 1593–1598. [4] K. Hara, T. Sakata, Electrocatalytic formation of CH4 from CO2 on a Pt gas diffusion electrolyte, J. Electrochem. Soc. 144 (2) (1997) 539–545. [5] R.L. Cook, R.C. Macduff, A.F. Sammells, High rate gas phase CO2 reduction to ethylene and methane using gas diffusion electrodes, J. Electrochem. Soc. 137 (1990) 607–608. [6] D.P. Summers, S. Leach, K.W. Frese, The electrochemical reduction of aqueous carbon-dioxide to methanol at molybdenum electrodes with low overpotentials, J. Electroanal. Chem. 205 (1986) 219–232. [7] Y. Hirata, N. Matsunaga, S. Sameshima, Reforming of biogas using electrochemical cell, J. Ceram. Soc. Jpn. 119 (11) (2011) 763–769. [8] Y. Hirata, Y. Terasawa, N. Matsunaga, S. Sameshima, Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–Reduction of CH4–CO2 mixed gases, Ceram. Int. 35 (2009) 2023–2028. [9] S. Matayoshi, Y. Hirata, S. Sameshima, N. Matsunaga, Y. Terasawa, Electrochemical reforming of CH4–CO2 gas using porous Gd-doped ceria electrolyte with Ni and Ru electrodes, J. Ceram. Soc. Jpn. 117 (11) (2009) 1147–1152. [10] M. Ando, Y. Hirata, S. Sameshima, N. Matsunaga, Electrochemical reforming of CH4–CO2 mixed gas using porous yttria-stabilized zirconia cell, J. Ceram. Soc. Jpn. 119 (11) (2011) 794–800. [11] Y. Suga, R. Yoshinaga, N. Matsunaga, Y. Hirata, S. Sameshima, Electrochemical reforming of CH4–CO2 mixed gas using Gd-doped ceria porous electrolyte with Cu electrode, Ceram. Int. 38 (2012) 6713–6721. [12] Y. Hirata, Y. Kisanuki, S. Sameshima, T. Shimonosono, Formation of hydrogen by electrochemical reaction of CO gas and H2O vapor using porous Gd-doped ceria electrolyte, Ceram. Int. 40 (2014) 10153–10157. [13] K. Ueda, Y. Hirata, S. Sameshima, T. Shimonosono, K. Yamaji, Formation of hydrogen from the CO–H2O system using porous Gd-doped ceria electrochemical cell with MnO cathode and Fe3O4 anode, J. Asian Ceram. Soc. 3 (2015) 82–87. [14] Y. Hirata, M. Ando, N. Matsunaga, S. Sameshima, Electrochemical decomposition of CO2 and CO gases using porous yttria-stabilized zirconia cell, Ceram. Int. 38 (2012) 6377–6387. [15] T. Shimonosono, M. Ueno, Y. Hirata, S. Sameshima, Decomposition of carbon monoxide and carbon dioxide using cathode-supported dense yttria-stabilized zirconia electrolyte cell, Ceram. Int. 40 (2014) 15957–15964. [16] S. Sameshima, H. Ono, K. Higashi, K. Sonoda, Y. Hirata, Y. Ikuma, Electrical conductivity and diffusion of oxygen ion in rare earth-doped ceria, J. Ceram. Soc. Jpn. 108 (12) (2000) 1060–1066. [17] M. Kamiya, E. Shimada, Y. Ikuma, M. Komatsu, H. Haneda, S. Sameshima, Y. Hirata, Oxygen self-diffusion in cerium oxide with Nd, J. Mater. Res. 16 (2001) 179–184. [18] Y. Ibuski, Y. Hirata, S. Sameshima, N. Matsunaga, Influence of composition of Gd-doped ceria electrolyte on performance of solid oxide fuel cell, Mater. Sci. Forum 724 (2012) 389–392. [19] Y. Ibuski, H. Kunigo, Y. Hirata, S. Sameshima, N. Matsunaga, Processing and performance of solid oxide fuel cell with Gd-doped ceria electrolyte, J. Eur. Ceram. Soc. 31 (2011) 2663–2669. [20] T. Shimonosono, Y. Hirata, S. Sameshima, T. Horita, Electronic conductivity of La-doped ceria ceramics, J. Am. Ceram. Soc. 88 (2005) 2114–2120. [21] T. Shimonosono, G. Hiramatsu, Y. Hirata, S. Sameshima, N. Matsunaga, T. Doi, T. Horita, Electrochemical properties of cathode for solid oxide fuel cell with Gd-doped ceria electrolyte, Adv. Mater. Res. 26–28 (2007) 275–278.
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Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system Yoshihiro Hirata a,n, Shinji Daio a, Ayaka Kai a, Taro Shimonosono a, Reiji Yano a, Soichiro Sameshima a, Katsuhiko Yamaji b a
Department of Chemistry, Biotechnology, and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Fuel Cell Group, Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-11 Higashi, Tsukuba, Ibaraki 305-8565, Japan
b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 August 2016 Accepted 27 August 2016
This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H2 fuel and circulate CO2 gas in the production process of electric power. A large electric power was measured in the H2–O2 gas system and the CO–O2 gas system at 1073 K. The formation process of O2 ions in the endothermic cathodic reaction (1/2O2 þ 2e -O2 ) controlled the cell performance in both the gas systems. The electric power of the H2–CO2 gas system, which allowed to change CO2 gas into a CO fuel (H2 þCO2-H2O þCO) in the cathode, was 1/31–1/11 of the maximum electric power for the H2–O2 gas system. This result is related to the larger endothermic energy for the formation of O2 ions from CO2 molecules at the cathode (CO2 þ2e -COþO2 ) than from O2 molecules. The CO–H2O gas system and the H2–H2O gas system was expected to produce a H2 fuel in the cathode (COþH2O-H2 þCO2, H2 þH2O-H2 þH2O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Electrical properties D. ZrO2 E. Fuel cells
1. Introduction The decrease in the CO2 concentration in atmosphere is an important target to stop the global warming [1]. To overcome this situation, many renewable energies such as wind power, solar energy, water power, geothermal energy, tidal power, and biomass energy have been developed instead of fossil fuels. The above energies are converted to electric power or in the form of clean H2 molecules. The produced H2 molecules are supplied as a fuel to several types of fuel cells [2]. The conversion efficiency from a chemical energy of H2 molecules to an electric power is closely related to the structures and materials of electrolyte, anode and cathode of a fuel cell [2]. On the other hand, the research on the circulation of CO2 gas or energy development using CO2 gas has been scarcely reported as compared with the papers and patents on a H2 fuel [3–7]. In our previous papers, (1) the electrochemical reforming of biogas (60% CH4–40% CO2) to produce a H2–CO mixed fuel n
Corresponding author. E-mail address: [email protected] (Y. Hirata).
(CH4 þCO2-2H2 þ2CO) [8–11], (2) the electrochemical shift reaction of CO gas to produce a H2 fuel (COþH2O-H2 þCO2) [12,13] and (3) electrochemical decomposition of CO gas into solid carbon and O2 gas (2CO-2C þO2) [14,15] were investigated to achieve a complete closed system (Eq. (1)) of H2 production and CO2 decomposition from a biogas. Eq. (1) shows the ideal combined chemical process and proceeds by the supply of an external electric power.
Biogas (3CH4 + 2CO2 ) + CO2 (circulated) + H2 O →7H2 + CO2 (circulated) + 5C + 5/2O2
(1)
Our next challenge is to establish the combined fuel cell systems which produce both the electric power and a H2 fuel during the decomposition process of CO2. As a first stage of this strategy, we analyze 6 reactions for H2 and CO fuels in Table 1. In a usual fuel cell, Eq. (2) for the H2–O2 gas system is applied to produce an electric power. Eq. (5) for the CO–O2 gas system is also possible in a solid oxide fuel cell to produce an electric power. CO gas in Eq. (5) is formed from Eq. (3) for the H2–CO2 gas system, which can also produce an electric power during the formation process of H2O and CO gas. Eq. (7) for the CO–H2O system is an attractive reaction
http://dx.doi.org/10.1016/j.ceramint.2016.08.170 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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2
Table 1 Possible fuel–oxidant gas systems for fuel cell. Fuel
Oxidant
Chemical reaction
H2
O2
H2 + 1/2O2 → H2O
(2)
H2
CO2
H2 + CO2 → H2O + CO
(3)
H2
H2O
H2 + H2O → H2O + H2
(4)
CO
O2
CO + 1/2O2 → CO2
(5)
CO
CO2
CO + CO2 → CO2 + CO
(6)
CO
H2O
CO + H2O → CO2 + H2
(7)
H2
CO2
CO2
electric power H2O
CO
H2
electric power CO
Eq. (3)
Eq. (7)
H2O
Combined system A CO2
H2
electric power
electric power H2O
CO
O2
CO2
CO
Eq. (3)
Eq. (5)
O2
Combined system B Fig. 1. Combined reaction systems for the production of a H2 fuel and the circulation of CO2 gas in the production process of electric power.
2. Standard Gibbs free energy change Fig. 2(a) shows the temperature dependence of the standard Gibbs free energy change of Eqs. (2), (3), (5) and (7) in Table 1. Eqs. (2) and (5) have the large negative ΔG° values in a wide temperature range of 273–1273 K. The corresponding emf (electromotive force, E°) is higher than 1 V (E° ¼ ΔG°/2F, F: Faraday constant). These chemical reactions are used to produce a great
Standard Gibbs free energy change, ΔGº (kJ/mol)
circulated
of possible chemical reactions produces electric power in addition to the useful chemical product which can be supplied as an active starting gas in the neighbor electric power generation process. The circulation of CO2 gas in the above combined system provides an ideal solution for both the global warming and clean energy development. In this paper, the evaluation of electric power was experimentally studied using an yttria-stabilized zirconia cell (YSZ) for the basic reactions of Eqs. (2)–(5) and (7) in Table 1 at 1073 K.
Electromotive force, Eº (V)
[12,13] which creates a H2 fuel during the production of an electric power. The produced CO2 gas is supplied to Eq. (3) to change again into CO gas during the production process of an electric power. Eqs. (4) and (6) do not seem to proceed in a fuel cell. Although the standard Gibbs free energy change (ΔG°) of Eq. (4) or (6) is 0 kJ/ mol, the ΔG value of Eq. (4) or (6) becomes a negative value by controlling the partial pressures of H2, H2O, CO and CO2 gases in the anode and cathode of a fuel cell. When the activation energy required for the decomposition of H2O and CO2 molecules is sufficiently low, Eqs. (4) and (6) are the possible reaction systems producing an electric power. Fig. 1 shows two types of the combined reaction systems for the production of a H2 fuel and the circulation of CO2 gas in the production process of electric power. The combined system A uses Eqs. (3) and (7) in Table 1. The two reactions can produce two types of electric power and the formed CO and H2 gases are circulated between Eqs. (3) and (7) as a closed system. Another combined system B includes Eqs. (3) and (5), and can produce two types of electric power. The produced CO2 gas is treated in a closed system. In the combined system B, a H2 fuel produced in another process is to be supplied. As shown in Fig. 1, the combined system
100 H2 + CO2 = H2O + CO (3) 0 CO + H2O = H2 + CO2 (7) -100 H2 + 1/2O2 = H2O (2) -200 CO + 1/2O2 = CO2 (5) 1.6 1.2 0.8
CO + 1/2O2 = CO2 (5)
(a) (b)
H2 + 1/2O2 = H2O (2)
0.4 CO + H2O = H2 + CO2 (7) 0.0 H2 + CO2 = H2O + CO (3) -0.4 273
673
1073
1473
Temperature (K) Fig. 2. (a) Standard Gibbs free energy change of Eqs. (2), (3), (5) and (7), and (b) the corresponding electromotive force (emf, E°).
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
3. Experimental procedure An end-closed tube electrolyte of 8 mol% Y2O3-stabilized zirconia (YSZ-8, Nikkato Co. Ltd., Japan) was used in this experiment. Its outer diameter and thickness are 15 and 2 mm, respectively. In the fabrication of anode electrode, a YSZ powder (92 mol% ZrO2– 8 mol% Y2O3, TZ-8Y, Tosoh Co. Ltd., Japan) was dispersed in a 0.4 M Ni(NO3)2 aqueous solution at a volume ratio of YSZ:Ni¼70:30. The prepared suspension was stirred for 24 h and then freeze-dried. The freeze-dried powder was calcined at 873 K for 1 h to form a NiO–YSZ mixed powder. In the fabrication of a cathode powder, the above-mentioned YSZ powder was dispersed in a RuCl3 aqueous solution at a volume ratio of YSZ:Ru ¼ 70:30. The pH value of the suspension was adjusted to 10 to precipitate Ru(OH)3. After stirring for 24 h, the suspension was freeze-dried. The freeze-dried powder was calcined at 1073 K for 1 h to form a RuO2–YSZ mixed powder. Each electrode powder was dispersed in a solvent of 75 vol% isopropyl alcohol–25 vol% toluene, and was mixed with polyethylene glycol (lubricant) of 40 mass% and polyvinyl butyral (binder) of 20 mass% against the mass of YSZ powder. The prepared non-aqueous pastes of anode and cathode powders were spread on the outer and inner surfaces of the YSZ tube electrolyte, respectively. The anode and cathode pastes on the YSZ electrolyte were heated at 1473 and 1173 K for 1 h, respectively. Then, a Pt mesh welded to a Pt lead wire was attached to each electrode with a Pt paste and then heated at 1273 K for 1 h. The fabricated cell was fixed in an Al2O3 tube using a brass fixater and then placed in an electric furnace. Before a cell test, NiO and RuO2 in the anode and cathode were reduced to metal Ni and Ru, respectively, by supplying a H2 gas at 1073 K for 1 h. The performance of the YSZ cell was examined for the H2–O2 gas system, H2–CO2 gas system, H2–H2O gas system, CO–O2 gas system and CO–H2O gas system at 1073 K. The relationship between terminal voltage and electric current was measured by a potentiostat (HAL-3001, Hokuto Denko Corp., Japan). Since it was difficult to measure the exact surface areas of both the electrodes, the measured current was not converted to the current density in this paper. The oxygen partial pressure of the outlet gas of anode side was measured by a YSZ oxygen sensor at 973 K. The gas compositions of the outlet gases of anode and cathode sides were measured by a gas chromatograph (GT7100T, J-ScienceLab Co. Ltd., Japan). The flow rates of outlet gases were measured by a flow meter (Soapfilm Flow Meter, 300111002, GL Science Inc., Japan).
4. Results and discussion 4.1. YSZ cell performance using the H2–O2 gas system Fig. 3 shows (a) the terminal voltage and (b) electric power of an yttria-stabilized zirconia cell (YSZ) at 1073 K using a 97% H2–3%
3
H2/3%H2O 50 ml/min, air 100 ml/min H2/3%H2O 50 ml/min, air 200 ml/min 1073K
1.0
0.5
0.0
Electric power (mW)
electric power. On the other hand, Eq. (7) provides a negative ΔG° value below 1097 K and is suitable to produce an electric power at a low temperature. Eq. (3) shows an opposite tendency of ΔG° value against heating temperature as compared with Eq. (7) and gives a negative ΔG° value above 1097 K. The E° values for Eqs. (3) and (7) are small as compared with the E° values for Eqs. (2) and (5). In addition to the ΔG° values of possible chemical reactions, the temperature dependence of the self diffusion coefficient of oxide ions in a solid electrolyte affects the electric power produced. The self diffusion coefficient of O2 ions decreases at a low temperature, causing the increase in the internal resistance of a fuel cell [16–19]. The combination of ΔG° and the self diffusion coefficient of O2 ions controls the electric power of a fuel cell.
Terminal voltage (V)
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300 200 100 0 0
200 400 Electric current (mA)
600
Fig. 3. (a) Terminal voltage and (b) electric power of an yttria-stabilized zirconia cell at 1073 K using a H2/3% H2O fuel and air.
H2O fuel and an O2 (air) oxidant. The results of Fig. 3 were used as reference data for comparison with the H2–CO2 gas system. The OCV (open circuit voltage) and maximum electric power were independent of the flow rate (100–200 ml/min) of the supplied air, and 1.075–1.100 V and 280–286 mW (at 595–597 mA of electric current), respectively. The measured result is basically explained by the ΔG° and E° values for the H2–O2 gas system in Fig. 2. The terminal voltage (E) of the cell for the H2–O2 gas system is expressed by Eq. (8),
ΔG 2F ⎧ ⎫ PH2O(anode) ⎪ 1⎪ ⎨ ΔG° + RT ln ⎬ =− 1/2 ⎪ 2F ⎪ P P ( ) ( ) anode cathode H2 ⎩ ⎭ O2
E=−
=E ° −
PH2O(a) RT ln 2F PH2(a)PO1/2(c) 2
(8)
Fig. 3(a) indicates a small influence of the flow rate of the supplied air (cathode) on E value, suggesting that the partial pressures of H2 (anode), H2O (anode) and O2 (cathode) are not sensitive to the flow rates of a H2 fuel and air. The decrease in E is caused by the increase of electric current as seen in Fig. 3(a). The electric current is transported by O2 ions through the YSZ electrolyte [16,17,19,20]. The flux (J) of O2 ions diffusing the YSZ electrolyte is given by Eq. (9),
J=−D
∂C ΔC ≈−D ∂x Δx
(9)
where D is the self diffusion coefficient of O2 ions, ΔC is the difference of the concentration of O2 ions between cathode side and anode side of the YSZ electrolyte, and Δx is the thickness of the electrolyte. The increased current in Fig. 3 is related to the increase in J in Eq. (9) and leads to the increase in ΔC at given D and Δx values. The increased ΔC accelerates the chemical reaction of H2 gas and O2 ions (H2 þO2 -H2Oþ2e ) at the anode and causes the decrease of PH2 (anode) and the increase of PH2O
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Terminal voltage (V)
1.0
Key Experiment Measured Calculated No.1-3 No.1-4 No.1-5
0.9 0.8
0.5 30
Electric power (mW)
10−3I (8.314 J/mol K)(298.15 K)(106 ml)(60 s) 2(9.6485 × 104 C/mol)(1.01325 × 105 Pa)(1 m3)(1 min)
PCO(c) = PCO2(c)
20 10
0
10
20 30 Electric current (mA)
40
50
Fig. 4. (a) Terminal voltage and (b) electric power of a YSZ cell using the H2/3% H2O (50 ml/min)–CO2 (100 ml/min) gas system at 1073 K.
(anode), resulting in the decrease of E (terminal voltage) in Eq. (8). 4.2. YSZ cell performance using the H2–CO2 gas system Fig. 4 shows (a) the terminal voltage and (b) electric power of the YSZ cell using the H2–CO2 gas system at 1073 K. The same YSZ cell used in Section 4.1 (the H2–O2 gas system) was examined for the H2–CO2 gas system. The OCV value was 0.790–0.916 V in the repeated experiments of three times. The maximum power was 9.1–26.6 mW at 14.4–41.0 mA of electric current. These electric powers were 1/31–1/11 of the maximum power (280–286 mW) for the H2–O2 gas system at 1073 K. The terminal voltage and electric power were almost independent of the flow rate (100–200 ml/ min) of supplied CO2 gas. When the terminal voltage decreased below about 0.6 V, it was impossible to get an electric current. The emf for Eq. (3) in Table 1 is expressed by Eq. (10).
PH O(a)PCO(c) ΔG° RT − ln 2 2F 2F PH2(a)PCO2(c)
(11)
The equilibrium constant (K) is given by Eq. (12).
PH2(a)PO1/2(a) 2
PH2O(a)
V (CO) V (CO) + V (CO2) V (CO2) V (CO) + V (CO2)
) = V (CO) ) V (CO ) 2
(15)
(10)
Resistance (Ω)
1 K H2 O⟷H2 + O2 2
( (
The terminal voltage E in Eq. (13) was calculated using the following ΔG°, K and PO2(a) measured by YSZ oxygen gas sensor for the anode outlet gas: ΔG° (1100 K) ¼0.107 kJ/mol, K (1100 K) ¼ 1.3129 10 9 atm1/2, PO2(a)¼ 1.1322 10 21 Pa. Fig. 4(a) shows the emf (E) as a function of measured current. The measured tendency of E is explained by the calculated E (Eq. (13)) which decreases with an increase in electric current. The ratio of PCO(c)/PCO2(c) was calculated to be 2.662 10 4– 3.241 10 3 for the electric current of 3.5–42.6 mA in the 1st experimental (No. 1–3). The composition of cathode outlet gas was analyzed by a gas chromatography but the formation of CO gas was not recognized. This result is due to the low CO concentration below the detection limit of the gas chromatography used. Therefore, the increase in the electric current reduces the terminal voltage which is related to the partial pressures of gases in the anode and cathode. As seen in Fig. 4(b), the measured electric power was also well explained by the calculation based on Eq. (13). The electric current is greatly influenced by the total resistance of transportation of O2 ions through the electrolyte and the overpotentials for the chemical reactions at the anode (H2 þO2 -H2Oþ2e ) and cathode (CO2 þ2e -COþO2 ) [21]. Fig. 5 compares the resistance of the YSZ cell for (a) the H2–O2 gas system and (b) the H2–CO2 gas system at 1073 K as a function of electric current. The resistance (R) was determined from the difference of electric current (ΔI) per 0.02 V of the drop of terminal voltage (ΔE) in the measured E–I relation (Figs. 3 and 4). The measured R values showed strong and weak dependence on electric current in the H2–CO2 gas system and the H2–O2 system, respectively. The magnitude of R was larger for the H2–CO2 gas
In the anode, the following equilibrium is assumed.
K=
(14)
That is, the PCO/PCO2 ratio at the cathode in Eq. (13) is expressed by the ratio of the flow rates of CO and CO2 gases (Eq. (15)) and the flow rate of CO gas is determined by the measured electric current (Eq. (14)).
0.6
E=−
=
10−3IRT 2FP
=7.607 × 10−3I ( mA)
0.7
0
V (CO, ml/ min) =
10 (b) H2/3% H2O –CO2 gas system
1
(a) H2/3% H2O –O2 gas system
(12)
The substitution of Eq. (12) for Eq. (10) results in Eq. (13).
E=−
PO1/2(a)PCO(c) ΔG° RT − ln 2 2F 2F KPCO2(c)
0.1 (13)
The flow rate (V(CO) ml/min, 298 K) of CO gas formed at the cathode by the reaction of CO2(c) þ2e -CO(c)þ O2 , is related to the measured current (I mA) by Eq. (14).
1
10
100
1000
Electric current (mA) Fig. 5. Resistance of a YSZ cell for (a) the H2/3% H2O–O2 gas system and (b) H2/3% H2O–CO2 gas system at 1073 K.
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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5
Terminal voltage (V)
1073K 1.0
Key
Flow rate Flow rate (ml/min) Oxidant (ml/min) 50 Air 100 H2 50 Air 100 H2/3% H2O 10%CO-90%Ar 50 Air 200 10%CO-90%Ar 100 200 Air 10%CO-90%Ar 200 Air 200 Fuel
0.5
Fig. 6. Enthalpy for the reactions at the cathode and anode in (a) the H2–O2 gas system and (b) the H2–CO2 gas system.
system rather than for the H2–O2 gas system. The same YSZ cell was used in both the experiments in Figs. 3 and 4, and the same kind of a H2/3% H2O fuel was supplied to the anode. The R values in Fig. 5 reflect mainly the resistance associated with the different chemical reactions at the cathode (1/2O2 þ2e -O2 , CO2 þ2e -COþO2 ). A large energy (a large drop of electric power) is required for the formation of O2 ions from CO2 oxidant rather than from O2 oxidant. Fig. 6 shows the enthalpy for the reactions at the cathode and anode in (a) the H2–O2 gas system. The anodic reaction with a H2 fuel (H2 þO2 -H2Oþ2e ) is accompanied by a large exothermic energy ( 1085 kJ/mol-H2, 298 K). The cathodic reaction in the H2– O2 gas system (1/2O2 þ 2e - O2 ) needs a large endothermic energy (948.8 kJ/mol-O2 ions). The total reaction forms H2O gas with the exothermic energy of 136.3 kJ/mol-H2O (298 K). The formation process of O2 ions from O2 gas and electrons controls the cell performance and the endothermic energy for the formation of O2 ions is treated as an activation energy for the electrochemical reaction of H2 and O2 gases. The cathodic reaction in the H2–CO2 gas system (CO2 þ2e -COþO2 ) needs a large endothermic energy of 1231.7 kJ/mol-O2 ions because CO2 is more chemically stable than O2 as seen in Fig. 6(b). The summation of the anodic and cathodic reactions gives an endothermic reaction of H2 þCO2-H2O þCO. That is, the H2–O2 gas system has a higher reactivity in a fuel cell than the H2–CO2 gas system because of (1) the lower endothermic energy for the formation of O2 ions at the cathode and (2) the exothermic reaction for the formation of H2O gas (H2 þ 1/2O2-H2O). As discussed above, the small electric power in the H2–CO2 gas system is mainly explained by the increased activation energy of the formation of O2 ions from CO2 at the cathode.
Electric power (mW)
0.0 60
40
20
0
0
50
100 150 Electric current (mA)
200
Fig. 7. (a) Terminal voltage and (b) electric power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K.
4.3. YSZ cell performance using the CO–O2 gas system Fig. 7 compares (a) the terminal voltage and (b) electric power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K. Since the YSZ cell examined in the experiments of Figs. 3 and 4 was broken by mixing an excess amount of liquid H2O with a N2 carrier gas in the cathode, another different YSZ cell was used in the experiment of Fig. 7. The OCV (1.050 V) of the H2/3% H2O fuel (reference gas system) was close to the OCV (1.075– 1.100 V) of the YSZ cell used in Fig. 3 but the electric current measured in Fig. 7 was small as compared with Fig. 3. The ΔG° for the CO–O2 gas system in Fig. 2 was comparable to the ΔG° for the H2–O2 gas system. The emf for the CO–O2 gas system is expressed by Eq. (16).
E = E° −
PCO2(a) RT ln 2F PCO(a)PO1/2(c)
(16)
2
Since PO2(c) in air is treated as a constant value (2.1225 104 Pa), the emf depends on the ratio of the partial pressures of CO2 and CO in the anode. The measured OCV (0.755– 0.778 V, Table 2) was lower than that for the H2–O2 gas system. However, the electric current for both the gas systems was measured in a similar magnitude range. The maximum power for the CO–O2 gas system was independent of the flow rate of a CO fuel and 59–66% of the maximum power for the H2–O2 gas system. This
Table 2 Summary of OCV and maximum power of a YSZ cell using the H2–O2 gas system and the CO–O2 gas system at 1073 K. Experiment
No. No. No. No. No.
2-1 2-2 2-3 2-4 2-5
Anode
Cathode
Gas
Flow rate (ml/min)
Gas
Flow rate (ml/min)
H2 H2-3%H2O 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar
50 50 50 100 200
Air Air Air Air Air
100 100 200 200 200
Volume ratio
OCV (V)
Maximum power (mW)
H2/O2 ¼ 1/0.42 H2/O2 ¼ 1/0.43 CO/O2 ¼1/8.38 CO/O2 ¼1/4.19 CO/O2 ¼1/2.09
1.267 1.050 0.778 0.758 0.755
63.19 58.10 38.56 37.37 37.18
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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1 2 5 10
1 2 5
H2O H2O H2O – – – – H2O H2O H2O H2O 99 98 95 20 20 20 20 99 98 95 90 0 2 4 10
Flow rate (ml/min) Liquid Flow rate (ml/min) Flow rate (ml/min)
N2 N2 N2 Air Air Air Air N2 N2 N2 N2 – – – H2O H2O H2O H2O – – – – 50 50 50 20 18 16 10 20 20 20 20 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar 10%CO-90%Ar H2 H2 H2 H2 3-1 3-2 3-3 4-1 4-2 4-3 4-4 5-1 5-2 5-3 5-4 No. No. No. No. No. No. No. No. No. No. No.
This cell is attractive because the cathode outlet gas includes a H2 fuel as seen in Fig. 2 [12,13]. This cell may be operated at a low temperature. However, the E° value is relatively small as compared with the E° for the H2–O2 gas system or the CO–O2 gas system. The increase in PCO(a) and PH2O(c) contributes to the enhancement of E and the resultant electric power. The experimental results using the CO–H2O gas system (experiment No. 3-1 to No. 3-3) are presented in Table 3. The same YSZ cell used in Table 2 was also examined in the experiments in Table 3. Liquid H2O was mixed with a N2 carrier gas and supplied to the YSZ cell heated at 1073 K. The OCV of 0.577–0.585 V was measured for the flow rates of 1–5 ml liq.H2O/min. However, it was impossible to get an electric current, indicating no decomposition of H2O vapor at the cathode. This result may be explained by a high endothermic energy for the formation of O2 ions at the cathode as seen in Fig. 6(a). As
Gas
(17)
Liquid
PCO2(a)PH2(c) RT ln 2F PCO(a)PH2O(c)
Flow rate (ml/min)
E = E° −
Gas
The terminal voltage of a YSZ cell using the CO–H2O gas system is expressed by Eq. (17).
Cathode
4.4. YSZ cell performance using the CO–H2O gas system and the H2– H2O gas system
Anode
CO–O2 gas system as well as the H2–O2 gas system, can produce a large power in a YSZ cell. Fig. 8 shows the enthalpy for the reactions at the cathode and anode in the CO–O2 gas system. The large exothermic energy ( 1231.74 kJ/mol-CO) at the anode (CO þO2 -CO2 þ2e ) is supplied to the cathodic reaction to form O2 ions (1/2O2 þ2e O2 ). The formation process (endothermic reaction) of O2 ions dominates the cell performance. This situation is the same as the H2–O2 gas system in Fig. 6(a). According to Eq. (16), the electric power ( ¼E(terminal voltage)I(electric current)) is enhanced by increasing PCO(a) and PO2(c). In this experiment, the mixed gas of 10% CO–90% Ar was supplied to keep a safe experiment. A fuel of a high PCO(a) may be used in the industrial application of the fuel cell. Similarly, the emf for the H2–O2 gas system is expressed by Eq. (8) and increases with an increase in PH2(a). The high E and electric power for the H2–O2 gas system shown in Fig. 7 and Table 2 are due to the high PH2(a) (97–100% H2, 0.9828 105–1.0132 105 Pa).
Experiment
Fig. 8. Enthalpy for the reactions at the cathode and anode in the CO–O2 gas system.
Table 3 Summary of OCV and maximum power of a YSZ cell using the CO–H2O gas system, the CO/H2O–O2 gas system and the H2–H2O gas system at 1073 K.
Volume ratio
CO/H2O(g) ¼ 1/2.71 102 CO/H2O(g) ¼ 1/5.41 102 CO/H2O(g) ¼ 1/1.35 103 CO/H2O(g)/O2 ¼1/0/2.09 CO/H2O(g)/O2 ¼1/1.50 103/2.33 CO/H2O(g)/O2 ¼1/3.38 103/2.62 CO/H2O(g)/O2 ¼1/1.35 104/4.19 H2/H2O(g) ¼ 1/6.77 10 H2/H2O(g) ¼ 1/1.35 102 H2/H2O(g) ¼ 1/3.38 102 H2/H2O(g) ¼ 1/6.77 102
0.580 0.585 0.577 0.765 0.767 0.768 0.772 0.855 0.846 0.831 0.867
OCV (V)
00.00 00.00 00.00 18.06 17.69 17.55 17.54 00.00 00.00 00.00 00.00
Maximum power (mW)
6
Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i
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compared with O2 oxidant, more endothermic energy of 136.3 kJ/ mol-H2O (298 K) is required to form O2 ions. However, the decomposition energy (136.3 kJ/mol H2O) of H2O vapor is about the half of the decomposition energy of CO2 gas (282.94 kJ/mol CO2, Fig. 6(b)). As discussed in Section 4.2, the H2–CO2 gas system produced a small electric power. Therefore, another factor except for the decomposition energy of H2O vapor is to be investigated to explain the results in Table 3. In the experiments of No. 4-1 to 4-4, liquid H2O was mixed with a CO fuel in the anode to accelerate the decomposition of H2O vapor and the formation of a H2 fuel. Air of 20 ml/min was supplied to the cathode. This cell system showed the OCV of 0.765– 0.772 V and it was possible to get a small electric power of about 18 mW at 1073 K. This reaction system was based on the CO–O2 gas system in Section 4.3. Basically, Eq. (5) in Fig. 2 (COþ 1/2O2CO2) is applied to produce an electric power. In addition, the excess CO was expected to produce a H2 fuel in the anode through the shift reaction with H2O vapor (Eq. (7), COþ H2O-H2 þCO2). The measured OCV for the CO/H2O–O2 gas system was comparable to that for the CO–O2 gas system. The addition of H2O vapor in the anode gave little influence on the maximum power. The H2 gas fraction of the anode outlet gas, which was analyzed by a gas chromatography, was about 0.2 vol% in the experiments No. 4-2 to 4-4. The low H2 fraction may be due to the small ΔG° value ( 0.7375 kJ/mol) at 1073 K in Fig. 2. In the experiments of No. 5-1 to 5-4, H2 and H2O vapor were supplied to the anode and cathode of the YSZ cell at 1073 K, respectively. The OCV of 0.831–0.867 V was measured but no electric power was measured. This situation was the same as the experiments of the CO–H2O gas system (No. 3-1 to 3-3). At this moment, it is difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.
5. Conclusions In this paper, fuel cell performance of five kinds of gas systems was examined in an yttria-stabilized zirconia cell (YSZ) to establish the combined fuel cell systems which can produce a H2 fuel and circulate CO2 gas in the production process of an electric power. As a reference gas system, the performance of the YSZ cell using the H2–O2 (air) gas system was measured at 1073 K. The maximum electric power for the H2–CO2 gas system, which allowed to change CO2 gas into a CO fuel (H2 þCO2-H2O þCO), was 1/31–1/ 11 of the maximum power for the H2–O2 gas system. The increase in the electric current reduced the terminal voltage which was closely related to the partial pressures of gases in the anode and cathode. In both the gas systems, the formation process of O2 ions from O2 or CO2 and electrons controlled the cell performance. The endothermic energy for the formation of O2 ions was treated as an activation energy for the electrochemical reaction of H2 gas and O2 or CO2 gas. The cathodic reaction in the H2–CO2 gas system (CO2 þ2e -COþO2 ) needs a large endothermic energy than the H2–O2 gas system because CO2 molecules are more chemically stable than O2 molecules. In addition, the summation of the anodic (H2 þ O2 -H2O þ2e ) and cathodic (CO2 þ 2e -COþ O2 ) reactions gives an endothermic reaction of H2 þCO2-H2O þCO. That is, the H2–O2 gas system has a higher reactivity in a fuel cell than the H2–CO2 gas system. On the other hand, the CO–O2 gas system as well as the H2–O2 gas system, produced a large electric power in
7
a YSZ cell. The maximum power for 10% CO/90% Ar–O2 gas system was independent of the flow rate of the CO fuel and 59–66% of the maximum power for the 97% H2/3% H2O–O2 gas system at 1073 K. The emf and electric power of the CO–O2 gas system is to be enhanced by increasing the partial pressure of CO in the anode. In both the H2–H2O gas system and the CO–H2O gas system, the OCV values in the range of 0.83–0.87 V and 0.58–0.59 V were measured at 1073 K, respectively. However, no electric current was measured. At this moment, it is difficult to apply H2O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.
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Please cite this article as: Y. Hirata, et al., Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.08.170i