Study of Operating Conditions for High Efficiency and Anode Safety of Industrial-Size Solid Oxide Fuel Cell

doi:10.6023/A22010041

Abstract

Solid oxide fuel cell (SOFC) has been regarded as one of the promising energy conversion technologies since it provides higher efficiency and lower pollution than conventional power systems. How to improve the SOFC efficiency is one of the focus issues in both industry and academia. Besides, when industrial-size SOFC is operated at high fuel utilization (U_fuel) to achieve high efficiency, the Ni anode in the downstream region may be locally oxidized due to the high oxygen partial pressure. Thus, operating conditions for both high efficiency and anode safety are required. In this work, a method for the measurement and evaluation of SOFC efficiency was established. By comparing the test results under different conditions (current, fuel flow rate, and temperature) with high fuel utilization, it was found that at the same fuel utilization, the voltage decreased as the current increased. Therefore, the lower current is beneficial for higher efficiency, while the higher current is beneficial for higher power. The relationship between voltage fluctuation and local oxidation of anode during operation at high fuel utilization was also studied. After analyzing the critical condition of Ni oxidation, the cell output voltage higher than the critical electromotive force of Ni oxidation was proposed as the safe operating condition to prevent local oxidation of anode. Based on the cell samples and test parameters used in this work, it was found a high efficiency of over 50% was corresponding to the fuel utilization range of 77% to 90%. And the maximum electrical efficiency was always corresponding to U_fuel=87.10%. Although the operating conditions corresponding to the maximum efficiency of SOFC cells or stacks with different materials, structures, and fabrication processes may vary, the measurement and evaluation methods, as well as the judgment of the safe operating condition proposed in this study, are still helpful. In practical application, the operating conditions could be determined based on such testing results according to the importance of efficiency, power and long-term stability.

Key words: solid oxide fuel cell, high efficiency, high fuel utilization, local oxidation of anode, safe operating condition

Cite this article

Yige Wang, Hangyue Li, Zewei Lyu, Minfang Han, Kaihua Sun. Study of Operating Conditions for High Efficiency and Anode Safety of Industrial-Size Solid Oxide Fuel Cell[J]. Acta Chimica Sinica, 2022, 80(8): 1091-1099.

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Fig. & Tab. 14

Figure 1. (a) I-V curve and (b) EIS result of the cell at 720 ℃ with 1 L•min–1 H2 and 3 L•min–1 air

Figure 2. Change of voltage with time at decreasing fuel flow rate under constant current operation of (a) 10, (b) 20 and (c) 30 A

Figure 3. Variations of voltage and efficiency with fuel utilization under currents of (a) 10, (b) 20 and (c) 30 A

Current/A	η_ele,max^a/%	U_fuel^b/%	Fuel flow rate/(L•min^–1)	Voltage/V	Power/W
10	53.57	87.10	0.08	0.771	7.71
20	52.92	87.10	0.16	0.761	15.23
30	50.47	87.10	0.24	0.726	21.78

Table 1. Operating conditions corresponding to the maximum electrical efficiency

Figure 4. Voltage test results under different fuel utilization compared to the iso-efficiency curves of 35%~55% [The shaded region is the operating conditions with high efficiency (>50%) and high power (>15 W)]

Figure 5. Change of voltage with time at decreasing fuel flow rate under constant current operation of (a) 20 and (b) 30 A at different temperatures (720, 750 and 800 ℃)

Figure 6. Variations of voltage and electrical efficiency with fuel utilization under different currents of (a) 20 and (b) 30 A at different temperatures (720, 750 and 800 ℃)

Figure 7. Voltage test results under different fuel utilization compared to the iso-efficiency curves of 35%~55% at different temperatures (720, 750 and 800 ℃) [The shaded region is the operating conditions with high efficiency (>50%) and high power (>15 W)]

Fuel utilization/%	ΔV/mV^a
Fuel utilization/%	10 A	20 A	30 A
63.34	7	2	2
69.68	15	3	3
77.42	40	14	8
87.10	57	49	35
99.54	73	85	100

Table 2. Voltage fluctuation under different fuel utilizations

Figure 8. Changes of voltage and the real part of 10 kHz impedance over time at a high fuel utilization (99.54%) The test conditions were 720 ℃, 10 A, and 0.07 L•min–1 H2.

Fuel utilization/%	20 A			30 A
Fuel utilization/%	800 ℃	750 ℃	720 ℃	800 ℃	750 ℃	720 ℃
69.68	5	5	6	4	4	4
77.42	8	15	23	5	5	7
87.10	13	41	53	19	33	45
99.54	38	61	73	45	62	63

Table 3. Voltage fluctuation (ΔV) under different fuel utilizations

Figure 9. Changes of voltage and the real part of 10 kHz impedance over time at a high fuel utilization (87.10%) The test conditions were 800 ℃, 30 A, and 0.24 L•min–1 H2.

No.	Temperature/℃	E*^a/V	Current/A	U_fuel^b/%	U^c/V	Power/W	η_ele^d/%	Evaluation of this condition
	Before degradation (1^st day)
1	720	0.744	20	77.42	0.814	16.28	50.31	U>E*, Higher power
2	720	0.744	20	87.10	0.761	15.23	52.92	U>E*, Higher efficiency
3	720	0.744	30	87.10	0.726	21.78	50.47	U<E*, not safe enough
	After degradation (7^th day)
4	750	0.730	20	87.10	0.760	15.20	52.83	U>E*, Better for stability
5	800	0.706	20	77.42	0.813	16.27	50.26	U>E*, Higher power
6	800	0.706	20	87.10	0.774	15.48	53.81	U>E*, Higher efficiency
7	800	0.706	30	87.10	0.750	22.49	52.10	U>E*, Higher power & efficiency

Table 4. Parameters and evaluation of conditions with high efficiency (>50%) and high power (>15 W)

Fuel utilization/%	Fuel flow rate of dry H₂/(L•min^–1)
Fuel utilization/%	10 A	20 A	30 A
63.34	0.11	0.22	0.33
69.68	0.10	0.20	0.30
77.42	0.09	0.18	0.27
87.10	0.08	0.16	0.24
99.54	0.07	0.14	0.21

Table 5. Testing parameters

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