Diffusion and Gas Conversion Analysis of Solid Oxide Fuel Cells at Loads via AC Impedance

Table 1

Values for a parallel RC as shown in Figure 5(a) are fitted to the low-frequency impedance loops of button cells.

(a) Varying hydrogen feed concentration at two temperatures

Physical elements	𝑅lf	𝐶lf	𝑌0	B	𝑌^∗0	B *	Note
xH (T = 750°C)	(mΩ cm^2)	(F cm^−2)	(S cm^−2 s^1/2)	(s^1/2)	(S cm^−2s^1/2)	(s^1/2)
47.3%	138.5	40.60	25.66	4.493	25.71	4.493	i = 175 mA cm^−2
63.7%	137.4	41.40	26.13	4.412	26.42	4.435
97.0%	120.1	51.85	31.25	4.700	31.43	4.713
xH (T = 800°C)
33.1%	172.3	58.45	26.76	5.441	27.53	5.532	i = 175 mA cm^−2
45.5%	146.9	78.55	35.56	6.090	33.32	5.831
97.0%	123.6	88.35	37.98	5.643	40.64	5.079

*The finite diffusion element (FDE), fitted in series with the Faradaic impedance as shown in Figure 5(b), is also listed in this Table 1(a), showing little difference between the parallel RC and the series fits.

(b) Varied load current density at two hydrogen feed concentrations

Physical elements	𝑅lf	𝐶lf	𝑌0	B	Note
i (mA cm^−2, 𝑥H=33.1%)	(mΩ cm^2)	(F cm^−2)	(S cm^−2 s^1/2)	(s^1/2)
0.000	638.8	16.99	7.215	6.078	T = 800°C,; xw = 3.0%,; Balance N2
175.0	172.3	58.45	26.76	5.441
300.0	126.4	75.65	35.28	5.313
i (mA cm^−2, 𝑥H=45.5%)
0.000	620.4	16.42	7.310	5.849	T = 800°C,; xw = 3.0%,; Balance N2
175.0	146.9	78.55	35.56	6.090
300.0	100.0	103.4	45.15	5.595