EIS results calculated from fitting the Nyquist plots in Fig. 4(A) with the modified Randles circuit.²⁰ Values and errors are averages and standard deviations of three measurements.

Fe at%	a	R s (Ω)	R ct (Ω)	C dl (μF cm−2)	k 0 (cm s−1)
ta-C	0.89 ± 0.00	16 ± 2	841 ± 160	0.3 ± 0.0	0.002 ± 3 × 10−4
2	0.76 ± 0.07	18 ± 1	4.0 ± 0.6	5.0 ± 0.2	0.37 ± 0.05
5	0.82 ± 0.05	18 ± 1	2.7 ± 0.5	5.9 ± 1.0	0.54 ± 0.11
10	0.84 ± 0.05	22 ± 8	1.6 ± 0.3	7.0 ± 0.4	0.94 ± 0.16