to the spectrum. The doublets used for fitting FeNC catalyst spectra are categorised by their Mössbauer parameters as well as the conditions under which they can be observed. The nomenclature here follows the assignment in the literature; note that the number and position of the doublets can depend on the fitting procedure used for interpreting the experiment, so that a degree of subjectivity is introduced. Two types of doublet are observed in (nearly) all literature Mössbauer experiments: D1/D1a (dark green), found at low isomer shifts and quadrupole splittings, and D2 (blue), found at low to intermediate isomer shifts and intermediate to high quadrupole splittings, both appearing under all measurement conditions. Depending on the fitting procedure, a third doublet, D3, can be used; this is an iron(II) high spin species with high isomer shifts not shown in Figure . If a D3 doublet is used in the fitting process, at least one D3/D3a doublet appears under all measurement conditions, and under in situ or operando conditions a second D3b signal can be observed. The D3 signals are shown in Figure . Ni, Gallenkamp et al. furthermore use a D1b doublet (light green) with low isomer shifts and low to intermediate quadrupole splittings for fitting, which is found to disappear for an electronically activated catalyst. The D4 doublet, finally, is only found at operando conditions and assigned to a species accumulating before the rate determining step of the ORR catalytic cycle.