Operando spectroscopy study of the carbon dioxide electro-reduction by iron species on nitrogen-doped carbon

The carbon–carbon coupling via electrochemical reduction of carbon dioxide represents the biggest challenge for using this route as platform for chemicals synthesis. Here we show that nanostructured iron (III) oxyhydroxide on nitrogen-doped carbon enables high Faraday efficiency (97.4%) and selectivity to acetic acid (61%) at very-low potential (−0.5 V vs silver/silver chloride). Using a combination of electron microscopy, operando X-ray spectroscopy techniques and density functional theory simulations, we correlate the activity to acetic acid at this potential to the formation of nitrogen-coordinated iron (II) sites as single atoms or polyatomic species at the interface between iron oxyhydroxide and the nitrogen-doped carbon. The evolution of hydrogen is correlated to the formation of metallic iron and observed as dominant reaction path over iron oxyhydroxide on oxygen-doped carbon in the overall range of negative potential investigated, whereas over iron oxyhydroxide on nitrogen-doped carbon it becomes important only at more negative potentials.


Supplementary
. Nanostructure of sample Fe/N-C a and b HRTEM images. Scale bar 5 nm. c Selected area electron diffraction pattern exhibiting two rings, corresponding to distances of 0.15 and 0.26 nm. 4 Scale bar 1 1/nm.

Supplementary Figure 7.
Structural and elemental composition dynamics for Fe/N-C sample studied by ambient pressure XPS a Absolute atomic abundance of the elements at different experimental conditions applied in the following order: UHV and 298 K (black); UHV and 473 K (red); 0.15 mbar H2O and 298 K (green); 0.15 mbar CO 2 and 298 K (orange) after evacuation of the chamber to 10 -7 mbar. Note that after dosing the water, the chamber was evacuated to a pressure of 10 -7 mbar in order to remove most of the chemisorbed water b Absolute atomic abundance of the different N components of the fitted N1s XP spectra at the different experimental conditions. c Relative abundance of the different N components of the fitted N1s XP spectra at the different experimental conditions. XP spectra were recorded collecting photoelectrons emitted with KE=150 eV corresponding to a depth of information of about 0.5 nm. Any changes of the N species due to their thermal decomposition can be disregarded because only a negligible amount of N is desorbed in the gas phase as NO molecules and measured by mass spectrometry. d Open circuit potential (OCP) as function of time measured for the sample Fe/N-C upon immersion in 0.05M KHCO3.  Figure 11. Fast-XANES spectra measured for the sample Fe/O-C in operando FY-mode Fe K edge XANES spectra measured in 0.05M KHCO3 at different potential (as indicated) for the Fe/O-C. Each spectrum is merged from 11 fast Fe K-edge XANES spectra. Each spectrum is edge jump normalised and then an offset in the y axis is applied for a better representation. During the experiment, the potential was varied in steps of 0.1 V and hold at constant potential for 3 minutes. The plots show the reproducible redox processes that the Fe/O-C undergoes upon polarization.   Subsequently, chronoamperometry was also performed with simultaneous quick-XANES measurements as described previously. The potential was hold for 3 minutes and then was stepwise increased from OCP to -2 V, in steps of 0.1 V each. During the operando study, at more negative potential, when HER is the main reaction path, the catalyst's instability manifested itself as a sharp cathodic current peak (Supplementary Figure 10a). Indeed, this is a characteristic behaviour of these catalysts, which was also observed during the bench electrochemical tests, more markedly at more negative potential (Supplementary Figure 10b). Such sharp current peaks were previously accounted for as a result of the leaching of metal species into the solution, or to the formation of gas bubbles sticking on the catalysts surface and their subsequent release. 8 We believe that certainly the latter phenomenon takes place in these experiments because we also observe a reduction of the fluorescence signal intensity at the point of maximum current. This suggests that the growth of H2 bubbles increases the thickness of the medium that the incoming and emitted X-rays travel through resulting in a reduced transmission. The intensity is however immediately restored as the bubbles are released and diffuse away from the electrode surface. The corresponding catalyst structural dynamics indicate that from an oxidized Fe-L species the catalyst becomes metallic at the point of maximum current.

Supplementary
The catalyst undergoes several turnovers between these two states starting from -1.25 V with non-defined time dependence. The oxidized Fe-L species which converts into Fe 0 upon H2 release is characterized by a very intense resonance at ca. 7130 eV.
Despite the presence of O species observed in the Fourier transform EXAFS spectrum relative to this species ( Figure 8b in the main text), such high intensity implies a different chemical environment than the Fe(II)-O species observed at less negative potential. A similar pronounced increase of the white line intensity was observed for Fe-Fe hydrogenase upon hydride bond formation and protonation. 9 Analogously, we can assume a similar situation occurs here.
In alkaline solution, the HER occurs from H + discharge. 10 Therefore one would assume that in the electrical double layer, H + species accumulate and chemisorb at more negative potential. As a consequence of the higher eavailability, the Fe(II)-L could be related to the formation of hydride species, which may originate from the dissociation of chemisorbed OH. We hypothesize that the dynamics observed are related to the discharge of H + leading to HER, via interaction with intermediate hydride species. Despite the mechanism is unclear, the results indicate that HER occurs on a metallic surface, which implies that, the CV previous to the bulk electrolysis is not an "innocent" treatment and can modify the selectivity of the electro-catalyst.