Easy access to Fe2N nanomaterials from Fe nanocrystals and investigation of their electrocatalytic properties for the water electrolysis and CO2 reduction

Abstract

The nanostructured iron nitride phases, comprising only non-toxic and abundant elements, show potential in a wide range of applications, and are thus of great interest in the context of sustainable development. Especially, the nanostructured ε-Fe _x N phase (x = 2-3) can play the role of electrocatalyst (in fuel cells or for hydrogen production), or of anode in Li-ion batteries. However, obtaining morphology and composition (x) controlled nanoparticles of this phase is challenging. Here we show that α-Fe nanoparticles produced by an organometallic approach can be nitridated by a simple exposure to an ammonia flow in mild conditions of temperature, either in the powder form or as thin layers on FTO electrodes while keeping their initial morphology. Structural and magnetic measurements, combined with chemical analysis, and spectroscopic investigations evidenced the formation of the pure ε-Fe₂N phase, with a preserved nanostructure. The electrocatalytic activity of this nanomaterial has been evaluated for CO₂ reduction and for the oxygen evolution reaction. These results may open new perspectives for studying the properties and reactivity of ε-Fe₂N nanomaterials, and promote their use.

Scheme 1. (A) Synthetic approach to ZVFeNPs; (B) activation and nitridation conditions leading to H and N samples, respectively.

Fig. 1. (a) Typical SEM image of the nitridated material (scale bar: 100 nm); (b) SAXS data and corresponding fit.

Fig. 2. (a) SEM image of the nitridated material and Fe/N atomic ratios derived from EDX analysis on the various surface areas showcased; (b) XRD diagram of the nitridated material (black line) in comparison to ε-Fe₂N (orange line, ICSD33575) and ζ-Fe₂N (blue line, ICSD152811) reference diagrams with inset showing an enlargement of the diagrams in the low angles window.

Fig. 3. Mössbauer spectrum of the nitridated material recorded at 80 K (dots) and best fit obtained (black line) from a combination of a paramagnetic Fe₂N contribution (PM Fe₂N, red line), and a ferromagnetic Fe₂N contribution (FM Fe₂N, blue line).

Fig. 4. Photographs of a typical modified FTO electrode before (a) and after (b) nitridation and (c) TEM image of the nitridated deposit after scratching from the electrode, being suspended in toluene and drop-casted on a carbon-coated copper grid (scale bar 50 nm).

Fig. 5. I–V curves recorded for a Fe₂N/FTO catalyst electrode immersed in a 1.0 M KOH electrolyte solution. Linear sweep voltammograms were recorded with a slow potential scan rate of 5 mV s⁻¹.

Fig. 6. (a) The 159th I–V curve recorded for the Fe₂N/FTO electrode (blue line) and the steady I–V curve recorded for the blank FTO electrode (black line) immersed in 1.0 M KOH solution with a slow potential rate of 5 mV s⁻¹ (b) Tafel plot from the I–V curve of Fe₂N/FTO electrode for the slow reaction rate region.

Fig. 7. Cyclic voltammograms recorded at different potential scan rates for: (a) an as-prepared Fe₂N/FTO catalyst electrode and (b) an activated Fe₂N/FTO catalyst electrode. Plotting of the current density obtained at 0.82 V vs. RHE in function of potential scan rates for the case of: (c) as-prepared Fe₂N/FTO catalyst electrode and (d) activated Fe₂N/FTO catalyst electrode. Electrolyte was a 1.0 M KOH solution.

Fig. 8. Raman spectra recorded for an as-prepared Fe₂N/FTO catalyst electrode (red trace) and an activated Fe₂N/FTO catalyst electrode (black trace). Excitation was made by a 532 nm laser.

Fig. 9. Subsequent I–V curves recorded on a ε-Fe₂N/graphite catalyst electrode immersed in a 0.1 M NaHCO₃ (pH 6.8 solution) saturated CO₂ gas. Potential scan rate was 5 mV s⁻¹.

Fig. 10. I–t curves recorded during the bulk electrolysis at −0.7 V, −0.8 V, −0.9 V and −1.0 V vs. RHE employing a ε-Fe₂N/graphite catalyst electrode. Electrolyte was a 0.1 M NaHCO₃ solution saturated with CO₂.

Fig. 11. Product selectivity obtained at different potentials employing a ε-Fe₂N/graphite catalyst electrode. Electrolyte was a 0.1 M NaHCO₃ solution saturated with CO₂ gas.

Fig. 12. SEM images taken on a ε-Fe₂N/graphite catalyst electrode after being conditioned at −0.7 V vs. RHE in a 0.1 M NaHCO₃ solution saturated with CO₂ for 1.5 hours. Scale bars 5 µm and 0.5 µm for middle and left images, respectively.

Fig. 13. EDX elemental mapping conducted on the ε-Fe₂N/graphite catalyst electrode after being conditioned at −0.7 V vs. RHE in a 0.1 M NaHCO₃ solution saturated with CO₂ for 1.5 hours.

Fig. 14. Raman spectra recorded on an as-prepared ε-Fe₂N/graphite electrode (red trace) and the same electrode after being conditioned at −0.7 V vs. RHE for 1.5 hours in a 0.1 M NaHCO₃ electrolyte solution saturated with CO₂ (black trace). Excitation was provided by employing a 532 nm laser.