Investigating Co and Fe Single-Atom (SA) Decorated NS-Doped Reduced Graphene Oxide (Co SA/NS-rGO, Fe SA/NS-rGO) Catalysts for NOx
- to NH3 Pulsed Electroreduction
Authors: Şimşek Kaan , Kaya Sarp 
Published: August 2024

Electrochemical NH3 production has attracted much attention due to its potential to compensate for and supplement the Haber-Bosch process that produces hundreds of billions of kg of NH3 annually.1 One of the proposed electrochemical pathways to produce NH3 is the N2 reduction reaction. However, this reaction suffers from a high bond dissociation energy of 945 kJ mol−1, low NH3 Faradaic efficiency (F.E.) at high overpotentials, and NH3 quantification problems, limiting the extent of its applications.2,3 On the other hand, other N-containing species, NO3
- and NO2
-, that are commonly found in sewages and agricultural water, hold great potential for electrochemical NH3 production owing to their high NH3 yield rate and F.E. as well as building a self-sustaining NH3 production cycle working tandem with denitrification processes.1,4

In this study, we investigated Co and Fe single-atom (SA) decorated NS-doped reduced graphene oxide (Co SA/NS-rGO and Fe SA/NS-rGO) catalysts for nitrate reduction reaction (NO3RR) and nitrite reduction reaction (NO2RR). Both materials have defect-rich properties owing to preserving the nature of doped graphene materials even after the metal incorporation. The presence of the layered structure of graphene and homogenous distribution of Fe and Co SAs in the doped graphene basal plane is shown through High-Resolution Scanning Transmission Electron Microscopy (HR-STEM) measurements. In addition to the Co and Fe SAs on the NS-rGO planes, SEM and X-ray Photoelectron Spectroscopy (XPS) investigations demonstrated that CoS and FeS could be simultaneously formed.

We performed NO3RR and NO2RR experiments in 0.1 M KOH + 0.1 M KNO3 and 0.1 M KOH + 0.1 M KNO2 electrolytes with constant Ar purging in a glass H-Cell with a Nafion proton exchange membrane. We obtained NH3 with 65% F.E. at -0.4 V and a maximum of 2 mol s-1 molM
-1 yield rate using Co SA/NS-rGO catalyst while with 60% F.E. at -0.5 V and 2.3 mol s-1 molM
-1 yield rate using Fe SA/NS-rGO catalyst for NO3RR. Furthermore, the ∼10% NO2
- F.E. that is obtained for NO3RR experiments at -0.3 V and a significant increase in NH3 F.E. after -0.3 V for NO2RR indicates that NO3
- to NO2
- reduction takes place at this potential and our catalysts follow a two-step electron transfer mechanism. Moreover, linear sweep voltammetry (LSV) measurements indicated an onset potential of -0.4 V and -0.5 V for Co SA/NS-rGO and Fe SA/NS-rGO, respectively, for the NO3RR which are in excellent agreement with NO3RR F.E. results. We demonstrate that after prereduction at high negative potentials and with optimizations in experimental conditions, one can obtain >80% NH3 F.E. for NO3RR.

To further increase the NH3 yield, three different pulsed electroreduction studies such as applying a potential at (1) the open circuit potential (OCP), (2) NO2
- reduction potential, (3) Co2+ or Fe2+ red-ox potentials and NH3 reduction potentials for 1-5 s intervals were performed. On Co SA/NS-rGO, an increased NH3 F.E. of ∼100% at -0.4 V was obtained under pulsed conditions. Also, by modulating NO3
- to NO2
- conversion to more negative potentials by pulsed electroreduction, significantly higher NH3 F.E. values were obtained for both catalysts. The higher NH3 F.E. and lower overpotential of Co SA/NS-rGO compared to Fe SA/NS-rGO could be attributed to the CoS formation that is more stable than FeS.

References:

(1) Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a Century of Ammonia Synthesis Changed the World. Nature Geosci
2008, 1 (10), 636–639. https://doi.org/10.1038/ngeo325.

(2) Suryanto, B. H. R.; Du, H.-L.; Wang, D.; Chen, J.; Simonov, A. N.; MacFarlane, D. R. Challenges and Prospects in the Catalysis of Electroreduction of Nitrogen to Ammonia. Nat Catal
2019, 2 (4), 290–296. https://doi.org/10.1038/s41929-019-0252-4.

(3) Andersen, S. Z.; Čolić, V.; Yang, S.; Schwalbe, J. A.; Nielander, A. C.; McEnaney, J. M.; Enemark-Rasmussen, K.; Baker, J. G.; Singh, A. R.; Rohr, B. A.; Statt, M. J.; Blair, S. J.; Mezzavilla, S.; Kibsgaard, J.; Vesborg, P. C. K.; Cargnello, M.; Bent, S. F.; Jaramillo, T. F.; Stephens, I. E. L.; Nørskov, J. K.; Chorkendorff, I. A Rigorous Electrochemical Ammonia Synthesis Protocol with Quantitative Isotope Measurements. Nature
2019, 570 (7762), 504–508. https://doi.org/10.1038/s41586-019-1260-x.

(4) Duca, M.; Koper, M. T. M. Powering Denitrification: The Perspectives of Electrocatalytic Nitrate Reduction. Energy Environ. Sci.
2012, 5 (12), 9726–9742. https://doi.org/10.1039/C2EE23062C.