(Invited) Understanding Metal-Based Catalysts for Electrochemical Ammonia Synthesis

Abstract

Electrochemical reduction of N2 to NH3 under ambient conditions may provide an alternative to the Haber−Bosch process for sustainable NH3 synthesis when powered by solar- or wind-generated electricity. However, the development of this process has been hindered by the lack of efficient electrocatalysts for the N2 reduction reaction (N2RR), due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). Here we present our recent studies of electrochemical ammonia synthesis on Pd and Fe/Fe-oxide catalysts. First, we found that Pd nanoparticles can catalyze the electrohydrogenation of N2 to NH3 in 0.1 M phosphate buffer solution (PBS) with a high yield rate and a Faradaic efficiency of 8.2% for NH3 production at 0.1 V vs the reversible hydrogen electrode (RHE). In operando X-ray absorption spectroscopy (XAS) study revealed the formation of α-phase Pd hydride during the reaction. The unique N2RR activity of Pd at low overpotentials outperforms that of Au and Pt nanoparticle catalysts, which is attributed to a new electrohydrogenation mechanism that involves an electrochemical formation of PdHx and a chemical hydride transfer from PdHx to *N2 to form *N2H, thus to lower the energy barrier for the rate-limiting step of N2RR. Second, we developed an Fe/Fe3O4 catalyst for N2RR, which was prepared by oxidizing an Fe foil at 300 oC in O2 atmosphere followed by in situ electrochemical reduction until a stable state was reached. The derived Fe/Fe3O4 catalyst shows greatly enhanced activity and selectivity for N2RR under ambient conditions than the original Fe foil, achieving a Faradaic efficiency of 8.29% for NH3 production at −0.3 V vs RHE in 0.1 M PBS electrolyte, which is around 120 times higher than that of the Fe foil. The high selectivity is enabled by an enhancement of the intrinsic (surface-area-normalized) N2RR activity as well as an effective suppression of the undesired HER. Comparative studies indicate that the N2RR activity may depend on the Fe/Fe-oxide ratio. Furthermore, the N2RR selectivity of the Fe/Fe3O4 catalyst is superior to that of Fe, Fe3O4, and Fe2O3 nanoparticle catalysts, which may provide new insights into the understanding and development of efficient electrocatalysts for ambient NH3 synthesis.