Electrocatalytic ammonia synthesis catalyzed by mesoporous nickel oxide nanosheets loaded with Pt nanoparticles

Abstract

Owing to its cost-effectiveness and adjustable eight-electron distribution in the 3d orbital, nickel oxide (NiO) is considered an effective electrocatalyst for an ambient electrochemical nitrogen reduction reaction (NRR). However, because of the low conductivity of the transition metal oxide electrocatalyst, its application in this field is limited. In this study, we found that the doping of NiO nanosheets with a small amount (3–10 nm) of Pt nanoparticles (Pt/NiO-NSs) leads to considerable improvements in the Faradaic efficiency (FE) and NH3 yield compared with those obtained using pure NiO, breaking the common perception that commercial Pt-based electrocatalysts demonstrate little potential for NRR due to their high hydrogen evolution tendency. In a 0.1 mol/L Na2SO4 solution at −0.2 V vs. RHE, a typical Pt/NiO-2 sample exhibits an optimum electrochemical NH3 yield of 20.59 μg h–1 mg–1cat. and an FE of 15.56%, which are approximately 5 and 3 times greater, respectively, than those of pure NiO nanosheets at the same applied potential. X-ray photoelectron spectroscopy analysis revealed that Pt in Pt/NiO-NSs exist as Pt0, Pt2+, and Pt4+ and that high-valence Pt ions are more electropositive, thereby favoring chemisorption and the activation of N2 molecules. Density function theory calculations showed that the d-band of Pt nanoparticles supported on NiO is significantly tuned compared to that of pure Pt, affording a more favorable electronic structure for NRR. The results of this study show that Pt can be an effective NRR electrochemical catalyst when loaded on an appropriate substrate. Most importantly, it provides a new synthetic avenue for the fabrication of highly active Pt-based NRR electrocatalysts.