Cooperative electrocatalytic N2 reduction based on WS2-covered Co9S8 hexagonal heterostructures derived from ZIF-67@POMs

Abstract

Electrochemical N2 reduction reaction (NRR) to produce NH3 at ambient conditions is a promising route to replace the energy-intensive Haber–Bosch process, but electrochemical NRR with satisfied efficiency remains challenging. Therefore, a number of efforts have been committed to develop high-performance NRR electrocatalysts for highly efficient NH3 synthesis. Herein, Co9S8 nanosheets with a WS2 outer layer (Co9S8@WS2) are designed and prepared from ZIF-67@polyoxometalates (polyoxometalates , H3PW12O40·xH2O) and used for NRR at ambient temperature and pressure. The successful introduction of S-vacancies can adjust the electronic structure of the material by forming a electron-deficient environment, can make N2 molecules easily adsorbed and activated on the W active site of the catalyst surface, and thus, can promote the electrocatalytic NRR activity of the catalyst. In 0.1 M KOH, the optimized Co9S8@WS2-900 exhibits enhanced Faradaic efficiency (46.81%) and good NH3 yield (50.25 μg h−1 mgcat.−1) at −0.4 V vs. a reversible hydrogen electrode, exceeding many recently reported cobalt- or tungsten-based materials. The superior electrocatalytic activity is attributed to the unique composite structures and synergistic effect of electronic couplings between Co9S8 and sulfur-rich vacancies WS2. A density functional theory calculation indicates that the inert N2 can be activated by bare W atoms on the rim of the S-vacancies in Co9S8@WS2. The protonation of N2 to form N–NH∗ species is the potential-limiting step (ΔG = 1.05 eV). Therefore, the unique bimetallic sulfide clad architecture and rich S vacancies of Co9S8@WS2 provide the reference and inspiration for better designing of high-efficiency nitrogen-fixing catalysts.