Work function regulation of nitrogen-doped carbon nanotubes triggered by metal nanoparticles for efficient electrocatalytic nitrogen fixation

Cost-effective carbon-based materials are appealing candidates for the electrochemical nitrogen reduction reaction (NRR), while the large bandgap results in a low reactivity and selectivity of the NRR. Work function (W) regulation, altering the electron-transfer ability of carbon materials, is regarded as an encouraging descriptor to enhance the reaction kinetics of the electrochemical NRR, but it has been rarely investigated. To address this issue, Mott-Schottky heterostructural metal/nitrogen-doped carbon nanotubes (M@NCNTs) were designed. Work function theory predicts that systems with a lower W value require smaller extra energy to activate adsorbed N-2 molecules. As expected, Ni@NCNTs with the lowest W value displayed the highest faradaic efficiency of 7.33% compared to the other samples. Theoretical simulations and orbital component analysis revealed that different encapsulated metals could tune the work function of M@NCNTs, especially by inducing an upshift of the Fermi level of the outer NCNTs and accelerating the transfer of electrons from the catalyst surface to the adsorbed N-2 through the upshifted C-N (pi) orbitals, thus guaranteeing the effective activation of N-2 molecules. This work provides a guideline for the rational design of NRR catalysts by tuning the work function.