General approach for the synthesis of nitrogen-doped carbon encapsulated Mo and W phosphide nanostructures for electrocatalytic hydrogen evolution

Abstract

A general strategy for the synthesis of the core-shell nanostructure of nitrogen-doped carbon encapsulated MoP and WP (MoP@NC and WP@NC) and the electrochemical hydrogen evolution reaction (HER) are demonstrated. The synthetic procedure involves the self-assembling of polyoxometalate (M = Mo/W) and phytic acid on a polyetheleneimine backbone and the subsequent pyrolysis of the self-assembled supramolecular aggregates in an inert atmosphere without a traditional phosphidating agent. MoP@NC has a quasi-spherical shape with a MoP core (57 nm) and nitrogen doped porous carbon shell, whereas WP@NC has a nitrogen-doped carbon coated rodlike nanostructure. MoP@NC has a large amount of pyridinic (∼59%) and Mo-bonded (∼33%) nitrogen. MoP@NC is highly active toward hydrogen evolution reaction (HER) and delivers the benchmark current density 10 mA/cm² at an overpotential of 52, 106, and 171 mV in acidic, alkaline and neutral pH, respectively. It shows a Tafel slope of 49 mV/dec, high turnover frequency (0.28 s^-1 at η = 100 mV), and faradaic efficiency (96%) in acidic electrolyte. MoP@NC has remarkable durability in acidic and alkaline pH with a negligible increase in overpotential after 1000 extensive repeated potential cycles. The encapsulating nitrogen-doped carbon shell protects the active catalyst from corrosion and the catalyst retains its phase purity and structural integrity even after 10 h of long-time hydrogen evolution at constant potential. The outstanding HER activity of MoP@NC is accounted for by the small particle size, large surface area, and strong chemical coupling between MoP and nitrogen-doped carbon.