The evolution of hierarchical porosity in self-templated nitrogen-doped carbons and its effect on oxygen reduction electrocatalysis

Abstract

Pyrolitic self-templating synthesis is an effective method for creating hierarchically porous N-doped carbons. We study the evolution of microstructure in self-templated carbons derived from magnesium nitrilotriacetate, in the 600–1000 °C temperature range. The materials are characterised using N₂ adsorption, Hg intrusion, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, elemental analysis, scanning electron microscopy and transmission electron microscopy. The carbons display high specific surface areas (up to 1830 m² g⁻¹), and high pore volumes (up to 3.1 mL g⁻¹). Interestingly, each porosity type – micro, meso, and macro – evolves along its own route. Micropore growth is most significant between 600 and 700 °C, yet it slows down and stops around 800 °C; this indicates that micropores form by removal of tarry matter from the interstices between graphitic sheets, rather than by physical/chemical etching of these sheets. Mesopores, templated by spontaneously forming MgO nanoparticles, become dominant at 800 °C; further agglomeration of these particles leads to macropore templating at 900 °C. The porosity evolution is explained by the growth of MgO particles, as monitored by XRD broadening. Furthermore, the degree of disorder decreases with the pyrolysis temperature, most significantly between 700 and 800 °C, with the Raman I_D/I_G ratio dropping from 1.36 to 1.17. Correspondingly, the in-plane length of graphitic crystallites increases along the series, from 14 to 17 nm. Although the nitrogen content decreases with pyrolysis temperature, from 6.9 to 4.1 at%, the ratio between graphitic and pyridinic nitrogens remains constant. We then measure the performance of these carbons as electrocatalysts in the oxygen reduction reaction (ORR) at pH 13 using rotating disk electrode voltammetry and electrochemical impedance spectroscopy. Remarkably, the ORR activity trend is independent of nitrogen concentration or degree of disorder. Instead, it is governed by the microstructural parameters, most importantly surface area and microporosity.