Systematic study of the N concentration effects on metal-free ORR electrocatalysts derived from corncob: Less is more

Abstract

We report nitrogen-doped biomass-derived porous carbon materials with great performance for the Oxygen Reduction Reaction (ORR) in alkaline media. The level of nitrogen doping in a simple pyrolysis of corncob (CC) was varied systematically, a 1:1 CC:urea ratio (CC_1U) gave the best performance in terms of onset potential (E_onset = 0.97 V vs. RHE), maximum current density (j_max = -3.22 mA cm⁻²), hydroperoxide ion yield (%HO₂^– = 1.18 % at 0.5 V), and electron transfer number (n = 3.86 at 0.5 V). Unexpectedly, for higher CC:urea ratios the doping decreases, instead of plateauing, with lower concentration of C-N sites and more sp² sites as determined by XPS, as well as lower specific surface area (SSA), while increasing both porosity and carbon (0 0 2) interplanar distance (d_(0 0 2)). These materials should be durable and robust, since their performance actually improved after accelerated degradation tests. This study proves that renewable “waste” can be upconverted into metal-free electrocatalysts for electrochemical energy conversion technologies and emphasizes the need for studying and controlling doping levels to enhance performance.