Free-Standing Carbon Electrodes for the Oxygen Evolution Reaction

Abstract

N-functionalized hydrothermal carbon materials (N-HTC) with spherical morphology were prepared by hydrothermal synthesis utilizing glucose and urotropine as starting materials, which makes the synthesis cost-efficient, environmental friendly and allows simple scale-up. The molar ratio of glucose and urotropine has been varied and the powder materials were pelletized, resulting in mechanically stable disc electrodes. By increasing the amount of nitrogen, N-contents from 5 wt% up to maximal 8 wt% were obtained after pyrolysis at 1000 °C. The pellet can then be used as an anode for alkaline water splitting. In literature, there are still no direct evidence of metal-free carbon as electrocatalyst for oxygen evolution reaction. By comparing the N-functionalized sample and without functionalization, we found that the former has superior stability under water splitting condition (relative high potential). Up to one week of stable oxygen production was achieved. The result indicates that nitrogen facilitates oxygen evolution reaction. Besides, the nitrogen species also hinders carbon corrosion, proved by a simple Hoffmann cell measurement. Pellets with different synthesis condition as well as nitrogen content are compared in order to get the optimized synthesis condition. For better understanding, methylene blue adsorption and mercury intrusion porosimetry are used to characterize the pore structure of the carbon materials. The relative work function and conductivity of the carbon pellet are determined by Kelvin-probe and Van-der-Pauw methods, respectively. A conventional three-electrode assembly is used to characterize the electrochemical performance, including activity, stability as well as mass transport property by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy (EIS). A transmission line equivalent circuit model is used to fit the EIS complex spectra, where the diffusion resistance and reaction resistance can be determined. The two resistances are very important reaction key performance indicator for oxygen evolution reaction, as it can distinguish a diffusion process from a reaction. The gas products as well as electrolyte after electrochemical test can be analysed by mass spectrometry, ESI-MS, UV-VIS spectroscopy, etc. From the mass spectrometry results, the mole fraction of oxygen in the total product gas can be determined, determining the selectivity of the reaction. Besides, water diffusion NMR measurements before and after electrochemical test shows changes in the water interaction, which can be correlated to Nitrogen species changes in the electrode. This study demonstrates that N-functionalization improves the stability of HTC for oxygen evolution reaction providing a promising pathway for the future energy conversion and storage.