Nitrogen‐containing hydrothermal carbon (N‐HTC) materials of spherical particle morphology were prepared by means of hydrothermal synthesis with glucose and urotropine as precursors. The molar ratio of glucose to urotropine has been varied to achieve a continuous increase in nitrogen content. By raising the ratio of urotropine to glucose, a maximal nitrogen fraction of about 19 wt % could be obtained. Decomposition products of both glucose and urotropine react with each other; this opens up a variety of possible reaction pathways. The pH has a pronounced effect on the reaction pathway of the corresponding reaction steps. For the first time, a comprehensive analytical investigation, comprising a multitude of analytical tools and instruments, of a series of nitrogen‐containing HTC materials was applied. Functional groups and structural motifs identified were analyzed by means of FTIR spectroscopy, thermogravimetric MS, and solid‐state NMR spectroscopy. Information on reaction mechanisms and structural details were obtained by electronic structure calculations that were compared with vibrational spectra of polyfuran or polypyrrole‐like groups, which represent structural motifs occurring in the present samples.

The thermal treatment of an activated carbon/chars with a nitrogen precursor is not a sustainable nor an efficient method for the incorporation of N into the carbon structure. This study proposes the use of hydrothermal carbonization (HTC) as an environmentally friendly method for the incorporation of N into the bulk of carbon materials. The authors propose the following sequence for the synthesis of Nenriched carbon materials (NCM) for energy storage applications: HTC of the N precursor and biomass ! activation of N-hydrochar (N-HC). To investigate the proposed method, HTCs of spent coffee grounds (SCG) with N precursors (urea and alanine) were conducted at 220 C for 5 hours. The resulted N-HCs were subjected to a mild thermal activation via pyrolysis at 600 C for 2 hours. The results showed that HTC enhances the incorporation of N into the carbon matrix via many reactions, for example, the Maillard reaction (MR) or the Mannich reaction, which may accompany the formation of HC via the solved-intermediate pathway or the solid-to-solid pathway, respectively. However, adjusting some parameters before HTC, for example, the concentration of the N precursor and the pH value of the slurry is important to avoid a significant reduction in the N-HC yield. The proposed method led to the synthesis of NCM with a N content of 10.3wt% The X-ray photoelectron spectroscopy (XPS) confirmed the incorporation of N into the bulk of NCM and showed a significant increase in the content of heterocyclic N compounds (pyridinic N, pyrrolic N, and graphitic N) in the NCM. The incorporation of N via the proposed method significantly improved the electrochemical properties of NCM as the values of the specific capacitance and the electrical conductivity in the NCM increased by five times and four times, respectively.

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The oxygen evolution reaction (OER) is one of the bottlenecks of electrochemical water splitting. Metal‐free carbons from biomass are highly abundant and can be easily synthesized. Their low price, high conductivity and functionalization makes them promising materials. Herein, we report about free‐standing carbon electrodes as electrocatalysts for the OER. In contrast to powder‐based catalysts, free‐standing electrodes not only avoid additives, but also facilitate post analysis and better reflect industrial conditions. Here, the performance of pure carbon electrodes is compared to those of N‐functionalized ones. Utilizing several analytical techniques, the difference in performance can be rationalized by physical properties. Especially, the analysis of the gaseous products is shown to be of crucial importance. It reveals that N‐doped carbons generate more oxygen and are more robust against carbon corrosion. This illustrates the importance of measuring selectivity especially for carbon electrocatalysts, as higher currents do not necessarily result in higher catalytic activity.