Abstract Potato peels (PPs) as waste biomass were selected as the biobased carbon source for this study, using urea as N precursor and boron trioxide as B precursor for the “in situ doping” via hydrothermal carbonization (HTC). During HTC, the feedstocks decompose over a wide range of complex chemical degradation mechanisms that finally form single B‐ and N‐ as well as B,N‐co‐doped hydrochars (HCs). Upon chemical ZnCl2 activation, the single B‐doped activated carbon (AC) possessed a maximum B content of 0.2 wt%, whereas co‐doped B,N‐AC had the highest N content of 5.7 wt% with a B content of 0.1 wt%. The influence of single and B,N‐co‐doping on the physical‐chemical material properties of the AC electrodes was analyzed and compared, in combination with its effect on the electrochemical performance for energy storage application. Compared to pristine AC derived from PPs, the B‐doped and B,N‐co‐doped AC depicted increased electrical conductivity (EC) values of 50.3 S ⋅ m−1 and 34.0 S ⋅ m−1, respectively. In addition, the B,N‐co‐doped AC unveiled the highest average specific capacitances of 51.7 F ⋅ g−1 at 100 mV ⋅ s−1 and of 71.9 F ⋅ g−1 at 5 mV ⋅ s−1 outperforming the specific capacitance values of the reference material AC from peat.

The synthesis of biobased N-doped carbon-based electrodes from the feedstocks sucrose and Miscanthus using pyrrole as the N precursor is reported. Apart from the biomasses, pyrrole plays a key factor in the material transformation. It has been investigated whether pyrrole is also suitable in conjunction with a biomass under subcritical water conditions. The N content increases with a higher mass ratio of pyrrole between 1.4 and 4.2%. The physical-chemical properties of the electrodes are analyzed, and the potential application in energy storage is further assessed.

The hydrothermal carbonization (HTC) technique and subsequent pyrolysis were applied. Herein, two different types of biobased feedstocks, sucrose (Suc) and Miscanthus (Mis), were chosen. Urea served as N precursor for the in situ doping. HTC of Suc and Mis with urea leads to N-doped hydrochars (N-HCs). Suc and Mis decompose in different, complex degradation pathways, which leads to the emergence of N-HCs consisting of distinct ratios of N-containing primary chars (N-PCs) and secondary chars (N-SCs). After pyrolysis, maximum N contents of 5.6 wt % and 4.8 wt % of the N-doped pyrolyzed hydrochar (N-PHC) electrodes from Suc and Mis, respectively were reached. The role of PC and SC formation on the impact of N-doping in the context of physicochemical properties of the N-PHCs was compared with each other. In this study, they were tested with respect to the influence of N-PCs and N-SCs on their electrochemical performance in energy storage application. It turned out that the electrical conductivity (EC) and specific capacitances increased. Highest EC value of 129.1 S & sdot; cm-1 was obtained with N-PHC based on Suc. Simultaneously, enhanced average specific capacitances of 47.0 F & sdot; g-1 at 5 mV & sdot; s-1 and 3.5 F & sdot; g-1 at 100 mV & sdot; s-1 are ascertained with N-PHCs from Suc and Mis, respectively. Sucrose (Suc) and Miscanthus (Mis) were each added with urea. Via hydrothermal carbonization (HTC) N-doped hydrochars (N-HCs) with primary char (PC) and secondary char (SC) phases were obtained. Pyrolysis results in N-doped pyrolyzed hydrochars (N-PHCs), which have promising physical-chemical properties for energy storage application. image

AbstractThe 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.

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 N-enriched 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 degrees C for 5 hours. The resulted N-HCs were subjected to a mild thermal activation via pyrolysis at 600 degrees 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.

The functionalization of sustainable carbon materials and their application in energy storage systems attract more and more relevancy. Bakery waste and spent coffee grounds were chosen as abundant organic residues and found to be suitable starting materials for hydrothermal carbonization and a subsequent chemical activation obtaining carbon contents of > 88%. In situ doping of the hydrochars during carbonization has proven to be a successful method for insertion of Fe2O3-, Fe3O4- and MnO2-Nanoparticles into the carbon matrix, supported by XRD analysis and SEM images. Chemical activation with K2CO3 led to iron contents up to 18% of iron and around 8% of manganese, respectively, in the corresponding activated carbon. Electrochemical characterization revealed overall higher specific capacitance for activated carbons derived from spent coffee grounds, with a highest of 87F*g−1. In contrast, the highest specific capacitance measured for activated carbons originated from bakery waste was 40,3F*g−1.

Despite large varieties of commercially available electrodes, only few are suitable for electro-active bacterial colonization during biofilm formation in microbial fuel cells (MFCs), and most of these electrodes are cost prohibitive. Hence there is need to search for low-cost alternative electrodes for MFCs. Pyrochars were produced in this study by pyrolysis (600 degrees C and a continuous flow rate of 3 L/min of nitrogen gas for 30 min) and subsequently steam and potassium hydroxide (KOH) activation of the pyrochar at 600 degrees C were carried out accordingly. Physicochemical, structural, and electrochemical properties of the activated and non-activated pyrochars were determined according to standardized analytical methods. According to BET, 1626 m(2) g(-1) surface area and 14.74 A pore diameter were obtained from the KOH-activated pyrochar which was also the most conductive (0.26 S m(-1)). Chemical activation of pyrochar with KOH resulted in increased electrical conductivity (EC), pore diameter, and most importantly the material's surface area according to the findings. In conclusion, KOH-activated corncob pyrochar holds potentials for producing electrode materials with desirable characteristics for successful application in MFC compared to the non-activated and steam-activated pyrochars of the same biomass.

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.