A Comparative Study on the Influence of N-Doped Primary and Secondary Hydrochars on the Electrochemical Performance of Biobased Carbon Electrodes for Energy Storage Application

Abstract

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 ⋅ cm−1 was obtained with N-PHC based on Suc. Simultaneously, enhanced average specific capacitances of 47.0 F ⋅ g−1 at 5 mV ⋅ s−1 and 3.5 F ⋅ g−1 at 100 mV ⋅ s−1 are ascertained with N-PHCs from Suc and Mis, respectively.