Recent Progress in the Influence of B- and N-Doped Primary and Secondary Hydrochar-Based Electrodes from Renewable Raw Materials on the Capacitive Performance for Energy Storage Applications

A comparative analysis was conducted to elucidate the roles of primary char (PC) and secondary char (SC) formation in modulating the effects of B- and N-doping on the physical-chemical characteristics of the resulting electrodes for energy storage applications. For the synthesis of biobased N-doped hydrochar-based electrodes, sucrose (Suc), Miscanthus (Mis) and potato peels (PPs) were utilized as carbon sources, whereas urea and pyrrole (Pyr) were chosen as N precursors; boron trioxide (B2O3) was selected as B precursor. In situ doping via hydrothermal carbonization (HTC) was used, followed by pyrolysis, obtaining N-doped hydrochars (N-HCs). During HTC, the feedstocks undergo a multitude of complex decomposition mechanisms leading to the formation of N-HCs, comprising different ratios of PCs and SCs. Subsequent pyrolysis and chemical ZnCl2 activation, produced N-doped pyrolyzed hydrochars (N-PHCs) with 5.6 wt % N from Suc and 4.8 wt % N from Mis applying urea. The N-PHCs from Suc and Mis employing Pyr, 3.1 and 4.2 wt % N, respectively were achieved. PPs-based syntheses focused mainly on activated carbons (ACs), B-doped AC yielded 0.2 wt% B, whereas co-doped B,N-AC reached 5.7 wt% N with 0.1 wt% B. Electrochemical testing revealed that specific capacitance increased, detecting 47.0 F·g−1 at 5 mV·s−1 for N-PHCs based on Suc and 3.5 F·g−1 at 100 mV·s−1 for those from Mis using urea. In case of PPs, the highest specific capacitances were recorded for B,N-co-doped AC, determining values of 51.7 F·g−1 at 100 mV·s−1 and 71.9 F·g−1 at 5 mV·s−1.