Engineering Porous Biochar for Electrochemical Energy Storage

In recent years, porous carbon-based materials have demonstrated their potential as electrode materials, particularly as supercapacitors for energy storage. The specific capacity of a carbon-based material is strongly influenced by its porosity. Herein, activated biochar (BCA) from millet was prepared using ZnCl2 as an activator at temperatures of 400, 700, and 900 °C. Activation was achieved through wet and dry impregnation of millet bran powder particles. The porosity of BCAs was assessed by determining the iodine and methylene blue numbers (NI and NMB, respectively), which provide information on microporosity and mesoporosity, respectively. Characterization of the BCAs was carried out using Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and cyclic voltammetry. The data show that the BCA prepared at 700 °C following dry impregnation, P700(p), has the highest NI and the highest geometric mean value (ñ=√(N_I ×N_MB ) ), a descriptor we introduce to characterize the overall porosity of the biochars. P700(p) biochar exhibited remarkable electrochemical properties and a maximum specific capacitance of 440 F.g-1 at a current density of 0.5 A/g. These performances are best correlated with ñ. Moreover, the capacitive retention increases with cycling, up to 130%, thus suggesting electrochemical activation of the biochar during the galvanostatic charge-discharge process. To sum up, the combination of pyrolysis temperature and the method of impregnation permitted to obtaining a porous biochar with excellent electrochemical properties, meeting the requirements of supercapacitors and batteries.