Deciphering electrochemically promising electron-shuttling characteristics of hydrolysable tannin-abundant Galla chinensis for bioenergy generation in microbial fuel cells

This first-attempt study revealed the feasibility assessment that medicinal herbs with abundant hydrolysable tannin (i.e., Galla chinensis (gallnut)) were highly promising to maximize bioenergy extraction. The cyclic voltammetric profiles of both gallnut and tannic acid (TA) showed significant redox potential peaks for comparison. Moreover, higher extraction temperature at 85 °C could enhance the hydrolysis efficiency of tannin contents and possibly further resulting in higher yields of gallic acid (GA). The power density analysis of microbial fuel cells (MFCs) indicated that the supplementation of gallnut water extract (GWE) could significantly stimulate the power generation of MFCs. This might be resulted from the sufficient hydrolysis of GWE into GA as electron shuttles (ESs) for power stimulation. Compared with other herbal extract, GWE owned not only the most abundant content of antioxidants, but also the best stimulation capability on power generation (e.g., 67.77 mW m⁻² at 1000 mg L⁻¹ GWE) for bioenergy/biorefinery applications.