Spent Tea Leaves and Coffee Grounds as Potential Biocathode for Improved Microbial Electrosynthesis Performance

Microbial electrosynthesis (MES) has emerged as a sustainable energy platform capable of simultaneous wastewater treatment and valuable chemical production. The performance of MES, like other bioelectrochemical systems, largely depends on its electrode (cathode), providing the platform for microbial growth as well as electron transfer. However, most of the electrodes are expensive, and their nonrenewable characteristics, cost, and poisoning nature are major bottlenecks in MES commercialization. Thus, several efforts have been made to explore the potential of waste carbon-based electrodes to reduce carbon footprints as well as electrode manufacturing costs. In this study, the feasibility of using spent tea leaves (STL) and spent coffee grounds (SCG) as MES biocathode was tested. Different bioelectrochemical tests suggested improved MES performance with STL and SCG biocathode along with reduced electrode resistance and improved current density. A 1.5- and 2.0-fold increase in cyclic voltammetry (CV) current output was observed for SCG and STL, respectively, with substantial mediator peaks of high intensity indicating enhanced electrocatalytic activity. Enrichment of some fermentative and exoelectrogenic microbial classes such as Clostridia, Bacteroidia, and Deltaproteobacteria led to a 1.3- and 1.4-fold increase in butyrate production for SCG and STL cathode, respectively. These results demonstrate the potential of STL and SCG as MES cathode for improved energy and chemical production.