Coupling biomass pretreatment for enzymatic hydrolysis and direct biomass-to-electricity conversion with molybdovanadophosphoric heteropolyacids as anode electron transfer carriers

Owing to their acidity, oxidizing ability and redox reversibility, molybdovanadophosphoric heteropolyacids (Hn+3PMo12-nVnO40, abbreviated as PMo12-nVn) were employed as electron transfer carriers for coupling biomass pretreatment for enzymatic hydrolysis and direct biomass-to-electricity conversion. In this novel coupled process, PMo12-nVn pretreatment that causes deconstruction of cell wall structure with PMo12-nVn being simultaneously reduced can be considered as the charging process. The reduced PMo12-nVn are further re-oxidized with release of electrons in a liquid flow fuel cell (LFFC) to generate electricity is the discharging process. Several Keggin-type PMo12-nVn with different degree of vanadium substitution (DSV, namely n) were prepared. Compared to Keggin-type phosphomolybdic acid (PMo12), PMo12-nVn (n = 1-6) showed higher oxidizing ability but poorer redox reversibility. The cellulose enzymatic digestibility of PMo12-nVn pretreated wheat straw generally decreased with increase in DSV, but xylan enzymatic digestibility generally increased with DSV. PMo12 pretreatment of wheat straw at 120 degrees C obtained the highest enzymatic glucan conversion (EGC) reaching 95%, followed by PMo11V1 pre-treatment (85%). Discharging of the reduced heteropolyacids in LFFC showed that vanadium substitution could improve the maximum output power density (P-max). The highest P-max was obtained by PMo9V3 (44.7 mW/cm(2)) when FeCl3 was used as a cathode electron carrier, while PMo12 achieved the lowest P-max (27.4 mW/cm(2)). All the heteropolyacids showed good electrode Faraday efficiency (>95%) and cell discharging efficiency (>93%). The energy efficiency of the coupled process based on the heat values of the products and generated electric energy was in the range of 18%-25% depending on DSV. PMo12 and PMo11V1 seem to be the most suitable heteropolyacids to mediate the coupled process. (C) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Automated summary

Molybdovanadophosphoric heteropolyacids can act as electron transfer carriers in biomass pretreatment, enabling charging and discharging processes that generate electricity when coupled with a liquid flow fuel cell. The degree of vanadium substitution affects enzymatic digestibility of biomass and can improve the maximum output power density of the cell.