Fig.1 Renewable energy drives electrochemical transformation of biomass to fine chemicals and fuels

Fig.2 Organic acids, platform molecules derived from lignocellulose, stand at the crossroad of lignocellulose conversion route to advanced biofuels[34]

Fig.3 Fufural and 5-hydroxylmethylfufural (HMF) are representative platform molecules that can be (electrochemically) converted to monomers for renewable polymers (FA, fufuranic acid; FDCA, 2,5-furandicarboxylic acid) and biofuels (MF, 2-methylfuran; DMF, 2,5-dimethylfuran)[23]. Fufural can also be converted to levulinic acid (the dashed arrow), then undergoes another route of transformation as shown in Fig.2

Fig.4 Potential profile in an electrolyzer for electrochemical conversion of biomass (a). CEM denotes cation exchange membrane, which is a representative separator between anolyte and catholyte[49]. Anodic overpotential (ηa) and cathodic overpential (ηc) as a function of cell current (b)[50]

Fig.5 General guidlines for the electrochemical reactor design[55]

Fig.6 A fishbone design scheme of electrochemical reactors for lignocellulose conversion

Fig.7 Electrocatalytic reduction of itaconic acid to methyl succinic acid, and competitive hydrogen evolution reaction(a); Cyclic voltagram of pure supporting electrolye (H2SO4, red curves) and itaconic acid solution (black curves) with Ni cathode(b), and with Pb cathode(c)[62]

Fig.8 3 representative types of electrochemical cells with different configurations

Fig.9 Single pass of electrolyte in electrochemical cell(a); multiple passes of electrolyte in cell, with a hydrocyclone as a representative separation unit(b)[58]

Fig.10 Consecutive reduction and oxidation of levulinic acid stream in a single electrochemical cell for the synthesis of octane

Fig.11 Schematic of a press-filter type electrochemical reactor for lignin depolymerization(The components of the reactor are all of commerical sources. The planar porous anode is drilled with holes of 3 mm diameter (lower left), allowing the electrolyte to flow through the anodes. Spacers with modified flow channels (lower right) could feed the electrolyte to the anodes in the interior of the reactor[84])

Fig.12 Flow scheme showing the electrochemical membrane reactor (ECMR) for lignin depolymerization(a)(AEM is anion exchange membrane, NFM is nanofiltration membrane); 3 D scheme of the anode compartment with static mixers right next to the Ni anodes to promote the liquid flow and in-situ product removal through the ceramic nanofiltration (NF) membrane(b); ECMR has better permeability through the NF membrane compared with post-reaction filtration of the reaction medium(c); Gel permeation chromotography shows that ECMR process has improvent in yield of small molecular components(d)[77]

Fig.13 Picture of the bench scale electrochemical cells for the continuous oxidation of HMF to FDCA, an important monomer for bioplastics[88]