Biomass liquid fuel cell

(A) Based on polyoxometalate catalyst

Catalysts play a key role in the process of low temperature biomass electrolysis conversion. The catalyst to be selected should have strong oxidation properties, which can oxidize various organic substrates and crack C-C bonds at low temperatures. POM is a kind of polyatomic structure formed by three or more transition metal oxygen ions connected by shared oxygen atoms. Due to its special structure, POM shows good physical and chemical properties. POM plays a good catalytic role in the hydrolysis and oxidation of organic matter and is considered as a promising catalyst for liquid fuel cells.

Recent work on POM-catalyzed biomass liquid fuel cells has been reported. Various types of POM (including Keggin-type and non-Keggin-type) are used for biomass oxidation and oxygen reduction reactions. Liu et al. studied a fuel cell using combustible agricultural waste (wheat straw and wine residue) as fuel and H3PMo12O40 as catalyst, and the corresponding power density of the fuel cell reached 111 mW· cm-2. Zhao and Zhu oxidized lignolsulfonates at 95~100 ℃ using H3PW12O40, H4PVW11O40, H4P-Mo11VO40, K5PV2Mo10O40 and H3PMo12O40, and reported high lignin conversion and power generation efficiency. In addition, The output power depends on the POM catalyst used, and the typical power density output ranges from 0.3 to 45 mW· cm-2.

The open circuit voltage is the key factor affecting the performance of LFFC. Obviously, a larger potential difference between the POM of the anode and cathode can improve the output performance of the fuel cell. For anodes, POM with strong oxidation capacity is preferred, but the electrode potential in the reduced state should be as low as possible. For the cathode, the POM should have a high electrode potential to ensure a large loop voltage between it and the anode, but should also be easily oxidized by oxygen for full regeneration.

Lewis acids (such as Sn4+, Fe3+, VO2+, and Cu2+) can be added to the reaction system as copromoters of POM. Liu et al. added Fe3+ and Cu2+ to cellulose-based fuel cells as POM facilitators, and found that the power density increased from 0.45 mW cm-2 to 0.72 mW cm-2. Metal ions that act as Lewis acids have been reported to help break the glycosidic bonds in cellulose, and are more effective than Bronster acids. Xu et al. studied the co-catalysis of FeCl3 and POM. The addition of Fe3+ significantly improves overall performance, as Fe3+ accelerates hydrolysis of the biomass and enhances electron transport.

(ii) Based on other REDOX ion pairs

In addition to POM, other REDOX ion pairs have also been reported for DBFC. Gong et al. reported on the biomass liquid fuel cells with Fe3+/Fe2+ REDOX pair for anode and VO2+ /VO2+ REDOX pair for cathode. The biomass is oxidized by Fe3+ on the anode side. The reduced Fe2+ releases electrons at the anode to become Fe3+ again. The researchers studied the oxidation of Fe3+/ Fe2+ ion pairs and biomass. FeCl3 acts as an oxidizing agent and catalyst for the oxidation of biomass and is reduced to Fe2+. Using glucose as a model compound for biomass, the reaction in the anodic solution can be written as:

Fe2+ then releases electrons to the anode.

The result is that Fe3+ is regenerated.

Electrons pass through an external circuit and are captured by VO2+ at the cathode, forming VO2+. The maximum current density of the battery reaches 100 mA· cm-2, and the energy conversion efficiency is as high as 76.5%.

Li and Song demonstrate a straw-based fuel cell that uses methyl violet as an electron carrier, nickel foam as an anode, and Pt/C as a cathode. When ZnCl2 solvent with a mass fraction of 65% is used, the fuel cell shows excellent performance. In addition, the addition of methyl violet in the battery system greatly improves the discharge performance, with a maximum power output of 0.3 mW· cm-2. Hibino et al. have developed a direct fuel cell based on cellulose. The battery uses SN0.9IN0.1P2O7-polytetrafluoroethylene (PTFE) composite electrolyte and Pt/C as the cathode and anode. The cellulose was pretreated with 85% H3PO4 and placed in the anode of the battery. The battery reaches a maximum power density of 32.7 mW· cm-2 at 250°C. In this electrochemical process, H2O acts as the main reactant and the end product of cellulose is CO2. Ding et al. reported another process using H3[PMo12O40] and FeCl3 as electron carriers and proton carriers to achieve integration of wheat grass pretreatment products into ethanol production and biomass conversion to electricity.

(3) Model-based biomass compounds

The properties of LFFC are closely related to the chemical structure of the biomass used. As shown in Table 1, a wide variety of biomass feedstocks are used as LFFC fuels. The study found that the use of polymerized biomass substances, such as cellulose, starch and hemicellulose, can produce a higher power density than the use of small molecules alcohols and acids. This is because most natural biomass polymers contain polyhydroxyl compounds, and hydroxyl groups play an important role in the photoredox reaction of POM and alcohols. In order to understand the effect of hydroxyl group on photoredox activity, Wu et al. studied the LFFC performance of a series of model biomass compounds with hydroxyl group number ranging from 1 to 6 as fuel, and found that the output power of the battery was strongly affected by the hydroxyl group content in the molecular structure of the biomass.