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.

The experimental results showed that the presence of more hydroxyl groups accelerated the photoelectrochemical reaction between POM and biomass, resulting in a higher degree of POM reduction, resulting in a higher power output. Therefore, biomass compounds with polyhydroxyl molecular structures such as starch, hemicellulose, cellulose, and even switchgrass and wood meal are ideal fuels for LFFC. Liu et al. used glucose as a biomass model compound to study the degradation mechanism of biomass by POM.

In fact, the POM degradation of biomass sugars by solar radiation is the result of two reactions: thermal effect and photocatalysis. Due to the strong absorption of visible and near-infrared light, the reduced POM can increase the overall temperature of the reaction system, thus significantly enhancing the oxidation of glucose. The glucose-fueled fuel cell generates electricity through photocatalytic oxidation, and at an operating voltage of 0.4 V, the battery has an energy efficiency of 36.7%, which means that consuming 1 kg of glucose will produce 1.43 kW· h of electricity.

(4) Based on non-biomass-based materials (sludge and coal)

In addition to conventional biomass, unconventional biomass fuels can also be used as fuel for LFFC, including sludge, inferior coal and grease. Sewage and sludge are hazardous wastes produced during the treatment of domestic and industrial wastewater, which contain proteins, lipids, humic acids, polysaccharides, and harmful organic and inorganic pollutants. Sludge treatment costs are high, accounting for more than 50% of the total cost of water treatment.

At present, research has reported different technical routes for treating sewage sludge to achieve energy reuse while reducing environmental impact. In the anaerobic digestion process, the sludge can be converted into biogas, liquid fuel or gaseous fuel. Two of the main technical challenges are the slow rate of methane production and the accumulation of volatile fatty acids that inhibit methane production. Sludge can also be burned directly or in conjunction with coal in coal-fired power plants, but at the same time can lead to fly ash production and toxic chemical emissions and other potential problems. Sewage and sludge power generation through MFC is an important way for municipal solid waste treatment and renewable energy development. However, due to the low power density of MFC, no practical applications have been seen so far. Zhang et al. studied the performance of sludge-based LFFC. After reacting at 100 ℃ for 24 h, most of the organic matter in the sludge was degraded. The power density of the cell can be as high as 50 mW· cm-2, which is 100 times higher than the output power of sludger-based MFC reported in the literature. Therefore, LFFC may be a promising sludge utilization method. The accumulation of soluble inorganic matter in sludge-based LFFC needs further study.

Low-quality coal (such as lignite) usually has only a lower calorific value of combustion and a lower commercial value. Therefore, there is an urgent need for technologies that can effectively utilize inferior coal and reduce environmental pollution. Weibel et al. use REDOX pair (Fe3+/Fe2+) as electrolyte to convert coal into electricity, but because the conversion efficiency is very low, the research results are not optimistic. Nunoura et al. developed an alkaline aqueous bio-carbon fuel cell that operates at high temperatures (250 ° C). However, the performance was not satisfactory and the equipment was quite complex. Zhang et al. use LFFC to obtain electricity from inferior coal without complex pretreatment. At 100~200 ℃, POM will gradually oxidize coal particles. The lignite based fuel cell has a power density of up to 120 mW· cm-2, which is the highest power density achieved by LFFC technology to date. The experimental results show that certain chemicals in low-quality coal can be converted into electricity. However, it is difficult to achieve complete oxidation and utilization of aromatic ring groups in coal by low temperature LFFC technology.

In addition to sludge and inferior coal, other types of organic matter, such as vegetable oils and animal fats, have also been used for the degradation of LFFC to generate electricity. The results show that LFFC oil treatment technology is basically impossible to achieve. There are two possible reasons for the limited degradation of oils and fats: first, the molecular bonds of oils and fats are relatively stable and difficult to crack; Secondly, the solubility of the oil in water is not very high, which increases the difficulty of contact between the catalyst and the organic matter in the water fuel cell. Liu et al. developed a system that combines anaerobic fermentation and LFFC, in which the fermentation products are used as fuel in the second step. This combined system with food residue fermentation has a high power generation efficiency (34%) and a short processing time. Since the chemicals in food residues and lignite are more complex than polyhydroxyl polymer biomass, their treatment requires the development of more efficient pretreatment methods, while new catalysts may also help improve the performance of fuel cells.

5. Strengths and Challenges

Biomass based LFFC technology is a new strategy for renewable energy development, showing very promising results. Direct biomass LFFC technology offers many advantages over conventional fuel cell technology.

First, with this pioneering fuel cell technology, biomass such as grass, wood, algae, agricultural waste and even sewage sludge can be directly converted into electricity at low temperatures.

Secondly, biomass-based LFFC uses liquid POM instead of precious metal-based electrocatalysts as biomass degradation catalysts. Because POM catalysts are very stable and have low sensitivity to most organic and inorganic contaminants, biomass can be used as fuel without the need for pre-purification or treatment.

Third, LFFC technology can achieve high power output.

Finally, LFFC technology has little impact on the environment. In theory, only CO2 and H2O are produced by biomass oxidation.

Therefore, in the future, biomass-based LFFC is expected to become a low-cost alternative to small power plants to achieve sustainable energy conversion and production from biomass.

LFFC still faces challenges in research and commercialization:

(1) There is still a need to develop efficient catalysts. The alternative catalyst should have a strong oxidation capacity, which can oxidize various organic substrates and crack C-C bonds at low temperatures.

(2) The performance of LFFC still needs improvement. The experimental results show that LFFC is difficult to convert 100% biomass to CO2, and the oxidation reaction rate is relatively slow. The kinetics of the electrode reaction must be studied to maximize the power output.

The separation of inorganic and organic residues from catalysts is also a challenge for the commercial use of LFFC.

(4) The life of LFFC should be considered. POM will corrode the graphite electrode plate and proton exchange membrane, shortening the service life of the battery pack.

(5) The structure of the battery and battery stack must be carefully designed to reduce the internal resistance.

One of the challenges for biomass-based LFFC is how to completely oxidize the biomass to improve overall energy efficiency and minimize the residue of organic waste. The chemical composition of biomass is very complex and may contain lignin, cellulose, hemicellulose, aliphatic resins, proteins, and many other organics. Therefore, it is often difficult to completely oxidize all the biomass components in the solution at low temperatures. However, complete oxidation will not only improve the utilization of biomass, but also significantly reduce the cost of residue treatment. The following methods can be used to improve the oxidation of biomass in the battery anode. First, the higher temperature is conducive to the deep oxidation of the biomass in the anode solution. It has been reported that lignin can be completely oxidized by POM at high temperatures. The second is the use of POM catalysts with high electrode potentials, such as vanadium-doped POM. A more effective option is to use adjuvants (such as Pt/C particles) to further increase the degree of reaction and improve the overall conversion of biomass to CO2. In the reaction using solar energy or light instead of heating, it is still necessary to increase the oxidation rate and degree.