Study On Low Temperature Biomass Based Redox Flow Fuel Cell

Biomass energy is one of the most important renewable energy sources in the world today and in the future.Biomass stores solar energy through photosynthesis,which can meet the energy demand for our world.The utilization of bio-energy can reduce emissions of CO₂ and other pollutants and solve environmental problems?such as global warming,climate change,air pollution and acid rain?.Lignocellulosic biomass,such as agricultural and forestry residues,by-products of biological materials,and wood,is an important biomass resource because such biomass has no conflict issue with human food.Converting lignocellulosic biomass to sustainable electricity has been received great attention recently.Current technologies for biomass-electricity conversion include microbial fuel cells?MFC?,solid oxide fuel cells?SOFC?,and polymer electrolyte membrane fuel cells?PEMFC?.However,the problems with these technologies are:?1?biomass needs to be pretreated and converted into usable substances for these fuel cells;?2?the output power of MFC and PEMFC is very low;?3?the fuel cell performances are sensitive to operating conditions and their working life are limited.These difficulties seriously hinder the application of biomass to electricity conversion.In this paper,a low-temperature biomass-based redox flow fuel cell which uses polyoxometalates?POM?as catalyst and charge carrier has been studied.Raw biomass,including leaves,grass and wood powder,was directly used as fuels in this biomass fuel cell for electricity generation with oxygen.The first chapter introduces the classification and advantages of biomass and technologies about biomass energy utilization.Biomass-based fuel cells were introduced in detail,including microbial fuel cell,solid oxide fuel cell,direct carbon cell,polymer electrolyte membrane fuel cell and photo fuel cell.In chapter 2,a low-temperature biomass fuel cell was designed and constructed.The graphite felt was used as the anode,POM and biomass reaction solution was used as the anode electrolyte,and a platinum-catalyzed oxygen reduction electrode was used as the cathode.The cathode and anode were separated by a Nafion membrane.The electron and proton transfer processes in fuel cell under light or/and heating conditions were investigated in detail,and the power output was evaluated when different biomass materials were used as fuel such as starch and cellulose.The results show that the output power of this biomass fuel cell reached 0.7 mw cm^-2 and the Faraday efficiency reached as high as 94%.In chapter 3,the structural design of low-temperature biomass fuel cell was improved.Vanadium containing POM was used as cathode electrolyte instead of platinum catalyst,and porous graphite felt was used as cathode.The current generated in fuel cell in terms of the discharge of POMs on the electrodes with different electrode potentials,while the biomass and oxygen can regenerate the POM at the cathode and cathode respectively.The power of the improved biomass fuel cell was verified by collected raw biomass?switchgrass powder and shrub leaves?as fuel,indicating that the power density of the cell could reach 44⁵1 mW cm^-2.The stability test of the fuel cell shows that it can be stabilized at the output of 30 mw cm^-2 within a continuous12 hours experimental time.Chapter 4 studied the synergistic promotion of two effects?photocatalysis and photothermal effect?of light irradiation on the biomass fuel cell.Because the light absorption coefficient of POM electrolyte solution is very high,under actual solar light irradiation,the electrolyte solution temperature could rise to 85^oC?in summer?,which is enough to cause thermal oxidation degradation of biomass by POM.It was confirmed that the photothermal conversion efficiency was 85%and the quantum yield was 14.4%in the study.Based on the"black box"theory,the energy flows of this biomass fuel cell were analyzed,and the energy efficiency was calculated 36.7%based on the enthalpy of biomass.In chapter 5,the electrolyte was characterized by cyclic voltammetry and linear scanning.A three-electrode system was constructed by using a reference electrode in fuel cell electrodes.The results show that the cathode discharging process has a high interface resistance/capacitance and charge transfer resistance/pseudocapacitance and thus a high overpotential.Moreover,the working conditions?such as temperature and concentration?have a great influence on the process of cathode discharge.However,the anode discharging has a low overpotential and is insensitivity to working conditions.In chapter 6,the oxidation and degradation products of biomass were characterized by NMR,GC,LC-MS,GPC,TOC.The chemical analysis shows that the biomass polymer was degraded into small molecules or oligomers with POM under light or heat.The C-C bond in the small molecule can also be broken until CO₂ formed.Experiments show that after 12 cycles of oxidation,85%of biomass was completely oxidized to CO₂.Chapter 7 summarized the experiment results and discussed research plans in the future which could be focused on the improvement of the cathode electrode and the improvement of the biomass oxidation and degradation efficiency.