Conversion And Utilization Of Biomass By The Redox Couples Catalyzed Electrochemical Processes

As biomass is a kind of renewable energy with extensive sources and cheap price,the conversion and utilization of biomass can not only help alleviate the global economic development crisis caused by the increasing exhausted fossil fuels,but also avoid the pollution to the environment.The existing methods of biomass utilization,such as pyrolysis,gasfication and microorganism degradation,have many disadvantages,such as high reaction temperature,expensive catalyst and short life span.By using electrochemistry method to transfer biomass to hydrogen and electricity is a high efficient,low energy consumed process.Therefore,this thesis aims at applying redox couples to biomass based electrochemical conversion process,and hope to realize efficient biomass utilization under mild conditions.A redox couple is a pair of oxidized and reduced substances that form a conjugated system.The oxidized substance of a redox couple can oxidize biomass and itself turns into a reduced substance.The reduced substance joins an electro-oxidation reaction with small over-potential on a simple graphite electrode.Thus,using redox couples can reduce consumed energy in electrolysis cell and improve the performance of fuel cell.Thus,this thesis explores the electro-chemical conversion process of biomass with redox couples and hope to achieve converting biomass in many fields with wild conodition.This thesis studys on biomass based hydrogen electrolysis evolution using Fe³⁺/Fe²⁺redox couple;CO₂ electro-reduction using PMo₁₂/H-PMo₁₂ redox couple and electricity production by fuel cell operation within two redox couples(PMo₁₂/H-PMo₁₂and VO₂⁺/VO²⁺;Si₁₂W/H-SiW₁₂ and Fe³⁺/Fe²⁺).The detailed main research contents are as follows:1.There are many methods for“biomass to hydrogen”,but the high input energy,expensive catalyst and limited service life make the cost of hydrogen production high.A low energy consumption electrolysis device that directly converts the primary biomass into hydrogen at low temperature was designed to realize high efficiency hydrogen production.By using Fe³⁺/Fe²⁺redox couple as a catalyst,the faraday efficiency of hydrogen production for this system is as high as 90.6%.Fe³⁺/Fe²⁺redox couple functions as an oxidation agent?oxidizing biomass?,charge carrier?transferring the charge to anode?and discharge agent?discharge on anode electrode?.The electro-catalytic activity of Fe³⁺/Fe²⁺ion was demonstrated by cyclic voltammetry and the rate of hydrogen evolution was measured at different current densities.At very low cell potential,smaller than 0.7 V,hydrogen begins to be produced,which is much less than noble metal catalytic water splitting hydrogen evolution.The electric energy consumption in our experiments for glucose-Fe ion system is 1.845 kW h Nm^-3 H₂ at100 mA cm^-2,which can save about 60.74%electric energy of typical alkaline water electrolysis(4.7 kWh Nm^-3 H₂ at U_H2O=2 V,the current density is 100 mA cm^-2).Different from reported alcohol electrolysis,the biomass based electrolysis process does not require any noble-metal catalyst on the anode.2.CO₂ is the main cause of greenhouse effect.CO₂ reduction can not only reduce CO₂ emission,but also prepare high-value reduction products.Among all the methods of CO₂ reduction,the electrolysis cell is easy to be produced in large scale.However,due to the limitation of catalyst,much bigger applied potential is needed,which makes the energy consumption of the whole process high.In a designed electrolysis cell,a chemical regenerative catalyst,phosphomolybdic acid(PMo₁₂),was used as an electron-coupled proton carrier,and glycerol was employed as a reducing agent to provide electrons to PMo₁₂2 under photo irradiation.Glycerol was firstly oxidized to value-added products such as acetic acid under the catalysis of PMo₁₂.After an electrical potential was applied,the reduced PMo₁₂(H-PMo₁₂)released electrons and was re-oxidized to PMo₁₂ on the anode,while CO₂ was reduced into valuable hydrocarbons by taking electrons on the cathode.The developed approach has multiple advantages over the conventional CO₂ electrolysis,e.g.low electrolysis potential,and a noble metal-free anode due to the low overpotential of H-PMo₁₂ oxidation on the anode.The electrolysis current densities of 0.23 and 0.29 A cm^-2 were achieved under 2.0 V of electrolysis voltage for the Cu and Ag cathode,respectively,which is 2-3 times of conventional electrolysis.The faradaic efficiency for CO₂ reduction is 63.53%and77.75%at 1.9 V,and the energy efficiency is 45.60%and 53.67%for the Cu and Ag electrode,respectively.3.A fuel cell is a device that converts chemical energy of fuel into electrical energy.By applying a fuel cell,biomass can be convertsed into clean electrical energy.High temperature and expensive catalysts are the main reasons for high price and low efficiency of fuel cell.Based on this,two biomass based fuel cell without any precious catalyst were built,and two redox couples,PMo₁₂/H-PMo₁₂(SiW₁₂/H-SiW₁₂)and VO₂⁺/VO²⁺(Fe³⁺/Fe²⁺)were applied in the fuel cell.Under heating?or light?,PMo₁₂(or SiW₁₂)captured electrons from biomass and combined protons in biomass,and then turned into reduced polyoxometallate complex(H-PMo₁₂ or H-SiW₁₂).H-PMo₁₂(or H-SiW₁₂)release electrons on graphite electrode,while VO₂⁺(or Fe³⁺)received electrons and reduced to VO²⁺(or Fe²⁺).VO²⁺(or Fe²⁺)was re-oxidized by oxygen.The maximum output power density of these two biomass fuel cell is 66.24mW cm^-2 and 81.36 mW cm^-2,respectively.