Strengthening Mechanism And Method Of Electrocatalytic Hydrogen Production From Coal-Water System

Hydrogen energy is considered as the most promising energy carrier in the 21st century.The most ideal way to produce hydrogen is the electrolysis of water.However,the power consumption of this process is 4.5-5 k Wh/Nm³,which has become a bottleneck restricting the industrialization of this technology.The oxygen evolution reaction(OER)of water electrolysis at the anode is a multi-step reaction with slow kinetics,which causes the high power consumption of water electrolysis.Replacing the oxidation of heavy carbon for the OER can reduce the power consumption of hydrogen production.Coal is a rich and cheap heavy carbon resource.If coal is introduced into the water electrolysis for hydrogen production,that is,coal-assisted water electrolysis for hydrogen production(CAWE),this can not only reduce the power consumption of water electrolysis for hydrogen production,but also realize the clean utilization of coal,which is important for the development of hydrogen storage technology and the innovative use of coal resources.This study focuses on the construction of a CAWE system with low power consumption and high current density,which involves revealing the law of coal oxidation conversion,developing cheap and high catalytic activity anodes,and verifying at the process and device levels.For CAWE,coal is used as a carbon source to undergo oxidation conversion in anode zone.For this reason,this article first studied H₂O₂ controllable oxidation of coal,and proposed a method to strengthen the reactivity of coal with H₂O₂,in order to understand the oxidation mechanism of coal and the composition of liquid phase products.In the research,the law of the reaction between H₂O₂ and coal was studied,including the influence of the main reaction parameters on the reaction and the changes in the physical and chemical properties of coal during the reaction.Then,the reactivity of H₂O₂ with coal was improved with the help of Fe²⁺,and studied the effects of the main reaction parameters on the oxidation of coal in the Fe²⁺/H₂O₂system.Finally,it was proposed that Fe²⁺and glacial acetic acid cooperate to catalyze the reaction of H₂O₂ with coal,which can not only improve the reactivity of H₂O₂with coal,but also obtain valuable liquid phase products with high yield.The results showed that for the reaction of coal with Fe²⁺/H₂O₂ system,the best reaction conditions were:[Fe²⁺]/[H₂O₂]molar ratio,0.00453;H₂O₂ concentration,3 mol/L;reaction temperature,60°C;reaction time,4 h.Fe²⁺and glacial acetic acid had a synergistic catalytic effect on the reaction of coal with H₂O₂,which can not only enhance the reactivity of H₂O₂ with coal,but also obtain valuable liquid phase products with high yield.Among the obtained liquid phase products,carboxylic acids were the most abundant,accounting for 47.22%.The macromolecular structure of coal contains both an organic carbon skeleton and inorganic components.Based on this,in order to lay a foundation for the study of the oxidation conversion and hydrogen evolution law of coal in the CAWE,this paper selected the simplest organic small molecules(oxalic acid)and inorganic sulfur-containing substances(sulfites)to study their electrochemical reaction characteristics.In the study,the influence of the main reaction parameters on the hydrogen production rate was studied;and water electrolysis and CAWE were compared under the best reaction conditions,including the analysis of the thermodynamic characteristics and power consumption of the reaction by linear sweep voltammetry and the chronopotentiometry method,respectively.The results showed that sulfite and oxalic acid can be used as reactants for chemical assisted water electrolysis for hydrogen production.The oxidation reaction of sulfite and oxalic acid replaced the oxygen evolution reaction of water electrolysis,respectively,which can reduce the power consumption of water electrolysis.For hydrogen production by sulfite electrolysis and oxalic acid electrolysis,respectively,the rate of hydrogen production increased with the increase of the concentration of reactants and the reaction temperature.The oxidation reaction of sulfite and the oxidation reaction of oxalic acid had lower activation energy than the oxidation reaction of water,and the reaction only required 2 electrons,so it can be quickly oxidized at the anode,generating high current density,generating more hydrogen and requiring lower power consumption.Further,the influence of coal rank,oxygen-containing groups and minerals in coal on the CAWE was studied,and the changes in the physical and chemical structure of coal were analyzed in the CAWE,so as to deepen the understanding of the reaction mechanism of the CAWE and provide theoretical guidance for seeking strategies that can solve the slow reaction rate.In the research,electrochemical methods were first used to study the performance of CAWE with different characteristics,and then a series of methods were used to analyze changes in physical and chemical properties of coal during the CAWE.The results showed that in the process of CAWE,the passivation of coal was mainly due to the enrichment of aliphatic carbon,Ar OH or Ar OR linked to oxygen,and changed in the microstructure,surface structure and microcrystalline structure of the coal.After the CAWE:for low-rank coals,the aromatic clusters of coals decreased,while the average substituents of aromatic rings increased;for middle-rank coals,the aromatic clusters of coals increased,and the average substituents of aromatic rings increased;For high-rank coals,the aromatic clusters of the coal increased,while the average substituents of the aromatic ring decreased;the minerals in the coal had a promoting effect on the reaction,and the oxygen-containing groups had an inhibiting effect on the reaction.Aiming at the problem of high cost of traditional anode materials,a cheap graphite felt electrode was proposed as the electrode for the CAWE,and the reaction law of CAWE with the graphite felt electrode was studied,which provided important guidance for the development of non-precious metal anodes that can be applied on a large scale.In the study,firstly,the feasibility of the CAWE with the graphite felt electrode was analyzed,and the performance differences between graphite felt electrodes and typical noble metal electrodes(Pt,Sn O₂/Ti,Pt/Ti,Ir O₂-Ru O₂/Ti and Ir O₂-Ta₂O₅/Ti)in the CAWE were compared.After then,the influence of the reaction conditions on the CAWE with the graphite felt electrode was studied.Finally,the performance of the CAWE with the graphite felt electrode was evaluated,including power consumption and stability.The results showed that cheap graphite felt electrodes had high catalytic activity and stability in the CAWE.For the CAWE with the graphite felt electrode,as the concentration of coal slurry and the reaction temperature increased,the reaction rate improved,energy consumption reduced and the precipitation of hydrogen enhanced.When the current density was 5 m A/cm²,10m A/cm² and 50 m A/cm²,compared with Pt?Ir O₂-Ru O₂/Ti water electrolysis for hydrogen production,the electricity consumption of Pt?graphite felt electrolysis for the CAWE was reduced by 39.09%,28.75%and 11.73%.Finally,a two-step method has been used for the CAWE to achieve low power consumption and high current density.In the study,in order to determine the implementation of the two-step electrolysis of coal for hydrogen production,firstly,the anodic oxidation of Fe²⁺was carried out to obtain the operating parameters of the electrolysis system;then the reaction of coal with Fe³⁺was studied to determine the operation parameters of the hydrothermal system;Based on the above research,the operating parameters of the two-step electrolysis of coal for hydrogen production were obtained,and its performance was evaluated.The results showed that graphite felt electrodes had the highest catalytic ability for the electrochemical oxidation of Fe²⁺.Increasing the concentration of Fe²⁺and the reaction temperature can enhance the electrochemical oxidation of Fe²⁺.The optimal reaction conditions for the reaction between coal and Fe³⁺were:reaction time,6 h;temperature,90°C;coal slurry concentration,50 g/L;Fe³⁺concentration,500 mmol/L.The two-step electrolysis of coal for hydrogen production can achieve high current and low power consumption hydrogen production and energy storage.When the cell voltage was 1.2 V,the current density generated can reach 100 m A/cm²,and the power consumption of this process is 2.63 k Wh/Nm³.