Study On The Reaction Path And Anodic Oxidation Mechanism Of Coal And Wastewater Assisted Water Electrolysis For Hydrogen Production

It is an inevitable trend that renewable energy becomes the main energy source in China,and the electrolysis of hydrogen from renewable energy is an inevitable choice to achieve the goal of“double carbon”in the medium and long term.It is urgent to reduce the energy consumption of hydrogen from electrolysis to make it have better application value.Coal-assisted water electrolysis changes the reaction path,cleverly bypassing the main source of high power consumption of water electrolysis,and significantly reduces the energy demand by lowering the theoretical decomposition voltage,which can reduce the power consumption of hydrogen production by more than half.At the same time,coal-assisted water electrolysis also provides a new idea for clean utilization and low-temperature conversion of coal.At present,coal-assisted water electrolysis is still in the initial stage of development,and there are certain shortcomings in the way of judging the electrochemical activity of coal,and a lot of research is still needed to deepen the understanding of kinetics and reaction mechanism.Based on the above background,this paper systematically investigates the reaction kinetics,oxidation mechanism,efficiency enhancement mode and regulation mechanism of coal electrolysis around the coal-assisted water electrolysis for hydrogen production,and realizes the accurate judgment of coal electrochemical activity from multiple perspectives,in order to provide theoretical basis and data support for revealing the reaction mechanism from the molecular level,further enhancing the reaction rate and promoting the popularization and application of this technology.Firstly,the reaction pathway of coal electrolysis was disentangled,and the kinetic characteristics of each step were investigated separately to obtain direct evidence that the coal electrolysis reaction is dominated by indirect oxidation.The key factors influencing the reaction activity were explored in conjunction with the structural characterization.It was found that Fe²⁺electrooxidation conforms to the kinetic characteristics of the primary reaction,and the error between the theoretical and actual values is less than 1%.Therefore,the Fe²⁺concentration in the electrolyte can be tested by the redox current.Coal particles will release Fe²⁺-based iron in the electrolyte,and these Fe²⁺will contribute to the current thus bringing a large error to the judgment of the real activity of coal.The measurement of the net concentration change of Fe²⁺produced by chemical oxidation can truly reflect the oxidation rate.Coal particles undergo a series of single-electron reactions during oxidation to produce alcohol,aldehyde and carboxylic acid structures on the surface in turn,where the relative content of C-O structures is a key factor in determining the reactivity of coal particles.Secondly,the evolution of the main functional groups,surface elements,carbon structure and microcrystalline structure of coal particles with reaction depth was comprehensively analyzed by combining various modern testing techniques to investigate the oxidation mechanism of coal molecules during the electrolysis process.It is shown that the oxidation of coal particles is more sensitive to temperature compared to Fe²⁺electrooxidation,and elevated temperature is more favorable to promote the oxidation of coal particles.The oxidation rate of C-O structure is faster and is the main source of reactivity of coal particles,and the-COOH structure is the final stage of the whole oxidation process.The further oxidation of this structure to CO₂ is slower and is the main control step of the oxidation of coal particles.Among the carbon structures of coal particles,the oxidation rate of aliphatic carbon is faster than that of aromatic carbon overall,with methylene and oxidized methylene having the highest reactivity.The C-C bond in the aromatic branch undergoes oxidative breakage in electrolysis and further oxidation to form oxygen-substituted aromatics.Subsequently,the regulatory mechanisms of the three variables of temperature,sulfuric acid concentration,and stirring rate were investigated simultaneously by factorial design method,which made the cross-sectional comparison between different studies possible,and the real effect of sulfuric acid concentration on coal oxidation activity was explored in depth.The results show that increasing the temperature,increasing the stirring,and decreasing the sulfuric acid concentration all had positive effects on the diffusion of Fe²⁺,thus promoting Fe²⁺electrooxidation,but decreasing the sulfuric acid concentration made the Fe²⁺electrooxidation less reversible.The electrolyte itself has a certain influence on the molecular structure and reaction process of coal,and there is a large error in judging the oxidation activity of coal particles only from the current density.In this paper,we excluded the interference of electrolyte by detailed control experiments and found that the oxidation activity of active sites in coal decreases with increasing sulfuric acid concentration,and more small organic molecules are stripped out.Unsaturated carbon-carbon double bonds are oxidatively broken in electrolysis thus introducing oxygen elements into the molecules.Then,the electrolysis characteristics and reaction mechanism of desulfurization wastewater and gas washing water synergistic coal electrolysis were studied in a pioneering way.The reaction types of wastewater electrolysis were determined in combination with composition tests,and the enhanced mechanism of wastewater promoting coal electrolysis was analyzed.The results show that the electro-oxidation processes of wastewater all have obvious stage characteristics.For desulfurization wastewater,the reaction rate of chlorine ions releasing electrons to generate chlorine gas is significantly higher than that of water molecules decomposing to generate oxygen,and the application processes with different advantages are proposed based on the research results.For gas washing water,with the increase of voltage,the oxidation of organic matter,the oxidation of bromine ions and the decomposition of water occur sequentially at the anode,and the COD of wastewater will be reduced after electrolysis,thus reducing the pollution.The enhanced mechanism of gas wash water for coal electrolysis is proposed by synthesizing the experimental results.Finally,the feasibility of heteroatom-doped carbon materials in coal electrolysis was studied for the first time.A series of self-supported metal-free carbon-based catalysts were prepared,and the catalytic activity and catalyst formation mechanism were analyzed by combining performance testing with morphological analysis and structural characterization.It is shown that the carbon material substrate will not be oxidized in the potential interval of coal electrolysis.The electron transfer rate of carbon cloth is faster than that of carbon paper,which is more suitable for use as a substrate.During the synthesis process,phytic acid molecules aggregate at the substrate,which induces the growth of polyaniline on the surface of the substrate.Structural analysis shows that N and P are successfully doped into the graphite lattice of the carbon material,introducing more defects thus leading to higher catalytic activity.The N and P doped carbon substrate have higher stability,while the substrate doped with only O elements are less stable.The oxidation charge of the best performing NPO/N/OCC is enhanced by 47.6%compared to that of Pt.