Electrochemical Hydrogen Production Coupled With Biomass Molecules Oxidation For Value-added Chemicals Synthesis

With the declining cost of renewable electricity,significant progress has been made in electrochemial water splitting for green hydrogen production,contributing to the imminent realization of low-carbon hydrogen energy.However,the sluggish oxygen evolution reaction（OER）at the anode in water splitting still remains as a key factor limiting its large-scale application due to its high energy consumption.The coupling of electrochemmial hydorgn production with anodic organic molecule oxidation,particularly biomass-derived molecules,has gained considerable attention and research interest.This approach not only significantly reduces the energy consumption for hydrogen production but also enables the production of higher value-added oxidation products at anode,such as 2,5-furandicarboxylic acid and formic acid.Nevertheless,this field is still in its developing stages and requires progress in the following aspects.（1）Design of efficient catalysts:The oxidation of organic biomass molecules in aqueous systems encounters challenges including low concentration of substrate molecules,limited diffusion,and sluggish kinetics of adsorption and activation on the catalyst surface.Designing suitable catalysts to achieve effective enrichment and efficient conversion of organic substrate molecules in heterogeneous phases is crucial;（2）Coupling of reaction control and product separation:Current research primarily focuses on alkaline electrocatalytic systems and the oxidation products are mainly organic acids.Consequently,the separation process is complex and energy-intensive;On the other hand,acidic systems can directly obtain organic acids but face issues such as low catalyst activity and structural instability.Therefore,achieving the coupling of reaction control and product separation is essential for improving the economic feasibility of electrochemical hydrogen production coupled with oxidation;（3）Understanding of reaction mechanism:Electrocatalytic conversion of biomass platform molecules under aqueous conditions involves multiphase catalytic systems,including water,oil,gas,and catalyst interfaces.Compared to direct electrocatalytic water splitting,the process becomes more complex.Therefore,gaining a deep understanding of the regulation mechanisms of oxidation reactions is of significant importance for the novel electrochemical hydrogen production coupled with biomass molecule oxidation.In addressing the challenges of electrochemical hydrogen production coupled with biomass molecule oxidation,this paper introduces a novel in-situ exfoliation strategy.Through this approach,the catalyst size is effectively reduced,while simultaneously enhancing the exposure of active sites.Consequently,the adsorption and oxidation processes of biomass molecules on the catalyst surface are substantially optimized.This leads to a marked improvement in both the current density of the catalyst and its selectivity towards the oxidation of biomass molecules.Considering the intricate products separation and associated high energy consumption,this paper proposes an acidic electrochemical hydrogen production coupled with oxidation reaction,which increases the product concentration and hydrogen generation rate.Moreover,it enables the direct separation of pure formic acid solution without acidification,potentially reducing the cost of product separation.In response to isuue of low reaction selectivity due to unclear reaction mechanisms,this paper reveals the insights into biomass molecule oxidation mediated by surface-active oxygen in alkaline electrooxidation.Additionally,it uncovers the distinct mechanisms of OER and biomass molecule oxidation under acidic conditions.Based on this understanding,an innovative strategy is developed to replace OER with glucose oxidation.This strategy effectively improves the catalyst’s stability under acidic conditions.The progress made in catalyst design,coupling reaction with separation,and mechanisms studies holds significant theoretical and practical value in advancing the efficient conversion of biomass platform molecules through electrochemical hydrogen production coupled with oxidation.The specific research content of the thesis is as follows:1.Ultrathin nanosheets array towards electrochemical hydrogen production coupled with 5-hydroxymethylfurfural oxidation.Layered double hydroxides（LDHs）have garnered significant attention in the field of alkaline electrochemical hydrogen production coupled with biomass molecules oxidation due to their flexibility in composition and structure.To address the issues of insufficient exposure of active sites for LDH catalysts,an in-situ exfoliation strategy was developed for the synthesis of ultrathin Co Al-LDH nanosheet array catalyst.In this paper,an in-situ delamination strategy is employed to synthesize an ultra-thin Co Al-LDH nanosheet array electrocatalyst for efficient hydrogen evolution coupled with5-hydroxymethylfurfural（HMF）oxidation to produce the high-value product2,5-furandicarboxylic acid（FDCA）.The catalyst exhibits excellent HMF oxidation activity and stability（with an onset potential of 1.30 V vs.RHE and a FDCA faradaic efficiency of 99.4%）due to the abundant exposed active sites and rich oxygen vacancies.Notably,the catalyst retains its performance without degradation after eight cycles.The introduction of oxygen vacancies is demonstrated,through theoretical calculations and in-situ infrared spectroscopy,to effectively optimize the adsorption and oxidation processes of HMF on the LDH surface.A dual-electrode electrolysis cell composed of this oxygen vacancy-rich ultra-thin Co Al-LDH nanosheet array catalyst achieves efficient HMF oxidation coupled with hydrogen production,with a hydrogen generation rate of 44.2 L h^–1 m^–2,four times higher than that of traditional electrolysis for hydrogen production.Simultaneously,the separation of the oxidized product FDCA is achieved,with a powder product purity of up to 99.5%.This work provides novel strategies and methods for designing electrocatalysts for efficient synthesis of high-value chemicals through electrolytic hydrogen evolution coupling.2.Amorphous LDHs nanosheets array electrodes for electrochemical hydrogen production coupled with glycerol oxidation.LDHs has demonstrated excellent electrocatalytic activity,but its selectivity towards organic compound oxidation is compromised due to the competing OER process.Addressing the low activity and selectivity of Ni Fe-LDH catalysts for the oxidation of biomass molecules,this study introduces an amorphous Ni Fe-LDH array catalyst designed for selective oxidation of glycerol to formic acid at lower reaction voltages.The results reveal that the glycerol oxidation current density of the amorphous Ni Fe-LDH is 10 times higher than that of the crystalline Ni Fe-LDH,achieving a formic acid faradaic efficiency of 99.0%and a formate yield of 4.3 mol m^-2 h^-1,and the hydrogen production rate improves 5 folds compared to conventional water splitting.Based on the amorphous Ni Fe-LDH anode,an asymmetric electrolytic cell comprising an alkaline anode compartment and an acidic cathode compartment is designed,enabling glycerol oxidation coupled with high current density hydrogen production at a low cell voltage（0.9 V）.Furthermore,a pathway for electrolytic hydrogen production coupled with waste oil recovery is devised,utilizing the amorphous Ni Fe-LDH catalyst to convert waste oils into high-value products such as potassium formate and clean fuels（biodiesel and hydrogen）,thereby expanding the range of feedstock sources and facilitating product separation and downstream value addition.This work provides a theoretical foundation for the design of catalysts for the electrooxidation of biomass small molecules and offers new insights for the valorization of alkaline electrolysis products.3.Research on strategies for enhancing the stability and acidic electrochemical hydrogen production coupled with glucose oxidation.In alkaline electrolysis,the oxidation products are usually organic acid salts,which require acidification for product separation,leading to high separation costs.Therefore,the development of acidic electrochemical hydrogen production coupled with oxidation systems and stable,inexpensive catalysts is of significant importance in reducing product separation costs.However,the stability of anode catalysts under acidic conditions,particularly non-precious metal catalysts,has been a limiting factor.Addressing the issue of poor stability of non-precious metal anode catalysts under acidic conditions,this work focuses onγ-Mn O₂ as a model catalyst fabricated by electrochemical deposition.The instability mechanism of the catalyst is investigated,and a strategy is proposed to enhance the catalyst’s stability by replacing OER with glucose oxidation,effectively improving the catalyst’s stability.Combining theoretical calculations with in-situ electrochemical mass spectrometry experiments involving ¹⁸O,it is revealed that the OER process ofγ-Mn O₂involves partial lattice oxygen oxidation,which accelerates the overoxidation and dissolution of Mn.By substituting the OER with glucose oxidation,the overoxidation of Mn is suppressed,reducing the catalyst’s deactivation rate from 123.9 m V h^-1 to 0.059 m V h^-1.Stable operation for 960 hours is achieved at 10 m A cm^-2.This work provides a new approach to enhance the stability of acidic anode catalysts and reduce the product separation costs in acidic electrochemical hydrogen production coupled with biomass oxidation.4.Highly efficient Mn O_x electrode for electrochemical hydrogen production coupled with biomass molecule oxidation.In the previous study,manganese oxide（Mn O_x）was identified as a promising catalyst for acidic electrolytic hydrogen evolution coupled with biomass oxidation reactions.To further enhance its activity and stability under acidic conditions,this work synthesizes a Mn O_x/Ti catalyst via pyrolysis and applies it for the electrocatalytic oxidation of various biomass molecules.In the glucose oxidation reaction for formic acid production,the Mn O_x/Ti catalyst achieves a current density of 200 m A cm^-2 at only 1.55 V vs.RHE,with a selectivity of 84.4%and a faradaic efficiency of 77.2%.The maximum formic acid concentration reaches 919.6 m M.Moreover,the catalyst demonstrates excellent stability,operating steadily for over 3500 hours at a current density of100 m A cm^-2,surpassing the performance of most previously reported acidic anode catalysts in terms of both current density and stability.By employing a direct distillation strategy,a formic acid solution of 37.2 g L^-1 is obtained in a 6L volume with a purity of～99%.Furthermore,a large-scale electrolytic cell（100 cm²）with a Mn O_x/Ti anode catalyst is assembled,which can achieve a high current of 30 A with a cell voltage of only 3.57 V and exhibit selectivity over 80%and formic acid faradaic efficiency over 60%.This work develops an efficient and stable anode catalyst under acidic conditions,providing a novel approach for low-cost separation of oxidation products.