Efficient Preparation And Hybrid Water Electrolysis Performance Of Biomass-Derived Carbon Catalyst

Electrocatalytic water splitting is considered to be a promising method for the preparation of clean hydrogen energy.Due to the high overpotential of hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode,resulting in low energy conversion efficiency of electrolytic water splitting,which greatly hinds the development of electrolytic water splitting for hydrogen production.Although the commercial noble metal based electrocatalyst（Pt/C,Ir O₂ and Ru O₂,etc.）exhibit excellent catalytic activity,their large-scale application is limited due to their high cost and resource scarcity.Therefore,the development of abundant,low-cost and efficient non-noble metal electrocatalysts plays a key role in improving the energy conversion efficiency of water electrolysis.In addition,during the electrocatalytic water splitting process,the kinetics of OER at the anode is seriously limits the efficiency of hydrogen production from electrolysis water.Therefore,it is of great significance to reduce the energy consumption of electrolysis water and improve the energy conversion efficiency by replacing the sluggish OER with electrochemical oxidation of organic molecules that are more easily oxidized in thermodynamics.Biomass has the advantages of abundant resources,low price,renewable and so on.Converting biomass into high-performance carbon-based catalysts is beneficial to expand the application range of high value-added utilization of it.In this work,a series of high efficiency biomass derived carbon-based electrocatalysts were prepared by carbonization,hydrothermal reaction,electrochemical deposition technology and other methods.The structure,conductivity and catalytic active sites of electrocatalysts were optimized,and the internal relationship between the microstructure and electrochemical performance of the catalysts was revealed.In addition,biomass-derived monosaccharide oxidation reaction（MOR）was used to replace the anodic sluggish OER,so as to reduce the energy consumption of electrolytic water and improve the energy conversion efficiency.The detailed researches are as follows:1.A core-shell structure bifunctional electrocatalyst（Co₂P@NPPC）was in situ synthesized by using biomass chitosan as carbon and nitrogen source,and phytic acid as phosphorus source through one-step carbonization and phosphating.The results show that Co₂P@NPPC exhibits bifunctional catalytic activity for both HER and OER with a low overpotential（147 m V）and Tafel slope(62 m V dec^-1)in acidic medium.Meanwhile,Co₂P@NPPC exhibits a high OER performance in alkaline solution,with overpotential of 316m V at a current density of 10 m A cm^-2,which is superior to the Ir O₂ catalyst.Density functional theory（DFT）calculation results show that P atom doping can optimize the hydrogen adsorption free energy(△G_H*),and N/P co-doped porous carbon can improve the electron state density of Co₂P,enhancing its electrical conductivity,and further improving the electrocatalytic performance of Co₂P@NPPC.2.The anodic sluggish kenictic OER limits the energy conversion efficiency of water electrolysis.In order to further reduce the energy consumption of water electrolysis,biomass-derived glucose oxidation reaction（GOR）was used to replace the anodic OER to promote the HER.Nitrogen doped porous carbon supported cobalt nanoparticles（Co@NPC）was in situ synthesized by one-step pyrolysis using a ternary deep eutectic solvent（DES）consists of gluconic acid,choline chloride and urea as carbon and nitrogen source and self-template.The results show that Co@NPC-800 catalyst shows excellent catalytic activity for GOR with a low potential of 1.46 V to reach the current density of 10 m A cm^-2,which is far lower than that of anodic OER at the same current density.Moreover,Co@NPC-800 also exhibits excellent bifunctional catalytic performance,requiring a low voltage of 1.56 V to drive current density of 10 m A cm^-2,which is nearly 180 m V less than conventional overall water splitting.It can not only reduce the voltage in hydrogen production from the electrolysis of water,but also can obtain high value-added chemicals at the anode,thus improving the efficiency of energy conversion.In addition,the DFT calculation results show that the adsorption energy of glucose is higher than that of H₂O molecule,revealing that the glucose takes precedence over H₂O molecules adsorbed on the catalytic active site during the electrochemical oxidation reaction process at the anode to replace OER,thus reducing the voltage of water electrolysis.3.On the basis of replacing OER with GOR to reduce the energy consumption of hydrogen production by electrolysis of water,and to further develop efficient electrocatalyst for water splitting to produce hydrogen under large current density,using glucose as carbon source,biomass derived carbon nanosheets（Fe-Ni₂P@C/NF）with three-dimensional hierarchical structure was prepared on nickel foam by hydrothermal and low temperature phosphating methods.The Fe-Ni₂P@C/NF was used as bifunctional electrocatalyst for water splitting and biomass oxidation under large current density.The results show that Fe-Ni₂P@C/NF exhibits high electrocatalytic HER activity at high current densities,with the overpotential of 75 and 313 m V to reach the current densities of 10 and 1000 m A cm^-2,respectively.In addition,biomass derived monosaccharide oxidation reaction（MOR）was used to replace the sluggish OER to further reduce the high overpotential of electrolytic water splitting.It is worth noting that Fe-Ni₂P@C/NF catalyst only requires a low voltage of 1.55 V to drive current density of 100 m A cm^-2 during the coupled MOR and HER for hybrid water splitting,which is lower than that of conventional overall water splitting.The yield of high added value chemicals,lactic acid,is up to 52.1%.DFT calculation results show that Fe doping can optimize the adsorption and dissociation energy of H₂O molecule.In addition,a synergistic effect between carbon layer and Fe-Ni₂P can be formed to weaken the hydrogen adsorption free energy,thus further improving the electrocatalytic activity.4.Based on previous studies,the in-situ support of carbon nanotubes coated with cobalt on the carbonized wood（Co@NCNT/CW）was prepared by electrochemical deposition and pyrolysis.As a self-supporting electrocatalyst,Co@NCNT/CW exhibits excellent electrocatalytic HER performance in both acidic and alkaline media,with overpotential of 263m V at the high current density of 500 m A cm^-2.In addition,during the hybrid water splitting for coupling HER with monosaccharide oxidation reaction（MOR）,Co@NCNT/CW shows excellent catalytic activity,requiring only a low voltage of 1.26 V to drive the current density of 50 m A cm^-2,significantly lower than the traditional overall water splitting（1.64 V）,which greatly reduces the energy consumption of electrolytic water splitting and improves the energy conversion efficiency.Moreover,DFT calculation results show that introducing nitrogen into the carbon nanotube can adjust the electronic structure of its surrounding carbon atoms,and further optimize the adsorption and dissociation energy of H₂O molecules.