Preparation Of Catalysts For Water Electrolysis And Key Materials For Hydrogen Fuel Cells

The specialty of chemical engineering and technology is the basic guarantee for the realization of chemical production.The discipline direction of environmental pollution control chemistry and industry is proposed to balance the serious pollution problems caused by the chemical industry and the combustion of fossil fuels.The research and development of clean energy fundamentally solves environmental problems which is one of the effective ways to balance chemical production,living needs and environmental pollution problems.Therefore,the research and development of electrocatalysts in clean energy conversion device is a branch of environmental pollution control chemistry and industry in chemical engineering and technology.The development of clean energy conversion devices comes from the limitation of uncontrollable natural factors such as solar energy,hydropower and wind energy,which hinders the storage and use of such clean energy.Based on this,a series of devices including fuel cells,metal-air batteries,and electrolyzed water have been developed to realize processes of energy conversion,storage,use,etc.Most electrochemical reactions involved in the above energy conversion devices use noble metal（Pt,Ru,Ir）-based catalysts to reduce the reaction activation energy,the high cost of catalysts restricts the development and application of related devices.Therefore,it is important to develop non-precious metal-based electrocatalysts to replace noble metal-based electrocatalysts for the development and utilization of new energy devices.This paper mainly uses cheap and easily available transition metals and carbon materials as the main body of the catalysts,and makes an in-depth study on the design,preparation,reaction mechanism and device application of oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）electrocatalysts used in electrolytic water devices and oxygen reduction reaction（ORR）electrocatalysts used in H₂-O₂fuel cell devices.A bimetallic amorphous Co Fe borate electrocatalyst was prepared on a current collector of carbon paper by a one-step solvothermal method,it exhibits excellent OER performance.After systematic characterization,it was found that the Co Fe borate would be irreversible phase transition from Co Fe borate to Fe-doped Co OOH after the OER process.The results of this study confirms the phase transition mechanism of Co Fe borate in OER process and clarifies the catalytic mechanism of Co OOH as the OER active center.A new method for the synthesis of OER electrocatalysts by physically adsorbed metal ions（PAMI）was proposed,that is,porous materials were used to directly adsorb metal ions as OER electrocatalysts.A series of Co_xFe_yCB（x+y=1）catalysts were prepared by this method.Among them,the Co_0.7Fe_0.3CB exhibits the best OER performance,it requires overpotentials of295 m V and 350 m V to achieve the current densities of 10 m A cm^-2 and 100m A cm^-2,respectively.The catalyst prepared by this method not only possesses high catalytic activity,but also has simple synthesis,does not involve complex chemical reaction and preparation process,and has the potential of scale-up.The method possesses wide universality and is suitable for a variety of carriers,such as carbon nanotubes,graphene,C₃N₄,Ca CO₃and porous organic polymers.A new method to achieve the long-term OER catalytic process was proposed.The metal salt solution was directly dispersed in the electrolyte to construct a homogeneous catalytic system.In this system,the active species will be in situ formed on the surface of the current collector at the OER potential.The catalytic system exhibits excellent OER performance,which requires an overpotential of 289 m V to achieve a current density of 10 m A cm^-2.The OER active substances can be regenerated by supplementing the metal salt solution during the OER process due to the homogeneous characteristics in this system.So a long-term OER operation process can be achieved in this system.For example,a continuous and stable OER stability test was achieved at a current density of 80 m A cm^-2 for 300 hours by 5 times of salt solution supplementation.The proposed catalytic system provides a new idea to solve the problem of OER catalytic stability.The NiFe bimetallic OER electrocatalyst（Ni SAs/Fe-Ni OOH）was prepared by self-proposed PAMI method.The Ni SAs/Fe-Ni OOH can achieve a current density of 10 m A cm^-2 with only an overpotential of 269 m V.After a series of in-situ and ex-situ characterization,it was found both Fe-Ni OOH nanoparticle and Ni C₄ single-atom in Ni SAs/Fe-Ni OOH.DFT calculations reveal a tandem catalytic mechanism for OER between the two active sites（i.e.,Fe-Ni OOH and Ni C₄）in this catalyst.Accurately,the first two steps of OER occur at the Fe-Ni OOH site,after the oxygen migration process,the last two-step elementary reactions occur at the Ni C₄ site.The tandem catalytic mechanism can effectively reduce the overall activation energy of the reaction,which provides a new idea for the design of catalysts with multiple active sites.The highly active HER electrocatalyst WN-Ni N/CFP was successfully prepared by procedures of hydrothermal and nitridation in ammonia gas.The WN-Ni N/CFP only requires an overpotential of 37.1 m V to reach a current density of 10 m A cm^-2 in alkaline.The Ni₃N/WN heterojunction structure was confirmed after a serious of characterization.DFT calculation results show that the heterointerface can effectively promote the adsorption and dissociation of water molecules and optimize the desorption of OH^*,thereby reducing the free energy of the HER process and improving the HER electrocatalytic activity.In view of the current tests of most non-precious metal-based ORR electrocatalysts are still limited to the RDE half-cell test system,which show poor activity and stability in fuel cells system.A new idea to use defect-rich metal-free N-doped porous carbon materials with fully covalent bond structure（NDPC-1000）as the ORR electrocatalyst was proposed.The peak power density of the AEMFC assembled with NDPC-1000 as the cathode is as high as 913 m W cm^-2.Importantly,this AEMFC was tested for stability at current densities of 0.25 A cm^-2 and 0.5 A cm^-2,the voltage decay was less than 25%after 100 h and 60 h respectively.DFT calculations also reveal the catalytic mechanism that the graphitic nitrogen-controlled armchair and pengaton defect structures can effectively enhance the ORR activity.The dissociation energy of the catalytic active site was calculated by DFT,it was found that the dissociation energy of the metal-free material in the form of covalent bonds is much larger than that of the Fe N₄ single-atom structure in the form of coordination bonds,revealing that the catalyst has an intrinsic mechanism of stability,which validates the experimental results.