Electrooxidation Of Small Organic Molecules Boosted Electrocatalytic Hydrogen Evolution/carbon Dioxide Reduction

With the rapid emergences of serious problems such as energy shortage,environmental pollution and greenhouse effect,technologies of electrocatalytic water splitting for hydrogen production and carbon dioxide(CO₂) reduction for value-added chemicals production have aroused tremendous attenions.However,the oxygen evolution reaction(OER),which occurs at the anodes of both traditional electrocatalytic water splitting and CO₂ conversion systems,is a process of rather slow kinetics.The OER needs a large overpotential to drive,resulting in much increased energy consumption of these systems.Additionally,O₂ produced at the anode can be obtained easily from the atmosphere,is therefore of low value.Moreover,the O₂ in electrolytic system will not only lead to the possible formation of membrane-degraded reactive oxygen species during the traditional water electrolysis process,but may also be mixed with H₂ generated at the cathode,signifying a potential but dangerous explosion risk.Various OER catalysts of enhanced performances have been developed by scientists in recent years and the efficiency of water electrolysis has been improved to some extent,which,however,is far from satisfying.Thus,it is critically important to develop more efficient electrocatalytic systems by replacing anodic OER with a more energy-efficient oxidation reaction of small organic molecules.In this paper,methanol and glycerol have been utilized to substitute for OER,which significantly reduce the required anode potential and the energy consumption of the reaction system,meanwhile,value-added products can be obtained at anode when equipped with designed catalysts and regulated electrolysis conditions.In particular,the same product of formic acid can be produced by coupling methanol partial oxidation and electrocatalytic CO₂ reduction at the anode and cathode,respectively.The relationship between catalyst structure and catalytic performance has also been investigated.The main results of the study are summarized in the following:(1)Metal-organic framework Cu Co-BCD ultrathin nanosheet catalyst has been prepared by a facile ultrasonic-assisted method.The XRD,SEM,TEM,XPS and other characterizations have been conducted to investigate the morphology and structure of the electrode.The catalytic activity of the Cu Co-BCD electrode for methanol oxidation was evaluated in alkaline aqueous solution of methanol.A rather low potential of 1.36 V vs.RHE is required to reach a current density of 10 m A/cm² in the methanol electrolyte,which is 200 mV lower than that required for OER.Moreover,the Faraday efficiency(FE)for hydrogen generation at the cathode is up to100%,manifesting the efficient cathodic hydrogen production assisted by methanol oxidation.Additionally,isotopic labeling experiment has been adopted to trace the origin of cathodic H₂,which validates the hydrogen origin by the cleavage of water molecules.(2)The Ni-doped hierarchically Beta zeolite(Ni-H-beta)electrode has been synthesized by one-step hydrothermal method using tetraethylammonium hydroxide(TEAOH)and cetyltrimethylammonium bromide(CTAB)as structural guide agents.The morphology,pore and electronic structures of the materials were investigated by SEM,TEM,BET and XPS techniques.Ni species is doped into the lattice framework of zeolite as demonstrated by XPS results,which act as the catalytic metal center exhibiting excellent electrocatalytic methanol oxidation performance at the anode.The ratio of Si/Al in zeolite can be adjusted by controlling the amount of Al input,so as the acidity of the catalyst.With the increase of Al concentration,the strong acid sites of the catalyst increased and the mass current density of catalyst are elevated significantly,as demonstrated by the catalytic performance and pyridine-adsorption IR spectroscopy of various samples of varied Si/Al ratios.A mass current density of1450 A/g can be achieved at the Si/Al ratio of 10.(3)Ni(OH)₂@Pd/NF catalyst with a sandwiched structure has been fabricated by a solvothermal method utilizing Ni foam as the substrate and its catalytic performances on the glycerol oxidation(GOR)and the hydrogen evolution(HER)reaction have been studied,which demonstrates a promotion effect of Ni(OH)₂ on GOR and HER when compared with those of Pd/NF.When equipped with the bifunctional Ni(OH)₂/Pd/NF catalyst at both anode and cathode for glycerol electrooxidation and HER,respectively,the established co-electrolysis system shows a rather low required onset voltage of 0.8 V,which is significantly 670 mV lower than that required for pure water electrolysis,greatly reducing the energy consumption of the electrolysis system.Moreover,value-added products such as formic acid,glyceric acid,lactic acid can be obtained by the oxidation of glycerol.In addition,the catalytic stability and regeneration properties of Ni(OH)₂/Pd/NF were investigated by applying the voltage of 1.1 V and-1.1 V for each electrode for 1 hour and repeating the cycle for 10 times.(4)The self-supported CuO nanosheets grown on copper foam(CuONS/CF)synthesized by one-step oxidation was prepared as an anodic catalyst for partial methanol oxidation,and meanwhile,mesoporous SnO₂ was grown on carbon cloth(SnO₂/CFC)and utilized as cathode electrode for CO₂ electrochemical reduction.Phase composition,morphology and electronic structure of these two catalysts,as well as their electrocatalytic performance for methanol partial oxidation and ECR,have been carefully studied.The Faraday efficiencies for methanol partial oxidation and CO₂ reduction are as high as 97%and 83%,respectively.The assembled co-electrolysis system exhibites a quite low onset voltage of 0.93 V,which is more than 500 mV lower than that needed for traditional ECR system.Impressively,value-added products of formate can be produced simultaneously on both sides of the electrode,largely improving the efficiencies of fomate production.Such a strategy may provide an innovative and promising method for the efficient production of formate.