Preparation And Performance Of Iron-based Catalysts For Electrochemical Water Oxidation

Electrocatalytic water splitting plays a significant role in conversion of solar energy into chemical fuels.Water oxidation is considered as the bottleneck of water splitting due to its high thermodynamic barriers and multistep proton-and electron-transferred processes.Therefore,the development of stable and highly efficient water oxidation catalysts is essential for realization of overall water splitting.Among the various water oxidation catalysts,Fe-based materials are regarded as promising water oxidation electrocatalysts owing to their low cost and natural abundance.In this thesis,FeOOH/NPC-NF(Nickel Foam)constructed by electrodeposition of FeOOH on the surface of zeolitic imidazolate framework(ZIF-8)derived porous carbon(NPC)was developed as electrode for catalytic water oxidation.Electrochemical tests indicated that FeOOH/NPC-NF exhibited an excellent catalytic activity in 1 M KOH with an overpotential of 230 mV to attaining a current density of 100 mA cm-2.The characterizations of electrochemical impedance spectroscopy(EIS),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and Raman suggested that ZIF-8 derived porous carbon not only improve the electronic conductivity of FeOOH,but also provide enough channels for the in situ generated NiO from the innermost NF substrate to diffuse into the surface of porous carbon.The interactionm of in-situ formed NiO with FeOOH can produce synergetic effects for the improvement of catalytic performance.Considering that the porous carbon derived from metal-organic frameworks(MOFs)with ordered structures as the precursors are generally prepared via carbonization process at high temperatures,Fe-Salen complex and Co2+were selected as the precursors for the synthesis of Co1.2Fe/C hybrid catalyst by solvothermal carbonization at 160? in this thesis.The use of molecular complex as the precursors for the preparation of metal-carbon materials could lead to the confined growth of the metal centers due to the existence of molecular subunit.This result not only facilitated the uniform distribution of Co1.2Fe hydroxide in the Salen-derived carbon matrix,but also enhanced the electron conductivity via the intimate contact between metal and nanocarbon.Electrochemical measurements manifested that Co1.2Fe/C exhibited superior catalytic activity to the recently reported CoFe-based materials in 1 M KOH with an overpotential of 260 mV at the current density of 10 mA cm-2.The FeCH(Iron carbonate hydroxide)decorated NF was adopted as a semisacrificial template to prepare NiFe-MOF/FeCH-NF electrode by solvothermal reaction,and the as-prepared NiFe-MOF/FeCH-NF was subsequently used directly as working electrode towards water oxidation without carbonization.In this approach,the FeCH nanosheets not only provide Fe sources for the synthesis of NiFe-MOF nanoplates,but also control the diffusion rate of etched Ni2+ions from the innermost NF substrate.Thus,the rapid electron transfer and mass transport can be achieved by the resultant NiFe-MOF nanoplates with reduced thickness.Electrochemical measurements suggested that the NiFe-MOF/FeCH-NF electrode presented the highest catalytic activity among the recently reported NiFe-based MOF electrocatalysts in 1 M KOH with an overpotential of 200 mV at the current density of 10 mA cm-2.Overall,the above work provides new strategies for rationally designing low-cost,highly efficient and stable electrocatalysts on the basis of deep understanding of electrocatalytic water oxidation kinetics and interfacial electron transfer.