Preparation And Performance Of NiFe-based Hydroxide-oxide Microspheres For Oxygen Evolution Reaction

Hydrogen energy is a carbon-free clean energy with advantages of abundant reserves, higher calorific value, and fully converted into pure water during utilization, therefore, hydrogen energy is regard as an ideal renewable non-fossil energy sources. However, the large scale application of hydrogen energy associated with three problems: low-cost hydrogen production, safety hydrogen storage and efficient hydrogen utilization. For the hydrogen production aspect, water electrolysis in alkaline environment has the advantages of high hydrogen purity, green environmental protection and simple craft, gradually attracting widespread attention and research, however,the large-scale industrialization of the electrolytic hydrogen technology is mainly limited to high oxygen overpotential, which consume large amounts of extra energy. Noble metals based catalysts such as Ru and Ir has excellent oxygen evolution activity, however, the high cost and deficient resource of noble metals limited the large-scale applications, therefore, the development of high efficiency and good stability non-noble metals based electrocatalyst for oxygen evolution is of great significance.Layered double hydroxides (LDHs) are a kind of layered structurematerial with controllable chemical compositions and turnable guest anion species between layers. Based on the research, the LDHs or mixed metal oxide(MMO) obtained by calcination treatment of LDHs materials contains Co, Ni,Fe or Mn has promising oxygen electrode catalytic performance (activity and stability). However, the LDHs powder is prone to aggregate, resulting in a decrease in electrochemically active surface area and stability, which is detrimental to reveal the active sites. Therefore, the controllable preparation of less agglomeration hierarchical hollow microspheres structure based on LDHs or MMO material is a key problem to be solved.In this work, Ni2Fe-LDH and CoxNi2-xFe-LDH hollow microspheres were prepared by one-step hydrothermal synthesis based on template-free method ,and the LDHs precursors were treated with calcination to obtain the mixed metal oxide hollow microspheres.ICP-AES, XRD, SEM, TEM were used to characterize the composition, structure and morphology of the samples, the electrochemical performance were characterized by rotating disc electrode and cyclic voltammograms and linear sweep voltammograms technique. The main results of this work are as follows:(1) Ni2Fe-LDH hollow microspheres were synthesized by one-step hydrothermal method with raw materials of Ni(NO3)2·6H2O, Fe(NO3)3·9H2O,and NH4F as morphology regulation agent while CO(NH2)2 as precipitant. The hollow microspheres with a diameter of ca. 7?9 ?m, and the size of the LDHs lamellar structures on the surface was about 1 ?m. NH4F plays a great role in the formation of the hollow microspheres: Act as morphological regulation agent and cause chemically induced self-transformation. The formation of the hollow microspheres was Ostwald ripening based on the reaction time experiment. Although the surface area was reduced during the formation of hollow microspheres, however, the electrochemical active surface area (ECSA)was increased due to the excellent hierarchical structure. Finally, the Ni2Fe-LDH hollow microspheres exhibits best OER activity, which is superior to the noble metal oxide RuO2. The ?10 overpotential of the Ni2Fe-LDH hollow microspheres in 1 mol·L-1 KOH solution is only 290 mV, the Tafel slope is 51 mV·dec-1, and only 12.9% of overpotential increases after 2000 CV cicles.(2) CoxNi2-xFe-LDH hollow microspheres were synthesized by one-step hydrothermal method with raw materials of Ni(NO3)2·6H2O, Co(NO3)2·6H2O and Fe(NO3)3·9H2O , and NH4F as morphology regulation agent while CO(NH2)2 as precipitant. The CoxNi2-xFe-LDH hollow microspheres exhibits high oxygen evolution activity, the introduction of the Co2+ leads to the high conductivity Ni?OOH generate at low potential more easily and increase the reversibility, and one of the outstanding OER performance is Co1Ni1Fe-LDH hollow microspheres. The ComNi2-mFeOx-MMO hollow microspheres was obtained by calcinating CoxNi2-xFe-LDH hollow microspheres. After calcination, the hollow microspheres morphology of the precursor was completely retained. With the calcination temperature increases, sintering phenomenon occurs at 700?, which reduced the ECS A so that OER and ORR activity decreased to some extent, electrochemical tests found that Co0.75Ni1.25FeOx-MMO hollow microspheres obtained at 600? has the lowest ?E (EOER-EORR) of 0.95 V, which outperform the noble metal oxide IrO2 (1.32 V) and close to commercial Pt/C (0.94 V).