Synthesis Of Fe-,Co-,Ni-Based Electrocatalysts For Water Splitting

The electrocatalytic hydrogen production system driven by green renewable energy（solar energy,wind energy,etc.）is the most promising way to develop hydrogen energy.However,at present,electrocatalytic hydrogen production by water splitting faces the problems of high cost and low hydrogen production efficiency.Therefore,the development of cheap and efficient electrocatalysts for the hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）is significant to break the bottleneck and realize the large-scale application of water splitting to hydrogen.In this paper,Fe-,Co-,Ni-based selenides and oxides are mainly studied.Firstly,a series of micro-and nano-structures with different shapes are designed and synthesized by a simple method,and then these materials are used as electrocatalysts for OER or HER in alkaline system.Combined with the experimental phenomena and characterization results,it is respectively discussed that the effects of composites and bonding interface on OER properties,the surface or deep reconstruction of the precatalysts in the process of anodic oxidation or cathodic reduction.After discussion and analysis,the structure-activity relationship and the catalytically active species are clarified.Meanwhile,the understanding of the underlying mechanisms for alkaline OER and HER was deepened.The main research results obtained are as follows:（1）A series of novel Co Se/Fe Se₂ composites with different atomic ratios of Co and Fe are synthesized by one-step hydrothermal method.When the atomic ratio of Co/Fe is about 3:1(Co_0.75Fe_0.25-Se),the Co_0.75Fe_0.25-Se exhibits the best OER activity that only the overpotential of 246 m V is required to reach the current density of 10 m A cm^-2,and the Tafel slope is as low as 41.4 m V dec^-1.Such good OER properties are mainly due to the rich interfaces and favorable electronic interaction between Co and Fe atoms.（2）A unique core-shell heterogeneous electrocatalyst（NF/Ni Se@Fe₂O₃）is successfully designed and synthesized using a simple two-step hydrothermal method.The as-synthesized electrocatalyst demonstrates remarkable OER activity in 1 M KOH with a low overpotential of 220 m V（or 282 m V）at 10 m A cm^-2(or 200 m A cm^-2),and only a small Tafel slope of 36.9 m V dec^-1.Significantly,the NF/Ni Se@Fe₂O₃ can sustain an extraordinarily long-term OER stability in 1 M KOH nearly 230 h.X-ray photoelectron spectroscopy and X-ray absorption spectroscopy show that the NF/Ni Se@Fe₂O₃ heterostructure has a highly hydroxylated surface and strong interface coupling due to the formation of Fe-Se bond at the interface between Ni Se and Fe₂O₃.Density functional theory calculation further confirms that the formation of Fe-Se bond leads to the movement d-band center and optimizes the electronic structure,which promotes the adsorption of oxygen-containing intermediates and the desorption of O₂in the OER process,thus greatly enhancing the OER activity of the electrocatalyst.Moreover,the unique core-shell structure and strong interface coupling effectively improve the long-term stability of the NF/Ni Se@Fe₂O₃.（3）A Ni-Mo-Se precatalyst with three-dimensional hierarchical structure is synthesized,and a facile on-site electrochemical activation strategy for achieving deep reconstruction of the precatalyst is developed successfully.Using the combination of multiple spectroscopic characterizations and high resolution electron microscopy techniques,it is unraveled that the Ni-Mo-Se precatalyst is deeply reconstructed intoγ-Ni OOH with co-leaching of Mo and Se after anodic oxidation.Such flower-likeγ-Ni OOH is constituted by distorted ultrathin nanosheets with a thickness of～4.5 nm and contains abundant intercalated species such as water or OH^-/CO₃^2-,thus offering a large quantity of accessible active sites.In purified KOH electrolyte,to reach the current density of 10 m A cm^-2,theγ-Ni OOH requires an overpotential of only 244 m V,outperforming almost all the reported Fe-free Ni OOH-based electrocatalysts.（4）The nickel foam is transformed into Ni₃Se₄/Ni Se₂ electrode by one-step thermal selenization.Electrochemical tests and subsequent characterization confirm that the Ni₃Se₄/Ni Se₂ electrode undergoes obvious activation in the process of alkaline HER,accompanied by the destruction of Ni Se₂ and the dissolution of Se.In situ Raman spectra and quasi in situ X-ray photoelectron spectroscopy show that the nano-nickel（Ni⁰）will be formed with the destruction of Ni Se₂ in the process of cathodic activation.Therefore,Ni₃Se₄/Ni is the real active species for HER,although the nano-nickel（Ni⁰）is very easy to be oxidized to Ni O in air.Because Ni₃Se₄/Ni can balance Volmer step and Heyrovsky/Tafel step in alkaline HER process,it shows good HER activity that the current density of-10 m A cm^-2 can be achieved with an overpotential of only 106 m V.（5）A Ni-doped Co₃O₄(Co_3-xNi_xO₄)prepared by a simple method exhibits favorable HER activity and stability（>300 hours,whether in 1 M KOH or realistic 30wt%KOH solution）after in situ electrochemical activation,which outperforms almost all the oxide-based electrocatalysts.More importantly,using the combination of in situ Raman spectroscopy and multiple high resolution electron microscopy techniques,it is identified that the surface of Co_3-xNi_xO₄ crystals is reduced into intertwined Co_yNi_1-yO nanoparticles with highly exposed{110}reactive planes.Density functional theory calculation further proves that the Ni-doped Co O component in Co_yNi_1-yO plays a major role during the alkaline HER,because the introduction of Ni atoms into Co-O octahedras can optimize the electrical conductivity and tailor the adsorption/desorption free energies of H_ad and OH_ad intermediates.