Study On Three-Dimensional Nanocomposite Structure And Catalytic Performance Of Transition Metal Hydroxide For Alkaline Seawater Electrolysis

Hydrogen（H₂）has advantages of high energy density,renewable,combustible and zero greenhouse gas emissions from combustion,and is considered to be the most ideal sustainable clean energy carrier.At present,water electrolysis is an ideal way to produce hydrogen,but the heavy reliance on fresh water limits its development.Seawater electrolysis can effectively solve the dependence on freshwater resources,and it is expected to obtain nearly infinite clean hydrogen energy.However,seawater electrolysis to produce hydrogen faces serious problems such as the competitive reaction of chloride ion oxidation on the anode and the corrosion of chloride ions to electrode materials.Therefore,the development of electrocatalysts for high-efficiency selective oxygen evolution reaction（OER）is of great theoretical and practical significance for seawater electrolysis.The noble metals of iridium dioxide（Ir O₂）and ruthenium dioxide（Ru O₂）have excellent OER catalytic performance,but their performance is unstable during seawater electrolysis.And they have the disadvantages of limited reserves and high price,which are difficult for commercial applications.Therefore,transition metal nickel-iron hydroxide（NiFe-LDH）-based catalyst materials were prepared in this paper and the performance has been studied in alkaline simulated seawater（1 M KOH+0.5 M NaCl）and alkaline natural seawater（1 M KOH+Seawater）.（1）Nickel molybdate nanorods（NiMoO₄/NF）were synthesized by a solvothermal method,and the nanorods were grown vertically on the nickel foam（NF）substrate.S-NiMoO₄/NF was obtained by sulfide modification of NiMoO₄/NF.Sulfur modification by a hydrothermal method can improve conductivity of the material,the resistance to chloride corrosion and the activity for hydrogen evolution reaction（HER）.S-NiMoO₄@NiFe-LDH with a three-dimensional core-shell heterostructure was prepared by electrodepositing NiFe-LDH on the surface of S-NiMoO₄/NF.In 1 M KOH+0.5 M NaCl,S-NiMoO₄@NiFe-LDH required overpotentials of 170 m V and273 m V to reach 100 m A cm^-2for HER and OER,respectively.That was less than 480m V to induce the oxidation of Cl^-to Cl O^-,and better than most of the catalysts reported.The result of chronoamperometry showed that the current of S-NiMoO₄@NiFe-LDH was not significantly attenuated after 20 h.Analyzing the XPS after the test,it showed that amounts of SO₄^2-was generated,which produced electrostatic repulsion to Cl^-,making S-NiMoO₄@NiFe-LDH have good resistance corrosion to chloride ion and keep stable.While the valence state of elements was not affected after HER stability,which was consistent with the results of the chronoamperometry.In a two-electrode system,S-NiMoO₄@NiFe-LDH was used as both anode and cathode,simultaneously.The result was that in 1 M KOH+0.5 M NaCl and 1 M KOH+Seawater,the current density of 100 m A cm^-2 were generated at the voltages of 1.68 V and 1.73 V,respectively.That were both better than Pt/C||Ir O₂（1.73 V and 1.81 V）.Moreover,the cell showed good stability in both alkaline simulated seawater and natural seawater.（2）NiFe-LDH with a nano-flower-like structure was synthesized on nickel foam by a hydrothermal method.CeO₂@NiFe-LDH was generated by depositing CeO₂nanoparticles on the surface of NiFe-LDH for oxygen evolution reaction（OER）in the chloride-containing electrolyte.In 1 M KOH+0.5 M NaCl,it was found that the overpotential of 265 m V could make CeO₂@NiFe-LDH generate a current density of100 m A cm^-2,and the value of Tafel slope was 47.7 m V dec^-1.That was better than the performance of NiFe-LDH and CeO₂@NF.Therefore,the oxygen vacancies provided by CeO₂ and the the formation of heterojunction with NiFe-LDH both enhanceed the catalyst activity.Due to the complex and diverse species presented in natural seawater,CeO₂@NiFe-LDH required the overpotential of 285 m V to achieve a current density of100 m A cm^-2 in 1 M KOH+Seawater.In 1 M KOH,the polarization curve of CeO₂@NiFe-LDH(overpotential of 262 m V to reach 100 m A cm^-2)and Tafel slope(42.9 m V dec^-1)were both close to those in 1 M KOH+0.5 M NaCl,but the performance of NiFe-LDH in the two electrolytes is quite different.Therefore,it showed that the performance of CeO₂@NiFe-LDH was not affected by Cl^-.That was,Cl^-didn’t not change the intrinsic kinetics of CeO₂@NiFe-LDH for OER.Moreover,the chronoamperometric curve of NiFe-LDH decreased significantly after 30 h in 1 M KOH+0.5 M NaCl.But the chronoamperometric curve of NiFe-LDH was stable in 1M KOH.In 1 M KOH+0.5 M NaCl electrolyte,the chronoamperometric curves of CeO₂@NiFe-LDH was also stable after 30 h.The phenomenon showed that the CeO₂layer could effectively alleviate the corrosion of Cl^-so that CeO₂@NiFe-LDH could exhibit superior OER performance and excellent stability in chloride-containing electrolytes.