Synthesis Of NiFe-Based Nanostructure Materials And Study On The Oxygen Evolution Performance

The global population increase in record and the increasing pressure on natural resources and energy demand are being exacerbated by industrialization in developing countries.Therefore,electrochemical water splitting reactions play a critical role in energy generation,conversion,and storage devices.For instance,the ability to store energy from intermittent renewable energy sources is an ongoing challenge.However,electrochemical energy storage and conversion devices play a key role in the advancement of pure,viable,and cost-effective energy systems to meet the industrial demand of our society.Therefore,to meet up industrial needs with electrolysis-produced hydrogen,the development of a low-price and cost-effective catalyst for the oxygen evolution reaction（OER）is crucial.Catalysts such as IrO2 and RuO2 are being well used with state-of-the-art OER electrocatalysts,but such precious-metal-based materials are severely limited by their scarcity and high cost.Therefore,it is important to develop low-cost and high efficiency electrocatalysts for OER.To replace these precious metals-based catalysts,transition metals are the best choice and exhibit exceptional electrochemical performance.Transition metal oxides and hydroxides based on nickel（Ni）,iron（Fe）,and cobalt（Co）have shown promising electrocatalytic activity towards the OER.However,the fabrication of a homogeneously mixed metal electrocatalyst is not a facile process.Amongst the various synthesis routes for electrocatalyst fabrication,the electrochemical approach provides a facile means to fabricate homogeneously mixed metal oxide nanostructures with the desired composition,morphology,and shape.The individual advantage is that the synthesis can be carried out rapidly with a simple setup under ambient conditions.In recent decades,Nickel-iron-based catalysts have shown the highest OER activity and might be used in the water oxidation process.NiFe-based（oxy）hydroxides and layered double hydroxides have been demonstrated to be amongst the highly active OER electrocatalysts in alkaline electrolyte solutions,competing and in many cases surpassing state of the art.However,a further enhanced catalytic activity is desired and challenges remain in the design of NiFe-based catalysts:（ⅰ）How to replace the content of precious metals with nonprecious transition metals?（ⅱ）How to optimize the electronic structure?（ⅲ）How to improve the strong electronic interaction and active sites of NiFe-based catalysts to make its OER performance close to IrO2 and RuO2.However,to solve the above problems,the modulating of electronic structure,improve the robust electronic interaction and increase the catalytic active sites of NiFe-based materials.The strength of basic OER reactants was regulated and optimized by doping and electrodeposited of foreign elements,such as S,Ce,and Co,which greatly improved the basic OER performance.1.We successfully synthesized a unique composite electrocatalyst that cobalt sulfide electrodepositing on NiFe hydroxide nanosheet（NiFe hydroxide@CoS）.The NiFe hydroxide@CoS exhibited a low overpotential of 259 mV at current density of 10 mAcm-2,a low Tafel slope of 45.8 mV dec-1 and excellent durability in 1 M KOH solution.The enhanced catalytic activity is due to the increased intrinsic catalytic activity and enlarged surface area caused by the electrodeposition of CoS over NiFe hydroxide nanosheet.Therefore,the facile electrodeposition of CoS benefits NiFe hydroxide nanosheet with the better activity toward OER,facilitating the low-cost and stable water electrolysis.The deposition of CoS improves catalytic activity by providing higher metal valence states for Ni and Fe species,which is expected to aid absorption to intermediates（*OH and*OOH）during the OER reaction.In short,the NiFe hydroxide@CoS electrocatalyst’s increased OER catalytic activity in this study was due to both intrinsic activity and the catalyst’s increased surfaced area.2.In second part,Ce@NiFeS was successfully prepared by a three-step hydrothermal method by interfaced Ce and addition tunable sulfurization,which elucidates their electronic structure modulation,introducing an additional active site,and achieving high catalytic performance toward OER.Both Ce interfacing and sulfidations significantly improved the catalyst OER performance in 1M KOH according to electrochemical tests.In alkaline media,it exhibits excellent performance,with a minimal overpotential of only 195 mV at a current density of 10 mA cm-2,compared to 270 mV of state-of-the-art NiFe-LDH.However,the electronic structure could be significantly modulated due to Ce nanoparticles and an additional Sulfurization step,resulting in increased intrinsic catalytic activity and improved water oxidation performance in an alkaline electrolyte.