Preparation Of Iron-cobalt Based Spinel Compound For Electrocatalytic Water Splitting

Increasing fossil fuel extraction and depletion have caused serious environmental problems,it is urgent to develop new clean energies to replace traditional fossil fuels.Due to its high energy density,light weight,abundance on Earth,and environment friendliness,hydrogen is one of the best alternatives of fossil fuels.Among many hydrogen production technologies,electrocatalytic water decomposition is regarded as the most effective method to produce hydrogen.However,the high energy consumption caused by the slow reaction kinetics of electrode and large overpotential seriously restrict the development of water electrolysis to produce hydrogen.Excellent electrocatalysts are very important for reducing the energy barrier and improving the kinetics of both OER and HER.At present,the most efficient electrocatalyst for HER and OER are Pt,RuO₂ and IrO₂,respectively.Nevertheless,the expensive price and scarcity hinder their practical applications.Thus,it is indispensable to develop the non-noble metal electrocatalysts with higher activity,good stability,and earth-abundant.Because of high catalytic activity and low cost,Ni/Co-based transition metal oxides are the research hotspot of OER catalysts.It is know that Fe has a positive effect on the catalytic activity of Ni-based compounds,but there are few studies about the effect of Fe in Co-based compounds.Inspired by this,we firstly prepared the FeCo₂O₄nanosheet arrays supported on Ni foam（FeCo₂O₄/NF）by hydrothermal method and investigated its OER performance.In order to further improve catalytic performance,the FeCo₂S₄/NF bifunctional catalyst was synthesized by secondary hydrothermal treatment of Fe-Co precursor using Na₂S·9H₂O.We systematically investigated the performance of overall water-splitting of FeCo₂S₄/NF,and preliminarily discussed its structure evolution during HER and OER.The main contents and conclusions of this paper are summarized as follows:（1）FeCo₂O₄/NF was prepared by hydrothermal method using FeCl₂·4H₂O as Fe source instead of part of Co in Co₃O₄/NF.The result of XRD showed that FeCo₂O₄/NF is pure phase and FE-SEM observation showed that FeCo₂O₄/NF catalyst present nanosheet structure.Tafel slope of FeCo₂O₄/NF is 79 mV dec^-1 and it requires an overpotential of 380 mV to deliver the current density of 50 mA cm^-2 in 1 M KOH,which is much superior than that of Co₃O₄/NF,suggesting that the introduction of Fe can improve the OER catalytic activity of Co₃O₄/NF.（2）FeCo₂S₄/NF was derived from a subsequent hydrothermal sulfuration of Fe-Co precursor using Na₂S·9H₂O.The result of XRD showed that FeCo₂S₄/NF has no impurity.The results of ICP-MS and XPS demonstrated that the atomic ratio of Fe,Co,S in FeCo₂S₄/NF is consistent with the ratio of starting materials and the stoichiometric ratio of FeCo₂S₄,and exist in the form of Fe²⁺,Fe³⁺,Co²⁺,Co³⁺and S^2-,respectively.The results of FE-SEM and TEM indicated it maintain the nanosheet structure of Fe-Co precursor.According to the results of electrochemical performance test of water-splitting,when serving as a catalyst for HER,it need an overpotential of 132 mV to deliver the current density of 10 mA cm^-2.Meanwhile,FeCo₂S₄/NF electrode shows high OER catalytic activity,which only requires an overpotential of 270 and 290 mV to achieve current densities of 50 and 100 mA cm^-2,respectively.The FeCo₂S₄/NF electrode exhibits considerable electrochemical stability at high current density,with negligible potential change in alkaline electrolyte.Furthermore,a two-electrode alkaline electrolyzer was constructed using FeCo₂S₄/NF as the anode and cathode,respectively.It could achieve a current density of 10 mA cm^-2 at a cell voltage of 1.63 V,which is a potential bifunctional catalyst.（3）To further characterize the structure evolution of FeCo₂S₄/NF after electrochemical stability test,XRD,XPS and Raman were performed.After the HER stability test,the crystal structure and chemical composition of FeCo₂S₄/NF did not change substantially.Although no obvious changes in the crystal structure,the surface of FeCo₂S₄/NF was transformed to the corresponding metal（oxy）hydroxides during the OER test,which could be the actual active species for OER.The above results indicate that the transition metal chalcogenides are unstable under alkaline condition as an OER catalyst,and the internal chalcogenide provide a good electron transport channel for the surface active species,which may be one of the reasons for its excellent catalytic performance.