Ni/Fe-based Non-precious Metal Catalysts Are Used As High-current Oxygen Evolution Electrodes

With the accelerated consumption of non-renewable fuels such as oil and natural gas,hydrogen energy is progressively coming into focus due to its advantages of the high energy density and zero CO₂ release.As one of the most attractive strategies of producing sustainable and high purity hydrogen,electrochemical water splitting involves oxygen evolution reaction(OER)at the anode and hydrogen evolution reaction(HER)at the cathode.The OER is a four electron-proton coupled process,demanding a higher overpotential to surmount the sluggish kinetics as compared with HER,which is only a two electro-transfer reaction.The high overpotential would lead to an increase in the cell voltage and superfluous power comsumption.This defect has become the bottleneck of industrial water electrolysis and greatly diminishes the efficiency of hydrogen production.At present,noble metal oxides(RuO₂ and Ir O₂)and noble metal(Pt)are commonly regarded as the state-of-the-art efficient electrocatalysts for OER and HER,respectively.Nevertheless,the shortcomings such as the rareness,unacceptable cost and poor stability of these materials severely hinder their widespread applications in industrial water electrolysis.Furthermore,these catalysts always show high catalytic performance at low current densities such as 10mA/cm².When the current density is increasd to the commercially required high values such as 500 and 1000 mA/cm²,the catalytic activity is always depressed.Hence,it is desirable and urgent to develop the cheap,highly active and highly stable electrocatalysts for overall water splitting,especially for OER at high current density.Based on the above considerations,four electrocatalysts with low cost,high activity and excellent stability are designed and prepared,the catalytic performances for OER are comprehensively investigated.The main contents are as follows.1.Ultrathin FeNiO_xH_y nanoflake arrays with the thickness of only 4.5 nm were prepared on Ni foam(NF)via a facile hydrothermal reaction.The oxygen evolution reaction(OER)properties of the obtained sample(FeNiO_xH_y/NF)were investigated under alkaline conditions(1.0 M KOH).The optimized FeNiO_xH_y/NF displays extremely small overpotentials of only 195 and 306 mV to achieve the current densities of 10 and 1000 mA/cm²,respectively,and shows almost no potential attenuation during the 160 hours of stability test even the current density is up to 1000mA/cm²,demonstrating brilliant OER catalytic activity and durability.FeOOH and the NiOOH produced from the in situ oxidation of the surface Ni atoms of the NF substrate are the active sites.The synergistic effect between FeOOH and NiOOH is responsible for the high performances.To our knowledge,the high activity and stability of FeNiO_xH_y/NF catalyst outperform almost all of the OER catalysts reported to date.2.In this part,we report nickel foam(NF)supported low-crystalline NiCoFe nanosheet array electrocatalysts derived from NiColayer double hydroxide via spontaneous cation exchange reaction.Driven by the redox properties of NiColayer double hydroxide and iron salt precursors,the entire cation exchange reaction process is completed without extra energy input.The chemical composition and the catalytic properties of the as-prepared NiCoFe/NF catalysts are strongly dependent on the valence state of the Fe cation in the iron salt precursors.The NiCoFe/NF-FeSO₄catalyst with FeSO₄as the iron salt precursor exhibits satisfactory OER activity with much lower overpotential of 293 mV to reach the current density of 100 mA/cm² as compared with NiCoFe/NF-Fe₂(SO₄)₃(?₁₀₀=333 mV),NiCo/NF(?₁₀₀=425 mV)and commercial RuO₂(?₁₀₀=399 mV)catalysts.Moreover,NiCoFe/NF-FeSO₄ also exhibits impressive long-term durability for OER.The current density is nearly unchanged during the 95 h of continuous OER test,demonstrating a promising application in the actual industrial water electrolysis.These findings indicate iron ion valence states in the iron salt precursors have a significant impact on the composition of the catalysts and thus affect their catalytic performances.3.Nickel foam(NF)-supported NiCoP nanorod array catalyst is prepared and used for HER and OER.Not only at a small current density(10 mA/cm²),but also at the commercially required high current density(?500 mA/cm²),NiCoP/NF catalyst shows excellent bifunctional properties with extremely low overpotential and high stability.Small overpotentials of 60 and 253 mV are needed to drive the current density of 10 mA/cm² for HER and OER,respectively,and the cell voltage of overall water splitting is only 1.55 V.At the high current density of 500 mA/cm²,the overpotentials are 180 and 365 mV,and the corresponding cell voltage is only 1.83 V.Furthermore,the cell voltage hardly changes within 24 h of overall water splitting test,which makes the catalyst have promising applications in the industrial water electrolysis.The high performance of NiCoP/NF outperforms the commercial RuO₂-Pt/C couple and most of the bifunctional Ni/Co-based catalysts reported so far.4.S,Fe-NiFeO_xH_y/NF catalysts with 3D nanoflower structure are prepared through surface modification of nickel foam(NF)with S and Fe elements first and then electrodeposition technique.This preparation process only needs 6 min.The catalytic performances of the series of S,Fe-NiFeO_xH_y/NF catalysts for OER in alkaline electrolyte are investigated.Results show that the S,Fe-NiFeO_xH_y/NF catalyst requires an overpotential of only 196 mV to achieve the current density of 10 mA/cm².Even at the high current density of 500 mA/cm²for industrial demands,the overpotential of OER is only 291 mV.In addition,the S,Fe-NiFeO_xH_y/NF catalyst also exhibits promising long-term stability for industrial applications,maintaining a high current density of 500 mA/cm² for at least 40 h without any degradation.The open nanoflower structure makes the catalyst expose more active sites,which is benificial to improve the catalytic activity.In addition,the strong interaction between the components of the catalyst is also responsible for its excellent catalytic performance.The novel and convenient synthesis strategy proposed in this work has promising applications in the design and preparation of low-cost,high-performance non-noble metal OER electrocatalysts.