Synthesis And Electrocatalytic Properties Of Ni-based Multi-element Transition Metal Hydroxides And Phosphides

The electrocatalytic water splitting technology uses electrical energy to split water into hydrogen and oxygen,which can convert the electrical energy to store into chemical energy for storage.Hydrogen energy has the advantages of clean,renewable,high combustion calorific value and so on.There are two half reactions for water splitting,namely,hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Both HER and OER exhibit low kinetic characteristics,hindering the progress of water splitting.Electrocatalysts can decrease the overpotentials of HER and OER and facilitate the water splitting progress.The scarce precious metal catalysts,such as Pt,Ru,Ir and their oxides,have excellent catalytic activity,but cannot meet the application of large-scale industrial production.Therefore,it is particularly important to develop non-noble metal catalysts with comparable catalytic activity and sufficient stability.In this study,Ni-based multiple transition metal hydroxides and phosphides were grown directly on the surface of nickel foam?NF?,with low cost,high catalytic activity and good stability.Zirconium doped nickel-iron layered double hydroxides nanosheets?NiFeZr LDHs?were deposited on NF substrate by the simple hydrothermal method.The doped Zr regulated the electronic structure of Ni and Fe and increased the electrochemical active surface area of NiFe LDHs.Therefore,NiFeZr LDHs showed excellent OER catalytic activity.NiFeZr LDHs only required a small overpotential of 198 mV to achieve the current density of 10 mA cm^-2 with a small Tafel slope of 53.1 mV dec^-1,which was better than NiFe LDHs and IrO₂/C catalyst.In addition,the catalyst also showed good electrochemical stability without significant activity decline after 12 hours of constant voltage test.First,NiMoO₄ nanorods were formed on NF by hydrothermal reaction,then cobalt-iron hydroxide was deposited on them,and finally they were converted into multi-metal phosphide nanorods?P-NM-CF HNRs?by high-temperature phosphating.The surface of P-NM-CF HNRs was highly pleated and porous,which was beneficial to the adsorption of electrolyte and the diffusion of gas products.The strongly electronegative P adjusted the electronic structure of the metal elements,which could enhance the ability of attracting and capturing electrons during the HER process.Therefore,P-NM-CF HNRs showed excellent electrocatalytic activity.For OER,P-NM-CF HNRs could reach the current density of 10 mA cm^-2 under a low overpotential of 195 m V in 1M KOH,displaying a small Tafel slope of 48.4 mV dec^-1.Meanwhile,P-NM-CF HNRs achieved high-efficiency wide-range pH hydrogen evolution,only requiring low overpotentials of100 and 130 mV to 10 mA cm^-2 in 1 M KOH and 0.5 M H₂SO₄,respectively.To perform overall water splitting,P-NM-CF HNRs only required a low voltage of 1.53 V to achieve10 mA cm^-2,with outstanding stability without activity degradation in the stability test for 36h.