Synthesis And Electrocatalytic Oxygen Evolution Reaction Of Three-dimensionally Ordered Hierarchical Porous Ni-Co Alloys

To alleviate the environmental issues and energy crisis,electrochemical water splitting has received more and more attention as a convenient and effective route to generate hydrogen.The efficiency of overall water splitting depends on the anode oxygen evolution reaction(OER).At present,noble metal-based catalysts with good OER catalytic activity have low reserves and poor stability,so transition metals(such as Ni,Co,Fe)with rich reserves have become the potential substitutes due to their high stability and activity.In this work,we prepared 3D ordered macro-/mesoporous Ni by using the dual-templating and chemical deposition technique and studied the performance for electrocatalytic OER.Then we extended the method to Ni-Co alloys and discussed the effects on the OER performance from the perspective of Ni-Co ratios and porous structure.Here are the main results in our work:(1)3D ordered macro-/mesoporous Ni(3DOM/m Ni)was prepared using PMMA and the surfactant Brij 58 as macroporous and mesoporous templates,respectively,and its OER performance was studied in 1.0 M KOH solution.The size of the PMMA template is about 340 nm.The 3DOM/m Ni prepared with this template has an interconnected ordered hierarchical porous structure.The size of macropore is the same as that of PMMA template(340 nm)and the mesoporous size is in the range of 3-5 nm.Compared with commercial RuO2 catalyst,the 3DOM/m Ni catalyst exhibits better OER performance.(2)In order to further improve the performance of 3DOM/m Ni,the second metal element Co was introduced in our work.3DOM/m NixCo100-x alloys(x=100,80,70,60,30 and 0)with six different ratios were prepared by the same method.Moreover,the effects of different Ni-Co ratios of 3DOM/m NixCo100-x catalysts on the OER performance were also investigated.The optimum ratio for 3DOM/m NixCo100-x catalyst is 70-30(Ni7oCo30)according to the physical characterization and electrochemical results.In this case,the macro-/mesoporous structure of the sample is the most ordered and the OER performance is the best.The overpotential is just 241 mV to reach the current density 10 mA cm-2 and the Tafel slope is 44 mV dec-1.The 3DOM/m Ni70Co30 exhibits good stability with only 8.0%current density decay for 25 h electrolysis.(3)Futhermore,the effects of the porous structure of catalysts on OER performance were also investigated.Based on the optimal alloy ratio of 70:30,three catalysts with different structures from 3DOM/m Ni70Co30 were prepared for comparison,such as ordered only macroporous structure,ordered only mesoporous structure and nanoparticles.Among them,3DOM/m Ni70Co30 exhibits the best OER catalytic performance which can be attributed to its distinct 3D hierarchical macro-/mesoporous structure.The macroporous structure can enhance the mass transfer,and the ordered mesopores can increase the specific surface area and the exposure of active sites.