Preparation And Water Electrolysis Performances Of CoNi-based Polymetallic Phosphides 3D Self-supporting Electrodes

Electrochemical water splitting is considered to be a potential way to produce hydrogen due to the advantages of simple technology,abundant raw materials,environmental benignity and high purity of prepared hydrogen.Noble metal-based catalysts are considered as the most efficient catalysts toward hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),however,the scarcity and highcost of noble metal-based catalysts hinder their widespread industrial use.In addition,Pt-group catalysts usually exhibit remarkable HER activities but poor OER performance,which is similar to Ir catalysts that only exhibit good OER activities.Bifunctional catalysts for HER and OER also have low activity and poor stability in the same electrolyte.Therefore,it is significant to design and fabricate efficient non-precious-metal based catalysts for HER and OER,especially the bifunctional overall water splitting.Nowadays,transition metal phosphides(TMPs)have received considerable interests as highly efficient,greatly active,greatly stable,and low-cost electrocatalysts for HER or OER.Among them,nickel and cobalt phosphides showed efficient bifunctional catalytic properties for water splitting in general.Based on the above research background,in this work,transition metal phosphide is taken as the research object,this work integrates the advantages of hydrogen evolution and oxygen evolution catalysts and improve the catalytic performance of electrolytic water by means of composition design,element doping,structure design and so on.We prepared CoxNiyP/NF and CoxNixMzP/NF(M is Fe,Mn)3D self-supporting electrodes in situ grown on the nickel foam via a hydrothermal method with subsequent phosphorization,which consists of phosphides and amorphous phosphates.Then,we studied the HER and OER performance of CoxNiyP/NF and CoxNixMzP/NF in alkaline electrolyte.On the basis of a large number of experiments,some valuable research results have been obtained.Firstly,CoxNiyP/NF 3D self-supporting electrodes with different compositions were prepared,and the effects of Co and Ni atomic ratio on the morphology and performance of the electrodes were studied.CoNiP/NF with Co and Ni atomic ratio of 1:1 has the best bifunctional catalytic performance.In alkaline media,NiCoP/NF requires an overpotentials of 101 mV to reach the current density of 10 mA·cm-2 for HER,and it can maintain good performance after a long-term stability test.Additionally,NiCoP/NF exihibits good performance toward OER,a low overpotential of 270 mV is needed to achieve a current density of 10 mA·cm-2.Using CoNiP/NF as both the cathode and anode of an electrolytic cell for overall water splitting,which needs a cell voltage of 1.64 V to achieve 20 mA·cm-2 current density.CoxNixMzP/NF 3D self-supporting electrode material was prepared by doping a third metal Mn or Fe into CoNiP/NF.Mn and Fe doping promote CoxNixMzP/NF to form a heterostructure in which phosphide and phosphate amorphous coexist.According to the experimental results,the CoxNixMnzP/NF F is inferior to CoNiP/NF due to the strong adsorption of Mn by hydroxyl,while the Co0.45Ni0.45Fe0.1P/NF doped with appropriate amount of Fe has better performance.Compared with CoNiP/NF,the HER stability and OER catalytic performance of Co0.45Ni0.45Fe0.1P/NF have been greatly improved.Co0.45Ni0.45Fe0.1P/NF exhibits high activity and excellent durable performance for HER with an overpotential of 107 mV at current density of 10 mA·cm2 in alkaline media,and the catalytic activity did not decline after 10,000 cycles of cyclic voltammetry test.It also shows superior catalytic activity toward the OER with a low overpotential of 250 mV at 10mA·cm-2 in alkaline media.Furthermore,the practical application of Co0.45Ni0.45Fe0.1 P/NF as a bifunctional catalyst for overall water splitting reaction yields current densities of 20 mA·cm-2 at a low cell voltage of 1.60 V.The performance for overall water splitting of Co0.45Ni0.45Fe0.1P/NF is better than most bifunctional catalysts reported so far.Reasonable composition design,good three-dimensional structure and multi-metal element doping make Co0.45Ni0.45Fe0.1P/NF have the extremely excellent catalytic performance.The heterostructure of the phosphide and phosphate amorphous layer formed by Fe doping has an important effect on the electrocatalytic performance of the sample,which improves the HER stability and the OER activity.The above work not only prepares excellent bifunctional catalytic electrode materials,but also provides new ideas for developing high-performance non-precious metal bifunctional catalysts.This research is of great significance to promote the practical application of electrochemical water splitting.