Preparation Of Porous Nickel-cobalt Phosphide-supported Ultra-thin Alloy Catalysts And Their High-efficiency Total Water Splitting Performance

As one of the promising green energy resources,hydrogen energy has a high combustion energy and abundant raw materials.For the manufacturing processes of hydrogen production,electrochemical water splitting has become one of the most prospective technologies for hydrogen generation.However,the overpotentials in the hydrogen evolution reaction（HER）and the oxygen evolution reaction（OER）processes result in great energy expenditure during in the electrolysis process and thus hinder the industrialized development.Hence,it is necessary to develop high catalytical active and low-cost dual-function catalysts for water electrolysis.In past decades,a lot of catalysts were developed for water splitting,most of them were not meet an efficiency both for HER and OER simultaneously.To realize well combination of HER active materials and OER active materials,we designed and fabricated an outstanding bifunctional electrocatalyst for overall water splitting by electrodepositing ultrathin NiFe film on the surface of NiCo phosphide supported by titanium mesh（NiCoP/TM）.In order to further elevate the performance of the catalyst,vanadium was doped in the former catalyst.Nickle-cobalt-based phosphides have good catalytic activity for HER,but have poor catalytic activity for OER.Herein,ultrathin NiFe alloy was electrodeposited on the surface of NiCo phosphide（NiCoP）supported by titanium mesh（TM）,NiFe/NiCoP/TM possess an excellent and outstanding catalytic performance for water splitting.In order to obtain high catalytic performance,the experimental conditions including the composite of the electrolyte and the electrochemical parameters involved were optimized.As prepared NiFe/NiCoP/TM electrodes were characterized by SEM,TEM,EDX and XRD.Morphology SEM observation suggested that the NeFe alloy were uniformly coated on the surface and the top of the holes of the NiCoP/TM substrate.The NiFe coat was approximately 0.5 μm to 1 μm thick,the average pore size was about 17 μm and the depth of the holes was about 15 μm.The EDX results confirmed the uniform distribution of the coated NiFe alloy.The phase composition of the NiFe coating was proved to be Ni0.36Fe0.64 by XRD.The XPS results indicated the slightly oxidation of the surface,generating nickel-iron hydroxides and oxides.The electrocatalytic performances of NiFe/NiCoP/TM for HER and OER were measured by linear sweep voltammetry（LSV）in a solution of 1.0 M KOH or 0.5 M phosphate buffer solution（PBS）.The overpotentials were 57 mV and 220 mV to afford the current density（j）of 10 mA cm-2 in alkaline solution for HER and OER,respectively.The cell voltage at 10 mA cm-2 was 1.55 V,which was superior to that of Pt/C/TM‖RuO2/TM.In 0.5 M neutral phosphate buffer solution,an overpotential of 458 mV for OER was required to achieve the same current density.The excellent bifunctional activity of the NiFe/NiCoP/TM electrocatalyst was attributed to four aspects.Firstly,after NiFe deposition,the original porous structure of the NiCoP/TM were well reserved,which ensures large active area and lots of activity sites.Secondly,NiFe alloy owns outstanding conductivity increases electron migration rate and reduces the resistance.Thirdly,the hydroxides and oxides yielded on the surface owing to slight oxidation reinforced the OER catalytic performance.Fourthly,synergistic effect between NiFe alloy and the NiCoP base coating co-contributed to the outstanding performance.In our second work,vanadium was introduced into the NiFe/NiCoP/TM system to further elevate the electrocatalytic performance.The suitable VOx@NiFe/NiCoP/TM electrocatalyst was prepared by adjusting the composite of the electrolyte and the electrochemical parameters.The prepared VOx@NiFe/NiCoP/TM electrocatalysts were analyzed by SEM,EDS,TEM,XRD and XPS.The analysis results confirmed that VOx@NiFe was uniformly deposited on surface of NiCoP/TM,and the morphology of sublayer were well reserved.The content of V in VOx@NiFe composite was about 1.5%（atomic ratio）,which also was proved by the XPS analysis.The prepared VOx@NiFe/NiCoP/TM electrodes possess an efficient electrocatalytic activity for water electrolysis.The LSV measurement demonstrated that only overpotentials of merely 45 mV and 215 mV were required to afford the j=10 mA cm-2 in 1.0 M KOH solution for the HER and OER processes,respectively.The cell voltage was measured to be just 1.52 V.The V-doped electrode was confirmed to be dual functional with long durability.Electrochemical double layer capacitance（Cdl）and EIS were measured to understand the advantages of the electrode from electrochemical active area aspect.The high catalytic performance of VOx@NiFe/NiCoP/TM compared with its precursor was attributed to of the enhance of the oxygen vacancies,the hydrophilicity of the material and the electrochemical active surface area（ECSA）.At the meantime,the porous structure of electrode increased the accessability of the active site and eased the mass transport of the generated gases in the water electrolysis process.Furthermore,the excellent conductivity not only enhanced the electron transfer rate but also reduced the resistance.Our research results indicated that the rational composite of electrocatalysts were promising for overall water splitting by combinating HER active and OER active catalytic materials together.