Synthesis Of Nickel-based Catalysts And Investigation Of Their Electrocatalytic Performance For Water Splitting

Hydrogen production by water electrolysis is considered to be a very effective way to solve the modern energy crisis,and hydrogen production by water electrolysis can alleviate the climate change and environmental crisis caused by conventional fossil fuel combustion.Water electrolysis involves two important semi reactions:cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.Noble metals such as Pt,Ir and Rh have been proved to be excellent HER materials due to their suitable Gibbs free energy of hydrogen adsorption.However,the high cost and very limited reserves greatly hinder their wide application.In addition,the OER process has inherent slow reaction kinetics,which greatly limits the efficiency of water electrolysis.Similarly,noble metal based oxides such as RuO2 and Ir O2 have excellent OER activity,but are also limited by cost and reserves.Based on the above considerations,we are committed to developing low-cost,highly active electrocatalysts.Transition metal（such as Fe,Co and Ni）based catalysts are expected to become substitutes for precious metal based electrocatalysts due to their low price,abundant earth resources,and good conductivity.Nickel based catalysts have a flexible and adjustable structure and good electrocatalytic performance,making them a promising electrocatalytic material.However,the intrinsic activity of nickel based catalysts is still low,the number of active sites is small,and the research on the catalytic mechanism is not deep enough.In this article,we start with transition metal nickel based materials and design and synthesize nickel based catalysts with high intrinsic activity and good stability through two different approaches:doping and alloying strategy.The sources of high activity and good stability of the catalysts were studied by combining theoretical calculations and experimental analysis.The specific research content includes the following two parts:（1）An iron doped NiS material（named Fe-NiS）was prepared using a one-step hydrothermal method.This material has excellent electrocatalytic oxygen evolution activity.It can provide ccurrent densities of 100 m A cm^-2 and 200 m A cm^-2 at low overpotentials of only 214 m V and 254 m V in 1.0 M KOH,and can stably undergo OER test for more than 140 hours.The experimental characterization and theoretical calculation results show that iron doping will lead to higher intrinsic activity of NiS materials,and also adjust the electronic structure of the catalyst to make it easier to generate more active NiOOH species in the test process.Iron doping also optimize the adsorption Gibbs free energy of intermediates in the OER process,which together promoted the OER activity of the material.Experimental analysis shows that the iron element always maintains Fe³⁺before and after OER test,indicating that the effect brought by doping can exist stably and continue to play a role.（2）Based on the shortcomings of the previous work:the electrochemical activity is single,the working pH range is relatively narrow,and the synthesis method is relatively cumbersome.We prepared a ternary alloy catalyst（named Ru-NiCo）,which is still based on the transition metal nickel based materials.Ru-NiCo0.5-600℃material with high HER activity in the full pH range and Ru-Ni0.75Co material with prominent OER activity under alkaline conditions were obtained by changing the metal ratio.Ru-NiCo0.5-600℃material only respectively requires 42 m V,77 m V and 93 m V for HER in 1.0 M KOH,1.0 M HCl O4 and 1.0 M PBS solutions to achieve a current density of 10 m A cm^-2.Ru-Ni0.75Co material only need an ultra-low overpotential of 176 m V to provide a current density of10 m A cm^-2 in 1.0 M KOH.Ru-NiCo0.5-600℃||Ru-Ni0.75Co electrolytic cell only needs a low battery voltage of 1.48 V to achieve a current density of 10 m A cm^-2 for overall water splitting,and can be stable for more than 120 hours.More importantly,we clearly proposed and comfirmed that the strong clectronic coupling and synergistic effect among the three metals in the process of water splitting are the source of excellent catalytic performance and good stability of materials based on experimental and theoretical calculations.This work has great reference significance for understanding the role of various components of multi-component materials in water electrolysis.