Self-catalyzed Synthesis Of Carbon Nanotube-based Electrocatalytic Materials For Overall Water Splitting

Developing clean and efficient new energy resources,reducing the exploitation and dependence of fossil energy are the best choice for sustainable development of the society.Hydrogen is one of the most promising new energy sources in future because of its wide availability,green and clean nature,pollution-free of the water combustion products and recyclable utilization.Hydrogen production from alkaline electrolysis can avoid the corrosion of equipment and impurity of products caused by acid mist in the past,and has become an effective way to produce high purity hydrogen.The two semi-reactions involved in electrolytic water:hydrogen evolution reaction（HER）in cathode and oxygen evolution reaction（OER）in anode,which also play an important role in determining the efficiency of electrolytic water.Nowadays,it is generally accepted that the most efficient catalyst is precious metal catalyst.The catalytic ability of commercial Pt/C for HER is hard to surpass.IrO₂ and RuO₂ are also recognized as the state-of-the-art catalysts with excellent catalytic activity for OER.Although precious metals have high activity,their high cost and easy agglomeration and inactivation in the reaction greatly limit their large-scale applications.Accordingly,we intend to synthesize a self-catalytic growth of carbon nanotubes encapsulated transition metal compounds,these materials with good conductivity and unique tubular structure,which are not only inexpensive and earth-abundant,but also have high active area.The electrochemical measurements show that the metal nanoparticles coated with tubular structure have excellent catalytic activity,meanwhile,the stability of catalytic process is guaranteed by their unique structural characteristics.When used as a bifunctional material in water electrolysis,it can effectively reduce the cell voltage of the electrolytic water and showed good stability under different current densities.The details are as follows:（1）Nitrogen-doped carbon nanotubes（CNTs）coated with metal Co have been prepared by self-catalytic growth of metal Co particles from Co salts at high temperatures,and then sulfurized to form S,N co-doped CNTs coated with core-shell structure CoS₂@Co nanoparticles.This rationally designed catalyst material has two kinds of heteroatom doping,which adjust its electronic structure,further improve its electrical conductivity,and thus expose a large amount of electrochemically active area.The special core-shell structure not only accelerates the cracking of water,but also promotes the desorption of O₂,which greatly improves the catalytic performance of HER and OER,and self-catalyzed CNTs as well provides corrosion-resistant space for the stable catalysis of active components.（2）N-doped CNTs coated with metal Ni were grown by self-catalytic growth process at high temperature using Ni salts.To solve the problem that metal Ni particles could not be completely vulcanized,Mo element was introduced into the growth of CNTs.Mo_xC particles grown together with CNTs could adjust the type of N-doping in CNTs,increase the content of pyrrole-N,and the electron cloud of pyrrole-N was biased inside the inner ring,in the state of electron absorption outside the ring,thus promoted the completely formation of NiS₂ nanoparticles by the electrons lost from the metal Ni.NCNTs/NiS₂@Mo₂C has better catalytic performance than NCNTs/NiS₂@Ni,the current density of electrolytic water can reach 10 mA cm^-2 with only 1.52 V,meanwhile,it also showed stronger stability under different current densities and the Faraday efficiency is close to 100%.（3）By adjusting the ratio of Ni and Fe salts,N-doped carbon nanotubes encapsulated alloy Ni₃Fe nanoparticles have been synthesized by a simple self-catalytic growth process.In addition,two anions,S and Se,were introduced to investigate the effect of different anions on the properties of precursors.The electrochemical test and mechanistic analysis confirmed that the NiS₂/Fe₇S₈heterojunction formed in the N-CNTs by vulcanization of Ni₃Fe nanoparticles accelerated the decomposition of water more intensely than NiSe₂/Fe₃Se₄ formed by introducing Se atom.The current density of 50 mA cm^-2 for OER reaction could be achieved with a small overpotential of 330 mV,at the same time,the introduction of S and Se atom could further promote the rate of desorption for O₂ product,and heterojunction interface could be beneficial for the excellent catalytic performance of OER.