The Synthesis And Performance Study Of Efficient Transition Metal-based Electrocatalysts For Water Splitting

Electrocatalytic water splitting to produce hydrogen is considered as one of the effective ways to solve the energy crisis and environmental pollution and plays a vital role in the development of sustainable energy systems in the future.The high energy barrier of water decomposition and the low catalytic efficiency of current catalysts greatly impede the large-scale commercial application of this renewable energy technology.How to design and prepare efficient and stable catalysts is a hot research topic in the field of electrocatalysis.Recent studies have shown that the catalytic performance of catalysts is dependent on the number of active sites and the intrinsic activity of each active site.Therefore,reasonable engineering of nano-catalysts is an effective strategy to improve their catalytic performance.In this paper,we take many strategies to prepare a series of advanced catalysts for water splitting.The detailed research contents and conclusions are as follows:1.We developed a one-step chemical vapor deposition method for synthesizing self-supported three-dimensional Cu3P/Cu nanowires by using copper mesh and phosphorus powder as copper and phosphorus sources,respectively.The optimized self-supported Cu3P/Cu exhibited excellent HER catalytic performance in acidic solution with a small onset overpotential of-44 mV,a low Tafel slope of 72 mV dec^-1and a low overpotential of 120 mV at a current density of 10 mA cm^-2.The self-supported Cu3P/Cu electrode also showed good catalytic activity in alkaline solution,which only required the overpotential of 252 mV for HER and 380 mV for OER at 10mA cm^-2,respectively.Finally,a two-electrode configuration was constructed with Cu3P NB/Cu as both the anode and the cathode to assess its catalytic activity for overall water splitting.The performance of Cu₃P/Cu（+）//Cu₃P/Cu（-）system was superior to that of the IrO₂（+）//Pt/C（-）catalyst.The superior catalytic performance of Cu₃P/Cu electrode was attributed to the following features:1)Cu3P has a good electrical conductivity that accelerates the charge transfer and decreases the resistance of the catalytic system;2)One-dimensional nanowire structure has high specific surface area and electronic directional capability;3)Three-dimensional structure provides more active sites owing to the high surface area and the gap between nanowires facilitates gas production and bubble release;4)Compared with planar electrode,this self-supported electrode doesn’t require the use of a polymer binder between the electrocatalysts and the electrodes,which avoids the blocking of active sites and in turn improve the catalytic performance.Meanwhile,the catalysts are not easy to shed during electrochemical process,which also improves the durability of materials and long-time operability.Therefore,it is expected to achieve the goal of electrochemical water splitting in industry by constructing efficient and stable catalysts self-supported catalysts.2.Plenty of current studies have proven that layered hydroxides or oxyhydroxides composed of edge-sharing octahedral MO6 layers are OER active materials.Layered double hydroxides（LDHs）,comprised of abundant edge-sharing octahedral MO6 layers,are potentially considered as the promising OER catalysts.Therefore,LDHs is a promising OER catalyst.However,inferior electrical conductivity and limited active sites greatly impede its commercial application in the field of electrocatalysis.Defect engineering of LDHs is considered as an effective strategy to improve the catalytic performance.We designed and prepared novel amorphous trimetallic LDHs nanocages for efficient OER.Extended X-ray absorption fine structure（EXAFS）and electron paramagnetic resonance（EPR）results suggested that the catalysts possessed abundant coordinatively unsaturated metal sites and abundant oxygen vacancies,which is beneficial to the intermediates absorption on catalysts surface and the decrease of reaction barrier.X-ray photoelectron spectroscopy（XPS）and X-ray absorption spectroscopy（XAS）revealed that the synergistic interaction of Co,Fe and Mn delicately modulate the electronic structure and the local coordination environment,which largely dictated the activity of a catalyst.Significantly,the as-prepared amorphous CoFeMn-LDH nanocages show relatively low overpotential of 280 mV at 10mA cm^-2,a small Tafel slope of 36 mV dec^-1,a small resistance of 3.45Ω,a high Faradaic efficiency of 99.3%and outstanding long-time durability for water oxidation in alkaline solution.3.Metallic nanostructure-decorated MoS2 nanohybrids have been widely used in the field of electrochemical catalysis,in which MoS2 serves as a two-dimensional support to enhance the performance of the metal nanoparticles by preventing their aggregation.However,the strong interaction of the counterpart metal species on the MoS2 substrate and the consequent effect of the resultant catalyst on alkaline catalytic performance have rarely been investigated.Therefore,based on phase transition and interface engineering,we firstly used DFT calculations to evaluate the phase transition of MoS2 induced via interacting with a series of metal atoms.The results indicated that the absorption of Ir or Cu atom on the surface of MoS2 may trigger phase transition.Detailed DFT calculation analysis showed that the phase transformation of MoS2 may occur when the absorbed Ir atom concentration was above 20%.Based on the above mentioned theoritical calculations,we designed and synthesized Ir/MoS2heterostructure with varying amount of Ir atomic concentration.Raman spectroscopy,X-ray photoelectron spectroscopy（XPS）and X-ray absorption spectroscopy（XAS）confirmed that the adsorption of Ir on MoS2 surface triggered the phase transition of MoS2 from 2H to 1T.The high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）images showed that the regions of MoS2 surrounded by Ir nanocrystals were successfully converted into the 1T phase.In addition,we also synthesized Cu-MoS2 and demonstrated that the adsorption of Cu ion on MoS2 surface also induced phase transition of MoS2,as confirmed by XPS and XAS characterization.The Ir/MoS2 heterostructures exhibit excellent catalytic activity with a low overpotential of-44 mV for the hydrogen evolution reaction（HER）and 330 mV for the oxygen evolution reaction（OER）at 10 mA cm^-2.These performances are superior to the commercial Pt/C and IrO2 in 1.0 M KOH.A two-electrode cell assembled with Ir/MoS2 as bifunctional catalysts showed excellent performance for overall water splitting,superior to IrO₂（+）//Pt/C（-）.We believe that the spontaneous phase transformation of this material not only opens up a new perspective for developing advanced catalysts for alkaline water splitting but also presents an efficient and intriguing method for the phase engineering of two-dimensional materials.