Preparations And Electrocatalytic Water Splitting Performances Of Transition-metal-based Hybrid Materials

Since the beginning of the first industrial revolution,human demand for non renewable energy has increased exponentially.Fossil fuel consumption has increased exponentially.The massive consumption of fossil fuels,in light of the harmful side products emitted to the environment and the extremely long generation time for fossil fuels,leads to the almost irremediable deterioration of the Earth’s ecology and the depletion of the fossil fuels.Consequently,development of clean,green,and sustainable energy sources has become the most time pressing research topic.Hydrogen,because of its environmental friendliness and high calorific valu,has long been considered as one of the alternative fuels in most developed countries.Among the many existing technologies,renewable energy driven electrochemical hydrogen production through decomposition of water appears to be the most promising one because of its cleanliness,low raw material（water）cost,and high product purity.Water electrolysis includes two and a half reactions:Anodic oxygen evolution（OER）and cathodic hydrogen evolution（HER）.However,its slow kinetic characteristics lead to high overpotential in the catalytic process.Thus we should develop efficient catalysts to lower the overpotential associated with the OER and the HER.The HER performance of noble metal based catalysts such as Pt and OER performance of IrO₂ and RuO₂ are excellent.However,due to the scarcity and high cost of these precious metals on the earth,it is desirable and also urgent to develop efficient and cost-effiective catalysts composed of Earth-abundant elements.The constituent elements of transition-metal phosphides are relatively abundant in reserves,which can reduce their economic cost,thus providing great possibilities for the large-scale applications in water electrolysis greatly.In addition,because of their outstanding electrical conductivity,attractive electronic structures,as well as adjustable valence states,they usually deliver HER or OER performances comparable to those of precious metals,therefore attracting extensive and intensive research attentions in recent years.Through reasonable precursor design and high temperature treatment,the precursor composed of transition metal elements can be designed and prepared into excellent electrocatalytic materials.It is of great practical significance to replace precious metals and reduce production costs.In this paper,a series of metal phosphide/carbon hybrid materials were synthesized by using metal complexes as precursors through high temperature phosphorization,and their electrocatalytic performance for water decomposition was systematically studied,and the possible mechanism of enhancing electrocatalytic activity was preliminarily revealed.The main elements are as follows:1.Since polydopamine has good coating properties and coordination ability to metal ions,we can use its unique "target transport" function to anchor iron ions and cobalt ions on the foam nickel substrate,and further transform the gold elements into corresponding metal phosphides by the high-temperature phosphating strategy.While polydopamine is carbonized into nitrogen doped carbon,and in situ.A mixed metal phosphide based armor catalyst is formed by coating the outer surface of metal phosphide nanoparticles.The catalyst exhibited excellent electrocatalytic performances in 1.0 M KOH,delivering 50,100,and 250 mA/cm² at ultralow overpotentials of 212,242,and 267 mV,respectively with a low Tafel slope of 63.3 mV/dec for the oxygen evolution reaction and achieving ultralow overpotentials of 200,232,and 281 mV for the hydrogen evolution reaction at 50,100,and 250 mA/cm²,respectively with a low Tafel slope of 75.7 mV/dec.As a bifunctional electrocatalyst,the product exhibited low cell voltages of 1.65,1.73,and 1.79 V at 50,100,and 250 mA/cm²,respectively for overall water splitting.In addition,the preparation method of catalytic electrode is relatively simple,especially suitable for the preparation of large size self-supporting electrode,so it has a good application prospect.2.Accordingly,open-mouth N-doped carbon nanoboxes with embedded mixed metal phosphides nanoparticles were fabricated from monodisperse Ni₃[Fe（CN）₆]₂·H₂O nanocubes through sequential conformal Ni₃[Co（CN）₆]₂·12H₂O layer coating,ammonia etching,and thermal phosphorization.The strategy of combining the open-mouth nanobox structure with the embedded mixed metal phosphide nanoparticles was proposed.Due to the synergistic effect between the mixed metal phosphides,the armor catalyst design provides protection for the active sites,and the large opening on the nano box shell makes the electrolyte more fully contact with the active sites inside the nano box,and provides a fast transport channel for the electrolyte and gas products,which makes the catalyst have excellent electrocatalytic activity.The product catalyst exhibited highly efficient electrocatalytic performances,achieving low overpotentials of 204 and 129 mV for the oxygen evolution reaction and hydrogen evolution reaction,respectively and a small working voltage of 1.57 V for the overall water splitting,all at 10 mA/cm².And it shows the super stable electrocatalytic long-term effect.This research work can provide a good idea and material model for the design and synthesis of other heterogeneous catalysts.3.A strategy of controllable assembly and synergistic catalysis based on hydrogen evolution reaction（HER）active and oxygen evolution reaction（OER）active single functional sites,was ingeniously assembled to high-performance catalysts for overall-water-splitting.Mono-disperse zeolitic imidazolate framework cubes were adopted as the reaction precursors to construct bi-functional electrocatalysts through conformal prussian blue analogue layer coating and the subsequent thermal phosphorization process.This double functionalization concept can simultaneously maximize the catalytic performances of HER and OER active sites,rather than the common case that can only fully perform one catalytic function and sacrifice another one.The catalyst delivered outstanding electrochemical activities in alkaline electrolyte,evidenced by the current density of 10 mA/cm² at low overpotentials of 174 and 53 mV for the OER and HER,respectively and a ultra-low operating cell voltage of 1.46 V for the overall-water-splitting.This work provides a new idea for the design and preparation of nano functional materials derived from metal complexes,and has important reference value for the development of high performance mixed metal phosphide electrocatalysts.Figure[95]table[12]reference[151]...