Preparation Of High-performance Metallic Phosphate-based Electrocatalysts And Their Water Splitting Activities

In recent years,the depletion of fossil energy and environmental pollution have become increasingly prominent.Therefore,seeking green renewable energy instead of fossil energy is the best way to solve the energy crisis and environmental pollution problem.Hydrogen is considered to be the most ideal new energy source because of its advantages of clean,pollution-free,efficient,and easy to store and transport.Electrocatalytic water splitting to produce hydrogen is highly promising in the development of hydrogen production technology due to its high efficiency,environmental friendliness,high purity of gas production and strong adaptability to energy fluctuations.At present,although precious metals and their oxides exhibited excellent electrocatalytic activity,its scarcity and high cost limit its wide application.Exploring efficient,stable,and low-cost non-precious metal electrocatalysts has become an urgent task.Transition metal phosphide is a kind of interstitial compound formed by the phosphorus atom entering the crystal lattice of the transition metal,and has the advantages of stable structure,high electronic conductivity and good chemical stability.Since the large electronegativity of the phosphorus,the transition metal phosphide shows the proper adsorption energy toward the reaction intermediates,resulting in superior electrocatalytic activity and stability.Compared with single metal phosphide,dual metal phosphide plays a synergistic role of two different metal elements,which not only could modulate the electronic structure of electrocatalyst,improve charge transport efficiency,not only provide more surface reactive sites,thereby improving the catalytic efficiency and stability of the catalyst.In this paper,we chosen nickel-based transitional metallic phosphide as the main research topic,and designed and synthesized a series of nickel-based transitional metallic phosphide.The electrocatalytic activity for the hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?as well as the catalytic mechanism were investigated.The details were illustrated as follows:1.Nickel-manganese bimetal phosphides porous nanosheet arrays growth on nickel foam were prepared by low-temperature phosphating reaction with NiMn-LDH as precursor.The electrochemical tests results indicate that optimal Ni₂Mn₁P as HER electrocatalyst delivers overpotential of 80 and 120 mV at a current density of 10 mA cm^-2 in the acidic and alkaline electrolyte,respectively.The Ni₂Mn₁P as OER electrocatalyst possesses a low overpotential of 250 mV at the current density is 20mA cm^-2 in the alkaline electrolyte.By using the Ni₂Mn₁P as the cathode and anode,the two-electrode electrolyzer shows a cell voltage of 1.60 V at 10 mA cm^-2 in alkaline electrolyte.The electrochemical active area and electrochemical impedance results indicates that the enhanced electrocatalytic activity can be attributed to the following aspects:?i?The synergistic effect between bimetals enhances the conductivity,optimize the adsorption energy toward reaction intermediates,and reduces the reaction energy barrier of the catalytic process;?ii?The porous nanosheet array structure could provide more active sites,resulting in enhanced electrocatalytic activity.2.Iron-dopednickel-cobalt/nickel-cobalt-phosphorushollownanocube electrocatalyst was developed by a facile reflux reaction.The doping amount of Fe element was achieved by changing the reactant amount.The electrochemical tests results indicate that optimal NiCo/NiCoP-2.5Fe exhibited the highest OER catalytic activity,with an overpotential of 290 mV at the current density was 10 mA cm^-2.The i-t test result indicates that the current density show good stability after 25 h,indicating its excellent stability.The enhanced OER activity can be attributed to the improved charge transfer activity and enhanced active sites induced by the Fe doping.3.A nickel-cobalt bimetal phosphides oxide/nitrogen-doped carbon composite electrocatalyst nanocage electrocatalyst was derived from a by a simple chemical etching,low-temperature phosphating reaction and calcination method using the nickel-cobalt bimetallic cube as the precursor,which was prepared by using cobalt cobalt cyanide-based Prussian blue as a precursor.The optimal nickel-cobalt bimetal phosphides oxide/nitrogen-doped carbon composite electrocatalyst synthesized by 7days of coprecipitation and 90 minutes of etching exhibited the highest OER performance,delivering an overpotential of 320 mV at 10 mA cm^-2.Electrochemical active area and electrochemical impedance test results indicate that the unique nanocage structure increases the specific surface area of the catalyst,which exposes more active sites and facilitates the contact between the catalyst the electrolyte,The synergistic effect of phosphating and nitrogen-doped carbon composite regulates the electronic structure and accelerates the electron transport rate,thus significantly improving the catalytic performance of OER.