Controlled Preparation Of Cobalt-based Catalysts And Their Performance In Alkaline Water Splitting

As industrialization and urbanization increase,the energy crisis intensifies around the world and environmental pollution becomes increasingly severe.Energy has become the lifeblood of countries’and peoples’economies.While the world’s energy sources face issues such as high carbon emissions,structures that need to be optimized and a fossil fuel shortage,and green energy,sustainable development and energy transition are now key areas of development.As an emerging energy source,hydrogen energy has been attracting a lot of attention because of its eco-friendly,high energy calorific value and renewable nature.Electrolytic hydrogen production is currently recognized as the cleanest hydrogen production technology in the world because of the abundance of feedstocks and the high efficiency of hydrogen production among the common methods used to produce hydrogen.The process of hydrogen production from electrolytic water consists of hydrogen evolution reaction（HER）at the cathode and oxygen evolution reaction（OER）at the anode.Electrolytic water reactions usually face a large energy barrier,resulting in a high overpotential and slow reaction kinetics.Precious metals and their oxides（e.g.Pt,Ru O₂ and Ir O₂）are often used as catalysts to reduce the reaction overpotential.However,the high cost and low availability of precious metals limit their widespread use;in addition,the single active site limits the reaction kinetics of catalytic conversion.Therefore,it is important to develop cheap,efficient and structurally tunable non-precious metal catalysts.Based on this,this paper takes transition metal cobalt-based compounds as the object of study and modulates the interaction between cobalt-based compounds and carriers,the electronic structure,metal valence states and d-band centers of cobalt-based compounds by means of heteroatom doping and composite strategies,thereby improving the reaction catalytic kinetics.The research reported in this article comprises the following three main areas.（1）A“one-step gel pyrolysis”strategy was proposed for the preparation of N-doped porous carbon nanosheets loaded with cobalt phosphide nanoparticles（Co₂P@N-C）by gelation and high-temperature carbonization.The effect of carbonization temperature on the catalytic performance of Co₂P@N-C composites was investigated.Based on the synergistic effect between the cobalt phosphide nanoparticles and the carbon substrate,the Co₂P@N-C composite（Co₂P@N-C-900）with a calcination temperature of 900°C has the optimal electrocatalytic performance and long cycle stability.The composite exhibits excellent HER and OER properties in 1.0 M KOH solution and can remain stable for up to 24 h.The experimental results show that the anchoring of the cobalt phosphide nanoparticles in the N-doped porous carbon substrate not only avoids the agglomeration of nanoparticles but also improves the electrical conductivity of the composite,which reduces the resistance to charge transfer during the catalytic process;the cobalt phosphide nanoparticles are wrapped by an amorphous carbon layer,which protects them in alkaline solutions for a long time;in addition,the porous carbon substrate contains a large number of mesopores with a large specific surface area.The porous carbon substrate also contains a large number of mesopores that have a high specific surface area,which can completely expose reactive sites to the electrolyte and significantly enhance the reaction kinetics of the catalytic process.（2）On the basis of work（1）,ruthenium-doped cobalt phosphide and ruthenium-doped carbon substrate composites（Ru-Co₂P@Ru-N-C）were prepared by one-step ruthenium（Ru）doping,in which the precious metal ruthenium was uniformly distributed as a single atom in N-doped porous carbon（Ru-N-C）and phosphide nanoparticle lattices（Ru-Co₂P）,leading to the construction of bimetallic active sites,for which an innovative“dual site-support interaction”synthesis strategy was proposed.Physical characterization and theoretical calculations showed that Ru single atoms were present in the carbon substrate via Ru-N₄ coordination,and the functionalized lattice Co and coordination-unsaturated Ru¹-N-C sites contributed to lowering the water dissociation energy barrier and optimizing the hydrogen desorption properties as the active site of HER,respectively;Ru single atoms were present in the lattice of cobalt phosphide via Ru²-Co-P coordination,and due to the difference in ionic radius and electronegativity,the the alternative doping of Ru into the Co₂P lattice triggers lattice expansion,which effectively modulates the d-band center and valence state of the transition metal Co,lowering the energy barrier of the OER rate-determining step,and the Ru in Ru²-Co-P acts as the active site of the OER.As a result,the Ru-Co₂P@Ru-N-C composite exhibited excellent HER and OER activities in alkaline conditions withη₁₀of 69 m V and 280 m V,respectively;a current density of 10 m A cm^-2 could be achieved with only 1.56 V in water electrolyzer with the Ru-Co₂P@Ru-N-C composite as cathode and anode,respectively.The structural stability of Ru-Co₂P@Ru-N-C composites during long cycling was investigated by non-in situ characterization.In addition,the synthetic strategy is universal,and the Ru-Ni₂P@Ru-N-C and Ru-Fe₂P@Ru-N-C composites were prepared by replacing the transition metal salt species,and both materials exhibited excellent catalytic performance for HER and OER by electrochemical tests.（3）The W-doped cobalt selenide nanosheets(W-Co_0.85Se)were successfully synthesized by a one-step hydrothermal method.The material has a special nanosheet morphology with a large specific surface area,and the active sites distributed on the nanosheets can be fully exposed to the electrolyte;cobalt selenide has high electrical conductivity,which facilitates the charge transfer during the catalytic conversion process.In addition,the two-dimensional structure is more conducive to the release of H₂ and O₂ generated during the catalytic process,avoiding the accumulation of gases on the electrode.Compared with cobalt selenide(Co_0.85Se)nanosheets,doping with the W element,which has a large atomic radius and high electronegativity,can promote the electronic interaction between W and Co_0.85Se,which plays a regulatory role in the electronic environment of Co_0.85Se,thus contributing to the improvement of the catalytic activity of the material HER and OER.Based on the doping modification of W elements,the W-Co_0.85Se nanosheets precipitated HER and OER atη₁₀ of 101 m V and 294 m V,respectively,in a solution of 1.0 M KOH,while exhibiting excellent stability.