Preparation And Electrocatalytic Performance Of Co-based Transition Metal Catalysts

In order to get rid of the shackles of traditional fossil energy and respond to environmental problems,people are constantly developing and utilizing new clean energy sources.Compared with solar energy,wind energy and water energy,which are usually restricted by geographical location and season,and nuclear energy,which will also bring serious environmental problems once leaked,hydrogen energy enters people’s vision due to its high calorific value,sustainability,abundant storage and zero pollution.The development of hydrogen energy can realize real green,clean and sustainable development.Currently,the proposed development goals of China’s carbon peak and carbon neutral will further speed up the process of carbon reduction.As a zero-carbon energy carrier,hydrogen is gaining more and more attention:it is expected that in 2050,more than 20%of the world’s CO₂ emission reduction can be accomplished through hydrogen energy substitution,and hydrogen energy consumption will account for 18%of the world energy market.Among the various ways of hydrogen production,water electrolysis technology not only has zero carbon emission throughout the process,but also has the potential to be marketed.The water-splitting reaction consists of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）,and according to the DFT calculation,noble metals such as Pt,Ir and Ru have the highest catalytic activity.However,the scarcity and high price of noble metal catalysts severely restrict the promotion of water electrolysis technology,and the search for alternative non-noble metal-based catalysts is the most critical issue in this field.The ideal electrocatalysts should possess the four characteristics:（1）Large reserves of raw materials,low price and no pollution.（2）Simple and environmentally friendly preparation method,and mass production is possible.（3）High catalytic activity,low onset potential and low overpotential.（4）High stability,long working time and stable activity.Therefore,in order to evaluate the activity of water-splitting catalysts,a series of tests and calculations are needed to obtain some parameters:overpotential at a given current(1 m A cm^-2,10 m A cm^-2,100 m A cm^-2,etc),exchange current density,Tafel slope,resistance,electrochemical activity area,activation energy,stability,etc.Co-based transition metal catalysts have many advantages,such as good electrical conductivity The Co-based transition metal catalytic materials have many advantages,such as good electrical conductivity,suitable energy band structure and more stable chemical stability,and are very suitable alternative catalytic materials.Therefore,this paper discusses the design and development of Co-based transition metal catalyst to improve the overall number of active sites and the intrinsic activity of each active site through interface engineering,defect engineering,and atomic doping,so as to achieve the dual functional requirements of HER and OER,and thus improve the water-splitting performance of non-precious metal-based catalysts.The specific research contents of this paper are as follows:（1）The water-splitting properties of petal-like CoP/Ni₂P catalysts prepared by hydrothermal and phosphating treatments based on interfacial engineering were investigated and found that:The interface can regulate the electronic state of the catalyst,expose the active site,adjust the binding energy and thus modulate the catalytic activity.In this work,an interface engineering strategy is proposed to prepare different phosphides using homogeneous hydroxide precursor.The effects of temperature and ratio on the interfacial structure and catalytic activity of Co P/Ni₂P catalysts are investigated,we finally find a proper condition to adjust the electronic state,which greatly improves the catalytic performance.As a proof of concept,Co P/Ni₂P biphasic catalyst showed great catalytic activity for HER,OER and water-splitting reactions（close to the commercial combination Pt/C||Ru O₂）with excellent cycling stability（continuous operation for 40 h with 98%retention of current density）in alkaline electrolyte.（2）CoP₃ nanoneedle array electrodes with amorphous and porous structures were prepared by hydrothermal,room temperature co-precipitation and phosphating treatments.The catalytic performance study revealed that the amorphous structure can effectively reduce the catalytic reaction energy barrier and increase the active site,which will help to modulate the adsorption energy of the catalytic material.In this work,we used 2-methylimidazole and ethanol to fabricate abundant nanopore defects（30 nm）without affecting the Co（OH）₂ structure,which greatly improve the geometric area,expose more active sites and further modulate the electronic state.Meanwhile,with the special 3D framework of nickel foam substrate and the nanoneedle structure of the catalyst itself,the water-splitting products（H₂ and O₂）can be rapidly discharged,which ensures the continuous and efficient operation of the electrode.As a result,the amorphous,porous Co P₃ electrode as anode and cathode exhibit excellent water-splitting activity exceeding the Pt/C||Ru O₂(A-P-Co P₃||A-P-Co P₃:E₁₀=1.57 V,E₁₀₀=1.64V;Pt/C||Ru O₂:E₁₀=1.57 V,E₁₀₀=1.66 V),and 96.1%of the initial current density was maintained after 40 h of stability test.This work provides a strong support for the exploration of novel defect engineering and amorphization treatment strategies,and provides a feasible strategy for the exploration of efficient water-splitting catalysts.（3）Ni₂P/CoP₃ self-supported electrodes were prepared based on interface engineering by hydrothermal and phosphating treatment.The catalytic performance study revealed that Ni₂P/Co P₃ 3D heterostructure possesses excellent catalytic activity as a water-splitting catalyst.The reasons for its activity enhancement were analyzed in depth with the help of DFT simulations.This heterostructure combines Co P₃ and Ni₂P with complementary hydrogen adsorption Gibbs free energy and modulates the electronic state through rich interfacial region,thus improving the electrochemical active area,conductivity and activation energy of the whole catalyst.As a result,Ni₂P/Co P₃ heterostructure shows impressive HER and OER performance over a wide p H range.In p H 7/13/14 electrolytes,only 57.3,81.4 and 99.6 m V are required for HER to reach 10 m A cm^-2;and 323,327 and 337 m V are required for OER to reach 100 m A cm^-2.More importantly,the whole water-splitting device assembled with Ni₂P/Co P₃ as cathode and anode also exhibits excellent catalytic activity(1.557 V@10 m A cm^-2)as well as work stability（over 40 h）,which is superior to the Pt/C||Ru O₂ noble metal combination dropped in NF.Thus,this work not only provides a design concept for the development of multifunctional electrocatalysts,but also provides new ideas to explore the performance of water electrolysis technology over a wide p H range by interface engineering.（4）The Ru-doped NiO/Co₃O₄ nanoneedle array heterostructure based on carbon cloth substrate were prepared by hydrothermal,room temperature immersion and oxidating treatment.The catalyst was able to modulate the overall electronic energy state through the synergistic interaction between the Ni O/Co₃O₄ heterostructures and the appropriate Ru doping,which improved the electrochemical active area and conductivity and reduced the activation energy required for the catalytic reaction.As a result,the catalyst electrode exhibits superior OER,ORR and HER performance in alkaline environment(only 138 m V and 269 m V overpotential are required for HER and OER reactions to reach 100 m A cm^-2,and the half-wave potential for ORR reaction is as high as 0.88 V),which exceeds the performance of Pt/C and Ru O₂ in the same type of reactions.More importantly,the water-splitting device assembled based on this catalyst electrode also exhibits high activity(1.555 V@10 m A cm^-2,1.650 V@100 m A cm^-2)and can operate stably for more than 40 h,better than Pt/C/CC||Ru O₂/CC.Therefore,this work not only provides a new design concept for the development of multifunctional electrocatalysts,but also makes a contribution to the exploration of Ru doping and structural engineering.