Synthesis And Electrocatalytic Water Splitting Performance Of Ni-based Transition Metal Phosphides/Selenides

The electrocatalytic water splitting for hydrogen production is a green and efficient way to prepare high purity hydrogen fuel.A high efficiency electrocatalyst for water splitting can greatly improve efficiency of energy conversion and reduce the cost of energy consumption.The electrolysis water splitting is divided into two half reactions:the electrocatalytic hydrogen evolution(HER)and the electrocatalytic oxygen evolution(OER).Currently,the noble metal Pt based material is the most effective catalyst for hydrogen evolution reaction(HER),while the noble metal Ir/Ru based material is the best catalyst for oxygen evolution reaction(OER).However,high costs and less natural reserves limit their extensive applications.Due to the unique d-orbital electronic structure,element diversity,adjustable atomic structure and abundant reserves,transition metal materials have become one of the most promising types of electrocatalysts to replace noble metal materials.Optimal design of transition-metal-based electrocatalysts can promote the development of new energy technologies.Based on the above point of views,this thesis takes transition metal phosphides/selenides as the main research objects,purposing to improve the electrocatalytic activity and stability by atomic structure adjustment,surface microstructure design,aerophobic performance optimization and recombination with conductive substrates.3D Ni-Co-OH with nanosheet arrays structure was prepared on the carbon fiber cloth substrate by electrodeposition method.Then,due to the low intrinsic electrocatalytic activity of hydroxide,Ni-Co-P nanosheets were further synthesized by low temperature phosphating method.The active material is directly grown on the carbon cloth,which can be directly used as a working electrode,to avoid the use of adhesive,reduce the resistance and improve the overall stability.Benefits from regular 3D porous nanosheet structure and excellent bimetallic effect,Ni-Co-P nanosheets maintained high performance under the condition of low active substance loading whether in alkaline or acidic electrolyte.To investigate the effect of bimetallic action on the material,we also examined the structure and electrocatalytic hydrogen evolution performance of Ni-Co-P nanosheets with different Ni/Co ratios.The Ni-Co-P-2 synthesized by the precursor solutions with Ni:Co=1:2 showed the best performance:The Ni-Co-P-2 electrode exhibited superior HER performance with a low overpotential of 57 and 147 mV to reach 10 and 100 m A·cm^-2 current density in 1 mol·L^-1KOH.In order to further optimize the design of surface interface microstructure,a layer of Ni Co₂O₄ nanowires was first grown on carbon substrate by hydrothermal method.Then a smaller scale Ni-Co hydroxide nanosheet array based on Ni Co₂O₄ nanowires was grown by electrodeposition.Finally,the morphology of Ni Co₂O₄ nanocoral(NC)precursor was retained by low temperature phosphating method and replaced by bimetallic NiCoP with higher intrinsic electrocatalytic activity.The C_dl results demonstrated that the nano-coral structure could greatly expand the active surface area of the material,thus facilitating the improvement of electrocatalytic activity.Furthermore,the layer-by-layer construction design makes the structure of NiCoP NC stable,the morphology was not damaged before and after the electrocatalytic performance test,and the structure was still stable after ultrasonic dispersion.The nanocoral structure possessed superhydrophilic/superaerophobic properties as well as extremely low bubble adhesion,which benefit to the rapid detachment of bubbles in electrocatalytic reactions and maintain the best performance.Benefits from these advantages,NiCoP NC exhibited excellent electrocatalytic performance.For HER in acidic condition,NiCoP NC only required overpotential of 55 and 121 mV to approach current density of 10and 100 m A·cm^-2.Moreover,the cell voltage required to achieve a current density of 10m A·cm^-2 and 100 m A·cm^-2 for overall water splitting is 1.62 and 1.88 V in alkaline condition,and it can work steadily for more than 25 h.The development of bifunctional electrocatalysts to achieve both efficient HER and OER catalysis is essential.In this work,nitrogen-doped carbon fibred skeleton was prepared by the electrospinning process followed by a carbonizing treatment.Then NiSe₂/Ni₅P₄ nanosheets were synthesized by hydrothermal method followed by a continual low temperature selenium-phosphating treatment supporting on the nitrogen-doped carbon fibred skeleton.The as-prepared electrocatalysts have nanosheets array structure which is conducive to exposing more active sites.Additionally,profit from the high intrinsic catalytic activity of the phosphide,high conductivity of selenides and carbon nanofibers,NiSe₂/Ni₅P₄ nanosheets not only exhibited efficient HER performance,but also possess favorable OER performance.Furthermore,the NiSe₂/Ni₅P₄ nanosheets were used as both cathode and anode to assemble a two-electrode overall water splitting device,attaining current density of 10 m A·cm^-2 by a low cell voltage of 1.65 V in alkaline condition,and can work stably for more than 24 h.Ni-Co-S-P nanoparticles were directly grown on carbon fiber cloth via a summary one-step electrodeposition process.The electronic structure and the electrocatalytic performance of the electrocatalyst were improved by the co-optimization of bimetallic effect and double-anionic components.The test results showed that the HER and OER performance of the Ni-Co-S-P were better than those of bimetallic Ni-Co-S,Ni-Co-P,and double-anionic Ni-S-P,Co-S-P.Moreover,the contact angle of Ni-Co-S-P underwater bubbles was 151.6°,and the contact angle of droplets in air was 0°.The underwater bubble adhesion test results further showed that the superaerophobic performance of the nanoparticles structure was beneficial to the rapid detachment of bubbles,which reduced the effect of bubbles on the exposure of active sites and the exfoliation of surface active materials,thus improving the stability and electrocatalytic activity of the Ni-Co-S-P.The interconnected nanoparticles were instrumental in increasing the specific surface area and accelerating mass transfer,thus enhancing the electrocatalytic overall water splitting performance.As a result,the Ni-Co-S-P displayed an excellent performance with a cell voltage of 1.61 V to reach 10 m A·cm^-2 for overall water splitting in alkaline condition.Ni-Co-S-P also exhibited robust stability.We presented the electrodeposition synthesis of Ni Co Se₂ nanosheets network on conductive carbon cloth with interconnected macropore structure.The intrinsically metallic property endowed Ni Co Se₂ with excellent conductivity,thus accelerating the electrochemical process.The crimped nanosheets configuration and highly open structure provided an effective path for ion and electron transport.The superhydrophilic property of the electrocatalyst leaded to the close affinity with electrolyte,expanding the specific surface area of the activity,increasing the exposure of the active sites,and then further improving the electrocatalytic activity of the material.Density functional theory(DFT)calculations revealed the intrinsically metallic nature of Ni Co Se₂,and the interaction between of Ni and Co could finetun the surface charge distribution of bimetallic selenides.Benefiting from these advantages,Ni Co Se₂ nanosheets required overpotential of 113 and 280 mV to reach 10 and 100 m A·cm^-2for HER,the corresponding tafel slope was 65 mV dec^-1,and for OER,it required overpotential of 256 and 340 mV to reach 10 and 100 m A·cm^-2,the corresponding tafel slope was 71 mV dec^-1 in alkaline condition.