Heteroatom Modified Ni-based Bifunctional Catalysts For Water Electrolysis

Hydrogen generation has drawn extensive attention as its relevance to zero carbon emission and renewable energy technologies.Developing cost-effective and stable bifunctional catalysts for overall water splitting is of enormous importance for the realization of sustainable clean-energy technologies.In particular,the key issue of these techniques relies heavily on active and low-cost catalysts to accelerate the sluggish kinetics progress of hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Nevertheless,to date,the state of the art HER and OER electrocatalysts are based on noble metals like Pt and Ir,but unfortunately,their high cost and natural scarcity greatly restrict its application in large-scale hydrogen production.Ni-based electrocatalysts have sparked great interest owing to their many excellent properties,including earth abundance,good conductivity and electroactivity.This dissertation is devoted to improving the electrocatalytic activity of Ni-based electrocatalysts by modifying the surface of Ni with heteroatom,which provides an efficient strategy to tune its surface chemical structure and achieve optimized electrocatalytic properties.DFT calculation further revealed the role of heteroatom in modulating chemisorption of hydrogen intermediates,and consequently how to improve the overall electrochemical water-splitting performance.In this work,a novel Ni/Ni₃S₂ nanosheet catalyst was successfully achieved via a facile hydrothermal-hydrogen reduction method.Owing to the three-dimensional?3D?self-supported architecture of Ni/Ni₃S₂ nanosheets,the Ni/Ni₃S₂ eletrode exhibited splendid activity toward overall water splitting.Utilizing Ni/Ni₃S₂ as both anode and cathode for overall water splitting,a current density of 10 mA cm^-2 was achieved at an electrolyzer voltage of as low as 1.57 V,and no decay after 50 h ageing at 10 mA cm^-2was obseved,both of which exceeded that of the integrated performance of Pt/C and RuO₂.To confirm the experimental results,a series of Ni-based catalysts with different sulfuration degrees were synthesized by changing the content of Na₂S during hydrothermal process,achieving the successful modulation of S in these samples.Electrochemical and XPS results showed that the HER catalytic activities of the Ni/Ni₃S₂ catalysts firstly increased with S content growth and then decreased gradually.The optimal surface S:Ni atomic ratio of the Ni/Ni₃S₂ catalyst is about 28.9%as measured by XPS.DFT calculation revealed that the incorporation of S atom have significantly modified the electronic structure of Ni,and bringed out the?G_H*of S/Ni?100?closer to zero.The S₃/Ni?100?model at a relatively low S coverage?S:Ni=1:3?exhibited best HER activity.Therefore,the theoretical predictionand the results from the experimental work were in good agreement.In the second work,NiN with nanosheets structure was constructed by hydrothermal-ammonia reduction method,which achieved the precise modification of N in the metallic Ni.Firstly,the precursor of Ni?OH?₂ grown on NF was prepared by simple hydrothermal strategy,and then the NiN nanosheets was synthesized by NH₃reduction under the specified temperature.The nanostructured NiN presented a low overpotential(?₁₀)of 63 mV for HER,and its OER overpotential measured at 20 mA cm^-2 was 320 mV.An alkaline electrolyzer established by two symmetric optimized NiN electrodes showed high-efficiency activity toward overall water splitting with a current density of 10 mA cm^-2 at only 1.59 V and alomost no deactivation after 50 h ageing at 100 mA cm^-2.In order to explore the essential reason of the promotion of catalyst activity and stability,a series of Ni-based catalysts with different nitridation degrees were synthesized by tuning the temperature of ammonia reduction method.With the temperature raising,the nitrogen content first increased and then decreased and the electrochemical catalytic activity showed the same varation rule.The optimized NiN-400 with atomic ratio of N/Ni is 28.07%presented the best catalytic activity.DFT calculation on the N/Ni?111?model showed that the incorporation of N atom can modulate the H_ads adsorption energy on Ni and bringed out the?G_H*of N/Ni?111?is getting to zero with atomic ratio of N:Ni 33%,which is condusive to hydrogen evolution reaction.This result was well matched with the experiment results?optimized NiN-400,28.07%?.