Synthesis Of Nickel Carbon Nanotubes And Nitrogen-doped Carbon Nanotubes For Hydrogen Evolution Reactions

The rapid boom in the worlds'population,incessant use of fossil fuels and their increasing depletion is causing grave environmental and energy challenges in this 21^st century.To overcome these challenges,hydrogen is being introduced as a green energy carrier and alternative to fossil fuels.This is as a result of its high gravimetric energy density and zero-carbon emission during combustion.A greener route for hydrogen production is through electrochemical water splitting in which hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?are the respective half-cell reactions.Platinum?Pt?and its based compounds are the ideal electrocatalysts for HER,however,their limited availability and exorbitant cost mars their large-scale use.Given these disadvantages,researchers are exploring cheaper and more available options in the form of transition metals like nickel?Ni?and its composites as alternatives.Ni is widely studied as a result of its eco-friendly nature,good activity and stability in alkaline media.In this research project,we present facile methods of enhancing the HER activity of the bulk Ni metal.This is carried out by fabricating electrocatalysts in the form of Ni nanoparticles encapsulated in carbon nanotubes/nitrogen doped carbon nanotubes as support.In the past,the HER activity in alkaline media of nanosized Ni nanoparticles encapsulated in carbon nanotubes/nitrogen doped carbon nanotubes has been scarcely reported.The carbon nanotubes support serves to ensure proper dispersion and circumvent aggregation of the Ni nanoparticles which are the active sites of the catalyst.Whereas,the nitrogen doping provides extra electrons and reduces the hydrogen adsorption Gibbs free energy for the HER.The fabricated electrocatalysts possesses good morphology,low overpotential at high current density,low Tafel slopes,low resistance,high surface area and defects.The main content of the research is summarized as follows.1.Nickel nanoparticles encapsulated in carbon nanotubes?CNTs?by a facile and novel one-step pyrolysis method are obtained from fumaric acid and nickel acetate as carbon and nickel sources respectively.The synthesized Ni encapsulated CNTs are characterized by various methods and are confirmed to possess large surface areas and numerous mesopores.They were applied as non-precious metal electrocatalyst for HER in 1 M KOH solution.The results show that the Ni encapsulated carbon nanotubes synthesized at 650? exhibited the best catalytic activity and stability with the smallest Tafel slope of 102 mV dec^-1,an onset potential of 0.110 V and overpotential of 0.266V to achieve a current density of 10 mA cm^-2.2.Nitrogen-doped Ni@CNTs with high concentration of pyridinic and graphitic nitrogen are fabricated by a cheap and one-step pyrolysis method.The most active catalyst synthesized at 800? exhibits an overpotential of 0.244 V to reach a current density of 10 mA cm^-2,Tafel slope of 93.3 mV dec^-1 and a satisfactory 10 h stability.Low resistance and large ECSA value of the sample also favor the competent response for HER in alkaline media.The robust HER activity of the catalyst is a result of the nickel nanoparticles which are the active spots of reaction,while the presence of well-developed nitrogen containing carbon nanotubes with large content of pyridinic and graphitic nitrogen may provide high electron density and feasible routes for its transportation to deliver an outstanding HER performance.3.The better activity of nitrogen doped carbon nanotubes as support for HER,attracted further exploration.As a result,nitrogen-enriched nickel carbon nanotubes for improved hydrogen evolution reaction was fabricated at various temperatures,nickel acetate and glucose concentrations.The samples are further examined by different characterization techniques.The study revealed that the sample synthesized at 800? with 2 mmol each of nickel and glucose possessed optimum performance with an overpotential of 0.147 V to achieve a current density of 10 mA cm^-2,Tafel slope of 57.6mV dec^-1 and a good 12 h stability.The high defects,well-formed and large density of the nitrogen-doped CNTs prevented agglomeration of the Ni nanoparticles and enabled their proper dispersion and availability leading to an efficient HER performance in 1 M KOH.This study provides a more economical route for the synthesis of nitrogen doped CNTs containing Ni nanoparticles since low concentrations of the nickel acetate and the glucose are employed.