Development Of In-situ Generated Electrocatalysts For Water Splitting,Oxygen Reduction Reaction And Methanol Oxidation Reactions

Fossil fuels have been vital to the advancement of developed society,but climate change,ozone depletion,environmental pollution,and energy crises remain the primary challenges for the eco-friendly modern world.Due to the rapidly growing environmental issues and global renewable energy demand in the 21st century,it is urgent to explore eco-friendly and renewable energy sources to replace traditional fossil fuels.Developing sustainable and renewable energy sources and different energy conversion and storage is the key challenge for researchers.Among the possible renewable energy technologies/electrocatalytic reactions,the hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）,overall water splitting,oxygen reduction reaction（ORR）and methanol oxidation reaction（MOR）are leading the energy and storage conversion processes owing to their potential with zero pollution.Hydrogen has potential energy due to its high gravimetric energy density,zero carbon dioxide emission,and eco-friendly advantages.Electrocatalytic water splitting is regarded as a promising approach for hydrogen and oxygen production and is composed of two half-reactions i.e.,HER and OER,on the cathode and anode,respectively.Noble Pt metal and noble-metal oxides（Ir O₂ and Ru O₂）are state-of-the-art electrocatalysts for HER and OER,respectively.Despite that,limited resource,high-priced,and poor operating stability retard their large-scale commercial applications.Searching for earth-abundant electrocatalysts with remarkable performance and high stability to replace precious metals plays a significant role in the commercial application of electrochemical water splitting.Among the transition metal-based electrocatalysts,phosphides and carbides have been considered remarkable for water splitting.In addition,proton exchange membrane fuel cells are intriguing systems that exhibit great potential as power sources for vehicle power supplies and portable devices.However,the sluggish electrochemical reaction kinetics of cathodic ORR and anodic MOR have impeded their widespread commercialization.In this regard,different electrocatalysts comprising noble and/or non-noble metals have been synthesized to boost the reaction kinetics of ORR and MOR.Although noble metal-based electrocatalysts are more expensive than non-noble metals,their negative overpotentials make them superior for use in fuel cells.Pt-based electrocatalysts are considered the state-of-the-art candidates for ORR and MOR in fuel cells.However,several obstacles associated with the high loading of Pt,including scarcity,high cost,insufficient activity,poor stability,and poisoning by intermediates,hinder the widespread deployment of fuel cells.As an alternative strategy,developing novel material supports is a promising way to lower the loading of Pt and furnish more cost-effective and high-performance electrocatalysts.Therefore,five detailed research works in this doctoral dissertation are planned for the development of low-cost,scalable,highly effective,and stable in-situ generated electrocatalysts for water splitting and ORR/MOR.1.During this research work,we initially introduced a novel facile one-step in-situ strategy through pyrolysis to synthesize Co₂P nanoparticles encapsulated boron,nitrogen,and phosphorous tri-doped carbon nanotubes（Co₂P/BNP-CNTs）.The as-prepared samples deal with the description of the different physical techniques such as field emission scanning electron microscopy（FESEM）,high-resolution transmission electron microscopy（HRTEM）,X-rays diffraction（XRD）,energy dispersive X-ray spectroscopy（EDX）,X-rays photoelectron spectroscopy（XPS）,Raman spectroscopy and surface analysis.A typical three-electrode workstation is used for the electrocatalytic HER performance of the Co₂P/BNP-CNTs and commercial Pt/C electrocatalysts.The Co₂P/BNP-CNT-900electrocatalyst shows a low overpotential of 133 m V at a current density of 10 m A cm^-2 and a small Tafel slope of 90 m V dec^-1 in 0.1 M KOH solution.The Co₂P/BNP-CNT-900 electrocatalyst exhibits superior long-term stability for 15 h and 1000 cycles with negligible performance loss.The synergetic effect between Co₂P nanoparticles and heteroatom-doped CNTs contributes to the remarkable HER performance.These uniquely synthesized nanocomposites with high conductivity enable remarkable access to active sites and provide a surface for the effective mass transport of medium or gases,which enhances electron transfer for the HER process.Our works suggest a one-pot facile and promising synthesis strategy to achieve inexpensive rational designing high-performance electrocatalysts with enhanced HER performance.2.Earth-abundant nickel-based composites are of deep concern as electrocatalysts for the OER are significant to many sustainable chemical and energy transformation technologies.However,the realization of effective OER is still far away by requiring a high sustainable driving potential above thermodynamic demand.An innovative one-pot in-situ synthesis approach was extended to synthesize a series of nitrogen（N）and phosphorous（P）co-doped carbon nanotubes（CNTs）embedded with Ni₂P nanoparticles.Benefiting from the in-situ designed Ni₂P nanoparticles,N,P co-doped carbon nanotubes（NP-CNTs）with larger surface area,pore rich structure,and optimized temperature,the as-prepared Ni₂P NPs/NP-CNT-850 exhibits remarkable electrocatalytic activity for the OER in 1.0 M alkaline solution.The optimized Ni₂P NPs/NP-CNT-850 can achieve a reference current density of 10 m A cm^-2 at a low overpotential of 298 m V,lower Tafel slope of 53 m V dec^-1,and long-term stability without significant degradation for 17 h.Our study opens innovative in-situ engineering to dramatically increase the electrocatalytic activity of Ni-based composite for other applications via rational design of architectures with multiple active sites.3.Industrial requirements demand large-scale production of earth-abundant OER-electrocatalysts.Optimizing electrocatalytically activity and recognizing the most reactive sites for OER electrocatalysts are significant for sustainable future energy technologies.The in-situ annealing technique was reported for the first time in the preparation of Fe₃C nanoparticles（NPs）encapsulated via nitrogen and phosphorous co-doped carbon nanotubes for OER.The synthesized Fe₃C NPs@NP-CNT-800 composite shows remarkable electrocatalytic performance and affords a benchmark current density of 10 m A cm^-2 at a low overpotential of 280m V in an alkaline solution.Furthermore,a novel Fe₃C NPs@NP-CNT-800 hybrid surpasses the standard Ru O₂ electrocatalyst in terms of OER performance,and it shows negligible degradation for chronoamperometric and chronopotentiometry investigations.The remarkable performance and stability are ascribed to the Fe₃C NPs,novel tubular bamboo-like morphology of its carbon materials and heteroatom doping.In addition,this morphology contributes to electrochemical interfaces,larger surface area,active catalytic sites,and rapid charge transfer kinetics.4.Developing cost-effective and remarkable electrocatalysts for OER and HER performs an excelling role in boosting the hydrogen energy application.We extended the above novel in-situ one-pot strategy for the first time to synthesize molybdenum carbide nanoparticles（Mo₂C NPs）incorporated on nitrogen（N）and phosphorous（P）co-doped stereotaxically carbon（SC）.The optimized Mo₂C NPs/N,P-SC-800 electrocatalyst exhibits lower overpotentials of 131 and 287 m V for HER and OER to deliver a current density of 10 m A cm^-2 in 1.0 M KOH medium with smaller Tafel slopes of 58.9 and 74.4 m V dec^-1,respectively.In addition,an electrolyzer using Mo₂C NPs/N,P-SC-800 electrode as cathode and anode delivers a current density of 10 m A cm^-2 at a small voltage of 1.64 V for overall water splitting.The excellent water splitting performance could be ascribed to optimum Mo₂C NPs for more accessible active sites,highly active N,P-SC networks for accelerated electron transfers,and synergetic effect between Mo₂C NPs and N,P-SC networks.The N,P-SC network not only enhances the overall dispersion of Mo₂C NPs but also contributes numerous electroactive edges to enhance the performance of HER,OER,and overall water splitting activity.5.Doped materials with well-defined morphologies have attracted substantial research interest due to their excellent activity and stability in fuel cells.Celery-like nitrogen and phosphorous co-doped carbon nanofibers frameworks（PNCNF）have been successfully fabricated via a novel in-situ doping and self-assembly strategy.The as-prepared PNCNF serves as a superior support for uniform Pt nanoparticles.Compared to the commercial Pt/C,the resulting Pt/PNCNF composite possesses a porous architecture and high surface area,exhibiting remarkable mass activity,durability,and anti-poisoning ability toward ORR and MOR.In addition,the high CO tolerance of Pt/PNCNF could be ascribed to a strong interaction between the Pt nanocrystals and PNCNF,which facilitates OH^-adsorption and removal of CO intermediate.The improved electrocatalytic properties benefit from the morphological and compositional advantages of the N and P co-doped carbon nanofiber frameworks.Specifically,the orientation of celery-like carbon nanofibers layers in Pt/PNCNF optimizes its mechanical properties.This synthetic strategy for preparing N and P co-doped carbon nanofibers frameworks is a new and general pathway for the rational engineering of heteroatom-doped carbon composites.