Design Of Non-noble Metal Nanomaterials And Their Electrocatalytic Performance

Along with ever-increasing economic development and energy demand,it will inevitably give rise to the rapid depletion of fossil fuels as well as the accompanying climate and environmental issues.Therefore,there is an urgency to explore promising renewable energy carriers,develop high-efficiency energy conversion technologies and fabricate clean energy systems.Among these clean energy sources,hydrogen energy has been considered as the most promising future energy carrier thanks to its merits of wide raw materials availability,abundant reserves,ultrahigh energy density and environmental friendliness.It plays a critical role in energy conversion devices such as fuel cells,water electrolyzers and metal-air batteries for developing and utilizing hydrogen energy.Among these,the rates of electrocatalytic oxygen reduction reaction(ORR),oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)determine the overall efficiency for energy conversion devices.It is highly desirable to employ highly efficient catalysts to speed up the rates of these reactions to obtain high energy conversion efficiency.Currently,Pt-based materials have been addressed as the most efficient ORR and HER electrocatalysts,while Ru-or Ir-based materials are the benchmarkable OER electrocatalysts.Although these noble metal catalysts show superior catalytic activities,the limited resource reserves and expensive prices have still seriously restricted their widespread applications.Therefore,for promoting the development of hydrogen energy economy,it is essential to design and develop the non-noble catalysts with high efficiency and low cost.This dissertation is mainly focused on the development of ORR,OER and HER electrocatalysts towards the target of enhancing their catalytic performance.Moreover,the effects of structure,morphology and chemical composition on catalytic performance were systematically studied.Moreover,the structure-activity relationships between the catalytic activity and structure/morphology/composition were explored.Furthermore,density functional theory(DFT)calculations have studied the catalytic mechanisms behind the superior activities.These research results in this dissertation will be expected to provide new ideas for the development of other advanced catalytic materials.This dissertation is mainly composed of the following aspects:(1)A multifunctional catalyst comprising of Fe₃C and Co nanoparticles encapsulated in a hierarchical structure of N-doped carbon(Fe₃C-Co/NC)was synthesized by a template-removal method.By regulating the synthesis conditions,we systematically explored the effect of the composition and hierarchical structure on the catalytic activity.Further studies have found that Fe₃C-Co/NC exhibits superior multifunctional catalytic performances.Specifically,the Fe₃C-Co/NC shows outstanding ORR activities with a high half-wave potential of 0.885V and good stability as well as tolerance to methanol in 0.1 M KOH,which are superior to commercial Pt/C catalyst.Also,the Fe₃C-Co/NC shows excellent bifunctional catalytic activity with small oxygen electrode activity parameter value of 0.72 V for ORR and OER in0.1 M KOH.Moreover,the Fe₃C-Co/NC also displays good OER and HER activities.The above superior performance probably is ascribed to the following points:(i)the carbon layers activated by Fe₃C-Co nanoparticles can afford efficient ORR active sites;(ii)the pyridinic N is beneficial to the adsorption for OER intermediates,thus accelerating the catalytic reaction kinetics;and(iii)hybridizing Fe₃C-Co nanoparticles into N-doped carbon can optimize the adsorption of hybrid for H atom,therefore improving the HER activity.(2)CoP has attracted ever-growing interest,which has been considered as one of the most promising electrocatalysts.However,its relatively poor electrical conductivity,few accessible active sites and low intrinsically catalytic activity make it challenging to achieve desired electrocatalytic performance.To overcome these shortcomings and inadequacies,the following strategies are adopted:(i)hybridizing with high-conductivity reduced graphene oxide(RGO)to increase the dispersion of the active phase and simultaneously to enhance the conductivity of the electrode material;(ii)preparing tiny active phase nanoparticles to afford abundant catalytically active sites;and(iii)p-metal Al and d-metal Fe codoping to delicately modulate the electronic structure of CoP to optimize the adsorption energy for reaction intermediates.As expected,the Al,Fe-codoped CoP/RGO hybrid as bifunctional electrocatalyst for overall water splitting shows good activity with an only cell potential of1.66 V to drive a current density of 10 mA cm^-2.Moreover,Al,Fe-codoped CoP/RGO shows no obvious potential increase after chronopotentiometric test for 10 h,suggesting its good durability.(3)As compared to the crystalline materials,low-crystallinity catalysts not only own the high conductivity but also have abundant coordinatively-unsaturated sites to activate oxidized intermediates,which always endow their superior OER activity.Therefore,we fabricate Ni_0.65Ga_0.30Fe_0.05 hydroxide self-supported electrode via directly electrodepositing low-crystallinity hydroxides onto the macroporous nickel foam.This electrode delivers superior alkaline OER electrocatalytic properties with a low overpotential of 200 mV to achieve a current density of 10 mA cm^-2,low Tafel slope of 42 mV dec^-1 and good OER selectivity with high Faradaic efficiency of 96%.Moreover,this electrode exhibits no obvious potential increase after chronopotentiometric measurement for 100 h,indicating its impressive stability.The above superior performances are probably ascribed to following reasons:(i)the electrode has unique hierarchical structure,which integrates macropores from nickel foam and mesopores from open interspaces among Ni_0.65Ga_0.30Fe_0.05 hydroxide nanosheets,bringing easy electrolyte penetration and favorable gas diffusion during the OER process;(ii)the low-crystallinity Ni_0.65Ga_0.30Fe_0.05 hydroxide can not only improve the conductivity of active materials to facilitate the electron transfer from current collector to active sites but also can provide abundant coordinatively-unsaturated sites to activate oxidized intermediates;and(iii)introduction of Ga and Fe can optimize the adsorption for OER intermediates,thus enhancing the electrocatalytic activity.(4)Based on the DFT simulation results,it was found that Ga doping can effectively modulate the electronic structure of CoP and optimize the adsorption between catalytic active sites and H₂O,H and OH,thus improving the alkaline HER performance.Combining the electrodeposition and phosphorization treatment methods,the ultrathin Ga-doped CoP nanosheets self-supported electrode is successfully fabricated onto the carbon fiber paper.Electrochemical measurement results indicate that the electrode delivers excellent alkaline HER performances with an overpotential of just 44 mV to drive a current density of 10 mA cm^-2,and good HER selectivity with high Faradaic efficiency of 99%.Furthermore,no obvious current density decay was observed after a 50 h chronoamperometric test,revealing its good stability.Experimental results suggest that the high conductivity of carbon fiber paper is favorable to electron transfer from the current collector to Ga-doped CoP nanosheets.Moreover,the ultrathin Ga-doped CoP nanosheets(thickness size of about 5?10 nm)can provide abundant catalytically active sites.