Structure Design And Electrocatalytic Water Splitting Properties Of Novel Electrospun Nanofibers Containing Noble Metals

The massive use of fossil fuels has caused serious environmental pollution and energy crises,promoting the development of clean energy sources.Hydrogen（H₂）with an energy density of 146 k J g^-1 is a green clean energy source owing to its abundant reserves and the fact that the combustion product is only water.H₂ plays an important role in advancing the technology of renewable energy and achieving carbon neutrality.Due to its simplicity and lack of CO₂ emissions,H₂ production from water electrolysis has successfully attracted global scientists and industry attention.A typical water electrolysis process includes the hydrogen evolution reaction（HER）at the cathode and the oxygen evolution reaction（OER）at the anode.The theoretical thermodynamic voltage required for the electrolytic water is 1.23 V.However,practical intrinsic energy barriers and complex reaction processes result in slow reaction kinetics,for which higher overpotentials are often required in practical applications to overcome the intrinsic activation barrier.Therefore,a number of highly active electrocatalysts are used to accelerate the kinetics of the reaction to improve catalytic activity.Currently,precious metals（Pt,Ru,Ir,Rh）are recognized as the best catalysts for the electrolysis of water.Nonetheless,their high price,scarcity and instability seriously limit practical applications.Hence,it is meaningful to develop highly active,resistant and stabilized electrocatalysts with low precious metal content.Nanofibrous materials prepared by electrosspinning technology with large specific surface area and flexible structure benefit to accelerate electron and mass transfer processes.Meanwhile,the large specific surface area favors the uniform dispersion of nanoparticles to avoid aggregation,thereby increasing the number of exposed active sites and achieving effective catalysis.In addition,carbon nanofibrous have excellent conductivity and serve as a substrate on which precious metal nanoparticles are loaded,achieving the construction of low content precious metal catalysts.In this paper,with the objective of developing highly efficient electrocatalytic water splitting catalysts,electrospun nanofibrous materials containing noble metals were constructed by combining electrospinning technology with calcination,carbonization,oxidative polymerization,chelate adsorption and chemical reduction.Then some efficient strategies such as doping,construction of amorphous defects and heterogeneous structures and the integration with conductive substrates are demonstrated to improve the electrocatalytic performance.The morphology,microstructure and elemental composition of these materials were comprehensively characterized.The experimental results showed that the prepared noble metal-containing nanofiber materials exhibited excellent HER and OER activities and stability.The details of the research for this paper are as follows:1.Iridium-based oxide nanofiber catalysts with homogeneous morphology were prepared by electrospinning technique integrated with calcination process,furthermore,the electrocatalytic OER properties of the prepared nanofiber catalysts were investigated.The details are as follows:（1）Iridium-molybdenum oxide（Ir Mo O_x）nanofibers with a low crystallinity were obtained by calcination the electrospun polyvinylpyrrolidone（PVP）/iridium salt/transition metal molybdenum salt composite nanofibers in air,which can catalyze OER under acidic conditions.The overpotential of Ir Mo O_x is 267 m V at a current density of 10 m A cm^-2.Due to the electron transfer between the high-valence Mo⁶⁺and Ir elements,the electronic structure of Ir Mo O_x is modulated to effectively reduce the reaction energy barrier and enhance the catalytic activity and stability.In addition,the low crystallinity Ir Mo O_x is also beneficial to accelerate the proton diffusion rate and further improve the OER activity.（2）Ir-Ce O₂-C heterostructured nanofibers were prepared through low-temperature controlled calcination of polyvinylpyrrolidone/iridium salt/ceria salt obtained from electrospinning technique.Ir-Ce O₂-C NFs exhibit optimal OER activity in both basic and acidic media.In particular,the current density of 10 m A cm^-2 can be reached in alkaline and acidic environments with the overpotential of only 279 and 283 m V.Experimental and theoretical calculations show that the catalytic active site of Ir-Ce O₂-C NFs is Ir.Meanwhile,some of the O in Ce O₂ combine with Ir to form Ir-O bonds,which optimize the electronic structure of Ir to enhance the OER performance.2.Ultra-small iridium nanoparticles anchored on the surface of the nitrogen-doped carbon nanofibers and Ni/carbon composite nanofibers were prepared via an electrospinning combined with chelate adsorption or chemical reduction deposition process,and their electrocatalytic HER properties were investigated.The details are as follows:（1）Ultra-small iridium nanoparticles were successfully loaded onto nitrogen-doped carbon nanofibers（NCNFs）by chelate adsorption and controlled carbonization process to achieve high-efficiency catalytic HER properties in alkaline and acidic electrolytes with overpotentials of 23 and 8 m V at a current density of 10 m A cm^-2,respectively.The introduction of nitrogen in NCNFs reduces the charge transfer resistance and increase the catalytic active site,simultaneously.Furthermore,the mass fraction of Ir in Ir-NCNFs-2 is only 14.5 wt%,indicating that this strategy achieves reduced precious metal content in some degree.（2）Ultra-small Ir nanoparticles loaded on nitrogen-doped nickel-carbon nanofibers（Ni-NCNFs-Ir）were obtained by chemical reduction method.With the optimized Ni and Ir feeding ratio,the sample 0.1Ni-NCNFs-5Ir with only 6.0 wt%Ir content has the highest HER performance,showing an overpotential of 25 and 22 m V for basic and acidic HER at a current density of 10m A cm^-2,respectively.In addition,the smaller Tafel slop also illustrated the faster kinetics of the catalyst.3.Amorphous rhodium（Rh）-based oxide nanofibers and Co/carbon composite nanofibers decorated with highly dispersed ultrafine Rh nanoparticles were prepared by electrospinning,calcination or combined with wet chemical reduction methods,and their electrocatalytic HER properties were investigated.The details are as follows:（1）Low crystallinity rhodium-molybdenum oxide（Rh Mo O_x）nanofibers were successfully synthesized by combining electrospinning technology with calcination process.Due to the optimization of the catalytic active site by electron transfer between Rh and Mo,the reaction energy barrier is greatly decreased,which allows Rh Mo O_x to have a low overpotential（35 and 22 m V）in both alkaline and acidic media.Meanwhile,it can be stabilized for a long time of 100 h in both media,indicating that Rh Mo O_x has excellent practical application in the field of water electrolysis for hydrogen generation.（2）Rh nanoparticles were loaded onto the nitrogen-doped Co/C nanofibers using a wet chemical reduction method.Due to the synergistic effect between Co and Rh,the HER activity of 0.15Co-NCNFs-5Rh was greatly enhanced with an overpotential of 13 and18 m V under alkaline and acidic conditions,respectively.This method provides a very effective method for the preparation of high-performance Rh-based HER electrocatalysts.In summary,we have prepared a series of electrospun nanofibers containing noble metals by combining electrospinning with other effective synthetic methods to achieve their applications in HER and OER.The morphology of the nanofibrous materials was modulated and the structure was optimized by some effective strategies such as doping,constructing low crystalline states and heterogeneous structures,and combining with conductive substrates,which improved their catalytic activity and stability.This paper provides a new strategy for the preparation of highly active and stable nanofibrous materials.