Application And Construction Of Highly Active And Stable Noble Metal Electrocatalysts

More attention has been paid to the development of renewable energy sources ascribed to the limited storage of fossil fuels and ever-increasing environmental pollution.Compared to other renewable energy sources,hydrogen has attracted considerable attention due to its merits,high energy density and zero emission in synthesis.Electrochemical splitting of water is widely recognized as the most promising technology to environmentally friendly generate hydrogen.Hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）is the cathodic and anodic reaction in water splitting,respectively;while,HER and OER are uphill reactions requiring high-performance electrocatalyst to efficiently lower the energy barrier to realize the industrialization of water splitting technology.Moreover,due to the high potential and corrosive condition in anodic side,the stability issue should be well addressed to prolong the longevity of water splitting devices.The electrochemical adsorption and conversion of reaction intermediates are normally involved in electrocatalysis;thus,modulating the electronic configuration of active site to reach a moderate binding strength between active site and reaction intermediate is an effective methodology to boost the intrinsic catalytic activity.Besides,adjusting the morphology to expose more active sites is also meaningful to enhance the catalytic activity.The strong interplay between two different metal atoms can significantly promote the stability of electrocatalyst.In this dissertation,we propose to enhance the activity and stability of electrocatalyst by constructing new phase,alloying,tailoring morphologies and single atom manufacturing.Since the strong interaction between palladium atom and hydrogen atoms to form Pd H species,Pd electrocatalyst usually shows a low HER catalytic activity;thus,to form a new phase to weaken the binding strength between Pd and H atoms is prominent for using Pd as efficient HER electrocatalyst.Pd P₂@CB electrocatalyst is prepared by heating Na H₂PO₂ and Pd@CB at 350 ^oC confirmed by XRD,XPS and TEM tests.Pd P₂@CB electrocatalyst demands only 27.5 m V to attain 10 m A cm^-2 in HER catalysis in acidic medium,115.6 m V lower than Pd@CB;moreover,the HER performance of Pd P₂@CB is also better than commercial Pt/C.A 1.4-fold higher electrochemical surface area is recorded for Pd P₂@CB than Pd@CB indicating that more active sites are created in Pd P₂@CB electrocatalyst.Similarly,the overpotential at 10 m A cm^-2 in alkaline is 35.4m V for Pd P₂@CB,lower than Pd@CB and Pt/C due to the higher electrochemical surface area calculated from double layer capacitances.Besides,ignorable degradation in HER performance is recorded after 5000 potential cycles ascribing to the strong electronic interaction between Pd and P atoms suppressing the aggregation during potential cycling.Furthermore,Pd P₂@CB also exhibits better OER performance than commercial Ir O₂ in1 M KOH electrolyte;thereby,only 1.72 V is needed for Pd P₂@CB to drive water splitting to reach 50 m A cm^-2 in 1 M KOH electrolyte.Normally,ruthenium and iridium have been utilized as anodic electrocatalyst in Solid polymer electrolyte water electrolysis（SPEWE）.Improvement in the utilization efficiency of noble metals is meaningful for the industrialization of SPEWE.Adjusting the morphology of electrocatalyst is to expose more active sites has been recognized as a promising method to enhance the utilization efficiency of electrocatalyst.Boosted utilization efficiency can be achieved by minimizing the thickness of shell in core-shell structure.Pd@Ir core-shell nanoparticles with particle size of 4 nm have been loaded on CNT via one-pot method proved by the EDS mapping.The thickness of Ir shell is only 1nm.The overpotential at 10 m A cm^-2 is 266 m V for Pd@Ir/CNT,much lower than Ir/CNT and Ir O₂;moreover,the mass activity at 1.6 V vs.RHE is 9.13 A g _Ir^-1 for Pd@Ir/CNT,which is 117 times higher than Ir O₂.Additionally,Pd@Ir/CNT shows a higher HER performance than Pt/C since a moderate hydrogen binding strength is achieved for Pd@Ir structure;thereby,1.51 V is needed to drive acidic water splitting for Pd@Ir/CNT,90 m V lower than Pt/C-Ir O₂.SPEWE has attracted much attention due to high purity of generated hydrogen and quick response.However,the stability of OER electrocatalyst plague the commercialization of SPEWE.The strong electronic interplay between two atoms in alloyed electrocatalyst can efficiently enhance the stability as well as catalytic activity due to the tailored electronic configuration of active center.Tungsten atoms with high electronegativity have been introduced to Ir nanoparticles to synthesize Ir W/CNT electrocatalyst.Tungsten atoms have been successfully doped into lattices of metallic Ir proved by XPS and TEM.The OER performance reaches its maximum with atomic ratio between W and Ir of 1:1.Ir W/CNT also performs a stable OER activity during 2000potential cycles due to the strong interaction between W and Ir;while,OER performance of commercial Ir O₂ and Ir/CNT is sharply decayed due to the formation of metastable HIr O₂ and soluble Ir O₄^2-;moreover,the hydrogen adsorption strength on metallic Ir has been enhanced by the introduced W atoms resulting in a superior HER performance,better than Ir/CNT and Pt/C.Ir W/CNT requires only 1.52 V to drive water splitting with10 m A cm^-2 and sustains for 10 h.100%utilization efficiency is achieved for single atom catalyst capable of efficiently catalyzing HER and OER due to the unsaturated coordination of metal single atoms.However,due to the harsh conditions in anodic side of SPEWE,a low stability is recorded for single atom electrocatalyst restricting the practical application of single atom electrocatalyst.Hydrazine oxidation reaction（Hz OR）with theoretical thermodynamic voltage of-0.33 V vs.RHE has been researched as substitute of OER in SPEWE to decrease the cell voltage.Iridium single atom electrocatalyst（Ir-SA/NC）is synthesized via a pyrolysis method.The FT-EXANES spectrum indicates the formed structure is Ir-N₄ species.0.39 V is demanded to drive Hz OR with current density of 10 m A cm^-2,90m V lower than Pt/C;furthermore,unobvious degradation in Hz OR performance is recorded after 2000 potential cycles.Ir-SA/NC requires only 31.9 m V overpotential to drive HER with 10 m A cm^-2;thereby,0.39 V is required to drive HER-Hz OR catalysis,1.13 V lower than overall water splitting.