Designed Synthesis And Performance Of Strained Pd And RuCo Alloy Electrocatalysts For Ammonia Synthesis Based On Nitrogen Oxidation-Reduction

Ammonia is a potential carbon-free energy carrier and is considered one of the most promising clean fuels because it exhibits high energy density,safety,and ease of liquefaction,storage,and transport.The traditional synthesis of ammonia is carried out under high-temperature and high-pressure conditions using nitrogen（obtained from air separation）and hydrogen（obtained from reforming of fossil fuels）as raw materials（the Haber-Bosch method）.In addition,the centralized and continuous production mode of the Haber-Bosch process makes it difficult to match it with currently distributed and intermittent renewable energy（wind power and photovoltaic,etc.）.Thus,two sustainable strategies have been developed to realize ammonia synthesis under ambient conditions:（1）coupling of nitrogen electrooxidation and nitrate electroreduction;and（2）coupling of nitrogen plasma oxidation and nitrogen oxide electroreduction.Between these,nitrogen electrooxidation and nitrate and nitrogen oxide electroreduction are involved.The critical issues with these electrocatalytic reactions are low activities,high energy consumption,and unclear active species and reaction mechanisms.Consequently,tensile strained palladium porous nanosheets（Pd-s PNSs）,Ru Co alloys,and Ru-doped Co nanosheets are designed to improve the electrocatalytic performance and energy efficiency.Combining the results of various in situ measurements,the actual active species and definite reaction mechanisms are revealed.The main research contents are summarized as follows:1.For the nitrogen electrooxidation reaction,Pd-s PNSs are designed and constructed to address the problems of low catalytic activity and unclear reaction mechanisms.Due to the tensile stress,Pd-s PNSs exhibited the highest reported nitrate formation rate of 18.56μg h^-1 mg_cat.^-1at 1.75 V vs.RHE（reversible hydrogen electrode）.¹⁵N isotopice labelling experiments and strict control experiments confirmed that the generated nitrate originated from nitrogen electrooxidation.Subsequently,a series of in situ electrochemical characterization studies proved that the Pd O₂ generated in situ during the electrocatalytic oxidation process was the real active species.Furthermore,the introduction of tensile stress on the Pd surfaces facilitated the conversion of Pd to Pd O₂.2.To address the issues of high overpotential and low energy efficiency for the nitrate electroreduction reaction,we proposed a new three-step relay reduction mechanism and synthesized Ru Co alloy hollow dodecahedral catalysts.The Ru Co catalyst with 15%Ru content showed the best catalytic performance with an onset potential of+0.4 V vs.RHE,an ammonia yield rate of 1.23 mmol cm^-2 h^-1,a Faradaic efficiency of 96.8%,and an energy efficiency of 42.1%.Subsequently,a series of in situ electrochemical tests,control experiments,and theoretical calculations confirmed that the encouraging performance originated from the as-proposed three relay mechanism,which included:（1）nitrate react spontaneously with Co to form nitrite and cobalt hydroxide,（2）cobalt hydroxide was reduced back to the metal state at a low potential with the assistance of ruthenium,and（3）nitrite continued to undergo hydrogenative conversion on the surface of Ru Co to produce the final ammonia product.3.For electroreduction of nitrogen oxides,Ru-doped Co nanosheets were adopted as model catalysts with which to probe the reaction type and reaction mechanism.The electrochemical measurements showed that Ru₉Co₉₁ exhibited the best catalytic performance.At-0.6 V vs.RHE,the rate for production of ammonia over Ru₉Co₉₁ was1.14 mmol mg^-1 h^-1.The optimal energy consumption for ammonia production was0.819 MJ mol_NH3^-1.Control experiments showed that nitrite electroreduction,nitrate electroreduction,and NO₂ electroreduction occurred simultaneously in the reaction system.Notably,nitrite electroreduction contributed substantially to the main partial current.Electrochemical in situ Fourier transform infrared spectroscopy and online differential mass spectrometry demonstrated that reduction of nitrogen oxides on Ru₉Co₉₁ occurred through N-terminal alternating hydrogenation pathway.