Cationic Regulation Of Perovskite Oxides For Efficient Electrocatalytic Synthesis Of Ammonia

Ammonia（NH₃）is a crucial chemical raw material that is widely used in agriculture,manufacturing,medical treatment,military applications,and other fields.The electrocatalytic ammonia synthesis can be realized in two ways:electrocatalytic nitrogen reduction reaction（eNRR）or electrocatalytic nitrate reduction reaction（eNITRR）.These two methods are expected to become alternative technologies for industrial NH₃ synthesis,providing new options for green and efficient NH₃ synthesis.However,the poor NH₃ yield rate and selectivity of electrocatalytic NH₃ synthesis continue to be the main bottleneck for the commercialization of these renewable energy technologies.Among many available catalyst candidates,perovskite oxide electrocatalysts have attracted much attention in the field of electrocatalytic synthesis of NH₃ due to their simple preparation method,low cost,flexible composition/structure,and abundant oxygen vacancy on the surface.However,a systematic and explicit catalytic mechanism for the electrocatalytic synthesis of NH₃ from perovskite is lacking,which makes it a challenge to design and synthesize new high efficiency perovskite type electrocatalysts for NH₃ synthesis by using reasonable strategies.Cationic regulation is an effective method to improve the electrocatalytic activity of perovskite oxides.The oxygen vacancies,the electronic structure of the active center,and the electron transport characteristics of perovskite oxides can be regulated by cationic regulation,so that perovskite has excellent electrocatalytic performance.Based on the cationic regulation strategy,this thesis aims to design high-efficiency perovskite electrocatalysts for electrocatalytic NH₃synthesis.While conducting in-depth research on the catalytic mechanism,effective strategies were employed to design the basic features of perovskite oxides in order to optimize their electrocatalytic NH₃ synthesis performance.The main research results obtained include the following four parts:（1）Inspired by d-band center theory,a series of double perovskite LaCo_xNi_1-xO₃（x=0.2,0.33,0.5,0.67,and 0.8,donated as LCNO-I,LCNO-II,LCNO-III,LCNO-IV,and LCNO-V）were chosen as theoretical models,where the d-band centers can be tuned and optimized by changing the stoichiometric ratios of Co and Ni elements.To explore the relationship between d-band center and eNRR performance of perovskite.Theoretically,the d-band center of the transition metal is calculated,and it is found that the d-band center of the transition metal can be significantly changed by the modulation of the B-site metal ratio.A series of LCNO nanorods catalysts matching the metal ratio of the theoretical models were prepared by hydrothermal-coprecipitation method,and the eNRR performance was tested.The experimental results show that LCNO-III（x=0.5）nanorods have attained the highest Faradaic efficiency（FE）and NH₃ yield rate among various LCNO nanorods,which are17.65%and 13.48μg h^–1 mg_cat.^–1,respectively.Theoretical calculations show that LCNO-III exhibits a d-band center closer to the Fermi level（ε_d=-0.96 e V）,indicating the strongest adsorption capacity for nitrogen molecules of LCNO-III（the calculated adsorption energy value is-2.01 e V）.This strong adsorption of nitrogen is beneficial to the further catalytic activation of nitrogen by perovskite materials.Therefore,the eNRR performance can be enhanced by modulating d-band center of perovskite.（2）The formation of more oxygen vacancies on the surface of perovskite oxides is beneficial to nitrogen adsorption and activation process,thus improving the eNRR performance.However,it is difficult to regulate oxygen vacancies reasonably by hydrogen heat treatment,and most perovskites are unstable under such conditions.To solve the above problems,La_xFe O_3-δ（L_xF,x=1,0.95,and 0.9）were used as model oxides,and the surface oxygen vacancy concentration was adjusted by changing concentration of A-site defects（La defects）,so that the relationship between oxygen vacancy concentration and eNRR performance of perovskite could be systematically studied.The experimental results show that the surface oxygen vacancy concentration is positively correlated with the La defect concentration.Meanwhile,compared with LF,the eNRR activity of L_0.95F and L_0.9F was significantly improved.At-0.5 V and-0.3 V,it delivers the highest NH₃ yield rate of 22.1μg h^-1 mg_cat.^-1and a FE of 25.6%,which are 2.2 and 1.6 times that of LF,respectively.Both experimental characterizations and theoretical calculations suggest that the enhanced eNRR activity can be mainly attributed to the favorable merits produced by the oxygen vacancies:the promoted adsorption/activation of reaction species,and thus optimized reaction pathways.Finally,the versatility of this strategy was further demonstrated by synthesizing other perovskite and testing them for oxygen vacancies and eNRR performance,which showed that the perovskite with rich oxygen vacancies exhibits superior eNRR performance.（3）Based on the above research conclusions,the optimization of the electronic structure of perovskite catalytic center metal and the construction of surface oxygen vacancy can promote the improvement of the catalytic performance of eNRR.A high entropy perovskite oxide catalyst with high eNRR activity can be designed by combining the synergistic effect between multiple components in high entropy materials and the abundant oxygen vacancy sites on the surface of perovskite oxides.Therefore,high-entropy perovskite materials Ba_xFe_0.2Co_0.2Ni_0.2Zr_0.2Y_0.2O_3-δ(B_x（FCNZY）_0.2（x=0.9 and 1）are designed and synthesized as efficient eNRR catalysts.The generation of additional surface oxygen vacancies and the optimization of the electronic structure of the catalytically active center can be induced by changing the A site stoichiometric ratio of metals and B-site high entropy engineering.At the same time,the valence state of Ni has changed obviously.TheNH₃ yield rate and FE for B_0.9（FCNZY）_0.2 are 1.51 and 1.95 times higher than those for B（FCNZY）_0.2,respectively.The d-band center theory is used to predict the B-site catalytic active center.Compared with other metals（Fe,Co,Zr,and Y）,Ni exhibits a d-band center closer to Fermi level,indicating the it possesses better adsorption of nitrogen than other metals.The free energy values of intermediate states in the optimal distal pathway show that the third protonation step（*NNH₂→*NNH₃）is the rate-determining step（RDS）.Designed by the A site non-stoichiometric ratio of metals and B-site high entropy engineering strategy,B_0.9（FCNZY）_0.2exhibits lower free energy of RDS,and thus has better eNRR performance.（4）Perovskite synthesized by solid phase method has large particles,irregular morphology,and small specific surface area,which leads to low catalytic activity.To solve the problem,La Fe O_3-δ（LF）perovskite with a one-dimensional nanofiber structure was prepared by electrospinning technology.The cross-linked network woven between fibers not only facilitates the diffusion of reactants and electron transport during electrocatalysis,but also avoids particle agglomeration to expose the largest active site.Based on this material,the electronic structure of perovskite catalytic active center was optimized by further combining with B-site substitution strategy.La Fe_0.9M_0.1O_3-δ(M=Co,Ni,and Cu,donated as LF_0.9Co_0.1,LF_0.9Ni_0.1,and LF_0.9Cu_0.1)perovskite materials were prepared and used as electrocatalysts for eNITRR.Compared to the LF nanofibers,the prepared LF_0.9Cu_0.1 nanofibers deliver a higher NH₃ yield rate of 348.53±14.63μg h^-1 mg_cat.^-1and a FE of 47.75±2.01%.Results show that after Fe in LF is partially replaced by Cu,Fe in LF_0.9Cu_0.1 exhibit higher charge redistribution ability.At the same time,a higher oxygen vacancy concentration is generated on the surface of LF_0.9Cu_0.1.Using in situ Fourier Transform infrared spectroscopy and online differential electrochemical mass spectrometry tests to capture the intermediates generated by the catalytic process,and further analyzing the mechanism of the eNITRR process combined with theoretical calculations,it is concluded that the partial substitution of Cu is beneficial to reduce the free energy of the RDS of eNITRR,thereby improving the performance of eNITRR.