Electrochemical N₂ Reduction For NH₃ Synthesis On Gold/Ruthenium-based Catalysts

Ammonia plays a crucial role in daily life as a vital fundamental feedstock in modern industry and agriculture.It has long been also regarded as an ideal hydrogen storage carrier,due to its high hydrogen content,high energy density and high boiling point.At present,industrial ammonia synthesis is dominated by the well-established Haber-Bosch（H-B）process feeding from reaction of N₂ and H₂ and operating under high temperature and pressure.As it involves the industrial H₂ synthesis,the H-B process consumes over 1%of the global annual energy supply and leads to huge CO₂emission.Therefore,it is desirable to develop ammonia synthesis processes driven by clean energy under mild conditions.Electrocatalytic nitrogen reduction（ENRR）is widely recognized as a promising alternative,which enables ammonia synthesis directly from N₂ and H₂O driven by renewable electricity available from solar sources.Nevertheless,poor kinetics and current efficiency of ENRR strictly limited the development for industrial application,which is mainly due to the difficulty of N₂molecular activation,the weak dissolution and diffusion in the electrolyte,and the strong interference of competitive hydrogen evolution reaction.Therefore,it is necessary to develop an efficient catalyst to increase the ammonia production rate and inhibit the hydrogen evolution reaction.Furthermore,reasonable electrode construction,electrolyte selection and electrolyzer designing,to provide enough N₂ molecules in the reaction system are also effective strategies to further improve the overall ENRR performance.In this paper,a series of gold/ruthenium-based catalysts were prepared via different synthesis methods.Rationally regulating the electronic structure on the catalyst surface promoted the chemisorption and activation of N₂ molecules at the active sites and inhibited the competitive HER process,thereby significantly improving the ammonia production rate and Faradaic efficiency（FE）.Meanwhile,we developed the PEM flow electrolyzer by spraying the catalyst on carbon paper as gas diffusion electrode（GDE）,and employing the commercial proton exchange membrane（PEM）as solid electrolyte,in which poor solubility and diffusion of N₂ in aqueous electrolyte was overcome.As a result,ammonia production rate was further improved.（1）Base on the weak adsorption of H*on the high-indexed surface of gold nanoparticle,we synthesized gold nanoparticle enclosed of multiple high-index facets via a typical seed growth method toward ENRR.The catalyst showed excellent ENRR performance in Li₂SO₄ electrolyte solution,achieving the highest FE of 73.22%and ammonia production rate of 46.1μg h^-1 mg^-1 at-0.3 V vs.RHE.DFT calculation results show that the high-index facets including Au（553）and Au（551）can effectively reduce the energy barrier of PDS（*N₂→*NNH）,thus favoring the N₂ activation.More importantly,the destabilized adsorption of competitive HER intermediate on these high-index facets dramatically inhibited the competitive HER,resulting in a remarkably increased FE.（2）Based on the strong N₂ adsorption on OVs that favors the cleavage of N≡N bond,We synthesized the amorphous Ru Au clusters anchored on Oxygen-vacancy-rich Ce O₂ nanorod（Ru Au/Ce O₂-OV）toward ENRR.The catalyst attained NH₃ yield rate at-0.2 V vs.RHE,superior to that over Ru Au/Ce O₂ with deficient OVs on Ce O₂ surface.The improved activity originated from the synergistic effect of numerous OVs as N₂adsorption sites and the adjacent Ru Au as N₂ activation sites.In addition,the low-coordinated Au surfaces could limit the competitive HER activity,while the N2activation was also difficult.To further enhance the ENRR efficiency,we combined the Auspecies with the Ru-based catalyst,which possess higher intrinsic ENRR activity.We further constructed the PEM electrolyzer using Ru Au/Ce O₂-OV catalyst toward NH₃synthesis.Under the applied N₂ pressure of 4 bar,the PEM afforded the NH₃ generation rate of 185.53μg mg^-1 h^-1 at-0.3 V vs.RHE,2～6 times of those obtained in PEM electrolyzer under N₂ pressure of 1 bar and in Li₂SO₄ electrolyte.The remarkably enhanced ENRR activity was mainly due to:1)the PEM electrolyzer enables the N₂directly transferring across the GDE and achieving the catalyst surface,avoiding the poor solubility and diffusion of N₂ in aqueous solution;2)increasing the N₂ pressure could further increase the number of molecules,facilitate the N₂ adsorption on catalyst surface and the effective collision,and favor the NH₃ desorption from catalyst surface.（3）Since the lead element is HER-inactive species,We incorporated Pb species into Ru/Ce O₂ nanorod as ENRR catalyst.In the PEM electrolyzer,the Ru Pb/Ce O₂catalyst achieved improved NH₃ yield rate and FE compared to pristine Ru/Ce O₂.The poisonation toward HER resulted in more Ru sites for N₂ hydrogenation,remarkably promoting the ENRR activity.Furthermore,Li⁺partially substituting the H⁺effectively reduced concentration and diffusion rate of H⁺in the PEM,thereby declined the competitive HER activity.Therefore,we used the Li⁺modified PEM as solid-state electrolyte,achieving a remarkably improved FE of 15.57%toward ENRR,with appropriately inhibited NH₃ yield rate(166.07μg mg^-1 h^-1)at-0.3 V vs.RHE using Ru Pb/Ce O₂ catalyst under N₂ pressure of 4 bar.DFT calculation results determined that the introduction of Pb into Ru/Ce O₂ limited the electron transfer from Ru species to Ce O₂ surface,which declined the required energy for the adsorption and the initial activation of N₂ on Ru sites.After Pb incorporation,the*H could not bind with Ru atoms,which restrictly restrained the HER activation.（4）Since the exposed edge of 2D MXene material possess higher intrinsic activity,we synthesized atomic dispersed Ru species on edge exposed Ti₃C₂O_x MXene(Ru _SA/e Ti₃C₂O_x)via ultraviolet light reduction method combined with ice confinement,in which Ru atoms served as active sites toward ENRR.At-0.2 V vs.RHE in 0.5 M Li₂SO₄ solution,an optimal NH₃ yield rate(59.26μg mg^-1 h^-1)and FE（45.11%）were achieved over the obtained catalyst,outperforming those over the Ru species on plane exposed Ti₃C₂O_x MXene(Ru _SA/p Ti₃C₂O_x).In the PEM electrolyzer using Li⁺modified membrane as electrolyte,the Ru _SA/e Ti₃C₂O_x catalyst exhibited further optimal ENRR performance,achieving the NH3 yield rate(272.04μg mg^-1 h^-1)and FE（5.23%）in-0.6V vs.RHE under the applied N₂ pressure of 4 bar.The greatly promoted ENRR performance at relatively higher potential also demonstrated the promsing potential of PEM electrolyzer for practical application.