| Ammonia(NH3)as a fundamental chemical material not only can be used as a raw material for fertilizer production in agriculture,but also considered as a high-quality clean energy and hydrogen storage carrier in industry.Due to its wide application prospects and industrial requirements,The ammonia synthesis technology is getting more and more attention.At present,the traditional ammonia production process still relies on the Haber-Bosch method invented in the beginning of the 20th century.However,with the limitations of thermodynamics and kinetics,the process needs to be carried out under severe conditions such as high temperature and high pressure.Besides this,the energy-intensive process also consumes 1 to 2%of the world’s annual energy with large tons of CO2 emissions.In recent years,electrochemical ammonia synthesis has attracted much attention because it provides an eco-friendly alternative to drive electric energy to synthesize NH3 under mild conditions.However,it remains challenging to design efficient catalysts with high activity and high stability in electrochemical ammonia synthesis area,which is believed to be the most difficult point in the NRR process.Mainly,the catalysts should activate inert N≡N triple bonds efficiently for lower free energy changes(ΔG)of process.Simultaneously,to get higher faradic efficiency(FE)toward nitrogen reduction reaction(NRR),the catalysts must get priority to the rival hydrogen evolution reaction(HER)occurred during NRR process.In view of this,based on the Co element,we synthesized two kinds of nanomaterials by doping to construct hole engineering and alloying strategies,which were applied into NRR process and obtained good performance.The specific works are as follows:(1)Synthesis of S-doped Co3O4 tetrahedron.As a transition metal element,the transition metal Co and its compounds are proved by theoretical calculation to be in the vantage point of the NRR volcanic map and have good NRR potential.Based on this,we synthesize Co3O4 regular tetrahedra by hydrothermal method,and in order to speed up the adsorption of N2 during electrocatalytic process and inhibiting the intrinsic HER activity of transition metal oxide,we modified Co3O4 by S-doping strategy to form S-doped Co3O4 with oxygen-rich vacancies.The S-doped Co3O4 prepared improved the electrochemical surface area and modulated the electronic structure of the catalyst,besides this,the Hydrogen Evolution Reaction(HER)was suppressed because of the existence of O vacancies,which greatly improves the activity and FE of NRR compared to the unmodified samples.Specifically,S-Co3O4 can achieve a maximum ammonia production rate of 10.68 μg h-1 cm-2 and a Faraday efficiency of 20.52%in the 0.5 M K2SO4 electrolyte system,which is nearly three times larger than pure Co3O4,besides this,the current stability of the material can reach 24 hours under the standard electrode potential of-0.25 V.In addition,the material has good performance in different electrolytes.(2)Synthesis of Au1Co1@GO.It’s believed the alloyed structure has special structural characteristics,and the combination between noble metals and transition metal elements can play a complementary effect in electrochemical catalysis.In this article,based on the Co element,we chose to combine with the Au element and synthesized the Au1Co1 alloy nanoparticles by annealed co-reduction method,the Au1Co1 NPs were loaded on the graphene oxide(GO)to achieve the uniform distribution.based on this,we optimized the composition of Au and Co elements,synthesized AuxCoy@GO samples with different element ratios,compared to the different atomic proportions of AuxCoy@GO samples,the Au1Co1@GO exhibited the highest electrochemical NRR performance.The maximum NH3 yield was calculated to be 36.68 μg h-1 mgcat.-1 and Faradic efficiency(FE)was 22.03%at-0.2 V(vs RHE)in 0.5 M K2SO4 electrolyte,which was much higher than monometallic Au@GO and Co@GO.Theoretical calculations proved that the alloyed Au1Co1 structure effectively reduced N2 adsorption energy and accelerate the activation of nitrogen molecules.The enhanced NRR performance was attributed to the synergistic effect of alloy and optimum elemental composition. |
