| Ammonia（NH3）plays a crucial role in contemporary society due to its significant impacts on medicine,agriculture,and chemical industries,and it is also a carbon-neutral energy carrier with high energy density.Currently,NH3 synthesis has mainly relied on the Haber-Bosch process developed successfully in the early 20th century,but this process consumes a huge amount of energy and emits a large amount of CO2 greenhouse gas.Therefore,since the 1960s,electrochemical nitrogen reduction reaction（NRR）directly from N2 and H2O has become an important strategy for ammonia production and a hot research topic in the academia and industry.Although significant progress has been made in the performance of ammonia production with the efforts of researchers,the bottleneck problem of industrial application of electrochemical nitrogen fixation technology is still relatively prominent due to the extremely low solubility of N2(0.675 mmol·L-1),high N≡N bond dissociation energy(941 k J·mol-1),and competitive hydrogen evolution reaction（HER）,resulting in low NH3 yield and Faradaic efficiency.Recent studies have shown that using nitrate（NO3-）as a nitrogen source instead of N2 can reduce energy consumption and alleviate the strong competition of HER due to its high-water solubility and low N=O bond dissociation energy(204 k J·mol-1),further improving the efficiency of NH3 synthesis.Electrochemical NO3-reduction reaction（NO3-RR）is a complicated eight-electron transfer pathway for NH3 production,involving multiple reaction mechanisms and intermediates（NO2-,NO,N2O,NH2OH,N2,and N2H4,etc.）.Therefore,there is an urgent need to design and develop NO3-RR electrocatalysts with high NH3selectivity.The research work of this dissertation focuses on improving the catalytic activity and selectivity of electrochemical NH3 synthesis through the modification of Ti,Zr-based oxide electrocatalysts（hybridization with carbon materials,atom doping,and vacancy control）and the use of different nitrogen sources（N2 and NO3-）for efficient ammonia production.The main research contents and conclusions of this dissertation are as follows:（1）A hybrid of Ti O2 and juncus effusus-derived carbon microtubes（Ti O2/JE-CMTs）was prepared as an active NRR electrocatalyst by combining the impregnation method and high-temperature pyrolysis method.The higher conductivity of JE-CMTs ensures the stability of electron transfer in this hybrid material during the electrocatalytic process.Meanwhile,the inner cavity and outer surface of the unique three-dimensional cross-linked hollow tubular structure of JE-CMTs serve as a carrier,effectively preventing the aggregation and detachment of Ti O2 nanoparticles,improving the dispersion of Ti O2 and exposing more catalytic active sites,further strengthening the binding/interaction of the catalyst with N2 molecules.In 0.1 mol·L-1 Na2SO4 electrolyte,the Ti O2/JE-CMTs hybrid material exhibited a NH3 yield of 20.03μg·h-1·mgcat.-1and a FE of 10.76%at-0.50 V versus the reversible hydrogen electrode（RHE）,outperforming many Ti-and carbon-based NRR catalysts recently reported.（2）Mn-doped Ti O2(Ti1-xMnxO2)nanospheres were prepared using the condensation reflux method and high-temperature pyrolysis method to enhance N2 electroreduction.The results showed that the NH3 yield of Ti1-xMnxO2 nanospheres(20.05μg·h-1·mgcat.-1)was significantly higher than that of undoped Ti O2(8.06μg·h-1·mgcat.-1),with a FE of11.93%at-0.50 V in 0.1 mol·L-1 Na2SO4 solution.Furthermore,this catalyst exhibited the electrochemical stability of at least 24 h.Density functional theory（DFT）calculations showed that the Ti4c3+site in the Ti1-xMnxO2 system was the true active site for N2 binding and activation,and the potential-limiting step was the*N2 to*NNH process through an alternating pathway.（3）Based on the mechanical ball milling method,Ti2O3 nanoparticles with pure Ti3+system were prepared as NRR catalysts.The narrow bandgap of Ti2O3（≈0.1 e V）endows it with high conductivity,which is beneficial for the charge transfer during the reduction process of N2 to NH3.Ti2O3 catalyst with pure Ti3+system achieved a NH3 yield of 26.01μg·h-1·mgcat.-1and a Faradaic efficiency of 9.16%at-0.25 V in 0.1 mol·L-1 HCl electrolyte,and exhibited the electrochemical stability of at least 40 h.DFT calculations revealed that the enhanced electrocatalytic activity of Ti2O3 originates from the Ti3+active sites,which significantly reduces the overpotential of the potential-determining step.Additionally,the self-assembled aqueous Ti2O3/CP-based Zn-N2 battery can simultaneously generate value-added NH3(NH3 yield of 5.3μg·h-1·mgcat.-1)and output electricity(power density of 0.8 m W·cm-2).（4）In view of the extremely low water solubility,ultra-high N≡N bond dissociation energy,and weak chemical adsorption capacity on catalysts associated with N2,NO3-with high water solubility and smaller N=O bond energy was selected as a nitrogen source for the bifunctional pathway of removing NO3-pollutions and synthesizing NH3.Defective Zr O2-x nanoparticles were prepared using H2 plasma etching for electrocatalytic NO3-reduction reaction（NO3-RR）to synthesize NH3.The optimized Zr O2-x catalyst exhibited better electrocatalytic activity for NO3--to-NH3 conversion in 0.1 mol·L-1Na OH aqueous electrolyte containing 0.1 mol·L-1 Na NO3,with a NH3 FE as high as91.75%and a NH3 yield of 3736.59μg·h-1·cm-2 at-0.6 V versus RHE.In addition,the constructed Zr O2-x/CP-based Zn-NO3-battery device realized the functionality of simultaneously removing NO3-pollutants,outputting electric energy and generating value-added NH3,with a power density of 2.48 m W·cm-2 and a NH3 yield of 21.88μmol·h-1·cm-2.In summary,the research in this paper provides a new approach for the design and preparation of other transition metal oxide catalysts,and electrochemical synthesis of ammonia using different nitrogen sources in the future.Additionally,the constructed aqueous Zn-N2 and Zn-NO3-batteries offer guidance and reference for the study of metal-N2,metal-NO,metal-NO2-,and metal-NO3–batteries in the future. |
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