Electrochemical Reduction Of Nitrogen/Oxygen By Graphdiyne And Graphene Based Catalysts

Electrocatalytic N₂ reduction reaction（NRR）and O₂ reduction reaction（ORR）offer a green path for N₂ to NH₃ and O₂ to new energy conversions,both of which are of great significance for human thrive and development.The present-day industrial ammonia is synthesized by a mature technology of the Harber-Bosch process,which requires harsh conditions of high temperature（300-500℃）and high pressure（>200 atm）.However,more than 2%of the global energy supply was inputted to the synthesis of ammonia,along with gigatons of annual CO₂ emissions.Unlike the energy-intensive Haber-Bosch process,ammonia synthesis under ambient conditions over NRR shows many advantages,while its efficiency achieved by far is still too low to meet the practical command.On the other hand,the large-scale application of the new energy technology is hampered by the expensive noble Pt catalysts utilization on ORR.In this scenario,rational engineering of efficient but cost-effective catalysts with further optimizing the electrocatalytic systems play key roles in promoting the NRR efficiency and reducing the cost of new energy technology.In this thesis,we constructed graphdiyne（GDY）based electrocatalysts and developed a pressurized electrocatalytic system to boost the electrochemical ammonia production efficiency.In addition,the synergies between transition metal catalysts with porous graphene lever a superior ORR performance,which would pave an avenue for effective and no-precious ORR catalysts design.The following results are achieved:A novel Cl doped ultrathin GDY nanosheets（Cl-GDY）was nanostructured by Cl₂ corrosion strategy,the process of which not only introduced a heteroatom of Cl dopant in the host but also etched the pristine GDY into ultrathin nanosheets.Morphology characterization showcased the ultrathin nature of as-prepared Cl-GDY with a thickness of～2 nm.Moreover,structural analysis verified the success of Cl doping on GDY,leading to more defect formation and electron density modulation.Detailed electrochemical NRR experiments confirmed that the Cl-GDY catalyst affords a fairly high NH₃ yield rate of 10.7 g h^-1 cm^-2 and Faradic efficiency of 8.7%at-0.45 V and-0.4 V versus reversible hydrogen electrode（RHE）,respectively,which shows 4-fold enhancement than that of pristine GDY.Furthermore,density functional theory revealed that Cl doping is beneficial for N₂ reduction on the GDY host by lowing the limiting potential by 0.11 e V to the pristine GDY.Theπ-backdonation ability of catalytic centers is vital to facilitating the sluggish kinetics of NRR.We put forward a versatile in-situ coordination approach to the synthesis of a series of single atoms anchored on GDY backbones（denoted as M SA/GDY,M=W,Mo,Re,Mn）to optimize theπ-backdonation ability of metal centers.The microscopy and synchrotron-based X-ray absorption spectroscopy verify the atomically dispersed M-C₄ moieties with a low metal valence state were well-established in the as-prepared M SA/GDY catalysts.Under rigorous ENRR protocol,an activity trend of Re SA/GDY>Mo SA/GDY>Cr SA/GDY>W SA/GDY>>Mn SA/GDY（no activity）was delivered.Remarkably,the Re SA/GDY displayed an optimal NH₃ yield rate of 15.3μg h^-1 cm^-2 with a Faradic efficiency of8.07%at-0.35 V versus reversible hydrogen electrode,which is also quantitatively confirmed by the isotope ¹⁵N₂ measurements.Furthermore,theoretical studies revealed that the strong M-to-N₂π-backdonation of Re SA/GDY renders a low energy requirement of+0.42 e V for the reductive hydrogenation of*N₂ to*NNH,which is considered as the bottleneck of NRR.And a novel NH₃ desorption mechanism through N₂ co-adsorption on a Re-NH₃ intermediate is proposed to facilitate the NH₃ desorption from Re SA/GDY with a low energy input of+0.82 e V for the distal and mix pathways.By positive cooperation of metal single-atom catalysts with a pressurized reaction system,the chemical kinetics and thermodynamic driving forces of the NRR were regulated.The applied catalysts were the densely populated single metal atoms on GDY（M SA/GDY,M=Rh,Ru and Co）by using a mild and one-pot approach,which features M-C₄ coordination.Under high N₂ partial pressure,the dissolved N₂ concentration in water increases and more N₂ could be delivered to the electrode surface,leading to the amplified NRR activity with a simultaneously retarded hydrogen reduction reaction.As a result,a fairly high ammonia yield rate of 74.15μg h^-1 cm^-2,a Faradic efficiency of 20.36%and an NH₃ partial current density of 0.35 m A cm^-2 were achieved for Rh SA/GDY at 55 atm of N₂,which shows 7.3-folds,4.9-folds and 9.2-folds enhancement in comparison with those obtained in ambient conditions.The isotope experiment using adequately cleaned¹⁵N₂ further ensured the ammonia electrosynthesis from N₂.Additionally,similar NRR activity promotion was observed on the as-prepared Ru SA/GDY and Co SA/GDY under the pressurized electrolysis systems,demonstrating a general and cooperative strategy of elevating N₂ partial pressure in reprogramming NRR.Theoretical calculations reveal that the N₂ adsorption and kinetic activity for Rh SA/GDY are enhanced under the pressurized conditions,facilitating the ENRR process.Integration of amorphous Co N_x with 3D porous nitrogen-doped graphene aerogel（NGA）forming the Co N_x/NGA hybrid inherits both prominent catalytic activity of Co N_x and excellent conductivity of NGA.Moreover,the hierarchical pores over the NGA substrate dramatically promote the catalyst’s mass and electron transfer process.As a result,the Co N_x/NGA exhibited an onset potential of 0.93 V,a half-wave potential of 0.83 V and a limited current density of 5.4 m A cm^-2,which is comparable to the noble Pt/C catalysts.Furthermore,the Co N_x/NGA drives a high mass-energy density of 638 wh kg^-1 in a Zn-air battery,signifying its potential practical applications.