Study On Highly-efficient Non-precious Metal Catalysts Towards Electrochemical Carbon Dioxide Reduction

The large-scale application of unsustainable fossil fuel not only has led to the global energy crisis but also continuously increases the carbon dioxide?CO₂?concentration in the atmosphere,which breaks the original of sustainable carbon cycle process in nature,and induces a series of climate problems,such as global warming.Therefore,energy-and environmental-related topics have become the focus of scientific research in recent years.Especially,the capture,storage and utilization of CO₂ have attracted tremendous attention.Thereinto,the electrochemical CO₂ reduction process driven by the renewable energy sources is considered to be one of the most promising ways for CO₂ conversion,as it can convert the CO₂ into value-added and useful chemicals and fuel.However,considering the linear CO₂ molecule is fully oxidized and extremely stable,efficient and robust electrocatalysts are urgently needed to design and prepare to promote this kinetically sluggish reduction process.Although precious metals,such as silver and gold,can exhibit superior CO₂ reduction selectivity,they are not suitable for practical applications because of their scarcity,high cost and questionable stability.Therefore,the development of low-cost,highly efficient and stable non-precious metal electrocatalysts for CO₂ reduction is of crucial importance to alleviate above problems and its further use.This thesis mainly focus on the non-precious metal materials,optimizing the structure to regulate the distribution of CO₂reduction products,conceptual putting forward the Zn-based electrocatalyst with achieving high faradaic efficiency?FE?and large current density of carbon monoxide?CO?simultaneously that may meet the need of potential practical application;discussing the effect of different zinc facets on CO₂ electroreduction to formate.The research content mainly includes the following aspects:1.The pyridinic-N-enriched self-supported carbon nanotubes?N-CNTs?have been fabricated facilely by growing N-CNTs in situ on the stainless steel mesh?SS?with only melamine as the nitrogen and carbon sources by one-step thermal treatment.This fabricated integrated N-CNTs/SS electrode exhibits the high catalytic performance of CO₂electroreduction to CO through optimizing the precursor dosage and thermal treatment temperature,including the low overpotential of 0.1 V,high 75%FE of CO.Contrast experiment demonstrates that the catalytic performance of CO₂ electroreduction to CO is attributed to the enriched pyridinic-N content.Furthermore,the gas products of CO₂electroreduction on this N-CNTs/SS electrode are only the CO and hydrogen?H₂?,which are the main component of syngas.The H₂/CO ratio in the clean syngas product can be easily tailored in a large range between 1:3 and 3:1 via tuning the thermal treatment temperature or applied potential.Different ratios of these components of syngas are importance for different fuel product applications,such as the methanation,Fischer–Tropsch process and hydroformylation of alkenes.This provides the potential new strategy for the preparation of syngas.2.The porous Zn nanosheets are fabricated by the in situ electrochemical reduction of porous ZnO nanosheets,and directly used as the electrocatalyst for the CO₂ reduction.As a result,this porous Zn nanosheets exhibit high FE and large partial current density of CO at the same time,benefiting from the porous architecture with increased exposure and accessibility of active sites.Furthermore,the theoretical calculation result demonstrates that the high catalytic performance of CO₂ electroreduction to CO is attributed to the favorable adsorption energy of the key intermediate,which can be comparable to the precious metal catalysts?Au and Ag?.All these results suggest that the high catalytic performance of CO₂electroreduction to CO on this Zn-based catalyst,especially achieving the high FE and large partial current density of CO simultaneously.This indicates the great potential of commercial applications for CO₂ electroreduction to CO on these porous Zn nanosheets.3.Metallic zinc,regarded as the typical CO selectivity catalyst in CO₂ electroreduction,can also exhibit the formate selectivity.To illustrate the real reasons for this,density functional theory is first used to calculate the adsorption energy for the key HCOO*intermediates to formate on main Zn facets.Finally,the Zn?002?exhibits the favorable adsorption energy for the key HCOO*intermediates.And then the Zn catalyst with predominant?002?facet is prepared on Zn foil via the electrodeposition,oxidization and electroreduction process,and exhibits the maximum formate FE of 90%,the partial current density of 40 mA cm^-2 and excellent stability.Detailed theoretical calculations suggest the high selectivity toward formate is due to its proper binding energy of the key intermediate?HCOO*?on Zn?002?surface relative to the other intermediates of COOH*and H*to produce CO and H₂,respectively.And the CO₂ electroreduction to formate on Zn?002?facet is the most energetically favorable among the three competing cathodic reactions,which explains well the observed high FE for formate.This work might provide a guidance to explore the reasons for the same electrocatalyst with different CO₂ reduction products selectivity.