Design,Synthesis And Performance Study Of Catalysts Towards CO₂ Electroreduction To HCOOH

The excessive consumption of fossil fuels has not only caused energy crisis,but also led to large amount emissions of greenhouse gas CO2.The ever-increasing concentration of CO2 in the atmosphere has seriously threatened the living environment on the earth.CO2 electroreduction technology is considered as a promising avenue to reduce the concentration of CO2,because it can achieve both the carbon-neutral energy cycle and the storage of renewable electricity.As an important liquid product from CO2 electroreduction reaction,formic acid(HCOOH)has wide applications in pharmaceutical synthesis,hydrogen storage,and proton exchange membrane fuel cells.In addition,recent technoeconomic analysis showed that HCOOH is one of the most economically viable products towards CO2 electroreduction.To this end,the electroreduction of CO2 to HCOOH is of great significance.However,the high reaction energy barrier and complicated multiproton-coupled electron transfer steps result in slow kinetics(low activity)and broad distribution of products(low selectivity)over most electrocatalysts.Therefore,the key scientific issues for CO2 electroreduction lies in how to design catalysts which can efficiently activate and selectively transform CO2.In this dissertation,we presented the structural engineering of cost-effective earth abundant Sn and Bi for simultaneously achieving high activity and selectivity towards CO2 electroreduction.The main contents are outlined as following:1.We developed a Bi@Sn catalyst with core-shell structure,and applied it in CO2 electroreduction reaction.In H-cell system,Bi@Sn catalyst achieved FEHCOOH of 91%and jHCOOH of-31.0 mA cm-2 at-1.1 V vs RHE.To overcome the dissolution limit of CO2 molecular,we further evaluated the catalytic performance of Bi@Sn catalyst in the flow cell system.At current density of-250.0 mA cm-2,the FEHCOOH of Bi@Sn up to 92%.Theoretical studies indicate that energy barrier of the potential-limiting step for the formation of HCOOH was decreased owing to the compressive strain in Sn shell,resulting in the enhanced catalytic performance.2.We designed an effective method for promoting CO2 activation by introducing grain boundaries into Bi catalysts.During the electrochemical transformation,the structural rearrangement of BiPO4 contributed to the formation of metallic Bi with abundant grain boundaries(GBs),denoted as PD-Bi.The potential application of PD-Bi was testified on gas diffusion electrode in flow-cell system.PD-Bi achieved an FE of 91.9%for HCOOH at high current density of-600.0 mA cm-2.CO2 adsorption capacity and kinetics studies demonstrated that the abundant GBs within PD-Bi promoted the adsorption of CO2 and stabilization of CO2*-intermediate,resulting in facilitated CO2 activation and accordingly enhanced performance.3.We fabricated a Na doped Bi catalyst via topotactic transformation of commercial NaBiO3 nanosheets.At current density of-900.0 mA cm-2,the NaBiO3 derived Bi(BD-Bi)showed an FEHCOOH of 92.1%,which was the best among all reported Bi-based catalysts.Moreover,BD-Bi could maintain its stability for 20 hours at-200.0 mA cm-2.We further adopted a solid-state-electrolyte cell to avoid the use of high-concentration electrolyte.Under this circumstance,BD-Bi showed a continuous production of 1.34 M pure HCOOH solution.