Construction And Study On Novel Cobalt-based Electrocatalysts For Oxygen-involving Reactions

Electrochemical energy conversion technology has been considered an effective way to alleviate the current energy problems owing to its cleanness and efficiency.However,these energy conversion systems are highly dependent on a series of electrochemical oxygen-involving reactions,such as anodic oxygen evolution reaction（OER）in electrocatalytic water splitting and cathodic oxygen reduction reaction（ORR）in fuel cells,which involve multi-electron transfer processes.These reactions are always limited by high energy barrier and complex steps,which leads to slow kinetics and low energy conversion efficiency.The present commercial noble metal（Ru,Ir and Pt）-based oxygen-involving catalysts show good catalytic activity,but their unsatisfactory stability,high price and resource scarcity limit their further large-scale application.With respect to noble metal-based catalysts,non-precious metal-based catalysts have been widely researched owing to their advantages of low cost and flexible design.Among them,cobalt（Co）-based catalysts are especially representative.However,there are still many challenges to be solved,such as their catalytic activity and stability are still needed to be improved.In addition,it is increasingly important to clarify the catalytic mechanism of catalysts to reveal its structure-activity relationship,thus ultimately guiding the preparation and application of catalysts.In this thesis,Co-based electrocatalysts were selected as research target.With the aming of constructing efficient and stable Co-based oxygen-involving electrocatalysts,electronic tuning and geometric structure design strategies were developed to increase the number and accessibility of active sites,so as to improve their activity and stability.Meanwhile,in-situ technique and theoretical simulation were used to monitor the conversion process of reaction intermediates to reveal the mechanism,providing important guidance for catalyst design.The details are as follows:The rare earth oxide CeO₂ was selected to modified cobalt carbonate hydroxides(Co（CO₃）_0.5（OH）·0.11H₂O,CoCH),forming the heterostructure of CeO₂-CoCH.The optimization of synthetic conditions confirmed that when the CeO₂ loading capacity was 15 wt%and the size was 7 nm,the catalyst（Cat-7-15%）showed the best OER catalytic performance,which reached a current density of 10 m A cm^-2 at an overvpotential of 281 m V.The heterogeneous interface of CeO₂ and CoCH was atomically characterized through the combination of HAADF-STEM and EELS characterization.It was confirmed that the electron interaction occurred between CeO₂ and CoCH via the heterogeneous interface,where Co lost electrons accompanied with the increased valence state of Co ion.The enhanced Lewis acid of high-valented Co ions helps the enrichment and adsorption of OH^-at the initial stage of the catalytic process.Meanwhile,the reaction intermediates were monitored by in-situ Raman.It was confirmed that the formation of Co-OOH intermediate was detected in CeO₂-CoCH at a lower oxidation potential than CoCH,thus accelerating the OER process.CoCH was doped with heterogeneous Fe elements,and the dandelion-like bimetallic Fe CoCH was synthesized by optimizing the reaction conditions.CeO₂ was modified onto the substrate to form a CeO₂-Fe CoCH heterostructure to further explore the different functional effects of CeO₂ for various ions for OER.The electrocatalytic results showed that the overpotential of CeO₂-Fe CoCH at 10 m A cm^-2 was 280 m V,which was 40 m V lower than that of Fe CoCH.Morphological and structural characterization indicated that CeO₂ showed obvious interaction with low-valented Co ion in the stem position,which increased the valence of low-valented Co and enhanced the Lewis acid.In-situ Raman also revealed that the formation of Co-OOH intermediates was firstly detected at a lower potential in CeO₂-Fe CoCH,suggesting the acceleration of OER process.The hollow double-shelled Co-based single atom catalyst（DS Co-N/C）was synthesized by ZIF epitaxial growth method.HAADF-STEM and XAFS characterization confirmed that Co was atomically distributed on the substrate in the form of Co-N₄ species.The experimental and DFT results confirmed that the double-shelled structure of DS Co-N/C could effectively increase the loading capacity of single atoms and was conducive to the mass and charge transport during the reaction process.Meanwhile,the existence of the outer layer could effectively prevent the corrosion of the active site in the inner layer.The above advantages enabled the ORR catalytic performance of DS Co-N/C significantly enhanced compared with that of SS Co-N/C.The half-wave potential of DS Co-N/C in alkaline and acidic media were0.87 V vs.RHE and 0.77 V vs.RHE,respectively.Moreover,after 100 h long-time stability test,the current densities of 95%and 91%were maintained respectively.On the basis of the above hierarchical structure design,by adjusting the ratio of Zn and Co salt in the precursor,the double-shelled Co-based single atom catalyst（Co-Catalyst-1）,nanocluster catalyst（Co-Catalyst-2）and nanoparticle catalyst（Co-Catalyst-3）were synthesized to explore the origin of different catalytic activity for different-sized active sites and its influence on the ORR catalytic performance.Combined with microstructural analysis and DFT calculation,it was confirmed that N in the active site of Co-N₄ in Co-Catalyst-1 was more electronegative than that of Co-Catalyst-2 and Co-Catalyst-3 respectively,which is more conducive to the transfer of electrode electrons to the Co sites,expediting the generation and transformation of the rate-determining step intermediates*OOH.Moreover,the largest specific surface area and the highest degree of disorder of Co-Catalyst-1 could improve the mass and charge transport efficiency during the reaction,thus accelerating the ORR process.The ORR catalytic results showed that the half-wave potential of Co-Catalyst-1 was 0.87 V vs.RHE,which was superior to Co-Catalyst-2（0.86 V vs.RHE）and Co-Catalyst-3（0.84 V vs.RHE）.