Preparation Of Non-Precious Metal ORR/OER Electrocatalysts And Their Applications In Rechargeable Zinc-Air Batteries And Fuel Cells

With the increasing depletion of fossil fuels and ever deteriorating environmental problems,the development and utilization of sustainable clean energy systems is of significant importance.Towards this end,rechargeable zinc air batteries(RZABs)and proton exchange membrane fuel cells(PEMFCs)are particularly important,especially for future mobile energy storage device,owing to their high conversion efficiency and energy density as well as environmental friendliness.For both devices,the air cathode which involves oxygen evolution reaction(OER)and/or oxygen reduction reaction(ORR)plays an essentially important role in determing their overall efficiency.The sluggish kinetics for OER and/or ORR reactions,the slow mass transfer and the poor electrode stability represent the typical technological challenges which impede their large-scale commercialization.To tackle these issues,the design of the cathode catalyst with desirable activity and stability as well as low cost is of paramount importance.Specifically,the RZABs requires bifunctional OER/ORR electrocatalysts in alkaline conditions,while the PEMFCs requires highly active ORR electrocatalysts in acidic conditions.In this regard,iron-based compounds,owing to their merits such as natural abundance,low cost,adjustable morphology,flexible composition and tunable active site,have attracted the most attention.Particularly,the iron-nitron-carbon compounds have been explored the most efficient non-noble ORR electrocatalysts under acidic conditions,while their oxides and hydroxides represent the most active non-noble OER electrocatalysts.However,despite the enormous research efforts,the performance of the non-noble bifunctional OER/ORR electrocatalysts for RZABs as well as ORR electrocatalysts for PEMFCs are still far from being satisfying for commercial applications.In this thesis,three projects have been carried out on design of iron-group-element-based cathode catalysts for OER and/or ORR in RZABs and PEMFCs.By manipulating the chemical composition,the meso-scale structure(including the dimension,the porosity and the morphology),and the local electronic structure,a series of non-noble electrocatalysts with high activity and stability were synthesized;they were characterized by various techniques;theoretical calculations were also employed to investigate the structure-activity relationship.The main results are summarized as follows:(1)In the first project,a highly active S-doped Fe/N/C(S-Fe/N/C)3D network with macro-meso-micro hierarchical porosity was prepared using metal-organic framework(MOFs)nanoparticles as the carbon source.The chemical composition and structural features have been well optimized and characterized;and the structural evolution mechanism from the starting discrete MOFs nanoparticles to the eventual 3D hierarchically porous networks was also revealed.Owing to the structural and the compositional merits,the optimal S-Fe/N/C catalyst exhibits remarkable oxygen reduction reaction catalytic activity with a half-wave potential of 0.82 V(versus RHE)and a mass activity of 18.3 A g-1 at 0.8 V in 0.1 M H2SO4 solution;and the PEMFCs test also confirmed its excellent catalytic activity,which gives a maximal power density as high as 800 mW cm-2 at 1 bar.(2)In the second project,by manipulating the interaction between the iron salt and the polydopamine-functionalized MOFs,a porous hollow nanocomposite(denoted as H-Co@FeCo/N/C)was obtained where numerous metallic Co nanoparticles(around 8 nm in diameter)are embedded within a porous N/C nanoshell doped with atomically dispersed Fe and Co metal sites.The chemical interactions among the dopamine,the polydopamine thin layer,the organic ligand in the MOFs,the metal cations and the solvents were found to play a profound role in regulating the size of the metallic Co nanoparticles and the local composition as well as the eventual structure of the hollow FeCo/N/C layer.The copresence of the CoNx and FeNx dual active sites were characterized by various techniques including XAFS.Due to the presence of the relatively small Co nanoparticles and the CoNx-FeNx in the carbonaceous layer,the H-Co@FeCo/N/C demonstrates superior OER/ORR bifunctional electrocatalytic activity(ΔE=0.698 V)and a mass activity of 6.8 A gcat.-1 at 0.9 V,outperforming the commercial Pt/C+RuO2 catalyst.When assembled for RZABs,the H-Co@FeCo/N/C cathode displays a long cycle life of 200 h(Egap of about 1.0 V@10 mA cm-2).DFT calculations were also employed to understand the synergistic catalytic effect of the CoNx and FeNx dual active sites towards ORR in the alkaline condition.The dual active site FeCoN6 was identified as the optimal geometric configuration on which the energy barrier for ORR is significantly lower than those on the site of FeN4 or CoN4 site.(3)Last but not least,towards the pursuit of high-performance non-noble-metal-based OER electrocatalysts,mesoporous NiFe-LDH nanosheets with affluent cation vancancy were prepared via a facile sovlethermal treatment of the prestine NiFe-LDH.It is speculated that the usage of DMF induced the partial dissolution or leakage of the cations(e.g.,Ni2+/Fe3+)from the prestine NiFe-LDH,which were then deposited on the surface of NiFe-LDH in the form of NiFe2O4 in an alkaline condition.Due to the presence of extensive metal cation vacancy,highly oxidative Fe(3+δ)+species with longer Fe-Ni bond length were found in thus treated NiFe-LDH according to the XPS and XAFS analyses,which could favor the adsorption of the oxygen intermediates O*and OH*involved during the OER process.Compared with the prestine NiFe LDH,thus solventhermally treated sample exhibits evidently improved OER and ORR catalytic activity;particularly,the overpotential for OER is as low as 230 mV and 510 mV@10 mA cm-2 when evaluated in the alkaline and neutral electrolyte,respectively;it also displays remarkable performance as a bifunctional electrocatalyst for RZABs,which could operate steadily for 500 h and 230 h in the alkaline and neutral electrolyte,respectively.This work provides a novel yet straightforward method to tune the local electronic structure and catalytic activity of NiFe-LDH materials.