Construction Of A Novel Three-dimensional Homogeneous ORR Catalytic System In Fuel Cells And Characterization Of Nano-scale Localized Impedance

As a type of energy conversion device,fuel cell directly converts chemical energy into electrical energy and has received extensive attention due to the non-carbon emission and high theoretical efficiency.However,the cathode sluggish kinetics of oxygen reduction reaction(ORR)and the demand for precious metal Pt catalysts hinder the further development of fuel cells.In addition,the uneven distribution of the traditional heterogeneous catalyst layer leads to deficient utilization of active sites and poor mass transfer efficiency of rectants.This thesis aims to construct a novel ORR homogeneous reaction system with low cost,high catalyst utilization and high mass transfer efficiency by covalently grafting the non-noble metal molecular catalyst sites to the side chains of ionomer.Firstly,the contributing factors for the intrinsic activity of transition metal complex catalysts were discussed:(i)In order to study the effect of the assembly mode and metal interaction,a series of mono-and binuclear metal phthalocyanine complexes(FePc,FePc-PcFe,FePc-PcCo,CoPc-PcCo and CoPc)were prepared and electrochemical characterization including cyclic voltammetry(CV)and rotating disk electrode(RDE)were carried out.The binuclear metal phthalocyanines(FePc-PcFe,CoPc-PcCo)have shown higher activity than their mononuclear analogues(FePc,CoPc);Fe metal active sites have displayed a better catalytic performance than Co sites;Interestingly,for the binuclear phthalocyanine FePc-PcCo with heterogeneous metals,its ORR half-wave potential is more closed to FePc-PcFe than CoPc-PcCo.X-ray absorption spectroscopy reveals that FePc-PcCo and FePc-PcFe have similar planar quadrilateral structures,while a non-planar structure is proved exist in CoPc-PcCo sample;Furthermore,the increasing of the ORR activity among those five catalysts is well in agreement with the descending LUMO energies in DFT calculations and the lower LUMO energy indicates a favorable adsorption of O2,which in turn affects the ORR performance.(ii)In order to further study the regulating role of the ligands on the electronic structure and the reactivity of metal complexes,a series of iron-based complexes with different ligands(denoted as FeL,L=TAA,Pc,TPP,Corrole,Tim and Salen)were synthesized as ORR catalysts.The electrocatalytic activity follows the order of FeTAA>FePc>FeTPP>FeCorrole>FeTim>FeSalen.An electron-transfer number close to 4 was derived for all these complexes except for FeTim and FeSalen,implying a near complete reduction of oxygen to water.X-ray absorption near edge structure spectroscopy(XANES)and Mossbauer spectroscopy were used to probe the nature of the distinct activities by investigating the iron-centre electron structures.Density function theory(DFT)calculations were carried out to study the charge redistribution across the iron complexes.Novel activity descriptors including the charge and spin densities on the Fe site were proposed and validated by available experimental data,presenting a strategy to design highly active nonprecious metal complex catalysts with specific supporting ligands.Based on the intrinsic activity and stability of metal complexes in the above studies,metalloporphyrin was selected as the active sites in the constructed homogeneous catalytic system.For the cathode catalyst layer in acidic media,5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin iron(TMPPFe)was covalently grafted into the side chain of Nafion to obtain Nafion-TMPPFe,which showed a positive half-wave potential shift compared with the traditional heterogeneous catalytic layer structure;for the ORR in the alkaline media,the molecular catalyst 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin cobalt(TMPPCo)was anchored to the side chain of the anion-conductive ionomer(polyfluorene,PF),thereby achieving a homogeneous catalysis environment inside ion-flow channels,with greatly improved mass transfer and turnover frequency as a result of 100%utilization of the catalyst molecules.The unique structure of the homogeneous catalysis system comprising interconnected nanoreactors exhibits advantages of low overpotential and high fuel-cell power density.This strategy of reshaping of the catalyst layer structure may serve as a new platform for applications of many molecular catalysts in fuel cells.In order to characterize the ionomer dispersity over the active site surface in the as-prepared homogeneous catalytic system,especially the phase-separation structure of the ionomer,an in-situ localized nano-scale impedance characterization(AFM-EIS)was carried out by coupling atomic force microscope(AFM)with the impedance tester in the shielding box by a customized procedure.By this method,the localized proton-transport resistance at different humidities was observed in spatially diverse locations,and the value decreased with the increasing humidity.In addition,the microstructure of Nafion was reconstructed by numerical simulation to examine the relationship between the microstructure of the ionomer and the localized proton transport resistance distribution.The results showed that the spatial diversity of proton-transport resistance arose from the variable concentration of hydrophilic groups at the contact location of the AFM tip and the ionomer,and from the heterogeneity of dry sulfonic acid groups in the ionomer that creates local variation in water content.The realization of this analytical technique is helpful for the microstructure characterization of the studied homogeneous catalytic system,and is expected to provide some insight into a proper design strategy for the catalyst layer structure in the future.