Pt-free Electrocatalytsfor The Oxygen Reduction Reaction

Polymer electrolyte membrane fuel cells (PEMFCs) are ideal candidates forstationary and mobile power generation due to their high energy conversionefficiency.Currently, Pt-based catalysts are the most efficient catalysts for the catalysisof the oxygen reduction reaction (ORR) in fuel-cell cathodes; however, they cannotmeet the demand for the widespread commercialization of fuel cells because of thescarcity of platinum. The development of high-performance and low-cost catalyticmaterials for the ORR has been the key issue for the large-scale application of fuel cells.In recent years, the catalytic activity of non-platinum catalysts have been greatlyimproved, but they are still less comparable to that of the Pt-based catalysts, especiallyat high current conditions in PEMFCs. The scientific challenges in this field are list asfollowing, the exploration of the activity center of non-Pt catalysts, the development ofadvanced technologies for increasing the density of active sites, and the developmentofefficient non-platinum catalytic electrode.In this thesis, we had curried out the following researcheson basis of theabovementioned object.(1) Palladium has been the focus of recent research on alternative Pt catalysts forthe oxygen reduction reaction (ORR). We show that the activity and stability of Pdtoward the ORR can be enhanced by Pd–O-oxides covalent bonding when Pd issupported on exfoliated montmorillonite (ex-MMT) nanoplatelets. It wasexperimentally observed that a thin PdOxlayer with a Pd–O–MMT junction is formed atthe particle/support interface. First-principle calculations further revealed that thejunction is formed between the Pd atoms and the O atoms in the AlO6octahedra of theex-MMT. Because of the influence of the ex-MMT on the electronic structure of Pd, theelectronic structure of Pd on ex-MMT is similar to that of Pt and the bonding strength ofOads is diminished. This diminished bond strength allows for the ORR on thePd/ex-MMT to proceed through an efficient pathway, as is the case with Pt. This studyopens a new pathway to modify of the electronic structure of Pd for enhanced ORRactivity and stability.(2) A novel strategy to selectively generate planar pyridine and pyrrolicnitrogen-doped graphene (NG) using layer-structured montmorillonite (MMT) as aquasi-closed flat nano-reactor was developed. We demonstrate that the selectivity for the formation of pyridine and pyrrolic N (planar N) is inversely proportional to theinterspace width (δ) of the MMT. The highest selectivity for planar N formation (up to90.27%) occurred in the case where the NG was formed inside H-MMT with a δ of0.46nm, followed by the cases where it was formed inside Na-MMT with a δ of0.53nm,between MMT particles with a δ of~1μm, and without MMT. As a catalyst, NGexhibited a low electrical resistance, high electrocatalytic activity, and good stabilitytoward the ORR in an acidic electrolyte. The half-wave potential of the ORR on anelectrode made of the most active NG prepared in the present work was less than that ofstate-of-the-art carbon-supported platinum by only60mV in an acidic electrolyte. Inaddition, the NG showed almost no deterioration after an accelerated stress test. Thisnew method avoids vacuum-based, elaborate fabrication methods and thus provides asimple but efficient and versatile approach to the low-cost mass production of NG forindustrial purposes that range from catalysis and sensors to super capacitors andlithium-ion batteries.(3) We report a “shape fixing via salt recrystallization” method to efficientlysynthesize N-doped carbon material with a large number of active sites exposed to thethree-phase zones, for used as a catalyst for the ORR. Self-assembled polyaniline with3D network structure was fixed and fully sealed inside the NaCl crystal fromrecrystallization of NaCl solution. During pyrolyzing, the NaCl crystal provides a fullyclosed nanoreactor which facilitates the N incorporation and graphitization. Thegasification in such a closed nanoreactor causes a large number of pores in the resultantsamples. The3D network structure, which is conducive for mass transport and highutilization of the active sites, was well preserved in the final N-doped carbon materialswith the dissolution of NaCl. The PEMFC with the invented cathode catalyst outputs apeak power of600mWcm-2, which is among the best non-precious metal catalysts forthe ORR having been reported so far.