Preparation Of Electrocatalysts For Oxygen Reduction Reaction And Investigation Of Their Electrochemical Performance

As potential candidates for sustainable energy conversion systems, fuel cells carry high expectations, particularly for mobile applications. The cathode is a key factor in optimizing the power output of fuel cells, due to a high overpotential for the sluggish oxygen reduction reaction (ORR) on this electrode. Therefore, highly active and stable electrocatalysts for the ORR are of great interest to engineers and scientists. At present, Pt-based materials have been proven to be the most efficient electrocatalysts for ORR in either acid or alkaline media. However, as one of the scarcest noble metals on earth, the most challenging issue encountered in pursuing the commercialization of Pt-based energy devices is the high cost associated with the heavy use of Pt. In addition, Pt-based electrocatalysts suffer from other limiting factors, such as declining activity caused by methanol poisoning and instability under extreme electrochemical condition.In order to tackle the above issues, some studies have been dedicated in this research thesis to screening more efficient and robust ORR electrocatalytic materials that do not contain platinum (Pt-free) or contain only small amounts of this noble metal (low-Pt). The main research contents are as follows:1) Pt nanoparticles supported within TiO2 mesoporous thin films are fabricated by a facile electrochemical deposition method. It is revealed that growth of Pt nanoparticles is a 3D progressive nucleation process. Due to the strong interaction between TiO2 and Pt, the as-prepared electrocatalyst exhibits remarkable catalytic activity towards ORR. Further study demonstrates higher stability of Pt/TiO2 than commercial Pt/C.2) Functional multilayer films containing electrochemically reduced graphene oxide (ERGO) are fabricated by the alternating layer-by-layer (LBL) assembly of negatively charged graphene oxide (GO) and positively charged poly(diallyldimethylammonium chloride) (PDDA) in combination with an electrochemical reduction procedure. As a metal-free catalyst, the resulting [PDDA@ERGO]n multilayer film possesses a remarkable electrocatalytic activity toward the ORR with superior methanol tolerance in alkaline media. Further research indicates that the unusual catalytic activity of the prepared hybrid films arises from synergetic chemical coupling effects between PDDA and ERGO. Importantly, the [PDDA@ERGO]n multilayer film reported here is easy to build up with the advantages of fine control of the film thickness, being energy effective, fast and green without using dangerous and corrosive substances.3) A highly promising CoO@Co/N-C electrocatalyst is prepraed using an economical and scalable method. The as-prepared CoO@Co/N-C achieves an onset potential of 0.99 V (vs. RHE) and a limiting current density of 7.07 mA cm-2 (at 0.3 V versus RHE) at a rotation rate of 2500 rpm in an O2-saturated 0.1 M KOH solution, comparable to a commercial Pt/C catalyst. Nitrogen sorption measurements revealed that the CoO@Co/N-C catalyst has a high specific surface area and a mesoporous structure, which can increase the density of the catalytically active sites accessible to reactants. Analysis of the XPS, XRD, and TEM measurements demonstrates that the as-prepared CoO@Co/N-C catalyst contained C-N, Co-N-C, and CoO@Co moieties. The synergistic effect among them makes the CoO@Co/N-C catalyst highly active towards the ORR in alkaline solution.4) Just like the CoO@Co/N-C electrocatalyst, Fe3O4@Fe/N-C electrocatalyst is obtained by impregnating in situ generated iron(?)-1,10-phenanthroline complex on commercially high specific surface area carbon blacks and subsequent pyrolysis at high temperature. High ORR limiting current density (5.79 mA cm-2) and positive half-wave potential (0.859 V versus RHE) at a rotation rate of 1600 rpm over this electrocatalyst with a mass loading of 0.2 mg cm-2 can be achieved in 0.1 M KOH solution. Detailed investigations have also revealed the existence of N-C, Fe-N-C, and Fe@Fe3O4 core-shell nanoparticles in this composite electrocatalyst. The high activity of this electrocatalyst is attributed to the synergistic effect among them.5) Composites based on helical carbon nanotube (HCNT) and Mn3O4 are synthesized by a hydrothermal approach. Since carbon materials have long been regared as effective electrocatalysts for converting O2 to HO2- in alkaline media and Mn3O4 have shown certain activity for catalyzing HO2- to OH-, the prepared HCNT/Mn3O4 composites present comparable electrochemical activity towards ORR with a four-electron oxygen reduction pathway in alkaline solution as expected. Introduction of an air calcination step at 300? for 2 h is found to improve both the ORR activity and the stability of the composite.