Year

2015

Degree Name

Doctor of Philosophy

Department

Faculty of Science, Medicine and Health

Abstract

Fuel cells (FCs) are believed to be promising energy conversion systems to satisfy today’s increasing energy demand because of their high energy output and zero environmental impact. The cathodic oxygen reduction reaction (ORR) is recognised to be kinetically limited step in fuel cell. As commonly used electrocatalyst, platinum (Pt) has been inhibited from large-scale production because of its high cost, susceptive to fuel poisons and low stability. Therefore, numerous efforts have been devoted in developing novel nanostructured materials with high efficiency, low cost and environmental friendliness for the ORR.

The main goal for this study is to design and develop novel nanostructured electrocatalysts for the ORR to reduce or replace the utilization of Pt, meanwhile with acceptable efficiency, better stability as well as high selectivity to resist the poison from the fuels. Such catalytic materials will would largely reduce the cost of catalysts in FCs and improve the cell performance by facilitating the ORR process and eliminating the by-products. In this study, two analogue FC systems, proton exchange membrane fuel cell (PEMFC) and anion exchange membrane fuel cell (AEMFC) were considered and different types of electrocatalysts were synthesized and examined in these systems respectively.

Pt alloy (Pt-Pd, Pt-Cu) electrocatalysts were firstly synthesized through a facile aqueous based galvanic replacement and their electrocatalytic performance was examined in acidic environment and in PEMFC.

One issue that cause the poor stability of Pt is the aggregation and dissolution of Pt nanoparticle on carbon black during the long term electrochemical process. In this study, using the in-situ localization method, Pt nanoparticles were grown on a palladium shells through a modified galvanic replacement forming a Pt-Pd alloyed nanostructure. This material showed enhanced ORR performance and better stability under the accelerated durability test (ADT) up to 10,000 cycles. The electrochemical performance of this materials were analysed through the rotating ring disk electrode (RRDE) and single H2/O2 proton exchange membrane fuel cell test. The improved electrocatalytic performance and better stability were attributed to the unique structure of Pt/Pd and the strain effects caused by the incorporation of Pd into the lattice of Pt.

A hollow platinum-copper (PtCu) nanoparticle with mesoporous features was prepared through a modified galvanic replacement method using copper nanoparticles as sacrificial templates. The bimetallic PtCu nanoparticle with much lower cost compared with the Pt, showed improved electrocatalytic performance towards the ORR and with extreme stability under the ADT up to 10,000 cycles. The performance of this material was investigated using various electrochemical testing methods including the rotating ring disk electrode (RRDE) and single H2/O2 proton exchange membrane fuel cell test. The improved performance of this material was attributed to the geometric and electronic changes of Pt surface due to the alloying of copper, abundant mass transfer channels and the high specific surface area of the mesoporous PtCu.

In the alkaline medium; oxygen could be reduced through a faster reaction kinetic thus offering more choices in selecting electrocatalysts. Moreover, with the development of anion exchange membrane (AEM) in recent decades, the AEMFC has been witnessed as a promising fuel cell system that could provide comparable electrocatalytic performance with the PEMFC. Therefore there is an urgent need for developing novel ORR electrocatalysts in alkaline medium and for AMEFC.

Following the development of this sacrificial template method, a palladium (Pd)- nickel (Ni) hollow nanoparticle using Pd and Ni to replace the utilization of Pt was synthesized and the electrocatalytic performance of this material was examined under the alkaline medium and in AMEFC. The PdNi alloyed hollow nanoparticles showed porous features on the shells and also exhibited much improved electrocatalytic performance and stability compared with the commercial Pt/C. The electrochemical tests were performed through the rotating disk electrode (RDE) system and AEMFC test. The improved performance of this material was ascribed to the changes of electronic structure and the “strain effect” of Pd when alloying Pd with Ni. In addition, porous structure of this material would also provide large surface area and active sites thus benefiting the reduction of oxygen and mass transfer through the nanoparticles.

Further work includes the synthesis of nitrogen doped graphene nanostructures for the oxygen reduction reaction in the alkaline medium. For this work, the thin polypyrrole (PPy) was deposited onto the graphene aerogel through vapor phase polymerisation (VPP) and the deposited PPy was used as a nitrogen source to provide nitrogen into graphene lattice when thermal treatment. The advantage of this method is that, the deposited PPy could effectively prevent graphene sheets from stacking during the drying and thermal annealing process and the robust crumpled graphene sheets could also be produced during this process. This 3-dimensional (3D) microporous structure could provide the electrocatalysts with ample active site and abundant ion and mass transfer channels thereby facilitating the ORR process. In this process, the nitrogen content and configuration could be well managed through varying the heating temperature thus providing further ways in regulating the ORR performance of the electrocatalysts. The electrochemical properties were tested using the rotating disk electrode (RDE) system and anion exchange membrane fuel cell test revealing the improved catalytic performance of this material compared with the traditional prepared nitrogen doped flat graphene materials. The improved electrocatalytic performance was suggested to arisen from the optimised n itrogen configuration and the robust 3D porous structure on the electrodes.

In addition, core shell cobalt and cobalt oxide nanostructure supported on nitrogen doped graphene aerogel was also prepared at last and used an comparable electrocatalysts with Pt for the AMEFC. The cobalt oxides coupled on graphene supports have been widely reported with improved catalytic ORR performance, however the relatively low electrical conductiv ities were thought to inhibit the electron transfer around the nanoparticle s. Inspiring the advantage of the core shell nanostructure, a core shell structured consis ting of cobalt core and cobalt oxide shell was prepared and anchored onto the graphene sheets through a feasible one-pot hydrothermal process. This material was characterised with sc anning electron microsc opy, transition electron microscopy and energy X-ray dispersive ma pping analysis to confirm the successful synthesis of core shell structure. The elec trochemical performan ce was tested through the rotating disk electrode (RDE) showing improved performance in comparison with the pure cobalt oxide or nitrogen doped graphe ne aerogel electrocatal ysts. The improved electrocatalytic performance wa s attributed to the well-defin ed core shell structure and the designed 3D structure of the supporting materials.

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