Catalyst electrode development from one-dimensional platinum silver-based alloy nanostructures for proton exchange membrane fuel cells

Fidiani, Elok (2021). Catalyst electrode development from one-dimensional platinum silver-based alloy nanostructures for proton exchange membrane fuel cells. University of Birmingham. Ph.D.

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Abstract

The primary research activity on proton exchange membrane fuel cells (PEMFCs) is still directed to reduce the system cost by using a low Pt catalyst loading while maintaining high power performance and durability. The development of advanced catalyst designs usually focuses on shape-controlled nanostructures and alloying to form bimetallic or multimetallic compositions. They have been demonstrated as effective strategies to boost the catalyst activities toward the sluggish oxygen reduction reaction (ORR) at the cathodes in PEMFCs. However, most of the advanced catalyst nanostructures with the superior catalytic activities are limited to half-cell electrochemical measurement in liquid electrolytes, and there is a big challenge to fully transferring the performance to electrodes in operating fuel cells.

Such a challenge motivates this PhD research to develop high power performance and durable catalyst electrodes from one-dimensional (1D) Pt-based alloy nanostructures for PEMFC application, advancing our previous achievements on 1D Pt nanostructure array gas diffusion electrodes (GDEs). The alloying of Pt with Ag is investigated to construct a cost-effective AgPt nanorod catalyst, based on the close lattice constant between Ag and Pt to minimise the atomic segregation and sustain catalyst stability. The influence mechanisms of the reaction process are systematically explored on the growth, crystal structure and morphology of 1D AgPt nanostructures considering their catalytic performance recorded in the membrane electrode assembly (MEA) test in single PEMFCs.

Firstly, a scalable preparation method is established for growing AgPt NRs on carbon support utilising formic acid reduction approach. Ultrafine single-crystal AgPt NRs with an average diameter of 3-4 nm and length of ~15 nm are obtained by controlling the ion reduction process to induce the nucleation and growth of Pt and Ag along the <111> direction. The optimal Ag1Pt1 NR/C catalyst shows 1.22 and 1.51-fold higher power density and mass activity, respectively than commercial Pt/C nanoparticles (NPs) as cathode catalysts in PEMFCs. After the accelerated degradation test (ADT), severe Ag redeposition is observed at the interface between the cathode and polymer electrolyte membrane.
The introduction of Au is therefore proceeded to improve the stability of AgPt NR/C forming Au-AgPt NR/C. The integration of 5 at% Au effects the metal ion reduction procedure, leading to the formation of longer NRs of up to ~20 nm and a high ratio of Pt depositing on the surface. Consequently, these improvements bring about 7% increase in the electrochemical surface area (ECSA) of the catalyst, and 1.15- and 1.70-fold higher ORR activity compared to AgPt NR/C and Pt/C, respectively. The stabilisation effect of the Au alloying is evaluated in PEMFCs, and even better stability is demonstrated than Pt NR/C.

The understanding and experience obtained from controlling the ion reduction process to grow AgPt NRs are transferred to fabricate GDE by growing AgPt NR arrays directly on GDLs. A different alloying mechanism between Ag and Pt is found for the NR growth on the GDL from that on the carbon support, ascribed to the inert GDL surface and moderate reaction temperature of 40 °C. The alloying turns less harmonised when the Ag content more than 10 at% as this leads to the formation of a Ag metal phase. Au is then introduced to construct Au-AgPt NR GDE, and its alloying effect is probed. It is found that the integration of Au plays a game-changing role in the NR structure, leading to self-rearrangement atomic deposition and improving Pt placement on the NR surface. The Au-AgPt NR GDE shows a higher peak power density of 0.61 W cm-2 than those of the Pt NR and Pt/C GDEs with ~20 wt% more of Pt loading and a less performance decline after the ADT.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):