Rational design of cathode catalysts for proton exchange membrane fuel cells

Abstract

Proton exchange membrane fuel cells (PEMFCs) have been regarded as a promising candidate for fuel cell electric vehicles (FCEVs). Yet, the wide application of such technology is greatly dependent on the cost, performance and durability of the oxygen reduction reaction (ORR) catalysts at cathode. Currently, Pt-based electrocatalyst is commonly used in PEMFCs. Despite being an effective catalyst for ORR, its high cost and inherent scarcity has made it one of the most expensive components in the fuel cell, accounting for over 40% of the total cost. Therefore, the search for cost effective, catalytically active and durable ORR catalysts is crucial for the advancement of PEMFCs. An attractive strategy is to alloy Pt with cheaper and more abundant transition metals (TM) to achieve comparable or higher ORR performance to Pt. However, stabilizing TM against acidic and oxidative environment under fuel cell operation is challenging. Dissolution of TM from the alloy will dramatically reduce the ORR performance and will also trigger membrane degradation in PEMFCs. In this study, a facile synthesis strategy was developed to construct dealloyed PtCo/C (denoted as d-PtCo/C). Dealloying of PtCo/C allowed partial removal of Co from the surface/near surface of the alloy nanoparticles, hence forming a 2-3 atomic layers-thick Pt skin, which could provide certain protection against Co leaching. I have also explored surface modification of the dealloyed-PtCo/C with Ir. Ir is quite stable to acidic medium and is resistant against surface segregation. Hence, addition of small amount of Ir to d-PtCo/C may further improve the durability of the catalyst. Apart from that, I have also explored an emerging class of alloy, high entropy alloys (HEAs), as a new type of ORR catalyst for PEMFCs. HEA catalyst that comprises of multiple principal elements may allow fine tuning of oxygen adsorption energy (E⁰_ads ) for outstanding ORR activity while minimizing precious Pt usage. High structural and chemical stability are also anticipated from its high entropy and lattice distortion effects, which may offer excellent durability under the harsh operating conditions in PEMFCs.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Minhua SHAO (Supervisor)