Advanced Pd-based nanomaterials for electro-catalytic oxygen reduction in fuel cells: A review

Devising cost-effective and high-performance nanocatalysts for the inherently slow oxygen reduction reaction (ORR) represents a critical hurdle in the commercial improvement of fuel cells for energy conversion. Recently, considerable attempts have concentrated on exploring Pd-based nanocatalysts with advanced stability to utilize as substitutes for Pt. In this review, we first describe ORR mechanisms and summarize research conducted with Pd electro-catalysts, including single and alloyed Pd nanostructures on different substrates. The application of Pd catalysts as cathode nanomaterials in proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs), and anion exchange membrane fuel cells (AEMFCs) is also reviewed. The insights into the connections between catalytic performance, structure, and preparation process are addressed. In particular, approaches for fabricating efficient Pd electro-catalysts, such as increasing the number of reactive centers and modifying nanoparticle-support interactions, are discussed. Challenges and prospects for upcoming investigations in developing desirable ORR nanocatalysts are highlighted.