Enhancing Oxygen Reduction Activity of Pt‐based Electrocatalysts: From Theoretical Mechanisms to Practical Methods

Abstract

Pt‐based electrocatalysts are considered as one of the most promising choices to facilitate the oxygen reduction reaction (ORR), and the key factor enabling their success is to reduce the required amount of platinum. Herein, we focus on illuminating both the theoretical mechanisms which enable enhanced and sustained ORR activity and the practical methods to achieve them in catalysts. The various multi‐step pathways of ORR are firstly reviewed and the rate‐determining steps based on the reaction intermediates and their binding energies are analyzed. We then explain the critical aspects of Pt‐based electrocatalysts to tune oxygen reduction properties from the viewpoints of active sites exposure and altering the surface electronic structure, and further summarize representative research progress towards practically achieving these activity enhancements with a focus on platinum size reduction, shape control and core Pt elimination methods. We finally outline the remaining challenges and provide our perspectives with regard to further enhancing their activities.

Limiting Current Density of Oxygen Reduction under Ultrathin Electrolyte Layers: From the Micrometer Range to Monolayers

“…While oxygen uptake limitation is in principle well known in corrosion science, in other fields of electrochemistry it is usually not much discussed. For instance, in order to reach the targeted high current densities as they occur in membrane electrode assemblies (MEAs), different experimental approaches beyond high speed rotating disk, are utilized, such as floating electrodes and gas diffusion electrodes [13–22] . The idea in these approaches is that electrolyte just wets the catalyst layer so that the reactant gas does not have to diffuse through bulk electrolyte and thus diffusion limitation will be significantly reduced.…”

“…Direct experimental access to such high kinetic current densities as achieved in fuel cells would require thicknesses of the depletion layer in the range of 100 nm or below. This is not achievable for RDE, but shrinking the electrolyte layer thickness is the idea behind the so called floating electrode concept that allows reaching geometric current densities in the A/cm 2 range, [13–22] where the catalyst is deposited onto a thin porous substrate that is floating on the electrolyte surface. The reactant gas is provided from above.…”

Modulating Metal–Organic Frameworks as Advanced Oxygen Electrocatalysts

“…Currently, noble metal‐based materials have been considered as benchmarks for oxygen electrocatalysis, such as Ir and Ru‐based materials for OER, and Pt‐based materials for ORR. [ 25,26 ] However, considering the high cost, scarcity and poor durability of these precious metal‐based electrocatalysts, developing next generation oxygen electrocatalysts with low cost, high electrocatalytic activity and superior durability seems to be crucial, but challenging. Thanks to the rapid development of nanotechnology in the past years, many novel oxygen electrocatalysts with reduced utilization of precious metals (such as precious metal‐based alloys), or even precious metals free electrocatalysts (including transitional metal‐based materials, heteroatom‐doped carbons, etc.)…”

Cathode Design for Proton Exchange Membrane Fuel Cells in Automotive Applications

“…> 2.5 A/ cm 2 ), the cathode catalyst layer undergoes a high water saturation level (i.e., flooding conditions). This thus reduces oxygen mass transport to reaction sites of the catalyst finally resulting in poor fuel cell power performance [13]. Accordingly, various technologies, including 3D mesh flow field plates [14,15], ordered structure electrodes [16] and hydrophobic GDLs [17], have been investigated to overcome the water management issues.…”