Bifunctional Pt-IrO 2 Catalysts for the Oxygen Evolution and Oxygen Reduction Reactions: Alloy Nanoparticles versus Nanocomposite Catalysts

In the present study, different concepts for the development of bifunctional oxygen reduction reaction/oxygen evolution reaction (ORR/OER) electrocatalysts are explored and compared. Bifunctional ORR/OER catalysts are often suggested to improve the stability during startup and shutdown of fuel cells. Furthermore, they have been proposed for the so-called unitized regenerative fuel cells (URFCs) that would allow a closed loop system to use and produce hydrogen on demand. We compare the electrocatalytic performance of conventional PtxIry alloy nanoparticles (NPs) with Pt–IrO2 NP composites (nanocomposites), both immobilized onto a commercial carbon support. The Pt–IrO2 nanocomposites thereby consist of a mixture of Pt NPs and IrO2 NPs. By probing the electrocatalytic performance before and after exposing the electrocatalysts to accelerated degradation tests (ADTs), it is shown that the Pt–IrO2 nanocomposite concept offers advantages but also some disadvantages over the conventional alloy concept. In particular, it is shown that while the nanocomposites are initially less active for the ORR because of an interparticle effect, their performance is less affected by the ADTs. However, all the tested catalysts experience a decline of the Ir/Pt ratio upon ADT treatment, highlighting the limiting value of Ir as an OER catalyst for startup–shutdown protection in fuel cells as well as the challenging stability requirements for URFCs.