However, despite the multifunctionality of Ir-based catalysts, the inconsistent active sites and irreversibility of Ir for HER, HOR, and OER have limitations in their application to real device systems with varying operating conditions. Thus, Ir-based materials possess good OER, HER and HOR catalytic activity and can be used as anodes and cathodes of water electrolyzers and as anodes of PEM fuel cells. For HER and HOR, it has been reported that metallic-Ir surfaces such as Ir (111) show significant catalytic activities 20, 21. Based on the X-ray photoemission spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), amorphous IrO x (Ir III) exhibits a higher oxidation state than rutile IrO 2 (Ir IV) due to abundant electrophilic oxygen species (O I‒) which cause nucleophilic attack of water, leading to enhanced OER catalytic activity 17, 18, 19. In particular, an amorphous IrO x surface is considered the best active site for OER 15, 16. In this regard, multifunctional catalysts can be a promising strategy in environments where the electrochemical reaction changes rapidly, such as the voltage reversal of water electrolysis and PEM fuel cell systems.Īmong the various catalysts, Ir-based materials are excellent candidates to fit this strategy owing to their remarkable OER activity as well as good HER and HOR catalytic activity 11, 12, 13, 14. A recent study proposed reducing the damage to the electrode by selectively promoting HOR catalysts by suppressing the ORR 10 and introducing a water oxidation catalyst to the anode of the PEM fuel cell to induce an OER, as it is a reaction that competes with the carbon corrosion reaction 9. In polymer electrolyte membrane (PEM) fuel cell operation, fuel starvation occurring at the anode side leads to a voltage reversal phenomenon, causing corrosion of carbon components like catalyst support, gas diffusion layer (GDL) and flow field plate on the anode 8, 9. When shut-down occurs in water electrolysis, the potential of the hydrogen electrode is increased, leading to degradation at the hydrogen electrode 6, 7. Recent studies have reported that unexpected operating conditions, such as a sudden shut-down and fuel starvation, induce voltage reversal to corrode hydrogen electrodes, degrading the durability of the systems. Fuel cells produce electricity through two-electrode reactions: hydrogen oxidation (HOR) and oxygen reduction (ORR) 5. Water electrolysis consists of a two-electrode reaction: a hydrogen evolution reaction (HERs) and an oxygen evolution reaction (OERs) 3, 4. In view of a clean and renewable society, there has been great interest in water electrolysis and fuel cells for producing and using hydrogen, which is considered a promising energy carrier 1, 2. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design. Our analysis reveals that a metallic Ir subsurface under thin IrO x layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrO x layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrO x at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation.
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