High crystallinity design of Ir-based catalysts drives catalytic reversibility for water electrolysis and fuel cells

Abstract

<jats:title>Abstract</jats:title><jats:p>The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrO<jats:sub>x</jats:sub> at potentials &gt; 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrO<jats:sub>x</jats:sub> layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrO<jats:sub>x</jats:sub> layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.</jats:p>