Recent Development of Low Iridium Electrocatalysts toward Efficient Water Oxidation

Developing high-performance and low-cost electrocatalysts for oxygen evolution reaction (OER) is the key to implementing polymer electrolyte membrane water electrolyzer (PEMWE) for hydrogen production. To date, iridium (Ir) is the state-of-the-art OER catalyst, but still suffers from the insufficient activity and scarce earth abundance, which results in high cost both in stack and electricity. Design low-Ir catalysts with enhanced activity and stability that can match the requirements of high current and long-term operation in PEMWE is thus highly desired, which necessitate a deep understanding of acidic OER mechanisms, unique insights of material design strategies, and reliable performance evaluation norm, especially for durability. With these demand in mind, we in this review firstly performed a systematic summary on the currently recognized acidic OER mechanism on both activity expression (i.e. the adsorbate evolution mechanism, the lattice oxygen mediated mechanism and the multi-active center mechanism) and inactivation (i.e. active species dissolution, evolution of crystal phase and morphology, as well as catalyst shedding and active site blocking), which can provide guidance for material structural engineering towards higher performance in PEMWE devices. Subsequently, we critically reviewed several types of low-Ir OER catalysts recently reported, i.e. multimetallic alloy oxide, supported, spatially structured and single site catalysts, focusing on how the performance has been regulated and the underlying structure-performance relationship. Lastly, the commonly used indicators for catalyst stability evaluation, wide accepted deactivation characterization techniques and the lifetime probing methods mimicking the practical operation condition of PEMWE are introduced, hoping to provide a basis for catalyst screening. In the end, few suggestions on exploring future low-Ir OER catalysts that can be applied in the PEMWE system are proposed.