Hydrogen is one of the clean energies for the human future. Electrochemical water splitting renders a promising process for hydrogen production. Water splitting in acidic media only requires a small overpotential for cathodic hydrogen evolution. The low anodic oxygen evolution activity and serious catalyst deactivation have limited the water splitting in acidic media. 

In a study published in ACS Energy Letters, a research group led by Prof. CAO Rong from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences successfully synthesized a series of Iridium nanocrystals as highly efficient and stable bifunctional electrocatalysts for water splitting in acidic media. 

The component of the active sites on the catalyst surface have been controlled via using the unique rigid macrocyclic compound, cucurbit[6]uril, as both support and stabilizer. 

Researchers found that the optimized CB[6]-Ir catalyst is among the highest activities reported in acidic media, which achieved a 10 mA/cm² overall water splitting current density at only 1.56 V. Moreover, the electrolytic cell exhibited a high stability of at least 20h of continuous operation at 5 mA/cm².  

Spectroscopic results gave the insight into the interaction between cucurbit[6]uril and Iridium as well as how the cucurbit[6]uril content variation control the surface component of the catalysts. 

Through density functional thoery (DFT) calculation, the co-author Prof. ZHUANG Wei from FJIRSM, who specializes in theoretical calculations on various catalytic systems, provided further proof to confirm that the electron transfer between cucurbit[6]uril and Iridium. This is the first report on the theoretical calculation on the cucurbit[n]uril-metal catalysts. 

This study not only provides a new strategy to prepare efficient water splitting electrocatalysts, but also proves that cucurbit[n]uril plays an important role in the improvement of electrocatalysts. 

Schematic illustration of the CB[6]-Ir composite, the interaction between CB[6] and Ir and the superior water splitting performance as bifunctional electrocatalyst (Image by Prof. CAO’s group)