The electrocatalytic conversion of CO₂ into value-added chemicals using renewable electricity is a promising approach to reduce atmospheric CO₂ concentration and realize carbon-energy balance. However, the low current density still limits CO₂ electroreduction reaction (CO₂RR) for commercial application.

Crystalline porous metal-organic frameworks (MOFs) are one class of promising alternatives for CO₂RR due to their high CO₂ adsorption uptakes and periodically arranged isolated metal active sites. However, the poor conductivity and slow electron-transfer capability of the traditional MOFs usually result in low current density in CO₂RR.  

In a study published in  Angew. Chem. Int. Ed., Prof. CAO Rong and Prof HUANG Yuan-Biao from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences (CAS) constructed one type of conductive two-dimensional phthalocyanine-based MOF nanosheets (NiPc-NiO₄).  

The researchers heated nickel phthalocyanin-2,3,9,10,16,17,23,24-octaol (NiPc-OH) and Ni(OAc)_2′4H₂O in water at 85 °C for 24 h, and fabricated ultrathin 2D NiPc-NiO₄ nanosheets by exfoliating from its bulk via high-frequency sonication at room temperature.

As expected, the researchers found that the electrical conductivity of NiPc-NiO₄ has a high value of 4.8 × 10^-5 S m^-1 measured at room temperature using a two-contact probe method. Such good electrical conductivity would be beneficial for the electron transfer to the active sites during CO₂RR, thereby improving energy conversion efficiency and electrochemical activity. 

The as-prepared 2D NiPc-NiO₄ nanosheets showed outstanding activity towards CO₂ electroreduction with nearly 100% CO selectivity, large CO partial current density up to 34.5 mA cm^-2, high turnover frequency up to 2603 h^-1, and excellent long-term durability. Such high turnover frequency (TOF) and CO partial current density is higher than any state-of-the-art MOF catalysts. 

The density functional theory calculation proved that the nickel of phthalocyanine center is the active sites and NiPc-NiO₄ performs better activity than the phthalocyanine molecules due to the fast electron transfer capacity and excellent reducibility, which is consistent with the experiment results. 

This study provides an effective way to improve the CO₂ electroreduction performance by designing conductive crystalline frameworks with uniformly distributed phthalocyanine active sites. It also builds a bridge between the homogeneous molecule catalysts and heterogeneous porous catalysts. 

Nickel phthalocyanine molecules as active sites were installed into 2D conductive metal-organic framework nanosheets for efficiently CO₂ electroreduction reaction with nearly 100% CO selectivity（Image by Prof. CAO’s group） 

Contact: 

Prof. CAO Rong 

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

Email: rcao@fjirsm.ac.cn