Usage of Metal Organic Frameworks as water-splitting catalysts

Introduction

Electrochemical water-splitting processes have arisen as an alternative for fighting energy source shortages through H₂ generation. However, this process requires the usage of catalysts in order to reduce the energy input. Metal organic frameworks (MOFs) are novel materials which may act as water-splitting catalysts due to their stability, crystal structure and high conductivity.

Methods

MOF NH₂-MIL-101(Fe) was synthetized through a solvothermal method (20 h, 110°C) using FeCl₃∙6H₂O, 2-aminoterephthalic acid and dimethylformamide. For the electrochemical water-splitting process assessment, different dosages of MOF-Fe were placed on the working electrode by different means (dropwise, dip-coating, ultrasonic dispersion), with Ni foam for alkaline and neutral pHs or carbon paper for acid pH (1 cm²), using graphite sticks and HgCl₂ electrodes as counter and reference electrodes, respectively. This three-electrode cell was connected to a PGSTAT302N potentiostat (Methrom).

Results

MOF-Fe was tested for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at neutral, acid, and alkaline pHs. MOF-Fe showed a high performance in the alkaline OER. Moreover, 0.25 mg/cm² was found to be the optimal MOF-Fe dosage. Thus, 10, 50 and 100 mA/cm² were obtained by applying low overpotentials of 300, 347 and 372 mV. This catalyst also exhibited high stability for 90 h at such current densities, defeating the benchmark catalyst (IrO₂) in both activity and stability. Characterization analysis showed that MOF-Fe's performance could be explained by its high crystallinity, elevated functional group presence, and great surface area.

Conclusions

The noble metal-free MOF-Fe was a suitable alternative for applying electrochemical water-splitting processes in a more environmentally friendly and economic approach, opening a path for the future application of these processes.

Acknowledgments

This work was supported by Project H2-ZeroWaste (AXA Research Fund). Aida Díez is grateful to Ramon y Cajal (RYC2023-044934-I)'s financial support (MICIU).