Nickel-Based Metal-Organic Framework Materials with Mixed Ferrocene-Based Ligands As Anodic Catalysts for Water Electrolysis and Urea Electrolysis
Authors: Hong Zhen Wei , Lee Chun-I , Pan Chun-Jern 
Published: August 2024

Water electrolysis is a highly promising technology for hydrogen production, generating green hydrogen with no greenhouse gas emissions. The water electrolysis reaction involves two half-reactions: hydrogen evolution and oxygen evolution. While oxygen evolution in water electrolysis involves a four-electron transfer requiring higher overpotential and thus leading to energy consumption. Urea electrolysis offers lower theoretical energy consumption than water electrolysis, making them a key technology in future electrolytic hydrogen production processes. Conventional anodic catalysts, such as Ir and Ru, exhibit good performance in oxygen evolution, but being precious metals, they face limitations due to high cost and limited abundance. Its is crucial to find an active, durable and cost-effective anode catalyst for realistic application.

Metal-organic framework (MOF) materials, due to their high porosity, large surface area, and unique structural features, have emerged as promising materials for water electrolysis. However, their relatively poor stability is a drawback that needs to be overcome. In herein, we design the mixed ligand strategy for synthesizing the MOF materials. Ferrocene-based ligands, ferrocenedicarboxylic acid (Fd), combing with terephthalic acid (H2BDC) as the mixed ligands for this study. The MOF catalysts, namely Ni-BDC and Ni-BDC-Fd, was synthesized using a simple one-step hydrothermal method with ferrocenedicarboxylic acid and terephthalic acid.

Raman spectra revealed the presence of Fe-O, originating from the iron in Fd, demonstrating a structural transformation during the hydrothermal process. XRD analysis also confirmed the existence of FeO. The SEM images showed nanosheets and nanoneedles morphologies for Ni-BDC, while a dense nanofiber structure for Ni-BDC-Fd demonstrating the effect of 2nd ligand on the MOF structure.

Electrochemical tests were conducted in 1 M KOH electrolyte and 1 M KOH + 0.5 M urea electrolyte, including oxygen evolution reaction (OER), urea oxidation reaction (UOR). The overall reaction comprising the same catalysts for the anode and the cathode side is performed to examine the real electrolysis performance.

Results showed that the addition of Fd enhanced OER and UOR activity in alkaline and urea electrolytes. Ni-BDC-Fd achieved 1.483 V vs. RHE and 1.386 V vs. RHE at 65 mA/cm-2 driving potential for OER and UOR, respectively, requiring a smaller potential than Ni-BDC to achieve the same current density. To verify that the performance improvement was not solely due to iron, NiFe-BDC was synthesized by adding iron nitrate, showing inferior UOR performance compared to Ni-BDC-Fd but superior OER performance to NiFe-BDC. This indicated that the performance improvement resulted from structural changes rather than just the presence of iron atoms. Ni-BDC-Fd exhibited the lowest Tafel slope in OER, indicating a faster reaction rate due to the increased active surface area during OER. After continuous catalysis for 80 hours in the alkaline water electrolysis cell and urea electrolysis cell, a significant improvement in the loss of potential was observed.

These results suggest that the addition of ferrocene dicarboxylic acid altered the structure of Ni-BDC, maintaining an appropriate interlayer distance. This not only enhanced its catalytic performance but also improved its catalytic stability, demonstrating a promising strategy for preparing metal-organic framework materials for electrocatalysis applications.

Figure 1