Bimetallic Nickel-based Metal-Organic Frameworks as Electrocatalysts for the Oxygen Evolution Reaction in Water Splitting

Abstract

The oxygen evolution reaction (OER) is the more difficult half-reaction in water splitting, making it a primary factor limiting the scalable green hydrogen economy. While catalysts made from the noble metals IrO2 and RuO2 exhibit excellent catalytic activity, they are too expensive and rare for scalable hydrogen applications. This thesis focuses on Ni-based metal-organic frameworks (MOFs) as potential electrocatalysts for OER that are less expensive and more abundant. This work investigated the effect of iron as a bulk component versus iron impurity in the electrolyte. The two MOF systems were studied: the bimetallic FeFFIVE-1-Ni (Ni-Fe-MOF) and Ni-MOF-74. Their electrocatalytic activity was assessed in alkaline conditions and compared in unpurified 1 M KOH and purified, 1 M Fe-free KOH.

The MOFs were characterized structurally and morphologically using X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. Then, they were fabricated as homogeneous electrocatalyst ink and drop-cast on fluorine-doped tin oxide (FTO) substrates. Their electrochemical performance toward OER was tested using cyclic voltammetry, steady-state Tafel analysis, chronopotentiometry, potential cycling, and electrochemical impedance spectroscopy. These methods were employed to assess catalytic activity, redox behavior, stability, and charge-transfer characteristics in the presence and absence of Fe impurities in the solution.

The findings reveal that OER catalytic activity at Ni-Fe-MOF, both in KOH and in Fe-free KOH, is the same and depends on the bulk iron rather than the presence of the surface iron in the electrolyte, with the same current density 24±3 mA cm-2 and Tafel slope of 48±3 mV/dec reaction kinetics. On the other hand, OER catalytic activity at Ni-MOF-74 displays strong dependence on the inclusion of Fe from the trace of the electrolyte that enhances the catalytic activity post-anodization, and this is only applicable at low Ni-MOF-74 loading; Tafel slope 75±2 mV/dec in 1M KOH and Higher Tafel slope113±3 mV/dec, with greater overpotential in Fe-free KOH. At higher Ni-MOF-74 loading, the effect of surface iron became less apparent.

Ni-Fe-MOF exhibits stability during long-term chronopotentiometry at 1 mA cm-2 and both in KOH and in Fe-free KOH with a small change in overpotential ±20mV during the 24h in KOH and 61 h in Fe-Free KOH.