Defence Faiz Sultan: "Synthesis and characterization of mixed metal oxides as electrocatalysts for the oxygen evolution reaction"

Promotors: 1^st promotor: Prof. Paolo P. Pescarmona, co-promotor: Dulce M. Morales

Abstract: Large-scale hydrogen (H₂) production via water electrolysis is hindered by the half-cell oxygen evolution reaction (OER) occurring at the anode, which involves a four-electron transfer process. This requires strategic synthesis of cost-effective and high-performance electrocatalysts through scalable methods. In this regard, CoFe₂O₄ and NiFe₂O₄ nanoparticles (NPs) were synthesized using a bicontinuous microemulsion method for the first time as electrocatalysts for the OER. The electrocatalysts were characterized using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption method. The OER measurements in a three-electrode system showed that CoFe₂O₄ has a higher catalytic activity than NiFe₂O₄, with an overpotential of 410 mV and a Tafel slope of 80 mV dec⁻¹ to reach a current density of 10 mA cm⁻². The superior performance of CoFe₂O₄ was attributed to its higher electrochemical surface area and higher electrical conductivity.
In addition to this, bimetallic NiMn oxides were synthesized by a simple co-precipitation method followed by calcination at two different temperatures, 350 °C and 550 °C, and labelled as NMO-1 and NMO-2, resulting in NiMnO₃/NiMn₂O₄ and NiMnO₃, respectively. Their electrochemical performance was evaluated using two different protocols: one based on non-optimized methods reported in the literature and the other following closely best practices, which involve the selection of the electrodes, electrode preparation, and conditioning step. The two methods showed that NMO-1 outperforms NMO-2 in terms of current density and overpotential. The study highlights the importance of following best practices in the electrochemical evaluation of the electrocatalysts.
Furthermore, NiCo2O4/NiO heterostructures with different Ni to Co ratios (NiCoOx and NiCo2Ox) were synthesized using a hydrothermal method. The catalysts were thoroughly characterized, and their OER activity was evaluated using a rotating disk electrode (RDE) setup in 1 M and 7 M KOH electrolyte solutions. The electrochemical performance evaluation was also carried out in purified 1 M and 7 M KOH electrolytes to investigate the impact of the presence of metal impurities in the electrolytes. NiCo2Ox demonstrated superior electrocatalytic activity in terms of overpotential compared to NiCoOx in all scenarios (all electrolyte compositions), likely due to the differences in phase and elemental composition between the two samples. The performance of Ni plate-supported NiCo2Ox was further investigated in an H-type electrolytic cell in industry-like conditions (40 ºC in 7 M KOH), demonstrating excellent stability at a constant current density of 200 mA cm−2 and outperforming Ni plate for over 24 h. These results highlight the importance of electrocatalyst investigation under industry-like conditions.

Dissertation