Doping and High-Entropy: Strategies for Nanostructured Spinel Oxides for the Oxygen Evolution Reaction

The urgent imperative to mitigate anthropogenic climate change has catalyzed a global transition toward sustainable energy systems, establishing green hydrogen produced via water electrolysis as a critical energy vector. However, the widespread deployment of this technology is currently hindered by the sluggish kinetics of the anodic Oxygen Evolution Reaction (OER), which typically necessitates the use of scarce and costly noble metal electrocatalysts, such as Ruthenium and Iridium oxides. This doctoral thesis addresses these limitations by developing Platinum Group Metal (PGM)-free anode materials based on Earth-abundant spinel oxides, specifically nickel ferrite (NiFe2O4). While NiFe2O4 offers chemical stability and precursor abundance, its practical application is constrained by low intrinsic electrical conductivity and a suboptimal electronic configuration of the active sites. To overcome these challenges, this study employs a low-temperature sub-critical hydrothermal synthesis method, validated herein as a versatile, energy-efficient, and reproducible route to produce phase-pure nanostructured spinels with maximized specific surface area (SSA). The research initially investigates the electronic and morphological modulation of NiFe2O4 through doping with Chromium and Manganese. Detailed physicochemical characterization indicated distinct behaviors for the two dopants: while high levels of Mn substitution led to phase segregation of Ni(OH)2, Cr was successfully incorporated into the spinel lattice. This incorporation induced lattice strain in the resulting NiFeCrO4 sample, effectively inhibiting particle growth and yielding ultrafine nanoparticles with a mean diameter below 5 nm and a high SSA of 226 m2 g-1. Electrochemically, the Cr-doped spinel demonstrated superior OER performance, matching the commercial IrO2 benchmark. This enhancement is primarily attributed to improved kinetics resulting from the electronic tuning of the eg​ orbital occupancy of Fe cations, as well as a surface reconstruction mechanism facilitated by the leaching of unstable high-valent Cr species, which promotes the formation of highly active Fe- and Ni-oxyhydroxides. Expanding upon these results, the study further explored medium- and high-entropy spinel oxides (HESOs) to evaluate the potential of the "cocktail effect" in enhancing catalytic activity. Despite successfully synthesizing single-phase multi-metallic nanoparticles for most compositions, the inclusion of Copper was found to cause secondary phase segregation due to severe Jahn-Teller distortions. Crucially, electrochemical analysis revealed that maximizing compositional complexity did not yield the anticipated synergistic benefits; instead, increasing the number of constituent elements resulted in a dilution of the active Cr-Fe-Ni sites, thereby diminishing the intrinsic kinetics of the materials. Consequently, the "medium-entropy" trimetallic system, (CrFeNi)O4, emerged as the optimal electrocatalyst, outperforming both the more complex HESO compositions and the IrO2 reference. Normalization of the current by the electrochemically active surface area confirmed that the superior activity of (CrFeNi)O4 derives mainly from favorable intrinsic reaction kinetics—evidenced by the lowest Tafel slope—rather than morphological factors alone. Furthermore, chronopotentiometry stability tests demonstrated a dynamic improvement in catalytic activity over time, indicative of a beneficial surface reconstruction mechanism. Ultimately, this work establishes hydrothermally synthesized (CrFeNi)O4 as a highly efficient PGM-free anode for alkaline water electrolysis, demonstrating that precise electronic tailoring yields superior results compared to the sole maximization of compositional complexity.