Oxygen-ion transport and electrocatalysis in high entropy oxides

Global average temperatures have been increasing since the onset of the Industrial Revolution, with the most pronounced rise occurring between 2023 and 2024, exceeding 1.5°C above pre-industrial levels. This temperature increase is primarily attributed to the elevated atmospheric concentrations of greenhouse gases, such as CO₂ and CH₄, largely resulting from fossil fuel combustion. To mitigate greenhouse gas emissions, significant research efforts have been devoted to sustainable energy solutions. One widely investigated strategy involves the utilization of renewable energy sources, such as solar and wind, to meet energy demands through efficient energy-to-fuel and fuel-to-energy conversion processes. The development of these technologies necessitates high-performance solar cells, electrolyzers, fuel cells, and batteries, all of which rely on advanced electrolyte and electrode materials. A fundamental challenge in materials science is the design and optimization of materials with superior functional properties to enhance the efficiency, stability, and performance of these energy conversion and storage systems.
High entropy oxides are a new class of promising materials known for their single-phase stability, compositional complexity, and exceptional functional properties. ... mehrThe main objective of the thesis is to explore the potential of high entropy oxides as oxygen ion conductors and electrocatalysts for oxygen evolution reaction. The initial step in assessing these functional properties involves synthesizing the high entropy oxides in various forms, such as powders, pellets, and films. The high entropy oxides are synthesized using mechanochemical synthesis, sol-gel processing, reverse co-precipitation, pulsed laser deposition, and conventional sintering techniques. The fluorite-structured high entropy oxide (Ce,La,Pr,Sm,Y)O2-δ is considered for comparison. Powders produced using mechanochemical synthesis, sol-gel processing, and reverse co-precipitation consistently exhibit the desired fluorite structure. Likewise, thin films synthesized using the sol-gel process and pulsed laser deposition also maintain the fluorite structure, with pulsed laser deposition allowing the production of films with various morphologies, such as polycrystalline, columnar, and epitaxial. In contrast, pellets synthesized through conventional sintering exhibit a mixture of fluorite and bixbyite structure due to the high sintering temperatures. These techniques are applied to various high entropy oxides to evaluate their potential as oxygen ion conductors and electrocatalysts for the oxygen evolution reaction.
Fluorite- and perovskite-type high entropy oxides are explored as oxygen ion conductors. The oxygen ion conduction is studied for the pellets of these high entropy oxides. However, the pellets of fluorite structured high entropy oxide - (Ce,La,Pr,Sm,Y)O2-δ transition to phase mixture of fluorite and bixbyite structure due to high sintering temperatures. In order to prevent the formation of the phase mixture during the sintering, Zr was added at different atomic fractions. Indeed, the fluorite structure is stabilized approximately at 10 at.% of Zr content. Additionally, (Ce,La,Pr,Sm,Y)1-xZrxO2-δ exhibit a homogenous distribution of elements with Pr exhibiting multivalency. As an additional benefit of adding Zr, the ionic conductivity increases with the addition of Zr and reaches a maximum at 8 at.% of Zr and decreases with further addition of Zr. Interestingly, the electronic conductivity in an oxidizing atmosphere expected from a multivalent Pr is suppressed, while high ionic conduction is observed in the fluorite-structured high entropy oxides. The thesis also investigates perovskite-type high entropy oxides, specifically high entropy rare-earth aluminates. Traditional perovskite-structured rare-earth aluminates are known oxygen ion conductors; however, they exhibit p-type electronic conduction in oxidizing environments due to oxygen exchange between the atmosphere and lattice oxygen at high temperatures. High entropy materials, with their elevated configurational entropies, offer stability at high temperatures, suggesting they could reduce unwanted electronic conduction. To explore this phenomenon, high entropy aluminates - (Gd0.2La0.2Nd0.2Pr0.2Sm0.2)1-xCaxAlO3 are considered. These high entropy aluminates exhibit an orthorhombic perovskite structure with a homogenous distribution of elements, with minor secondary phases appearing at x = 0.15 and x = 0.2. Conductivity increases with Ca content which peaks at x = 0.1 and stays constant with further addition of Ca. At the same time, the p-type conduction typically observed in doped rare-earth aluminates is effectively mitigated. High entropy oxides offer promising potential to constrict the electronic conduction observed in traditional oxygen ion conductors. Furthermore, the conductivities achieved in this research are comparable to the state-of-the-art oxygen ion conductor, yttria-stabilized zirconia.
High entropy materials are promising for developing high-activity electrocatalysts. Their inherent tunability and multiple active sites create opportunities for designing earth-abundant catalytic materials that support energy-efficient electrochemical energy storage. In this thesis, the high entropy perovskite-type oxides are shown to significantly enhance catalytic activity for the oxygen evolution reaction (OER), the kinetically limiting half-reaction in electrochemical energy conversion, including green hydrogen production. Specifically, the catalytic performance of the (001) facet of La(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3-δ was compared to its parent compounds with a single B-site in the ABO3 perovskite structure. While these single B-site perovskites generally follow expected volcano-type activity trends, the high entropy oxide outperforms all parent compounds, achieving 17 to 680 times higher currents at a fixed overpotential. Since all samples were grown as epitaxial layers, these results confirm an intrinsic composition-function relationship independent of geometry or surface variability. X-ray photoemission studies reveal a synergistic effect from the simultaneous oxidation and reduction of transition metal cations during intermediate adsorption, highlighting high entropy oxides as a highly attractive class of earth-abundant materials for OER catalysis with activity potential beyond the scaling limits of traditional mono- or bimetallic oxides.