The design of first-row transition metal oxides as electrocatalysts for the oxygen evolution reaction

Abstract

Chapter 1.The threat of climate change continues to be amplified by the global reliance on traditional carbon-based fuels for electricity and power generation, whose combustion leads to greenhouse gas emissions. Transforming our current energy model to a green hydrogen economy will greatly reduce the environmental burden by moving towards a carbon-free cycle. However, an efficient method for generating renewable hydrogen on an industrial scale is necessary to realize this vision. The electrochemical splitting of water into hydrogen and oxygen is viewed as an ideal approach that utilizes an abundant chemical resource, water, to obtain high-purity hydrogen and is directly compatible with renewable electricity sources, such as wind and solar. Of the two half-reactions that comprise water splitting, the oxygen evolution reaction is the energy-intensive bottleneck as it involves the removal of four protons and four electrons. Iridium oxide is widely regarded as the state-of-the-art electrocatalyst to promote this reaction. However, the high cost and scarcity of iridium prevents its scalability for hydrogen production. In this chapter, we therefore turn our attention to the more earth-abundant first-row transition metal oxides. Several materials in this broad class have been shown to be efficient at promoting oxygen evolution. Herein, select examples of first-row transition metal oxide materials are discussed in the context of their application as electrocatalysts for the oxygen evolution reaction. Chapter 2.The large-scale viability of hydrogen as a renewable fuel has been limited by the high energetic cost of the oxygen evolution reaction (OER). However, efficient water oxidation is ubiquitous in nature, catalyzed by Mn(II/III/IV)-containing metal-oxide clusters in the Photosystem II plant protein complex. In pursuit of low cost, high-activity OER catalysis, we have synthesized nanopowders of CoxMn2−xO3 (at.% Co = 0, 0.2, 0.5, 1, 2, 5, and 10) having a bixbyite mineral structure, as confirmed by powder X-ray diffraction on the as-made powders and single crystal X-ray diffraction on a recrystallized sample. Neutron powder diffraction indicates that cobalt preferentially occupies the trigonal 8a site in bixbyite at lower cobalt loadings. The application of this material as an electrocatalyst for the OER will be discussed in Chapter 3. Chapter 3.In pursuit of low-cost, high-activity electrocatalysts for the oxygen evolution reaction (OER), we report the performance of a series of CoxMn2−xO3 nanopowders (at.% Co = 0, 0.2, 0.5, 1, 2, 5, 10) having the bixbyite mineral structure. The synthesis and characterization of these materials are described in Chapter 2. In Chapter 3, we show that increasing the cobalt content in the bixbyite lattice results in an increase in the current density for OER with concurrent decreases in the overpotential and Tafel slope. The 10 at.% cobalt-modified bixbyite (Co0.2Mn1.8O3) delivers the highest activity with an overpotential of 486 mV at 10 mA·cm−2 and a Tafel slope of 90 mV·dec−1. The beneficial role of cobalt incorporation into bixbyite for the OER activity is attributed to the stronger M−O bonding interactions of low-spin Co3+, resulting in greater oxidizing power compared to manganese in the bixbyite lattice. These effects are anticipated to be greater in the trigonal site, suggesting that the selective incorporation of cobalt at this site may be largely responsible for the improved electrocatalytic activity of these materials. Chapter 4.Obtaining hydrogen as a renewable fuel through water splitting is severely hindered by the energy-intensive oxygen evolution reaction (OER). Transition metal oxides based on low-cost and earth-abundant elements have been shown to provide high OER activity rivaling that of commercial IrO2, with nickel iron oxide/oxyhydroxide systems exhibiting some of the lowest reported overpotentials. Here, we report a NiFeOx material with a nanospike morphology synthesized via a hydrothermal method, which contains a uniform distribution of nickel and iron centers. This unique material displays high catalytic activity for OER, requiring an overpotential of only 284 mV at 10 mA·cm−2, which is lower than that of its amorphous counterpart and commercial IrO2 (326 and 308 mV, respectively) under identical conditions. This material also exhibits excellent long-term stability with no major loss in electrocatalytic current density and retention of the nanospike morphology after several hours under OER conditions. The high activity of this catalyst is attributed to its large number of active sites as well as an increased oxidation state of the nickel centers, which is favorable for the OER. Chapter 5.The fabrication of electrodes for electrocatalytic analysis requires the preparation of inks containing the active catalyst. The composition and properties of a catalyst ink can dramatically impact the measured activity for an electrochemical reaction, and thus catalyst ink fabrication is invariably tied to electrocatalyst performance. To obtain optimal ink behavior, it is critical that the agglomerates of catalyst nanoparticles plus conductive carbon support are sufficiently small to maximize contact with the conductive ionomer (i.e., Nafion). However, the synthesis of heterogeneous electrocatalyst nanoparticles is often performed at high temperatures, which can readily lead to undesirable sintering of the nanoparticles and the formation of larger particulates. This can lead to poor ink performance in electrocatalytic testing, preventing accurate characterization of the catalyst properties. Herein, we demonstrate that tip sonication can be used as a quick and easy post-synthesis method to reduce the particle size of catalyst materials. We apply this approach to a series of brownmillerite Sr2GaCoO5 samples and show that less than one minute of tip sonication provides catalyst particles that deliver optimal activity for the oxygen evolution reaction. Chapter 6.Brownmillerites have gained recent attention as electrocatalytic materials for the oxygen evolution reaction (OER). Also known as oxygen-deficient perovskites, these materials exhibit ordered oxygen vacancies that act as adsorption and reaction sites for hydroxide ions to achieve OER in alkaline media. A cobalt-based brownmillerite, Sr2GaCoO5, was recently reported to exhibit high OER activity under both neutral and alkaline electrolyte conditions. In this chapter, we report the electrocatalytic OER behavior of new manganese-doped brownmillerites, Sr2GaMnxCo1−xO5, in which the Mn3+ ions partially replace Co3+ in the lattice. The addition of manganese is found to be beneficial for the OER activity, although the measured current density does not scale linearly with the manganese content. Indeed, the sample with the mid-value manganese incorporation in this series, namely Sr2GaMn0.1Co0.9O5, yields the highest electrocatalytic activity. While the oxidative power of Mn is weaker compared to Co, manganese may be able to better maintain the charge buildup necessary for the OER turnover. We propose further experimental and computational studies to help establish the role of manganese in these electrocatalytic materials.