Mixed Metal Oxide Electrodes for Oxygen Evolution Reaction

Abstract

This thesis explores transition metal oxide catalysts for the oxygen evolution reaction with a specific focus on cobalt and copper oxides. CoOx is well recognized for it’s OER activity, while CuOx received less attention. This work explores the effects of doping additional elements, such as Cr (into CoOx) and Ce and La into CuOx-type electrodes. Catalyst design and optimization often rely on a trial-and-error approach. Gaining real-time insights into the structure-activity relationship and identifying reaction intermediates is crucial for developing improved catalysts. This thesis utilizes in situ Raman spectroscopy and optical imaging to investigate potential-dependent catalyst transformations and detect reaction intermediates, enabling more effective catalyst design. Co[Cr]Ox/Au catalysts have been explored in Chapter 2. The catalyst identities show extreme dependence on solution environment and bias. The catalyst development takes place in situ at open circuit potential (OCP) and was confirmed with optical imaging and Raman spectroscopy. The catalysts were prepared by submerging the substrate in the precursor solutions containing Co and Cr ions, followed by annealing. The presence of Cr significantly influences the oxidation state of Co in the deposited electrode material and its redox behavior, preferring Co2+ species (Co(OH)2). Our work seems to be the first one to show that even in pure CoOx electrodes, Co4+ formation occurs, at much lower potentials (~1.14 V) than the OER onset. While for the Cr-modified catalyst, it occurs at a relatively higher potential ~1.44 V, demonstrating that the presence of Cr resists the formation of Co4+ species. Co3+ is known to be the most active Co species for OER, presence of Cr in catalyst film improves OER by stabilizing this species. Besides this, in situ Raman spectroscopy and isotopic substitution experiments identified a metastable metal hydroperoxo species (Au-O-OH) species which serves as an indicator of OER onset. The origin of this species is suggested to be a spillover species from the catalytically active phase (MOx/Au), which migrates to the Au surface and gives rise to Au-O-OH. Incorporation of lanthanides like Ce and La into CuOx was explored in Chapters 3 and 4. The inherent coordination mismatch between Cu (square planar) and lanthanides (cubic co-ordination), introduced lattice strain, modifying the electronic structure of active sites, thereby enhancing OER activity by tuning the binding energies of adsorbed intermediates (*OH, *O, *OOH). Cu[M]Ox/Au were prepared by a single-step electrodeposition method. Incorporating Ce and La enhances catalytic activity, and the maximum activity was achieved at 40% Ce and 20% La in Cu[M]Ox/Au catalysts. Investigation of Cu[Ce]Ox/Au showed that the samples are typically heterogeneous. The regions responsible for enhanced OER activity could be identified using a combination of optical microscopy and Raman spectroscopy. The Cu[M]Ox catalysts remain structurally dynamic with respect to potential. A band at 590 cm-1 was observed under OER potential conditions for both CuOx and Cu[M]Ox catalysts. The band was assigned to a highly oxidized copper species (labeled as Cu3+ species). We characterized this band using Raman and isotope-labeled studies. The species exists within an extended -O-Cu-O-Cu-O- type framework and is strongly correlated with OER activity. A band at ~ 816 cm-1 was observed only at the site of O2 bubble formation and was characterized as a terminal -OOH* species, indicating the importance of such an intermediate within the OER mechanism. In the Cu[M]Ox systems, Ce remains redox active and was found to influence the oxidation state of Cu. Lanthanum, on the other hand, is not redox active. The Cu[Ce]Ox catalyst showed steady and sustained high activities, while the activity of Cu[La]Ox approached that of CuOx with time, indicating that a non-redox active dopant such as La tends to segregate out within a structurally dynamic OER process. In the last part of the work (Chapter 5), to assess the performance under conditions relevant to commercial water electrolyzers, the OER activity of optimized Cr-doped cobalt oxide (Chapter 2) and Ce and La-doped copper oxide catalysts (Chapters 3 and 4) was evaluated at elevated temperatures. Kinetic parameters, including apparent activation energy (Eₐ/) and Tafel slopes, were measured to elucidate temperature-dependent behavior. The enhanced OER performance of these doped catalysts is attributed to better reaction kinetics. We believe Co and Cu oxides present a model scenario to serve as the basis for catalyst improvement using doping. We have illustrated that doping has a significant influence on redox properties and catalytic activities (currents and apparent activation energies). Cu[M]Ox type system could also be extended to other redox active transition metals (TM[M]Ox systems), as a similar coordination mismatch between TM e.g., Co, Fe, Ni, Mn etc. (preferring octahedral coordination, CN=6) and lanthanides (preferring cubic coordination, CN= 8) also exists, akin to Cu-lanthanide system. This type of doping could be systematically explored to enhance activities of well-known oxides such as NiOx, CoOx, and FeOx.