Villalobos Porras, Javier Francisco: Strategies for stabilization and activation of Mn- and Co-based catalysts for the oxygen evolution reaction. , Berlin, Technische Universität, Diss., 2022
10.14279/depositonce-16407
Open Access Version
Abstract:
The complex kinetics of the oxygen evolution reaction (OER) makes this reaction one of the most considerable challenges for implementing water splitting as a solution to efficiently storage renewable energy, like sunlight. The sluggish kinetics of OER catalysis make high catalytic activity and stability under harsh conditions fundamental requirements for the overall efficiency of the process. While many studies are focused on the inherent activity of the electrocatalysts, a better understanding of the material transformations relating to degradation, stabilization, restructuring and activation is highly desirable, both from a scientific perspective and for applications. Na-containing Co- and Mn-based oxides were electrodeposited by a new electrodeposition method in alkaline media. Tartrate is included in the electrodeposition electrolyte to complex and stabilized the metallic (Co and Mn) ions, preventing their spontaneous precipitation. The CoOx films showed higher catalytic activity than the MnOx films, but both had a current drop over cycling. By an applying a 30-minute open circuit break, the CoOx current fully recovered, whereas the MnOx recovered partially, suggesting that some changes are irreversible. The Tafel slope of both films were 120 mV dec-1 or higher and increased over cycling, indicating a chemical rate-limiting step or poor conductivity of the films. The changes in the catalytic current were related to a partial oxidation at the near-surface region of MnOx (and CoOx), proved by X-ray absorption spectroscopy (XAS). By introducing 30 % of MnOx into CoOx, a new material, (Co0.7Mn0.3)Ox, was formed. The (Co0.7Mn0.3)Ox films proved to be more stable than the single oxides under similar conditions (no current drop was observed at catalytic potentials). The catalytic activity of (Co0.7Mn0.3)Ox was not significantly different than CoOx, in terms of overpotential (η ≈ 500 mV at 10 mA) and current density (j ≈ 12 mAcm-2 at 1.74 V vs. RHE). The increase of the stability (without affecting the catalytic activity significantly) was assigned to various factors, namely, prevention of MnO4- ions dissolution, a higher conductivity and an optimization of the metal oxygen binding energy. To understand how to use the structural changes beneficially, the electrochemical restructuring of Ery (a synthetized crystalline mineral: Co3(AsO4)2·8H2O) was studied in different electrolytes (with same pH and concentration). The restructuring to a Co-based amorphous oxide was deconvoluted in three processes: amorphization, anionic exchange and redox activity change, using X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and the electrochemical redox-charge (ERC), respectively. The rate of these processes and the structural order of the final material (proved by EXAFS) were directly affected by the electrolyte´s nature. The slowest amorphization was observed in carbonate electrolyte, where the ERC increased the most. Interestingly, the current was only significantly activated (100 % of the initial value) in carbonate electrolyte, demonstrating that the electrochemical restructuring and the current activation are two different processes, where activation has further requirements, namely, an adequate cluster size, a high Co oxidation state and high redox activity. Developing strategies to extend the lifetime and increase the efficiency of OER electrocatalysts is essential for their future bigger-scale application in energy storage. Stability, activation and reactivation are desirable features when coupling OER electrocatalysts to renewable sources for sustainable energy production.