Mesoporous transition metal oxides for oxygen electrocatalysis in energy conversion technologies

Sammanfattning: Electrocatalysis, the foundation of electrical to chemical energy transformation, enables the mitigation of the electrical energy losses during reactions and the control of selectivity of the process to certain chemical products. The slow rate and the multi-step complexity of oxygen-associated reactions, namely oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), motivate the use of platinum group metals (PGM) catalysts, which significantly increase the price of the technologies due to the cost and scarcity of PGM-based materials. This thesis aims to fundamentally understand the electrocatalytical aspects of oxygen-associated reactions and their relevance to sustainable technologies by development of cheap and abundant materials.In this work, hydrothermal treatment routes are developed for synthesis of mesoporous MOx (M = Cr, Fe, Co, Ni, Ce) and NiCo2O4 as water-processable oxygen electrocatalysts. Firstly, anionic surfactant templated mesoporous NiO shows the lowest voltage loss with the highest turnover frequency for OER in consequence of the most accessible active sites among of nanoporous nickel (II) oxide (Paper I). It is observed that nickel and cobalt oxides are efficient bifunctional oxygen electrocatalysts compared to other investigated metal oxides. This stems from the lower voltage loss and by the presence of surface adsorbed hydroxyl species. In situ quantification shows that hydrogen peroxide is either the terminal product or the intermediate for ORR on meso-Cr2O3 and on other electrocatalysts, respectively (Paper II). In Paper IV, mesoporous NiO and NiCo2O4 are synthesized by using a template-free hydrothermal route, and NiCo2O4 performs more efficient bifunctional oxygen catalysts compared to NiO. It is found that ORR on mesoporous NiO and NiCo2O4 follow (2+1)e- and 4e-ORR path, with hydroxyl radical and hydroxyl ion as terminal products, respectively.Integrating the ORR and OER in electrochemical cells enables the study and development of energy conversion technologies. The bifunctional oxygen activity of meso-NiO is demonstrated in a PGM-free oxygen pump fed with air and water, resulting in a low faradic efficiency due to limited triple reaction points (Paper II). The performance of the oxygen pump has been significantly improved by exchanging the catalyst to mesoporous NiCo2O4 and the anolyte to concentrated KOH. The same setup is used for synthesis of the hydroxyl radical using mesoporous NiO. The hydroxyl radical is identified using degradation of rhodamine B, and a degradation rate of 0.034 min−1 is obtained in Paper IV. Additionally, two effective 2e-ORR electrocatalysts of porous organic conducting polymer poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) (Paper III) and mesoporous chromium (Paper V) have been studied for electrochemical refinery H2O2 by electrocatalysis-controlled comproportionation reaction (2?2? + ?2 → 2?2?2). It is observed that the hydrogen peroxide as terminal product of oxygen reduction shows ~70% Faradic efficiency on these two materials. The optimization of operation conditions on PEDOT: PSS-based hydrogen peroxide electrolyzer allows the conversion efficiency of 80% below 1V cell voltage. The optimized meso-Cr2O3-based hydrogen peroxide electrolyzer enables the conversion efficiency up to 90% that can be assigned to the suppressed of deterioration of catalyst.To summarize, this thesis has developed mesoporous metal oxides use as PGM-free electrocatalysts for investigating oxygen-associated reactions in the alkaline condition. Furthermore, the work has explored the energy conversion applications using the functionality of the developed oxygen electrocatalysts.