Studies of manganese-based electrocatalysts for oxygen reactions

Abstract

Bifunctional catalysts capable of catalyzing both oxygen reduction (ORR) and oxygen evolution (OER) reactions are extremely valuable for oxygen-based energy conversion devices such as regenerative fuel cells and metal-air batteries. However, the underlying property of such catalysts that gives rise to their bifunctionality is not yet known nor explored. With the first use of near infrared photoluminescence spectroscopy for tracking the changes in the individual metal cation valence states during electrocatalysis in combination with in-situ gravimetric and resistance measurements, we show the underlying correlation between catalytic activity, potential-dependent resistance and nature of reaction intermediates on various bifunctional and non-bifunctional surfaces. Our results show that bifunctional Mn2O3 reversibly switches electrical polarity from p-type to n-type along with the formation of high-valent cationic Mn4+ active sites as well as low-valent cationic Mn2+ active sites during OER and ORR, respectively, which is absent in non-bifunctional NiO and Co3O4. Results also show other key process steps such as lattice hydration/dehydration other occur during polarization. These results are rationalized in terms of a band structure framework that correlates electrochemical activity with the formation energy of various metal cation intermediates. In the second study, we correlate electronic phase transitions, formation of reaction intermediates, and gravimetric results to the mechanism by which oxygen reactions occur on different crystal structures of MnO2. OER that occurs via adsorbate evolution mechanism (AEM) involves the formation of higher oxidation states that stabilize higher order reaction intermediates, whereas OER that occurs via lattice oxygen mechanism (LOM) involves the abstraction of oxygen from the lattice. Despite having the same composition, ????-MnO2 and ????-MnO2 have different activities for OER, which we report is due to their differing electronic structures with respect to the oxygen redox couple. We show that the formation of higher oxidation states (Mn4+, Mn5+), Mn-OH reaction intermediates, and electronic phase transition in the form of n-type to p-type switching occurs in . ????-MnO2 confirming AEM-OER. Moreover, we report consumption of lower oxidation states (Mn2+) and inability to switch type of electronic conductivity in ????-MnO2 and attribute these results to LOM-OER. In the third study, we use interlayer constraining in order to determine the key active sites of ORR in ????-MnO2. Since ????-MnO2 has a layered structure, the intercalation of cations from either LiOH or KOH electrolyte affect the ORR activity, as indicated by Tafel polarization measurements. Furthermore, the electrocatalytic activity was correlated with changes in potential-dependent resistance as well as changes in key Mn oxidation states. Most importantly, the electrode showed increases in resistance during ORR during K+ intercalation, whereas the resistance did not change appreciably during Li+ intercalation. In addition, we show a higher formation of Mn2+ species in ????-MnO2 in LiOH than in KOH. This suggests that Mn2+ is a key ORR active site, which has also been shown in the literature. While the electrode in both electrolytes show formation of Mn2+, the retention of these active sites aided by interlayer constraining as a result of Li+ intercalation may explain the increased ORR activity in LiOH. We propose an interlayer constraining model that has yet to be corroborated with further experiments, along with an electronic band framework in order to explain the differences in activity. The goal of this thesis is create structure-defect-property correlations between manganese-based electrocatalysts to ultimately be used in energy conversion and energy storage applications.