Fundamental Investigation Of The Electrocatalytic Activity Of Layered Mixed Metal Oxides For Low Temperature Oxygen Electrocatalysis

Li–O2 (Li–air) batteries are among the most promising energy storage technologies due to their high theoretical specific capacity and energy density. Key challenges with this technology include high overpotential losses associated with catalyzing the electrochemical reactions (i.e., oxygen reduction and evolution reactions) at the cathode of the battery. One way to address this challenge is to incorporate an active electrocatalyst, such as first-order Ruddlesden-Popper series of layered oxides. We show that the composition of the A-site in first-order Ruddlesden-Popper series of layered oxides (A2BO4) has a significant effect in the electrochemical activity of Li-O2 cathodes. Among the oxides composed of lanthanides (La, Pr, Nd) that form stable structures, La2NiO4 exhibits the best electrochemical performance when incorporated in Li-O2 cathodes. Furthermore, we find that the electrochemical performance of La2NiO4 could be further improved by doping the La site with an alkaline earth metal, such as Ba. We show that Ba0.25La1.75NiO4 exhibits the best discharge capacity and lowest OER potential when compared to undoped La2NiO4, Sr0.25La1.75NiO4 and Ca0.25La1.75NiO4. Stability of these oxide electrocatalysts is demonstrated under electrochemical conditions. We anticipate that these findings will further enhance the driving force for utilizing first-order Ruddlesden-Popper series of layered oxides as efficient non-precious metal-based cathode electrocatalysts for high-energy storage systems. In the second portion of this study, we report through the example of La2NiO4+δ that layered nickelate oxide materials with rod-shaped nanostructure exhibit promising electrochemical performance as cathode electrocatalysts for Li–O2 batteries. We demonstrate the ability to control the nanostructure of La2NiO4+δ electrocatalyst at the nanoscale level using a reverse-microemulsion synthesis approach. We show that Li–O2 batteries with cathodes containing rod-shaped La2NiO4+δ electrocatalyst exhibit lower charging potentials and higher reversible capacities when compared to batteries with carbon-only cathodes. Our studies indicate that the enhancement in the battery performance induced by the rod-shaped La2NiO4+δ electrocatalyst can be attributed to the fact that La2NiO4+δ nanorods (i) facilitate the formation of nanosized Li2O2 particles during discharge, and (ii) promote the electrocatalytic activity toward the oxygen evolution reaction during charging. These findings open up avenues for the utilization of (i) reverse-microemulsion method for controlling the nanostructure of layered oxide materials, and (ii) nanorod-structured nickelate oxides as efficient cathode electrocatalysts for Li–O2 batteries. In the third part of this thesis, we explore the potential of the aforementioned electrocatalysts as promising, non-precious metal based electrocatalysts for ORR in alkaline media. We systematically study the effect of the transition metal site composition using well-defined nanostructures of these oxides terminated by (001) surface facets. Using rotating ring disk electrode voltammetry studies, we show that doping the Ni site with Mn (La2Ni0.875Mn0.125O4+δ) leads to the best ORR activity among all the oxide compositions considered. Detailed kinetic analyses demonstrate that nanostructured Mn-doped LNO also exhibits the highest selectivity toward the desired, direct 4e- pathway for ORR. Furthermore, stability tests via cyclic voltammetry scans, show that Mn-LNO is stable over the course of cycling with minimal change in activity induced by degradation of the carbon support