(614g) In-Situ XAS Study of Self-Switching Electrocatalyst for Oer and ORR

Oxygen evolution and reduction through heterogeneous catalysis have attracted much attention because they play important roles in renewable and sustainable energy applications. Traditional battery technology is based on intercalation mechanism and no catalytic process is involved. Recent breaking-through in rechargeable lithium oxygen batteries introduces OER and ORR into battery system for the first time. After intensive screening active catalysts in the past few years, α-MnO₂ have been identified as the most active electrocatalyst for both reactions, while the fundamentals of how this catalyst is involved in OER/ORR during electrochemical charging/discharging haven’t yet been explored.

Our previous studies in photocatalytic oxygen evolution from water suggest that Mn³⁺ exhibits much higher activity in oxygen evolution than Mn⁴⁺ and it is known that Mn⁴⁺ is only active for oxygen reduction reaction from the literature. Our hypothesis is that α-MnO₂ in the composite oxygen cathode switching its oxidation state during charging and discharging due to lithium intercalation/de-intercalation. At initial discharging, Mn is +4 in α-MnO₂ and it acts as ORR electrocatalyst. At the end of discharging, the oxidation state of Mn is close to +3 because lithium intercalation into α-MnO₂ significantly reduces the oxidation state of Mn. When the battery is on charging mode, Mn³⁺ in Li_x-α-MnO₂ acts as oxygen evolution site to decomposition of lithium peroxide to lithium and oxygen. During deep charging to 4.5V, the lithium ions intercalated into α-MnO₂ structure are removed and Mn atoms are back to Mn⁴⁺ again (see the figure). Such switching mechanism greatly promotes the electrocatalytic activity of α-MnO₂ nanowires in both ORR and OER during cycling at the same system.

Because the catalyst in the electrochemical cell is in a complicated environment (gas, liquid and solid, three phases), in-situ techniques are appreciated. In this presentation, we will discuss our recent progress in using in-situ X-ray absorption techniques to monitor the oxidation state of Mn during the electrocatalytic processes at real time and to elucidate the electrocatlytic mechanism of ORR/OER on the surface of α-MnO₂. We have successfully fabricated a custom electrochemical cell, which allows us to monitor the electrocatalyst structure evolution in real time when subjected to high flux synchrotron X-ray source. This represents the first attempt to utilize in-situ XAS techniques to study the mechanism of OER and ORR in an electrochemical cell at real time. The XAS data clearly show the oxidation state of Mn shifted to 3+ during discharging and returned to 4+ when the cell was charged. This in-situ technique will help us reveal the detailed mechanism in electrocatalytic OER/ORR and advance our understanding of the fundamentals.