(614g) In-Situ XAS Study of Self-Switching Electrocatalyst for Oer and ORR
AIChE Annual Meeting
2013
2013 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Electrocatalysis and Photoelectrocatalysis: Fundamentals and Applications
Thursday, November 7, 2013 - 10:30am to 10:50am
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 Lix-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.