Structural Basis for Differing Electrocatalytic Water Oxidation by the Cubic, Layered and Spinel Forms of Lithium Cobalt Oxides

Gardner, G, Al-Sharab J, Danilovic N, Go Y B, Ayers KE, Greenblatt M, Dismukes G C.  2015.  

Journal:

Energy Environ. Sci.

Pages:

-

Abstract:

The two polymorphs of lithium cobalt oxide, LiCoO2, present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs catalysis of water oxidation/oxygen evolution (OER; 2H2O[rightward arrow]O2 + 4H+ + 4e- ). Previously, we reported high OER electrocatalytic activity from nanocrystals of the cubic phase vs. poor activity from the layered phase - the archetypal lithium-ion battery cathode. Here we apply transmission electron microscopy, electron diffraction, voltammetry and elemental analysis under OER electrolysis condition to show that labile Li+ ions (de)intercalate from layered LiCoO2, initiating structural reorganization to the cubic spinel LiCo2O4, in parallel with formation of an active catalytic phase. Comparison of cubic LiCoO2 (50nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec-1), overpotential ([small eta] =  420 mV @ 10 mA cm-2 at pH = 14), Faradic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO2 and spinel LiCo2O4 is attributable to their [Co4O4]n+ cubane structural units, which provides lower oxidation potential to Co4+ and lower inter-cubane hole mobility. By contrast, the layered phase which lacks cubanes exhibits extensive intra-planar hole delocalization which entropically disfavors the four electron/hole concerted OER reaction.

Related External URL:

http://dx.doi.org/10.1039/C5EE02195B
Citation:
Gardner, G, Al-Sharab J, Danilovic N, Go Y B, Ayers KE, Greenblatt M, Dismukes G C.  2015.  Structural Basis for Differing Electrocatalytic Water Oxidation by the Cubic, Layered and Spinel Forms of Lithium Cobalt Oxides. Energy Environ. Sci.. :-.