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Title: Structural basis for differing electrocatalytic water oxidation by the cubic, layered and spinel forms of lithium cobalt oxides
The two polymorphs of lithium cobalt oxide, LiCoO 2 , present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H 2 O → O 2 + 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 conditions to show that labile Li + ions partially deintercalate from layered LiCoO 2 , initiating structural reorganization to the cubic spinel LiCo 2 O 4 , in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO 2 (50 nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark for membrane-based systems) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec −1 ), overpotential ( η = ∼420 mV@10 mA cm −2 at pH = 14), faradaic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO 2 and spinel LiCo 2 O 4 is attributed more » to the presence of [Co 4 O 4 ] n+ cubane structural units, which provide lower oxidation potential to Co 4+ and lower inter-cubane hole mobility. By contrast, the layered phase, which lacks cubane units, exhibits extensive intra-planar hole delocalization which entropically hinders the four electron/hole concerted OER reaction. An essential distinguishing trait of a truly relevant catalyst is efficient continuous operation in a real electrolyzer stack. Initial trials of cubic LiCoO 2 in a solid electrolyte alkaline membrane electrolyzer indicate continuous operation for 1000 hours (without failure) at current densities up to 400 mA cm −2 and overpotential lower than proven PGM (platinum group metal) catalysts. « less
Authors:
; ; ; ; ; ;
Award ID(s):
1433492
Publication Date:
NSF-PAR ID:
10075619
Journal Name:
Energy & Environmental Science
Volume:
9
Issue:
1
Page Range or eLocation-ID:
184 to 192
ISSN:
1754-5692
Sponsoring Org:
National Science Foundation
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