Journal
ENERGY & ENVIRONMENTAL SCIENCE
Volume 9, Issue 1, Pages 184-192Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c5ee02195b
Keywords
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Funding
- National Science Foundation, Division of Chemical, Bioengineering, Environmental, and Transport Systems (CBET)
- U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office [CBET-1433492]
- DOE Small Business Innovation Research (SBIR) [DE-SC0013179]
- Directorate For Engineering
- Div Of Chem, Bioeng, Env, & Transp Sys [1433492] Funding Source: National Science Foundation
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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; 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 LiCoO2, initiating structural reorganization to the cubic spinel LiCo2O4, in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO2 (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 (eta = similar to 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 LiCoO2 and spinel LiCo2O4 is attributed to the presence of [Co4O4](n+) cubane structural units, which provide lower oxidation potential to Co4+ and lower intercubane 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 LiCoO2 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.
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