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, LiCoO₂, present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H₂O → 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 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 (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 LiCoO₂ and spinel LiCo₂O₄ is attributed to the presence of [Co₄O₄]ⁿ⁺ cubane structural units, which provide lower oxidation potential to Co⁴⁺ 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 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.