Structural basis for differing electrocatalytic water oxidation by the cubic, layered and spinel forms of lithium cobalt oxides. Issue 1 (9th November 2015)

• Record Type: Journal Article
• Authors: Gardner, Graeme; Al-Sharab, Jafar; Danilovic, Nemanja; Go, Yong Bok; Ayers, Katherine; Greenblatt, Martha; Charles Dismukes, G.
• Is Part Of: Energy & environmental science. Volume 9:Issue 1(2016)
• Page Start: 184
• Page End: 192
• Publication Date: 2015-11-09
• Subjects: Energy conversion -- Periodicals; Fuel switching -- Periodicals; Environmental sciences -- Periodicals; Environmental chemistry -- Periodicals; 333.79
• DOI: 10.1039/c5ee02195b
• Languages: English
• ISSNs: 1754-5692

Abstract

Abstract : Comparison of the OER activity of cubic and layered LiCoO2 shows a convergence to the catalytically active LiCo2 O4 spinel phase, which demonstrates excellent performance in membrane-based alkaline electrolysis. 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; 2H2 O → 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 LiCo2 O4, 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 LiCo2 O4 is attributed to the presence of [Co4 O4 ] 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 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.