posted on 2015-06-05, 00:00authored byClyde
W. Cady, Graeme Gardner, Zachary O. Maron, Maria Retuerto, Yong Bok Go, Shreeda Segan, Martha Greenblatt, G. Charles Dismukes
Transition metal oxides containing cubic B4O4 subcores are noted for their catalytic activity in water oxidation
(OER). We synthesized a series of ternary spinel oxides, AB2O4, derived from LiMn2O4 by either
replacement at the tetrahedral A site or Co substitution at the octahedral
B site and measured their electrocatalytic OER activity. Atomic emission
and powder X-ray diffraction confirmed spinel structure type and purity.
Weak activation of the OER occurs upon A-site substitution: Zn2+ > Mg2+ > A-vacancy > Li+ =
0. Zn and
Mg substitution is accompanied by (1) B-site conversion of Mn(IV)
to Mn(III), resulting in expansion and higher symmetry of the [Mn4O4]4+ core relative to LiMn2O4 (inducing greater flexibility of the core and lower
reorganization barrier to distortions), and (2) the electrochemical
oxidation potential for Mn(III)/IV) increases by 0.15–0.2 V,
producing a stronger driving force for water oxidation. Progressive
replacement of Mn(III/IV) by Co(III) at the B site (LiMn2–xCoxO4, 0 ≤ x ≤ 1.5) both symmetrizes the [Mn4–xCoxO4] core
and increases the oxidation potential for Co(III/IV), resulting in
the highest OER activity within the spinel structure type. These observations
point to two predictors of OER catalysis: (1) Among AMn2O4 spinels, those starting with Mn(III) in the resting
lattice (prior to oxidation) result in longer, weaker Mn–O
bonds for this eg1 antibonding electronic configuration,
yielding greater core flexibility and a higher oxidation potential
to Mn(IV), and (2) a linear free energy relationship exists between
the electrocatalytic rate and the binding affinity of the substrate
oxygen (*OH and *OOH) to the B site.