ja3000084_si_001.pdf (390.08 kB)
Nucleation, Growth, and Repair of a Cobalt-Based Oxygen Evolving Catalyst
journal contribution
posted on 2012-04-11, 00:00 authored by Yogesh Surendranath, Daniel A. Lutterman, Yi Liu, Daniel G. NoceraThe mechanism of nucleation, steady-state growth, and
repair is
investigated for an oxygen evolving catalyst prepared by electrodeposition
from Co2+ solutions in weakly basic electrolytes (Co-OEC).
Potential step chronoamperometry and atomic force microscopy reveal
that nucleation of Co-OEC is progressive and reaches a saturation
surface coverage of ca. 70% on highly oriented pyrolytic graphite
substrates. Steady-state electrodeposition of Co-OEC exhibits a Tafel
slope approximately equal to 2.3 × RT/F. The electrochemical rate law exhibits a first order dependence
on Co2+ and inverse orders on proton (third order) and
proton acceptor, methylphosphonate (first order for 1.8 mM ≤
[MePi] ≤ 18 mM and second order dependence for 32
mM ≤ [MePi] ≤ 180 mM). These electrokinetic
studies, combined with recent XAS studies of catalyst structure, suggest
a mechanism for steady state growth at intermediate MePi concentration (1.8–18 mM) involving a rapid solution equilibrium
between aquo Co(II) and Co(III) hydroxo species accompanied with a
rapid surface equilibrium involving electrolyte dissociation and deprotonation
of surface bound water. These equilibria are followed by a chemical
rate-limiting step for incorporation of Co(III) into the growing cobaltate
clusters comprising Co-OEC. At higher concentrations of MePi ([MePi] ≥ 32 mM), MePO32– equilibrium binding to Co(II) in solution is suggested by the kinetic
data. Consistent with the disparate pH profiles for oxygen evolution
electrocatalysis and catalyst formation, NMR-based quantification
of catalyst dissolution as a function of pH demonstrates functional
stability and repair at pH values >6 whereas catalyst corrosion
prevails
at lower pH values. These kinetic insights provide a basis for developing
and operating functional water oxidation (photo)anodes under benign
pH conditions.