10.1021/jacs.6b00332.s001
Mikaela Görlin
Mikaela
Görlin
Petko Chernev
Petko
Chernev
Jorge Ferreira de Araújo
Jorge
Ferreira de Araújo
Tobias Reier
Tobias
Reier
Sören Dresp
Sören
Dresp
Benjamin Paul
Benjamin
Paul
Ralph Krähnert
Ralph
Krähnert
Holger Dau
Holger
Dau
Peter Strasser
Peter
Strasser
Oxygen
Evolution Reaction Dynamics, Faradaic Charge
Efficiency, and the Active Metal Redox States of Ni–Fe Oxide
Water Splitting Electrocatalysts
American Chemical Society
2016
metal reduction step
faradaic efficiency
Ni centers increase
metal redox state
Active Metal Redox States
O 2 release
anode catalysts
XAS data
structure motifs
fuels reactors
metal oxidation process
OER catalysis
water splitting
OER conditions
O 2
DEMS
NiOOH catalyst
oxidation states
faradaic charge efficiencies
Fe K
OER activity
reaction rate
oxygen evolution reaction dynamics
electrochemical mass spectrometry
Faradaic Charge Efficiency
Ni atoms
Oxygen Evolution Reaction Dynamics
surface catalysis
Fe centers
oxidation state
reaction product molecules
2016-03-31 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/Oxygen_Evolution_Reaction_Dynamics_Faradaic_Charge_Efficiency_and_the_Active_Metal_Redox_States_of_Ni_Fe_Oxide_Water_Splitting_Electrocatalysts/3201397
Mixed Ni–Fe oxides are attractive
anode catalysts for efficient
water splitting in solar fuels reactors. Because of conflicting past
reports, the catalytically active metal redox state of the catalyst
has remained under debate. Here, we report an in operando quantitative
deconvolution of the charge injected into the nanostructured Ni–Fe
oxyhydroxide OER catalysts or into reaction product molecules. To
achieve this, we explore the oxygen evolution reaction dynamics and
the individual faradaic charge efficiencies using operando differential
electrochemical mass spectrometry (DEMS). We further use X-ray absorption
spectroscopy (XAS) under OER conditions at the Ni and Fe <i>K</i>-edges of the electrocatalysts to evaluate oxidation states and local
atomic structure motifs. DEMS and XAS data consistently reveal that
up to 75% of the Ni centers increase their oxidation state from +2
to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst
under OER catalysis. The Fe centers consistently remain in the +3
state, regardless of potential and composition. For mixed Ni<sub>100–<i>x</i></sub>Fe<sub><i>x</i></sub> catalysts, where <i>x</i> exceeds 9 atomic %, the faradaic efficiency of O<sub>2</sub> sharply increases from ∼30% to 90%, suggesting that Ni atoms
largely remain in the oxidation state +2 under catalytic conditions.
To reconcile the apparent low level of oxidized Ni in mixed Ni–Fe
catalysts, we hypothesize that a kinetic competition between the (i)
metal oxidation process and the (ii) metal reduction step during O<sub>2</sub> release may account for an insignificant accumulation of
detectable high-valent metal states if the reaction rate of process
(ii) outweighs that of (i). We conclude that a discussion of the superior
catalytic OER activity of Ni–FeOOH electrocatalysts in terms
of surface catalysis and redox-inactive metal sites likely represents
an oversimplification that fails to capture essential aspects of the
synergisms at highly active Ni–Fe sites.