%0 Journal Article
%A Görlin, Mikaela
%A Chernev, Petko
%A Ferreira de Araújo, Jorge
%A Reier, Tobias
%A Dresp, Sören
%A Paul, Benjamin
%A Krähnert, Ralph
%A Dau, Holger
%A Strasser, Peter
%D 2016
%T Oxygen
Evolution Reaction Dynamics, Faradaic Charge
Efficiency, and the Active Metal Redox States of Ni–Fe Oxide
Water Splitting Electrocatalysts
%U 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
%R 10.1021/jacs.6b00332.s001
%2 https://ndownloader.figshare.com/files/5025430
%K metal reduction step
%K faradaic efficiency
%K Ni centers increase
%K metal redox state
%K Active Metal Redox States
%K O 2 release
%K anode catalysts
%K XAS data
%K structure motifs
%K fuels reactors
%K metal oxidation process
%K OER catalysis
%K water splitting
%K OER conditions
%K O 2
%K DEMS
%K NiOOH catalyst
%K oxidation states
%K faradaic charge efficiencies
%K Fe K
%K OER activity
%K reaction rate
%K oxygen evolution reaction dynamics
%K electrochemical mass spectrometry
%K Faradaic Charge Efficiency
%K Ni atoms
%K Oxygen Evolution Reaction Dynamics
%K surface catalysis
%K Fe centers
%K oxidation state
%K reaction product molecules
%X 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 K-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 Ni100–xFex catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 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 O2 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.
%I ACS Publications