DOI: 10.1002/cctc.201402756
Enhancing Activity for the Oxygen Evolution Reaction: The
Beneficial Interaction of Gold with Manganese and Cobalt
Oxides
Rasmus Frydendal,a Michael Busch,b Niels B. Halck,b Elisa A. Paoli,a Petr Krtil,c
Ib Chorkendorff,a and Jan Rossmeisl*b
Introduction
The sustainable production of hydrogen is a promising route
for intermittent energy sources such as wind and solar
power.1 The electrochemical splitting of water is facilitated in
electrolyzers, in which hydrogen is evolved at the cathode and
oxygen at the anode. The overall efficiency of such cells is severely
hindered by losses at the anode,2, 3 at which the complicated
oxygen evolution reaction (OER) introduces a large overpotential.
This process has been the focus of many studies,
both theoretical4–10 and experimental,11–17 but despite the
keen attention, the state-of-the-art OER catalysts still exhibit
large overpotentials. In OER, four electrons and protons are removed
from two water molecules. The proton and electron
most difficult to remove determine the overpotential. The four
reaction steps are shown in Equations (1)–(4).
*OH2 ! *OH þ Hþ þ e ð1Þ
*OH ! *¼O þ Hþ þ e ð2Þ
*¼O þ H2O ! *OOH þ Hþ þ e ð3Þ
*OOH ! * þ O2 þ Hþ þ e ð4Þ
in which * represents an active surface site. This reaction pathway
is valid for acidic solutions but changing to an alkaline environment
does not change the thermodynamic analysis presented
herein. The potential-determining step indicates the
potential needed to have all steps downhill in free energy.
From a thermodynamic point of view, the potential for removing
protons and electrons is given by differences in free
energy between reaction intermediates.18 Therefore, minimizing
the overpotential is firstly a matter of binding the reaction
intermediates with the right strength to the catalyst surface,
making the largest free energy difference for any oxidation
step as small as possible. Unfortunately, the binding energies
of the different intermediates cannot be varied independently
from each other; in general binding energies of similar intermediates
scale with each other.19–21 This phenomenon has
been established for reaction intermediates such as SHx, NHx
,
and OHx on a variety of surfaces including metals, sulfides, nitrides,
and oxides and is known as scaling relations. The two
OER intermediates *OH and *OOH therefore interact in
a similar way with any catalyst surface, which limits the activity
of even the best performing catalysts. For the OER (proceeding
via *OH and *OOH intermediates), a minimal overpotential
of 0.3–0.4 V is needed owing to a constant difference in free
energy of 3.2 eV for the 2 e/H+ oxidation from *OH to
*OOH.9, 10 The minimum overpotential can be found from the
energy difference of 3.2 eV by dividing with two elementary
charges and subtracting the equilibrium potential for oxygen
Electrochemical production of hydrogen, facilitated in electrolyzers,
holds great promise for energy storage and solar fuel
production. A bottleneck in the process is the catalysis of the
oxygen evolution reaction, involving the transfer of four electrons.
The challenge is that the binding energies of all reaction
intermediates cannot be optimized individually. However, experimental
investigations have shown that drastic improvements
can be realized for manganese and cobalt-based oxides
if gold is added to the surface or used as substrate. We propose
an explanation for these enhancements based on a hydrogen
acceptor concept. This concept comprises a stabilization
of an *OOH intermediate, which effectively lowers the potential
needed for breaking bonds to the surface. On this basis,
we investigate the interactions between the oxides and gold
by using DFT calculations. The results suggest that the oxygen
evolution reaction overpotential decreases by 100–300 mV for
manganese oxides and 100 mV for cobalt oxides.
a R. Frydendal, E. A. Paoli, Prof. I. Chorkendorff
Center for Individual Nanoparticle Functionality
Department of Physics
Technical University of Denmark
DK-2800 Kongens Lyngby (Denmark)
b Dr. M. Busch, N. B. Halck, Prof. J. Rossmeisl
Center for Atomic-Scale Materials Design
Department of Physics
Technical University of Denmark
DK-2800 Kgs. Lyngby (Denmark)
E-mail: jross@fysik.dtu.dk
c Dr. P. Krtil
Department of Electrocatalysis
J. Heyrovsky´ Institute of Physical Chemistry
Academy of Sciences of the Czech Republic
Dolejsˇkova 3, 18223 Prague (Czech Republic)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402756.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 0000, 00, 1–7 &1&
These are not the final page numbers!
CHEMCATCHEM
FULL PAPERS