catalysts
Communication
Electrodeposition of Fe-Complexes on Oxide Surfaces for
Efficient OER Catalysis
Sahir M. Al-Zuraiji
1,2
,Tímea Benkó
2
, Krisztina Frey
2
, Zsolt Kerner
2
and József S. Pap
2,
*
Citation: Al-Zuraiji, S.M.; Benkó, T.;
Frey, K.; Kerner, Z.; Pap, J.S.
Electrodeposition of Fe-Complexes
on Oxide Surfaces for Efficient OER
Catalysis. Catalysts 2021, 11, 577.
https://doi.org/10.3390
/catal11050577
Academic Editor: Vincenzo Baglio
Received: 29 March 2021
Accepted: 28 April 2021
Published: 30 April 2021
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1
Doctoral School on Materials Sciences and Technologies, Óbuda University, H-1034 96/b Bécsi Street,
H-1034 Budapest, Hungary; sahir.aziz@energia.mta.hu
2
Centre for Energy Research, Surface Chemistry and Catalysis Department, 29-33 Konkoly-Thege Street,
H-1121 Budapest, Hungary; benko.timea@ek-cer.hu (T.B.); frey.krisztina@ek-cer.hu (K.F.);
kerner.zsolt@ek-cer.hu (Z.K.)
* Correspondence: pap.jozsef@ek-cer.hu; Tel.: +36-1-392-2222 (ext. 3284)
Abstract: Progress in non-covalent/self-assembled immobilization methods on (photo)electrode
materials for molecular catalysts could broaden the scope of attainable systems. While covalent
linkage (though considered more stable) necessitates functional groups introduced by means of
often cumbersome synthetic procedures, non-covalent assemblies require sufficient propensity of
the molecular unit for surface adsorption, thus set less rigorous pre-requisites. Herein, we report
efficient electrodeposition (ED) of two Fe(III) complexes prepared with closely related NN’N pincer
ligands yielding stable and active ad-layers for the electrocatalysis of the oxygen-evolving reaction
(OER). The ED method is based on the utilization of a chloride precursor complex [Fe
III
Cl
2
(NN’N)],
which is dissolved in an organic electrolyte undergoes chloride/aqua ligand exchange upon addition
of water. ED provides patchy distribution of a chloride-depleted catalyst layer on indium tin oxide
(ITO) and fluorine-doped tin oxide (FTO) surfaces, which can be applied for long periods as OER
electrocatalysts. Compared to drop-casting or layering of [Fe
III
Cl
2
(NN’N)] with Nafion (a commonly
used support for molecular electrocatalysts), the surface modification by ED is a material saving and
efficient method to immobilize catalysts.
Keywords: oxygen-evolving reaction; molecular electrocatalyst; immobilization; electrodeposition;
electrochemical impedance spectroscopy
1. Introduction
Electrochemical water splitting (electrolysis of water) is considered to be an important
technology, consented to suppress the usage of fossil fuels since it can help to overcome the
storage-reuse obstacles of renewable-based energy systems by producing the green energy
carrier H
2
[1,2]. The complete reaction in Equation (1) necessitates efficient water oxidation
catalysts (WOCs) as functional components of the overall system. A WOC is meant to
enhance the efficiency of the oxygen-evolving reaction (OER) thus, aiding solar-to-chemical
energy conversion as in the artificial photosynthesis concept [3]. However, the OER from
water in Equation (2) is kinetically sluggish, involving the transfer of four electrons and
four protons. As a consequence, it means a bottleneck for the overall splitting reaction,
where the electrons and protons generated by the anodic OER are used up for the respective
formation of H
2
in the cathodic reaction in Equation (3), requiring much lower kinetic
overpotential [4]:
2 H
2
O
(l )
→ O
2(g)
+ 2 H
2(g)
(1)
2 H
2
O
(l )
→ O
2(g)
+ 4e
−
+ 4 H
+
(aq)
; E
o
=+1.23 V vs. NHE (2)
4 H
+
(aq)
+ 4e
−
→ 2 H
2(g)
; E
o
= 0 V vs. NHE (3)
Catalysts 2021, 11, 577. https://doi.org/10.3390/catal11050577 https://www.mdpi.com/journal/catalysts