PHOTOELECTROCHEMICAL WATER SPLITTING: CASE

MOKSLINĖ KONFERENCIJA
SCIENTIFIC CONFERENCE
2014
PHOTOELECTROCHEMICAL WATER SPLITTING: CASE STUDIES ON p-CuxO
AND n-InxGa1-xN PHOTOELECTRODES
Jurga Juodkazytė, Benjaminas Šebeka, Irena Savickaja
Center for Physical Sciences and Technology, Department of Electrochemical Material Science
el. p.: [email protected]
Splitting of water into molecular
hydrogen and oxygen in the photoelectrochemical cell (PEC) using solar light and
semiconductor electrodes is very attractive
from the viewpoint of sustainable hydrogenbased energy economy.
Copper oxides Cu2O and CuO are
among the most investigated p-type metal
oxide solar absorbers due to their direct band
gaps of ~ 2 eV and ~ 1.3 eV, respectively, which
are close to the optimal energy for sunlight
absorption, as well as due to the position of the
conduction band edge, which is more negative
+
than the potential of H reduction, making
these
photocathodes
the
promising
candidates for solar hydrogen generation [1].
n-GaN is wide band gap semiconductor,
the Eg of which can be reduced by adding
indium [2]. The InxGa1-xN is chemically stable
material with the band gap ranging from 0.7
to 3.4 eV depending on indium content.
Small Eg is beneficial for larger amount of
light absorption, however, narrower potential
range
becomes
available
for
ox/red
reactions. As a result, water splitting and H2
evolution might be out of the potential range
when photo-electrode is illuminated.
Our studies were aimed at the
investigation of the potential of these
semiconductor materials for photoelectrochemical generation of hydrogen.
Copper oxide electrode was prepared
by means of chemical or electrochemical
oxidation of copper foil. The procedure
resulted in formation of nanostructures (Fig.
1a), the electrochemically active surface area
of which exceeded geometric one by a factor
of ~250 [3].
Samples of GaN were grown on 2-in
(001) sapphire substrates in a close-coupled
3×2-in flip-top showerhead MOCVD reactor
(Aixtron Ltd). 70 – 100 nm thick InxGa1-xN
were grown on ~ 3.5 μm t hick GaN substrate.
A thin 2–3 nm InGaN layer with constantly
changing In concentration (1 to 10%) was
used as a buffer to relax lattice mismatch
induced stress.
Performance of Cu/CuxO photocathode
was studied in PEC with nanotubular titania
photoanode, whereas in the case of InxGa1-xN
photoanodes, Pt cathode was used. In both
cases photoelectrochemical experiments
were performed without applying external
bias [4, 5]. High intensity discharge Xe-lamp
with 6000 K spectrum and calibrated with a
silicon diode to simulate AM 1.5 Sun
-2
illumination (100 mW cm ) was used.
The techniques employed in the
investigations included X-ray photoelectron
spectroscopy, Raman spectroscopy, X-ray
diffraction, scanning electron microscopy
and cyclic voltammetry.
The
processes
involved
in
the
performance of photoelectrochemical cells
investigated along with the issues of photocorrosion of CuxO and GaN photoelectrodes (Fig.
1) as well as strategies for optimization of
water photoelectrolysis will be discussed.
a)
b)
c)
d)
Fig. 1. Evidence of photocorrosion: SEM images of
Cu/CuxO photocathode before (a) and after (b)
photoelectrolysis in PEC with Ti/TiO2 photoanode
in 0.1 M KOH; SEM images of GaN photoanode
before (c) and after (d) exploitation in PEC with
Pt cathode in 0.1 M KOH; scale b ar: 1 μm
References
1.
2.
3.
4.
5.
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