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Metal oxide electrodes for 3

photo-activated water splitting

†
†
Davide Barreca*, Giorgio Carraro , Alberto Gasparotto , Chiara Maccato †
†
*Padova University, Padova, Italy, Padova University and INSTM, Padova, Italy
3.1 Introduction

The utilization of sunlight can provide helpful power necessary to supply to energy
needs in the framework of an improved environmental sustainability, provided that it
is efficiently harvested, converted, and stored [1–6]. To this aim, an attractive route
involves the conversion of radiant energy into chemical fuels, mimicking the ele-
gant example provided by natural photosynthesis [7–10]. In this wide context, over
the last two decades, harnessing solar energy for the clean production of molecular
hydrogen (H 2 ) by PEC water splitting over inorganic semiconductors has received a
great attention [2,3,11–18]. In fact, this route represents a key strategy for sustainable
energy generation [19–28], thanks to the possibility of driving the target processes
with virtually zero ecological footprint starting from largely available resources, i.e.,
water, extremely abundant on Earth, and sunlight, inexhaustible and intrinsically re-
newable [29–32]. Indeed, PEC solar water splitting could be the basis for a sustain-
able, carbon-free, and cost-effective hydrogen-based economy, paving the way to the
progressive substitution of fossil sources, with relevant economic and environmental
advantages [2,15,30,33–35]. In fact, hydrogen, the cleanest energy fuel, has a very
−1
high energy density (≈130 MJ × kg ) and is considered as one of the most promising
energy carriers for the near future [28,36–39]. At variance with direct photocatalytic
systems, which typically involve simultaneous generation of H 2 and O 2 at the photo-
catalyst surface, in PEC systems O 2 and H 2 evolution occurs at the anode and cathode,
respectively (see Section 3.2) [37,40]. The collection of the two gases at separated
electrodes offers important technical advantages for practical utilization [32].
To date, the development of a promising H 2 O splitting device is still directly depen-
dent on the use of photoanodes possessing certain essential requirements [1,8,13,21]:
(i) low cost and good stability in aqueous solutions; (ii) a band gap (E G ) suitable for
solar light absorption (Fig. 3.1); (iii) conduction (CB) and valence band (VB) edges
matching water oxidation and reduction potentials; and (iv) high conversion efficiency
of photogenerated electrons and holes, implying an efficient charge carrier separation
and transport [15].
So far, numerous efforts have been devoted to the preparation of various photoelec-
trodes endowed with suitable properties [5,8,29,35,41,42], particularly n-type semi-
conducting materials working as photoanodes, because the oxygen evolution reaction
(OER) from water remains a main bottleneck in the H 2 O splitting process. Since
the active material is subjected to severe oxidizing conditions [9,10,15], the most

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