Page 32 - Multifunctional Photocatalytic Materials for Energy

Metal oxide electrodes for photo-activated water splitting 21

testing of nanostructured metal oxide photoanodes for PEC water splitting applica-
tions. After a brief overview of the basic concepts of photoelectrochemical water
splitting, the chapter summarizes the most recent and cutting-edge results obtained
for photoanode materials based on selected metal oxides. Rather than providing a
comprehensive review on the topic, the main goal is to offer a general overview of
important achievements in the field, with particular regard to the most effective ac-
tions carried out to improve material photoefficiency: (i) tailoring of morphology/
nano-organization; (ii) doping and functionalization with suitable catalysts/activators;
and (iii) fabrication of nanocomposites and heterostructured systems [42,63]. In the
discussion of selected experimental results, particular efforts are dedicated to high-
lighting the interplay between the system features and the ultimate material activity,
examined in terms of photocurrent density/potential curves. For additional technical
details, the reader is referred to specialized review papers available in the literature
[5,14,31,33,35,37,39,40,42,63,64].

3.2 Fundamentals of photoelectrochemical water
splitting: An overview

In general, photoelectrochemical water splitting occurs through the following half-
reactions, which, in a PEC cell, take place at two interconnected, albeit physically
separated, electrodes [4,6,40,65]:

Oxidation2HO ® O + 4H + 4e - E ° = . 1 23 V vs RHE (3.1)
+
.
:
2 2 ox
+
:
Reduction4H + 4e - ® 2H E ° = . 0 00 V vs RHE (3.2)
.
2 red
Overall reaction2HO ® 2 H + O 2 (3.3)
:
2
2
where RHE denotes the reversible hydrogen electrode. Among the possible configura-
tions of PEC water splitting cells [4,33], in the most common case, the photoanode is
based on an n-type semiconductor, capable of absorbing light with energy ≥E G to gen-
erate electron-hole pairs [6,31,40]. Upon light absorption, photo-excited holes move
toward the semiconductor surface and are transferred to the electrolyte, oxidizing wa-
ter molecules to yield O 2 (Eq. 3.1 and Fig. 3.1A) [5,33,40]. Concomitantly, photo-
excited electrons are forced to the back contact and transferred through the external
circuit to the counterelectrode, a metal or a p-type semiconductor (cathode) [18,32].
At the interface of the latter with the electrolyte, electrons can react with protons and
directly take part in H 2 evolution (Eq. 3.2) [37,40]. The photocurrent generated in the
external circuit and registered as a function of the applied potential (Fig. 3.1B) is an
important parameter for the evaluation and comparison of photoanode performances.
Despite being very attractive for sustainable energy generation, PEC water splitting
is indeed a difficult approach [29]. As a matter of fact, hydrogen production is hindered
by the remarkable stability of water, because of the large positive change in the Gibbs