Page 67 - Multifunctional Photocatalytic Materials for Energy

56 Multifunctional Photocatalytic Materials for Energy

possible like hydrogen evolution by water splitting owing to these reactions have sim-
ilar thermal dynamic potentials, as shown in Table 4.1. However, precise control over
this multiple- step process remains a huge kinetic challenge. The main challenges for
photo/catalytic CO 2 reduction include (i) low conversion efficiency/current of cata-
lysts, (ii) poor product selectivity of the catalysts, (iii) lack of a fundamental under-
standing of the real reactions during the CO 2 multistep reactions, and (iv) lack of
reliable way to precisely predict the possible reaction pathways.

4.3.3 Photocatalysts requirements for catalytic reactions

An efficient photocatalyst must meet all requirements simultaneously in order to op-
timize the five steps presented in Section 4.3.1. First, the semiconductor must have a
small band gap in order to harvest the sunlight as much as possible. At the same time,
regarding the energy band, the CB and the VB of the semiconductor must straddle
the redox potentials of the desired photocatalytic reactions (Fig. 4.5A). To minimize
the charge recombination, fast charge mobility and long charge diffusion length are
essential as well [6,15,38–41].
The configuration commonly used for photoelectrochemical water splitting is to
use a single photoelectrode (n-type photoanode for oxidation shown in Fig. 4.5B
or p-type photocathode for reduction shown in Fig. 4.5C) and an electrochemically
active material, generally platinum, as the counter electrode, which is called a half
PEC cell. In half-cell configurations, however, the photogenerated voltage usu-
ally is not high enough to initiate these reactions, and an external bias is required
regardless of the band gap and light absorption capability of the semiconductor
photocatalyst. The ultimate goal of this field is to produce chemical fuels with
solar energy input only by coupling the photoanode and photocathode together
in a full PEC tandem cell (Fig. 4.5D), which requires consideration of both light
absorption and band alignment for two photoelectrodes (see more on this topic in
Section 4.5.5).

4.3.4 Solar to chemical conversion efficiency
Not all the photogenerated charge carriers can survive and reach the surface for chem-
ical reactions. Photocatalytic solar fuel generation competes with many side processes
that consume the charge carriers and dramatically decrease the carriers’ population.
Practical solar-to-chemical (STC) conversion efficiency is therefore far below the
theoretical values. Primarily, the STC efficiency η is determined by the following
processes: (i) light absorption to generate charge carriers in photoelectrode (light ab-
sorption efficiency, η abs ), (ii) charge separation and migration to the photoelectrode
surface (charge separation efficiency, η sep ), and (iii) charge injection and reaction at
the photoelectrode/electrolyte interface (charge injection efficiency, η inj ) [14,42–45].
The overall STC efficiency η can be illustrated as

h µ h ´ h ´ h (4.6)
abs sep inj