Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 268

electric potential (Bockris et al., 1983):

4H + ( aq ) 4e − , A = 2H 2 O( l ) = O 2 ( g ) + + 1 . 23 V on surface of anode (oxidation)

(9.2.11)

2H + ( aq ) 2e − = + H 2 ( g ), C = 0 . 00 V on surface of cathode (reduction) (9.2.12)

where 1.23 V is the standard potential of the anode that indicates the theoretical
minimum requirement. If viewed from the level of molecules, water electrolysis, pho-
toelectrolysis, and photoelectrochemical methods can be categorized as the same type.
However, their engineering approaches may be very different, which will be discussed
in detail in the following sections.
As to splitting of water molecules with electricity as indicated by Equations (9.2.11)
and (9.2.12), an electrode efficiency defined on the basis of the gap between the actual
potential bias and the theoretical minimum value of 1.23 V is often used to assess
the performance of the electrode (Licht, 2005; Bockris et al., 1983). This will not be
discussed in detail in this chapter.
In water electrolysis, water is split with an electric current to produce hydrogen.
Direct current (DC) passes through two electrodes immersed in water, i.e., anode and
cathode, and hydrogen is produced on the surface of the cathode when the electric
potential is sufficiently high. The electrodes can be shaped to rods or plates, and the
reactions taking place on the surface of the electrodes are shown in Equations (9.2.11)
and (9.2.12). In order to avoid confusion with other terminology such as photoeletrol-
ysis and photoelctrochemical methods, this paper suggests that the terminology “water
electrolysis'' is only used when the electricity is fully obtained from an external power
generated from photovoltaic panels or turbines driven by solar-generated steam or
other gases. Therefore, the electrolyzer and water do not receive the sunlight for
water splitting. This categorization considered the engineering flexibility and engi-
neering practicality for the integration of independent power sources and various
electrolyzers.
As shown in Figure 9.2.4, the basic components of the hydrogen production
unit include two electrodes (anode and cathode) and one external power source. It
can be found that the overall efficiency of the hydrogen production depends on the
electricity-to-hydrogen efficiency of the electrolyzer, and the solar-to-electricity conver-
sion efficiency. Table 9.2.4 shows the power consumption and efficiency of different
industrial electrolysis systems in the U.S. (Ivy, 2004), Europe (European Commision,
2001) and China (CSPCS, 2009).
Since conventional electrolysis is mature technology and the electricity-to-
hydrogen efficiency of a commercially available electrolyzer lies in the range of
50%-80% either using alkaline or polymer electrolyte membrane electrolyzers, the
electricity generation dominates the overall efficiency of the hydrogen production.
Currently, the power generation efficiency with photovoltaic panels is about 10-20%
(van Helden et al., 2004; Yamada et al., 2011; Hanna et al., 2006). Therefore, the
maximum overall efficiency of hydrogen production is below 16% (Khaselev et al.,
2001). It is anticipated that the power generation efficiency of photovoltaic panels can
be enhanced in the future by use of new materials to accommodate more irradiation
of the solar spectrum.