IEEE Electrification Magazine - March 2018 - 22

Figure 2(b), helps the higher final solar-H 2 conversion efficiency. In contrast, a catalyst with a higher overpotential
[the red line at Figure 2(b)] gives rise to a lower PH2 value.
the ec composed on Figure 2(a) and the yellow line in
(b) does not utilize the full amount of PV power, considering that PH2 + Pkin is still smaller than PPV,max. therefore, the
role of converter can help an ec to fully use the PPV,max [Figure 2(c)]. By the mPPt procedure, extra PH2 gain can be
expected, indicating a much higher H 2 gain even with
same PV and ec systems, as shown by the yellow line in
Figure 2(b). Unless the efficiency of a converter is small
enough to reduce the PH2 to a conventional PV-ec system
without a converter, the converter utilization is important
part needed to maximize the H 2 generation.

25
EC I-V
PV I-V

Current (mA)

20

PPV,max

15
10

PH2

Pkin

5
0

25

1
1.5
Voltage (V)
(a)

2

EC with Good Catalyst
EC with Bad Catalyst
PV I-V

20
Current (mA)

0.5

The advantages of a PV-EC System

Higher PH2

15

Smaller
PH2

10
5
0

0.5

1
1.5
Voltage (V)
(b)

2

25

P

PV

Current (mA)

20

,m

ax

=

I×

15
PH2

10

V

Pkin

EC I-V
PV I-V
Extra PH2

5
0

0.5

1
1.5
Voltage (V)
(c)

2

Figure 2. An analysis method for understanding the PV-EC system:
(a) the H 2 power density ^PH2h and kinetic loss ^Pkinh at a given current-voltage (I-V) curve of the PV and EC and (b) the contrast of PH2
with a good and bad EC catalyst. (c) PH2 and Pkin after the dc-dc
converter assistance on (a) (Chang et al. 2017). (Image courtesy of
the American Chemical Society.)

and VPV = VEC, as stated in the intersection of Figures 1(a)
and 2(a). We could estimate the amount of power stored
as H 2 as PH2 and the amount of solar-driven electrical
power loss as Ploss. PH2 can be the generated current from
the PV-ec system multiplied by 1.23 V, since chemical
energy stored as H 2 is as same as the free-energy change
of a H 2 O-splitting reaction.
At a fixed solar cell condition, the catalyst materials
with lower overpotential, displayed as the yellow line in

22

I EEE E l e c t r i f i c a t i on M a gaz ine / march 2018

Based on the concept suggested in Figure 2, continuous
research has been made to improve the efficiency, stableness, scalability and economic viability of a PV-ec
system. For instance, the concept of connecting solar
cell with H 2 O electrolysis catalysts was suggested by the
turner group at national renewable energy Laboratory
(nreL). this research, also known as PV-PEC system, provided insight to the researchers to develop PV-ec system. nreL's continuous research increased the efficiency
of the PV-Pec system and recently broke 16% Sth efficiency [Figure 3(a)].
Additionally, researchers have developed the novel
configuration: a topology and material designing process
to increase the Sth efficiency and sustain its stability during photoelectrolysis. For example, the nocera group pioneered the compact PV-ec system, known as an artificial
leaf. Based on the compact configuration and control of
the number of PV cells, his group utilized four series-connected silicon PVs and transition metal-based catalysts to
achieve 10% Sth efficiency [Figure 3(b)].
to keep abreast of the fast-growing organic and inorganic perovskite solar cell, the gratzel group applied a PV
and state-of-the-art transition metal-based catalyst for
H 2 O splitting. this research reaps benefits in terms of cost
efficiency because PV and ec materials are famously inexpensive. Additionally, this system shows 12.3% Sth efficiency, even with less expensive PV and ec materials,
which have comparable efficiency with the PV-ec system
using novel materials.
recently, several PV-ec systems with more than 20%
Sth efficiency have been reported. By controlling the surface area of an ec and PV, and by applying concentrator on
a PV, the Fuji group reported a PV-ec system with 24.4%
Sth efficiency [Figure 3(d)]. In 2016, the Jaramillo group
broke 30% Sth efficiency by utilizing a state-of-the-art
catalyst and an extremely efficient tandem solar cell. not
only for the subcompartments, they heated the ec and
optimized the light and series connection of each subcompartment to reap highest efficiency. even if the external electrical energy was utilized for heating the cell, this



Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2018

Contents
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