IEEE Electrification Magazine - March 2018 - 21

operation is required for the PV voltage range that is less
than the voltage at the intersection of two PV curves,
while the buck operation is needed for a higher voltage
range. If VPV,max is higher than VEC,MPPT, as displayed in Figure 1(c), the maximum power point of PV is in the buck
operation range, and a buck converter is then required for
achieving PPV,max.
In contrast, if VPV,max is less than VEC,MPPT, as in
Figure 1(d), a boost converter is needed to operate at the
maximum power point. In most cases, VPV,max is higher
than VEC,MPPT because a low-voltage ec is efficient and easily implemented. In addition, PV cells can be connected inseries for high open-circuit voltage. consequently, many
researchers use buck-type converters, i.e., a buck converter
in the low-voltage PV-ec system and push-pull topology
in the high-voltage application for galvanic isolation and
high step-down ratio. Soft-switching converter topologies,
such as quasi-resonant converter and phase shift fullbridge converter, are utilized for achieving higher converter efficiency at a high PV voltage.

The EC System
the ec system is composed of catalyst-decorated electrodes (cathode and anode) and a membrane between two
electrodes. to deeply understand the catalyst, we should
first understand the concept of overpotential, which is the
extra potential that should be applied in addition to thermodynamic potential. this is the result of activation

IPV, PPV

PV
Cell

energy of the chemical potential, which results in higher
energy costs. even though the thermodynamic energy
cannot be changed at a fixed temperature and pressure
condition, overpotential can be varied by the utilization of
a catalyst. Lowering the overpotential requires the proper
catalyst materials.
One reaction of H 2 O splitting can be divided into O 2
and H 2 generation, indicating that we need both cathode
and anode catalyst materials to decrease the overpotential. In the case of H 2 O oxidation catalyst for O 2 generation, researchers have first used rare metal oxides
(iridium oxide or ruthenium oxide) but, recently, have
transitioned to cheaper metal-based catalysts, i.e., compositions mainly based on nickel, manganese, iron, and
cobalt are often developed.
the membrane, usually the polymer membrane
between two electrodes, should be less resistant and
should have a good separation property of gaseous products. to keep away from a short of this system, the membrane should only transfer the ion form, e.g., proton or
hydroxide, to balance the ec system. Since the conductivity
of ions through the membrane is not ultrahigh, resistance
along the membrane is not negligible, especially under a
high rate of reaction.
Figure 2 shows the overlaid I-V relationship of a PV and
ec component. If a PV and ec are directly connected, the
photogenerated voltage and current from PV should
match the applied voltage and current from ec as I PV = I EC

IEC, PEC

VPV

VEC

IPV, PPV

IEC, PEC

+
EC

PV
Cell

VPV

+
dc-dc
Converter

VPV = VEC IPV = IEC
PPV = PEC

VPV ≠ VEC IPV ≠ IEC
PPV ≠ PEC/ηconv

(a)

(b)

Power

Maximum
Power Point

PPV,max

Power
PPV,max

Boost
Operation

Buck
Operation

Maximum
Power Point EC
Boost
Operation
Buck
Operation

PV
VEC,MPPT

VEC

VPV,max Voltage

VPV,max VEC,MPPT

(c)

Voltage

(d)

Figure 1. The two configurations of the PV-EC system. (a) A direct coupling for a PV-EC system. (b) A dc-dc converter connection to construct a
PV-conv-EC system. Power-voltage curves of the PV-EC system when (c) VPV,max > VEC,MPPT and (d) VPV,max < VEC,MPPT.

IEEE Electrific ation Magazine / ma r c h 201 8

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