H2Tech - Q3 2021 - 31

HYDROGEN INFRASTRUCTURE DEVELOPMENT
SPECIAL FOCUS
FIG. 8. Flow diagram of the electrolysis process.
The significant advantages of PEM over alkaline electrolyzers
are higher turndown ratio, increased safety due to the absence
of KOH electrolyte, more compact design due to higher current
densities, and higher differential pressures. The system cost of
PEM electrolyzers is approximately 1.3-1.8 times that of alkaline
systems. The main drawbacks of this technology are the limited
lifetime of the membranes and the high cost of the electrolyzers.
Alkaline electrolysis is a mature technology for H2
production
up to MW scale and is the most widely used type of electrolysis
technology. The electrodes consist of non-noble metals like
nickel with an electrocatalytic coating. The alkaline electrolyzer
is relatively lower cost, as less rare material is required and water
purity is less stringent. Zero-gap alkaline electrolyzers hold the
key to cheap and efficient renewable energy storage. They use
concentrated lye solution as the electrolyte and require a gas-impermeable
separator to prevent the product gases from mixing.
The advantages to the bipolar (filter press) design are reduced
stack footprint, higher current density and the ability to
produce higher-pressure gas. Pressurized alkaline electrolyzers
have lower efficiency and produce a lower-purity product
than atmospheric alkaline (AEL). The foremost advantage of
pressurized AEL compared to atmospheric AEL is that pressurized
AEL produces compressed H2
(either for grid injection
or further use) with less additional energy input. This happens
because the reduction in electric efficiency of the electrolysis
with increased pressure is lower than the energy needed to
compress the produced H2
.
Nevertheless, alkaline electrolyzer cells do not operate well
at very low-current densities. They are limited in terms of flexibility
in the load-following operation required for use with
renewable energy sources. In alkaline electrolyzers, gases can
blend across porous separators when the current fluctuates,
making them unsuitable for powering directly from intermittent
renewable power sources.
Another issue is that it takes 30 min-60 min to restart the system
following a shutdown. The most significant disadvantage of
AEL electrolyzers is that the utilized electrolytes (alkaline solution,
e. g. 20%-30% KOH solution) are highly corrosive, necessitating
high maintenance costs. A general overhaul of the system
is necessary every 56,000 hr-90,000 hr of operation. AEL
electrolyzers also have a large footprint and low output pressure.
SOEC electrolyzers operate at high temperatures of 800°C-
1,000°C. SOEC has the highest overall efficiency of 70% and
utilizes waste heat. It runs in regenerative mode to achieve water
electrolysis using solid oxide/ceramic electrolyte to produce
H2
and O2
. Energy demand from SOEC is reduced due to Joule
heating of an electrolysis cell, which is utilized in the electrolysis
process at high temperature.
Advantages of SOEC include long-term stability, fuel flexibility,
low emissions and low operating costs. However, a disadvantage
is the high operating temperature, which results in long
startup times and break-in times. The high operating temperature
also leads to mechanical compatibility issues, such as thermal
expansion mismatch and chemical instability, such as diffusion
between layers of material in the cell. The cost of an SOEC
electrolyzer is 2-4 times higher than alkaline, and the operating
pressure range is approximately 1 bar.
AEM electrolyzers are equipped with an anion exchange
membrane (AEM) and are also known as alkaline PEM electrolyzers.
They require pure water feed and cheap components,
such as platinum group metal-free catalysts and stainless steel
bipolar plates. AEM electrolyzers are simple, robust and easy
to operate. No water is transported to the cathode, and there is
no need for an H2
-water separator, which makes for a simpler
H2Tech | Q3 2021 31
H2Tech - Q3 2021

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