H2Tech - Q2 2021 - 23

PATHWAYS FOR SUSTAINABLE HYDROGEN
The issue of sustainability is a topic of significant discussion
today. However, it has not been addressed thoroughly due to its
multifactor outcome affecting the human population, the ecosystem and the capacity of the environment to sustain a given
activity or process with minimal or manageable detrimental effects. At some point in the near future, sustainability will need
to be precisely defined, with metrics in place to truly assess the
sustainability of H2 production.
Electrolytic methods. One method for green H2 production-and probably the most popular at the moment-is water
(H2 O) electrolysis using renewable energy sources. This method breaks apart (splits) H2 O into H2 gas and oxygen gas (O2 ).
On a fundamental level, this process is water-intensive with
approximately 2.4 gal of water needed to produce 1.8 lb of H2
gas, assuming minimal losses. In addition, the electrodes for the
process use specific metals from mining operations that are not
only water-intensive but also carbon-intensive, generating considerable waste. The overall sustainability for this method still
requires improvement. FIG. 1 shows an example of a common
electrolytic cell used for splitting water into H2 gas and O2 gas.
Pathways to obtain the most sustainable H2 to date (green
H2 ) via water electrolysis generate the purest H2 at > 99.9%
purity. This process alone can be conducted using several different methods and can be carried out at numerous different
geographical locations. The most important technologies for
water electrolysis are alkaline electrolysis, proton exchange
membrane (PEM) electrolysis and solid oxide cell (SOEC)
high-temperature electrolysis. FIG. 2 shows a common scheme
for a PEM electrolyzer cell. The membrane material is a key feature of PEM cell technology.
At present, water electrolysis is the most developed method
for green H2 globally and is sold commercially. A number of companies have been installing large-scale electrolyzers in different
locations worldwide. The use of renewable energy coupled with
water-splitting technology is how this method is considered, to
a certain extent, sustainable. Nevertheless, water electrolysis is
the shortest-term pathway for achieving highly sustainable H2
production. Corporations and research groups are investigating
how PEM can be used more efficiently by lowering the energy
requirements for water splitting. This area of research has been
more active compared to H2 storage and transportation, which
are still carbon-intensive. However, ammonia (NH3) and other
alternatives are receiving attention for their potential use as H2
carriers. This is an area of ongoing development that is characterized by a number of challenges, starting with the fact that ammonia has an extremely pungent odor and must be first cracked
at the point of use to produce nitrogen and H2 .
An alternative method for H2 production is photo-electrochemical (PEC) water splitting. This method uses specialized
semiconductors (PEC materials) and light energy to directly
dissociate the water molecule into H2 gas and O2 gas. This
technology is a long-term pathway due to present technology
limitations; however, it holds significant potential for commercial use.
Furthermore, thermochemical water splitting uses high temperature from a concentrated solar power farm to split the water
molecule. Water, liquid and vapor are used in this method, in
addition to turbines, to create a loop that consumes only water.

SPECIAL FOCUS

Solar and wind pathways. The use of solar energy to produce
H2 can be carried out in two main ways:
1. Water electrolysis, using solar-generated electricity
2. Water splitting with direct solar energy.
When considering solar-generated electricity, PV cells to
promote water electrolysis often come to mind. To be practical and for large-scale deployment, the cost of H2 generation
via solar energy must be significantly reduced. Previous studies
have predicted that achieving a high solar-to-H2 efficiency is a
significant driving force for reducing H2 generation costs.
To date, the highest efficiency using a PV water-splitting system is around 12%. Theoretical studies suggest that 25%-30%
efficiency can be achieved. Solar-thermal methods via direct dissociation of water employ the high temperatures generated by
solar collectors to split water molecules into H2 gas and O2 gas.
PEC water splitting is a form of electrolysis, but direct sunlight is
used to irradiate a semiconductor immersed in water, which then
produces the current used to split water into H2 gas and O2 gas.
Solar energy is not free of challenges. The technology must
overcome several hurdles to achieve better sustainability:

DC supply

Cathode

Anode

Electrolyte water
FIG. 1. Example of a common electrolytic cell for water splitting.

DC source

C
a
t
h
o
d
e

PEM
membrane
H+

H2O

A
n
o
d
e

Catalyst
FIG. 2. Diagram of a simplified PEM electrolytic cell.
H2Tech | Q2 2021

H2Tech - Q2 2021

Table of Contents for the Digital Edition of H2Tech - Q2 2021