applied

sciences
Review
Recent Developments on Hydrogen Production Technologies:
State-of-the-Art Review with a Focus on Green-Electrolysis
Leonardo Vidas 1 and Rui Castro 2, *

1 Instituto Superior Técnico, University of Lisbon, 1049-001 Lisboa, Portugal; leonardo.vidas@tecnico.ulisboa.pt

2 INESC-ID/IST, University of Lisbon, 1000-029 Lisboa, Portugal
* Correspondence: rcastro@tecnico.ulisboa.pt

Abstract: Growing human activity has led to a critical rise in global energy consumption; since
the current main sources of energy production are still fossil fuels, this is an industry linked to the
generation of harmful byproducts that contribute to environmental deterioration and climate change.
One pivotal element with the potential to take over fossil fuels as a global energy vector is renewable
hydrogen; but, for this to happen, reliable solutions must be developed for its carbon-free production.
The objective of this study was to perform a comprehensive review on several hydrogen production
technologies, mainly focusing on water splitting by green-electrolysis, integrated on hydrogen’s
value chain. The review further deepened into three leading electrolysis methods, depending on
the type of electrolyzer used—alkaline, proton-exchange membrane, and solid oxide—assessing
their characteristics, advantages, and disadvantages. Based on the conclusions of this study, further



developments in applications like the efficient production of renewable hydrogen will require the


consideration of other types of electrolysis (like microbial cells), other sets of materials such as in
Citation: Vidas, L.; Castro, R. Recent anion-exchange membrane water electrolysis, and even the use of artificial intelligence and neural
Developments on Hydrogen networks to help design, plan, and control the operation of these new types of systems.
Production Technologies:
State-of-the-Art Review with Focus
Keywords: hydrogen value chain; hydrogen storage methods; hydrogen production technologies;
on Green-Electrolysis. Appl. Sci. 2021,
water electrolysis technologies; alkaline water electrolysis; proton-exchange membrane electrolysis;
11, 11363. https://doi.org/10.3390/
solid oxide electrolysis
app112311363

Academic Editors: Pooya Davari,

Enrico Cagno, Mohsen Soltani and
Edris Pouresmaeil 1. Introduction
Nowadays, an ever-expanding human population coupled with a growth in anthro-
Received: 31 July 2021 pogenic activities and general better standards of living have led to a significant surge in
Accepted: 19 November 2021 overall energy consumption [1,2]. Presently, most of the energy generation comes from
Published: 1 December 2021 fossil fuel sources; Figures 1 and 2 show how the widespread use of coal, oil, and natural
gas since the beginning of the 19th century has led to the continued emission of greenhouse
Publisher’s Note: MDPI stays neutral gases—such as carbon dioxide, methane, and nitrous oxide—causing a gradual increase in
with regard to jurisdictional claims in the concentration of these gases in the Earth’s atmosphere and contributing to environmen-
published maps and institutional affil- tal degradation and climate change (it should be noted that the most recent measurement
iations.
has already peaked at 419 ppm, in May of this year) [3,4].
As is well known, the presence of these gases in the atmosphere traps heat radiating
from the Earth toward space, effectively warming it. Figure 3 shows how there is mounting
evidence that this global warming is man-made, namely, by observing the rise of world
Copyright: © 2021 by the authors. temperatures, the warming of the oceans, shrinking ice sheets, glacial retreats, decreased
Licensee MDPI, Basel, Switzerland. snow cover, the declining of the Arctic Sea ice, a broad sea level rise, widespread ocean
This article is an open access article acidification, and more extreme weather events in general [5–7].
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).

Appl. Sci. 2021, 11, 11363. https://doi.org/10.3390/app112311363 https://www.mdpi.com/journal/applsci

Appl. Sci. 2021, 11, 11363 2 of 27

Figure 1. CO2 atmospheric concentration: long-term overview. Based on data retrieved from [8].

Figure 2. Relationship between fossil fuel consumption and CO2 concentration in the atmosphere.
Based on data retrieved from [8].

Figure 3. Relationship between CO2 concentration in the atmosphere and global temperature
anomaly. Based on data retrieved from [9].
Appl. Sci. 2021, 11, 11363 3 of 27

Furthermore, fossil fuels are naturally a finite resource; so, using them is inherently lim-
iting the use of such energy sources by future generations. While these factors are enough
to motivate a total replacement to alternative sources of energy, it turns out that we are
actually increasing the use of these conventional fuels, whose impacts are already rapidly
approaching tipping-points that will bring disastrous consequences for humanity [10–12].
Fighting climate change might be the greatest challenge of this generation; it all boils
down to halting the temperature rise, which in turn means decreasing the atmospheric
concentration of greenhouse gases, which, again, in turn means finding solutions to replace
fossil fuels as our primary energy source; to reverse the on-going global warming, we
urgently need to decarbonize the world economy—hence, the development of renewable
energy sources has become essential. While such renewable sources like solar and wind
can provide environmentally friendly alternatives to fossil fuels, their intermittent nature
brings the need of an energy storage medium that allows for the continual provision
of energy; as there is no one-size-fits-all solution, we need a multi-faceted approach to
accomplish that. For instance, instead of using common batteries, these sources could
grant the energy needed to produce hydrogen from water, which can then be stored as
a means to generate electrical and mechanical energy, as well as heat—thus ensuring
the continuous production of emissions-free energy, which is necessary to fulfill modern
society’s consumption requirements [13,14]. The push for environmentally friendly energy
solutions has renewed the interest to accelerate the development of hydrogen production
methods. Currently, around 96% of global hydrogen production comes from non-renewable
fossil fuels [15,16]. However, besides releasing harmful greenhouse gases to the atmosphere,
these methods can only produce low-purity hydrogen [17–19].
This article focused on studying green hydrogen production methods, namely, through
the analysis of different types of water electrolysis technologies currently being developed
and used in modern industry—including their characteristics and modes of operation,
their advantages and disadvantages, and their similarities and differences. It does so
integrated on hydrogen’s value chain, therefore adding to this growing body of research.
However, an extensive review of the state of the art in general hydrogen production
methods is given, including all the current main methods of producing hydrogen—either
by renewable or non-renewable sources: hydrocarbon reforming, thermochemical biomass
processes, biological biomass processes, and water splitting. The research regarding these
several production technologies is deepened, referring to the respective detailed challenges
and future trends on related published work; this review addresses not only current and
commercial technologies but also future technologies presently in the research phase (but
which are expected to be of interest for the coming years). This analysis is one of this
article’s most valuable assets, as, to the best knowledge of the authors, no such review is
available in the literature.
The remainder of the article is arranged as follows: Section 2 gives a general descrip-
tion of hydrogen’s value chain, addressing the main end-use strategic configurations and
the leading prevailing forms of storage. Section 3 then delivers an overview on hydrogen
production technologies, starting with a background explanation of some important con-
cepts and then moving to the in-depth study of electrolysis. A literature review takes place
in Section 4, and the article ends with some conclusions in Section 5.

2. Hydrogen Value Chain

Some now argue that a true decarbonized economy cannot even exist without hydro-
gen. The European Commission is particularly determined to make Europe deliver on
its ambitious climate promise to achieve carbon neutrality by 2050; in its main document
related to the introduction of green hydrogen technology, it aims to give a boost to clean pro-
duction, since this immensely versatile energy vector could find many possible applications
in all major sectors of modern society—from industry to buildings to transportation [20].
Figure 4 shows a generic scheme of hydrogen’s value chain, hinting that to frame the
implementation of this strategy, it is important to first define the configurations considered
Appl. Sci. 2021, 11, 11363 4 of 27

to be priorities in the hydrogen value chain—from production to final consumption. This

comprises, in practice, three phases:
• Production. The first stage of the hydrogen value chain consists in its production, with
different pathways, processes, and associated technologies already identified. Depend-
ing on the required scale, large-scale (centralized) production is distinguished from
small-scale (decentralized) production—ideally, close to the place of consumption;
• Storage and Distribution. The second stage starts with storage and ends with de-
livery for final use. This stage includes processes that generally break down into
sub-processes; a sub-process can be, for instance, underground gas storage, lique-
faction, compression, storage and distribution in gas networks, road and maritime
transport, or refueling. Naturally, all these have their own risk and safety concerns
associated with the operation at very low temperatures and/or very high pressures,
requiring tanks with higher thicknesses to guarantee insulation levels; this topic will
be addressed with more detail in Section 2.2.2.
Likely combinations of hydrogen fueling processes could be:
– Road distribution in the form of liquefied/compressed gas, ending with a liquid-
to-liquid refueling process, liquid-to-gaseous cryogenic storage systems, or gas-
to-gas at various scales;
– Distribution by ships in the form of liquefied hydrogen, including delivery for
end-use in oil pipelines and road transport;
– Distribution of gaseous hydrogen through pipeline systems;
– Blending of hydrogen with natural gas in the current natural gas infrastructure.
• End-use. In the third stage, the hydrogen value chain is addressed to the main
end-use applications in the mobility/transport and industrial sectors. In residential
and industrial stationary applications, mixtures of hydrogen and natural gas can be
applied to generate heat and electricity.

Figure 4. General flowchart of hydrogen’s value chain.

An utmost important matter on this last phase is with regards to the strategic con-
figuration of the hydrogen value chain, i.e., how hydrogen going to be used as an energy
vector.

2.1. End-Use on Hydrogen Value Chain

Generally speaking, the current characteristics of any generic energy system allows
five main end-use configurations: power-to-gas (P2G), power-to-mobility (P2M), power-to-
industry (P2I), power-to-synfuel (P2FUEL), and power-to-power (P2P) [21].
Appl. Sci. 2021, 11, 11363 5 of 27

2.1.1. Power-to-Gas Configuration

Figure 5 shows a general arrangement of a typical P2G value chain, alluding to be
mainly directed towards the decarbonization of the current natural gas system.

Figure 5. Generic scheme of a power-to-gas hydrogen value chain configuration.

This approach involves mixing (blending) hydrogen with natural gas, aiming for the
gas networks to transport more energy from renewable sources than from fossil origin
in the medium to long term [22]. Considering that both the technical characteristics of
the end-use equipment (furnaces, turbines, boilers...) and those of the gas network itself
impose limitations on the percentage composition of hydrogen in the mixture, there is also
the option—or the need—to build dedicated hydrogen networks.

2.1.2. Power-to-Mobility Configuration

P2M aims at mobility and transport, considering the technical design and installation
of refilling station networks resorting to: geo-optimization tools that help find the best
locations to make accurate mapping of those places, cost-minimization analysis of fuel cells
on board of vehicles—with a particular focus on light vehicles (taxis, company fleets, and
shared mobility), heavy vehicles (goods and passengers), rail (non-electrified trains), ships,
and even airplanes [23,24]—and profitability analysis of on-site hydrogen production. The
scheme on Figure 6 displays an overview of a typical P2M value chain.

Figure 6. Generic scheme of a power-to-mobility hydrogen value chain configuration.

Currently, there are already several companies and promoters with P2M projects in
progress, namely, involving the modeling, optimization, and performance simulations
of energy consumption related to the hydrogen refilling stations for light and heavy
vehicles; these are mainly associated with logistics centers, industries, transport fleets, and
Appl. Sci. 2021, 11, 11363 6 of 27

cruise ships—clearly showing the research interest and dynamic already generated in this
particular field of hydrogen’s value chain [25,26].

2.1.3. Power-to-Industry Configuration

P2I refers to hydrogen usage in modern industry; presently, it is an essential chem-
ical agent for the production of ammonia, methanol, various polymers, and many other
compounds and materials in oil refining and also as a process byproduct in some sub-
sectors of the inorganic chemical industry (many more applications can be identified with
a wide-level range of technological maturity) [27].
Looking at Figure 7, which depicts a generic scheme of P2I, hydrogen has primar-
ily the potential to replace natural gas as a heat source in industry, in processes where
electrification is not possible or is economically inefficient—such as sectors that use high
temperatures (e.g., steel and cement)—which may require the adaptation or replacement
of equipment but do not need highly pure hydrogen [28,29].

Figure 7. Generic scheme of a power-to-industry hydrogen value chain configuration.

2.1.4. Power-to-Fuel Configuration

Synthetic fuels are currently produced by steam reforming of fossil hydrocarbons
(mainly methane) and by coal or biomass gasification. Other technologies include coal
liquefaction, namely, through direct processes like co-processing and dry hydrogenation,
where coal is directly converted to liquid synfuel avoiding the initial conversion to syngas.
Figure 8 presents a general overview of what a P2FUEL value chain looks like, indicat-
ing how different renewable technologies can enable the production of these fuels, leading to
the perspective that all crude oil products could at first be produced synthetically—resulting
in true renewable fuels [30]. P2FUEL thus states that, in conjunction with the electrification
of the economy and previous value chains, this route could in principle lead to the total
decarbonization of the energy sector [31].

Figure 8. Generic scheme of a power-to-fuel hydrogen value chain configuration.

Appl. Sci. 2021, 11, 11363 7 of 27

2.1.5. Power-to-Power Configuration

Finally, if its purpose is to participate in the electricity supply market, the production of
electrical energy using properly adapted gas turbines or hydrogen fuel cell stations—which
in itself is produced with electricity—may be, at first glance, an energy-inefficient option.
Nonetheless, Figure 9 shows a possible design scheme of P2P, revealing how it may
be interesting from a system’s services point of view, especially with regards to storage
(in addition to batteries and dams with a pumping mode) and presents an option that
strengthens the security of supply in a context of accelerated decarbonization of the elec-
tricity sector. For instance, when dealing with dry hydrological years, in which there is
a lower availability of water resources, large amounts of stored hydrogen could be used
to feed high-power fuel cells (or possibly even hydrogen turbines), in this way ensuring
supply security [32,33].

Figure 9. Generic scheme of a power-to-power hydrogen value chain configuration.

2.2. Storage on Hydrogen Value Chain

A new question now arises about this stored hydrogen; current storage options consist
mainly of compressed gas, cryogenically frozen or liquefied gas, and chemical storage
(such as in metal hydrides and ammonia) [34].

2.2.1. Storage by Compression

Although compressing hydrogen requires less energy than liquifying it, storing hy-
drogen in a compressed gaseous state requires substantially more storage space; that is
disadvantageous. Even so, compressed hydrogen gas is presently the most commonly
used storage technology. This system has many advantages, such as a simple operation
(resulting in low costs), rapid (dis)charge cycles at a wide range of temperatures, and
low energy requirements, as mentioned before [34]. Nonetheless, due to the overall low
density of hydrogen as a gas, the main obstacle of this kind of storage is possibly that
it requires bulky systems (over two times the volume that of natural gas with the same
energy output) [35–37]. Hydrogen as a compressed gas is also more volatile than in all other
storage options; leaks due to impact or compartment failure may result in a rapid discharge
of a highly explosive gas. Moreover, these issues may include material weakening, which
causes premature degradation of mechanical properties and results in cracking and loss of
the storage cylinders’ rigidity [38].

2.2.2. Storage by Liquefaction

The process of liquefying hydrogen requires large amounts of energy, since this gas
has extremely low boiling and melting points (−252.9 ◦ C and −259.2 ◦ C, respectively).
This results in a high energy consumption required for solid and liquid hydrogen storage
options, needing up to 30% of its potential stored energy; additionally, the insulation
Appl. Sci. 2021, 11, 11363 8 of 27

requirements for such storage are not easy to obtain [34], despite tanks of cryogenic
hydrogen being much lighter than, say, tanks that can hold pressurized hydrogen.
Studies on the optimization process of large-scale hydrogen liquefaction have found
that a wide range of lower-cost, highly efficient designs are heavily dependent on the plant’s
capacity; selecting the optimal process also depends on other relevant conditions such as
the plant’s location, utility costs, and customer needs [39]. Moreover, case-study analysis
of advanced liquefaction systems have shown how there is still room for improvement on
what concerns overall efficiency challenges. These may include changes in the hydrogen’s
feed temperature and the catalysts themselves; the total yield could further benefit from
design adjustments that reduce environmental impact and waste production [40].

2.2.3. Storage by Chemical Processes

Regarding chemical storage methods, metal hydride systems in particular store hy-
drogen in a solid-state—that happens when hydrogen molecularly bonds to the metal—
resulting in a much safer and high-volume efficient tankage when compared to liquid
or gas [34,38]; although this being still an emerging technology, it has already reached
high levels of safety—proven to be non-reactive even to bullets [41]. This presents as
a major advantage over, say, battery storage, which nowadays still has associated risks
with explosiveness. Current research on metal hydrides is focused on improving the
adsorption properties and decreasing the cost of this system [34,38], with magnesium
hydride appearing as a promising solution due to its favorable properties; however, this
system requires relatively high ambient temperatures to (dis-)charge, which may pose
a drawback to the transportation sector. Such drawbacks are currently being addressed
through research on several different production techniques, as well as the study of catalyst
inclusion to the systems [34].
On the other hand, hydrogen can also be stored in ammonia. This compound is well
known and used within the fuel industry, making it easily one highly valued hydrogen
storage option. It has a relatively high hydrogen density and a high utilization flexibility—
both in stationary and mobile applications. As ammonia’s capabilities ensure long-term
stable storage and transportation, it can cope with the needs to store energy in time and
in space (stationary and export/import energy, respectively) [42]. When comparing these
storage methods, liquid ammonia was found able to store volumes of hydrogen larger
than, say, liquefied hydrogen itself. Besides, it can also be stored at normal pressure and
temperature conditions (1 bar and 25 ◦ C, respectively)—much lower than compressed
hydrogen gas, for example. However, this very high density also means heavier storage
and a special transportation demand. Moreover, the process of releasing hydrogen from
ammonia downstream consumes great amounts of energy, requiring even more in the
case where high-purity hydrogen is needed; this could arguably be the most challenging
aspect of high-efficiency hydrogen storage in ammonia, particularly considering that both
compressed and liquid hydrogen easily deliver pure hydrogen from design [27]. Methanol
could be seen as competitor of ammonia-based hydrogen storage, as it yields higher energy
density; however, it has a lower hydrogen content by weight and volume [43].
Table 1 offers a summary of the described so far, compiling the major advantages and
disadvantages of the leading hydrogen storage technologies.

Table 1. Summary of current hydrogen storage methods.

Storage Technology Compression Liquefaction Chemical Ref.

Basic Properties
Storage temperature (◦ C) ∼25 <−252.90 ∼25 [27,35]
Storage pressure (bar) 690 1 9.90 [27,35]
Density (kg/m3 ) 39 70.80 600 [27,35]
Hydrogen Content
Gravimetric (wt%) 100 100 17.80 [27,38]
Volumetric (kg-H2 /m3 ) 39 70.80 106.80 [27,38]
Appl. Sci. 2021, 11, 11363 9 of 27

Table 1. Cont.

Storage Technology Compression Liquefaction Chemical Ref.

Energy Density (LHV)
Gravimetric (MJ/kg) 120 120 18.60 [27,34]
Volumetric (MJ/L) 4.50 8.49 12.70 [27,34]
Transfer Mechanism
Catalytic
Discharge method Pressure release Evaporation decomposition [27,41]
(T = 400 ◦ C)
Extraction energy
- 0.91 30.60 [27,41]
(kJ/mol-H2 )
Miscellaneous
Ambient temperature
Safer and smaller
operation; low cost;
Ambient pressure storage; ambient
rapid (dis-)charge
Advantages operation; lighter temperature and [34,35]
cycles at wide range
cheaper storage tanks pressure operation;
of temperatures;
non-reactive
simple operation
Very high pressures High energy
Emerging technology,
needed; large and heavy consumption; very low
high cost; high
Disadvantages storage tanks required; temperatures needed; [38,41]
temperature
gas volatility; premature severe insulation
requirements
degradation of tanks requirements

Nowadays, the most advanced technologies for storing hydrogen are cryogenically
liquefied and compressed gas [38]; but these methods may not be completely suitable for
future widespread hydrogen applications, mainly due to leakage and safety concerns in
their pressurized form and energy requirements in the case of liquefaction. Even so, as
the push for environmentally friendly solutions is gaining traction, new technologies are
constantly being researched and developed to overcome these issues [38].

3. Hydrogen Production Technologies

Hydrogen has been produced, in one form or another, for a long time. Most of
that time, and to this day, it has been produced through environmentally unsustainable
methods; thus, one of the most important and urgent goals of the scientific community
today is to decarbonize hydrogen production.

3.1. Background Concepts

Some of these processes are briefly described below; they are further discussed with
more detail in Section 4 below.

3.1.1. Steam Methane Reforming

This is a process in which methane is heated, with steam (usually also with a catalyst),
to produce a mixture of carbon monoxide and hydrogen [44]. Methane, coming from
natural gas, reacts with steam under a pressure up to 25 bar, splitting into carbon monoxide
(later removed) and hydrogen molecules—as shown in Equation (1). Because this is an
endothermic reaction, heat must be supplied to the process for it to occur:
Heat
CH4 + H2 O −−−→ CO + 3 H2 (1)

3.1.2. Oil and Naphtha Reforming

Also known as catalytic reforming, this is a complex chemical process used to con-
vert petroleum refinery naphthas (distilled from crude oil) into high-octane liquid refor-
Appl. Sci. 2021, 11, 11363 10 of 27

mates, which are stocks for gasoline [45]. The process converts linear hydrocarbons into
branched alkanes and cyclic naphthenes, which are then partially dehydrogenated to pro-
duce high-octane aromatic hydrocarbons—and also significant amounts of hydrogen gas,
as a byproduct.

3.1.3. Coal Gasification

Coal is a chemically complex and highly variable substance, which can be converted
into a variety of products. The gasification process of coal is one method to produce power,
liquid fuels, chemicals, and hydrogen [46].
Specifically, hydrogen is produced by first reacting coal with oxygen and steam—under
high pressures and temperatures—to form synthesis gas (a mixture consisting primarily of
carbon monoxide and hydrogen), like is shown in Equation (2).

Heat + Pressure
2 CH + O2 + H2 O −−−−−−−−−−→ CO + 4 H2 + CO2 (2)

After removing impurities from the synthesis gas, the carbon monoxide present in the
gas mixture reacts again with steam to produce additional hydrogen and carbon dioxide,
following the reaction of Equation (3).

CO + H2 O −→ H2 + CO2 (3)

Hydrogen is removed in a separation system, and the highly-concentrated carbon

dioxide stream is subsequently captured and stored.

3.1.4. Biomass
Being a renewable organic resource, biomass usually includes forest and agriculture
crop residues and animal and other organic solid waste [47], and it can be used to produce
hydrogen, along with other byproducts, by gasification. As seen below, in Equation (4), this
process converts organic carbonaceous materials into carbon mono-/dioxide and hydrogen,
at high temperatures, without combustion and with a controlled amount of oxygen or
steam intake.

C6 H12 O6 + O2 + 2 H2 O −→ 2 CO + 4 CO2 + 8 H2 (4)

Note: Actual biomass has a highly variable composition and complexity, with cellulose being one major
component; the reaction above uses glucose as a substitute.

Carbon monoxide then reacts with water to form more carbon dioxide and more
hydrogen via a water–gas shift reaction (Equation (3)), and special membranes separate
the hydrogen from this gas stream.
Pyrolysis is a particular type of biomass gasification technology that uses no oxygen.
This is because, in general, biomass does not gasify as easily as coal, producing other
hydrocarbon compounds in the gas mixture exiting the gasifier.
As a result, an extra step must typically be taken to reform these hydrocarbons to
yield a clean syngas mixture of hydrogen, carbon monoxide, and carbon dioxide. Then,
just as in the gasification process for hydrogen production, a shift reaction happens (with
steam) that converts the carbon monoxide to carbon dioxide—hydrogen is produced and
then separated and purified.

3.1.5. Biological Hydrogen Production

Photobiological processes use live microorganisms and sunlight to turn water—and
sometimes organic matter—into hydrogen. In photolytic biological systems, microorgan-
isms such as green microalgae or cyanobacteria use sunlight as a mechanism to split water
into oxygen and hydrogen ions; these hydrogen ions are later combined through direct
or indirect routes and released as hydrogen gas [48]. Some photosynthetic microbes use
sunlight as the driver to break down organic matter, releasing hydrogen; this mechanism is
Appl. Sci. 2021, 11, 11363 11 of 27

commonly known as “photofermentative hydrogen production,” and some of its major

challenges include a very low production rate and a low solar-to-hydrogen efficiency—
something researchers are currently looking at, expecting to make the microbes better at
collecting and using energy, thus becoming more suited overall for hydrogen production.

3.1.6. Water Electrolysis

This method refers to the oldest method of producing hydrogen, dating back to the
19th century [49]; it generally refers to a DC electrical power source connected to two
electrodes, which are then placed in water. The flow of an electric current splits water
molecules into its constituents, making hydrogen appear at the cathode side and oxygen
appear at the anode side. The efficiency of electrolysis is increased through the addition of
an electrolyte to the solution (such as a salt, an acid, or a base) and the use of electrocatalysts,
which raise the reaction’s rate too.
The article now focuses on recent developments of this technology, discussing its
efficiency, durability, cost, and overall challenges.

3.2. Water Electrolysis Technologies

While there are many different methods to produce hydrogen, currently the vast
majority of the global hydrogen production comes from non-renewable fossil fuels—in
particular, steam reforming of methane—mainly due to its low cost and high efficiency.
However, these processes also tend to produce less-pure hydrogen, besides obviously
releasing harmful greenhouse gasses to the atmosphere [17].
So, today, new environmentally friendly energy strategies are sought to replace the
current system, namely through water electrolysis; this method enables the production of
eco-friendly, high-purity hydrogen, in addition to still releasing oxygen as a byproduct—as
seen in Equation (5).
Electricity + Heat 1
H2 O −−−−−−−−−−→ H2 + O2 (5)
2
However, water electrolysis is still not economically competitive due to high energy
consumption costs and low hydrogen yield rates [15]. In order to increase overall efficiency,
many researchers have been looking to develop alternatives with low-cost electrocatalysts
and less energy consumption.
Equation (5) shows that, in electrolysis processes, water molecules are the reactant,
which—under the influence of electricity and heat—dissociate into hydrogen and oxygen,
in an oxidation–reduction process. This process can be then classified into three main
types, based on its operating conditions, the electrolyte and electrolyzer used, and the ionic
agent present (OH− , H+ , O2− ): alkaline electrolysis (AEL), proton-exchange membrane
electrolysis (PEMEL), and solid oxide electrolysis (SOEL), respectively.

3.2.1. Alkaline Water Electrolysis

Hydrogen production by alkaline water electrolysis was first introduced by Troostwijk
and Diemann in 1789 [50], so it is by now a well-established technology. Figure 10 shows a
generic depiction of an alkaline water electrolyzer.
This electrolyzer is characterized by having two electrodes operating in a liquid
alkaline electrolyte solution, generally potassium or sodium hydroxide (KOH or NaOH,
respectively); the electrodes are kept apart by a diaphragm, where the transport of the
hydroxide ions (OH− ) occurs, from one electrode to the other [51].
So, the process initiates at the cathode, the site of reduction, where two molecules of
an alkaline water solution are reduced to one molecule of hydrogen (H2 ), and two hydroxyl
ions are produced (OH− ). The produced hydrogen emanates from the cathode surface
to recombine in a gaseous form, while the hydroxyl ions transfer through the porous
diaphragm to the anode, under the influence of the imposed electrical potential. Here a
Appl. Sci. 2021, 11, 11363 12 of 27

similar reaction occurs: two water molecules are oxidized, forming one diatomic oxygen
(O2 ) molecule and four hydrogen atoms. These half-reactions are shown below:

Cathode: 2 H2 O + 2 e− −→ H2 + 2 OH− (6)

1
Anode: 2 OH− −→ H2 O + O2 + 2 e− (7)
2
To obtain the overall cell equation, one just has to add both half-equations:

1
H2 O −→ H2 + O2 (8)
2
which is the same as Equation (5).

Figure 10. Schematic illustration of an alkaline water electrolyzer.

Alkaline electrolysis normally operates at low temperatures, around 30–80 ◦ C, and,

as seen, uses an aqueous solution as the electrolyte [52,53]. The diaphragm is usually
made of asbestos [54] and electrodes made of nickel; this diaphragm in the middle of
the cell is what separates the cathode side from the anode side, avoiding the mixing of
the produced gases [55]. This method does have some negative aspects, such as limited
current densities, low operating pressure, and low energy efficiency, making this type of
electrolyzer just suited to operate at almost constant power while connected to the grid; one
of the highest-powered electrolyzers of this type, however, has shown to have a dynamic
response fast enough to track the production of a renewable power plant [56].
An investment cost analysis from the past 30 years [57] revealed that capital expen-
diture (CAPEX) values for large alkaline electrolyzers can vary significantly over plant
capacity, ranging from EUR 1250 /kWel to around EUR 700/kWel —depending on whether
the plant is small (<1 MW) or large (>40 MW), respectively. The annual operational ex-
penditures (OPEX) are generally around 7% for medium-sized plants, but, due to superior
technical approaches and a higher lifespan, it is expected to decrease to 2% in 2030 [57].
Appl. Sci. 2021, 11, 11363 13 of 27

3.2.2. Proton-Exchange Membrane Electrolysis

The first PEMEL was idealized by Grubb in the early 1950s, having been later de-
veloped by General Electric Co. (Boston, MA, USA) in 1966 to overcome the drawbacks
of AEL [58–60]. Figure 11 displays a schematic illustration of a proton-exchange membrane
water electrolyzer, showing how the working principle of this technology is very similar
to that of PEM fuel cells—actually being the exact reverse—with solid polysulfated mem-
branes used as an electrolyte (proton conductor), through which the ionic agents move
during the process [61–63].

Figure 11. Schematic illustration of a proton-exchange membrane water electrolyzer.

Finally, the protons and electrons re-combine at the cathode side to produce hydrogen,
as shown in the following half-reactions:

1
Anode: H2 O −→ 2 H+ + O2 + 2 e − (9)
2
Cathode: 2 H+ + 2 e− −→ H2 (10)

These proton-exchange membranes have many important characteristics, such as low

gas permeability, high proton conductivity, small thickness, and the potential to operate at
high pressures and ambient temperatures. In terms of sustainability and environmental
impact, PEMEL is also found to be one of the most favorable methods for conversion
of renewable energy to highly pure hydrogen; this is mainly due to other promising
advantages like its compact design, high current density (meaning higher efficiencies),
fast response, and small footprint [63–66]. Additionally, PEMEL plants are very simple,
Appl. Sci. 2021, 11, 11363 14 of 27

which is more attractive for industrial applications; these applications might include
offshore wind parks [67], grid-independent/grid-assisted solar hydrogen generation and
grid-independent integrated solar hydrogen energy systems [68].
Electrocatalysts used in this method are usually noble metals such as platinum or
palladium for the cathode [65,69] and iridium/ruthenium oxide for the anode [70–73],
which makes the whole process more expensive than, say, alkaline water electrolysis. Here
water is accrued by being pumped on the anode side, where it is electrochemically split into
oxygen, hydrogen protons, and single electrons; these protons then travel via the proton-
exchange membrane to the cathode side, while the electrons exit from the anode through
the external power circuit, which provides the driving force to the chemical reaction.
So, one of the main challenges of proton-exchange membrane water electrolysis is to
reduce production cost while maintaining high efficiency. Substantial research has been
devoted to this matter, namely, to tackle issues like relative electrolyzer sizing, operation
intermittence, output pressure, oxygen generation, and water consumption. If such barriers
are overcome, together with a strong investment in R&D, PEMEL capital costs could see a
substantial reduction from around EUR 2000/kWel in 2020 to around EUR 900/kWel in
2030 [74]—with operational costs following the same path. The levelized cost of hydrogen
(LCOH) is also expected to decreased, especially over the increase of PEMEL plant scales; a
growth from 1 MW to 40 MW could represent a drop in LCOH values from EUR 7.37/kg
to EUR 4.49/kg [75].
Several authors have proposed a large number of different methods to increase the
efficiency of PEM water electrolysis [76], and, as a result, this technology is ever approach-
ing sustainable commercial market establishment [61]. Moreover, a new approach to this
method is currently under development, which promises to combine AEL’s low cost with
PEMEL’s high efficiency: anion-exchange membranes, made of polymers with anionic
conductivity, which are set to replace the asbestos diaphragm and help improve overall
electrolysis yield rates [77,78].

3.2.3. Solid Oxide Electrolysis

Dönitz and Erdle were the first to develop solid oxide electrolysis, in the 1980s [79,80].
This method has attracted significant attention due to the conversion process of electrical
into chemical energy, along with the high-efficiency production of pure hydrogen [61,81].
Solid oxide electrolysis operates at high pressures and temperatures, being novel by using
water in the form of steam—as seen in Figure 12, showing a schematic illustration of the
process; it conventionally uses O2− as the ionic agent, which mostly come from yttria-
stabilized zirconia [82].
SOEL’s operating principle is very similar to AEL’s, only slightly differing the half-
reaction equations:

Cathode: H2 O + 2 e− −→ H2 + O2− (11)

1
Anode: O2− −→ O2 + 2 e− (12)
2
Nowadays, some proton ceramic conducting materials have been studied to replace
regular ionic agents on solid oxide fuel cells, due to these showing higher thermodynamic
efficiency and superior ionic conductivity at the operating temperatures [83]. Proton
ceramic electrolysers could allegedly deliver pure dry hydrogen straight from steam, in this
way averting costly processes downstream—like further gas separation and compression.
Yet, the development of such technology has undergone some constraints linked to limited
electrical efficiency, mainly due to poor electrode kinetics and electronic leakage [84].
Proton ceramic electrochemical cells, on the other hand, produce hydrogen at interme-
diate temperatures through solid oxide proton conductors. Some reliable and highly robust
electrodes are needed to increase the electrochemical efficiency of this process, as well as
to ease the conduction of stable lower-temperature electrolysis; these cells, coupled with
custom catalysts and a specific ceramic architecture could operate reversibly with great
Appl. Sci. 2021, 11, 11363 15 of 27

performance [85]. Recent studies [86] have successfully achieved self-sustainable reversible
hydrogen operations, having confirmedly credited the remarkable electrocatalytic activity
to superior proton conduction. This lower-temperature operation grants a set of numerous
benefits, namely, lower heat losses, the possibility of using lower-heat-grade materials, and
reduced capital costs due to a decrease in surface-area needs [87]. Others confirmed this
trend [88], showing how PCECs can perform with extremely high Faradaic efficiencies
and low long-term degradation, while inherently providing CO2 sequestration and H2
with purity levels suited for natural gas use—presenting as a very positive alternative
to conventional electrolysis. Besides, insufficient long-term stability leading to serious
deterioration caused by electrolysis—which was considered to be irreversible before—has
been found to be completely eliminated through reversible cycling between electrolysis
and fuel-cell modes [89].
Solid oxide electrolysis thus presents as an advantageous method to produce hydro-
gen, although still having some issues preventing it to be commercialized on a large scale,
namely related to a lack of stability, degradation, and very high temperatures require-
ments [90–92]. This is also why it is especially not adequate for coupling with intermittent
power sources but more with nuclear or combined cycle power plants [67].

Figure 12. Schematic illustration of a solid oxide water electrolyzer.

Currently, SOEL capital costs still fluctuate considerably and are quite uncertain,
mainly due to its pre-commercial status; although being surely situated above
EUR 3000/kWel [93], experts suggest that solid oxide systems could experience the strongest
Appl. Sci. 2021, 11, 11363 16 of 27

relative cost reduction by 2030, reaching values as low as EUR 750/kWel by 2030 with
production scale-up [74].
Table 2 shows a summary comparison between all the processes described so far,
analyzing different aspects of each technology—from operation to economic parameters
and from system details to some nominal features.

Table 2. Characteristics of different water electrolysis technologies. Adapted from [76].

Characteristics AEL 1 PEMEL 2 SOEL 3 Ref.

Operation Parameters
Temperature (◦ C) 40–90 20–100 650–1000 [79,90]
Pressure (bar) <30 <200 <20 [93,94]
Current density (A/cm2 ) 0.20–0.40 0.60–2.00 0.30–2.00 [79,93]
Voltage (V) 1.80–2.40 1.80–2.20 0.70–1.50 [90,93]
Nominal Features
Cell area (m2 ) <4 <0.13 <0.06 [93,94]
Production rate (m3 /h) <1400 <400 <10 [93]
Gas purity (%) >99.50 >99.99 >99.90 [77,94]
System Details
Energy consumption
∼5.55 ∼5.40 ∼3.80 [93,95]
(kWh/m3 )
Efficiency (%) 51–60 46–60 76–81 [93]
Stack lifetime (kh) 60–120 60–100 8–20 [79,95]
Degradation (%/y) 0.25–1.50 0.50–2.50 3–50 [93]
Economic Parameters
Capital cost
740–1390 1300–2140 >2000 [93,95]
(EUR/kWh)
Maintenance cost
2–3 3–5 n.a. [93]
(% of investment/year)
Miscellaneous
High efficiency; low
Low capital cost; cheap Highest purity; compact
pressure; low energy
Advantages catalysts; high durabil- design; high production [83,96]
consumption; no need
ity; stable operation rate
for noble metal catalyst
High cost of rare Large design;
Corrosive system; low-
components; low durability;
Disadvantages est purity; high energy [97,98]
acidic environment; small cell area;
consumption
high pressure high temperature
Maturity Commercial Near commercial Demonstration [74,99]
1 2 3
Alkalyne electrolysis; proton-exchange membrane electrolysis; solid oxide electrolysis. Note: The characteristics reported in this
table are not inherent to each technology but merely what has been demonstrated (among the cited research). Often, the most mature
technologies are the ones most developed, thus seeming like they have better inherent properties; that might not be the case—for instance,
cell area is mainly dependent on development state, so it is safe to assume that it will naturally increase with time for PEMEL and SOEL.

4. State-of-the-Art Review
Green hydrogen is one of the most promising clean and sustainable energy carriers,
emitting only water as a byproduct of its production and no carbon emissions [100]. Having
many attractive properties as an energy carrier, namely, a high energy density (which
is more than double that of typical solid fuels [99]), hydrogen is mostly used today in
industrial applications such as fertilizers [101], petroleum refining processes [102], chemical
and petrochemical industries [103,104], and fuel cells.
Several authors have previously studied and described the various forms in which
hydrogen can be produced, from renewable and non-renewable energy resources.
Appl. Sci. 2021, 11, 11363 17 of 27

4.1. Steam Methane Reforming

A. Boyano et al. argue that steam methane reforming is one of the most promising pro-
cesses for hydrogen production [105]. They performed a study on SMR from the viewpoint
of overall environmental impact, using an exergo-environmental analysis (the combination
of exergy analysis and life cycle assessment), having found that the components in which
chemical reactions occur tend to have high exergy destruction. The analysis further shows
that the environmental impact of exergy destruction within all components in the SMR
plant is significantly higher than the single-component environmental impact—meaning
that the overall impact can be reduced simply by reducing the exergy destruction within
the components. The downsize is then the request for more efficient and modern equip-
ment, which is expensive; thus, the authors refer to an exergo-economic analysis [106,107]
conducted in parallel, to provide important information on cost reduction.
Other studies analyzed the possibility of increasing the efficiency of steam methane
reforming. Jing Xu et al. studied the hypothesis of adding boron atoms into the reaction
to increase the stability of nickel catalysts; they ended up achieving a 5% increase in the
conversion efficiency [108]. Lightheart et al., in turn, addressed the influence of particle
size on the activity and stability in SMR of supported rhodium nanoparticles; the study
concluded that catalysts with rhodium nanoparticles smaller than 2.5 nm deactivate more
strongly than catalysts with larger nanoparticles [109].
Moreover, Harald Malerød-Fjeld and his co-authors presented a protonic membrane
reformer that yields productions of high-purity electrochemically compressed hydrogen
from SMR—in a single-stage process with almost no energy loss [110]. This technology
has shown to allow great intensification of compressed hydrogen production with high
energy efficiency and very low net carbon emission, particularly when renewable electricity
is used. Furthermore, as this technology is scalabe, when performed in locations with
access to effective carbon capture, protonic membrane reformers also allow for a true
distributed carbon neutral footprint by producing an almost pure carbon dioxide stream as
a byproduct.

4.2. Oil and Naphtha Reforming

On the other hand, R. Trane and his co-authors state that hydrogen can be produced
in an environmentally friendly and sustainable method through steam reforming of bio-
oil—although it is still in an early phase of its development [111]. Many different catalytic
systems were investigated, and the most promising metals, like we saw for SMR, seem to
be nickel, rhodium, or ruthenium. This study then concludes that, if to be used industrially,
steam reforming of bio-oil needs further investigation and optimization.
D. Iranshahi et al. state that refineries can be considered as alternative sources of
hydrogen production, and they proposed a novel membrane technology configuration for
a radial-flow naphtha reformer [112]. They investigated and compared different types of
tubular membrane reactors (TMR), as well as the effect of varying certain parameters, and
found that radial-flow TMR improves by applying a pattern that lessens the pressure drop,
leading to an increase in percentage conversion of reactants and products yield. With this,
the team managed to increase the hydrogen production efficiency by 1.50%.
In an effort to gather the scattered information of several articles published on this
subject, M. Rahimpour, M. Jafari, and D. Iranshahi collected a series of studies regarding
catalytic naphtha-reforming processes [113] and concluded that, in general, the reactors
used today are either tubular or spherical, and the feedstock may flow in axial or radial
direction; additionally, to improve the performance of the whole process, thus increasing
the output and reducing waste, all new units are to be designed based on continuous
catalyst regeneration (CCR) reformers.

4.3. Coal Gasification

In the continuous demand for more efficient and more environmentally friendly meth-
ods to produce hydrogen, S. S. Seyitoglu and his co-authors point to coal gasification [114].
Appl. Sci. 2021, 11, 11363 18 of 27

This thermochemistry transformation generates gas from coal [115], i.e., converts solid fuel
to gas fuel; the aim of this process is mainly to decrease harmful emission occurring during
the traditional burning of coal and also to increase the fuel’s density. Their study in partic-
ular analyzes the gasification process’ performance of different types of coal, concluding
that the gasification of Tuncbilek coal followed by Soma coal provide the highest energy
efficiency processes, with 41% and 38%, respectively. Other studies [116] have analyzed
the impact of moisture contents on the gasification process, concluding through numerical
simulations that coal gasification time increases with increasing moisture content—since
high moisture content causes a decrease in temperature, which reduces the reaction rates.
Piotr Burmistrz et al. have gone a step further and carried out a deep analysis of
the carbon footprint of hydrogen production from sub-bituminous coal and lignite, using
two gasification technologies—GE Energy/Texaco and Shell [117]. Among the analyzed
variants of hydrogen production, sub-bituminous coal gasified with Shell technology was
the one holding the lowest carbon footprint, at around 19 kg CO2e /kg H2 ; on the other
hand, Shell technology used to gasify lignite held the highest, at 25.30 kg CO2e /kg H2 . This
technology was included in this analysis despite not being renewable—and not comparable
with SMR, which can already be used to produce blue hydrogen—because it is on the verge
of doing it too: as expected, the authors concluded that the use of capture and sequestration
of CO2 decreases the overall carbon footprint of all the processes.
J. Huang and I. Dincer take yet another approach and, in their study, conducted a para-
metric study to find the best steam-to-carbon ratio that yields the maximum performance of
an integrated gasifier system for hydrogen production [118]. They found evidence that, in
general, increasing this value makes the system work at its most optimal performance; at a
0.9 steam-to-carbon ratio, the maximum energy efficiency is reached: 53.80%. The authors
then conclude that further increasing the proportion does not yield much more performance
improvements (only incurring in higher costs). The same conclusion is reached regarding
ambient temperature—it is best to operate this system in low-temperature climate areas. If
that is achieved, the authors state, gasification of coal presents itself as the cleanest and
most efficient method of utilizing coal for hydrogen production.

4.4. Biomass
Y. Kalinci and his co-authors took a different route and chose to review the various
processes for conversion of biomass into hydrogen, first dividing them into two main
groups: thermo-chemical processes and biological conversions [119]. They went on and
discussed the various systems in terms of their energetic and exergetic aspects and also
summarized potential methods for comparison purposes. Carrying out a simulation with
a wide range of pressure and temperature conditions brought as a conclusion that the
maximum energy efficiency values for the gasification reaction is around 46.54%.
B. Zhao et al. chose to address the impact of temperature on biomass combustion and
gasification, in terms of SO2 /NOx emissions [120]. They found that, for three different
algae biomass species, both emissions increase with an increase in combustion temperature;
particularly, NOx peak formation was further accelerated with this increase in temperature.
On the other hand, SO2 emissions were significantly higher at 900 ◦ C when compared with
700 ◦ C and 800 ◦ C, but no second-peak formation was particularly relevant.
M. Mujeebu explored hydrogen and syngas production by superadiabatic combus-
tion (SAC), stating at the outset that, at present, the most effective method of hydrogen
production is the conversion of the hydrocarbon sources [121]. The author deduces that
even though there are diverse kinds of techniques being explored for hydrogen production,
unfortunately thermal reforming of methane and other fossil fuels (seen before) will still
continue, until alternative clean technologies are popularized. Superadiabatic combustion
of biomass may just be one of those alternatives, as decomposition and biomass gasification
has demonstrated excellent performance. However, M. Mujeebu concludes, research has yet
a long way to go before materializing SAC systems for practical applications—particularly
Appl. Sci. 2021, 11, 11363 19 of 27

by considering the risks associated with storage and transportation of hydrogen (the reason
why onsite production is receiving more attention).
A. Abuadala and I. Dincer have conducted a detailed review in their study [122],
discussing mainly sawdust wood biomass-based hydrogen production systems and their
applications. They performed a comprehensive sensitivity analysis on the hydrogen
yield from steam biomass gasification, concluding in general that there are various key
parameters affecting the hydrogen production process and system performance: pressure,
temperature, current density, and the fuel utilization factor. At a particular set of values for
these parameters, the authors found a strong potential to increase energy efficiency from
45% to 55%.

4.5. Biological Sources

D. Das and his co-author state that, adding to hydrogen being the fuel of the future
mainly due to its high conversion efficiency, recyclability, and nonpolluting nature, bi-
ological hydrogen production processes (mostly controlled by either photosynthetic or
fermentative organisms) are more environment friendly and less energy intensive as com-
pared to thermochemical and electrochemical processes [123]. They concluded that the
rate of fermentative hydrogen production is always faster than that of the photosynthetic
production. They found that most of the biological processes are operated at ambient
temperature and pressure, so rgwt are not energy-intensive processes.
O. Elsharnouby et al. defend biohydrogen as having the potential to replace current
hydrogen production technologies heavily relying on fossil fuels [124]. In their study,
it was found that attaining technical and economic efficiencies is the main drive behind
employing co-cultures of pure bacteria in fermentative hydrogen production. These are the
ones with the best performance at generation rates, although it is essential to first determine
specific optimal operational conditions. Additionally, as seen in the previous study, side
products of biohydrogen can be useful too; biodiesel wastes, oil industry wastewaters, and
microalgal biomass have significant potential as sustainable feed stocks.
However, the usefulness and practical application of biohydrogen to everyday energy
problems is still unclear, according to D.B. Levin et al. [125]. By first standardizing the units
of hydrogen production, the authors intended to calculate the size of biohydrogen systems
that would be required to power proton-exchange-membrane fuel cells of various sizes.
They undoubtedly concluded that biohydrogen technologies are still in their infancy,
and if they are to become commercially competitive with steam reforming and electrolysis,
they must be able to synthesize hydrogen at rates that are sufficient to power reasonably
sized fuel cells. So, further research and development aimed at increasing the rates of
synthesis, optimizing bioreactor designs to rapidly remove and purify side-product gases,
genetically modify enzyme pathways, and increasing overall final yields of hydrogen
productionareis essential.

4.6. Water Electrolysis

F. Barbir approaches the subject of water electrolysis, especially PEMEL, as a viable
alternative to produce hydrogen from renewable energy sources; in his study [68], several
possible applications are discussed, including grid-independent and grid-assisted hydrogen
generation, the use of support-electrolyzers, and integrated systems where electrolytically
generated hydrogen is stored and then—via fuel cell—converted back to electricity when
needed. It goes even deeper, by addressing specific issues regarding the use of PEMEL
electrolyzers in renewable energy systems, such as sizing of equipment, the issue of inter-
mittent operation, output pressure, oxygen generation, water consumption, and, naturally,
efficiency. His findings are indeed interesting: PEMEL is a viable alternative for hydrogen
generation in conjunction with renewable energy sources, particularly solar photovoltaics.
PEMEL electrolyzers are simpler than conventional alkaline electrolyzers, being able to
generate hydrogen (and optionally even oxygen) at pressures up to 200 bar with very little
additional power consumption (which may be attractive for applications where it needs to
Appl. Sci. 2021, 11, 11363 20 of 27

be stored). This method is also capable of producing hydrogen with very high purity, at
very high efficiencies—between 70% and 90% depending on the generation rate.
O. Atlam and M. Kolhe decided to approach this thematic from another perspective,
developing an electrical equivalent model for a PEMEL electrolyser [126]. Using experi-
mental results, the authors managed to model the input current–voltage characteristic for a
single PEM electrolyser cell under steady-state conditions; useful power conversion and
losses were taken into account, following Faraday’s Law. They found that the developed
model matches very closely the experimental results in the active operating electrolysis
region, and, using the developed model and a simplified equivalent circuit, the hydrogen
production rate and electrolysis efficiency can be estimated. It was observed that the
hydrogen production rate is proportional to the input current, and efficiency decreases
with input voltage, being up to 68% in this study.
M. Balat agrees on hydrogen as a future energy carrier having a number of advan-
tages [35]. One of them is that it can be produced from a variety of primary resources,
through water electrolysis; another important advantage is that its only major oxidation
product is water vapor—so its use produces no CO2 , if generated from renewable energy
sources and nuclear energy. The author asserts that hydrogen also has good properties
as a fuel for internal combustion engines in automobiles, being able to be used as a fuel
directly (not much different from engines using gasoline nowadays). The main problem
here is that while hydrogen supplies three times the energy per kilogram of gasoline, it has
only one tenth the density (when in a liquid form—very much less when it is stored as a
compressed gas).
N.A. Burton and his co-authors presented an extensive literature review on increasing
the efficiency of hydrogen production, stating that “although hydrogen presents an excel-
lent option as an energy carrier, much of hydrogen’s current uses are based on its ability to
chemically react with other molecules” [76]. Some examples its uses as a reactant include
petroleum processing, the production of petrochemicals, and the process for recycling
plastics [127]. Besides, over 96% of the presently produced hydrogen is still generated
using fossil fuels, only 4% coming from commercial electrolysis (yet with low efficiency
and high production costs).
J. Joy, J. Mathew, and S. C. George have studied the impact of nanomaterials in
photoelectrochemical water splitting, a technique that could effectively couple solar energy
with hydrogen production [128]. This promising recent technology has the potential
to become an easy, cheap, and sustainable method of generating hydrogen, simply by
adjusting the bandwidth of the photocatalyst material. By regulating the size and shape
of their structure, materials such as nanotubes, nanowires, nanorods, and nanosheets can
boost the overall conversion from solar light to hydrogen in terms of energy efficiency;
with the inclusion of these nanomaterials in semiconductors, one observes a clear increase
in the absorption of solar light. The biggest drawback of such technology resides in its
efficiency—which is still very low—and the need to develop cost-effective materials to
overcome said performance.
Finally, S. A. Grigoriev et al., in their study regarding current status and research
trends in water electrolysis science and technology, give us the future outlook of the next
generation of electrolyzers: increasing the operating current density while improving
efficiency [129]. Ideally, the authors believe water electrolyzers could be used for grid-
balancing services and energy storage systems, with market applications foreseen in the
short-term period. Artificial intelligence and neural network methods could even be
used for efficiently designing, planning, and controlling the operation of these types of
systems, something that poses very interesting and daring challenges for the future of these
technologies [130–132].
The various hydrogen production methods along with their advantages, disadvan-
tages, efficiency, and capital costs—based on the literature review done so far—are provided
in Table 3.
Appl. Sci. 2021, 11, 11363 21 of 27

Table 3. Hydrogen production technologies summary.

Production Efficiency Cost

Advantages Disadvantages Ref.
Method (%) (EUR/kg)
Hydrocarbon Reforming
Steam methane Developed technology; Unstable supply; production
74–85 1.90 [105–109]
reforming existing infrastructure of greenhouse gases
Production of heavy oils and
Partial oxidation Established technology 60–75 1.24 [111]
petroleum coke
Use of fossil fuels;
Auto-thermal Well established technology;
production of greenhouse 60–75 1.24 [112,113]
reforming existing infrastructure
gases
Hydrocarbon Gasification
Fluctuating H2 yields
Abundant, cheap feedstock; because of feedstock impu-
Coal gasification 35–55 1.60 [114,115]
less GHG emissions rities, seasonal availability
and formation of tar
↑ Non-Renewable ↑
↓ Renewable ↓
Thermochemical Biomass Processes
Tar formation; fluctuating
Abundant; cheap feedstock; H2 amount because of feed-
Pyrolysis 35–50 1.38 [119]
CO2 -neutral stock impurities; seasonal
availability
Biological Biomass Processes
Low yields of H2 ; sunlight
Consumption of CO2 ;
Bio-photolysis required; large reactor 10–11 1.79 [119]
production of O2
required; high cost
Simple method; no light
Fatty acids elimination; large
Dark fermentation required; CO2 -neutral; waste 60–80 2.15 [121]
reactor required
recycling
Low efficiency; low
Photo Waste water recycling; production rates, large
10 2.37 [122]
fermentation CO2 -neutral reactor and sunlight re-
quired
Water Splitting
Clean and sustainable;
High capital costs; element’s
Thermolysis production of O2 ; abundant 20–45 6.96 [122]
toxicity; corrosion problems
feedstock
Low efficiency, non-effective
Production of O2 ; abundant
Photolysis photocatalytic material; 6 7.54 [121]
feedstock; no emissions
sunlight required
Established technology;
zero-emissions; Storage and transportation
Electrolysis 60–80 8.63 [68,126]
existing infrastructure; problem
production of O2

5. Conclusions
The development of renewable hydrogen production technologies is a vital step
moving forward into a truly sustainable human existence; the use of renewable resources
for energy generation is pivotal. Although renewable hydrogen production technologies
have made some very important advances lately—increasing its feasibility as a broad-
scale energy generation method—there remains the need to develop methods with greater
Appl. Sci. 2021, 11, 11363 22 of 27

efficiency for them to be economically competitive with current hydrogen production

methods sourced on fossil fuels.
In this review, a short introduction was made about the various strategic configurations
for the hydrogen value chain and the best storage systems currently available. Some
background concepts were discussed regarding hydrogen production technologies, with
particular attention given to water electrolysis. In addition, research related to alkaline
water, solid oxide, and proton-exchange membrane electrolysis was deepened, giving focus
to their respective characteristics, operational and economical parameters, nominal features,
system details, advantages/disadvantages, and market maturity. The contributions of
this study come not only from the discussion of the present state of the art but also from
the elaboration of the in-depth investigation of historical research, challenges, and recent
achievements on several renewable and non-renewable hydrogen production techniques,
from established steam, methane/oil, and naphtha reforming and coal gasification to
developing ones such as from biomass and biological sources. The main issue with these
emerging technologies, it was concluded, has to do with efficiency; so, further research into
the enhancement of biomass processes like bio-photolysis, dark-/photo-fermentation, and
pyrolosis but also water-splitting processes such as thermolysis, photolysis, and mainly
electrolysis will be essential to increase the efficiency of renewable hydrogen production.
As highlighted in this study, future research on electrolysis in particular should seek to
resolve issues related to the cost of some rare materials used, the acidity and corrosiveness
of the environment in which it occurs, the high operating temperatures and pressures
needed, as well as improving the durability of the components and increasing the cells’
area—thus increasing current densities and production rates.
Subjects of interest to the scientific community will be, for instance, to dive deeper
into the field of anion-exchange membranes, made of polymers with anionic conductivity,
or microbial electrolysis, where hydrogen is produced from organic matter (including
renewable biomass and wastewaters) through microbe oxidation. Moreover, possible future
outcomes of these studies may include the fields or artificial intelligence and computer
neural networks, which could also find some use in sustainable hydrogen production,
helping design and plan the operation of such processes in a more efficient way.

Author Contributions: Conceptualization, L.V. and R.C.; methodology, L.V. and R.C.; software,
L.V.; validation, L.V. and R.C.; formal analysis, L.V. and R.C.; investigation, L.V.; resources, L.V.;
data curation, L.V.; writing—original draft preparation, L.V.; writing—review and editing, R.C.;
visualization, L.V. and R.C.; supervision, R.C.; project administration, R.C.; funding acquisition, R.C.
All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Fundação para a Ciência e a Tecnologia (FCT), grant number
UIDB/50021/2020.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or
in the decision to publish the results.

Abbreviations
The following abbreviations are used in this manuscript:

P2G Power-to-gas
P2M Power-to-mobility
P2I Power-to-industry
P2FUEL Power-to-synfuel
Appl. Sci. 2021, 11, 11363 23 of 27

P2P Power-to-power
AEL Alkaline electrolysis
PEMEL Proton-exchange membrane electrolysis
SOEL Solid oxide electrolysis
CAPEX Capital expenditures
OPEX Operational expenditures
LCOH Levelized cost of hydrogen
SMR Steam methane reforming
TMR Tubular membrane reactors
CCR Continuous catalyst regeneration
SAC Super adiabatic combustion

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