PC Eif VNK

https://dx.doi.org/10.21577/0103-5053.
20220052
J. Braz. Chem. Soc., Vol. 33, No. 8, 844-869, 2022

©2022 Sociedade Brasileira de Química Review
Technological Perspectives and Economic Aspects of Green Hydrogen in the

Energetic Transition: Challenges for Chemistry
Lilian L. N. Guarieiro,a,b,c Jeancarlo P. dos Anjos,b,c Luciana A. da Silva, c,d
Alex Á. B. Santos,b Ewerton E. S. Calixto,b Fernando L. P. Pessoa,b

Jose L. G. de Almeida,b Miguel Andrade Filho,b Flavio S. Marinho, b
Gisele O. da Rochac,d,e and Jailson B. de Andrade *,a,b,c,d
a
Instituto de Estudos Avançados, Centro Universitário SENAI CIMATEC, 41650-010 Salvador-BA, Brazil
b
Centro Universitário SENAI CIMATEC, 41650-010 Salvador-BA, Brazil
c
Instituto Nacional de Ciência e Tecnologia em Energia e Ambiente (INCT),
Universidade Federal da Bahia (UFBA), 40170-110 Salvador-BA, Brazil
d
Instituto de Química, Universidade Federal da Bahia (UFBA), Campus Ondina,
40170-270 Salvador-BA, Brazil
e
Centro Interdisciplinar em Energia e Ambiente (CIEnAm), Universidade Federal da Bahia (UFBA),
40170-115 Salvador-BA, Brazil
Green hydrogen is a fuel capable of promoting sustainable energy development and is gaining
attention in the current global energy transition framework. The global shift toward decarbonization
has triggered a substantial boost in the hydrogen industry. This study presents an overview of the
current status of hydrogen production, regulation of the green hydrogen segment, hydrogen storage,
distribution, and transportation, and final use and application. We also critically discuss the viability
of adopting green hydrogen in terms of possible economic and environmental impacts as well as
the main challenges and opportunities it represents for the Chemical Sciences.
Keywords: green hydrogen, energy transition, sustainable development, decarbonization,

defossilization, clean and affordable energy
1. Introduction hydrogen (H2) industry. This industry, in turn, offers several

alternatives for the use of hydrogen in the transition toward
Since the Paris Climate Agreement in 2015, governments a low-carbon economy: as an energy carrier and storage
and businesses worldwide have stepped up, taking the medium for conversion back to electricity, as a fuel for
actions needed to reach a global average temperature all modes of transport and mobility, and as a potential
target well below 2 °C above the pre-industrial levels.1 substitute for fossil hydrocarbons in industries such as steel
Several initiatives are related to reducing greenhouse gas and petrochemicals.4 H2 has gained importance because it
(GHG) emissions (CO2, CO, NOx, among others) that can is a carbon-free fuel with high heating value and a high
occur with the use of technologies aimed at decarbonizing energy carrier with less volume, which can significantly
various sectors. Many initiatives have been taken to solve reduce carbon emissions.4
this problem, including encouraging and using biofuels In the last ten years, there has been an apparent
such as biodiesel2,3 and ethanol. However, green H2 has increase in interest in research on hydrogen. From 2010
been standing out in recent years due to its properties to 2021, the number of published studies on hydrogen
and application possibilities. The global shift toward more than doubled (Figure 1). VOSviewer software 6
decarbonization has triggered a substantial boost in the was used to construct networks of scientific publications
about “hydrogen” through a search on the Science Direct
*e-mail: jailsondeandrade@gmail.com platform5 clusters which were built using the words used in
Editor handled this article: Luiz Ramos (Guest) the titles and abstracts of the works obtained on hydrogen
Vol. 33, No. 8, 2022 Guarieiro et al. 845
(Figure 2a). The formed word clusters show the trends in

hydrogen research: as a renewable energy source (signed
in red; Figure S1, Supplementary Information (SI) section),
as a reagent or product of chemical and physical reactions
(green color; Figure S2, SI section), its use in photocatalytic
production methods (dark blue; Figure S3, SI section), its
use in engines and impact on pollutant emissions (yellow;
Figure S4, SI section), its interaction with other materials
(purple; Figure S5, SI section), its production through
electrochemical reactions using electrocatalysts (light
blue; Figure S6, SI section), and ways of storage (orange;
Figure S7, SI section). It is interesting to observe in
Figure 1. Number of publications on hydrogen per year from January 2010
Figure 2b an organization of the clusters formed in relation to September 2021. Source: Science Direct Platform5 using “hydrogen”
to the years of publication. The most consolidated themes as keyword.
on hydrogen are closer to the yellow clusters, and the state-
of-the-art research is the purple clusters. It means that the with a single proton was the precursor for all other nuclei.
topics covered in this review are at the frontier of knowledge Hydrogen is, by far, the most abundant chemical element
about hydrogen, which will be discussed in detail below. in the universe, accounting for around 90% of the matter.
It is already well established that the stars’ radiant energy
2. The Hydrogen Atom and Molecule derives from internal sources caused by nuclear events in
which lighter nuclei as protium are fused to create heavier
After the Big Bang, the universe was populated by nuclei releasing the energy stored in the nucleus. In nature,
particles such as protons, neutrons, and electrons. In the the most common type of explosion with nuclear fusion
Nucleosynthesis Era, a sequence of thermonuclear reactions is the hydrogen-rich core-collapse supernovae, known as
rapidly combined protons and neutrons to give rise to type II supernovae, observed in star-forming galaxies by
deuterium nuclei and then helium.7 So, the hydrogen nucleus the presence of prominent hydrogen lines in their spectra.8,9
Figure 2. (a) Seven clusters of 725 items using “hydrogen” as the search term and (b) clusters of the evolution of terms over time, which the yellow
represents the most consolidated topics.
846 Technological Perspectives and Economic Aspects of Green Hydrogen in the Energetic Transition J. Braz. Chem. Soc.
On Earth, hydrogen is the third most abundant element on internal combustion device powered by a mixture of
the surface when considering the number of atoms and hydrogen and oxygen. The combustion reaction produces
the tenth most abundant by mass. Hydrogen is found in water and releases –286 kJ mol–1 of energy, equivalent to
minerals, oceans, fossil fuels, and all living organisms. the energy content of ca. 140 MJ kg–1, which is greater
Hydrogen predominantly occurs as protium, an isotope than any other fuel.12
with a single proton in the nucleus (11H or P), accounting The cold electrochemical combustion of hydrogen,
for 99.986%, followed by deuterium atoms (12H or D), in or any other gaseous fuel, is the principle of a fuel cell
which the nucleus also contains a neutron, accounting for discovered by Sir Grove more than 180 years ago. However,
0.014%. The addition of a second neutron in the nucleus commercial interest only started in the 1960s, when NASA
forms tritium (31H or T), a very unstable isotope. used fuel cells to provide electrical power for spacecraft in
Hydrogen was first isolated and identified by Henry the space program.13
Cavendish10 in 1766 by reacting several acids with iron, A direct current is produced in a fuel cell when
zinc, and tin, showing that it was much lighter than air. hydrogen is oxidized to protons in a gaseous diffusion
Hydrogen was already part of Lavoisier’s table of 33 electrode, releasing electrons to the opposite gaseous
elements, published in his book, Traité Élémentaire de diffusion electrode, where oxygen is reduced, with water as
Chimie, in 1789.11 Currently, hydrogen occupies the first the final product. The device converts the chemical energy
position in the periodic table of the elements with the of the hydrogen fuel into electricity. It will continue to
simplest electronic configuration, 1s1, with a small radius generate electricity as long as external fuel and an oxidant
allowing to form more compounds than any other element. are supplied and do not require recharging. In this way,
When hydrogen is combined as a diatomic molecule, the versatility of hydrogen in energy conversion with zero
it forms dihydrogen (H2), a stable, colorless, odorless, carbon emission makes it a perfect energy vector when
tasteless, and nontoxic gas. As a homonuclear diatomic extracted from a decarbonized source. Although it has a
molecule, dihydrogen shows nuclear spin isomers, flame temperature similar to other fuels, under the presence
ortho- and para-hydrogen, with parallel and antiparallel of ambient oxygen, its propagation, buoyancy, and laminar
spins, respectively. The different internal energies in burning speed are higher compared with other fuels.14
o-H2 and p-H2 lead to significant differences in physical Among the main advantages of the use of hydrogen
properties. At low temperatures, the lower energy p-H2 is its property of producing carbon-free emissions during
state is favored, and the equilibrium concentration of o-H2 combustion. In addition to its high calorific power, hydrogen
gradually increases until it reaches room temperature, is the lightest reactive gas, making it very economical to be
when it contains 75% o-H2 and 25% p-H2.7 The main produced from specific routes, becoming an attractive green
physical property affected by the nuclear-spin isomerism of energy alternative compared with fossil fuels.15
hydrogen is thermal conductivity, which is more than 50%
greater than for the p-H2. Dihydrogen is also a nonpolar and 3. Possible Sources of Hydrogen
poorly polarizable molecule, implying virtually nonexistent
intermolecular forces. In nature, hydrogen is associated with other elements
Thus, molecular hydrogen is a gas in the standard state such as oxygen in water, carbon in hydrocarbons, and other
with a very low boiling point. When compressed, the gas elements in biomass. Nature has developed its own set of
deviates from the ideal behavior, with a compressibility catalysts to produce hydrogen or use it as an energy source,
such as hydrogenases. A hypothesis is that the genesis of
factor higher than 1 (Z > 1, where , P = pressure;
hydrogenase enzymes probably occurred when the Earth
V = volume; T = temperature; R = universal gas constant), had a hydrogen-rich atmosphere, during the first stages
indicating that the H2 molecules repel each other when they of life on our planet, and existing primitive organisms
are too close. In addition to the low density, these properties depended on this molecule as an energy source. This
make it challenging to store hydrogen gas. Table S1 (SI enzyme is found in many microorganisms, mainly archaea,
section) summarizes some important physical properties and bacteria, but also in some eukaryotic organisms.16
of hydrogen gas. Conventional H 2 production technologies include
The energy within the chemical bond between the steam reforming of natural gas (methane) and petroleum,
hydrogen atoms (Table S1) can easily be transformed into catalytic decomposition of natural gas, partial oxidation
either heat or electricity. In Lavoisier’s table, hydrogen of heavy hydrocarbons, and coal or coke gasification.
was already described as flammable gas, and in the early These technologies are energy-demanding since high
19th century, François Isaac de Rivaz developed the first temperatures are applied. In addition, 96% of these H2
sources come from fossil fuels.14,17 Nowadays, the most are biomass and biogas, which lead to CO2 formation in
used and affordable technology to produce H2 is natural gasification and biofuel reforming processes, among others.
gas reforming, which will likely persist in the near future, However, there are cleaner processes (e.g., electrolysis) that
although it is linked to various GHG emissions, including can use a renewable energy source and use it to separate the
carbon dioxide and NOx.14 H2 present in water without forming CO2. Considering how
For a long time, H2 has been produced using fossil clean a particular technology is in relation to the H2 and
fuels as its primary raw material. A classic example is the CO2 formed has motivated several discussions worldwide
steam reforming of natural gas (NG), which accounts for to establish international standards that indicate acceptable
48% of global production.16,18 In this process, the methane limits of CO2 emissions in the production of H2 by different
(CH4) present in greater amount in NG reacts with water technological routes.20
(steam) and produces, in addition to H2, carbon monoxide The possibility of obtaining “clean” or renewable H2
(CO), in a second reaction (water-gas shift) reacts with with zero or minimal GHG emissions led to the designation
steam producing carbon dioxide (CO2) and hydrogen “green H2”. The first reference on this subject is a 1995
(equations 1 and 2). publication21 that used the term “renewable hydrogen”
(hydrogen produced from renewables). As early as 2006,
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g), California22 defined green H2 as the one produced cleanly
ΔH0 = +206 kJ mol–1 CH4 (1) and sustainably, using renewable energy sources such
CO(g) + H2O(g) ⇌ CO2(g) + H2(g), as solar or wind. The first mention of green H2 in the
ΔH0 = –41 kJ mol–1 CH4 (2) European Union (EU) was in a document establishing
the green H2 economy in Europe.23 It is possible to find
On the other hand, other methods can be used to produce several definitions of green H2 in the literature, according
hydrogen with low carbon content, such as thermochemical to the type of energy source and GHG emissions as any:20
biomass conversion, water by electrolysis, and biochemical (i) renewable source; (ii) renewable energy source with an
methods involving bacteria and algae through anaerobic explicit mention of air pollution, safety, and global climate
digestion and fermentation.17 Thus, the conventional methods issues; (iii) renewable energy source with an explicit
used for the production of H2 are (i) thermochemical, mention of low GHG intensity factors; (iv) renewable
(ii) electrochemical, (iii) photobiological, and (iv) source or other zero carbon energy grid with carbon capture
photoelectrochemical processes.13 Additionally, energy from and sequestration and/or emissions offsets; (v) renewable
sustainable resources and water must be used to produce and nuclear source; (vi) source (renewable or not) with low-
green H2, also known as H2G. Moreover, hydrogen allows intensity GHG emissions; (vii) low carbon energy source
using sustainable energy sources, namely biomass, hydro, with low environmental impact.
geothermal, solar, and wind.14,19 There is a better agreement in recognizing that green
A carbon-free hydrogen society has been the goal of the H2 requires using renewable sources, either with or without
hydrogen energy transition. Thus, the production of the so- paying attention to GHG emissions. “Renewable sources”
called green H2 has been approached preferentially based on are well defined in the EU Directive 2018/2001/EC (or
hydrogen production through the electrolysis of water using RED 2).23 However, from a legal point of view, additional
renewable energy sources. These processes have been in eligibility criteria such as carbon emission intensity
progress for some time, and the production of hydrogen via limits may be interpreted differently by governments and
alkaline water electrolysis is now a mature technology with standards or certification bodies.
commercially available megawatt (MW) scale installations.19 Furthermore, there is still no consensus around the
In this sense, there are two paths to produce “clean” world regarding criteria, standards, norms, and legislation
H2: (i) production from nonrenewable raw material sources that define and certify how green or sustainable a particular
(e.g., NG) together with carbon capture and sequestration, or route or process is for H2 production.20 The lack of a global
(ii) H2 production, avoiding and/or minimizing the formation green H2 market has been driving several national and
of CO2 and making use of renewable sources of raw materials international standardization entities intending to facilitate
(e.g., biomass, biogas, among others) and energy (e.g., solar, a green H2 market with a regional and global scope.
wind, among others). The latter include the so-called zero-
carbon routes, without CO2 generation and with low carbon 3.1. Color-coded hydrogen classification
(with CO2 generation reduced to acceptable levels).20
Using renewable raw material energy sources does Just as the term “green hydrogen” has been used, there is
not guarantee that CO2 will not be formed. Examples an expanded color-based hydrogen nomenclature according
to its energy source, production route, and GHG footprint renewables (such as hydroelectric or solar power, among
(Figure 3).24-26 Green hydrogen, the main focus of the others) in their grid, in terms of carbon footprint, the yellow
present study, is produced from water electrolysis powered hydrogen generated may be considered “greener” than the
by electric energy generated from renewable sources, such yellow hydrogen generated from a grid electricity which is
as wind, solar, hydro, geothermal, and tidal. It is considered derived from other energy sources (i.e., thermoelectric). In
to be zero or minimal carbon emissions. turn, pink hydrogen is also produced by water electrolysis,
By the Figure 3 we can identify a palette of at least but powered by nuclear energy. Since the current nuclear
eight different colors of hydrogen, and among them, energy technology is based in nuclear fission of radioactive
green hydrogen. Keeping in mind there is no harmonized elements, it is not a carbon-related energy source. For this
systematically defined color-classified hydrogen gas reason, even though there is a negative sense associated
internationally, some definitions may be overlapping to each to nuclear power, mainly derived from the associated to
other or even ambiguous, depending on the interpretation of long-lived radioactive waste and the Chernobyl (1986) and
a given stakeholder20,27-30 or researcher. In this classification Fukushima (2006) accidents, nuclear power is considered
(Figure 3), the black/gray, brown, blue, and turquoise a clean and sustainable way of producing hydrogen. Some
hydrogen are generated from fossil fuels while the green, advantages of using nuclear fuel are: (i) it is a kind of very
yellow, and pink/purple/red hydrogen are generated from dense energy, so it produces minimal waste;32 and (ii) its
water electrolysis. In addition, white hydrogen is produced GHG emission is virtually inexistent.31,33,34 In terms of
as a byproduct of industrial processes. decarbonization and climate neutrality, nuclear-produced
Accordingly, brown hydrogen is a product of coal hydrogen is considered sustainable if the radioactive waste
gasification, and black/gray hydrogen is a product of natural is disposed correctly (following the strictest international
gas reforming. Both of them are related to high GHG legislation). Indeed, in February 2022 the European
footprints.30 In turn, although blue and turquoise hydrogen Commission35-37 has agreed with this argument although
also are derived from fossil sources, they are considered some critics have declared it as “greenwashing”.38
low-carbon emission if about 90% of the generated GHG From the color classification for hydrogen (Figure 3),
is sequestered by carbon capture, and storage (CCS).28,31 probably the most confuse denomination is the one from
Indeed, if applying an efficient CCS step, blue hydrogen the white hydrogen due to multiple concepts found in the
becomes sustainable, possibly achieving negative-carbon literature. According to NACFE,24 OceanBased Perpetual
emissions.29 Turquoise hydrogen is produced by natural Energy,25 and GEI,26 the definition of white hydrogen is
gas pyrolysis, yielding solid carbon as byproduct, which the one formed as byproduct of industrial processes. For
can be repurposed in the industry sector. instance, startups are developing a closed loop plastic
Yellow hydrogen is produced by water electrolysis depolymerization industrial process. This process yields a
powered by grid electricity of a given country or region. syngas.39 This syngas can be transformed into a variety of
Grid electricity may be variable (mixed-origin) energy, substances, and among them, methane, and white hydrogen.
depending on the electric energy matrix considered. This innovative route is environmentally beneficial in
For instance, for a country which has a high share of different ways: (i) since it is a closed-loop process,
Figure 3. Color spectrum classification for hydrogen gas according to its energy source, production route, and GHG footprint (adapted from NACFE,24
OceanBased Perpetual Energy,25 and GEI).26
virtually no emissions are released; (ii) it reutilizes end- to install plants that produce green hydrogen using the
of-life plastic residues accumulated in the environment; infrastructure available in the country (or even prevent their
and (iii) it generates methane and hydrogen which can installation). The vision must be comprehensive regarding
be repurposed to generate energy in different ways. The the green hydrogen production chain and its current and
hydrogen produced by the mentioned route is classified as future uses to create an environment of greater legal security
white since it is an industrial byproduct. On the other hand, for investors in this new market.
white hydrogen is also defined as “a naturally-occurring Among the gaps that already exist, the lack of criteria
geological hydrogen found in underground deposits and for clean and renewable energy injected into the national
created through fracking” by some other entities and interconnected system (SIN) can be removed from the
organizations.40-42 The search for viable strategies for same submarket, maintaining its original characteristics
storage of H2 in underground deposits is still a challenge. and giving the products derived from it all the attributes
However, considering fracking is an industrial process to of sustainability, subsequently enabling the certification of
exploit unconventional fossil fuels from shale formations, a green product.44
the first definition of white hydrogen is correctly applied The fiscal and tax aspects should deserve special
for both cases. attention, compatible with the effort to carry out an energy
transition without losing sight of the fact that eventual trade
4. Regulation of the Green Hydrogen Segment barriers can strongly affect the current Brazilian export
agenda, which is still largely carbonized. Encouraging
The transition from a carbon-based energy matrix to policies for the decarbonization of our production has
matrix-supported hydrogen energy from a renewable source become urgent, in line with the expansion of green
brings a discussion of a regulatory framework to guarantee hydrogen production in Brazil.
the sustainable character of these primary energy sources.
In addition, it involves the inclusive participation of all 5. Hydrogen Preparation in the Bench and
regions of a country. Industrial Scales
The design of the developmental model of this new
era, with a focus on energy and socio-environmental Hydrogen preparation methods including low- to zero-
sustainability, is under construction. It will demand from carbon energy sources is gaining the interests of the energy
all of us (academia, governments, industry, and society) the sector. Worldwide, it has been achieved when the hydrogen
elaboration of a regulatory framework that gives direction, production plant is linked with renewable or nuclear
pace, and long-term vision compatible with environmental, energy.20,27-30 Yet, biological processes are proving to be
social, and governance (ESG) criteria, in a circular promising alternatives for sustainable hydrogen production
economy with conscientious consumption. In addition to using renewable energy sources, especially concerning
the criteria described above, this new regulatory framework waste use and management.
must consider the technical and safety peculiarities of this Moreover, the so-called biohydrogen is produced from
vector (H2) with its positive externalities and aspects to be the conversion of water molecules and organic substrates
mitigated, always with broad participation from organized into hydrogen, by the action of microorganisms, through
society. the catalytic activity of two main enzymes, hydrogenase
In global terms, this discussion demands each and nitrogenase.19 The possibility to use waste resources
established form of government interacting with their such as wastewater significantly contributes to sustainable
regulatory agencies, 43 with the legislative houses, H2 production. In addition to the use of low-cost substrates
universities, research institutions, and representative and energy sources, this route can contribute to the major
associations of the industrial and commercial segments environmental issue of waste management.19 Although
to give legitimacy to the new standards and ensure that the most suitable energy sources for H2 production are
they will be accepted. In Brazil, notably the regulatory wind and solar energy, as they have been considered more
agencies ANEEL (Brazilian Electric Energy Agency) and suitable for this purpose, studies have been conducted
ANP (Brazilian Agency for Petroleum, Natural Gas and regarding combining renewable energy sources (such
Biofuels) together with the other relevant actors (federal as solar and geothermal).19 In general, green H2 can be
government, legislative representants, private sector, and produced from different energy sources and renewable
research institutions) are central for those discussions. raw materials.
It is immediately necessary to identify regulatory gaps In general lines, Table 1 presents the main green H2
that inhibit investment and rules that make it challenging production methods with their respective classifications.
Currently, more than 95% of H2 is produced from H2O(l) → H2(g) + 1/2O2(g) (3)
fossil fuel thermochemical reformation, accounting
for around 3% of global CO2 emissions per year. But The primary energy source supplied to break the O–H
there are relatively unused technologies to produce bond will determine how clean the hydrogen produced
clean hydrogen from water that show great promise by water splitting will be. Electrical, thermal, nuclear, or
in the near term.45 These, so far, relatively small-scale photonic energy can be used to generate hydrogen from
hydrogen production technologies based on water water (equation 3).
splitting comprise different methods, such as electrolytic,
thermochemical, photoelectrochemical, photocatalytic, 5.1. Electrochemical method
and photobiological methods.46 Water is the most abundant
hydrogen decarbonized source. However, extracting Electrolysis was first performed in 1802 by Sir
hydrogen from a very stable molecule demands a great Humphrey Davy, who noted that a galvanic cell could
amount of energy, since hydrogen and oxygen are strongly produce hydrogen and oxygen.47 Many different electrolyzer
bonded to each other (O–H bond, ∆H = 464 kJ mol‑1). designs were developed in the 1920s and 1930s. 48
Splitting water into H2 and O2 is an endergonic process Nowadays, electrolysis is the most well-established
(reaction 3), which means it is a thermodynamically non- technology for producing hydrogen from water. Water
spontaneous reaction where ∆H° (enthalpy) = 286 kJ mol–1, electrolysis involves the hydrogen (H2) evolution reaction
∆G° (Gibbs free energy) = 237 kJ mol–1, E°overall (overall on a cathode and the oxygen (O2) evolution reaction on an
standard potential) = –1.23 V, and hν (λ ≅ 1000 nm). anode driven by electrical energy in an electrolytic cell.
Table 1. Classification of green H2 production methods
Type of energy used H2 production method Raw material

electrolysis water
Electric
plasma arc decomposition natural gas
thermochemical processes –
H2S splitting H2S
with catalysis water splitting water
biomass conversion biomass
Thermal
water splitting water
gasification biomass
without catalysis
reform biofuels
H2S splitting H2S
photovoltaic electrolysis water
photocatalysis water
Photonics
photoelectrochemical water
biophotolysis water
dark fermentation biomass
Biochemical
enzyme water
high temperature electrolysis water
hybrid thermochemical cycles water
Electric + thermal catalytic term cracking of fossil fuels (with CO2 capture and storage) fossil fuel
coal gasification (with CO2 capture and storage) water
fossil fuel reform fossil fuel
Electrical + photonics + photoelectrolysis water
biochemistry + thermal thermophilic digestion biomass
biophotolysis biomass, water
Photonics + biochemistry photofermentation biomass
artificial photosynthesis biomass, water
Source: adapted from Dincer31 and El-Emam and Özcan.33
The half-reactions and respective standard potential (E°) and developed by General Electric in 1966, using
at 25 °C and 1 atm in acid and alkaline electrolytes are proton conductor membranes (Figure 4).54 PEM water
written as follows (equations 4 to 7): electrolysis has significant advantages over AEC, such
as high current density (above 2 A cm–2), high efficiency,
In acidic electrolyte and fast response. Even though PEM overcomes the
Cathode: 2H+(aq) + 2e– → H2(g) E° = 0 V (4) drawbacks of alkaline water electrolysis, it is more
Anode: H2O(l) → 1/2O2(g) + 2H+(aq) + 2e– E° = 1.23 V (5) expensive, mainly due to the high cost of the electrode
materials based on platinum for the cathode and iridium
In alkaline electrolyte for the anode (Figure 4).52
Cathode: 2H2O(l) + 2e– → H2(g) + 2OH–(aq) E° = –0.83 V (6) Besides AEC, AEM, and PEM technologies, solid
Anode: 2OH–(aq) → O2(g) + 2H+(aq) + 2e– E° = 0.40 V (7) oxide (SOC) electrolysis and microbial electrolysis (MEC)
cells are also in development. The SOC cathode and
In both acidic and alkaline electrolytes, the overall anode are isolated by solid electrolyte and operate at high
reaction, and the theoretical decomposition voltage temperatures (600-1000 °C). In SOC, water is decomposed
(1.23 V) are the same (equation 3). However, in practice, to H2 and O2– on the cathode, and oxide ions are transported
when the theoretical decomposition voltage is applied through the solid electrolyte to the anode, where they
to the electrolytic cell, water electrolysis is kinetically are oxidized to O2.50 MEC can produce hydrogen gas
reversible, and additional overpotentials (η) are required from organic matter in wastewater using exoelectrogenic
to overcome the energy barrier for both cathode and anode microorganisms on the anode.55
reactions.49,50 The magnitude of the overpotential is highly Although water electrolysis has been known for
dependent on the cathode material and thus can be divided more than 200 years and is the most popular among all
into three classes: (i) metals with high overpotential, such water-splitting methods, it still contributes to only 4%
as Cd, Tl, Hg, Pb, Zn, Sn, among others; (ii) metals with of the total hydrogen production worldwide because the
medium overpotential, Fe, Co, Ni, Cu, Au, Ag, and W; and production cost is much higher than that for reforming
(iii) metals with low overpotential, Pt, and Pd.51 fossil fuels. So, extracting hydrogen from water at a large
The first technology available on the market for water scale in a sustainable way is still a challenge for chemistry.
electrolysis was the alkaline electrolysis cell (AEC) Sustainable electrolysis based on solar and wind energy
because alkaline electrolytes are less corrosive than can produce hydrogen with high purity and is a green and
acidic ones (Figure 4). Presently, the most widely used straightforward process (Figure 4). Undoubtedly, using
cathode material in AECs is nickel or its alloys with renewable energy sources to produce clean H2 with water
molybdenum.50 AEC technology is the most commercially electrolysis is the main path in the energy transition to the
established. However, the limited current densities (below so-called hydrogen economy.
400 mA cm–2), low operating pressure, and low energy
efficiency led to a new approach with the development of 5.2. Thermochemical method
an anion exchange membrane (AEM) built of polymers
with anionic conductivity (Figure 4).52 With the same The endothermic reaction of water splitting also can be
operating principles, proton exchange membrane (PEM) driven by heat. However, for water splitting (equation 1)
electrolysis was conceived by Grubb 53 in the 1950s to become a spontaneous reaction, it is required to raise
Figure 4. Schematics of electrolytic cells main types, alkaline electrolysis cell (AEC), alkaline electrolysis cell with anion exchange membrane (AEM),
electrolysis cell with proton exchange membrane (PEM).
the temperature to at least 2000 K (1 atm), when free A three-step thermochemical cycle can be derived from
energy variation is negative and temperatures up to a two-step thermochemical cycle in which the highest
4260 K are necessary to obtain a 50% hydrogen yield.49 temperature reaction is replaced by a two-step reaction
Moreover, at such temperatures, it is challenging to process as follows:
separate hydrogen and oxygen in time, which results in a
mixed gas prone to explosion. The thermochemical cycle AB → A + B (low temperature) (10)
technology introduced by Funk and Reinstrom56 in the H2O + A → 0.5 H2 + AO (mild temperature) (11)
1960s is an alternative to the high temperatures required AO + B → 0.5 O2 + AB (high temperature) (12)
for water thermolysis.56,57 The reaction temperature in
thermochemical cycles for hydrogen production, around Currently, many three-step thermochemical cycles are
1100 K, matches the high-temperature thermal energy under research and development. Among them are the sulfur-
provided by nuclear power plants or solar energy.58 The iodine (S-I), copper-chlorine (Cu-Cl), and magnesium-
thermochemical cycle process is still in the research stage, chlorine (Mg-Cl) cycles.60 In the most advanced stage is
in which hydrogen and oxygen are indirectly obtained the iodine-sulfur cycle, initially proposed in the 1970s by
through water splitting through cyclic chemical reactions General Atomic.61 Countries such as Japan, the USA, France,
that require heat. Except for water, all the other chemicals and China have conducted investigations of a scaled-up S-I
in a thermochemical cycle can be continuously recycled thermochemical cycle. The results predict an efficiency of
through repetitive series of chemical reactions using 52% when a high-temperature nuclear reactor is integrated
intermediate reactions and substances recycled during the with this cycle.57 Figure 6 illustrates the main steps of the
process.59 Among the thermochemical methods for water S-I thermochemical cycle, in which the maximum reaction
splitting, the most studied are two-step and three-step temperature is 850 °C, compared with 530 °C in the Cu-Cl
cycles. thermochemical cycle, but the latter involves an electrolysis
In a two-step thermochemical cycle, a metal oxide (MxOy) step. The Mg-Cl thermochemical cycle is another low-
acts as an intermediate medium for the decomposition of temperature cycle that operates at maximum temperatures
water. In the first step, the higher-valence metal oxide is within 450‑550 °C.47 Cu-Cl, Mg‑Cl, and S-I thermochemical
initially thermally reduced to a lower-valence metal oxide cycles are considered more promising than others, but further
in an endothermic process to produce oxygen, which research is needed to address the higher efficiency and lower
requires temperatures higher than 1500 °C. In the second cost to produce hydrogen on a large scale.
step, the lower-valence (or zero) metal oxide is oxidized in
an exothermic process to produce hydrogen through water
splitting, which generally requires temperatures lower than
1000 °C (Figure 5). The reduction and oxidation reactions
are represented in equations 8 and 9, respectively.
Figure 5. General scheme of a two-step thermochemical cycle. Figure 6. Scheme of a three-step S-I thermochemical cycle.
Reduction: MxOy → MxOy-δ + δ/2O2 (T > 1500 °C) (8)

5.3. Photocatalytic method
Oxidation: MxOy-δ + δH2O → MxOy + δH2 (T < 1000 °C) (9)
Several candidates have been proposed for use as redox Artificial photosynthesis is considered ideal since
material in the two-step thermochemical cycle for water this process can capture solar energy and directly convert
splitting metal oxide based redox pair reactions, including it into strategic fuels such as hydrogen and non-fossil
ZnO/Zn, Fe3O4/Fe, SnO2/SnO, CeO2/Ce2O3, Mn2O3/MnO, hydrocarbons.62 Photocatalytic H2 production is a form of
Co3O4/CoO, CdO/Cd, and GeO2/GeO.57-59 artificial photosynthesis inspired by nature that is widely
exploited in the generation of H2 based on water splitting The overall reaction is:
assisted by a semiconductor particle, which acts as an H2O + hν (< 1,000 nm) → H2 + 1/2O2 E° = −1.23 V (16)
artificial leaf. The photoelectrolysis of water was first
reported by Fujishima and Honda63 in 1972 using TiO2 Thus, water splitting using particulate photocatalysts is
(semiconductor electrode) connected to a platinum counter- a possible mean of achieving solar hydrogen production.
electrode exposed to ultraviolet radiation. A semiconductor On the other hand, the complete oxidation of an organic
particle with a small amount of a noble metal, such as compound to CO2 and H2O in the presence of oxygen is
platinum, deposited on the surface is essentially a miniature a down-hill reaction (ΔG < 0). However, the free energies
photoelectrochemical cell; water is oxidized directly on of the reforming reaction can be positive, as is the case
the semiconductor surface and reduced on the noble metal of reforming glycerol and ethanol (4 and 97 kJ mol–1,
surface (Figure 7). respectively). Such reactions, and the reforming reaction of
other biomass derivatives, can also be induced by photons
using semiconductor particles irradiated with ultraviolet
or visible light. The combination of water photolysis and
photodecomposition reactions of organic compounds in a
photocatalytic cell under anaerobic conditions leads to H2
and CO2 production.65 Several studies demonstrate that it is
possible to obtain H2 from the photocatalytic reforming of
an aqueous solution of biomass derivatives under ambient
conditions.65,67,68 In addition to biomass derivatives, it is
also possible to use wastewater with a high organic matter
content and reduced sulfur compounds as sacrificial agents
Figure 7. Schematic representation of photocatalytic water splitting
on a semiconductor powder particle with co-catalyst (noble metal (hole scavengers).68-71 Hole scavengers increase the half-
nanoparticle). life of electrons in the conduction band, increasing the
efficiency of photocatalytic hydrogen production.
T h e d e c o m p o s i t i o n o f wa t e r m o l e c u l e s t o A wide variety of photocatalysts semiconductor has
produce hydrogen and oxygen is an uphill reaction been investigated so far, such as chalcogenides (ZnS, CdS,
(ΔG° = +238 kJ mol–1; E°overall = –1.23 V; equation 3), a CdSe, MoS2),72-75 metal oxides (TiO2, SnO2, BiTaO4, ZnO,
semiconductor can conduct it with suitable band potential LaFeO3, ZrO2),76-79 carbonaceous materials (g-C3N4),80,81
and incident light with a wavelength shorter than 1000 nm solid solutions (CdxZn(1–x)SySe(1–y))81 and metal-doped
(i.e., 1.23 eV ca. 1000 nm), the energy required to split water. semiconductors (SrTiO3:Al, Cd-SnO2/CdO/CdS).68,69 The
In order to achieve the overall water splitting, the bottom of best choice to produce photocatalysts for water splitting
the semiconductor’s conduction band must be located at a is not to use toxic chemical methods or metals, so the
more negative potential than the reduction potential of H+ to hydrogen produced will, in fact, be green.
H2 (E° = 0 V vs. NHE (normal hydrogen electrode) at pH 0). Several methods have been proposed and applied for
On the other hand, the top of the valence band must be more hydrogen production, as mentioned. Recently, studies82,83
positively positioned than the oxidation potential of H2O to have highlighted the possibility of H2 production via the
O2 (E° = 1.23 V vs. NHE).64,65 The energy required to split photocatalytic route from glycerol, a byproduct of biodiesel
the water molecule matches the UV-visible-near-infrared production.
(NIR) light found in sunlight, composed of 5% ultraviolet,
43% visible, and 52% infrared light.66 The photocatalytic 5.4. Gray, blue, and green hydrogen
process allows water to be decomposed into oxygen and
hydrogen at room temperature without the application of As presented in Figure 8, according to CertifHy,
external voltage, according to the following equations: one possible certification for hydrogen production, 18
blue H2, is produced by non-renewable raw materials
and energy (e.g., NG) and whose CO2 emissions are
Semiconductor + hν (< 1,000 nm) → e–CB + h+VB (13)
below 36.4 g CO2 eq MJ–1 H2. Interestingly, in Figure 8
H2O + 2 h+VB → 1/2O2 + 2H+ (at semiconductor surface)
it is presented that by replacing gray hydrogen by blue
E° = 1.23 V (14)
hydrogen, there is an estimative of 60% reduction in the
2H+ + 2 e–CB → H2 (at Pt nanoparticle or other noble metal)
GHG emissions, with a low-carbon threshold value of
E° = 0 V (15)
36.4 g CO2 eq MJ–1 H2 has been proposed by some experts
Figure 8. Categorization of hydrogen according to its source of energy and emission intensity (adapted from Velazquez and Dodds).20
as a “quantitative criteria” to a given hydrogen production In many regions, the scarcity of this resource does not
route to be classified as a “low-carbon” technology.20,34 allow for the production necessary to meet local demand.
The production of green hydrogen in the short and In addition to the higher water consumption, green H2
medium-term has been debated, and many companies requires approximately 11 times more energy per unit of
favor its use for several reasons. Initially, it was argued that H2 produced when compared with non-renewable routes
the high cost and lack of standardization and definitions (before carbon capture), which need to be cheap.85-87
regarding the demand price limited the end-use sectors Greening the global H 2 supply would require
where H2 can be more impactful. Without the necessary approximately 3.90 TWh of electricity annually, representing
demand, investments in new or upgraded production lines 60% more than the combined energy generated globally by
and distribution networks, and new storage infrastructure, wind and solar photovoltaics in 2020 (2.44 TWh).86
become unjustified.20 Currently, electrolyzers can only reach the competitive
International consortia such as the Hydrogen Council84 limit of USD 2 kg–1 when operating on “free” electricity
and industries, and intergovernmental strategies such 30% of the time or more, dramatically limiting the supply
as the European Union Hydrogen Backbone Project, of H2. In 2020, without the necessary demand, 1.6 TWh
have already started investment actions to transform the of renewable energy had to be wasted by the California
hydrogen economy. Each believes that this transformation Independent System Operator (CAISO), an amount
should occur gradually in the order: gray → blue → green corresponding to 28.9 kt H2. However, CAISO data show
(Figure 8). This strategy has three limitations that can be that only 50 MW electrolyzers could achieve a capacity
overcome with continued use of blue H2: (i) the availability factor of 30% of the wasted energy, enough to produce
of resources varies geographically; (ii) there are energy only 451 t of H2.86
inefficiencies in the electrolysis process; (iii) the H2 supply While H2 is seen as an alternative to reduce the waste of
is intermittent due to the constant demand for use of the excess renewable energy (for more information, read about
product. renewable curtailment), converting the excess energy into
Furthermore, the production of H2 requires an abundant H2 may subsequently reflect losses of up to 70% in turbines
amount of low-cost water or simple hydrocarbons or fuel cells (energy → gas → energy).
(preferably methane). Green H2 consumes at least 9 kg of The simplest solution to avoid wasting energy would be
water per kg of H2, while gray and blue H2 require half to increase demand. The problem lies in a business model
this amount (when produced by steam reforming methane that focuses on the worldwide use of electrolyzers, whose
with a gas-water exchange reaction). As already discussed, production cost is not yet appealing for industries to start
water is a limited resource in many regions, with uses in buying energy to produce green H2 continuously, thus
several sectors, such as energy, agriculture, and sanitation. causing intermittent production. On the other hand, blue H2
is not associated with the electricity supply network, has a A very promising route is hydrogen production by
low cost, and can be continuously produced, as NG is also anaerobic digestion (AD).90-92 Among the many other
continuously produced in several locations.88 biotechnological processes for H2 production, also known
In summary, the main purpose of using H2 is to as dark fermentation, it is the most robust process, as it can
decarbonize the economy, and it makes sense to use the convert all types of biomass, including food and agricultural
H2 that offers the most significant environmental benefit. waste, sewage sludge, and distillery waste, among others,
However, there are cases where the use of blue H2 is more into biogas that contains CH4 and H2.93
appropriate. For instance, if a natural gas plant has access The AD process does not require sunlight, requires
to the entire transport infrastructure, it makes sense to use moderate process conditions, and has lower energy
this H2, contributing to emission reductions with a carbon demands. Furthermore, this process’ hydrogen yield and
capture process. Regardless of whether H2 is green or blue, hydrogen production rate are more attractive than other
caution must be exercised when embarking on projects processes.94 A complete review of AD applied to biogas
aimed at building new transport networks or H2 plants based production covered all aspects, such as factors affecting
on non-renewable sources, due to the high construction cost efficiency (temperature, pH, C/N ratio, organic loading
of new infrastructure and the long-time horizon of these rate (OLR), and retention time), accelerators (green
projects. It is necessary to include in the equation whether biomass, pure biological culture, and inorganic additives),
the option for blue H2 with carbon capture will justify the reactors (conventional anaerobic reactors, sludge retention
investment in the long term.88 reactors, and anaerobic membrane reactors), and biogas
processes.95
5.5. Biohydrogen
6. Hydrogen Storage
The use of a wide variety of biologically available raw
materials contributes to improvements in the energy and Storage of chemical energy (including H2) represents
socioeconomic sphere of developing countries, such as the only technically feasible and scalable approach to
Brazil. interseason storage of renewable energy, but this only
The H2 production can take place with virtually any raw makes sense if the renewable energy supply exceeds 70%
biomaterial. In Brazil, due to its wide availability, ease of of demand. Thus, it is unlikely that the excess supply of
production, operation, and high conversion to H2, ethanol renewable energy will somehow accelerate the use of green
has been the most promising raw biomaterial. Table 2 H2 in the short term.96
presents the top ten ethanol producers in the world. There are different ways of storing H2: compressed
The production of pure H2 (99.99 vol%) has been shown hydrogen gas reservoirs, liquid hydrogen reservoirs, metal
to be an interesting alternative through the introduction of hydrides, carbon adsorption, and microspheres. Compressed
ethanol pre-treatment and pre-reformation to the reforming hydrogen gas reservoirs are the most commonly used for
methane steam process. This route can be intensified by high-pressure gas storage. Most vehicles powered by fuel
including palladium (Pd) membranes, which are selective to cells use this form of hydrogen storage in cylinders, similar
H2, as already reported in the literature.89 It has also proved to those used with compressed natural gas.97,98
to be a competitive route as the carbon market consolidates High-pressure cylinders typically store hydrogen at
and membrane costs decrease. a pressure of 3600 psi (250 bar), but some cylinders can
handle 5000 psi (350 bar). However, there has been the
Table 2. Ten largest bioethanol producers in the world development of new materials in order to increase the
storage capacity, and consequently the pressure, so that
Country Production in 2016 / (× 109 L)
explosions do not occur.99 Currently, cylinders are made
USA 57730
with thin plates using highly resistant materials with
Brazil 27616
excellent durability. In general, these are classified into
Europe 5213
China 3199
four types, according to the material used: type 1, made
Canada 1651 entirely of aluminum or steel; type 2, made with a thin layer
Thailand 1219 of aluminum or steel surrounded by another composite,
Argentina 1113 usually carbon fibers, in a circular shape; type 3, made with
India 852 a thin layer of steel or aluminum fully enveloped by other
Rest of the world 1855 compounds such as carbon fibers; and type 4, made with a
Source: adapted from Mosca.89 layer of tough plastic-wrapped in another tough compound.
Table 3 presents some examples of applications of the Hydrogen storage in vehicles can be done in two forms:
different types of storage that can be used for hydrogen. liquid and gaseous hydrogen fuels. Figure 9 presents
The use of cylinders is indicated for hydrogen storage at hydrogen-based systems in vehicles.
small and medium scale. At larger scales, hydrogen can
be stored cryogenically as a liquid and in pipes or porous
storage underground, as an underground geological method
(most economical),103 or in interactions between hydrogen
and hydrocarbons dissolved in reservoir rocks through
microbial metabolism, for example.104
Underground storage is possible in different types of
reservoirs, but what is more viable are salt caves, which
are also used for storing fossil gases. This type of storage
is best suited for large volumes and long periods (weeks or
seasons), and it has low operating pressure (50-250 bar).
Caves of this type are found worldwide with different
levels of storage ability. Although they are found in the
Asia Pacific, South America, Southern Europe, and the
Figure 9. Systems of (a) internal combustion engine vehicle; (b) hybrid
west coast of North America, only the US and UK use this electric vehicle; (c) fuel cell vehicle. LH2: liquid hydrogen (adapted from
type of cave to store H2.100 Ugurlu and Oztuna).108
In terms of technology, some research aims to develop
new materials that can be used in the H2 storage process, 7. Hydrogen Transport
such as H2 adsorption using carbon nanostructures.105 One
nanostructure that has been heavily investigated is graphene- The energy sector has extensive experience with
based.106,107 The use of this material can significantly increase transporting gases over long distances. However, H2 has
storage efficiency, offering high stability. additional challenges due to its physical properties. Despite
Table 3. Overview of hydrogen storage types and their peculiarities
Reference Scale Type of storage Type of tank Requirements Note

5-10 kg compressed gas high pressures require tanks to be reinforced
97,98 small board vehicles 2, 3 or 4
at 350-950 bar with carbon fibers or pre-stressed concrete
100 small buffer hydrogen 2 or 4 100 kg –
tube trailers for hydrogen
91,101 small delivery to refueling 4 1000 kg at 540 bar –
stations
spherical vessels or
protection from adverse weather conditions and
100 medium underground pipe – ca. 20 bar
no requirement for shaft structures
facilities
hydrogen liquefaction process requires very
LH2 at 20 K
high energy and is costly
93,102 large cryogenically as a liquid spherical tanks two 3200-m3 spheres,

21.3 m in diameter, each largest LH2 storage facility at NASA’s
capable of storing 230 t Kennedy Space Center
of LH2
type-1, API 5L X52 more economical than geological caverns but
100 large underground pipes < 20 t H2
pipes estimated capital cost is high
> 750 t useable H2
underground lined rock installed capital cost of lined rock caverns
100 large – requires multiple
caverns decreases with increased H2 storage
caverns
installed capital cost of salt caverns decreases
with increased H2 storage
100 large underground salt caverns – 50-250 bar
salt caverns are more economical than lined
rock caverns
LH2: liquid hytrogen.
having a high energy density by mass of 33.3 kWh kg–1 (the 2700 km.110 Another modality is to transport H2 via ships.
value for CH4 is 13.9 kWh kg–1), it has a low energy density This option favors the transport of liquid H2, ammonia,
by volume of 3 kWh m–3 (the value for CH4 is 10 kWh m–3 LOHC, methanol, and synthetic liquids. The Japanese
under standard conditions).109 Thus, to transport the same government has started feasibility studies for these options.
amount of energy, larger volumes of H2 must be considered.
For this reason, H2 is treated to reduce its volume for 8. Uses of Hydrogen
transport. Currently, the solutions for H2 transport are
compression, liquefaction, use of liquid organic hydrogen In terms of renewable H2 applications, its energy
carriers (LOHCs), or conversion to ammonia. Each of these conversion can be highlighted in different types of
solutions increases the energy density by volume. energy such as power-to-X (P2X), power-to-power (P2P),
Compressed H2 can be transported by truck or gas power-to-chemical, and power-to-mining. The process
cylinders with 200 to 700 bar pressures. However, this of converting energy generated from solar (photovoltaic)
modality is only viable for short distances (a few hundred and wind sources into different types of energy for use in
kilometers) and reduced volumes. For long distances, H2 is various sectors of the economy, or even to be reconverted
usually transported in its liquid form, and the liquefaction into energy, has the potential to increase the flexibility of
process requires cooling it to a temperature of –253 °C or the electricity grid significantly. Optional locations can
lower. Up to 3500 kg of H2 can be transported by truck.109 be built to place temporary surplus energy by displacing
However, as the distance increases, the trucks become a fossil fuel energy sources, contributing to net-zero
less viable option. carbon emissions.111 Green H2, produced by electrolysis
For longer distances, where it is unfeasible to use trucks from renewable energy sources, appears to be a relevant
for transport, compressed H2 pipes are used, thus increasing alternative to obtain much-needed carbon neutrality in
the volume transported to thousands of tons per day. The H2 various sectors of the economy, such as the transport,
distribution network is still short (approximately 5000 km) chemical, and general industries. A representation of the
compared with natural gas (3 million km) in Asia, Europe, possible applications of H2 produced by electrolysis from
and North America. An alternative is to inject green H2 into renewable energy sources is presented in Figure 10.
natural gas lines, but it requires replacing valves, regulators, In the P2P application, producing H2 by electrolysis
compressors, and measuring devices. In some cases, from renewable energy sources and storing and reconverting
complete replacement of piping is necessary. Pipes can also hydrogen into electricity by fuel cells or gas turbines are
transport ammonia, and some are already in operation. An promising processes for applications outside of the grid,
example is the Togliatti-Odessa pipeline in Russia, with in remote locations or rural communities, or as a power
Figure 10. Possible applications of green hydrogen (adapted from IRENA).112

reserve. It is not yet a competitive process for connection

to the grid.113 This is due to the low efficiency of the entire
system, as it currently varies in the range of 30 to 40%.114
In this case, underground storage in salt caves, aquifers,
or depleted oil and gas fields is mentioned as the most
economical way to store large volumes of hydrogen.115
Another possibility is the use of combined H2 storage to
compensate for the variations in electrical energy produced
by wind and photovoltaic generators, minimizing the use
of thermal generating plants, which use fossil fuels. It Figure 11. Steel-producing countries in 2020.110
could also be an alternative to reduce the adverse effects
caused by the uncertainty of climate forecasts regarding The steel industry is heavily dependent on coal and
hydroelectric plants. accounts for approximately 8% of global CO2 emissions.120,121
Globally, the industry already produces and uses H2 in With increasing pressure on the carbon-intensive mining
a variety of catalysts and catalytic processes, ranging from industry to reduce emissions, the world’s leading players are
the synthesis of ammonia to asymmetric hydrogenation working hard to decarbonize the sector. Several options are
processes for pharmaceuticals.16 being considered for the decarbonization process: increasing
The largest share of H2 demand is from the chemical the efficiency of current production processes, recycling
and refining sectors, and steam methane reforming is steel, using carbon capture and storage, and using hydrogen.
the principal option, although coal gasification is also Regarding the last option, H2 can reduce emissions in the
used for metal and petrochemical production. Other steel production process to participate in the decarbonization
industrial sectors also use H2, but their cumulative share process in two ways: using it as an auxiliary reducing agent
of global demand is small, just 1%,116 and include the in the production process called the basic oxygen furnace
manufacturing of glass, food products (hydrogenation of route in a blast furnace (BF-BOF) or as an agent for reducing
fats), bulk and specialty chemicals, and semiconductors, iron in the process known as direct iron reduction (DRI or
the cooling of large stationary electrical generators, the H2-DRI).122
production of propellant fuel for aerospace vehicles, and Ito et al.120 evaluated the situation in the steel industry
others. Hydrogen produced by electrolysis can be mainly and examined the possible technologies that could be
used as a raw material in industrial processes such as used to achieve carbon neutrality. Most steel production
the following: (i) chemical: ammonia (Haber-Bosch is carried out by the primary route, in which the iron ore
process), methanol, and urea (with carbon capture, use is processed to obtain a sintered material or iron pellets,
and/or storage (CCUS)); Fischer-Tropsch products such which are subsequently melted in a blast furnace with coke
as light/heavy naphtha, kerosene, gas, diesel, and others to make pig iron and then processed in a basic oxygen
(with CCUS); polymers, and resins are relevant markets furnace to produce steel. The remaining steel production
for industrial hydrogen;117 (ii) refining: hydrogen is used comes from the secondary route by heating scrap metal in
for hydrocracking and hydrotreating processes (removal an electric arc furnace. Processes in the primary route emit
of sulfur from fuels). Refineries represent very important mainly direct greenhouse gases, and those in the secondary
consumers of industrial hydrogen; (iii) HEFA/HVO route emit mainly indirect greenhouse gases, which vary
process: hydrogen is used in hydrotreating vegetable depending on the electricity mix used in the electric arc
oil (HVO), commonly referred to as renewable diesel, furnace.
and in hydroprocessed esters and fatty acids (HEFAs). Carbon capture, use, and storage (CCUS) is a method to
Hydroprocessing of oils and fats is an alternative to capture CO2 emissions and either process them for further
esterification to produce diesel from biomass. The utilization as fuel or store them in geological formations
HVO/HEFA process produces straight chain paraffinic such as exhausted undersea gas reservoirs. CCUS alone
hydrocarbons free of aromatics, oxygen, and sulfur and cannot achieve carbon neutrality, but it could mitigate
has high cetane numbers. HEFAs can typically be used in emissions into the atmosphere.
all diesel engines as aviation bio-jet fuel.118,119 The second kind of potential technology involves
As a permanent material that can be continuously replacing coke or natural gas with alternative reductants of
recycled, steel is a fundamental material for industry iron ore, including hydrogen and direct electric current. The
and society in general. Its production has been growing advantage is that this can probably make steel production
substantially, reaching 1.9 billion tons in 2020 (Figure 11).111 fully green. However, developing these technologies will
require much more time and money than what is required water together with oxygen electrolysis, from the reform
for CCUS. or biotechnological processes of biogas, and from the
The method of direct hydrogen reduction of iron uses the combustion of products formed from green H2, such as
element to reduce iron ore in a shaft furnace or a fluidized green ammonia.
bed reactor. The shaft furnace still has an inconvenience: it Along these lines, work was carried out to evaluate the
requires iron ore pellets, and the process to produce them energy and environmental impacts, flame structure and
can cause significant emissions depending on the heat stability, and association with modern techniques used in
source of the pellet plant. A great amount of hydrogen is combustion. Studies110,123-133 have analyzed the influence
also required for both reactors, requiring a large amount of of parameters on the characteristics of hydrogen flames
electrolyzers. In the case of the fluidized bed reactor, the use or mixtures of hydrogen with other fuels, evaluating
of iron fines removes the need to pelletize, cutting down the their stability and efficiency. The flame flow aspects of
CO2 emitted during this process. Additionally, it can reach CH4/H2 mixtures with the rotational flow were evaluated,
higher levels of metallization. On the other hand, fluidized verifying the velocity stability characteristics in the studied
bed reactors in steelmaking are less developed than shaft mixtures.126 The curvature in the flow for flame development
furnaces and require a larger investment. and its propagation were investigated to understand better
Some other processes are already in development, joint combustion between H2 and NH3.110 Flame velocity
producing carbon-neutral steel, such as biomass-based and mass diffusion aspects as well as propagation stability
ironmaking with CCUS, suspension ironmaking, plasma were studied in the combustion of a mixture of CH4, H2,
direct steel production, and electrolytic processes. and air, allowing a better understanding under the evaluated
These last three technologies are at the early stages of conditions.130
development. Their technical and economic viability in A modern technique called low-oxygen dilution
large-scale production must be proved, and there is a high (moderate or intense low oxygen dilution, MILD)
degree of uncertainty over their industrial utilization. combustion was analyzed with several mixtures of CH4 and
Hydrogen-based reduction technologies are more H2, evaluating aspects of NOx emissions and their routes,
developed and have lower technological risks, but there supporting the environmental analysis of associations of
is no sufficiently large-scale hydrogen electrolyzer modern techniques for the combustion of H2.125
available, a prerequisite to produce sufficient green H2 for Combustion propagation studies evaluating the velocity
the reduction process. Furthermore, these energy sources uncertainty of hydrogen flame were developed, allowing
are still costly, and the whole process is not competitive a better understanding of the operation under specific
compared with the coking process. Much time and great conditions of feeding gases to the burning.131
effort in research and development are still needed to reach Stabilization of rotational flows, very common in
competitiveness for these new direct reduction processes. industrial combustion equipment, was analyzed in synthetic
One of the mechanisms still underway for energy natural gas flames mixed with H2, evaluating aspects of
conversion systems for thermal use for heating or its flame velocity, NOx formation, and combustion stability.110
generation comes from combustion. Fossil fuels are still Studies with ammonia and dimethyl ether (DME) as
used in liquid, solid or gaseous form, including hydrogen e-fuels for energy applications have been an important
via unsustainable routes (gray H2). research area. Experimental and numerical studies were
By looking ahead to the following decades, combustion better carried out to understand combustion and its
will still be used in energy conversion, and given the energetic and emission aspects.131,133-139
decarbonization in progress, the use of green H2 should be The use of polyoxymethylene dimethyl ether (OMEn),
considered, especially as an energy source in the chemical a promising e-fuel, was studied for possible applications in
industry. Recently, research has been carried out (2018-2021) engines.135 The flame structure and CO and NOx emissions
regarding using hydrogen in combustion, considering its were evaluated for its use as a possible fuel for the green
importance for the H2 economy that is expected to develop. H2 chain.
The possibility of its use in thermoelectric plants, chemical The combustion of ammonia, CH4, and H2 mixture was
processes (such as ammonia and green cement production), investigated through experimental and numerical studies.
and the production of fuels (e-fuels) for vehicles and heavy The stability of flames with swirling flow with the addition
transport opens up an important field of research. of ammonia under different conditions and the emissions
The combustion of green H2 follows some of the main of NOx and CO were evaluated.137
routes as the origin of its formation: mixed with CO and The combustion of e-fuel oxymethylene ether 1
fractions of biomass gasification (syngas), as a result of (OME1) and its blends with n-dodecane was also studied,138
evaluating the performance in engines. The process of Although H2 does not emit any dangerous by-products
ignition and soot formation was analyzed, verifying the during combustion, some points still need to be considered
fundamental characteristics for possible expansion of its in its production and storage, as it is highly reactive in the
use with the decarbonization process. presence of oxygen, which can lead to explosive action.
Turbulent ammonia flames were evaluated using the Therefore, H2 sensors must be attached to vehicles to detect
oxygen-enriched combustion technique at high pressures. any hydrogen leakage that might happen.143,144 Internal
Turbulence effects on flame stability and propagation combustion engines accompanied by hydrogen can also
velocity and the comparative influence of turbulence be seen in the automotive market.14
and reaction effects were evaluated. 139 This line of In-vehicle systems that use H2 (Figure 9), the pressure
study supports the possibility of developing combustion produced by the combustion of the H2 pushes down the
equipment for the industrial use of ammonia coming from pistons in the cylinders of the internal combustion engine
the green H2 chain. (system I, Figure 9a). In hybrid vehicles (system II,
With the same objective, studies of syngas and biogas Figure 9b), hydrogen is the combustion engine fuel
combustion, either enriched with H2 or not, are being responsible for rotating the wheels, which works as an
carried out to understand better combustion and possible energy source to charge the vehicle’s battery. Unlike
applications in energy production and industrial thermal previous vehicles, fuel cell vehicles (system III, Figure 9c)
applications.110,138,140-142 do not have internal combustion engines. They are powered
A review140 of the fundamental characteristics of syngas by hydrogen through fuel cells that create electricity for
combustion was conducted, identifying important points of the battery. Based on the three types of vehicles with a
stability and efficiency of syngas combustion and indicating hydrogen engine, system III has the lowest emissions of
gaps in the development of combustion devices for better volatile organic compounds, followed by II and I.145
industrial use of this fuel, which will play an essential role
in the green H2 chain. 9. Economic and Environmental Impacts
Comparative studies of gas turbines between natural
gas and biogas were carried out. The influence of swirling Cost reduction and improvements in novel technologies
flow, combustion efficiency, and CO and NOx emissions encourage companies and governments to invest time
were analyzed.141 and financial resources in new regulations, policies, and
Syngas flames were analyzed under the conditions of alternatives to decarbonize industries, especially the
the modern oxy-fuel combustion technique, evaluating transport and heating sectors and most society’s activities.
combustion stability and emissions and verifying the One possible solution to reach this goal is to use H2
environmental behavior of the syngas burner operation.138 as one of the leading energy vectors, especially green and
Turbulent syngas flames were also studied.110 The clean H2, when it comes from clean energy sources such as
influence of the equivalent ratio and turbulence intensity solar, wind, and hydroelectric power. Although currently
on the flame stability and velocity were analyzed, there are many established ways to produce hydrogen, none
supporting the research on turbulent syngas combustion of them can help to achieve the goals of the Paris Agreement
and its operating conditions for further industrial to 2030 and 2050. In addition, it is estimated that up to
applications. 2020, the industry sector demand for hydrogen was 51 Mt,
By presenting these recent studies, a vital interest with chemical production consuming ca. 46 Mt, attesting
of the academic community in the role of combustion to the importance of the hydrogen market and this activity
in the green H2 chain is identified. Therefore, important worldwide.132 To understand the importance, 90% of all
possibilities open up for a better understanding of the hydrogen produced is directed to industrial use, especially
benefits and impacts of this chain with the technological ammonia, methanol, and steel plants.124
route of combustion. For this reason, the implementation of a production
Due to the properties highlighted above, H2 was chosen chain based on green H2 has brought about relevant
as one of the alternatives for the main automotive industry discussions among governments to define strategic plans
sources in relation to the transition from electric vehicles for the sector. In this context, both the economic aspects
to fuel cells. Although many power systems have been of the benefits and the bottlenecks in the chain and the
implemented in electrical system vehicles, the hydrogen- environmental aspects associated with decarbonization have
based energy system is favored by consumers. This is been discussed in the literature. Recent works88,146-159 have
because it offers many advantages, as saving excellent discussed green H2 in several countries and the value chain.
efficiency and being environmentally friendly. On the other hand, the implementation of green hydrogen
production has been researched based on the potential of are considered. The industry currently has better resources
generating renewable energy, verifying the application to deal with this type of gas. Furthermore, it can reduce
of this new energy source for the decarbonization of the GHG emissions by replacing gray H2 with the green one.
transport and industrial sectors.88,146-149,153,155,157,159 For this reason, in the short term, most projects should be
The use of hydroelectric energy and its expansion for aimed at the needs of industry and foreign markets. Sectors
the production of green hydrogen and the replacement such as heavy road transport, naval, aviation, chemical,
of fossil fuels, and its impact on decarbonization, was cement, and steel, when combined, account for 30% of
studied in Nepal.149 In Pakistan, a study148 was carried out global GHG emissions and involve a complicated electrical
on implementing the green hydrogen economy using solar transition. Despite the emissions, each sector has or will
energy, biomass, and municipal solid waste as sources, have a continuous demand for H2. This would reduce the
supporting the country’s decarbonization and enabling associated renewable energy waste due to prolonged storage
it to be a global supplier. The formation of an e-fuel based on a possible imbalance in supply/demand.
production chain between countries was also studied,88 The realization of the green H2 economy also depends
with the connection of renewable generation in Iceland on advances in technology such as electrolyzers. The
(via geothermal energy and hydropower) and the surplus installed capacity of electrolyzers is currently around
of renewable energy production in Germany. 200 MW, still, far from the capacity required for the
Studies have also evaluated the prospects of, and desired consumption of green H2.109 However, this capacity
obstacles to, green H2 chain deployment. For example, in is expected to increase considerably as new electrolyzer
Russia, studies146,153 identified the potential for generating projects are launched. For example, according to a 2021
renewable energy and producing green hydrogen and the report released by the International Renewable Energy
bottlenecks and analyzed the costs and production payback. Agency (IRENA),109 over a 5 month period (November
A similar study156 was carried out in the Philippines, 2019 to March 2020), the expected capacity for 2030
verifying the possibility of hydrogen production and how increased from 3.2 to 8.2 GW. In addition, an electrolyzer
the country can participate in the decarbonization of the with a capacity of 22 GW was announced in Australia.
economy. The implementation of green H2 production sites Further, new investments are being directed to the Arabian
and production capacity in different regions of Uzbekistan Peninsula. Three industries have announced a USD 5 billion
was analyzed, using a hierarchical methodology multi- project for a 4 GW green ammonia plant expected to start
criteria decision-making (MCDM) method in addition to operating in Saudi Arabia in 2025.162 At the same time, a
financial feasibility analysis.155 company at Al Duqm signed an understanding agreement
In Brazil, the pros and cons and economic feasibility with one group to invest USD 2.5 billion to start a green
of producing green H2 were studied based on the surplus H2 and green ammonia plant. China, in turn, expects to
production of hydroelectric and wind energy, identifying start operating electrolyzers of 70-80 GW by 2030.163 In
the possibility of production at costs comparable with the 2020 alone, 28 projects were announced across China. It is
current production of gray hydrogen.147,157 believed that by around 2030, the production of green H2
Studies on the implementation of green H2 production will become competitive, reaching a level of USD 2.60 kg–1
and its value chain have begun to be intensely discussed and less than USD 1.50 in 2050.86 Until then, the blue H2,
in the literature. However, there is a need for further with carbon capture, along with the production of green H2
studies, including analyses of more consolidated data on is the way to cleaner hydrogen production. This summarizes
production prices, coupling of production chains and uses, the main expansion projects in electrolyzer capacity by
and regulation of the sector. companies worldwide.
Projects related to green H2 production are still expensive
compared with those using fossil fuels. According to the 10. Green Hydrogen and Challenges for
International Energy Agency (IEA),160 green H2 from wind Chemistry
energy costs between USD 4-10 kg–1 and USD 7-17 kg–1
when solar energy is used, while gray H2 produced from In the current global scenario, humankind is confronted
fossil fuels costs USD 1-2.50 kg–1. However, the production with an oncoming scarcity of fossil fuel resources together
of gray H2 generates close to 9.5 kg CO2 kg–1 H2. For this with the ever-increasing need for energy. Energy is
reason, the carbon market becomes an ally for projects essential for human development and prosperity and a
involving green H2 to become economically viable.161 critical factor in achieving sustainable development.152-155
The current domestic market demand for green H2 is Hydrogen is considered by several experts, the energy
focused on industrial use, especially when safety issues carrier for revolutionizing the energy system since it is
a carbon-free fuel. Green hydrogen is a clean alternative management of chemicals and waste”), among others
for generating energy for the electricity, industry, heating, (Table S2).
and transportation sectors and to meet the 2030 and 2050 By transitioning from a carbon-based economy to a
agendas and, consequently, sustainable development.164-169 hydrogen economy, there will be a drastic reduction in
Considering hydrogen is a clean fuel capable of greenhouse compounds (GHGs) and the elevated-toxicity
promoting sustainable energy development, a hydrogen chemicals either exhausted from fossil fuel burning
economy is rising worldwide. Hydrogen fuel is crucial for or during petroleum exploitation to all environmental
decarbonisation and meeting climate neutrality (net zero) up compartments. Therefore, pollution-related diseases will
to 2050. In regard to the UN Sustainable Development Goals also decrease proportionally. Goals 14 and 15 will also be
(SDGs),169 those goals are, by conception, indivisible and met in a few decades by ceasing or drastically reducing
interlinked to each other. Or in other terms, when considering onshore and offshore fossil fuel exploitation. Considering
one specific SDG we should also consider many synergims that 2021-2030 is the decade of the Ocean designed by the
and some trade-offs among them, which are fully described UN, the transitioning to a hydrogen economy could also
by a matrix of 17 goals by 169 targets. At the Table S2 (SI synergistically go along with this movement and its actions.
section) we point out six SDGs and their associated targets In fact, by decreasing the global dependency on fossil
closely related to the sustainable nature of adopting green fuels for energy generation, it would alleviate considerably
hydrogen as energy resource in the future. Among the SDGs, geopolitical tensions which historically occur among
at first, the most closely related is the Goal 7 (affordable and different countries. Ultimately, it would also corroborates
clean energy) and all its targets (Table S2), such as “ensure with the Goal 16 (peace, justice, and strong institutions)
universal access to affordable and modern energy services”, and Goal 17 (artinership for the goals), which are critical
including a “substantial increase of the share of renewable to enable all the other SDGs.
energy in the global energy energy mix” by “promoting Launching a hydrogen-based economy will demand
investments in clean energy research technology”, and from Chemistry, Engineering, and associated fields fast and
“ensuring modern and sustainable energy services for solid actions to fill the gaps needed to become practical.134
developing countries”. Accordingly, Goal 13 (climate action) The bottlenecks in hydrogen-based energy open several
is also met since transitioning from a carbon-based to a green opportunities for those fields. Furthermore, enabling
hydrogen-based economy would consirerably decrease conditions to adopt green hydrogen as an energy carrier
GHG emissions, which are responsible for global warming. for humankind will synergistically match Industry 4.0 and
Synergistically, by meeting Goal 13, major improvements the Circular Chemistry/Circular Economy frameworks.
are also likely to be reached in regard to the Goals 14 (life However, there are some challenges ahead of us to be
below water) and 15 (life on land), not presented in Table S2. solved in the near future: (I) although the obvious benefits
Additionally, green hydrogen is also transversally of employing green hydrogen as an energy source, it remains
related to some other SDGs, such as Goal 3 (good health prohibitively expensive. New approaches for obtaining
and well-being, principally in relation to “the reduction efficient and low-cost energy are required. Hydrogen as fuel
of deaths and illnesses from harzardous chemicals and needs to be equally affordable and readily available for every
pollutants from air, water, and soil” and “global health country around the world, regardless of its current degree of
risk reduction and management”) and Goal 9 (industry, development; (II) hydrogen fuel plans are currently being
innovation, and infrastructure by “developing sustainable implemented to address energy needs in some countries.
and inclusive infraestructures and industrialization to On the other hand, those countries have varying maturity
significantly raise the industry share of employment and levels in using hydrogen in various contexts, using green
gross domestic product globally, but with special attention hydrogen, fossil-based hydrogen (with or without carbon
to the least developed countries”). sequestration), or a combination of both at various scales.
Green hydrogen economy is also associated to Widespread discussion and policy development are critical
Goal 11 (sustainable cities and communities, in the way in order to determine how much green hydrogen will be
it corroborates to “affordable and sustainable transport genuinely adopted by society, hence determining how much
systems”, and “to support positive economic, social, and infrastructure, research, development, and innovation will
environmental links to development planning, and also be done in the coming decades; (III) since green hydrogen
promoting inclusions, mitigation and adaptation to climate relies on electrolysis, new electrolyzers and cost-effective
change”) as well as the Goal 12 (responsible consumption and more efficient electrolyzer technologies (including the
and production, by “achieving the sustainable management development of new materials) are critical to making green
and efficient use of natural resources” and “responsible hydrogen competitive when compared with blue and brown
hydrogen; (IV) hydrogen storage and distribution is a key and Environment, PRH27.1 ANP/FINEP and FAPESB,
enabling factor to promote the hydrogen economy, and it is a Project MEPHYSTO (Biocomplexidade e Interações
real issue nowadays; (V) there is an urgency to transform the Físico-Químico-Biológicas em Múltiplas Escalas no
current infrastructure and materials for the (i) transportation Atlântico Sudoeste) from the Brazilian Antarctica Program
(automotive, train, maritime, and aviation) system, (ii) (CNPq No. 442695/2018-7). Special thanks to Ms Patricia
industrial plants and processes; (iii) electricity grid (jointly Resende for the big help with the figures.
to renewables, such as wind, biomass, solar PV, and tidal),
and (iv) domestic sector by research and development Author Contributions
and innovation (R&D&I); (VI) with the adoption of green
hydrogen, some regions that are already facing freshwater Lilian L. N. Guarieiro was responsible for conceptualization, formal
scarcity will have to surpass this obstacle. It provides a analysis, investigation, visualization, writing original draft, writing
new avenue for study into new methods for creating green review and editing; Jeancarlo P. dos Anjos, Luciana A. da Silva, Alex
hydrogen from seawater or wastewater. A. B. Santos, Ewerton E. S. Calixto, Fernando L. P. Pessoa, Jose L.
G. de Almeida, Miguel Andrade Filho, Flavio S. Marinho, and Gisele
11. Concluding Remarks O. da Rocha were responsible for formal analysis, investigation,
visualization, writing original draft and writing review; Jailson B.
Humanity is assuming a significant change of direction de Andrade was responsible for conceptualization, formal analysis,
towards conscient energy production and consumption, investigation, visualization, writing original draft, writing review
enabling the hydrogen economy and sustainable and supervision.
development. Although green hydrogen is still cost-
effective and has important drawbacks, the development of
new processes for guaranteeing the hydrogen chain seems Lílian L. N. Guarieiro received a degree
feasible within the following decades. A large array of in Chemistry from Centro Universitário
opportunities and challenges for the Chemical Sciences is de Lavras (2003), a master’s degree in
ahead of us right now. We need to plan the energy matrix Organic Chemistry and a specialization
transition up to 2050, and the real clean hydrogen economy in Petroleum Chemistry from Federal
fully applied in all continents up to 2070. University of Rio de Janeiro (2006), and a
Enabling sustainability necessitates ongoing efforts PhD in Analytical Chemistry from Federal University of
from various stakeholders in the political, industrial, Bahia (2010); she was in the sandwich doctorate program
economic, and social realms. Despite the numerous at Virginia Polytechnic Institute in Blacksburg, Virginia,
obstacles in (i) production, (ii) storage and distribution, USA; and was a post-doc at the National Institute of Science
and (iii) use and applications, the role of green hydrogen and Technology for Energy and Environment (2011). She
in the new world energy grid is conceivable and viable. is currently the coordinator of the professional master’s
On a regional basis, South America plays a critical program in sustainable development at SENAI CIMATEC.
role in the green hydrogen chain. As a result, Brazil has She has experience in environmental chemistry, analytical
a high chance of becoming a market leader in green H2. chemistry, and organic chemistry, with an emphasis on oil,
The diversity of Brazil’s renewable energy matrix (wind, gas, and biofuels.
solar, hydropower, and biofuels) creates opportunities
for several projects in the short- and medium-terms, Jeancarlo P. dos Anjos received a
including production via mature technologies such as water degree in Chemistry (2008) and a master’s
electrolysis and use of renewable electricity via the existing degree in agrochemistry (2010) from
grid, as well as storage and distribution of green H2 in the UFLA, and a PhD in Chemistry from UFBA
context of a new grid. (2014) with an emphasis on Analytical
Chemistry. He was a postdoctoral fellow at
Supplementary Information the National Institute of Science and Technology in Energy
and Environment (2014-2016). He is an adjunct professor
Supplementary information is available free of charge and vice-coordinator of the professional master’s degree
at http://jbcs.sbq.org.br as PDF file. program in sustainable development at SENAI CIMATEC
University Center. He has experience in chemistry,
Acknowledgments working mainly with the development and validation of
analytical methods, liquid and gas chromatography, sample
The authors would like to thank CNPq, INCT of Energy
preparation techniques, quality control of beverages, and Professor of the Chemical Engineering course, collider
environmental pollutants. and researcher of the Chemical and Biochemical Process
Intensification Group (GIPQB) in Brazil of University
Luciana A. da Silva received a PhD Center SENAI CIMATEC in Salvador-BA, Brazil.
from the Universidade Federal da Bahia
(UFBA) in 2001, and during the 2006 Fernando L. P. Pessoa received a PhD
academic year, she was a postdoctoral in Chemical Engineering from the Federal
fellow in Environmental Science and University of Rio de Janeiro and Lyngby
Engineering in the Division of Engineering University (Denmark) (1992). He worked
and Applied Science at the California Institute of at the Federal University of Bahia as a
Technology (CALTECH), USA, working with hybrid researcher (4 years), at the Camaçari-Bahia
photocatalytic systems for hydrogen production. She was Petrochemical Complex (6 years) and at the UFRJ Chemistry
an affiliate member of the Brazilian Academy of Sciences School for 25 years, where he became Full Professor. He
(2010-2014) and is currently a full professor at UFBA. is currently a Volunteer Professor at EQ-UFRJ and Full
Her research interests are in the areas of photocatalytic Professor at the SENAI CIMATEC University Center. He
hydrogen production and the synthesis and optical is Coordinator of the Competence Center for Process
properties of new semiconductor materials applied to Intensification at SENAI-CIMATEC. He has experience
photocatalytic reactions. in the field of Chemical Engineering, with an emphasis on
Applied Thermodynamics and Process Engineering, working
Alex Á. B. Santos received a DSc from mainly on the following subjects: oil, petrochemistry, natural
the Energy and Environment Program of products, supercritical fluid and phase balance.
the Interdisciplinary Center for Energy and
Environment (CiEnAm), Federal University José L. G. de Almeida graduated with
of Bahia (2010); a degree in Mechanical a degree in Chemical Engineering from
Engineering from the Federal University UFRJ (1979); received a master’s degree
of Bahia (1998); and a master’s degree in Mechanical in Chemical Engineering from Coppe
Engineering from the State University of Campinas (2001). UFRJ (1983) and a doctorate in catalysis
He is a professor and researcher at SENAI CIMATEC and from Université Claude Bernard, Lyon,
a senior member of the Brazilian Society of Engineering France (1995); graduated from the Advanced Business
and Mechanical Sciences (ABCM). He is also coordinator Management Program, Instituto Internacional San Telmo,
of the Stricto Sensu Graduate Program in Computational Spain (2007); and achieved specialization in data sciences
Modeling and Industrial Technology. His research interests at UFBA (2020). Fluent in English, French, and Spanish,
are in the areas of industrial combustion, formation and and intermediate level in German. General director of
control of soot and NOx, energy, thermal engineering, DETEN Química S.A. for 12 years, with experience in
industrial maintenance, energy efficiency of processes and project management (R&D). Currently works at SENAI
industrial equipment. CIMATEC as an executive manager, responsible for the
areas of chemistry, energy efficiency, pulp and paper,
Ewerton E. S. Calixto graduated environment, food, and laboratories.
in Chemical Engineering from Federal
University of Campina Grande (UFCG) Miguel Andrade Filho received
in 2007, concluded the master’s degree in a master’s degree in energy industry
the area of Chemical Process Integration regulation from Universidade Salvador
from Federal University of Rio de Janeiro (2007); graduated with a degree in
(UFRJ) in 2011 and concluded the doctorate in the same Mechanical Engineering from the Federal
area also in UFRJ. Professional experience in process University of Bahia (1976); specialized in
intensification of chemical and biochemical processes petroleum and petrochemical equipment (PETROBRAS,
(process integration, mass and energy integration), process 1977); received an MBA in business administration from
analysis (modeling, simulation and process optimization) Universidade Federal da Bahia; and currently works as
and also experience in onshore and offshore chemical/oil a consultant, researcher, and new business manager at
process units. Ewerton worked for large companies such SENAI CIMATEC University Center. He has experience
as Chemtech, Amec and Siemens. He is currently Adjunct in the areas of industrial maintenance, marketing, and
management in the oil and natural gas processing References

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https://dx.doi.org/10.21577/0103-5053.

J. Braz. Chem. Soc., Vol. 33, No. 8, 844-869, 2022

Technological Perspectives and Economic Aspects of Green Hydrogen in the

Lilian L. N. Guarieiro,a,b,c Jeancarlo P. dos Anjos,b,c Luciana A. da Silva, c,d

Alex Á. B. Santos,b Ewerton E. S. Calixto,b Fernando L. P. Pessoa,b

Keywords: green hydrogen, energy transition, sustainable development, decarbonization,

1. Introduction hydrogen (H2) industry. This industry, in turn, offers several

(Figure 2a). The formed word clusters show the trends in

Table 1. Classification of green H2 production methods

Type of energy used H2 production method Raw material

Reduction: MxOy → MxOy-δ + δ/2O2 (T > 1500 °C) (8)

Table 3. Overview of hydrogen storage types and their peculiarities

Reference Scale Type of storage Type of tank Requirements Note

93,102 large cryogenically as a liquid spherical tanks two 3200-m3 spheres,

Figure 10. Possible applications of green hydrogen (adapted from IRENA).112

reserve. It is not yet a competitive process for connection

management in the oil and natural gas processing References

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