3. The role of photophysics and photochemistry on water split process

## 3.1. Principles

Alternatives for purification of the reformate for the removal of CO and feed in PEMFC have been proposed by several researchers [3–10]. According to Rosseti et al. [5, 6], there are wellestablished routes, such as high- and low-temperature water-gas shift (WGS) and methanation, which can be integrated into the hydrogen production unit. Chen et al. [8] experimentally investigated a reaction system composed of two stages, an ethanol vapor reforming reactor (Ni/Al2O3 catalyst) followed by a water-gas shift reactor (Fe/Cr2O3 catalyst) to purify the hydrogen stream. In this study, four operational parameters including liquid flow, H2O/C molar ratio, reactor temperature and water-gas shift (WGS) reactor temperature were evaluated. The results indicated that the molar ratio H2O/C is the factor that most influences the

Another alternative widely evaluated in the available literature [9, 10] considers the use of reactive systems of hydrogen-permeable catalytic membranes, which can lead to the production of highly pure hydrogen and therefore enable direct integration between the reformer unit and PEMFC. Koch et al. [11] studied the ethanol-steam reforming process aiming to feed a PEM fuel cell to produce clean energy. The process consists of two stages as shown in Figure 1; the first stage produces a high hydrogen content gas via ethanol steam reformation. The second stage, a palladium-based membrane, separates the hydrogen from the rest of the reformed gas, producing high-purity hydrogen (>99.9999%), which prevents poisoning produced by impurities or fuel shortage. Koch et al. concluded that ethanol-steam reformer process was able to generate a pure hydrogen stream of up to 100 mm/min to feed the PEM

Based on the feasibility of energy cogeneration through fuel cells from biomasses such as ethanol, Rossetti et al. [6] performed the simulation and optimization of the H2 production process from the ethanol reformation with water vapor. The layout of the system was inspired by an existing unit in combined heat and power generation, with the purpose of evaluating the

performance of the system, which can be optimized to minimize CO formation.

fuel cell [11].

66 Advances In Hydrogen Generation Technologies

Figure 1. Simplified scheme of the reformer processes [11].

Solar energy is the unique renewable source that can fulfill the world's needs for the future [12]. The direct conversion of solar energy into renewable hydrogen fuel is done basically by two methods, photocatalysis and photoelectrochemical (PEC) water splitting. The first method relies on photocatalytically active particles suspended in aqueous electrolyte solutions, where one or both water-splitting half reactions take place. The second method uses photocatalytically active particles or thin films deposited on electrodes [13].

Photocatalysis involves photophysical processes, initiated by photon absorption, followed by the generation of excited states and finalized as a photochemical or electrochemical redox reaction. These excited states permit that a prohibitive reaction under certain conditions can occur by the use of a photocatalyst, and this reason makes photocatalysis interesting for solar energy conversion technologies [14].

On search (and development) of new materials/catalysts for water-splitting processes, a common approach is to mimic natural processes and/or analogue materials. In case of water splitting, the natural process is photosynthesis. Under this point of view, the central role of natural water-splitting process is occupied by an enzyme complex, known as photosystem II (PS II), capable to split water using sunlight [15].

Photons absorbed by this enzymatic complex are transferred to the catalyst core, where a single charge separation takes place [4]. This catalyst core in PS II is a Mn4CaO5 oxo-bridged complex, represented by two similar models in Figure 2, but its exact reaction mechanism is still obscure [16].

Chlorophyll fluorescence is used to provide information on many aspects of photosynthesis. There are two different quenching mechanisms for chlorophyll fluorescence, a photochemical and a non-photochemical quenching. The first one is caused by charge separation at PS II reaction centers and can be considered a reliable measure of the PS II charge separation rate.

Figure 2. Representations of PS II core catalyst [16].

The second one may be due to a number of other non-radiative de-excitation processes in PS II [17].

Pijpers et al. punctuate that is necessary to separate light collection/conversion from catalysis. Whereas light collection/conversion generates one electron/hole pair at a time, water splitting is a four-electron/hole global process 4 as shown in reaction (1). This part of the process is particularly demanding once it involves the formation of double bonds between oxygen, four protons and four electrons [18]. Reaction (1) is known as oxygen evolution reaction (OER).

$$2\text{H}\_2\text{O} \xrightarrow{} \text{O}\_2 + 4\text{H}^+ + 4\text{e}^- \tag{1}$$

3.2. Artificial design

represented in Figure 3.

3.3. Catalysts

water splitting [21].

following the reactions represented in Eqs. (3) and (4).

Figure 3. General representation of a photocatalyst [14].

Pijpers et al. affirm that an artificial photosynthesis design must guarantee that one electronhole pair of a semiconductor be integrated with the catalyst to perform OER [4]. A general representation of an ideal photocatalyst, as proposed by Hisatomi, Takanabe and Domen, is

Focusing on artificial processes, the evolution of oxygen, by UV-illuminated single crystals of TiO2, suspended in water, was firstly reported by Frank and Honda et al. [22], in 1972. Further investigations in the photoelectrochemical behavior of TiO2 leads to an increase in the interest on metal-oxide-based materials such as catalysts and with some time the development of a mixed catalyst for the mediation of water cleavage by visible light (Pt/RuO2 is cited as an example). In parallel, some earth-abundant (Mn, Fe, Co, Ni) 3d-metal-based materials were developed [20]. By now, the interest on TiO2 particles resides in its use as support material for

Photoanodes of CdS were also used to cleavage water molecules induced by visible light 11

One associated issue of these electrodes is the photocorrosion in the time which deactivates these electrodes, increasing cost and causing maintenance to be difficult. These photoanodes were improved by coating with polypirrole-inhibiting photocorrosion of CdS anodes into Cd2+

PHOTOANODE : CdS <sup>þ</sup> 2H<sup>þ</sup> ➔ Cd2<sup>þ</sup> <sup>þ</sup> <sup>S</sup> (3)

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PHOTOCATHODE : 2e� þ 2H2O➔H2 þ 2OH� (4)

The subsequent, less demanding process is the reduction of H<sup>+</sup> into two hydrogen molecules, as shown by reaction (2), and known as hydrogen evolution reaction (HER).

$$4\text{H}^+ + 4\text{e}^- \bullet 2\text{H}\_2 \tag{2}$$

It is important to point that, different from artificial processes, that usually have high overpotentials, this natural multi-protein complex uses only small driving forces and moderate activation energies [19].

#### 3.2. Artificial design

Pijpers et al. affirm that an artificial photosynthesis design must guarantee that one electronhole pair of a semiconductor be integrated with the catalyst to perform OER [4]. A general representation of an ideal photocatalyst, as proposed by Hisatomi, Takanabe and Domen, is represented in Figure 3.

### 3.3. Catalysts

The second one may be due to a number of other non-radiative de-excitation processes in

Pijpers et al. punctuate that is necessary to separate light collection/conversion from catalysis. Whereas light collection/conversion generates one electron/hole pair at a time, water splitting is a four-electron/hole global process 4 as shown in reaction (1). This part of the process is particularly demanding once it involves the formation of double bonds between oxygen, four protons and four electrons [18]. Reaction (1) is known as oxygen evolution reaction (OER).

The subsequent, less demanding process is the reduction of H<sup>+</sup> into two hydrogen molecules,

It is important to point that, different from artificial processes, that usually have high overpotentials, this natural multi-protein complex uses only small driving forces and moderate

as shown by reaction (2), and known as hydrogen evolution reaction (HER).

2H2O ➔ O2 þ 4H<sup>þ</sup> þ 4e� (1)

4H<sup>þ</sup> þ 4e� ➔ 2H2 (2)

PS II [17].

Figure 2. Representations of PS II core catalyst [16].

68 Advances In Hydrogen Generation Technologies

activation energies [19].

Focusing on artificial processes, the evolution of oxygen, by UV-illuminated single crystals of TiO2, suspended in water, was firstly reported by Frank and Honda et al. [22], in 1972. Further investigations in the photoelectrochemical behavior of TiO2 leads to an increase in the interest on metal-oxide-based materials such as catalysts and with some time the development of a mixed catalyst for the mediation of water cleavage by visible light (Pt/RuO2 is cited as an example). In parallel, some earth-abundant (Mn, Fe, Co, Ni) 3d-metal-based materials were developed [20]. By now, the interest on TiO2 particles resides in its use as support material for water splitting [21].

Photoanodes of CdS were also used to cleavage water molecules induced by visible light 11 following the reactions represented in Eqs. (3) and (4).

$$\text{PHOTOANODE}: \text{CdS} + 2\text{H}^+ \blackrightarrow \text{Cd}^{2+} + \text{S} \tag{3}$$

$$\text{PHOTOCATION} : 2\text{e}^- + 2\text{H}\_2\text{O} \blackheadrightarrow 2\text{OH}^- \tag{4}$$

One associated issue of these electrodes is the photocorrosion in the time which deactivates these electrodes, increasing cost and causing maintenance to be difficult. These photoanodes were improved by coating with polypirrole-inhibiting photocorrosion of CdS anodes into Cd2+

Figure 3. General representation of a photocatalyst [14].

ions, following the equations represented in reaction (5), more complex kinetically and thermodynamically less favorable [22].

$$2\mathrm{H}\_{2}\mathrm{O} + 4\mathrm{H}^{+} \bullet \mathrm{O}\_{2} + 4\mathrm{H}^{+} \tag{5}$$

electrodeposited on hematite, capable of evolving oxygen and practically transparent. This optical transparency property permits the composition of the oxide catalyst into a hematite/ perovskite tandem water-splitting cell. Solar-to-hydrogen conversion efficiencies are reported to be around 2%. The development of the catalyst and its integration into photoanodes open

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Following a nanostructuring and morphology control approach [13], numerous efforts have been taken to investigate morphologies and nanostructures of the NiCo-oxide and also the relationship between the nanostructure and the electrocatalytic activity. Compared to NiCo oxides, NiCo sulfides appear as promising electrocatalysts [13]. More recent works show nickel cobalt selenide (NiCo-selenide) as a better electrocatalyst candidate when compared to

Cocatalysts are combined to catalysts in order to increase its efficiency [13]. It is concluded that the role of cocatalysts is different in photocatalysis and in photoelectrochemical water splitting for LaTiO2N/Co3O4 systems. Cocatalyst is not only material and concentration dependant, but it also depends of the size distribution of particles. Small particles are favorable for photocatalysis due to kinetic effects. In PEC, larger cocatalyst particles are better, probably due to avoiding charge recombination. Under this point of view, the design of a cocatalyst size

Recently, Garcia-Esparza et al. [27] report the manufacture of an oxygen insensitive system (MoOx/Pt/SrTiO3), that, when coated on metal surfaces, can evolute hydrogen and avoid recombination of H2 and O2 onto water, maintaining H2 evolution at high taxes. Catalyst candidates can also be proposed using computational chemistry tools. Many distinct properties such as optical and transport properties electronic-band structure, density of states, and others, can be accurately determined by distinct levels of theory. Recently, Reshak [28] proposed that Tl10Hg3Cl16 single crystals can be used as a catalyst in water splitting due to electronic and optical properties calculated by density functional theory/mBJ. Its results are compared to experimental data to conclude that these computational tools can be used with

The proton exchange membrane fuel cell (PEMFC) is a device that converts chemical energy into electrical energy through an electrochemical reaction [29]. At the anode, the hydrogen or other fuel that may produce pure hydrogen like the ethanol-steam reforming process is oxidized and releases protons and electrons. The protons are carried out to the cathode side through electrolyte and the electrons produce electrical current in an external circuit. At the cathode, the electrons and the protons are associated with oxygen (often from atmospheric air) producing water and energy as a final product from PEMFC; thus, the fuel cell system does not

new possibilities in solar fuel production [26].

its oxide-based and sulfide-based species [20].

4. Proton exchange membrane fuel cell

produce pollutants and is ambient compatible.

must be function to the solar light energy conversion process [13].

good levels of precision, helping to elucidate internal details of these systems.

Inert anodes are also applied on water oxidation, usually coated by a catalyst. One example is the use of ITO or FTO electrodes, coated by a self-assembled amorphous film, generated by electrodeposition of cobalt salts into phosphate, methyl phosphonate or borate electrolytes [18]. That approach has as a characteristic the need of an electrolyte medium that is a key factor for activity, selectivity and formation of the self-assembled amorphous film. The selection for cobalt resides on the fact that its tetranuclear oxo core mimics the natural oxygenevolving complex (OEC) of PSII [23]. Regarding cobalt, thin films of Co-Pi (cobalt-phosphate) are deposited on Fe2O3, WO3 and ZnO electrodes, focusing on reducing the onset potential for water oxidation leading to performing the process in neutral pH conditions. A posterior approach deposits Co-Pi thin films on the ITO film attached to an np-Si solar cell, directing the voltage produced by the solar cell to reduce the cited over-potential [15].

Cobalt is also reported to be associated with molybdenum-based polyoxometalates with the limitations that this system must contain tris(2,2<sup>0</sup> -bipyridine)ruthenium (II) (Ru(bpy)3 2+) and sodium persulfate (S2O8 <sup>2</sup>�) in an aqueous borate buffer solution at pH 8.0 [24]. Berardi et al. [23] presents a new class of isostructural cubane-shape catalysts Cobalt-based, Hydrogen substituted by Me, t-Bu, OMe, Br, COOMe and CN, capable to water oxidation under dark or illuminated conditions, unfortunately again under highly basic conditions (pH = 8.0) in the presence of (Ru(bpy)3 2+) as photosensitizer and sodium persulfate (S2O8 <sup>2</sup>�) as electrolyte. The best quantum efficiency (QE) decrescent substituent order is determined as OMe > COOMe > Me ≈ H ≈ Br ≈ CN > t-Bu. In conclusion, the combination of semiconductor-electrocatalystelectrolyte interfaces is mandatory on water-splitting photocatalysis.

Yellow scheelite monoclinic BiVO4 is also used as a photocatalyst for O2 evolution under visible light, in the presence of an appropriate electron acceptor. But due to its conduction band bottom limit is located on a more positive potential than the potential of water reduction; it is incapable of evolving into hydrogen [25]. However, BiVO4 doped with In and Mo produces a Bi(1-X)In(X)V(1-X)Mo(X)O4 catalyst, with a more negative conduction band than H+ /H2, making it capable of water splitting at neutral pH-evolving hydrogen with no use of any sacrificial agent [25].

Following additional layers' approach, Si electrodes gain focus again in 2014, with Kaiser and Jaegermann's [12] work. The electrochemical properties of single-crystalline p-type 3C-SiC films on p-Si and n-Si substrates are investigated as electrodes in H2SO4 aqueous solutions, under dark and light conditions. The photoelectrochemical measurements on different wavelengths indicate the p-SiC film on p-Si substrate which can generate a cathodic photocurrent, corresponding with hydrogen production, and can also generate an anodic photocurrent, for oxygen evolution. Iron nickel oxide (FeNiOx) is also used for water splitting. One remarkable example is reported by Morales et al. on which an amorphous layer of the oxide is oxidatively electrodeposited on hematite, capable of evolving oxygen and practically transparent. This optical transparency property permits the composition of the oxide catalyst into a hematite/ perovskite tandem water-splitting cell. Solar-to-hydrogen conversion efficiencies are reported to be around 2%. The development of the catalyst and its integration into photoanodes open new possibilities in solar fuel production [26].

Following a nanostructuring and morphology control approach [13], numerous efforts have been taken to investigate morphologies and nanostructures of the NiCo-oxide and also the relationship between the nanostructure and the electrocatalytic activity. Compared to NiCo oxides, NiCo sulfides appear as promising electrocatalysts [13]. More recent works show nickel cobalt selenide (NiCo-selenide) as a better electrocatalyst candidate when compared to its oxide-based and sulfide-based species [20].

Cocatalysts are combined to catalysts in order to increase its efficiency [13]. It is concluded that the role of cocatalysts is different in photocatalysis and in photoelectrochemical water splitting for LaTiO2N/Co3O4 systems. Cocatalyst is not only material and concentration dependant, but it also depends of the size distribution of particles. Small particles are favorable for photocatalysis due to kinetic effects. In PEC, larger cocatalyst particles are better, probably due to avoiding charge recombination. Under this point of view, the design of a cocatalyst size must be function to the solar light energy conversion process [13].

Recently, Garcia-Esparza et al. [27] report the manufacture of an oxygen insensitive system (MoOx/Pt/SrTiO3), that, when coated on metal surfaces, can evolute hydrogen and avoid recombination of H2 and O2 onto water, maintaining H2 evolution at high taxes. Catalyst candidates can also be proposed using computational chemistry tools. Many distinct properties such as optical and transport properties electronic-band structure, density of states, and others, can be accurately determined by distinct levels of theory. Recently, Reshak [28] proposed that Tl10Hg3Cl16 single crystals can be used as a catalyst in water splitting due to electronic and optical properties calculated by density functional theory/mBJ. Its results are compared to experimental data to conclude that these computational tools can be used with good levels of precision, helping to elucidate internal details of these systems.
