Fundamentals of Water-Splitting Reaction: Comparison
Please note this is a comparison between Version 1 by Parnapalle Ravi and Version 3 by Sirius Huang.

A viable energy source, hydrogen has the advantages of clean energy, high conversion efficiency, and environmental friendliness. One of the possible routes for the generation of hydrogen in this context is the solar water splitting reaction under direct sunlight. To succeed on a commercial basis, though, there remains a very long way to go. 

  • solar energy
  • photocatalytic water splitting
  • hydrogen production

1. Introduction

The water-splitting process entails several photophysical and photochemical stages. Initially, the first process is started by photon absorption, which creates an electron–hole (e-h+) pair. The electrons will move to the conduction band and the holes remain at the valance band. The position of the CB minimum (CBM) should be more negative than the water reduction potential of H+/H2 (0 V vs. RHE), while the VB maximum (VBM) should be more positive than the water oxidation potential of H2O/O2 (1.23 V). In order to generate more electron–hole pairs, effective light absorption with minimal light scattering is the crucial factor in addition to bandgap engineering of the material [1][2][3][35,37,38]. Moreover, it is the fact that particle catalysts have a considerable light scattering issue rather than an absorption one. The electron–hole pair should be separated and diffuse towards the respective redox co-catalyst sites present in the photocatalyst. Due to (a) charge carriers’ short lifespan and (b) chemical processes’ lengthy time scales, a significant portion of charge carriers (>90%) undergo recombination; this leads to poor net redox reaction, which is one of the major issues of photocatalysis. It should be stressed that the photocatalyst’s function is essential for efficient light harvesting.
General redox reactions that take place to produce oxygen and hydrogen as follows:
2 H + + 2 e H 2
2 H 2 O + 4 h + 4 H + + O 2
The main factors increasing SWS efficiency are electron–hole pair separation and diffusion. It should be emphasized once more that charge recombination occurs more quickly than charge usage. To address this issue, the scientific community has created a number of materials and techniques [4][16].

2. Hydrogen Generation Using Scavengers/Sacrificial Agents

As previously mentioned, it is essential to separate the electron-hole pairs and diffuse them towards the redox co-catalyst sites to reduce the amount of recombination. In this context, use of sacrificial agents is one of the strategies involved to improve the efficiency. In order to devour the holes, organic molecules are often utilized as the sacrificial agents and involved in photooxidation. In the literature, a plethora of sacrificial agents such as alcohols, amines, and sulphides are used for better HERs [5][39]. The choice of the sacrificial agent must be carefully considered in order to achieve effective HER. To facilitate the simple electron/hole transfer between light-harvesting photocatalysts and sacrificial reagents, the energy levels (VBM and CBM) of the two materials must match for the sacrificial agent to be chosen. In this context, Na2S/Na2SO3 is used for high HERs in the majority of chalcogenide-based photocatalysts, including CdS, PbS, ZnS, and MoS2, which are also known to be visible light-active materials. However, for improved hole utilization and thus superior HERs, TiO2 and titania-based composites employ alcohol molecules including methanol, ethanol, and glycerol. However, triethanol amine (TEOA) is used as a sacrificial agent for enhanced HERs in the instance of graphitic carbon nitride (g-C3N4), which is recognized to be one of the intriguing materials that functions as a visible light-active material with a bandgap of 2.4 eV [6][8]. However, the quantity of hydrogen produced from the sacrificial agent and water is still up for discussion along with oxidized byproducts such as CO2.
Here is the detailed hole utilization mechanism of categorized sacrificial reagents such as alcohols, amines, and sulphides as follows [5][7][39,40].

2.1. Alcohols (Glycerol)

2.1. Alcohols (Glycerol):

Glycerol adsorbed on the surface of photocatalyst will interact with holes and forms radicals as shown in Equation (5). Once glycerol engages the holes, the excited electrons on the conduction band consequently produce H2 gas as shown Equations (3)–(9). The hydroxyl glycerol in the second stage is unstable, and it interacts with holes, resulting in the formation of glyceraldehyde as shown in Equation (6). Glyceraldehyde dissociates further producing an unstable CO radical that combines with a few holes and eventually converts to CO2 as shown in Equations (7) and (8).
M e t a l   o x i d e   N P   h v h + + e
3 H 2 O + 3 e 3 H + + 3 O H *
C 3 H 8 O 3 + 3 h + C 3 H 5 O 3 * + 3 H +
C 3 H 5 O 3 * + 3 h + C 3 H 2 O 3 * + 3 H +
C 3 H 2 O 3 * + 2 h + 3 C O * + 2 H +
3 C O * + 3 O H * 3 C O 2 + 3 H +
Overall net reaction
14 H + + 14 e 7 H 2

2.2. Triethanolamine (TEOA)

2.2. Triethanolamine (TEOA):

In the case of TEOA as sacrificial reagent, TEOA becomes formaldehyde and then to hydrogen as shown in Equations (14) and (15).
P h o t o c a t a l y s t   h v h + + e
2 H + + 2 e   H 2  
h + + H 2 O     O H * + H +
h + + O H     O H *
T E O A + O H * H C H O + N H 3
H C H O + O H * H 2 + b y   p r o d u c t s C O 2

2.3. Sodium Sulphide and Sodium Sulphite Mixture (Na


S and Na





P h o t o c a t a l y s t   2 h v 2 h + + 2 e  
A t C B 2 e + 2 H 2 O   H 2 + 2 O H
A t V B 2 h + + S O 3 2 + 2 O H S O 4 2 + H 2 O  
2 S O 3 2 + 2 h +     S 2 O 6 2
2 S 2 + 2 h + S 2 2  
S 2 2 + S O 3 2   S 2 O 3 2 + S 2
In this type of systems, sulphide (S2−) and sulphite (SO32−) can act as sacrificial inorganic reagents for the photocatalytic hydrogen generation because they are very efficient hole acceptors, enabling the effective separation of the charge carriers. The oxidation of S2− and SO32− can occur either by a two-electron transfer process (16) to (18) or one-electron oxidation (19) to (21).

3. Overall Water Splitting

Overall water splitting (OWS) is an act of producing hydrogen that involves utilizing water as the lone reactant at a pH of 7 with the right photocatalyst under full sunshine. Even if HER and OER happen as shown in Equations (1) and (2), the rate of HER is much lower than it would be with a sacrificial agent. As a matter of fact, the HER values reported from OWS are at least 2–3 orders of magnitude less than those reported using sacrificial agents. The OER (Equation (2)) is a four electron process that cause sluggish kinetics and lowers the total rate of OWS. In this regard, several catalyst materials have been published in the literature with the aim of enhancing the overall kinetics of OWS and so achieving efficient hydrogen production. In this scenario, Domen and colleagues as well as several others have made contributions using a Z-scheme method to effective hydrogen production [8][9][10][11][12][13][14][15][17,29,41,42,43,44,45,46]. In which BiVO4:Mo and SrTiO3:La,Rh used as photocatalyst-I (PS-I) and photocatalyst-II (PS-II), and Au used as electron mediator. The CB of PS-I is closer to the VB of PS-II via Au metallic electron mediator to reduce the resistance to the migration of electrons and holes. Whenever the composite got excited with light energy, both PS-I and PS-II generates photoexcitons and the electrons at CB of PS-I will combined with the holes at VB of PS-II via Au mediator. The terminal electrons and holes will take place in redox reaction. However, compared to the anticipated STH (solar to hydrogen) conversion efficiency values (10% and higher), the efficiency (1%) that has been recorded so far is quite low for commercial-scale hydrogen generation. There is a chance to raise the total rate of OWS by employing alternate strategy such as the thin film technique. The efficiency of converting solar energy into hydrogen is often measured using one of two techniques as follows.
Apparent   quantum   yield   AQY = 2 × no   of   hydrogen   molecules no   of   photons × 100
From the above equation, the number of incident photons can be measured using a radiant power energy meter, which considers the following equation (Equation (23)):
E = nhv = nh C λ
where E stands for incoming light energy, n stands for photon number, h for Planck’s constant (6.634×1034 j S), c for light velocity, and λ for incident light wavelength.

4. Factors for Achieving the High Efficiency of SWS

It is practically impossible for a single semiconductor material to function as an effective SWS photocatalyst and to carry out all three of the key processes of (a) light absorption from the broad spectrum of sunlight, (b) charge separation and diffusion to redox sites, and (c) actual redox reactions.
Habitually, a composite photocatalyst made up of at least two (for example, Pt and TiO2), three (for example, Au, rGO, and TiO2), or more (for Z-scheme photocatalysts) components are effective to move closer to overcome the above-mentioned key processes. Very few researchers have tried to combine the three basic photocatalytic processes using effective synthetic techniques [2][16][17][37,47,48]. Furthermore, a charge diffusion is one approach that might be used to improve the activity by seamlessly combining the charge generating sites and charge usage (or redox) sites [4][16]. To accomplish this, designing a highly integrated single composite material for effective hydrogen generation from SWS is required. A systematic increase in the complexity of the photocatalyst design of Au-gC3N4/TiO2 (221 mmol h−1·g−1) outperforms SWS kinetics from g-C3N4/TiO2 (91 mmol h−1·g−1) [18][49]. When an integrated Au–N–TiO2–graphene composite was used instead of titania alone, the HER of 525 mmol h−1·g−1 was found [19][50]. The improvement in activity with 1275 mmol h−1·g−1 and sustained activity for 125 h were attributed to the electronic integration of an Au–Pt bimetal cluster with titania [20][51]. On titania, a similar observation was achieved using Au–Ag or Au–nanorod [21][22][52,53]. According to a recent study, a single titania nanotube material with heterojunctions between native and nonnative structures, such as the presence of the anatase, rutile, and brookite phases, and Pt as the co-catalyst exhibited higher HER activity from SWS under one sun conditions than anatase-rutile (2.5 mmol h−1·g−1) or bare anatase (0.15 mmol h−1·g−1) [23][24][54,55]. The difference in activity is attributable to the existence of bulk heterojunctions in a composite material as opposed to a single junction (anatase-rutile) or bare anatase nanotubes, which can aid in effective charge transfer between the particles. Independent of the photocatalyst systems, the outcomes given here suggest that SWS activity increased from a negligible value to 7.6 mmol h−1·g−1 [16][17][18][19][20][21][22][23][47,48,49,50,51,52,53,54]. Furthermore, the HER was measured using a photocatalyst made of Au–Pd/rGO/TiO2 both in particulate and thin-film forms by easy casting on a glass plate. The measured HERs from thin-film and particulate versions are 21.5 and 0.50 mmol h−1·g−1, respectively. It is specifically due to the difference of light absorption. Despite having all of the necessary photocatalyst components, the HER values are incredibly low for particulate photocatalyst systems and the evaluation of HER in the particulate form of the photocatalyst composite acts as a unifying factor among all these findings. Simple comparisons between photocatalyst suspension and the identical photocatalyst preserved in an aqueous methanol solution as a thin film demonstrate that the former exhibits low light penetration while the later exhibits high light penetration [25][56]. Regardless of the angle of light incidence, light penetration would be inadequate with photocatalyst suspension.
Although there are multiple reasons that prevent SWS from operating at a high efficiency, major SWS limiting issues and some potential solutions are provided as listed below:
The primary limiting element is the very different time scales between photophysical and photochemical processes and how to connect them. Structural and electronic components integration is crucial to be optimized for efficient diffusion of charge carriers to redox sites and their exploitation for redox reactions. Quantum dots (QDs) and 2D-layered materials might be used to overcome the aforementioned issue. It is also important to note that the nanoscience, which only involves photophysical processes, has made a significant contribution to the rapid expansion of applications involving light emission. It is very desirable to use synthesis methods that would result in bulk heterojunctions in a photocatalyst composite. For instance, the assembly of QDs in the pores of wide bandgap materials using techniques such as SILAR (successive ionic layer adsorption and reaction) results in bulk heterojunctions.
In general, scaling up the catalyst quantity, even from 10 to 100 to 1000 mg at a laboratory level, substantially reduces the effectiveness of any photocatalyst system. This is largely because of poor light absorption combined with excessive charge recombination [26][57]
Since OWS is often carried out in extremely acidic environments [27][58], it is essential to be able to conduct the tests at a pH close to neutral (pH = 7). There is definitely a need for greater study in this area.
Noble metals are frequently used, which is not a cost-effective solution, and there is no reasonable consideration given to the selection of a certain co-catalyst for a specific semiconductor [27][58]. More research must be done to examine more affordable and plentiful co-catalysts, with a focus on rational selection.
It is necessary to switch to using environmentally beneficial and/or biomass-derived materials such as glycerol and cellulose instead of sacrificial ones such as methanol. Sacrificial agent use in water splitting may be a temporary fix, and OWS will be the long-term fix [5][39].
Despite the fact that CdS, PbS, and other chalcogenide QDs have very strong visible light absorption qualities and the capacity to control the bandgap, they are also vulnerable to photo-corrosion and are unfriendly to the environment [28][29][59,60]. The oxide-based QDs should be the focus of additional efforts.
Until now, a lot of work has gone into creating effective systems for the SWS process, which produces hydrogen. However, none of the photocatalysts can produce hydrogen in a practical manner. Indeed, under sunshine or one-sun circumstances, none of the powder catalyst systems have been studied at the gram scale. Table 1 lists some of the top hydrogen production activity values obtained using several powder-based catalysts.
Table 1. The highest recorded solar hydrogen production using several photocatalyst systems that have been thoroughly studied in powder form.
The current analysis concentrates on the additional crucial factors that contribute to improving the overall effectiveness of solar light-driven water splitting. The current study stresses a thin film-based technique. It is advantageous to boost efficiency as opposed to the particulate-based research that is commonly used by many researchers worldwide.

4.1. Mechanical Stirring Is Unfavorable

It is difficult to analyze the photocatalyst powder on a big scale, and continual stirring of the powder solution is necessary to enhance light harvesting. It is an energy expensive when mechanical churning is used at such huge loads. Following the conclusion of reaction investigations utilizing photocatalyst powders, the collection of the catalyst using centrifugation and filtering is another time-consuming and energy-intensive step [40][71]. Above all, the essential notion of better light absorption does not appear to happen with an increase in the photocatalyst quantity in the suspension. Additionally, when the catalyst concentration rises, light cannot reach all areas of the solution due to the turbidity of the solution. Moreover, the price of hydrogen generated using traditional steam reforming techniques is $3–4 per kg, therefore any new approach needs to at least be competitive with that to be taken into consideration. The typical thin film method of producing solar cells would be advantageous for better light absorption, and many benefits and drawbacks of photocatalysts in their thin-film and particulate forms, respectively, will be examined in this context.

4.2. Loading Effect

It is anticipated that more catalyst will need to be loaded into the solution in order to produce solar hydrogen on a wide scale. In this context, Maeda et al. examined the impact of loading on the quantum yield of hydrogen for a Z-scheme photocatalyst using Pt/ZrO2/TaON as the hydrogen evolution catalyst and Pt/WO3 as the oxygen evolution catalyst [26][57]. It was discovered that the AQY drops from 6.3 to 2.7% when the total catalyst dosage rises from 75 to 150 mg. In contrast to the predicted rise in light absorption with an increase in the quantity of powder catalyst loading, only a portion of the particles are exposed to photons at a given moment, and the remainder particles are inactive. However, when there is only a tiny quantity of catalyst present in the test solution, the majority of the particles are exposed to photons and form a greater number of charge carriers, which helps to achieve high efficiency. This is directly corroborated by the research done by Nalajala et al., who found that using 25 mg of powder Pd/TiO2 in 40 mL of aqueous methanol produced less H2 (9 mmol h−1·g−1) than using 1 mg of the same catalyst under the same circumstances (32 mmol h−1·g−1) [41][34]. The hydrogen yield (HY) per gram of catalyst in water splitting studies with a tiny quantity of catalyst (1 mg) would appear to be quite high (Table 2). Reporting the hydrogen evolution rate for a few different weights for a certain volume of water or aqueous solution is the preferred method, since it enables other labs to replicate the findings. However, this raises the unavoidable question of what is the best method to use for handling massive quantities of catalyst for SWS.
Table 2. Highest recorded solar hydrogen production using several photocatalysts that were tested in powder form at low concentrations.

4.3. Scale-Up and Disintegration Issues of Photocatalysts

Under reaction circumstances, the catalyst system must remain intact in order to produce hydrogen from SWS sustainably. The catalyst system in particular is anticipated to be stable for prolonged exposure to light and a wet environment. The development of photocatalyst materials in the particulate form is now the focus of several initiatives. The greatest recorded hydrogen production activity using various catalyst systems are shown in Table 1; However, they were selected because (a) they could demonstrate sustainability for at least 10 h in one day of sunlight and (b) the catalyst quantity used in real studies needed to be at least 10 mg. Whatever the provided catalysts were confirmed to be active, only a small number of investigations have demonstrated their stability for 100 h or more [34][39][65,70]. A contrasting truth may be found by simply comparing the findings provided in Table 1 and Table 2. Higher HER values were seen with 1 mg of catalyst from the actual data (Table 2) compared to those shown in Table 1, which use 10–200 mg of catalyst. A suspension form of a catalyst cannot increase activity linearly with an increase in catalyst amount. A few studies also indicate a decline in activity within 10 h after the reaction, raising the likelihood of catalyst breakdown [30][61]. The activity and stability of the thin film were established by Schroder et al. [1][35], during the 30 days active light period, and Goto et al. investigated the films for 1000 h [52][36]. A stable photocatalyst would sustain HER activity for a very long time.
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