The Application of Nano Titanium Dioxide for Hydrogen Production and Storage Enhancement

Abstract

The utilization of hydrogen (H₂) as a renewable and clean energy carrier, free from the reliance on fossil fuels, represents a significant technological challenge. The use of renewable energy sources for hydrogen production, such as photocatalytic hydrogen generation from water under solar radiation, has garnered significant interest. Indeed, the storage of hydrogen presents another hurdle to the ongoing advancement of hydrogen energy. Concerning solid-state hydrogen storage, magnesium hydride (MgH₂) has emerged as a promising option due to its high capacity, excellent reversibility, and cost-effectiveness. Nevertheless, its storage performance needs improvement to make it suitable for practical applications. Titanium dioxide (TiO₂) has distinguished itself as the most extensively researched photocatalyst owing to its high photo-activity, good chemical and thermal stability, low toxicity, and affordability. This review highlights the application of TiO₂ for hydrogen production under visible and solar light, with a particular focus both on its modification without the use of noble metals and its utilization as a catalyst to enhance the hydrogen storage performance of MgH₂.

1. Introduction

The extensive consumption of fossil fuels, particularly over the past century, has significantly contributed to environmental pollution. As environmental and socio-economic consciousness continues to grow, governments and legislative bodies worldwide are expressing concern and examining the factors associated with pollution that affect the energy landscape. Consequently, there is an urgent requirement to replace fossil fuels with renewable, eco-friendly, and alternative energy sources [1,2,3,4,5].

Therefore, to date, the most pressing scientific and technological challenge is the development of a renewable and clean energy that does not rely on fossil fuels. From this perspective, one of the most appealing options for large-scale utilization is hydrogen as a recyclable energy carrier. Consequently, green approaches to hydrogen production, such as the photocatalytic process from water under solar radiation, has garnered considerable interest [6,7]. For photocatalytic hydrogen production, titanium dioxide (TiO₂) represents a promising candidate among various inorganic semiconductor photocatalysts [8,9] due to its high efficiency, high stability, non-toxicity, and low cost. However, the practical application remains constrained, since TiO₂ responds solely to ultraviolet light, which constitutes approximately 4% of the solar spectrum. Therefore, it is mandatory to design and synthesize TiO₂-based photocatalytic systems capable of reacting to the whole visible light range, accounting for ~43% of the solar spectrum, thus improving the efficiency [10,11]. The storage of hydrogen to provide grid energy from intermittent energy sources is another major challenge for the further development of hydrogen energy [12]. On the other hand, magnesium hydride (MgH₂) stands out as one of the most promising candidates in solid-state hydrogen storage thanks to its high capacity (7.6 wt%), excellent reversibility, and cost-effectiveness. Nonetheless, its practical application is constrained by its high operating temperature requirements and suboptimal kinetic performance [13]. An effective approach to enhance the hydrogen storage performance of MgH₂ involves the introduction of catalysts, which may encompass metals [14,15,16], metal oxides [16,17,18,19], metal sulfides [20,21], and metal halides [19,22].

Focusing on work published in the last decade, this review firstly delves into the utilization of TiO₂ for hydrogen production under visible and solar light, as well as its role in enhancing the hydrogen storage performance of MgH₂. Successively, in the domain of hydrogen production, various strategies aimed at enhancing TiO₂ performance without the use of noble metals are explored.

2. Titanium Dioxide

TiO₂ is the most widely investigated photocatalyst due to its remarkable photo-activity, strong chemical and thermal stability, low toxicity, and cost-effectiveness [23]. In nature, four polymorphs of TiO₂ are found: anatase (tetragonal), rutile (tetragonal), brookite (orthorhombic), and TiO₂(B) (monoclinic). Additionally, two high-pressure forms have been synthesized starting from rutile: TiO₂(II) and TiO₂(H) [24]. However, rutile and anatase are indeed the two most common crystalline forms of TiO₂, and they find numerous applications owing to their unique properties [25]. TiO₂ is an n-type semiconductor, and for bulk materials, anatase has a band gap of 3.20 eV, which corresponds to an absorption threshold at a wavelength of 384 nm, while rutile has a band gap of 3.02 eV, which corresponds to an absorption threshold at the 410 nm wavelength [24]. Therefore, near-ultraviolet (UV) radiation is necessary to excite anatase, whereas the photo-response of rutile slightly extends into the visible light spectrum [26]. These varying band gap values can be attributed to differences in the lattice structures of anatase and rutile TiO₂, resulting in distinct densities and electronic band structures [24]. The TiO₂ photocatalytic activity depends on several factors, including phase structure, specific surface areas, crystallite size, and pore structure. Moreover, the photocatalytic activity of anatase is generally considerably higher than that of rutile [27]. The absorption of photons with energy equal to or greater than the band gap results in the generation of electron–hole pairs through the excitation of electrons from the valence band to the conduction band of the semiconductor. When both electrons and holes migrate to the surface of semiconductor without recombination, the photogenerated electrons can participate in reduction reactions, while the holes engage in oxidation reactions. These processes form the basis for photocatalytic water splitting and the photodegradation of organic pollutants [28].

2.1. Hydrogen Production Exploiting TiO₂ under Visible and Solar Light

Water splitting is an uphill reaction that needs the standard Gibbs free energy change of 237 kJ mol⁻¹ [2]. It consists of two half reactions [3]:

2H⁺ + 2e⁻ → H₂

(1)

2H₂O + 4h⁺ → O₂ + 4H⁺

(2)

Therefore, the overall reaction is [2]:

H₂O → H₂ +1/2 O₂

(3)

In order for a semiconductor to function as a photocatalyst for water splitting, it is imperative that the bottom of its conduction band exhibits a more negative potential than the reduction potential of H⁺/H₂ at 0 V versus the normal hydrogen electrode (NHE). Simultaneously, the top of its valence band should possess a more positive potential than the oxidation potential of O₂/H₂O at 1.23 V versus NHE. Therefore, for efficient water splitting, the band gap of the semiconductor should exceed 1.23 eV; conversely, to harness visible light, the band gap should be less than 3.0 eV [29]. Accordingly, a band gap falling within the range of (1.23–3.0) eV is a necessary requisite. In addition, photocatalytic H₂ generation is influenced by some factors such as surface area and particle size, band gap energy, corrosion resistance, and the presence of a sacrificial agent [3] such as glycerol [1], triethanolamine (TEOA) [10], ethanol [30], or methanol [11].

For H₂ production, the performance of TiO₂ is generally limited by the high recombination rate of the photogenerated electrons and holes, an inability to utilize visible light, and fast backward reactions [31]. Loading TiO₂ with noble metals such as Pt, Au [32,33,34], or Pd [35] allows the photocatalytic efficiency of TiO₂ to be enhanced by reducing the fast recombination of photogenerated charge carriers [11] and allows the TiO₂ response to the entire visible light spectral range to be extended [3]. However, the use of noble metals is expensive, and their availability is limited; consequently, their use is unsuitable for large-scale energy production [11]. For these reasons, other strategies have been optimized to improve the photocatalytic efficiency of TiO₂, such as employing non-noble metals, metal oxides, cadmium sulfide (CdS), and molybdenum disulfide (MoS₂).

Below, different TiO₂ modification strategies that have been evaluated under visible and solar light are presented.

2.1.1. Modification of TiO₂ with Non-Noble Metals and Metallic Oxides

Numerous approaches have been developed to improve the photocatalytic response of TiO₂, in terms of both broadening the reaction spectrum to the entire visible spectral range [11] and mitigating the rapid recombination phenomena of electron–lacuna pairs [36].

The incorporation of non-noble metals (such as Fe, Cu, Co, Ni, and Cr) or metallic oxides onto TiO₂ represents a valid alternative to noble metals. In particular, the use of this approach can effectively hinder the recombination of electron–hole pairs and promote the separation of charge carriers [37].

Subha et al. [38] synthesized a photocatalyst system based on TiO₂ doped with Cu and Zn (CuZn-TiO₂). TEM images and XPS spectra showed the presence of Cu₂O, CuO, ZnO, and Cu⁰ on the TiO₂ surface. Metal–semiconductor junctions and heterojunctions between semiconductors were formed due to the growth of Cu₂O, CuO, ZnO, and Cu⁰ on the surface of TiO₂. Effective charge separation due to the heterojunctions and high electron mobility due to Cu⁰ improved the H₂ production. In addition, Cu⁰, CuO, and Cu₂O allowed the absorption of TiO₂ to shift to visible light. In particular, the CuZn-TiO₂ with an optimum loading of Cu and Zn of 0.5 wt% achieved a H₂ production of about 14,521 μmol h⁻¹ g⁻¹_cat under solar light, using glycerol as sacrificial reagent. A plausible mechanism for H₂ production is reported in the work of Subha et al. [38] and is schematically represented in Figure 1.

A cobalt (Co)-doped TiO₂ photocatalyst was synthesized by Sadanandam et al. [36] through the impregnation method. The presence of cobalt species on the TiO₂ surface of the synthesized photocatalyst was observed. The Co/TiO₂ photocatalyst, with 1 wt% of Co doped on TiO₂ P25, achieved a maximum hydrogen production of 11,021 μmol h⁻¹ g⁻¹ from aqueous glycerol solutions under solar light irradiation, showing that Co²⁺ ions can extend the photo-response of TiO₂ into the visible region [36].

Díaz et al. [11] prepared a series of photocatalyst systems consisting of TiO₂ P25 combined with Fe, Co, Ni, Cu, and Zn (denotated as Fe/TiO₂, Co/TiO₂, Ni/TiO₂, Cu/TiO₂, and Zn/TiO₂, respectively) using the impregnation method. The TiO₂ was loaded with 2 wt% of the selected transition metal and evaluated under both visible and UV light irradiation. During visible light exposure, with methanol serving as a sacrificial reagent, the photocatalysts Ni/TiO₂, Co/TiO₂, Zn/TiO₂, and Fe/TiO₂ exhibited lower H₂ production rates than the pure TiO₂. Conversely, Cu/TiO₂ demonstrated a substantial enhancement in photocatalytic activity, yielding H₂ production rates equal to 220 μmol h⁻¹ g⁻¹ [11].

Kotesh Kumar et al. [39] developed a photocatalyst system comprising bimetallic Cu-Ni alloy NPs decorated on TiO₂ P25. The Cu-Ni/TiO₂ photocatalyst, with a Cu loading of 2 mol% and Ni loading of 5 mol%, achieved a remarkable H₂ evolution rate of 35.4 mmol g⁻¹ h⁻¹ under solar light conditions, employing methanol as a sacrificial reagent. Additionally, the monometallic photocatalysts Cu-doped TiO₂ (Cu/TiO₂) and Ni-doped TiO₂ (Ni/TiO₂) were also synthesized and assessed. It was observed that the monometallic photocatalysts exhibited lower H₂ yields compared to the Cu-Ni/TiO₂ systems [39]. Reddy et al. [40] fabricated mesoporous nano TiO₂ and introduced Fe/TiO₂ catalysts by synthesizing and introducing Fe dopants onto the TiO₂ surface through the impregnation process. These catalysts were tested under solar light irradiation and the H₂ production rate of 270 μmol/h was attained with 0.5 wt% of Fe³⁺ loading. A comparison between Fe doping on the TiO₂ surface and Fe doping into the TiO₂ lattice revealed that metal ions should be doped in the proximity to the TiO₂ surface to achieve efficient photocatalytic activity through enhanced charge transfer [40]. Police et al. [1] prepared a Cu₂O/TiO₂/Bi₂O₃ ternary photocatalyst: the presence of Cu₂O and Bi₂O₃ allows the visible-light absorption to be extended and helps minimize the electron–hole recombination of TiO₂, respectively. When exposed to solar light irradiation, the ternary photocatalyst, with 2 wt% Cu and 2 wt% Bi, demonstrated an impressive H₂ production rate of 3678 μmol/h in glycerol aqueous solution. Furthermore, the dependence of photocatalytic H₂ production on the glycerol and catalyst content was investigated. It was observed that under an optimal glycerol concentration and catalyst amount, a H₂ production rate of 6727 μmol/h was achieved [1].

A photocatalyst consisting of mesoporous ZrO₂-TiO₂ NPss anchored on reduced graphene oxide (rGO) was synthesized by Subha et al. [41]. The ZrO₂-TiO₂/rGO photocatalyst, with a loading of 1.0 wt% of ZrO₂ and rGO on TiO₂, exhibited remarkable H₂ production (~7773 μmol h⁻¹ g⁻¹) in a glycerol aqueous solution under solar light irradiation. Furthermore, a ZrO₂-TiO₂ photocatalyst, with 1.0 wt% of ZrO₂ loading on TiO₂, was also examined, and it achieved an H₂ production rate of 5781 μmol h⁻¹ g⁻¹. The ZrO₂-TiO₂ heterojunction anchored on rGO sheets demonstrated the suppression of electron–hole pair recombination and an enhancement in the efficiency of interfacial charge transfer [41].

Praveen Kumar et al. [42] synthesized a photocatalyst composed of CuO deposited onto TiO₂ nanotubes. The photocatalyst, with a copper loading of 1.5 wt%, exhibited the highest photocatalytic hydrogen evolution rate under solar light using glycerol as a sacrificial agent. It achieved an impressive H₂ production rate of 99,823 μmol h⁻¹ g⁻¹ [42].

Subha et al. [43] synthesized a Cu_xO/TiO₂ nanocomposite loaded with layered Ni(OH)₂. Under solar light irradiation and in the presence of glycerol (5 vol%), the Ni(OH)₂-Cu_xO-TiO₂ photocatalyst, with an optimal loading of 1.0 wt% Ni(OH)₂ and 0.5 wt% Cu_xO, showed a H₂ production of ~15,789 μmol h⁻¹g⁻¹. Furthermore, Cu-doped TiO₂ photocatalysts (Cu_xO/TiO₂) were tested. It was shown that the photocatalyst with 0.5 wt% Cu loading on TiO₂ allowed an advancement in H₂ production (~7679 μmol h⁻¹ g⁻¹) [43].

2.1.2. Coupling TiO₂ with CdS

The CdS semiconductor has been extensively studied as a sensitizer for photocatalytic water splitting due to its narrow band gap and strong absorption in the visible-light spectral region [44]. Combining TiO₂ with a CdS semiconductor can further enhance the photocatalytic performance of TiO₂ [45]. Luo et al. [46] synthesized a CdS/TiO₂(B) type II heterojunction via photo deposition, with CdS dots anchored on TiO₂(B) nanosheets. Under visible light, a H₂ evolution rate of 1577 μmol g⁻¹ h⁻¹ was achieved. Additionally, a CdS/TiO₂(B) type I heterojunction was synthesized via the hydrothermal method; however, it resulted in a H₂ evolution rate of 48 μmol g⁻¹ h⁻¹ under visible light, significantly lower than that obtained with the type II heterojunction. The proposed mechanism for H₂ production by CdS/TiO₂(B) reported by Luo et al. [46] is schematically represented in Figure 2.

Zhu et al. [47] utilized TiO₂ nanotube arrays, obtained by anodizing titanium foils, in conjunction with CdS quantum dots to create CdS quantum dots decorated TiO₂ nanotube arrays. Under visible light and in the presence of Na₂SO₃ and Na₂S as sacrificial reagents, a H₂ production rate of 1.89 μmol h⁻¹ cm⁻² was achieved [47].

Gao et al. [44] prepared TiO₂ microspheres with highly exposed (001) facets loaded with CdS. They observed that CdS/TiO₂ microsphere composites with a CdS:TiO₂ molar ratio of 1 indicated the highest activity, resulting in a H₂ production rate of 76.55 μmol/h when lactic acid was used as the scavenger for photo-generated holes under visible light [44].

A porous TiO₂ co-doped with CdS and WO₃ was synthesized by Qian et al. [45]. The ternary porous CdS/WO₃/TiO₂ photocatalyst, with a molar ratio of CdS:WO₃:TiO₂ equal to 8:8:100, achieved a H₂ evolution rate of 2106 μmol g⁻¹ h⁻¹ under visible light using Na₂S and Na₂SO₃.

Furthermore, the binary CdS/TiO₂ and WO₃/TiO₂ porous photocatalysts were tested, resulting a H₂ evolution rate of 1056 μmol g⁻¹ h⁻¹ and 981 μmol g⁻¹ h⁻¹ for CdS/TiO₂ and WO₃/TiO₂, respectively. The possible mechanism for H₂ production using the CdS/WO₃/TiO₂ photocatalyst proposed by Qian et al. [45] is schematically represented in Figure 3.

2.1.3. Coupling TiO₂ with MoS₂

The efficiency of photocatalytic hydrogen production can also be improved by exploiting MoS₂ as a co-catalyst for TiO₂ [48]. Several strategies have been developed to improve the performance of MoS₂/TiO₂ photocatalysts under visible light. Yuan et al. [49] introduced a composite photocatalyst system based on TiO₂ nanosheets and MoS₂, in which the visible light absorption of TiO₂/MoS₂ was expanded by incorporating CuInS₂ quantum dots as a light-harvesting inorganic dye. The ternary CuInS₂/TiO₂/MoS₂ photocatalyst with 0.6 mmol g⁻¹ of CuInS₂ and 0.5 wt% of MoS₂ exhibited a H₂ evolution rate of 1034 μmol h⁻¹ g⁻¹ under visible light in the presence of Na₂S and Na₂SO₃ as sacrificial reagents. A possible mechanism for H₂ production is reported in the work of Yuan et al. [49] and is schematically represented in Figure 4. A Zn(II)-5,10,15,20-tetrakis(4-carboxyphenyl)-porphyrin (ZnTCPP)-sensitized MoS₂/TiO₂ photocatalyst was synthesized by Yuan et al. [10], exploiting the ZnTCPP to improve the absorption in visible light and the MoS₂ to enhance the photocatalytic activity. The ZnTCPP-MoS₂/TiO₂ photocatalyst was prepared by synthesizing MoS₂/TiO₂ composites, and then adsorbing the ZnTCPP dye on the TiO₂ surface. TiO₂ P25 was used. A H₂ production rate of 10.2 μmol h⁻¹, under visible light, was achieved using the photocatalyst loaded with 1.00 wt% of MoS₂. TEOA was used as the sacrificial electron donor in the photocatalytic hydrogen evolution experiments. Shen et al. [50] synthesized MoS₂ nanosheet–porous TiO₂ nanowire hybrid nanostructures using the hydrothermal method. A H₂ evolution rate of 16.7 mmol h⁻¹ g⁻¹ was achieved under visible light in the presence of Eosin Y dye in TEOA aqueous solution. Flower-like MoS₂@TiO₂ catalysts were synthesized by Ma et al. [51] using a hydrothermal method, using a metal–organic framework (MOF) as a precursor. The MoS₂@TiO₂ photocatalysts, with a loading of 14.6 wt% of MoS₂, allowed a H₂ production rate of 10,046 μmol h⁻¹ g⁻¹ under visible light to be achieved in the presence of fluorescein as photosensitizer in an aqueous acetone solution and using TEOA as a sacrificial donor.

2.1.4. Coupling TiO₂ with MoS₂ and CdS

Du et al. [52] developed a photocatalyst based on a porous TiO₂ monolith with MoS₂ nanosheets and CdS nanoparticles grown on the porous TiO₂. The MoS₂-CdS-TiO₂ photocatalyst was designed to exploit both CdS and MoS₂ to extend the absorption to visible light and to improve the hydrogen production efficiency. The MoS₂-CdS-TiO₂ photocatalyst with 3% of MoS₂ and 10% of CdS (molar ratio to TiO₂) showed a H₂ generation rate of 4146 μmol h⁻¹ g⁻¹ using Na₂S and Na₂SO₃ as sacrificial reagents under visible light. In addition, CdS-TiO₂ and MoS₂-TiO₂ photocatalysts were also tested. A H₂ generation rate of 875 μmol h⁻¹ g⁻¹ and 1360 μmol h⁻¹ g⁻¹ was obtained with CdS-TiO₂ and MoS₂-TiO₂, respectively. Qin et al. [53] prepared MoS₂/CdS-TiO₂ nanofibers via an electrospinning-mediated photo-deposition method. The MoS₂/CdS-TiO₂ photocatalyst, with a loading of 1 wt% of MoS₂, showed a H₂ evolution rate of 280.0 μmol h⁻¹ in a lactic acid aqueous solution under visible light.

The main works that describes TiO₂-based photocatalysts modified with non-noble metals, metallic oxides, CdS, and MoS₂ for H₂ production under visible and solar light were summarized in

Table 1. TiO₂-based photocatalysts modified with non-noble metals, metallic oxides, CdS, and MoS₂ for H₂ production under visible and solar light.

Photocatalyst	Crystalline Phases of TiO2	Loading	Irradiation	Sacrificial Reagents	H2 Production	Ref.
Cu2O/TiO2/Bi2O3	anatase	2 wt% of Cu	solar light	glycerol	6727 μmol h−1	[1]
		2 wt% of Bi
ZnTCPP-MoS2/TiO2	anatase/rutile	1.00 wt% of MoS2	visible light	TEOA	10.2 μmol h−1	[10]
Cu/TiO2	anatase/rutile	2 wt% of Cu	visible light	methanol	220 μmol h−1 g−1	[11]
Co/TiO2	anatase/rutile	1 wt% of Co	solar light	glycerol	11,021 μmol h−1 g−1	[36]
CuZn-TiO2	anatase	0.5 wt% of Cu	solar light	glycerol	14,521 μmol h−1 g−1	[38]
		0.5 wt% of Zn
Cu-Ni/TiO2	anatase/rutile	2 mol% of Cu	solar light	methanol	35.4 mmol h−1 g−1	[39]
		5 mol% of Ni
Fe/TiO2	anatase	0.5 wt% of Fe3+	solar light	-	270 μmol h−1	[40]
ZrO2-TiO2/rGO	anatase	1.0 wt% of ZrO2	solar light	glycerol	7773 μmol h−1 g−1	[41]
		1.0 wt% of rGO
CuO/TiO2	anatase/rutile	1.5 wt% of Cu	solar light	glycerol	99,823 μmol h−1 g−1	[42]
Ni(OH)2-CuxO-TiO2	anatase	1.0 wt% of Ni(OH)2	solar light	glycerol	7679 μmol h−1 g−1	[43]
		0.5 wt% of CuxO
CdS/TiO2	anatase	CdS:TiO2 molar ratio of 1	visible light	lactic acid	76.55 μmol h−1	[44]
CdS/WO3/TiO2	anatase	molar ratio 8:8:100	visible light	Na2S and Na2SO3	2106 μmol h−1 g−1	[45]
CdS/TiO2		-			1056 μmol h−1 g−1
WO3/TiO2		-			981 μmol h−1 g−1
CdS/TiO2(B)	TiO2(B)	5% of CdS	visible light	Na2S and Na2SO3	1577 μmol h−1 g−1	[46]
CdS/TiO2	anatase	-	visible light	Na2S and Na2SO3	1.89 μmol h−1 cm−2	[47]
CuInS2/TiO2/MoS2	anatase	0.5 wt% of MoS2	visible light	Na2S and Na2SO3	1034 μmol h−1 g−1	[49]
MoS2/TiO2 nanowire and Eosin Y	anatase	-	visible light	TEOA	16.7 mmol h−1 g−1	[50]
Flower-like MoS2@TiO2 and fluorescein	anatase	14.6 wt% of MoS2	visible light	TEOA	10,046 μmol h−1 g−1	[51]
MoS2-CdS-TiO2	anatase	3% of MoS2 and 10% of CdS molar ratio to TiO2	visible light	Na2S and Na2SO3	4146 μmol h−1 g−1	[52]
CdS-TiO2		-			875 μmol h−1 g−1
MoS2-TiO2		-			1360 μmol h−1 g−1
MoS2/CdS-TiO2 nanofibers	anatase	1 wt% of MoS2	visible light	lactic acid	280.0 μmol h−1	[53]

.

2.2. Exploiting the TiO₂ Properties for Hydrogen Storage

For solid-state hydrogen storage, several materials such as MgH₂ [13,54,55,56], NaAlH₄ [57], LiAlH₄ [58], and LiBH₄ [59,60] have been considered due to their higher gravimetric and volumetric hydrogen densities [61]. However, their practical applications are limited due to their sluggish kinetic and high dehydrogenation temperature [54,57,58,62]. Concerning MgH₂ [13,54,55,56,63,64,65,66,67,68], NaAlH₄ [57,69,70], LiAlH₄ [71,72] and LiBH₄ [60,62], the use of TiO₂ as a catalyst has been investigated in order to improve the dehydrogenation/rehydrogenation kinetics of these materials, showing an improvement in performance by adding TiO₂. Moreover, the use of TiO₂ has also been investigated for MgH₂-NaAlH₄ composite [61] and 4MgH₂-LiAlH₄ composite [73]. Both composites showed significant improvements in the dehydrogenation/rehydrogenation kinetics with the introduction of TiO₂.

Application of TiO₂ to Enhance the Hydrogen Storage Performance of MgH₂

Several strategies have been adopted to improve the hydrogen storage performance of MgH₂ by exploiting TiO₂ as catalyst. A carbon-supported nanocrystalline TiO₂ was synthesized by Zhang et al. [63] as a catalyst precursor. The carbon-supported nanocrystalline TiO₂ (TiO₂@C) exhibited good catalytic activity in the hydrogen storage reaction of MgH₂. It was observed that the TiO₂@C-containing MgH₂ composite with a TiO₂@C content of 10 wt% released 6.5 wt% hydrogen within 7 min at 300 °C. Moreover, the dehydrogenated sample took up approximately 6.6 wt% hydrogen within 10 min at 140 °C under 50 bar of H₂. Chen et al. [54] explored the catalytic effect of synthesized Co/TiO₂ nanocomposite on the hydrogen desorption/absorption properties of MgH₂. It was observed that using Co/TiO₂ highly improved the hydrogen desorption/absorption kinetics of MgH₂ in comparison to using Co or TiO₂ separately. The MgH₂-Co/TiO₂ composite showed an absorption of 6.07 wt% H₂ within 10 min at 165 °C under 60 bar of H₂ pressure and a dehydrogenation peak temperature of 235.2 °C. A sandwich-like structure composed of Ti₃C₂ and carbon-supported anatase TiO₂ (Ti₃C₂/TiO₂(A)-C) was developed by Gao et al. [55] in order to exploit the laminar MXene and the corresponding metal oxide to enhance the hydrogen absorption and desorption kinetics of MgH₂. The Ti₃C₂/TiO₂(A)-C structure was prepared using a heat treatment. The catalyst-doped MgH₂ composites with 5 wt% of Ti₃C₂/TiO₂(A)-C released approximately 5 wt% hydrogen within 1700 s at 250 °C under 0.05 MPa of hydrogen pressure, with a larger rate constant of 0.258 wt% min⁻¹. An uptake of approximately 4 wt% hydrogen within 800 s at 125 °C, under 3 MPa hydrogen pressure, was also observed. Unlike the preparation of the TiO₂/MXene heterostructure via the heat treatment of MXene, Gao et al. [13] also developed self-assembled TiO₂/Ti₃C₂T_x heterostructures through a one-step ultrasonic method at room temperature. The TiO₂ nanoparticles were self-assembled onto a few layers of Ti₃C₂T_x. The TiO₂/Ti₃C₂T_x-MgH₂ composite with 5 wt% of TiO₂/Ti₃C₂T_x achieved the fastest dehydrogenation and hydrogenation kinetics, showing a hydrogen desorption of 5.98 wt% within 600 s at 300 °C and a hydrogen absorption of 5.90 wt% within 1200 s at 175 °C. A catalyst composed of TiO₂ nanoparticles deposited on the pore wall of a three-dimensionally ordered macroporous (3DOM) structure was synthesized by Shao et al. [56]. It was observed that the 3DOM TiO₂ catalyst improved the hydrogen storage properties of MgH₂. The MgH₂-3DOM TiO₂ composite showed a hydrogen desorption of 5.75 wt% within 1000 s at 300 °C. A hydrogen absorption of 4.17 wt% within 1800 s even at 100 °C was observed. Jardim et al. [64] investigated the effects of TiO₂ nanorods on the absorption/desorption properties of MgH₂. The TiO₂ nanorods were produced through titanate nanotube heat treatment, whereas titanate nanotubes were synthesized via the alkaline hydrothermal route using commercial TiO₂ anatase as a starting material. At 350 °C, the MgH₂-TiO₂ nanorod composite showed a hydrogen absorption of 5.5 wt% after 10 min under 10 bar of hydrogen pressure. Furthermore, it was observed that after 5 min at 350 °C under 0.1 bar of hydrogen pressure, the composite desorbed around 100% of the absorbed hydrogen. A Ni/TiO₂ cocatalyst with a Ni@TiO₂ core–shell structure was synthesized by Zhang et al. [65] via a modified hydrothermal synthesis method. The MgH₂-Ni/TiO₂ composite with 5 wt% of Ni/TiO₂ allowed a hydrogen absorption of 4.50 wt% to be obtained at 50 °C under an initial hydrogen pressure of 3.0 MPa and a hydrogen desorption of 5.24 wt% in 1800 s at 250 °C under 0.005 MPa of hydrogen pressure [65]. Zhang et al. [66] synthesized and investigated three-dimensional flower-like TiO₂ (fl-TiO₂) and three-dimensional flower-like carbon-wrapped TiO₂ (fl-TiO₂@C) as the catalysts for MgH₂. Both catalysts enhanced the hydrogen sorption kinetics of MgH₂. Superior desorption kinetics of the MgH₂-fl-TiO₂@C composite compared to the MgH₂-fl-TiO₂ composite were observed. The MgH₂-fl-TiO₂@C composite allowed a hydrogen desorption of 6.0 wt% to be obtained in 7 min at 250 °C under a hydrogen pressure of 1 kPa. Concerning the absorption, the MgH₂-fl-TiO₂@C composite showed a hydrogen absorption of 6.3 wt% at 150 °C within 40 min under 5 MPa of hydrogen pressure. Liu et al. [67] synthesized graphene-supported TiO₂ nanoparticles (TiO₂@rGO) using the solvothermal method. It was observed that the MgH₂-TiO₂@rGO composite desorbed 6.0 wt% hydrogen within 6 min at 300 °C and absorbed 5.9 wt% hydrogen within 2 min at 200 °C. The enhancement of the catalytic activity can be due to fine, uniform TiO₂ nanoparticles that were obtained by exploiting the combined effect of ethylene glycol and graphene during the solvothermal process. In addition, rGO can act as an electronic conductive channel to enhance the catalytic effect. Zhang et al. [68] synthesized TiO₂ nanosheets with exposed {001} facets in order to exploit the nanometer-size and highly active {001} facets of anatase TiO₂ to enhance the reaction kinetics of MgH₂. The MgH₂-TiO₂ nanosheet composite allowed a hydrogen desorption of 6.0 wt% to be obtained at 260 °C within 192 s and, in addition, a hydrogen desorption of 1.2 wt% within 300 min at 180 °C was also observed. Furthermore, the composite absorbed 6.1 wt% hydrogen within 10 s at 150 °C. Also, a hydrogen absorption of 3.3 wt% hydrogen at 50 °C within 10 s was obtained.

The general strategy to improve H₂ storage performance of MgH₂ using TiO₂-based catalysts are reported in

Table 2. Summary of strategies adopted to improve H₂ storage performance of MgH₂ using TiO₂-based catalysts.

Composite	Hydrogen Desorption	Hydrogen Desorption Properties (Time, Temp., Pressure)	Hydrogen Absorption	Hydrogen Absorption Properties (Time, Temp., Pressure)	Ref.
MgH2-TiO2/Ti3C2Tx	5.98 wt%	600 s, 300 °C, 0.05 MPa	5.90 wt%	1200 s, 175 °C, 3 MPa	[13]
MgH2-Co/TiO2	6.25 wt%	10 min, 265 °C, 0.02 bar	6.07 wt%	10 min, 165 °C, 60 bar	[54]
	6.2 wt%	15 min, 250 °C, 0.02 bar	5.56 wt%	10 min, 130 °C, 60 bar
	5.4 wt%	100 min, 220 °C, 0.02 bar	4.24 wt%	10 min, 100 °C, 60 bar
	4.6 wt%	100 min, 210 °C, 0.02 bar	~ 1 wt%	10 min, 50 °C, 60 bar
MgH2-Ti3C2/TiO2(A)-C	5 wt%	1700 s, 250 °C, 0.05 MPa	4 wt%	800 s, 125 °C, 3 MPa	[55]
MgH2-3DOM TiO2	5.75 wt%	1000 s, 300 °C, 0.005 MPa	4.17 wt%	1800 s, 100 °C, 3 MPa	[56]
	5.74 wt%	3000 s, 275 °C, 0.005 MPa	5.40 wt%	1000 s, 175 °C, 3 MPa
MgH2-TiO2@C	6.5 wt%	7 min, 300 °C, -	6.6 wt%	10 min, 140 °C, 50 bar	[63]
MgH2-TiO2 nanorods	~5.5 wt%	5 min, 350 °C, 0.1 bar	5.5 wt%	10 min, 350 °C, 10 bar	[64]
MgH2-Ni/TiO2	5.24 wt%	1800 s, 250 °C, 0.005 MPa	4.50 wt%	120 min, 50 °C, 3.0 MPa	[65]
	6.08 wt%	600 s, 300 °C, 0.005 MPa
MgH2-fl-TiO2@C	6.0 wt%	7 min, 250 °C, 1 kPa	6.3 wt%	40 min, 150 °C, 5 MPa	[66]
	4.9 wt%	60 min, 225 °C, 1 kPa	5.0 wt%	40 min, 100 °C, 5 MPa
	3.0 wt%	100 min, 200 °C, 1 kPa	3.9 wt%	40 min, 50 °C, 5 MPa
MgH2-fl-TiO2	6.0 wt%	8 min, 250 °C, 1 kPa	6.0 wt%	40 min, 150 °C, 5 MPa	[66]
	3.9 wt%	60 min, 225 °C, 1 kPa
	0.9 wt%	100 min, 200 °C, 1 kPa
MgH2-TiO2@rGO	6.0 wt%	6 min, 300 °C, 0.0004 MPa	5.9 wt%	2 min, 200 °C, 3 MPa	[67]
MgH2-TiO2 nanosheets	6.0 wt%	192 s, 260 °C, 1 kPa	6.1 wt%	10 s, 150 °C, 5 MPa	[68]
	1.2 wt%	300 min, 180 °C, 1 kPa	3.3 wt%	10 s, 50 °C, 5 MPa

.

3. Conclusions

This review has predominantly focused on the utilization of titanium dioxide (TiO₂) in the dual capacity of hydrogen (H₂) production and H₂ storage. Concerning the aspect of H₂ production, extensive efforts have been dedicated to enhancing the photocatalytic efficiency of TiO₂ under both visible and solar light, all while abstaining from the utilization of precious metals. In the realm of solar-driven H₂ production, TiO₂ has frequently undergone modifications involving non-precious metals and metal oxides. Glycerol has consistently featured as the sacrificial agent in the majority of the documented research endeavors.

In the context of visible-light-driven H₂ production, the strategies have commonly entailed the amalgamation of TiO₂ with MoS₂, complemented by the incorporation of a photosensitizer, and the fusion of TiO₂ with CdS. Turning our attention to the subject of H₂ storage, this review has unequivocally demonstrated the catalytic prowess of TiO₂ in augmenting the H₂ storage performance of magnesium hydride (MgH₂). A myriad of strategies has been employed, encompassing the synthesis of TiO₂ nanosheets, flower-like TiO₂ structures, TiO₂ nanorods, TiO₂ nanoparticles, TiO₂/Ti₃C₂Tx heterostructures, graphene-supported TiO₂ nanoparticles, carbon-enveloped nanocrystalline TiO₂, Co/TiO₂ nanocomposites, and Ni@TiO₂ core–shell structures.

The integration of MgH₂ with TiO₂-based catalysts has yielded notable enhancements in H₂ absorption and desorption kinetics, leading to lower operating temperatures for H₂ desorption and absorption processes, especially when compared to pure MgH₂.