Overview of the Recent Findings in the Perovskite-Type Structures Used for Solar Cells and Hydrogen Storage

Abstract

Perovskite-type structures have unique crystal architecture and chemical composition, which make them highly attractive for the design of solar cells. For instance, perovskite-based solar cells have been shown to perform better than silicon cells, capable of adsorbing a wide range of light wavelengths, and they can be relatively easily manufactured at a low cost. Importantly, the perovskite-based structures can also adsorb a significant amount of hydrogen atoms into their own structure; therefore, perovskite holds promise in the solid-state storage of hydrogen. It is widely expected by the scientific community that the controlled adsorption/desorption of the hydrogen atoms into/from perovskite-based structures can help to overcome the main hydrogen storage issues such as a low volumetric density and the safety concerns (i.e., the hydrogen embrittlement affects strongly the mechanical properties of metals and, as such, the storage or transport of the gaseous hydrogen in the vessels is, especially for large vessel volumes, challenging). The purpose of this review is to provide an updated overview of the recent results and studies focusing on the perovskite materials used for both solar cells and hydrogen storage applications. Particular attention is given to (i) the preparation and the achievable efficiency and stability of the perovskite solar cells and (ii) the structural, thermodynamic, and storage properties of perovskite hydrides and oxides. We show that the perovskite materials can not only reach the efficiency above current Si-based solar cells but also, due to good stability and reasonable price, can be preferable in the solid-state storage of hydrogen. Then, the future trends and directions in the research and application of perovskite in both solar cells and hydrogen storage are also highlighted.

1. Introduction

The combination of the enormous demand for energy caused by rapid global economic development, population growth, and recent environmental challenges such as pollution or global warming yields the pursuit of suitable renewable energy sources such as hydropower, wind, geothermal, tidal, solar thermal, photovoltaics and/or hydrogen [1,2,3,4,5]. Hydropower, which generates electricity from fast-running or falling water, can be used for both electricity production and energy storage. However, despite the apparent advantages of hydrogen power over other renewable energy sources (e.g., relatively large and stable source of energy), it has many serious limitations, such as a high construction cost (i.e., it can be built on only specific land areas) and a large impact on the local ecosystem [6]. High construction and/or maintenance costs are also one of the main drawbacks of tidal or geothermal energy sources [7,8]. Then, it is particularly the relatively low installation and maintenance costs that make the photovoltaic systems a highly attractive source of renewable energy [9]. In general, photovoltaic systems utilize solar modules; each consists of a large number of solar cells [10,11]. Interestingly, the first solar cells were silicon-based and were developed at Bell Laboratories in 1954. They utilized the photoelectric effect in the p-n junction of the doped silicon and could reach an efficiency of ~6% [12]. We remind the reader that silicon is the second most common element in the Earth‘s lithosphere. It is also non-toxic and has exceptional stability and optimal bandgap for photovoltaic conversion. In addition, the current semiconductor technology is silicon-based. Therefore, it enables the easy preparation of high-quality silicon wafers at a reasonable cost. As a result, silicon has become fundamental to the development of photovoltaic technology [13,14,15]. Note that in the past 50 years, the efficiency of silicon solar cells has improved more than three times [16,17]. Nowadays, silicon-based technology is approaching its own intrinsic efficiency limits of ~29.1% [17]. For example, by combining the Si heterojunction architecture (i.e., amorphous Si/crystalline Si) with integrated back contacts, the superior photoconversion efficiency of ~26.7% for silicon solar cells has been reported [18]. On the other hand, silicon-based solar cells have several limitations, such as the high environmental impact (i.e., their fabrication requires a significant amount of energy, as well as their recycling is complicated) and rigidity (i.e., no possible bending of cells yielding a limited shape design), the production cost cannot be significantly reduced and, finally, the efficiency which is close to their theoretical limits (i.e., efficiency above 30% is not easily being reachable) is also strongly weather condition dependent (i.e., the amount of produced electricity notably reduces for lower light exposure) [19,20,21].

Hence, to overcome these limitations, next-generation materials such as quantum dots [22], organic [23], dye-sensitized [24], copper zinc tin sulfide [25], and perovskite [26], often referred to as the “third generation” of solar cells, have been proposed and extensively studied. Among them, perovskite solar cells (PSCs) have attracted significant attention from the community due to their excellent optoelectronic properties such as high absorption coefficients [27,28], long charge-carrier diffusion lengths [29,30], low exciton binding energies [31,32], tuneable bandgaps [33], and power conversion efficiency (PCE) [34,35] but also a low manufacturing cost (i.e., can be prepared by roll-to-roll technology), lightweight and flexibility [36,37,38]. In general, the PSCs can be separated based on the used materials (compounds) into two categories: all-inorganic perovskites [39,40] and hybrid organic-inorganic perovskites [41,42]. The standard formulation of these perovskite compounds is ABX₃, where both A and B represent cations, and X denotes an anion (see Figure 1) [43,44,45]. Note that a variety of perovskite compounds are available in different forms, including carbides, nitrides, fluorides, chlorides, bromides, oxides, hydrides, and iodides [46,47]. When hydrogen, oxygen, halide atom, nitrogen, or fluorine replaces element X within a compound, the resulting entities are known as perovskite hydride, perovskite oxide, perovskite halide, perovskite nitride, and perovskite fluoride, respectively [48,49,50,51,52].

In 2009, the conversion efficiency of PSCs of 3.8% was reported in the pioneer study of Kojima et al. [53]. Since then, the efficiency of PSCs has increased several times. Recently, Yoo et al. [54] have shown that by utilizing the fluorine-doped tin dioxide with improved charge carrier management, an efficiency of 25.2% (i.e., efficiency is above other thin-film solar cells [55]) has been demonstrated. This study presented charge carrier management, and the optical advances will soon enable it to reach efficiency above 26% [56], that is, more than for Si-based solar cells. Another important use of perovskite is in the area of flexible solar cells [19]. We remind the reader that it is, particularly, a combination of lightweight and flexibility that makes flexible solar cells highly attractive for a large number of applications, including portable electrical chargers, electrical production on large-scale industrial roofs, powering various vehicles, bendable/wearable electronics, etc. In the flexible perovskite solar cells (F-PSCs), the main difficulties are related to the fabrication (i.e., the formation of the electron transport layer (ETL) is at a low temperature, below 150 °C, highly challenging) [57]. Hence, different fabrication approaches and strategies have been developed to design durable F-PSCs with high efficiency [58,59,60]. For example, the TiO₂ ETL prepared by the plasma-enhanced atomic layer deposition has enabled it to achieve good durability and a conversion efficiency of 12.2% [58]. More recently, Chung et al. [59] developed a low-temperature fabrication process of the porous planar ETL, which, in turn, enabled it to reach a superior certified efficiency of F-PSCs of 19.9%. Another approach to achieving high efficiency of F-PSCs is utilizing hybrid electrodes. For example, Paik et al. [60] have demonstrated an efficiency of 21.02% when using the SnO₂-TiO₂ hybrid electrodes.

In addition, perovskite materials have also been extensively studied due to their excellent hydrogen storage capacity (i.e., solid-state storage) [61,62,63]. It is noteworthy that hydrogen is the most common element in the universe with a large energy density, non(low)-toxic, carbon-free, and can also be used with some modifications within the current combustion engines [64,65,66,67,68]. In contrast to batteries, hydrogen does not have deep discharge and other negative effects, and, as such, the storage capacity does not degrade in time [69]. The storage and transportation of hydrogen remain challenging, and researchers have been diligently working on the development of novel materials for hydrogen storage [70,71]. It can be stored in either a liquid state through liquefaction or a gaseous state via gas compression or a solid state [72,73,74,75]. The gravimetric density and hydrogen desorption temperature are two important performance indicators for evaluating hydrogen storage materials [76,77]. Higher gravimetric density means more hydrogen can be stored in a smaller volume or mass of the storage material [78,79]. Hydrogen desorption temperature refers to the temperature at which hydrogen molecules are released from a solid storage material [80]. To achieve a controlled release of hydrogen, maintaining a moderate hydrogen desorption temperature is crucial. Hence, the advancement of hydrogen storage materials possessing high gravimetric density along with moderate hydrogen desorption temperatures is the key to enhancing the storage and utilization of hydrogen energy [77,81,82]. Among various perovskite compounds, the oxides and hydrides are the predominant structural configurations with potential applicability for hydrogen storage (e.g., they have good stability and can be prepared at a relatively reasonable cost) [83,84,85,86,87].

It is evident from the present discussion that the field of perovskite materials is growing exponentially in terms of both the published papers and research groups being involved. The purpose of this review is to provide an overview of the recent findings and trends in perovskite materials used in both emergent areas: photovoltaic and solid-state hydrogen storage. The review is organized as follows: Section 2 focuses on the perovskite materials for solar cell applications. The solid-state storage of hydrogen by utilizing perovskite is given in Section 3. The fundamental properties of perovskite that are of importance in hydrogen storage are also discussed in Section 4 and Section 5. Finally, a brief discussion on possible future trends in the field of perovskite is given in Section 6. Importantly, to our knowledge, this is the first review that combines both areas of research in perovskite materials. As a result, our review would help researchers keep tracking the current trends and findings in the perovskite materials and, correspondingly, can help researchers share their knowledge in two separate studies of perovskite.

2. Perovskite Materials Used in Photovoltaic

2.1. Perovskite Solar Cells

As mentioned previously in the Introduction, a large number of perovskite compounds have already been prepared. Then, it is the outstanding optical properties compared to all-inorganic perovskites that made hybrid organic-inorganic being predominantly used in the PSCs [88,89]. Recent research has shown that metal halide perovskites are highly potential compounds of hybrid organic-inorganic perovskites [90,91]. It is because of a combination of the energy tunable bandgap [92,93] and a highly flexible crystal lattice, which allows the formation of dimensionalities ranging from three-dimensional down to zero-dimensional [94,95]. Noticing that in these compounds, cation A stands for cesium, methylammonium (MA), and/or formamidinium (FA), whereas B is a divalent metal cation such as lead or tin; and X is a halide such as iodide, bromide and/or chloride as also depicted in Figure 1 [96,97].

Importantly, all of these perovskite materials can be prepared by either wet or dry synthesis [98,99]. Because of its simplicity, high repeatability, and precise control over stoichiometry, wet synthesis is more often used in PSCs [100,101]. However, there are several issues associated with wet synthesis, such as solubility, toxicity, solvent compatibility, and higher cost [50,102]. In contrast, dry synthesis enables the insolubility problem to be overcome through solid-state reactions, mechanochemical synthesis, and vapor phase deposition methods [103,104]. The PSCs generally consist of five fundamental layers: the perovskite light-absorber layer, the transparent conductive oxide as a conducting substrate (typically tin-doped indium oxide or fluorine-doped tin oxide), the metal (mainly gold, silver, or copper), and two charge transport layers, typically the ETL and the hole transport layer [105]. PSCs have a similar operating principle as conventional heterojunction or p-i-n solar cells. The light absorbed by the perovskite absorber layer produces free electron-hole pairs (excitons are weakly bound), which are subsequently transported to the charge transport layer interfaces, producing an open-circuit voltage and a photocurrent [106,107,108]. Restricting the analysis to the function of the hole transport material in optimizing PCE, its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels must be strategically aligned with those of the perovskite and the back electrode within the device. With vacuum level as the reference, the HOMO level of hole transport materials should be higher in energy than the perovskite valence band in order to facilitate hole transportation from the perovskite. Secondly, the LUMO of the hole transport materials should be as high as possible with respect to the perovskite conduction band, as shown in Figure 2 [109]. The latter conduction band offset would avoid electrons traveling from the perovskite to the hole transport materials and, as a result, reduce the current and voltage due to the recombination with holes in the hole-transporting layer or at the interfaces. Furthermore, the HOMO level of the hole transport materials should be positioned just below the Fermi level of the back electrode to ensure rapid charge collection [106,110,111].

PSC devices can also be divided based on their planar architecture into conventional (n-i-p) PSCs and inverted (p-i-n) PSCs. (see Figure 3) [112]. Many recent studies on high-performance conventional PSCs [113] utilize zinc oxide (ZnO) [114,115], tin oxide (SnO₂) [116,117,118], or spiro-OMeTAD [119,120], whereas for high-performance p-i-n devices, the Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) [119,121] or self-assembled molecules [122,123], and PC₆₁BM [124,125] or C₆₀ [126,127] are primarily employed. In recent years, the research has shifted from the conventional PSCs to inverted PSCs due to their superior stability and efficiency approaching to of conventional PSCs [128]. Advances in the charge transport materials, investigating the impact of interface engineering, and defect passivation strategies are crucial for both the enhancement of PCE and the stability of PSCs. For example, Tan et al. [34] have proposed a dimethylacridine-based molecular doping method that can achieve a well-matched perovskite/tin-doped indium oxide (ITO) interface with comprehensive passivation of grain boundaries. This approach has enabled the achievement of a certified PCE of 25.39% and excellent stability by retaining 96.6% of the starting efficiency after 1000 h light soaking in inverted PSCs. Tang et al. [129] have revealed that the deposition of additional indium oxide on the ITO substrate by the atomic layer deposition enhances the anchoring of the self-assembled monolayers (SAMs), making them more resistant to detachment from the ITO substrate. The SAMs, containing a trimethoxysilane group, showed great anchoring to the substrate. These inverted PSCs have achieved a certified PCE of 24.8% and retained 98.9% of their initial PCE after 1200 h of operation at 85 °C. It has been demonstrated that by using a cyanoacrylic-acid-based molecular additive for NiOx-based inverted PSCs, their efficiency and stability can be easily improved [130]. In this study, a PCE of 23.48% and maintained 95.4% of its original performance after 1960 h of continuous operation at 65 °C have been reported. Caprioglio et al. [131] have studied the impact of different charge transport layers and passivation methods on Voc and short-circuit current density (Jsc) of wide-bandgap Br-rich PSCs. They demonstrated that adjustment of the perovskite n-interface layer (PCBM) can help reduce the quasi-fermi level splitting and the Voc mismatch. As a result, it increased the Voc. Then, by altering the perovskite p-interface layer (SAMs), the field screening effects can be suppressed, and correspondingly, the charge extraction and the Jsc can be enhanced. Namely, the PSCs had Voc up to 1.29 V, a fill factor of 80%, and Jsc of 17 mA/cm². Importantly, their thermal stability with a T80 lifetime exceeding 3500 h at 85 °C. Imran et al. [132] have shown that by combining interface engineering and additive/passivation techniques, the highly efficient and stable RbCsFAPbI₃-based inverted PSCs can be prepared. Here, the optimal additive content (2% GuHCl) improved film morphology and optoelectronic properties, while NiOx/PTAA bi-interfacial engineering and PEAI passivation helped to minimize defects and enhanced charge transfer resulting in Jsc of 24.52 mA/cm², PCE of 22.78% and retained 95% of their initial performance after 500 h. It is important to note that an alternative strategy of PSC fabrication that enables the achieve an extraordinarily high certified PCE of ~25.8% has only recently been reported by Zhou et al. [35]. This strategy utilizes a modulation of anion–π interaction with halide in the AX component. As a result, it helps to establish the so-called dual-site regulation, and correspondingly, it improves phase and component purity at the nanoscale [133].

2.2. Other Solar Cells Systems Utilizing Properties of Perovskite Materials

First of all, we remind the reader that the open-circuit voltage (V_oc) of a single-junction solar cell is limited by the ratio of the bandgap of the absorber over the elementary charge. Hence, in a single junction cell, an absorber with a narrower bandgap is unable to generate a high V_oc due to limitations imposed by its bandgap. Conversely, an absorber with a wider bandgap has the potential to yield a higher V_oc; however, the short-circuit current is restricted because photons with energy lower than the absorber’s bandgap do not contribute to the photocurrent. The limited efficiency of these solar cells (i.e., one p-n junction) can also be overcome by combining more cells together with different bandgap energy (i.e., two/multiple p-n junctions), and they are known as tandem/triple, etc. solar cells [134,135].

These tandem solar cells usually consist of two types of solar cells: perovskite-silicon (or perovskite—CIGS) and all-perovskite solar cells. In this case, the top layer is usually made of a wide-bandgap perovskite material that absorbs high-energy photons, while the bottom layer consists of a narrow-bandgap material like silicon or copper indium gallium diselenide (CIGS) that absorbs lower-energy photons (see Figure 3) [112,136,137]. For instance, Tockhorn et al. [138] fabricated perovskite-silicon tandem solar cells with periodic nanotextures that reduced the reflection losses compared to planar tandems and increased fabrication yield from 50% to 95%. Moreover, the enhanced optoelectronic properties of the perovskite top cell resulted in a significant improvement in V_oc by 15 mV. Finally, they have reported a certified power conversion efficiency of 29.8%. To improve the perovskite interface, Liu et al. [139] employed a sequential interface engineering technique that enhanced the conduction band offset and recombination rate at the perovskite/C₆₀ interface. This process involved initially depositing ethylenediamine diodide, followed by a sequential deposition of 4-Fluoro-Phenethylammonium chloride. This approach resulted in a wide bandgap (1.67 eV) PSC with a Voc of 1.26 V, and PCE of 21.8%. Furthermore, the perovskite-silicon tandem solar cell achieved a certified PCE of 29.0%. Moreover, Zheng et al. [140] found that the solvent’s properties substantially impact the degree of moisture interference. They employed n-Butanol as part of a solvent engineering strategy, leveraging its low polarity and moderate volatilization rate to improve perovskite film uniformity. The resulting wide bandgap (1.68 eV) PSC achieved 20.79% of PCE, and the perovskite-silicon tandem solar cell achieved 25.9% of PCE for 16 cm² by slot-die deposition. In addition, Priyanka et al. [141] demonstrated that incorporating a barium stannate (BaSnO₃) charge transport layer enhanced the performance of perovskite/CIGS tandem solar cells. They reported that using a high work function metal/Cs_0.15MA_0.15FA_0.70Pb(I_0.85Br_0.15)₃/BaSnO₃/ITO configuration as an HTL-free top perovskite solar cell resulted in an impressive PCE of 39%.

Interface engineering of perovskite layers plays a crucial role in the performance of all-perovskite tandem solar cells. Yu et al. [142] revealed that the energy levels and redox potentials of tin adducts are significantly influenced by the presence of electron-withdrawing ligands. As a result, they investigated both a narrow bandgap FA_0.7MA_0.25Cs_0.05Sn_0.5Pb_0.5I₃ (1.26 eV) perovskite and a wide bandgap Cs_0.2FA_0.8PbI_1.8Br_1.2 (1.77 eV) perovskite, resulting in all-perovskite tandem solar cells with a certified PCE of 26.96%. Li et al. [143] reported that the solution-processed ETL ink, which consists of hybrid fullerenes (i.e., C₆₀, PCBM, and indene-C₆₀ bisadduct), enhanced interface morphology, conductivity, and passivation. Correspondingly, it led to the development of an all-perovskite tandem solar cell with a narrow bandgap FA_0.7MA_0.3Sn_0.5Pb_0.5I₃ (~1.25 eV) perovskite and a wide bandgap Cs_0.35FA_0.65PbI_1.8Br_1.2 (~1.80 eV) perovskite, achieving a PCE of 23.3% with an active area of 20.25 cm².

As we mentioned in the Introduction, perovskite materials are also used in flexible, semi-transparent, or space solar cells. In the case of flexible PSCs, the perovskite is fabricated on a flexible substrate such as poly(ethylene terephthalate) (PET) using thermal radiation annealing [144]. It has shown that a high power conversion efficiency of ~22.61% and a remarkable fill factor of 83.42% can be achieved by means of dual-sided annealing [145]. Briefly, dual-side annealing allows the PET to remain undistorted during high-temperature annealing, reduces the grain boundaries, decreases lattice mismatch, and achieves cleaner vertical stacking of grains in perovskite. Defects in the perovskite films can be suppressed by utilizing the ETL in SnO₂:OH ETL processed at room temperature [146]. In this study, the ETL showed a lower defect density, especially a reduced concentration of oxygen vacancies, yielding improved energy band alignment and enhanced surface wettability. Then, by using a MAPbI₃ base perovskite, they achieved a PCE of 18.71 % (active area: 36.50 cm²) and retained over 83% of their initial PCE after multiple flexing test cycles of flexible PSCs. Another method to suppress defects from the interface utilized the proline hydrochloride, which is rich in –NH₃⁺ groups and possesses a conjugated rigid structure [137]. It not only stabilized the α-phase FAPbI₃ template but also protected it from degradation caused by the phase transitions. By means of this method, the flexible FAPbI₃ perovskite solar cell achieved a certified 23.51% of PCE (active area of 0.0601 cm²) and retained 83% of its initial PCE undergoing a flexing test involving 6000 bending cycles at a 5 mm radius [137].

Semi-transparent solar cells are used in building windows and/or cladding, as well as vehicle integration. The transparency of perovskite solar cells relies on both the bandgap of the perovskite layer and the transmittance of the electrode. A thin metal layer frequently serves as the transparent electrode in semi-transparent solar cells. For instance, Ju et al. [147] reported a semi-transparent PSC with a 3D-structured fluorine-doped tin oxide enhanced the diffuse transmittance and shortened the carrier travel distance. They have reported that transparent PSC devices achieved a power conversion efficiency between 12.0% and 14.6% and an average visible transmittance (AVT) of 13.4–17.0%. Jin et al. [148] reported that using Spiro-OMeTAD and PBDB-T as the HTL can achieve higher performance, stable thermal stability, and moisture resistance in semi-transparent solar cells. PBDB-T improved the stability of the perovskite/HTL interface via interacting with lead ions in the perovskite through its thiophene units and carbonyl functional groups, effectively passivating interfacial defects. They obtained PCE, AVT, and light utilization efficiency of ~13.71%, 36.04%, and 4.94%, respectively. Yoo et al. [149] demonstrated that the potassium pyrophosphate interlayer between the ETL SnO₂ and the perovskite layer enhanced both the performance and the stability. This improvement was attributed to the passivation of the SnO₂ surface, better perovskite film morphology, and reduced non-radiative recombination at the perovskite interface. Their semi-transparent perovskite solar cells achieved a PCE of ~16%, an AVT of ~36%, and long-term stability.

Solar cells face numerous challenges in the space environment, particularly from cosmic radiation. PSCs have demonstrated impressive resilience against different types of radiation, including electrons, protons, ultraviolet rays, and gamma rays [150,151,152]. The effect of high doses of 1 MeV e-beam radiation up to an accumulated fluence to 10¹⁶ e⁻cm⁻² on perovskite thin films and solar cells have been reported by Pérez-del-Rey et al. [153]. They found that PSCs based on quartz substrates remain stable even under high doses of 1 MeV e-beam irradiation. Analysis of time-resolved microwave conductivity in both pristine and irradiated films revealed a slight decrease in the charge carrier diffusion length following irradiation.

3. Perovskite-Type Hydrogen Storage—Overview

In the case of hydrogen storage, it is necessary that materials can fulfill the following conditions: (1) the hydrogen bonds in these materials are of remarkable strength; (2) these materials encompass a substantial volume, facilitating the storage of a significant amount of hydrogen; (3) these materials exhibit catalytic properties that enhance hydrogen uptake; and, (4) also exhibit acceptable gravimetric hydrogen storage capabilities, contributing to enhanced hydrogen storage capacity [154,155]. The perovskite for hydrogen storage utilizes hydride, oxide, and halide.

3.1. Perovskite-Type Hydride

The compounds formulated as ABH₃, referred to as perovskite-type hydrides, have stable structures and distinctive properties; therefore, they have gained considerable attention from the scientific community [156,157]. The perovskite hydrides are usually categorized into two groups based on the selected elements A and B in the perovskite structure. In the first group, the elements A and B could be selected from groups I and II of the periodic table [158]. Typical members of this structured group are BaLiH₃, CsCaH₃, KMgH₃, NaMgH₃, RbCaH₃, or SrLiH₃. Among different types of ABH₃, the magnesium-based perovskite hydrides have recently received particular attention from researchers due to their high hydrogen storage capacity. For example, the superior gravimetric and volumetric hydrogen storage densities of 6% and 88 kg/m³ of NaMgH₃ have already been demonstrated [74]. The second group consists of those of perovskite-type hydrides, where element A is either a monovalent alkali metal or a divalent metal, and element B is a transition metal. Some examples of the compounds of this structural group include CaCoH₃, CaNiH₃, LiTH₃ (T: Fe, Co, Ni, Cu), SrPdH₃, and SrLiH₃. Another area of research involves studying the elemental contributions towards the band structure of S-block perovskites, which exhibit similarities to Pb-perovskites [159]. Building on previous efforts, Adeyinka et al. [160] utilized density functional theory calculations to determine the hydrogen storage capacities of KBeH₃, KMgH₃, and KCaH₃, which were found to be 5.866%, 4.516% and 3.649%, respectively.

3.2. Perovskite-Type Oxides

The formula of typically perovskite-type oxides is composed of ABO₃. The ABO₃ perovskite-type oxides possess not only a remarkable hydrogen storage capacity but also long-term stability [161,162]. Nowadays, perovskite-type oxides have emerged as suitable materials for efficient electrocatalysis in hydrogen conversion and storage, owing to their distinctive physical and chemical attributes, that is, superior redox activity, flexible structure, remarkable ionic/electronic conductivity, and outstanding thermal and chemical stability [163,164]. Researchers are devoted to studying the performance of perovskite-type oxides for hydrogen storage at room temperature [165]. The pioneering study of Sakaguchi et al. [166] has revealed that perovskites (SrCe_0.95Yb_0.05O₃) can store hydrogen at even room temperature in the context of Ni/MH systems. A few years later, the maximum hydrogen absorption capacity of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ was determined to be 1.72 wt% at 333 K and 6 mol∙L⁻¹ KOH, as recalculated from the electrochemical capacity [167].

4. Perovskite-Type Hydrides Structural, Thermodynamic, and Hydrogen Storage Properties

The fundamental properties of perovskite-type hydrides are routinely obtained from the first-principles calculations using the density-functional theory (DFT). In this section, we will present the recent data on the properties of the currently considered perovskite-hydrides materials, that is, Mg-based perovskite-type hydrides MgXH₃ (X = Co, Cu, Ni) [168], Ca-based perovskite-type hydrides CaXH₃ (X = Mn, Fe, Co) [47], and metal base perovskite-hydrides AeSiH₃ (Ae = K, Li, Na, Mg) [169], AeVH₃ (Ae = Be, Mg, Ca, Sr) [170], XSrH₃ (X = K and Rb) [73], XCuH₃ (X = Co, Ni, Zn) [171], XCoH₃ (X = In, Mn, Sr, Sn, Cd) [172], XPtH₃ (X = Li, Na, K, Rb) [173], XAlH₃ (X = Na, K) [174], XTiH₃ (X = K, Rb, Cs) [77] and KXH₃ (X = Be, Ca, Mg) [160].

The calculated structure of MgXH₃ (X = Co, Cu, Ni) hydride perovskite is given in Figure 4. In the unit cell, the Mg atoms are located at the corners with coordinates, the metal atoms X- are in the center of the unit cell with coordinates, and the H atoms are at the octahedral sites at the center of faces. The assessment of the stability of materials MgCoH₃, MgCuH₃, and MgNiH₃ has been determined utilizing the Birch-Murnaghan equation of state in conjugation with energy volume cures [175,176]. Mathematically, it can be obtained from

$$E_{total}\left(V\right)=E_0\left(V\right)+{\displaystyle \frac{B_{0}V}{B_0^{\prime }\left(-1\right)}}\left[B_0\left(1-{\displaystyle \frac{V_0}{V}}\right)+{\left({\displaystyle \frac{V_0}{V}}\right)}^{B_0^{\prime }}-1\right],$$

(1)

where E₀(V) is the total energy of the ground state of a unite cell at a given volume V, V₀ is the equilibrium volume, B₀ is the bulk modulus and $B_0^{\prime }$ stands its derivate with respect to pressure. Then, the gravimetric storage capacity of hydride perovskite, which is of importance in hydrogen storage applications, is estimated from the gravimetric ratio [177]

$$\mathrm{Cwt} [\%] =\left({\displaystyle \frac{\frac{A_H}{A_M}{m}_H}{m_M+\frac{A_H}{A_M}{m}_H}}\times 100\right)\left[\mathrm{\%}\right].$$

(2)

Here m and A stand for the molar mass and atoms, respectively; and subscript H stands for hydrogen and subscript M means the host perovskite material. Theoretically achievable amounts of hydrogen stored per unit mass of the host perovskite for all three Mg-based perovskites (i.e., Co, Cu, Ni) are summarized in

Table 1. The calculated lattice constant (a = b = c in Å), volume (V in ų), density (ρ in g/cm³), formation enthalpy (ΔH in kJ/mol·H₂), gravimetric hydrogen storage capacity (Cwt% in wt%), and desorption temperature (T_des in K) for MgXH₃, where X = Co, Cu, Ni, respectively. Results are reproduced from [168] with permission from Springer Nature.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\mathit{\rho }$ · (g/cm3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)	${\mathit{T}}_{\mathit{d}\mathit{e}\mathit{s}}$ · (K)
MgCoH3	3.64	3.32	36.44	3.93	−70.93	542.69
MgCuH3	3.32	3.49	42.42	3.56	−63.27	484.08
MgNiH3	3.49	3.36	37.97	3.76	−68.54	524.40

. Note that the hydrogen desorption temperature (T_des), which is an essential parameter for assessing the temperature required to release the stored hydrogen, can be determined from

$$T_{des}={\displaystyle \frac{\Delta H}{\Delta S}},$$

(3)

where ΔH and ΔS are the formation energy and entropy change of hydrogen (130.7 J mol⁻¹ K⁻¹), respectively.

The structure of CaXH₃ is similar to the above-discussed perovskite-type hydride (i.e., in the present case, Mg is just replaced by Ca in Figure 3). The fundamental properties of the CaXH₃ are given in

Table 2. The optimized lattice constants (a₀), volumes (V), densities (ρ), formation enthalpies (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of CaXH₃ (X: Mn, Fe, or Co) compounds. Results are reproduced from [47] with permission from Wiley.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\mathit{\rho }$ · (g/cm3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
CaMnH3	3.09	3.60	46.58	3.50	−0.25
CaFeH3	3.06	3.50	42.99	3.82	−0.42
CaCoH3	2.97	3.48	42.16	4.03	−0.44

. After calculation, the lattice constant (Å), volume (V in ų), density (ρ in g/cm³), formation enthalpies (ΔH_f), and gravimetric hydrogen storage capacity (Cwt% in wt%) of the hydrides studied are given in Table 2. The negative values of the formation enthalpies listed in Table 2 indicate that CaXH₃ (X: Mn, Fe, Co) perovskite-type hydrides are thermodynamically stable and can be synthesized. Among them, CaCoH₃ has the highest stability, which is given by

$$\Delta H_f=E_t\left(\mathrm{C}\mathrm{a}\mathrm{X}{\mathrm{H}}_3\right)-[E\left(\mathrm{C}\mathrm{a}\right)+E\left(\mathrm{X}\right)+3\times E\left(\mathrm{H}\right)],$$

(4)

where E_t (CaXH₃) symbolize total energy of CaXH₃ and E(Ca), E(X), and E(H) are the ground state energies for one Ca atom, one X atom, and one H atom, respectively [47].

The crystal structure of cubic AeSiH₃ (Ae = K, Li, Na, Mg) compounds is shown in Figure 5. In the unit cell, the atoms Ae-are at the corners with coordinates, the Si atoms are located in the center of the unit cell with coordinates, and the H atoms are at the octahedral sites at the center of faces. The formation enthalpies of AeSiH₃ hydrides are estimated as follows [169]:

$$\Delta H_f\left(\mathrm{A}\mathrm{e}\mathrm{S}\mathrm{i}{\mathrm{H}}_3\right)={[E}_t\left(\mathrm{A}\mathrm{e}\mathrm{S}\mathrm{i}{\mathrm{H}}_3\right)-[E_s\left(\mathrm{A}\mathrm{e}\right)+E_s\left(\mathrm{S}\mathrm{i}\right)-3E_s\left(\mathrm{H}\right)]$$

(5)

where E_s (Ae) shows the energy of Ae (Ae = Li, K, Na, Mg) atoms, E_s (Si) shows the energy of Si atoms, and E_s(H) shows the ground state energy of H atoms. E_t shows the total energy of the compound. After calculation, the lattice constant (Å), volume (V in ų), density (ρ in g/cm³), formation energy (ΔH_f in eV/atom), and gravimetric hydrogen storage capacity (Cwt% in wt%) of the hydrides studied are summarized in

Table 3. The optimized lattice constants (a₀), volumes (V), densities (ρ), formation energy (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of AeSiH₃ (Ae = Li, K, Na, Mg) compounds. Results are reproduced from [169] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\mathit{\rho }$ · (g/cm3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
LiSiH3	7.946	4.001	64.079	1.046	−17.372
KSiH3	4.306	3.917	60.122	1.952	−15.760
NaSiH3	5.588	3.986	63.363	1.429	−16.134
MgSiH3	5.456	3.977	62.933	1.476	−15.063

. We emphasize here that the higher entropy values contribute to the release of hydrogen from perovskite materials. Perovskite materials with lower hydrogen adsorption-free energy values have a higher affinity for hydrogen gas, making them more likely to store hydrogen efficiently.

We only note that the structure of AeVH₃, where Ae = Be, Mg, Ca, and Sr, is similar to the perovskite-type hydride presented in Figure 5 [170]. The lattice parameters (Å) are calculated as 3.48, 3.66, 3.73, and 3.83; The volumes (ų) are calculated as 42.21, 49.22, 53.28, and 56.22, respectively, for AeVH₃ with Ae = Be, Mg, Ca, and Sr. For the perovskite hydrides AeVH₃ (where Ae = Be, Mg, Ca, Sr), the gravimetric hydrogen storage capacity decreases with increasing alkaline metal (Ae) mass, with values of 4.6, 3.7, 3.1, and 2.0 wt%, respectively. This trend shows that the decreasing gravimetric hydrogen storage capacity is due to the increasing mass of alkaline metals. Consequently, BeVH₃ stands out as the most suitable material for hydrogen storage applications, with a higher gravimetric density of 4.6 wt% compared to the other three materials studied [170].

The following are perovskite-type hybrids with identical structures but with different atomic arrangements. A brief description of their structures is given below, followed by the calculated values. All calculated gravimetric hydrogen storage capacities of XSrH₃ (X = K and Rb) are given in

Table 4. The optimized lattice constants (a₀), volumes (V), formation energy (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of XSrH₃ (X = K and Rb) compounds. Results are reproduced from [73] with permission from Wiley.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
KSrH3	2.33	4.77	108.50	−6.60
RbSrH3	1.71	4.99	124.77	−5.67

. KSrH₃ has a greater capacity to store hydrogen (2.33%) compared to RbSrH₃ (1.71%). In the context of illustrating thermodynamic properties, The Debye temperature is a crucial parameter in the discussion of the quasiharmonic Debye model and various thermodynamic properties. As the temperature increases, the Debye temperature decreases. Similarly, the Debye temperature increases at higher pressures. KSrH₃ has a higher Debye temperature value compared to RbSrH₃. It can, therefore, be said that KSrH₃ is more stable than RbSrH₃ [73].

All calculated gravimetric hydrogen storage capacities of XCuH₃ (X = Co, Ni, Zn) are given in

Table 5. The optimized lattice constants (a₀), volumes (V), free energy (F), and gravimetric hydrogen storage capacities (Cwt%) of XCuH₃ (X = Co, Ni, Zn) compounds. Results are reproduced from [171] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\mathit{F}$
CoCuH3	2.8	3.3287	36.882	−1895.3
NiCuH3	3.0	3.3245	36.742	−5499.0
ZnCuH3	2.7	3.6129	47.160	−2512.5

. NiCuH₃ has a greater capacity to store hydrogen (3.0%) compared to CoCuH₃ (2.8%) and ZnCuH₃ (2.7%). In the thermodynamic properties, the negative values of free energy deliberate the thermodynamic stability of any material; the free energy of these three materials is all negative, indicating that they are stable materials in relation to each other. CoCuH₃ is more stable than NiCuH₃ and ZnCuH₃ because CoCuH₃ has the lowest free energy of them [171].

All calculated gravimetric hydrogen storage capacities of XCoH₃ (X = In, Mn, Sr, Sn, Cd) are given in

Table 6. The optimized lattice constants (a₀), volumes (V), formation energy (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of XCoH₃ (X = In, Mn, Sr, Sn, Cd) compounds. Results are reproduced from [172] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
CdCoH3	1.74	3.42	20.00	−0.93
InCoH3	1.71	3.52	43.61	−1.09
MnCoH3	2.59	3.60	46.66	−0.87
SnCoH3	1.68	3.59	46.27	−1.31
SrCoH3	2.03	3.66	49.03	−0.78

. MnCoH₃ is most favored for hydrogen storage properties because its gravimetric ratio for hydrogen storage is greater than all these studied materials [172].

All calculated gravimetric hydrogen storage capacities of XPtH₃ (X = Li, Na, K, Rb) are given in

Table 7. The calculated lattice constant (a₀), volume (V), formation energy (ΔH_f), gravimetric hydrogen storage capacity (Cwt%), and desorption temperature (T_des) for XPtH₃ (X = Li, Na, K, Rb). Results are reproduced from [173] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)	${\mathit{T}}_{\mathit{d}\mathit{e}\mathit{s}}$ · (K)
LiPtH3	1.45	3.54	44.35	−0.32	237.77
NaPtH3	1.35	3.63	47.96	−0.31	225.39
KPtH3	1.26	3.80	54.83	−0.22	162.43
RbPtH3	1.06	3.90	59.25	−0.11	80.11

. Compared to other materials, LiPtH₃ exhibits the highest gravimetric hydrogen storage capacities and stability (T_des) [173].

All calculated gravimetric hydrogen storage capacities of XAlH₃ (X = Na, K) are given in

Table 8. The optimized lattice constants (a₀), volumes (V), formation energy (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of XAlH₃ (X = Na, K) compounds. Results are reproduced from [174] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
NaAlH3	5.40	3.792	54.526	−0.903
KAlH3	4.19	3.938	61.070	−1.250

. NaAlH₃ has a higher gravimetric hydrogen storage capacity than KAlH₃. The formation energies of these compounds are negative, demonstrating their thermodynamic stability [174].

All calculated gravimetric hydrogen storage capacities of XTiH₃ (X = K, Rb, Cs) are given in

Table 9. The optimized lattice constants (a₀), bulk modulus B₀ (GPa), the derivative of bulk modulus B₀’ (GPa), formation energy (ΔH_f), and gravimetric hydrogen storage capacities (Cwt%) of XTiH₃ (X = K, Rb, Cs) compounds. Results are reproduced from [77] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	${\mathit{B}}_0$ · (GPa)	${\mathit{B}}_0\prime$ · (GPa)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
KTiH3	3.36	3.999	44.938	1.767	−0.285
RbTiH3	2.22	4.103	42.083	1.887	−0.220
CsTuH3	1.65	4.233	37.176	1.704	−0.146

. The weight hydrogen storage capacities of KTiH₃, RbTiH₃, and CsTiH₃ were determined to be 3.36, 2.22, and 1.65 wt%, respectively. The hydrogen desorption temperatures for KTiH₃, RbTiH₃, and CsTiH₃ were determined to be 209 K, 161 K, and 107 K, respectively. These results indicate that KTiH₃ has the potential to be a hydrogen storage material [77].

Mbonu et al. [160] used DFT to calculate the structural properties of the S-block halide perovskite KXCl₃ (X = Be, Ca, Mg). In terms of thermodynamic performance, KMgCl₃ has the highest heat capacity, while KCaCl₃ has the lowest. In order to investigate the hydrogen storage capabilities of this perovskite system, functionalization was carried out by replacing chlorine (Cl) atoms in the crystal lattice with hydrogen (H) atoms. The resulting functionalized compounds, KBeH₃, KMgH₃, and KCaH₃, are detailed in

Table 10. The calculated lattice constant (a = b = c in Å), volume (V in ų), bulk modulus B₀ (GPa), formation enthalpy ($\Delta H_f$ in kJ/mol·H₂), and gravimetric hydrogen storage capacity (Cwt% in wt%) for KXH₃ (X = Be, Ca, Mg). Results are reproduced from [160] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	${\mathit{a}}_0$ · (Å)	$\mathit{V}$ · (Å3)	${\mathit{B}}_0$ · (GPa)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)
KBeH3	5.866	4.41	158.385	2.650	−1.226
KMgH3	4.516	4.74	297.538	0.916	−2.523
KCaH3	3.649	5.10	89.719	27.902	−1.845

. KBeH₃ has the best gravimetric hydrogen storage capacity of 5.866%, surpassing KMgH₃ and KCaH₃.

5. Perovskite-Type Oxides Structural, Thermodynamic, and Hydrogen Storage Properties

Ostadebrahim et al. [163] used a single-phase sol-gel method to synthesize ternary metal perovskite-type oxides LaMO₃ (M = Cr, Mn, Fe, Co, Ni) and investigated their surface area, pore size, and electrochemical properties. The nanostructured materials parameters of the surface area and the pore size are crucial reference indicators for the electrochemical storage of hydrogen. According to the analysis results of BET-BJH, the BET surface area (SA_BET), total pore volume (V_t), average pore diameter (D_avg), and S_BJH of the samples are shown in

Table 11. BET and BJH data of LaMO₃ (M = Cr, Mn, Fe, Co, Ni) nanocrystals. Results are reproduced from [163] with permission from Elsevier.

Samples	${\mathit{S}\mathit{A}}_{\mathit{B}\mathit{E}\mathit{T}}({\mathbf{m}}^2/\mathbf{g})$	${\mathit{V}}_{\mathit{t}}({\mathbf{c}\mathbf{m}}^2/\mathbf{g})$	${\mathit{D}}_{\mathit{a}\mathit{v}\mathit{g}}$ · (nm)	${\mathit{S}}_{\mathit{B}\mathit{J}\mathit{H}}({\mathbf{m}}^2/\mathbf{g})$
LaCrO3	1.4838	0.0044	12.060	5.8201
LaMnO3	8.8778	0.0302	13.616	6.7078
LaFeO3	20.997	0.1201	10.721	20.698
LaCoO3	8.2064	0.0693	33.807	10.929
LaNiO3	4.8519	0.0562	98.998	16.537

. In the electrochemical performance of LaMO₃ in a three-electrode electrochemical cell, LaFeO₃ achieved the highest current density at the anodic peak. As shown in Table 10, LaFeO₃ exhibited a morphology characterized by a spherical shape, the maximum number of nanoparticles per unit area, the highest total pore volume (V_t), and the lowest average pore diameter (D_avg). This morphology is indicative of superior electrode response.

The electrochemical hydrogen storage capacity of LaMO₃ nanocrystals is shown in

Table 12. Electrochemical hydrogen storage capacity of LaMO₃ (M = Cr, Mn, Fe, Co, Ni) nanocrystals. Results are reproduced from [163] with permission from Elsevier.

Active Material	Substrate	Electrolyte Solution	Reference Electrode	Counter Electrode	Discharge Capacity
LaCrO3	-	-	-	-	6790 mAh/g
LaMnO3	-	-	-	-	10,500 mAh/g
LaFeO3	Cu electrode	6M KOH	Ag/AGCl	Pt	13,500 mAh/g
LaCoO3	-	-	-	-	8800 mAh/g
LaNiO3	-	-	-	-	7000 mAh/g

. LaFeO₃ shows the most favorable performance in terms of electrochemical hydrogen storage capacity. The lifetime and stability of the electrochemical hydrogen storage were investigated by galvanostatic charge-discharge tests and Tafel polarisation analysis. The LaFeO₃ electrode exhibited stable discharge capacity over a current range of 1 to 4 mA. The Tafel test results showed the lowest corrosion rate for LaFeO₃ nano-perovskites. These results show that LaFeO₃ nano-perovskites are a promising candidate for energy storage applications [163].

Bhardwaj et al. [178] used perovskite oxide materials, specifically the proton insertion anode Sm_1−xSr_xCoO_3−δ (x = 0, 0.5, 1), for nickel/oxide rechargeable batteries. Sm_1−xSr_xCoO_3−δ (x = 0, 0.5, 1) was synthesized via solid-state reactions and its electrochemical hydrogen storage performance in alkaline electrolytes. The oxygen-deficient Sm_0.5Sr_0.5CoO_3−δ (SSC) exhibits a maximum reversible discharge capacity of 182 mAh g⁻¹, which increases with increasing temperature. Partial substitution of the acceptor dopant strontium with samarium induces lattice expansion, a greater extent of Co-O interactions, and increases the reducibility of cobalt ions, thereby promoting higher hydrogen storage.

Nabil et al. [179] studied the electronic, mechanical, and elastic properties of LaCrO₃H_x (x = 0, 6) for hydrogen storage applications, which have been investigated. The determination included cell parameters, crystal structure, and mechanical performance. In terms of electronic properties, the Fermi level density of states (DOS) for LaCrO₃ and LaCrO₃H₆ is exceptionally low. This indicates that the lower the DOS of the Fermi level, the more stable the compound.

Table 13. Cell parameters, mechanical properties of LaCrO₃ and LaCrO₃H₆ [179].

Compounds	Calculated Cell Parameters (A)	${\mathit{B}}_0$ · (GPa)	${\mathit{B}}_0\prime$ · (GPa)
LaCrO3	3.85	128.07	3.2
LaCrO3H6	4.43	62.26	23.48

shows that LaCrO₃ has a higher bulk modulus value, and when hydrogen binds to LaCrO₃, the bulk modulus decreases, indicating greater material compressibility. In conclusion, the LaCrO₃ compound has a remarkable hydrogen storage capacity of about 4 wt%.

All calculated gravimetric hydrogen storage capacities of perovskite-type hydrides/oxide KMO_3−xH_x (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, and 3.0) are given in

Table 14. The calculated lattice constant (a = b = c in Å), volume (V in ų), formation enthalpy (ΔH_f in kJ/mol·H₂), gravimetric hydrogen storage capacity (Cwt% in wt%), and desorption temperature (T_des in K) for KMO_3−xH_x (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, 2.4, 2.7, and 3.0). Results are reproduced from [180] with permission from Elsevier.

Compounds	$\mathbf{C}\mathbf{w}\mathbf{t}$%	a	${\mathit{a}}_0$ · (Å) b	c	$\mathit{V}$ · (Å3)	$\Delta {\mathit{H}}_{\mathit{f}}$ · (eV/Atom)	${\mathit{T}}_{\mathit{d}\mathit{e}\mathit{s}}$ · (K)
KMgO3	-	4.109	-	-	69.37	−0.023	-
KMg2.7O0.3	0.28	4.102	4.102	4.112	69.19	−11.122	825
KMg2.4O0.6	0.59	4.092	4.067	4.140	68.90	−10.826	803
KMg2.1O0.9	0.92	4.059	4.057	4.162	68.54	−10.529	781
KMg1.8O1.2	1.28	4.034	4.038	4.196	68.35	−10.223	759
KMg1.5O1.5	1.67	4.107	3.989	4.182	68.51	−9.937	737
KMg1.2O1.8	2.10	4.510	4.653	4.437	92.90	−9.648	716
KMg0.9O2.1	2.58	4.450	3.975	4.108	72.60	−9.347	694
KMg0.6O2.4	3.10	4.156	3.966	4.091	67.43	−8.624	640
KMg0.3O2.7	3.69	4.058	4.084	4.004	66.35	−7.904	587
KMgH3	4.35	4.069	-	-	67.36	−7.606	564

. The calculated lattice parameters indicate that KMg has a cubic crystal structure in KMgO₃ and KMgH₃. However, for other doping concentrations, the cubic crystal structure is distorted when anions are doped on the anion side [180].

6. Future Trends

Perovskite solar cells show immense potential to become the next photovoltaic commercial technology. Future trends in perovskite solar cells focus on enhancing their efficiency, stability, scalability, and sustainability. A possible way to improve efficiency is by using tandem solar cells. For instance, there is a trend in integrating perovskite layers with silicon or materials like CIGS (copper indium gallium selenide) to form tandem solar cells. These configurations have already surpassed 30% efficiency. Another challenge for PSCs is their stability. To address this, researchers must focus on interface engineering to overcome various defects, non-radiative recombination, and energy band alignment. Scalability might be overcome by using roll-to-roll processing, which is a high-speed and low-cost process and results in the fabrication of flexible and lightweight perovskite modules [181]. The printing technique could also be optimized for the large-area production of perovskite solar cells [182]. Regarding sustainability and toxic reduction, reducing the amount of lead content in PSCs is crucial for minimizing environmental impact. There is a development of lead-free alternatives like tin-based perovskites. Ultimately, achieving high efficiency, long-term stability, and reduced environmental impact in PSCs will provide significant societal benefits [183,184].

In perovskite for hydrogen storage, the future trends focus on expanding their applications in hydrogen production, storage, and conversion processes. The key areas that need special focus and further development are perovskites for Hydrogen Evolution Reactions (HER), hydrogen storage materials, and hydrogen sensors. Perovskites are being explored as efficient, low-cost catalysts for hydrogen production through water splitting. In the future, improving the catalytic activity of perovskites by tuning their composition and surface properties is crucial. The next step, is developing new perovskite compositions and structures and exploring the integration of perovskite with other materials to optimize hydrogen storage capacities [185].

7. Conclusion Remarks

In this review, we have presented not only the applications of perovskite solar cells but also the hydrogen storage capacities of perovskite-type materials. Due to their excellent structural properties and cost-effectiveness, perovskite materials offer substantial research potential in the field of hydrogen storage.

The main future advancements for perovskite solar cells include the development of tandem solar cells, flexible solar cells, and space solar cells. Among perovskite-type hydrides, LiSiH₃ exhibits the highest gravimetric hydrogen storage capacity at 7.946%, highlighting the significant potential of this compound series for hydrogen storage. Additionally, perovskite-type oxides demonstrate excellent hydrogen adsorption and storage capabilities in electrode applications, leading to increased electrode capacity and robust cycling stability. The low application cost of these materials in energy storage systems further emphasizes the development potential of perovskite-type oxides for hydrogen storage. These findings collectively underscore the promising prospects of perovskite-type hydrides and oxides in the field of hydrogen storage. Overall, the present results are highly significant for future research on perovskite-type hydrogen storage compounds and the development of perovskite solar cells.