Microsoft Word - CAN-5834 Characterization and Application of Nanomaterials 2024, 7(1), 5834. https://doi.org/10.24294/can.v7i1.5834 1 Review Advancements in water splitting for sustainable energy generation: A review Razu Shahazi1, Amirul Islam Saddam1, Srabani Majumdar1, Md. Rakibul Islam1, Mohammed Muzibur Rahman2,3, Md. Mahmud Alam1,2,*, Ajoy Kumer4, Giti Paimard5 1 Department of Chemical Engineering, Z. H. Sikder University of Science and Technology (ZHSUST), Shariatpur 8024, Bangladesh 2 Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah 21589, Saudi Arabia 3 Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia 4 Department of Chemistry, College of Arts and Sciences, IUBAT-International University of Business Agriculture and Technology, Dhaka 1230, Bangladesh 5 Laboratory of Nanoscale Biosensing and Bioimaging (NBAB), School of Ophthalmology and Optometry, School of Biomedical Engineering, State Key Laboratory of Ophthalmology Optometry, and Vision Science, Wenzhou Medical University, Wenzhou 325027, China * Corresponding author: Md. Mahmud Alam, alam-mahmud@hotmail.com, mmalam@zhsust.ac.bd Abstract: Water splitting, the process of converting water into hydrogen and oxygen gases, has garnered significant attention as a promising avenue for sustainable energy production. One area of focus has been the development of efficient and cost-effective catalysts for water splitting. Researchers have explored catalysts based on abundant and inexpensive materials such as nickel, iron, and cobalt, which have demonstrated improved performance and stability. These catalysts show promise for large-scale implementation and offer potential for reducing the reliance on expensive and scarce materials. Another avenue of research involves photoelectrochemical (PEC) cells, which utilize solar energy to drive the water-splitting reaction. Scientists have been working on designing novel materials, including metal oxides and semiconductors, to enhance light absorption and charge separation properties. These advancements in PEC technology aim to maximize the conversion of sunlight into chemical energy. Inspired by natural photosynthesis, artificial photosynthesis approaches have also gained traction. By integrating light-absorbing materials, catalysts, and membranes, these systems aim to mimic the complex processes of natural photosynthesis and produce hydrogen fuel from water. The development of efficient and stable artificial photosynthesis systems holds promise for sustainable and clean energy production. Tandem cells, which combine multiple light-absorbing materials with different bandgaps, have emerged as a strategy to enhance the efficiency of water-splitting systems. By capturing a broader range of the solar spectrum, tandem cells optimize light absorption and improve overall system performance. Lastly, advancements in electrocatalysis have played a critical role in water splitting. Researchers have focused on developing advanced electrocatalysts with high activity, selectivity, and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). These electrocatalysts contribute to overall water-splitting efficiency and pave the way for practical implementation. Keywords: water splitting; oxygen evolution reaction (OER); hydrogen evolution reaction (HER); photoelectrochemical (PEC) cells; scarce materials; catalyst 1. Introduction In the quest for sustainable energy generation, the development of efficient and clean technologies is of paramount importance. Among the various renewable energy sources, hydrogen has emerged as a promising candidate due to its high energy content and versatility. Water splitting, a process that involves separating water into its constituent elements, hydrogen and oxygen, offers a viable pathway for the production CITATION Shahazi R, Saddam AI, Majumdar S, et al. Advancements in water splitting for sustainable energy generation: A review. Characterization and Application of Nanomaterials. 2024; 7(1): 5834. https://doi.org/10.24294/can.v7i1.5834 ARTICLE INFO Received: 16 April 2024 Accepted: 6 May 2024 Available online: 31 May 2024 COPYRIGHT Copyright © 2024 by author(s). Characterization and Application of Nanomaterials is published by EnPress Publisher, LLC. This work is licensed under the Creative Commons Attribution (CC BY) license. https://creativecommons.org/licenses/ by/4.0/ Characterization and Application of Nanomaterials 2024, 7(1), 5834. 2 of hydrogen as a clean fuel [1,2]. Traditionally, water splitting has relied on electrolysis, a process that utilizes electricity to drive the reaction. However, electrolysis methods have faced challenges in terms of energy efficiency and cost- effectiveness, limiting their large-scale implementation [3,4]. To overcome these barriers, researchers and scientists around the world have been working diligently to break new ground in water splitting technology. In recent years, significant advancements have been made in the field of water splitting, leading to the development of novel and efficient approaches. These breakthroughs have the potential to revolutionize the renewable energy landscape and pave the way for a sustainable future [5,6]. One of the key areas of focus in water splitting research has been the development of catalysts. Catalysts play a crucial role in facilitating the water splitting reaction by reducing the energy requirements and increasing the reaction rates. Traditional catalysts, such as platinum, are effective but expensive, hindering their widespread adoption. However, researchers have made remarkable progress in developing low- cost and earth-abundant catalysts, such as transition metal oxides and molecular catalysts, which exhibit excellent catalytic activity and stability [7–9]. Another significant advancement in water splitting technology is the exploration of photoelectrochemical (PEC) cells. PEC cells utilize semiconductor materials to harness solar energy and drive the water splitting reaction. By combining light absorption and catalytic activity in a single device, PEC cells offer a promising approach to achieve solar-driven water splitting. Researchers have been actively investigating various semiconductor materials, such as metal oxides and perovskites, to enhance the efficiency and stability of PEC cells [10–12]. Furthermore, advancements in nanotechnology have opened up new avenues for improving water splitting efficiency. Nanostructured materials provide a high surface area, improved charge transport, and enhanced light absorption, making them ideal candidates for water splitting applications. Nanoparticles, nanowires, and nanotubes have demonstrated remarkable performance in catalyzing the water splitting reaction, offering unparalleled opportunities for efficient and cost-effective hydrogen production [13–15]. Moreover, the integration of water splitting technologies with renewable energy sources, such as wind and solar, holds tremendous potential for sustainable energy generation. By utilizing excess electricity generated from renewable sources during off-peak hours, water splitting can store the energy in the form of hydrogen, which can be used later for power generation or as a clean fuel for transportation [16–18]. Countries such as Japan, the United States, Germany, China, and South Korea have been actively researching and developing water splitting technologies [19]. They have made notable advancements in terms of increasing efficiency, reducing costs, and developing new materials for electrocatalysts [20]. Japan has a strong research community and has been actively collaborating with universities, research institutes, and industries to advance sunlight-driven water splitting technology [21]. Institutions such as the University of Tokyo, Kyoto University, and the National Institute of Advanced Industrial Science and Technology (AIST) have been at the forefront of this research. Japanese researchers have been working on the development of efficient and stable photoelectrochemical (PEC) cells and photoelectrodes for water splitting. The Characterization and Application of Nanomaterials 2024, 7(1), 5834. 3 United States has a vibrant research community dedicated to advancing water splitting technology. Many universities, national labs, and private research institutions have been conducting research to improve the efficiency, durability, and cost-effectiveness of water splitting systems [22]. Electrolysis, particularly proton exchange membrane (PEM) electrolysis and solid oxide electrolysis cells (SOEC), has been an area of focus in the USA. Besides this, researchers have been working on developing efficient and stable photoelectrodes, exploring new materials, and improving light absorption and charge separation processes. In addition, Germany, China, and South Korea have been actively researching and developing water splitting technologies [23,24]. In a short, advancements in water splitting technology are breaking the barriers that have hindered its widespread implementation for sustainable energy generation. The development of efficient catalysts, exploration of photoelectrochemical cells, utilization of nanostructured materials, and integration with renewable energy sources are propelling the field forward. These advancements offer a promising pathway towards a clean and sustainable future, where hydrogen can play a vital role in meeting our energy needs while minimizing environmental impact. 2. Development of efficient and cost-effective catalysts for water splitting Water splitting is a promising technology for producing clean and renewable hydrogen fuel. It involves the separation of water into hydrogen and oxygen gases through electrochemical reactions. The process typically requires the use of catalysts to enhance the reaction rates and efficiency. Over the years, researchers have been working on developing efficient and cost-effective catalysts for water splitting. 2.1. Platinum group metals (PGMs) PGMs, particularly platinum and iridium, have traditionally been used as catalysts for water splitting. However, their high cost and limited availability hinder large-scale applications. Researchers are exploring ways to reduce or replace the use of PGMs with more abundant and cost-effective materials [25–27]. 2.2. Earth-abundant catalysts Efforts have been focused on developing catalysts based on earth-abundant elements, such as transition metal oxides, sulfides, phosphides, and nitrides. These materials offer the advantages of low cost and scalability. For example, metal oxides like iron oxide (Fe2O3) and cobalt oxide (Co3O4) have shown promising catalytic activity [28–30]. An illustration of catalysis by earth-abundant materials is shown in Figure 1. Figure 1. Water splitting with earth-abundant elements. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 4 2.3. Bimetallic and alloy catalysts Combining different metals into bimetallic or alloy catalysts can enhance their catalytic properties. For instance, combining nickel (Ni) with iron (Fe) or cobalt (Co) has shown improved activity for water splitting. These catalysts can be synthesized using various methods, including electrochemical deposition, sol-gel techniques, and physical mixing [31–33]. 2.4. Molecular catalysts Researchers are also exploring molecular catalysts, especially based on abundant and inexpensive organic compounds. These catalysts typically contain metal complexes with ligands that facilitate the water splitting reactions. A molecular catalytic reaction is demonstrated in Figure 2. Molecular catalysts offer precise control over the catalytic properties and can be designed to optimize efficiency [34– 36]. Figure 2. Schematic diagram of homogeneous catalysis with soluble molecular catalyst. 2.5. Nanostructured catalysts Nanostructured catalysts, such as nanoparticles, nanowires, and nanotubes, have attracted attention due to their high surface area and unique electronic properties. These structures can enhance catalytic activity by providing more active sites and improving charge transport. Examples include metal nanoparticles supported on conductive substrates or semiconductor nanomaterials [37–40]. Several nanoparticles and nanowires are shown in Figure 3. Figure 3. An illustration of nanoparticles and nanowire. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 5 2.6. Computational design Advances in computational modeling and machine learning have enabled the rational design of catalysts with enhanced activity. By simulating the electronic structure and reaction kinetics, researchers can identify promising catalyst candidates for experimental validation, accelerating the discovery process [41–43]. 3. Solar energy to drive the water-splitting reaction Utilizing solar energy to drive the water-splitting reaction is a promising approach for sustainable hydrogen production. It involves harnessing the energy from sunlight and converting it into chemical energy stored in the form of hydrogen gas. There are two common methods for using solar energy in water splitting: 3.1. Photovoltaic (PV) electrolysis This method involves using photovoltaic cells, commonly known as solar cells, to directly convert solar energy into electricity. The generated electricity is then used to power an electrolyzer, which splits water into hydrogen and oxygen gases. The electrolyzer consists of two electrodes (cathode and anode) immersed in an electrolyte solution. When an electric current is applied, water molecules at the cathode are reduced to produce hydrogen gas (H2), while water molecules at the anode are oxidized to produce oxygen gas (O2). Catalysts are employed at the electrodes to enhance the reaction rates and improve overall efficiency [44–46]. 3.2. Photoelectrochemical (PEC) water splitting PEC water splitting combines the principles of solar cells and electrolysis into a single device. A photoelectrochemical cell is used, which typically consists of a semiconductor electrode immersed in an electrolyte solution [47]. A schematic diagram of photoelectrochemical water splitting is depicted in Figure 4. The semiconductor electrode absorbs photons from sunlight, generating electron-hole pairs. The excited electrons participate in the reduction reaction (hydrogen evolution), while the holes contribute to the oxidation reaction (oxygen evolution) [48–51]. Catalysts are essential in PEC cells to facilitate the reaction kinetics and improve efficiency. Both PV electrolysis and PEC water splitting have their advantages and challenges: Figure 4. Schematic diagram of photoelectrochemical water splitting. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 6 3.2.1. Advantages • Utilization of abundant and renewable solar energy. • Production of clean and sustainable hydrogen fuel. • Compatibility with existing infrastructure for hydrogen storage and utilization. • Potential for decentralized hydrogen production. 3.2.2. Challenges • Efficiency: Maximizing the efficiency of solar energy conversion and the water- splitting reaction to maximize hydrogen production. • Catalysts: Developing efficient and stable catalysts that can enhance the reaction rates and reduce energy losses. • Materials: Exploring and optimizing semiconductor materials with desirable properties for efficient solar energy absorption and charge separation. • Durability: Ensuring the long-term stability and durability of the materials and catalysts under harsh operating conditions. • Cost: Reducing the cost of materials, catalysts, and system components to enable widespread adoption. Ongoing research and development efforts are focused on improving the efficiency, stability, and cost-effectiveness of solar-driven water-splitting technologies. By addressing these challenges, solar energy can be harnessed to drive the water-splitting reaction, enabling the production of clean and sustainable hydrogen fuel. 4. Natural photosynthesis to hydrogen fuel By integrating light-absorbing materials, catalysts, and membranes, artificial photosynthetic systems aim to mimic the complex processes of natural photosynthesis and produce hydrogen fuel from water. These systems, often referred to as artificial photosynthesis or artificial leaf systems, seek to harness solar energy and use it to drive the water-splitting reaction, generating hydrogen gas (H2) as a clean and renewable fuel. Here’s a breakdown of the key components: 4.1. Light-absorbing materials Light-absorbing materials, such as semiconductors or molecular dyes, capture sunlight and convert it into usable energy. These materials should have a broad absorption spectrum, efficient light harvesting, and good charge separation properties to generate the necessary energetic electrons [52–54]. 4.2. Catalysts Catalysts facilitate the water-splitting reaction by reducing the energy barriers and increasing the reaction rates. They are typically used at the cathode (hydrogen- evolving reaction, HER) and anode (oxygen-evolving reaction, OER) to promote the respective electrochemical reactions. Catalysts can be based on various materials, including earth-abundant metals, metal oxides, molecular complexes, or even biological enzymes [55–57]. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 7 4.3. Membranes Membranes are employed to separate the HER and OER compartments, preventing the mixing of hydrogen and oxygen gases and enhancing the overall system efficiency. Proton-exchange membranes (PEMs) or other selective ion-conductive membranes are used to enable the transport of protons while blocking the crossover of gases [58–60]. 4.4. Electron transfer pathways Efficient pathways for electron transfer are essential to transport the generated electrons from the light-absorbing materials to the catalytic sites. Electron-conductive materials or structures, such as conductive electrodes or nanowires, are used to facilitate the movement of electrons to the respective electrodes [61–63]. By integrating these components, artificial photosynthetic systems emulate the fundamental processes of natural photosynthesis, where plants and algae convert sunlight, water, and carbon dioxide into chemical energy in the form of carbohydrates. In the case of artificial photosynthesis for hydrogen production, the focus is on generating hydrogen fuel from water using sunlight as the primary energy source. These systems hold promise for sustainable and carbon-neutral energy production, but there are still challenges to overcome, such as improving the efficiency, stability, and scalability of the components, as well as reducing costs. Extensive research and development efforts are ongoing to advance the field of artificial photosynthesis and enable its practical implementation as a viable technology for hydrogen production and energy storage. 5. Tandem cells to enhance the efficiency of water-splitting systems Tandem cells have emerged as a strategy to enhance the efficiency of water- splitting systems in artificial photosynthesis. Tandem cells are multi-junction devices that combine multiple light-absorbing materials with different bandgaps in a stacked configuration [64–66]. This configuration allows for the efficient capture of a broader range of the solar spectrum, thereby increasing the overall energy conversion efficiency. Here’s a closer look at how tandem cells work: 5.1. Bandgap combinations Different semiconductor materials have different bandgaps, which determine the range of light wavelengths they can efficiently absorb. In tandem cells, materials with varying bandgaps are carefully selected and arranged in a series to create a cascade of absorption layers. The bandgap of each layer is tailored to match the energy level of a specific portion of the solar spectrum, enabling efficient utilization of a wider range of photons [67,68]. 5.2. Efficient light harvesting As sunlight passes through the tandem cell, each layer absorbs a specific portion of the solar spectrum. The absorbed photons generate electron-hole pairs (excitons) in the respective layers, leading to the production of electrical current [69,70]. The light harvesting technique is demonstrated in Figure 5. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 8 Figure 5. Light harvesting technique. 5.3. Charge separation and collection The excited electrons and holes generated in each layer are rapidly separated due to the different bandgaps and internal electric fields. Efficient charge collection mechanisms are employed to extract the electrons and holes from each layer and direct them to their respective contacts or electrodes. 5.4. Water-splitting reactions The separated electrons and holes can be utilized for the water-splitting reaction. The excited electrons are directed to the cathode, where they participate in the reduction reaction (hydrogen evolution) by converting protons (H+) from water into hydrogen gas (H2). The holes are directed to the anode, where they participate in the oxidation reaction (oxygen evolution) by oxidizing water molecules (H2O) to produce oxygen gas (O2). By combining materials with different bandgaps in tandem cells, a larger portion of the solar spectrum can be effectively harvested, leading to improved light-to- hydrogen conversion efficiency. This approach allows for better utilization of solar energy and has the potential to achieve higher efficiencies compared to single-junction devices. Tandem cells are an active area of research, and scientists are exploring various material combinations, device architectures, and fabrication techniques to optimize their performance. The development of efficient and stable tandem cells is crucial for advancing the field of artificial photosynthesis and enabling more efficient solar-driven water-splitting systems for sustainable hydrogen production. 6. Development and optimization of OER and HER The development and optimization of the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrocatalysts are crucial for advancing various energy conversion and storage technologies. Here are some key aspects involved in the development and optimization of OER and HER processes: 6.1. Catalyst screening and design Initial stages involve screening and evaluation of various catalyst materials to Characterization and Application of Nanomaterials 2024, 7(1), 5834. 9 identify candidates with high activity for OER and HER. The catalyst design considers factors such as electronic structure, surface area, crystal structure, and surface chemistry to enhance catalytic activity and stability. Computational modeling and high-throughput screening techniques are often employed to accelerate catalyst discovery [71,72]. 6.2. Nano-structuring and surface modifications Nano-structuring techniques, such as nanoparticle synthesis, thin-film deposition, or nanowire fabrication, are employed to increase the surface area and expose more active sites. Surface modifications, such as doping, alloying, or surface functionalization, can tailor the catalyst’s electronic properties and surface reactivity, leading to improved performance [73–75]. Surface functionalization of catalysts is illustrated in Figure 6. Figure 6. Surface functionalization of catalyst. 6.3. Interface engineering The catalyst-support interface plays a crucial role in the overall catalytic activity and stability. Interface engineering techniques, such as optimizing the catalyst-support interaction, introducing interlayers, or using conductive substrates, can enhance electron transfer kinetics and catalytic performance [76–78]. 6.4. Co-catalysts and synergy effects Co-catalysts, such as metal nanoparticles, metal oxides, or molecular complexes, can be combined with the primary catalyst to enhance catalytic performance. Synergistic effects between different catalyst components can promote electron transfer, modify reaction kinetics, and improve overall efficiency [79,80]. 6.5. Ion and mass transport Efficient ion and mass transport within the electrochemical system is crucial for optimizing OER and HER. Strategies to enhance mass transport include designing porous electrode structures, optimizing electrolyte composition, and improving gas diffusion pathways [81,82]. 6.6. Stability and durability Long-term stability and durability of OER and HER catalysts are essential for practical applications. Researchers focus on understanding degradation mechanisms, developing strategies to mitigate catalyst degradation (e.g., corrosion resistance), and exploring protective coatings or encapsulation techniques [83,84]. Characterization and Application of Nanomaterials 2024, 7(1), 5834. 10 6.7. Advanced characterization techniques Advanced characterization techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and in-situ spectroscopy, provide insights into catalyst structures, active sites, and reaction mechanisms. These techniques help in understanding the structure- activity relationships and guide catalyst optimization efforts. The development and optimization of OER and HER catalysts involve a multidisciplinary approach, combining materials science, surface chemistry, electrochemistry, and computational modeling. Continued research efforts aim to enhance catalytic activity, selectivity, stability, and cost-effectiveness to enable efficient and sustainable energy conversion and storage systems. 7. Catalyst for OER and HER Researchers have indeed focused on developing advanced electrocatalysts with high activity, selectivity, and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). These electrocatalysts play a critical role in facilitating efficient and sustainable water splitting, which is essential for various applications, including artificial photosynthesis and renewable energy storage. Here’s an overview of the advancements in electrocatalyst development for the OER and HER: 7.1. Oxygen evolution reaction (OER) 7.1.1. Metal oxides and mixed metal oxides Metal oxides, such as ruthenium oxide (RuO2), iridium oxide (IrO2), and manganese oxide (MnOx), have shown excellent catalytic activity for the OER. Researchers have been exploring the synthesis of nanostructured and well-defined metal oxide catalysts to enhance their surface area and expose more active sites. Mixed metal oxides, combining different elements, can exhibit improved OER activity and stability compared to single-metal oxides [85–87] (Table 1). Table 1. Comparison of oxygen evolution reaction (OER) performance with various transition metal oxide and hydroxide [88]. Materials pH Overpotential for 10 mA cm−2/V Tafel Slope/mV decade−1 MnCo-G 14 0.33 48 RuO2 14 0.3 42 Ni5Mn-LDH-MWCNT 14 0.35 (iR-corrected) 83 Co5Mn-LDH-MWCNT 14 0.3 (iR-corrected) 74 CoNi-LDH/Fe-PP-M 14 0.32 53 CuCo2O4/N-rGO 14 0.36 64 Co3S4@MoS2 14 0.33 59 CoMoO4 14 0.31 56 CoP 14 0.36 66 CoFe LDH 13 0.36 49 NiFe LDH 14 0.33 41 Characterization and Application of Nanomaterials 2024, 7(1), 5834. 11 7.1.2. Perovskite oxides Perovskite oxides, with a general formula of ABO3, have garnered significant attention for OER electrocatalysis. Materials such as strontium titanate (SrTiO3), strontium iridate (SrIrO3), and barium strontium cobalt iron oxide (BSCF) have demonstrated promising OER activity. Doping, surface modification, and nano structuring techniques are employed to optimize the performance of perovskite oxides [89–92]. 7.1.3. Earth-Abundant talysts To overcome the cost and scarcity associated with noble metals, researchers are actively exploring earth-abundant catalysts for the OER. Materials like cobalt-based compounds (e.g., Co3O4), nickel-iron-based compounds (e.g., NiFe layered double hydroxides), and metal phosphides (e.g., nickel phosphide, cobalt phosphide) have shown promising OER activity [93,94]. 7.2. Hydrogen evolution reaction (HER) 7.2.1. Platinum group metals (PGMs) PGMs, particularly platinum (Pt) and palladium (Pd), are highly efficient HER catalysts due to their excellent activity and stability. Researchers are working on developing advanced Pt- and Pd-based catalysts with enhanced activity through alloying, nano structuring, and developing hybrid materials. 7.2.2. Earth-abundant catalysts To address the cost and sustainability issues associated with PGMs, researchers are actively exploring earth-abundant alternatives for HER. Materials such as transition metal sulfides (e.g., molybdenum sulfide, nickel-molybdenum sulfide) and metal phosphides (e.g., nickel phosphide, cobalt phosphide) have shown promising HER activity [93,94]. Several earth-abundant catalysts and their properties are shown in Table 2. Table 2. Examples of earth-abundant HER electrocatalysts. Catalyst material η at −10mAcm−2 (mV) Tafel slope (mV per decade) pH Faradaic yield NiMo 200 (100 mAcm−2) 122 14.8 NA CoMo 170 (100 mAcm−2) 92 14.8 NA NiMo 185 (100 mAcm−2) 112 14.8 NA NiMo 70 (20 mAcm−2) NA 14.3 NA NiMo 34 (20 mAcm−2) NA 14 NA MoS2 260 50 0 NA Pt 50 140–150 13 NA Ni 58 81.6 14 NA Mo 65 76 14 NA MoS2 200 (15 mAcm−2) 40 −0.3 100%* MoS2 ~150 41 0 NA MoS2 170 60 0.2 NA CoS2 145 51 0 NA Characterization and Application of Nanomaterials 2024, 7(1), 5834. 12 Table 2. (Continued). Catalyst material η at −10 mAcm−2(mV) Tafel slope (mV per decade) pH Faradaic yield CoS2 ~175 93 7 100% CoMoSx 250 85 7 −100% WS2 ~250 60 0 NA CoSe2 90 39 0 NA MoS1.0Sel.0 ~200 56 0 100%* NiSe2 ~140 49 0 NA Ni2P 130 (20 mAcm−2) 46 0 100%* CoP 85 (20 mAcm−2) 50 0 100%* FeP 55 38 0 100% MoP 64 NA 0 100% CoNx 170 75 14 NA CoNx 140 30 0 NA NiMoNx 225 (5 mAcm−2) 35.9 1 NA α-MoB ~225(20 mAcm−2) 55 −0.3 100% Mo2C 130 53 0 NA MoC 124 43 0 NA MoC 77 50 14 NA Ni/C 34 41 0 100%* Cu95Ti5 60 110 13 NA 7.2.3. Molecular catalysts Molecular catalysts, typically based on metal complexes or metalloporphyrins, offer precise control over the active sites and electronic properties. Researchers are designing and synthesizing molecular catalysts with tailored structures to optimize HER activity, selectivity, and stability. The development of advanced electrocatalysts with high activity, selectivity, and stability is crucial for improving the overall efficiency and commercial viability of water-splitting technologies. Researchers continue to explore new materials, catalyst designs, and strategies to enhance the performance of electrocatalysts for the OER and HER, aiming to enable efficient and sustainable hydrogen production. 8. Conclusion In conclusion, significant progress has been made in the development and optimization of catalysts and technologies for water splitting, which has advanced the production of sustainable energy. Catalysts based on abundant and inexpensive materials, such as nickel, iron, and cobalt, have shown improved performance and stability, reducing the need for costly materials. Photoelectrochemical (PEC) cells, which utilize novel materials like metal oxides and semiconductors, aim to maximize the conversion of solar energy into chemical energy for water splitting. Artificial photosynthesis approaches, inspired by natural photosynthesis, integrate light- absorbing materials, catalysts, and membranes to produce hydrogen fuel from water, offering a potential solution for clean energy production. Tandem cells, which Characterization and Application of Nanomaterials 2024, 7(1), 5834. 13 combine multiple light-absorbing materials, optimize light absorption and enhance system efficiency. Furthermore, advancements in electrocatalysis have led to the development of advanced electrocatalysts with high activity, selectivity, and stability for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). These advancements collectively pave the way for the practical implementation of water splitting in various energy conversion and storage systems, bringing us closer to a sustainable and clean energy future. Conflict of interest: The authors declare no conflict of interest. References 1. Hota P, Das A, Maiti DK. A short review on generation of green fuel hydrogen through water splitting. International Journal of Hydrogen Energy. 2023; 48(2): 523-541. doi: 10.1016/j.ijhydene.2022.09.264 2. Mohsin M, Ishaq T, Bhatti IA, et al. Semiconductor Nanomaterial Photocatalysts for Water-Splitting Hydrogen Production: The Holy Grail of Converting Solar Energy to Fuel. Nanomaterials. 2023; 13(3): 546. doi: 10.3390/nano13030546 3. Gong Y, Yao J, Wang P, et al. Perspective of hydrogen energy and recent progress in electrocatalytic water splitting. Chinese Journal of Chemical Engineering. 2022; 43: 282-296. doi: 10.1016/j.cjche.2022.02.010 4. Rafique M, Mubashar R, Irshad M, et al. A Comprehensive Study on Methods and Materials for Photocatalytic Water Splitting and Hydrogen Production as a Renewable Energy Resource. Journal of Inorganic and Organometallic Polymers and Materials. 2020; 30(10): 3837-3861. doi: 10.1007/s10904-020-01611-9 5. Li Y, Sun Y, Qin Y, et al. Recent Advances on Water‐Splitting Electrocatalysis Mediated by Noble‐Metal‐Based Nanostructured Materials. Advanced Energy Materials. 2020; 10(11). doi: 10.1002/aenm.201903120 6. Wang YZ, Yang M, Ding Y, et al. Recent Advances in Complex Hollow Electrocatalysts for Water Splitting. Advanced Functional Materials. 2021; 32(6). doi: 10.1002/adfm.202108681 7. Shamsah SMI. Earth-Abundant Electrocatalysts for Water Splitting: Current and Future Directions. Catalysts. 2021; 11(4): 429. doi: 10.3390/catal11040429 8. Hayat A, Sohail M, Ali H, et al. Recent Advances and Future Perspectives of Metal‐Based Electrocatalysts for Overall Electrochemical Water Splitting. The Chemical Record. 2022; 23(2). doi: 10.1002/tcr.202200149 9. Li S, Li E, An X, et al. Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives. Nanoscale. 2021; 13(30): 12788-12817. doi: 10.1039/d1nr02592a 10. Hamdani IR, Bhaskarwar AN. Recent progress in material selection and device designs for photoelectrochemical water- splitting. Renewable and Sustainable Energy Reviews. 2021; 138: 110503. doi: 10.1016/j.rser.2020.110503 11. Sivagurunathan AT, Adhikari S, Kim DH. Strategies and implications of atomic layer deposition in photoelectrochemical water splitting: Recent advances and prospects. Nano Energy. 2021; 83: 105802. doi: 10.1016/j.nanoen.2021.105802 12. Ali M, Pervaiz E, Noor T, et al. Recent advancements in MOF‐ based catalysts for applications in electrochemical and photoelectrochemical water splitting: A review. International Journal of Energy Research. 2020; 45(2): 1190-1226. doi: 10.1002/er.5807 13. Pratibha, Kapoor A, Rajput JK. Nanostructured materials for the visible-light driven hydrogen evolution by water splitting: A review. International Journal of Hydrogen Energy. 2022; 47(40): 17544-17582. doi: 10.1016/j.ijhydene.2022.03.232 14. Wang Y, Zhang J, Liang W, et al. Plasmonic Metal Nanostructures as Efficient Light Absorbers for Solar Water Splitting. Advanced Energy and Sustainability Research. 2021; 2(11). doi: 10.1002/aesr.202100092 15. Samanta B, Morales-García Á, Illas F, et al. Challenges of modeling nanostructured materials for photocatalytic water splitting. Chemical Society Reviews. 2022; 51(9): 3794-3818. doi: 10.1039/d1cs00648g 16. Mohamed HH. Green processes and sustainable materials for renewable energy production via water splitting. In: Cheong KY, Apblett A (editors). Sustainable Materials and Green Processing for Energy Conversion. Elsevier; 2022. pp. 169-212. doi: 10.1016/b978-0-12-822838-8.00007-7 17. Hosseini SE, Wahid MA. Hydrogen from solar energy, a clean energy carrier from a sustainable source of energy. International Journal of Energy Research. 2020; 44(6): 4110-4131. doi: 10.1002/er.4930 18. Ashraf M, Ayaz M, Khan M, et al. Recent Trends in Sustainable Solar Energy Conversion Technologies: Mechanisms, Characterization and Application of Nanomaterials 2024, 7(1), 5834. 14 Prospects, and Challenges. Energy & Fuels. 2023; 37(9): 6283-6301. doi: 10.1021/acs.energyfuels.2c04077 19. Han N, Race M, Zhang W, et al. Perovskite and related oxide based electrodes for water splitting. Journal of Cleaner Production. 2021; 318: 128544. doi: 10.1016/j.jclepro.2021.128544 20. Wang Y, Seo B, Wang B, et al. Fundamentals, materials, and machine learning of polymer electrolyte membrane fuel cell technology. Energy and AI. 2020; 1: 100014. doi: 10.1016/j.egyai.2020.100014 21. Kawawaki T, Kawachi M, Yazaki D, et al. Development and Functionalization of Visible-Light-Driven Water-Splitting Photocatalysts. Nanomaterials. 2022; 12(3): 344. doi: 10.3390/nano12030344 22. Vilanova A, Dias P, Lopes T, et al. The route for commercial photoelectrochemical water splitting: a review of large-area devices and key upscaling challenges. Chemical Society Reviews. 2024; 53(5): 2388-2434. doi: 10.1039/d1cs01069g 23. Jolaoso LA, Duan C, Kazempoor P. Life cycle analysis of a hydrogen production system based on solid oxide electrolysis cells integrated with different energy and wastewater sources. International Journal of Hydrogen Energy. 2024; 52: 485-501. doi: 10.1016/j.ijhydene.2023.07.129 24. Qahtan TF, Alade IO, Rahaman MS, et al. Mapping the research landscape of hydrogen production through electrocatalysis: A decade of progress and key trends. Renewable and Sustainable Energy Reviews. 2023; 184: 113490. doi: 10.1016/j.rser.2023.113490 25. Salonen LM, Petrovykh DY, Kolen’ko YuV. Sustainable catalysts for water electrolysis: Selected strategies for reduction and replacement of platinum-group metals. Materials Today Sustainability. 2021; 11-12: 100060. doi: 10.1016/j.mtsust.2021.100060 26. Hughes AE, Haque N, Northey SA, et al. Platinum Group Metals: A Review of Resources, Production and Usage with a Focus on Catalysts. Resources. 2021; 10(9): 93. doi: 10.3390/resources10090093 27. Liu J, Li Y, Zhou X, et al. Positively charged Pt-based cocatalysts: an orientation for achieving efficient photocatalytic water splitting. Journal of Materials Chemistry A. 2020; 8(1): 17-26. doi: 10.1039/c9ta10568a 28. Karuppasamy L, Gurusamy L, Ananan S, et al. Metal-organic frameworks derived interfacing Fe2O3/ZnCo2O4 multimetal oxides as a bifunctional electrocatalyst for overall water splitting. Electrochimica Acta. 2023; 449: 142242. doi: 10.1016/j.electacta.2022.142242 29. Warsi MF, Shaheen N, Sarwar MI, et al. A comparative study on photocatalytic activities of various transition metal oxides nanoparticles synthesized by wet chemical route. Desalination And Water Treatment. 2021; 211: 181-195. doi: 10.5004/dwt.2021.26463 30. Cao Q, Li Q, Pi Z, et al. Metal–Organic-Framework-Derived Ball-Flower-like Porous Co3O4/Fe2O3 Heterostructure with Enhanced Visible-Light-Driven Photocatalytic Activity. Nanomaterials. 2022; 12(6): 904. doi: 10.3390/nano12060904 31. Jeghan SMN, Kim D, Lee Y, et al. Designing a smart heterojunction coupling of cobalt-iron layered double hydroxide on nickel selenide nanosheets for highly efficient overall water splitting kinetics. Applied Catalysis B: Environmental. 2022; 308: 121221. doi: 10.1016/j.apcatb.2022.121221 32. Yu M, Budiyanto E, Tüysüz H. Principles of Water Electrolysis and Recent Progress in Cobalt‐, Nickel‐, and Iron‐Based Oxides for the Oxygen Evolution Reaction. Angewandte Chemie International Edition. 2021; 61(1). doi: 10.1002/anie.202103824 33. Yaseen W, Ullah N, Xie M, et al. Ni-Fe-Co based mixed metal/metal-oxides nanoparticles encapsulated in ultrathin carbon nanosheets: A bifunctional electrocatalyst for overall water splitting. Surfaces and Interfaces. 2021; 26: 101361. doi: 10.1016/j.surfin.2021.101361 34. Zhang B, Zheng Y, Ma T, et al. Designing MOF Nanoarchitectures for Electrochemical Water Splitting. Advanced Materials. 2021; 33(17). doi: 10.1002/adma.202006042 35. Yao D, Gu L, Zuo B, et al. A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting. Nanoscale. 2021; 13(24): 10624-10648. doi: 10.1039/d1nr02307a 36. Li Z, Hu M, Wang P, et al. Heterojunction catalyst in electrocatalytic water splitting. Coordination Chemistry Reviews. 2021; 439: 213953. doi: 10.1016/j.ccr.2021.213953 37. Huang J, Jiang Y, An T, et al. Increasing the active sites and intrinsic activity of transition metal chalcogenide electrocatalysts for enhanced water splitting. Journal of Materials Chemistry A. 2020; 8(48): 25465-25498. doi: 10.1039/d0ta08802a 38. Huang H, Cho A, Kim S, et al. Structural Design of Amorphous CoMoPx with Abundant Active Sites and Synergistic Catalysis Effect for Effective Water Splitting. Advanced Functional Materials. 2020; 30(43). doi: 10.1002/adfm.202003889 Characterization and Application of Nanomaterials 2024, 7(1), 5834. 15 39. Raheema MH, Jaber GS. Synthesis of Carbon Nanotubes Using Modified Hummers Method for Cathode Electrodes in Dye- Sensitized Solar Cell. Baghdad Science Journal. 2023; 20(6): 2290-2299. doi: 10.21123/bsj.2023.7150 40. Shahazi R, Majumdar S, Saddam AI, et al. Carbon nanomaterials for biomedical applications: A comprehensive review. Nano Carbons. 2023; 1(1): 448. doi: 10.59400/n-c.v1i1.448 41. Chen Y, Zheng W, Murcia-López S, et al. Light management in photoelectrochemical water splitting – from materials to device engineering. Journal of Materials Chemistry C. 2021; 9(11): 3726-3748. doi: 10.1039/d0tc06071b 42. Moon C, Shin B. Review on light absorbing materials for unassisted photoelectrochemical water splitting and systematic classifications of device architectures. Discover Materials. 2022; 2(1). doi: 10.1007/s43939-022-00026-2 43. Kawase Y, Higashi T, Domen K, et al. Recent Developments in Visible‐Light‐Absorbing Semitransparent Photoanodes for Tandem Cells Driving Solar Water Splitting. Advanced Energy and Sustainability Research. 2021; 2(7). doi: 10.1002/aesr.202100023 44. Liu HY, Cody CC, Jayworth JA, et al. Surface-Attached Molecular Catalysts on Visible-Light-Absorbing Semiconductors: Opportunities and Challenges for a Stable Hybrid Water-Splitting Photoanode. ACS Energy Letters. 2020; 5(10): 3195-3202. doi: 10.1021/acsenergylett.0c01719 45. Yang G, Yu S, Kang Z, et al. Building Electron/Proton Nanohighways for Full Utilization of Water Splitting Catalysts. Advanced Energy Materials. 2020; 10(16). doi: 10.1002/aenm.201903871 46. Liu PF, Yin H, Fu HQ, et al. Activation strategies of water-splitting electrocatalysts. Journal of Materials Chemistry A. 2020; 8(20): 10096-10129. doi: 10.1039/d0ta01680b 47. Zeng C, Dai L, Jin Y, et al. Design strategies toward transition metal selenide-based catalysts for electrochemical water splitting. Sustainable Energy & Fuels. 2021; 5(5): 1347-1365. doi: 10.1039/d0se01722a 48. Gahlot S, Kulshrestha V. Graphene based polymer electrolyte membranes for electro-chemical energy applications. International Journal of Hydrogen Energy. 2020; 45(34): 17029-17056. doi: 10.1016/j.ijhydene.2019.06.047 49. Li C, Baek JB. The promise of hydrogen production from alkaline anion exchange membrane electrolyzers. Nano Energy. 2021; 87: 106162. doi: 10.1016/j.nanoen.2021.106162 50. Du N, Roy C, Peach R, et al. Anion-Exchange Membrane Water Electrolyzers. Chemical Reviews. 2022; 122(13): 11830- 11895. doi: 10.1021/acs.chemrev.1c00854 51. Tiwari JN, Singh AN, Sultan S, et al. Recent Advancement of p‐ and d‐Block Elements, Single Atoms, and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting. Advanced Energy Materials. 2020; 10(24). doi: 10.1002/aenm.202000280 52. Li B, Tian Z, Li L, et al. Directional Charge Transfer Channels in a Monolithically Integrated Electrode for Photoassisted Overall Water Splitting. ACS Nano. 2023; 17(4): 3465-3482. doi: 10.1021/acsnano.2c09659 53. Ng WC, Chong MN. Organic-inorganic p-type PEDOT: PSS/CuO/MoS2 photocathode with in-built antipodal photogenerated holes and electrons transfer pathways for efficient solar-driven photoelectrochemical water splitting. Sustainable Materials and Technologies. 2023; 38: e00749. doi: 10.1016/j.susmat.2023.e00749 54. Ašmontas S, Mujahid M. Recent Progress in Perovskite Tandem Solar Cells. Nanomaterials. 2023; 13(12): 1886. doi: 10.3390/nano13121886 55. Martinho F. Challenges for the future of tandem photovoltaics on the path to terawatt levels: a technology review. Energy & Environmental Science. 2021; 14(7): 3840-3871. doi: 10.1039/d1ee00540e 56. Kumar P, Thokala S, Singh SP, et al. Research progress and challenges in extending the infra-red absorption of perovskite tandem solar cells. Nano Energy. 2024; 121: 109175. doi: 10.1016/j.nanoen.2023.109175 57. He R, Ren S, Chen C, et al. Wide-bandgap organic–inorganic hybrid and all-inorganic perovskite solar cells and their application in all-perovskite tandem solar cells. Energy & Environmental Science. 2021; 14(11): 5723-5759. doi: 10.1039/d1ee01562a 58. Ullah F, Chen CC, Choy WCH. Recent Developments in Organic Tandem Solar Cells toward High Efficiency. Advanced Energy and Sustainability Research. 2021; 2(4). doi: 10.1002/aesr.202000050 59. Wang Y, Shi H, Cui K, et al. Reversible electron storage in tandem photoelectrochemical cell for light driven unassisted overall water splitting. Applied Catalysis B: Environmental. 2020; 275: 119094. doi: 10.1016/j.apcatb.2020.119094 60. Zhang D, Cho H, Yum J, et al. An Organic Semiconductor Photoelectrochemical Tandem Cell for Solar Water Splitting. Advanced Energy Materials. 2022; 12(42). doi: 10.1002/aenm.202202363 61. Zhou B, Gao R, Zou J, et al. Surface Design Strategy of Catalysts for Water Electrolysis. Small. 2022; 18(27). doi: Characterization and Application of Nanomaterials 2024, 7(1), 5834. 16 10.1002/smll.202202336 62. Li J, Liu Y, Chen H, et al. Design of a Multilayered Oxygen‐Evolution Electrode with High Catalytic Activity and Corrosion Resistance for Saline Water Splitting. Advanced Functional Materials. 2021; 31(27). doi: 10.1002/adfm.202101820 63. Almomani F, Shawaqfah M, Alkasrawi M. Solar-driven hydrogen production from a water-splitting cycle based on carbon- TiO2 nano-tubes. International Journal of Hydrogen Energy. 2022; 47(5): 3294-3305. doi: 10.1016/j.ijhydene.2020.12.19 64. Lee JU, Kim JH, Kang K, et al. Bulk and surface modified polycrystalline CuWO4 films for photoelectrochemical water oxidation. Renewable Energy. 2023; 203: 779-787. doi: 10.1016/j.renene.2022.12.129 65. Joseph M, Kumar M, Haridas S, et al. A review on the advancements of graphitic carbon nitride-based photoelectrodes for photoelectrochemical water splitting. Energy Advances. 2024; 3(1): 30-59. doi: 10.1039/d3ya00506b 66. Singh B, Indra A. Surface and interface engineering in transition metal–based catalysts for electrochemical water oxidation. Materials Today Chemistry. 2020; 16: 100239. doi: 10.1016/j.mtchem.2019.100239 67. Gu H, Shi G, Chen HC, et al. Strong Catalyst–Support Interactions in Electrochemical Oxygen Evolution on Ni–Fe Layered Double Hydroxide. ACS Energy Letters. 2020; 5(10): 3185-3194. doi: 10.1021/acsenergylett.0c01584 68. Yang X, Guo R, Cai R, et al. Engineering transition metal catalysts for large-current-density water splitting. Dalton Transactions. 2022; 51(12): 4590-4607. doi: 10.1039/d2dt00037g 69. Zhou X, Wang P, Li M, et al. Synergistic effect of phosphorus doping and MoS2 co-catalysts on g-C3N4 photocatalysts for enhanced solar water splitting. Journal of Materials Science & Technology. 2023; 158: 171-179. doi: 10.1016/j.jmst.2023.02.041 70. Xiao N, Li S, Li X, et al. The roles and mechanism of cocatalysts in photocatalytic water splitting to produce hydrogen. Chinese Journal of Catalysis. 2020; 41(4): 642-671. doi: 10.1016/S1872-2067(19)63469-8 71. Luo Y, Zhang Z, Chhowalla M, et al. Recent Advances in Design of Electrocatalysts for High‐Current‐Density Water Splitting. Advanced Materials. 2022; 34(16). doi: 10.1002/adma.202108133 72. Sun H, Xu X, Kim H, et al. Electrochemical Water Splitting: Bridging the Gaps Between Fundamental Research and Industrial Applications. Energy & Environmental Materials. 2023; 6(5): 12441. doi: 10.1002/eem2.12441 73. Luo F, Guo L, Xie Y, et al. Iridium nanorods as a robust and stable bifunctional electrocatalyst for pH-universal water splitting. Applied Catalysis B: Environmental. 2020; 279: 119394. doi: 10.1016/j.apcatb.2020.119394 74. Luo F, Hu H, Zhao X, et al. Robust and Stable Acidic Overall Water Splitting on Ir Single Atoms. Nano Letters. 2020; 20(3): 2120-2128. doi: 10.1021/acs.nanolett.0c00127 75. Qin R, Chen G, Feng X, et al. Ru/Ir‐Based Electrocatalysts for Oxygen Evolution Reaction in Acidic Conditions: From Mechanisms, Optimizations to Challenges. Advanced Science. 2024; 11(21): 2309364. doi: 10.1002/advs.202309364 76. Pascuzzi MEC, Goryachev A, Hofmann JP, et al. Mn promotion of rutile TiO2-RuO2 anodes for water oxidation in acidic media. Applied Catalysis B: Environmental. 2020; 261: 118225. doi: 10.1016/j.apcatb.2019.118225 77. Zhang Y, Yan R, Xu X, et al. Next Generation Noble Metal‐Engineered Catalysts: From Structure Evolution to Structure‐ Reactivity Correlation in Water Splitting. Advanced Functional Materials. 2023; 34(4). doi: 10.1002/adfm.202308813 78. Bao J, Xie J, Lei F, et al. Two-Dimensional Mn-Co LDH/Graphene Composite towards High-Performance Water Splitting. Catalysts. 2018; 8(9): 350. doi: 10.3390/catal8090350 79. Patial S, Hasija V, Raizada P, et al. Tunable photocatalytic activity of SrTiO3 for water splitting: Strategies and future scenario. Journal of Environmental Chemical Engineering. 2020; 8(3): 103791. doi: 10.1016/j.jece.2020.103791 80. Yu J, Wu X, Guan D, et al. Monoclinic SrIrO3: An Easily Synthesized Conductive Perovskite Oxide with Outstanding Performance for Overall Water Splitting in Alkaline Solution. Chemistry of Materials. 2020; 32(11): 4509-4517. doi: 10.1021/acs.chemmater.0c00149 81. Zhang L, Jang H, Li Z, et al. SrIrO3 modified with laminar Sr2IrO4 as a robust bifunctional electrocatalyst for overall water splitting in acidic media. Chemical Engineering Journal. 2021; 419: 129604. doi: 10.1016/j.cej.2021.129604 82. Aegerter D, Borlaf M, Fabbri E, et al. Tuning the Co Oxidation State in Ba0.5Sr0.5Co0.8Fe0.2O3-δ by Flame Spray Synthesis Towards High Oxygen Evolution Reaction Activity. Catalysts. 2020; 10(9): 984. doi: 10.3390/catal10090984 83. Peng X, Jin X, Gao B, et al. Strategies to improve cobalt-based electrocatalysts for electrochemical water splitting. Journal of Catalysis. 2021; 398: 54-66. doi: 10.1016/j.jcat.2021.04.003 84. Lei L, Huang D, Zhou C, et al. Demystifying the active roles of NiFe-based oxides/(oxy)hydroxides for electrochemical water splitting under alkaline conditions. Coordination Chemistry Reviews. 2020; 408: 213177. doi: 10.1016/j.ccr.2019.213177 Characterization and Application of Nanomaterials 2024, 7(1), 5834. 17 85. Bodhankar PM, Sarawade PB, Kumar P, et al. Nanostructured Metal Phosphide Based Catalysts for Electrochemical Water Splitting: A Review. Small. 2022; 18(21). doi: 10.1002/smll.202107572 86. Feng Y, Zhu L, Pei A, et al. Platinum–palladium-on-reduced graphene oxide as bifunctional electrocatalysts for highly active and stable hydrogen evolution and methanol oxidation reaction. Nanoscale. 2023; 15(42): 16904-16913. doi: 10.1039/d3nr04014c 87. Jebaslinhepzybai BT, Prabu N, Sasidharan M. Facile galvanic replacement method for porous Pd@Pt nanoparticles as an efficient HER electrocatalyst. International Journal of Hydrogen Energy. 2020; 45(19): 11127-11137. doi: 10.1016/j.ijhydene.2020.02.059 88. Lyu Z, Zhang X, Liao X, et al. Two-Dimensionally Assembled Pd–Pt–Ir Supernanosheets with Subnanometer Interlayer Spacings toward High-Efficiency and Durable Water Splitting. ACS Catalysis. 2022; 12(9): 5305-5315. doi: 10.1021/acscatal.2c00859 89. Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nature Reviews Chemistry. 2017; 1(1). doi: 10.1038/s41570-016-0003 90. Lobinsky AA, Tolstoy VP, Kodinzev IA. Electrocatalytic properties of γ-NiOOH nanolayers, synthesized by successive ionic layer deposition, during the oxygen evolution reaction upon water splitting in the alkaline medium. Nanosystems: Physics, Chemistry, Mathematics. 2018; 9(5): 669-675. doi: 10.17586/2220-8054-2018-9-5-669-675 91. Shinagawa T, Garcia-Esparza AT, Takanabe K. Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion. Scientific Reports. 2015; 5(1). doi: 10.1038/srep13801 92. Antipin D, Risch M. Calculation of the Tafel slope and reaction order of the oxygen evolution reaction between pH 12 and pH 14 for the adsorbate mechanism. Electrochemical Science Advances. 2022; 3(6). doi: 10.1002/elsa.202100213 93. Lin L, Lin Z, Zhang J, et al. Molecular-level insights on the reactive facet of carbon nitride single crystals photocatalysing overall water splitting. Nature Catalysis. 2020; 3(8): 649-655. doi: 10.1038/s41929-020-0476-3 94. Liu W, Zhang H, Li C, et al. Non-noble metal single-atom catalysts prepared by wet chemical method and their applications in electrochemical water splitting. Journal of Energy Chemistry. 2020; 47: 333-345. doi: 10.1016/j.jechem.2020.02.020