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Formulating a strategy to prepare catalytic electrodes that are both cost-effective and exhibit high efficiency in driving overall water splitting

Nickel Foam
Nickel Foam

Currently, the strong guarantee of energy is indispensable for the sustained and rapid development of the economy. As fossil fuels continue to deplete and environmental issues arise, researchers are increasingly focusing on the development of renewable energy sources. Among these, hydrogen energy is considered the most promising and ideal energy carrier for the future. Hydrogen serves as an energy storage medium with notable characteristics such as a high fuel value (142 kJ mol−1), efficiency, and cleanliness. Various industrial methods exist for hydrogen production, including the synthesis gas and natural gas cracked by petroleum, refrigeration of coke oven gas, and fermentation of organic wastewater. However, water electrolysis stands out as a highly efficient approach to produce high-purity hydrogen compared to these methods. Its simplicity in terms of equipment requirements, convenience, and the abundance of water resources, particularly seawater, aligns well with the concept of green and pollution-free energy production. If electric energy is utilized judiciously and catalytic electrodes with exceptional performance and low cost are chosen, water electrolysis has the potential to become the most popular method for hydrogen production. In fact, it could emerge as one of the most promising approaches. Despite being invented over a century ago in 1893, the application of hydrogen energy in various fields only accounts for a mere 4% of all energy sources. This is primarily due to its high energy consumption and cost. Consequently, the development of efficient, stable, and affordable catalytic electrodes takes precedence in order to overcome these challenges and propel the utilization of hydrogen energy forward.

The current catalyst materials used in commercial water electrolysis primarily consist of precious metals such as platinum, iridium, and ruthenium. These precious metals exhibit excellent catalytic performances in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, their high cost hinders their widespread application. In response to this limitation, researchers have discovered that low-cost and abundant transition metals like iron, nickel, and cobalt, when doped with different elements such as oxygen, boron, carbon, nitrogen, phosphorus, sulfur, and selenium, can also exhibit relatively good catalytic effects during HER and OER. This finding opens up the possibility of large-scale hydrogen production. Over the course of several decades of research, scientists have identified several strategies for preparing high-efficiency catalytic electrodes. Firstly, they focus on selecting catalytic electrode support base materials that possess a high specific surface area, high stability, and low cost. Examples of such materials include iron foam, nickel, carbon cloth supports, as well as unconventional base materials like sponges, paper, and fiber cloth. Secondly, researchers aim to enhance the intrinsic activity of the catalytic material by incorporating transition metal elements (such as iron, cobalt, and nickel) with non-metallic elements (such as carbon, nitrogen, oxygen, boron, and selenium) and altering the lattice composition. Lastly, they design the structure and morphology of the catalytic materials to achieve a larger catalytic area under specific circumstances, thereby increasing their catalytic activity. However, recent advancements have introduced new strategies to maximize the catalytic activity of catalytic electrodes while reducing energy consumption. For instance, the utilization of the local magnetocaloric effect and local photothermal effect has shown promise in achieving the highest possible catalytic activity. These innovative approaches hold potential for further improving the efficiency and cost-effectiveness of water electrolysis for hydrogen production.

Nickel Foam (8)

In this review, we examine recent advancements in the field of electrocatalytic water splitting. The focus of these advancements has been twofold. Firstly, researchers have extensively studied the use of various support materials, including iron, nickel foil/foam, and non-metallic conductive carbon materials, in the creation of catalytic electrodes containing both metal and non-metal components. These electrodes have been designed to achieve high efficiency in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Secondly, researchers have investigated the use of catalytic materials with high-efficiency intrinsic activity, such as metal oxides, phosphides, sulfides, and nitrides. These materials exhibit different catalytic effects on the HER and OER depending on the pH of the electrolyte. To achieve high-efficiency catalytic materials, specific preparation methods and control strategies are necessary. Catalytic electrodes are a crucial component of the electrocatalytic process, and the choice of electrode material can greatly impact the efficiency of hydrogen production. Therefore, the development of high-efficiency, environmentally friendly, and cost-effective electrode materials is essential for the advancement of hydrogen production technology through water electrolysis.

Catalytic electrodes of exceptional quality consist of both catalytic materials and support base materials. Nevertheless, it is the catalytic materials that exert the greatest impact on the overpotential of the hydrogen evolution reaction (HER). The selection of appropriate catalysts for the HER can effectively diminish the overpotential and enhance the efficiency of electric energy utilization during the water splitting process.

A stable base material is crucial for an electrode to exhibit excellent catalytic effects. In recent years, researchers have opted for conductive media like nickel foam, titanium sheets, carbon cloth, and carbon nanotubes as supports for the preparation of self-supporting catalytic electrodes. This has led to the development of many highly efficient catalytically active electrodes. The support material plays a vital role in not only providing a stable position for the catalytic material but also ensuring corrosion resistance and durability of the catalytic electrode. This is achieved through its processability, acid and alkali resistance, and good mechanical properties. The subsequent section highlights the advantages of nickel foam as support materials for high-efficiency catalytic electrodes.

Nickel foam (NF) is an innovative functional material characterized by its hollow and interconnected metallurgical skeleton. The pores within the foam range from 0.3 to 2 mm, providing it with high porosity, a significant specific surface area, and a low bulk density. N ickel foam exhibits remarkable resistance to alkali corrosion due to its unique network structure, ensuring its stability. Additionally, nickel foam possesses excellent mechanical processing and hydrodynamic properties. Notably, nickel foam demonstrates a low overpotential for the hydrogen evolution reaction (HER) during water splitting.

In recent times, numerous scientific research teams have employed a range of techniques such as electrodeposition, hydrothermal, acid activation, chemical etching, and others to fabricate diverse materials on nickel foam. However, a research team has recently introduced a novel and remarkably rapid synthesis method. In this method, a solution of mercaptoethanol was carefully dripped onto nickel foam and subjected to high temperature annealing, resulting in the formation of a nickel sulfide (Ni3S2) film (Ni3S2/NF). The three-dimensional structure of the nickel foam possesses a significantly large specific surface area, thereby providing ample sites for the growth of catalytic materials. Consequently, the Ni3S2/NF obtained exhibited commendable activity and stability for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Impressively, it achieved a current density of 10 mA cm−2 in alkaline media with remarkably low overpotentials of 131 and 312 mV, respectively. The success of this work can be attributed to the stability of the nickel foam and its excellent mechanical processing capabilities, which facilitated the preparation of cost-effective, high-performance, and stable catalysts.

Nickel Foam (6)

In 2020, a group of researchers published a study on a monolithic electrode made of nickel that demonstrated exceptional performance in both HER and OER in a neutral medium with a pH of 7.0. The study utilized a combination of chemical and electrochemical etching methods, utilizing a pre-treated NF as the working electrode in 0.5 M H2SO4 for 5 minutes, resulting in the formation of NF with karst landform characteristics. The karst NF exhibited low overpotentials of 110 mV (HER) and 432 mV (OER) at 10 mA cm−2 in 1 M PBS. Another research team employed an in situ Na2S-induced etching method to convert NF into atomically thin nickel sulfide nanosheets with doped Fe, resulting in the synthesis of FexNi3−xS2@NF, which exhibited good bifunctional activity, remarkable stability, and good water adsorption/dissociation kinetics. The FexNi3−xS2@NF required overpotentials of 72 mV at 10 mA cm−2 for the HER and 252 mV at 100 mA cm−2 for the OER in 1.0 M KOH.

Recent research has yielded significant advancements in the field of NF. Notably, catalytic electrodes utilizing nickel foam as a base material have demonstrated increasingly potent catalytic effects. The implementation of NF has proven instrumental in driving progress in the realm of hydrogen production via water splitting, effectively mitigating overpotential in reactions.

Nickel foam possesses strong resistance to alkali corrosion and a distinctive network structure due to its high porosity, large specific surface area, low bulk density, and high stability. Additionally, nickel foam exhibits excellent mechanical processing capabilities, hydrodynamic performance, and a significant electrochemical reaction interface. Consequently, it holds immense potential as an electrode material for electrochemical hydrogen evolution. The foam base material’s three-dimensional structure offers an expanded specific surface area and active sites, thereby enhancing the electrochemical performance of the catalytic electrode. As a result, it has garnered extensive attention in the field of electrolysis. Apart from Ni foam, other three-dimensional base materials like Co foam and Fe foam have also been employed for similar purposes.

In recent years, the field of electrochemistry has witnessed significant advancements and progress, thanks to the dedicated efforts of numerous scientific researchers. However, there are still several areas that require further improvement. Firstly, the choice of base material plays a crucial role in determining the catalytic performance as it serves as a supporting foundation. In addition to meeting the basic requirements of durability and stability, the cost of the base material should also be taken into consideration. Secondly, sulfur is often used as a catalyst despite its low conductivity. To enhance its electrochemical activity, researchers can manipulate its electronic structure through element doping, create a loose and porous structure through topography design, and develop multidimensional structures through structural engineering. Furthermore, extensive research has been conducted on alkaline electrolytes, and researchers are now exploring neutral solutions. In terms of practical applications, the electrolysis of domestic wastewater and sewage for hydrogen production has emerged as a promising research direction. This approach aligns with the concept of sustainable development by simultaneously addressing the need for hydrogen energy and wastewater treatment. Moreover, desalination of sewage and wastewater can be achieved during the production of hydrogen energy, further supporting the ecological concept of sustainable development. Additionally, utilizing seawater as an electrolyte presents a significant research objective. The electrolysis of seawater is a global challenge, and leveraging the corrosion resistance of nitrides and the high catalytic efficiency of phosphides at full pH can potentially lead to breakthroughs in this complex task. However, it is evident that more extensive research is required to accomplish this goal.

 

 

 

Picture of Lu

Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

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WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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