Utilization of nanostructured materials on three-dimensional nickel foam materials as electrocatalysts for water cracking

The depletion of fossil fuels and growing concerns about energy and environmental issues have led to extensive research efforts to develop alternative energy sources. Electrochemical electrolysisof water is considered a promising strategy for hydrogen fuel production. However, efficient water cracking requires efficient and durable electrocatalysts that can accelerate the kinetics of the oxygen-extraction reaction (OER) and hydrogen-extraction reaction (HER). Noble Metal-containing catalysts, such as Pt and its alloys, RuO2 and IrO2, are currently the most efficient catalysts for catalyzing OER and HER. However, the high cost and scarcity of noble metals have hindered their large-scale application and the development of renewable energy technologies. To overcome these limitations, considerable efforts have been made to design and synthesize non-precious metal electrocatalysts using earth-abundant materials as economical alternatives to OER and OERHER. Transition metal oxides, sulfides, phosphides, carbon materials, selenides, and mixed-metal complexes have been extensively studied and have shown promising performance for OER and HER. However, most of these electrocatalysts require higher overpotentials than noble metal-based catalysts, and improving their stability remains crucial. Therefore, there is an urgent need to develop a low-cost and efficient alternative electrode structure with high activity and long-term stability for efficient water splitting.

In the field of electrochemical applications, two main strategies have been used to prepare electrodes. The first and most widely used technique involves the use of catalysts in powdered form. Typically, electrodes are constructed by utilizing slurries of electroactive materials, conductivity enhancers, and adhesives on conductive substrates. However, this approach is not without drawbacks. The main disadvantage is the requirement for an electrically insulating binder, which reduces the contact area between the electrolyte and the catalyst. This may lead to blocking of the catalytically active sites, resulting in high resistance and reduced electrocatalytic performance. In addition, the electrodes are relatively unstable, as the attached catalyst tends to peel off from the conducting substrate at high current densities. The second main strategy for electrode preparation is the use of noble metal-based materials which are electrodeposited directly onto the conductive substrate, such as nickel Foam, copper foil, carbon cloth or paper, FTO, stainless steel and nickel foil. However, this approach is not without its limitations. It is difficult to precisely control the accessible space between the deposited active materials and, therefore, the electrode performance diminishes with increasing film thickness due to the inaccessibility of the substrate to the catalytically active sites inside. In addition, the complexity and high cost of the method greatly hinder its practical application. Therefore, the development of cost-effective fabrication techniques forthree-dimensional (3D) electrodes is necessary for successful electrochemical applications.

The development of new three-dimensional metallic materials with porous structures and high specific surface areas has attracted considerable attention due to their potential to reduce ion diffusion length and increase ionic and electronic conductivity. Therefore, a promising direction for electrocatalyst design is to combine different structural dimensions in order to create a nanostructured composite of catalyst carriers that provide high conductivity, large surface area and high stability. Nickel foam (NF), a commercially available and inexpensive material, has been widely used as a substrate and carrier for electrode materials due to its desirable three-dimensional open-pore structure, high electrical conductivity, and large specific surface area.The micropores and jagged flow channels within the NFs also provide good mass transfer and large surface area per unit area. Various substrates, including different metal foams, meshes, foils and fabrics, have been explored as collectors for electrochemical applications such as lithium-ion batteries, supercapacitors, solar cells and water splitting. In particular, porous NFs have attracted attention due to their low cost, electrical conductivity, and large electroactive surface area, which is ideal for loading catalysts and increasing electrochemical active sites. In addition, porous NFs have advantages in enhancing the mass transport of electrolytes, making them suitable candidates for high surface area current collectors in energy applications. Direct growth of active materials on nickel foam also enhances catalyst-substrate contact for efficient electron transfer in water cracking reactions. While supercapacitor and battery electrodes remain the main applications for NFs, recent studies have shown that electrode materials deposited on this material have superior OER activity than Ni foils and nets. These materials can be applied either in their natural form or decorated with active materials, and the foams can be used both as collectors and support substrates. Thus, the growth of a nanostructured, earth-rich catalytic material on NF substrates holds promise for the development of advanced electrode materials for energy storage/conversion devices. Although earth-rich electrocatalysts grown directly on nickel foam are commonly available, three-dimensional electrodes on nickel foam for OERs and HERs have not been extensively investigated, although they are porous hierarchical structures that are low-cost and easy to fabricate.