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Three-Dimensional Electrocatalysts: Enabling Sustainable Water Splitting Reactions

Silver Foam (2)

In recent years, there has been a growing interest in the research and development of clean energy alternatives that are abundant on Earth. This is mainly due to the increasing global energy demand and the negative impact of traditional energy resources on both human health and the environment. One potential alternative to fossil fuels is hydrogen (H2), which is considered a promising energy carrier due to its high mass-specific energy density. However, the efficient production of hydrogen is crucial in order to address the impending energy crisis. Currently, industrial hydrogen production relies heavily on steam reforming methods, which still rely on fossil fuels and result in low purity and high costs. To overcome these challenges, water splitting through electricity or solar energy has emerged as a convenient solution. This technology involves two main reactions: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). The HER is a relatively easy two-electron transfer process, while the OER is more complex and sluggish due to the involvement of multiple intermediates in the four-electron transfer process. Extensive research has been conducted on these reactions, leading to the development of electrocatalysts that can enhance reaction rates and reduce overpotential. The literature reveals a wide range of electrocatalysts with different compositions and structures. Currently, Pt-based materials are considered state-of-the-art catalysts for the HER, while iridium oxides serve as the benchmark for the OER. However, the scarcity and high cost of these noble metals make them unsustainable resources. In order to achieve a hydrogen economy that is accessible and sustainable, it is crucial to explore new and efficient electrocatalysts based on earth-abundant elements. This research will play a vital role in finding alternative energy sources that are both environmentally friendly and economically viable.

A wide range of non-noble-metal electrocatalysts have been reported to improve performance, including metal oxides and spinels, as well as nitrogen/phosphorus-coordinated metal on carbon and nonmetal carbon materials. Nanostructuring is a critical method to enhance electrocatalytic performance by exposing more active sites to the electrolyte. Porous materials with large surface areas and high conductivity are also beneficial for catalytic activity. These materials are often supported on porous supports and coated on conductive substrates, but the use of polymeric binders may decrease catalytic activity. Therefore, the development of binder-free electrocatalysts, such as free-standing thin films, aerogels, or hydrogels, is crucial for achieving enhanced catalytic performance.

Three-dimensional nanostructured materials have the potential to fulfill the requirements for catalysts. These electrocatalysts can be used as working electrodes without the need for a polymer binder. They can be directly grown on current collectors or formed into free-standing films or monoliths. Compared to one- and two-dimensional materials, they offer larger surface areas and three-dimensional electron transport pathways. Additionally, they possess favorable mechanical and antipoisoning properties. In this review, we examine the recent advancements in the field of three-dimensional electrocatalysts, specifically focusing on substrate-assisted and substrate-free catalysts for sustainable water splitting reactions. The discussion highlights the different substrates and fabrication methods employed for these three-dimensional catalysts. Furthermore, we propose and discuss potential future directions based on the achieved results in this area.

Copper Foam
Copper Foam

Extensive research has been conducted on substrate-assisted 3D electrocatalysts. These electrocatalysts are created by applying active species onto different substrates like nickel foam, Cu foil, fluorine-doped tin oxide (FTO), or carbon fiber paper. The active species, which can have diverse structures, are either grown on or bound to these substrates using methods such as chemical vapor deposition (CVD), electrodeposition, hydrothermal reaction, and more. This process results in the formation of a 3D framework.

Nickel foam (NF) possesses a highly porous dendritic structure composed of pure nickel, which demonstrates superior electrocatalytic activity compared to non-noble materials. Moreover, its conductive and continuous 3D framework enables the expansion of the electrode’s surface area, thereby promoting electrocatalytic activity. Consequently, NF serves as an excellent candidate for an electrode substrate. It is well-established that Pt and Pt-based materials exhibit exceptional activity with high current density and minimal overpotential during the HER. Qian et al. introduced a method involving the decoration of 3D porous nickel electrodes with micro-/nanoscale noble metal (Ag, Au, and Pt) particles through electrodepositing. This approach minimizes the usage of noble metals, thereby enhancing the intrinsic activity of the electrode materials at a minimal cost. The resulting nanoscale Pt-particle-decorated 3D porous nickel electrode showcases a substantial exchange current density of 9.47 mA cm–2, a low overpotential of –0.045 V for the HER, remarkable durability, and minimal consumption of Pt. The distinctive 3D porous structure of NF not only amplifies the actual surface area of the catalyst but also facilitates the uniform distribution of electrodeposited noble metal particles, thereby promoting direct contact with the alkaline solution.

Chen et al. conducted a study where they produced a 3D N-doped NiFe double-layered hydroxide (LDH) film by growing N-doped NiFe LDH nanolayers on a 3D NF substrate framework. This film consists of numerous thin nanolayers, each several hundred nanometers in size and approximately 0.8 nm thick, vertically grown on the framework. The doping ratio of N in this material is remarkably high, around 17.8%. The resulting electrode exhibits excellent activity in catalyzing the OER, with an overpotential of 0.23 V at a current density of 10 mA cm–2, a high Faradaic efficiency of about 98%, and stable operation for over 60 hours in 0.1 M KOH. The enhanced catalytic performance can be attributed to the nanolayer thickness, high N doping content, and the 3D framework.

The text also mentions that 3D NF is an ideal template for hosting catalysts and increasing the number of reaction sites due to its cost-effectiveness and large surface area. However, it suffers from instability in acidic electrolytes, making it unsuitable for the catalytic HER. To address this issue, the researchers fabricated a 3D NF with graphene layers deposited on its surface as a support for loading MoSx catalysts for the HER. The graphene layers provide robust protection and effectively enhance the stability of the catalyst in acidic conditions. In the presence of a 0.5 M H2SO4 solution, the hydrogen evolution rate reaches 302 mL g–1 cm–2 h–1 at an overpotential of V = 0.2 V. Additionally, the graphene layers on NF can influence the growth behavior and catalytic activities of active species. The study also proposes a graphene-barrier-mediated LDH catalyst, which proves to be an efficient catalyst for the OER.

In order to achieve efficient water splitting, it is necessary to use catalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in the same electrolyte. A strongly acidic or basic electrolyte is required to minimize overpotentials. However, current approaches often face challenges in integrating the two catalysts effectively, resulting in subpar overall performance. Therefore, the development of bifunctional catalysts that are active for both the HER and the OER is highly desirable.

One promising approach presented by Feng et al. involves the growth of a Ni3S2 nanosheet array on a nickel foam (NF) through in situ sulfidization using thiourea as a sulfur source. The resulting 3D Ni3S2/NF electrode exhibits excellent activity and durability for the OER and the HER in both alkaline and neutral electrolytes. Importantly, the Ni3S2/NF electrode remains flexible without any structural damage and contains metallic nickel as well as the hexagonal phase of Ni3S2. The 3D structure of the electrode remains intact, with Ni3S2 nanosheets evenly distributed on the NF surface. This electrode demonstrates a remarkable Faradaic yield of approximately 100% in both the HER and the OER, along with superior stability exceeding 200 hours.

Nickel Foam (6)

In a recent study, our research group successfully fabricated a 3D self-supported electrode by growing cobalt phosphorus (CoP) mesoporous nanorod arrays on a nickel foam using an electrodeposition method. This CoP-based electrode can be directly utilized as a working electrode for both the HER and the OER, resulting in a low overpotential of 390 mV to achieve a current density of 10 mA cm–2, while maintaining strong durability. This advancement holds great potential for efficient water splitting applications.

This review highlights recent significant advancements in the field of three-dimensional catalysts for water splitting. The incorporation of well-connected 3D conductive networks in 3D electrocatalysts enables efficient charge transfer and gas transport, resulting in high conductivity. Moreover, these catalysts can be directly utilized as electrodes without the need for binders, thanks to their exceptional mechanical strength. This characteristic allows for the exposure of more active sites compared to powdery catalysts. The use of 3D materials holds great promise in enhancing catalytic performance for sustainable water splitting in the near future. Substantial progress has already been made in this pursuit, with the development of various substrates such as metallic, carbon-based, rigid, flexible, and substrate-free catalysts. Notably, graphene-based electrode materials have shown significant potential in integrating with 3D electrodes. However, despite these advancements, research on 3D electrocatalysts is still in its early stages, and there are several challenges that need to be addressed for their practical application in water splitting.

(1)The electrocatalytic performance of a 3D catalyst heavily relies on its composition and structure. Catalysts made of earth-abundant elements and constructed in a complex hierarchical architecture are gaining significant attention in sustainable water splitting reactions. Therefore, it is imperative to design catalysts based on both experimental and theoretical electrochemistry for future advancements.

(2)The selection of appropriate substrates plays a crucial role in substrate-assisted catalysts. While metallic substrates have higher catalyst loading weights, they are more expensive than carbon-based substrates. Flexible substrates are more desirable than rigid ones due to the emergence of flexible energy devices. 3D substrates offer larger surface areas than 2D substrates, which can expose more active sites and facilitate the transport of reactant species. Therefore, it is essential to meticulously select various kinds of substrates.

(3)The activity and durability of a catalyst are significantly influenced by the interaction between active species and substrates. Therefore, it is crucial to develop methods that improve the distribution of active species and enhance the interaction between active species and substrates.

 

 

 

 

 

 

 

 

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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