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Advancements in Self-supported Catalysts for Sturdy and Optimal Water Splitting in Commercial Electrolyzers

Coil to coil electroplating production line (1)

Electrochemical water splitting is a highly promising technology for generating hydrogen as a sustainable and clean alternative to fossil fuels. This process involves two half reactions, namely the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER). To enhance the efficiency of the electrolyzer system, high-performance catalysts are required on both the anode and cathode to reduce the overpotential of these reactions. However, the use of noble metal-based catalysts, such as Pt-based materials for HER or Ir- and Ru-based materials for OER, poses challenges for the rapid commercialization and scalability of electrolyzers due to their high cost and limited availability. Therefore, it is crucial to develop a cost-effective and abundant catalyst that can match or even surpass the catalytic activity of noble metal-based catalysts.

Long-term stability of a catalyst is a crucial factor for practical applications, in addition to its catalytic activity. Non-noble metal based catalysts tend to perform well only at low current densities, such as 10 mA cm-2, and often experience degradation in catalytic performance at higher current densities, such as 1 A cm-2, which is the actual operating current density for commercial industrial electrolyzers. The catalytically active phase must possess high corrosion resistance under strong acidic or alkaline conditions, as well as strong adhesion to the current collector, to ensure sufficient catalyst robustness. This is particularly important for powdered electrocatalysts, which require immobilization on a current collector using a conducting polymer binder, such as Nafion. However, achieving long-term electrocatalysis with a steady catalytic output, especially at high current densities, is challenging due to catalyst peeling and high interfacial resistance between the catalyst and current collector. Nickel-based plates are commonly used as electrodes for commercial electrolyzers due to their acceptable activity, good resistance under corrosive alkaline conditions, and easy and inexpensive fabrication. However, coating nickel-based plates with catalyst powder can create a bad pathway for generated gas bubbles, leading to significant adhesion to the catalyst. This gas-release behavior becomes more pronounced as the current density increases, which can accelerate the catalyst peeling problem.

In order to tackle these challenges, a potential solution has been proposed in the form of self-supported catalysts. These catalysts are directly constructed on 3D architectured electrodes such as nickel foam, copper foam, and carbon cloth. They offer several advantages that make them a promising option for commercialized electrolyzers. Firstly, they can be used directly as electrodes for oxygen evolution reaction (OER) or hydrogen evolution reaction (HER) without the need for complex processes. Secondly, they eliminate the use of polymer binders and carbon additives as conducting agents. Thirdly, they prevent issues like peeling phenomenon and catalyst aggregation that are commonly associated with powdered catalysts. Fourthly, they ensure good electric contact between the current collector and the catalysts, facilitating fast charge transfer. Fifthly, they provide a large surface area through the 3D scaffold and open pores. Lastly, they exhibit excellent catalytic performance due to the synergistic effect between the intrinsic and extrinsic catalytic activity of the catalysts and substrates. Numerous studies have been dedicated to developing self-supported catalysts that are grown on a structurally stable and highly catalytic open structured current collector. This article presents cutting-edge approaches in designing self-supported electrocatalysts, taking into account their catalytic properties that closely resemble real-world commercial applications.

The anodic half reaction of water splitting, known as OER, is often considered a bottleneck in the overall reaction due to its slow 4-electron charge transfer kinetics and complex reaction pathway involving various intermediate species. To overcome this, electrocatalysts are applied to the anode to facilitate OER kinetics. While noble platinum group metals like Ir or Ru are currently the state-of-the-art electrocatalysts for OER, their scarcity and high cost limit their practical use in industrial settings. As a result, researchers have turned to transition metal-based catalysts, which are more abundant and cost-effective. Transition metal oxide, (oxy)hydroxide, chalcogenide (S, Se, Te), nitride, phosphide, boride, and carbide have all shown high potential as alternative catalysts to replace noble-metal based ones.

Copper Foam
Copper Foam

Excellent stability is a crucial requirement for the practical application of electrocatalysts, in addition to their high catalytic activity. Alkaline electrolyzers, which are widely used in commercial water electrolysis, utilize a 30% KOH (6 M KOH) solution as the electrolyte. However, under these harsh conditions, most transition metal-based electrocatalysts tend to degrade due to various factors such as phase decomposition, structure reconstruction, physical detachment, or aggregation of the catalyst at the electrode. Consequently, their initial electrocatalytic properties are compromised. This stability issue becomes even more severe when operating at high current densities (500 mA cm-2), which are necessary for practical applications. In the case of powdered catalysts, the main challenge in terms of stability lies in the peeling of catalyst powder from the electrode. This occurs due to the destruction of the catalyst-binder-electrode configuration in the corrosive electrolyte. To address this issue, self-supported catalysts have emerged as a highly promising alternative. These catalysts are designed to be robust and durable, even at high current densities. By eliminating the need for a separate binder, self-supported catalysts offer improved stability and longevity, making them suitable for practical applications.

Water splitting involves two electrochemical reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). The OER oxidizes water to produce oxygen, while the HER reduces water to produce hydrogen. The kinetics of the OER are slow due to the requirement of transferring four electrons, whereas the HER is relatively fast as only two electrons need to be transferred. However, the use of platinum as an HER electrode is expensive and unstable, prompting researchers to search for low-cost and stable catalysts as alternatives. Transition metal-based catalysts have been explored due to their relatively lower cost and high activity. However, most of these catalysts exhibit lower activity compared to platinum. To enhance the electrochemical performances and stability, researchers have focused on engineering the morphology, surface properties, and linkage of these catalysts with the substrates. Substrates also play a crucial role in improving electrochemical performances. By engineering substrates, conductivity, stability, and electrocatalytic activity can be enhanced. Commonly used substrates include copper foam (Cu foam), carbon materials, and nickel foam (Ni foam) due to their affordability and high conductivity. Although few transition metal catalysts surpass commercial platinum in terms of electrocatalytic activity, there have been notable advancements that warrant consideration.

Nickel Foam (4)

The effective connection between the Ni foam substrate and the catalyst that grows directly on it offers the catalytic system the advantages of easy electron transport and strong electrochemical stability.

The objective of this study was to investigate transition metal-based materials that are abundant on Earth in order to achieve a specific goal. Primarily, the focus was on the utilization of first-row transition metals such as nickel (Ni), cobalt (Co), and iron (Fe) in various forms including oxides, hydroxides, chalcogenides, phosphides, nitrides, carbides, and borides. These materials were employed to fabricate electrodes that are both cost-effective and efficient for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and bi-functional catalysis. Additionally, the incorporation of bimetallic and trimetallic compounds has been recognized for their ability to enhance catalytic activity by improving electrical conductivity and modifying the electronic configuration. Recently, there has been a growing interest in developing bi-functional catalysts for overall water splitting due to their cost-effectiveness, which simplifies the fabrication of electrode systems for commercial water splitting and reduces manufacturing costs.

Another area of research that has gained significant attention is nanostructure engineering. Many researchers have focused on studying nanostructures in the form of nanowires, nanorods, and nanosheets, which possess a large surface area. These nanostructures offer numerous adsorption sites for reactants in the electrolyte. Furthermore, self-supported catalyst forms in the form of catalyst foams have been extensively investigated. These catalyst foams, which are catalysts grown on a conductive substrate, offer several advantages: firstly, they eliminate the need for carbon additives as conducting agents and binder polymers; secondly, they ensure good electrical contact between the catalyst and the conductive substrate; thirdly, they prevent catalyst aggregation during HER or OER; and finally, they enhance catalytic performance through a synergistic effect with the intrinsic catalytic activity of the substrate. Therefore, if self-supported catalyst forms can be synthesized in a simple and scalable manner, they would be the optimal choice for practical applications.

Nickel Foam (6)

Despite significant progress and extensive research, the practical application of many catalyst materials for hydrogen production still poses challenges. While improving electrocatalytic performance with low overpotential at a specific current density has been the primary focus of many studies, the durability of catalysts is crucial for sustainable hydrogen production. Recent research has highlighted the importance of evaluating catalytic properties based on SEM or TEM images and XPS data after long-term tests to assess durability. However, the current testing conditions, which involve 1M KOH at a current density of 10 mAcm-2, are not suitable for assessing stability in commercial electrolyzers that operate in strong alkaline or acid conditions (over 30 wt% solution) at high current densities (over 1 A cm-2) for water splitting. Furthermore, most studies on electrocatalysts for water splitting have been one-sided in alkaline electrolysis, with little consideration given to the possible availability of proton exchange membrane (PEM) electrolysis in acidic media. PEM electrolysis boasts a high heating value (HHV) efficiency of over 90%, surpassing the 70-80% HHV efficiency of alkaline electrolysis. Additionally, it can operate at a relatively high current density of 1 A cm-2, which is superior to alkaline electrolysis. However, the use of noble metals such as Pt, Ir, and Ru is limited due to their high corrosion resistance against acidic conditions, while transition metal-based materials easily dissolve in acid conditions, losing their catalytic properties. Therefore, there is an urgent need to develop an electrocatalyst that can be both durable and efficient in acidic conditions to commercialize PEM electrolysis. As a result, future research on electrocatalysts should focus on their durability in both strong alkaline and acidic conditions, even at high current densities, to ensure efficient catalytic performance. With current efforts and the development of efficient and durable electrocatalysts, a new era of sustainable hydrogen energy may be on the horizon.

 

 

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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