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Alkaline electrolysis cells with zero gap design for hydrogen gas storage in renewable energy systems

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For more than two centuries, the process of electrolyzing water has been known. This involves applying an electric potential across two electrodes immersed in water, which causes the water molecules to break down into two parts hydrogen and one part oxygen. The hydrogen produced can be stored as a green fuel, which, when combined with oxygen, releases energy and produces only water as a byproduct. However, pure water is a poor conductor of electricity, making electrolysis inefficient. To improve conductivity and efficiency, water-soluble electrolytes are added. Electrolysis can be performed under acidic, neutral, or basic conditions, depending on the electrolyte used. Acidic conditions cause severe corrosion of common metals, necessitating the use of expensive precious metals as electrodes, resulting in high capital costs. Neutral electrolysis using sodium chloride is energetically expensive and produces environmentally questionable side reactions such as the production of hydroxide and chlorine gas. Alkaline conditions allow for the use of cheaper, earth-abundant metals as electrodes, but generally operate at a lower efficiency, requiring larger devices and thus higher costs. Highly efficient alkaline electrolysis could hold the key to cheap and efficient water splitting.

The electrolysis of water is a topic of great importance in contemporary society due to its potential to store large amounts of renewable energy in the form of hydrogen gas. Hydrogen can serve as a universal energy carrier, facilitating the distribution of renewable energy throughout a network while also balancing supply and demand. When hydrogen is utilized, it combines with oxygen to produce only water and release its stored energy, thus completing a carbon-free cycle. This process is crucial for the modern energy infrastructure as we transition towards a more diverse renewable energy landscape. This transition will involve a shift from a centralized power distribution network, where power is generated at a central station and delivered to households, to a decentralized network where multiple energy sources, such as solar panels on individual houses, contribute to the network from various locations. Therefore, a universal and time-independent energy carrier like hydrogen is essential for maintaining network stability.

Traditional alkaline electrolysis, which involves two electrode plates separated by a liquid alkaline electrolyte, suffers from low current densities (less than 0.25 A·cm-2) and typically achieves efficiencies of less than 60%. These relatively low efficiencies have prompted the development of alternative water splitting technologies, most notably acidic Proton Exchange Membrane (PEM) Electrolysis and more recently Solid Oxide Electrolysis. PEM cells, operating at approximately 2 A·cm-2 and 1.7 V, have achieved an efficiency of 72%. However, the benefits of this high performance are offset by the high costs associated with the Nafion membrane and the noble catalysts, such as platinum and iridium, which are necessary due to the acidic environment. Solid oxide electrolysis, on the other hand, requires significantly higher operating temperatures, resulting in additional energy inputs. PEM electrolysers employ a proton exchange membrane as the electrolyte and utilize a zero gap cell design, where the electrodes are directly deposited onto the membrane.

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The alkaline environment presents a significant advantage in the use of inexpensive and abundant metals for catalysts and other cell components. On the other hand, Proton Exchange Membrane (PEM) offers high-performing electrolysis cells at the expense of capital cost. By combining the benefits of both alkaline and PEM electrolysis, electrolysers that operate at high current densities and efficiencies can be developed at low cost. An important step towards these “Advanced Alkaline Electrolysers” is the use of a cell design based on the zero gap concept.

In alkaline electrolysis, the zero gap cell design involves compressing two porous electrodes on either side of a hydroxide ion conducting membrane or gas separator. This creates a gap between the two electrodes that is equal to the thickness of the membrane (<0.5 mm) rather than the traditional setup (>2 mm), significantly reducing the Ohmic resistance contribution from the electrolyte between the two electrodes. A gas diffusion layer provides an electrical connection from the porous electrode to the bipolar plate, while simultaneously allowing a feed of electrolytic solution and the removal of gas products.

The main difference between the traditional setup and the zero gap design is the use of porous electrodes instead of solid metal plates. This allows for cells with a very small inter-electrode gap, compact design, and high efficiency. It forces gas bubbles to be released from the backside of the electrodes, reducing their contribution to the cell voltage.

The design of the CCS cell involves the deposition of a catalyst layer directly onto a porous substrate, which is then compressed onto both sides of the membrane. This substrate serves as both the electrode and the gas diffusion layer and can take various forms. The initial research on the zero gap setup utilized a catalyst coated substrate setup, employing steel and nickel mesh electrodes due to their widespread availability and relatively low cost. Recent research has explored different substrates and geometries, including porous carbon paper and porous nickel foam.

Incorporating mesh electrodes into a zero gap cell allows for a robust and structurally sound design. The mesh is compressed on both sides of the membrane or gas separator using a bipolar plate with an integrated flow-field. This arrangement not only provides a pathway for the electrolyte but also facilitates efficient removal of product gases from the cell. Additionally, the flow of electrolyte can help dissipate heat from the cell, particularly when operating at high current densities. This excess heat can be managed through a heat exchange system in the external cell setup. The components of the setup are compressed together to ensure reliable connections, and gaskets are employed to prevent leakage. However, caution must be exercised during compression to avoid membrane deformation.

A modified version of the CCS setup can be achieved by utilizing high surface area electrodes, such as Nickel foam. This type of electrode offers the advantage of a significantly larger active surface area compared to mesh substrates. As a result, the cell design is slightly altered, and flow-field etched bipolar plates are no longer necessary due to the electrolyte flow through the porous material. However, these plates are still commonly used.

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Gas management becomes a crucial factor when using metal foam due to its small pore size. When high current densities are applied, effective gas removal is necessary to prevent gas bubbles from covering parts of the material and reducing its available surface area. The large surface area of the nickel foam provides a high number of sites for catalyst deposition, and one of the most effective anodic electrodes reported is Ni/Fe (OH)2 deposited onto a Nickel foam substrate.

To further enhance the performance of the nickel foam electrode, we modified it to fabricate an asymmetric porous nickel electrode. This electrode has small pores in contact with the membrane to provide the maximum active surface area, and a more open structure on the backside to facilitate gas bubble removal from the bulk. A performance of 0.5 A·cm-2 was reported at a cell voltage of 1.8 V and 80 °C. The gaskets were used as multifunctional sealants and electrolyte flow channels, allowing the foam to be compressed directly onto un-etched bipolar plates.

The advancement of zero gap alkaline electrolysis to achieve comparable performance to that of PEM electrolysis necessitates research in three key areas: catalysts, membranes, and cell design. The implementation of the zero gap design has resulted in significantly enhanced performance in comparison to the conventional arrangement, thereby enabling alkaline electrolysers to narrow the performance gap with PEM electrolysers. However, it is imperative to undertake further development to enhance cell design. It is essential to comprehend each contribution to cell resistance, particularly interfacial contact resistances and resistances arising from bubble formation and removal, and to implement solutions to minimize these contributions to cell resistance. The use of high surface electrodes must be explored to quantify their performance improvements, and novel geometries must be developed to achieve further performance enhancements.

 

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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.

About HGP

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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