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Application of metal foam in flow batteries

Silver Foam (2)

An essential component in the development of large-scale energy storage systems. Energy is a fundamental aspect of our daily lives, and the majority of today’s electrical energy is derived from fossil fuels, including coal, natural gas, and oil. Nuclear, hydro, and renewable energy technologies, such as solar, wind, and biomass, also contribute to our electricity supply. However, renewable energy sources are generally site-specific, intermittent, and unstable, making it challenging to integrate them into the existing grid. Wind energy, for example, provided the highest proportion of renewable power capacity in 2008, but only half of this power can be used for the electrical grid, with the rest being wasted due to inadequate storage solutions.

 

Experts suggest that a conventional electric grid without an effective energy storage system could become destabilized if renewable energy exceeds 20% of the energy-generation capacity. Therefore, large-scale energy storage technologies are of great importance to make good use of renewable energy resources. Several large-scale energy storage technologies have been developed, including pumped hydro, compressed air, superconducting magnetic energy, flywheels, and electrochemical energy storage systems. Among these, electrochemical methods are the best choice due to their reliability, easy operation, lack of special requirements, and high recovery rate. Many battery technologies have been proposed and developed for electrical energy storage applications, including traditional rechargeable batteries such as lead-acid, nickel-cadmium, and lithium-ion batteries. However, the redox flow battery (RFB) shows the most promising future compared to other batteries. The RFB is an essential component in the development of large-scale energy storage systems.

Copper Foam 1 (11)

The Redox Flow Battery (RFB) is a highly promising energy storage device that utilizes two redox couples within the electrolytes to facilitate the process of charge and discharge. A reactor of redox flow cell is comprised of two compartments that are separated by an anion exchange membrane. Each compartment is linked to a reservoir tank and a pump, thereby forming an electrolyte circuit.

In a conventional Redox Flow Battery (RFB) configuration, the electrolytes traverse the electrode surface, where electrochemical reactions occur. The active species undergo oxidation or reduction, and the resultant electrons traverse an external circuit. In order to preserve the neutrality of all electrolytes, ions from the supporting electrolyte migrate across a membrane to the opposite side of the RFB.

In the context of electrochemical systems, the migration of active species from one electrolyte to another is commonly referred to as cross-contamination. This phenomenon can result in the self-discharge or degradation of a battery. To prevent cross-contamination, it is essential to employ a membrane that facilitates the transport of charged ions and solvent while maintaining electro-neutrality and electrolyte balance.

Copper Foam
Copper Foam

In contrast to conventional batteries, redox flow batteries (RFBs) store their active species in the electrolyte rather than the electrode. The power output of RFBs is independent of energy and is determined by the electrode area and the number of cells in a stack. However, the energy capacity of RFBs is influenced by the concentration of active species, the number of transferred electrons per mole of active redox ions during discharge, and the solution volume in a reservoir tank. As a result, RFBs exhibit a greater range of variability and can achieve higher energy capacity by increasing the volume of electrolyte or the concentration of active species, without the need for additional expensive stacks.

Redox flow batteries have been the subject of research for nearly four decades, dating back to the first concept introduced by Thaller in 1976. The NASA-Lewis Research Centre developed the initial complete redox energy storage system, which utilized the Fe(III)/Fe(II) and Cr(III)/Cr(II) redox couples as the positive and negative active species, respectively. Since then, redox flow batteries have undergone significant development, resulting in various systems. RFBs can generally be classified into two major categories based on their principles.

Redox flow batteries can be defined as either an aqueous system or a nonaqueous system, depending on the solvents employed in the electrolyte. Aqueous RFBs utilize water as an electrolyte solvent, which is cheaper and safer than nonaqueous systems. However, their operating potential is limited by the electrochemical potential window of water, which is generally lower than 2.0 V depending on pH. Consequently, aqueous RFBs have a low energy density, which is their primary disadvantage for current practical applications. On the other hand, nonaqueous RFBs employ organic electrolyte solvents that offer a much higher potential window, such as 5.0 V for acetonitrile (CH3CN). As a result, high power and energy output can be achieved, making nonaqueous RFBs increasingly attractive.

An optimal electrode for use in a redox flow battery (RFB) must possess several key characteristics, including a high surface area, low electronic resistance, strong chemical resistance, reasonable cost, and high electrochemical activity towards the electrochemical reactions. However, due to the typically acidic and corrosive environment of RFBs, only a limited number of electrode materials are capable of withstanding such conditions.

Copper Foam 1 (7)Electrode materials for RFBs can be categorized into two types: two-dimensional (2D) and three-dimensional (3D). Carbon felt (CF) is commonly employed as a 3D electrode, while carbon cloth, carbon black, and activated carbon (AC) are utilized as 2D electrodes. In a sodium polysulfide/bromine RFB, CF, carbon cloth, carbon black, and AC are the primary electrode materials for an aqueous bromide/bromine couple, while AC, metal foam(nickel foam,etc.) are the primary electrode materials for a negative half-cell.

The differences between these two types of materials are attributed to their distinct structural characteristics, which correspond to different electrolyte flow modes. These configurations are commonly referred to as “flow-through” and “flow-by” electrodes, respectively, as the electrolyte flows through a porous electrode in the former and flows past other types of electrodes in the latter.

In a flow-through configuration, the liquid electrolyte flows through a porous matrix electrode, where electrochemical reactions occur, and electrons move through the network of metal foam to the current collector. This flow-through electrode provides a uniform concentration of electroactive species and enhances mass transfer. However, it requires an extremely low flow rate and high scale-up costs, which limit its applications. In contrast, most flow batteries in the literature are of the flow-by configuration, where electrodes are generally made of 2D or 3D materials corresponding to different electrolyte flow modes.

The selection of 2D or 3D electrodes is contingent upon various factors, including the physical state of the flowing reactant, the electrode reaction taking place, and the conductivity of the electrolyte phase. In the case of liquid-phase reactants, the conventional approach involves the use of planar flow-by electrodes. However, 3D electrodes with a large surface area per unit volume have been extensively employed in Redox Flow Batteries (RFBs) due to their ability to facilitate fast electrochemical reactions and reduce polarization. These 3D electrodes are primarily composed of carbon-based materials and metal foams.

Copper Foam 1 (8)

Carbon-based materials possessing a large surface area, suitable porosity, and low electronic resistance are typically utilized as 3D electrode materials for RFBs, including zinc/bromine, all-vanadium, bromine-polysulfide, and zinc-cerium RFBs. Nevertheless, carbon-based electrodes often exhibit inadequate electrochemical activity and kinetic reversibility towards electrochemical reactions. Consequently, researchers have turned to metal foam. In addition to noble metals, certain metals with a high overpotential for gas evolution, such as Ni foam, have been employed as substrate electrodes in several RFBs. Ni foam was first employed as a negative electrode and demonstrated 77.4% energy efficiency during 48 charge-discharge cycles in a sodium polysulfide/bromine RFB. Metal foam electrodes have exhibited commendable performance and are progressively being employed on a significant magnitude.

 

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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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