Refrigerators just became ‘cooler’ and more environmentally friendly

The rising demand for cooling worldwide comes with significant energy consumption and environmental concerns. Traditional cooling methods rely on vapor compression refrigeration, which uses harmful refrigerants that contribute to global warming.

The search for efficient, eco-friendly alternatives has led to new approaches, including Peltier semiconductors, caloric materials, and thermogalvanic systems.

Among these, thermogalvanic systems offer a unique advantage. They cool through an endothermic redox reaction, where a chemical process absorbs heat while reversing the reaction releases it. This cycle continuously pumps heat, enabling a sustainable refrigeration process.

Unlike solid-state thermoelectric cooling, thermogalvanic systems use liquid electrolytes, making them more flexible, scalable, and cost-effective. This technology could be adapted for applications ranging from battery cooling to wearable climate control and industrial-scale refrigeration.

Despite their promise, thermogalvanic systems have historically faced challenges. The typical temperature drop achieved in experiments has been limited to just 0.1 K, making practical use difficult. Most studies have focused on improving system design, but little progress has been made in optimizing the electrolytes that drive these reactions. Researchers are now tackling this problem head-on to unlock the full potential of electrochemical cooling.

The efficiency of any cooling technology is measured by the coefficient of performance (COP), which compares the amount of cooling generated to the energy required to achieve it. In thermogalvanic refrigeration, this depends on a key property of the electrolyte known as the temperature coefficient (α).

This value is linked to the entropy change during the redox reaction. The ideal electrolyte would have a high α to maximize cooling while also maintaining a low specific heat capacity to enable noticeable temperature reductions.

Iron-based thermogalvanic electrolytes, particularly those using Fe²⁺/³⁺ or ferricyanide (Fe(CN)₆³⁻/⁴⁻), are well known for their cost-effectiveness and ability to produce significant entropy changes.

Theoretically, hydrated iron ions in aqueous solutions could generate large entropy shifts, but strong interactions between these ions and anions have historically reduced efficiency. The α value has remained low, between 0.2 and 1.5 mV K⁻¹, limiting the cooling power of these systems.

To overcome these limitations, researchers have attempted to dissolve iron salts in organic solvents instead of water. While this improved some properties, it also reduced ion concentration, leading to poor conductivity and low cooling power.

Water itself presents another challenge—its high specific heat capacity (4.2 J g⁻¹ K⁻¹) prevents significant temperature drops during cooling cycles. Finding the right combination of solutes and solvents to enhance cooling remains a critical challenge.

A recent study published in Joule introduces a major advancement in thermogalvanic refrigeration by optimizing the electrolyte composition. Researchers from Huazhong University of Science and Technology developed a new strategy to enhance the performance of iron-based electrolytes, achieving a record-breaking cooling performance.

The team used a dual approach to improve entropy changes and reduce ion interactions. First, they introduced perchlorate (ClO₄⁻) as a counterion, which helps dissociate the iron salts more effectively. This promotes hydration of Fe²⁺/³⁺, leading to a more favorable solvation structure. Second, they developed a binary solvent system using nitriles—organic compounds known for their low specific heat capacity and unique solvation properties.

Among six different nitrile solvents tested, acetonitrile (MeCN) produced the best results. It reduced heat retention due to its low specific heat capacity (1.75 J g⁻¹ K⁻¹) while also reorienting the solvent dipoles around Fe²⁺ ions. This rearrangement significantly enhanced the entropy change of the redox reaction.

Spectroscopic analysis and molecular dynamics simulations confirmed that nitrile solvents improved solvation dynamics, leading to an unprecedented α value of 3.73 mV K⁻¹—more than double previous records. The optimized electrolyte increased cooling power from 28.0 to 48.6 mW cm⁻², and the researchers estimate that, in a complete system, a COP of 14.3 could be achieved.

To demonstrate the effectiveness of their improved thermogalvanic system, the researchers conducted real-world tests. The optimized system successfully cooled the surrounding electrolyte by 1.42 K, a significant improvement over the 0.1 K achieved in previous studies. This marks a major step toward making electrochemical refrigeration viable for everyday use.

Lead researcher Jiangjiang Duan highlighted the broader implications of the discovery. “Thermogalvanic technology is on its way to our lives, either in the form of clean electricity or low-power cooling, and both research and commercial communities should be paying attention.”

Looking ahead, the team is focused on refining the system’s design for stability, scalability, and efficiency. They are also exploring new materials and mechanisms to push performance even further. Potential applications range from personal cooling devices to large-scale industrial refrigeration systems, all operating with significantly lower energy consumption compared to conventional methods.

“Though our advanced electrolyte is commercially viable, further efforts in system-level design, scalability, and stability are required to promote the practical application of this technology,” Duan explained. The team is actively seeking industry partners to help bring thermogalvanic refrigeration from the lab to the market.

As global cooling demands continue to rise, sustainable solutions like thermogalvanic refrigeration could play a critical role in reducing energy consumption and environmental impact.

With recent breakthroughs in electrolyte optimization, this technology is moving closer to real-world applications, promising a greener and more efficient future for refrigeration.

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