In a world in desperate need of a cheap, reliable and renewable energy source, cold fusion seems to provide an ideal solution, but achieving it remains just out of reach. Cold fusion is a hypothesised nuclear process that mimics the reactions in stars at room temperature. It promises energy on scales far beyond those given by fossil fuels, using a clean source of heavy water – two deuterium atoms bonded to an oxygen atom. However, cold fusion is a theoretical concept that many scientists believe is implausible. There remains no mechanism to explain the process, and no research has found a reproducible experiment that would prove its possibility.
Stellar Reactions at Room Temperature
Stars undergo fusion in the core, where the high gravitational pressure forces protons to interact. The plasma is at a high enough temperature to provide the energy to bind four protons and form helium nuclei. The mass of the resulting atom is less than the sum of the individual unbound protons. This mass defect is released as electromagnetic radiation. With the success of nuclear fission in the early 20th century, scientists turned to the cheaper and more sustainable process of nuclear fusion as a potential energy source, providing 4 million times more energy per kilogram than coal, gas and oil . However, achieving the suns temperature of 27 million Kelvin requires such a large energy input that the resultant output is not economically viable .
The proposition of fusion at room temperature seemed a shot in the dark, which is why 20th-century scientists were so eager to be the first to find it. The term cold fusion dates back to 1956 when muons were suggested as a method to catalyse fusion reactions at low temperatures . The physical process is based on the same mechanism that was later discovered to occur in stars. Two deuterons; each comprising a proton and a neutron, combine with sufficient energy to produce a more massive particle, and a single by-product. The best understanding of a physical mechanism that scientists have is to allow thin strips of palladium acting as a cathode, to absorb deuterium atoms from an electrolyte solution in heavy water. Many researchers argue that the palladium catalyses the third reaction in Fig. 1, such that deuterons are fused to helium nuclei at low temperatures. However, we are yet to find a widely accepted mechanism for this to occur, let alone a formal explanation of the physical process.
Have we achieved cold fusion?
In 1989, Pons and Fleischmann created a table-top experiment to create helium from heavy water. Using an electric current to ionise the deuterium atoms in the water, they expected to see helium nuclei form on the palladium cathode, along with heat radiation; arising from the mass defect. The experiment attracted global media attention as Stanley Pons and Martin Fleischmann claimed the discovery of a cold fusion reaction. The results from the experiment showed energy production far greater than could be explained by Chemistry. Scientists across the globe rushed to replicate the ground-breaking experiment, but it was unachievable on the same scale in every case .
The Pons-Fleischmann experiment lacked the scientific rigour required to validate its results. Unfortunately, few experiments using similar methods since it made headlines have come close to showing signs of fusion at room temperature. The experiment received a lot of criticism about its irreproducibility before it was discovered that the energy produced is inconsistent with the theoretical energy from nuclear mass defect. It seems we are still a long way from understanding cold fusion, and it may be a while before a miracle energy source like this is discovered in the lab.
Why should we continue searching?
For many scientists, the Pons-Fleischmann experiment was not a failure, but a promising beginning to research into cold fusion. In the United States, Italy and Japan, huge funding has been available to university departments and institutes since 1989 to continue investigating possible cold fusion mechanisms . Many small private companies around the globe are carrying out their research, to use fusion methods as a clean and sustainable energy source to replace fossil fuels. In 2015, Google started its mission to develop room-temperature fusion methods with an initial budget of £10 million .
Scientists have not given up on cold fusion. Despite worldwide criticism and many institutes and governments deciding that it is a waste of funding, things have evolved since the experiment in 1989. Perhaps cold fusion in the way Pons and Fleischmann had aspired to achieve is not possible with modern science, but fusion reactors such as ITER’s First Plasma (2025) promise to provide the first steps in making fusion a sustainable and affordable method of energy production .
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Figure 1: Berlinguette, C., Chiang, Y., Munday, J., Schenkel, T., Fork, D., Koningstein, R. and Trevithick, M., 2019. Revisiting the cold case of cold fusion. Nature, 570(7759), pp.45-51.