This doesn't aim to improve efficiency. Most refrigerators use phase change of volatile liquids to gases and back again. Most suitable liquids were toxic and/or

This doesn't aim to improve efficiency. Most refrigerators use phase change of volatile liquids to gases and back again. Most suitable liquids were toxic and/or flammable, so chemists invented the much safer replacement of fully halogenated hydrocarbons (CFCs). The first to gain widespread use was R-12: dichlorodifluoromethane (a.k.a. Freon). CFCs turned out to break down in the upper atmosphere, releasing chlorine that catalyzed destruction of ozone. The worst ones were banned under the Montreal Protocol treaty, and less damaging partially halogenated hydrocarbons (HCFCs and HFCs) were developed as a replacement, e.g. R-22. Modern HFCs like R-134a have even lower ozone depleting potential. However, ozone depletion isn't the only problem. CFCs/HCFCs/HFCs are also very effective greenhouse gases, often trapping thousands of times more heat than CO2. In an attempt to solve this problem, another class of refrigerants was invented: the hydrofluoroolefins (HFOs). These have low ozone depletion potential, and low global warming potential, but they are somewhat controversial because they compromise the excellent safety of the earlier halogenated refrigerants. Solid state refrigeration avoids the whole problem, so if it's possible without harming efficiency, as the article suggests, then it's an obviously good idea.