Palladium is a metal with the unique property of being able to store up to 900 times its volume of hydrogen within its crystal lattice. Palladium is also currently being used in catalytic converters to convert the more harmful carbon emissions into carbon dioxide, water, and nitrogen gas. However, palladium's uses are limited due to its extreme scarcity and high cost.[As of August 2020] 1 kilogram of palladium costs almost £52,000, making its future usage in hydrogen storage less likely.
The government of Japan is planning to switch Japan's energy use almost entirely to hydrogen over the next few decades, creating a "hydrogen society" (see my blog post on this for more information). Currently, both methods of hydrogen storage (pressurised or liquid) use a significant amount of energy, reducing the efficiency of using hydrogen as an energy store. Using palladium would greatly reduce the energy needed to store hydrogen, while also storing it at a similar density to liquid hydrogen. Like with many other transition metals, palladium absorbs hydrogen, forming palladium hydride. Palladium hydride is actually not a compound, but an alloy composed of palladium and metallic hydrogen (normally only found when under very high pressure).
As is normally the case with uses for palladium, the prohibitive cost has prevented palladium from being widely used as hydrogen storage. However, an industrial use has been found which relies on palladium's absorption of hydrogen. Palladium alloys are used to separate hydrogen from other gases. A thin sheet of the alloy is used, and only the hydrogen is capable of passing through the palladium alloy to the other side.
Palladium is used in catalytic converters as a catalyst, along with platinum, rhodium and sometimes iridium, all of which are very expensive metals. Three-way oxidation-reduction catalytic converters are the standard in the UK, due to strict EU standards. They oxidise carbon monoxide to carbon dioxide, unburnt hydrocarbons into carbon dioxide and water, and also, unlike two-way oxidation catalytic converters, convert nitrogen oxides into nitrogen and oxygen gas. Catalytic converters help to significantly reduce air pollution in cities, saving thousands of lives. Unfortunately, due to the high cost of the metals used in catalytic converters, they have become a target for thieves, and in recent years catalytic converter theft has increased dramatically.
One of the main uses of palladium (accounting for 16% of palladium demand) is in jewellery, where people are prepared to pay high prices, so the cost of palladium is less consequential. Palladium is not normally used in pure form, but instead as an alloy with gold to form "white gold". While nickel, a much cheaper metal, can also be used, it causes an allergic reaction in roughly one in eight people, meaning that some people can't wear white gold with nickel. Grey gold, much less commonly used, is also an alloy of gold and palladium.
However, jewellers using palladium must be careful when heating it. At temperatures in excess of 400°C palladium is oxidised rapidly, becoming more brittle and discolouring. The palladium would then no longer be able to be shaped properly, and anyway would be less in demand due to the change in appearance. If the alloy is not the right colour, but still has the right malleability and hardness, rhodium plating can be used to make it look a silvery colour
Multi-layer ceramic capacitors (MLCCs) are another major use of palladium. As the name suggests, multi-layer ceramic capacitors are composed of many individual capacitors, arranged end to end. Capacitors are components of electrical circuits used to store charge and are one of the key components of almost all complex electronics. For this reason, the electronics industry consumes 15% of all palladium produced.
In recent years, due to the increased demand for palladium in catalytic converters, palladium mining has increased drastically, and in 2017 214 tonnes were produced, as compared to only 175 tonnes for gold. Palladium is not normally the main export of a mine, instead, it is merely a by-product found during the mining of other metals, especially other platinum group metals. This tends to lead to large price fluctuations, as supply generally isn't dependent on the demand for palladium, but is instead dependent on the demand for platinum, copper, and other metals. The mines are largely based in Russia and southern Africa, with smaller operations in North America.
When a car is scrapped, the platinum group metals within it are then recycled, mostly to be used again in catalytic converters. Typically, they are extracted using a solvent, such as hydrochloric acid, before being separated. It is a very lucrative process, with the platinum alone in new catalytic converters being worth $200, and the catalytic converters of old cars being worth $600 or more.
Overall, palladium has many useful properties, but its rarity and cost limit it only to applications where tiny amounts of it are used (multi-layer ceramic capacitors), luxury items (jewellery), or circumstances where regulations necessitate its use (catalytic converters). It is a shame that palladium is so rare, as its ability to store vast quantities of hydrogen is very useful for efficient storage, and would certainly be widely used, especially in hydrogen fuel cell cars, if it was not so expensive.
Sources:
https://web.archive.org/web/20061206003556/http://www.unctad.org/infocomm/anglais/palladium/uses.htm
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