In 1839 the first ever hydrogen fuel cell was created. It was already well known that when enough energy was put into water it could be broken into the hydrogen and oxygen that form it. One William Robert Grove had the idea that if hydrogen and oxygen could be recombined with an electrolyte present to reproduce the water but also gain a flow of charge. Now vehicles can be fuelled with hydrogen but there is a catch. How is the least dense element going to be contained in a reasonable volume. It could be cooled into a liquid but this doesn’t seem practical for a car or perhaps out under high pressure but there are quite a lot of safety concerns about putting high pressure tanks in road vehicles. One of the suggestions that is being looked into is that of using metal hydrides as a way of chemically storing hydrogen by reacting it with a metal from which it can be extracted when used in the car.
The primary set of hydrides that have been put under scrutiny is the platinum group which contains platinum (obviously) hydride, iridium hydride, ruthenium hydride, rhodium hydride and palladium hydride. It has been reported that hydrogen can occupy the gaps between the metal lattice in palladium with an efficiency of 70%, meaning that 70% of all the holes in the octahedral structure are filled with hydrogen. This seems to be the maximum that can be achieved when palladium is in standard conditions. As for rhodium, it shows quite poor ability for containing hydrogen with only small amounts being adsorbed to the metal’s surface and only when the conditions are extreme pressure of many gigapascal. Recently rhodium dihydride was discovered, the first dihydride of the platinum group, when rhodium was put under intense pressure in liquid hydrogen.
This study has looked at the formation enthalpies of these metal hydrides including the combined alloy metal hydride of palladium and rhodium. It was found that palladium dihydride can exist, but is very unstable and breaks down by releasing hydrogen back into simple palladium hydride even when high pressures were applied. It was also shown that the bimetal alloy demonstrated purely metallic properties while interestingly what could be gathered from palladium dihydride implies that it is a semimetal.