Curie and Debierne used mercury as the cathode material when first preparing radium metal.
In September 1910, Marie Curie and André-Louis Debierne electrolyzed a solution of purified radium chloride. At the mercury cathode, radium was deposited into the mercury, forming a radium–mercury amalgam rather than appearing immediately as a separate lump of metal.
They then heated the amalgam in hydrogen to drive off the mercury, leaving metallic radium. This was the first successful isolation of radium in its elemental, metallic state, more than a decade after the Curies had identified radium compounds.
Mercury was not merely an electrode support: it made the separation practical by capturing the newly produced radium in an amalgam. Graphite, copper, and platinum are all familiar electrode materials, but they were not the cathode material used in this landmark preparation.