Vanadium, and obscure metal, provides the latest installment in our journey through the economics of the periodic table. This element has hardened steel since ancient times, and today it lies at the heart giant batteries that could be vital to the future of solar energy. Our regular chemistry maestro, professor Andrea Sella of University College London, demonstrates vanadium's surprisingly colourful properties.

And, Justin Rowlatt meets Bill Radvak, chief executive of American Vanadium - the only vanadium company in the US - and asks what a 'redox flow battery' could do for the BBC's headquarters in London. We also hear from solar energy entrepreneur Alexander Voigt about the particular niche that vanadium will fill in the future ecosystem of electricity grid storage.