Hydrogen derived from water as a sustainable solar fuel: learning from biology

The United Nations Climate Change Conference (COP21) held in Paris in 2015 and the follow-up conferences in Marrakesh (COP22) and very recently in Bonn (COP23) have established an unprecedented international agreement that during this century human society must break from its reliance on energy from fossil fuels to energy sources, which do not release greenhouse gases, particularly carbon dioxide. This is a very hard call. It will not only involve improving more efficient use of energy but also the establishment of new technologies and infrastructures for its generation and distribution. Among renewable energy resources able to tackle these challenges significantly are nuclear or those derived from solar radiation. However in the case of nuclear fusion, a technological breakthrough is required while nuclear fission is limited in the long run by the finite supply of uranium fuel. Here I focus on solar energy which is already contributing to the challenge of COP21. In particular, I discuss its capture and storage through the splitting of water to produce oxygen and hydrogen reducing equivalents, which is essentially the route taken by biology about 3 billion years ago. In so doing I will describe how Nature goes about achieving this fundamental energy converting reaction as a back drop to efforts to achieve the same goal using non-biological photo-electrochemical technology.