State of the art research

Hydrogen can be produced efficiently and cheaply using a tandem cell [16, 17]. This device (see Figure below) connects a photoelectrochemical cell to a Grätzel solar cell. The Grätzel cell converts energy in red light to electricity, providing the small extra bias to drive oxygen production over the metaloxide electrode which absorbs blue light in the photoelectrochemical cell. Hydrogen Solar, which aims to bring the tandem cell to the market, reports an efficiency of 8% for a tandem cell based on a WO3 photoanode in the photoelectrochemical cell [18].

Schematic representation of a water cleavage device (left); the energy level diagram for the tandem device, Z-scheme (middle) and a working tandem device.

A promising material for on-board hydrogen storage is NaAlH4 [7, 13, 19]. Thermodynamics suggests this material to release 5.5 wt% of H2 at temperatures close to 110 °C. Kinetics requires higher temperatures, but reasonably fast and reversible adsorption/desorption can be achieved by adding Ti-based (and other) catalysts and by ball milling. In particular, a reversible capacity of 4 wt% over 40 adsorption-desorption cycles has been demonstrated [20]. Other promising materials for hydrogen storage are borohydrides like LiBH4, and metal ammine complexes like Mg(NH3)6Cl2. LiBH4 has a high weight capacity, and the major hydrogen desorption (13.5 wt%) starts at approximately 200 °C when SiO2 is added as a catalyst. Uncertainties concern the crystal structure of the high temperature phase of this material, and the reversibility of this material. Mg(NH3)6Cl2 stores 9.1 wt% H2 which can be released below 350 °C by combining it with an ammonia decomposition catalyst.