NSF/DOE Solar Hydrogen Fuel: Tunable Semiconductor/Catalyst Interfaces for Efficient Solar WaterSplitting

Much research has gone into creating new materials and processes for making solar fuels cell devices that convert and store the energy in sunlight as fuels (hydrogen and hydrocarbons) from renewable feedstocks (water and carbon dioxide). However, no commercial products of this type yet exist due to the high cost of fuel cell materials, low solar to chemical conversion efficiencies, instability and short lifetime. An award is being made to Profs. Charles Dismukes, Eric Garfunkel and Martha Greenblatt of Rutgers University through the joint NSF/DOE Solar H2 Fuel Solicitation to create new nanomaterials and combine them in a special fashion to produce a new device that circumvents these limitations, while increasing the solar efficiency by a factor of 2. The new materials in the solar photovoltaic cells will be integrated with existing catalysts to make standalone light-driven electrolyzers for splitting water into its elements and, separately, reducing carbon dioxide to a liquid fuel. The device will feature earth abundant catalysts that rival the efficiency of existing platinum group catalysts, yet are economical and scalable. Syntheses of new composite photovoltaic materials to be used as solar absorbers for the anode and cathode that feature complementary spectral coverage are predicted to deliver a two-fold increase in solar-to-hydrogen conversion efficiency relative to current systems. New methods for integration of catalysts with solar absorbers will be devised. Attainment of these goals read directly on environmental and energy security benefits to the USA.

The goals of this award require the design of new materials and the coupling of them into new working devices. The specific milestones to be achieved are: 1) To achieve a two-fold increase to 10% of the current state-of-the-art solar-to-hydrogen efficiency by constructing a tandem photoelectrochemical cell with catalysts for Oxygen evolution (cubic LiCoO2/ionomer) and Hydrogen evolution (Ni5P4/ionomer) coupled to (dual) bandgap absorbers as photoanode (perovskite oxynitrides ABO3-xNx) and photocathode (pn-silicon), using an alkaline exchange membrane as a hydroxide conducting medium. 2) Synthesize selected members of the perovskite oxynitride series ABO3-xNx where A = alkaline earth or rare earth cation and B = Ti, V, Zr, Nb, Ta, and with B site substitutions Nb or Ta for Ti. 3) Prepare thin films of the foregoing perovskites, first by pulsed laser deposition (PLD), and, in subsequent years, explore other methods to grow gradient films. 4) Characterize the optical bandgaps and photoinduced carrier lifetimes of these materials prior to attaching O2 evolution catalysts. 5) Using PLD, prepare thin films of cubic LiCoO2 both in direct contact with the photoanode and as a buried junction with intervening transparent conductors. 6) Produce stable interfaces between Ni5P4 with various photocathodes (silicon and other low bandgap semiconductors) to achieve efficient electron capture for H2 evolution. 7) Produce stable interfaces between Ni5P4 and alkaline exchange ionomers to achieve efficient hydroxide transport, slower charge recombination, and long term stability. 8) Investigate two electrolyte systems for connecting the photoelectrodes: aqueous alkaline electrolyte solution (pH 14), and alkaline exchange membrane. Achievement of these milestones will represent a significant advance in the technology.