Combined Catalysis and Optical Screening for High Throughput Discovery of Solar Fuels Catalysts

Abstract

Considerable research and development efforts are being devoted to the efficient generation of solar fuels. A solar fuels device couples a solar photoabsorber with catalysts to convert solar energy to chemical energy via reactions such as oxygen evolution (water splitting). Widespread deployment of this technology hinges upon discovery of new materials through efforts such as the high throughput screening of oxygen evolution catalysts, as discussed in this manuscript. We derive an expression for the efficiency of the oxygen evolution catalyst that combines catalytic and optical properties. Using this hybrid efficiency, we screen 5456 samples in a (Fe-Co-Ni-Ti)O x pseudo-quaternary catalyst library using automated, high throughput electrochemical and optical experiments. The observed compositional trends in this catalyst efficiency lead to the discovery of a new high performance composition region. 1,2 The widespread deployment of this technology requires a device fabricated with high-performance, robust materials made from earth-abundant elements. 3 The realization of this device relies on the discovery of new materials, prompting an aggressive high-throughput materials discovery effort within the Joint Center for Artificial Photosynthesis (JCAP, http://solarfuelshub.org/). This effort includes searches for new photoanodes, photocathodes, and catalysts for several solar fuels reactions, of which the oxygen evolution catalyst is discussed in this manuscript.One established solar fuel generator concept combines a solar photoabsorber with an optically inactive heterogeneous catalyst to evolve oxygen (water splitting). 3,4 That is, the oxygen evolution reaction (OER) is mediated by a "dark" catalyst that is coupled to a photoabsorber, which provides the required potential and current for the electrocatalytic reaction through the photovoltaic effect. This anodic reaction balances a fuel-producing cathodic reaction such as hydrogen evolution or CO 2 reduction. Variations within this solar fuel design concept include the use of a tandem absorber containing a coupled photoanode and photocathode instead of a single photoabsorber [5][6][7] or the use of a homogeneous OER catalyst instead of the heterogeneous catalyst. 3,[8][9][10][11] Homogeneous catalysts are not considered in this manuscript, and the general discussion of optimizing the OER catalyst is applicable to any photoabsorber device. Our primary assumptions concerning device architecture are that the OER catalyst coats the photoanode and this anode is exposed to solar illumination. These assumptions are based upon a tandem photoabsorber system in which the photoanode is the top cell and has a larger bandgap than the photocathode.While heterogeneous catalysts are used in a variety of technologies, the OER catalyst in this solar application has a special optical performance requirement. Since the solar radiation impinges the catalyst before reaching the photoabsorber, the catalyst layer must be sufficiently transparent. Since the optical properties of the electroc...

An Integrated Device View on Photo-Electrochemical Solar-Hydrogen Generation

“…Large efforts to discover material systems with appropriate properties can result in new sets of practical and deployable components that can be incorporated into practical devices. Combinatorial approaches to identify promising materials as both photoabsorbers and electrocatalysts have already started to show some promising directions for materials development (103)(104)(105)(106). Transport challenges identified for the three device configurations referred to above can be tackled mostly by device design.…”

Identification of optimal solar fuel electrocatalysts via high throughput in situ optical measurements

“…The array of composition samples was deposited by inkjet printing onto the FTO-coated side of a 10 cm  15 cm glass plate at a resolution of 2880  1440 dpi, as described previously. 27,43 Each composition was synthesized as a 1 mm  1 mm composition sample containing 3.8 nmol of the metal on a 2 mm pitch. Elemental precursor inks, of the type previously described, 12,[21][22][23]27,43,44 were prepared by mixing 5 mmol of each metal precursor with 0.80 g Pluronic F127 (Aldrich), 1.0 mL glacial acetic acid (T.J. Baker, Inc.), 0.…”

“…27 To date, the combined efficiency has been calculated using optical data acquired in ambient conditions, but Fig. 5 clearly shows the importance of in situ optical measurements.…”

“…As described in detail elsewhere, 27 the operational current density and overpotential of the OER catalyst in an integrated solar fuel device is ultimately determined by the light absorber and current-matched cathode. The spectral absorption of the photoabsorber stack is of particular importance and can be added as an additional weighting factor in Eq.…”

“…However, the transparency of the catalyst under operating potential is a critical parameter that can be measured as a function of catalyst composition and then combined with the band gap or absorption spectrum of a potential light absorber to generate an improved figure of merit for catalyst performance. 27 Herein we report high throughput capabilities for acquisition of the requisite spectral data on catalyst libraries under OER operating conditions.…”