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...

Discovering Ce-rich oxygen evolution catalysts, from high throughput screening to water electrolysis

“…22,23 The compositions were produced by inkjet printing four separate metal precursor inks (3.8 nanomoles of metal in each 1 mm 2 spot), prepared by a previously described method, 11,24 onto a conducting substrate. The inks were dried and the metal precursors converted to oxides by calcination in air at 350…”

Advanced and In Situ Analytical Methods for Solar Fuel Materials

“…Defining reasonable figures of merit allow one to determine which catalysts offer the best combination of high turnover frequency (mol product/mol active site/s) at low overpotential and low parasitic optical absorption, without the need to test each particular catalyst individually in a complete solar water-splitting device as a function of catalyst loading. Trotochaud et al and Gregoire et al described figures of merit based on experimentally measured data [188,208]. The approach described by Trotochaud et al is summarized here, and later compared to that of Gregoire et al The amount of light at a given wavelength λ reaching the semiconductor surface is a function of the incoming light flux, F(λ), multiplied by the corresponding catalyst transmission probability, T c (λ, t).…”

“…Recently, an increase in both modeling and experimental work has taken place to see how these components fare in composite systems [182,[191][192][193][194][195][196][197][198][199][200][201][202][203][204][205][206][207]. Although high-throughput screening has been used to predict which catalysts may contain ideal optical properties (low absorption) [208], little work has directly investigated the photocatalytic properties of the current state-of-the-art catalysts. In situ ultraviolet/visible (UV-Vis) spectroscopy is a valuable method for quantitatively evaluating the optical properties of catalysts under working conditions.…”

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

“…The cell design, described in detail by Gregoire et al, 16 provides the capability to perform standard 3-electrode electrochemical characterization of each sample in the catalyst library. The cell was outfit with a Pt wire counter electrode and custom made Ag/AgCl reference electrode.…”