Discovery of Fe-Ce Oxide/BiVO4 Photoanodes through Combinatorial Exploration of Ni-Fe-Co-Ce Oxide Coatings

The Materials Provenance Store

We present a database resulting from high throughput experimentation, primarily on metal oxide solid state materials. The central relational database, the Materials Provenance Store (MPS), manages the metadata and experimental provenance from acquisition of raw materials, through synthesis, to a broad range of materials characterization techniques. Given the primary research goal of materials discovery of solar fuels materials, many of the characterization experiments involve electrochemistry, along with optical, structural, and compositional characterizations. The MPS is populated with all information required for executing common data queries, which typically do not involve direct query of raw data. The result is a database file that can be distributed to users so that they can independently execute queries and subsequently download the data of interest. We propose this strategy as an approach to manage the highly heterogeneous and distributed data that arises from materials science experiments, as demonstrated by the management of over 30 million experiments run on over 12 million samples in the present MPS release.

Discovery of complex oxides via automated experiments and data science

The quest to identify materials with tailored properties is increasingly expanding into high-order composition spaces, with a corresponding combinatorial explosion in the number of candidate materials. A key challenge is to discover regions in composition space where materials have novel properties. Traditional predictive models for material properties are not accurate enough to guide the search. Herein, we use high-throughput measurements of optical properties to identify novel regions in three-cation metal oxide composition spaces by identifying compositions whose optical trends cannot be explained by simple phase mixtures. We screen 376,752 distinct compositions from 108 three-cation oxide systems based on the cation elements Mg, Fe, Co, Ni, Cu, Y, In, Sn, Ce, and Ta. Data models for candidate phase diagrams and three-cation compositions with emergent optical properties guide the discovery of materials with complex phase-dependent properties, as demonstrated by the discovery of a Co-Ta-Sn substitutional alloy oxide with tunable transparency, catalytic activity, and stability in strong acid electrolytes. These results required close coupling of data validation to experiment design to generate a reliable end-to-end high-throughput workflow for accelerating scientific discovery.

Surface Modification of Hematite Photoanodes with CeOx Cocatalyst for Improved Photoelectrochemical Water Oxidation Kinetics

Hematite is a promising photoanode for solar water splitting by photoelectrochemical (PEC) cells, but its performance is limited by the slow kinetics of water oxidation reaction or oxygen evolution reaction (OER). Surface modification of hematite photoanodes with a suitable water oxidation cocatalyst is a key strategy for improving the kinetics of water oxidation. In this study, a CeO_x overlayer is deposited on the surface of the hematite photoanode by a water-based solution method with ceric ammonium nitrate (CAN) followed by heat treatment. The photocurrent of CeO_x -modified hematite is 3 times higher than that of pristine hematite (at 1.23 V vs. RHE) under AM 1.5G, 1 sun conditions. Through hole-scavenger measurements, Tafel plot analysis, and electrochemical impedance spectroscopy, it is concluded that CeO_x overlayer increases the hole injection efficiency, improves the surface catalytic activity, and enhances charge transfer across the photoanode/electrolyte interface. These observations are attributed to the synergistic effects of Ce³⁺ /Ce⁴⁺ redox species in CeO_x and the oxygen vacancies. This work elucidates the role of CeO_x as an efficient cocatalyst overlayer to improve the OER kinetics of photoanodes.

Photocurrent Recombination Through Surface Segregation in Al–Cr–Fe–O Photocathodes

Chemical surface segregation is a design variable in the optimization of phocathodes but has largely been investigated through surface passivation or decoration. In this study a long charge carrier lifetime material, Al–Cr–Fe–O, exhibiting strong photocurrent recombination is investigated for its atomic scale crystallographic and chemical inhomogeneity. Combined scanning transmission electron microscopy and atom probe tomography unveils that insulating Al- and Cr-rich surface layers form during processing. These are discussed to be the primary reason for experimentally observed charge carrier recombination. This study highlights the importance of processing in the design, discovery and optimization of new light absorber materials for photoelectrochemical water splitting.

Spray Pyrolysis as a Combinatorial Method for the Generation of Photocatalyst Libraries

An inexpensive, combinatorial method to evaluate an array of metal oxide materials as photocatalysts for solar fuel production utilizing spray pyrolysis is presented. This new approach capitalizes on the inherent properties of spray pyrolysis. We take advantage of the natural lateral gradient produced in a spray cone to fashion four-metal-three-at-a-time compositional triangular patterns on conductive glass substrates from simple nitrate salt precursor solutions. Subsequent annealing produces thin-film electrodes that are readily screened for photochemical activity using a simple laser scanner system. The apparatus employed is constructed from readily available commercial components, making it accessible to a wide number of laboratories. Our method complements other combinatorial methods in that it provides a chemically different environment for the formation of materials that might produce different morphologies and metal oxidation states and it allows for easy evaluation of layered structures, as well single-phase materials, thereby expanding the number of unique materials tested as potential photocatalysts. As a proof of principle, the discovery and optimization of a new Na-doped CuBi₂O₄ photocatalyst is described.

Rational Design and Construction of Cocatalysts for Semiconductor-Based Photo-Electrochemical Oxygen Evolution: A Comprehensive Review

Photo-electrochemical (PEC) water splitting, as an essential and indispensable research branch of solar energy applications, has achieved increasing attention in the past decades. Between the two photoelectrodes, the photoanodes for PEC water oxidation are mostly studied for the facile selection of n-type semiconductors. Initially, the efficiency of the PEC process is rather limited, which mainly results from the existing drawbacks of photoanodes such as instability and serious charge-carrier recombination. To improve PEC performances, researchers gradually focus on exploring many strategies, among which engineering photoelectrodes with suitable cocatalysts is one of the most feasible and promising methods to lower reaction obstacles and boost PEC water splitting ability. Here, the basic principles, modules of the PEC system, evaluation parameters in PEC water oxidation reactions occurring on the surface of photoanodes, and the basic functions of cocatalysts on the promotion of PEC performance are demonstrated. Then, the key progress of cocatalyst design and construction applied to photoanodes for PEC oxygen evolution is emphatically introduced and the influences of different kinds of water oxidation cocatalysts are elucidated in detail. Finally, the outlook of highly active cocatalysts for the photosynthesis process is also included.

Single-Source Bismuth (Transition Metal) Polyoxovanadate Precursors for the Scalable Synthesis of Doped BiVO4 Photoanodes

Single-source precursors are used to produce nanostructured BiVO₄ photoanodes for water oxidation in a straightforward and scalable drop-casting synthetic process. Polyoxometallate precursors, which contain both Bi and V, are produced in a one-step reaction from commercially available starting materials. Simple annealing of the molecular precursor produces nanocrystalline BiVO₄ films. The precursor can be designed to incorporate a third metal (Co, Ni, Cu, or Zn), enabling the direct formation of doped BiVO₄ films. In particular, the Co- and Zn-doped photoanodes show promise for photoelectrochemical water oxidation, with photocurrent densities >1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode (RHE). Using this simple synthetic process, a 300 cm² Co-BiVO₄ photoanode is produced, which generates a photocurrent of up to 67 mA at 1.23 V vs RHE and demonstrates the scalability of this approach.

Interface Engineering and its Effect on WO3-Based Photoanode and Tandem Cell

During photoelectrochemical (PEC) water splitting, the reactions occur on the surface of the photoelectrode. Therefore, the properties of the interfaces between the various components of the electrode (semiconductor/semiconductor, semiconductor/catalyst, or photoelectrode/electrolyte) affect the PEC performance of the composite material. Notably, surface trap states may hinder charge transfer and transport properties, and also cause Fermi pinning, affecting the quasi-Fermi level and onset potential under illumination, which may in turn influence the PEC performance of the corresponding tandem cells. In this study, plate-like WO₃ array films prepared by an aqueous chemical growth method were employed to highlight the effect of interfacial properties on the performance of a WO₃-based photoanode. The Mott-Schottky and linear sweep voltammetry experiments prove the existence of surface trap states and Fermi pinning for pristine WO₃, which are alleviated after an "etching" treatment and disappeared after surface passivation by a Ga₂O₃ layer. Both etching and passivation increase the oxygen evolution activity and the Faradaic efficiency for the oxygen evolution reaction (OER). After loading a permeable catalyst (FeOOH), the photocurrent is further increased, and there is a synergistic effect between loading of the electrocatalyst with etching or passivation. The onset potentials of the samples follow the trends: etch-WO₃/FeOOH < WO₃/FeOOH ≤ WO₃/Ga₂O₃/FeOOH < etch-WO₃ < WO₃ < WO₃/Ga₂O₃, indicating that the interfacial properties have a significant effect on the PEC performance. Meanwhile, the modified WO₃-based electrode was combined with a dye-sensitized solar cell to fabricate tandem cell, which showed 2.42-fold photocurrent density compared with the pristine WO₃-based tandem cell.