Joel A. Haber

Multigram synthesis of copper nanowires using ac electrodeposition into porous aluminium oxide templates

Multigram quantities of Cu nanowires ca. 25 nm in diameter and µm in length have been produced by AC electrodeposition into porous aluminium oxide (PAO) templates. Multiple, large-area Al electrodes (5 × 11 cm or 10 × 25 cm) are anodized in parallel at 25.0 V in 0.3 M H2SO4(aq) using custom built baths. The pores are efficiently filled by applying 200 Hz sine waves at 10 Vrms between the anodized Al and Cu plate counter electrodes immersed in a 0.50 M CuSO4(aq) solution. Dissolution of the PAO template in 0.6 M H3PO4 to free the Cu nanowires results in significant coarsening of the nanowires, whereas dissolution of the PAO template in 1.0 M NaOH(aq) results in retention of the Cu nanowire diameters corresponding to the pore diameter of the PAO template. Liberated Cu nanowires were characterized by scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

High-throughput bubble screening method for combinatorial discovery of electrocatalysts for water splitting.

Combinatorial synthesis and screening for discovery of electrocatalysts has received increasing attention, particularly for energy-related technologies. High-throughput discovery strategies typically employ a fast, reliable initial screening technique that is able to identify active catalyst composition regions. Traditional electrochemical characterization via current-voltage measurements is inherently throughput-limited, as such measurements are most readily performed by serial screening. Parallel screening methods can yield much higher throughput and generally require the use of an indirect measurement of catalytic activity. In a water-splitting reaction, the change of local pH or the presence of oxygen and hydrogen in the solution can be utilized for parallel screening of active electrocatalysts. Previously reported techniques for measuring these signals typically function in a narrow pH range and are not suitable for both strong acidic and basic environments. A simple approach to screen the electrocatalytic activities by imaging the oxygen and hydrogen bubbles produced by the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is reported here. A custom built electrochemical cell was employed to record the bubble evolution during the screening, where the testing materials were subject to desired electrochemical potentials. The transient of the bubble intensity obtained from the screening was quantitatively analyzed to yield a bubble figure of merit (FOM) that represents the reaction rate. Active catalysts in a pseudoternary material library, (Ni-Fe-Co)Ox, which contains 231 unique compositions, were identified in less than one minute using the bubble screening method. An independent, serial screening method on the same material library exhibited excellent agreement with the parallel bubble screening. This general approach is highly parallel and is independent of solution pH.

Generating information-rich high-throughput experimental materials genomes using functional clustering via multitree genetic programming and information theory.

High-throughput experimental methodologies are capable of synthesizing, screening and characterizing vast arrays of combinatorial material libraries at a very rapid rate. These methodologies strategically employ tiered screening wherein the number of compositions screened decreases as the complexity, and very often the scientific information obtained from a screening experiment, increases. The algorithm used for down-selection of samples from higher throughput screening experiment to a lower throughput screening experiment is vital in achieving information-rich experimental materials genomes. The fundamental science of material discovery lies in the establishment of composition-structure-property relationships, motivating the development of advanced down-selection algorithms which consider the information value of the selected compositions, as opposed to simply selecting the best performing compositions from a high throughput experiment. Identification of property fields (composition regions with distinct composition-property relationships) in high throughput data enables down-selection algorithms to employ advanced selection strategies, such as the selection of representative compositions from each field or selection of compositions that span the composition space of the highest performing field. Such strategies would greatly enhance the generation of data-driven discoveries. We introduce an informatics-based clustering of composition-property functional relationships using a combination of information theory and multitree genetic programming concepts for identification of property fields in a composition library. We demonstrate our approach using a complex synthetic composition-property map for a 5 at. % step ternary library consisting of four distinct property fields and finally explore the application of this methodology for capturing relationships between composition and catalytic activity for the oxygen evolution reaction for 5429 catalyst compositions in a (Ni-Fe-Co-Ce)Ox library.

Mapping quantum yield for (Fe-Zn-Sn-Ti)Ox photoabsorbers using a high throughput photoelectrochemical screening system.

Combinatorial synthesis and screening of light absorbers are critical to material discoveries for photovoltaic and photoelectrochemical applications. One of the most effective ways to evaluate the energy-conversion properties of a semiconducting light absorber is to form an asymmetric junction and investigate the photogeneration, transport and recombination processes at the semiconductor interface. This standard photoelectrochemical measurement is readily made on a semiconductor sample with a back-side metallic contact (working electrode) and front-side solution contact. In a typical combinatorial material library, each sample shares a common back contact, requiring novel instrumentation to provide spatially resolved and thus sample-resolved measurements. We developed a multiplexing counter electrode with a thin layer assembly, in which a rectifying semiconductor/liquid junction was formed and the short-circuit photocurrent was measured under chopped illumination for each sample in a material library. The multiplexing counter electrode assembly demonstrated a photocurrent sensitivity of sub-10 μA cm(-2) with an external quantum yield sensitivity of 0.5% for each semiconductor sample under a monochromatic ultraviolet illumination source. The combination of cell architecture and multiplexing allows high-throughput modes of operation, including both fast-serial and parallel measurements. To demonstrate the performance of the instrument, the external quantum yields of 1819 different compositions from a pseudoquaternary metal oxide library, (Fe-Zn-Sn-Ti)Ox, at 385 nm were collected in scanning serial mode with a throughput of as fast as 1 s per sample. Preliminary screening results identified a promising ternary composition region centered at Fe0.894Sn0.103Ti0.0034Ox, with an external quantum yield of 6.7% at 385 nm.

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

An efficient photoanode is a prerequisite for a viable solar fuels technology. The challenges to realizing an efficient photoanode include the integration of a semiconductor light absorber and a metal oxide electrocatalyst to optimize corrosion protection, light trapping, hole transport, and photocarrier recombination sites. To efficiently explore metal oxide coatings, we employ a high-throughput methodology wherein a uniform BiVO4 film is coated with 858 unique metal oxide coatings covering a range of metal oxide loadings and the full (Ni-Fe-Co-Ce)Ox pseudoquaternary composition space. Photoelectrochemical characterization of the photoanodes reveals that specific combinations of metal oxide composition and loading provide up to a 13-fold increase in the maximum photoelectrochemical power generation for oxygen evolution in pH 13 electrolyte. Through mining of the high-throughput data we identify composition regions that form improved interfaces with BiVO4. Of particular note, integrated photoanodes with catalyst compositions in the range Fe(0.4-0.6)Ce(0.6-0.4)Ox exhibit high interface quality and excellent photoelectrochemical power conversion. Scaled-up inkjet-printed electrodes and photoanodic electrodeposition of this composition on BiVO4 confirms the discovery and the synthesis-independent interface improvement of (Fe-Ce)Ox coatings on BiVO4.

Parallel electrochemical treatment system and application for identifying acid-stable oxygen evolution electrocatalysts.

Many energy technologies require electrochemical stability or preactivation of functional materials. Due to the long experiment duration required for either electrochemical preactivation or evaluation of operational stability, parallel screening is required to enable high throughput experimentation. Imposing operational electrochemical conditions to a library of materials in parallel creates several opportunities for experimental artifacts. We discuss the electrochemical engineering principles and operational parameters that mitigate artifacts in the parallel electrochemical treatment system. We also demonstrate the effects of resistive losses within the planar working electrode through a combination of finite element modeling and illustrative experiments. Operation of the parallel-plate, membrane-separated electrochemical treatment system is demonstrated by exposing a composition library of mixed-metal oxides to oxygen evolution conditions in 1 M sulfuric acid for 2 h. This application is particularly important because the electrolysis and photoelectrolysis of water are promising future energy technologies inhibited by the lack of highly active, acid-stable catalysts containing only earth abundant elements.

Manufacturing of ‘ink based’ CIGS solar cells/modules

The rapid growth of the PV business demands increased production capacity and lowered cost of manufacturing PV modules. ISET has developed a low cost ‘ink-based’ process by which high efficiency CIGS modules can be manufactured at very low costs. ISET is currently in the process of commercializing this technology. This paper describes ISETs progress in setting up the pilot plant of 3MW/Yr nominal capacity for CIGS modules and plans to expand it into a full scale (≫50 MW/Yr) manufacturing facility.

Combinatorial discovery of new thin film photovoltaics

A combinatorial approach to discover new types of thin film photovoltaic devices containing only abundant, inexpensive, and relatively nontoxic elements is described. A large number of compound semiconductors with band-gaps suitable for solar energy conversion (1.0-2.0 eV) are known, including many sulfide compounds, but have not yet been used in efficient devices. Thin films of several sulfide semiconductors will be prepared and their microstructure and optical and electrical properties characterized. Combinatorial methods will be employed to simultaneously prepare many combinations of back contacts, absorber layers, buffer layers, heterojunction window layers, and top contacts. The combinatorial approach is necessary, because existing thin film technologies have largely been selected and improved empirically. The combinatorial approach will enable us to greatly accelerate the rate of empirical discovery.

Colorimetric Screening for High-Throughput Discovery of Light Absorbers

High-throughput screening is a powerful approach for identifying new functional materials in unexplored material spaces. With library synthesis capable of producing 10(5) to 10(6) samples per day, methods for material screening at rates greater than 1 Hz must be developed. For the discovery of new solar light absorbers, this throughput cannot be attained using standard instrumentation. Screening certain properties, such as the bandgap, are of interest only for phase pure materials, which comprise a small fraction of the samples in a typical solid-state material library. We demonstrate the utility of colorimetric screening based on processing photoscanned images of combinatorial libraries to quickly identify distinct phase regions, isolate samples with desired bandgap, and qualitatively identify samples that are suitable for complementary measurements. Using multiple quaternary oxide libraries containing thousands of materials, we compare colorimetric screening and UV-vis spectroscopy results, demonstrating successful identification of compounds with bandgap suitable for solar applications.

‘Ink-based’ CIGS solar cells on lightweight Titanium foil

Fabrication of Copper Indium Gallium Selenide (CIGS) solar cells on Ti foil using ISETs ink-based process is challenging, because a high temperature annealing step in hydrogen ambient causes embrittlement of the Ti substrate. To ameliorate this situation, ISET has developed a proprietary process that encapsulates the Ti foil with a thin protective layer which enables the Ti foil to survive the ink-based cell fabrication process, without compromising its flexibility. Using this approach, ISET has achieved a champion cell efficiency of 11.7% (AM1.5). In addition, the insulating nature of this protective layer has enabled us to fabricate monolithically integrated modules on titanium substrate and potentially reduce the overall cost of manufacturing CIGS modules on flexible metal foil substrates.