Efficient and durable catalysis of the oxygen evolution reaction in acidic media remains a grand challenge at the intersection of electrochemistry and materials discovery. Antimony-based rutile oxides have shown great promise but suffer from poor durability at high concentration of activity-promoting elements such as Mn. We use combinatorial methods to realize a new family of catalysts wherein combinations of Sn, Ti, and Sb enable activity in rutile oxides with Mn concentration less than 40%.

Non-precious metal catalysts for acidic OER typically require a high concentration of activity-promoting elements, e.g., Mn. We describe the high throughput discovery of quinary oxide catalysts with low Mn concentration via mixing with Sb, Sn, and Ti.

Properties can be tailored by tuning composition in high-order composition spaces. For spaces with complex phase behavior, modeling the properties as a function of composition and phase distribution remains a formidable challenge. We present materials structure–property factorization (MSPF) as an approach to automate modeling of such data and identify synergistic phase interactions. MSPF is an interpretable machine learning algorithm that couples phase mapping via Deep Reasoning Networks (DRNets) to matrix factorization-based modeling of the representative properties of each phase in a dataset. MSPF is demonstrated for Bi–Cu–V oxide photoanodes for solar fuel generation, which contains 25 different phase combinations and correspondingly exhibits complex composition-structure-photoactivity relationships. Comparing the measured photoactivity to a learned model for non-interacting phases, synergistic phase interactions are identified to guide further photoactivity optimization and understanding. MSPF identifies synergistic interactions of a BiVO4-like phase with both Cu2V2O7-like and CuV2O6-like phases, creating avenues for understanding complex photoelectrocatalysts.

: In the search for photoanode materials with band gaps suitable for utilization in solar fuel generation, approximately 1.2 − 2.8 eV, theory-guided experiments have identi fi ed a variety of materials that meet the band gap requirements and exhibit operational stability in harsh photoelectrochemical environments. In particular, M-V-O compounds (M is a transition metal or main group element) with VO 4 structural motifs were predicted to show a remarkably wide range of band energetics (>3 eV variation in the energy of valence band maximum) and characteristics, depending on the M and crystal structure, which is beyond the extent of electronic structured tuning observed in previously studied families of metal oxide photoanodes. While this fi nding guided experimental discovery of new photoanode materials, explicit experimental veri fi cation of the theoretical prediction of the tunable electronic structure of these materials has been lacking to date. In this study, we use X-ray photoelectron spectroscopy and Kelvin probe microscopy to experimentally investigate the electronic structure of M-V-O photoanodes, enabling comparison to theory on a common absolute energy scale. The results con fi rm the prediction that band edge energies of ternary vanadates vary signi fi cantly with metal cations. The valence band variation of approximately 1 eV observed here is larger than that reported in any analogous class of metal oxide semiconductors and demonstrates the promise of tuning the metal oxide electronic structure to enable e ffi cient photoelectrocatalysis of the oxygen evolution reaction and beyond. Because midgap states can hamper realization of the high photovoltage sought by band edge tuning, we analyze the electronic contributions of oxygen vacancies for the representative photoanode V 4 Cr 2 O 13 to guide future research on the development of high-e ffi ciency metal oxide photoanodes for solar fuel technology.

The development of efficient, stable photoanodes and photocathodes remains a primary materials challenge in the establishment of a scalable technology for artificial photosynthesis. The typical photoelectrode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, carrier transport, and elimination of deleterious surface recombination sites. We describe high throughput investigations of the variation in performance and photo-response of integrated photoanode and photocathode libraries. The photoanode assembly libraries consist of uniform BiVO4 light absorbers coated with films varying in composition and loading and evaluated at pH 9 and 13. The photocathode assembly libraries, for example, consist of uniform CuBi2O4 light absorbers coated with film libraries varying in composition and loading. Photoelectrode assemblies based on promising coating compositions and loadings were characterized in detail.

Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. In this work, a data‐driven materials science approach utilizing high‐throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi‐element coating libraries to discover new classes of candidate passivation and electron‐selective contact materials for p‐type CuBi2O4. The optimized overlayer (Cu1.5TiOz) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon‐to‐current efficiency by 15.5% compared to non‐coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high‐throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high‐performance photoelectrodes for incorporation into solar‐fuel generation systems.

The importance of metal oxide photoanodes in solar fuels technology has garnered concerted efforts in photoanode discovery in recent decades, which complement parallel efforts in development of analytical techniques and optimization strategies using standard photoanodes such as TiO2, Fe2O3 and BiVO4. Theoretical guidance of high-throughput experiments has been particularly effective in dramatically increasing the portfolio of metal oxide photoanodes, motivating a new era of photoanode development where the characterization and optimization techniques developed on traditional materials are applied to nascent photoanodes that exhibit visible light photoresponse. The compendium of metal oxide photoanodes presented in the present work can also serve as the basis for further technique development, with a primary goal to establish workflows for discovery of materials that perform better against the critical criteria of operational stability, visible light photoresponse, and photovoltage suitable for tandem absorber architectures.

Copper vanadates are a promising class of solar fuel photoanodes with broad spectral response and excellent operational stability. The present performance limitations of these photoanodes are most evident in the rapid decrease in photoactivity with decrease in either photon energy or applied bias, the former limiting efficient utilization of the solar spectrum and the latter limiting photovoltage. We designed high-throughput photoelectrochemical screening to characterize these two aspects of improving Cu-V-based photoanodes in quaternary oxide composition spaces of the form Cu-V-X-O, where X = Mg, Ca, Sr, Fe. The results reveal that alloying of 2+ cations onto the Cu site of copper vanadates can improve photoelectrochemical properties, and Sr-alloyed Cu5V2O10 emerges as the most promising photoanode providing the best combination of photovoltage and spectral response. Six quaternary oxide phases are discovered as photoanodes with visible light activity below 1.23 V versus reversible hydrogen electrode, highlighting the high-throughput photoanode discovery.

Combinatorial synthesis combined with high throughput electrochemistry enabled discovery of 29 ternary oxide photoanodes, 14 with visible light response for oxygen evolution. Y3Fe5O12 and trigonal V2CoO6 emerge as particularly promising candidates due to their photorepsonse at sub-2.4 eV illumination.

Analysis and visualization of over 2121 OER catalyst compositions over pH 3–13 identifies mechanistic clusters.

With the increasing energy demand, developing renewable fuel production strategies such as photoelectrocatalytic hydrogen production is critical to mitigating the global climate change. In this work, we experimentally validate a new stable and photoactive material, Mg2MnO4, from the exhaustive theoretical exploration of the chemical space of X (=Mg and Ca), Mn and O.

The development of an efficient, stable photoanode to provide protons and electrons to the (photo)cathode remains a primary materials challenge in the establishment of a scalable technol. for artificial photosynthesis. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious surface recombination sites. To provide a more efficient exploration of metal oxide coatings for a given light absorber, we introduce a high throughput methodol. wherein a uniform BiVO4 library is coated with multi-metal oxide coatings. This presentation will focus on the variation in performance and photo-response of integrated photoanodes consisting of BiVO4 coated with a sputter deposited Fe-Ce oxide film as a function of compn. and loading. Parallel photoanode libraries were prepd. and evaluated at pH 13 and pH 9.

Electrocatalysis of the oxygen evolution reaction is central to several energy technologies including electrolyzers, solar fuel generators, and air breathing batteries. Strong acid electrolytes are desirable for many implementations of these technologies, although the deployment of such device designs is often hampered by the lack of non-precious-metal oxygen evolution electrocatalysts, with Ir-based oxides comprising the only known catalysts that exhibit stable activity at low overpotential. During our exploration of the Mn-Sb-O system for precious-metal-free electrocatalysts, we discovered that Mn can be incorporated into the rutile oxide structure at much higher concentrations than previously known, and that these Mn-rich rutile alloys exhibit great catalytic activity with current densities exceeding 50 mA cm(-2) at 0.58 V overpotential and catalysis onset at 0.3 V overpotential. While this activity does not surpass that of IrO2, Pourbaix analysis reveals that the Mn-Sb rutile oxide alloys have the same or better thermodynamic stability under operational conditions. By combining combinatorial composition, structure, and activity mapping with synchrotron X-ray absorption measurements and first principles materials chemistry calculations, we provide a comprehensive understanding of these oxide alloys and identify the critical role of Sb in stabilizing the trivalent Mn octahedra that have been shown to be effective oxygen evolution reaction (OER) catalysts.

Combinatorial (photo)electrochemical studies of the (Ni-Mn)Ox system reveal a range of promising materials for oxygen evolution photoanodes. X-ray diffraction, quantum efficiency, and optical spectroscopy mapping reveal stable photoactivity of NiMnO3 in alkaline conditions with photocurrent onset commensurate with its 1.9 eV direct band gap. The photoactivity increases upon mixture with 10-60% Ni6MnO8 providing an example of enhanced charge separation via heterojunction formation in mixed-phase thin film photoelectrodes. Density functional theory-based hybrid functional calculations of the band edge energies in this oxide reveal that a somewhat smaller than typical fraction of exact exchange is required to explain the favorable valence band alignment for water oxidation.

Identifying new catalyst materials for complex reactions such as the electrochemical reduction of CO2 poses substantial instrumentation challenges due to the need to integrate reactor control with electrochemical and analytical instrumentation. Performing accelerated screening to enable exploration of a broad span of catalyst materials poses additional challenges due to the long time scales associated with accumulation of reaction products and the detection of the reaction products with traditional separation-based analytical methods. The catalyst screening techniques that have been reported for combinatorial studies of (photo)electrocatalysts do not meet the needs of CO2 reduction catalyst research, prompting our development of a new electrochemical cell design and its integration to gas and liquid chromatography instruments. To enable rapid chromatography measurements while maintaining sensitivity to minor products, the electrochemical cell features low electrolyte and head space volumes compared to the catalyst surface area. Additionally, the cell is operated as a batch reactor with electrolyte recirculation to rapidly concentrate reaction products, which serves the present needs for rapidly detecting minor products and has additional implications for enabling product separations in industrial CO2 electrolysis systems. To maintain near-saturation of CO2 in aqueous electrolytes, we employ electrolyte nebulization through a CO2-rich headspace, achieving similar gas-liquid equilibration as vigorous CO2 bubbling but without gas flow. The instrument is demonstrated with a series of electrochemical experiments on an Au-Pd combinatorial library, revealing non-monotonic variations in product distribution with respect to catalyst composition. The highly integrated analytical electrochemistry system is engineered to enable automation for rapid catalyst screening as well as deployment for a broad range of electrochemical reactions where product distribution is critical to the assessment of catalyst performance.

The performance of oxygen-evolving photoanodes based on bismuth vanadate (BiVO4) is critically determined by the surface coating. While these coatings passivate surface defects, transport photogenerated holes, protect against corrosion, and aid catalysis, their optimal composition changes with operating pH, thus affecting overall performance. We use high-throughput photoelectrochemistry methods to map photoanode performance to enable the discovery of optimal composition and loading of Ce-rich sputter-deposited (Fe-Ce)O-x overlayers on undoped BiVO4 in pH 9 borate buffer electrolyte. The optimal composition is found to be 20% Fe and 80% Ce with an optimal Fe + Ce metal loading of 0.9 nmol mm(-2). Analysis of the composition and loading dependence of (i) the photocurrent transients upon illumination toggling, (ii) stabilized photocurrent densities, and (iii) photogenerated hole-transfer efficiency reveals the confluence of phenomena that gives rise to the optimal performance yielding nearly perfect transfer efficiency over a narrow composition window.

Photoelectrocatalysis of the oxygen evolution reaction remains a primary challenge for development of tandem-absorber solar Bi fuel generators due to the lack of a photoanode with broad solar spectrum utilization, a large photovoltage, and stable operation. Bismuth vanadate with a 2.4-2.5 eV band gap has shown the most promise becauses its photoactivity down to 0.4 V vs RHE is sufficiently low to couple to a lower-gap photocathode for fuel synthesis. Through development of photoanodes based on the FeWO4 structure, in particular, Fe-rich variants with addition of about 6% Bi, we demonstrate the same 0.4 V vs RHE turn-on voltage with a 2 eV band gap metal oxide, enabling a 2-fold increase in the device efficiency limit. Combinatorial exploration of materials composition and processing facilitated synthesis of n-type variants of this typical p-type semiconductor that exhibit much higher photoactivity than previous implementations of FeWO4 in solar photochemistry. The photoanodes are particularly promising for solar fuel applications given their stable operation in acid and base.

Solar-driven oxygen evolution is a critical technology for renewably synthesizing hydrogenand carboncontaining fuels in solar fuel generators. New photoanode materials are needed to meet efficiency and stability requirements, motivating materials explorations for semiconductors with (i) band-gap energy in the visible spectrum and (ii) stable operation in aqueous electrolyte at the electrochemical potential needed to evolve oxygen from water. Motivated by the oxygen evolution competency of many Mn-based oxides, the existence of several Bi-containing ternary oxide photoanode materials, and the variety of known oxide materials combining these elements with Sm, we explore the Bi−Mn−Sm oxide system for new photoanodes. Through the use of a ferri/ferrocyanide redox couple in high-throughput screening, BiMn2O5 and its alloy with Sm are identified as photoanode materials with a nearideal optical band gap of 1.8 eV. Using density functional theory-based calculations of the mullite BiMnMnO5 phase, we identify electronic analogues to the well-known BiVO4 photoanode and demonstrate excellent Pourbaix stability above the oxygen evolution Nernstian potential from pH 4.5 to 15. Our suite of experimental and computational characterization indicates that BiMn2O5 is a complex oxide with the necessary optical and chemical properties to be an efficient, stable solar fuel photoanode.