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)­O_<i>x</i> 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)O_<i>x</i> 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)­O_<i>x</i> coatings on BiVO₄

Electrochemical Stability of Metastable Materials

We present a first-principles-based formalism to provide a quantitative measure of the thermodynamic instability and propensity for electrochemical stabilization, passivation, or corrosion of metastable materials in aqueous media. We demonstrate that this formalism can assess the relative Gibbs free energy of candidate materials in aqueous media as well as their decomposition products, combining solid and aqueous phases, as a function of pH and potential. On the basis of benchmarking against 20 stable as well as metastable materials reported in the literature and also our experimental characterization of metastable triclinic-FeVO₄, we present quantitative estimates for the relative Gibbs free energy and corresponding aqueous regimes where these materials are most likely to be stable, form inert passivating films, or steadily corrode to aqueous species. Furthermore, we show that the structure and composition of the passivating films formed on triclinic-FeVO₄ are also in excellent agreement with the Point Defect Model, as proposed by the corrosion community. An open-source web application based on the formalism is made available at https://materialsproject.org

Discovery of Manganese-Based Solar Fuel Photoanodes via Integration of Electronic Structure Calculations, Pourbaix Stability Modeling, and High-Throughput Experiments

The solar photoelectrochemical generation of hydrogen and carbon-containing fuels comprises a critical energy technology for establishing sustainable energy resources. The photoanode, which is responsible for solar-driven oxygen evolution, has persistently limited technology advancement due to the lack of materials that exhibit both the requisite electronic properties and operational stability. Efforts to extend the lifetime of solar fuel devices increasingly focus on mitigating corrosion in the highly oxidizing oxygen evolution environment, motivating our development of a photoanode discovery pipeline that combines electronic structure calculations, Pourbaix stability screening, and high-throughput experiments. By applying the pipeline to ternary metal oxides containing manganese, we identify a promising class of corrosion-resistant materials and discover five oxygen evolution photoanodes, including the first demonstration of photoelectrocatalysis with Mn-based ternary oxides and the introduction of alkaline earth manganates as promising photoanodes for establishing a durable solar fuels technology

Discovery and Characterization of a Pourbaix-Stable, 1.8 eV Direct Gap Bismuth Manganate Photoanode

Solar-driven oxygen evolution is a critical technology for renewably synthesizing hydrogen- and carbon-containing 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 near-ideal optical band gap of 1.8 eV. Using density functional theory-based calculations of the mullite Bi³⁺Mn³⁺Mn⁴⁺O₅ 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