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Photoelectrochemical (PEC) CO₂reduction (PEC CO₂R) is a prospective approach for utilizing solar energy to synthesize a variety of carbon-containing chemicals and fuels, the most valuable of which are multicarbon (C₂₊) products, such as ethylene and ethanol. While these products can be produced with high faradaic efficiency using Cu, this occurs over a relatively narrow potential range, which, in turn, imposes constraints on the design of a device for PEC CO₂R. Herein, we used continuum-scale modeling to simulate the solar-to-C₂₊(STC₂₊) efficiency of PEC CO₂R devices fed with CO₂-saturated, 0.1 M CsHCO₃. We then explored how cell architecture and the use of single or dual photoelectrode(s) alters the optimal combination of photoelectrode bandgaps for high STC₂₊efficiency. Ultimately, this work provides guidance for the co-design of the device architecture and photoelectrode bandgaps required to achieve high STC₂₊efficiency. The insights gained are then used to identify systems that yield the highest amount of C₂₊products throughout the day and year.

Noble metals supported on reducible oxides, like CoO_xand TiO_x, exhibit superior activity in many chemical reactions, but the origin of the increased activity is not well understood. To answer this question we studied thin films of CoO_xsupported on an Au(111) single crystal surface as a model for the CO oxidation reaction. We show that three reaction regimes exist in response to chemical and topographic restructuring of the CoO_xcatalyst as a function of reactant gas phase CO/O₂stoichiometry and temperature. Under oxygen-lean conditions and moderate temperatures (≤150 °C), partially oxidized films (CoO_x<1) containing Co⁰were found to be efficient catalysts. In contrast, stoichiometric CoO films containing only Co²⁺form carbonates in the presence of CO that poison the reaction below 300 °C. Under oxygen-rich conditions a more oxidized catalyst phase (CoO_x>1) forms containing Co³⁺species that are effective in a wide temperature range. Resonant photoemission spectroscopy (ResPES) revealed the unique role of Co³⁺sites in catalyzing the CO oxidation. Density function theory (DFT) calculations provided deeper insights into the pathway and free energy barriers for the reactions on these oxide phases. These findings in this work highlight the versatility of catalysts and their evolution to form different active phases, both topological and chemically, in response to reaction conditions exposing a new paradigm in the catalyst structure during operation.

Water-vapor fed electrolysis, a simplified single-phase electrolyzer using a proton-exchange membrane electrode assembly, achieved >100 mA cm⁻²performance at <1.7 V, the best for water-vapor electrolysis to date, and was tested under various operating conditions (temperature and inlet relative humidity (RH)). To further probe the limitations of the electrolyzer, a mathematical model was used to identify the overpotentials, local water activity, water content values, and temperature within the cell at these various conditions. The major limitations within the water-vapor electrolyzer are caused by a decreased water content within the membrane phase, indicated by increased Ohmic and mass transport losses seen in applied voltage breakdowns. Further investigations show the water content (λ, mole of water/mole of sulfonic acid) can decrease from 13 at low current densities down to 6 at high current densities. Increasing the temperature or decreasing RH exacerbates this dry-out effect. Using our mathematical model, we show how these mass transport limitations can be alleviated by considering the role of water as both a reactant and a hydrating agent. We show that low cathode RH can be tolerated as long as the anode RH remains high, showing equivalent performance as symmetric RH feeds.

Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO₂reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field.

Anionic molecular models for nonhydrolyzed and partially hydrolyzed aluminum and gallium framework sites on silica, M[OSi(OtBu)₃]₄⁻and HOM[OSi(OtBu)₃]₃⁻(where M=Al or Ga), were synthesized from anionic chlorides Li{M[OSi(OtBu)₃]₃Cl} in salt metathesis reactions. Sequestration of lithium cations with [12]crown‐4 afforded charge‐separated ion pairs composed of monomeric anions M[OSi(OtBu)₃]₄⁻with outer‐sphere [([12]crown‐4)₂Li]⁺cations, and hydroxides {HOM[OSi(OtBu)₃]₃} with pendant [([12]crown‐4)Li]⁺cations. These molecular models were characterized by single‐crystal X‐ray diffraction, vibrational spectroscopy, mass spectrometry and NMR spectroscopy. Upon treatment of monomeric [([12]crown‐4)Li]{HOM[OSi(OtBu)₃]₃} complexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of active sites for Meerwin–Ponndorf–Verley (MPV) transfer hydrogenations with Al/Ga‐doped silica catalysts.

Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively.

Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively.