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1. Understanding and Controlling the Performance-Limiting Steps of Catalyst-Modified Semiconductors

   https://doi.org/10.1021/acs.jpclett.0c02386  

   Nguyen, Nghi P.; Wadsworth, Brian L.; Nishiori, Daiki; Reyes Cruz, Edgar A.; Moore, Gary F. (January 2020, The Journal of Physical Chemistry Letters)

   null (Ed.)

   Understanding and controlling factors that restrict the rates of fuel-forming reactions are essential to designing effective catalyst-modified semiconductors for applications in solar-to-fuel technologies. Herein, we describe GaAs semiconductors featuring a polymeric coating that contains cobaloxime-type catalysts for photoelectrochemically powering hydrogen production. The activities of these electrodes (limiting current densities >20 mA cm–2 under 1-sun illumination) enable identification of fundamental performance-limiting bottlenecks encountered at relatively high rates of fuel formation. Experiments conducted under varying bias potential, pH, illumination intensity, and scan rate reveal two distinct mechanisms of photoelectrochemical hydrogen production. At relatively low polarization and pH, the limiting photoactivity is independent of illumination conditions and is attributed to a mechanism involving reduction of substrate protons. At relatively high polarization or pH, the limiting photoactivity shows a linear response to increasing photon flux and is attributed to a mechanism involving reduction of substrate water. This work illustrates the complex interplay between transport of photons, electrons, and chemical substrates in photoelectrosynthetic reactions and highlights diagnostic tools for better understanding these processes. 

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2. Water Splitting Electrocatalysis within Layered Inorganic Nanomaterials

   https://doi.org/10.5772/intechopen.88116  

   Ramos-Garcés, Mario V; Sanchez, Joel; Barraza_Alvarez, Isabel; Wu, Yanyu; Villagrán, Dino; Jaramillo, Thomas F; Colón, Jorge L (February 2020, IntechOpen)

   Eyvaz, M; Yüksel, E (Ed.)

   The conversion of solar energy into chemical fuel is one of the “Holy Grails” of 21st century chemistry. Solar energy can be used to split water into oxygen and protons, which are then used to make hydrogen fuel. Nature is able to catalyze both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) required for the conversion of solar energy into chemical fuel through the employment of enzymes that are composed of inexpensive transition metals Instead of using expensive catalysts such as platinum, cheaper alternatives (such as cobalt, iron, or nickel) would provide the opportunity to make solar energy competitive with fossil fuels. However, obtaining efficient catalysts based on earth abundant materials is still a daunting task. Progress in finding an ideal catalyst for the OER has been challenging as it appears that the overpotential for these catalysts have plateaued. Recent theory has shown that nanoscopic confinement of catalysts into 3D frameworks increases stability and efficiency of catalysts for OER. We are studying the use of the layered inorganic nanomaterial zirconium phosphate (ZrP) for water splitting. In this chapter we review the advancements made with ZrP as a support for transition metals for the OER. Our studies have found that ZrP is a suitable support for transition metals as it provides an accessible surface where the OER can occur. Further findings have also show that exfoliation of ZrP increases the availability of sites where active species can be adsorbed and performance is improved with this strategy. 

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3. Integrated Photocatalytic Materials for Fuel Production

   Khusnutdinova, D; Beiler, A; Wadsworth, B; Moore, G. (January 2018, Materials Research Society symposia proceedings)

   Controlling matter and information across the nano-, meso-, and macro-scales is a challenge for science and the imagination. In this presentation, we highlight recent advances in our research efforts to develop synthetic methodologies for constructing an integrated photocathode for light activating chemical transformations that include capturing, converting, and storing solar energy as fuel. A recent example involves development of a direct one-step method to chemically graft porphyrin catalysts that chemically transform water to hydrogen as well as carbon dioxide to carbon monoxide onto a visible-light-absorbing gallium phosphide (GaP) semiconductor. The porphyrin complexes are prepared using a synthetic strategy that yields a tetrapyrrole macrocycle with a pendant 4-vinylphenyl attachment group. This structural modification allows use of the UV-induced immobilization chemistry of olefins to attach intact metalloporphyrin complexes to the semiconductor surface. Solar hydrogen production is demonstrated via photoelectrochemical testing in pH neutral aqueous solutions under simulated solar illumination. Key features of the constructs presented here include use of metalloporphyrins with built-in chemical sites for direct grafting to a GaP semiconductor, creating novel hybrid photoactive assemblies capable of converting photonic energy to fuel. 

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4. Molecular Surface Coatings for Semiconductor Photoelectrochemistry and Photocatalysis

   Moore, G. F.; Beiler, A. M.; Khusnutdinova, D.; Wadsworth, B. L. (January 2017, ACS 253rd National Meeting & Exposition)

   Catalysts accelerate chemical transformations, but the ability to effectively interface them with surfaces for driving industrially relevant reactions using electricity or sunlight as a power source remains a major challenge. This presentation will report on recent efforts from our research group aimed at developing molecular surface coatings for photoactivating chemical transformations that include capturing, converting, and storing solar energy as a fuel. Addressing this obstacle improves fundamental understanding of catalysis in complex environments and enables technological advancements that depend on the precise control and selectivity of nanoscale components. By designing extended environments for the coordination of molecular catalysts, key features of biological enzymes such as extended ligation spheres, channels for substrate delivery and product removal, as well as regeneration strategies can be integrated with the design and synthesis of human-engineered catalysts. Functionality of these hybrid materials for applications in semiconductor photoelectrochemistry and photocatalysis are examined using electrochemical characterization techniques and an improved understanding of structure and function relationships is achieved using surface-sensitive characterization methods, including grazing angle Fourier transform infrared and X-ray photoelectron spectroscopies. 

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5. Guiding Principles for Designing Highly Efficient Metal‐Free Carbon Catalysts

   https://doi.org/10.1002/adma.201805252  

   Zhang, Lipeng; Lin, Chun‐Yu; Zhang, Detao; Gong, Lele; Zhu, Yonghao; Zhao, Zhenghang; Xu, Quan; Li, Hejun; Xia, Zhenhai (December 2018, Advanced Materials)

   Abstract Carbon nanomaterials are promising metal‐free catalysts for energy conversion and storage, but the catalysts are usually developed via traditional trial‐and‐error methods. To rationally design and accelerate the search for the highly efficient catalysts, it is necessary to establish design principles for the carbon‐based catalysts. Here, theoretical analysis and material design of metal‐free carbon nanomaterials as efficient photo‐/electrocatalysts to facilitate the critical chemical reactions in clean and sustainable energy technologies are reviewed. These reactions include the oxygen reduction reaction in fuel cells, the oxygen evolution reaction in metal–air batteries, the iodine reduction reaction in dye‐sensitized solar cells, the hydrogen evolution reaction in water splitting, and the carbon dioxide reduction in artificial photosynthesis. Basic catalytic principles, computationally guided design approaches and intrinsic descriptors, catalytic material design strategies, and future directions are discussed for the rational design and synthesis of highly efficient carbon‐based catalysts for clean energy technologies. 

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