Chemical Water Splitting Research Articles

(Invited) Group III-Nitride Nanostructures and Devices: From Classical Optoelectronics to Artificial Photosynthesis

Third generation, group III-nitrides (InN, GaN, AlN and their alloys) semiconductor nanostructures have emerged as an important class of materials for a host of critical applications ranging from classical optoelectronics to quantum to artificial photosynthesis (i.e., photo(electro) chemical water splitting and CO2 reduction). Group III-nitrides exhibit unique properties including broad, tunable, direct energy bandgap, large exciton binding energy, giant spontaneous and piezoelectric polarization field, perfect conduction/valence band edge positioning to drive the redox reactions for fuel productions, interesting surface/interface properties, making them the only materials of choice for the afore-mentioned applications. In this talk, I will first present some recent advances of their epitaxy growth/synthesis, electronic, optical, and quantum properties, including the development of high efficiency visible and deep UV LEDs/laser diodes, and the realization of monolithic micro-LEDs.A highly efficient and robust artificial photosynthesis integrated device and system (APIDS) is the key for achieving carbon-neutral alternative to fossil fuels. III-nitrides can be used as an ideal building block of APIDS because of their recently discovered photocatalytic properties demonstrating great potential in artificial photosynthesis compared to decade long studied photocatalysts. Herein, the most recent advance in the utilization of III-nitrides-based artificial leaves for building a carbon-neutral substance and energy recycling system will be presented, focusing on solar water splitting toward H2 and solar CO2 reduction toward fuels and chemicals. I will also present on III-nitrides nanostructures-based non-classical on-demand quantum light sources (single photon and entangled photons sources) generation and their usage in quantum communication and optical quantum computing protocols. Overall, the talk focuses on the unique potential of group III-nitrides materials for emerging applications.

Rational synthesis of highly efficient dual–Z–scheme InVO4/FeVO4/Ex–CQDs–g–C3N4 heterojunction for photo(electro)chemical water splitting and pollutant removal applications

Rational synthesis of highly efficient dual–Z–scheme InVO4/FeVO4/Ex–CQDs–g–C3N4 heterojunction for photo(electro)chemical water splitting and pollutant removal applications

Effective thermal-electric control system for hydrogen production based on renewable solar energy

Effective thermal-electric control system for hydrogen production based on renewable solar energy

Need for Rationally Designed SnWO4 Photo(electro)catalysts to Overcome the Performance Limitations for O2 and H2 Evolution Reactions

Although the α-SnWO4 material has recently been considered as a new good candidate for visible-light-driven photo(electro)chemical water splitting, the performance is still low and requires further improvement. Here, we present a deep fundamental work on the influence of the various possible facets exposed on this material for oxygen and hydrogen evolution reactions using hybrid density functional theory. The energetic, electronic, water redox, and charge carrier transport features of the four possible (100), (010), (001), and (110) facets (low-Miller index surfaces) are investigated, and significant anisotropic nature is revealed. The relevant properties of each facet to the water oxidation/reduction reactions are correlated with the surface W coordination number. Taking into account the stability and combining optoelectronic and water redox features together of each surface, our work demonstrates that the (110) facet is photocatalytically the best candidate for the OER, while the (100) facet is the best candidate for the HER. Their transport characteristics are found to be much better than those obtained for the three major (121), (210), and (111) facets of synthesized α-SnWO4 samples. Substitutional Ge at the Sn site and Mo at the W site on the two (110) and (100) facets are expected to increase the rates of the water oxidation/reduction reactions. An analysis of the reaction mechanism for the OER in (110)-oriented α-SnWO4 reveals a promising performance of this facet for electrocatalytic water oxidation. These outcomes will greatly motivate experimentalists for carefully designing (110)- and (100)-oriented α-SnWO4 samples to enhance the photo(electro)catalytic OER and photocatalytic HER performances.

Transition metal-based layered double hydroxides for photo(electro)chemical water splitting: a mini review.

The conversion of solar energy into usable chemical fuels, such as hydrogen gas, via photo(electro)chemical water splitting is a promising approach for creating a carbon neutral energy ecosystem. The deployment of this technology industrially and at scale requires photoelectrodes that are highly active, cost-effective, and stable. To create these new photoelectrodes, transition metal-based electrocatalysts have been proposed as potential cocatalysts for improving the performance of water splitting catalysts. Layered double hydroxides (LDHs) are a class of clays with brucite like layers and intercalated anions. Transition metal-based LDHs are increasingly popular in the field of photo(electro)chemical water splitting due to their unique physicochemical properties. This article aims to review recent advances in transition metal-based LDHs for photo(electro)chemical water splitting. This article provides a brief overview of the research in a format approachable for the general scientific audience. Specifically, this review examines the following areas: (i) routes for synthesis of transition metal-based LDHs, (ii) recent developments in transition metal-based LDHs for photo(electro)chemical water splitting, and (iii) an overview of the structure-property relationships therein.

Pinning of the Fermi Level in CuFeO2 by Polaron Formation Limiting the Photovoltage for Photochemical Water Splitting

AbstractCuFeO2 is recognized as a potential photocathode for photo(electro)chemical water splitting. However, photocurrents with CuFeO2‐based systems are rather low so far. In order to optimize charge carrier separation and water reduction kinetics, defined CuFeO2/Pt, CuFeO2/Ag, and CuFeO2/NiOx(OH)y heterostructures are made in this work through a photodeposition procedure based on a 2H CuFeO2 hexagonal nanoplatelet shaped powder. However, water splitting performance tests in a closed batch photoreactor show that these heterostructured powders exhibit limited water reduction efficiencies. To test whether Fermi level pinning intrinsically limits the water reduction capacity of CuFeO2, the Fermi level tunability in CuFeO2 is evaluated by creating CuFeO2/ITO and CuFeO2/H2O interfaces and analyzing the electronic and chemical properties of the interfaces through photoelectron spectroscopy. The results indicate that Fermi level pinning at the Fe3+/Fe2+ electron polaron formation level may intrinsically prohibit CuFeO2 from acquiring enough photovoltage to reach the water reduction potential. This result is complemented with density functional theory calculations as well.

(Invited) HydroGEN Benchmarking: Developing Best Practices for Water Splitting Technologies

The need for renewable sources of hydrogen is increasing rapidly based on global goals for decarbonization. Interest and investment has also increased with the advent of the Hydrogen Council and the DOE H2@Scale initiative. There are several pathways which could eventually be part of the overall mix, but are currently at different maturities and have different challenges. Low cost renewable energy provides a potential pathway for low temperature electrolysis to make a large impact in the short term, while high temperature baseload such as nuclear matches well with high temperature electrolysis based on solid oxide technology. Photoelectrochemcal and solar thermal chemical water splitting are earlier stage technologies, but could be viable in the long term especially in areas without current infrastructure, if the materials targets can be achieved and manufacturing capability is developed. In the nearer term, understanding of these challenges can also provide valuable perspective for the more mature pathways. In any of these technologies, appropriate comparisons of test results is important to measure overall progress in the field and identify promising directions, as well as to provide credibility to published results. The DOE Fuel Cell Technologies Office previously funded the development of a detailed reference guide of common methodologies, protocols, and important limitations for measuring critical performance properties of advanced hydrogen storage materials, which has served as a resource to the hydrogen storage materials development community to aid in clearly communicating the relevant performance properties of new materials as they are discovered and tested. The project was coordinated with the IEA and involved 46 co-authors and contributors from 9 countries. The final documents are available to the public on-line: https://www.energy.gov/eere/fuelcells/downloads/recommended-best-practices-characterization-storage-properties-hydrogen-0 A similar effort is now underway for water splitting technologies, framed by questionnaires distributed to the community, frameworks for materials and device testing, and workshops and discussions for feedback. A series of protocols is being drafted to provide easy to follow procedures for materials characterization. The team is also identifying gaps in methods and materials for possible development, and drafting technology roadmaps to highlight research needs. This talk will describe the success of the storage effort, and status for the water splitting benchmarking project.

Recent advances in nanostructured metal nitrides for water splitting

This review summarizes the recent research progress made in nanostructured metal nitrides for electrochemical and photo(electro)chemical water splitting.

Boron Nanoparticles for Room‐Temperature Hydrogen Generation from Water

Boron nanoparticles (BNPs) are of great interest for applications such as neutron capture therapy of cancer cells, hydrogen generation from water, and high energy density fuels. Boron is particularly interesting for chemical water splitting, because of its high gravimetric hydrogen generation potential of 277 g H2 per kg B. However, only a few studies of water splitting by reaction with boron are available, and those have used high temperature steam with external heating. Room‐temperature boron hydrolysis is of great interest from both scientific and practical perspectives. The studies presented here demonstrate that high purity amorphous BNPs can be oxidized by water to produce H2 at room temperature, without external energy input, in the presence of catalytic quantities of an alkali metal or alkali metal hydride. The BNPs are produced in a single step gas phase process via CO2 laser‐induced pyrolysis of mixtures of B2H6 and SF6. The BNPs are spherical with a primary particle diameter of 10–15 nm, narrow size distribution, and specific surface area exceeding 250 m2 g−1. This first demonstration of room‐temperature chemical splitting of liquid water using boron opens up exciting new possibilities for on‐demand hydrogen generation at high gravimetric capacity.

Mixed Conductors under Light: On the Way to Solid Oxide Photo-Electrochemical Cells

Photon driven electrochemical reactions may contribute significantly to future sustainable energy supply. Many different oxides (e.g. TiO2 or Fe2O3) have been used so far to run the most prominent photon driven reaction, namely photo-(electro-)chemical water splitting. However, despite strong efforts still efficiency and/or materials stability are not sufficient for widespread application. In the vast majority of studies the photo-electrochemical reaction takes place in aqueous solution close to room temperature. Photo-electrochemistry based on solid oxide electrolytes, on the other hand, is an almost unexplored field. In such cells, operation temperatures of several hundred °C are required to enable sufficient oxide ion conduction. Very little knowledge is available on the experimental realizability of such cells and on their problems. UV light was shown to accelerate oxygen incorporation into SrTiO3 [1] and a theoretical model of a solid state photo-electrochemical cell including a mixed conducting electrode was described in Ref. [2]. In this contributions, experimental results are presented which introduce two approaches to solid oxide photo-electrochemical cells. i) In one approach, first an oxide based high temperature solar cell was developed. A thin film of Sr-doped LaCrO3 on an undoped SrTiO3 single crystal leads to a heterojunction with large space charge potential. Upon UV illumination this can be used to gain photo-voltages as high as 900 mV at 400°C. Operation at 500°C still leads to 600 mV and short circuit currents in the several mA/cm2 range [3]. Interestingly, the entire SrTiO3 single crystal (0.5 mm thickness) becomes conducting upon illumination. The photon induced driving force was then used to pump oxygen from low to high oxygen partial pressures and thus to chemically store photon energy [3]. In the corresponding solid oxide photo-electrochemical cell, the bottom electrode of the oxide solar cell (Sr-doped LaFeO3) was employed as the top electrode of a zirconia based solid electrochemical cell. By short circuiting the outer electrodes of this oxide hetero-structure electrochemical oxygen pumping became possible. ii) In the second approach, a mixed conducting oxide (TiO2 or SrTiO3) with Pt current collector was deposited directly on an oxide ion conductor (YSZ). Upon UV light, voltages in the 200 mV range result at ca. 400°C. Oxygen exchange of the illuminated mixed conductor upon UV light modified this voltage such that even after switching off the light a voltage remained. Thus, two effects come into play: a photo-voltage that is coupled to the external circuit via a solid electrolyte and a voltage due to the oxygen chemical potential difference which is built up upon illumination. The latter can be considered as the result of a kind of light driven battery charging. [1] X. Ye, J. Melas-Kyriazi, Z. A. Feng, N. A. Melosh, W. C. Chueh, Phys. Chem. Chem. Phys. 2013, 15, 15459-15469. [2] R. Merkle, R.A. De Souza, J. Maier, Angew. Chem. Int. Ed. 2001, 40, 2126-2129 [3] G. C. Brunauer, B. Rotter, G. Walch, A. K. Opitz, G. Ponweiser, J. Summhammer, J. Fleig, Adv. Funct. Mat. DOI: 10.1002/adfm.201503597

Studies on the preparation of active oxygen-deficient copper ferrite and its application for hydrogen production through thermal chemical water splitting

Hydrogen generation through thermal chemical water splitting technology has recently received increasingly international interest in the nuclear hydrogen production field. Besides the main known sulfur-iodine (S-I) cycle developed by the General Atomics Company and the UT3 cycle (iron, calcium, and bromine) developed at the University of Tokyo, the thermal cycle based on metal oxide two-step water splitting methods is also receiving research and development attention worldwide. In this work, copper ferrite was prepared by the co-precipitation method and oxygen-deficient copper ferrite was synthesized through first and second calcination steps for the application of hydrogen production by a two-step water splitting process. The crystal structure, properties, chemical composition and δ were investigated in detail by utilizing X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA), atomic absorption spectrometer (AAS), ultraviolet spectrophotometry (UV), gas chromatography (GC), and so on. The experimental two-step thermal chemical cycle reactor for hydrogen generation was designed and developed in this lab. The hydrogen generation process of water splitting through CuFe2O4−δ and the cycle performance of copper ferrite regeneration were firstly studied and discussed.