Development of an efficient yet durable photoelectrode is of paramount importance for deployment of solar-fuel production. Here, we report the photoelectrochemically self-improving behaviour of a ...silicon/gallium nitride photocathode active for hydrogen production with a Faradaic efficiency approaching ~100%. By using a correlative approach based on different spectroscopic and microscopic techniques, as well as density functional theory calculations, we provide a mechanistic understanding of the chemical transformation that is the origin of the self-improving behaviour. A thin layer of gallium oxynitride forms on the side walls of the gallium nitride grains, via a partial oxygen substitution at nitrogen sites, and displays a higher density of catalytic sites for the hydrogen-evolving reaction. This work demonstrates that the chemical transformation of gallium nitride into gallium oxynitride leads to sustained operation and enhanced catalytic activity, thus showing promise for oxynitride layers as protective catalytic coatings for hydrogen evolution.Development of efficient yet durable photoelectrodes is of paramount importance for deployment of solar-fuel production. The photoelectrochemically self-improving behaviour of a silicon/gallium nitride photocathode highly efficient for hydrogen production is now reported.
Full text
Available for:
GEOZS, IJS, IMTLJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK, ZAGLJ
An understanding and control of complex physiochemical processes at the photoelectrode/electrolyte interface in photoelectrochemical cells (PECs) are essential for developing advanced solar-driven ...water-splitting technology. Here, we integrate ambient pressure X-ray photoelectron spectroscopy (APXPS) and high-level first-principles calculations to elucidate the evolution of the H2O/InP (001) interfacial chemistry under in situ and ambient conditions. In addition to molecular H2O, OH and H are the only two species found on InP (001) at room temperature. Under elevated temperatures, although the formation of In–O–P is thermodynamically more favorable over In–O–In, the latter can be preferentially generated in a kinetically driven and nonequilibrated environment such as ultrahigh vacuum (UHV); however, when InP is exposed to H2O at both elevated pressures and temperatures, its surface chemistry becomes thermodynamically driven and only In–O–P (or PO x ) oxygen bridges form. Our simulations suggest that In–O–In, rather than In–O–P, constitutes a charge carrier (hole) trap that causes photocorrosion in PEC devices. Therefore, understanding and modulating the chemical nature of oxygen bridges at the H2O/InP (001) interface will shed light on the fabrication of InP-based photoelectrodes with simultaneously enhanced stability and efficiency.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Improving the stability of semiconductor materials is one of the major challenges for sustainable and economic photoelectrochemical water splitting. N-terminated GaN nanostructures have emerged as a ...practical protective layer for conventional high efficiency but unstable Si and III-V photoelectrodes due to their near-perfect conduction band-alignment, which enables efficient extraction of photo-generated electrons, and N-terminated surfaces, which protects against chemical and photo-corrosion. Here, we demonstrate that Pt-decorated GaN nanostructures on an n
+
-p Si photocathode can exhibit an ultrahigh stability of 3000 h (
i.e.
, over 500 days for usable sunlight ∼5.5 h per day) at a large photocurrent density (>35 mA cm
−2
) in three-electrode configuration under AM 1.5G one-sun illumination. The measured applied bias photon-to-current efficiency of 11.9%, with an excellent onset potential of ∼0.56 V
vs.
RHE, is one of the highest values reported for a Si photocathode under AM 1.5G one-sun illumination. This study provides a paradigm shift for the design and development of semiconductor photoelectrodes for PEC water splitting: stability is no longer limited by the light absorber, but rather by co-catalyst particles.
GaN/Si photocathode exhibits an ultrahigh stability of 3000 h under AM 1.5G one-sun illumination.
Many energy storage and conversion devices rely on processes that take place at complex interfaces, where structural and chemical properties are often difficult to probe under operating conditions. A ...primary example is solar water splitting using high-performance photoelectrochemical cells, where surface chemistry, including native oxide formation, affects hydrogen generation. In this Perspective, we discuss some of the challenges associated with interrogating interface chemistry, and how they may be overcome by integrating high-level first-principles calculations of explicit interfaces with ambient pressure X-ray photoelectron spectroscopy and direct spectroscopic simulations. We illustrate the benefit of this combined approach toward insights into native oxide chemistry at prototypical InP/water and GaP/water interfaces. This example suggests a more general roadmap for obtaining a realistic and reliable description of the chemistry of complex interfaces by combining state-of-the-art computational and experimental techniques.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
All elements, except for helium, appear to solidify into crystalline forms at zero temperature, and it is generally assumed that the introduction of lattice defects results in an increase in internal ...energy. β-Rhombohedral boron, a thermodynamically stable form of elemental boron at high temperature, is known to have a large amount of partial occupied sites, seemingly in conflict with our common knowledge. By using lattice Monte Carlo techniques combined with ab initio calculations, we find that the β-phase is stabilized by a macroscopic amount of intrinsic defects that are responsible not only for entropic effects but also for a reduction in internal energy. These defects enable the conversion of two-center to three-center bonds and are accompanied by the presence of localized, nonconductive electronic states in the optical gap. In addition we find that the ab initio Ising model describing the partial occupancy of β-boron has macroscopic residual entropy, suggesting that boron is a frustrated system analogous to ice and spin ice.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Determination of Phase Stability of Elemental Boron White, Mary Anne; Cerqueira, Anthony B.; Whitman, Catherine A. ...
Angewandte Chemie (International ed.),
March 16, 2015, Volume:
54, Issue:
12
Journal Article
Peer reviewed
Boron is an important element, used in applications from superhard materials to superconductors. Boron exists in several forms (allotropes) and, surprisingly, it was not known which form (α or β) is ...stable at ambient conditions. Through experiment, we quantify the relative stability of α‐boron and β‐boron as a function of temperature. The ground‐state energies of α‐boron and β‐boron are nearly identical. For all temperatures up to 2000 K, the complicated β‐boron structure is more stable than the simpler α‐boron structure at ambient pressure. Below 1000 K, β‐boron is entropically stabilized with respect to α‐boron owing to its partially occupied sites, whereas at higher temperatures β‐boron is enthalpically stabilized with respect to α‐boron. We show that α‐boron only becomes stable on application of pressure.
Elementary information: Through experimental thermodynamics, the fundamental question of which structure of elemental boron is most stable has been answered: it is the unique disordered β‐form.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Magnetic diffusion plays an important role in inertial confinement fusion with strong magnetic fields. In this paper, we improve a previous analysis of the generation and diffusion of the magnetic ...field Morita et al., Phys. Plasmas 25, 094505 (2018). For the generation process, we calculate the temporal evolution of the coil current using a self-consistent circuit model. The results show that the peak of the calculated magnetic field is delayed by 1.2 ns compared with that of the incident laser pulse. For the diffusion process, we evaluate the electrical conductivity of warm dense gold over a wide temperature range (300 K–100 eV) by combining the Kubo–Greenwood formula based on a quantum molecular dynamics simulation with the modified Spitzer model. Our simulation shows that the maximum magnetic field (530 T) that penetrates the cone is delayed by 2.5 ns compared with the laser peak. This result is consistent with experiments Sakata et al., Nat. Commun. 9, 3937 (2018) that showed that applying a strong magnetic field improved the heating efficiency of fusion fuel.
Solid-state metal hydrides are prime candidates to replace compressed hydrogen for fuel cell vehicles due to their high volumetric capacities. Sodium aluminum hydride has long been studied as an ...archetype for higher-capacity metal hydrides, with improved reversibility demonstrated through the addition of titanium catalysts; however, atomistic mechanisms for surface processes, including hydrogen desorption, are still uncertain. Here, operando and ex situ measurements from a suite of diagnostic tools probing multiple length scales are combined with ab initio simulations to provide a detailed and unbiased view of the evolution of the surface chemistry during hydrogen release. In contrast to some previously proposed mechanisms, the titanium dopant does not directly facilitate desorption at the surface. Instead, oxidized surface species, even on well-protected NaAlH4 samples, evolve during dehydrogenation to form surface hydroxides with differing levels of hydrogen saturation. Additionally, the presence of these oxidized species leads to considerably lower computed barriers for H2 formation compared to pristine hydride surfaces, suggesting that oxygen may actively participate in hydrogen release, rather than merely inhibiting diffusion as is commonly presumed. These results demonstrate how close experiment–theory feedback can elucidate mechanistic understanding of complex metal hydride chemistry and potentially impactful roles of unavoidable surface impurities.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
PDF
